Alterations in dopamine metabolism after chronic administration of haloperidol. Possible role of increased autoreceptor sensitivity

Behavioral evidence of depolarization block of dopamine neurons after chronic treatment with haloperidol and clozapine

Electrophysiological studies have shown that chronic treatment with haloperidol causes depolarization block (DB) of dopamine cells in anesthetized and paralyzed rats. It has been proposed that the emergence of DB underlies the therapeutic and side effects of this drug. However, the relevance of DB to the clinical actions of haloperidol has been questioned on the grounds that chronic drug-induced DB has not yet been demonstrated in freely moving animals. In this study, responding for rewarding electrical brain stimulation was used to assess the occurrence of DB in rats chronically treated with haloperidol or clozapine. The time course of the effects of acute haloperidol (7.8-500 microg/kg) and clozapine (5-40 mg/kg) and of withdrawal from chronic drug treatment on reward and performance measures were also characterized. Haloperidol and clozapine dose-dependently attenuated reward and performance, haloperidol producing a predominant suppression of performance, and clozapine preferentially attenuating reward. Chronic (21 d) treatment with haloperidol (500 microg/kg) caused responding to cease in the six rats tested, and repeated injection with apomorphine restored the behavior in all of them; such an effect of apomorphine was observed in only two of six rats treated acutely with the same dose of haloperidol. Chronic treatment with clozapine (20 mg/kg) increased reward thresholds, an effect that was reversed by apomorphine in chronically, but not acutely, treated rats. The times at which chronic haloperidol-treated rats resumed responding was positively correlated with indices of behavioral supersensitivity after withdrawal, suggesting that the effect of apomorphine was not caused by direct stimulation of upregulated postsynaptic receptors. These findings constitute the first behavioral evidence of DB in unanesthetized, freely moving animals treated chronically with antipsychotics. They also demonstrate that the neural substrates mediating reward and performance are functionally independent and differentially sensitive to haloperidol and clozapine.

Tyrosine hydroxylase, aromatic L-amino acid decarboxylase and dopamine metabolism after chronic treatment with dopaminergic drugs

Mice were treated with dopamine (DA) receptor agonist and antagonist drugs: Agonists: (+/-)-SKF 38393 ((+/-)-1-phenyl-2,3,4, 5-tetrahydro-(1H)-3-benzazepine-7,8-diol) [DA D1-like]; bromocriptine, [DA D2 selective]; quinpirole, [DA D2/D3 preferring]; (+/-)-7-hydroxy-dipropylamino-tetralin (7-OH-DPAT), [DA D3/D2 preferring], Antagonists: R(+)-SCH 23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine), [DA D1-like]; and haloperidol, [DA D2-like]. All drugs were administered intraperitoneally, two injections daily 8 h apart for 30 days. Aromatic L-amino acid decarboxylase (AAAD) and tyrosine hydroxylase (TH) activity, protein and mRNA, as well as DA metabolism were followed with time thereafter in the nigrostriatal neurons. We observed that chronic administration of D1-like agonists had no effect on TH or AAAD activity, while D2-like agonists decreased AAAD, but not TH activity. Additionally, chronic blockade of DA D2-like receptors resulted in prolonged induction of TH and AAAD, while chronic blockade of DA D1-like receptors induced changes of AAAD only. Compared to TH the induction of AAAD was longer lasting. DA metabolism was altered by chronic administration of drugs acting on DA D2-like, but not DA D1-like receptors, and in general the patterns of change did not follow those for TH or AAAD. When studied 48 h after the last dose of the chronic haloperidol schedule TH displayed tolerance to acute drug challenge. At the same time interval, there was tolerance to the enhancing effects of haloperidol and SCH 23390 on DA metabolism. The induction of AAAD by haloperidol or SCH 23990 did not appear to develop tolerance after chronic administration. These observations complement existing knowledge, and provide novel information about AAAD that may have practical importance for Parkinson's patients on L-DOPA therapy.

Effect of apomorphine infusion on dopamine synthesis rate relates to dopaminergic tone

The effects of apomorphine on the striatal L-[11C]DOPA influx rate was examined in anaesthetized Rhesus monkeys using positron emission tomography (PET). In comparison with baseline conditions, the addition of a continuous infusion of apomorphine produced decreases in the striatal L-[11C]DOPA influx rate in all the monkeys examined. The effect of apomorphine infusion also showed a dose-dependent trend. In individual monkeys, the magnitude of the effect showed a baseline dopaminergic tone-dependency; that is, the effect of apomorphine was most pronounced in monkeys with high baseline influx rates, and in monkeys with lower baseline values apomorphine induced a weaker effect. Studies of radiolabeled tracer and radiolabeled metabolites formed in plasma confirmed that apomorphine infusion did not induce any change in the peripheral elimination or metabolite formation of L-[11C]DOPA. The decreased striatal L-[11C]DOPA influx rate induced by apomorphine was interpreted as an agonist effect on dopamine autoreceptors regulating the dopamine synthesis rate. The observation of a baseline dopaminergic tone-dependent effect is in agreement with earlier results showing this influence on the striatal influx rate as measured with the tracer L-[11C]DOPA. A priori, it can be established that L-[11C]DOPA and PET provide a method not only to study the structural integrity of the presynaptic dopaminergic system but also to study the homeostasis-regulating mechanisms of this neurotransmitter system in vivo. The ability to measure condition-dependent effects in individuals should be of great importance in determining specific pathophysiological mechanisms underlying degenerative and functional disorders affecting the dopaminergic system.

Chronic clozapine versus chronic haloperidol treatment: differential effects on electrically evoked dopamine efflux in the rat caudate putamen, but not in the nucleus accumbens

Fast cyclic voltammetry at carbon-fibre micro-electrodes was used to investigate the effects of chronic clozapine or haloperidol administration on electrically evoked dopamine efflux in the nucleus accumbens and caudate putamen of the anaesthetized rat. Stimulation trains were delivered to the median forebrain bundle (60 pulses, 350 microns duration) every 5 min, and the evoked dopamine efflux measured as a function of a) the applied stimulus intensity (range 0.2 mA-1.0 mA), and b) the applied stimulus frequency (range 10 Hz-250 Hz). Chronic administration of either clozapine (20 mg/kg x 21 days, p.o.) or haloperidol (1 mg/kg x 21 days, p.o.) significantly reduced electrically evoked dopamine efflux in the nucleus accumbens over the range of stimulus intensities and frequencies tested. The reduction in evoked dopamine efflux observed in the nucleus accumbens of clozapine- and haloperidol-treated rats showed no statistically significant difference. In contrast, only chronic haloperidol treatment significantly reduced evoked dopamine efflux in the caudate putamen. These findings demonstrate that chronic treatment with either the atypical neuroleptic, clozapine, or the typical neuroleptic, haloperidol, produce long-term changes in mesolimbic dopamine function; actions which may underlie their antipsychotic efficacy. They also provide further evidence that the sparing action of clozapine on nigrostriatal dopamine activity may underlie the lower incidence of extrapyramidal side effects associated with its long-term administration.

Stimulus-evoked dopamine overflow in the rat nucleus accumbens is decreased following chronic haloperidol administration: an in vivo voltammetric study

Fast cyclic voltammetry was used to investigate the effects of chronic haloperidol (HAL) treatment on electrically evoked dopamine (DA) overflow in the nucleus accumbens of the anaesthetized rat in vivo. Evoked DA efflux was significantly reduced in rats treated with 1.0 mg/kg per day HAL for 21 days. In rats treated with 0.5 mg/kg per day, evoked DA overflow was reduced, but did not differ significantly from control values. In untreated animals, injection of a single dose of HAL resulted in a significant increase in the DA overflow evoked by subsequent stimulus trains. In contrast, this HAL challenge did not produce a significant enhancement in evoked DA overflow in any of the HAL-treated groups. These results are consistent with the previous reports that basal DA release is reduced after chronic HAL treatment, and show for the first time that chronic HAL administration decreases stimulus-evoked DA overflow in the rat nucleus accumbens in vivo.

Neurochemical effects of prolonged treatment with remoxipride as assessed by intracerebral microdialysis in freely moving rats

1. At three microdialysis sessions, dialysates were collected from the striatum of the same rats. 2. Microdialysis session 1. A single s.c. injection of remoxipride (40 mumol/kg), resulted in increased dialysate concentrations of dopamine, DOPAC and HVA. 3. Microdialysis session 2. Continuous administration of remoxipride (8.6 mumol/rat/day) for 14 days, using mini-osmotic pumps, produced maintained elevated levels of dopamine, DOPAC and HVA. 4. Microdialysis session 3. A challenge dose of remoxipride (40 mumol/kg s.c.), given to the rats after a 48-hour wash-out period following the continuous remoxipride treatment, increased the dialysate concentrations of dopamine, DOPAC and HVA to similar extent as at dialysis session 1. 5. It is concluded that after long-term treatment of remoxipride, an adaptation of the basal state of the DA system appears to take place, implying a lowering of basal DA release and DA metabolism. However, the capacity to respond with increased DA release and DA metabolism to renewed remoxipride treatment is retained, indicating little, if any, tolerance.

Subchronic treatment with the neuroleptic-like peptide desenkephalin-gamma-endorphin may decrease dopaminergic neurotransmission in the nucleus accumbens of rats

In rats, subchronic administration of desenkephalin-gamma-endorphin (DE gamma E) into the nucleus accumbens or subcutaneously for 10 days resulted in hypoactivity. Intra-accumbens administration caused a significant reduction in the nucleus accumbens tissue levels of the dopamine (DA) metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). Systemic administration of DE gamma E decreased DOPAC and 5-hydroxyindoleacetic acid (5-HIAA) levels in nucleus accumbens tissue. Subchronic subcutaneous DE gamma E treatment reduced the basal release of [3H]DA from rat nucleus accumbens slices in vitro and the basal release of endogenous DA and DOPAC in vivo as assessed with on-line dialysis in the nucleus accumbens of freely moving rats. The DA agonist N,N-dipropyl-7-hydroxy-2-aminotetralin (DP-7-ATN) was equally effective in inhibiting [3H]DA release elicited by electrical stimulation from slices of subchronically DE gamma E and placebo treated rats. Administration of a small dose of apomorphine caused similar reductions of the in vivo release of DA and DOPAC in both placebo and DE gamma E treated rats. These results indicate that subchronic DE gamma E treatment may decrease dopaminergic neurotransmission in the nucleus accumbens. This effect is probably not due to alterations in the sensitivity of presynaptically located DA autoreceptors mediating DA release in vitro and in vivo.

Chronic neuroleptic treatment does not suppress the dynamic characteristics of the dopaminergic receptor D2 system

1. Rats were treated with either haloperidol (0.5 mg/kg) or haloperidol plus an anticholinergic drug (0.5 and 0.15 mg/kg/day respectively) for 3 days, 7 days and 16 months. 2. Estimates made twenty hours after the last doses showed that haloperidol reduced the concentrations of the dopamine metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the striatum and the olfactory tubercle. 3. A challenge dose of either haloperidol or haloperidol plus an anticholinergic drug was administered to rats pretreated with haloperidol or haloperidol plus an anticholinergic drug; this challenge dose reversed the reduction in dopamine metabolites caused by neuroleptic administration. 4. After sixteen months of haloperidol administration dopamine levels were reduced, but adding an anticholinergic drug to haloperidol treatment prevented this reduction in dopamine concentration.

Chronic treatment with haloperidol modifies the sensitivity of autoreceptors that modulate dopamine release in rat striatum

The effects of apomorphine or sulpiride on electrically evoked dopamine release from striatal slices of rats pretreated with haloperidol were investigated. Chronic haloperidol treatment (1 mg/kg per day for 21 days) significantly reduced electrically evoked dopamine release from striatal slices until 72 h after the last injection. The apomorphine-induced reduction and the sulpiride-induced increase in evoked dopamine release were significantly enhanced by the chronic treatment with haloperidol at 72 h after the last injection. The enhancement of the sulpiride-induced increase in evoked dopamine release was inversely correlated with the dopamine release evoked by the first stimulation in striatal slices from haloperidol-treated (r = -0.85, n = 12, P < 0.01) but not from saline-treated rats. These results suggest that an increase in the sensitivity of dopamine autoreceptors due to chronic treatment with haloperidol could partially account for the reduction in dopamine release from striatal slices of rats.

Seroquel: biochemical profile of a potential atypical antipsychotic

Seroquel and the atypical antipsychotic clozapine were compared using a number of biochemical measures in rats which are indicative of potential antipsychotic activity and possible extrapyramidal side effect liability. Both in vitro and in vivo, these compounds are low potency D-2 dopamine (DA) receptor antagonists and are relatively more potent 5-HT2 antagonists than typical antipsychotic drugs. Seroquel also exhibited low affinity for D-1 DA receptors in vitro, but D-1 receptor occupancy was not detectable in vivo. Unlike clozapine, Seroquel lacks appreciable activity at either D-1 DA or muscarinic receptors. Following IP administration, both compounds produce similar elevations in DA metabolite concentrations. Following 1 month of daily administration, at doses which produce large increases in striatal DA metabolite concentrations, both Seroquel and clozapine fail, unlike typical antipsychotics, to increase the number of striatal D-2 receptors, but do decrease the number of 5-HT2 receptors in frontal cortex. ICI 204,636 produces a short-lasting increase in plasma prolactin levels, but these increases are much greater than those that are produced by clozapine. One day after 3 weeks of daily administration, tolerance, to the ability of Seroquel to elevate DA metabolite and plasma PRL concentrations is not observed. These biochemical observations are discussed with regard to the atypical profile of Seroquel in behavioral and electrophysiological studies.

Schizophrenia and the D1 receptor: focus on negative symptoms

Negative symptoms have been associated with structural impairment in the PFC, and hypothesized to arise from a central hypodopaminergic substrate. Corticofugal PFC neurons, which are inhibited by VTA DA innervation, exert a tonic excitatory modulation on DA activity in the NAS. Lesions of ascending DA forebrain projections "uncouple" the functional link between D1 and D2 receptors, permitting independent activation of D1 sites in generating behavioral output. A previously identified absence of this D1/D2 link in schizophrenic brain suggests that functional activation of PFC D1 receptors may induce hyperinhibition of descending corticofugal efferents to the NAS. Consequent hypoactivity of DA in the NAS is proposed to give rise to negative symptoms of schizophrenia, and low dose DA agonist treatments may mimic behavioral features of this symptom profile via direct PFC D1 stimulation. It follows that clozapine's efficacy for negative symptoms may be attributable, in part, to blockade of PFC D1 receptors, with subsequent enhancement of glutamate-facilitated NAS DA activity.

Effect of Gi protein ADP-ribosylation induced by pertussis toxin on dopamine-mediated behaviors

The effect of Gi protein modification produced by intrastriatal pertussis toxin injection on dopamine (DA)-mediated behaviors was studied. Administration of the selective D2 agonist quinpirole induced ipsilateral rotation but the selective D1 agonist SKF 38393 did not. However, SKF 38393 was able to increase the rotation induced by quinpirole. The selective D2 antagonist raclopride and the selective D1 antagonist SCH 23390 both blocked the effect of quinpirole. Striatal levels of cAMP were measured in both intact and pertussis toxin injected striatum. SKF 38393 induced a significant increase in cAMP, but quinpirole had no effect. When both drugs were administered together, quinpirole attenuated the SKF 38393-induced increase in cAMP levels. Moreover, quinpirole-induced attenuation of SKF 38393 effect was greater in intact striatum. In pertussis toxin-injected striatum, quinpirole only attenuated SKF 38393-induced increase of cAMP to control levels. This imbalance between intact and injected striatum might be the cause of the rotation in pertussis toxin-injected rats suggesting an important role for G proteins in DA receptor interactions.

Genotype-dependent effects of chronic stress on apomorphine-induced alterations of striatal and mesolimbic dopamine metabolism

After 10 daily consecutive restraint experiences, DBA/2 (DBA) mice showed an increase of climbing behavior after injection of 0.25 mg/kg of the dopamine (DA) agonist apomorphine (APO), while no changes were observed following vehicle or 1 mg/kg of APO. By contrast, chronically stressed C57BL/6 (C57) mice showed a clear-cut decrease of climbing behavior at the dose of 0.25 mg/kg of APO and a similar, although less pronounced, effect of stress on the behavior of mice injected either with vehicle or with 1 mg/kg APO. The DA agonist at these same doses decreased 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 3-methoxytyramine (3-MT) concentrations in the caudatus putamen (CP) and nucleus accumbens septi (NAS) of both strains. Higher DOPAC, HVA and 3-MT concentrations were evident in stressed DBA mice receiving 0.25 mg/kg but not 1 mg/kg of APO, in both CP and NAS. Concerning C57 mice, lower concentrations of the 3 metabolites were present at both doses of APO in the NAS of stressed mice in comparison with non-stressed animals, while no significant stress-related effects were evident in the CP. Non-significant differences between control and stressed mice of both strains were evident as regards DA concentrations in CP and NAS. These results suggest that repeated stressful experiences lead to a hyposensitivity of DA presynaptic receptors in DBA mice while they produce a sensitization of mesolimbic DA presynaptic receptors possibly accompanied by down-regulation of postsynaptic DA receptors in the C57 strain.

Alterations in dopamine release from striatal slices of rats after chronic treatment with haloperidol

The effects of chronic treatment with haloperidol on spontaneous and electrically evoked dopamine (DA) release from striatal slices of rats were investigated in vitro. DA was measured by high-performance liquid chromatography coupled to an electrochemical detector. The superfusion with haloperidol caused a dose-dependent (100 nM-100 microM) reduction in the electrically evoked DA release from striatal slices of rats, which was not antagonized by the superfusion with apomorphine. Chronic administration of haloperidol (1 mg/kg per day for 21 days) caused a significant reduction in electrically evoked DA release as well as in spontaneous DA release from striatal slices 24 h after the last injection. Moreover, pretreatment with haloperidol prevented the reduction of the DA release evoked in response to haloperidol superfusion (1 microM). These results indicated that chronic administration of haloperidol reduced DA release from striatal slices of rats, accompanied by tolerance for the inhibitory effect of drug superfusion on evoked DA release.

Enhanced haloperidol-induced prolactin stimulation with chronic neuroleptic treatment in the rat

Animals were treated either acutely, or chronically for 21 days, with a low dose (0.1 mg/kg) of haloperidol, then sacrificed to obtain trunk blood for radioimmunoassay of prolactin (PRL) level. PRL concentrations on day 21 of chronic treatment were greater than two-fold those produced by acute neuroleptic. Challenge with apomorphine to rats withdrawn for 48 hours revealed similar PRL reductions as a group withdrawn from chronic vehicle injections.

Influence of acute and chronic haloperidol treatment on dopamine metabolism in the rat caudate-putamen, prefrontal cortex and amygdala

The present study investigated the actions of single and repeated injections of the classical antipsychotic drug, haloperidol (1 mg.kg-1 IP), on dopamine (DA) metabolism in three distinct rat brain regions, namely the prefrontal cortex, amygdala and caudate-putamen (CP), using a high-performance liquid chromatographic assay. Acute administration of the drug caused significant elevations in concentrations of two major DA metabolites in all three areas studied. Less marked acute increases were seen in the CP following 10 days of repeated haloperidol treatment. However, in both the prefrontal cortex and the amygdala, the development of such "tolerance" was somewhat delayed in comparison, occurring only after a 22-day treatment schedule. The amygdala displayed the greatest degree of neurochemical tolerance, returning to control values by day 22 of chronic treatment. When allowance was made for the withdrawal effects of antipsychotic drug administration, a genuine tolerance phenomenon was observed in all three areas examined. These data suggest that if neurochemical tolerance is a prerequisite for functional DA receptor blockade and hence therapeutic efficacy, then both the prefrontal cortex and amygdala should be considered as potential therapeutic targets of haloperidol and perhaps antipsychotic drugs in general.

Decrease in the evoked release of endogenous dopamine and dihydroxyphenylacetic acid from rat striatal slices after withdrawal from repeated haloperidol

The release of endogenous DA (dopamine) and DOPAC (3,4-dihydroxyphenylacetic acid) from rat striatal slices was measured after withdrawal from a prior long-term treatment of the rat with haloperidol to investigate adaptive changes in striatal DA and DOPAC release induced by chronic haloperidol treatment. Striatal slices prepared 24 h after the last injection of daily treatment with haloperidol for up to 14 days (2.5 mg/kg per day) were superfused and stimulated for 5 min with K+ (50 mM). Haloperidol treatment for 3 or 7 days decreased K(+)-stimulated DA release by maximally 35%, but a 14-day treatment was not effective. The K(+)-stimulated release of DOPAC, which occurred after the change in DA release, was reduced significantly by the treatment for 7 or 14 days. A higher daily dose of haloperidol (10 mg/kg per day) produced a more pronounced decrease in stimulated DA release after a 14-day treatment without having an effect after 3 days. However, the stimulated release of DOPAC decreased markedly after both 3 and 14 days of haloperidol treatment. The slight reduction in the DA content of the slices after K+ stimulation was seen in some haloperidol-treated tissues, although this change did not always parallel the simultaneous decrease in DA release. These results indicate that the K(+)-induced stimulation of endogenous DA release and the synthesis of DA are impaired after withdrawal from repeated haloperidol treatment.

Effects of chronic SCH 23390 treatment on dopamine autoreceptor function in rat brain

The effects of chronic treatment with SCH 23390 (0.5 mg/kg per day, 18 days), a selective (DA) D-1 receptor antagonist, on dopamine autoreceptor function in rats were investigated. The decrease of DA metabolism in mesolimbic and nigrostriatal systems and inhibition of spontaneous explorative locomotor activity by low doses of the selective D-2 receptor agonist, quinpirole, were used as behavioral and biochemical indices of DA autoreceptor function. The DA metabolism decreasing effect of quinpirole (20 and 50 micrograms/kg) in mesolimbic or nigrostriatal DA systems was similar in SCH 23390- and vehicle-treated rats. Two-way analysis of variance showed that rats treated chronically with SCH 23390 were significantly more active than vehicle-treated rats. However, there were no statistically significant differences in the quinpirole-induced hypomotility response between these groups. These results indicate that chronic D-1 blockade, unlike classical antipsychotic drugs, does not modulate the biochemical and behavioral indices of DA autoreceptor activation, suggesting a lack of interaction between DA autoreceptors and D-1 receptors in the central nervous system.

The effect of chronic clozapine and haloperidol on basal dopamine release and metabolism in rat striatum and nucleus accumbens studied by in vivo microdialysis

The effects of chronic treatment with clozapine and haloperidol on basal dopamine release and metabolism were studied in the striatum and the nucleus accumbens of awake, freely moving rats using in vivo microdialysis. Chronic haloperidol (2 mg/kg X 21 days) but not chronic clozapine (20 mg/kg X 21 days) decreased basal dopamine release and metabolism in both the striatum and the nucleus accumbens. These results differ significantly from those previously reported with in vivo voltammetry. Possible explanations are discussed.

Sensitization versus tolerance to the dopamine turnover-elevating effects of haloperidol: the effect of regular/intermittent dosing

Recent clinical research suggests that particular patterns of changes in presynaptic dopamine (DA) turnover accompany the therapeutic response to neuroleptics. We sought to determine whether daily versus weekly dosing of haloperidol for 3 weeks produced distinct effects on DA, dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) concentrations in multiple brain areas. Daily dosing favored the development of tolerance to the DA-turnover elevating effects of haloperidol in the striatum and nucleus accumbens. Weekly dosing favored the development of sensitization in the striatum, posterior olfactory tubercle, and ventral tegmental area. These results suggest that dosing schedules may determine, at least in part, the effects of chronic neuroleptic administration on presynaptic DA function.

Chronic low-dose haloperidol effects on self-stimulation rate-intensity functions

Animals responding for biphasic square wave stimulation to the VTA were treated for 26 days with a low dose (0.07 mg/kg) of the neuroleptic haloperidol and tested at 1 h post-injection. Initially the drug induced a pronounced lateral displacement of the baseline rate-intensity function, concomitant with a depression in slope. Over the course of chronic treatment, partial tolerance was observed to the drug-induced increases in threshold concomitant with the onset of a significant suppression in peak response rate. Biochemical tolerance to stimulated dopamine metabolism (as per cent non-drug control) was significant only for mesolimbic (versus neostriatal) regions, in animals receiving haloperidol according to pre- and post-test administration schedules. The observation of sensitization to peak rate reductions parallels previous reports for spontaneous locomotor activity measures and is compatible with depolarization inactivation mechanisms proposed to account for delayed-onset clinical effects. Further, selective biochemical tolerance in mesolimbic regions supports suggestions that mesolimbic dopamine is important as a substrate for subtle low dose neuroleptic effects which may be relevant for studying pharmacotherapeutic treatment issues.

Neurochemical effects of chronic haloperidol and lithium assessed by brain microdialysis in rats

1. Psychotropic drugs ameliorate psychotic symptoms only after repeated administration. 2. To assess the neurochemical effects of chronic haloperidol and lithium administration, microdialysis was performed simultaneously in the prefrontal cortex, the nucleus accumbens, and the striatum after haloperidol, and separately in the lateral hypothalamus and the hippocampus after lithium. 3. Chronic administration of haloperidol decreased dopamine turnover in the prefrontal cortex and the striatum. It did not affect the nucleus accumbens detectably. 4. No tolerance to haloperidol developed in any of the three regions. 5. Lithium enhanced the response of the serotonergic system to amphetamine in the lateral hypothalamus but not in the hippocampus. 6. The antipsychotic effect of haloperidol might be related to dopamine turnover decrease in the prefrontal cortex. 7. The antidepressant effect of lithium might be related to enhancement of serotonin responsiveness in the hypothalamus.

Chronic haloperidol treatment with low doses may enhance the increase of homovanillic acid in rat brain

Homovanillic acid (HVA) levels were determined by high performance liquid chromatography with electrochemical detection in the striatum and prefrontal cortex of rats that had received single or repeated injections of various doses of haloperidol. Haloperidol increased the HVA concentrations in both brain regions after both acute and chronic treatment with doses of 0.01-1 mg/kg. The increase in the HVA concentrations in the striatum was blunted after repeated haloperidol injections with doses of 0.5-1 mg/kg, suggesting that haloperidol pretreatment results in a decreased responsiveness to the drug at high doses (tolerance). Tolerance also developed to the effect of long-term haloperidol treatment on the HVA concentrations in the prefrontal cortex at the highest dose used (1 mg/kg). This suggests that the differences in the development of tolerance between the striatum and prefrontal cortex are not qualitative but quantitative. However, repeated haloperidol injections at doses of 0.01-0.05 mg/kg enhanced the increase in HVA concentrations. This suggests that tolerance does not develop after chronic haloperidol treatment with low doses. Decreased HVA concentrations were also found after withdrawal from chronic haloperidol treatment (rebound decrease). However, this rebound decrease was much smaller than the decrease in response of the HVA concentrations to repeated haloperidol injections, suggesting that different mechanisms are involved.

Can the supersensitivity of rodents to dopamine be regarded as a model of tardive dyskinesia?

1. The paper presents arguments derived from both, clinical work and animal experiments, for or against the traditional hypothesis suggesting that tardive dyskinesia (TD) is caused by supersensitivity to dopamine. The main aim of this study was to answer the question posed in the title - whether the supersensitivity to dopamine evoked in rodents by neuroleptics can be regarded as an adequate pharmacological model of TD. 2. The data presented here prove that chronic administration of neuroleptics to schizophrenic patients cannot be the only factor inducing TD; furthermore, symptoms similar or identical to those of TD are also observed in the course of other disorders, not connected with neuroleptics, e.g. aging or schizophrenia itself. 3. Clinical data offer no clear evidence for the existence of a direct cause-effect relationship between super-sensitivity to dopamine and occurrence of TD. 4. The role of brain degeneration, caused by different factors but in particular by the process of aging, in the pathogenesis of dyskinetic disorders, including TD, has been stressed. 5. Pharmacological and biochemical data show that chronic administration of classic neuroleptics to animals induces an increase in the density of dopamine D-2 receptors (Bmax). It seems that this receptor-mediated supersensitivity may concern both the postsynaptic and the presynaptic D-2 dopamine receptors. On the other hand, it is not clear enough whether a dopamine D-1 receptor-mediated supersensitivity might also be a causal factor of TD. 6. The analysis in animals, of biochemical and pharmacological effects of neuroleptics which do not induce TD showed that in some situations these drugs may also evoke the receptor-mediated supersensitivity concerning dopamine D-2 receptors. 7. The method of a prolonged (approx. 1 year) oral administration of neuroleptics seems to differentiate those which induce TD from those which do not, at least regarding the induction of an increase of Bmax for butyrophenone neuroleptics and an increase of apomorphine-induced stereotypy, however, some exceptions are noted. 8. The above analysis of clinical and experimental data suggests that the supersensitivity to dopamine in rats treated chronically with neuroleptics cannot be accepted as a model which reflects the etiopathogenesis of TD. Neither a positive nor a negative result obtained in this test is reliable enough, and either depends on the tested parameters (apomorphine stereotypy and [3H]spiperon binding seem to be the most reliable), route of neuroleptic administration, duration of treatment and, probably, a number of other, still unknown factors.(ABSTRACT TRUNCATED AT 400 WORDS)

Persistence of cyclicity of the plasma dopamine metabolite, homovanillic acid, in neuroleptic treated schizophrenic patients

The dopamine metabolite, homovanillic acid, decreases in concentration in plasma between 8:30 A.M. and 12:30 P.M. In patients with schizophrenia this cyclic change is attenuated by chronic neuroleptic treatment; however, if the 8 A.M. dose of neuroleptic is omitted, the decrease in level occurs. Presuming that neuroleptics attenuate the decline through a receptor mediated compensatory increase in dopamine release, it would appear that receptors are not fully occupied by neuroleptics even at therapeutically effective doses. The usual morning decrease in plasma cortisol levels was unaffected by neuroleptics.

Effects of haloperidol on neurotransmitter activity in the rat vas deferens

1. The effects of haloperidol on the responses of the isolated rat vasa deferentia to catecholamines and ACh were studied. 2. Haloperidol produced a competitive antagonism to responses elicited by NA and DA in the vas deferens. 3. The M1 and M2 muscarinic responses to ACh of the vas deferens were potentiated by this neuroleptic. 4. The AChE activity of the vas deferens was significantly depressed by pretreatment with haloperidol. 5. The ability of haloperidol to lower AChE activity was compared with that of neostigmine and it may be due to a similar molecular mechanism. 6. The present results suggest that haloperidol has anti-AChE properties that may be responsible for the potentiation of the responses to ACh. 7. The study indicates that haloperidol has a wider range of pharmacological actions than previously reported.

Tolerance to haloperidol-induced increases in dopamine metabolites: fact or artifact?

Haloperidol increased 3,4-dihydroxyphenylacetic acid and homovanillic acid concentrations in the striatum, nucleus accumbens and olfactory tubercle of both drug-naive rats and rats pretreated with haloperidol (10 injections). The increases in metabolite concentrations were greater in all brain regions of the naive rats, suggesting that haloperidol pretreatment resulted in a decreased responsiveness to the drug (tolerance). However, subchronic haloperidol injections also resulted in decreased basal metabolite concentrations in rats killed 48 h after the last injection. While the response of drug-experienced rats to haloperidol was attenuated relative to that of drug-naive rats, this difference could be accounted for entirely by the decreased basal metabolite concentrations that occur after repeated haloperidol injections.

Repeated atypical neuroleptic administration: effects on central dopamine metabolism monitored by in vivo voltammetry

Changes in extracellular DOPAC levels were monitored simultaneously in the nucleus accumbens and striatum of halothane/N2O anaesthetised rats using in vivo differential pulse voltammetry with carbon fibre electrodes following repeated administration of the atypical neuroleptics thioridazine and clozapine. Thioridazine (20 mg/kg s.c.) increased the DOPAC peak in the nucleus accumbens and striatum of rats treated with saline for the previous 21 days by 66% +/- 5 S.E.M. and 91% +/- 16 respectively. No such increase was recorded in the nucleus accumbens of rats previously treated with thioridazine (20 mg/kg s.c.) for 21 days. Similarly the increase in the striatum produced by a challenge dose on day 22 was markedly attenuated compared to controls although analysis of absolute DOPAC peak heights revealed extracellular DOPAC to be elevated above basal levels in this region (but not the nucleus accumbens) indicating a possible selective action of this drug to induce absolute tolerance to its acute effects in the nucleus accumbens after repeated administration. Administration of increasing doses of apomorphine (0.05, 0.1, 0.25 mg/kg s.c.) 1 h after a challenge dose of thioridazine (20 mg/kg s.c.) on day 22 to rats treated with the neuroleptic for the previous 21 days produced a progressive decrease in extracellular DOPAC levels both in the nucleus accumbens and striatum. Repeated administration of clozapine (50 mg/kg s.c.) for 21 days failed to induce tolerance to the acute effects of this drug, extracellular DOPAC levels increasing by 60% +/- 8 and 90% +/- 18 in the nucleus accumbens and striatum respectively following challenge with the drug on day 22.(ABSTRACT TRUNCATED AT 250 WORDS)

Apomorphine enantiomers' effects on dopamine metabolism: receptor and non-receptor related actions

The enantiomers of apomorphine (APO) inhibited dopamine synthesis in rat striatal synaptosomes, with R(-)-APO being about twice as potent as S(+)-APO. Sulpiride, a DA receptor antagonist, partially antagonized the inhibitory effects of only (-)-APO, suggesting that (-)-APO's, but not (+)-APO's, effects on dopamine synthesis may be at least partially receptor-mediated. The addition of 6-methyl-5,6,7,8-tetrahydrobiopterine (6-MPH4), an artificial cofactor for tyrosine hydroxylase, partially antagonized the inhibitory effects of both enantiomers, being considerably more effective against the (+)enantiomer. These data suggest that the APO enantiomers may directly inhibit enzymes within the synaptosome which regulate dopamine synthesis. Furthermore, investigations measuring DA synthesis rates in synaptosomes that had been pre-incubated with (-)-APO and then washed to remove the (-)-APO in the medium, indicate that (-)-APO may be retained by synaptosomes. Preliminary studies measuring the accumulation of [3H](-)-APO by synaptosomes also suggest that synaptosomes can accumulate APO. Although both APO enantiomers suppressed DA synthesis in vitro, only (-)-APO reduced striatal DA metabolite concentrations in vivo, and this reduction was prevented by haloperidol, a DA receptor antagonist. In addition, 6-MPH4 prevented the decrease in the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) produced by (-)-APO but not the decrease in the DA metabolite homovanillic acid (HVA).

Homocysteine prevents apomorphine-induced decreases in dopamine metabolites

Low doses of apomorphine reduced dopamine (DA) metabolite concentrations in mouse and rat striata. These reductions were prevented by pretreatment with homocysteine at a dosage which had been shown to inhibit protein carboxyl and other methylation reactions in brain. By itself, homocysteine did not alter DA metabolite concentrations. These findings are discussed with regard to the possible role of cerebral methylation reactions in mediating DA autoreceptor functions.