Salmela et al. BMC Neuroscience 2012, 13:93http://www.biomedcentral.com/1471-2202/13/93
RESEARCH ARTICLE Open Access
Cellular elements for seeing in the dark:voltage-dependent conductances in cockroachphotoreceptorsIikka Salmela, Esa-Ville Immonen, Roman Frolov, Stephan Krause, Yani Krause, Mikko Vähäsöyrinki andMatti Weckström*
Abstract
Background: The importance of voltage-dependent conductances in sensory information processing is well-established in insect photoreceptors. Here we present the characterization of electrical properties in photoreceptorsof the cockroach (Periplaneta americana), a nocturnal insect with a visual system adapted for dim light.
Results: Whole-cell patch-clamped photoreceptors had high capacitances and input resistances, indicating largephotosensitive rhabdomeres suitable for efficient photon capture and amplification of small photocurrents at lowlight levels. Two voltage-dependent potassium conductances were found in the photoreceptors: a delayed rectifiertype (KDR) and a fast transient inactivating type (KA). Activation of KDR occurred during physiological voltageresponses induced by light stimulation, whereas KA was nearly fully inactivated already at the dark resting potential.In addition, hyperpolarization of photoreceptors activated a small-amplitude inward-rectifying (IR) current mediatedat least partially by chloride. Computer simulations showed that KDR shapes light responses by opposing the light-induced depolarization and speeding up the membrane time constant, whereas KA and IR have a negligible role inthe majority of cells. However, larger KA conductances were found in smaller and rapidly adapting photoreceptors,where KA could have a functional role.
Conclusions: The relative expression of KA and KDR in cockroach photoreceptors was opposite to the previouslyhypothesized framework for dark-active insects, necessitating further comparative work on the conductances. Ingeneral, the varying deployment of stereotypical K+ conductances in insect photoreceptors highlights theirfunctional flexibility in neural coding.
BackgroundIn sensory cells, voltage-gated ion channels shape thevoltage responses arising from currents generated in sen-sory transduction processes. The biophysical propertiesof the channels allow them to change the electrical prop-erties of the membrane in a voltage- and time-dependentmanner, which in graded potential neurons and sensorycells lead to amplification of relevant and attenuation ofirrelevant signals. In this way ion channels can effectivelyregulate the membrane according to the requirementsset by the input (e.g. the transduction currents) and, in
* Correspondence: [email protected] of Physics, University of Oulu, Oulu, Finland
general, the sensory ecology of the animal [1]. Expressinga suitable composition of specific channel types enablestuning of the information coding performance versus themetabolic cost of voltage signalling [2-4].Photoreceptors form a well-established model system
for examining the specific molecular mechanismsinvolved in processing sensory information that is carriedby graded voltage signals in both vertebrates [5-7] andinsects [8,9]. In flies, photoreceptors of fast-flying diurnalspecies possess a distinctively different set of voltage-gated potassium channels (Kv-channels) than those ofslower and crepuscular species [10,11]. Photoreceptors ofdiurnal flies rely on non- or slowly inactivating delayedrectifier (DR) channels, whereas nocturnal or crepuscular
l Ltd. This is an Open Access article distributed under the terms of the Creativeommons.org/licenses/by/2.0), which permits unrestricted use, distribution, andiginal work is properly cited.
Salmela et al. BMC Neuroscience 2012, 13:93 Page 2 of 14http://www.biomedcentral.com/1471-2202/13/93
flies express mainly inactivating Kv channels [10-12]. TheDR channels in the dawn and dusk active fruit fly (Dros-ophila melanogaster) are responsible for attenuatingthe light-dependent depolarization and speeding up themembrane filter at higher light levels [13-15], whereasthe rapidly inactivating transient A-type channels dy-namically shape the transient signals to enable full use ofthe available voltage range [3]. This allows Drosophilaphotoreceptors to extract information efficiently fromthe dynamic light stimuli and to encode it into voltageresponses of limited amplitude and speed without exces-sive metabolic costs [3,4,13].Cockroaches are mainly dark-active, but can also aggre-
gate in daylight [16]. While they rely heavily on mechano-and chemosensory systems when gathering informationabout their surroundings [17,18], vision also appears toplay a significant role in their behaviour [19-21]. The im-portance of vision can also be inferred from the largecompound eyes (over 3000 ommatidia per eye [22]) thathave been maintained through evolution for hundreds ofmillions of years, despite the associated metabolic cost[23,24]. Therefore, cockroaches form an interesting modelsystem for studying mechanisms of vision under darkconditions when the rate of photons arriving in the eye issmall.While several studies on vision of nocturnal insects
have been published [25], a detailed characterization ofthe biophysical properties of photoreceptors has not beenpreviously performed in any dark-active insect. Our earl-ier investigations have revealed several peculiar featuresof the cockroach photoreceptors, e.g. exceptional actionpotential coding in the axons [26] and nearly randomlyvarying functional properties [27], both of which wereinterpreted as adaptations to nocturnal vision. In thisstudy, we have characterized the biophysical propertiesof the voltage-dependent conductances in the somata ofdissociated cockroach photoreceptors using the patch-clamp method. Mathematical modelling was performedto circumvent experimental limitations in monitoring thesimultaneous interplay of different conductances duringvoltage responses to light. Relative contributions of thecharacterized conductances in shaping physiologicallyrelevant signals were calculated and discussed with re-spect to the previously proposed hypothesis for the rolesof different types of Kv-channels in photoreceptors of in-sect species with varying visual ecology.
MethodsElectrophysiologyAll experiments were performed using adult male cock-roaches Periplaneta americana obtained from BladesBiological Ltd (Edenbridge, Kent, UK). The animals werekept at 25°C in a 12 h day-night rhythm. The ommatidiadissociation procedure was similar as described
previously for Drosophila [15]. In brief, after decapitationand removal of antennae, eyes were cut off with a sharprazor blade. Retinas were scooped out and cut into sev-eral pieces. The retinal fragments were then incubatedfor 8-10 min in extracellular solution supplemented with0.2 mg/ml collagenase type 2 (Worthington BiochemicalCorp., Lakewood, NJ USA) and 0.2 mg/ml Pankreatin(Sigma-Aldrich) followed by gentle trituration with sys-tematically varying the tips of the trituration pipettes,until ommatidia started to fall off. Separate ommatidiawere allowed to settle in the recording chamber on thestage of an inverted microscope (Axiovert 35 M, Zeiss,Germany). The preparation and the recordings weredone at room temperature (20-23°C).Patch-clamp recordings were performed using an
Axopatch 1-D amplifier (Molecular Devices, USA) andpCLAMP 9 software (Molecular Devices, USA). Patchmicroelectrodes were made from borosilicate glass (Har-vard Apparatus Ltd, UK) using a P-87 electrode puller(Sutter Instrument Company, Ca, USA) and had resis-tances between 5 and 15 MΩ. Access resistances weremonitored throughout the experiment and after 80-90%compensation they were typically well below 10 MΩ.Voltage errors caused by access resistance were correctedoffline for currents larger than ± 200 pA.The standard bath solution contained (in mM): 120
NaCl, 5 KCl, 4 MgCl2, 1.5 CaCl2, 10 N-Tris-(hydroxy-methyl)-methyl-2-amino-ethanesulfoncic acid (TES),25 L-proline and 5 β-alanine, pH 7.15 (NaOH). Forexperiments involving K+ gradients we prepared a highK+ concentration bath solution containing (in mM): 120NaCl, 50 KCl, 4 MgCl2, 1.5 CaCl2, 10 TES, pH wasadjusted to 7.15 (NaOH). The standard and high K+ bathsolutions were mixed in relevant proportions to receiveK+ concentrations of 5, 15, 25 and 50 mM.Electrode solutions contained (in mM) either
140 K-gluconate (referred to as Cl-free) or 140 KCl (re-ferred to as Cl-containing) together with 10 TES, 2MgCl2, 4 Mg-ATP, 0.4 Na-GTP and 1 NAD, pH wasadjusted to 7.15 (KOH). Properties of K+ currents werestudied using Cl-free solutions, while experiments in-volving the study of the inward current or voltage andcurrent responses to light stimuli were performed withthe Cl-containing electrode solution. All chemicals werepurchased from Sigma-Aldrich.All recordings were done from green-sensitive photo-
receptors, identified by their response to stimulationwith a green LED (525 nm). Whole-cell input resis-tances, capacitances and access resistances were deter-mined offline from voltage clamp experiments with ahyperpolarizing voltage step, using the test pulse methoddescribed in pCLAMP 9 manual.Light responses were recorded by stimulating the
photoreceptors with an LED through the fluorescence
Salmela et al. BMC Neuroscience 2012, 13:93 Page 3 of 14http://www.biomedcentral.com/1471-2202/13/93
port of the microscope. LED intensity was controlledwith a voltage-current converter and the acquisitionhardware and software. Voltage light responses wererecorded in the amplifier’s current clamp mode (I = 0)and the light-induced currents (LIC) in the voltage-clamp mode, with a holding potential of 74 mV. Lightstimuli were either pulses or a dynamic waveform takenfrom the van Hateren naturalistic time series intensity(NTSI) database [28].
Data analysis and mathematical modelingData were analyzed using OriginPro 8.5 (Originlab, US).Conductances were calculated from currents recorded indifferent holding potentials V as g= I/(V -Erev), where I isthe current and Erev is the reversal potential. The voltagevalues presented in the text were corrected for the liquidjunction potential (LJP) unless stated otherwise.Kinetic parameters of gating of K+ currents, recordings
of light-induced current in response to a 10 s naturalisticlight contrast sequence [28], and the corresponding
0 200 4000
2
4
6
8
10
12
Cou
nt
Capacit
1 s
20m
V
0 pA
Voltage re
Current re
NTSI stimulu
1nA
-56 mV
60 µm
D
BA
Figure 1 General features of isolated photoreceptors. A) Isolated ommtheir fused rhabdom concentrically around the longitudinal axis of the omLenses at the distal end (top) and axons at the proximal end (bottom) areB) Whole-cell capacitances of green-sensitive photoreceptor cells (n = 45). Cthan 1 photon on average showed both single photon absorptions (+) andthe random properties of phototransduction. The holding potential in voltacorresponding light-induced-currents (middle) to a 10 s long naturalistic ligRelative intensities were 1 (gray traces) and 10 (black trace). E) In whole-ceinjections (bottom) exhibited both inward- and outward rectification, indicamplitude depolarizations are single photon responses to ambient light.
voltage responses obtained from patch clamp experi-ments were used in a Hodgkin-Huxley type mathemat-ical model implemented in Matlab (Mathworks, USA).The model was then used to study the relative contribu-tion of ionic conductances during simulated lightresponses. The model is described in detail in theAppendix.
ResultsGeneral properties of cockroach photoreceptorsIsolated cockroach ommatidia were between 100 and150 μm long and ca. 30 μm wide (Figure 1A) and nor-mally did not contain the photoreceptor axons. In patchclamp experiments, the seal resistance was typicallygreater than 10 GΩ and the whole-cell input resistance(Rin) in darkness varied between 200 MΩ and 10 GΩ(Rin= 1.6 ± 2.4 GΩ, n = 32). The Rin values were largerthan the previous estimates from in vivo intracellularrecordings [26,27], possibly because of the absence of amembrane leak due to membrane piercing with a sharp
600 800 1000
ance
sponse
sponse
s waveform
+
+
+
-+
-
-
-
-
-
100 ms
50 pA1 ms flash
-0.1 nA
+0.1 nA
-63 mV
0.2 s10 mV
E
C
atidia of Periplaneta americana. Single photoreceptors cluster withmatidium. The rhabdom can be recognized as a dark central structure.ripped off during isolation. Note the pigmentation of the ommatidia.) Current responses to 1 ms dim light flash stimuli containing lessfailures (-). The variability of response latency and amplitude reflectsge clamp mode was -77 mV. D) Voltage responses (top) andht intensity series (bottom) recorded in whole-cell patch clamp.ll current clamp recordings, voltage responses (top) to currentating the presence of voltage-dependent conductances. The small-
Salmela et al. BMC Neuroscience 2012, 13:93 Page 4 of 14http://www.biomedcentral.com/1471-2202/13/93
glass microelectrode [29]. Whole-cell membrane capaci-tance was measured as a proxy for the membrane areaand cell size, which are known to vary within single om-matidia [30]. The capacitances ranged from 100 to800 pF (Figure 1B) and did not follow a normal distribu-tion, which may reflect different photoreceptor sizegroups in the ommatidia [30]. We cannot rule out thepossibility that some recordings with the largest capaci-tances could contain more than one cell. However, suchoccurrences have not been reported before with insectphotoreceptors in patch clamp, although in intracellularrecordings it is possible [31].Photoreceptors in isolated ommatidia were function-
ally robust, with light responses occasionally recordedfor over one hour of continuous light stimulation.Figure 1C shows quantum bumps, which are currentresponses to single-photon stimulation. Quantum bumpscould be recorded from all the cells used in the analysesand their presence was used as an indicator of photo-receptor health. Examples of macroscopic current and
0 100 2000
2
4
00
2
4
Cur
rent
(nA
)
Time (ms) Time
+3
-117
-77ED
BA
-77-
Figure 2 Outward currents in photoreceptors. Voltage-clamp recordingactivated with depolarization the cell. The outward current consisted of atA photoreceptor with large transient current B) A photoreceptor with bothinactivate during a 10 s long voltage clamp. Voltage was clamped from -47Depolarizing pulses from -57 to +3 mV given after a hyperpolarizing -117 mE) Positive prepulse inactivated the transient component, and subsequentcurrent could be isolated by subtraction of the currents from protocols in D
voltage responses elicited by a 10 s naturalistic lightstimulus [28] are shown in Figure 1D. Hyperpolarizingand depolarizing current steps in darkness producedvoltage responses characterized by a slow passive mem-brane time constant (170 ms for the -50 pA trace inFigure 1E). The rectification, i.e. the nonlinear, asymmet-ric behaviour of voltage in response to depolarizing andhyperpolarizing current injections (Figure 1E), demon-strated the presence of voltage-dependent conductances,which were then studied further.
Voltage-activated K+ (Kv) currentsVoltage clamp experiments revealed a voltage-activatedoutward current (Figure 2) responsible for the rectifica-tion observed at depolarized voltages in current clamprecordings (Figure 1E). Kinetics and amplitude of theoutward current varied from cell to cell (Figure 2A-B).In approximately half of the photoreceptors, the currentclearly consisted of two components: a fast-activatingtransient current and a slow-activating sustained current
100 200
(ms)0 2 4 6 8 10
0
2
4
Time (s)
F = D - E
F
C
10 ms
1 nA57
+3
s revealed of voltage-dependent outward currents that could beleast two components, the amount of which varied between cells. A)transient and sustained currents C) The sustained current did notmV to +3 mV with 10 mV intervals after a -117 mV prepulse. D)V pre-pulse elicited both a sustained and a transient outward current
depolarization activated only the sustained current. F) The transient) and E). The scale bar applies for panels D, E, and F.
Salmela et al. BMC Neuroscience 2012, 13:93 Page 5 of 14http://www.biomedcentral.com/1471-2202/13/93
(Figure 2B). The sustained current showed no inactiva-tion during prolonged voltage pulses (Figure 2C).The transient current displayed voltage-dependent in-
activation whereas the sustained current did not, thusallowing separation of the two current components byvoltage clamp protocols. Depolarizing pulses given aftera prepulse of -117 mV activated both the transient andthe sustained current (Figure 2D). Depolarizing pulsesfollowing a -57 mV prepulse evoked only the sustainedcurrent (Figure 2E), due to the inactivation of the transi-ent current by the pre-pulse. The current obtained bysubtraction of the currents evoked by the two protocolswas taken as the transient current (Figure 2F). Thevoltage-dependences of the sustained and the transientcurrent resembled delayed-rectifier and A-type Kv cur-rents, respectively, both of which are commonly foundin neurons [32,33], including insect photoreceptors[3,11-13,15].The sustained current’s identity and selectivity was
examined with tail currents recorded under different ex-ternal potassium concentrations (Figure 3). The fittedNernst slope was 52 ± 4 mV/mM (mean± SE), close tothe theoretical value of 58 mV/mM for potassium withthe solutions used. Under standard K+ concentrations([K]in/K]out = 5 mM/140 mM) the reversal potential ofthe current was -68 ± 5 mV (mean ± SD, n = 17). Theor-etical Nernst potential for potassium was -84 mV, imply-ing that the measured conductance was not entirelypotassium-specific under multi-ionic conditions. The re-versal potential was similar for chloride-containing and
1 10 100
-100
-80
-60
-40
-20
0
-59 mV-63 mV-67 mV-71 mV
10 ms100 pA-55 mV
Ere
v(mV
)
[K+]out
(mM)Figure 3 Reversal potential of the sustained current followedthe Nernst slope for potassium. The reversal potential wasmeasured from tail currents with varying external K+ concentrations(inset), resulting in Erev = -68 mV under the standard 5 mMconcentration (n = 17 (5 mM), 6 (15 and 50 mM) or 5 (25 mM), dataare mean± SD). The fitted Nernst slope (solid line) was 52 mV/mMand the theoretical Nernst slope (dashed line) for potassium was58 mV/mM. Theoretical Erev with 5 mM [K+]out was -84 mV.
chloride-free solutions (see Methods for a description ofsolutions). Because of the overlap with the sustainedcurrent, the transient current’s reversal potential couldnot be measured reliably but was assumed to be the sameas for the non-inactivating current, i.e. -68 mV, based onthe close resemblance of both to K+ currents in insects[33]. The similarity of the voltage-dependent behaviourof the currents to previous findings in insects(Figure 2D-F) and the Nernst slope of the sustained cur-rent’s reversal potential (following K+ concentration andthus indicating a mainly K+ permeant channels; Figure 3)indicate that the currents are generated by voltage-dependent potassium (Kv) channels. Based on their simi-larities to delayed-rectifier and A-type Kv currents, thenon-inactivating sustained current will be referred asKDR and the inactivating transient current as KA in thefollowing.KDR was isolated by giving voltage pulses from -47 to
+23 mV in 10 mV steps after a -57 mV prepulse thatinactivated the KA conductance (Figure 2E). Conduc-tances were calculated from the steady-state currentsand fitted with first order Boltzmann function g(V) =gmax/(1 + exp((V50 - V)/slope)), corresponding to 1st
order kinetics for the activation. The resulting half-activation voltage (V50) was -31 ± 9 mV with slope factorof 12.0 ± 2.0 mV, and the maximum conductance (gmax)was 78 ± 22 nS (mean ± SD, n = 6), ranging between 40and 90 nS. A normalized KDR activation profile isshown in Figure 4A (black squares and curve). Activa-tion and deactivation kinetics were determined fromsingle-exponential fits to activating currents or deactivat-ing tail currents. At physiologically relevant voltagesfrom -70 to -10 mV, KDR activation time constants fellbetween 20 and 11 ms (Figure 4B black squares).Voltage-dependence of KA activation was determined
with the voltage clamp subtraction protocol (Figure 2D-E).Peak conductances were then calculated from the peakcurrents and fitted with a 2nd order Boltzmann functiong(V) = gmax/(1 + exp((V50 - V)/slope))2 , corresponding to2nd order activation kinetics for the KA channels. Thehalf-activation potential for the 2nd order Boltzmann was-43± 4 mV with slope of 8.4 ± 1.6 mV (mean± SD,n = 5). The normalized activation profile for KA is shownin Figure 4A (gray circles and curve; note that becausethe activation function is of the 2nd order, the V50 valuein the equation did not here translate into the 50% valueof the activation). Voltage-dependence of KA inactiva-tion was determined from the peak currents, elicited bya -7 mV command pulse following an inactivating pre-pulse. A first order Boltzmann fit to the peak currentsgave a half-inactivation potential of -85± 1 mV and aslope factor of -11.3 ± 2.9 mV (Figure 4A, gray trianglesand curve, mean ± SD, n = 4). Activation and inactivationtime constants were fitted to the subtraction protocol
-120 -80 -40 0 400
5
10
15
20
25
-120 -80 -40 0 400
10
20
30
40
50
60
70
-120 -80 -40 0 40
0.0
0.5
1.0
Voltage (mV) Voltage (mV)τ ac
t(m
s)
τ inac
t(m
s)
CB
I/Im
ax
g/g m
ax
Voltage (mV)
A
Figure 4 Voltage-dependent properties of the Kv currents. A) Steady state activation and inactivation properties of the Kv currents (all dataare mean± SD). Black squares represent activation of the sustained KDR current (n = 6); gray symbols represent transient KA current’s activation(circles, n = 5) and inactivation (triangles, n = 4). The curves are the corresponding Boltzmann fits: KDR activation is a 1st order Boltzmann withV50 = -31 mV and slope = 12 mV. KA activation is a 2nd order Boltzmann with V50= -43 mV and slope= 8.4 mV. KA inactivation is a 1st orderBoltzmann with V50= -85 mV and slope= -11 mV. B) Activation time constants of KDR (black squares, n = 8 to 14) and KA (gray circles, n = 5). KDRactivation time constant was fitted with a bell-function τKDR = 1/(4�exp(-43*V) + 156�exp(43*V)) s, where V is voltage in volts. KA activation timeconstant was nearly voltage-independent and was thus set to constant 1.5 ms for the simulations. C) Time constant of the KA inactivation (n = 3to 7). Bell function is τKA= 1/(341 �exp(44�V) +0.211�exp(-44�V)) s, where V is voltage in volts. Inset: the inactivation recovery protocol used forvoltages below -80 mV.
Salmela et al. BMC Neuroscience 2012, 13:93 Page 6 of 14http://www.biomedcentral.com/1471-2202/13/93
currents with a pulse function, I= Imax�(1 -exp(-t/τact)2�exp(-t/τinact)). Due to the large capacitive transient inwhole-cell voltage clamp recordings, the rapid activationkinetics of KA could not be acquired reliably. None-theless, the activation time constant was fast at around1-2 ms (Figure 4B, gray circles, mean ± SD, n= 5), andthus several fold faster than activation of KDR. KA inacti-vation time constants were obtained from either subtrac-tion protocol currents or from the inactivation recovery(Figure 4C, inset). KA inactivation time constants(Figure 4C) ranged between 50 and 5 ms at physiologic-ally relevant voltages (-70 to -10 mV). The maximum KAconductance was 36± 29 nS (mean± SD, n= 5) and ran-ged between 11 and 79 nS.
Kv currents and photoreceptor sizeThe whole-cell capacitance results from the photorecep-tor membrane, which includes the folded microvillarmembrane of the rhabdomere and the unfolded mem-brane of the soma. The differences in measured capaci-tances (Figure 1B) could thus reflect the variable size ofphotoreceptors, or alternatively the sizes of their rhabdo-meres or somas, or both. If the differences in capaci-tance were produced by rhabdomere or soma sizevariation, there should be other differences as well.To test if the measured capacitances were linked to
other photoreceptor properties, we looked at Kv conduc-tances and voltage light responses recorded in the samecells. Kv currents were elicited by a -4 mV voltage stepgiven after a hyperpolarizing inactivation removal pulse.KDR steady-state and KA peak conductances were thencalculated from the currents after series resistance cor-rection. Larger KA conductances were found in small
cells, whereas no KA conductance could be observed inlarge cells (Figure 5A). However, the capacitive transientarising from the whole-cell capacitance and access resist-ance might partly conceal the transient KA current inthe cells with large capacitance. KDR conductances werefound in all cells, and conductance values showed apositive trend with increasing capacitance (Figure 5B),and KDR conductance density was 0.14 ± 0.06 nS/pF(mean ± SD, n = 23). Voltage responses to a saturating10 s long light pulse were recorded from the samephotoreceptors. The depolarization at the end of the re-sponse was taken as a measure of light-induced voltagechange that is the result of the interplay between thedepolarizing light-induced current and the hyperpolariz-ing Kv currents that are activated by the depolarization.Light-induced steady-state depolarization was smallerin cells with lower than those with higher capaci-tances (Figure 5C) and the relationship between thedepolarization whole-cell capacitance resembled thevariability of light responses as reported by Heimonenet al. (2006). This suggests that the variability reportedearlier is related to cell size, possibly due to fewer ormore numerous microvilli in the smaller or larger rhab-domeres and, consequently,a smaller or larger amount oftransducing channels.
Pharmacology of Kv channelsPharmacological properties of Kv currents were testedwith a number of Kv channel blockers in whole-cell volt-age clamp (Figure 6A-D). 4-aminopyridine (4-AP) typic-ally blocks A-type Kv currents at high micro- tomillimolar concentrations [34]. Application of 1 mM 4-AP in the extracellular solution inhibited the transient
0 200 400 600 800 10000
20
40
60
80
0 200 400 600 800 10000
10
20
30
40
0 200 400 600 800 10000
10
20
30
40
gKD
R(n
S)
Capacitance (pF)
gKA
(nS
)
Capacitance (pF)
CB
ΔV(m
V)
Capacitance (pF)
A
Figure 5 Small cells have large KA conductances and small depolarization in response to light. A) KA and B) KDR conductances versusthe whole-cell capacitances (n = 23). Conductances were calculated from the currents elicited by a voltage jump to -4 mV from -104 mV C) Thelight-induced steady-state depolarization was smaller in cells with lower whole-cell capacitances.
Salmela et al. BMC Neuroscience 2012, 13:93 Page 7 of 14http://www.biomedcentral.com/1471-2202/13/93
KA current, but had no effect on the KDR current(Figure 6A). Tetraethylammonium (TEA), a commonblocker of delayed rectifier Kv currents, inhibited thenon-inactivating KDR only partially at a high concentra-tion of 50 mM (Figure 6B). Quinidine, a non-specificblocker of various Kv currents in insect neurons [15,34],inhibited the transient KA current at 1 mM extracellularconcentration (Figure 6C) and the slow-activating KDR(Figure 6D) with half-maximum inhibitory concentrationof IC50 = 32 ± 3 μM and Hill coefficient of 0.97 ± 0.08(mean ± SE). Although quinidine inhibited the KDR
0
1
2
3
4
5
Cur
rent
(nA
)
50 ms
0
1
2
3
DC
BA
0
1
2
3
4
Cur
rent
(nA
)
0.0 1 10 100 1000
0.0
0.2
0.4
0.6
0.8
1.0
I KD
R/I
KD
Rm
ax
[quin] (μM)
Figure 6 Pharmacological properties of the Kv currents. Kvcurrents were elicited with a positive command potential, givenafter a negative inactivation-removal prepulse. Black traces arecontrols; gray traces are currents recorded after drug application. A)1 mM 4-AP suppressed the transient KA component. B) 50 mM TEAblocked KDR partially. C) 1 mM quinidine blocked both KDR and KA.D) Concentration dependence of the quinidine block for the KDR(mean± SD, n = 3 to 5). The fitted curve is a logistical function 1/(1 + ([quin]/IC50)
p), with half-inhibition concentration IC50 = 32 ± 3 μMand Hill slope p= 0.97 ± 0.08 (mean± SE).
current well, it had other effects, possibly mediated byinterference with the light-gated channels or their activa-tion; thus using it during light responses produced in-conclusive results and was not investigated further.Application of 100 nM α-dendrotoxin, a potent blockerof Shaker A-type Kv channels [35], did not block KA orKDR (data not shown).
Hyperpolarization-activated inward-rectifying (IR) currentIn the Cl-containing bath solution the current clamprecordings exhibited an inwardly-rectifying response tonegative current injections (Figure 1E). In the whole-cellvoltage-clamp experiments under similar ionic condi-tions, hyperpolarization of the photoreceptors activateda small inward current (IR, Figure 7A). Increasing theextracellular K+ concentration from 5 mM to 20 mMhad no effect on the IR current (Figure 7B), indicatingthat it is neither carried nor modulated by potassium.Substituting the extracellular NaCl with Na-gluconatereduced the IR current amplitude (Figure 7C). Thevoltage-dependence of the current and the Na-gluconateeffect were similar to the CLC-2 channels [36], suggest-ing chloride as the main current carrier. Because the acti-vation of this current took place in a negative voltageregime compared to the dark resting potential, it was notinvestigated further here.
Roles of K+ conductances in light responsesResponses to 10 s naturalistic light intensity series wererecorded in both voltage and current clamp modes(Figure 1D). Recorded light currents were then used forestimation of the light-induced conductance required forthe light response in simulations. The light-inducedcurrent (LIC) is determined by the light-induced con-ductance and its driving force (voltage difference be-tween the membrane potential and the LIC reversalpotential). Thus, although possible to determine in
-130 -120-110 -100 -90 -80 -70
-80
-60
-40
-20
0-130 -120-110 -100 -90 -80 -70
-40
-30
-20
-10
0
1 s
10 pA
0 pA
Cur
rent
(pA
)
Voltage (mV)A C
Cur
rent
(pA
)
Voltage (mV)B
Figure 7 Hyperpolarization-activated inward rectifying (IR) current. A) Hyperpolarizing voltage clamps from -74 mV to -124 mV elicited asmall-amplitude IR current that activated slowly and showed no inactivation. The sharp transients at the end of the clamp currents are capacitivetransients. B) The IR current was insensitive to changes in the external potassium concentration, ruling out potassium as the current carrier. Blackcircles with [K]o = 5 mM, white squares [K]o = 20 mM (mean± SEM, n = 4). C) Replacing the NaCl in the bath with Na-gluconate reduced the IRcurrent, indicating that the IR current is carried at least partly by chloride. Black circles are controls with NaCl, white squares are substitutionexperiments with Na-gluconate (mean± SEM, n = 4).
Salmela et al. BMC Neuroscience 2012, 13:93 Page 8 of 14http://www.biomedcentral.com/1471-2202/13/93
voltage clamp, it was not possible to measure the actualLIC driving the photoreceptor’s voltage response, whenrecording in the current clamp mode when the voltageis varying freely. With the model, however, we could in-directly estimate the currents contributing to the voltageresponses to moderate intensity light stimulations, basedon the experimentally determined light-induced andvoltage-gated conductances [13,37].To determine the roles of KDR and KA during light
responses, a Hodgkin-Huxley type model of a cockroachphotoreceptor was implemented following similarapproach as previously described in Drosophila [3]. Themodel was based on the experimentally determinedlight-induced conductance (Figure 1D), the voltage- and
Model
C = A-B
B
A
Figure 8 Validation of voltage-dependent properties in the simulatiosimilar current responses as in experiments (right column, c.f. Figure 2D-F iThe current elicited from voltage jump from -117 mV prepulse up to + 3 m+3 mV. C) The subtraction current A -B.
time-dependent properties of KA and KDR (Figure 4),and the values of the resting potential, the capacitanceand the input resistance. Simulated voltage clamps(Appendix, Figure 8) and light responses to a 10 s natur-alistic contrast stimulus (Figure 9A, c.f. Figure 1D)behaved similarly to the experiments, and thus the vari-ous currents underlying the voltage responses could beestimated with the model. During the simulated lightresponses KDR activated strongly, producing currents upto 1 nA (Figure 9B). Conversely, KA currents during thelight responses were small and the maximal KA currentduring the initial voltage transient was only ca. 40 pA(Figure 9C). The strong inactivation at physiologicalvoltages kept KA currents very small throughout the
10 ms1 nA
10 ms1 nA
Experiment
10 ms
0.5 nA
ns. Voltage clamp simulations with the model (left column) resulted inn Results). The insets on the left show the voltage protocols used. A)V. B) The current elicited by voltage jump from -57 mV prepulse up to
Vrest
= -60 mV
1 s
10 mV
Irest
= 0.6 pA
C
B
A
1 s250 pA
Irest
= 51 pA
1 s5 pA
Figure 9 Simulated light responses and underlying Kv currents.A) Voltage responses to the naturalistic light stimulus weresimulated with the model (Appendix), using the light-inducedconductances calculated from the light-induced currents recorded involtage clamp (Figure 1D) as input. Relative stimulus intensities were1 (black) and 10 (grey). B) KDR current was activated already at restand increased further during the light-induced depolarization. C) KAcurrents before and during the light response were small.
Salmela et al. BMC Neuroscience 2012, 13:93 Page 9 of 14http://www.biomedcentral.com/1471-2202/13/93
simulations. As a test of the significance of the KDR, itspartial removal from the model down to 10% of themean experimentally determined conductance valuesincreased the depolarization level of the light responses(Figure 10A). Conversely, increasing the KDR
200%100%
1000%
10%
B
1 s10 mV
A
1 s
10 mV
Figure 10 Effect of varying the maximum conductances of theKDR and KA in the simulations. A) Light responses were simulatedwith 10% (top trace), 100% (gray trace), 200% and 1000% (lowesttraces) of the experimentally determined mean KDR maximumconductance. B) Modifying the maximal KA conductance had noeffect on the simulated light response. The responses weresimulated with 0%, 100%, 200% and 1000% KA conductancesrelative to the standard simulation value of 60 nS. Due to minimaldifferences in the responses the traces overlap.
conductance up to 10-fold decreased the amplitude andspeeded up the voltage response (Figure 10A). Varyingthe KA maximum conductance from zero to ten-foldfrom the experimentally determined value had no visibleeffect on the simulated voltage responses (Figure 10B).Similarly, no effect was found when other steady-stateKA parameters were varied within their minimum/max-imum experimental ranges, i.e. V50 of activation (-43 to-40 mV) and inactivation (-88 to -84 mV) and theslopes of activation (6.2 to 10.5 mV) and inactivation(-14.3 to -8.5 mV). The possible influence of the darkresting potential on these results was checked by run-ning simulations using resting potentials ranging from-80 to -50 mV. With more hyperpolarized restingpotentials the initial KA transients became larger butsoon after the onset of the light stimulation the KAquickly inactivated to very low levels, similar to thestandard simulation conditions. The overlap betweenthe KA steady-state activation and inactivation(Figure 4D) indicated that the channel is partially acti-vated already at dark resting potential at ca. -60 mV,although the conductance is small (ca. 0.08 nS). Since thegraded voltage changes were slow compared to the kinet-ics of KA, inactivation always dominated and KA thusremained mostly inactivated after the initial transient.
DiscussionPhotoreceptors can be used as model systems for signalprocessing, involving input signals in the form of light-gated current, modulation of the resulting voltage sig-nals by voltage-dependent channels, and output by syn-aptic transmission to interneurons [7]. Nocturnal orcrepuscular insects such as the cockroach have to copewith dim environments, where the reliability of visualinformation decreases due to the stochastic nature ofphoton arrival and relatively large transduction noise[38]. We have for the first time characterized in detailthe electrical membrane properties in photoreceptors ofan insect adapted to dark, the cockroach Periplanetaamericana.Several anatomical and physiological strategies for
improving dim light vision exist in the insect visualsystems. Compound eye optics, photoreceptor propertiesand the later neural processes can be optimized for effi-cient light capture and signal transmission [25]. At thephotoreceptor level, common strategies used by noctur-nal arthropods include temporal summation by thelow-pass properties of phototransduction and thephoto-insensitive membrane, large-amplitude single pho-ton responses and a large rhabdomere that increases thephoton catch [39,40]. Spatial summation of signals fromseveral photoreceptors in the 2nd order neurons of theLamina can further improve vision under conditionswhere the number of photons is very small [41-43]. A
Salmela et al. BMC Neuroscience 2012, 13:93 Page 10 of 14http://www.biomedcentral.com/1471-2202/13/93
conclusion that can be made from these studies is that,when only a small number of photons are absorbed bythe photoreceptors, sufficient visual information isacquired through sacrificing either spatial or temporalresolution, or both. Our new results show how thevoltage-dependent channels fit into this big picture inthe cockroach photoreceptors. Both the experimentalapproach, patch-clamping of the photoreceptors in theisolated ommatidia, and the theoretical approach, math-ematical modelling, are used to gain mechanistic under-standing. Interestingly, our results show that the Kvchannel composition in the cockroach photoreceptorsdoes not follow the pattern found previously in theDiptera [10,11].As can be expected, temporal integration in the
cockroach photoreceptors is clearly one of the mainvisual adaptations for life in the dark [27]. It limitscoding to the relatively slow visual signals comparedto the fast-flying diurnal flies, whose photoreceptorsrespond to much higher stimulus frequencies [44,45].In cockroach photoreceptors low-pass filtering arisesfrom the slow phototransduction (Figure 1C-D) andthe electrical properties of the membrane (Figure 1E),manifested in the large whole-cell resistance and cap-acitance. The capacitance ensues from the rhabdo-mere’s large microvillar area that increases the photoncapture efficiency of the cells [46]. The high resistancein the dark and at rest combined with the large cap-acitance yields a slow time-constant: ca. 60 ms withtypical membrane capacitance of 400 pF and resistanceof 150 MΩ at -60 mV. This corresponds to a temporallow-pass filter with a cut-off frequency of ca. 3 Hz.For comparison, in dark-adapted photoreceptors of thediurnal blowfly Calliphora vicina, membrane resistanceand time constant are 32 MΩ and 4 ms, resulting ina corner frequency of 25 Hz, which in light-adaptationis nearly tripled to 72 Hz [12]. In cockroach photore-ceptors the high input resistance near the resting po-tential amplifies single photon responses, enablinglarge amplitude voltage bumps.During light responses the membrane gain and speed
are strongly modulated by voltage-gated channels. Thedark- and day-active Dipteran fly species possess varyingKv channel compositions dominated by either A-type orDR channels, respectively [10,11]. This is considered toresult from the optimization between the need for fastervision and the subsequent increase in the metaboliccosts [2,4]. Moreover, some insects that are active duringboth day and night demonstrate circadian changes in theKv channels, with transient currents expressed at thenight and sustained currents during the day [47,48]. Itwas therefore surprising that the dominant Kv current inthe nocturnal cockroach was the noninactivating KDR(Figure 2).
Our results show that the conductances activated withthe depolarization are relatively specific for K+, and thusit is plausible that they are created by the Kv-channels(Figure 3). The voltage-activated currents could be sep-arated into two components, the sustained non-inactivating KDR and the transient inactivating currentKA (Figure 4. and the Appendix). Although both KDRand KA showed some of the typical characteristics ofpreviously described insect Kv-channels, most import-antly the voltage-dependence of activation and inactiva-tion and the sensitivity to various Kv blockers (Figure 6;compare to channels in [33].), the molecular identities ofthe channels remain unknown. However, the insensitiv-ity of KA to the Drosophila Shaker blocker αDTX sug-gests that KA is not coded by the Shaker gene. KDR wasactivated already at the -60 mV resting potential in dark,contributing almost 90% of total membrane conduct-ance. Therefore KDR participates in shaping even thesmallest light responses, the quantum bumps, and maybe required to prevent or attenuate saturation with tran-sient increases of light. Simulations showed that simi-larly to the sustained Kv currents in the photoreceptorsof other species, KDR adjusts the speed and amplitudeof the light-induced voltage responses [10,11,13].The role of the transient Kv conductance, KA, is more
difficult to assess. KA conductance at the dark restingpotential was small (< 0.1 nS) and computer simulationsdemonstrated that KA had no significant role in pro-longed light responses. Because of its rapid inactivation,even a 10-fold simulated increase in the KA conduct-ance or various manipulations of voltage-dependentproperties of KA had no significant effect on the lightresponse (Figure 10). An increase in the KA current dur-ing the initial voltage transient was observed when thedark resting potential was set below the experimentallydetermined values. However, the physiological relevanceof this finding is likely to be very small because themembrane voltage is rarely below the dark resting po-tential during the light stimuli. For high intensity lightstimuli, the cation influx through the light-sensitivechannels may induce exchanger and pump activity,which can lead to a brief hyperpolarization below theresting potential [49]. It is possible that some of the cellsin vivo could have more negative resting potentialswhere KA channels could be more effectively activated.However, no systematic variation of the resting potentialwas found during this work, neither has this beenreported in earlier studies [27].Besides the inter-species differences in Dipteran
photoreceptor Kv channels [10], Kv channel expressioncan vary within the same species between photorecep-tors with different functional and structural properties.In Drosophila, blue- and UV-sensitive photoreceptorswith longer axons express larger transient Kv
Salmela et al. BMC Neuroscience 2012, 13:93 Page 11 of 14http://www.biomedcentral.com/1471-2202/13/93
conductances than green-sensitive cells with short axons[50]. The transient conductance (as opposed to the alter-native sustained DR type) has been suggested to de-crease the attenuation of the voltage signals duringpropagation to the 2nd order cells. Since cockroachphotoreceptors have exceptionally long axons reachingover 1 mm [51], KA could serve a similar function.Previous studies [27] and the data presented here
(Figure 5A&C) indicate that a fraction of cockroachphotoreceptors exhibit a particular strongly-adaptingphenotype, characterized by higher KA conductance andsmaller whole-cell capacitance than photoreceptors onaverage. In vivo intracellular recordings with sharp elec-trodes have earlier demonstrated the presence of actionpotential-like signals in the photoreceptor axons [26]and simulations of spiking hyper-adapting photorecep-tors have shown that such combination efficientlyencodes transient light intensity changes [27]. Generally,A-type Kv conductances regulate several aspects in spik-ing neurons [32,33,52]. We therefore hypothesize thatthe KA conductance could be important in tuning thetransient responses in a sub-population of photorecep-tors, related to signalling with either transient gradedpotentials or spikes in the axons. However, good qualityintracellular recordings from the thin axons, 0.5 - 1 μmin diameter are lacking at present.The functional role of the hyperpolarization activated
IR current (Figure 7) is enigmatic, because its activationrange is well below physiological signalling range. Itcould be related to transient hyperpolarization of themembrane after strong light stimulation [53]. Althougha detailed study of its possible physiological function isbeyond the scope of this paper, our results show that itis not carried by potassium and that substitution experi-ments are in accordance to its being a chloride conduct-ance. These findings resemble a chloride currentmediated by the CLC-2 channels [54], which have alsobeen reported in the Drosophila photoreceptors [55].
ConclusionsIn conclusion, we have characterized three types ofvoltage-dependent conductances in the cockroachphotoreceptors, two Kv and one (putative) chloride con-ductance. The Kv-conductance composition does notconform to the previously formulated hypothesis of theroles of KDR and KA types of Kv channels in the insectphotoreceptors of varying visual ecology. This earlier hy-pothesis was based on the studies of Dipteran flies and amore comprehensive comparative study should be con-ducted spanning all major insect groups. Results of suchwork would complement our current understandingon the different roles that Kv channels may have inphotoreceptor signalling or in graded voltage signallingin general.
AppendixMathematical model of the cockroach photoreceptorGlossary: model variables and parameters
V membrane voltage (volts)Ilight, KDR, KA, leak light-induced, KDR, KA or leak current
(amperes)C membrane capacitance (farads)t time (seconds)glight,leak light-dependent- or leak conductance
(siemens)GKDR, KA maximum conductance of KDR or KA
(siemens)Elight, K reversal potential of light-induced or
potassium current (volts)KDRact, KAact activation parameter for KDR or KA
(unitless)KAinact inactivation parameter for KA (unitless)τXact, τXinact activation or inactivation time constant
for X (seconds)
An isopotential Hodgkin-Huxley-like model of thephotoreceptor soma was constructed in Matlab pro-gramming environment (Mathworks, USA), using themeasured passive, light- and voltage-dependent proper-ties. Since we simulated the photoreceptor depolariza-tions arising from the light stimulation, thehyperpolarization-activated IR current was not includedin the model.
Passive propertiesThe experimentally derived average whole-cell capaci-tance of C= 380 pF was used in the simulations. A pas-sive leak conductance gleak, with reversal potentialEleak = 0 mV, was added in the model to give a restingpotential of -60 mV in simulations to match the experi-mental conditions. The value for gleak was calculated togive a zero net current at the resting potential. Withstandard Kv conductances and resting potential of-60 mV, the gleak was 0.9 nS and resulted in a whole-cellresistance of 136 MΩ at rest and 940 MΩ at -84 mV,where the experimental input resistances were measuredwith the voltage clamp.
Light-dependent conductanceThe light-dependent conductance, glight(t), caused by lightstimulation, was determined in voltage clamp. A 10 s longwaveform taken from the van Hateren naturalistic stimu-lus database [28] was used to control the intensity of agreen LED (Figure 1D bottom trace). Light-induced cur-rents (LIC, Figure 1D) were recorded from the photore-ceptors clamped to a holding potential of -74 mV inwhole-cell mode. Light-dependent conductances were cal-culated by dividing the LIC recorded at -74 mV by thedriving force of -84 mV, assuming a reversal potential of
Table 1 Parameters for Kv conductances in the model
Steady-state conductance (mean± SD) Time constant (mean±SE)
*) KA activation time constant was fixed at 1.5 ms.gmax=maximum conductance, V50 = half activation/inactivation voltage, slope= slope factors for the steady-state parameters (Eq. 2) or time constant (Eq. 3),P= order for the steady-state parameter (Eq.2), α and β= activation and deactivation rates for Eq. 3, τ0=offset for bell function (Eq. 3).
Salmela et al. BMC Neuroscience 2012, 13:93 Page 12 of 14http://www.biomedcentral.com/1471-2202/13/93
Elight = +10 mV (determined in a separate set of experi-ments with identical solutions).
Voltage-dependent propertiesThe voltage-dependent potassium conductances KDRand KA were incorporated in the model. The inwardcurrent was not included, since we modelled only vol-tages where the inward current is inactive. We used theexperimentally determined potassium reversal potentialof EK = -68 mV for both the KDR and KA (Figure 3).Parameters for the voltage-dependent properties of KDRand KA are listed in Table 1. Maximum KDR conduct-ance in the model was 78 nS, which corresponds to themean value in the experiments. 60 nS maximum con-ductance value was used for the KA, which matched thevalue of the representative cell that was used in themodel validation (Figures 8, 2D-F). KA maximum con-ductance was varied in the simulations without signifi-cant effect on the voltage responses, which makes thisparameter non-critical (Figure 10B).
Hodgkin-Huxley-like equationsThe model followed modified Hodgkin-Huxley formal-ism [56] where the membrane voltage V and activation/inactivation properties of the voltage-dependent conduc-tances KDR and KA are described with a group of non-linear ordinary differential equations (Eq. 1).
ð1ÞThe non-inactivating KDR conductance was modelled
with a single activation parameter (KDRact), whereas theinactivating KA conductance had a 2nd order activation(KAact) and an inactivation (KAinact) parameter. The
activation/inactivation differential equations are func-tions of the voltage-dependent steady-state activation/in-activation parameters (Eq. 2) and the correspondingtime constants. Parameters for the equations are listedin Table 1.
Kxs�sact=inact Vð Þ ¼ 1þ e�V�V50ð Þslope
� ��P
ð2Þ
τxact=inact Vð Þ ¼ 1αe�slope⋅V þ βeslope⋅V
þ τ0 ð3Þ
The model was validated by simulating the voltageclamp subtraction protocol shown in Figure 2D-F. Forvalidation, we used the mean activation and inactivationparameters (Figure 4A-C) and the maximum conduc-tances for the example cell shown in Figure 2D-F (52 nSfor KDR and 60 nS for KA). The voltage-clamp subtrac-tion protocol was simulated by setting the Hodgkin-Huxley differential equation dV/dt to zero and solvingthe net current (Itot= IKDR+ IKA+ ILEAK) with the voltageclamp protocol V(t) (Figure 8).Light responses were simulated using light-dependent
conductances glight(t). Solving the differential equationgroup with the Matlab ode solver ode23s gave the photo-receptor voltage and activation/inactivation parametersfor the conductances, which were then used to calculatethe corresponding currents.Matlab code for the simulations is available from the
authors upon request.
Competing interestsThe authors declare that they have no competing interests.
Authors’ contributionsIS analyzed the data, performed the simulations and drafted the manuscript.EI, RF, SK and YK performed experiments, analyzed the data and drafted themanuscript. MV and MW coordinated the study and drafted the manuscript.All authors read and approved the final manuscript.
AcknowledgementsKyösti Heimonen helped in many discussions during this work. We aregrateful to Roger Hardie for his help in establishing the patch clamping ofinsect photoreceptors in our laboratory. The work was supported by grantsto IS: Finnish Graduate School of Neuroscience, Biocenter Oulu; to MW andto MV: Academy of Finland (grants no. 118480 and 129762), Sigrid JuseliusFoundation; and to SK: Academy of Finland.
Salmela et al. BMC Neuroscience 2012, 13:93 Page 13 of 14http://www.biomedcentral.com/1471-2202/13/93
Received: 3 April 2012 Accepted: 12 July 2012Published: 6 August 2012
References1. Laughlin SB: Matched filtering by a photoreceptor membrane. Vision Res
1996, 36(11):1529–1541.2. Niven JE, Laughlin SB: Energy limitation as a selective pressure on the
evolution of sensory systems. J Exp Biol 2008, 211(11):1792–804.3. Niven JE, Vähäsöyrinki M, Kauranen M, Hardie RC, Juusola M, Weckström M:
The contribution of Shaker K+ channels to the information capacity ofDrosophila photoreceptors. Nature 2003, 421(6923):630–634.
4. Niven JE, Vähäsöyrinki M, Juusola M: Shaker K+-channels are predicted toreduce the metabolic cost of neural information in Drosophilaphotoreceptors. Proc R Soc Lond B Biol Sci 2003, 270(Suppl 1):S58–S61.
5. Barrow AJ, Wu SM: Low-conductance HCN1 ion channels augment thefrequency response of rod and cone photoreceptors. J Neurosci 2009,29(18):5841–5853.
6. Beech DJ, Barnes S: Characterization of a voltage-gated K channel thataccelerates the rod response to dim light. Neuron 1989, 3(5):573–581.
7. Barnes S: After transduction: response shaping and control oftransmission by ion channels of the photoreceptor inner segments.Neuroscience 1994, 58(3):447–459.
9. Fain GL, Hardie R, Laughlin SB: Phototransduction and the evolution ofphotoreceptors. Curr Biol 2010, 20(3):R114–R124.
10. Laughlin SB, Weckström M: Fast and Slow Photoreceptors - aComparative-Study of the Functional Diversity of Coding andConductances in the Diptera. J Comp Physiol A 1993, 172(5):593–609.
11. Weckström M, Laughlin SB: Visual Ecology and Voltage-Gated IonChannels in Insect Photoreceptors. Trends Neurosci 1995, 18(1):17–21.
12. Weckström M, Hardie RC, Laughlin SB: Voltage-activated potassiumchannels in blowfly photoreceptors and their role in light adaptation.J Physiol (Lond ) 1991, 440(1):635–657.
13. Vähäsöyrinki M, Niven JE, Hardie RC, Weckström M, Juusola M: Robustnessof neural coding in Drosophila photoreceptors in the absence of slowdelayed rectifier K+ channels. J Neurosci 2006, 26(10):2652–2660.
14. Krause Y, Krause S, Huang J, Liu CH, Hardie RC, Weckström M: Light-dependent modulation of shab channels via phosphoinositide depletionin Drosophila photoreceptors. Neuron 2008, 59(4):596–607.
16. Bell WJ, Adiyodi KG (Eds): The American Cockroach. London: Chapman andHall; 1981.
17. Zill SN: Mechanoreceptors: Exteroceptors and proprioceptors. InCockroaches as models for neurobiology: applications in biomedical research,Volume Volume II. Edited by Huber I, Masler EP, Rao BR. Boca Raton, Florida:CRC Press, Inc; 1990:247–267.
18. Seelinger G: Chemoreception. In Cockroaches as models for neurobiology:applications in biomedical research, Volume Volume II. Edited by Huber I,Masler EP, Rao BR. Boca Raton, Florida: CRC Press, Inc; 1990:269–284.
19. Brown S, Strausfeld N: The effect of age on a visual learning task in theAmerican cockroach. Learn Mem 2009, 16(3):210–223.
20. Mizunami M, Weibrecht JM, Strausfeld NJ: Mushroom bodies of thecockroach: their participation in place memory. J Comp Neurol 1998,402(4):520–537.
21. Ye S, Leung V, Khan A, Baba Y, Comer CM: The antennal system andcockroach evasive behavior. I. Roles for visual and mechanosensory cuesin the response. J Comp Physiol A 2003, 189(2):89–96.
22. Füller H, Eckert M, Blechschmidt K: Distribution of GABA-likeimmunoreactive neurons in the optic lobes of Periplaneta americana. CellTissue Res 1989, 255(1):225–233.
23. Laughlin SB: Energy as a constraint on the coding and processing ofsensory information. Curr Opin Neurobiol 2001, 11(4):475–480.
24. Laughlin SB, van Steveninck RR dR, Anderson JC: The metabolic cost ofneural information. Nat Neurosci 1998, 1(1):36–41.
25. Warrant EJ, Dacke M: Vision and Visual Navigation in Nocturnal Insects.Annu Rev Entomol 2011, 56(1):239–254.
26. Weckström M, Järvilehto M, Heimonen K: Spike-like potentials in the axonsof nonspiking photoreceptors. J Neurophysiol 1993, 69(1):293–296.
27. Heimonen K, Salmela I, Kontiokari P, Weckström M: Large functionalvariability in cockroach photoreceptors: optimization to low light levels.J Neurosci 2006, 26(52):13454–13462.
28. van Hateren JH: Processing of natural time series of intensities by thevisual system of the blowfly. Vision Res 1997, 37(23):3407–3416.
29. Li WC, Soffe S, Roberts A: A direct comparison of whole cell patch andsharp electrodes by simultaneous recording from single spinal neuronsin frog tadpoles. J Neurophysiol 2004, 92(1):380–386.
30. Butler R: The identification and mapping of spectral cell types in theretina of Periplaneta americana. J Comp Physiol A 1971, 72(1):67–80.
31. Smakman JGJ, Stavenga DG: Angular Sensitivity of Blowfly Photoreceptors- Broadening by Artificial Electrical Coupling. J Comp Physiol A 1987,160(4):501–507.
32. Hille B: Ion channels of excitable membranes. Sunderland, MA: SinauerAssociates; 2001.
33. Wicher D, Walther C, Wicher C: Non-synaptic ion channels in insects–basicproperties of currents and their modulation in neurons and skeletalmuscles. Prog Neurobiol 2001, 64(5):431–525.
34. Schafer S, Rosenboom H, Menzel R: Ionic currents of Kenyon cells fromthe mushroom body of the honeybee. J Neurosci 1994, 14(8):4600–4612.
35. Harvey AL: Twenty years of dendrotoxins. Toxicon 2001, 39(1):15–26.36. Rose U, Derst C, Wanischeck M, Marinc C, Walther C: Properties and
possible function of a hyperpolarisation-activated chloride current inDrosophila. J Exp Biol 2007, 210(14):2489–2500.
37. Niven JE, Vahasöyrinki M, Juusola M, French AS: Interactions between light-induced currents, voltage-gated currents, and input signal properties inDrosophila photoreceptors. J Neurophysiol 2004, 91(6):2696–2706.
38. Lillywhite PG, Laughlin SB: Transducer noise in a photoreceptor. Nature1979, 277(5697):569–572.
39. Laughlin S, Blest AD, Stowe S: The sensitivity of receptors in the posteriormedian eye of the nocturnal spider, Dinopis. Journal of ComparativePhysiology A: Neuroethology, Sensory, Neural, and Behavioral Physiology 1980,141(1):53–65.
40. Frederiksen R, Wcislo WT, Warrant EJ: Visual reliability and information ratein the retina of a nocturnal bee. Curr Biol 2008, 18(5):349–353.
41. Greiner B, Ribi WA, Warrant EJ: A neural network to improve dim-lightvision? Dendritic fields of first-order interneurons in the nocturnal beeMegalopta genalis. Cell Tissue Res 2005, 322(2):313–320.
42. Theobald JC, Greiner B, Wcislo WT, Warrant EJ: Visual summation in night-flying sweat bees: a theoretical study. Vision Res 2006, 46(14):2298–2309.
43. Klaus A, Warrant EJ: Optimum spatiotemporal receptive fields for vision indim light. J Vis 2009, 9(4):18 1–16.
44. Gonzalez-Bellido PT, Wardill TJ, Juusola M: Compound eyes and retinalinformation processing in miniature dipteran species match theirspecific ecological demands. Proc Natl Acad Sci USA 2011, 108(10):4224–9.
45. Niven JE, Anderson JC, Laughlin SB: Fly photoreceptors demonstrateenergy-information trade-offs in neural coding. PLoS Biol 2007, 5(4):e116.
46. Snyder AW: Physics of vision in compound eyes. In Handbook of sensoryphysiology, Volume VII/6A. Volume 7. Edited by Autrum H. Berlin, Germany:Springer; 1979:225–313.
48. Hevers W, Hardie RC: Serotonin modulates the voltage dependence ofdelayed rectifier and Shaker potassium channels in Drosophilaphotoreceptors. Neuron 1995, 14(4):845–856.
49. Jansonius NM: Properties of the sodium pump in the blowflyphotoreceptor cell. J Comp Physiol A 1990, 167(4):461–467.
50. Anderson J, Hardie RC: Different photoreceptors within the same retinaexpress unique combinations of potassium channels. J Comp Physiol A1996, 178(4):513–522.
51. Butler R: The anatomy of the compound eye of Periplaneta americana L.2. Fine structure. J Comp Physiol A 1973, 83(3):239–262.
52. Connor J, Stevens C: Voltage clamp studies of a transient outwardmembrane current in gastropod neural somata. J Physiol (Lond ) 1971,213(1):21–30.
53. Uusitalo RO, Weckström M: Potentiation in the first visual synapse of thefly compound eye. J Neurophysiol 2000, 83(4):2103–2112.
54. Enz R, Ross BJ, Cutting GR: Expression of the voltage-gated chloridechannel ClC-2 in rod bipolar cells of the rat retina. J Neurosci 1999,19(22):9841–7.
Salmela et al. BMC Neuroscience 2012, 13:93 Page 14 of 14http://www.biomedcentral.com/1471-2202/13/93
56. Yamada WM, Koch C, Adams PR: Multiple channels and calcium dynamics.In Methods in neuronal modeling: From synapses to networks. 2nd edition.Edited by Koch C, Segev I. Cambridge, MA: MIT Press; 1998.
doi:10.1186/1471-2202-13-93Cite this article as: Salmela et al.: Cellular elements for seeing in thedark: voltage-dependent conductances in cockroach photoreceptors.BMC Neuroscience 2012 13:93.
Submit your next manuscript to BioMed Centraland take full advantage of:
• Convenient online submission
• Thorough peer review
• No space constraints or color figure charges
• Immediate publication on acceptance
• Inclusion in PubMed, CAS, Scopus and Google Scholar
• Research which is freely available for redistribution
Submit your manuscript at www.biomedcentral.com/submit
