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.     

Implication of Rho-associated kinase in the elevation of extracellular dopamine levels and its related behaviors induced by methamphetamine in rats

A growing body of evidence suggests that several protein kinases are involved in the expression of pharmacological actions induced by a psychostimulant methamphetamine. The present study was designed to investigate the role of the Rho/Rho-associated kinase (ROCK)-dependent pathway in the expression of the increase in extracellular levels of dopamine in the nucleus accumbens and its related behaviors induced by methamphetamine in rats. Methamphetamine (1 mg/kg, subcutaneously) produced a substantial increase in extracellular levels of dopamine in the nucleus accumbens, with a progressive augmentation of dopamine-related behaviors including rearing and sniffing. Methamphetamine also induced the decrease in levels of its major metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanilic acid (HVA). Both the increase in extracellular levels of dopamine and the induction of dopamine-related behaviors by methamphetamine were significantly suppressed by pretreatment with an intranucleus accumbens injection of a selective ROCK inhibitor Y-27632. In contrast, Y-27632 had no effect on the decrease in levels of DOPAC and HVA induced by methamphetamine. Under these conditions, there were no changes in protein levels of membrane-bound RhoA in the nucleus accumbens following methamphetamine treatment. It is of interest to note that the microinjection of Y-27632 into the nucleus accumbens failed to suppress the increases in extracellular levels of dopamine, DOPAC, and HVA in the nucleus accumbens induced by subcutaneous injection of a prototype of micro -opioid receptor agonist morphine (10 mg/kg). Furthermore, perfusion of a selective blocker of voltage-dependent Na+ channels, tetrodotoxin (TTx) into the rat nucleus accumbens did not affect the increase in extracellular levels of dopamine in the rat nucleus accumbens by methamphetamine, whereas the morphine-induced dopamine elevation was eliminated by this application of TTx. The extracellular level of dopamine in the nucleus accumbens was also increased by perfusion of a selective dopamine re-uptake inhibitor 1-[2-[bis(4-fluorophenyl)methoxy]-4-(3-phenylpropyl)piperazine (GBR-12909) in the nucleus accumbens. This effect was not affected by pretreatment with intranucleus accumbens injection of Y-27632. These findings provide first evidence that Rho/ROCK pathway in the nucleus accumbens may contribute to the increase in extracellular levels of dopamine in the nucleus accumbens evoked by a single subcutaneous injection of methamphetamine. In contrast, this pathway is not essential for the increased level of dopamine in this region induced by morphine, providing further evidence for the different mechanisms of dopamine release by methamphetamine and morphine in rats.     

Dopaminergic innervation of the amygdala is highly responsive to stress

The amygdala has been implicated in the neuronal sequelae of stress, although little is known about the neurochemical mechanisms underlying amygdala transmission. In vivo microdialysis was employed to measure extracellular levels of dopamine in the basolateral nucleus of the amygdala in awake rats. Once it was established that impulse-dependent release of dopamine could be measured reliably in the amygdala, the effect of stress, induced by mild handling, on amygdala dopamine release was compared with that in three other dopamine-innervated regions, the medial prefrontal cortex, nucleus accumbens, and caudate nucleus. The magnitude of increase in dopamine in response to the handling stimulus was significantly greater in the amygdala than in the nucleus accumbens and prefrontal cortex. This increase was maximal during the application of stress and diminished after the cessation of stress. In contrast, the increases in extracellular dopamine levels in other regions, in particular the nucleus accumbens, were prolonged, reaching maximal values after the cessation of stress. These results suggest that dopaminergic innervation of the amygdala may be more responsive to stress than that of other dopamine-innervated regions of the limbic system, including the prefrontal cortex, and implicate amygdalar dopamine in normal and pathophysiological processes subserving an organism's response to stress.     

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.     

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.     

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.     

L-tryptophan attenuation of the dopaminergic and behavioral responses to cocaine.

This study assessed the effects of acute intravenous L-tryptophan (neutral amino acid precursor for serotonin) administration on cocaine-induced dopaminergic responses. Male Sprague-Dawley rats were surgically implanted with guide cannulas in the nucleus accumbens 5 days prior to the study and with vascular catheters (carotid artery and jugular vein) on the day prior to the study. Using microdialysis, extracellular nucleus accumbens dopamine levels were measured in freely moving rats. Following a 2 h equilibration period, animals were randomized (n=7-8 per group) to receive either a constant intravenous (IV) infusion of L-tryptophan (200 mg/kg/h) or an equal volume (2 ml/h) of saline. Ninety minutes into the infusion, cocaine (20 mg/kg) was injected intra-peritoneally. Cocaine increased nucleus accumbens microdialysate dopamine levels (500% at 30 min). This was associated with marked hyperactivity. Tryptophan infusion elevated plasma tryptophan (8-fold), and blunted the cocaine-induced increase in nucleus accumbens microdialysate dopamine levels by approximately 60%. Furthermore, tryptophan attenuated the cocaine-induced locomotor activity. These neurochemical and behavioral effects of tryptophan were associated with a marked increase in brain tissue serotonin content. The results of these studies demonstrate the feasibility of acute dietary manipulation of neurochemical and behavioral responses to cocaine. The duration, adaptation and tolerance to these effects remain to be elucidated.     

Differential Effect of Stress on In Vivo Dopamine Release in Striatum, Nucleus Accumbens, and Medial Frontal Cortex

Microdialysis was used to assess extracellular dopamine in striatum, nucleus accumbens, and medial frontal cortex of unanesthetized rats both under resting conditions and in response to intermittent tail-shock stress. The dopamine metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid also were measured. The resting extracellular concentration of dopamine was estimated to be approximately 10 nM in striatum, 11 nM in nucleus accumbens, and 3 nM in medial frontal cortex. In contrast, the resting extracellular levels of 3,4-dihydroxyphenylacetic acid and homovanillic acid were in the low micromolar range. Intermittent tail-shock stress increased extracellular dopamine relative to baseline by 25% in striatum, 39% in nucleus accumbens, and 95% in medial frontal cortex. 3,4-Dihydroxyphenylacetic acid and homovanillic acid also were generally increased by stress, although there was a great deal of variability in these responses. These data provide direct in vivo evidence for the global activation of dopaminergic systems by stress and support the concept that there exist regional variations in the regulation of dopamine release.     

In vivo CREB phosphorylation mediated by dopamine and NMDA receptor activation in mouse hippocampus and caudate nucleus

The pattern of CREB phosphorylation was investigated in the caudate nucleus and hippocampus 10 min or 3 h after i.p. injection of dopamine or NMDA receptor agonists alone, or in combination with antagonists. Ten minutes after C57BL/6 J mice were injected with either the dopamine D1 receptor agonist SKF-38393 hydrobromide or NMDA, immunoreactivity of phosphorylated CREB (pCREB) was significantly increased in all parts of the caudate nucleus but not in hippocampal regions. However, 3 h after the injection of SKF-38393, pCREB levels in the caudate nucleus did not differ significantly from the pCREB levels in control animals, whereas pCREB levels were still elevated 3 h after NMDA injection. Except for the D1 receptor antagonist SCH-23390, which induced CREB phosphorylation in the caudate nucleus, dopamine and NMDA receptor antagonists had little effect on pCREB levels by themselves. However, the NMDA receptor antagonist CGS-19755 injected i.p. blocked both the NMDA- and SKF-38393-induced rise of pCREB levels in the caudate nucleus. Similarly, the D1 receptor antagonist SCH-23390 inhibited the effects produced by SKF-38393 or NMDA. Interestingly, the D2 receptor antagonist sulpiride also blocked the SKF-38393-triggered rise of pCREB. The results demonstrated that NMDA and dopamine receptors modulate pCREB levels in the caudate nucleus and suggest mutual permissive roles for both receptors.     

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.     

Comparison of Dopamine Uptake in the Basolateral Amygdaloid Nucleus, Caudate-Putamen, and Nucleus Accumbens of the Rat

Abstract: Regional differences in the kinetics and pharmacological inhibition of dopamine uptake were investigated with fast-scan cyclic voltammetry in both the intact rat brain and a brain slice preparation. The regions compared were the basolateral amygdaloid nucleus, caudate-putamen, and nucleus accumbens. The frequency dependence of dopamine efflux evoked in vivo by electrical stimulation of the medial forebrain bundle was evaluated by nonlinear curve fitting with a Michaelis-Menten-based kinetic model. The K _m for dopamine uptake was found to be significantly higher in the basolateral amygdala (0.6 µ M ) than in the other two regions (0.2 µ M ), whereas the V _max value for dopamine uptake in the basolateral amygdala was significantly lower (0.49 µ M /s vs. 3.8 and 2.4 µ M /s in the caudate and accumbens, respectively). Similar kinetics were also obtained in brain slices. Addition of a dopamine uptake inhibitor, cocaine or nomifensine (10 µ M ), to the perfusion buffer increased the apparent K _m value >25-fold in slices of both the caudate-putamen and nucleus accumbens. In contrast, neither uptake inhibitor had an observable effect in the basolateral amygdaloid nucleus. Thus, dopamine uptake in the rat brain is regionally distinct with regard to rate, affinity, and sensitivity to competitive inhibition.     

D(3) receptor ligands modulate extracellular dopamine clearance in the nucleus accumbens

An involvement of the D(3) dopamine receptor in the regulation of extracellular dopamine has been suggested. However, the mechanisms mediating this effect are unclear. We have used the technique of no net flux microdialysis under transient conditions to examine the influence of the D(3) -preferring agonist (+)-PD128907 upon extracellular dopamine levels in the nucleus accumbens of the mouse. (+)-PD 128907 (0.1 mg/kg intraperitoneally) significantly decreased extracellular dopamine. This decrease was associated with a marked increase in the extraction fraction, which suggests an increase in dopamine clearance. The ability of D(3) -preferring compounds to modulate dopamine uptake was investigated in vitro using rotating disk electrode voltammetry. (+)-PD 128907 (10 nm) significantly increased the initial clearance rate of 3 microm dopamine in rat nucleus accumbens tissue suspensions. Kinetic analysis revealed no change in the apparent K (m) of uptake but it showed a 33% increase in V (max). In contrast, the D(3) antagonist GR 103691 (10 nm) significantly decreased dopamine uptake. Consistent with the low levels of D(3) receptors in the dorsal striatum, neither compound affected uptake in tissue suspensions from this brain region. These data indicate that D(3) receptor activation increases dopamine uptake in the nucleus accumbens and suggest that this receptor subtype can regulate extracellular dopamine by modulating the DA transporter activity.     

Alterations in the Function of Cerebral Dopaminergic and Serotonergic Systems Following Electroacupuncture and Moxibustion Applications: Possible Correlates with Their Antistress and Psychosomatic Actions

Alterations in cerebral monoamines following application of electroacupuncture were investigated using conscious rats with and without application of restraining stress. The dopamine and serotonin levels were significantly decreased in the nucleus accumbens, caudate putamen, and lateral hypothalamus and increased in the dorsal raphe nucleus by restraining stress. On the other hand, application of electroacupuncture on the lumbar and hindlimb segments eliminated the above changes in dopamine, while the changes in serotonin were attenuated by lumbar and hindlimb electroacupuncture. However, the effects of hindlimb electroacupuncture were greater than those of lumbar electroacupuncture. These results clearly indicate that lumbar and hindlimb electroacupuncture stimulations have differential effects on brain monoaminergic neurons in rats exposed to restraining stress. Moxa burning stimulation was applied to the lumbar and hindlimb segments of rats without restraining stress. The dopamine level was significantly increased in the midbrain substantia nigra-ventrotegmental area by hindlimb moxibusion. On the other hand, the serotonin levels were significantly increased in the nucleus amygdala by lumber moxibusion and decreased in the nucleus accumbens by hindlimb moxibusion. The present results indicate that electroacupuncture applied to the lumbar and hindlimb segments has an antistress effect, while the application of moxibustion to the lumbar and hindlimb segments was likely to stimulate the functions of mesocortical and mesolimbic dopaminergic and serotonergic neurons. We suggest that functional alterations in cerebral dopaminergic and serotonergic neurons are involved in the clinical efficacy of electroacupuncture and moxibustion, especially because of their antistress and psychosomatic actions.     

Effects of Aging on the Interaction Between Glutamate, Dopamine, and GABA in Striatum and Nucleus Accumbens of the Awake Rat

The aim of the present study was to investigate, using microdialysis, the effects of aging on the glutamate/dopamine/GABA interaction in striatum and nucleus accumbens of the awake rat. For that, the effects of an increase of the endogenous concentration of glutamate on the extracellular concentration of dopamine and GABA in striatum and nucleus accumbens of young (2-4 months), middle-aged (12-14 months), aged (27-33 months), and very aged (37 months) male Wistar rats were studied. Endogenous extracellular glutamate was selectively increased by perfusing the glutamate uptake inhibitor L-trans-pyrrolidine-3,4-dicarboxylic acid (PDC) through the microdialysis probe. In young rats, PDC (1, 2, and 4 mM) produced a dose-related increase of dialysate concentrations of glutamate in both striatum and nucleus accumbens. PDC also increased dialysate dopamine and GABA in both structures. These increases were significantly correlated with the increases of glutamate but not with the PDC dose used, which strongly suggests that the increases of dopamine and GABA were produced by glutamate. In striatum, there were no significant differences in the dopamine/glutamate and GABA/glutamate correlations between young and aged rats. This means that the effects of glutamate on dopamine and GABA do not change during aging. On the contrary, in the nucleus accumbens of aged rats, the increases of dopamine, when correlated with the increases of glutamate, were significantly lower than in young rats. Moreover, the ratio of dopamine to glutamate increases at maximal increases of glutamate was negatively correlated with aging. On the contrary, the ratio of GABA to glutamate increases in nucleus accumbens was positively correlated with aging, which suggests that the effects of endogenous glutamate on GABA tend to be higher in the nucleus accumbens of aged rats. The findings of this study suggest that aging changes the interaction between endogenous glutamate, dopamine, and GABA in nucleus accumbens, but not in striatum, of the awake rat.     

Rats that differentially respond to cocaine differ in their dopaminergic storage capacity of the nucleus accumbens

Cocaine (COC) inhibits the re-uptake of dopamine. However, the dopamine response to COC also depends on dopamine inside storage vesicles. The aim of this study was to investigate whether rats that differentially respond to COC differ in their dopaminergic storage capacity of the nucleus accumbens. Total and vesicular levels of accumbal dopamine as well as accumbal vesicular monoamine transporter-2 levels were established in high (HR) and low responders (LR) to novelty rats. Moreover, the effects of reserpine (RES) on the COC-induced increase of extracellular accumbal dopamine were investigated. HR displayed higher accumbal levels of total and vesicular dopamine than LR. Moreover, HR displayed more accumbal vesicular monoamine transporters-2 than LR. COC increased extracellular accumbal dopamine more strongly in HR than in LR. A low dose of RES prevented the COC-induced increase of accumbal dopamine in LR, but not in HR. A higher dose of RES was required to inhibit the COC-induced increase of accumbal dopamine in HR. These data demonstrate that HR were marked by a larger accumbal dopaminergic storage pool than LR. It is hypothesized that HR are more sensitive to COC than LR, because COC can release more dopamine from accumbal storage vesicles in HR than in LR.     

Intromissive Stimulation from the Male Increases Extracellular Dopamine Release from Fluoro-Gold-Identified Neurons within the Midbrain of Female Hamsters

Extracellular concentrations of dopamine in the nucleus accumbens were monitored using microdialysis in ovariectomized female Syrian hamsters hormonally primed with estradiol and progesterone or with a similar regimen of oil injections. Some females in each of these groups had their vaginas occluded with tape, whereas the remaining females' vaginas stayed unoccluded. When exposed to a male, both groups of hormonally primed females showed high levels of lordosis. However, only in the hormone-primed, unoccluded females were there significant elevations of dialysate dopamine during the sexual interactions with the male. There were no significant elevations in dopamine levels in the oil-treated females during interactions with the male. These data suggest that nucleus accumbens dopamine is responsive to stimuli associated with the vaginocervical stimulation received by the female during intromissions by the male. Histological analyses were based on Fluoro-Gold efflux through the probes combined with immunocytochemistry for tyrosine hydroxylase. Probe placements in the rostral accumbens, caudal accumbens, or rostral bed nucleus of the stria terminalis were not distinguishable based on analyses of basal dopamine levels, volume of Fluoro-Gold injection sites, or Fluoro-Gold labeling of midbrain, tyrosine hydroxylase-stained neurons. The number of midbrain neurons containing Fluoro-Gold was positively related to basal dopamine levels, indicating that the amount of dopamine recovered from the nucleus accumbens in microdialysis studies is a function of the number of neurons contributing to the terminal field in the region of the probe.     

Differences in effects of sultopride and sulpiride on dopamine turnover in rat brain

Sultopride and sulpiride are both chemically similar benzamide derivatives and selective antagonists of dopamine D2 receptors. However, these drugs differ in clinical properties. We compared the effects of sultopride and sulpiride on dopamine turnover in rats following the administration of these drugs alone or in combination with apomorphine. The administration of sultopride or sulpiride markedly accelerated dopamine turnover in the rat brain. The increase in the level of dopamine metabolites in the striatum was more marked in the sultopride-treated rats. Sulpiride affected the limbic dopamine receptors preferentially, whereas sultopride affected the striatal and the limoic dopamine receptors equally. A low dose of apomorphine induced a reduction in the concentration of dopamine metabolites in the striatum and the nucleus accumbens by approximately 55%, but not in the medial prefrontal cortex. Sultopride was more effective in preventing an apomorphine-induced reduction in dopamine metabolite levels. These results from rat experiments would model the pharmacological differences observed between sultopride and sulpiride in clinical use.     

Effect of direct and indirect dopamine agonists on brain extracellular ascorbate levels in the striatum and nucleus accumbens of awake rats

Systemic administration of direct and indirect dopamine agonists resulted in increased extracellular ascorbic acid levels in the striatum and, to a lesser degree, in the nucleus accumbens as measured by in vivo voltammetry. Intraperitoneal d-amphetamine sulfate (5mg/kg) increased ascorbate concentrations in striatal extracellular fluid. Amphetamine also increased extracellular ascorbate levels in the nucleus accumbens although more gradually and to a lesser extent. Intraperitoneal phenethylamine hydrochloride (20 mg/kg) following pargyline hydrochloride pretreatment (20 mg/kg) increased extracellular ascorbate levels in the striatum significantly above the small increase seen in the nucleus accumbens. The direct acting dopamine agonists Ly-141865 and Ly-163502 when given i.p. at 1 mg/kg, resulted in increased extracellular ascorbate concentrations in both brain areas, again with a significantly greater effect in the striatum. These results indicate that brain extracellular ascorbate levels can be modulated by dopaminergic neuro-transmission and that this modulation is quantitatively different in different dopamine-containing brain structures.     

Stimulation of prefrontal cortex at physiologically relevant frequencies inhibits dopamine release in the nucleus accumbens

The prefrontal cortex (PFC) is thought to provide an excitatory influence on the output of mesoaccumbens dopamine neurons. The evidence for this influence primarily arises from findings in the rat that chemical or high-intensity and high-frequency (60-200 Hz) electrical stimulations of PFC increase burst activity of midbrain dopamine neurons, and augment terminal release of dopamine in the nucleus accumbens. However, PFC neurons in animals that are engaged in PFC-dependent cognitive tasks increase their firing frequency from a baseline of 1-3 Hz to 7-10 Hz, suggesting that the commonly used high-frequency stimulation parameters of the PFC may not be relevant to the behavioral states that are associated with PFC activation. We investigated the influence of PFC activation at lower physiologically relevant frequencies on the release of dopamine in the nucleus accumbens. Using rapid (5-min) microdialysis measures of extracellular dopamine in the nucleus accumbens, we found that although PFC stimulation at 60 Hz produces the expected increases in accumbal dopamine release, the same amplitude of PFC stimulation at 10 Hz significantly decreased these levels. These results indicate that activation of PFC, at frequencies that are associated with increased cognitive demand on this region, inhibits the mesoaccumbens dopamine system.     

Clearance of Exogenous Dopamine in Rat Dorsal Striatum and Nucleus Accumbens: Role of Metabolism and Effects of Locally Applied Uptake Inhibitors

In vivo electrochemistry was used to investigate the mechanisms contributing to the clearance of locally applied dopamine in the dorsal striatum and nucleus accumbens of urethane-anesthetized rats. Chronoamperometric recordings were continuously made at 5 Hz using Nafion-coated carbon fiber electrodes. When a finite amount of dopamine was pressure-ejected at 5-min intervals from a micropipette adjacent to the electrode, transient and reproducible dopamine signals were detected. Substitution of L-a-methyldopamine, a substrate for the dopamine transporter but not for monoamine oxidase, for dopamine in the micropipette did not substantially alter the time course of the resulting signals. This indicates that metabolism of locally applied dopamine to 3,4-dihydroxyphenylacetic acid is not responsible for the decline in the dopamine signal. Similarly, changing the applied oxidation potential from ±0.45 to ±0.80 V, which allows for detection of 3-methoxytyramine formed from dopamine via catechol-O-methyltransferase, had little effect on signal amplitude or time course. In contrast, lesioning the dopamine terminals with 6-hydroxydopamine, or locally applying the dopamine uptake inhibitors cocaine or nomifensine before pressure ejection of dopamine, significantly increased the amplitude and time course of the dopamine signals in both regions. The effects of cocaine and nomifensine were greater in the nucleus accumbens than in the dorsal striatum. Local application of lidocaine and procaine had no effect on the dopamine signals. Initial attempts at modeling resulted in curves that were in qualitative agreement with our experimental findings. Taken together, these data indicate that (1) uptake of dopamine by the neuronal dopamine transporter, rather than metabolism or diffusion, is the major mechanism for clearing locally applied dopamine from the extracellular milieu of the dorsal striatum and nucleus accumbens, and (2) the nucleus accumbens is more sensitive to the effects of inhibitors of dopamine uptake than is the dorsal striatum.