Enhancement of self-stimulation behavior in rats and monkeys after chronic neuroleptic treatment: Evidence for mesolimbic supersensitivity

The effect of chronic neuroleptic drug treatment on self-stimulation of the mesolimbic dopamine system was tested. Rats with electrodes implanted into the ventral tegmental nucleus (A10 cell body area) were treated with haloperidol for three weeks. Afterwards, the rats showed a 35% increase in self-stimulation rate, as compared to pre-drug control rates. This increase persisted for three weeks after drug withdrawal before returning to baseline rates. Rats treated for three weeks with the atypical neuroleptic, clozapine, also showed an increase, the duration and magnitude of which was similar to that seen in the haloperidol group. In addition, four rhesus monkeys with electrodes in the nucleus accumbens (one of the terminal projection areas of the A10 mesolimbic dopamine system) were given a three week treatment with haloperidol, after which all animals showed a significant, long-lasting decrease in self-stimulation threshold, as measured by a rate-independent reward paradigm. Taken together, these results suggest the induction of receptor supersensitivity in the mesolimbic dopamine system by long-term treatment with neuroleptic drugs.     

Increase in mesolimbic electrical self-stimulation after chronic haloperidol: reversal by L-DOPA or lithium

After chronic neuroleptic drug treatment, an increase in electrical intracranial self-stimulation (ICSS) rate is seen from electrodes in the A10 dopaminergic nucleus. This increase, which persists for approximately 3 weeks following drug withdrawal, is believed to represent a behavioral manifestation of drug-induced dopaminergic synaptic supersensitivity. Chronic L-DOPA caused a partial reversal of haloperidol-induced ICSS increase. Lithium carbonate, given concurrently with the haloperidol, partially prevented the development of ICSS supersensitivity. It is concluded that dopaminergic synaptic sensitivity has a two-way modulatory capability.     

Mesolimbic dopamine function is not altered during continuous chronic treatment of rats with typical or atypical neuroleptic drugs

Summary Rats were treated continuously for up to 20 months with either haloperidol (1.4–1.6 mg/kg/day), sulpiride (102–109 mg/kg/day) or clozapine (24–27 mg/kg/day). B_max for specific mesolimbic binding of³H-spiperone,³H-N,n-propylnorapomorphine or³H-piflutixol did not differ in tissue taken from animals treated for up to 12 months with haloperidol, sulpiride or clozapine by comparison to age-matched control rats. Mesolimbic dopamine (50 M)-stimulated adenylate cyclase activity was not altered in any drug treatment group. Spontaneous locomotor activity was transiently decreased during treatment with haloperidol for 1 or 3 months, but not by chronic sulpiride or clozapine treatment. Locomotor activity was not consistently increased in any drug treatment group. After 20 months of continuous drug treatment, focal bilateral application of dopamine (12.5 or 25 g) into the nucleus accumbens caused equivalent increases in locomotor activity in control rats and in animals receiving haloperidol, sulpiride or clozapine.These findings suggest that dopamine receptor blockade is not maintained in the mesolimbic area following chronic treatment with haloperidol, sulpiride or clozapine, and indicate that, under these conditions, clozapine and sulpiride may not act selectively on mesolimbic dopamine receptors.     

Tyrosine hydroxylase mRNA and protein are down-regulated by chronic clozapine in both the mesocorticolimbic and the nigrostriatal systems

The dopaminergic system is one of the most important targets for pharmacological treatment of schizophrenia. Despite substantial work on mechanisms of action, it is not clear which dopaminergic pathways mediate the therapeutic effects of antipsychotic drugs. It has been shown that chronic clozapine, an atypical antipsychotic, decreases dopamine levels in the mesocorticolimbic system but not in the nigrostriatal system. Because tyrosine hydroxylase is the rate-limiting enzyme in the biosynthesis of dopamine, we studied the effect of chronic clozapine in both dopaminergic systems. We demonstrated a decrease of tyrosine hydroxylase mRNA not only in the ventral tegmental area but also in the substantia nigra, the cell body areas of the mesocorticolimbic and the nigrostriatal systems, respectively. The reduced tyrosine hydroxylase mRNA level in these areas is accompanied by an ample reduction in the tyrosine hydroxylase protein level in their corresponding axonal terminal fields, the nucleus accumbens and the striatum. There was thus discordance between the clozapine-induced decrease of tyrosine hydroxylase mRNA and protein and the absence of an effect on dopamine levels in the nigrostriatal system. It has been suggested that reduced levels of dopamine in the mesocorticolimbic system are required for the antipsychotic effect of the drug. Therefore, the modulation of tyrosine hydroxylase gene expression by clozapine in the mesocorticolimbic system might be necessary for its antipsychotic effect; this effect might be of relevance when considering new atypical agents.     

Effects of acute and chronic clozapine on dopaminergic function in medial prefrontal cortex of awake, freely moving rats

We previously showed that chronic administration of the clinically atypical and clinically superior antipsychotic drug clozapine selectively reduces dopamine (DA) release in the nucleus accumbens but not neostriatum, and that this effect appears mediated by anatomically selective mesolimbic DA depolarization blockade. The present study extends that research to another mesocorticolimbic DA locus, the medial prefrontal cortex. Acute clozapine challenge (5-40 mg/kg i.p.) produced dose-dependent increased extracellular levels of DA and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), in the medial prefrontal cortex of awake, free-moving rats as measured by in vivo brain microdialysis. Chronic clozapine treatment (20 mg/kg/day for 21 days) did not significantly change basal extracellular levels of DA, DOPAC or HVA. Acute clozapine challenge on day 22 in the chronic clozapine-treated animals produced no significant differences in medial prefrontal cortex DA, DOPAC or HVA as compared to chronic vehicle-treated animals, indicating that tolerance to clozapine does not develop in the mesocortical DA system, in contrast to the mesolimbic system. The DA agonist apomorphine (100 micrograms/kg) produced decreased basal extracellular levels of DA, DOPAC and HVA in medial prefrontal cortex of both chronic clozapine-treated and chronic vehicle-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Neurochemical correlates of brain-stimulation reward measured by ex vivo and in vivo analyses

Evidence from ex vivo analyses of dopaminergic function following self-stimulation behavior is reviewed and compared to in vivo analyses of extracellular dopamine measured by chronoamperometry during self-stimulation. Both data bases provide strong support for a dopaminergic substrate for brain-stimulation reward obtained by electrical stimulation of the ventral tegmental area (VTA). Data obtained from in vivo measures of dopamine release are particularly compelling as a positive correlation was observed between the rate/intensity function for self-stimulation and increments in the oxidation current for dopamine. An examination of the effects of the dopamine uptake blockers, cocaine and GBR 12909 on self-stimulation and stimulated release of dopamine revealed a facilitation of both measures. In contrast, the noradrenaline uptake blocker desipramine had no effect on either self-stimulation or extracellular dopamine. These pharmacological experiments also are consistent with a dopaminergic substrate of brain-stimulation reward at electrode sites in the VTA.     

Dopaminergic substrates of intracranial self-stimulation in the caudate-putamen.

An extensive mapping of the caudate-putamen in rat for intracranial self-stimulation (ICS) site was undertaken to provide addtional support for the role of dopamine in brain-stimulation reward. Eight-seven per cent of the placements in the neostriatum supported ICS, with self-stimulation rates greater than 250/15 min at 56% of these sites. Electrical stimulation also elicited rearing and clonus, and contralateral body turn, both of which varied in magnitude between animals. In a second experiment, animals were prepared with electrodes aimed at the lateral caudateputamen. Those subjects displaying ICS subsequently received 6-hydroxydopamine lesions to the dopamine cell bodies in the substantia nigra pars compacta, either ipsilateral or contralateral to the electrode. The destruction of the dopamine cell bodies attenuated ICS in both groups during the first post-lesion test sessions. However, the rates in the ipsilateral group declined to between 2 and 9% of control scores, whereas the rate in the contralateral group improved over testing to 72% of control values, 28 days after the lesion. On the basis of these data, it was concluded that unilateral destruction of the dopaminergic nigro-neostriatal bundle (NSB) has two effects on ICS behavior. First, unilateral reduction of neostriatal dopamine is accompanied by a loss of brain-stimulation reward at sites normally innervated by the NSB, specifically the caudate-putamen. Secondly, lesions of the NSB produce a general disruption in bar-pressing behavior, as evidenced by the attenuation of ICS following contralateral lesions.     

Characterization of [3H]clozapine binding sites in rat brain

Summary We examined the characteristics of [³H]clozapine binding sites in four rat brain regions (frontal cortex, limbic area, hippocampus and striatum) in order to elucidate the pharmacological profile of this unique atypical antipsychotic drug. The specific [³H]clozapine binding was found to be saturable and reversible in all these brain regions. Scatchard analysis of the saturation data indicated that the specific binding consisted of high- and low-affinity components. Displacement experiments showed that the muscarinic cholinergic receptor represented about 50% of [³H]clozapine binding in each brain area. Serotonin 5-HT₂ and dopamine D₄ receptor binding sites could also be detected by displacement experiments using ketanserin and nemonapride, respectively, in frontal cortex and limbic area, but not in hippocampus or striatum. Alpha-1, alpha-2, histamine H₁, dopamine D₁, D₂, or D₃ receptor components could not be determined within the high-affinity [³H]clozapine binding sites in any brain region. It is possible that the atypical property of clozapine may depend on the modulatory effect on dopaminergic function via 5-HT₂ receptor blockade and/or may be mediated via D₄ receptor blockade in the mesocortical and mesolimbic area.     

Prefrontal cortex and neostriatum self-stimulation in the rat: Differential effects produced by apomorphine

In a dose-response experiment, the effects of intraperitoneal injections of the dopamine receptor agonist, apomorphine (0.075, 0.15, 0.3, 0.6 and 1.2 mg/kg) were studied on self-stimulation elicited from electrodes implanted in the medial and sulcal prefrontal cortex and caudate-putamen in the rat. From the medial and sulcal prefrontal cortex electrodes, apomorphine produced a dose-related decrease of self-stimulation rate which was consistent across animals. From the caudate-putamen electrodes, on the contrary, apomorphine produced a facilitatory effect in the majority of the animals at one or more doses, however, at other doses a decreased self-stimulation rate was observed. The clear and consistent effects of apomorphine on self-stimulation of the prefrontal cortex, together with other experimental evidence in the same line, suggest that dopamine is mediating self-stimulation of this cortical area.     

Antagonism of footshock stress-induced inhibition of intracranial self-stimulation by naloxone or methamphetamine

Rats were trained to lever-press for intracranial self-stimulation (ICSS) with electrodes implanted in the ventral tegmental area (VTA). The effect of inescapable footshock on response rates to ICSS was examined in the present study. Markedly decreased response rates to ICSS were observed 15 min to 24 h following inescapable footshock. Naloxone (10.0 mg/kg) itself was without effect on response rates to ICSS, but completely antagonized the decreased response rates by the stressor treatment. A relatively low dose of methamphetamine (0.5 mg/kg), which showed no effect on ICSS rates in naive rats, also antagonized the decreased response rates to ICSS. The present results suggest that inescapable footshock may release endorphin in the mesolimbic or mesocortical area; the released endorphin may act on dopaminergic nerve endings and interrupt dopaminergic transmission. The decreased activity of dopaminergic neurons may cause the decreased response rates to ICSS.     

Chronic clozapine, but not haloperidol, alters the response of mesoprefrontal dopamine neurons to stress and clozapine challenges in rats.

Previously, we demonstrated that serotonin-lesioned rats had an enhanced mesoprefrontal dopaminergic response to restraint stress. This study attempted to extend our knowledge regarding this serotonin/dopamine interaction by seeing if suppression of serotonin metabolism by chronic administration of the atypical antipsychotic, clozapine, would have similar effects. Both typical and atypical neuroleptics require chronic administration in humans before antipsychotic activity is seen. Rats treated for 21 days with clozapine or haloperidol, a typical antipsychotic without significant binding affinity for serotonergic receptors, showed lowered basal dopamine metabolism in the medial prefrontal cortex, the nucleus accumbens, and the striatum, as expected. Basal serotonin metabolism in the prefrontal cortex was also lowered by clozapine treatment, but not haloperidol. One of two challenges were given to chronically treated rats: 30 min of restraint stress or an acute challenge of clozapine. When corrected for baseline differences, both challenges significantly elevated dopamine metabolism in the prefrontal cortex of the clozapine group more than the saline or haloperidol groups. No hyperresponsiveness was seen with serotonin metabolism in the prefrontal cortex or either dopamine or serotonin metabolism in the nucleus accumbens in clozapine-treated, challenged rats. Additionally, this augmentation of the dopaminergic stress response was not seen with a single, acute administration of clozapine. The significance of the clozapine-induced hyperresponsiveness of the mesoprefrontal dopamine system is discussed with regard to clinical efficacy of clozapine.     

Dopamine and serotonin metabolism in response to chronic administration of fluvoxamine and haloperidol combined treatment

Summary Treating primary ‘negative symptoms’ of schizophrenia with a combination of a typical antipsychotic and a selective serotonin reuptake inhibitor, is more effective than with antipsychotic alone and is similar to the effect of the atypical antipsychotic, clozapine. The mechanism of this treatment combination is unknown and may involve changes in dopaminergic and serotonin systems. We studied dopamine and serotonin metabolism in different rat brain areas at 1.5 and 24 h after the last dosage of chronic treatment (30 days), with haloperidol plus fluvoxamine, each drug alone, and clozapine. Haloperidol-fluvoxamine combination, haloperidol, and clozapine treatments increased striatal and frontal cortex dopamine turnover and reduced striatal tyrosine hydroxylase activity at 1.5 h. At 24 h both dopamine turnover and tyrosine hydroxylase activity were reduced. Thus, in chronically treated animals, release of striatal dopamine increases following a drug pulse and returns to baseline by 24 h. Serotonin and 5-hydroxyindoleacetic acid concentrations were decreased at 1.5 h in haloperidol-fluvoxamine and clozapine groups and returned to normal levels by 24 h. A limited behavioral assessment showed that treatment with haloperidol plus fluvoxamine reduced motor activity compared to haloperidol, and increased sniffing compared to haloperidol, fluvoxamine and clozapine. These findings indicate that combining antipsychotic with SSRI results in specific changes in dopaminergic and serotonergic systems and in behavior. The possibility that these may be relevant to the mechanism underlying the clinical effectiveness of augmentation treatment warrant further study. An erratum to this article is available at .     

Self-stimulation of the nucleus accumbens and ventral tegmental area of tsai attenuated by microinjections of spiroperidol into the nucleus accumbens

The contribution of dopaminergic neurons to self-stimulation of the ventral tegmental area, nucleus accumbens and prefrontal cortex was investigated. The ventral tegmental area is the site of non-striatal dopaminergic neurons and their axons project to the nucleus accumbens and prefrontal cortex. Injections of spiroperidol, a dopamine antagonist, into the nucleus accumbens significantly reduced self-stimulation of the ipsilateral ventral tegmental area but did not influence self-stimulation of the contralateral ventral tegmental area. Injections of spiroperidol into the prefrontal cortex did not reduce self-stimulation of the ipsilateral or contralateral ventral tegmental area. Electrical stimulation of sites in the nucleus accumbens positive for self-stimulation antidromically activated neurons of the ventral tegmental area, and a reduction of discharge of these neurons following administration of apomorphine suggested that they were dopaminergic neurons. These observations provide additional evidence implicating dopaminergic neurons in brain-stimulation reward and suggest that dopaminergic neurons contribute to self-stimulation of the nucleus accumbens but not the prefrontal cortex.     

Hippocampus modulates self-stimulation reward from the ventral tegmental area in the rat

Rats with electrodes in the ventral tegmental area were tested for the threshold of intracranial reward using a rate insensitive self-stimulation procedure. It was found that an electrolytic lesion of a part of the dorsal hippocampus induced a marked decrease in the variation of thresholds across rats, while the mean reward level did not change. This indicates that a factor is removed, by the hippocampal lesion, which causes differences in reward between individual rats. It is suggested that the mesolimbic dopaminergic system is involved in this modulatory influence of the hippocampus on reward.     

Preferential alterations in the mesolimbic dopamine pathway of heterozygous reeler mice: an emerging animal-based model of schizophrenia

Based on a number of neuroanatomical and behavioural similarities, recent evidence suggests that heterozygous reeler mice, haploinsufficient for reelin expression, represent a useful model of psychosis vulnerability. As brain mesolimbic dopamine pathways have been proposed to be associated with the pathophysiology of psychotic disorders, we thought it would be of interest to examine whether these animals present disturbances in the mesolimbic dopamine system. To this end we studied by immunocytochemical, in situ hybridization procedures and receptor autoradiography, several markers of the mesotelencephalic dopamine pathway in heterozygous reeler mice and controls. We report that heterozygous reeler mice exhibit a reduction in the number of tyrosine hydroxylase-immunoreactive cell bodies and tyrosine hydroxylase mRNA levels in the ventral tegmental area, as well as a reduction of tyrosine hydroxylase and dopamine transporter immunoreactivity in the dopamine terminal fields of the limbic striatum. In these areas we also observed a reduction of dopamine D2 receptor mRNA. Finally, a marked increase in D3 receptor mRNA levels was observed concomitant with a significant increase in D3 binding sites. On the contrary, the nigrostriatal pathway did not show any significant alteration in heterozygous reeler mice with regards to the dopaminergic markers examined in substantia nigra cell bodies and dorsal striatum dopamine terminal fields. These results suggest a specific link between reelin-related neuronal pathology and dopamine involvement in the pathophysiology of psychotic disorders.     

Feeding and body weight regulation after 6-OHDA application into the preoptic area

Our previous results showed that neurochemical destruction of the amygdaloid terminal field of the mesolimbic dopaminergic system caused disturbances in body weight regulation and feeding. In the present experiments, it was studied whether 6-hydroxydopamine (6-OHDA)-induced bilateral lesions of the mesolimbic dopaminergic system in the lateral preoptic area produce similar symptoms in rats. To enhance the selectivity of the neurotoxin, 6-OHDA was also used after desmethylimipramine (DMI) premedication. Both 6-OHDA and 6-OHDA + DMI treatments resulted in hypophagia, hypodipsia and body weight decrease. A significant increase of water intake was found in sham-operated controls and lesioned animals, in response to extracellular dehydration caused by polyethylene glycol. Intracellular dehydration induced by hypertonic saline resulted in increase of water intake of all animals; however, 6-OHDA- and 6-OHDA + DMI-treated rats drank less than the controls. Similar observation has been made when food intakes were compared after 2-deoxy-D-glucose treatment. Results show that mesolimbic dopaminergic elements play an essential role in the regulation of feeding.     

Evidence for a role of the preoptic area in lateral hypothalamic self-stimulation

We examined the effects of unilateral radiofrequency lesions in the preoptic area on lateral hypothalamic self-stimulation in 15 rats. The animals were tested for self-stimulation in the lateral hypothalamus at 3 different current intensities from electrodes placed in both hemispheres, and then received a unilateral lesion in the preoptic area. Four hours later they were again tested for self-stimulation at the 3 current intensities and then daily over the following 14 days, or until they recovered their presurgical rates of self-stimulation. Rate of self-stimulation decreased in the damaged hemisphere, and recovered to prelesion levels within 2 weeks in 6 of 9 rats. In the intact hemisphere rate of self-stimulation increased above the prelesion level during a period from 1 to 2 weeks after the lesion. These results suggest that the preoptic area is involved in lateral hypothalamic self-stimulation. The effects of d-amphetamine (1 mg/kg) and apomorphine (2 mg/kg) injections on turning behavior were also studied in an open field and in a rotometer. Apomorphine induced contraversive turning to the lesion side in the open field and d-amphetamine induced ipsiversive turning in the rotometer.     

Inhibition of stress-induced dopamine output in the rat prefrontal cortex by chronic treatment with olanzapine.

BACKGROUND: Chronic exposure to stressful events precipitates or exacerbates many neuropsychiatric disorders, including depression and schizophrenia. Evidence suggests that treatment with the atypical antipsychotic drugs olanzapine or clozapine results in a superior amelioration of the anxious and depressive symptoms that accompany schizophrenia relative to therapy with classical antipsychotics such as haloperidol. Moreover, olanzapine and clozapine, but not haloperidol, increase the brain content of neuroactive steroids. The effects of olanzapine and clozapine on the stress-induced increase in dopamine output in the rat cerebral cortex have now been compared with that of haloperidol. METHODS: Rats chronically treated (3 weeks, once a day) with each drug were exposed to foot-shock stress or injected with a single dose of the anxiogenic benzodiazepine receptor ligand FG7142, and dopamine release was then measured in the prefrontal cortex by vertical microdialysis. RESULTS: Long-term administration of olanzapine or clozapine prevented or markedly inhibited, respectively, the increase in the extracellular dopamine concentration induced by foot shock; haloperidol had no such effect. Chronic olanzapine treatment also blocked the effect of FG7142 on dopamine output. CONCLUSIONS: The reduction in the sensitivity of cortical dopaminergic neurons to stress shown to be elicited by treatment with olanzapine or clozapine may contribute to the anxiolytic actions of these drugs.     

Accelerated maternal responding following intra-VTA pertussis toxin treatment

Prior studies have supported a role for mesolimbic dopaminergic mechanisms in the regulation of maternal behavior. Accordingly, the ventral tegmental area (VTA) and its dopaminergic projections to the nucleus accumbens (NAc) and medial prefrontal cortex (mPFC) have been implicated in both the onset and maintenance of normal maternal behavior. To date, studies of direct manipulation of VTA neurochemistry at the onset of maternal behavior have been limited. The current study was undertaken to directly test the hypothesis that enhancement of dopaminergic transmission in the mesolimbic dopamine system can stimulate maternal activity using a pup-induced virgin model. Nulliparous female rats were stereotaxically infused with pertussis toxin (PTX 0, 0.1, or 0.3 μg/hemisphere) into the VTA to chronically stimulate the activity of dopaminergic projection neurons. After 3 days of recovery, maternal responding to donor pups was tested daily, and latency (in days) to full maternal behavior was recorded. Intra-VTA PTX treatment produced a robust dose-dependent decrease in maternal behavior latency, and a long-lasting increase in locomotor activity. These effects were associated with significantly decreased dopamine D1 receptor mRNA expression in the NAc. No effects of PTX treatment on mesolimbic dopamine utilization or mPFC receptor expression were observed. The findings indicate that chronic neural activation in the VTA accelerates the onset of maternal behavior in virgin female rats via modification of the NAc dopamine D1 receptor.