Ž . Brain Research 753 1997 69–79 Research report Dopamine depletion in the rostral nucleus accumbens alters the cerebral metabolic response to cocaine in the rat David Lyons, Linda J. Porrino ) Center for the Neurobiological InÕestigation of Abused Drugs, Department of Physiology and Pharmacology, Bowman Gray School of Medicine, Medical Center BouleÕard, Winston-Salem, NC 27157, USA Accepted 10 December 1996 Abstract The functional consequences of dopamine depletion in the rostral nucleus accumbens were examined using the quantitative w 14 x 2- C deoxyglucose method for determining rates of local cerebral glucose utilization. Cerebral metabolism was determined in 35 brain structures of Sprague–Dawley rats with unilateral 6-hydroxydopamine or sham lesions of the rostral accumbens. The effect of the lesion was assessed in cocaine-naive animals treated systemically with cocaine or saline. In saline-treated animals, the lesion increased cerebral metabolism in typical basal ganglia regions, such as the globus pallidus and entopeduncular nucleus, as well as portions of the extended amygdala that included the bed nucleus of the stria terminalis and the hypothalamic preoptic area. Cerebral metabolism was affected bilaterally in a subset of all affected structures which demonstrated that the functional consequences of the lesion extended beyond the primary monosynaptic output zones of the rostral accumbens. The lesion also changed the topography of the normal cocaine response such that cocaine effects were blunted in the shell of the nucleus accumbens, globus pallidus and the medial ventral pallidum. Thus, the present study describes functional evidence of the link between the rostral accumbens and the extended amygdala and demonstrates that dopamine in the rostral accumbens plays an important role in the central response to cocaine. q 1997 Elsevier Science B.V. All rights reserved. Keywords: Basal forebrain; Amygdala; Local cerebral glucose utilization; Ventral striatum 1. Introduction Ž . The nucleus accumbens Acb and the olfactory tuber- w x cle together comprise the ventral striatum 15 . This region of forebrain is thought to function as a site of convergence w x of information from limbic and motor systems 37 and as a site for the processing of information related to the motivational and reinforcing properties of natural rein- w x forcers and drugs of abuse 27,57 . The Acb is a heteroge- nous structure that can be divided into medial and lateral w x parts 18 , now commonly called the shell and core, re- w x spectively 59 . Distinctions between shell and core are w x made based on immunohistochemistry 15,18,22,56,62,63 , w x w x receptor binding 15,59 , cytoarchitecture 34,35 , mor- w x w x phology 34 , anatomical connectivity 3,4,25,60 and sus- w x ceptibility to neurotoxins 23,36,55,61 . Recent studies have ) Ž . Corresponding author. Fax: q1 910 716 8501; E-mail: [email protected]also identified functional distinctions between the shell and core of the Acb in that, for example, the shell appears to be more sensitive to the effects of cocaine and other drugs w x of abuse 40 , and the blockade of AMPA or kainate glutamate receptors in the medial shell, but not the core, w x elicits ingestive behavior 32 . Although the shell and core can be readily differentiated from one another in caudal portions of the Acb, such a distinction is far more difficult in rostral regions. The density of immunoreactivity for substance P and acetyl- cholinesterase activity, for example, which clearly delin- eate core from shell more caudally, does not confer clear borders between subdivisions in the more rostral portions w x of the Acb 22,62 . Furthermore, the patterns of afferent and efferent connectivity of the shell and core are quite distinct from one another in more caudal regions, but exhibit a considerable degree of overlap more rostrally w x 56,60 . The rostral pole of the accumbens, therefore, appears to form a separate subdivision of this nucleus, exhibiting both core- and shell-like characteristics. This 0006-8993r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. Ž . PII S0006-8993 96 01493-X
Transcript
Ž .Brain Research 753 1997 69–79
Research report
Dopamine depletion in the rostral nucleus accumbens alters the cerebralmetabolic response to cocaine in the rat
David Lyons, Linda J. Porrino )
Center for the Neurobiological InÕestigation of Abused Drugs, Department of Physiology and Pharmacology, Bowman Gray School of Medicine, MedicalCenter BouleÕard, Winston-Salem, NC 27157, USA
Accepted 10 December 1996
Abstract
The functional consequences of dopamine depletion in the rostral nucleus accumbens were examined using the quantitativew14 x2- C deoxyglucose method for determining rates of local cerebral glucose utilization. Cerebral metabolism was determined in 35 brain
structures of Sprague–Dawley rats with unilateral 6-hydroxydopamine or sham lesions of the rostral accumbens. The effect of the lesionwas assessed in cocaine-naive animals treated systemically with cocaine or saline. In saline-treated animals, the lesion increased cerebralmetabolism in typical basal ganglia regions, such as the globus pallidus and entopeduncular nucleus, as well as portions of the extendedamygdala that included the bed nucleus of the stria terminalis and the hypothalamic preoptic area. Cerebral metabolism was affectedbilaterally in a subset of all affected structures which demonstrated that the functional consequences of the lesion extended beyond theprimary monosynaptic output zones of the rostral accumbens. The lesion also changed the topography of the normal cocaine responsesuch that cocaine effects were blunted in the shell of the nucleus accumbens, globus pallidus and the medial ventral pallidum. Thus, thepresent study describes functional evidence of the link between the rostral accumbens and the extended amygdala and demonstrates thatdopamine in the rostral accumbens plays an important role in the central response to cocaine.
q 1997 Elsevier Science B.V. All rights reserved.
Keywords: Basal forebrain; Amygdala; Local cerebral glucose utilization; Ventral striatum
1. Introduction
Ž .The nucleus accumbens Acb and the olfactory tuber-w xcle together comprise the ventral striatum 15 . This region
of forebrain is thought to function as a site of convergencew xof information from limbic and motor systems 37 and as
a site for the processing of information related to themotivational and reinforcing properties of natural rein-
w xforcers and drugs of abuse 27,57 . The Acb is a heteroge-nous structure that can be divided into medial and lateral
w xparts 18 , now commonly called the shell and core, re-w xspectively 59 . Distinctions between shell and core are
w xmade based on immunohistochemistry 15,18,22,56,62,63 ,w x w xreceptor binding 15,59 , cytoarchitecture 34,35 , mor-
w x w xphology 34 , anatomical connectivity 3,4,25,60 and sus-w xceptibility to neurotoxins 23,36,55,61 . Recent studies have
also identified functional distinctions between the shell andcore of the Acb in that, for example, the shell appears tobe more sensitive to the effects of cocaine and other drugs
w xof abuse 40 , and the blockade of AMPA or kainateglutamate receptors in the medial shell, but not the core,
w xelicits ingestive behavior 32 .Although the shell and core can be readily differentiated
from one another in caudal portions of the Acb, such adistinction is far more difficult in rostral regions. Thedensity of immunoreactivity for substance P and acetyl-cholinesterase activity, for example, which clearly delin-eate core from shell more caudally, does not confer clearborders between subdivisions in the more rostral portions
w xof the Acb 22,62 . Furthermore, the patterns of afferentand efferent connectivity of the shell and core are quitedistinct from one another in more caudal regions, butexhibit a considerable degree of overlap more rostrallyw x56,60 . The rostral pole of the accumbens, therefore,appears to form a separate subdivision of this nucleus,exhibiting both core- and shell-like characteristics. This
0006-8993r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved.Ž .PII S0006-8993 96 01493-X
( )D. Lyons, L.J. PorrinorBrain Research 753 1997 69–7970
suggests that this portion of the Acb can be viewed as aunique subregion and not merely as a rostral extension of
w xeither the shell or core 64 . It further raises the possibilitythat the rostral pole may not only be anatomically, but alsofunctionally, distinct from other portions of the nucleusaccumbens.
In support of this view is growing evidence for theimportance of the rostrocaudal dimension within the nu-cleus accumbens. The expression of dopamine D recep-3
tors within the accumbens, for example, follows both arostrocaudal and mediolateral gradient, with the highestdensity of receptors expressed in the rostral pole regionw x10 . Rostrocaudal gradients can also be observed in the
w xdistribution of D receptors 2,7,22 , as well as for the2w xexpression of mRNA for dopamine D receptors 22 ,1
w xenkephalin, dynorphin, and substance P 55 . Chronic re-peated cocaine administration in rats produces an up-regu-lation of m opioid receptors in the rostral accumbens atlower doses than that needed to up-regulate these receptors
w xthroughout the Acb 53,54 . In addition, the regulation ofacetylcholine release as evoked by the activation of D2
receptors in the accumbens differs along a rostrocaudalgradient, suggesting differential interactions betweendopaminergic and cholinergic neurons along this dimen-
w xsion 17 . In studies of cerebral metabolism, the rostralaccumbens appears to be exceptionally sensitive to theeffects of cocaine and other psychostimulants in that acti-vation in the rostral pole occurs at lower doses than thatnecessary to increase metabolism in more caudal regions
w xof the accumbens 41,43 . Furthermore, prolonged expo-sure to cocaine results in decreased rates of cerebralmetabolism restricted to the rostralmost portions of the
w xnucleus 12,13 . These data form the basis of the hypothe-sis that the rostral accumbens may play a unique role inthe functional response to cocaine. Further functional stud-ies of this portion of the nucleus accumbens, however, arenecessary to establish the specific significance of the ros-tral pole.
Numerous studies have established a critical role fordopamine, particularly within the nucleus accumbens, as asubstrate for the reinforcing effects of appetitive stimuli,
w xincluding drugs of abuse 27,37,57 . Destruction of thedopaminergic inputs to the Acb by the neurotoxin, 6-hy-
Ž .droxydopamine 6-OHDA , decreases intravenous self-ad-w xministration maintained by cocaine 45 . Furthermore, the
administration of dopaminergic antagonists directly intow xthe Acb alters cocaine self-administration 33,38,44 . The
purpose of the present series of experiments was to iden-tify the distribution of changes in functional activity thataccompany the selective depletion of dopamine in the
w14 xrostral Acb by 6-OHDA as determined by the 2- C de-Ž .oxyglucose 2-DG method. A second focus of this study
was to characterize the specific contribution of thedopaminergic innervation of the rostral Acb to the patternsof functional activity produced by systemic administrationof cocaine.
2. Materials and methods
2.1. Animals
Experiments were performed on male Sprague–Dawleyrats weighing between 225–300 g at the start of theexperiment. Rats were pair housed in standard hangingrodent cages with food and water available ad libitum
Žunder standard controlled lighting 12 h light–dark cycle;.lights on 05:00–17:00 h and standard temperature condi-
tions. All procedures were carried out according to estab-lished practices as described in the NIH guide for care anduse of laboratory animals. In addition, all procedures werereviewed and approved by the Animal Care and UseCommittee of the Bowman Gray School of Medicine ofWake Forest University.
2.2. Experimental procedure
In order to evaluate the functional consequences ofdopamine depletion of the rostral pole of the nucleusaccumbens, rates of local cerebral glucose utilization werecompared in two groups of rats: those with a unilateral
Ž .6-OHDA lesion of the rostral Acb ns6 and thoseŽ .receiving sham lesions of the rostral pole ns6 . The
unilateral lesions permitted both within-subject compar-isons, as well as comparison to sham-lesioned controls.Seven days following rostral pole lesions, rats were pre-pared for the 2-DG procedure. The procedure was initiatedby bolus injection of radiotracer 3 min after intravenous
Ž .injections of saline 1 mlrkg and preceded as describedbelow.
The second experiment was undertaken to determine therole of rostral Acb dopamine in mediating the acute effectsof cocaine. The effects of the acute intravenous administra-
Ž .tion of cocaine 5 mgrkg on rates of local cerebralglucose utilization were measured in two groups of rats:those with unilateral 6-OHDA lesions of the rostral AcbŽ .ns4 and those receiving sham lesions of the rostral poleŽ .ns4 . As in experiment 1, 7 days following rostral polelesions, rats were prepared for the 2-DG procedure. Theprocedure was also initiated 3 min after the intravenousinjection of cocaine.
2.3. Drugs
Cocaine hydrochloride was obtained from the NationalŽ .Institute on Drug Abuse Research Triangle, NC , and
Ž .dissolved in physiological saline 1.0 mlrkg . Equivalentvolumes of saline were administered to sham-lesionedcontrol subjects. Doses were calculated as the salt.
ŽDesmethylimipramine DMI, Sigma Chemical Co., St..Louis, MO was also dissolved in saline. 6-Hydroxy-
Ž .dopamine HBr 6-OHDA; Sigma Chemical Co. was dis-
( )D. Lyons, L.J. PorrinorBrain Research 753 1997 69–79 71
solved in physiological saline containing 0.25% ascorbateŽ .Sigma Chemical Co. .
2.4. 6-Hydroxydopamine lesions
ŽRats were anesthetized with pentobarbital 50 mgrkg,.i.p. and unilateral lesions were made by micro-injection of
Ž .4 mg 0.5 or 1.0 ml of 6-OHDA in a solution that wasŽinfused over a 5 min period into the rostral Acb coordi-
nates: 2.9 mm anterior to bregma, 0.8 mm lateral from the.midline, and 7.2 mm from the surface of the skull . Thirty
Žminutes before the infusion of 6-OHDA, DMI 15 mgrkg,.i.p. was injected to protect noradrenergic uptake sites. In
animals receiving neurotoxin lesions, an equal volume ofvehicle was injected into the contralateral hemisphere atthe same coordinates. Sham-lesioned control rats wereinjected bilaterally with vehicle into the rostral pole of thenucleus accumbens. All animals were allowed to recoverfor 1 week before further testing.
2.5. Local cerebral glucose utilization
On the day of the measurement of rates of local cerebralglucose metabolism rats were anesthetized with a mixtureof halothane and nitrous oxide. Polyethylene catheterswere inserted into a femoral vein and artery and runsubcutaneously exiting at the nape of the neck. Suchcatheter placement allows the intravenous administrationof drug or tracer and permits animals to move freely
w xthroughout the experimental procedure 8 . Animals wereallowed at least 4 h to recover from the catheter placementbefore the initiation of the 2-DG procedure.
The 2-DG experimental procedure was initiated by theinjection of an intravenous pulse of 125 mCirkg of 2-de-
w 14 x Žoxy-D- 1- C glucose New England Nuclear, Boston, MA;.spec. act. 50–55 mCirmmol followed by a flush of
heparinized saline. Timed arterial blood samples weredrawn thereafter at a schedule sufficient to define the timecourse of the concentrations of arterial 2-DG and plasmaglucose. Arterial blood samples were centrifuged immedi-ately. Plasma concentrations of 2-DG were determined by
Žliquid scintillation spectrophotometry Beckman Instru-.ments, Fullerton, CA and plasma glucose concentrations
Žassessed with a glucose analyzer Beckman Instruments,.Fullerton, CA . Approximately 45 min after tracer injec-
tion, the animals were killed by an intravenous overdose ofŽ .sodium pentobarbital 100 mgrkg, i.v. . Brains were
Ž .rapidly removed, frozen in isopentane y458C and storedŽ .at y708C. Coronal sections 20 mm thick were cut in a
cryostat maintained at y228C. Five of every 10 sectionswere thaw-mounted on glass coverslips, dried on a hot-plate, and autoradiographed with Kodak EMC or MIN-R
w14 xX-ray film, along with a set of C methylmethacrylateŽ .standards Amersham, Arlington Heights, IL previously
calibrated for their equivalent 14C concentration in 20 mmbrain sections.
[ 3 ]2.6. H Mazindol autoradiography
w3 xDesmethylimipramine-insensitive H mazindol auto-radiography was carried out based on procedures described
w x w xby Javitch et al. 20 as adapted by Sharpe et al. 48 .Sections were preincubated for 2 min in each of 6 succes-sive rinses of 100 mM Tris-HCl, pH 7.4 at 08C. Theseconditions were selected to insure removal of the 2-DGtracer and its metabolite from sections before incubationw x14 . Briefly, following preincubation sections were thenincubated for 40 min at 48C in buffer containing 5 nMw3 x Ž . ŽH mazindol 19 mCirmmol New England Nuclear,
.Boston, MA in the presence of 30 mM DMI. Sectionswere then rinsed in two consecutive 3 min washes inbuffer at 48C, with a final 10 s rinse in cold water.Non-specific binding was defined in the presence of 600mM unlabeled benztropine. Tissue sections were immedi-ately dried under a stream of cold air. The sections were
Žthen placed in apposition to Ultrofilm Leica, Cambridge,.MA for 4 weeks in the presence of tritium standards
Ž .Amersham, Arlington Heights, IL .
2.7. Densitometry
Autoradiograms were analyzed by quantitativedensitometry with a computer-assisted image processing
Ž .system Imaging Research Inc., St. Catharine’s, Ontario .Optical density measurements of tissue 14C concentrationsfor each structure were made in a minimum of five brain
w3 xsections. Measurements of tissue H mazindol concentra-tions were made in a minimum of two brain sections.Tissue 14C and 3H concentrations were determined fromthe optical densities of tissue as compared to calibrationcurves obtained by densitometric analysis of the autoradio-grams of calibrated standards. Glucose utilization wascalculated from the local tissue 14C concentrations, the
w14 xtime course of the plasma C deoxyglucose and glucoseconcentrations by the operational equation of the methodw x 350 . Tissue H concentrations were determined asfmolrmg tissue in individual brain regions.
2.8. Statistics
ŽStandard statistics software SPSS for Windows,.Chicago, IL was used for statistical analysis. The extent
of dopamine depletion following unilateral 6-OHDA le-sions was evaluated by comparing the density ofw3 xH mazindol binding in nine individual regions of thestriatum both ipsilateral and contralateral to the lesion tothe density found in sham-lesioned rats. Comparisons weremade by a one-way analysis of variance followed by aBonferroni test for multiple comparisons.
Rates of local cerebral glucose utilization were deter-mined in 35 discrete brain regions, and statistical analysiswas carried out on each structure individually. To evaluatethe effects of dopaminergic depletion of the rostral pole of
( )D. Lyons, L.J. PorrinorBrain Research 753 1997 69–7972
the nucleus accumbens, side to side differences in rates ofglucose utilization were compared in 6-OHDA and sham-lesioned groups by means of Student’s t-tests for pairedcomparisons. Because no side to side differences wereobserved in sham-lesioned animals, and statistically signif-
Ž .icant differences each of a magnitude less than 5% werenoted in only 4 of 35 structures in lesioned rats, rates ofglucose utilization ipsilateral and contralateral to the sideof the lesions were combined and compared by means ofStudent’s t-tests for independent samples.
The effects of cocaine administration on rates of localcerebral glucose utilization in 6-OHDA- and sham-le-sioned rats were also measured in 35 structures. Side toside differences were assessed as in the first experiment bymeans of Student’s t-tests for paired comparisons, andrates combined as above. The effects of cocaine in sham-and 6-OHDA-lesioned rats were first determined by com-parison of combined bilateral rates of glucose utilizationwith their respective saline-treated groups by means ofStudent’s t-tests for independent samples. To further com-pare the effects of cocaine administration in these groups,the percent change in rates of metabolism for each co-
Ž .caine-treated rat sham- and 6-OHDA-lesioned was calcu-lated as a function of the mean rate in their respectivesaline-treated control groups. This procedure was followedin order to control for changes in baseline rates of glucoseutilization that resulted from 6-OHDA-lesioned rats. Thepercent change in sham- and 6-OHDA-lesioned groupswas then compared by means of Student’s t-tests forindependent samples.
3. Results
3.1. Effects of unilateral 6-OHDA lesions of the rostral[ 3 ]accumbens on H mazindol binding sites
The extent of each 6-OHDA lesion was evaluated auto-radiographically by determining the degree of loss ofdopamine uptake sites in regions of the striatum as shown
w3 xin Fig. 1. Densities of H mazindol binding in theseregions are shown in Table 1. Quantitative analysis ofw3 xH mazindol binding sites in sham-lesioned rats showedno side to side differences in density, hence, values werecombined and compared to levels of binding sites ipsilat-eral and contralateral to the lesion. Measurement of bind-ing sites demonstrated that at 7 days post lesion, bindingsite density ipsilateral to the lesion in the rostral pole ofthe Acb was significantly reduced to 69% of the contralat-
Ž .eral side P-0.05 as well as significantly decreased ascompared to the densities of sham-lesioned controls. No
w3 xdifferences between the densities of H mazindol bindingcontralateral to the side of the lesion and values in sham-lesioned controls were noted. Although the greatest loss ofsites was restricted to the rostral pole in all animals, thedensity of binding sites in the ipsilateral anterior portion ofthe central accumbens was also significantly reduced to
Ž .50% of the contralateral side P-0.05 , as well as com-pared to densities in sham-lesioned rats. Small reductionswere observed in the shell and core portions of the accum-bens at more caudal levels and the olfactory tubercle, butthese were not statistically significant. No differences were
w3 xFig. 1. Schematic diagram of regions examined for H mazindol binding as a measure of the extent of DA terminal damage in lesioned animals. The6-OHDA lesion reduced binding primarily in the rostral accumbens. The numbers refer to the distance of the section from bregma. aCPsanteriorcaudaterputamen; DLsdorsolateral caudate; DMsdorsomedial caudate; Corescore of accumbens; OTsolfactory tubercle; Shellsshell of theaccumbens, Vsventral caudate.
( )D. Lyons, L.J. PorrinorBrain Research 753 1997 69–79 73
Table 1w3 x Ž .H Mazindol binding fmolrmg tissue in sham-lesioned and unilaterally6-OHDA-lesioned animals showing percent depletion of binding in theipsilateral hemisphere compared to the contralateral hemisphere of le-
a Significant difference between ipsilateral levels compared to contralat-Žeral levels in lesioned animals and levels in sham-treated animals one-way
.ANOVA, P -0.0001; Bonferroni, P -0.05 .
evident in the dorsal striatum. Two animals were notincluded in the analysis because lesions were not localizedwithin the rostral pole of the accumbens.
3.2. Experiment 1. Effects of unilateral 6-OHDA lesions ofthe rostral accumbens on rates of local cerebral glucoseutilization
Rates of local cerebral glucose utilization in sham-le-sioned and unilaterally 6-OHDA-lesioned rats measured in35 individual structures are shown in Table 2. To aid thereader in understanding the precise location of importantregions measured in the present study, Fig. 2 contains anatlas illustrating regions of interest for several sites in thebasal forebrain superimposed on deoxyglucose autoradio-grams from normal rat brain cut in the coronal plane.Because significant side to side differences in rates ofglucose utilization in sham-lesioned rats were not observedin any structure, bilateral values are reported. Side to sidecomparisons of rates of glucose utilization in unilaterallydopamine-depleted rats revealed small, but significant, de-creases in metabolic rates at the site of the lesion in the
Ž .rostral pole of the Acb ipsilateral to the lesion y4% .Other side to side decreases in metabolic rate were notedin the agranular insular cortex adjacent to the rhinal fissureŽ .y5% . Increased rates of glucose utilization ipsilateral tothe lesion were observed in the entopeduncular nucleusŽ . Ž .q6% and in medial aspects of the habenula q5% . Itshould be noted that these statistically significant asymme-tries in metabolic rates were extremely small, but were inkeeping with the circumscribed nature of the lesions whichwhere limited to the rostral pole of the nucleus accumbens.
In contrast to the limited unilateral changes in cerebralmetabolism, unilateral 6-OHDA lesions of the rostral poleof the Acb also produced robust bilateral alterations inrates of glucose utilization, as compared to sham-treatedcontrols. At 7 days post lesion, rates of glucose utilization
Ž .were increased bilaterally in the globus pallidus q15% ,Ž .bed nucleus of the stria terminalis q10% , and the preop-
Ž .tic area of the hypothalamus q16% .
Table 2Ž .Local cerebral metabolic rates for glucose mmolr100 grmin following
Ža unilateral 6-OHDA lesion of the rostral nucleus accumbens mean"
a ŽSignificant side to side effect of lesion in lesioned animals paired.t-test, P -0.05 .
b Significant difference between bilateral metabolism in sham and le-Ž .sioned animals t-test, P -0.05 .
( )D. Lyons, L.J. PorrinorBrain Research 753 1997 69–7974
3.3. Experiment 2. Effects of the administration of cocainein 6-OHDA-lesioned animals
The administration of cocaine to sham-lesioned animalsincreased cerebral metabolism in portions of the mesocorti-colimbic system including all portions of the nucleusaccumbens, ventral pallidum, and lateral septum, as wellportions of the nigrostriatal system including the caudate,substantia nigra reticulata, subthalamic nucleus and globus
Ž .pallidus data not shown . These changes were essentiallyw xidentical to those noted previously 41 .
The effects of cocaine administration in rats receiving6-OHDA lesions of the rostral pole of the Acb on rates ofglucose utilization were largely bilateral. Few side to sidedifferences were found in lesioned animals that receivedcocaine and as in the previous experiments were relativelysmall in magnitude. These included reduced cerebral
Ž .metabolism in agranular insular cortex y4% , ventralŽ . Žcaudate y4% and the basolateral amygdala y8%, data.not shown .
Because baseline rates of glucose utilization were al-tered by 6-OHDA lesions as compared to rates in sham-le-sioned rats, in order to compare the effects of cocaineadministration in these groups, rates of glucose utilizationwere expressed as a percent of rates in saline-treatedcontrols. These data are shown in Table 3 and Fig. 3. Inmost regions within the nigrostriatal and mesocorticolim-
Table 3Ž .Effect of cocaine 5 mgrkg, i.v. on local cerebral metabolic rates for
Ž .glucose mmolr100 grmin in animals with a unilateral 6-OHDA lesionŽ . Ž . Žof the rostral accumbens ns4 and sham-treated ns4 rats mean"
Only those regions are shown that were affected by cocaine or showed aninteraction between cocaine and lesion. Data are presented as percentchange from saline-treated animals with the same lesion treatment.a Significant difference between bilateral metabolism in sham and le-
Ž .sioned animals t-test, P -0.05 .
w14 xFig. 2. Autoradiograms of coronal sections of normal rat brain from animals used according to the 2- C deoxyglucose procedure. Darker areas indicate aŽ .greater regional accumulation of radiotracer. Each numbered oval or set of ovals represents a distinct region of interest as identified in the present study.
( )D. Lyons, L.J. PorrinorBrain Research 753 1997 69–79 75
Fig. 3. Percent change in bilateral local cerebral glucose utilization,Žnormalized to saline treatment, following cocaine administration 5
.mgrkg, i.v. in sham-treated rats and in animals with a unilateral6-OHDA lesion. Asterisks indicate a significant attenuation of the effect
Ž .of cocaine in lesioned animals as compared to shams t-test, P -0.05 .GPsglobus pallidus; mVPsmedial ventral pallidum; Acbsnucleusaccumbens; OTsolfactory tubercle.
bic systems, cocaine administration had similar effects insham- and 6-OHDA-lesioned rats. These included the coreportion of the nucleus accumbens, olfactory tubercle, lat-eral septum, substantia nigra reticulata and subthalamus.Unilateral 6-OHDA lesions of the rostral pole of thenucleus accumbens, however, attenuated the effects ofcocaine in the shell of the nucleus accumbens, the medialaspect of the ventral pallidum, and the globus pallidus bothipsilateral and contralateral to the lesion. Diminished ratesof glucose utilization were also noted in the rostral accum-bens, although these did not reach statistical significance.In addition, rates of glucose utilization in the anteriorcingulate cortex, although not altered by cocaine adminis-tration in sham-lesioned rats, were significantly increasedin 6-OHDA-lesioned animals.
4. Discussion
The present study evaluated the functional conse-quences of the unilateral loss of dopamine within therostral pole of the nucleus accumbens. This study wasconducted to determine the contribution of dopamine withinthis specific site to basal levels of functional activity invarious brain regions, as well as to the functional responseelicited by cocaine administration. Unilateral 6-OHDAlesions aimed at the rostralmost portions of the nucleusaccumbens produced a loss of dopamine terminals that inturn resulted in significantly reduced rates of cerebralmetabolism at the site of the lesion, whereas metabolismwas unaffected in other portions of the accumbens. Thehighly restricted nature of the changes in cerebralmetabolism within the nucleus accumbens suggests thatthe lesions were largely limited to the rostral pole. Al-though dopaminergic terminals make up a minority of the
total striatal synapses and the loss of these terminals in thepresent study was partial, with maximal depletion of about70%, taken together these data indicate that dopamine is amajor determinant of functional activity within this striatalregion.
Although the changes in glucose utilization produced byunilateral dopaminergic denervation of the rostral polewere relatively circumscribed, affecting only 8 out of 35measured regions, the restricted nature of these changes isconsistent with previous studies of dopaminergic deple-
w xtions in the nigrostriatal system 11,29,52,58 . In thesestudies in which profound striatal depletions were on theorder of 95–98%, far larger than 60–70% depletions of thepresent study, functional alterations were generally limitedto discrete portions of basal ganglia such as the globuspallidus and to the lateral habenula. An alternative expla-nation for the discrete nature of the changes observed herethat should be considered, as well, is the degree of deple-tion. More complete loss of dopamine may have led tomore widespread effects on functional activity. It is alsopossible that denervation of a broader anatomical extent ofthe accumbens involving other subdivisions is necessary toalter functional activity in a greater number of the projec-tion fields.
Changes in glucose utilization in specific brain regionshave been primarily attributed to changes in the afferentinputs to that region, in contrast to the activity of cell
w xbodies 24,47,49 . Therefore, alterations in glucose utiliza-tion following dopaminergic depletion can be expected tobe largely localized to the projection sites of the affectedarea. In addition, changes in metabolism are not necessar-ily limited solely to primary output areas, but may alsoinvolve regions that receive secondary and tertiary trans-synaptic connections from the area of origin. The topogra-phy of the functional response to the unilateral 6-OHDAlesion of the rostral pole included some areas with knowndirect neuroanatomical connections of the rostral Acbw x4,5,62,64 , but alterations were not restricted to theseareas. Included were several regions with only limited orno connections with the rostral nucleus accumbens. Forexample, metabolism was altered in the preoptic area ofthe hypothalamus which receives direct projections fromthe rostral pole, but was altered as well in the bed nucleusof the stria terminalis and the more caudal portions of theglobus pallidus, despite the lack of direct projections fromthe rostral pole.
It is also important to emphasize that changes occurredonly in a subset of all projection sites of the rostral pole ofthe nucleus accumbens. There are strong projections fromthe rostral pole of the accumbens to the lateral hypothala-mus and the medial portion of the ventral pallidum, butmetabolism in these regions was unaffected. These datasuggest then that the strength of efferent projections alonedoes not predict the degree of metabolic change found in aparticular region of interest. Other factors such as thenature of the neurotransmitter and the temporal character-
( )D. Lyons, L.J. PorrinorBrain Research 753 1997 69–7976
istics of the firing pattern can also influence the size ofmetabolic changes that follow a given manipulation. Thisreport does, however, provide evidence of a functional linkbetween dopaminergic transmission in the rostral accum-bens and activity in both basal ganglia-like projections ofthe ventral striatum, e.g., globus pallidus and the entope-duncular nucleus, and limbic zones associated with theventral striatum, e.g., preoptic region and the bed nucleusof the stria terminalis.
Further evidence that the effects of dopamine depletionof the rostral accumbens on cerebral metabolism do notoccur solely in the direct anatomical connections of thisportion of the accumbens is provided by the bilateralnature of many of the changes in glucose utilization.Because the anatomical projections from the accumbens
w xare thought to be principally unilateral 16,64 , the bilateraleffects observed here must be attributed to multisynapticinter-hemispheric interactions. Physiological evidence forsuch bilateral interaction exists as reported in a number ofstudies of unilateral dopaminergic depletion of the caudatew x6,36 , self-stimulation to the medial forebrain bundlew x28,42 , as well as pharmacologic manipulation of the
w xsubstantia nigra 9,46 . In each case a unilateral manipula-tion also resulted in significant bilateral effects on func-tional activity. The bilateral nature of the consequences ofdopaminergic denervation of the rostral nucleus accum-bens is in sharp contrast to the strictly unilateral nature ofthe metabolic changes following dopaminergic depletion
w xof the nigrostriatal system 11,29,52,58 . This suggests thatthere may be differences in the role of dopamine in theprocessing of motivational and reward-related informationwithin the limbic system as compared to the processing ofsensory and motor information in the basal ganglia. Fur-thermore, the bilateral nature of the effects of unilateraldopaminergic denervation questions the assumption thatthe contralateral hemisphere can be used as a valid controlin experiments in which the effects of unilateral manipula-tions of DA in the nucleus accumbens are investigated, a
w xpoint emphasized by others 31 .There are several possible means by which the spread
of lesion effects on cerebral metabolism beyond the bound-aries of the efferent terminations of the rostral accumbenscan be envisioned. Although the lesion was primarilylocalized to the rostral pole of the accumbens, dopaminetransporter binding in more caudal regions of the nucleuswas altered as well, albeit to a much lesser extent than tothe pole. Adjacent regions may have been affected to asufficient degree to produce alterations in metabolism inthe terminal fields of other accumbens subdivisions. Inaddition, it has been reported that the axons of efferentspiny projection neurons within the accumbens collateral-ize extensively and may project beyond the dendritic fields
w xof these neurons 16,39 . This intrinsic circuitry demon-strates the existence of an intrastriatal association systemthat does not seem to respect the boundaries of the stan-dard subdivisions of the nucleus accumbens. Anatomical
organization of this sort may provide the basis for ways inwhich activity in circumscribed portions of the accumbenscan influence neural activity in multiple brain areas thatmay include regions that are not necessarily direct anatom-ical targets of the subregion of origin.
Another potential explanation is altered transmission incircuits via basal ganglia-thalamo-corticostriatal loopsw x3,4,62 . Although these loops were originally thought to
w xbe closed 1 , there is growing evidence, especially inventral striatal circuits, that these loops overlap with oneanother providing a means for integration of information
w xacross subdivisions of the striatum 21 . Such spiralinginterconnected loops, involving cortex, pallidum, and tha-lamus in addition to striatum, may help explain both thebilateral nature of the changes in cerebral metabolism aswell as the spread of these changes among terminations ofthe ventral striatum beyond sites directly linked to therostral accumbens.
In trying to place these data in perspective, however, itis important to consider information processing via thenucleus accumbens not simply in terms of serial wiringdiagrams. The complexity of the circuitry, both intrinsicand extrinsic, suggests that changes in neural activity ineven specific subregions of the nucleus accumbens maybring about a cascade of alterations in functional activityin multiple targets beyond the direct connections of the
w xnucleus accumbens 39 . Metabolic mapping with the 2-DGmethod is not an anatomical tract tracing technique inwhich specific point-to-point projections are identified fromone brain regions to another, but rather it maps changes infunction that may extend by various means to associatedneural systems. We should not expect changes in functionto be confined to defined anatomical circuitry, but we aremore likely to find effects that encompass a number ofcoordinated neural networks. Viewed in this manner andplacing aside attempts to define all of the potential circuitsinvolved, these data support the functional involvement ofsubcortical basal ganglia-like and ventral striatum-associ-ated limbic brain regions with dopaminergic activity in therostral nucleus accumbens.
The absence of dopaminergic inputs to the nucleusaccumbens also resulted in depressed rates of glucoseutilization in the agranular insular cortex. This regionprovides direct afferent projections to the nucleus accum-bens as do anterior cingulate, orbital and medial prefrontal
w xcortices 3,5 , yet metabolism in these latter regions wasunaltered by dopamine depletion. These data suggest thatprojections from agranular insular cortex play an importantrole in basal functional activity of the rostral pole. More-over, this diminished cerebral metabolism in the insula isnot likely to be the result of DA denervation in prefrontalcortex, caused by diffusion of 6-OHDA into prefrontalregions, because profound 95% DA depletion in medialprefrontal cortex produced no change in prefrontal cerebral
w xmetabolism 30 .The effects of the intravenous administration of cocaine
( )D. Lyons, L.J. PorrinorBrain Research 753 1997 69–79 77
in intact sham-lesioned animals were essentially identicalw xto those observed in previous studies 41 and were com-
prised of increased cerebral metabolism in the mesolimbicand nigrostriatal systems, motor cortex and cerebellar graymatter. The direct effects of the administration of the highdose of cocaine used in this study on rates of glucose
Ž .utilization were far greater in intensity 15–96% than theŽeffects of the unilateral dopaminergic depletion itself 4–
.16% . For this reason it was difficult to determine theconsequences of the lesion on the central cocaine responsebased on the absolute rates of glucose utilization. When,however, rates of glucose utilization following cocaineadministration in lesioned and sham-treated rats were nor-malized by comparison to there respective saline-treatedcontrol groups, it was evident that the topography of thefunctional response to drug was significantly different in
Ž .the two groups of rats Table 3 . When analyzed in thismanner, it is clear that the response to cocaine was signifi-cantly attenuated in rats with dopaminergic depletion ofthe rostral pole of the nucleus accumbens, as compared tointact rats, in the shell of the nucleus accumbens, themedial portion of the ventral pallidum, and the globuspallidus.
The pallidum, consisting of both dorsal and ventralaspects, is one of the major outflows of the nucleusaccumbens. A prominent hypothesis of psychostimulant
w xaction in the brain 26 asserts that the increased levels ofdopamine in the nucleus accumbens following psychostim-ulant administration are responsible for the locomotor-stimulating and reinforcing properties of this class ofdrugs, and one of the critical consequences of this in-creased dopaminergic transmission is altered output fromthe accumbens to the pallidum. There is functional evi-dence for this substrate in that rates of cerebral metabolismin the globus pallidus are correlated with psychostimulant-
w xinduced locomotor activity 41 . Furthermore, the nucleusaccumbens-ventral pallidal pathway is a critical substrate
w xfor the behavioral expression of locomotion 51 and thew xself-administration of cocaine or heroin 19 . The present
data then provide further evidence that an important actionof cocaine is the modification of the activity from theaccumbens to the pallidum by showing that whendopaminergic transmission was compromised in the rostralaccumbens the normal pallidal response to cocaine wassignificantly reduced.
Cocaine, in the present study, also produced significantalterations in glucose utilization in the anterior cingulatecortex of rats with 6-OHDA lesions of the rostral pole,although metabolism in this brain region was unaltered inintact rats. Similar findings have been reported by Engber
w xet al. 11 in which unilateral 6-OHDA injections into themedial forebrain bundle had no effect on cerebralmetabolism in the anterior cingulate cortex, but the admin-istration of the D agonist SKF 38393 or the D agonist1 2
quinpirole, however, altered metabolism in the cingulateon the lesioned side. These and the present data suggest
that ventral striatal dopaminergic denervation may disin-hibit functional activity in the ipsilateral anterior cingulatecortex, which only becomes evident following dopaminer-gic challenge.
In summary, the present study found that partial unilat-eral dopamine depletion of the rostral accumbens results ina highly discrete pattern of changes in functional activityin the rat. The configuration of functional changes ob-served in the present study suggests that the rostral pole ofthe accumbens can influence both basal ganglia and limbiccircuitry on a functional level. Moreover, the topographyof the activation of rates of metabolism by cocaine in theintact animal was changed by the lesion in such a mannerthat suggests that the rostral pole, the shell of the nucleusaccumbens and the ventral pallidum are important media-tors of the action of cocaine in the central nervous system.
Acknowledgements
This work was supported by NIDA Grants P50DA06634 and DA07522.
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