Chronic desipramine enhances amphetamine-induced increases in interstitial concentrations of dopamine in the nucleus accumbens

Vitamin D attenuated 6-OHDA-induced behavioural deficits, dopamine dysmetabolism, oxidative stress, and neuro-inflammation in mice

Background: L-DOPA, the predominant therapy for Parkinson's disease (PD) is associated with motor deficits after prolonged use. The nigrostriatal tract, a primary target of neurodegeneration in PD, contains abundant Vitamin-D receptors, suggesting a potential role for VD in the disease. Therefore, we tested the impact of Vitamin D3 (VD3) in a mouse model of PD.Methods: PD was induced in adult male C57BL6 mice by a single intrastriatal injection of 6-hydroxydopamine. Two weeks post lesion, these mice received injections of a vehicle, VD3, L-DOPA, or a combination of VD3/L-DOPA and compared with sham controls. Treatment lasted three weeks, during which motor-cognitive neurobehaviour was assessed. Five weeks post lesion, brains were collected and striatal levels of the following proteins assessed: tyrosine hydroxylase (TH), dopamine decarboxylase (DDC), monoamine oxidase (MAO-B), Catechol-O-methyl transferase (COMT), dopamine transporter (DAT), brain-derived neurotrophic factor (BDNF), microglia marker (CD11b), inflammation (IL-1β), apoptotic signaling (BAX) and oxidative stress (p47phox).Results: Treatment with VD3 attenuated behavioural deficits induced by 6-OHDA, protein associated with dopamine metabolism and biomarkers of oxidative stress. VD3 significantly increased contralateral wall touches, exploratory motor and cognitive activities. VD3 significantly enhanced the expression of TH, DAT, BDNF, while significantly reducing expression of MAO-B, CD11b, IL-I β and p47phox.Conclusion: VD3 reversed some of the 6-OHDA induced changes in proteins involved in modulating the dopamine system, behavioural deficits and oxidative stress biomarkers. The data suggests that VD3 might be beneficial in reducing L-DOPA dosage, thereby reducing problems associated with dosage and prolonged use of L-DOPA in PD management.

Dysthymia and depressive disorders: dopamine hypothesis

This paper reviews the recent literature supporting the hypothesis that reduced neurotransmission in the mesolimbic dopamine (DA) system may sustain some of the symptoms of depressive conditions including dysthymia. Experimental evidences indicate that mesolimbic DA plays a crucial role in controlling incentive, motivation and reward. Additionally, in different models of depression, a reduced DA activity in the limbic system, reversed by chronic antidepressant treatment, is observed. Finally, different antidepressants, irrespective of their acute action on the uptake of norepinephrine or serotonin, have the common property when given chronically to potentiate behavioural responses to DA agonists. The DA hypothesis of depression offers an explanation for the antidepressive effect of drugs such as sulpiride and amisulpride given at low doses, that preferentially block DA autoreceptors and thereby increase DA output.

Experimental confirmation of new drug-target interactions predicted by Drug Profile Matching

We recently introduced Drug Profile Matching (DPM), a novel affinity fingerprinting-based in silico drug repositioning approach. DPM is able to quantitatively predict the complete effect profiles of compounds via probability scores. In the present work, in order to investigate the predictive power of DPM, three effect categories, namely, angiotensin-converting enzyme inhibitor, cyclooxygenase inhibitor, and dopamine agent, were selected and predictions were verified by literature analysis as well as experimentally. A total of 72% of the newly predicted and tested dopaminergic compounds were confirmed by tests on D1 and D2 expressing cell cultures. 33% and 23% of the ACE and COX inhibitory predictions were confirmed by in vitro tests, respectively. Dose-dependent inhibition curves were measured for seven drugs, and their inhibitory constants (Ki) were determined. Our study overall demonstrates that DPM is an effective approach to reveal novel drug-target pairs that may result in repositioning these drugs.

Increased alcohol consumption in rats after subchronic antidepressant treatment

The use of antidepressants for alcoholism in humans has been a matter of controversy in recent years. Despite the existence of an important co-morbidity for depression and alcoholism, some studies suggest that the use of antidepressants could worsen the prognosis of alcoholism. However, there is a lack of studies in animal models exploring this phenomenon. In the present study, we show how the 15-d treatment with fluoxetine (10 mg/kg) or venlafaxine (50 mg/kg) affected alcohol deprivation effect (ADE) and subsequent alcohol consumption. Initially, fluoxetine reduced ADE and venlafaxine did not affect it. However, in the following days, both antidepressants increased alcohol consumption, an effect that was found to last at least 5 wk. Fluoxetine treatment was shown to cause a locomotor sensitized response to a challenge dose of amphetamine (0.5 mg/kg), indicating the presence of a supersensitive dopaminergic transmission. In summary, antidepressant treatment may increase alcohol consumption in rats after a period of alcohol deprivation and this could be related to alterations in the reward circuitry. This finding confirms in an animal model previous reports in humans that may limit the use of antidepressants for alcoholism.

Effects of chronic antidepressant drug administration and electroconvulsive shock on activity of dopaminergic neurons in the ventral tegmentum

Increasing attention is now focused on reduced dopaminergic neurotransmission in the forebrain as participating in depression. The present paper assessed whether effective antidepressant (AD) treatments might counteract, or compensate for, such a change by altering the neuronal activity of dopaminergic neurons in the ventral tegmental area (VTA-DA neurons), the cell bodies of the mesocorticolimbic dopaminergic system. Eight AD drugs or vehicle were administered to rats for 14 d via subcutaneously implanted minipumps, at which time single-unit electrophysiological activity of VTA-DA neurons was recorded under anaesthesia. Further, animals received a series of five electroconvulsive shocks (ECS) or control procedures, after which VTA-DA activity was measured either 3 d or 5 d after the last ECS. Results showed that the chronic administration of all AD drugs tested except for the monoamine oxidase inhibitor increased the spontaneous firing rate of VTA-DA neurons, while effects on 'burst' firing activity were found to be considerably less notable or consistent. ECS increased both spontaneous firing rate and burst firing of VTA-DA neurons. It is suggested that the effects observed are consistent with reports of increased dopamine release in regions to which VTA neurons project after effective AD treatment. However, it is further suggested that changes in VTA-DA neuronal activity in response to AD treatment should be most appropriately assessed under conditions associated with depression, such as stressful conditions.

Short- and long-term functional consequences of fluoxetine exposure during adolescence in male rats

BACKGROUND

Fluoxetine (FLX), a selective serotonin reuptake inhibitor, is prescribed for the treatment of major depressive disorder in young populations. Here, we explore the short- and long-term consequences of adolescent exposure to FLX on behavioral reactivity to emotion-eliciting stimuli.

METHODS

Adolescent male rats received FLX (10 mg/kg) twice daily for 15 consecutive days (postnatal days 35-49). The influence of FLX on behavioral reactivity to rewarding and aversive stimuli was assessed 24 hours (short-term) or 3 weeks after FLX treatment (long-term). A separate group of adult rats was also treated with FLX (postnatal days 65-79) and responsiveness to forced swimming was assessed at identical time intervals as with the adolescents.

RESULTS

Fluoxetine exposure during adolescence resulted in long-lasting decreases in behavioral reactivity to forced swimming stress and enhanced sensitivity to sucrose and to anxiety-eliciting situations in adulthood. The FLX-induced anxiety-like behavior was alleviated by re-exposure to FLX in adulthood. Fluoxetine treatment during adolescence also impaired sexual copulatory behaviors in adulthood. Fluoxetine-treated adult rats did not show changes in behavioral reactivity to forced swim stress as observed in those treated during adolescence and tested in adulthood.

CONCLUSIONS

Treating adolescent rats with FLX results in long-lived complex outputs regulated by the emotional valence of the stimulus, the environment in which it is experienced, and the brain circuitry likely being engaged by it. Our findings highlight the need for further research to improve our understanding of the alterations that psychotropic exposure may induce on the developing nervous system and the potential enduring effects resulting from such treatments.

Dopamine dysregulation syndrome: an overview of its epidemiology, mechanisms and management

Dopamine dysregulation syndrome (DDS) is a relatively recently described iatrogenic disturbance that may complicate long-term symptomatic therapy of Parkinson's disease. Patients with DDS develop an addictive pattern of dopamine replacement therapy (DRT) use, administering doses in excess of those required to control their motor symptoms. The prevalence of DDS in patients attending specialist Parkinson's disease centres is 3-4%. Amongst the behavioural disturbances associated with DDS are punding, which is a complex stereotyped behaviour, and impulse control disorders (ICDs), such as pathological gambling, hypersexuality, compulsive shopping and compulsive eating. We review the risk factors and potential mechanisms for the development of DDS, including personality traits, potential genetic influences and Parkinson's disease-related cognitive deficits. Impulsive personality traits are prominent in patients developing DDS, and have been previously associated with the development of substance dependence. Candidate genes affecting the dopamine 'D(2)-like' receptor family have been associated with impulsive personality traits in addition to drug and nondrug addictions. Impaired decision making is implicated in addictive behaviours, and decision-making abilities can be influenced by dopaminergic medications. In Parkinson's disease, disruption of the reciprocal loops between the striatum and structures in the prefrontal cortex following dopamine depletion may predispose to DDS. The role of DRT in DDS is discussed, with particular reference to models of addiction, suggesting that compulsive drug use is due to progressive neuroadaptations in dopamine projections to the accumbens-related circuitry. Evidence for neuroadaptations and sensitization occurring in DDS include enhanced levodopa-induced ventral striatal dopamine release. Levodopa is still considered the most potent trigger for DDS in Parkinson's disease, but subcutaneous apomorphine and oral dopamine agonists may also be responsible. In the management of DDS, further research is needed to identify at-risk groups, thereby facilitating more effective early intervention. Therefore, an increased awareness of the syndrome amongst treating physicians is vital. Medication reduction strategies are employed, particularly with regard to avoiding rapidly acting 'booster' DRT formulations. Psychosocial treatments, including cognitive-behavioural therapy, have been beneficial in treating substance use disorders and ICDs in non-Parkinson's disease patients, but there are currently no published trials of psychological interventions in DDS. Further studies are also required to identify factors that can predict those patients with DDS or ICDs who will derive benefit from surgical interventions such as deep brain stimulation.

Antidepressant treatment can normalize adult behavioral deficits induced by early-life exposure to methylphenidate

BACKGROUND

Methylphenidate (MPH) is prescribed for the treatment of attention-deficit/hyperactivity disorder. Exposure to MPH before adulthood causes behavioral deficits later in life, including anxiety- and depression-like behaviors and decreased responding to natural and drug rewards. We examined the ability of fluoxetine (FLX), a selective serotonin reuptake blocker, to normalize these MPH-induced behavioral deficits.

METHODS

Male rats received MPH (2.0 mg/kg) or saline (VEH) during preadolescence (postnatal day [PD] 20-35). When adults, rats were divided into groups receiving no treatment, acute or chronic FLX, and behavioral reactivity to several emotion-eliciting stimuli were assessed.

RESULTS

The MPH-treated rats were significantly less responsive to natural (i.e., sucrose) and drug (i.e., morphine) rewards and more sensitive to stress- and anxiety-eliciting situations. These MPH-induced deficits were reversed by exposure to FLX.

CONCLUSIONS

These results indicate that exposure to MPH during preadolescence leads to behavioral alterations that endure into adulthood and that these behavioral deficits can be normalized by antidepressant treatment. These results highlight the need for further research to better understand the effects of stimulants on the developing nervous system and the potential enduring effects resulting from early-life drug exposure.

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.

Chronic antidepressant treatment modulates the release of somatostatin in the rat nucleus accumbens

This study investigated the in vivo neuronal release of somatostatin in the rat nucleus accumbens (NAc), and the effect of chronic administration of antidepressants. Microdialysis studies were performed on male Sprague-Dawley rats, in accordance with the EU guidelines (EEC Council 86/609). Somatostatin levels were quantified by radioimmunoassay (RIA) or enzyme linked immuno sorbent assay (ELISA). A high concentration of potassium ions (K(+), 100 mM) was used to ascertain the neuronal release of somatostatin. Antidepressant treatments involved the administration of citalopram (20 mg/2 ml/kg, i.p., once daily) or desipramine (DMI, 5 mg/2 ml/kg, i.p., twice daily) for 21 days. Control groups received saline (2 ml/kg for 21 days, i.p.) once or twice daily respective of the antidepressant treatment. Basal levels of somatostatin released were found to be 20.01+/-0.52 fmol/sample. K(+) (100 mM) increased somatostatin levels at 205% of basal. Chronic citalopram and desipramine treatments also increased the somatostatin levels by 83+/-32% and 40+/-6% of basal, respectively. These findings indicate that somatostatin is released neuronally in the NAc. Antidepressants influence its release in a positive manner, suggesting the necessity of further studies for the elucidation of the involvement of somatostatin in the putative therapeutic effects of these agents.

Nicotine produces antidepressant-like actions: Behavioral and neurochemical evidence

Converging lines of evidence indicate the involvement of nicotinic acetylcholine receptors in depressive illness and antidepressant drug action. We investigated the effects of sub-chronic and chronic treatment with imipramine, nicotine and their combination on: (a) the ability of a dopamine-mimetic challenge to produce locomotor stimulation and (b) cortical density of beta-adrenoceptors. One week of treatment with imipramine (10 mg/kg, twice daily) did not result in an altered response to the apomorphine (0.15 mg/kg) challenge, but after 2 weeks, the imipramine-treated rats demonstrated hyperactivity. Conversely, such increased locomotor response was observed in rats treated with nicotine (0.4 mg/kg, twice daily) for 1 but not for 2 weeks. Groups treated with nicotine+imipramine for 1 and 2 weeks demonstrated equally high hyperactivity in response to the apomorphine challenge. This effect was not different from the effects of 1-week treatment with nicotine or 2-week treatment with imipramine. The density of beta-adrenoceptors was equally decreased by 2 (but not 1) weeks of the treatment with imipramine, nicotine and their combination. The present behavioral and neurochemical data suggest the antidepressant-like effect of the chronic treatment with nicotine. It appears that the potentiation of the dopamine-mimetic-induced hyperactivity cannot be explained by beta-adrenoceptor down-regulation.

Animal models of depression: are there any?

Simple tests for antidepressant-like activity, such as 5-HTP-induced syndrome or reserpine-induced hypomotility, are often mechanism-based tests, pharmacologically specific for certain known classes of therapeutically successful antidepressant agents. Many of these behavioural assays have been superseded by neurochemical techniques such as in vivo microdialysis. In contrast to these mechanistic-based models, investigators have also endeavoured to reproduce in the laboratory, factors that are believed to precipitate depression in people. It is a strong assumption in this approach that depression is a response to stress. This strategy profiles the consequences of chronic stress particularly psychosocial stress or early life events, in order to reproduce in animals the behavioural signs and pathologies associated with depression. The advances in the social psychological, clinical pathological and new areas such as neuroimaging research offer the possibility of establishing more sophisticated models for depression in animals with a broader range of biomarkers from the immunological and endocrinological to neurochemical and behavioural. Combining these novel insights with more traditional tests of depression may not only increase our understanding of the neurobiology of depression but also afford more precise and predictive preclinical models of depression. The responsiveness of different strains or genetically modified animals to stress is likely to be a key area of study. Furthermore we must look to individual differences in subjects, even within the same strain, to more fully understand why some individuals show pathological responses to stress whereas others appear unaffected. Conversely in validating our models using currently available treatments we must include the concept of non-responders so as not to disregard models that may extend therapeutic possibilities in these patients.

Effects of acute citalopram treatment on the methamphetamine-induced locomotor activity

1. Previously the authors have shown that acute citalopram treatment increased the dopamine D2 receptor expression in rat brain striatum (Kameda et al., 2000). In the present study, the authors attempted to determine whether these effects of citalopram influence the methamphetamine-induced locomotor activity. 2. The pretreatment with a single administration of citalopram (10 mg/kg, i.p.) resulted in the significant enhancement of the locomoter activity induced by methamphetamine treatment (1 mg/kg, i.p.). The enhancement was observed 30 min, 12 hours, 24 hours, but not 7 days after withdrawal of citalopram administration. 3. Then the authors determined the methamphetamine concentration in rat brain striatum by gas chromatography-mass spectrometry (GC-MS) The results showed that the concentration of methamphetamine wars significantly higher in the rats 24 hours, and also 7 days after withdrawal of citalopram administration, compared to the control rats. 4. These results emphasized the involvement of the high methamphetamine concentration, caused by the pretreatment with citalopram, in the enhancement of the methamphetamine-induced locomotor activity. However high methamphetamine concentration alone could not account for this enhancement, since the high concentration of methamphetamine observed 7 days after withdrawal of citalopram administration did not appear to enhance the methamphetamine-induced locomotor activity. Another mechanism through which the pretreatment with citalopram enhanced the methamphetamine-induced locomotor activity, such as the increased expression of the dopamine D2 receptors, could not be excluded.

Chronic desipramine treatment selectively potentiates somatostatin-induced dopamine release in the nucleus accumbens

Dopamine and somatostatin have been implicated in the pathophysiology of depression. We have employed in vivo microdialysis to investigate the regulation of dopamine release by somatostatin in the nucleus accumbens and the striatum of awake, freely moving rats, and to ascertain how this regulation may be affected by desipramine treatment. Somatostatin-14 (10(-4) M) infusion induced an increase in the release of dopamine and a decrease in the release of its metabolites in both the nucleus accumbens (568% of basal) and the striatum (546% of basal). Chronic desipramine treatment resulted in an exaggerated somatostatin-induced increase of dopamine levels, specifically in the nucleus accumbens (3542% compared with 564% of basal in the striatum), whereas acute desipramine treatment had no effect (582% of basal) compared with saline treated rats. Basal concentrations of dopamine and metabolites were not influenced by either chronic or acute treatment of desipramine in either brain area. These results demonstrate that somatostatin regulates dopamine release in the nucleus accumbens and the striatum. Chronic antidepressant treatment influences somatostatin's actions on dopamine function selectively in the nucleus accumbens.

Do antidepressants affect motivation in conditioned place preference?

The positive motivational effects of a range of antidepressants/neurotransmitter reuptake inhibitor compounds were studied using conditioned place preference. These agents included amitriptyline (2.5-10 mg/kg), venlafaxine (5 and 10 mg/kg), sibutramine (5 and 10 mg/kg), fluoxetine (2.5-10 mg/kg), paroxetine (5-15 mg/kg) and sertraline (2.5-10 mg/kg). Male Wistar rats were place conditioned in a three-compartment box to vehicle or drug alternately for 8 days using a 30-min pretreatment time. Control animals received vehicle only. Cocaine (5 mg/kg) was used as a positive control for the procedure. Significant place preference (P<0.05) was observed with paroxetine (15 mg/kg), fluoxetine (5 and 10 mg/kg), sertraline (2.5-10 mg/kg) and cocaine. Venlafaxine and sibutramine, serotonin/noradrenaline reuptake inhibitors, produced no place conditioning, while the highest dose of the tricyclic antidepressant, amitriptyline (10 mg/kg), produced signs of place aversion. The role of serotonin in reward pathways and differences in serotonin, noradrenaline and dopamine reuptake-inhibiting properties of these compounds may explain why only the serotonin-selective reuptake inhibitors produced place preference in this study.

The role of dopamine in the mechanism of action of antidepressant drugs

The present paper reviews evidence on the effect of antidepressant treatments on dopamine transmission. Chronic treatment with antidepressant drugs potentiates the behavioural stimulant responses elicited by the stimulation of dopamine receptors, including reward-related behaviours. Moreover, antidepressants affect dopamine release in several brain areas. The reviewed literature is discussed in terms of the possible mechanisms underlying antidepressant-induced supersensitivity to dopamine-mediated behavioural responses, and of the possible implications for the therapeutic effect of these drugs. It is concluded that the potentiation of dopaminergic neurotransmission induced by chronic antidepressant treatments might contribute to their therapeutic effect.

The effects of nicotine on neural pathways implicated in depression: a factor in nicotine addiction?

The prevalence of tobacco smoking varies considerably between different groups within the community, tobacco smoking being particularly prevalent in patients with depressive disorder. This review will focus on results, derived from animal studies, which suggest that, in addition to its primary reinforcing properties, nicotine also exerts effects in stressful environments, which may account for its enhanced addictive potential in depressed patients. It focuses on the evidence that depression sensitises patients to the adverse effects of stressful stimuli, and that this can be relieved by drugs that stimulate dopamine release in the forebrain. This mechanism, it is proposed, contributes to the increased craving to smoke in abstinent smokers exposed to such stimuli, because they become conditioned to use this property of nicotine to produce rapid alleviation of the adverse effects of the stress. The review also explores the possibility that chronic exposure to nicotine elicits changes in 5-HT formation and release in the hippocampus which are depressogenic. It is postulated that smokers are protected from the consequences of these changes, while they continue to smoke, by the antidepressant properties of nicotine. However, they contribute to the symptoms of depression experienced by many smokers when they first quit the habit.

Subchronic fluoxetine treatment induces a transient potentiation of amphetamine-induced hyperlocomotion: possible pharmacokinetic interaction

The results of the present study show that 5 days of systemic treatment with fluoxetine (5 mg/kg) resulted in an augmented locomotor response to amphetamine (0.5 mg/kg). This augmented response to amphetamine was observed 24 and 48 h, but not 5 days, after the cessation of fluoxetine treatment. Subchronic fluoxetine treatment also produced an increase in the brain concentration of amphetamine when rats were challenged with amphetamine 48 h, but not 5 days, after the cessation of fluoxetine treatment. Thus, the effect of subchronic fluoxetine in augmenting amphetamine-induced hyperactivity was consistent with the effect of subchronic fluoxetine in augmenting the amphetamine concentration in the brain. This pattern of results indicates that subchronic fluoxetine potentiates the response to amphetamine within a limited time-window, and that this potentiating effect is likely to be due to the reduced metabolism of amphetamine via the inhibition of cytochrome P450 by fluoxetine and/or its metabolite norfluoxetine.

Reciprocal changes in prefrontal and limbic dopamine responsiveness to aversive and rewarding stimuli after chronic mild stress: implications for the psychobiology of depression

BACKGROUND

Chronic mild stress (CMS) has been reported to induce behavioral abnormalities that model human depression. To investigate the role in depression of phasic dopamine transmission in cortical and limbic areas, we studied the effect of CMS on the responsiveness of dopamine (DA) transmission to aversive and rewarding stimuli in rats by microdialysis of the nucleus accumbens (NAc) shell and of the medial prefrontal cortex (PFCX).

METHODS

Rats were subjected for 30 days to CMS and administered two trials of tail pinch as aversive stimulus and two feeding sessions of a highly palatable food as rewarding stimulus. Concentric microdialysis probes were implanted in the NAc shell and in the medial PFCX.

RESULTS

In unstressed rats, DA decreased in the NAc and increased in the PFCX on the first tail-pinch trial; on the 1st feeding trial, DA increased in the NAc and to a larger extent in the PFCX. In the second tail-pinch trial or feeding trial, these responses were maintained in the PFCX but underwent habituation in the NAc. CMS did not affect basal dialysate DA in the NAc or in the PFCX but influenced the responsiveness of Da transmission to tail pinches and to feeding in a reciprocal manner. Thus, in the tail-pinch trial, CMS reversed the inhibitory response of NAc DA transmission into a stimulatory one and potentiated the stimulatory response in the PFCX. By contrast, in the feeding trial, CMS blunted the stimulatory response of DA transmission in the NAc in the first trial and in the PFCX in the second trial.

CONCLUSIONS

CMS reciprocally affected DA responsiveness to motivational stimuli, facilitating or inducing a stimulatory DA response to aversive stimuli but blunting stimulatory responses to rewarding stimuli. Given the postulated role of phasic DA responsiveness in the NAc shell for learning and of DA transmission in the PFCX for expression of motivation, we hypothesize that depression is the result of defective learning and expression of aversive and appetitive motivation.

Depression and stimulant dependence: neurobiology and pharmacotherapy

Depressive disorder rates in stimulant-dependent individuals are substantially higher than community rates. Further, depressive symptoms are considered a major component of stimulant withdrawal. The comorbidity of these disorders may reflect shared neurochemical alterations in the function of serotonin, dopamine, and peptide systems, such as corticotropin releasing factor (CRF) and neuropeptide Y (NPY). These alterations are observed in patients, and in animal models of depression and stimulant dependence, particularly in limbic brain structures. This shared neurobiology does not seem to result from significant shared heritability or genetic linkage; stimulants may induce changes in neurobiology that are similar to those found in depression, and these changes might provide a therapeutic target. Stimulant-dependent patients with a depressive disorder may be a specific subpopulation for antidepressant trials, and they might reduce their stimulant abuse when treated with antidepressants. Nevertheless, concomitant dependence on alcohol or opioids may influence this response, and antidepressants appear to be more effective for depression in combined stimulant and opioid dependence than in combined stimulant and alcohol dependence.

Involvement of serotonin and dopamine in the mechanism of action of novel antidepressant drugs: a review

Several hypotheses regarding the physiopathology of major depression exist. Attention has been focused on cerebral monoaminergic systems, the dysfunction of which is thought to underlie various aspects of depressive symptomatology. There is extensive literature describing the involvement of serotonergic and dopaminergic systems in the mechanism of action of antidepressant drugs. However, a unitary analysis of the data in terms of interaction between different monoaminergic systems is still lacking. In this article, studies reporting the biochemical, behavioral, and clinical effects of tricyclic antidepressants (TCAs), monoamine oxidase inhibitors (MAOIs), selective serotonin reuptake inhibitors (SSRIs), selective blockers of presynaptic dopamine (DA) receptors, and antagonists of serotonin-2 (5-hydroxytryptamine-2 [5-HT2]) receptors were reviewed. Analysis of the current literature indicates that long-term treatment with antidepressants causes adaptive changes of the serotonergic and dopaminergic systems. In particular, long-term administration of TCAs enhances the responsiveness of postsynaptic serotonin receptors to iontophoretically applied serotonin and potentiates the behavioral responses to both direct and indirect dopaminergic agonists. Repeated administration of SSRIs and MAOIs increases serotonergic transmission by desensitizing the inhibitory 5-HT1A somatodendritic and terminal 5-HT1B/1D autoreceptors. Selective blockers of DA autoreceptors exert their antidepressant effect by enhancing DA release. A similar mechanism of action could be hypothesized for 5-HT2 receptor antagonists. There is general agreement that the clinical effect of antidepressant drugs, which becomes evident only after long-term treatment, is caused by their ability to induce adaptive changes of the monoaminergic systems. Increases in both serotonergic and dopaminergic function have been consistently found after long-term treatment with various classes of antidepressant drugs. Recent studies have focused on the functional interaction between the serotonergic and dopaminergic systems to explain the mechanism of the antidepressant action of SSRIs and 5-HT2 antagonists.

Modifications in brain cAMP- and calcium/calmodulin-dependent protein kinases induced by treatment with S-adenosylmethionine

Several lines of evidence suggest that the mechanism of action of antidepressant drugs (AD) involves adaptive changes occurring in intraneuronal post-receptor signal transduction cascades. Protein phosphorylation has a key role in signal transduction and was previously found to be a target in the action of AD (5-HT and/or NA reuptake blockers). Several studies showed that cAMP- and type II Ca2+/calmodulin-dependent protein kinases (PKA and CaMKII) are markedly affected by typical AD in two different and complementary cellular districts, respectively microtubules (a somatodendritic compartment) and synaptic vesicles (a presynaptic terminal compartment). In order to investigate whether the effect on protein kinases may be involved in the therapeutic action of drugs it is interesting to compare the effect of atypical AD with that of typical drugs. In this study the effect of the atypical AD S-adenosylmethionine (SAMe) was tested. Repeated (12 days) SAMe treatment induced in cerebrocortical microtubules an increase in the binding of cAMP to the RII PKA regulatory subunit and an increase in the endogenous phosphorylation of microtubule-associated protein 2, an effect resembling that of typical AD. In synaptic terminals the treatment induced an increase in the activity of CaMKII and in the endogenous phosphorylation of vesicular substrates. However, this modification was found in the cerebral cortex rather than in the hippocampus, where typical AD affect CaMKII. In addition the synapsin I level was decreased in the hippocampus and increased in the cerebral cortex, an effect not detected with typical AD.

Differential effects of chronic imipramine and fluoxetine on basal and amphetamine-induced extracellular dopamine levels in rat nucleus accumbens

The effect of chronic treatment with the tricyclic antidepressant drug, imipramine (10 mg/kg per day), the selective serotonin (5-HT) reuptake inhibitor, fluoxetine hydrochloride (10 mg/kg per day), and vehicle, in drinking water for 24-28 days followed by 3-5 days withdrawal, on extracellular dopamine levels was studied in rat nucleus accumbens by in vivo microdialysis. Basal extracellular dopamine levels in the nucleus accumbens were increased after chronic imipramine (12.7 +/- 1.5 fmol/20 microl per 30 min, P = 0.019), and moderately decreased after chronic fluoxetine (6.5 +/- 0.6, P = 0.047), as compared to the vehicle controls (9.1 +/- 0.7), determined by one-way analysis of variance (ANOVA). Repeated measure ANOVA indicated that the D-amphetamine sulfate (0.5 mg/kg, s.c.)-induced increase in extracellular dopamine levels in the nucleus accumbens was potentiated after chronic imipramine (P = 0.002), but unchanged after chronic fluoxetine (P = 0.83). The difference in the effect of amphetamine could be influenced by the significant differences in basal levels. However, these results were also confirmed by analysis of the net area under the curve (net-AUC) for a 180-min period (six samples): for chronic imipramine (337 +/- 45 fmol/180 min, P = 0.005) and chronic fluoxetine (249 +/- 38, P = 0.57), as compared to the vehicle controls (178 +/- 29), determined by one-way ANOVA. We suggest that the effect of treatment with these agents on mesolimbic dopamine is unlikely to be involved in their shared antidepressant action, but may be relevant to other aspects of the therapeutic profile of these two drugs, e.g. the switch into mania which is more common after treatment with imipramine than fluoxetine and exacerbation of positive symptoms in patients with schizophrenia or schizoaffective disorder.

Effects of a psychostimulant drug sydnocarb on rat brain dopaminergic transmission in vivo

Transcerebral microdialysis was used to evaluate the effect of a psychostimulant drug, sydnocarb (3-(beta-phenylisopropyl)-N-phenylcarbamoylsydnonimine), on the extracellular levels of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the dorsal striatum and nucleus accumbens of freely moving rats. Sydnocarb dose dependently (4.4, 8.75 and 17.5 mg/kg, i.p.) induced a relatively modest (up to 350% of control) and long-lasting (up to 6 h) increase in dopamine extracellular level in the rat dorsal striatum. The drug at 8.75 mg/kg, i.p., produced an approximately similar increase in dopamine efflux in the dorsal striatum and in the nucleus accumbens of freely moving rats. Sydnocarb had no effect on DOPAC or HVA extracellular levels in the rat basal ganglia in vivo at any dose studied. It is important that the drug increased the efflux of dopamine in a tetrodotoxin-sensitive and Ca2+-dependent manner. Measurements of behavioral parameters in non-operated rats revealed that sydnocarb markedly increased locomotor activity and induced stereotyped behavior. These data suggest that the stimulant action of sydnocarb is accompanied by a facilitation of central dopaminergic transmission involving an increase in Ca2+-dependent vesicular dopamine efflux.

Chronic administration of l-sulpiride at low doses reduces A10 but not A9 somatodentritic dopamine autoreceptor sensitivity

The effect of chronic treatment (twice daily for 21 days) with low doses of l-sulpiride (2 mg/kg i.p.) on the apomorphine-induced inhibition of A10 and A9 dopaminergic neurons was compared with the effect of chronic administration of the classic antidepressant desipramine (20 mg/kg i.p. daily for 21 days). Intravenous administration of apomorphine (0.01-0.04 mg/kg), to rats treated chronically with l-sulpiride, produced a reduction of the spontaneous firing rate of A9 dopaminergic neurons not significantly different from that observed in control (saline-treated) rats. In contrast, apomorphine at the same doses was more potent in inhibiting A10 firing in control rats than in l-sulpiride-treated subjects. On the other hand, desipramine-treated rats were found normosensitive (as compared to saline-treated rats) to the inhibitory properties of apomorphine in both A9 and A10 dopaminergic neurons. It is suggested that chronic l-sulpiride-induced reduction of autoreceptor sensitivity in the A10 region may contribute to its clinical antidepressant effect.

The 5-HT1A receptor antagonist (S)-UH-301 decreases dopamine release in the rat nucleus accumbens and striatum

In this study we employed in vivo microdialysis to examine the effects of the selective 5-HT1A receptor antagonist (S)-5-fluoro-8-hydroxy-2-(dipropylamino)tetralin [(S)-UH-301] on extracellular concentrations of dopamine (DA) and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the nucleus accumbens (NAC) and dorsal striatum of awake freely moving rats. Systemic administration of (S)-UH-301 (1.25, 2.5, 5.0 mg/kg s.c.) dose-dependently decreased extracellular concentrations of DA, DOPAC and HVA in the NAC. (S)-UH-301 (2.5 mg/kg s.c.) also decreased DA, but not DOPAC and HVA, concentrations in the striatum. Infusion of low concentrations (1, 10 microM) of (S)-UH-301 into either the NAC or the striatum did not affect DA levels, while only the highest concentration (1,000 microM) significantly decreased DA levels in both areas. Similarly, infusion of the selective 5-HT1A receptor agonist (R)-8-hydroxy-2-(di-n-propylamino)tetralin [(R)-8-OH-DPAT] only in high concentrations (100, 1,000 microM) decreased DA levels in both regions. These data suggest that (S)-UH-301 decreases DA release both in the NAC and the striatum probably indirectly via its purported DA-D2/D3 receptor agonistic properties. However, the observed inhibitory effect of (S)-UH-301 on DA release in the studied brain regions may also be explained, at least partly, by a serotonergic influence on the DA systems, acting at 5-HT1A receptor sites located elsewhere in the brain.

Effects of D-amphetamine and phencyclidine on behavior and extracellular concentrations of neurotensin and dopamine in the ventral striatum and the medial prefrontal cortex of the rat

The effects of systemically administered phencyclidine (PCP; 2.5 mg/kg, s.c.) and D-amphetamine (1.5 mg/kg, s.c.) on the extracellular concentrations of neurotensin-like immunoreactivity (NT-LI) and dopamine (DA) in the ventral striatum (vSTR) and the medial prefrontal cortex (mPFC) were studied in freely moving rats using microdialysis. In separate animals, the effects of PCP and D-amphetamine on open field activity were also analyzed. PCP, but not D-amphetamine, caused a significant increase (156% over baseline) of NT-LI levels in the vSTR which was relatively short lasting, i.e., of less than 2 h duration. In contrast, both drugs significantly increased NT-LI concentrations in the mPFC by almost 100% during the same period. PCP and D-amphetamine also significantly increased extracellular levels of DA in the vSTR by 83 and 364%, respectively. However, the peak effect of PCP on DA appeared later than that of D-amphetamine, i.e., at 150 and 60 min, respectively, after drug administration. Also in the mPFC, both PCP and D-amphetamine significantly increased DA concentrations by 98 and 284%, respectively. Generally, effects on DA levels of both PCP and D-amphetamine were, in contrast to their effects on NT-LI levels, clearly more long-lasting, i.e., of 3-4 h duration. Behaviorally, D-amphetamine produced a more pronounced, general activation than PCP, with a faster onset of activation, i.e. within 30 vs 90 min after administration. However, both drugs produced long-lasting effects on the spatial organization of behavioral activity, which lasted for 3-4 h. In conclusion, the more pronounced behavioral stimulation by D-amphetamine (1.5 mg/kg, s.c.) vs PCP (2.5 mg/kg, s.c.) in the rat may largely be explained by its more potent DA-releasing effect in the brain. Initial behavioral suppression by PCP, e.g., of rearing, as well as its rather poor locomotor stimulant action in general, might relate to release of NT in the vSTR. The long-lasting, behavioral disorganization by both PCP and D-amphetamine may, however, be related to increased release of DA rather than NT in the mesolimbocortical areas.

On the significance of brain extracellular uric acid detected with in-vivo monitoring techniques: a review

The concentration of uric acid [UA] in the extracellular fluid (ECF) estimated with in-vivo voltammetry and microdialysis data is compared for probes of different diameters from the day of implantation (acute) to several days (chronic) or even months after surgery. For small probes (diameter < 160 microns) the acute [UA] of ca. 5 microM decreased significantly to ca. 1 microM under chronic conditions. For larger probes (e.g., 320-microns diameter) the acute [UA] was also ca. 5 microM, but this value significantly increased to ca. 50 microM under chronic conditions. Associated with this difference in [UA], there were parallel differences in the extent of gliosis around the probes. These findings are discussed in terms of possible sources of extracellular UA and their implications for in-vivo monitoring techniques in behaving animals.

Effect of antidepressants on striatal and accumbens extracellular dopamine levels

The effect of the selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitor, fluoxetine (10 mg/kg s.c.), two tricyclic antidepressants, clomipramine (10 mg/kg s.c.) and imipramine (10 mg/kg s.c.), and vehicle on extracellular dopamine levels was studied in rat nucleus accumbens and striatum by in vivo microdialysis. Fluoxetine produced significant decreases in extracellular dopamine levels in both the nucleus accumbens and striatum (mean maximum percentage decrease: 58% and 57% of pre-drug baseline, respectively). In contrast, imipramine and clomipramine significantly increased extracellular dopamine in the striatum (148% and 150%, respectively) compared to the effect of vehicle alone (118%). These results suggest that the selective serotonin reuptake inhibitor, fluoxetine, and the tricyclic antidepressants, clomipramine and imipramine, affect dopaminergic activity in diverse ways and in a region-specific manner. Thus, the antidepressant effect of these drugs is unlikely to be related to their acute effects on dopaminergic neurotransmission. The differential effects of the selective serotonin reuptake inhibitor and tricyclic antidepressants on extracellular dopamine could account for other differences in their clinical and side effect profiles. Further studies of the chronic effects of the selective serotonin reuptake inhibitor and the tricyclic antidepressants on dopaminergic activity are required to elucidate the role of dopamine in the antidepressant effect.

Behavioural response to SKF 38393 and quinpirole following chronic antidepressant treatment

The effects of chronic administration of antidepressant drugs (21-22 days s.c. via osmotic mini-pumps) on the behavioural responses of male Sprague-Dawley rats to (-)-quinpirole hydrochloride (0.05 mg kg-1 s.c., 5 min) and (+/-)-SKF 38393 hydrochloride (10 mg kg-1 s.c., 5 min) were investigated. Desipramine hydrochloride (10 mg kg-1 per day), phenelzine sulphate (10 mg kg-1 per day) and clorgyline hydrochloride (1 mg kg-1 per day) attenuated the suppression of locomotor activity induced by quinpirole, a dopamine D2-like receptor agonist, while clomipramine hydrochloride (10 mg kg-1 per day) was without effect. Yawning elicited by quinpirole was absent in phenelzine- and clorgyline-treated rats, but unaffected in rats treated chronically with desipramine and clomipramine. SKF 38393, a dopamine D1-like receptor agonist, significantly increased locomotor activity and time spent grooming in control animals. There were no significant effects of antidepressants on the behavioural responses to SKF 38393.

Antidepressant-like effect of selective dopamine D1 receptor agonists in the behavioural despair animal model of depression

We compared the effect of two selective dopamine D1 receptor agonists, SKF 38393 ((+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol.HCl) and A68930 ((1R,3S)-1-aminomethyl-5,6-dihydroxy-3-phenylisochroman.HCl), and that of imipramine in the behavioural despair model of depression. The dopamine D1 receptor agonists and imipramine showed an anti-immobility effect. Moreover we found that the 'antidepressant' effect of imipramine in the behavioural despair test was antagonized by SCH 23390, a selective dopamine D1 receptor blocker. The results further support the hypothesis that dopamine D1 receptor stimulation plays an important role in the mechanism of action of antidepressants and suggest that dopamine D1 receptor agonists might be considered as potential antidepressant drugs.

Local effects of ibogaine on extracellular levels of dopamine and its metabolites in nucleus accumbens and striatum: interactions with D-amphetamine

Systemic administration of ibogaine (40 mg/kg, i.p.) has been reported to induce both acute (1-3 h) and persistent (19-20 h) changes in extracellular levels of dopamine and its metabolites in the nucleus accumbens and striatum. In the present study, local administration of ibogaine to the striatum and nucleus accumbens produced effects that mimicked both the acute and persistent effects of systemic administration: perfusion with high concentrations (200 and 400 microM) of ibogaine mimicked the acute effects (decreased extracellular dopamine levels and increased extracellular metabolite levels) whereas perfusion with a low concentration (10 microM) of ibogaine mimicked the persistent effects (decreased extracellular levels of DOPAC). These results indicate that ibogaine acts directly in brain regions containing dopaminergic nerve terminals and that long-lasting effects of systemically administered ibogaine might be mediated by persisting low levels of ibogaine. Locally administered ibogaine (10 microM) was also found to enhance the effects of systemically administered D-amphetamine (1.25 mg/kg, i.p.) on extracellular dopamine levels, and conversely, systemically administered ibogaine (40 mg/kg, i.p.; 19 h pretreatment) enhanced the effects of locally administered D-amphetamine (1-10 microM). These results indicate that, in addition to a metabolic mechanism implicated previously, a pharmacodynamic mechanism contributes to the interaction between ibogaine and D-amphetamine. The relevance of such mechanisms to claims regarding ibogaine's anti-addictive properties is unclear.

Chronic antidepressant drug treatment attenuates motor-suppressant effects of apomorphine without changing [3H]GBR 12935 binding

The effects of chronic administration (28 days s.c. via osmotic minipumps) of the antidepressants phenelzine sulphate, desipramine hydrochloride and clomipramine hydrochloride (each at 10 mg/kg per day) on dopamine function have been measured in rats. Both phenelzine and desipramine attenuated the suppression of locomotor activity induced by apomorphine hydrochloride (0.05 mg/kg s.c. 15 min). Clomipramine did not affect the behavioural response to apomorphine. Analyses of brain tissue from these animals using the radioligand [3H]GBR 12935 revealed that there were no changes in dopamine uptake site density or affinity following the administration of phenelzine, desipramine or clomipramine. Analyses of brain monoamine oxidase activity and tricyclic levels were used to confirm the efficacy of the drug administration protocol. These data indicate that changes in dopamine uptake site density do not mediate antidepressant-induced changes in behavioural responses to apomorphine.

Depletion of mesolimbic dopamine during behavioral despair: partial reversal by chronic imipramine

Exposure of rate to the behavioral despair test (an animal model of depression) for 40 min resulted in a long-lasting depletion of mesolimbic dopamine output to about 40% of baseline values. The decrease in extracellular dopamine was partially prevented by chronic pretreatment with imipramine (20 mg/kg per day i.p. for 21 days). The results suggest that a fall in mesolimbic dopamine output may be associated with depressive states and indicate that changes in the functional status of the dopamine system contribute to the mechanism of action of imipramine.

Effects of amperozide on psychostimulant-induced hyperlocomotion and dopamine release in the nucleus accumbens

N-Ethyl-4-[4',4'-bis(p-fluorophenyl)-butyl]-1-piperazine carboxamide [amperozide (APZ)] is a novel atypical neuroleptic that appears to selectively act on the limbic system. The present study investigated behavioral and biochemical effects of APZ on either d-amphetamine (AMPH)- or cocaine (COC)-treated rats. Behavior was assessed by locomotor activity measurements. Compared to saline controls, APZ (5 and 10 mg/kg, SC) decreased spontaneous locomotion. AMPH (1.0 mg/kg, SC)- or COC (10 mg/kg, IP)-induced hyperlocomotion was markedly reduced by APZ administered 20 min earlier. Biochemical data were obtained by in vivo microdialysis in freely moving animals. APZ dose dependently increased interstitial concentrations of dopamine (DA, +25%) and its metabolite, homovanillic acid (HVA, +20%), in the nucleus accumbens (NAC). While either AMPH or COC alone increased DA levels (450 and 270%, respectively), pretreatment with APZ had no effect on these increases. In contrast, APZ pretreatment dose dependently attenuated the reduction of DA metabolites induced by both AMPH and COC. Thus, APZ blocked hyperlocomotion induced by psychostimulants without producing correlative changes in DA concentrations in the NAC.

Effects of chronic antidepressant treatment on dopamine-related [3H]SCH 23390 and [3H]spiperone binding in the rat striatum

1. The effects of chronic administration of antidepressants on dopamine-related [3H]SCH 23390 and [3H]spiperone binding to rat striatal membranes were assessed. 2. The monoamine oxidase inhibitors phenelzine (5 or 10 mg kg-1/day) and tranylcypromine (1 mg kg-1/day) and the tricyclic desipramine (10 mg kg-1/day) were administered for 28 days by constant subcutaneous infusion using Alzet (2ML4) osmotic minipumps. 3. These treatments did not alter Kd estimates for either [3H]SCH 23390 or [3H]spiperone binding sites. The monoamine oxidase inhibitors induced a decrease in the Bmax values for both [3H]SCH 23990 and [3H]spiperone binding sites. Desipramine induced a decrease in the Bmax value for [3H]SCH 23390 binding but had no effect on the Bmax value for [3H]spiperone binding.

The NMDA receptor antagonist MK-801 prevents imipramine-induced supersensitivity to quinpirole

Chronic treatment with imipramine enhanced the locomotor stimulant response to quinpirole (0.3 mg/kg s.c.), a dopamine D2/D3 receptor agonist. Chronic, but not acute, blockade of the NMDA receptor with the non-competitive antagonist MK-801 (0.3 mg/kg i.p.) prevented the imipramine-induced potentiation of the quinpirole effect. The results suggest that NMDA receptors play a role in the development of supersensitivity to dopamine receptor agonists produced by chronic antidepressant treatment.

Effects of acute and chronic desipramine treatment on somatostatin receptors in brain

The effects of acute (5 mg/kg, IP twice daily for 2 days) and chronic (5 mg/kg IP twice daily for 21 days) administration of desipramine (DMI) on [125I]-Tyr11-somatostatin binding sites in brain were examined. There was no change in [125I]Tyr11-somatostatin binding in membranes prepared from the frontal cortex, striatum, and hippocampus of rats acutely or chronically treated with DMI as compared to non treated animals. [125I]Tyr11-somatostatin binding was increased in membranes prepared from the rat nucleus accumbens only after chronic DMI administration. Scatchard analysis of the binding data from the nucleus accumbens showed that [125I]Tyr11-somatostatin labels a single population of somatostatin binding sites with an affinity constant, Kd, of 1.8 +/- 0.60 nM and a Bmax of 330 +/- 90 fmol/mg protein. Chronic treatment with DMI increased the Bmax (500 +/- 140 fmol/mg protein) but had no effect on the Kd. This finding shows a regional effect of DMI on [125I]Tyr11-somatostatin binding sites in rat brain and suggests that somatostatin may play a role in the pathophysiology of depression.

Chronic desipramine enhances the effect of locally applied amphetamine on interstitial concentrations of dopamine in the nucleus accumbens

In vivo microdialysis was used to study the effect of chronic desipramine (DMI, 5 mg/kg, twice daily for 21 days) on increases in interstitial dopamine (DA) produced by local administration of d-amphetamine (1.0, 3.3 and 10.0 microM) in the nucleus accumbens. Locally applied amphetamine increased interstitial DA in a dose-dependent manner. The amphetamine-induced increase was significantly greater in the DMI treated animals. These data suggest that chronic DMI may directly influence the functional status of the DA terminals in the nucleus accumbens.

Effects of chronic electroconvulsive shock on interstitial concentrations of dopamine in the nucleus accumbens

There is accumulating evidence that chronic electroconvulsive shock (ECS) can increase the functional output of central dopaminergic systems. The present experiments investigated the effects of acute and chronic ECS on interstitial concentrations of dopamine (DA) in the nucleus accumbens (NAC) using in vivo microdialysis in awake freely moving rats. ECS (150 V, 0.75 s) increased interstitial concentrations of DA, DOPAC and HVA to approximately 130% of baseline values. The magnitude of the ECS-induced increase in DA was not affected by chronic ECS. In contrast, the response of the DA metabolites was attenuated in the chronic ECS group. Chronic ECS did not influence apomorphine (25 micrograms/kg, SC)-induced decreases in extracellular concentrations of DA or its metabolites in the NAC, thus providing no support for the hypothesis that chronic ECS produces subsensitivity of DA autoreceptors. d-Amphetamine (1.5 mg/kg SC)-induced increases in extracellular DA were significantly prolonged in the NAC of the chronic ECS group. In accordance with previous reports, the locomotor stimulant effects of d-amphetamine were also enhanced in the chronic ECS group. These data provide further evidence that chronic ECS can increase certain behavioral and neurochemical indices of meso-accumbens DA function.