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Inhibition of the glutamate transporter by L-trans-PDC in the nucleus accumbens prevents the locomotor response to amphetamine
Neuropharmacology 41 (2001) 409–411 www.elsevier.com/locate/neuropharm
Rapid communication
Inhibition of the glutamate transporter by L-trans-PDC in the nucleus accumbens prevents the locomotor response to amphetamine He´le`ne N. David, Anne The´venoux, Jacques H. Abraini
*
Universite´ de Caen, UMR CNRS 6551, Centre CYCERON, Boulevard Henri Becquerel, BP 5229, 14074 Caen Cedex, France Received 5 April 2001; received in revised form 4 June 2001; accepted 7 June 2001
Abstract Infusion in the nucleus accumbens of the glutamate uptake inhibitor L-trans-PDC prevented the amphetamine-induced locomotor response. Since L-trans-PDC has been shown to block the amphetamine-induced increase in glutamate but not in DA release, our result indicates that the glutamate transporter is an obligatory target for the activating properties of amphetamine. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Glutamate; Dopamine; Amphetamine; Locomotion
The psychoactive drug amphetamine is usually thought to stimulate locomotion through an activation of the dopamine (DA) mesolimbic system. Compelling evidence has implicated DA release in the nucleus accumbens (NAcc) as a critical condition for the generation of locomotor activity in response to d-amphetamine. In fact, infusion of d-amphetamine results in a profound increase of accumbal DA release that is associated with locomotion, while blockade of DA receptors or destruction of DA terminals reverses these activating properties of amphetamine (Koob and Nestler, 1997). In addition, an indifference to amphetamine has been also evidenced in mice lacking the DA transporter (Giros et al., 1996). Alternatively, several reports have suggested a role of glutamate in the neurotoxic damage induced by amphetamine. In vivo studies have shown that systemic or intracerebral injections of amphetamine increased glutamate release in the striatum–accumbens complex (Del Arco et al., 1999), and the contribution of glutamate in the locomotor-activating properties of d-amphetamine has been further demonstrated in several behavioral studies (Burns et al., 1994; Vezina and Kim, 1999). Interestingly, infusion of the competitive glutamate
* Corresponding author. Tel./fax: +33-231-566-035. E-mail address: [email protected] (J.H. Abraini).
uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (L-trans-PDC), which enhances both glutamateand DA release (Del Arco et al., 1999) and further increases locomotion (Kim and Vezina, 1999) when administered alone in the striatum–accumbens complex of untreated animals, has been recently evidenced to alter the increase in glutamate but not in DA release produced by amphetamine (Del Arco et al., 1999). This finding questions the potential effect of L-trans-PDC on the locomotor response induced by d-amphetamine and further provides the opportunity to investigate the contribution of accumbal glutamate in the behavioral expression produced by d-amphetamine. This prompted us, in the present study, to compare the effects of Ltrans-PDC on both basal locomotor activity and the amphetamine-induced locomotor hyperactivity. Male adult Sprague–Dawley rats (Iffa Credo, France) weighing 200–220 g at the time of surgery were used in accordance with the declaration of Helsinki. They were housed at 21±0.5°C, in a 12:12 h light:dark cycle (lights on from 7 am to 7 pm) with food and water available ad libitum. At least three days after arrival, rats were anesthetized (pentobarbital sodium 30 mg kg⫺1 ip and ketamine 100 mg kg⫺1 i.m.), mounted on a stereotaxic apparatus with the incisive bar set 3.9 mm below the horizontal zero and implanted in the nucleus accumbens 1 mm above the aimed injection site (A: 1.7 mm, L: 1.4
0028-3908/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 8 - 3 9 0 8 ( 0 1 ) 0 0 0 8 4 - 3
410
H.N. David et al. / Neuropharmacology 41 (2001) 409–411
mm, V: 6.1 mm from Bregma; Paxinos and Watson, 1998) with 12 mm chronic bilateral stainless steel guide cannulae (21 gauge) that were anchored to the skull with stainless steel screws and dental cement. At the end of surgery, stainless steel wire stylets were inserted into the guide cannulae to prevent occlusion, and the animals housed individually and allowed to recover for at least 5 days before any pharmacological treatment. One week before testing, rats were handled and placed individually for 2 h (three times a week) in activity cages measuring 30×20×20 cm in order to familiarize them with the experimental procedure. On the day of testing, rats were placed in the activity cages for 60 min before drug treatment, infused bilaterally into the NAcc to a depth of 1 mm below the guide cannula tips, and recorded for 90 min post-injection. Two groups of rats were used and treated as follows: group 1 (n=6) was infused with L-trans-PDC (0, 0.05, 0.2 g/side randomly; Kim and Vezina, 1999; purchased from Tocris France); group 2 (n=6) was injected with either saline, d-amphetamine (5 g/side; dose chosen from the doses of amphetamine and the recovery of the dialysis probe used by Del Arco et al., 1999 and brain tissue diffusion; purchased from Calaire Chimie, France) and a mixed solution of d-amphetamine (5 g/side)+L-trans-PDC (0.2 g/side). Drug treatments were made in 1 l/side at 0.3 l min⫺1 using injection cannulae 13 mm long (30 gauge) connected via microtubing to microsyringes placed on a microdrive pump. All experiments were conducted between 8 am and 11 am; a minimum time interval of 7 days was spent between each drug treatment. Locomotor activity was quantified using horizontal infrared beams located 3 cm above the floor across the long axis of the activity cage (Imetronic, France), recorded over 1-min period intervals on a computer situated in an adjacent room and pooled every 10 min. Data analysis was made using non-parametric statistics. Within-group comparisons were carried out using a Friedman analysis of variance; following a significant Fvalue, post hoc comparisons were made using the Wilcoxon paired t-test. No significant difference was seen during the habituation periods prior drug treatments (group 1: F10,75=2.347, n.s.; group 2: F15,100=5.058, n.s.). Bilateral infusion in the NAcc of L-trans-PDC at 0.05 and 0.2 g/side induced a dose-dependent moderate and short-lasting locomotor activation (dose×time interactions: F16,120=11.676; P⬍0.05), which post hoc statistical analysis between post-injection total scores revealed to be significant at both 0.05 and 0.2 g/side [Fig. 1(A), n=6; P⬍0.05). In contrast, bilateral administration in the NAcc of d-amphetamine at 5 g/side induced a dramatic and long-lasting increase in locomotor activity, which behavioral activation was blocked when d-amphetamine was co-injected with L-trans-PDC at 0.2 g/side (drug×time interactions: F16,120=51.361; P⬍0.01). Post hoc comparison between post-injection total scores
Fig. 1. Effects of intra-accumbens infusion of L-trans-PDC on basal locomotor activity and the locomotor response to d-amphetamine. (A) Infusion of L-trans-PDC (n=6 per dose) at 0.05 g/side (䊐) and 0.2 g/side (䊏) resulted compared to saline (䊊) in a moderate dose-dependent increase in locomotor activity. (B) Infusion of d-amphetamine (5 g/side, 䊏) resulted compared to saline (䊊) in a dramatic increase in locomotor activity. Co-infusion (䊐) with d-amphetamine of L-transPDC at 0.2 g/side prevented the locomotor response to amphetamine. Infusion one week later of d-amphetamine, in order to investigate any functional disruption, further resulted in an increase in locomotor activity (왖). Insets show total activity counts ±SEM for the 90-min testing period following drug infusion. *P⬍0.05 vs saline solution; + P⬍0.05 vs d-amphetamine.
revealed a significant effect of d-amphetamine on locomotor activity [Fig. 1(B), n=6; P⬍0.05) that was significantly reduced by L-trans-PDC (Fig. 1, n=6; P⬍ 0.05). No other behaviors, such as stereotyped behaviors that could lead to a decrease in locomotor activity but may indicate a heightened effect of amphetamine, was seen following co-infusion of d-amphetamine with L-
H.N. David et al. / Neuropharmacology 41 (2001) 409–411
trans-PDC in the NAcc. Although continuous infusion of L-trans-PDC at 25 nmol/h for 14 days has been reported to induce dramatic neuronal loss and gliosis (Lievens et al., 1997), histological control using cresylviolet stained brain sections showed no evidence of neurotoxicity in the tissue surrounding the cannula tracks after drug injection. In addition, administration one week later of a second infusion of d-amphetamine, in order to investigate any functional disruption in the NAcc, further resulted in an increase in locomotor activity (F16,120=47.028; P⬍0.01), which post hoc analysis revealed to be significant (Fig. 1, n=6; P⬍0.05) compared to saline, but not different from the locomotor response produced by the first injection of d-amphetamine (Fig. 1, n=6; n.s.). This agrees with previous studies that have reported no neurotoxicity following acute injection of L-trans-PDC in the striatum at similar or much higher doses than those used herein (Massieu et al., 1995; Kim and Vezina, 1999). Our results confirm that, in untreated rats, administration in the NAcc of L-trans-PDC (a transportable glutamate analogue that produces the most selective and potent inhibition of the glutamate transporter without interfering with glutamate receptors; Bridges et al., 1991) results in a dose-dependent increase in locomotor activity (Kim and Vezina, 1999), a behavioral response that may be related to the increase in accumbal glutamate and DA release produced by L-trans-PDC when administered alone in untreated rats (Del Arco et al., 1999). In contrast, the present study further demonstrates, for the first time to the best of our knowledge, that co-infusion in the NAcc of L-trans-PDC with d-amphetamine abolished the locomotor response to d-amphetamine. Administration of d-amphetamine in the NAcc has been demonstrated to produce a facilitatory action on glutamate and DA release through a reversion of the Na+-dependent neurotransmitter transporters as a result of a vasoconstriction-induced reduction in oxygen availability and ATP synthesis for the cell (Del Arco et al., 1999). Interestingly, in contrast with its effect on basal glutamate release, infusion of L-trans-PDC in the NAcc has been shown to block the increase in glutamate, but not in DA, release induced by d-amphetamine (Del Arco et al., 1999). Therefore, consistent with this selective action of L-trans-PDC on the glutamate, but not DA, release produced by d-amphetamine in the NAcc, our results demonstrate that glutamate release in the NAcc of intact animals plays a crucial role in the expression
411
of the locomotor response to d-amphetamine, and further indicate that the glutamate transporter is an obligatory target for the locomotor-activating properties of amphetamine. Although the mechanisms of action of amphetamine at the glutamate and DA transporter are thought to differ somewhat, as amphetamine is only believed to act indirectly at the glutamate transporter and both directly and indirectly at the DA transporter (Del Arco et al., 1999), the findings of the present study, together with the indifference to amphetamine reported in mice lacking the dopamine transporter (Giros et al., 1996), indicate that the effects of amphetamine at the Na+-dependent neurotransmitter transporters would be a key mechanism for the behavioral expression and locomotoractivating properties of amphetamine. References Bridges, R.J., Stanley, M.S., Anderson, M.W., Cotman, C.W., Chamberlin, A.R., 1991. Conformationally defined neurotransmitter analogues. Selective inhibition of glutamate uptake by one pyrrolidine2,4-dicarboxylate diastereomer. Journal of Medicinal Chemistry 34, 717–725. Burns, L.H., Everitt, B.J., Kelley, A.E., Robbins, T.W., 1994. Glutamate–dopamine interactions in the ventral striatum: role in locomotor activity and responding with conditioned reinforcement. Psychopharmacology 115, 516–528. Del Arco, A., Gonzales-Mora, J.L., Armas, V.R., Mora, F., 1999. Amphetamine increases the extracellular concentration of glutamate in the striatum of the awake rat: involvement of high affinity transporter mechanisms. Neuropharmacology 38, 943–954. Giros, B., Jaber, M., Jones, S.R., Wightman, M., Caron, M.G., 1996. Hyperlocomotion and indifference to cocaine and amphetamine in mice lacking the dopamine transporter. Nature 379, 606–612. Kim, J.H., Vezina, P., 1999. Blockade of glutamate reuptake in the rat nucleus accumbens increases locomotor activity. Brain Research 819, 165–169. Koob, G.F., Nestler, E.J., 1997. The neurobiology of drug addiction. Journal of Neuropsychiatry and Clinical Neuroscience 9, 482–497. Lievens, J.-C., Dutertre, M., Forni, C., Salin, P., Kerkerian-LeGoff, L., 1997. Continuous administration of the glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylate produces striatal lesion. Molecular Brain Research 50, 181–189. Massieu, L., Morales-Villagran, A., Tapia, R., 1995. Accumulation of extracellular glutamate by inhibition of its uptake is not sufficient for inducing neuronal damage: an in vivo microdialysis study. Journal of Neurochemistry 64, 2262–2272. Paxinos, G., Watson, C., 1998. The Rat Brain in Stereotaxic Coordinates. Academic Press, New York. Vezina, P., Kim, J.H., 1999. Metabotropic glutamate receptors and the generation of locomotor activity: interactions with midbrain dopamine. Neuroscience and Biobehavioral Reviews 23, 577–589.
Rapid communication
Inhibition of the glutamate transporter by L-trans-PDC in the nucleus accumbens prevents the locomotor response to amphetamine He´le`ne N. David, Anne The´venoux, Jacques H. Abraini
*
Universite´ de Caen, UMR CNRS 6551, Centre CYCERON, Boulevard Henri Becquerel, BP 5229, 14074 Caen Cedex, France Received 5 April 2001; received in revised form 4 June 2001; accepted 7 June 2001
Abstract Infusion in the nucleus accumbens of the glutamate uptake inhibitor L-trans-PDC prevented the amphetamine-induced locomotor response. Since L-trans-PDC has been shown to block the amphetamine-induced increase in glutamate but not in DA release, our result indicates that the glutamate transporter is an obligatory target for the activating properties of amphetamine. 2001 Elsevier Science Ltd. All rights reserved. Keywords: Glutamate; Dopamine; Amphetamine; Locomotion
The psychoactive drug amphetamine is usually thought to stimulate locomotion through an activation of the dopamine (DA) mesolimbic system. Compelling evidence has implicated DA release in the nucleus accumbens (NAcc) as a critical condition for the generation of locomotor activity in response to d-amphetamine. In fact, infusion of d-amphetamine results in a profound increase of accumbal DA release that is associated with locomotion, while blockade of DA receptors or destruction of DA terminals reverses these activating properties of amphetamine (Koob and Nestler, 1997). In addition, an indifference to amphetamine has been also evidenced in mice lacking the DA transporter (Giros et al., 1996). Alternatively, several reports have suggested a role of glutamate in the neurotoxic damage induced by amphetamine. In vivo studies have shown that systemic or intracerebral injections of amphetamine increased glutamate release in the striatum–accumbens complex (Del Arco et al., 1999), and the contribution of glutamate in the locomotor-activating properties of d-amphetamine has been further demonstrated in several behavioral studies (Burns et al., 1994; Vezina and Kim, 1999). Interestingly, infusion of the competitive glutamate
* Corresponding author. Tel./fax: +33-231-566-035. E-mail address: [email protected] (J.H. Abraini).
uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (L-trans-PDC), which enhances both glutamateand DA release (Del Arco et al., 1999) and further increases locomotion (Kim and Vezina, 1999) when administered alone in the striatum–accumbens complex of untreated animals, has been recently evidenced to alter the increase in glutamate but not in DA release produced by amphetamine (Del Arco et al., 1999). This finding questions the potential effect of L-trans-PDC on the locomotor response induced by d-amphetamine and further provides the opportunity to investigate the contribution of accumbal glutamate in the behavioral expression produced by d-amphetamine. This prompted us, in the present study, to compare the effects of Ltrans-PDC on both basal locomotor activity and the amphetamine-induced locomotor hyperactivity. Male adult Sprague–Dawley rats (Iffa Credo, France) weighing 200–220 g at the time of surgery were used in accordance with the declaration of Helsinki. They were housed at 21±0.5°C, in a 12:12 h light:dark cycle (lights on from 7 am to 7 pm) with food and water available ad libitum. At least three days after arrival, rats were anesthetized (pentobarbital sodium 30 mg kg⫺1 ip and ketamine 100 mg kg⫺1 i.m.), mounted on a stereotaxic apparatus with the incisive bar set 3.9 mm below the horizontal zero and implanted in the nucleus accumbens 1 mm above the aimed injection site (A: 1.7 mm, L: 1.4
0028-3908/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 8 - 3 9 0 8 ( 0 1 ) 0 0 0 8 4 - 3
410
H.N. David et al. / Neuropharmacology 41 (2001) 409–411
mm, V: 6.1 mm from Bregma; Paxinos and Watson, 1998) with 12 mm chronic bilateral stainless steel guide cannulae (21 gauge) that were anchored to the skull with stainless steel screws and dental cement. At the end of surgery, stainless steel wire stylets were inserted into the guide cannulae to prevent occlusion, and the animals housed individually and allowed to recover for at least 5 days before any pharmacological treatment. One week before testing, rats were handled and placed individually for 2 h (three times a week) in activity cages measuring 30×20×20 cm in order to familiarize them with the experimental procedure. On the day of testing, rats were placed in the activity cages for 60 min before drug treatment, infused bilaterally into the NAcc to a depth of 1 mm below the guide cannula tips, and recorded for 90 min post-injection. Two groups of rats were used and treated as follows: group 1 (n=6) was infused with L-trans-PDC (0, 0.05, 0.2 g/side randomly; Kim and Vezina, 1999; purchased from Tocris France); group 2 (n=6) was injected with either saline, d-amphetamine (5 g/side; dose chosen from the doses of amphetamine and the recovery of the dialysis probe used by Del Arco et al., 1999 and brain tissue diffusion; purchased from Calaire Chimie, France) and a mixed solution of d-amphetamine (5 g/side)+L-trans-PDC (0.2 g/side). Drug treatments were made in 1 l/side at 0.3 l min⫺1 using injection cannulae 13 mm long (30 gauge) connected via microtubing to microsyringes placed on a microdrive pump. All experiments were conducted between 8 am and 11 am; a minimum time interval of 7 days was spent between each drug treatment. Locomotor activity was quantified using horizontal infrared beams located 3 cm above the floor across the long axis of the activity cage (Imetronic, France), recorded over 1-min period intervals on a computer situated in an adjacent room and pooled every 10 min. Data analysis was made using non-parametric statistics. Within-group comparisons were carried out using a Friedman analysis of variance; following a significant Fvalue, post hoc comparisons were made using the Wilcoxon paired t-test. No significant difference was seen during the habituation periods prior drug treatments (group 1: F10,75=2.347, n.s.; group 2: F15,100=5.058, n.s.). Bilateral infusion in the NAcc of L-trans-PDC at 0.05 and 0.2 g/side induced a dose-dependent moderate and short-lasting locomotor activation (dose×time interactions: F16,120=11.676; P⬍0.05), which post hoc statistical analysis between post-injection total scores revealed to be significant at both 0.05 and 0.2 g/side [Fig. 1(A), n=6; P⬍0.05). In contrast, bilateral administration in the NAcc of d-amphetamine at 5 g/side induced a dramatic and long-lasting increase in locomotor activity, which behavioral activation was blocked when d-amphetamine was co-injected with L-trans-PDC at 0.2 g/side (drug×time interactions: F16,120=51.361; P⬍0.01). Post hoc comparison between post-injection total scores
Fig. 1. Effects of intra-accumbens infusion of L-trans-PDC on basal locomotor activity and the locomotor response to d-amphetamine. (A) Infusion of L-trans-PDC (n=6 per dose) at 0.05 g/side (䊐) and 0.2 g/side (䊏) resulted compared to saline (䊊) in a moderate dose-dependent increase in locomotor activity. (B) Infusion of d-amphetamine (5 g/side, 䊏) resulted compared to saline (䊊) in a dramatic increase in locomotor activity. Co-infusion (䊐) with d-amphetamine of L-transPDC at 0.2 g/side prevented the locomotor response to amphetamine. Infusion one week later of d-amphetamine, in order to investigate any functional disruption, further resulted in an increase in locomotor activity (왖). Insets show total activity counts ±SEM for the 90-min testing period following drug infusion. *P⬍0.05 vs saline solution; + P⬍0.05 vs d-amphetamine.
revealed a significant effect of d-amphetamine on locomotor activity [Fig. 1(B), n=6; P⬍0.05) that was significantly reduced by L-trans-PDC (Fig. 1, n=6; P⬍ 0.05). No other behaviors, such as stereotyped behaviors that could lead to a decrease in locomotor activity but may indicate a heightened effect of amphetamine, was seen following co-infusion of d-amphetamine with L-
H.N. David et al. / Neuropharmacology 41 (2001) 409–411
trans-PDC in the NAcc. Although continuous infusion of L-trans-PDC at 25 nmol/h for 14 days has been reported to induce dramatic neuronal loss and gliosis (Lievens et al., 1997), histological control using cresylviolet stained brain sections showed no evidence of neurotoxicity in the tissue surrounding the cannula tracks after drug injection. In addition, administration one week later of a second infusion of d-amphetamine, in order to investigate any functional disruption in the NAcc, further resulted in an increase in locomotor activity (F16,120=47.028; P⬍0.01), which post hoc analysis revealed to be significant (Fig. 1, n=6; P⬍0.05) compared to saline, but not different from the locomotor response produced by the first injection of d-amphetamine (Fig. 1, n=6; n.s.). This agrees with previous studies that have reported no neurotoxicity following acute injection of L-trans-PDC in the striatum at similar or much higher doses than those used herein (Massieu et al., 1995; Kim and Vezina, 1999). Our results confirm that, in untreated rats, administration in the NAcc of L-trans-PDC (a transportable glutamate analogue that produces the most selective and potent inhibition of the glutamate transporter without interfering with glutamate receptors; Bridges et al., 1991) results in a dose-dependent increase in locomotor activity (Kim and Vezina, 1999), a behavioral response that may be related to the increase in accumbal glutamate and DA release produced by L-trans-PDC when administered alone in untreated rats (Del Arco et al., 1999). In contrast, the present study further demonstrates, for the first time to the best of our knowledge, that co-infusion in the NAcc of L-trans-PDC with d-amphetamine abolished the locomotor response to d-amphetamine. Administration of d-amphetamine in the NAcc has been demonstrated to produce a facilitatory action on glutamate and DA release through a reversion of the Na+-dependent neurotransmitter transporters as a result of a vasoconstriction-induced reduction in oxygen availability and ATP synthesis for the cell (Del Arco et al., 1999). Interestingly, in contrast with its effect on basal glutamate release, infusion of L-trans-PDC in the NAcc has been shown to block the increase in glutamate, but not in DA, release induced by d-amphetamine (Del Arco et al., 1999). Therefore, consistent with this selective action of L-trans-PDC on the glutamate, but not DA, release produced by d-amphetamine in the NAcc, our results demonstrate that glutamate release in the NAcc of intact animals plays a crucial role in the expression
411
of the locomotor response to d-amphetamine, and further indicate that the glutamate transporter is an obligatory target for the locomotor-activating properties of amphetamine. Although the mechanisms of action of amphetamine at the glutamate and DA transporter are thought to differ somewhat, as amphetamine is only believed to act indirectly at the glutamate transporter and both directly and indirectly at the DA transporter (Del Arco et al., 1999), the findings of the present study, together with the indifference to amphetamine reported in mice lacking the dopamine transporter (Giros et al., 1996), indicate that the effects of amphetamine at the Na+-dependent neurotransmitter transporters would be a key mechanism for the behavioral expression and locomotoractivating properties of amphetamine. References Bridges, R.J., Stanley, M.S., Anderson, M.W., Cotman, C.W., Chamberlin, A.R., 1991. Conformationally defined neurotransmitter analogues. Selective inhibition of glutamate uptake by one pyrrolidine2,4-dicarboxylate diastereomer. Journal of Medicinal Chemistry 34, 717–725. Burns, L.H., Everitt, B.J., Kelley, A.E., Robbins, T.W., 1994. Glutamate–dopamine interactions in the ventral striatum: role in locomotor activity and responding with conditioned reinforcement. Psychopharmacology 115, 516–528. Del Arco, A., Gonzales-Mora, J.L., Armas, V.R., Mora, F., 1999. Amphetamine increases the extracellular concentration of glutamate in the striatum of the awake rat: involvement of high affinity transporter mechanisms. Neuropharmacology 38, 943–954. Giros, B., Jaber, M., Jones, S.R., Wightman, M., Caron, M.G., 1996. Hyperlocomotion and indifference to cocaine and amphetamine in mice lacking the dopamine transporter. Nature 379, 606–612. Kim, J.H., Vezina, P., 1999. Blockade of glutamate reuptake in the rat nucleus accumbens increases locomotor activity. Brain Research 819, 165–169. Koob, G.F., Nestler, E.J., 1997. The neurobiology of drug addiction. Journal of Neuropsychiatry and Clinical Neuroscience 9, 482–497. Lievens, J.-C., Dutertre, M., Forni, C., Salin, P., Kerkerian-LeGoff, L., 1997. Continuous administration of the glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylate produces striatal lesion. Molecular Brain Research 50, 181–189. Massieu, L., Morales-Villagran, A., Tapia, R., 1995. Accumulation of extracellular glutamate by inhibition of its uptake is not sufficient for inducing neuronal damage: an in vivo microdialysis study. Journal of Neurochemistry 64, 2262–2272. Paxinos, G., Watson, C., 1998. The Rat Brain in Stereotaxic Coordinates. Academic Press, New York. Vezina, P., Kim, J.H., 1999. Metabotropic glutamate receptors and the generation of locomotor activity: interactions with midbrain dopamine. Neuroscience and Biobehavioral Reviews 23, 577–589.