Changes in striatal dopamine release and metabolism during and after subchronic haloperidol administration in rats

What were they thinking? Cognitive states may influence [11C]raclopride binding potential in the striatum

[(11)C]Raclopride ([(11)C]RAC) is a selective dopamine D(2)/D(3) antagonist that is commonly used in positron emission tomography (PET) studies to assess both basal levels of receptor availability and changes in availability caused by alterations in striatal dopamine concentration. When designing [(11)C]RAC studies, it is important to understand what variables may affect the results. Here, we examined differences in baseline striatal [(11)C]RAC binding potential (BP(ND)) under two different "rest" conditions. Thirteen subjects received [(11)C]RAC scans. Eight subjects were aware prior to initiation of scanning that they would receive a "baseline" scan, and that no additional procedures would take place during the scan ("certain rest" group, CER). Five subjects were informed that they might or might not receive an i.v. alcohol infusion during the scan ("uncertain rest" group, UNC). This group was informed five min after scan start that they would not receive alcohol. Voxel-wise analyses of binding potential (BP(ND)) images generated for both "rest" conditions indicated that receptor availability was higher in UNC than in CER. This result was confirmed by a region-of-interest analysis, which indicated that the average BP(ND) in right and left putamen was statistically higher in UNC. There were no differences in groups with respect to age or raclopride mass dose that could account for the difference in D(2)/D(3) receptor availability. Our findings suggest that even slight differences in cognitive states between groups can have an effect on BP(ND), presumably mediated by changes in endogenous dopamine concentration.

Sequential versus nonsequential measurement of density and affinity of dopamine D2 receptors with [11C]raclopride: 2: effects of DAT inhibitors

The multiple ligand concentration receptor assays (MLCRA) method allows, in a stable condition, reliable and reproducible measurements of the density and affinity of the dopamine (DA) D2 receptors with [11C]raclopride, using either a sequential method (two or more scans in one day) or a nonsequential method (two or more scans over days or weeks). We have shown that measurement of receptor density and affinity is also possible after an acute pharmacological challenge with methamphetamine and that both scanning protocols yield similar values. However, our attempts to measure receptor density and affinity after a pharmacological challenge with another class of drugs that lead to the same outcome, increase in synaptic DA concentrations, revealed opposite results with the two scanning methods: a decrease in receptor density with the sequential method and an increase in affinity with a nonsequential method. These results show the impact of the time-dependency of the effects of an 'acute' pharmacological challenge on MLCRA studies. A theoretical simulation is presented to account for the discrepancy in the sequential and nonsequential data. A possible alternate scanning paradigm is proposed to avoid the confounding effect of time variability of the endogenous ligand synaptic concentrations in the sequential condition.

Chronic haloperidol-induced alterations in pallidal GABA and striatal D(1)-mediated dopamine turnover as measured by dual probe microdialysis in rats

Using dual probe microdialysis, assessment of extracellular neurotransmitter levels in the corpus striatum and globus pallidus was performed in ovariectomized and gonadally intact female, Sprague-Dawley rats following chronic (24 weeks) oral haloperidol administration. Vacuous chewing movements, an animal analog of orofacial dyskinesia, were also recorded at several time points during haloperidol administration and throughout the dialysis sampling session. Basal GABA levels were significantly elevated in the globus pallidus of haloperidol-treated rats compared with vehicle animals. Injection of the dopamine D(1) agonist dihydrexidine (3mg/kg, s.c.) decreased striatal dopamine levels in both vehicle and haloperidol-treated rats, with a larger decrease seen in haloperidol-treated rats. Furthermore, dihydrexidine reduced striatal 3,4-dihydroxyphenylacetic acid and homovanillic acid levels only in haloperidol-treated rats. Gonadal status had no effect on any neurochemical measure. Vacuous chewing movements were significantly elevated in haloperidol-treated groups by the sixth week of treatment, with higher counts seen in gonadally intact rats. Vacuous chewing movements were significantly elevated above baseline in all groups following dihydrexidine, with no differential effect of prior haloperidol treatment or gonadal status. These results indicate a tonic increase in pallidal GABA levels and a hypersensitivity of D(1)-mediated striatal dopamine and dopamine metabolite decreases following chronic haloperidol treatment. While not found to be correlated with neurochemical measures, the heightened vacuous chewing movements in gonadally intact vs ovariectomized rats may serve as a model of hormone-mediated differences in neuroleptic-induced oral dyskinesia.

Decreased choline acetyltransferase immunoreactivity in discrete striatal subregions following chronic haloperidol in rats

Neuronal loss within the basal ganglia has been hypothesized to play a role in movement disorders (e.g., tardive dyskinesia) that often occur following chronic neuroleptic treatment. Previous studies in animal models have provided some support to this possibility, but have not assessed regionally specific changes after chronic neuroleptic administration. The present study examined whether counts of neurons containing acetylcholine, described as large aspiny type II neurons, were altered in subregions of the corpus striatum and nucleus accumbens following chronic haloperidol administration in rats. Rats were administered haloperidol decanoate (21 mg/kg, i.m.) or vehicle every third week for 24 weeks. Following 4 weeks of withdrawal from the drug, predefined regions were examined for choline acetyltransferase (ChAT) immunoreactive (ir) cells. Compared to the vehicle group, the haloperidol group showed significant reductions in ChAT-ir cell counts in the ventrolateral striatum, nucleus accumbens core, and nucleus accumbens lateral shell. No significant differences were found in the other regions examined: dorsolateral striatum, dorsomedial striatum, ventromedial striatum, nucleus accumbens medial shell, and horizontal limb of the diagonal band. These findings indicate that there may be regionally specific alterations in ChAT-ir cells following chronic haloperidol treatment, supporting previous hypotheses of striatal cholinergic cell loss resulting from chronic neuroleptic treatment. More importantly, the regions affected (ventrolateral striatum and nucleus accumbens) are critical in the regulation of oral movements, thus suggesting that alterations in cholinergic cell activity, and perhaps actual loss of cholinergic cells in these regions, may be important in the manifestation of late-onset oral dyskinesia.

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.

An evaluation of the role of 5-HT(2) receptor antagonism during subchronic antipsychotic drug administration in rats

A widely postulated mechanism of action for the atypical profile of many novel antipsychotic drugs (APDs) is their relatively high affinity for 5-HT(2) receptors. The present study investigated motor function and striatal dopamine (DA) efflux and metabolism in rats given 21 daily injections of drugs that differed in 5-HT(2) affinity. These drugs included: risperidone (high 5-HT(2A/2C)/high D(2)), clozapine (high 5-HT(2A/2C)/low D(2)), haloperidol (low 5-HT(2A/2C)/high D(2)), haloperidol+ritanserin (selective 5-HT(2A/2C)), or vehicle. Rats injected with haloperidol (0.5 mg/kg) or haloperidol+ritanserin (0.5 mg/kg and 1.0 mg/kg, respectively) showed extreme catalepsy on day 1, but significantly decreased catalepsy when tested again on days 7 and 21. Acute or subchronic risperidone (0.05 or 0.5 mg/kg), clozapine (20 mg/kg), or vehicle did not induce significant catalepsy. Microdialysis performed 24 h after the last injection demonstrated that rats treated with risperidone, clozapine, or vehicle showed similar increases in DA efflux and metabolism following an acute injection of a selective DA D(2/3) antagonist (raclopride, 0.5 mg/kg). DA efflux showed an attenuated response to raclopride in the haloperidol alone group; this effect was less apparent in the haloperidol+ritanserin group. However, both of these groups showed a similar tolerance effect to the raclopride-induced increase in DA metabolites. These results suggest that the profile seen after subchronic risperidone more closely resembles clozapine than haloperidol. While ritanserin reduced the tolerance-like effects of haloperidol on striatal DA efflux, the overall results demonstrate that potent 5-HT(2) blockade alone may not entirely account for the distinctive profile of novel APDs.

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.

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

Antipsychotic medications appear to exert their therapeutic effects by blocking D2 receptors. While D2 blockade occurs rapidly, reduction in psychotic symptoms is often delayed. This time discrepancy has been attributed to the relatively slow development of depolarization inactivation (DI) of dopaminergic neurons. The reduced firing rates associated with DI has been hypothesized to reduce dopamine release and thus psychotic symptoms. Studies assessing changes in dopamine release during chronic neuroleptic treatment, using microdialysis and voltammetry, have been inconsistent. This may be due to methodological differences between studies, the invasive nature of these procedures, or other confounds. To investigate the effects of DI on dopamine release, 3-MT accumulation, an index of dopamine release that does not involve disruption of brain tissue, was measured during acute and chronic neuroleptic treatment. These results are compared with those using other techniques. 3-MT levels remained elevated after chronic treatment, suggesting that DI does not markedly reduce release. Regulation of dopamine release during DI was examined using two techniques known to block dopamine neuronal impulse flow. 3-MT levels were markedly reduced by both, implying that DI does not alter the portion of dopamine release mediated by neuronal impulse flow. Overall, studies to date suggest that the delayed therapeutic effects of neuroleptics are not due to reductions in impulse dependent dopamine release. Recent studies using a neurodevelopmental animal model of schizophrenia have pointed to altered pre- and post-synaptic indices of dopamine neurotransmission. The results suggest that neuroleptics may exert their therapeutic effects, in part, by limiting the fluctuations in dopamine release, and raise new issues for future research.

Chronic haloperidol-induced changes in regional dopamine release and metabolism and neurotensin content in rats

Chronic neuroleptic administration has previously been shown to alter in vivo measures of dopaminergic function and lead to regionally selective increases in neurotensin levels. In the current study, female rats were administered chronic haloperidol for 6 months via subcutaneous silastic implants. After 24 weeks of administration, microdialysis probes were inserted into the lateral caudate putamen and the medial prefrontal cortex. Basal samples were collected prior to infusion of a high K+ concentration (100 mM KCl). Extracellular concentrations of dopamine, 3,4-dihydroxyphenylacetic acid, homovanillic acid, and 5-hydroxyindoleacetic acid were assessed using HPLC. Chronic haloperidol-treated rats showed increased basal dopamine metabolite levels in the caudate putamen and an altered response to the effects of high K+ on 3,4-dihydroxyphenylacetic acid; no significant differences were seen with other analytes in the caudate putamen. Although basal concentrations were not different between groups in the prefrontal cortex, haloperidol-treated rats showed a significant attenuation of response to the effects of high K+ infusion on dopamine metabolite concentrations. Radioimmunoassay measurement of tissue neurotensin content showed highly significant elevations of neurotensin concentrations in the caudate putamen and nucleus accumbens, but not in other brain regions analyzed. These results suggest a confluence of altered dopamine and neurotensin function in the caudate putamen which may be related to motor side effects of haloperidol, whereas changes in prefrontal dopamine function are not associated with altered neurotensin levels.

Effect of haloperidol and its metabolites on dopamine and noradrenaline uptake in rat brain slices

The effects of haloperidol and its metabolites on dopamine (DA) and noradrenaline (NA) uptake were investigated. Both direct uptake of [3H]DA and [3H]NA into the rat striatal and hippocampus slices and binding of a specific DA uptake inhibitor [3H]GBR-12935 were employed in the present study. Haloperidol pyridinium (HP+), haloperidol 1,2,3,6-tetrahydropyridine (HTP), 4-(4-chlorophenyl)-1,2,3,6-tetrahydropyridine (CPTP) and reduced haloperidol (RHAL) are potent inhibitors of DA uptake. HTP N-oxide (HTPNO) exhibits a relatively weak effect on DA uptake. Other metabolites of haloperidol, i.e. 4-(4-chlorophenyl)-4-hydroxypyridine (CPHP) and haloperidol N-oxide (HNO), as well as haloperidol itself possess negligible inhibitory effect on DA uptake. HP+ has been shown to be an amine releaser. It is possible that HP+ may induce amphetamine-like neurotoxicity. The effects of the metabolites of haloperidol on [3H]NA uptake are similar to those on [3H]DA uptake. HP+ appears to be different from MPP+, which is a more potent [3H]NA uptake blocker than on [3H]DA uptake. Although haloperidol exhibits no DA uptake inhibitory effect, it has a high affinity for the [3H]GBR-12935 binding site. The possible pharmacological implications such inhibitory effects on amine uptake are discussed.

Repeated treatment with ascorbate or haloperidol, but not clozapine, elevates extracellular ascorbate in the neostriatum of freely moving rats

Acute administration of neuroleptic drugs alters the extracellular level of ascorbate in the neostriatum, and increasing evidence suggests a role for this vitamin in the behavioral, and possibly therapeutic, effects of these drugs. To shed further light on this issue, extracellular ascorbate was recorded in the neostriatum and nucleus accumbens of awake, behaving rats following chronic treatment with either classical (haloperidol) or atypical (clozapine) neuroleptics or ascorbate itself. Electrochemically modified, carbon-fiber microelectrodes were lowered in place the day after the last of 21 daily injections of either haloperidol (0.5 mg/kg, SC), clozapine (20 mg/kg, IP), sodium ascorbate (500 mg/kg, IP) or vehicle. Voltammetric measurements were obtained during quiet rest and following administration of d-amphetamine (2.5 mg/kg). Repeated treatment with either haloperidol or ascorbate elevated basal extracellular ascorbate and potentiated the amphetamine-induced increase in ascorbate release in neostriatum but not nucleus accumbens. Both treatment groups also showed a significant increase in amphetamine-induced sniffing and repetitive head movements compared to vehicle-treated animals. In contrast, repeated clozapine had no effect on extracellular ascorbate in either neostriatum or nucleus accumbens, but increased the locomotor response to an amphetamine challenge. Thus, to the extent that increases in neostriatal ascorbate exert neuroleptic-like effects, such effects are likely to parallel haloperidol rather than clozapine.