Neuroadaptations in Ionotropic and Metabotropic Glutamate Receptor mRNA Produced by Cocaine Treatment

Abstract : The expression of glutamate receptor/subunit mRNAs was examined 3 weeks after discontinuing 1 week of daily injections of saline or cocaine. The level of mRNA for GluR1-4, NMDAR1, and mGluR5 receptors was measured with in situ hybridization and RT-PCR. In nucleus accumbens, acute cocaine treatment significantly reduced the mRNA level for GluR3, GluR4, and NMDAR1 subunits, whereas repeated cocaine reduced the level for GluR3 mRNA. Acute cocaine treatment also reduced the NMDAR1 mRNA level in dorsolateral striatum and ventral tegmental area. In prefrontal cortex, repeated cocaine treatment significantly increased the level of GluR2 mRNA. The GluR2 mRNA level was not changed by acute or repeated cocaine in any other brain regions examined. Repeated cocaine treatment also significantly increased mGluR5 mRNA levels in nucleus accumbens shell and dorsolateral striatum. Functional properties of the ionotropic glutamate receptors are determined by subunit composition. In addition, metabotropic glutamate receptors can modulate synaptic transmission and the response to stimulation of ionotropic receptors. Thus, the observed changes in levels of AMPA and NMDA receptor subunits and the mGluR5 metabotropic receptor may alter excitatory neurotransmission in the mesocorticolimbic dopamine system, which could play a significant role in the enduring biochemical and behavioral effects of cocaine.     

Repeated exposure to cocaine alters medial prefrontal cortex dopamine D₂-like receptor modulation of glutamate and dopamine neurotransmission within the mesocorticolimbic system

Repeated exposure to cocaine progressively increases drug-induced locomotor activity, which is termed behavioral sensitization. Previous studies have demonstrated that sensitization to cocaine is associated with a decrease in dopamine D? receptor function in the medial prefrontal cortex. The present report tested the hypothesis that reduced medial prefrontal cortex D? receptor function as a result of repeated cocaine exposure results in augmented excitatory transmission to the nucleus accumbens and ventral tegmental area, possibly as a partial result of enhanced inhibition of local dopamine release. Dual probe microdialysis experiments were conducted in male Sprague-Dawley rats 1, 7 or 30 days following the last of four daily injections of saline (1.0 mL/kg) or cocaine (15 mg/kg). Infusion of quinpirole (0.01, 1.0 and 100 μM), a D?-like receptor agonist, into the medial prefrontal cortex produced a dose-dependent decrease in cortical, nucleus accumbens and ventral tegmental area extracellular glutamate levels in control but not sensitized animals. Quinpirole also reduced basal dopamine levels in the medial prefrontal cortex in sensitized animals following 1 day of withdrawal from cocaine. Following 30 days of withdrawal, quinpirole also reduced dopamine levels in sensitized animals relative to saline controls, but not relative to baseline levels. These findings indicate that the expression of sensitization to cocaine is associated with altered modulation of mesocorticolimbic glutamatergic transmission at the level of the medial prefrontal cortex.     

Molecular neuroadaptations in the accumbens and ventral tegmental area during the first 90 days of forced abstinence from cocaine self-administration in rats

Cocaine self-administration is associated with a propensity to relapse in humans and reinstatement of drug seeking in rats after prolonged withdrawal periods. These behaviors are hypothesized to be mediated by molecular neuroadaptations within the mesolimbic dopamine system. However, in most studies of drug-induced neuroadaptations, cocaine was experimenter-delivered and molecular measurements were performed after short withdrawal periods. In the present study, rats were trained to self-administer intravenous cocaine or oral sucrose (a control non-drug reward) for 10 days (6-h/day) and were killed following 1, 30, or 90 days of reward withdrawal. Tissues from the accumbens and ventral tegmental area (VTA) were assayed for candidate molecular neuroadaptations, including enzyme activities of cAMP-dependent protein kinase (PKA) and adenylate cyclase (AC), and protein expression of cyclin-dependent kinase 5 (cdk5), tyrosine hydroxylase (TH) and glutamate receptor subunits (GluR1, GluR2 and NMDAR1). In the accumbens of cocaine-trained rats, GluR1 and NMDAR1 levels were increased on days 1 and 90, while GluR2 levels were increased on days 1 and 30, but not day 90; PKA activity levels were increased on days 1 and 30, but not day 90, while AC activity, TH and cdk5 levels were unaltered. In the VTA of cocaine-trained rats, NMDAR1 levels were increased for up to 90 days, while GluR2 levels were increased only on day 1; TH and Cdk5 levels were increased only on day 1, while PKA and AC activity levels were unaltered. Cocaine self-administration produces long-lasting molecular neuroadaptations in the VTA and accumbens that may underlie cocaine relapse during periods of abstinence.     

Behavioral sensitization to amphetamine is not accompanied by changes in glutamate receptor surface expression in the rat nucleus accumbens

We examined whether behavioral sensitization to amphetamine is associated with redistribution of glutamate receptors (GluR) in the rat nucleus accumbens (NAc) or dorsolateral striatum (DLSTR). Following repeated amphetamine treatment and 21 days of withdrawal, surface and intracellular levels of α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) or NMDA receptor subunits were determined using a protein cross-linking assay. In contrast to our previous results in cocaine-sensitized rats, we did not observe redistribution of GluR1 or GluR2 to the cell surface in the NAc after amphetamine withdrawal, although a small increase in total GluR1 was found in the shell subregion. Nor did we observe activation of signaling pathways associated with cocaine-induced AMPA receptor trafficking or changes in NMDA receptor subunits. No significant changes were observed in the DLSTR. We also investigated the effect of administering a challenge injection of amphetamine to amphetamine-sensitized rats 24 h prior to biochemical analysis based on prior studies showing that cocaine challenge decreases AMPA receptor surface expression in the NAc of cocaine-sensitized rats. GluR1 and GluR2 were not significantly altered in either NAc or DLSTR, although a modest effect on GluR3 cannot be ruled out. Our results suggest that glutamate transmission in the NAc is dramatically different in rats sensitized to amphetamine versus cocaine.     

Repeated Cocaine Administration Alters Extracellular Glutamate in the Ventral Tegmental Area

Abstract: The present study determined if repeated cocaine injections alter the effect of cocaine on extracellular glutamate in the ventral tegmental area (VTA). All rats were treated with daily cocaine (15 mg/kg i.p. × 2 days, 30 mg/kg i.p. × 5 days) or saline for 7 days. At 21 days after discontinuing the daily injections, a dialysis probe was placed into the VTA and the extracellular levels of glutamate were estimated. A systemic injection of cocaine (15 mg/kg i.p.) elevated extracellular glutamate in the VTA of rats pretreated with daily cocaine but not in the daily saline-pretreated subjects. No significant change in glutamate was produced by a saline injection in either pretreatment group. In a group of rats pretreated with daily cocaine, the D₁ antagonist SCH-23390 (30 µ M ) was infused through the dialysis probe prior to the acute injections of saline and cocaine. SCH-23390 prevented the increase in extracellular glutamate associated with the acute administration of cocaine. Behavioral data were collected simultaneously with the measures of extracellular glutamate. The behavioral stimulant effect of cocaine was greater in cocaine-pretreated than saline-pretreated subjects, and the behavioral augmentation in cocaine-pretreated rats was partly blocked by SCH-23390. These data support the hypotheses that repeated cocaine administration produces an increase in the capacity of D₁ receptor stimulation to release glutamate in the VTA and that this mechanism partly mediates behavioral sensitization produced in rats treated with daily cocaine injections.     

Effects of repeated cocaine on medial prefrontal cortical GABAB receptor modulation of neurotransmission in the mesocorticolimbic dopamine system

Increased excitatory output from medial prefrontal cortex is an important component in the development of cocaine sensitization. Activation of GABAergic systems in the prefrontal cortex can decrease glutamatergic activity. A recent study suggested that sensitization might be associated with a decrease in GABAB receptor responsiveness in the medial prefrontal cortex. Therefore, the present study examined whether repeated exposure to cocaine-modified neurochemical changes in the mesocorticolimbic dopamine system induced by infusion of baclofen into the medial prefrontal cortex. In vivo microdialysis studies were conducted to monitor dopamine, glutamate and GABA levels in the medial prefrontal cortex and glutamate levels in the ipsilateral nucleus accumbens and ventral tegmental area during the infusion of baclofen into medial prefrontal cortex. Baclofen minimally affected glutamate levels in the medial prefrontal cortex, nucleus accumbens or ventral tegmental area of control animals, but dose-dependently increased glutamate levels in each of these regions in animals sensitized to cocaine. This effect was not the result of changes in GABAB receptor-mediated modulation of dopamine or GABA in the medial prefrontal cortex. The data suggest that alterations in GABAB receptor modulation of medial prefrontal cortical excitatory output may play an important role in the development of sensitization to cocaine.     

Differential long-term neuroadaptations of glutamate receptors in the basolateral and central amygdala after withdrawal from cocaine self-administration in rats

Humans and laboratory animals remain highly vulnerable to relapse to cocaine-seeking after prolonged periods of withdrawal from the drug. It has been hypothesized that this persistent cocaine relapse vulnerability involves drug-induced alterations in glutamatergic synapses within the mesolimbic dopamine reward system. Previous studies have shown that cocaine self-administration induces long-lasting neuroadaptations in glutamate neurons of the ventral tegmental area and nucleus accumbens. Here, we determined the effect of cocaine self-administration and subsequent withdrawal on glutamate receptor expression in the amygdala, a component of the mesolimbic dopamine system that is involved in cocaine seeking and craving induced by drug-associated cues. Rats were trained for 10 days to self-administer intravenous cocaine (6 h/day) or saline (a control condition) and were killed after one or 30 withdrawal days. Basolateral and central amygdala tissues were assayed for protein expression of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunits (GluR1 and GluR2) and the NMDA receptor subunits (NR1, NR2A and NR2B). In the basolateral amygdala, GluR1 but not GluR2 levels were increased on days 1 and 30, NR2A levels were increased on day 1, and NR2B levels were decreased on day 30 of withdrawal from cocaine. In the central amygdala, GluR2 but not GluR1 levels were increased on days 1 and 30, NR1 levels were increased on day 30 and NR2A or NR2B levels were not altered after withdrawal from cocaine. These results indicate that cocaine self-administration and subsequent withdrawal induces long-lasting and differential neuroadaptations in basolateral and central amygdala glutamate receptors.     

Repeated cocaine administration alters the expression of genes in corticolimbic circuitry after a 3-week withdrawal: a DNA macroarray study

Addiction to psychostimulants elicits behavioral and biochemical changes that are assumed to be mediated by alterations of gene expression in the brain. The changes in gene expression after 3 weeks of withdrawal from chronic cocaine treatment were evaluated in the nucleus accumbens core and shell, dorsal prefrontal cortex and caudate using a complementary DNA (cDNA) array. The level of mRNA encoded by several genes was identified as being up- or down-regulated in repeated cocaine versus saline subjects. The results from the cDNA array were subsequently confirmed at the protein level with immunoblotting. Of particular interest, parallel up-regulation in protein and mRNA was found for the adenosine A1 receptor in the accumbens core, neuroglycan C in the accumbens shell, and the GluR5 glutamate receptor subtype in dorsal prefrontal cortex. However, there was an increase in TrkB protein in the nucleus accumbens core of cocaine-treated rats without a corresponding alteration in mRNA. These changes of gene expression in corticolimbic circuitry may contribute to the psychostimulant-induced behavioral changes associated with addiction.     

Behavioral expression of cocaine sensitization in rats is accompanied by a distinct pattern of modifications in the PKA/DARPP-32 signaling pathway

Repeated cocaine administration induces behavioral sensitization and modifications in the phosphorylation pattern of dopamine and cAMP-regulated phosphoprotein of Mr 32,000 (DARPP-32), characterized by a tonic increase in the Thr75 phosphorylated form, and a decrease in the Thr34 phosphorylated form. This study further investigated the correlations between cocaine sensitization and modifications in the DARPP-32 phosphorylation pattern, cAMP-dependent protein kinase (PKA) activity, and mGluR5 tone in the medial prefrontal cortex and nucleus accumbens. Behavioral sensitization and modifications in these neurochemical markers followed a similar temporal pattern. Moreover, in sensitized rats acute cocaine administration modified phosphorylation levels of Thr75- and Thr34-DARPP-32, GluR1, and NR1 subunits in the nucleus accumbens only at a dose double the efficacious dose in control rats. These results suggest that the high levels of phospho-Thr75 DARPP-32 maintain PKA in a prevalent inhibited state. Furthermore, in sensitized rats the acute administration of 6-methyl-2-(phenylethynyl)-pyridine, a mGluR5 antagonist, reinstated the phosphorylation levels of Thr75- and Thr34-DARPP-32, GluR1, and NR1 to control values, and a subsequent cocaine challenge did not elicit a sensitized response. These data suggest that a tonic increase in mGluR5 transmission in cocaine-sensitized rats sustains both the increase in phospho-Thr75 DARPP-32 levels and the expression of behavioral sensitization.     

Repeated exposure to cocaine alters the modulation of mesocorticolimbic glutamate transmission by medial prefrontal cortex Group II metabotropic glutamate receptors

Repeated cocaine exposure enhances glutamatergic output from the medial prefrontal cortex to subcortical brain regions. Loss of inhibitory control of cortical pyramidal neurons may partly account for this augmented cortical glutamate output. Recent research indicated that repeated cocaine exposure reduced the ability of cortical Group II metabotropic glutamate receptors to modulate behavioral and neurochemical responses to cocaine. Thus, experiments described below examined whether repeated cocaine exposure alters metabotropic glutamate receptor regulation of mesocorticolimbic glutamatergic transmission using in vivo microdialysis. Infusion of the Group II metabotropic glutamate receptor antagonist LY341495 into the medial prefrontal cortex enhanced glutamate release in this region, the nucleus accumbens and the ventral tegmental area in sensitized animals, compared to controls, following short-term withdrawal but not after long-term withdrawal. Additional studies demonstrated that vesicular (K(+)-evoked) and non-vesicular (cystine-evoked) glutamate release in the medial prefrontal cortex was enhanced in sensitized animals, compared to controls, that resulted in part from a reduction in Group II metabotropic glutamate receptor modulation of these pools of glutamate. In summary, these findings indicate that the expression of sensitization to cocaine is correlated with an altered modulation of mesocorticolimbic glutamatergic transmission via reduction of Group II metabotropic glutamate receptor function.     

Neuroadaptations of total levels of adenylate cyclase, protein kinase A, tyrosine hydroxylase, cdk5 and neurofilaments in the nucleus accumbens and ventral tegmental area do not correlate with expression of sensitized or tolerant locomotor responses to cocaine

Neuroadaptations induced by high-dose cocaine treatment have been hypothesized to persist after the cessation of drug treatment and mediate the expression of sensitization and tolerance to cocaine. We looked for evidence of these neuroadaptations in rats receiving more modest behaviorally effective cocaine treatments. Rats were exposed to either a sensitizing regimen of seven once-daily injections of 15 mg/kg cocaine or a tolerance-producing regimen involving a continuous infusion of the same daily dose. We assessed enzyme activity levels of protein kinase A and adenylate cyclase, and protein levels of tyrosine hydroxylase, cdk5 and neurofilaments in the nucleus accumbens and ventral tegmental area. Only protein kinase A activity levels were altered by cocaine treatment, but this alteration persisted for only 7 days, whereas a sensitized locomotor response was still evident at 21 days. Although behavioral tolerance to cocaine was seen the day after the termination of treatment, none of the molecular measures was altered on this or any other day. Thus, although increased protein kinase A activity can temporarily modulate sensitized responses to cocaine, alterations in total levels of the molecules assessed in our study do not correlate with the expression of sensitized or tolerant locomotor responses to cocaine.     

Alterations in ionotropic glutamate receptor subunits during binge cocaine self-administration and withdrawal in rats

Chronic cocaine use in humans and animal models is known to lead to pronounced alterations in glutamatergic function in brain regions associated with reinforcement. Previous studies have examined ionotropic glutamate receptor (iGluR) subunit protein level changes following acute and chronic experimenter-administered cocaine or after withdrawal periods from experimenter-administered cocaine. To evaluate whether alterations in expression of iGluRs are associated with cocaine reinforcement, protein levels were assessed after binge (8 h/day, 15 days; 24-h access, days 16-21) cocaine self-administration and following 2 weeks of abstinence from this binge. Western blotting was used to compare levels of iGluR protein expression (NR1-3B, GluR1-7, KA2) in the ventral tegmental area (VTA), substantia nigra (SN), nucleus accumbens (NAc), striatum and prefrontal cortex (PFC) of rats. iGluR subunits were altered in a time-dependent manner in all brain regions studied; however, selective alterations in certain iGluR subtypes appeared to be associated with binge cocaine self-administration and withdrawal in a region-specific manner. In the SN and VTA, alterations in iGluR protein levels compared with controls occurred only following binge access, whereas in the striatum and PFC, iGluR alterations occurred with binge access and following withdrawal. In the NAc, GluR2/3 levels were increased following withdrawal compared with binge access, and were the only changes observed in this region. Because subunit composition determines the functional properties of iGluRs, the observed changes may indicate alterations in the excitability of dopamine transmission underlying long-term biochemical and behavioral effects of cocaine.     

Acute and Repeated Systemic Amphetamine Administration: Effects on Extracellular Glutamate, Aspartate, and Serine Levels in Rat Ventral Tegmental Area and Nucleus Accumbens

Abstract: Recent work indicates an important role for excitatory amino acids in behavioral sensitization to amphetamine. We therefore examined, using in vivo microdialysis in awake rats, the effects of amphetamine on efflux of glutamate, aspartate, and serine in the ventral tegmental area and nucleus accumbens, brain regions important for the initiation and expression of amphetamine sensitization, respectively. Water-pretreated and amphetamine-pretreated rats were compared to determine if sensitization altered such effects. In both brain regions, Ca²⁺-dependent efflux of glutamate accounted for ∼20% of basal glutamate efflux. A challenge injection of water or 2.5 mg/kg of amphetamine did not significantly alter glutamate, aspartate, or serine efflux in the ventral tegmental area or nucleus accumbens of water- or amphetamine-pretreated rats. However, 5 mg/kg of amphetamine produced a gradual increase in glutamate efflux in both regions that did not reverse, was observed in both water- and amphetamine-pretreated rats, and was prevented by haloperidol. Although increased glutamate efflux occurred with too great a delay to mediate acute behavioral responses to amphetamine, it is possible that repeated augmentation of glutamate efflux during repeated amphetamine administration results in compensatory changes in levels of excitatory amino acid receptors in the ventral tegmental area and nucleus accumbens that contribute to development or expression of amphetamine sensitization.     

Chronic cocaine administration decreases the functional coupling of GABA(B) receptors in the rat ventral tegmental area as measured by baclofen-stimulated 35S-GTPgammaS binding

Results of numerous studies indicate that the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) modulates central dopamine systems, and that GABA(B) receptors may play a primary role in decreasing dopamine release. To determine if chronic cocaine administration alters the functional coupling of GABA(B) receptors to G-proteins in central dopamine systems, male F-344 rats received cocaine (15 mg/kg/injection) or saline three times a day at hourly intervals for fourteen consecutive days. Rats were decapitated one hour after the last injection and crude membrane preparations were made from the substantia nigra, caudate-putamen, ventral tegmental area, nucleus accumbens, and frontal cortex of individual rats. The ability of the specific GABA(B) receptor agonist baclofen to stimulate 35S-GTPgammaS binding in each of these regions was determined for individual animals. Additionally, baclofen-stimulated 35S-GTPgammaS binding in each of these regions in rats that received cocaine was compared to baclofen-stimulated 35S-GTPgammaS binding in rats that received control injections of saline. The EC50 of baclofen and maximal baclofen-stimulated 35S-GTPgammaS binding over basal levels were determined in each brain region in the saline group and in the cocaine group. Two-way ANOVA revealed a significant decrease in GABA(B) receptor-stimulated 35S-GTPgammaS binding in the ventral tegmental area of the cocaine group compared to the saline group. These data suggest that chronic exposure to cocaine decreases the functional coupling of GABA(B) receptors to G-proteins selectively in the ventral tegmental area. This finding may have implications in the augmented extracellular dopamine levels seen in the nucleus accumbens of rats that have been sensitized to cocaine.     

Src-protein tyrosine kinases are required for cocaine-induced increase in the expression and function of the NMDA receptor in the ventral tegmental area

Cocaine-induced long-term potentiation of glutamatergic synapses in the ventral tegmental area (VTA) has been proposed as a key process that contributes to the development of addictive behaviors. In particular, the activation of ionotrophic glutamate NMDA receptor (NMDAR) in the VTA is critical for the initiation of cocaine sensitization. Here we show that application of cocaine both in slices and in vivo induced an increase in tyrosine phosphorylation of the NR2A, but not the NR2B subunit of the NMDAR in juvenile rats. Cocaine induced an increase in the activity of both Fyn and Src kinases, and the Src-protein tyrosine kinase (Src-PTKs) inhibitor, 4-amino-5-(4-chlorophenyl)-7-( t -butyl)pyrazolo[3,4-d]pyrimidine (PP2), abolished both cocaine-induced increase in tyrosine phosphorylation of the NR2A subunit and the increase in the expression of NR1, NR2A, and NR2B in the VTA. Moreover, cocaine-induced enhancement in NMDAR-mediated excitatory post-synaptic currents was completely abolished by PP2. Taken together, these results suggest that acute cocaine induced an increase in the expression of NMDAR subunits and enhanced tyrosine phosphorylation of NR2A-containing NMDAR through members of the Src-PTKs. This in turn, increased NMDAR-mediated currents in VTA dopamine neurons. These results provide a potential cellular mechanism by which cocaine triggers NMDAR-dependent synaptic plasticity of VTA neurons that may underlie the development of behavioral sensitization.     

Cocaine-induced alterations in nucleus accumbens ionotropic glutamate receptor subunits in human and non-human primates

Chronic cocaine and withdrawal induce significant alterations in nucleus accumbens (NAc) glutamatergic function in humans and rodent models of cocaine addiction. Dysregulation of glutamatergic function of the prefrontal cortical-NAc pathway has been proposed as a critical substrate for unmanageable drug seeking. Previously, we demonstrated significant up-regulation of NMDA, (+/-)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate receptor subunit mRNAs and protein levels in the ventral tegmental area (VTA), but not the substantia nigra, of cocaine overdose victims (COD). The present study was undertaken to examine the extent of altered ionotropic glutamate receptor (iGluR) subunit expression in the NAc and the putamen in cocaine overdose victims. Results revealed statistically significant increases in the NAc, but not in the putamen, of NMDA receptor subunit (NR)1 and glutamate receptor subunit (GluR)2/3 wit trends in GluR1 and GluR5 in COD. These results extend our previous finding and indicate pathway-specific alterations in iGluRs in COD. In order to determine that changes were related to cocaine intake and not to other factors in the COD victims, we examined the effects of cocaine intravenous self-administration in rhesus monkeys for 18 months (unit dose of 0.1 mg/kg/injection and daily drug intake of 0.5 mg/kg/session). Total drug intake for the group of four monkeys was 37.9 +/- 4.6 mg/kg. Statistically significant elevations were observed for NR1, GluR1, GluR2/3 and GluR5 (p < 0.05) and a trend towards increased NR1 phosphorylated at serine 896 (p = 0.07) in the NAc but not putamen of monkeys self-administering cocaine compared with controls. These results extend previous results by demonstrating an up-regulation of NR1, GluR2/3 and GluR5 in the NAc and suggest these alterations are pathway specific. Furthermore, these changes may mediate persistent drug intake and craving in the human cocaine abuser.     

Reduced Clearance of Exogenous Dopamine in Rat Nucleus Accumbens, but Not in Dorsal Striatum, Following Cocaine Challenge in Rats Withdrawn from Repeated Cocaine Administration

Abstract: We investigated whether changes in the dopamine transporter in the nucleus accumbens or striatum are involved in cocaine-induced behavioral sensitization by using in vivo electrochemistry to monitor the clearance of locally applied dopamine in anesthetized rats. Rats were injected with cocaine-HCI (10 mg/kg i.p.) or saline daily for 7 consecutive days and then withdrawn for 7 days. Pressure ejection of a finite amount of dopamine at 5-min intervals from a micropipette adjacent to the electrochemical recording electrode produced transient and reproducible dopamine signals. After a challenge injection of cocaine (10 mg/kg i.p.), the signals in the nucleus accumbens of cocaine-treated animals became prolonged and the clearance rate of the dopamine decreased, indicating significant inhibition of the dopamine transporter. In contrast, simultaneous measurements in the dorsal striatum indicated a transient increase in both the amplitude of the signals and the clearance rate of the dopamine. The signals in both brain regions in the saline-treated animals given the cocaine challenge were similar to those in untreated animals given an acute injection of cocaine (10 mg/ kg i.p.) or saline. Behaviorally, not all of the cocaine- treated animals were sensitized; however, both sensitized and nonsensitized animals displayed similar changes in dopamine clearance rate. Quantitative autoradiography with [³H]mazindol revealed that the affinity of the dopamine transporter for cocaine and the density of binding sites were similar in cocaine- and saline-treated rats. The decrease in dopamine clearance rate observed in the nucleus accumbens of the cocaine-treated rats after a challenge injection of cocaine is consistent with increased do- paminergic transmission, but does not appear to be sufficient in itself for producing behavioral sensitization.     

Activator of G protein signaling 3: a gatekeeper of cocaine sensitization and drug seeking

Chronic cocaine administration reduces G protein signaling efficacy. Here, we report that the expression of AGS3, which binds to GialphaGDP and inhibits GDP dissociation, was upregulated in the prefrontal cortex (PFC) during late withdrawal from repeated cocaine administration. Increased AGS3 was mimicked in the PFC of drug-naive rats by microinjecting a peptide containing the Gialpha binding domain (GPR) of AGS3 fused to the cell permeability domain of HIV-Tat. Infusion of Tat-GPR mimicked the phenotype of chronic cocaine-treated rats by manifesting sensitized locomotor behavior and drug seeking and by increasing glutamate transmission in nucleus accumbens. By preventing cocaine withdrawal-induced AGS3 expression with antisense oligonucleotides, signaling through Gialpha was normalized, and both cocaine-induced relapse to drug seeking and locomotor sensitization were prevented. When antisense oligonucleotide infusion was discontinued, drug seeking and sensitization were restored. It is proposed that AGS3 gates the expression of cocaine-induced plasticity by regulating G protein signaling in the PFC.