Developmental regulation of GABA(B) receptor function in rat spinal cord

GABA(B) receptors are heterodimers coupled to G-proteins. The present study was undertaken to investigate activation of GABA(B) receptors in cerebral cortex and spinal cord using [35S]GTPgammaS binding assays, a direct measure of G-protein activity. The results revealed that the GABA(B) agonist baclofen stimulates GTPgammaS binding in cerebral cortex, with an ED50 of 50microM. This response is blocked by the GABA(B) receptor antagonist CGP 55845A (100nM). In contrast, baclofen-stimulated GTPgammaS binding was not observed in adult spinal cord tissue under similar incubation conditions, or after varying magnesium, calcium, GDP, [35S]GTPgammaS, or membrane concentrations in the assay medium. Stimulation of adult rat spinal cord muscarinic receptors did result in a concentration-related increase in [35S]GTPgammaS binding. Baclofen-stimulated GTPgammaS binding in adult spinal cord did not appear after peripheral inflammation, despite significant increases in GABA(B) subunit mRNA levels. As opposed to adult, appreciable GTPgammaS binding was observed in membranes prepared from spinal cords of rats within the first 14 days of postnatal development, suggesting that GABA(B) receptor function in the rat spinal cord is developmentally regulated. The results indicate that GABA(B) receptors may not be coupled to G-proteins in the adult rat spinal cord, or couple in a way that differs from that in newborns or adult cerebral cortex.     

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.     

Receptors in the Ventral Tegmental Area Mediating Nicotine-Induced Dopamine Release in the Nucleus Accumbens

Nicotine or cocaine, when administered intravenously, induces an increase of extracellular dopamine in the nucleus accumbens. The nicotine-mediated increase was shown to occur at least in part through increase of the activity of dopamine neurons in the ventral tegmental area. As part of our continuing studies of the mechanisms of nicotine effects in the brain, in particular, effects on reward and cognitive mechanisms, in the present study we examined the role of various receptors in the ventral tegmental area in nicotine and cocaine reward. We assayed inhibition of the increase of dopamine in the nucleus accumbens induced by intravenous nicotine or cocaine administration by antagonists administered into the ventral tegmental area. Nicotine-induced increase of accumbal dopamine release was inhibited by intrategmental nicotinic (mecamylamine), muscarinic (atropine), dopaminergic (D₁: SCH 23390, D₂: eticlopride), and NMDA glutamatergic (MK 801) and GABA_B (saclofen) antagonists, but not by AMPA-kainate (CNQX, GYKI-52466) antagonists under our experimental circumstances. The intravenous cocaine-induced increase of dopamine in the nucleus accumbens was inhibited by muscarinic (atropine), dopamine 2 (eticlopride), and GABA_B (saclofen) antagonists but not by antagonists to nicotinic (mecamylamine), dopamine D₁ (SCH 23390), glutamate (MK 801), or AMPA-kainate (CNQX, GYKI-52466) receptors. Antagonists administered in the ventral tegmental area in the present study had somewhat different effects when they were previously administered intravenously. When administered intravenously atropine did not inhibit cocaine effects. The inhibition by atropine may be indirect, since this compound, when administered intrategmentally, decreased basal dopamine levels in the accumbens. The findings indicate that a number of receptors in the ventral tegmental area mediate nicotine-induced dopamine changes in the nucleus accumbens, a major component of the nicotine reward mechanism. Some, but not all, of these receptors in the ventral tegmental area also seem to participate in the reward mechanism of cocaine. The importance of local receptors in the ventral tegmental area was further indicated by the increase in accumbal dopamine levels after intrategmental administration of nicotine or also cocaine.     

Eating-Induced Dopamine Release from Mesolimbic Neurons Is Mediated by NMDA Receptors in the Ventral Tegmental Area: A Dual-Probe Microdialysis Study

Abstract: This study was aimed at identifying the neuronal pathways that mediate the eating-induced increase in the release of dopamine in the nucleus accumbens of the rat brain. For that purpose, a microdialysis probe was implanted in the ventral tegmental area and a second probe was placed in the ipsilateral nucleus accumbens. Receptor-specific compounds acting on GABA_A (40 µ M muscimol; 50 µ M bicuculline), GABA_B (50 µ M baclofen), acetylcholine (50 µ M carbachol), NMDA [30 µ M (±)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP)], and non-NMDA [300 µ M 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)] receptors were infused into the ventral tegmental area by retrograde dialysis, whereas extracellular dopamine was recorded in the ipsilateral nucleus accumbens. Intrategmental infusion of muscimol or baclofen decreased extracellular dopamine in the ipsilateral nucleus accumbens; CPP and CNQX were without effect, and bicuculline and carbachol increased dopamine release. During infusion of the various compounds, food-deprived rats were allowed to eat for 10 min. The infusions of muscimol, bicuculline, baclofen, carbachol, and CNQX did not prevent the eating-induced increase in extracellular dopamine in the nucleus accumbens. However, during intrategmental infusion of CPP, the eating-induced increase in extracellular dopamine in the nucleus accumbens was suppressed. These results indicate that a glutamatergic projection to the ventral tegmental area mediates, via an NMDA receptor, the eating-induced increase in dopamine release from mesolimbic dopamine neurons.     

Orphanin FQ/Nociceptin Modulation of Mesolimbic Dopamine Transmission Determined by Microdialysis

Orphanin FQ has been reported to suppress extracellular dopamine levels in the nucleus accumbens after intracerebroventricular administration. This study sought to provide evidence for an intra-ventral tegmental site of action for this effect using a dual-probe microdialysis experimental design. Orphanin FQ was applied to the ventral tegmental area of anesthetized rats by reverse dialysis while extracellular dopamine was sampled with a second dialysis probe in the nucleus accumbens. Orphanin FQ at a probe concentration of 1 mM (but not at 0.1 mM) significantly reduced nucleus accumbens dialysate dopamine levels. The receptor-inactive analogue, des-Phe1-orphanin FQ (1 mM), produced a small but significant increase in nucleus accumbens dialysate dopamine levels. Simultaneous measurement of ventral tegmental area dialysate amino acid content revealed significant increases in both GABA and glutamate during infusion of orphanin FQ (1 mM). To determine if increased GABA overflow mediates the action of orphanin FQ on mesolimbic neurons, orphanin FQ (10 nmol) was microinjected directly into the ventral tegmental area in the presence or absence of the GABA(A) receptor antagonist, bicuculline (1 nmol). Bicuculline transiently blocked the suppressive action of orphanin FQ on accumbens dialysate dopamine levels. These data indicate that orphanin FQ decreases dopamine transmission in the nucleus accumbens by inhibiting dopamine neuronal activity in the ventral tegmental area through a mechanism that may involve an increased overflow of GABA.     

Estrogen-dependent alterations in D2/D3-induced G protein activation in cocaine-sensitized female rats

Estrogen potentiates behavioral sensitization to cocaine in the female rat by mechanisms that remain undetermined. In this study, functional receptor autoradiography was used to investigate estrogen modulation of D2/D3 receptor-induced G protein activation in components of the reward pathway of female rats treated acutely and repeatedly with cocaine. Rats were ovariectomized and given an empty (OVX group) or estradiol benzoate-filled (OVX-EB group) implant. After a week, animals received a daily saline or cocaine injection (15 mg/kg, i.p.) for 5 days, and again on day 13. Animals were killed, and brains were removed and cryosectioned. D2/D3-stimulated [35S]guanosine 5'-(gamma-thio) triphosphate (GTPgammaS) binding was assessed in the cingulate cortex area 2 (Cg2), striatum (STR), nucleus accumbens (NAc) and ventral tegmental area (VTA). OVX-EB rats showed more [35S]GTPgammaS binding in the Cg2 and lower binding in the VTA than OVX rats; in the VTA this effect was reversed by a single cocaine injection. Repeated cocaine administration had opposite effects in OVX and OVX-EB rats. [35S]GTPgammaS binding was decreased in the Cg2, NAc and STR of OVX-EB rats, and increased in OVX rats. The present results support the hypothesis that cocaine-induced changes in D2/D3 receptor activation are regulated by estrogen. These data suggest that changes in D2/D3 receptor function represent one mechanism by which estrogen regulates behavioral sensitization to 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.     

Strain differences in the distribution of dopamine (DA-2 and DA-3) receptor sites in rat brain

The dopamine (DA) pathway mediates numerous neuronal functions which are implicated in psychiatric disorders. Previously, our lab investigated the status of the dopamine transporter in the Wistar-Kyoto rat, a purported rodent model of depressive behavior, and reported significant alterations in transporter binding sites in several brain regions when compared to control rat strains. Given that DA-2 and DA-3 receptors belong to the same class of DA receptors, are co-localized in the mesolimbic and nigrostriatal regions of the brain and function as autoreceptors, this study mapped the distribution of central DA-2 and DA-3 receptors in Wistar-Kyoto and Wistar rats. The results indicated that while the binding of 125I-sulpride to DA-2 receptors was higher in the nucleus accumbens (shell) and ventral tegmental area, it was lower in the nucleus accumbens (core), caudate putamen and hypothalamus in Wistar-Kyoto compared to Wistar rats. In contrast, the binding of 125I-sulpride to DA-3 receptors was higher in the caudate putamen, nucleus accumbens (shell and core) and islands of Calleja in Wistar-Kyoto compared to Wistar rats. Given that DA-2 like receptors in the ventral tegmental area function as autoreceptors, it is possible that the greater inhibitory effects exerted by DA-2 and DA-3 receptors in Wistar-Kyoto rats may lead to a net deficit in DA levels in areas receiving projection from this cell body area.     

Repeated cocaine administration changes the function and subcellular distribution of adenosine A1 receptor in the rat nucleus accumbens

Adenosine A1 receptor (A1) protein and mRNA is increased in the nucleus accumbens following repeated cocaine treatment. In spite of this protein up-regulation, A1 agonist-stimulated [35S]GTPgammaS binding was attenuated in accumbens homogenates of rats withdrawn for 3 weeks from 1 week of daily cocaine injections. Cellular subfractionation revealed that the discrepancy between total A1 protein and G protein coupling resulted from a smaller proportion of receptors in the plasma membrane. The decrease in functional receptor in the plasma membrane was further indicated by diminished formation of heteromeric receptor complex consisting of A1 and dopamine D1A receptors. To explore the functional significance of the altered distribution of A1 receptors, at 3 weeks after discontinuing repeated cocaine or saline, animals were injected with cocaine and 45 min later the subcellular distribution of A1 receptors quantified. Whereas a cocaine challenge in repeated saline-treated animals induced a marked increase in membrane localization of the A1 receptor, the relative distribution of receptors in repeated cocaine rats was not affected by acute cocaine. These data suggest that the sorting and recycling of A1 receptors is dysregulated in the nucleus accumbens as the consequence of repeated cocaine administration.     

Central integration of cardiovascular and drinking responses elicited by central administration of angiotensin II: divergence of regulation by the ventral tegmental area and nucleus accumbens

Previous studies had implicated the involvement of the ventral tegmental area and its dopamine projections to the nucleus accumbens in goal-directed behavior. This study investigated whether or not the GABAergic inputs to the ventral tegmental area and, in turn, dopaminergic input to the nucleus accumbens from the ventral tegmental area modify drinking and cardiovascular responses elicited by central administration of angiotensin II. Injections of 25 ng of angiotensin II into a lateral cerebral ventricle of the rat elicited water intakes averaging 7-8 mL in 15 min with latencies usually less than 3 min. Pretreatment of the nucleus accumbens with spiperone, a dopamine antagonist, or the ventral tegmental area with gamma-amino butyric acid (GABA) produced dose-dependent reductions in water intake and number of laps taken while increasing the latency to drink. The spiperone injection did not alter the pressor response. On the other hand, the GABA injections attenuated the pressor responses to central angiotensin II administration. These findings suggest that GABA input to the ventral tegmental area modifies both the cardiovascular and drinking responses elicited following central administration of angiotensin II. However, the dopamine projections to the nucleus accumbens appear to be involved only in the drinking responses elicited by central injections of angiotensin II. Divergence for the coordination of the skeletal motor behavioral component and the cardiovascular component elicited by central administration of angiotensin II must occur before the involvement of these dopamine pathways.     

Intra-accumbens baclofen, but not muscimol, increases second order instrumental responding for food reward in rats

Stimulation of either GABA(A) or GABA(B) receptors within the nucleus accumbens shell strongly enhances food intake in rats. However the effects of subtype-selective stimulation of GABA receptors on instrumental responses for food reward are less well characterized. Here we contrast the effects of the GABA(A) receptor agonist muscimol and GABA(B) receptor agonist baclofen on instrumental responding for food using a second order reinforcement schedule. Bilateral intra-accumbens administration of baclofen (220-440 pmol) stimulated responding but a higher dose (660 pmol) induced stereotyped oral behaviour that interfered with responding. Baclofen (220-660 pmol) also stimulated intake of freely available chow. Muscimol (220-660 pmol) was without effect on responding for food on this schedule but did stimulate intake of freely available chow. Unilateral administration of either baclofen or muscimol (220 pmol) induced similar patterns of c-fos immunoreactivity in several hypothalamic sites but differed in its induction in the central nucleus of the amygdala. We conclude that stimulation of GABA(A) or GABA(B) receptors in the nucleus accumbens shell of rats produces clearly distinguishable effects on operant responding for food.     

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 Alters Glutamate Receptor Subunit Levels in the Nucleus Accumbens and Ventral Tegmental Area of Rats that Develop Behavioral Sensitization

Increased glutamate transmission in the nucleus accumbens and ventral tegmental area has been proposed as a mechanism underlying sensitized behavioral responses to repeated cocaine administration. GluR1, GluR2/3, and NMDAR1 subunits of glutamate receptors were quantified from immunoblots in these brain nuclei in rats at 24 h and 3 weeks after discontinuing 1 week of daily cocaine injections. Motor behavior was monitored after the first and last injections of daily cocaine, and those rats that showed >20% increase in motor activity after the last compared with the first injection were considered to have developed behavioral sensitization. The subjects that developed behavioral sensitization showed a significant increase in GluR1 levels in the nucleus accumbens at 3 weeks but not at 24 h of withdrawal. Conversely, sensitized animals showed a significant increase in NMDAR1 and GluR1 levels in the ventral tegmental area at 1 day but not at 3 weeks of withdrawal. None of these increases occurred in the rats exposed to daily cocaine that did not develop behavioral sensitization (<20% increase in motor activity), and no changes were measured in the level of GluR2/3 in any treatment group. The functional importance of the increases in glutamate receptor subunit levels is suggested by the fact that the changes were present only in rats that developed behavioral sensitization to repeated cocaine administration.     

Repeated cocaine enhances ventral hippocampal‐stimulated dopamine efflux in the nucleus accumbens and alters ventral hippocampal NMDA receptor subunit expression

Dopaminergic neurotransmission in the nucleus accumbens is important for various reward‐related cognitive processes including reinforcement learning. Repeated cocaine enhances hippocampal synaptic plasticity, and phasic elevations of accumbal dopamine evoked by unconditioned stimuli are dependent on impulse flow from the ventral hippocampus. Therefore, sensitized hippocampal activity may be one mechanism by which drugs of abuse enhance limbic dopaminergic activity. In this study, in vivo microdialysis in freely moving adult male Sprague–Dawley rats was used to investigate the effect of repeated cocaine on ventral hippocampus‐mediated dopaminergic transmission within the medial shell of the nucleus accumbens. Following seven daily injections of saline or cocaine (20 mg/kg, ip), unilateral infusion of N‐methyl‐d ‐aspartate (NMDA, 0.5 μg) into the ventral hippocampus transiently increased both motoric activity and ipsilateral dopamine efflux in the medial shell of the nucleus accumbens, and this effect was greater in rats that received repeated cocaine compared to controls that received repeated saline. In addition, repeated cocaine altered NMDA receptor subunit expression in the ventral hippocampus, reducing the NR2A : NR2B subunit ratio. Together, these results suggest that repeated exposure to cocaine produces maladaptive ventral hippocampal‐nucleus accumbens communication, in part through changes in glutamate receptor composition.

     

Altered Dopamine Receptor and Dopamine Transporter Binding and Tyrosine Hydroxylase mRNA Expression Following Perinatal NMDA Receptor Blockade

This study examined how perinatal phencyclidine (PCP) treatment would affect dopamine D2 receptor and dopamine transporter (DAT) binding at different stages after treatment cessation. Female rat pups received injections of PCP (10 mg/kg, s.c.) or saline on postnatal day (PN)7, 9 and 11. D2 receptor and transporter binding was examined at four time-points (PN12, 18, 32 and 96) following injections. PCP treatment altered D2 receptor binding throughout development, with a final end-point of 22-33% decreased binding at adulthood in the nucleus accumbens and caudate putamen (P < 0.01), accompanied by a small but significant increase in DAT binding in the caudate putamen. Tyrosine hydroxylase mRNA expression was also significantly increased by 25% (P < 0.05) in the ventral tegmental area of adult rats, suggesting that this model may produce a long-term increase in dopamine output. This study demonstrates that early insult to the brain from NMDA receptor hypofunction alters the dopaminergic system at different stages of development.     

Contribution of Ventral Tegmental GABA Receptors to Cocaine Self-administration in Rats

Recent evidence has suggested that compounds affecting GABAergic transmission may provide useful pharmacological tools for the treatment of cocaine addiction. Using a rat model of self-administration, the present study examined the effects of GABA agonists and antagonists injected directly into the ventral tegmental area (VTA) on cocaine intake in rats trained to self-administer cocaine (0, 125, 250 and 500 μg/infusion) under an FR5 schedule of reinforcement. Separate groups of rats received bilateral intra-VTA injections of the GABA-A antagonist picrotoxin (34 ng/side, n = 7; 68 ng/side, n = 8), GABA-A agonist muscimol (14 ng/side, n = 8), GABA-B agonist baclofen (56 ng/side, n = 7; 100 ng/side, n = 6), picrotoxin (68 ng/side) co-injected with the GABA-B antagonist 2-hydroxysaclofen (100 ng/side, n = 7; 2 μg/side, n = 8) or artificial cerebrospinal fluid (aCSF, n = 6) to assess the effects of the various compounds on the cocaine self-administration dose-response curve. Both picrotoxin and baclofen reduced responding maintained by cocaine, whereas muscimol had no effect on responding. In contrast, neither picrotoxin (n = 6) nor baclofen (n = 8) affected responding maintained by food. Interestingly, 2-hydroxysaclofen effectively blocked the suppression of responding produced by picrotoxin, suggesting that both picrotoxin and baclofen exert their effects via activation of GABA-B receptors. Additionally, these effects appear to be specific to cocaine reinforcement, supporting current investigation of baclofen as a treatment for cocaine addiction.     

Dopamine-dependent responses to cocaine depend on corticotropin-releasing factor receptor subtypes

The effects on locomotor response to cocaine challenge, acquisition of cocaine conditioned place preference and cocaine-induced dopamine (DA) release in nucleus accumbens and ventral tegmental area by the non-specific corticotropin-releasing factor (CRF) receptors antagonist alpha-helical CRF, the selective CRF receptor subtype 1 antagonist CP-154,526 and the selective CRF receptor subtype 2 antagonist anti-sauvagine-30 (AS-30) were investigated in rats. Both alpha-helical CRF (10 microg, i.c.v.) and CP-154,526 (3 microg, i.c.v.) decreased the cocaine-induced distance travelled, whereas AS-30 (3 microg, i.c.v.) did not show such an effect. The CRF receptor antagonists also have significant effects on stereotype counts induced by cocaine injection, in which the alpha-helical CRF or CP-154,526 but not AS-30 did significantly reduce the stereotype counts. alpha-Helical CRF (10 microg) prior to each injection of cocaine blocked cocaine conditioned place preference with no significant difference observed in the time spent in the drug-paired side between post- and pre-training and both 1 and 3 microg CP-154,526 also had significant inhibitory effects on cocaine-induced place preference. However, pre-treatment with an i.c.v. infusion of AS-30 (1 or 3 microg) prior to each injection of cocaine did not affect the acquisition of conditioned place preference. The alpha-helical CRF and CP-154,526 reduced extracellular DA levels of nucleus accumbens and ventral tegmental area in response to the injection of cocaine. However, both alpha-helical CRF and CP-154,526 did not modify extracellular DA levels under basal conditions. In contrast, the i.c.v. infusion of AS-30 had no effects on either the basal DA or the cocaine-induced increase in DA release in nucleus accumbens and ventral tegmental area. These findings demonstrate that activation of the CRF receptor is involved in behavioral and neurochemical effects of cocaine challenge and cocaine reward and that the role of CRF receptor subtypes 1 and 2 in cocaine-induced locomotion, reward and DA release is not identical. The CRF receptor subtype 1 is largely responsible for the action of the CRF system on cocaine locomotion and reward. These results suggest that the CRF receptor antagonist, particularly the CRF receptor subtype 1 antagonist, might be of some value in the treatment of cocaine addiction and cocaine-related behavioral disorders.     

Repeated exposure to methamphetamine, cocaine or morphine induces augmentation of dopamine release in rat mesocorticolimbic slice co-cultures

Repeated intermittent exposure to psychostimulants and morphine leads to progressive augmentation of its locomotor activating effects in rodents. Accumulating evidence suggests the critical involvement of the mesocorticolimbic dopaminergic neurons, which project from the ventral tegmental area to the nucleus accumbens and the medial prefrontal cortex, in the behavioral sensitization. Here, we examined the acute and chronic effects of psychostimulants and morphine on dopamine release in a reconstructed mesocorticolimbic system comprised of a rat triple organotypic slice co-culture of the ventral tegmental area, nucleus accumbens and medial prefrontal cortex regions. Tyrosine hydroxylase-positive cell bodies were localized in the ventral tegmental area, and their neurites projected to the nucleus accumbens and medial prefrontal cortex regions. Acute treatment with methamphetamine (0.1-1000 μM), cocaine (0.1-300 μM) or morphine (0.1-100 μM) for 30 min increased extracellular dopamine levels in a concentration-dependent manner, while 3,4-methylenedioxyamphetamine (0.1-1000 μM) had little effect. Following repeated exposure to methamphetamine (10 μM) for 30 min every day for 6 days, the dopamine release gradually increased during the 30-min treatment. The augmentation of dopamine release was maintained even after the withdrawal of methamphetamine for 7 days. Similar augmentation was observed by repeated exposure to cocaine (1-300 μM) or morphine (10 and 100 μM). Furthermore, methamphetamine-induced augmentation of dopamine release was prevented by an NMDA receptor antagonist, MK-801 (10 μM), and was not observed in double slice co-cultures that excluded the medial prefrontal cortex slice. These results suggest that repeated psychostimulant- or morphine-induced augmentation of dopamine release, i.e. dopaminergic sensitization, was reproduced in a rat triple organotypic slice co-cultures. In addition, the slice co-culture system revealed that the NMDA receptors and the medial prefrontal cortex play an essential role in the dopaminergic sensitization. This in vitro sensitization model provides a unique approach for studying mechanisms underlying behavioral sensitization to drugs of abuse.     

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.