Augmented behavioral response and enhanced synaptosomal calcium transport induced by repeated cocaine administration are decreased by calcium channel blockers

Recent studies suggest that calcium influx via L-type calcium channels is necessary for psychostimulant-induced behavioral sensitization. In addition, chronic amphetamine upregulates subtype Cav1.2-containing L-type calcium channels. In the present studies, we assessed the effect of calcium channel blockers (CCBs) on cocaine-induced behavioral sensitization and determined whether the functional activity of L-type calcium channels is altered after repeated cocaine administration. Rats were administered daily intraperitoneal injections of either flunarizine (40 mg/kg), diltiazem (40 mg/kg) or cocaine (20 mg/kg) and the combination of the CCBs and cocaine for 30 days. Motor activities were monitored on Day 1, and every 6th day during the 30-day treatment period. Daily cocaine administration produced increased locomotor activity. Maximal augmentation of behavioral response to repeated cocaine administration was observed on Day 18. Flunarizine pretreatment abolished the augmented behavioral response to repeated cocaine administration while diltiazem was less effective. Measurement of tissue monoamine levels on Day 18 revealed cocaine-induced increases in DA and 5-HT in the nucleus accumbens. By contrast to behavioral response, diltiazem was more effective in attenuating increases in monoamine levels than flunarizine. Cocaine administration for 18 days produced increases in calcium uptake in synaptosomes prepared from the nucleus accumbens and frontal cortex. Increases in calcium uptake were abolished by flunarizine and diltiazem pretreatment. Taken together, the augmented cocaine-induced behavioral response on Day 18 may be due to increased calcium uptake in the nucleus accumbens leading to increased dopamine (DA) and serotonin (5-HT) release. Flunarizine and diltiazem attenuated the behavioral response by decreasing calcium uptake and decreasing neurochemical release.     

Cocaine and Cocaethylene: Microdialysis Comparison of Brain Drug Levels and Effects on Dopamine and Serotonin

Abstract: Cocaethylene is a pharmacologically active metabolite resulting from concurrent cocaine and ethanol consumption. The effects of cocaine and cocaethylene on extracellular levels of dopamine in the nucleus accumbens, and serotonin in the striatum were characterized in vivo in the anesthetized rat. Both intravenous (3 μmol/kg) and intraperitoneal (44 μmol/kg) routes of administration were used. In addition to monitoring neurotransmitter levels, microdialysate levels of cocaine and cocaethylene were determined at 4-min intervals after intravenous administration, and at 20-min intervals after intraperitoneal administration. Extracellular levels of dopamine in the nucleus accumbens were increased to ∼400% of preinjection value by both cocaine and cocaethylene when administered intravenously. Cocaine caused a significant increase of striatal serotonin to 200% preinjection value, whereas cocaethylene had no effect. Brain levels of cocaine and cocaethylene after intravenous administration did not differ. After intraperitoneal administration, extracellular levels of dopamine in the nucleus accumbens were increased to 400% of preinjection levels by cocaine, but were only increased to 200% of preinjection levels by cocaethylene, the difference being statistically significant. Serotonin levels were increased to 360% of preinjection levels by cocaine, but only to 175% of preinjection value by cocaethylene. Levels of cocaine attained in brain were significantly higher than those for cocaethylene, suggesting pharmacokinetic differences with the intraperitoneal route. These results confirm in vivo that cocaethylene is more selective in its actions than cocaine with respect to dopamine and serotonin uptake. In addition, route-dependent differences in attainment of brain drug levels have been observed that may impact on interpretations of the relative potency of the reinforcement value of these compounds.     

Region-specific enhancement of basal extracellular and cocaine-evoked dopamine levels following constitutive deletion of the Serotonin(1B) receptor

The behavioral effects of cocaine are enhanced following constitutive deletion of the serotonin(1B) receptor. The neural substrates mediating the enhanced response to cocaine are unknown. The present studies determined whether basal dopamine dynamics or cocaine-evoked dopamine levels are altered in projection areas of mesostriatal or mesoaccumbens dopamine neurons following serotonin(1B) receptor deletion. Male wild-type and serotonin(1B) knockout mice were implanted with microdialysis guide cannulas aimed at the dorsal striatum or nucleus accumbens. The zero net flux method of quantitative microdialysis was used to quantify basal extracellular dopamine concentrations (DA(ext)) and the extraction fraction of dopamine (E(d)), which provides an index of dopamine uptake. Conventional microdialysis techniques were used to quantify cocaine (0, 5.0, and 20.0 mg/kg)-evoked dopamine overflow. Basal DA(ext) and E(d) did not differ in striatum of wild-type and knockout mice. Similarly, cocaine-stimulated dopamine overflow did not differ between genotype. The basal E(d) did not differ in the nucleus accumbens of wild-type and knockout mice. However, DA(ext) was significantly elevated in the nucleus accumbens of knockout mice. Cocaine-evoked dopamine overflow (nM) was also enhanced in the nucleus accumbens of knockout mice. However, the cocaine-induced increase in dopamine levels, relative to basal values, did not differ between genotype. These data demonstrate that deletion of the serotonin(1B) receptor is associated with increases in basal DA(ext) in the nucleus accumbens. This increase is not associated with an alteration in E(d), suggesting increased basal dopamine release in these animals. It is hypothesized that these alterations in presynaptic neuronal activity are a compensatory response to constitutive deletion of the serotonin(1B) receptor and may contribute to the enhanced behavioral effects of psychostimulants observed in knockout mice.     

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.     

Neurochemical Evidence that Postsynaptic Nucleus Accumbens D3 Receptor Stimulation Enhances Cocaine Reinforcement

Abstract: The mechanism by which two D₃ receptor-preferring agonists, 7-hydroxydipropylaminotetralin (7-OH-DPAT) and quinelorane, modulate cocaine reinforcement was examined by monitoring nucleus accumbens dopamine levels with in vivo microdialysis while rats intravenously self-administered the following four different drug solutions consecutively: (1) cocaine; (2) a combination of cocaine plus a low dose of either agonist; (3) either agonist alone; and finally, (4) a physiological saline solution. Both 7-OH-DPAT (4 µg/infusion) and quinelorane (0.25 µg/infusion) decreased cocaine (0.25 mg/infusion) intake in a manner indicating an enhancement of cocaine reinforcement and simultaneously decreased the cocaine-induced elevations in nucleus accumbens dopamine levels by >50%. Subsequent self-administration of either 7-OH-DPAT (4 µg/infusion) or quinelorane (0.25 µg/infusion) alone resulted in significant, but stable, increases in drug intake, with a concurrent decrease in nucleus accumbens dopamine levels to ∼50% below nondrug baseline levels. These findings indicate that postsynaptic D₃ receptor stimulation in the nucleus accumbens enhances the reinforcing properties of cocaine. In a second experiment, local application of 7-OH-DPAT via reverse dialysis (30 and 100 n M perfusate concentrations) dose-dependently decreased nucleus accumbens dopamine efflux to 76 ± 3.9 and 61 ± 6.3% of baseline, respectively, whereas there was no effect of this agonist on dopamine efflux in the ipsilateral striatum of these same animals. Coperfusion with the D₃ receptor-preferring antagonist nafadotride dose-dependently blocked the effect of 7-OH-DPAT on nucleus accumbens dopamine efflux. These results suggest that, at low concentrations, 7-OH-DPAT selectively activates D₃ receptors in vivo.     

Site and mechanism of behavioral tolerance to cocaine: A study of dopamine release in Wistar-Kyoto and spontaneously hypertensive rats

Wistar-Kyoto and spontaneously hypertensive rats received i.v. infusions of cocaine hydrochloride (60 mg/kg per day) for 3, 7, and 14 days, or saline for 7 days. Acute cocaine challenge (40 mg/kg, s.c.) was given to treated and control rats 24 hr after the termination of each infusion period. There were no strain differences in brain levels of cocaine during cocaine infusion, nor after cocaine challenges. There were no strain differences in resting levels of [³H]dopamine release. Release of [³H]dopamine decreased in nuclei accumbens of 7- and 14-day cocaine-infused animals. Release of [³H]dopamine was maximal in both brain regions 2 hr after acute cocaine challenge. After 14 days of cocaine infusion, cocaine challenge in both strains reduced [³H]dopamine release in the nucleus accumbens, but not in the striatum; the reduction being greater in Wistar-Kyoto rats. The behavioral tolerance which accompanies similar cocaine infusion regimens may be related to striatal tolerance to cocaine-induced dopamine release.     

Effect of experimenter-delivered and self-administered cocaine on extracellular beta-endorphin levels in the nucleus accumbens

Beta-endorphin is an endogenous opioid peptide that has been hypothesized to be involved in the behavioral effects of drugs of abuse including psychostimulants. Using microdialysis, we studied the effect of cocaine on extracellular levels of beta-endorphin in the nucleus accumbens, a brain region involved in the reinforcing effects of psychostimulant drugs. Experimenter-delivered cocaine (2 mg/kg, i.v.) increased extracellular beta-endorphin immunoreactive levels in the nucleus accumbens, an effect attenuated by 6-hydroxy-dopamine lesions or systemic administration of the D1-like receptor antagonist, SCH-23390 (0.25 mg/kg, i.p.). The effect of cocaine on beta-endorphin release in the nucleus accumbens was mimicked by a local perfusion of dopamine (5 microm) and was blocked by coadministration of SCH-23390 (10 microm). Self-administered cocaine (1 mg/kg/infusion, i.v.) also increased extracellular beta-endorphin levels in the nucleus accumbens. In addition, using functional magnetic resonance imaging, we found that cocaine (1 mg/kg, i.v.) increases regional brain activity in the nucleus accumbens and arcuate nucleus. We demonstrate an increase in beta-endorphin release in the nucleus accumbens following experimenter-delivered and self-administered cocaine mediated by the local dopaminergic system. These findings suggest that activation of the beta-endorphin neurons within the arcuate nucleus-nucleus accumbens pathway may be important in the neurobiological mechanisms underlying the behavioral effects of 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.     

Homer proteins regulate sensitivity to cocaine

Drug addiction involves complex interactions between pharmacology and learning in genetically susceptible individuals. Members of the Homer gene family are regulated by acute and chronic cocaine administration. Here, we report that deletion of Homer1 or Homer2 in mice caused the same increase in sensitivity to cocaine-induced locomotion, conditioned reward, and augmented extracellular glutamate in nucleus accumbens as that elicited by withdrawal from repeated cocaine administration. Moreover, adeno-associated virus-mediated restoration of Homer2 in the accumbens of Homer2 KO mice reversed the cocaine-sensitized phenotype. Further analysis of Homer2 KO mice revealed extensive additional behavioral and neurochemical similarities to cocaine-sensitized animals, including accelerated acquisition of cocaine self-administration and altered regulation of glutamate by metabotropic glutamate receptors and cystine/glutamate exchange. These data show that Homer deletion mimics the behavioral and neurochemical phenotype produced by repeated cocaine administration and implicate Homer in regulating addiction to cocaine.     

Additive effects of intra-accumbens infusion of the cAMP-specific phosphodiesterase inhibitor, rolipram and cocaine on brain stimulation reward

Evidence from cocaine self-administration studies suggests that increasing the activity of cyclic AMP (cAMP) pathways within the nucleus accumbens may produce a reduction in cocaine's reinforcing effects. Rolipram may increase intra-cellular levels of cAMP by selectively inhibiting Type IV phosphodiesterases, enzymes that catalyze cAMP breakdown. The present study was undertaken to test the hypothesis that infusion of rolipram into the nucleus accumbens would decrease cocaine-induced enhancement of the sensitivity of brain stimulation reward (BSR) pathways. BSR thresholds were determined in rats after the systemic administration of cocaine (4 mg/kg IP) and the infusion of rolipram (0.2 microg/side) into the nucleus accumbens both alone and in combination. Thresholds also were determined after the systemic administration of rolipram alone and, as a positive control, for amphetamine (10 microg/side) infused into the nucleus accumbens. BSR thresholds were significantly lowered below baseline levels following d-amphetamine administration suggesting that cannulae were in place to allow perfusion of reward pathways. Compared to values for saline alone, thresholds were lower after the injection of cocaine (4 mg/kg IP) or the infusion of rolipram (0.2 microg/side) into the nucleus accumbens. Treatment with the combination of cocaine and intra-nucleus accumbens rolipram produced a greater lowering of the BSR threshold than did administration of either rolipram or cocaine alone. Systemic administration of rolipram (0.5 mg/kg IP) either blocked the effects of BSR or raised BSR thresholds and produced stimulation-induced head jerking in most of the test animals. These results suggest that infusion into the nucleus accumbens of rolipram, an agent that putatively elevates cAMP levels in this structure, can enhance the sensitivity of reward pathways to BSR and can augment cocaine's actions on these pathways.     

Differences in Dopamine Clearance and Diffusion in Rat Striatum and Nucleus Accumbens Following Systemic Cocaine Administration

Acute cocaine administration preferentially increases extracellular dopamine levels in nucleus accumbens as compared with striatum. To investigate whether a differential effect of cocaine on dopamine uptake could explain this observation, we used in vivo electrochemical recordings in anesthetized rats in conjunction with a paradigm that measures dopamine clearance and diffusion without the confounding effects of release. When a finite amount of dopamine was pressure-ejected at 5-min intervals from a micropipette adjacent to the electrode, transient and reproducible increases in dopamine levels were detected. In response to 15 mg/kg of cocaine-HCl (i.p.), these signals increased in nucleus accumbens, indicating significant inhibition of the dopamine transporter. The time course of the dopamine signal increase paralleled that of behavioral changes in unanesthetized rats receiving the same dose of cocaine. In contrast, no change in the dopamine signal was detected in dorsal striatum; however, when the dose of cocaine was increased to 20 mg/kg, enhancement of the dopamine signal occurred in both brain areas. Quantitative autoradiography with [3H]mazindol revealed that the affinity of the dopamine transporter for cocaine was similar in both brain areas but that the density of [3H]mazindol binding sites in nucleus accumbens was 60% lower than in dorsal striatum. Tissue dopamine levels in nucleus accumbens were 44% lower. Our results suggest that a difference in dopamine uptake may explain the greater sensitivity of nucleus accumbens to cocaine as compared with dorsal striatum. Furthermore, this difference may be due to fewer dopamine transporter molecules in nucleus accumbens for cocaine to inhibit, rather than to a higher affinity of the transporter for cocaine.     

Workshop 3: 2

For over 20 years, the focus of studies examining the neurochemical and behavioral effects of cocaine and other psychostimulants has been on dopamine. Many behavioral studies have shown that dopamine plays an important role in the reinforcing and behavioral effects of cocaine. Cocaine binds to the dopamine transporter and inhibits dopamine uptake. While there are some effects of cocaine on dopamine receptors, dopamine levels, and the dopamine transporter, these neurochemical studies have not been able to account fully for the altered behavioral effects of cocaine following chronic cocaine administration. Recent studies by Kantak et al. have shown that the reinforcing effects of psychostimulants depend upon activation of brain nitric oxide synthase. In addition, Rocha et al. have reported that cocaine is self‐administered in animals lack dopamine transporters. This finding suggests that other neurochemical components are necessary for the reinforcing effects (and hence the abuse) of cocaine. Since cocaine binds to dopamine, norepinephrine and serotonin transporters, it is likely that a combination of effects on these systems may be responsible for the behavioral effects of cocaine. Mu‐ and kappa‐opioids regulate dopamine and serotonin and this regulation plays a role in the effects of cocaine (Izenwasser et al.). Unterwald and colleagues have shown that there are large effects of cocaine on opioid receptors and second messenger regulation. These studies show that there are interactions between multiple systems and that these interactions are important factors in the effects of abused drugs, perhaps more important than activation of dopaminergic systems alone. These findings will be discussed in terms of the implications for the development of treatments for cocaine abuse.     

Differential regulation of accumbal dopamine transmission in rats following cocaine, heroin and speedball self-administration

Cocaine/heroin combinations (speedball) exert synergistic neurochemical and behavioral effects that are thought to contribute to the increased abuse potential and subjective effects reported by polydrug users. In vivo fast-scan cyclic voltammetry was used to examine the effects of chronic intravenous self-administration (25 consecutive sessions) of cocaine (250 μg/inf), heroin (4.95 μg/inf) and speedball (250/4.95 μg/inf cocaine/heroin) on changes in electrically evoked dopamine (DA) efflux, maximal rate of DA uptake (V(max)) and the apparent affinity (K(m)) of the DA transporter in the nucleus accumbens. The increase in electrically evoked DA was comparable following cocaine and speedball injection; however, heroin did not increase evoked DA. DA transporter K(m) values were similarly elevated following cocaine and speedball, but unaffected by heroin. However, speedball self-administration significantly increased baseline V(max), while heroin and cocaine did not change baseline V(max), compared with the baseline V(max) values of drug-na?ve animals. Overall, elevated DA clearance is a likely consequence of synergistic elevations of nucleus accumbens extracellular DA concentrations by chronic speedball self-administration, as reported previously in microdialysis studies. The present results indicate neuroadaptive processes that are unique to cocaine/heroin combinations and cannot be readily explained by simple additivity of changes observed with cocaine and heroin alone.     

Neurochemical effects of the endocannabinoid uptake inhibitor UCM707 in various rat brain regions

To date, UCM707, (5Z,8Z,11Z,14Z)-N-(3-furylmethyl)eicosa-5,8,11,14-tetraenamide, has the highest potency and selectivity in vitro and in vivo as inhibitor of the endocannabinoid uptake. Its biochemical, pharmacological and therapeutic properties have been intensely studied recently, but the information on its capability to modify neurotransmitter activity, which obviously underlies the above properties, is still limited. In the present study, we conducted a time-course experiment in rats aimed at examining the neurochemical effects of UCM707 in several brain regions following a subchronic administration (5 injections during 2.5 days) of this inhibitor in a dose of 5 mg/kg weight. In the hypothalamus, the administration of UCM707 did not modify GABA contents but reduced norepinephrine levels at 5 h after administration, followed by an increase at 12 h. Similar trends were observed for dopamine, whereas serotonin content remained elevated at 1 and, in particular, 5 and 12 h after administration. In the case of the basal ganglia, UCM707 reduced GABA content in the substantia nigra but only at longer (5 or 12 h) times after administration. There were no changes in serotonin content, but a marked reduction in its metabolite 5HIAA was recorded in the substantia nigra. The same pattern was found for dopamine, contents of which were not altered by UCM707 in the caudate-putamen, but its major metabolite DOPAC exhibited a marked decrease at 5 h. In the cerebellum, UCM707 reduced GABA, serotonin and norepinephrine content, but only the reduction found for norepinephrine at 5 h reached statistical significance. The administration of UCM707 did not modify the contents of these neurotransmitters in the hippocampus and the frontal cortex. Lastly, in the case of limbic structures, the administration of UCM707 markedly reduced dopamine content in the nucleus accumbens at 5 h, whereas GABA content remained unchanged in this structure and also in the ventral-tegmental area and the amygdala. By contrast, norepinephrine and serotonin content increased at 5 h in the nucleus accumbens, but not in the other two limbic structures. In summary, UCM707 administered subchronically modified the contents of serotonin, GABA, dopamine and/or norepinephrine with a pattern strongly different in each brain region. So, changes in GABA transmission (decrease) were restricted to the substantia nigra, but did not appear in other regions, whereas dopamine transmission was also altered in the caudate-putamen and the nucleus accumbens. By contrast, norepinephrine and serotonin were altered by UCM707 in the hypothalamus, cerebellum (only norepinephrine), and nucleus accumbens, exhibiting biphasic effects in some cases.     

Chronic Cocaine Administration Increases CNS Tyrosine Hydroxylase Enzyme Activity and mRNA Levels and Tryptophan Hydroxylase Enzyme Activity Levels

Cocaine is an inhibitor of dopamine and serotonin reuptake by synaptic terminals and has potent reinforcing effects that lead to its abuse. Tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH) catalyze the rate-limiting steps in dopamine and serotonin biosynthesis, respectively, and are the subject of dynamic regulatory mechanisms that could be sensitive to the actions of cocaine. This study assessed the effects of chronic cocaine on brain TH and TPH activities. Cocaine was administered (0.33 mg/infusion, i.v.) to rats for 7 days every 8 min for 6 h per day. This administration schedule is similar to patterns of self-administration by rats when given ad libitum access to this dose. This chronic, response-independent administration increased TH enzyme activity in the substantia nigra (30%) and ventral tegmental area (43%). Moreover, TH mRNA levels were also increased (45 and 50%, respectively). In contrast to the enzymatic and molecular biological changes in the cell bodies, TH activity was unchanged in the terminal fields (corpus striaturn and nucleus accumbens). Similarly, TPH activity was increased by 50% in the raphe nucleus (serotonergic cell bodies). In summary, the chronic response-independent administration of cocaine produces increases in the expression of TH mRNA and activity in both the cell bodies of motor (nigrostriatal) and reinforcement (mesolimbic) dopamine pathways. These increases are not manifested in the terminal fields of these pathways.     

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.     

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.     

Cocaine treatment increases extracellular cholecystokinin (CCK) in the nucleus accumbens shell of awake,freely moving rats,an effect that is enhanced in rats that are behaviorally sensitized to cocaine

Cholecystokinin (CCK) is co-localized with dopamine, is known to modulate dopamine neurotransmission and is involved in behavioral sensitization to psychostimulants. To better understand its role, CCK was measured by microdialysis in the nucleus accumbens (NAC) shell in response to cocaine in drug-naive rats and in rats that are behaviorally sensitized to cocaine. Basal extracellular levels of CCK in drug-naive rats were 0.17 pg/20 min fraction, while in cocaine-sensitized rats, they were significantly higher (0.56 pg). Treating drug-naive rats with cocaine caused a significant increase in CCK to 0.58 pg. Cocaine treatment of cocaine-sensitized rats increased CCK to 0.98. When analyzed as a function of time after cocaine treatment, these increases were sustained and were significantly different from CCK levels of saline-treated rats. In cocaine-sensitized rats, CCK levels following cocaine treatment were also significantly higher than levels in drug-naive animals receiving a single injection of cocaine. These results provide evidence for an activation of the mesolimbic and/or cerebral cortical CCK system in response to repeated cocaine administration. These results provide a neurochemical basis for an important role of CCK (via modulation of dopamine neurotransmission) in expression of cocaine sensitization.     

Stereoselective inhibition of dopaminergic activity by gamma vinyl-GABA following a nicotine or cocaine challenge: a PET/microdialysis study

Elevation of endogenous GABA by the racemic mixture of gamma vinyl-GABA (GVG, Vigabatrin) decreases extracellular nucleus accumbens (NAc) dopamine (DA) levels and diminishes the response to many drugs of abuse known to elevate DA in the mesocorticolimbic system. We investigated the effects of the individual enantiomers (S(+)-GVG, R(-)-GVG) on cocaine-induced NAc DA in rodents as well as the effects of nicotine-induced increases in primates. In a series of microdialysis experiments in freely moving animals, S(+)-GVG (150 mg/kg), R(-)-GVG (150 mg/kg) or racemic (R, S) GVG (300 mg/kg) was administered 2.5 hours prior to cocaine (20 mg/kg) administration. When compared with cocaine alone, the R(-) enantiomer did not significantly inhibit cocaine induced NAc DA release. S(+)-GVG, at half the dose of the racemic mixture (150 mg/kg), inhibited cocaine-induced DA elevation by 40%, while the racemic mixture (300 mg/kg) inhibited cocaine-induced DA release by 31%. In addition, our PET studies in primates demonstrated that S(+)-GVG completely inhibits nicotine-induced increases in the corpus striatum, again at half the dose of the racemic mixture. The R(-) enantiomer was ineffective. Although the S(+) enantiomer has been well established as the active compound in the treatment of epilepsy, the efficacy of this enantiomer with regard to mesolimbic DA inhibition generates a complex series of clinical and neurochemical issues. Further investigations will determine the locus of action and physiologic properties of each enantiomer.