Local preoptic/anterior hypothalamic warming alters spontaneous and evoked neuronal activity in the magno-cellular basal forebrain
Md. Noor Alam a,b, Ronald Szymusiak a.c,*, Dennis McGinty a.b a Research Serrice (151A3), Department of Veterans Affairs Medical Center, Sepuh,eda, CA 91343, USA
Department of Psychology, School of Medicine, University of California, Los Angeles, CA, USA c Department of Medicine, School of Medicine, UniL,ersity of California, Los Angeles. CA, USA
Accepted 28 June 1995
Abstract
Local warming of the medial preoptic/anterior hypothalamus (POAH) promotes sleep, enhances EEG slow-wave activity during sleep, and suppresses arousal-related discharge in neurons of the midbrain reticular formation (MRF) and the posterior lateral hypothalamic area (PLHa). Another important site of sleep and arousal regulation, and a potential site of POAH thermal modulation, is the magnocellular basal forebrain (BF). We examined the ability of local POAH warming during wakefulness to influence the spontaneous and evoked discharge of neurons recorded in the BF of unanesthetized, unrestrained cats. Seventy of 174 BF neurons responded to 60-90 s periods of POAH warming with either increases or decreases in discharge rate. Forty-one of the 70 responsive cells displayed suppression of waking discharge during warming. Discharge rate in these cells declined by an average of 26.04 + 2.76%/°C of POAH temperature increase. The majority of warming-suppressed BF cells (73%) displayed higher rates of discharge during periods of wakefulness compared to periods of sleep. Twenty-nine of 70 responsive cells responded to POAH warming with an average increase in discharge rate of 43.81 + 6.26%/°C. A majority of these neurons (62%) exhibited higher spontaneous discharge rates during sleep compared to waking. Orthodromic excitatory responses were evoked in 29 BF cells by electrical stimulation of the MRF or PLHa. Thirteen of 29 cells displayed a waking-related discharge pattern, and responded to POAH warming with a significant suppression of evoked excitation. For a group of 15 behavioral state-indifferent cells (i.e., cells displaying no modulation of spontaneous discharge rate across the sleep-waking cycle), POAH warming had no effect on evoked excitatory responses. These results support the hypothesis that thermosensitive neurons of the POAH exert control of sleep-waking state, in part, via modulation of arousal- and sleep-regulating cell types within the magnocellular BF.
The medial preopt ic /anter ior hypothalamic area (POAH) is a major thermosensitive region of the mam- malian brain (see [4,36] for review). In addition to a critical role in thermoregulation, POAH thermosensitive neurons participate in the control of sleep and wakefulness. Local warming of the POAH promotes sleep onset and acutely enhances non-rapid-eye movement (NREM) sleep [33,34]. Neocortical EEG slow-wave activity within NREM sleep is augmented during POAH warming [24,43]. Elec- trolytic or neurotoxin-induced POAH damage causes both thermoregulatory deficits and chronic reductions in NREM
sleep [3,50]. Within the POAH of cats, NREM sleep onset is associated with increased spontaneous discharge rate of warm-sensitive neurons, as well as decreased discharge rate of cold-sensitive cell types [1]. Collectively, these findings support the hypothesis that activation of POAH warm-sensitive neurons is sleep-promoting, and cold-sensi- tive neuronal activation suppresses sleep [23].
POAH thermosensitive neurons appear to influence the s leep-waking state, in part, via modulation of arousal mechanisms located in the brainstem and diencephalon. POAH warming during waking has been shown to sup- press spontaneous discharge of arousal-related cell types in the midbrain reticular formation (MRF) [8] and in the posterior lateral hypothalamic area (PLHa) [19].
A critical forebrain site of both sleep and arousal regu- lation lies lateral to the POAH, within magnocellular re-
222 Md.N. Alam et al. / Brain Research 696 (1995) 221-230
gions of the lateral preoptic area and basal forebrain (BF). A majority of the neurons in these regions display peak discharge rates during waking [5,9,10,45,48]. BF choliner- gic neurons project monosynaptically to the neocortex [3,25,35], exert excitatory effects on cortical targets [13,26,30], and participate in the regulation of activated EEG patterns [5,32,37]. In addition, one or more non- cholinergic BF cell types may exert sleep-promoting ef- fects. Neurons that display elevated discharge rates during transitions from waking to NREM sleep and during stable NREM sleep [45,48] have been recorded in BF sites where electrical stimulation evokes sleep [41] and experimental lesions cause insomnia [22,46].
We examined the ability of medial POAH warming to modulate spontaneous and evoked BF neuronal discharge in unanesthetized adult cats. We found that during local POAH warming discharge of waking-related cell types was suppressed, and discharge of sleep-related neurons was increased. In addition, orthodromic excitatory re- sponses evoked in waking-related BF cell types by brain- stem and caudal hypothalamic stimulation were diminished during POAH warming.
2. Materials and methods
Subjects were five adult cats weighing between 2.7 kg to 3.7 kg. Under anesthesia (Nembutal, 35 mg/kg, i.p.) and employing aseptic conditions, cats were surgically prepared for chronic recording of BF neuronal activity and standard electrodes for monitoring sleep-wake states (see [48] for details). Two mechanical microdrives with 2-3 guide cannulae were implanted such that their tips rested 6 mm above the BF target sites (A, 12.7-15; L, 4.0-6.5). Three to four microwires (25-32 /xm insulated stainless steel wires) were passed through each cannula such that the tips projected into the BF. A stainless steel water perfused thermode was implanted into medial POAH (A, 13.5-14; L, 1.5; H, - 4 . 0 ) for the manipulation of local preoptic/anterior hypothalamic temperature (TpoAH). One micro-thermocouple was placed 1-2 mm adjacent to the thermode to quantify the local preoptic thermal stimulus and another thermocouple was placed in the central barrel of one microdrive to determine if the medial POAH ther- mal stimuli influenced local temperature in the BF (TBF). In three cats, bipolar stimulating electrodes were implanted in the MRF (A, 2.0-4.0; L, 4.0; H, - 2 . 0 ) and PLHa (A, 8.0-7.0; L, 5.5-4.0; H, - 2 . 0 ) to examine orthodromic responses of recorded cells.
Experiments were started at least seven days after surgery. On the day of the experiment, the cat was put in a sound attenuated recording chamber (ambient temperature 25 + I°C) and connected to the cables for the recording of neurophysiological variables and temperature. A heat ex- changer, located about 15 cm above the animal's head, was connected to a water reservoir outside the chamber.
The thermode was connected to a pump through silastic tubing. The medial POAH temperature was manipulated by 1.5-2.0°C by pumping hot water through the thermode at a regulated speed.
The microdrives were advanced in 25-50 /zm steps, until isolated single units were found, as confirmed in oscilloscope traces. Window discriminator and spike inte- grator outputs were displayed on a polygraph along with cortical electroencephalogram (EEG), electrooculogram (EOG), neck electromyogram (EMG), pontine-geniculate- occipital waves (PGO) and temperature signals. These waveforms were also digitized (Cambridge Electronic De- sign 1401, London, UK) and stored on disc.
The response of medial POAH warming on isolated BF neurons was tested during wakefulness by recording the spontaneous discharge rate of the neuron at (a) normal TpOAH for 30-60 s; (b) after warming POAH by 1.5-2°C for 30-90 s; and (c) after the withdrawal of the warming stimulus, i.e., during recovery for 30-90 s. During these tests, the behavioral state of the cat was carefully moni- tored both visually and polygraphically. During warming trials, cats were generally alert and sitting with eyes open. If necessary, sleep was prevented by mild auditory stimuli. Head and eye movements were typically present during trials. However, trials were not conducted during periods of vigorous locomotion and grooming due to the possibil- ity of movement artifact. Trials in which animals could not be kept awake or in which movement artifact was present were discarded. Most of the neurons were tested for their responsiveness at least twice. After warming trials, sponta- neous discharge rate of all responsive and some randomly selected indifferent neurons was recorded through 1-3 normal sleep-waking cycles.
Evoked responses to MRF and PLHa stimulation were also examined (monophasic square wave pulses; duration, 0.2 ms; intensity, 0.2-0.8 mA; rate, 1.5 Hz). When pre- sent, evoked responses were examined during normal wak- ing and during waking with medial POAH warming. Dur- ing these tests the sleep-waking state of the animals was carefully monitored to ensure that they remained in appro- priate arousal state throughout the period of the stimulus delivery. Evoked excitability (number of action potentials evoked per stimulus pulse) and the duration of post-excita- tory discharge suppression following the period of excita- tion were determined for each test conducted as described in detail previously [44].
At the end of the recording session, the location of the thermode, microwire positions, and the locations of the stimulating electrodes were histologically verified. Under deep Nembutal anesthesia (50 mg/kg), the brain was perfused with saline followed by 2% formaldehyde. The position of the thermode and microwires were identified in Nissl stained sections and reconstructed according to the cat brain atlas of Snider and Neimer [40].
Neurons were considered to be responsive to medial POAH warming if they showed at least a change of 10%
Md.N. Alam et al. /Bra in Research 696 (1995) 221-230 223
iil ii i i i i i i i l i i i i i i i i i i i l I IlU',iiiiiiiiiiiiiiiiiiil
EOG ~ / - ~ - ~ - - - ~ - -
+ i . ~ l ,
l iili llli ;,,,,, 30 See
Fig. 1. Recordings from a BF waking-related cell that displayed reduction in spontaneous discharge rate during POAH warming. A: record of a POAH warming trial conducted during waking. Top three panels show the cortical electroencephalogram (EEG), the electro-oculogram (EOG) and the dorsal neck electromyogram (EMG). The fourth panel is a rate histogram displaying the number of discriminated spikes occurring in successive 100 ms bins. The numbers above indicate the discharge rate (in Hz) calculated from the histogram during the time period indicated by the lines under the numbers. Bottom two panels show temperatures measured at the basal forebrain recording site (TBF) and adjacent to the preoptic/anterior hypothalamic thermode (TpoAH). Note that, while the animal remained electrographically and behaviorally awake throughout, discharge rate declined to 0.8 Hz during the TpOAH plateau, compared to rates of 3.5 Hz during the baseline period and 3.3 Hz during post-warming recovery. B and C: polygraph recordings of the spontaneous activity of the same cell during episodes of normal waking (B) and NREM sleep (C), demonstrating waking-related discharge. Each pen deflection in the SU channel represents the output of the spike discriminator. All other abbreviations as in A.
224 Md.N. Alam et al. /Brain Research 696 (1995) 221-230
in discharge rate/C change in T,,,, compared to their basal waking firing rates (see [ll). Wakefulness and NREM sleep were identified on the basis of EEG, EOG and EMG patterns using standard criteria. Mean discharge rates/s
were calculated from 60-300 s blocks of stable wakeful- ness and NREM sleep from l-3 normal sleep-wake cy- cles. Thus, state dependent discharge rates were typically
based on several minutes of recording for each cell in each
Fig. 2. Recordings from a BF sleep-related cell that displayed an increase in spontaneous discharge rate during POAB warming. A: record of a warming
trial conducted during waking. Abbreviations as in Fig. 1. Discharge rate increased to 1.1 Hz during the T roAH plateau, compared to 0.1 Hz during both the baseline and the post-warming recovery periods. B and C: polygraph recordings of the spontaneous activity of the same cell during waking (B) and NREM sleep (C), demonstrating sleep-related discharge.
Md.N. Alam et al. /Brain Research 696 (1995) 221-230 225
state. Neurons were classified as wake-related if the N R E M / W a k e ratio was < 0.8, N R E M sleep-related if the N R E M / W a k e ratio was > 1.2, and state-indifferent when the ratio was > 0.8 and < 1.2.
3. Results
3.1. Effects o f P O A H warming on spontaneous BF neu- ronal activity
The responsiveness of 174 BF neurons to medial POAH warming was tested during wakefulness. Of these, 70 (40%) responded and 104 (60%) did not respond to POAH warming. Of the responsive neurons, 41 (59%) exhibited a suppression of waking discharge and 29 (41%) showed a facilitation in discharge rate in response to warming (Figs. 1 and 2). The average %change /°C (_+ S.E.M.) of neurons displaying suppression of spontaneous discharge during POAH warming was - 26.04 _+ 2.76%. For the cells dis- playing increased waking discharge during warming, the average %change /°C was 43.81 _+ 6.26%. A majority of responsive neurons, 43 (61%), were slow firing ( < 5 impulses /s during waking). Ten (14%) were intermediate (5 -10 impulses /s during waking) and 17 (24%) were fast firing ( > 10 impulses /s during waking).
The spontaneous discharge rates of all responsive neu- rons and 82 unresponsive neurons were recorded across s leep-wake cycles. The populations of neurons that in- creased activity and those that decreased activity in re- sponse to POAH warming had different state-dependent
Fig. 3. Different distributions of sleep-waking discharge profiles for cells displaying suppression or facilitation of waking discharge during POAH warming. The ratio of NREM sleep to waking discharge rates is plotted against the % change in discharge rate/°C change in POAH temperature for each of the 70 responsive BF cells. The majority of cells that displayed suppression of discharge during warming (%change/°C < -10%) had NREM waking ratios < 1. The majority of cells that increased discharge during POAH warming (%change/°C > 10%) had NREM/waking ratios > 1. The mean NREM/waking ratios for the warming-suppressed and warming facilitated cells (0.64 + 0.07 vs. 2.35 _+ 0.53) were significantly different (independent t-test: t(68)=-3.74, P < 0.001).
1 WARMING-SUPPRESSED NEURONS
WARMING-FACILITATED NEURONS
WARMING-INDIFFERENT NEURONS
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0 WAKE-RELATED SLEEP-RELATED STATE-INDIFFERENT
Fig. 4. Different distributions of responses to POAH warming among groups of waking-related, sleep-related and state-indifferent cells. Ap- proximately equal numbers of waking-related cells were suppressed by or unresponsive to POAH warming. However, < 10% of all waking-related cells displayed facilitation of discharge during warming. Of sleep-related cells, the majority were warming-facilitated, while only 15% exhibited discharge suppression during warming. The majority of state-indifferent cells (78%) were also unresponsive to POAH warming.
discharge profiles (Fig. 3). A majority (73%) of neurons that exhibited decreased discharge in response to POAH warming also exhibited decreased discharge during NREM sleep and were, therefore, wake-related (Fig. 1B, C). The remaining cells were either sleep-related or state-indiffer- ent (Fig. 4). For the group of 41 neurons that displayed reduced activity during POAH warming, the mean sponta- neous discharge rate (±S .E .M. ) during waking, 10.12 + 2.37, was significantly different from the mean discharge rate during NREM sleep, 7.58 + 2.24. The mean N R E M / W a k e ratio of these neurons was 0.64 + 0.07.
Twenty-nine of seventy BF neurons responded to POAH warming with increases in discharge rate (Fig. 2A). A majority of these neurons (62%) also exhibited increased discharge during NREM sleep compared to waking (Fig. 2B, C). The remaining cells were either wake-related or state-indifferent (Fig. 4). As a group, these 29 neurons had higher mean discharge rates during NREM sleep (6.15 + 1.93) compared to waking rates (5.12 + 1.80). The mean N R E M / W a k e ratio was 2.35 ___ 0.53. This latter value was significantly different from the mean N R E M / w a k e ratio for the warming-suppressed cell group (Fig. 3).
104 neurons exhibited changes of less than 10%/°C in their basal waking discharge rate in response to POAH warming. The spontaneous discharge rates of 82 (79%) of these neurons were recorded during spontaneous episodes of wakefulness and NREM sleep. Of these neurons, 31 (38%) were wake-related, 11 (13%) were NREM sleep-re- lated and 40 (49%) were state-indifferent (Fig. 4).
3.2. Effects o f P O A H warming on stimulation-evoked or- thodromic responses
Of 69 cells tested, orthodromic excitatory responses were evoked in 29 BF neurons (42%) following stimula- tion of the MRF (n = 22) or PLHa (n = 7). Typical re-
226 Md.N. Alam et al. / Brain Research 696 (1995) 221-230
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Fig. 5. Examples of the effects of POAH warming on MRF stimulation-evoked responses in waking-related BF neurons. A and B: per±stimulus histograms from a waking-related cell that displayed suppression of spontaneous discharge during POAH warming. At baseline levels of TpoAu (A), MRF stimulation evoked an initial excitatory response followed by a period of suppressed discharge. Stimulus onset is indicated at time zero, and the histogram is based on 100 pulses delivered at a rate of 1.5 Hz. TpOAH is the average temperature recorded near the POAH thermode during the 150 s when pulses were delivered. During POAH warming (B), spontaneous activity was reduced, the magnitude of evoked excitation diminished, and the duration of post-excitatory discharge suppression increased. C and D: evoked responses from a waking-related cell that did not respond to POAH warming with a change in spontaneous activity; note the similarity in the pre-stimulus portions of the histograms in C and D. POAH warming in this cell was associated with both reductions in the evoked excitation and enhanced post-excitatory discharge suppression.
sponses consisted of an initial excitation followed by a period of discharge suppression (Fig. 5). In 5 cells, the response consisted of excitation only, without discharge suppression. Thirteen of 29 cells exhibiting evoked ortho- dromic responses were waking-related (Table 1). Four of these 13 cells also displayed suppression of spontaneous discharge rates during waking in response to POAH warm- ing.
Effects of POAH warming on evoked responses are shown in Fig. 5 for a waking-related cell that displayed suppression of spontaneous discharge during POAH warm- ing (Fig. 5A, B), and a waking-related cell that did not respond to POAH warming with a change in spontaneous
discharge rate (Fig. 5C, D). In both cases, POAH warming was accompanied by a reduction in the magnitude of the evoked excitatory response, and enhanced discharge sup- pression. For this group of waking-related cells, POAH warming resulted in significant reductions in evoked exci- tation (average number of spikes evoked per stimulus pulse), and significant increases in the duration of post-ex- citatory discharge suppression (Table 1).
Fifteen of 29 BF cells exhibiting evoked responses were sleep-waking state indifferent. In contrast to waking-re- lated cell types, there was no significant effect of POAH warming on stimulation-evoked response measures in the state-indifferent cell group (Table 1).
Table 1 Effects of POAH warming on stimulation-evoked responses in waking-related and state-indifferent basal forebrain neurons
Tar is the average hypothalamic temperature during the delivery of 100 stimulus pulses on which per±stimulus histograms were based. Latency is the latency from the stimulus pulse to the peak of the excitatory response (bin in the poststimulus period of the histogram containing the most spikes). Spikes/pulse is the average number of spikes evoked in the excitatory period per stimulus pulse. Duration of peds is the duration of the period of post-excitatory discharge suppression. P values based on results of paired t-tests, baseline wakefulness versus THy warming wakefulness values. NS, not significant.
Md.N. Alam et aL / Brain Research 696 (1995) 221-230 227
A single sleep-related cell displayed a brief, short la- tency excitation, followed by discharge suppression in response to MRF stimulation. This response was not al- tered by POAH warming.
3.3. Locations of BF recording sites
Shown in Fig. 6 are the locations of recorded cells, coded by sleep-wake discharge profile type, and response to POAH warming. The stippled area in the left panels of Fig. 6 indicate representative locations of the heat ex- change portion of POAH thermodes. Recording sites were in rostral portions of the magnocellular BF, ventral to the anterior commissure, at levels ranging from rostral to the optic chiasm, to the anterior hypothalamus. The majority of the recording sites were located in the horizontal limb of the diagonal band of Broca, the magnocellular preoptic area and rostral portions of the subpallidal substantia in- nominata. Some recording sites were located at the board-
ers of the nucleus accumbens, the supraoptic nucleus, globus pallidus, entopenduncular nucleus, and amygdala.
PLHa stimulating electrodes were located lateral and dorsal to the mammillary bodies and medial to the subtha- lamic nucleus and zona incerta. Brainstem electrodes were located in the midbrain reticular core, ventral and lateral to the central gray matter, and dorsal to the red nucleus.
4. Discussion
We found that 40% of a sample of neurons recorded in magnocellular regions of the BF responded to medial POAH warming with either suppression or facilitation of waking discharge rate. BF neurons that were suppressed by POAH warming were predominately wake-related, i.e., these cells displayed higher spontaneous discharge rates during waking compared to sleep. The majority of neurons that responded to POAH warming with an increase in discharge rate were NREM sleep-related.
A
)
B
°
Fig. 6. Locations of recorded cells, reconstructed on planes of section from the atlas of Snider and Neimer [40]. Numbers in the lower right portion of each coronal section indicate the distance (in ram) rostral to the intra-aural line. A: the stippled area denotes representative locations of the maximum heat exchange portion of POAH thermodes. Symbols indicate recording sites for sleep- and waking-related cell types. Solid triangles: sleep-related, warming facilitated cells; open triangles: sleep-related, warming suppressed cells; open diamonds: sleep-related, unresponsive to warming; solid circles: waking-related, warming-suppressed cells; open circles: waking-related, warming-facilitated cells; open hexagon: waking-related, unresponsive to warming. Abbreviations: AC, anterior commissure; OC, optic chiasm. B: symbols indicate approximate locations of sleep-waking state-indifferent cells, plotted on only two A - P sections for simplicity. Solid squares: state-indifferent, warming-suppressed cells; open squares: state-indifferent, warming-facili- tated cells; solid stars: state-indifferent, unresponsive to warming.
228 Md.N. Alam et al. / Brain Research 696 (1995) 221-230
Local warming of the POAH is known to acutely enhance NREM sleep occurrence [24,33,34]. Therefore, changes in BF discharge rate in response to POAH warm- ing could have been a consequence of state change, rather than a synaptic modulation of BF neurons by POAH thermosensitive cells. However, our experimental design controlled for this possibility. In every instance, the sleep-waking state of the animal during warming trials was carefully monitored behaviorally and electrographi- cally. Trials in which there was any evidence of state change were discarded.
Heat conduction from the medial POAH thermode to adjacent magnocellular BF regions could have influenced BF neuronal discharge via direct thermal effects. While this possibility cannot be completely excluded, we con- sider that this mechanism is unlikely to be of primary importance. We recorded temperature both within the me- dial POAH and at the BF recording site. Changes of 1.5-2.0°C in the medial POAH were associated with an average change of only 0.4°C at the recording sites. Within the BF, local temperature increases were higher at medial versus lateral sites due to proximity to the thermode, yet we did not find a medial to lateral gradation in cells responding to POAH warming (see Fig. 6). In many instances, changes in BF neuronal discharge were tempo- rally more closely related to changes in POAH temperature than to local BF temperature (e.g., see Figs. 1 and 2). For all warming trials, BF temperature reached peak values at an average of 13.2 + 0.63 s following the POAH tempera- ture plateau.
Cholinergic neurons within the magnocellular BF are critically involved in the regulation of electrocortical and behavioral arousal, although noncholinergic BF cell types may also participate in these functions. A direct projection from the BF to the neocortex is well-characterized [3,16,25,35] and spontaneous release of cortical acetyl- choline is elevated during wakefulness compared to NREM sleep [42]. BF electrical stimulation enhances cortical re- lease of acetylcholine [20,31] and exerts excitatory effects on cortical cells [26,30,55]. Neurotoxin-induced cell loss in acetylcholine-rich areas of the BF results in the appearance of EEG slow wave activity in the cortical EEG during waking [5,32,37], an effect reversed by intracortical trans- plantation of fetal BF tissue [51]. BF neurons that display peak discharge rates during wakefulness and reduced dis- charge during NREM sleep have been described [5,9,10,45,48]. A subgroup of waking-related BF neurons was found to project directly to cortex [5,11,48,49]. Cholinergic neurons in the rostral BF of the cat do not occur as a compact nucleus, but are interspersed with noncholinergic cell types [17,52]. Therefore, the population of waking-related cells described here probably included both cholinergic and noncholinergic neurons. The finding that 45% of a population of waking-related BF cells demonstrated suppression of waking discharge in response to medial POAH warming, can be taken as evidence for a
functionally significant POAH thermosensitive modulation of cholinergic and noncholinergic arousal mechanisms within the BF.
A group of 13 waking-related neurons that were ortho- dromically driven by PLHa or MRF electrical stimulation displayed significant reductions in stimulation-evoked ex- citation and prolongation of post-excitatory discharge sup- pression during POAH warming (Fig. 5 and Table 1). This effect was observed only in waking-related cell types. Evoked excitatory responses were not altered by POAH warming in a group of sleep-wake state indifferent cells. BF arousal-related cell types are hypothesized to relay ascending activating influences to neocortical, limbic sys- tem, and thalamic sites [6,15,38]. Our findings indicate that activation of POAH warm-sensing neurons may act to impede this flow of ascending activation in a subpopula- tion of arousal-related cells, and thereby promote sleep.
Our results do not demonstrate whether the effects of POAH warming on BF waking-related neurons reflect a direct inhibitory input from POAH warm-sensitive neurons (and/or excitatory inputs from cold-sensitive neurons), or are due to, indirect, multisynaptic pathways. The finding that BF waking-related cells exhibited diminished evoked excitability in response to PLHa and MRF stimulation (Fig. 5; Table 1) suggests that POAH warming was associ- ated with hyperpolarization of these cells. Direct projec- tions from the medial POAH to magnocellular portions of the BF have been described in rats [7]. POAH afferents make symmetrical synaptic contacts on BF cholinergic neurons, indicative of inhibitory inputs [57]. Excitation of POAH warm-sensing neurons by local warming could exert either presynaptic inhibitory effects, or local post- synaptic dendritic inhibition at sites of brainstem synaptic contact, resulting in a suppression of stimulation-evoked excitation. That POAH warming was further associated with prolongation of the period of post-excitatory dis- charge suppression suggests that somatic inhibition was augmented in some cells.
The magnocellular BF contains sleep-promoting as well as arousal-promoting cell populations. Electrical stimula- tion of the rostral ventral BF, in the vicinity of the horizontal limb of the diagonal band of Broca and the magnocellular preoptic area, has acute sleep-promoting effects [2,41] and suppresses arousal-related neuronal dis- charge in the MRF [21,47]. Electrolytic- or neurotoxin-in- duced lesions of these BF sites causes persistent insomnia in cats [22,46]. Neurons that display elevations in sponta- neous discharge rate during NREM sleep compared to waking have been recorded in the BF of cats [45,48] and rats [11,18,29]. The neurochemical phenotype(s) of BF sleep-related neurons is unknown, but GABAergic neurons with descending projections are potential candidates for mediating the brainstem inhibitory effects of BF stimula- tion [12].
We found that 49% of a population of BF sleep-related cells displayed increased discharge in response to local
Md.N. Alam et al. / Brain Research 696 (1995) 221-230 22t. ~
P O A H warming . These f indings indicate that a funct ion-
ally s ignif icant thermosensi t ive modula t ion of BF sleep-
promot ing mechan i sms originates wi thin the POAH. Again , the results of the present study cannot determine if this thermosensi t ive modula t ion is relat ively direct, or is due to indirect effects on BF sleep-related cell types via modula-
t ion of bra ins tem and PLHa arousal mechanisms. While we have emphasized the role of changes in BF
unit activity evoked by P O A H warming in the control of
s l e e p - w a k i n g state, they may also reflect thermoregulatory funct ions of the POAH. Regulatory adjustments made in response to mild heat exposure, i.e., reduct ions in motor
activity, reduced metabol ic rate, reduced respiratory rate, and augmented peripheral heat loss, also normal ly accom- pany sleep onset (see [14,28] for review). Exper imenta l
manipula t ions of the BF and PLHa inf luence locomotor activity [27,39,54,56]. The PLHa also participates in the
control of respiration [53,54]. P O A H thermosensi t ive influ- ences on cells in these brain areas may funct ion to activate
appropriate thermoregulatory responses to heat loads. In many instances, exposure to mild to moderate heat
loads evokes the state of N R E M sleep as an initial regula- tory response [28]. The abil i ty of natural thermal st imuli to evoke sleep, and the effectiveness of N R E M sleep as an
adaptive response to mild thermal loads points to the close association of hypothalamic thermoregulatory and sleep
regulatory mechan i sms [23]. Recent work from our labora- tory has demonstra ted that identif ied warm-sens i t ive neu-
rons in the medial P O A H of cats undergo spontaneous increases in discharge rate dur ing transi t ions from wakeful-
ness to sleep [1]. Cold-sensi t ive P O A H neurons decrease
spontaneous discharge dur ing sleep compared to waking. These spontaneous changes in the discharge rate of POAH temperature-sensi t ive neurons are quali tat ively and quanti- tatively s imilar to those induced by local P O A H warming , such as performed in this study. Therefore, the modula t ion
by P O A H thermosensi t ive neurons of arousal and sleep mechan i sms within the BF, PLHa, and bra ins tem can be hypothesized to contr ibute not only to sleep evoked by
central or whole-body warming , but to spontaneous transi-
t ions between waking and N R E M sleep as well.
Acknowledgements
Supported by the Depar tment of Veterans Affairs and
USPHS Grant MH47480.
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