Increased extracellular dopamine in the nucleus accumbens of the rat during associative learning of neutral stimuli

Brain microdialysis was used to study changes in dopamine in the nucleus accumbens and the dorsal striatum during associative learning between two neutral stimuli, flashing light and tone, presented on a paired schedule during stage 1 of a sensory preconditioning paradigm. The tone was subsequently paired with mild footshock using standard aversive conditioning procedures and the formation of a conditioned association between the flashing light and the tone in stage 1 was assessed by measuring the ability of the flashing light to elicit the same conditioned response as the tone when presented at test. The first experiment used behavioural monitoring only, to establish stimulus parameters for subsequent microdialysis experiments. Animals receiving paired presentation of the light and tone in stage 1 showed a conditioned suppression of licking to the light as well as to the tone, indicating that associative learning between the flashing light and the tone had occurred during stage 1, whilst in a separate group of animals given the same stimuli over the same time period but on an explicitly non-paired schedule, the conditioned emotional response was seen to the tone, but not to the light, showing that no association had been formed between the two stimuli during stage 1. In dialysis experiments using the same procedure, we measured a two-fold rise in dopamine in the nucleus accumbens during paired presentation of flashing light and tone, but not during non-paired presentation of the two stimuli. On subsequent test presentation of the two stimuli, we saw increases in accumbal dopamine on presentation of the tone in both groups, reflecting the formation of an association with the footshock in both. However the flashing light elicited an increase in dopamine only in the group which had received paired presentation at stage 1. Thus accumbal dopamine release at test is correlated to the ability of the stimulus to evoke a conditioned response measured behaviourally. Hypotheses of the behavioural function of the mesolimbic dopamine system centre on its role in mediating the effects of biological reinforcers, both rewarding and aversive, conditioned and unconditioned. The present results, showing increases in extracellular dopamine in the nucleus accumbens when an association is formed between two stimuli of which neither is a biological reinforcer nor, prior to formation of the association, affects dopamine levels, suggest a role for accumbal dopamine in the modulation of associative learning in general, not only that involving reinforcement.     

Effects of ibotenic acid lesions of the nucleus accumbens on paced mating behavior in the female rat.

The present study investigated the role of the nucleus accumbens (NAcc) in paced mating behavior in female rats. A sexually receptive female rat will approach and withdraw from a sexually active male, thereby controlling the timing of the receipt of sexual stimulation (e.g., mounts, intromissions, ejaculations). In this study, ibotenic acid lesions in the NAcc core increased the likelihood that a female rat would withdraw from a male rat after a mount but did not affect contact return latency or sexual receptivity. lbotenic acid lesions in the NAcc shell did not affect paced mating behavior or sexual receptivity. The results suggest that the NAcc core plays a role in suppressing withdrawal behavior in response to less intense mating stimulation. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Monoamine metabolism in the rat striatum during actions on the nucleus accumbens

In vivo microdialysis in conscious rats combined with HPLC-EC analysis was used to monitor extracellular levels of 3, 4-dihydroxyphenilacetic acid (DOPAC), homovanillic acid (HVA), and 5-hydroxyindoleacetic acid (5-HIAA) in the dorsal striatum (STR) during infusions of procain and apomorphine into the nucleus accumbens (N.acc). It was shown that apomorphine infused into the N.acc (2 x 10(-5) M) caused a decrease in striatal extracellular levels of DOPAC, HVA, and 5-HIAA. Infusions of procain into the N.acc (10(-5) M) produced an increase in extracellular DOPAC, and HVA in the STR. Data indicated that the N.acc exerts an inhibitory influence on the metabolism of dopamine in the STR, the influence being under control of dopaminergic system of the N.acc.     

Sexual behavior increases dopamine transmission in the nucleus accumbens and striatum of male rats: Comparison with novelty and locomotion.

Extracellular concentrations of dopamine (DA) and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were examined concurrently, using in vivo microdialysis, in the nucleus accumbens and dorsal striatum of sexually active male rats during tests of locomotor activity, exposure to a novel chamber, exposure to sex odors, the presentation of a sexually receptive female, and copulation. DA increased significantly in the nucleus accumbens when the males were presented with a sexually receptive female behind a screen and increased further during copulation. Although DA also increased significantly in the dorsal striatum during copulation, the magnitude of the effect was significantly lower than that observed in the nucleus accumbens. In contrast, forced locomotion on a rotating drum, exposure to a novel chamber, and exposure to sex odors did not increase DA significantly in either region. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Real-time dopamine efflux in the nucleus accumbens core during Pavlovian conditioning.

To assess the role of dopamine input to the nucleus accumbens core in anticipatory learning, fast-scan cyclic voltammetry was combined with appetitive Pavlovian conditioning. One group of rats (Paired) received 16 tone-food pairings for at least four daily sessions while the control group (Unpaired) received the same number of unpaired tone and food presentations. Both groups showed transient dopamine responses during food presentation throughout training, confirming dopamine involvement in reward processing. Only the Paired Group, however, showed consistently timed dopamine transients during the 10-s tone presentation. Transients first appeared near the end of the tone period as each animal acquired the tone-food association and then occurred progressively sooner on subsequent sessions. Later sessions also revealed a consistently timed dopamine response soon after food delivery in Paired animals. Collectively, these results implicate phasic dopamine release in the acquisition of Pavlovian learning and also suggest an early dopamine response to the unconditioned stimulus as training continues. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Nicotine effects on dopamine clearance in rat nucleus accumbens

In vivo voltammetry was used to measure the clearance to exogenously applied dopamine (DA) in the nucleus accumbens following acute systemic nicotine administration in urethane-anesthetized rats. The IVEC-5 system was used for continuous in vivo electrochemical measurements. A finite amount of DA was pressure-ejected (25-100 nl, 200 microM barrel concentration) at 5-min intervals from micropipettes (tip diameter, 10-15 microns) positioned 250 +/- 50 microns from the recording electrode. The peak DA concentration after each DA ejection was significantly decreased in rats following nicotine, but not in rats given saline. In addition, when mecamylamine was administered 20 min before nicotine it clearly antagonized nicotine effects. These results suggest that nicotine may actually facilitate DA transporter systems within the nucleus accumbens.     

A microdialysis study of nucleus accumbens core and shell dopamine during operant responding in the rat

This investigation examined dopamine release and metabolism in nucleus accumbens core and shell during three operant tasks in the rat. Rats were trained to lever press on a fixed-ratio 5, variable-interval 30 s, or a tandem variable interval 30/fixed-ratio 5 schedules; these three schedules were chosen because they generate a wide range of response and reinforcement rates. After several weeks of training, dialysis probes were implanted into nucleus accumbens core or shell subregions. A single 30 min behavioural session was conducted during the dialysis test session. Rats lever pressing on each of the three operant schedules showed a significant increase in extracellular dopamine relative to the food-deprived control group during the behavioural session. In addition, increases in dopamine in nucleus accumbens shell were found to be significantly greater than in the core during the lever pressing period. Across all three schedules, extracellular dopamine in the nucleus accumbens was significantly correlated with the number of lever presses performed, but was not correlated with the number of food pellets delivered. Analysis of covariance, which used amount of food consumed as the covariate, showed an overall group difference, indicating that dopamine levels increased in lever pressing animals even if one corrected for the amount of food consumed. These results indicate that dopamine release was more responsive in the nucleus accumbens shell than in the core during operant responding, and that increases in extracellular dopamine in nucleus accumbens are related to response rate rather than reinforcement magnitude.     

Voltammetry of extracellular dopamine in rat striatum during ICSS-like electrical stimulation of the medial forebrain bundle

Fast-scan cyclic voltammetry in the striatum of anesthetized rats has been used to monitor extracellular dopamine during forced electrical stimulation of the media forebrain bundle using parameters that mimic intracranial self-stimulation. The temporal resolution provided by microelectrodes positioned very near sites of dopamine release allows resolution of the response to individual 500-ms stimulation trains separated by 500-ms intervals. Uptake inhibition by Nomifensine alters the resolution obtained at short times after initiation of stimulation.     

Evidence for a role of dopamine in self-stimulation of the nucleus accumbens of the rat.

Investigated the effects of centrally administered spiroperidol, a dopamine receptor blocking agent, on self-stimulation of the nucleus accumbens and medial prefrontal cortex in the rat. Spiroperidol in a volume of 1 μl was microinjected into the region of the stimulating electrodes. Self-stimulation of the nucleus accumbens was significantly attenuated by .75, 1.0, and 2.0 μg spiroperidol. Control microinjections of the drug vehicle had no effect. Spiroperidol microinjected into the nucleus accumbens contralateral to the stimulating electrode, as a control for possible motor or nonspecific effects, did not attenuate self-stimulation. Microinjections of spiroperidol into the region of the stimulating electrodes in the prefrontal cortex had no consistent effect on self-stimulation with the two lower doses, but did result in attenuation at the 2.0 μg dose. Self-stimulation of the nucleus accumbens was not changed by microinjections of spiroperidol into the ipsilateral or contralateral prefrontal cortex. Similarly, self-stimulation of the prefrontal cortex was not altered by microinjections of spiroperidol into the nucleus accumbens. By controlling for nonspecific effects of spiroperidol, the results provide further evidence that dopaminergic neurons contribute to self-stimulation of the nucleus accumbens. (French summary) (38 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Context-dependent acquisition of copulatory behavior in the male rat: Role of female availability.

The effect of the environment in which sexual experience is acquired was examined on patterns of male rat copulatory behavior. Males trained in a pacing chamber with a 4-hole partition had significantly shorter ejaculation latencies compared with males trained in chambers with a 1-hole partition. Those differences persisted when males were switched into the other pacing condition, suggesting that the pattern of copulation in these males had become "fixed." In the second experiment, males were trained to associate an almond odor with copulation in either the 1-hole or 4-hole condition. Males ejaculated preferentially with females associated with the 4-hole pacing condition. Copulatory behavior in male rats is sensitive to female availability, and females associated with greater availability are preferred. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Different effects of opiate withdrawal on dopamine turnover, uptake, and release in the striatum and nucleus accumbens

The purpose of these experiments was to further characterize changes in dopaminergic function that follow withdrawal from chronic opiate treatment. Withdrawal after treatment to a maximum dose of 120 mg/kg of morphine did not alter dopamine concentrations in the substantia nigra, ventral tegmental area, striatum, or nucleus accumbens; but did decrease concentrations of DOPAC and the ratio of DOPAC to dopamine in the lateral striatum and nucleus accumbens. Uptake of tritiated dopamine was diminished for withdrawn slices obtained from the striatum with no effect observed for tissue from the nucleus accumbens. Deficits of in vitro release of tritiated dopamine also occurred following withdrawal, with the nucleus accumbens being sensitive to dependence produced by a lower dose of morphine. In conclusion, opiate withdrawal produces a complex pattern of effects on dopaminergic function that is specific for the striatum and nucleus accumbens.     

Synergistic elevations in nucleus accumbens extracellular dopamine concentrations during self-administration of cocaine/heroin combinations (Speedball) in rats

The abuse of cocaine/opiate combinations (speedball) represents a growing trend in illicit drug use. Delineation of neurobiological substrates mediating the reinforcing effects of the combination may increase our knowledge of reinforcement mechanisms and provide useful new information for the development of pharmacotherapies. Several studies suggest dopaminergic innervations of the nucleus accumbens (NAc) have a central role in the brain processes underlying drug reinforcement. The present study was undertaken to determine the relationship between the self-administration of cocaine/heroin combinations and NAc extracellular dopamine concentrations ([DA]e) using in vivo microdialysis and microbore high-pressure liquid chromatography. Rats were assigned randomly to one of three groups to self-administer i.v. cocaine (125, 250, and 500 micrograms/infusion; n = 5), heroin (4.5, 9, and 18 micrograms/infusion; n = 5), or cocaine/heroin combinations (125/4.5; 250/9, and 500/18 micrograms/infusion; n = 4) under a fixed ratio (FR) 10: 20-s time-out schedule of reinforcement/multicomponent dosing session. After stable rates of responding were engendered and maintained, microdialysis samples were collected in 10-min intervals during the self-administration session. Self-administration of cocaine/heroin combinations produced synergisitic elevations in NAc [DA]e (1000% baseline) compared with cocaine (400% baseline) and heroin (not significantly different from baseline levels). Neither the number of infusions nor the interinfusion intervals was significantly different between the groups across the self-administration session. Moreover, cocaine concentrations were not significantly different between the cocaine and cocaine/heroin groups. These results demonstrate that heroin interacts with cocaine to produce synergistic elevations in [DA]e, providing a neurochemical basis for understanding the abuse liability of cocaine/opiate combinations.     

Increased opioid inhibition of GABA release in nucleus accumbens during morphine withdrawal

The nucleus accumbens is a key component of the reward pathway that plays a role in addiction to many drugs of abuse, including psychostimulants and opioids. The effects of withdrawal from chronic morphine were examined in the nucleus accumbens using brain slices from morphine-treated animals. Recordings were made from interneurons in the shell of nucleus accumbens, and the presynaptic inhibition of GABA-A IPSCs by opioids was examined. In slices from control animals, opioids caused a maximal inhibition of 50%, forskolin increased the IPSC amplitude by less than twofold, and the maximal inhibition by opioids in the presence of forskolin was not changed. During withdrawal, however, forskolin caused approximately a fourfold increase in the amplitude of the IPSC, and the maximal inhibition by opioids was increased to 80%. The results indicate that transmitter release is increased during opioid withdrawal, particularly after the activation of adenylyl cyclase. The cAMP-dependent increase in transmitter release is potently inhibited by opioids, such that the overall effect of opioids is augmented during withdrawal. The induction of an opioid-sensitive cAMP-dependent mechanism that regulates transmitter release may be a critical component of acute opioid withdrawal.     

Ultrastructural immunocytochemical localization of neurotensin and the dopamine D2 receptor in the rat nucleus accumbens

The neuroleptic-like effects of neurotensin (NT) are thought to be due to interactions with dopamine (DA) acting primarily at D2 receptors within the nucleus accumbens septi (Acb). Using electron microscopic dual labeling immunocytochemistry, we sought to demonstrate cellular substrates for functional interactions involving NT and DA D2 receptors in the adult rat Acb. Peroxidase reaction product representing D2 receptor-like immunoreactivity (D2-LI) was seen along membranes of Golgi lamellae and multivesicular bodies of perikarya containing immunogold labeling representing NT-LI. Dually labeled somata usually contained highly indented nuclei, a characteristic of aspiny neurons. Dendrites also occasionally colocalized the two immunomarkers. Other somata, dendrites, and all axon terminals were singly labeled with either NT-LI or D2-LI. In distinct sets of terminals, NT-LI was commonly associated with large, dense-cored vesicles, whereas D2-LI was found along the plasmalemma and over nearby small clear vesicles. Each type of terminal comprised approximately 20% of synaptic input to NT-immunoreactive dendrites. Similar proportions of terminals containing NT-LI or D2-LI contacted unlabeled (approximately 55%) or NT-labeled (approximately 35%) dendrites and, occasionally, were observed converging onto common dendrites. Terminals containing NT-LI or D2-LI also were often closely apposed. These findings provide the first ultrastructural evidence that: (1) NT and D2 receptors are colocalized in aspiny neurons and dendrites, (2) NT may produce a direct postsynaptic effect on neurons receiving input from terminals which are presynaptically modulated by DA via D2 receptors, and (3) NT and DA acting at D2 receptors may interact through presynaptic modulation of common axon terminals.     

Persistent increase in dopamine release following activation of metabotropic glutamate receptors in the rat nucleus accumbens

We report two cases of severe hypertension and unilateral renal dysplasia. No renal artery stenosis and no other urogenital malformations were found. In both cases we found substantially enhanced secretion of renin from the dysplastic kidney. After nephrectomy both patients obtained a distinctive and permanent reduction or normalization of blood pressure. In the two cases reported, regional renin release induced by ischemia is a very likely etiological factor.     

Basolateral amygdala stimulation evokes glutamate receptor-dependent dopamine efflux in the nucleus accumbens of the anaesthetized rat

Afferents from the basolateral amygdala and dopamine projections from the ventral tegmental area to the nucleus accumbens have both been implicated in reward-related processes. The present study used in vivo chronoamperometry with stearate-graphite paste electrodes in urethane-anaesthetized rats to determine how basolateral amygdala efferents to the nucleus accumbens synaptically regulate dopamine efflux. Repetitive-pulse (20 Hz for 10 s) electrical stimulation of the basolateral amygdala evoked a complex pattern of changes in monitored dopamine oxidation currents in the nucleus accumbens related to dopamine efflux. These changes were characterized by an initial increase that was time-locked to stimulation, a secondary decrease below baseline, followed by a prolonged increase in the dopamine signal above baseline. The effects of burst-patterned stimulation (100 Hz, 5 pulses/burst, 1-s interburst interval, 40 s) of the basolateral amygdala on the basal accumbens dopamine signal were similar to those evoked by 20 Hz stimulation, with the lack of a secondary suppressive component. Infusions of the ionotropic glutamate receptor antagonists (+/-)-2-amino-5-phosphonopentanoic acid (APV) or 6,7-dinitroquinoxaline-2,3-dione (DNQX) into the nucleus accumbens dose-dependently blocked or attenuated the initial and prolonged increases in the dopamine signal following 20 Hz or burst-patterned basolateral amygdala stimulation. Infusions of the metabotropic glutamate receptor antagonist (+)-alpha-methyl-4-carboxyphenylglycine selectively blocked the intermediate suppressive effect of 20 Hz basolateral amygdala stimulation on dopamine oxidation currents. Blockade of glutamate receptors or inhibition of dopamine neuronal activity via infusions of either APV + DNQX, lidocaine or gamma-hydroxybutyric acid, respectively, into the ventral tegmental area did not effect the pattern of changes in the accumbens dopamine signal evoked by basolateral amygdala stimulation. These data suggest that the glutamatergic basolateral amygdala inputs to nucleus accumbens dopamine terminals synaptically facilitate or depress dopamine efflux, and these effects are independent of dopamine neuronal firing activity. Moreover, these results imply that changes in nucleus accumbens dopamine levels following presentation of reward-related stimuli may be mediated, in part, by the basolateral amygdala.     

Antagonism by locally applied dopamine into the nucleus accumbens or the corpus striatum of alpha-methyltyrosine-induced disruption of conditioned avoidance behaviour

Male Sprague-Dawley rats were trained to avoid an electric shock in a two-way shuttle-box. The local application of dopamine into the nucleus accumbens or the corpus striatum was found to antagonize the suppression of conditioned avoidance behaviour induced by systemically administered alpha-methyltyrosine, emphasizing the importance of these dopamine-rich brain structures in mediating conditioned avoidance behaviour.     

Electrical stimulation of dorsal and ventral striatum differentially alters the copulatory behavior of male rats.

Sexual behavior is a natural reward that activates striatal dopaminergic (DA) circuits, and dopamine exerts a facilitative influence on copulation. Electrical stimulation of the striatum has been shown to be rewarding, but its effect on male sexual behavior display has not been established. The objective of the present work was to assess the effects of low- and high-frequency electrical stimulation of the dorsal and ventral striatum on male rat sexual behavior expression. To this aim, copulatory activity of sexually experienced male rats was recorded during electrical stimulation of the nucleus accumbens (NAcc) or caudate-putamen (CP), at each stimulation frequency, before and after sexual exhaustion. Results showed that electrical stimulation of the NAcc at both frequencies increased the number of ejaculations that male rats were able to show in a 30-min period. By contrast, stimulation delivered to the CP inhibited sexual behavior by slowing its display. Each effect was more pronounced at low than at high stimulation frequencies. In the same rats, once sexually exhausted, electrical stimulation of these brain areas did not reverse the sexual behavior inhibition that characterizes the sexual exhaustion state. It is concluded that dorsal and ventral striatal DA brain regions exert opposite influences on copulatory behavior expression of sexually experienced male rats. Also, that the facilitative effect of NAcc electrical stimulation on sexual activity, with the stimulation parameters used, cannot surmount the sexual behavior inhibition resulting from copulation to satiation. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Metabotropic glutamate receptors in the rat nucleus accumbens

The effects of glutamate metabotropic receptors (mGluRs) on excitatory transmission in the nucleus accumbens were investigated using electrophysiological techniques in rat nucleus accumbens slices. The broad-spectrum mGluR agonist (1S,3R)-1-aminocyclopentyl-1,3-dicarboxylate, the mGluR group 2 selective agonists (S)-4-carboxy-3-hydroxyphenylglycine, (1S,3S)-ACPD) and (2S,1'S,2'S)-2-(2'-carboxycyclopropyl)glycine (L-CCG1), and the mGluR group 3 specific agonist L-2-amino-4-phosphonobutyrate (L-AP4) all reversibly inhibited evoked excitatory synaptic responses. The specific group 1 mGluR agonist (R,S)-3,5-dihydroxyphenylglycine [(R,S)-DHPG] did not depress transmission. Dose-response curves showed that the rank order of agonist potencies was: L-CCG1 > L-AP4 > (1S,3S)-ACPD. Group 2 and 3 mGluRs inhibited transmission via a presynaptic mechanism, as they increased paired-pulse facilitation, decreased the frequency of miniature excitatory postsynaptic currents and had no effect on their amplitude. The mGluRs did not inhibit transmitter release by reducing voltage-dependent Ca2+ currents through N- or P-type Ca2+ channels, as inhibition persisted in the presence of omega-conotoxin-GVIA or omega-Aga-IVA. The depression induced by mGluRs was not affected by specific antagonists of dopamine D1, GABA-B or adenosine A1 receptors, indicating direct effects. Finally, (R,S)-DHPG specifically blocked the postsynaptic afterhyperpolarization current (I(AHP)). Our results represent the first direct demonstration of functional mGluRs in the nucleus accumbens of the rat.     

Effect of antidepressant drugs on dopamine D1 and D2 receptor expression and dopamine release in the nucleus accumbens of the rat

This study examined the effect of repeated treatment with the antidepressant drugs, fluoxetine, desipramine and tranylcypromine, on dopamine receptor expression (mRNA and binding site density) in sub-regions of the nucleus accumbens and striatum of the rat. The effect of these treatments on extracellular levels of dopamine in the nucleus accumbens was also measured. Experiments using in situ hybridisation showed that the antidepressants caused a region-specific increase in D2 mRNA, this effect being most prominent in the nucleus accumbens shell. In contrast, none of the treatments increased D1 mRNA in any of the regions examined. Measurement of D2-like binding by receptor autoradiography, using the ligand [3H]YM-09151-2, revealed that both fluoxetine and desipramine increased D2-like binding in the nucleus accumbens shell; fluoxetine had a similar effect in the nucleus accumbens core. Tranylcypromine, however, had no effect on D2-like binding in the nucleus accumbens but decreased binding in the striatum. In micro-dialysis experiments, our data showed that levels of extracellular dopamine in the nucleus accumbens were not altered in rats treated with either fluoxetine or desipramine, but increased by tranylcypromine. From our findings, we propose that the antidepressant drugs tested enhance dopamine function in the nucleus accumbens through either increased expression of post-synaptic D2 receptors (fluoxetine and desipramine) or increased dopamine release (tranylcypromine).