Effects of chronic neuroleptic treatment on dopamine release: insights from studies using 3-methoxytyramine

Chronic Exposure to WIN55,212-2 During Adolescence Alters Prefrontal Dopamine Turnover and Induces Sensorimotor Deficits in Adult Rats

Several lines of evidence suggest that chronic exposure to cannabinoids during adolescence may increase the risk of schizophrenia. Studies of the disorder have identified altered cortical dopaminergic neurotransmission. In this study, we hypothesised that heightened endocannabinoid system activation via chronic exposure to a highly potent cannabinoid receptors agonist in adolescent rats would cause long-lasting neurobiological changes that may dramatically alter expression and functions of dopamine metabolising enzymes, comethyl-o-transferase (COMT) and monoamine oxidases MAO-A and MAO-B. To test this hypothesis, adult male rats (70 PND) undergoing chronic treatment of the highly potent and non-selective CB agonist WIN55,212-2 (1.2 mg/kg) during adolescence (PND 30-50) were subjected after 20 days washout period to prepulse inhibition of acoustic startle test (PPI) to confirm cannabinoid-induced sensorimotor-gating impairments and afterwards examined for COMT, MAO-A and MAO-B expression and activity in the prefrontal cortex. Chronic WIN55,212-2 exposure during adolescence caused disruption of PPI, increased cortical dopamine level, decreased COMT mRNA expression and decreased MAO-A and MAO-B enzymatic activities. These results indicate that chronic exposure to cannabinoids during adolescence induces sensorimotor-gating alterations which likely result from changes in the prefrontal cortex dopaminergic signalling. This has important implications for developing methods of targeting dopamine metabolising enzymes and/or sequelae of its dysregulation in cannabinoid-induced schizoaffective-like behaviour.

Nur77 gene knockout alters dopamine neuron biochemical activity and dopamine turnover

BACKGROUND

Transcription factors of the Nur family (Nurr1, Nur77, and Nor-1) are orphan nuclear receptors closely associated with dopamine neurotransmission in the central nervous system. Nur77 expression is strongly modulated by antipsychotic and ant-parkinsonian drugs in dopaminoceptive brain areas. However, the role of Nur77 in dopamine neuron activity and turnover remains elusive.

METHODS

We compared various behavioral and biochemical parameters between Nur77 knockout -/- and wild-type +/+ mice in basal and haloperidol-challenged conditions.

RESULTS

We report here that Nur77-deficient mice display enhanced spontaneous locomotor activity, greater sensitivity to a small dose of the dopamine D2 receptor agonist quinpirole acting mainly at autoreceptor sites, and higher levels of the dopamine metabolite DOPAC relative to wild-type mice. Dopamine turnover disturbances are also found after acute challenge with haloperidol, a dopamine D2 receptor antagonist. These alterations are associated with increased tyrosine hydroxylase expression and activity, and reduced catechol-O-methyltransferase expression.

CONCLUSION

Taken together, these results are consistent with the involvement of Nur77 in dopamine neuron biochemical activity and dopamine turnover.

Genetic susceptibility of mesocortical dopamine to stress determines liability to inhibition of mesoaccumbens dopamine and to behavioral 'despair' in a mouse model of depression

Clinical and preclinical research suggests a major role of mesocortical dopamine (DA) in psychopathology through regulation of subcortical, especially mesoaccumbens, DA functioning. In these experiments we demonstrate that the high vulnerability to stress-induced 'despair' and mesoaccumbens DA inhibition, exhibited by mice of the inbred strain C57BL/6 (C57) in a common animal model of depression, depends on their being highly susceptible to stress-induced mesocortical DA activation. Thus, C57 mice but not mice of the DBA/2 strain showed an extremely high level of immobility on their first experience with the forced swimming test (FST) as well as immediate and strong activation of mesocortical DA metabolism and inhibition of mesoaccumbens DA metabolism and release. In addition, the behavioral and the mesoaccumbens DA responses to FST in C57 mice were reduced and reversed, respectively, by bilateral mesocortical DA depletion. Finally, chronic treatment with the antidepressant clomipramine reduced immobility and eliminated both mesocortical DA activation and mesoaccumbens DA inhibition in response to FST. These results suggest that a genetically determined susceptibility to stress by the mesocortical DA system may favor the development of pathological behavioral responses through inhibition of subcortical DA transmission.

Predictable stress promotes place preference and low mesoaccumbens dopamine response

Aversive stimuli that are signaled, and therefore predictable, are preferred to unsignaled ones and promote less severe stress-related disturbances. Since stressful events are known to activate mesoaccumbens dopamine (DA) transmission, in the present experiments, we evaluated possible differences in mesoaccumbens DA response to predictable and unpredictable footshocks. Mice of the inbred strain DBA/2 were trained for conditioned place preference (CPP) in shuttle boxes. The procedure promoted significant preference for the compartment previously paired with predictable shocks (PR) to that paired with unpredictable shocks (NP). Mesoaccumbens levels of DA and its metabolites were therefore evaluated either after the first or the last (third) training session. A significant increase of 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) levels were observed in animals exposed for the first time to the apparatus without shock delivery (SHAM) or to the PR and NP conditions compared with unhandled mice. There was no difference between PR and NP values, and DOPAC and HVA levels in both groups differed from those observable in the SHAM-exposed group. However, trained mice exposed to NP showed a significant elevation of DOPAC and HVA levels in comparison with those exposed to PR. These results show that information about predictability of aversive stimuli reduces central stress responses and suggest a possible relationship between reduced stressfulness and preference for predictable aversive experiences.

Opposite imbalances between mesocortical and mesoaccumbens dopamine responses to stress by the same genotype depending on living conditions

The balance between mesocortical and mesoaccumbens dopamine (DA) response to stress may represent a major diathesis in psychopathology. These experiments evaluated the influence of variable living conditions on this phenotype and on behavioral coping. Mesocortical and mesoaccumbens DA responses to stress challenge (restraint) were analyzed in individually housed or food restricted mice of an inbred strain to control for genotype-dependent variability. Mice housed in groups with free access to food were used as controls. Little or no differences among the three conditions were found for basal mesoaccumbens and mesocortical DA and metabolite levels. Stress challenge promoted parallel activation of mesocortical and mesoaccumbens DA metabolism and release in group-housed mice. Individually housed mice showed enhanced mesocortical and reduced mesoaccumbens response to stress challenge. Instead, food restricted mice showed no response by the mesocortical DA system and enhanced mesoaccumbens DA response. Finally, the two differential housing conditions promoted opposite alterations of the behavioral profile exhibited by mice exposed to the forced swimming test. These results indicate opposite imbalances between mesocortical and mesoaccumbens DA responses to stress in intact, drug-naive animals, point to a strict relationship between these unbalanced responses and behavioral coping with aversive events and indicate that central and behavioral responses to stress are highly dependent on individual experiences.

Opposite genotype-dependent mesocorticolimbic dopamine response to stress

Identification of relevant phenotypes related to neural functioning has yet to receive the needed attention, although behavioral phenotyping, through comparative studies in inbred strains of mice, has produced some major findings (Cabib et al., 2000; Crabbe, 1999; Gerlai, 1996; Lathe, 1996). Central responses to stress play a major psychopathogenic role in the presence of a genetic liability (Fowels, 1992), and mesocortical and mesoacumbens dopamine metabolism and release are the most relevant among these responses (Abercrombie et al., 1989; Cabib and Puglisi-Allegra, 1994; Chrapusta et al., 1997; Di Chiara et al., 1999; Hervé et al., 1979; Imperato et al., 1991). Therefore, in the present study, we assessed strain-dependent differences in mesocortical and mesoaccumbens dopamine responses to a widely utilized stressful procedure (restraint), by comparing mice of the oldest and most studied inbred strains (Cabib et al., 2000): the C57BL/6JIco and DBA/2JIco. We found that stress produced inhibition of mesoaccumbens dopamine release accompanied by a very fast and strong activation of mesocortical dopamine metabolism in C57BL/6JIco mice, and the opposite in mice of the DBA/2JIco strain. These results suggest a genetic control over the balance between mesocortical and mesoaccumbens dopamine responses to stress, and provide a model for pre-clinical studies on molecular genetics of depression.

Changes in the pattern of brain-derived neurotrophic factor immunoreactivity in the rat brain after acute and subchronic haloperidol treatment

Our earlier work has shown that repeated administration of classical neuroleptic drugs gives rise to structural alterations in target regions of the mesolimbic pathway, most notably, nucleus accumbens. Such changes could be responsible for the efficacious or motor side effects associated with these drugs. Growth factors such as brain-derived neurotrophic factor (BDNF) provide trophic support for dopaminergic neurons during development and mediate synaptic and morphological plasticity in numerous regions of the adult CNS. The present study examines whether BDNF is altered in the mesolimbic pathway by classical neuroleptic treatment. Animals were administered haloperidol, 0.5 mg/kg, or vehicle, i.p., for either 3 or 21 days, followed by transcardiac perfusion with fixative. Three days of haloperidol administration dramatically decreased BDNF immunostaining in the neurons and fibers of the prefrontal cortex, hippocampus (dentate gyrus, CA2, and CA3), extended amygdala, and ventral tegmental area. BDNF-immunoreactive fibers virtually disappeared from the neostriatum and nucleus accumbens. Subchronic (21 days) treatment led to a rebound in BDNF immunoreactivity in most cell bodies but not in fibers. These results show that blockade of dopaminergic receptors with haloperidol rapidly downregulates BDNF in reward and emotional centers of the brain. Such rapid inactivation and subsequent reappearance of BDNF immunoreactivity could affect synaptic strength and plasticity and therefore be important preliminary steps in the cascade of neuronal events that lead to the efficacious or detrimental side effects of classical neuroleptic drugs.

Effects of smoking during antipsychotic withdrawal in patients with chronic schizophrenia

A number of studies have shown that patients with schizophrenia smoke more than other psychiatric patients and more than the general population. Also, medicated schizophrenics who smoke present more positive symptoms of schizophrenia than non-smokers. The objective of the present study was to assess the effect of smoking on ratings of psychopathology for 30 days following discontinuation of antipsychotic medication. The subjects were 101 treatment-resistant patients with schizophrenia who had been admitted to the inpatient service of Neuroscience Research Hospital (NRH), National Institute of Mental Health, between 1982 and 1994 to undergo studies involving discontinuation of antipsychotic medication. Patients were rated independently on a daily basis on the 22-item Psychiatric Symptom Assessment Scale (PSAS), an extended version of the Brief Psychiatric Rating Scale (BPRS). At baseline, ratings for Verbal Positive, Paranoia and Loss of Function were higher in smokers (n=65) than non-smokers (n=36), but a statistically significant difference was observed only for the Verbal Positive cluster. Analysis by gender revealed that male non-smokers had the lowest psychopathology ratings at baseline. There were no differences in Anxiety/depression, Behavior Positive, Deficit Symptoms or Mannerisms (a measure for abnormal involuntary movements). Following medication discontinuation, repeated-measure analysis demonstrated a 'time' effect for all the variables studied and a 'group' (smokers vs. non-smokers) effect for Verbal Positive, Paranoia, and Loss of Function. Post-hoc comparisons at individual time points showed significantly higher ratings for smokers at week 1 for Paranoia. No differences were observed at later time points. In conclusion, at baseline, smokers had more positive symptoms and were apparently more functionally impaired than non-smokers. This difference was no longer evident after a 30 day medication discontinuation period.

Behavioral and mesocorticolimbic dopamine responses to non aggressive social interactions depend on previous social experiences and on the opponent's sex

In these experiments we evaluated the relationship between behavioral and brain dopamine (DA) responses to social interactions. Subjects were group housed male mice confronted with a non aggressive male or female conspecific following either repeated defeat (defeated) or repeated non aggressive experiences (social). Defeated mice showed more defensive/submissive reactions then mice of the social group regardless of the opponent sex. However, mice defeated by females showed reduced social exploration without significant differences in non social exploration whilst the opposite was true for mice defeated by male opponents. Non aggressive social interactions enhanced dopamine metabolism in the prefrontal cortex (pFC) of DEFEATED mice regardless of opponent sex. However, only mice defeated by females showed enhanced dopamine metabolism and release in the nucleus accumbens septi (NAS) and olfactory tubercle (OT) following interaction with the non aggressive opponent. Finally, correlation between central and behavioral responses evidenced that 3,4-dihydroxiphenilacetic acid levels in the pFC were positively correlated with defensive behaviors and negatively correlated with non social exploration in mice confronted with male opponents but not in those confronted with females. The latter, showed a significant positive correlation between 3-methoxytyramine (3-MT) levels in the OT and defensive responses and significant negative correlation between social investigation and 3-MT levels in the OT and in the NAS. These results indicate a strict relationship between mesocorticolimbic dopamine transmission and behavior responses to social cues. Moreover, they strongly support the view that mesocorticolimbic DA modulates social behavior by affecting perceptive processing.

Striatal extracellular dopamine levels in rats with haloperidol-induced depolarization block of substantia nigra dopamine neurons

Correlations between substantia nigra (SN) dopamine (DA) cell activity and striatal extracellular DA were examined using simultaneous extracellular single-unit recordings and in vivo microdialysis performed in drug-naive rats and in rats treated repeatedly with haloperidol (HAL). Intact rats treated with HAL for 21-28 d exhibited significantly fewer active DA cells, indicating the presence of depolarization block (DB) in these cells. However, in rats that received surgical implantation of the microdialysis probe followed by a 24 hr recovery period, HAL-induced DA cell DB was reversed, as evidenced by a number of active DA neurons that was significantly higher than that in HAL-treated intact rats and similar to that of drug-naive rats. In contrast, using a modified probe implantation procedure that did not reverse SN DA neuron DB, we found striatal DA efflux to be significantly lower than in controls and significantly correlated with the reduction in DA neuron spike activity. Furthermore, although basal striatal DA efflux was independent of SN DA cell burst-firing activity in control rats, these variables were significantly correlated in rats with HAL-induced DA cell DB. Therefore, HAL-induced DB of SN DA neurons is disrupted by implantation of a microdialysis probe into the striatum using standard procedures. However, a modified microdialysis method that allowed reinstatement of DA neuron DB revealed that the HAL-induced inactivation of SN DA neurons was associated with significantly lower extracellular DA levels in the striatum. Moreover, the residual extracellular DA maintained in the presence of DB may, in part, depend on the burst-firing pattern of the noninactivated DA neurons in the SN.

Dose-response relationships for the antipsychotic effects and Parkinsonian side-effects of typical neuroleptic drugs: practical and theoretical implications

1. From a review of published literature it is concluded that the minimum dose of a neuroleptic drug (NLD) required to alleviate psychosis is very similar to that producing minimal parkinsonian side effects (PSE). This conclusion is reached both from group comparisons and individual comparisons of dose/response relations (DRR) for the two effects. 2. A lower dose of NLD is usually sufficient to prevent relapse in well stabilized patients than is needed to check an active psychotic state. 3. Anticholinergic agents used to reduce side effects of typical NLD can retard the therapeutic process during neuroleptic treatment of acute psychosis. Although it is not fully established that this is a central interaction, it is consistent with the idea that minimal side effects are a necessary condition for therapeutic effectiveness with typical antipsychotic drugs. 4. In relapse-free maintenance of psychosis-prone patients, tolerance occurs to PSE. Thus few patients need experience prolonged side effects during maintenance treatment with neuroleptics. 5. The evidence reviewed is discussed with respect to a previous hypothesis of the supposedly "indirect" action of typical neuroleptic drugs in therapy for psychosis. The evidence is consistent with the idea of a close causal relation between minimal PSE of these drugs, and their therapeutic effectiveness in the acute stage of treatment.