Stressor controllability modulates stress-induced serotonin but not dopamine efflux in the nucleus accumbens shell

The ability of an organism to control a stressor can modulate many of the consequences of stress. Previous research indicates that uncontrollable stress (inescapable shock, IS) activates serotonergic (5-HT) neurons of the dorsal raphe nucleus (DRN) to a greater degree than controllable stress (escapable shock, ES), potentiating 5-HT efflux in the DRN and in target regions. Previous research also indicates that IS selectively affects dopamine (DA) efflux. The present study measured 5-HT and DA efflux in the NAc shell during IS or ES using in vivo microdialysis. Male Sprague Dawley rats with probes in the NAc shell were dialysed while subjected to 100 1.0 mA tailshocks. Rats were run in yoked pairs in wheel-turn boxes such that one rat (ES) in the pair could terminate the shock received by both himself and his yoked (IS) partner by a behavioral response. No stress controls remained in the dialysis bowls. 5-HT efflux selectively increased during IS, and remained increased throughout as well as after the stress session. There was no effect of stress on DA efflux. These results indicate that the NAc 5-HT response is preferentially sensitive to stress and can be modulated by stressor controllability.     

SR141716A reduces the reinforcing properties of heroin but not heroin-induced increases in nucleus accumbens dopamine in rats

The present experiments tested the hypothesis that the selective CB1 receptor antagonist SR141716A alters heroin self-administration by attenuating heroin-induced increases in nucleus accumbens dopamine levels. SR141716A pretreatment dose-dependently (0.3-3 mg/kg, i.p.) reduced operant heroin self-administration by male Wistar rats under a fixed ratio schedule of reinforcement, and significantly lowered the breaking point of responding for heroin under a progressive ratio schedule of reinforcement. These observations are consistent with recent reports that CB1 receptor inactivation reduces the rewarding properties of opiates. Operant responding for water reinforcement by water-restricted rats was unaltered by these SR141716A doses. Microdialysis tests revealed that heroin self-administration significantly increases interstitial dopamine levels in the nucleus accumbens shell of vehicle-pretreated control rats. However, whereas SR141716A pretreatment dose-dependently reduced heroin self-administration, it did not alter the heroin-associated increase in nucleus accumbens dopamine. These findings suggest that the CB1 antagonist-induced attenuation of heroin reward does not involve dopaminergic mechanisms in the nucleus accumbens shell.     

Lesions of nucleus accumbens reduce instrumental but not consummatory negative contrast in rats

In Experiment 1, rats provided with brief daily access to 4% sucrose which preceded brief access to 32% sucrose (4-32) suppressed licking the 4% solution relative to 4-4 controls. This anticipatory negative contrast (ANC) was diminished when the 32% solution was downshifted to 4%. Licking the second 4% solution in shifted rats (4-32-4) was lower than licking of the second 4% solution in 4-4 control rats - a successive negative contrast (SNC) effect. Neither SNC nor ANC or their recovery were influenced by electrolytic lesions of the nucleus accumbens (NAC). Latency to initiate licking followed a concentration function, with rats initiating licking faster for 32 than 4% sucrose, but was not affected by the lesion. In Experiment 2, rats showed clear SNC in consummatory behavior when shifted from 32 to 4% sucrose and also showed SNC in running speed when shifted from a 12- to 1-pellet reward in a straight runway. As in Experiment 1, consummatory SNC was not affected by the lesion. However, in the runway, lesioned animals showed contrast later (after more trials) than the sham-lesioned rats and did not show contrast in the goal section, when goal speed was averaged across the postshift period. Reward downshift also increased the animals' tendency to backtrack in the runway and backtracking was greater in the lesioned rats during both the preshift and postshift periods. These data suggest that the NAC is not a necessary structure for the generation of expectancies, the comparison of rewards or the modulation of ingestive behavior. However, the NAC may be involved in responding to unmet expectancies when the task involves approach or instrumental behavior.     

Gene variants of brain dopamine pathways and smoking-induced dopamine release in the ventral caudate/nucleus accumbens

CONTEXT: Preclinical studies demonstrate that nicotine administration leads to dopamine release in the ventral striatum. However, human studies reveal considerable interindividual variability in the extent of smoking-induced dopamine release. OBJECTIVE: To determine whether common gene variants of the brain dopamine pathway explain this observed phenotypic variability in humans. DESIGN: Blood samples were drawn to determine gene variants of dopamine system components, and positron emission tomography scanning with the radiotracer raclopride labeled with radioactive carbon (11C) was performed to measure smoking-induced dopamine release. SETTING: Academic brain imaging center. PARTICIPANTS: Forty-five tobacco-dependent smokers. INTERVENTIONS: Subjects either smoked a cigarette (n = 35) or did not smoke (n = 10) during positron emission tomography scanning. MAIN OUTCOME MEASURES: Gene variants of dopamine system components (the dopamine transporter variable nucleotide tandem repeat, D2 receptor Taq A1/A2, D4 receptor variable nucleotide tandem repeat, and catechol-O-methyltransferase Val158Met polymorphisms) and change in [11C]raclopride binding potential in the ventral caudate/nucleus accumbens on positron emission tomography scans. RESULTS: For subjects who smoked during scanning, those with at least one 9 allele of the dopamine transporter variable nucleotide tandem repeat, fewer than 7 repeats of the D4 variable nucleotide tandem repeat, and the Val/Val catechol-O-methyltransferase genotype had greater decreases in binding potential (an indirect measure of dopamine release) with smoking than those with the alternate genotypes. An overall decrease in ventral caudate/nucleus accumbens binding potential in those who smoked compared with those who did not smoke was also found but was smaller in magnitude than previously reported. CONCLUSIONS: Smokers with genes associated with low resting dopamine tone have greater smoking-induced (phasic) dopamine release than those with alternate genotypes. These findings suggest that dopamine system genotype variabilities explain a significant proportion of the interindividual variability in smoking-induced dopamine release and indicate that smoking-induced dopamine release has a genetic predisposition.     

Reduced dopamine output in the nucleus accumbens but not in the medial prefrontal cortex in rats displaying a mecamylamine-precipitated nicotine withdrawal syndrome

Mesolimbocortical dopamine (DA) neurotransmission is important in the mediation of the dependence-producing actions of nicotine and other drugs of abuse. Withdrawal from chronic treatment with various types of addictive drugs, including amphetamine, cocaine, ethanol and morphine is associated with a decrease in dopaminergic output in the nucleus accumbens (NAC), whereas the effects of withdrawal from these drugs on dopaminergic output in the medial prefrontal cortex (PFC), as yet, remain largely unknown. This study examined putative changes in the extracellular levels of dopamine and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the NAC and in the PFC of rats displaying behavioral signs of nicotine withdrawal. Rats were infused for 7 days with nicotine via subcutaneously implanted minipumps, whereas control animals carried saline-containing pumps. On the fifth day of infusion a microdialysis probe was implanted in the NAC or the PFC of the rats. Forty-eight hours later the levels of DA and the monoamine metabolites were assessed in the dialysate. The behavioral and biochemical effects of a saline injection and a subsequent challenge with the nicotinic receptor antagonist mecamylamine (1 mg/kg s.c.) were determined. Following mecamylamine challenge in nicotine-treated animals, the levels of DA, DOPAC and HVA in the NAC, but not in the PFC, decreased below pre-injection levels and in relation to control animals. The score of abstinence signs increased in the nicotine-treated rats, as compared both to the score after saline and to that in control animals. The decreased DA output in the NAC in animals displaying nicotine withdrawal signs is similar to that seen after withdrawal of several other drugs of abuse, and may have bearing on motivational deficits associated with the abstinence reactions.     

Repeated administration of high doses of amphetamine increases release of ascorbic acid in caudate but not nucleus accumbens

Linear sweep voltammetry with carbon paste electrodes was used to monitor extracellular ascorbic acid (AA) in the caudate nucleus and nucleus accumbens of behaving rats. Amphetamine (2 or 5 mg/kg) was administered 4, 6 and 8 days after surgery. In general the amphetamine-induced increase in AA was greater in the caudate than in the nucleus accumbens. Furthermore, in the nucleus accumbens the amphetamine-induced increase in AA was very similar on all test days, but in the caudate the increase in AA produced by 5 mg/kg amphetamine was progressively larger on each test day. Thus AA seems to be regulated differently in the caudate and nucleus accumbens.     

The C-terminal fragment of substance P enhances dopamine release in nucleus accumbens but not in neostriatum in freely moving rats.

The in vivo microdialysis technique was used to study the effects of carboxyl or amino terminal sequences of substance P on the extracellular concentrations of dopamine, its metabolites dihydroxyphenylacetic acid and homovanillic acid, as well as on 5-hydroxyindoleacetic acid, in neostriatum and nucleus accumbens of freely moving rats. The i.p. administration of 37 nmol/kg of the substance P C-terminal heptapeptide analog [pGlu5, MePhe8, Sar9]SP5-11 (DiMe-C7) caused an increase in extracellular dopamine concentrations in nucleus accumbens but not in neostriatum. The administration of the equimolar dose of the heptapeptide N-terminal fragment substance P 1-7 (SP1-7) did not have an effect in either structure. No changes were observed in the extracellular concentrations of the metabolites after the administration of either substance. These results are discussed with respect to the reinforcing effects of substance P and its C-terminal sequence, which may be mediated via dopamine release in the nucleus accumbens.     

Intraaccumbal mecamylamine infusion does not affect dopamine output in the nucleus accumbens of chronically nicotine-treated rats

Summary. Recently we have shown that the nicotinic receptor (nAChR) antagonist mecamylamine both when administered systemically and locally into the ventral tegmental area (VTA) to chronically nicotine-treated rats reduces dopamine (DA) output in the nucleus accumbens (NAC) and elicits behavioral withdrawal signs. However, the putative contributory role of nAChRs in the NAC in mediating these effects of systemic mecamylamine has not been clarified. Therefore, we here investigated the effect on extracellular levels of DA in the NAC of local intraaccumbal administration of mecamylamine to chronically nicotine-treated rats and its putative behavioral correlates. In these experiments local application of mecamylamine into the NAC, in a concentration that increased NAC DA levels in control rats, did not affect DA output or behavior in the nicotine-treated animals. These results provide further support for the contention that nAChRs in the VTA, but not in the NAC, are of major importance for the mesolimbic DA reduction and associated behavioral signs in nicotine withdrawal. Received December 10, 1999; accepted February 8, 2000     

Injection of oxotremorine in nucleus accumbens shell reduces cocaine but not food self-administration in rats

Mesencephalic dopamine neurons form synapses with acetylcholine (ACh)-containing interneurons in the nucleus accumbens (NAcc). Although their involvement in drug reward has not been systematically investigated, these large aspiny interneurons may serve an important integrative function. We previously found that repeated activation of nicotinic cholinergic receptors enhanced cocaine intake in rats but the role of muscarinic receptors in drug reward is less clear. Here we examined the impact of local changes in muscarinic receptor activation within the NAcc on cocaine and food self-administration in rats trained on a progressive ratio (PR) schedule of reinforcement. Animals were given a minimum of 9 continuous days of drug access before testing in order to establish a stable breaking point (BP) for intravenous cocaine infusions (0.75 mg/kg/infusion). Rats in the food group acquired stable responding on the PR schedule within 7 days. On the test day, rats were bilaterally infused in the NAcc with the muscarinic receptor agonist oxotremorine methiodide (OXO: 0.1, 0.3 or 1 nmol/side), OXO plus the M(1) selective antagonist pirenzepine (PIRENZ; 0.3 nmol/side) or aCSF 15 min before cocaine or food access. OXO dose dependently reduced BP values for cocaine reinforcement (-17%, -44% [p<0.05] and -91% [p<0.0001] for 0.1, 0.3 and 1.0 nmol, respectively) and these reductions dissipated by the following session. Pretreatment with PIRENZ blocked the BP-reducing effect of 0.3 nmol OXO. Notably, OXO (0.1, 0.3 and 1.0 nmol/side) injection in the NAcc did not affect BP for food reward. The results suggest that muscarinic ACh receptors in the caudomedial NAcc may play a role in mediating the behavior reinforcing effects of cocaine.     

In vivo comparison of the regulation of releasable dopamine in the caudate nucleus and the nucleus accumbens of the rat brain

In vivo voltammetry has been used to measure the release of dopamine evoked by electrical stimulation of the medial forebrain bundle (MFB). Simultaneous measurements have been made with voltammetric-sensing electrodes ipsilateral to the stimulating electrode in the nucleus accumbens and the caudate nucleus of the anesthetized rat. During the stimulation, the species observed in both regions is voltammetrically identical to dopamine. Further evidence for the identity of dopamine is provided by anatomical, physiological, pharmacological, and postmortem data. Postmortem analysis of these brain regions after a single stimulation demonstrates that dopamine levels are unchanged, while dihydroxyphenylacetic acid (DOPAC) levels are increased in both regions. Systemic application of synthesis inhibitors results in a decrease in evoked release for each brain region. Amfonelic acid results in a restoration of stimulated release after synthesis inhibition. Evoked release is affected differently by pargyline in the two brain regions. The evoked release of dopamine is significantly elevated in the nucleus accumbens as a result of pargyline administration, but similar effects are not seen in the caudate nucleus. Tissue levels of dopamine are increased in both brain regions by pargyline, but the increase is significantly greater in the accumbens. Electrolytic lesions of the striatonigral pathway or systemic administration of picrotoxin eliminates the pargyline-induced difference in evoked release of dopamine. Amphetamine causes a reduction in stimulated release in the caudate nucleus with little effect on that observed in the nucleus accumbens.(ABSTRACT TRUNCATED AT 250 WORDS)     

Serotonin denervation enhances responsiveness of presynaptic dopamine efflux to acute clozapine in nucleus accumbens but not in caudate-putamen

Clozapine alters mesolimbic dopamine (DA) function but spares nigrostriatal DA function in laboratory animals, but the underlying mechanism is unknown. In the present study, acute intraperitoneal injection of clozapine (5-40 mg/kg) increased extracellular DA levels in nucleus accumbens (Acb) and caudate-putamen (CPu) of awake, freely moving rats as measured by in vivo brain microdialysis, without anatomic selectivity. However, in serotonin (5HT)-denervated rats acute clozapine preferentially enhanced DA levels in Acb as compared to CPu. Since (i) up-regulation of 5HT receptors on DA neurons may result from 5HT denervation, (ii) clozapine has potent anti-5HT action, and (iii) 5HT receptors are more dense in Acb than CPu, these data appear to add additional weight to previous suggestions that a serotonergic mechanism may partly underlie clozapine's mesolimbic selectivity.     

Chronic neuroleptic administration decreases extracellular GABA in the nucleus accumbens but not in the caudate-putamen of rats.

The extracellular levels of gamma-aminobutyric acid (GABA) in the caudate-putamen and the nucleus accumbens of rats following administration of haloperidol decanoate, fluphenazine decanoate, or vehicle for 8 months were assessed using intracranial microdialysis. Basal levels of extracellular GABA were significantly decreased in the nucleus accumbens of both neuroleptic-treated groups while levels of GABA in the caudate-putamen were not significantly different between groups. These results provide evidence for selective chronic neuroleptic-induced effects on in vivo GABA function in different terminal regions containing dopamine receptors.     

Anticonvulsant effects of ipsilateral but not contralateral microinjections of the dopamine D2 agonist LY 171555 into the nucleus accumbens of amygdala-kindled rats.

Recent radioligand binding studies demonstrated an increase in the density of dopamine D2 receptors in the nucleus accumbens ipsilateral to the stimulating electrode in amygdala- or hippocampal-kindled rats. In the present study we examined the anticonvulsant effect of dopamine agonists by unilateral microinjections into the nucleus accumbens in rats kindled from the right basolateral amygdaloid nucleus. Microinjections of the D2 agonist LY 171555 into the ipsilateral nucleus accumbens 15 min prior to the kindling stimulation in fully kindled rats decreased significantly kindling parameters such as seizure severity, seizure duration and afterdischarge duration, whereas the D1 agonist SKF 38393 had no anticonvulsant effects. After ipsilateral microinjection of 40 pmol LY 171555 focal and generalized kindled seizures were totally blocked in almost 50% of the rats. The anticonvulsant effect of LY 171555 could be completely antagonised by systemic administration of the D2 antagonist sulpiride. Microinjection of the D1 or D2 agonist into the nucleus accumbens contralateral to the stimulating electrode had no anticonvulsant effects. In accordance with other reports our data indicate a possible topographic limitation of D2 receptor mediated anticonvulsant effects to specific regions of the basal ganglia.     

Cortical cholinergic deficiency enhances amphetamine-induced dopamine release in the accumbens but not striatum

Cholinergic dysfunction has been implicated as a putative contributing factor in the pathogenesis of schizophrenia. Recently, we showed that cholinergic denervation of the neocortex in adult rats leads to a marked increase in the behavioral response to amphetamine. The main objective of this study was to investigate if the enhanced locomotor response to amphetamine seen after cortical cholinergic denervation was paralleled by an increased amphetamine-induced release of dopamine in the nucleus accumbens and/or striatum. The corticopetal cholinergic projections were lesioned by intraparenchymal infusion of 192 IgG-saporin into the nucleus basalis magnocellularis of adult rats. Amphetamine-induced dopamine release in the nucleus accumbens or striatum was monitored by in vivo microdialysis 2 to 3 weeks after lesioning. We found that cholinergic denervation of the rat neocortex leads to a significantly increased amphetamine-induced dopamine release in the nucleus accumbens. Interestingly, the cholinergic lesion did not affect amphetamine-induced release of dopamine in the striatum. The enhanced amphetamine-induced dopamine release in the nucleus accumbens in the cholinergically denervated rats could be reversed by administration of the muscarinic agonist oxotremorine, but not nicotine, prior to the amphetamine challenge, suggesting that loss of muscarinic receptor stimulation is likely to have caused the observed effect. The results suggest that abnormal responsiveness of dopamine neurons can be secondary to cortical cholinergic deficiency. This, in turn, might be of relevance for the pathophysiology of schizophrenia and provides a possible link between cholinergic disturbances and alteration of dopamine transmission.     

Activation of dopamine D1-like receptors in nucleus accumbens is critical for the acquisition, but not the expression, of nutrient-conditioned flavor preferences in rats

The present study investigated the role of dopamine neurotransmission within the nucleus accumbens (NAc) in flavor preference learning induced by the postoral consequences of carbohydrates. In Experiment 1, rats fitted with a gastric catheter were trained with a flavor (CS+) paired with intragastric (IG) infusions of 8% glucose and a different flavor (CS–) paired with IG water infusions. The CS+ preference was then evaluated in two-bottle preference tests following bilateral injection of the dopamine D1-like receptor antagonist SCH23390 into the NAc shell at total doses of 0, 12, 24 and 48 nmol. SCH23390 produced dose-related reductions in CS+ intake but did not block the CS+ preference except at the highest dose, which also greatly suppressed the CS intakes. In Experiment 2, new rats were injected daily in the NAc shell with either saline or SCH23390 (12 nmol), 10 min prior to training sessions with CS+ with IG glucose and CS– with IG water. In the two-bottle preference tests, the drug-treated rats, unlike the control rats, did not significantly prefer the CS+ (61 vs. 83% preference). In Experiment 3, new rats were trained with the same procedures as Experiment 2, except that brain injections were in the NAc core. In contrast to control rats, SCH-treated rats failed to prefer the CS+ to the CS– in two-bottle tests (55% vs. 89% preference). These results demonstrate that D1-like receptors in the NAc shell and core are greatly involved in the acquisition, but less so in the expression, of a flavor preference conditioned by postingestive effects of glucose.     

Lack of glucocorticoids attenuates the self-stimulation-induced increase in the in vivo synthesis rate of dopamine but not serotonin in the rat nucleus accumbens

Our previous study demonstrated that intracranial self-stimulation of the medial forebrain bundle can increase the in vivo synthesis turnover rate of dopamine (DA) and serotonin (5-HT) in the nucleus accumbens of adrenal-intact rats. The present study examined using microdialysis whether such increases in DA and 5-HT syntheses are influenced by adrenal hormones, which are also activated following intracranial self-stimulation. A decarboxylase inhibitor, NSD-1015, was perfused through reversed microdialysis which enabled the simultaneous measurement of 3,4-dihydroxyphenylalanine (DOPA) and 5-hydroxytryptophan (5-HTP) as an index of the in vivo turnover rate of DA and 5-HT syntheses. Adrenalectomy (ADX) attenuated significantly the self-stimulation-induced increase in dialysate levels of DOPA but not 5-HTP. Corticosterone (Cort) replacement reversed the attenuation in DOPA levels in adrenalectomized rats. The finding indicates that activation of DA synthesis in vivo in the nucleus accumbens during intracranial self-stimulation is dependent on, whereas that of 5-HT synthesis is independent of glucocorticoid modulation.     

Anticonvulsant effects of ipsilateral but not contralateral microinjections of the dopamine D2 agonist LY 171555 into the nucleus accumbens of amygdala-kindled rats

Recent radioligand binding studies demonstrated an increase in the density of dopamine D₂ receptors in the nucleus accumbens ipsilateral to the stimulating electrode in amygdala- or hippocampal-kindled rats. In the present study we examined the anticonvulsant effect of dopamine agonists by unilateral microinjections into the nucleus accumbens in rats kindled from the right basolateral amygdaloid nucleus. Microinjections of the D₂ agonist LY 171555 into the ipsilateral nucleus accumbens 15 min prior to the kindling stimulation in fully kindled rats decreased significantly kindling parameters such as seizure severity, seizure duration and afterdischarge duration, whereas the D₁ agonist SKF 38393 had no anticonvulsant effects. After ipsilateral microinjection of 40 pmol LY 171555, focal and generalized kindled seizures were totally blocked in almost 50% of the rats. The anticonvulsant effect of LY 171555 could be completely antagonised by systemic administration of the D₂ antagonist sulpiride. Microinjection of the D₁ or D₂ agonist into the nucleus accumbens contralateral to the stimulating electrode had no anticonvulsant effects. In accordance with other reports our data indicate a possible topographic limitation of D₂ receptor mediated anticonvulsant effects to specific regions of the basal ganglia.     

Colocalization of CART with substance P but not enkephalin in the rat nucleus accumbens

CART peptide is a novel neurotransmitter that, due to its distribution in the brain and its modulation of dopamine systems, may be involved in aspects of reward and drug abuse. In the nucleus accumbens (NAcc), CART peptide immunoreactivity (IR) is colocalized with substance P-IR in neurons. Approximately 86% of CART-IR cells colocalize with substance P, while only 19% of substance P-IR neurons contain CART. CART peptide does not colocalize with enkephalin-IR in this region. The substance P-CART colocalization exists in a rostro-caudal gradient with more colocalization in rostral regions. The presence of CART in substance P NAcc neurons suggests that CART neurons may be a subset of the basal ganglia direct pathway or that CART neurons are involved in limbic projections of the NAcc, such as to the ventral pallidum.     

Modafinil enhances extracellular levels of dopamine in the nucleus accumbens and increases wakefulness in rats

Modafinil (MOD) is a wakefulness-promoting drug that improves the alertness levels in narcolepsy; however, the molecular mechanism of action remains to be elucidated. We found that after a single icv injection of MOD (10 microg/5 microl) the extracellular levels of dopamine (DA) and l-DOPA collected from the nucleus accumbens were increased and decreased, respectively. Separately, the icv administration of MOD (10 microg/5 microl) to rats enhanced wakefulness (W) whereas diminished sleep during 4h. Lastly, the alertness induced by MOD was partially antagonized by the sleep-inducing endocannabinoid anandamide (ANA). We conclude that MOD enhances the extracellular levels of DA, promotes W and its effects on sleep are partially blocked by ANA.