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© 2005 Nature Publishing Group http://www.nature.com/natureneuroscience ARTICLES The subthalamic nucleus exerts opposite control on cocaine and ‘natural’ rewards Christelle Baunez1, Carine Dias2, Martine Cador2 & Marianne Amalric1 A challenge in treating drug addicts is preventing their pathological motivation for the drug without impairing their general affective state toward natural reinforcers. Here we have shown that discrete lesions of the subthalamic nucleus greatly decreased the motivation of rats for cocaine while increasing it for food reward. The subthalamic nucleus, a key structure controlling basal ganglia outputs, is therefore able to oppositely modulate the effect of ‘natural’ rewards and drugs of abuse on behavior. Modulating the activity of the subthalamic nucleus might prove to be a new target for the treatment of cocaine addiction. Drug dependence is a major worldwide public health problem. In drug addicts, one of the main unsolved challenges is eradicating the pathological motivation for the drug without impairing the general affective state of subjects and their motivation for ‘natural’ reinforcers. As drugs of abuse and natural reinforcers mostly recruit common biological substrates, the possibility of obtaining such dissociation remains an open issue. Structures of the basal ganglia classically considered motor structures can be involved in coding reward1,2 and in the reinforcing effects of cocaine3,4. Among these structures, the subthalamic nucleus (STN) has received much attention as a therapeutic target because its inactivation by high-frequency stimulation has been used successfully for the treatment of Parkinson disease5. Although inactivation of the STN is used mainly to improve motor deficits5–9, preliminary experimental and clinical observations have suggested that this structure may also have considerable involvement in the modulation of motivation10–15. Accordingly, this structure seems to be sensitive to drugs of abuse, as repeated injections of cocaine result in a decreased metabolic activity in the STN16,17. To address whether the STN could differentially affect motivation for conventional (‘natural’) or pharmacological (‘drug’) reinforcers, we have studied the effects of excitotoxic lesions of this nucleus, which have effects functionally similar to high-frequency stimulation7,18. We analyzed motivation for food and cocaine rewards using two complementary behavioral approaches: an ‘operant paradigm’ (progressive ratio) that measures the ability of the animals to provide an increased rate of responding to obtain food or drugs, and a ‘place-conditioning schedule’ that measures preference or aversion for an environment specifically paired with food or cocaine. The combined results of these two approaches indicate that excitotoxic lesions of the STN increase the motivation of rats for a food reward while greatly decreasing the motivation for cocaine. These data demonstrate that motivation for natural rewards and drugs of abuse can be oppositely modulated in part via the STN. This possibility indicates the STN may be a potential target for developing specific and efficient treatments for drug abusers. RESULTS Histology Only rats with a bilateral lesion of the STN and an injection site in the STN were included in the data analysis. As in previous studies8,15, the bilateral STN lesion selectively damaged all sectors of the STN bilaterally, sparing only a few cells in the most lateral portions of the nucleus (Fig. 1). We found no damage in the lateral hypothalamus. We eliminated from the behavioral data analyses eight rats with lesions above the nucleus. The location of the injection cannula was in the STN in four rats and in three it was located outside the STN. The performance of the eliminated rats was similar to that of control rats (‘sham-lesioned’ rats), thus confirming that the behavioral effects noted were specifically due to STN lesions or infusions in the STN. Behavioral results To assess the effects of STN lesions on food or cocaine intake and to ensure that STN-lesioned rats could acquire drug self-administration, we first tested the rats with a continuous reinforcement schedule (fixed ratio 1), in which each lever press was followed by the reward. Access to the reward was thus fairly easy and required a minimum of work. In this task, whatever the reward (a sucrose pellet or an intravenous cocaine injection), STN-lesioned rats obtained as many rewards as the sham-lesioned control rats did (approximately 100 pellets per session and 25 cocaine injections per session; P 4 0.05; Fig. 2a,b). In both procedures, with food and with cocaine reinforcement, there was no significant difference between the sham-lesioned and STN-lesioned groups in the rate of lever presses (P 4 0.05; Fig. 2c,d). The typical response records showed that both the sham-lesioned and STNlesioned groups had a stable pattern of evenly spaced responses on 1Laboratoire de Neurobiologie de la Cognition, Unité Mixte de Recherche (UMR) 6155 Centre National de la Recherche Scientifique Université de Provence, 13402 Marseille Cedex 20, France. 2Laboratoire de Neuropsychobiologie des Désadaptations, UMR5541 Centre National de la Recherche Scientifique, Université de Bordeaux 2, 33076 Bordeaux Cedex, France. Correspondence should be addressed to C.B. ([email protected]). Published online 27 March 2005; doi:10.1038/nn1429 484 VOLUME 8 [ NUMBER 4 [ APRIL 2005 NATURE NEUROSCIENCE ARTICLES Figure 1 Frontal sections at the level of the STN, stained with cresyl violet. Dashed lines outline the STN in a sham-lesioned rat (a) and an STN-lesioned rat (b). Inset, coronal section of the rat brain at the level of the subthalamic nucleus, from ref. 49. CP, cerebral peduncle. the active lever with a short delay before the first active injection in STN-lesioned rats (Fig. 2e). As all rats reached similar levels of intake with the same rate during acquisition (data not shown), the primary reinforcing properties of food and cocaine seemed to be identical in both sham-lesioned and STN-lesioned rats. As both groups worked similarly for both rewards when they could easily be obtained, the next issue was to determine whether STN lesions could differentially affect the behavior of the rats in a highly demanding task such as those with progressive ratio schedules of reinforcement, in which the work demand required to obtain the reward is progressively increased. When no difference is found in a continuous reinforcement schedule, a difference can be unmasked using a progressive ratio19,20. With progressive ratio schedules of reinforcement, the measure of the last ratio reached (also called the ‘breaking point’) allows assessment of the amount of effort a rat is willing to expend to obtain the reinforcer21. With this schedule, rats with bilateral STN lesions reached a higher breaking point than did sham-lesioned control rats and thus obtained a significantly higher number of reinforcers when working for one or two sucrose pellets (group effect: F1,13 ¼ 6.07 and 7.05, P o 0.05; Fig. 3a). We found a similar effect after intra-STN infusion of muscimol (3 ng/side) in rats working for one sucrose pellet (drug effect: F2,6 ¼ 20.04, P o 0.01; Fig. 3b). In contrast, when the reinforcer was a single intravenous injection of cocaine (250 mg/injection), Figure 2 Effects of bilateral STN lesions on continuous reinforcement tasks for food and cocaine. Each lever press is reinforced by one sucrose pellet (a,c) or by an intravenous injection of 250 mg cocaine (b,d). Each dot represents the mean number of pellets or injections during the 30-minute (a) or 120-minute (b) session (7 s.e.m.) for the last five daily sessions with this schedule (abscissa) or the mean rate of lever presses during the same sessions (c,d). Data are for sham-lesioned control rats (n ¼ 8 (a,c) and n ¼ 9 (b,d)) and for STN-lesioned lesion rats (n ¼ 7 (a,c) and n ¼ 9 (b,d)). (e) Kinetics of cocaine injections for a representative 2-hour session using the ‘fixed ratio 1’ schedule for one representative rat of each group. Each bar represents a cocaine infusion. NATURE NEUROSCIENCE VOLUME 8 [ NUMBER 4 [ APRIL 2005 a b Food intake 100 Cocaine intake 30 Number of injections b 80 60 40 20 20 10 1 c Sham STN 0 0 2 3 4 5 12 10 8 6 4 2 0 1 2 3 4 5 d 1 Number of lever presses/min © 2005 Nature Publishing Group http://www.nature.com/natureneuroscience CP Number of pellets STN Number of lever presses/min a STN-lesioned rats worked less than sham-lesioned rats. They reached a lower breaking point and gained a lower number of injections (group effect: F1,16 ¼ 8.69, P o 0.01) (Fig. 3c). The amount of locomotor activity recorded during the sessions for cocaine self-administration was equivalent in both groups (P 4 0.05; data not shown), ruling out the possibility of a motor effect of cocaine at this dose on the performance of the rats. As STN-lesioned rats showed decreased motivation to obtain one cocaine injection when the work load was increased, we then progressively decreased the cocaine dose in the continuous reinforcement schedule. In these conditions, we were able to evaluate any shift in the sensitivity of the rats to the drug and how much they were able to work to adjust the concentration of cocaine in their blood circulation. Each dose was available for a 1-h period. As classically described, the first 10 min correspond to the ‘loading period’ during which the animals adjust to the dose available. When the doses of cocaine were decreased, sham-lesioned rats consequently increased their number of injections as a function of the dose during these first 10 min (dose effect: F3,15 ¼ 30.93, P o 0.01; Fig. 4a). The STN-lesioned group showed a poorer adaptation to the lower doses of cocaine and obtained significantly fewer injections than control rats (group dose interaction: F3,30 ¼ 5.47, P o 0.01) during the first 10 min of the schedule, as demonstrated by the response pattern (Fig. 4c). However, when given the 1-h period of access, STN-lesioned rats were able to adapt their response to the diminished effect of cocaine reward value in the long term, as the total number of injections obtained during the session was not significantly different for the sham-lesioned and STN-lesioned groups (P 4 0.05; Fig. 4b). STN-lesioned rats clearly showed a slower rate of injection for each dose except the standard one (250 mg), especially during the first minutes (Fig. 4c). The fact that there was no difference between the groups at this last dose rules out the possibility of a motor impairment produced by the lesion. To assess whether the reinforcing property of food or cocaine was modified by the STN lesions, we used the place-conditioning paradigm to quantify their reward efficacy. In this paradigm, the reward effects of a given reinforcer (food or cocaine) are inferred by measurement of the time spent in a specific environment previously paired with the 2 3 4 5 0.30 0.25 0.20 0.15 0.10 Sham 0.05 STN 0.00 1 2 3 4 5 Session e Sham STN 0 Time (min) 120 485 ARTICLES 30 * 1 2 Number of pellets * 40 30 20 10 0 c 16 0 2 3 Muscimol (ng) 12 Cocaine Sham STN ** 8 4 0 Figure 3 Effects of bilateral STN lesions on performance in the progressive ratio task for food and cocaine. The willingness of the rats to work for the reward is evaluated by the mean number (7 s.e.m.) of reinforcers acquired during each session (averaged over five sessions). (a) Mean number of pellets obtained with one or two sucrose pellets as a reinforcer. (b) Mean number of pellets obtained with one sucrose pellet as a reinforcer after infusion of saline (0) or muscimol (2 or 3 ng per 0.5 ml) into the STN (n ¼ 4). (c) Mean number of intravenous cocaine injections obtained (250 mg/injection). Sham-lesioned control rats, n ¼ 8 (a) and n ¼ 9 (c); STN-lesioned lesion rats, n ¼ 7 (a) and n ¼ 9 (c). *, P o 0.05, compared with the sham-lesioned group. **, P o 0.01. reinforcer in comparison with time spent in another environment paired with no food or with vehicle injection. In a ‘choice’ situation and in the absence of the reinforcer, measurement of the time spent in these environments gives an indication of the positive or negative affective memory that the animal has of its food or cocaine experience. Both groups showed a preference for the environment associated with food, with the STN-lesioned group showing a stronger preference than the sham-lesioned group (conditioning effect: P o 0.05 (sign test); group effect: P o 0.05 (Mann-Whitney test); Fig. 5a). In contrast, after conditioning with two different doses of cocaine (5 mg/kg and 10 mg/kg; Fig. 5b), the sham-lesioned group showed a clear preference for the compartment associated with cocaine whatever the dose, but the STN-lesioned group showed a weaker preference for the compartment associated with cocaine (conditioning effect: P o 0.05 for both doses (sign test); group effect: Po 0.01 for the 10-mg/kg dose (Mann-Whitney test)). DISCUSSION In this study we have presented converging evidence showing that bilateral lesions of the STN have opposite effects on motivation for food and cocaine. Rats with excitotoxic lesions of the STN did indeed work harder for a food reward but less for cocaine. In parallel, STN lesions enhanced the preference for an environment previously associated with food while decreasing the preference for a cocaineassociated environment. Reversible inactivation of the STN by the GABAA receptor agonist muscimol increased the progressive ratio performance for food reinforcement to an extent similar to that of the STN excitotoxic lesion, thus confirming the specificity of the STN site itself in mediating the behavioral effects seen here. Either a muscimol infusion or a permanent lesion of the STN disrupts the performance of an attentional task, emphasizing the critical involvement of the STN in mediating reward information and attentional processes22,23. Our results confirm involvement of the STN in food reinforcement and further suggest that the effects of STN lesions on motivation are reinforcer dependent. They extend previous findings15 showing that in addition to its classical control of motor output, the STN is associated with motivational associative processes. The effects produced by STN lesions cannot be related to a nonspecific motor impairment, as we found no motor disinhibition (reflecting impulsive behavior)15 or changes in cocaine-induced locomotion in the STN-lesioned rats. STN lesions usually produce hyperkinetic-like disorders (such as ballism in primates) rather than motor depression and cannot therefore account for the lower number of lever presses for cocaine with the progressive ratio schedule8,24,25. As found here, a dissociation between natural and drug reinforcement has been demonstrated recently by electrophysiological studies. Selective neuronal subpopulation of the nucleus accumbens (ventral striatum) recorded in freely moving rats were found to be differentially activated by natural reward (water and food) versus cocaine reinforcement26,27. In addition, the activity of ventral striatal neurons was notably different for cocaine and juice reward in monkeys performing a reaction-time task28. These findings have important functional implications, as they suggest that different parallel microcircuits mediate responses for natural reward versus cocaine. As the nucleus accumbens and the STN participate in a cortico-basal ganglia-cortical ‘limbic loop’ encompassing the prefrontal cortex, nucleus accumbens, ventral pallidum area and STN29,30, which is analogous to the ‘motor loop’ involving the dorsal striatum31, these microcircuits are likely to be represented at each level of the loop. Specifically, the core region of the nucleus accumbens projects to the ventral pallidal area that projects to the medial part of the STN. There are successive GABAergic connections from the core region to the STN (via the ventral pallidum). Reciprocally, by its glutamatergic projections to the ventral pallidum32, the STN controls limbic information outflow33. Therefore, via this connection, STN may exert an opposite effect on these microcircuits, exerting an inhibitory control on the ‘natural reinforcer circuit’ while activating the ‘cocaine circuit’. This interpretation is consistent with the a b Loading period (first 10 min) Mean number of injections 50 Food 50 Number of injections * 70 10 © 2005 Nature Publishing Group http://www.nature.com/natureneuroscience b Food 90 Number of pellets Number of pellets a 20 18 16 14 12 10 8 6 4 2 0 STN Sham Overall period (60 min) 80 ** 70 60 * 50 40 # ** 30 # 20 10 0 10 c 30 80 Dose (µg) 0 250 0 Sham 10 30 80 Dose (µg) 250 STN 250 µg Figure 4 Effect of a change in dose during continuous reinforcement for cocaine self-administration. (a,b) The mean number of injections (7 s.e.m.) during the first 10 min (a) and the whole 60-minute session (b) of selfadministration for each dose for the sham-lesioned group (n ¼ 6) and the STN-lesioned group (n ¼ 6). *, P o 0.05, compared with the sham-lesioned group; #, P o 0.05 and **, P o 0.01, compared with the 250-mg dose. (c) Representative injection patterns of representative rats from the shamlesioned and STN-lesioned groups for each dose of cocaine during a 1-hour session. 486 80 µg 30 µg 10 µg 0 µg 0 60 Time (min) VOLUME 8 [ NUMBER 4 0 60 Time (min) [ APRIL 2005 NATURE NEUROSCIENCE ARTICLES Conditioned place preference 300 Preference score (s) © 2005 Nature Publishing Group http://www.nature.com/natureneuroscience a 250 b Food * Cocaine 300 STN Sham 250 200 200 150 150 100 100 50 50 0 0 ** 5 10 Dose (mg/kg) Figure 5 Effects of bilateral STN lesions on conditioned place preference for food or cocaine. The preference score (7 s.e.m.) is presented as the time spent in the compartment associated with food (a) or cocaine (5 mg/kg and 10 mg/kg intraperitoneally; b) on the test day minus the time spent in this compartment on the preconditioning day for sham-lesioned rats (n ¼ 6, 8 and 6, respectively) and STN-lesioned rats (n ¼ 6, 5 and 7, respectively). *, P r 0.05, and **, P r 0.01, compared with the sham-lesioned group. fact that via its connections to the various subloops of the topographically organized cortico–basal ganglia conveying motor, cognitive and limbic information, the STN lesion differentially affects goaldirected behavior for natural or drug reward. It is unlikely that STN lesions affect stimulus-reward association processes. STN lesions do not disrupt the association between a conditioned stimulus and a food reward15. In our experiments here, the fact that STN-lesioned rats showed a place preference indicates an intact association between the reinforcer and the environment. In contrast, some of our results presented here indicate that the STN can differentially modify the salience of the reward. This was indicated by the increased willingness of STN-lesioned rats to work for food reward with the progressive ratio schedule and by the vertical downward shift in the dose-response curve for cocaine. Such a shift in cocaine dose-response function reflects a change in reward efficacy, such as that seen in low-responder rats compared with high responders to novelty34 or for rats with short versus long access to cocaine35 or after lesioning of the sublenticular region of the extended amygdala, which includes part of the shell of the accumbens36. In the conditioned place-preference paradigm, the resulting preference-aversion provides an indirect measure of the incentive salience (euphoria-dysphoria) produced by a drug. In parallel, protocols such as the progressive ratio schedule allow measurement of the motivational value of a reinforcer. Our study here using these paradigms has indicated that STN lesions increase the salience of food and decrease that of cocaine. Modulation of the salience and the reinforcing and response-invigorating effects of natural reward or psychostimulant drugs is often attributed to the nucleus accumbens shell area37–40. The effects of STN lesions on the salience of the reward could thus be explained by its connections with the nucleus accumbens, within the limbic loop. Selective areas of this loop including the ventral striatum and the STN are indeed involved in the prediction and evaluation of reward, as has been found in studies of humans using functional magnetic resonance imaging41. The effects noted after STN lesions could also reflect a loss of the direct prefrontal influence on the STN. The prelimbic–medial orbital areas of the prefrontal cortex are indeed connected to the medial part of the STN30, and this specific region is involved in different aspects of reward processing, such as reward value, in nonhuman primates42,43. Furthermore, imaging studies of cocaine addicts have also emphasized dysfunctions in these regions44,45. As addiction is increasingly NATURE NEUROSCIENCE VOLUME 8 [ NUMBER 4 [ APRIL 2005 regarded as a compulsive disorder generating inappropriate responses and involving the prefrontal cortex46, it might well be that the orbitofrontal-subthalamic connection is critical in regulating of the faulty decision-making associated with addictive behavior. Accordingly, direct evidence of functional involvement of this corticosubthalamic connection has been found in attentional and perserverative processes in rats47. Our results have thus important clinical implications, as clinical evidence has shown that STN inactivation reduces obsessive-compulsive disorders48. In conclusion, our study has demonstrated that motivation for natural rewards and drug abuse can be oppositely modulated. This possibility opens new perspectives for the development of specific and efficient treatments for drug abuse. In this context, manipulations that decrease the activity of the STN may represent a new therapeutic strategy for drug addiction. METHODS Animals. Male Wistar rats (n ¼ 20; Iffa Credo) and Long-Evans rats (n ¼ 67; Janvier) were housed in pairs and were maintained on a 12-h light-dark cycle. During the food experiments, they were maintained at 80–85% of their free-feeding weight by restriction of their food to 15–17 g/rat per day. For cocaine experiments, the rats had no food restriction. Water was provided ad libitum, except during experimental sessions. All procedures were conducted in accordance with the French Agriculture and Forestry Ministry decree 87-849. Surgery. Rats were anaesthetized with xylazine (15 mg/kg intramuscularly) and ketamine (100 mg/kg intramuscularly) and were secured in a Kopf stereotaxic apparatus. Rats received bilateral injection of ibotenic acid (9.4 mg/ml (53 mM); n ¼ 42, STN-lesioned) or vehicle solution (phosphate buffer; 0.1 M; n ¼ 38, sham-lesioned) at the following coordinates49: anteroposterior, 3.8 mm (from bregma); lateral, 7 2.4 mm; dorsoventral, 8.35 mm (from skull). The volume injected was 0.5 ml per side infused over 3 min with a 10-ml Hamilton microsyringe connected by Tygon tubing fitted to the 30-gauge stainless steel injector needles and fixed on a micropump. The injectors were left in place for 3 min to allow diffusion. Twenty rats (n ¼ 10, sham-lesioned; n ¼ 10, STNlesioned) were further subjected to implantation of a catheter in the jugular vein. Seven rats were implanted with bilateral cannulae above the STN (dorosventral, 3 mm dorsal) so that the injector would protrude 3 mm below the cannulae. Cannulae were fixed with dental cement, and wire stylets 10 mm in length were inserted in the guide cannulae to prevent occlusion. Microinfusions. Rats were tested in the progressive ratio task for a couple of sessions and then received the vehicle injection. Rats were injected with a solution of vehicle or muscimol (2 or 3 ng per 0.5 ml per side) over a 3-min period as described above. One injection per week was done. Muscimol (Sigma-Aldrich) was dissolved in 0.9% NaCl saline solution at doses that have been shown to affect attentional processes in the STN23. Apparatus. Food reward experiments were done in four standard operant boxes (MedAssociates) with a retractable lever, a food magazine connected with a food pellet dispenser and a stimulus light located above the food magazine. The apparatus and online data collection were controlled by a computer and an interface (MedPC.). Cocaine self-administration experiments were done in eight boxes (Imetronics) equipped with two retractable levers above which was situated a stimulus light. The apparatus and online data collection were controlled by a computer and an interface (Imetronics). Two Plexiglas boxes (90 5 33 cm) divided into two main compartments, A and B (40 35 33 cm), with a small ‘intermediate’ chamber between (10 cm in length), were used for the conditioned place-preference test. Compartments A and B had different color patterns on the wall and differed in material on the floor. Behavioral protocol for the continuous and progressive ratio schedule of reinforcement. For the food experiment, as described15, rats (n ¼ 8, shamlesioned; n ¼ 8, STN-lesioned; n ¼ 7, cannulated) were trained to press a lever for one food pellet with a continuous reinforcement schedule (fixed ratio 1). The session ended when 100 pellets were delivered or 30 min had elapsed. After 487 © 2005 Nature Publishing Group http://www.nature.com/natureneuroscience ARTICLES acquisition of stable responding, all rats were subjected to the progressive ratio schedule, in which the number of presses required (fixed ratio) was arithmetically increased by steps of five with three repetitions of each step. The lever press that completed each fixed ratio within this progressive ratio schedule was rewarded. A session ended if the rat failed to press the lever for 5 consecutive minutes or 90 min had elapsed. The ‘fixed ratio 1’ procedure started 1 week after surgery for 10 consecutive days and the progressive ratio schedule was then undertaken for ten consecutive daily sessions. After ten sessions of progressive ratio for one pellet, five sessions of progressive ratio were conducted with two pellets as the reinforcer. For each session, the value of the last reinforced ratio reached (the breaking point) was recorded, as well as the number of pellets earned. The mean number of pellets earned by session over the ten sessions for one pellet and the five sessions for two pellets was measured for each rat. In the cocaine experiment, for the acquisition procedure (continuous reinforcement fixed ratio 1), 2 weeks after surgery, rats (n ¼ 10, sham-lesioned; n ¼ 10, STN-lesioned) were trained to self-administer cocaine at a dose classically used in this procedure (250 mg per 90-ml infusion in 3 s) during 13–15 daily 2-h sessions in the operant chambers. Pressing one lever (the active lever) switched on the light above the lever for 3 s and delivered cocaine to the blood stream. A 20-s ‘time-out’ was imposed during which any further presses were recorded but had no consequence. Pressing the other lever (the inactive lever) had no further consequence. For the ‘progressive ratio schedule of reinforcement’ part of the cocaine experiment, when the session started, a ‘free’ injection was delivered to fill the catheter. Pressing of the active lever after increasing ‘ratios’ (the number of lever presses required for a reward) was required for receipt of a 250-mg injection of cocaine. The ratios followed the modified equation of Roberts50 (number of lever presses required: 1, 3, 6, 10, 15, 20, 25, 32, 40, 50, 62, 77, 95, 118, 145, 178, 219 and so on). The session ended when the rat failed to complete a ratio within the hour after an injection. The rats were submitted to this schedule for ten consecutive daily sessions. For each session, the last ratio reached was recorded, as well as the number of reinforcers obtained and lever presses as well as locomotor activity. For the dose-response part of the cocaine experiment, after the progressive ratio schedule, the rats were subjected to a ‘fixed ratio 1’ schedule for four additional sessions. On the fifth and sixth sessions, after 1 h of testing at the 250-mg dose, the dose was decreased every hour (80 and 10 mg on the fifth day and 30 and 0 mg on the sixth day). The number of injections obtained during the adaptation phase (first 10 min after the dose switch) was analyzed, as well as the total number of injections during the entire 1-h session. Behavioral protocol for the conditioned place-preference experiment. For preconditioning, 44 rats (sham-lesioned, n ¼ 20; STN-lesioned, n ¼ 24) were placed for 10 min in the place-preference apparatus. Time spent in each compartment was measured in seconds. Rats were then subdivided into six groups and tested for food or cocaine (5 or 10 mg/kg) place preference. For conditioned place-preference experiment using food, two groups (n ¼ 6, sham-lesioned; n ¼ 8, STN-lesioned) were subjected to food restriction (15 g of standard lab chow per rat). For conditioning, on days 1, 3, 5 and 7, rats had access to sucrose pellets (4.5 g total; 100 pellets, 45 mg each) for a 30-min period, with half of the rats in compartment A and the other half in compartment B. On days 2, 4, 6 and 8, rats were placed for 30 min in the opposite compartment without food. For testing, on day 9, all rats were placed in the middle chamber from which both compartments were accessible. The time spent in each compartment was measured for 10 min. For the conditioned place-preference experiment using cocaine, a similar procedure was followed with four other groups (n ¼ 8 and n ¼ 6, shamlesioned, and n ¼ 8 and n ¼ 8, STN-lesioned, for the 5- and 10-mg/kg doses, respectively) injection of cocaine (5 10 mg/kg or 10 mg/kg intraperitioneally) replaced the sucrose pellets, and on alternate days, rats received an injection of vehicle. Statistical analysis. Results are expressed as means for each variable (i.e., number of pellets or injections, last ratio reached in the progressive ratio procedures, score of place preference and so on) in the various groups of rats. For each variable, the data were submitted to mixed-design analyses of variance 488 (Statview; SAS) with group (sham-lesioned versus STN-lesioned) as the between-subject factor and session or drug as the within-subject factor. When significant effects were found, post-hoc comparisons were made using simple main effects analysis. The nonparametric Mann-Whitney test was used for the conditioned place-preference experiment for group effects and the nonparametric sign test was used for conditioning effect. Histology. At the completion of testing, all rats were deeply sedated by chloral anesthesia (400 mg/kg intraperitoneally) and were perfused with a 4% paraformaldehyde solution (Sigma). Brains were removed and were kept in a 10% sucrose solution and frozen to be further cut with a cryostat. Frontal sections of the STN 30–40 mm in thickness were cut and were stained with cresyl violet for assessment of the extent and location of the lesions and injection sites. ACKNOWLEDGMENTS This study was supported by the Centre National de la Recherche Scientifique, a European Community 5th PCRDT program funding (QLK6-1999-02173), and by the Fondation France Parkinson and Conseil Régional d’Aquitaine. The authors thank P.V. Piazza for critical reading and corrections of the manuscript; B. J. Everitt, T.W. Robbins, S.H. Ahmed and L. Vanderschuren for discussion of the results and manuscript; and Y. Darbaky and R. LeCozannet for help. COMPETING INTERESTS STATEMENT The authors declare that they have no competing financial interests. Received 16 February; accepted 2 March 2005 Published online at http://www.nature.com/natureneuroscience/ 1. Hollerman, J.R., Tremblay, L. & Schultz, W. Influence of reward expectation on behavior-related neuronal activity in primate striatum. J. Neurophysiol. 80, 947–963 (1998). 2. Hassani, O.K., Cromwell, H.C. & Schultz, W. Influence of expectation of different rewards on behavior-related neuronal activity in the striatum. J. Neurophysiol. 85, 2477–2489 (2001). 3. Ito, R., Dalley, J.W., Robbins, T.W. & Everitt, B.J. Dopamine release in the dorsal striatum during cocaine-seeking behavior under the control of a drug-associated cue. J. Neurosci. 22, 6247–6253 (2002). 4. Porrino, L.J., Lyons, D., Smith, H.R., Daunais, J.B. & Nader, M.A. Cocaine self-administration produces a progressive involvement of limbic, association, and sensorimotor striatal domains. J. Neurosci. 24, 3554–3562 (2004). 5. Limousin, P. et al. Effects on parkinsonian signs and symptoms of bilateral subthalamic nucleus stimulation. Lancet 345, 91–95 (1995). 6. Bergman, H., Wichmann, T. & DeLong, M.R. Reversal of experimental parkinsonism by lesion of the subthalamic nucleus. Science 249, 1436–1438 (1990). 7. Benazzouz, A., Gross, C., Féger, J., Boraud, T. & Bioulac, B. Reversal of rigidity and improvement in motor performance by subthalamic high-frequency stimulation in MPTP-treated monkeys. Eur. J. Neurosci. 5, 382–389 (1993). 8. Baunez, C., Nieoullon, A. & Amalric, M. In a rat model of parkinsonism, lesions of the subthalamic nucleus reverse increases of reaction time, but induce a dramatic premature responding deficit. J. Neurosci. 15, 6531–6541 (1995). 9. Henderson, J.M. et al. Subthalamic nucleus lesions induce deficits as well as benefits in the hemiparkinsonian rat. Eur. J. Neurosci. 11, 2749–2757 (1999). 10. Krack, P. et al. What is the influence of subthalamic nucleus stimulation on the limbic loop? in Basal Ganglia and Thalamus in Health and Movement Disorders (eds. Kultas-Ilinsky, K. & Ilinsky, I.A.) 333–340 (Kluwer Academic/Plenum, New York, 2001). 11. Trépanier, L.L., Kumar, R., Lozano, A.M., Lang, A.E. & Saint-Cyr, J.A. Neuropsychological outcome of GPi pallidotomy and GPi or STN deep brain stimulation in Parkinson’s Disease. Brain Cogn. 42, 324–347 (2000). 12. Trillet, M., Vighetto, A., Croisile, B., Charles, N. & Aimard, G. Hemiballismus with logorrhea and thymo-affective disinhibition caused by hematoma of the left subthalamic nucleus. Rev. Neurol. (Paris) 151, 416–419 (1995). 13. Absher, J.R. et al. Hypersexuality and hemiballism due to subthalamic infarction. Neuropsychiatry Neuropsychol. Behav. Neurol. 13, 220–229 (2000). 14. Moro, E. et al. Chronic subthalamic nucleus stimulation reduces medication requirements in Parkinson’s disease. Neurology 53, 85–90 (1999). 15. Baunez, C., Amalric, M. & Robbins, T.W. Enhanced food-related motivation after bilateral lesions of the subthalamic nucleus. J. Neurosci. 22, 562–568 (2002). 16. Pontieri, F.E., Mainero, C., La Riccia, M., Passarelli, F. & Orzi, F. Functional correlates of repeated administration of cocaine and apomorphine in the rat. Eur. J. Pharmacol. 284, 205–209 (1995). 17. Uslaner, J.M., Crombag, H.S., Ferguson, S.M. & Robinson, T.E. Cocaine-induced psychomotor activity is associated with its ability to induce c-fos mRNA expression in the subthalamic nucleus: effects of dose and repeated treatment. Eur. J. Neurosci. 17, 2180–2186 (2003). 18. Darbaky, Y., Forni, C., Amalric, M. & Baunez, C. High frequency stimulation of the subthalamic nucleus has beneficial antiparkinsonian effects on motor functions in rats, VOLUME 8 [ NUMBER 4 [ APRIL 2005 NATURE NEUROSCIENCE © 2005 Nature Publishing Group http://www.nature.com/natureneuroscience ARTICLES but less efficiency in a choice-reaction time task. Eur. J. Neurosci. 18, 951–956 (2003). 19. Lorrain, D.S., Arnold, G.M. & Vezina, P. Previous exposure to amphetamine increases incentive to obtain the drug: long-lasting effects revealed by the progressive ratio schedule. Behav. Brain Res. 107, 9–19 (2000). 20. Vezina, P. Sensitization of midbrain dopamine neuron reactivity and the self-administration of psychomotor stimulant drugs. Neurosci. Biobehav. Rev. 27, 827–839 (2004). 21. Hodos, W. Progressive ratio as a measure of reward strength. Science 134, 943–944 (1961). 22. Baunez, C. & Robbins, T.W. Bilateral lesions of the subthalamic nucleus induce multiple deficits in attentional performance in rats. Eur. J. Neurosci. 9, 2086–2099 (1997). 23. Baunez, C. & Robbins, T.W. Effects of transient inactivation of the subthalamic nucleus by local muscimol and APV infusions on performance on the five-choice serial reaction time task in rats. Psychopharmacology (Berl.) 141, 57–65 (1999). 24. Whittier, J.R. Ballism and the subthalamic nucleus (nucleus hypothalamicus; corpus Luysii). Arch. Neurol. Psychiatry 58, 672–692 (1947). 25. Phillips, J.M. & Brown, V.J. Reaction time performance following unilateral striatal dopamine depletion and lesions of the subthalamic nucleus in the rat. Eur. J. Neurosci. 11, 1003–1010 (1999). 26. Carelli, R.M., Ijames, S.G. & Crumling, A.J. Evidence that separate neural circuits in the nucleus accumbens encode cocaine versus ‘‘natural’’ (water and food) reward. J. Neurosci. 20, 4255–4266 (2000). 27. Carelli, R.M. Nucleus accumbens cell firing during goal-directed behaviors for cocaine vs. ‘natural’ reinforcement. Physiol. Behav. 76, 379–387 (2002). 28. Bowman, E.M., Aigner, T.G. & Richmond, B.J. Neural signals in the monkey ventral striatum related to motivation for juice and cocaine rewards. J. Neurophysiol. 75, 1061– 1073 (1996). 29. Maurice, N., Deniau, J.M., Menetrey, A., Glowinski, J. & Thierry, A.M. Prefrontal cortexbasal ganglia circuits in the rat: involvement of ventral pallidum and subthalamic nucleus. Synapse 29, 363–370 (1998). 30. Maurice, N., Deniau, J.M., Glowinski, J. & Thierry, A.M. Relationships between the prefrontal cortex and the basal ganglia in the rat: physiology of the corticosubthalamic circuits. J. Neurosci. 18, 9539–9546 (1998). 31. Alexander, G.E., Crutcher, M.D. & DeLong, M.R. Basal ganglia-thalamocortical circuits: parallel substrates for motor, oculomotor, ‘‘prefrontal’’ and ‘‘limbic’’ functions. Prog. Brain Res. 85, 119–146 (1990). 32. Groenewegen, H.J. & Berendse, H.W. Connections of the subthalamic nucleus with ventral striatopallidal parts of the basal ganglia in the rat. J. Comp. Neurol. 294, 607–622 (1990). 33. Turner, M.S., Lavin, A., Grace, A.A. & Napier, T.C. Regulation of limbic information outflow by the subthalamic nucleus: excitatory amino acid projections to the ventral pallidum. J. Neurosci. 21, 2820–2832 (2001). NATURE NEUROSCIENCE VOLUME 8 [ NUMBER 4 [ APRIL 2005 34. Piazza, P.V., Deroche-Gamonet, V., Rouge-Pont, F. & Le Moal, M. Vertical shifts in self-administration dose-response functions predict a drug-vulnerable phenotype predisposed to addiction. J. Neurosci. 20, 4226–4232 (2000). 35. Ahmed, S.H. & Koob, G.F. Transition from moderate to excessive drug intake: change in hedonic set point. Science 282, 298–300 (1998). 36. Robledo, P. & Koob, G.F. Two discrete nucleus accumbens projection areas differentially mediate cocaine self-administration in the rat. Behav. Brain Res. 55, 159–166 (1993). 37. Pontieri, F.E., Tanda, G. & Di Chiara, G. Intravenous cocaine, morphine, and amphetamine preferentially increase extracellular dopamine in the ‘‘shell’’ as compared with the ‘‘core’’ of the rat nucleus accumbens. Proc. Natl. Acad. Sci. USA 92, 12304–12308 (1995). 38. Cardinal, R.N., Parkinson, J.A., Hall, J. & Everitt, B.J. Emotion and motivation: the role of the amygdala, ventral striatum, and prefrontal cortex. Neurosci. Biobehav. Rev. 26, 321–352 (2002). 39. Di Chiara, G. Nucleus accumbens shell and core dopamine: differential role in behavior and addiction. Behav. Brain Res. 137, 75–114 (2002). 40. Ito, R., Robbins, T.W. & Everitt, B.J. Differential control over cocaine-seeking behavior by nucleus accumbens core and shell. Nat. Neurosci. 7, 389–397 (2004). 41. Tanaka, S.C. et al. Prediction of immediate and future rewards differentially recruits cortico-basal ganglia loops. Nat. Neurosci. 7, 887–893 (2004). 42. Schultz, W., Tremblay, L. & Hollerman, J.R. Reward processing in primate orbitofrontal cortex and basal ganglia. Cereb. Cortex 10, 272–283 (2000). 43. Izquierdo, A., Suda, R.K. & Murray, E.A. Bilateral orbital prefrontal cortex lesions in rhesus monkeys disrupt choices guided by both reward value and reward contingency. J. Neurosci. 24, 7540–7548 (2004). 44. Bolla, K.I. et al. Orbitofrontal dysfunction in abstinent cocaine abusers performing a decision-making task. Neuroimage 19, 1085–1094 (2003). 45. Volkow, N.D. & Fowler, J.S. Addiction, a disease of compulsion and drive: involvement of the orbitofrontal cortex. Cereb. Cortex 10, 318–325 (2000). 46. Volkow, N.D., Fowler, J.S., Wang, G-J. & Goldstein, R. Role of dopamine, the frontal cortex and memory circuits in drug addiction: insight from imaging studies. Neurobiol. Learn. Mem. 78, 610–624 (2002). 47. Chudasama, Y., Baunez, C. & Robbins, T.W. Functional disconnection of the medial prefrontal cortex and subthalamic nucleus in attentional performance: Evidence for cortico-subthalamic interaction. J. Neurosci. 23, 5477–5485 (2003). 48. Mallet, L. et al. Compulsions, Parkinson’s disease, and stimulation. Lancet 360, 1302– 1304 (2002). 49. Paxinos, G. & Watson, C. The Rat Brain in Stereotaxic Coordinates 2nd edn. (Academic, Sydney, 1986). 50. Depoortere, R.Y., Li, D.H., Lane, J.D. & Emmett-Oglesby, M.W. Parameters of selfadministration of cocaine in rats under a progressive-ratio schedule. Pharmacol. Biochem. Behav. 45, 539–548 (1993). 489