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

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

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.

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.

Effects of intermittent and continuous subchronic administration of raclopride on motor activity, dopamine turnover and receptor occupancy in the rat

With the purpose of finding means to circumvent the marked pharmacokinetic differences of raclopride between rats and man, the effects of intermittent and continuous administration of raclopride were compared in rats. Intermittent administration of raclopride via subcutaneous injections resulted in a prompt increase of dopamine (DA) turnover and decrease of motor activity but these effects were of short duration, probably due to rapidly decreasing raclopride DA D2 receptor occupancy. In contrast, but similar to schizophrenic patients on raclopride treatment, stable plasma raclopride levels and a steady DA D2 receptor occupancy above 70% were produced in the caudate-putamen and nucleus accumbens/olfactory tubercle, when raclopride was administered continuously via minipumps at daily doses above 2 mg/kg. Tolerance to the acute effects of raclopride on DA turnover and locomotion was found with both routes of administration but it was more marked with continuous administration. At continuous raclopride administration, tolerance to the effects of raclopride on DA turnover and spontaneous motor activity as well as supersensitivity to amphetamine-induced motor activity occurred when 70% or more of DA D2 receptor sites were occupied, i.e. the same degree of receptor occupancy as found in patients given therapeutic doses of raclopride.

Modulatory role of dopamine on excitatory amino acid receptors

1. In extensively washed synaptic membrane preparations from rat prefrontal cortex, the "in vitro" addition of either the D1 (SKF 38393) or the D2 (LY 171555) specific agonists markedly decreased the apparent affinity of the NMDA receptor antagonist [3H]-MK801 specific binding. In the same membrane preparation, the concentration of L-glutamate required to produce half maximal enhancement of [3H]-MK801 binding was approximately the same both in the presence or in the absence of dopaminergic drugs. 2. I.c.v. administration of the neurotoxin 6-OHDA resulted in a dramatic reduction of dopamine (DA) prefrontal cortex levels, whilst repeated administrations (21 consecutive days) with either the D1 (SCH 23390) or the D2 (YM 09151-2) selective antagonist failed to change DA and DOPAC contents. 3. Repeated administrations with the D1 receptor blocker SCH 23390 selectively increased the Bmax values of [3H]-SCH 23390 binding while [3H]-spiroperidol binding was increased both by repeated administrations of YM 09151-2 and by i.c.v. injection of 6-OHDA. 4. Although both chronic D2 blockade and 6-OHDA lesions consistently increased D2 receptor number, in extensively washed synaptic plasma membranes (SPM) of rats repeatedly administered with YM 09151-2 but not with 6-OHDA, the [3H]-MK801 binding was increased. 5. It is concluded that the effects of NMDA receptor activation could not be directly mediated by stimulation of DA release, but are highly dependent upon the presence of DA axon terminals.

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.

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.

Depolarization inactivation of dopamine neurons: terminal release characteristics

The functional consequences of chronic treatment with haloperidol (0.5 mg/kg s.c. for 21-23 days) on striatal extracellular levels of dopamine and excitatory amino acids, aspartate and glutamate, were examined using microdialysis techniques. Our studies indicate that, in both awake and anesthetized animals, chronic haloperidol treatment does not appear to change basal outflow of dopamine and its response to an exogenous antagonist (i.e., a challenge dose of haloperidol). Furthermore, in chronic haloperidol and vehicle-treated animals, extracellular dopamine levels were decreased below our limit of detection following perfusion of tetrodotoxin through the probe, or into the medial forebrain bundle, suggesting that in both groups of animals extracellular dopamine levels are neuronally derived and seemed to depend equally on impulse flow. However, some differences were observed between the vehicle and haloperidol-treated animals: the excitatory action of 30 mM K+ on extracellular dopamine levels was decreased, and extracellular levels of glutamate were significantly increased, in animals treated chronically with haloperidol. The alterations in extracellular glutamate levels suggests that events at the terminal may be involved in maintaining the "normal" extracellular dopamine levels. Furthermore, the decrease in response to stimulation by K+ suggests that chronic haloperidol treatment may decrease the responsivity of the striatal dopamine system to stimuli.

Cortical regulation of subcortical dopamine systems and its possible relevance to schizophrenia

A unique model of DA system regulation is presented, in which tonic steady-state DA levels in the ECF act to down-regulate the response of the system to pulsatile DA released by DA cell action potential generation. This type of regulation is similar in many respects to the phenomenon proposed to mediate the action of norepinephrine on target neurons; i.e., an increase in the "signal-to-noise" ratio as measured by postsynaptic cell firing (Freedman et al., 1977; Woodward et al., 1979). However, in this model the signal and the noise are neurochemical rather than electrophysiological. Furthermore, the "noise" (tonic DA in the ECF) actually down-regulates the "signal" (phasic DA release) directly, and thereby provides a "signal" of its own that affects the system over a longer time-course. Therefore, the difference between signal and noise may also depend on the time frame under which such determinations are made.

The depolarization block hypothesis of neuroleptic action: implications for the etiology and treatment of schizophrenia

Antipsychotic drugs are known to block dopamine receptors soon after their administration, resulting in an increase in dopamine neuron firing and dopamine turnover. Nonetheless, antipsychotic drugs must be administered repeatedly to schizophrenics before therapeutic benefits are produced. Recordings from dopamine neurons in rats have revealed that chronic antipsychotic drug treatment results in the time-dependent inactivation of dopamine neuron firing via over-excitation, or depolarization block. Furthermore, the clinical profile of the response to antipsychotic drugs appears to correspond to the dopamine system affected: antipsychotic drugs that exert therapeutic actions in schizophrenics inactivate dopamine neuron firing in the limbic-related ventral tegmental area, whereas drugs that precipitate extrapyramidal side effects cause depolarization block of the motor-related substantia nigra dopamine cells. One factor that remains unresolved with regard to the actions of antipsychotic drugs is the relationship between dopamine turnover and depolarization block--i.e., why does a significant level of dopamine release or turnover remain after antipsychotic drug treatment if dopamine cells are no longer firing? We addressed this question using an acute model of neuroleptic-induced depolarization block. In this model, dopamine cells recorded in rats one month after partial dopamine lesions could be driven into depolarization block by the acute administration of moderate doses of haloperidol. However, similar doses of haloperidol, which were effective at increasing dopamine levels in the striatum of intact rats, failed to change dopamine levels in lesioned rats. This is consistent with a model in which neuroleptic drugs exert their therapeutic effects in schizophrenics by causing depolarization block in DA cells, thereby preventing further activation of dopamine neuron firing in response to external stimuli. Thus, attenuating the responsivity of the dopamine system to stimuli may be more relevant to the therapeutic actions of antipsychotic drugs than receptor blockade or decreases in absolute levels of dopamine, which could presumably be circumvented by homeostatic adaptations in this highly plastic system.

Effects of D1 and D2 dopamine antagonists on behavior of polydipsic rats

The behavioral and neurochemical effects of SCH3390 (SCH), a dopamine (DA) D1 antagonist, and haloperidol (HAL), a DA D2 receptor antagonist, on schedule-induced polydipsia (SIP) were examined. Once animals were made polydipsic, a vehicle or one of three doses of SCH or HAL were administered to seven groups of rats in a series of three five-session blocks in a drug condition, no-drug condition, drug condition design. Detailed behavioral measures and brain regional levels of monoamine neurotransmitters and their major acidic metabolites were analyzed. The volume of water consumed and the percent of time spent drinking was reduced dose dependently by both SCH and HAL. As drinking decreased, the time spent chewing increased for both drugs. The total amount of time animals engaged in all oral behaviors was not changed, suggesting that chewing was substituted for drinking. Neurochemical analysis revealed that HAL increased striatal DA significantly. The polydipsic paradigm may be an advantageous model for examining neuroleptics due to SIP's sensitivity to extrapyramidal side effects.

Striatal dopamine metabolism increases during long-term haloperidol administration in rats but shows tolerance in response to acute challenge with raclopride

The release and metabolism of dopamine (DA) in the striatum of rats during long-term haloperidol administration (32 weeks) was assessed using in vivo microdialysis. Basal levels of homovanillic acid (HVA) and dihydroxyphenylacetic acid (DOPAC) were significantly elevated over control values, while basal DA release was not significantly increased. The specific DA D2 receptor antagonist, raclopride (0.5 mg/kg, i.p.), increased DA release and metabolism in control animals, but this effect was profoundly blocked in the haloperidol treated group. These results suggest that chronic haloperidol treatment may induce compensatory increases in basal DA activity even though response to an acute D2 antagonist shows significant tolerance.

The enhancement of the hypomotility induced by small doses of haloperidol in the phase of dopaminergic supersensitivity in mice

Dopaminergic supersensitivity in mice was induced by pretreatment with a single injection of haloperidol (4.8 mg/kg). After the pretreatment, further treatment with haloperidol (0.6 or 0.01 mg/kg) was made at varying intervals, and catalepsy, locomotor activity and homovanillic acid (HVA) were measured. The intensity of the supersensitivity was evaluated by enhanced apomorphine (1 mg/kg)-induced climbing behavior. Supersensitivity was displayed on the 2nd and the 4th day. The cataleptogenic effect of haloperidol (0.6 mg/kg) was significantly weakened on the 1st, 2nd and 4th days. The motor inhibitory effect of haloperidol (0.01 mg/kg) increased on the 1st, 2nd and 4th days. Homovanillic acid was measured in the striatum and the prefrontal cortex on the 2nd day. Haloperidol (0.6 mg/kg) increased the concentrations of HVA in both regions of the brain. The increase in the concentrations of HVA in the striatum was blunted after the pretreatment, but such tolerance did not develop in the prefrontal cortex. Haloperidol (0.01 mg/kg) did not influence the concentration of HVA in both regions. These results suggest that the behavioral effect of a small dose of haloperidol may be enhanced, rather than reduced, in the phase of supersensitivity.

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.

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.

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.

Effects of long-term administration of antidepressants and neuroleptics on receptors in the central nervous system

1. A review of the effects of long-term administration of antidepressants and neuroleptics on receptors in the central nervous system is presented. 2. The effects of antidepressants on adenylate cyclase activity and on receptor binding in brain tissue are discussed. Effects on a variety of receptor types are considered. 3. The utilization of electrophysiological, behavioral, and neurochemical studies to assess receptor function after chronic antidepressant administration is discussed, as is the use of peripheral receptor estimations in clinical studies. 4. Animal studies on the actions of chronic administration of neuroleptics on pre- and postsynaptic dopamine receptors are reviewed. Effects of these drugs on dopamine receptors in humans are considered from the following perspectives: postmortem and in vivo binding studies in schizophrenia, tardive dyskinesia, and central versus peripheral receptor estimation.