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Differential effects of ventral striatal lesions on the conditioned place preference induced by morphine or amphetamine
Neuroscience Vol. 71, No. 3, pp. 701-708, 1996
Pergamon
0306-4522(95)00486-6
ElsevierScienceLtd Copyright © 1996 IBRO Printed in Great Britain. All rights reserved 0306-4522/96 $15.00 + 0.00
D I F F E R E N T I A L EFFECTS OF V E N T R A L STRIATAL LESIONS ON T H E C O N D I T I O N E D PLACE P R E F E R E N C E I N D U C E D BY M O R P H I N E OR A M P H E T A M I N E M. C. O L M S T E A D * and K. B. J. F R A N K L I N t Department of Psychology, McGill University, 1205 Ave: Dr. Penfield, Montreal, Quebec, Canada H3A 1B1 Abstract--The present experiment examined the role of the ventral striatum in the rewarding effect of morphine and amphetamine by testing whether lesions of cell bodies within this region disrupt the development of a conditioned place preference to either drug. Bilateral, N-methyl-D-aspartate- or kainic acid-induced lesions of the ventral striatum block a conditioned place preference to amphetamine (1.5 mg/kg x 3 pairings) but not to morphine (2 mg/kg x 3 pairings). Because both lesions spared anterior portions of the ventral striatum, we examined the effect of larger or more selective ventral striatal lesions on a conditioned place preference induced by morphine. Destruction of the entire ventral striatum reduced, but did not eliminate, a conditioned place preference to morphine, whereas selective lesions of the anterior ventral striatum were ineffective. These results indicate that the ventral striatum is not critically involved in morphine's rewarding effect and support the suggestion that the rewarding effects of opiates and stimulants do not involve identical neural substrates. Key words: reward, reinforcement, opiates, psychostimulants, excitotoxin lesions, drug dependence.
A variety of abused drugs appear to exert their rewarding effect via activation of a common neural substrate) 3'29'57 In particular, the mesolimbic dopamine system has been implicated in drug-induced reward because drug administration activates the system ~3 and its destruction disrupts the rewarding effect of both opiates and stimulants. 32"43'44"46-4s One of the primary targets of mesolimbic dopaminergic fibers is the ventral striatum, ~1'5~ a region which includes the nucleus accumbens septi (NAS), the medium sized cells of the olfactory tubercle and the ventromedial portions of the caudate-putamen. 2~ Although the rewarding effect of amphetamine and other dopaminergic agonists appears to involve the ventral striatum, 7,24,62 its role in opiate-induced reward is controversial. Lesions of the ventral striatum are reported to block a conditioned place preference to morphine 2s but the electrolytic lesions employed in this study would have destroyed fibers passing through the ventral striatum. Although lesions of ventral striatal cell bodies disrupt responding for heroin self-administration, 62 the effect is not as pronounced as the effect of the same lesions on cocaine self-administration. 62 Microinjection studies examining the role of ventral striatal cell bodies in opiate-in*Present address: Department of Experimental Psychology, University of Cambridge, Downing Street, Cambridge CB2 3EB, U.K. tTo whom correspondence should be addressed. Abbreviations: NAS, nucleus accumbens septi; NMDA, N-methyl-D-aspartate.
duced reward have also produced inconsistent results. Morphine microinjections into the ventral striatum produced a conditioned place preference in one study: 4 but the high dose used may have produced behavioral effects through an action at other neural sites) s Some reports indicate that animals will selfadminister opiates into the ventral striatum ~9'35 although one study suggested that they will n o t ) Finally, there is disagreement whether microinjections of opioid antagonists into the ventral striatum disrupt heroin self-administration. 4,52 The present study was designed to assess the role of ventral striatal cell bodies in opiate- and stimulantinduced reward. In order to avoid some of the practical and theoretical problems associated with other behavioral measures, 34 the conditioned place preference paradigm was used to assess drug reward. In this test, animals are given the choice between spending time in an environment previously paired with the presentation of rewarding stimuli (drug administration) and one associated with neutral stimulii. Preference for the drug paired cues is a measure of the rewarding effect of the drug. If opiate- and stimulant-induced reward are mediated through a common neural substrate in the ventral striatum, destruction of cell bodies in this region should block the development of a CPP to these drugs. In the following experiments separate groups of animals received neurotoxic or vehicle lesions of the ventral striatum and were tested for the development of a conditioned place preference to either morphine
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M . C . Olmstead and K. B. J. Franklin
or amphetamine. Lesions were p r o d u c e d by bilateral infusions of kainic acid or N M D A , b o t h o f which destroy neuronal elements while leaving fibers o f passage intact, j4"62
EXPERIMENTAL PROCEDURES
Subjects and surgery The subjects were 73 male Long-Evans rats (Charles River, Quebec) weighing between 300 and 350 g at the start of the experiment. Animals were housed individually on a 12h on and 12h off light/dark schedule, with ad libitum food and water except during experimental procedures. Before surgery, animals were anesthetized with Nembutal (60 mg/kg) and secured in a standard stereotaxic apparatus. Injection cannulae (30 gauge), aimed at the ventral striatum were inserted through two holes drilled in the skull. Bilateral lesions were induced by infusing NMDA (0.1 M solution) or kainic acid (0.5 #g per/11 of solution) over 10 min. Sham lesions were produced by infusing equal volumes of drug vehicle alone (phosphate buffer, pH 7.4). Cannulae were left in place for 5 min following the infusion. Animals were allowed to recover for two weeks before behavioral testing began. During this period, they were handled for a few min each day. Three sets of animals were prepared with lesions aimed at the different parts of the ventral striatum. In the first set, the injection was aimed at the caudal ventral striatum. Coordinates were 2.0 mm anterior to bregma, + / - 1.8 mm lateral to the midline, and 7.5mm ventral to the skull surface, according to the atlas of Paxinos and Watson (1986). 38 Extensive ventral striatum lesions were induced by infusing kainic acid at two sites: 0.7 #g in 0.7/tl of solution was injected over 10 min at coordinates 1.2 mm anterior to bregma, 1.8 mm lateral to the midline and 7.3 mm ventral to the skull' surface; 0.5 #g in 0.5/al of solution was injected over 5 min at 2.5 mm anterior to bregma, 1.5 mm lateral to the midline, and 7.0 mm ventral to the skull surface. Lesions restricted to the anterior ventral striatum were induced by infusing kainic acid (0.5 pg in 0.5 #1 of solution) over 5 min. Coordinates were 2.5 mm anterior to bregma, + / - 1 . 5 mm lateral to the midline, and 7.0 mm ventral to the skull surface.
Place conditioning The conditioned place preference apparatus consists of three compartments, made of wood with a plexiglass front wall. Two of the compartments were identical in size (45 × 45 × 30 cm), but differed in shading (white or black), texture (smooth floor or wire mesh floor), and olfactory cues (wood chips or vinegar). The third compartment, an unpainted tunnel (36 x 18 × 20cm), connected the other two compartments. In this apparatus rats do not exhibit a natural preference for either large compartment. 7 The conditioned place preference procedure required eight days. On day 1, entrances to the tunnels remained open and animals were allowed to freely explore the three compartments for 30 min. On each of the following six days, they were injected subcutaneously with drug (2 mg/kg morphine or 1.5 mg/kg amphetamine) or vehicle and were immediately confined to one of the large compartments for 30min. The order of injection and drug-paired compartment were counterbalanced within groups. On day 8, the connecting tunnel was opened, animals (in a drug free state) were placed in the tunnel, and the amount of time they spent in each compartment was recorded for 20 min. For the first set of rats with caudally placed lesions of the ventral striatum, half of the animals with NMDA-induced lesions, half of those with kainic acid-induced lesions and half the controls, were conditioned with morphine (2 mg/kg). The remaining animals were conditioned with
amphetamine (1.5 mg/kg). All rats in the other two sets were conditioned with morphine (2 mg/kg).
Histology The location and extent of each lesion was identified on Cresyl Violet stained slides by determining the area where cells were lost or gliosis was present. Only animals with bilateral lesions destroying a significant portion of the ventral striatum, but not extending beyond its boundaries, 2~ were included in the analysis.
Statistical analysis Behavioral data were analysed using ANOVAs with planned comparisons. The independent variables were group and compartment (drug-paired and saline-paired) and the dependent variable was time spent in each compartment. Because subjects spend time in both large compartments, the compartment factor was considered to be a repeated measure. Two types of comparisons were applied to the data. In the first, the time spent in the drug-paired compartment was compared to the time spent in the saline-paired compartment. This test determined whether individual groups displayed a conditioned place preference. The second comparison was between difference scores (time in drugpaired minus time in saline-paired compartments) across groups. This analysis determined whether the conditioned place preference (or lack thereof) exhibited by a given group differed from its control group. RESULTS
Histology The location and extent o f the largest (open circles) and smallest (hatched circles) N M D A - or kainic acid-induced lesions o f the caudal ventral striatum are presented in Fig. 1. Lesions were concentrated in the caudal end o f the ventral striatum typically d a m a g i n g m o s t o f the N A S . Both N M D A and kainic acid infusions p r o d u c e d neuronal damage throughout the core o f the N A S . P o r t i o n s o f the surrounding shell were also destroyed, but in general the rostral pole was spared. There was little or no damage in the rostral o n e - t h i r d o f the ventral striatum or in the ventromedial p o r t i o n o f the c a u d a t e - p u t a m e n . The olfactory tubercle was also frequently spared. Consistent with previous reports, ~4'4°kainic acid infusions p r o d u c e d a larger area o f cell loss a n d / o r destruction than did N M D A , destroying cells in more lateral and ventral p o r t i o n s o f the ventral striatum. W h e n d a t a f r o m animals with misplaced lesions were discarded, the groups consisted o f the following n u m b e r o f animals: 16 animals (seven lesion and nine sham) conditioned with morphine following N M D A lesions, 18 animals (eight lesion and 10 sham) c o n d i t i o n e d with a m p h e t a m i n e following N M D A lesions; 18 animals (eight lesion and 10 sham) c o n d i t i o n e d with m o r p h i n e following kainic acid lesions; and 19 animals (nine lesion and 10 sham) conditioned with a m p h e t a m i n e following kainic acid lesions. Large ventral striatum lesions (Fig. 2) destroyed those regions d a m a g e d in rats with caudal lesions, as well as m o r e rostral and ventral areas including the olfactory tubercle and, to a varying degree, neurons
Ventral striatum and the CPP to morphine and amphetamine in the ventral pallidum. The ventromedial caudate-putamen was damaged in two animals. On the basis of this analysis, data from 22 animals (10 lesion and 12 sham) were included in this group. Kainic acid-induced lesions aimed at the anterior ventral striatum destroyed approximately the rostral half of the ventral striatum. In all animals, olfactory tubercles and ventral pallidum neurons were spared. Neither large nor anterior ventral striatum lesions destroyed septal nuclei. Following histological analyses, data from 16 animals (eight lesion and eight sham) were included in the anterior ventral striatum lesioned group.
Conditioned place preference As shown in Table 1, N M D A lesions of the caudal ventral striatum had no effect on the development of a conditioned place preference to morphine. Animals with sham and NMDA-induced lesions showed a conditioned place preference for the morphine-paired compartment, sham [F(I,16)= 19.7; P <0.01] and lesion [F(1,16)=35.2; P < 0 . 0 1 ] . There was no difference in the size of the conditioned place preference between these two groups [F(1,32)=0.95; P > 0.05]. Kainic acid-induced lesions of the caudal ventral striatum also had no effect on the development of a conditioned place preference to morphine. Sham and
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lesioned groups showed a significant preference for the morphine-paired compartment (sham IF(l,18) = 13.9; P < 0.01] and lesion [F(1,18) = 16.4; P < 0.01], and the size of preference did not differ between the two groups [F(1,36)= 0.36; P > 0.05]. In contrast, caudal ventral striatal lesions blocked an amphetamine-induced conditioned place preference. Sham lesioned animals showed a significant preference for the drug-paired compartment [F(1,18)--- 22.39; P < 0.01]. whereas NMDA-lesioned animals did not [F(1,18)=0.32; P >0.05]. Difference scores (time in drug-paired minus time in saline-paired compartments) were significantly reduced by the lesions [F(1,36) = 8.2; P < 0.05]. Similarly, kainic acid-induced lesions of the caudal ventral striatum blocked the development of a conditioned place preference to amphetamine. Sham lesioned animals displayed a conditioned place preference [F(1,18) = 21.8; P < 0.01], whereas lesioned animals did not [F(1,18) --- 0.04; P > 0.05]. The size of the preference for the amphetaminepaired compartment was significantly reduced by the lesion [F(1,36) = 6.9; P > 0.05]. As shown in Table 2, kainic acid-induced lesions which destroyed the entire ventral striatum reduced a conditioned place preference to morphine. Both groups of animals developed a conditioned place preference, sham [F(1,22)=36.8; P < 0 . 0 1 ] and
+2.2
+I
.
÷i
.0
-+0
.
6
48
Fig. 1. The location and extent of the largest (open circles) and smallest (hatched circles) lesions of the caudal ventral striatum induced by NMDA or kainic acid infusions. Numbers to the right represent the anterior-posterior level in the atlas of Paxinos and Watson (1986).
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M.C. Olmstead and K. B. J. Franklin
~. 2
.
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I 1.O
+0-2
Fig. 2. The location and extent of the largest (open circles) and smallest (hatched circles) lesions of the ventral striatum induced by kainic acid infusions at two sites. Numbers to the right represent the anterior-posterior level in the atlas of Paxinos and Watson (1986).
lesion [F(1,22)= 7.8; P < 0.05], but the size of the preference for the morphine-paired compartment was significantly smaller in the lesioned group [F(1,44) = 3.9; P < 0.05]. Lesions confined to the anterior portion of the ventral striatum had no effect of the development of a conditioned place preference to morphine. Sham [F(I,14)= 10.9; P <0.01] and lesioned [F(I,14)= 13.8; P <0.01] groups developed a conditioned place preference to the drug-paired compartment, and there was no difference in the size of the preference between groups [F(1,28)=0.8; P > 0.05]. DISCUSSION
In the first experiment, excitotoxin-induced lesions of the caudal two-thirds of the ventral striatum completely eliminated a preference for an amphetamine-paired compartment. This finding supports previous reports that an action at dopaminergic receptors in the ventral striatum is both necessary 47 and sufficient TM for the rewarding effect of amphetamine in the conditioned place preference paradigm. In contrast, similar lesions had no effect on the development of a conditioned place preference to morphine. These results do not prove that the ventral
striatum is not involved in opiate-induced reward because both N M D A - and kainic acid-induced lesions spared anterior portions of the nucleus. It is possible that this region mediates morphine's rewarding effect and that sufficient ventral striatum cells remained intact to produce a conditioned place preference. To test this hypothesis, animals in subsequent experiments received neurotoxin infusions that lesioned the greater part of the ventral striatum, or only its anterior portion. Animals with extensive ventral striatum lesions exhibited a preference for the morphine-paired compartment, but compared to their sham lesioned controls, the size of the effect was reduced. Selective lesions of the anterior ventral striatum were ineffective. The behavioral results must be considered in light of the anatomical characteristics of the ventral striatum. In particular, the NAS is now viewed as three distinct territories: the core, shell, and the rostral pole. The core, located in the caudal threequarters of-the NAS, ensheathes the anterior limb of the anterior commissure and was originally separated from the surrounding shell on the basis of differential distribution of cholecystokinin immunoreactivity 59 and a number of other neurochemical markers. 12'39'6°'61 The N A S may also be partitioned on the basis of anatomical connections: afferents from
Ventral striatum and the CPP to morphine and amphetamine
705
~2.7
41
~O.
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~'
Fig. 3. The location and extent of the largest (open circles) and smallest (hatched circles) lesions of the anterior ventral striatum. Numbers to the right represent the anterior-posterior level in the atlas of Paxinos and Watson (1986). different neural sites preferentially (but apparently not selectively) innervate the core and shell regions of the nucleus. 5'42'6~ This topographical relationship is maintained in efferent targets. 8'n'25'26'6~ Characteristics of the core and shell regions merge in the rostral one-quarter of the N A S and cannot be distinguished; this third sub-region is referred to as the rostral pole. 6° Findings in the present results demonstrate that the development of a conditioned place preference to amphetamine is disrupted by lesions that destroy the core of the N A S and portions of the surrounding shell. They also suggest that extent, not location, of neuronal damage within the ventral striatum determines whether lesions will disrupt a conditioned place preference to morphine. Ventral striatal lesions reduced the preference for the morphine-paired compartment when they encompassed the entire ventral striatum, but not when they were confined to
either anterior or posterior regions of the nucleus. N o n e the less, both interpretations should be accepted with caution because none of the subjects had neuronal destruction confined to either the core, shell, or rostral pole regions of the NAS. Because the shell is more closely associated with the limbic system and the core is associated with the m o t o r system, 6° ventral striatal lesions that destroy the shell could produce more pronounced effects on reward related behavior than lesions confined to the core. The most striking finding in the present study is the differential effect of ventral striatal lesions on the development of a conditioned place preference to morphine or amphetamine. Results showing that lesions which block an amphetamine-induced conditioned place preference have no effect on a morphine-induced conditioned place preference are consistent with suggestions that neural substrates mediating the rewarding effect of opiates and stimu-
Table 1. Mean number of seconds (+ S.E.M.) spent in drug (morphine or amphetamine) and saline-paired compartments during conditioned place preference testing for animals with vehicle or neurotoxin-induced lesions of the caudal ventral striatum. Lesion NMDA Kainate
Group sham lesion sham lesion
Morphine-paired
Saline-paired
Amphetamine-paired
572.1 +__49.5 629.6 + 42.2 622.7 + 65.1 540.9 _ 61.2
267.4 -t- 36.6 226.9 __+42.0 249.3 _ 41.4 227.1 ___40.4
568.7 ___46.1 381.3 + 43.2 467.7 + 39.3 414.4 ___61.7
Saline-paired 186.5 _ 343.8 _ 256.3 + 424.2 +
39.6 34.9 20.7 80.8
706
M.C. Olmstead and K. B. J. Franklin Table 2. Mean number of seconds (+ S.E.M.) spent in morphine- and saline-paired compartments during conditioned place preference testing for animals with kainate-induced lesions of the ventral striatum (extensive or anterior). Lesion Extensive Anterior
Group sham lesion sham lesion
Morphine-paired 614.6 + 64.4 438.3 __51.2 510.8 + 25.0 407.7 4- 55.7
lants differ. For example, in the conditioned place preference paradigm, lesions of the fornix disrupt the rewarding effect of morphine but not of amphetamine, and lesions of the lateral nucleus of the amygdala have the opposite effect} 3'37 Furthermore, pretreatment with serotonin, dopaminergic, or glutamatergic antagonists block a conditioned place preference to amphetamine, but have no effect on a morphine-induced conditioned place preference. 3°m'33 Differences are also apparent in the self-administration paradigm: 6-hydroxydopamine lesions attenuate stimulant, but not opiate self-administration;4j opioid antagonists do not effect stimulant self-administration and dopaminergic antagonists have no effect on heroin self-administration;~7opioid receptor blockade in the ventral striatum affects intravenous heroin, but not cocaine, self-administration;9 and although ventral striatum lesions reduce responding for both cocaine and heroin, the postlesion response rate decrements are different for the two drugs. 62 Finally, microinjections of amphetamine into the ventral striatum increase responding for conditioned reward, 5° whereas microinjections of opioid agonist do not) ° The findings in this experiment suggesting that ventral striatum lesions reduce, but do not eliminate, a conditioned place preference to morphine appear to contradict a previous report. 28 It is difficult to evaluate whether ventral striatum lesions in the Kelsey et al. (1989) study attenuated or completely blocked preferences for the morphine-paired compartment because the experiment used a biased conditioned place preference procedure in which reward is assessed as an increase in time spent in the drug-paired compartment between pre- and postconditioning test sessions. Non-specific factors including stress and anxiety may confound interpretations of data using a biased conditioned place preference procedure. 6'53 Interpretations of the Kelsey et al. (1989) results are also problematic because electrolytic lesions in that study would have destroyed fibers passing through the ventral striatum. In contrast, excitotoxin-induced lesions used in the present study destroy ventral striatal cells while leaving fibers of passage intact. .9'62 It is likely, therefore, that the present study more accurately assesses the role of ventral striatal cell bodies in a morphineinduced conditioned place preference. It should also be noted that the present findings support reports
Saline-paired 178.3 + 30.2 211.6 _+23.9 290.8 + 51.7 160.2 + 34.1
that ventral striatal lesions reduce, but do not eliminate, self-administration of morphine, m~8 The present results do not necessarily contradict studies implicating dopaminergic processes within the ventral striatum in the rewarding effect of opiates: 5'46'48 Infusions of 6-hydroxydopamine in these studies would have destroyed dopaminergic fibers passing through the ventral striatum, rather than neurons within the region. It is not surprising that lesions of cell bodies of dopaminergic fibers in the ventral striatum have different effects on opiateinduced reward because the two classes of lesions have different effects on intravenous opiate selfadministration,m~6'4~'62activity levels, 27'55 and spatial learning in the Morris water maze) ,2° It is possible that the multiple cannula insertions and high concentrations of excitotoxin used to produce extensive lesions in Experiment 2 caused sufficient damage to fibers of passage to mimic the effect of electrolytic lesions. Given that both opiateand stimulant-induced reward are associated with dopaminergic activity in the mesolimbic syst e m , 7"16'24'62 a parsimonious interpretation of our results would be that dopaminergic and/or other fibers passing through the ventral striatum, but not ventral striatum cell bodies, are involved in opiateinduced reward. CONCLUSIONS Although the ventral striatal cell bodies have been implicated in the rewarding effect of opiates and stimulants, the present study is the first to investigate the effect of ventral striatum cell body lesions on the development of a conditioned place preference to morphine and amphetamine. Findings support previous evidence that stimulant-induced reward is mediated through the ventral striatum, but suggest that the region is not critically involved in the rewarding effect of opiates. These results are consistent with the suggestion that the neural substrates mediating the rewarding properties of opiates and stimulants are not identical, although they may share common elements. Given that morphine microinjections into different neural sites produce a conditioned place preference, 2'36'54 morphine's rewarding effect (induced by systemic injections) may be mediated through more than one neural substrate. Ventral striatum lesions, therefore, may attenuate but not
Ventral striatum and the CPP to morphine and amphetamine eliminate a conditioned place preference to m o r p h i n e because m o r p h i n e ' s rewarding effect could also be produced through a neural system that does not include the ventral striatum.
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That is, the ventral striatum may contribute to opiate-induced reward, but it is neither a necessary (present results) nor a sufficient36 c o m p o n e n t in systems which mediate the effect.
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31. Layer R. T., Uretsky N. J. and Wallace L. J. (1993) Effects of the AMPA/kainate receptor antagonist DNOX in the nucleus accumbens on drug-induced conditioned place preference. Brain Res. 617, 267-273. 32. Lyness W. H., Friedle N. M. and Moore K. E. (1979) Destruction of dopaminergic nerve terminals in nucleus accumbens: effect on d-amphetamine self-administration. Pharmacol. Biochem. Behav. 11, 553-556. 33. Mackey W. B. and van der Kooy D. (1985) Neuroleptics block the positive reinforcing effects of amphetamine but not of morphine as measured by place conditioning. Pharmacol. Biochem. Behav. 22, 101-105. 34. Mucha R. F., van der Kooy D., O'Shaughnessy M. and Bucenieks P. (1982) Drug reinforcement studied by the use of place conditioning in rat. Brain Res. 243, 91 105. 35. Olds M. E. (1982) Reinforcing effects of morphine in the nucleus accumbens. Brain Res. 237, 429 440. 36. Olmstead M. C. and Franklin K. B. J. (1994) Development of a conditioned place preference to morphine: effects of microinjections into various CNS sites. Soc. Neurosci. Abstr. 20, 1231. 37. Olmstead M. C. and Franklin K. B. J. (1994) Development of a conditioned place preference to morphine: effects of lesions of various CNS sites. Soc. Neurosci. Abstr. 20, 1232. 38. Paxinos, G. and Watson, C. (1986) The Rat Brain in Stereotaxic Coordinates. Academic Press, New York. 39. Pennartz C. M. A., Dolleman-Van der Weel M. J. and Lopes da Silva F. H. (1992) Differential membrane properties and dopamine effects in the shell and core of the rat nucleus accumbens studied in vitro. Neurosci. Lett. 136, 109-112. 40. Peterson G. M. and Moore R. Y. (1980) Selective effects of kainic acid on diencephalic neurons. Brain Res. 202, 165 182. 41. Pettit H. O., Ettenberg A., Bloom F. E. and Koob G. F. (1984) Destruction of dopamine in the nucleus accumbens selectively attenuates cocaine but not heroin self-administration in rats. Psychopharmacology 84, 167-173. 42. Phillipson O. T. and Griffiths A. C. (1985) The topographic order of inputs to nucleus accumbens in the rat. Neuroscience 16, 275-296. 43. Roberts D. C. S., Corcoran M. E. and Fibiger H. C. (1977) On the role of ascending catecholaminergic systems in intravenous self-administration of cocaine. Pharmacol. Biochem. Behav. 6, 615-620. 44. Roberts D. C. S. and Koob G. F. (1982) Disruption of cocaine self-administration following 6-hydroxydopamine lesions of the ventral tegmental area in rats. Pharmacol. Biochem. Behav. 17, 901-904. 45. Shippenberg T. S., Bals-Kubik R. and Herz A. (1993) Examination of the neurochemical substrates mediating the motivational effects of opioids: Role of the mesolimbic dopamine system and D-1 vs D-2 dopamine receptors. J. Pharmacol. exp. Ther. 265, 53-59. 46. Smith J. E., Guerin G. F., Co C., Barr T. S. and Lane J. D. (1985) Effects of 6-OHDA lesions of the central medial nucleus accumbens on rat intravenous morphine self-administration. Pharmacol. Biochem. Behav. 23, 843-849. 47. Spyraki C., Fibiger H. C. and Phillips A. G. (1982) Dopaminergic substrates of amphetamine-induced place preference conditioning. Brain Res. 253, 185-193. 48. Spyraki C., Fibiger H. C. and Phillips A. G. (1983) Attenuation of heroin reward in rats by disruption of the mesolimbic dopamine system. Psychopharmacology 79, 278-283. 49. Stewart G. R., Price M., Olney J. W., Hartman B. K. and Cozzari C. (1986) N-methylaspartate: an effective tool for lesioning basal forebrain cholinergic neurons of the rat. Brain Res. 369, 377 382. 50. Taylor J. R. and Robbins T. W. (1984) Enhanced behavioural control by conditioned reinforcers following microinjections of d-amphetamine into nucleus accumbens. Psychopharmacology 90, 390-397. 51. Ungerstedt U. (1971) Stereotaxic mapping of the monoamine pathways in the rat brain. Acta Physiol. Scandan. 197, 1-48. 52. Vaccarino F. J., Bloom F. E. and Koob G. F. (1985) Blockade of nucleus accumbens opiate receptors attenuates intravenous heroin reward in the rat. Psychopharmacology 86, 37-42. 53. van der Kooy D. (1987) Place conditioning: a simple and effective method for assessing the motivational properties of drugs. In Methods for Assessing the Reinforcing Properties of Abused Drugs (ed. Bozarth M. A.) pp. 229-240. Springer, New York. 54. van der Kooy, D., Mucha R. F., O'Shaughnessy M. and Bucenieks P. (1982) Reinforcing effects of brain microinjections of morphine revealed by conditioned place preference. Brain Res. 243, 107-117. 55. Weissenborn R. and Winn P. (1992) Regulatory behaviour, exploration and locomotion following NMDA or 6-OHDA lesions in the rat nucleus accumbens. Behav. Brain Res. 51, 127-137. 56. White N. M., Packard M. G. and Hiroi N. (1991) Place conditioning with dopamine D1 and D2 agonists injected peripherally or into the nucleus accumbens. Psychopharmacology 103, 271-276. 57. Wise R. A. and Bozarth M. A. (1987) A psychomotor stimulant theory of addiction. Psychol. Rev. 94, 469-492. 58. Wise R. A. and Hoffman D. C. (1992) Localization of drug reward mechanisms by intracranial injections. Synapse 10, 247-263. 59. Zaborszky L., Alheid G. F., Beinfeld M. C., Eidens L. E., Heimer L. and Palkovits M. (1985) Cholecystokinin innervation of the ventral striatum: a morphological and radioimmunological study. Neuroscience 14, 428-453. 60. Zahm D. S. and Brog J. S. (1992) On the significance of the core-shell boundary in the rat nucleus accumbens. Neuroscience 50, 751-767. 61. Zahm D. S. and Heimer L. (1993) The efferent projections of the rostral pole of the nucleus accumbens of the rat: Comparison with the core and shell projection patterns. J. comp. Neurol. 327, 220-232. 62. Zito K. A., Vickers G. and Roberts D. C. S. (1985) Disruption of cocaine and heroin self-administration following kainic acid lesions of the nucleus accumbens. Pharmacol. Bioehem. Behav. 23, 1029-1036. (Accepted 19 October 1995)
Pergamon
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D I F F E R E N T I A L EFFECTS OF V E N T R A L STRIATAL LESIONS ON T H E C O N D I T I O N E D PLACE P R E F E R E N C E I N D U C E D BY M O R P H I N E OR A M P H E T A M I N E M. C. O L M S T E A D * and K. B. J. F R A N K L I N t Department of Psychology, McGill University, 1205 Ave: Dr. Penfield, Montreal, Quebec, Canada H3A 1B1 Abstract--The present experiment examined the role of the ventral striatum in the rewarding effect of morphine and amphetamine by testing whether lesions of cell bodies within this region disrupt the development of a conditioned place preference to either drug. Bilateral, N-methyl-D-aspartate- or kainic acid-induced lesions of the ventral striatum block a conditioned place preference to amphetamine (1.5 mg/kg x 3 pairings) but not to morphine (2 mg/kg x 3 pairings). Because both lesions spared anterior portions of the ventral striatum, we examined the effect of larger or more selective ventral striatal lesions on a conditioned place preference induced by morphine. Destruction of the entire ventral striatum reduced, but did not eliminate, a conditioned place preference to morphine, whereas selective lesions of the anterior ventral striatum were ineffective. These results indicate that the ventral striatum is not critically involved in morphine's rewarding effect and support the suggestion that the rewarding effects of opiates and stimulants do not involve identical neural substrates. Key words: reward, reinforcement, opiates, psychostimulants, excitotoxin lesions, drug dependence.
A variety of abused drugs appear to exert their rewarding effect via activation of a common neural substrate) 3'29'57 In particular, the mesolimbic dopamine system has been implicated in drug-induced reward because drug administration activates the system ~3 and its destruction disrupts the rewarding effect of both opiates and stimulants. 32"43'44"46-4s One of the primary targets of mesolimbic dopaminergic fibers is the ventral striatum, ~1'5~ a region which includes the nucleus accumbens septi (NAS), the medium sized cells of the olfactory tubercle and the ventromedial portions of the caudate-putamen. 2~ Although the rewarding effect of amphetamine and other dopaminergic agonists appears to involve the ventral striatum, 7,24,62 its role in opiate-induced reward is controversial. Lesions of the ventral striatum are reported to block a conditioned place preference to morphine 2s but the electrolytic lesions employed in this study would have destroyed fibers passing through the ventral striatum. Although lesions of ventral striatal cell bodies disrupt responding for heroin self-administration, 62 the effect is not as pronounced as the effect of the same lesions on cocaine self-administration. 62 Microinjection studies examining the role of ventral striatal cell bodies in opiate-in*Present address: Department of Experimental Psychology, University of Cambridge, Downing Street, Cambridge CB2 3EB, U.K. tTo whom correspondence should be addressed. Abbreviations: NAS, nucleus accumbens septi; NMDA, N-methyl-D-aspartate.
duced reward have also produced inconsistent results. Morphine microinjections into the ventral striatum produced a conditioned place preference in one study: 4 but the high dose used may have produced behavioral effects through an action at other neural sites) s Some reports indicate that animals will selfadminister opiates into the ventral striatum ~9'35 although one study suggested that they will n o t ) Finally, there is disagreement whether microinjections of opioid antagonists into the ventral striatum disrupt heroin self-administration. 4,52 The present study was designed to assess the role of ventral striatal cell bodies in opiate- and stimulantinduced reward. In order to avoid some of the practical and theoretical problems associated with other behavioral measures, 34 the conditioned place preference paradigm was used to assess drug reward. In this test, animals are given the choice between spending time in an environment previously paired with the presentation of rewarding stimuli (drug administration) and one associated with neutral stimulii. Preference for the drug paired cues is a measure of the rewarding effect of the drug. If opiate- and stimulant-induced reward are mediated through a common neural substrate in the ventral striatum, destruction of cell bodies in this region should block the development of a CPP to these drugs. In the following experiments separate groups of animals received neurotoxic or vehicle lesions of the ventral striatum and were tested for the development of a conditioned place preference to either morphine
701
702
M . C . Olmstead and K. B. J. Franklin
or amphetamine. Lesions were p r o d u c e d by bilateral infusions of kainic acid or N M D A , b o t h o f which destroy neuronal elements while leaving fibers o f passage intact, j4"62
EXPERIMENTAL PROCEDURES
Subjects and surgery The subjects were 73 male Long-Evans rats (Charles River, Quebec) weighing between 300 and 350 g at the start of the experiment. Animals were housed individually on a 12h on and 12h off light/dark schedule, with ad libitum food and water except during experimental procedures. Before surgery, animals were anesthetized with Nembutal (60 mg/kg) and secured in a standard stereotaxic apparatus. Injection cannulae (30 gauge), aimed at the ventral striatum were inserted through two holes drilled in the skull. Bilateral lesions were induced by infusing NMDA (0.1 M solution) or kainic acid (0.5 #g per/11 of solution) over 10 min. Sham lesions were produced by infusing equal volumes of drug vehicle alone (phosphate buffer, pH 7.4). Cannulae were left in place for 5 min following the infusion. Animals were allowed to recover for two weeks before behavioral testing began. During this period, they were handled for a few min each day. Three sets of animals were prepared with lesions aimed at the different parts of the ventral striatum. In the first set, the injection was aimed at the caudal ventral striatum. Coordinates were 2.0 mm anterior to bregma, + / - 1.8 mm lateral to the midline, and 7.5mm ventral to the skull surface, according to the atlas of Paxinos and Watson (1986). 38 Extensive ventral striatum lesions were induced by infusing kainic acid at two sites: 0.7 #g in 0.7/tl of solution was injected over 10 min at coordinates 1.2 mm anterior to bregma, 1.8 mm lateral to the midline and 7.3 mm ventral to the skull' surface; 0.5 #g in 0.5/al of solution was injected over 5 min at 2.5 mm anterior to bregma, 1.5 mm lateral to the midline, and 7.0 mm ventral to the skull surface. Lesions restricted to the anterior ventral striatum were induced by infusing kainic acid (0.5 pg in 0.5 #1 of solution) over 5 min. Coordinates were 2.5 mm anterior to bregma, + / - 1 . 5 mm lateral to the midline, and 7.0 mm ventral to the skull surface.
Place conditioning The conditioned place preference apparatus consists of three compartments, made of wood with a plexiglass front wall. Two of the compartments were identical in size (45 × 45 × 30 cm), but differed in shading (white or black), texture (smooth floor or wire mesh floor), and olfactory cues (wood chips or vinegar). The third compartment, an unpainted tunnel (36 x 18 × 20cm), connected the other two compartments. In this apparatus rats do not exhibit a natural preference for either large compartment. 7 The conditioned place preference procedure required eight days. On day 1, entrances to the tunnels remained open and animals were allowed to freely explore the three compartments for 30 min. On each of the following six days, they were injected subcutaneously with drug (2 mg/kg morphine or 1.5 mg/kg amphetamine) or vehicle and were immediately confined to one of the large compartments for 30min. The order of injection and drug-paired compartment were counterbalanced within groups. On day 8, the connecting tunnel was opened, animals (in a drug free state) were placed in the tunnel, and the amount of time they spent in each compartment was recorded for 20 min. For the first set of rats with caudally placed lesions of the ventral striatum, half of the animals with NMDA-induced lesions, half of those with kainic acid-induced lesions and half the controls, were conditioned with morphine (2 mg/kg). The remaining animals were conditioned with
amphetamine (1.5 mg/kg). All rats in the other two sets were conditioned with morphine (2 mg/kg).
Histology The location and extent of each lesion was identified on Cresyl Violet stained slides by determining the area where cells were lost or gliosis was present. Only animals with bilateral lesions destroying a significant portion of the ventral striatum, but not extending beyond its boundaries, 2~ were included in the analysis.
Statistical analysis Behavioral data were analysed using ANOVAs with planned comparisons. The independent variables were group and compartment (drug-paired and saline-paired) and the dependent variable was time spent in each compartment. Because subjects spend time in both large compartments, the compartment factor was considered to be a repeated measure. Two types of comparisons were applied to the data. In the first, the time spent in the drug-paired compartment was compared to the time spent in the saline-paired compartment. This test determined whether individual groups displayed a conditioned place preference. The second comparison was between difference scores (time in drugpaired minus time in saline-paired compartments) across groups. This analysis determined whether the conditioned place preference (or lack thereof) exhibited by a given group differed from its control group. RESULTS
Histology The location and extent o f the largest (open circles) and smallest (hatched circles) N M D A - or kainic acid-induced lesions o f the caudal ventral striatum are presented in Fig. 1. Lesions were concentrated in the caudal end o f the ventral striatum typically d a m a g i n g m o s t o f the N A S . Both N M D A and kainic acid infusions p r o d u c e d neuronal damage throughout the core o f the N A S . P o r t i o n s o f the surrounding shell were also destroyed, but in general the rostral pole was spared. There was little or no damage in the rostral o n e - t h i r d o f the ventral striatum or in the ventromedial p o r t i o n o f the c a u d a t e - p u t a m e n . The olfactory tubercle was also frequently spared. Consistent with previous reports, ~4'4°kainic acid infusions p r o d u c e d a larger area o f cell loss a n d / o r destruction than did N M D A , destroying cells in more lateral and ventral p o r t i o n s o f the ventral striatum. W h e n d a t a f r o m animals with misplaced lesions were discarded, the groups consisted o f the following n u m b e r o f animals: 16 animals (seven lesion and nine sham) conditioned with morphine following N M D A lesions, 18 animals (eight lesion and 10 sham) c o n d i t i o n e d with a m p h e t a m i n e following N M D A lesions; 18 animals (eight lesion and 10 sham) c o n d i t i o n e d with m o r p h i n e following kainic acid lesions; and 19 animals (nine lesion and 10 sham) conditioned with a m p h e t a m i n e following kainic acid lesions. Large ventral striatum lesions (Fig. 2) destroyed those regions d a m a g e d in rats with caudal lesions, as well as m o r e rostral and ventral areas including the olfactory tubercle and, to a varying degree, neurons
Ventral striatum and the CPP to morphine and amphetamine in the ventral pallidum. The ventromedial caudate-putamen was damaged in two animals. On the basis of this analysis, data from 22 animals (10 lesion and 12 sham) were included in this group. Kainic acid-induced lesions aimed at the anterior ventral striatum destroyed approximately the rostral half of the ventral striatum. In all animals, olfactory tubercles and ventral pallidum neurons were spared. Neither large nor anterior ventral striatum lesions destroyed septal nuclei. Following histological analyses, data from 16 animals (eight lesion and eight sham) were included in the anterior ventral striatum lesioned group.
Conditioned place preference As shown in Table 1, N M D A lesions of the caudal ventral striatum had no effect on the development of a conditioned place preference to morphine. Animals with sham and NMDA-induced lesions showed a conditioned place preference for the morphine-paired compartment, sham [F(I,16)= 19.7; P <0.01] and lesion [F(1,16)=35.2; P < 0 . 0 1 ] . There was no difference in the size of the conditioned place preference between these two groups [F(1,32)=0.95; P > 0.05]. Kainic acid-induced lesions of the caudal ventral striatum also had no effect on the development of a conditioned place preference to morphine. Sham and
703
lesioned groups showed a significant preference for the morphine-paired compartment (sham IF(l,18) = 13.9; P < 0.01] and lesion [F(1,18) = 16.4; P < 0.01], and the size of preference did not differ between the two groups [F(1,36)= 0.36; P > 0.05]. In contrast, caudal ventral striatal lesions blocked an amphetamine-induced conditioned place preference. Sham lesioned animals showed a significant preference for the drug-paired compartment [F(1,18)--- 22.39; P < 0.01]. whereas NMDA-lesioned animals did not [F(1,18)=0.32; P >0.05]. Difference scores (time in drug-paired minus time in saline-paired compartments) were significantly reduced by the lesions [F(1,36) = 8.2; P < 0.05]. Similarly, kainic acid-induced lesions of the caudal ventral striatum blocked the development of a conditioned place preference to amphetamine. Sham lesioned animals displayed a conditioned place preference [F(1,18) = 21.8; P < 0.01], whereas lesioned animals did not [F(1,18) --- 0.04; P > 0.05]. The size of the preference for the amphetaminepaired compartment was significantly reduced by the lesion [F(1,36) = 6.9; P > 0.05]. As shown in Table 2, kainic acid-induced lesions which destroyed the entire ventral striatum reduced a conditioned place preference to morphine. Both groups of animals developed a conditioned place preference, sham [F(1,22)=36.8; P < 0 . 0 1 ] and
+2.2
+I
.
÷i
.0
-+0
.
6
48
Fig. 1. The location and extent of the largest (open circles) and smallest (hatched circles) lesions of the caudal ventral striatum induced by NMDA or kainic acid infusions. Numbers to the right represent the anterior-posterior level in the atlas of Paxinos and Watson (1986).
704
M.C. Olmstead and K. B. J. Franklin
~. 2
.
"7
~-I.G
I 1.O
+0-2
Fig. 2. The location and extent of the largest (open circles) and smallest (hatched circles) lesions of the ventral striatum induced by kainic acid infusions at two sites. Numbers to the right represent the anterior-posterior level in the atlas of Paxinos and Watson (1986).
lesion [F(1,22)= 7.8; P < 0.05], but the size of the preference for the morphine-paired compartment was significantly smaller in the lesioned group [F(1,44) = 3.9; P < 0.05]. Lesions confined to the anterior portion of the ventral striatum had no effect of the development of a conditioned place preference to morphine. Sham [F(I,14)= 10.9; P <0.01] and lesioned [F(I,14)= 13.8; P <0.01] groups developed a conditioned place preference to the drug-paired compartment, and there was no difference in the size of the preference between groups [F(1,28)=0.8; P > 0.05]. DISCUSSION
In the first experiment, excitotoxin-induced lesions of the caudal two-thirds of the ventral striatum completely eliminated a preference for an amphetamine-paired compartment. This finding supports previous reports that an action at dopaminergic receptors in the ventral striatum is both necessary 47 and sufficient TM for the rewarding effect of amphetamine in the conditioned place preference paradigm. In contrast, similar lesions had no effect on the development of a conditioned place preference to morphine. These results do not prove that the ventral
striatum is not involved in opiate-induced reward because both N M D A - and kainic acid-induced lesions spared anterior portions of the nucleus. It is possible that this region mediates morphine's rewarding effect and that sufficient ventral striatum cells remained intact to produce a conditioned place preference. To test this hypothesis, animals in subsequent experiments received neurotoxin infusions that lesioned the greater part of the ventral striatum, or only its anterior portion. Animals with extensive ventral striatum lesions exhibited a preference for the morphine-paired compartment, but compared to their sham lesioned controls, the size of the effect was reduced. Selective lesions of the anterior ventral striatum were ineffective. The behavioral results must be considered in light of the anatomical characteristics of the ventral striatum. In particular, the NAS is now viewed as three distinct territories: the core, shell, and the rostral pole. The core, located in the caudal threequarters of-the NAS, ensheathes the anterior limb of the anterior commissure and was originally separated from the surrounding shell on the basis of differential distribution of cholecystokinin immunoreactivity 59 and a number of other neurochemical markers. 12'39'6°'61 The N A S may also be partitioned on the basis of anatomical connections: afferents from
Ventral striatum and the CPP to morphine and amphetamine
705
~2.7
41
~O.
.O
~'
Fig. 3. The location and extent of the largest (open circles) and smallest (hatched circles) lesions of the anterior ventral striatum. Numbers to the right represent the anterior-posterior level in the atlas of Paxinos and Watson (1986). different neural sites preferentially (but apparently not selectively) innervate the core and shell regions of the nucleus. 5'42'6~ This topographical relationship is maintained in efferent targets. 8'n'25'26'6~ Characteristics of the core and shell regions merge in the rostral one-quarter of the N A S and cannot be distinguished; this third sub-region is referred to as the rostral pole. 6° Findings in the present results demonstrate that the development of a conditioned place preference to amphetamine is disrupted by lesions that destroy the core of the N A S and portions of the surrounding shell. They also suggest that extent, not location, of neuronal damage within the ventral striatum determines whether lesions will disrupt a conditioned place preference to morphine. Ventral striatal lesions reduced the preference for the morphine-paired compartment when they encompassed the entire ventral striatum, but not when they were confined to
either anterior or posterior regions of the nucleus. N o n e the less, both interpretations should be accepted with caution because none of the subjects had neuronal destruction confined to either the core, shell, or rostral pole regions of the NAS. Because the shell is more closely associated with the limbic system and the core is associated with the m o t o r system, 6° ventral striatal lesions that destroy the shell could produce more pronounced effects on reward related behavior than lesions confined to the core. The most striking finding in the present study is the differential effect of ventral striatal lesions on the development of a conditioned place preference to morphine or amphetamine. Results showing that lesions which block an amphetamine-induced conditioned place preference have no effect on a morphine-induced conditioned place preference are consistent with suggestions that neural substrates mediating the rewarding effect of opiates and stimu-
Table 1. Mean number of seconds (+ S.E.M.) spent in drug (morphine or amphetamine) and saline-paired compartments during conditioned place preference testing for animals with vehicle or neurotoxin-induced lesions of the caudal ventral striatum. Lesion NMDA Kainate
Group sham lesion sham lesion
Morphine-paired
Saline-paired
Amphetamine-paired
572.1 +__49.5 629.6 + 42.2 622.7 + 65.1 540.9 _ 61.2
267.4 -t- 36.6 226.9 __+42.0 249.3 _ 41.4 227.1 ___40.4
568.7 ___46.1 381.3 + 43.2 467.7 + 39.3 414.4 ___61.7
Saline-paired 186.5 _ 343.8 _ 256.3 + 424.2 +
39.6 34.9 20.7 80.8
706
M.C. Olmstead and K. B. J. Franklin Table 2. Mean number of seconds (+ S.E.M.) spent in morphine- and saline-paired compartments during conditioned place preference testing for animals with kainate-induced lesions of the ventral striatum (extensive or anterior). Lesion Extensive Anterior
Group sham lesion sham lesion
Morphine-paired 614.6 + 64.4 438.3 __51.2 510.8 + 25.0 407.7 4- 55.7
lants differ. For example, in the conditioned place preference paradigm, lesions of the fornix disrupt the rewarding effect of morphine but not of amphetamine, and lesions of the lateral nucleus of the amygdala have the opposite effect} 3'37 Furthermore, pretreatment with serotonin, dopaminergic, or glutamatergic antagonists block a conditioned place preference to amphetamine, but have no effect on a morphine-induced conditioned place preference. 3°m'33 Differences are also apparent in the self-administration paradigm: 6-hydroxydopamine lesions attenuate stimulant, but not opiate self-administration;4j opioid antagonists do not effect stimulant self-administration and dopaminergic antagonists have no effect on heroin self-administration;~7opioid receptor blockade in the ventral striatum affects intravenous heroin, but not cocaine, self-administration;9 and although ventral striatum lesions reduce responding for both cocaine and heroin, the postlesion response rate decrements are different for the two drugs. 62 Finally, microinjections of amphetamine into the ventral striatum increase responding for conditioned reward, 5° whereas microinjections of opioid agonist do not) ° The findings in this experiment suggesting that ventral striatum lesions reduce, but do not eliminate, a conditioned place preference to morphine appear to contradict a previous report. 28 It is difficult to evaluate whether ventral striatum lesions in the Kelsey et al. (1989) study attenuated or completely blocked preferences for the morphine-paired compartment because the experiment used a biased conditioned place preference procedure in which reward is assessed as an increase in time spent in the drug-paired compartment between pre- and postconditioning test sessions. Non-specific factors including stress and anxiety may confound interpretations of data using a biased conditioned place preference procedure. 6'53 Interpretations of the Kelsey et al. (1989) results are also problematic because electrolytic lesions in that study would have destroyed fibers passing through the ventral striatum. In contrast, excitotoxin-induced lesions used in the present study destroy ventral striatal cells while leaving fibers of passage intact. .9'62 It is likely, therefore, that the present study more accurately assesses the role of ventral striatal cell bodies in a morphineinduced conditioned place preference. It should also be noted that the present findings support reports
Saline-paired 178.3 + 30.2 211.6 _+23.9 290.8 + 51.7 160.2 + 34.1
that ventral striatal lesions reduce, but do not eliminate, self-administration of morphine, m~8 The present results do not necessarily contradict studies implicating dopaminergic processes within the ventral striatum in the rewarding effect of opiates: 5'46'48 Infusions of 6-hydroxydopamine in these studies would have destroyed dopaminergic fibers passing through the ventral striatum, rather than neurons within the region. It is not surprising that lesions of cell bodies of dopaminergic fibers in the ventral striatum have different effects on opiateinduced reward because the two classes of lesions have different effects on intravenous opiate selfadministration,m~6'4~'62activity levels, 27'55 and spatial learning in the Morris water maze) ,2° It is possible that the multiple cannula insertions and high concentrations of excitotoxin used to produce extensive lesions in Experiment 2 caused sufficient damage to fibers of passage to mimic the effect of electrolytic lesions. Given that both opiateand stimulant-induced reward are associated with dopaminergic activity in the mesolimbic syst e m , 7"16'24'62 a parsimonious interpretation of our results would be that dopaminergic and/or other fibers passing through the ventral striatum, but not ventral striatum cell bodies, are involved in opiateinduced reward. CONCLUSIONS Although the ventral striatal cell bodies have been implicated in the rewarding effect of opiates and stimulants, the present study is the first to investigate the effect of ventral striatum cell body lesions on the development of a conditioned place preference to morphine and amphetamine. Findings support previous evidence that stimulant-induced reward is mediated through the ventral striatum, but suggest that the region is not critically involved in the rewarding effect of opiates. These results are consistent with the suggestion that the neural substrates mediating the rewarding properties of opiates and stimulants are not identical, although they may share common elements. Given that morphine microinjections into different neural sites produce a conditioned place preference, 2'36'54 morphine's rewarding effect (induced by systemic injections) may be mediated through more than one neural substrate. Ventral striatum lesions, therefore, may attenuate but not
Ventral striatum and the CPP to morphine and amphetamine eliminate a conditioned place preference to m o r p h i n e because m o r p h i n e ' s rewarding effect could also be produced through a neural system that does not include the ventral striatum.
707
That is, the ventral striatum may contribute to opiate-induced reward, but it is neither a necessary (present results) nor a sufficient36 c o m p o n e n t in systems which mediate the effect.
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