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

Olfactory bulb monoamine concentrations vary with time of day

The olfactory bulb (OB) has been recently identified as a circadian oscillator capable of operating independently of the master circadian pacemaker, the suprachiasmatic nuclei of the hypothalamus. OB oscillations manifest as rhythms in clock genes, electrical activity, and odor sensitivity. Dopamine, norepinephrine, and serotonin have been shown to modulate olfactory information processing by the OB and may be part of the mechanism that underlies diurnal changes in olfactory sensitivity. Rhythmic release of these neurotransmitters could generate OB rhythms in electrical activity and olfactory sensitivity. We hypothesized that these monoamines were rhythmically released in the OB. To test our hypotheses, we examined monoamine levels in the OB, over the course of a day, by high-performance liquid chromatography coupled to electrochemical detection. We observed that dopamine and its metabolite, 3-4-dihydroxyphenylacetic acid, rhythmically fluctuate over the day. In contrast, norepinephrine is arrhythmic. Serotonin and its metabolite hydroxyindoleacetic acid appear to rhythmically fluctuate. Each of these monoamines has been shown to alter OB circuit behavior and influence odor processing. Rhythmic release of serotonin may be a mechanism by which the suprachiasmatic nuclei communicate, indirectly, with the OB.

Parametric studies of antipsychotic-induced sensitization in the conditioned avoidance response model: roles of number of drug exposure, drug dose, and test-retest interval

Repeated haloperidol and olanzapine treatment produces an enhanced disruption of avoidance responding, a validated measure of antipsychotic activity. Experimental parameters affecting this sensitization-like effect have not been thoroughly examined. The present study investigated the role of three parameters (number of injections, dose, and interval between initial exposure and challenge) in antipsychotic sensitization in the conditioned avoidance response paradigm. Well-trained Sprague-Dawley rats received different numbers of drug treatment (1-5 days) or different doses of haloperidol (0.025-0.10 mg/kg, subcutaneously) or olanzapine (0.5-2.0 mg/kg, subcutaneously). After certain time intervals (4, 10 or 17 days), they were tested for the expression of haloperidol or olanzapine sensitization in a challenge test in which all rats were injected with a lower dose of haloperidol (0.025 mg/kg) or olanzapine (0.5 mg/kg). Throughout the drug-treatment period, both haloperidol and olanzapine dose-dependently enhanced their disruption of avoidance responding. Three days later, the sensitization induced by a low dose of haloperidol (0.025 mg/kg) or olanzapine (0.5 mg/kg) was only apparent in rats that received treatment for 5 days, but not in those that received treatment for 1-4 days. The sensitization induced by the medium and high doses of haloperidol (0.05 and 0.10 mg/kg) or olanzapine (1.0 and 2.0 mg/kg) was still robust even with only 3 days of treatment. The sensitization induced by a 3-day haloperidol (0.10 mg/kg) and olanzapine (2.0 mg/kg) treatment was long-lasting, still detectable 17 days after the last drug treatment. This study suggests that antipsychotic sensitization is a robust behavioral phenomenon. Its induction and expression are strongly influenced by parameters such as number of drug exposures, drug dose, and test-retest interval. Given the importance of antipsychotic sensitization in the maintenance of antipsychotic effects in the clinic, this study introduces a paradigm that can be used to investigate the behavioral and neurobiological mechanisms underlying antipsychotic sensitization.

Delivery systems and dosing for antipsychotics

Schizophrenia is a devastating illness, affecting approximately 1-2 % of the world population. Age of onset is generally between 20 and 30 years of age with a chronic, unremitting course for the duration of the patient's life. Although schizophrenia is among the most severe and debilitating illnesses known to medicine, its treatment has remained virtually unchanged for over 50 years. This chapter covers several major concepts in experimental drug development and delivery: (1) the concept of "typical" vs. "atypical" classifications for antipsychotic drugs as it relates to dosing; (2) the development of depot formulations for improved medication adherence; and (3) several promising areas for future therapeutic advances related to the methods and duration of drug administration. These areas include sublingual, injectable, and implantable drug delivery strategies that have the potential to effect rapid and dramatic improvements in schizophrenia outcomes.

Antipsychotic dosing and drug delivery

This chapter addresses the current state of affairs regarding proposed mechanism of action for antipsychotic medications and how this mechanism relates to dosing and delivery strategies. The initial portion describes the history of antipsychotic medication, including key discoveries that contribute to the dopamine hypothesis of schizophrenia and provide evidence that dopamine D2 receptor antagonism remains the most copasetic explanation for both determination of dose and degree of efficacy for current antipsychotic medications. Early observations regarding the unique properties of clozapine and how those observations led to the misconception and misnomer of atypicality are also discussed. Subsequent sections relate the dosing of available medications using chlorpromazine equivalents, with a discussion of non-D2-related mechanisms to antipsychotic effects. The balance of the chapter explores the temporal pattern of receptor occupancy as a key determinant of antipsychotic effectiveness, noting that continuous infusion would present the optimal method of treatment. In addition to the pharmacodynamic benefits of continuous long-term delivery systems, the incidence, causes, and clinical consequences of poor adherence are addressed. These observations are then discussed in the context of clinical studies and meta-analyses, demonstrating superiority of long-term depot preparations over oral administration. However, despite overwhelming evidence in favor of long-term delivery systems, few options are available to provide such ideal medication delivery profiles. Barriers to creating traditional depot preparations for a large number of antipsychotic agents, as well as efforts to address these limitations with polymer-based microspheres are described. The potential extension of current formulations to very long-term delivery implants using biodegradable and nonbiodegradable platforms is then described. Benefits as well as limitations of such systems are discussed with respect to clinical and ethical issues as well as a brief description of potential regulatory and logistic barriers to developing better delivery options. In summary, this chapter describes the basis for relating the dose of all existing antipsychotic medications to dopamine D2 receptor affinity and the potential contribution of continuous occupancy to enhanced efficacy through superior biological effects and improved adherence.

D2-receptor upregulation is dependent upon temporal course of D2-occupancy: a longitudinal [11C]-raclopride PET study in cats

Long-term occupancy of dopamine D(2)-receptors, as achieved by chronic treatment with antipsychotics, leads to D(2)-receptor upregulation, and this upregulation is thought to be responsible for loss of efficacy and development of tardive dyskinesia. However, little is known about the parameters of D(2)-receptor blockade (duration and percentage of blockade) that lead to upregulation. In this study, we investigated the effects of different degrees (60 vs >80%) and durations (a transient peak vs 24 h/day) of D(2)-receptor blockade on inducing this upregulation. These different patterns of D(2)-receptor occupancy kinetics were produced in cats using bolus vs constant infusion of haloperidol for 4 weeks. D(2)-receptors were measured using positron emission tomography and Scatchard analyses of [(11)C]raclopride binding, before and after withdrawal of treatment. Continuously high (80% for 24 h/day) D(2)-receptor blockade led to a robust upregulation of striatal D(2)-receptors that was maximal at 1-week withdrawal (35+/-5%) and still detectable at 2-week withdrawal (20+/-3%). This pattern of D(2)-receptor blockade also induced behavioral tolerance to the effect of haloperidol on spontaneous locomotor activity. Continuously moderate (60% for 24 h/day) or transiently high (80% for a few hours/day) D(2)-receptor blockade did not produce any of these effects. The long-term effect of haloperidol on D(2)-receptor density and behavioral tolerance thus appears to be dependent not only on a critical threshold of D(2)-receptor blockade but also on the daily duration of D(2)-receptors blockade. This suggests that as far as antipsychotics are concerned, not only dose but disbursment throughout the day have an impact on eventual pharmacodynamic and behavioral outcomes.

Less is more: antipsychotic drug effects are greater with transient rather than continuous delivery

BACKGROUND

Most studies on the effects of antipsychotics focus on achieving threshold levels of the drug. The speed and frequency with which drug concentrations reach threshold levels and rise and fall within the day are generally ignored. Based on prior data, we predicted that variations in the within-day kinetics of antipsychotic drug delivery would produce different outcomes, even if we held achieved dose, route, and total duration of treatment constant.

METHODS

We compared the effects of within-day continuous (via minipump) versus transient (via subcutaneous injection) haloperidol treatment (n = 4-9/condition/experiment) at doses that yield equivalent peak levels of striatal D2 receptor occupancy (approximately 74%).

RESULTS

Over time, transient haloperidol gained efficacy, while continuous haloperidol lost efficacy in two animal models of antipsychotic-like effects (the suppression of amphetamine-induced locomotion and conditioned avoidance responding). This was related to the fact that continuous treatment led to a greater increase in striatal D2 receptor numbers--particularly D2 receptors in a high-affinity state for dopamine--relative to transient treatment and produced behavioral dopamine supersensitivity (as indicated by an enhanced locomotor response to amphetamine following antipsychotic treatment cessation). Treatment kinetics also influenced the postsynaptic response to haloperidol. Transient treatment increased striatal c-fos messenger RNA (mRNA) expression, while continuous treatment did not.

CONCLUSIONS

Relative to continuous antipsychotic exposure, within-day transient exposure is more efficacious behaviorally and is associated with a distinct molecular and gene expression profile. Thus, differences in the within-day kinetics of antipsychotic treatment can have different efficacy, and the potential clinical implications of this should be explored further.

Low dose quetiapine reverses deficits in contextual and cued fear conditioning in rats with excitotoxin-induced hippocampal neuropathy

Previous studies have demonstrated that adult rats with excitotoxic lesions of the hippocampus display deficits in memory-related behaviors similar to the memory deficits associated with schizophrenia. In this study, we assessed the sub-chronic effects of quetiapine, risperidone and haloperidol on performance deficits after intracerebroventricular administration of the excitotoxin, kainic acid, using paradigms for contextual and cued fear conditioning and spatial reversal learning in rats. The effects of three doses of quetiapine (5, 10 and 20 mg/kg) and single doses of risperidone (0.5 mg/kg) and haloperidol (0.15 mg/kg) were compared. Quetiapine administration at the lowest dose (5 mg/kg) reversed deficits in contextual and cued fear conditioning, but not deficits in spatial reversal learning, in kainic acid-treated animals. However, the two higher doses of quetiapine, and the single doses of risperidone and haloperidol, did not reverse any of the kainic acid-induced behavioral deficits. These results may be relevant to the effects of quetiapine and other antipsychotic drugs on memory deficits in patients with schizophrenia.

Poor evidence for depolarization block but uncoupling of nigral from striatal dopamine metabolism after chronic haloperidol treatment in the rat

Chronic haloperidol treatment induces depolarization block in midbrain dopamine neuronal systems. We studied the effect of this treatment on nigrostriatal dopamine catabolism using microwave fixation in situ of the brain to prevent post-mortem changes. Male Sprague-Dawley rats were given haloperidol (0.4 mg/kg/day, i.p.) or vehicle for 21 days. On day 22, some rats in each group received a haloperidol challenge (0.4 mg/kg, i.p.), and the remaining rats were given the vehicle. Dopamine metabolite levels 60 min after the challenge were assayed by combined gas chromatography-mass fragmentography. Haloperidol pretreatment significantly modified haloperidol challenge effect on regional dopamine metabolite contents. The challenge elevated all striatal metabolites studied similarly in the chronic vehicle- or chronic haloperidol-pretreated rats. In contrast, it did not significantly affect nigral dopamine metabolites except it elevated 3,4-dihydroxyphenylacetic acid in the haloperidol-pretreated rats. A linear correlation between the nigral and striatal contents of 3-methoxytyramine (R = 0.72, p = 0.03), and a trend for correlation (R = 0.65, p = 0.06) between the respective 3,4-dihydroxyphenylacetic acid contents were found after the haloperidol challenge in the vehicle-pretreated rats only. These results suggest that chronic haloperidol treatment uncouples somatodendritic dopamine turnover and release from those in the axon terminals of nigrostriatal dopamine neurons.

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.

An integrated view of pathophysiological models of schizophrenia

Pathophysiological processes that underlie the profound neuropsychiatric disturbances in schizophrenia are poorly understood. However, the clinical course of the disease, and a number of clinical and basic science observations, provide direction for formulating pathophysiological models that could be empirically tested. For example, repeated psychostimulant administration to healthy subjects can induce psychotic symptoms, and acute stimulant challenge in schizophrenia patients can precipitate psychosis. Also, NMDA antagonists induce positive, negative, and cognitive schizophrenic-like symptoms in healthy volunteers and precipitate thought disorder and delusions in schizophrenia patients. These human studies provide support for the dopamine and NMDA receptor hypofunction hypotheses of schizophrenia. Well-documented effects of NMDA antagonists on dopamine systems provide a basis to integrate the dopamine and NMDA receptor hypofunction hypotheses. Furthermore, it has become apparent that prominent actions of antipsychotic drugs, especially those with 'atypical' properties, involve antagonism of behavioral, electrophysiological and brain metabolic effects produced by administration of NMDA receptor antagonists. A confluence of clinical and basic science data suggests that an early developmental insult, potentially involving reduced NMDA receptor function, could facilitate sensitization of dopamine systems, leading to the formal onset of schizophrenia in late adolescence and early adulthood. Although clearly speculative, this conceptual model is consistent with existing evidence and suggests lines of future experimental investigation.

Schizophrenia and psychostimulant abuse: a review and re-analysis of clinical evidence

The authors selected articles from those published between 1975 and 1994 that specifically documented psychostimulant abuse in patients determined to be schizophrenic according to recent and relatively uniform diagnostic criteria. These articles indicated that the incidence of psychostimulant abuse in schizophrenics is 2-5 times higher than that of the general public. Additionally, unlike the decline in stimulant use seen in older adults in the general population, high rates of abuse appeared to be maintained in schizophrenics. Although the incidence of abuse in this group was high, comparisons of abuse rates generated by self report with those obtained by urinalysis indicated that the frequency of abuse is being underestimated by 15-21%. Potential factors contributing to stimulant abuse in schizophrenics, including the disease process, and the influence of chronic neuroleptic medication, were evaluated. Results indicated that the incidence of psychostimulant abuse was neither a common property of psychiatric patients, nor exclusive to schizophrenics, but appeared to be related to chronic treatment with neuroleptic drugs. Symptom severity was generally similar in schizophrenic abusers and non-abusers, which also suggested a degree of independence from the disease process. In a majority of the studies surveyed, abuse of stimulants followed disease onset. It was also found that stimulant abuse was associated with marked increases in hospitalization in this patient group, including those known to be neuroleptic medication compliant. Possible explanations for the initiation and maintenance of psychostimulant abuse in schizophrenics are discussed in relation to clinical and preclinical evidence on drug addiction.

Irreversible blockade of D2 dopamine receptors by fluphenazine-N-mustard increases D2 dopamine receptor mRNA and proenkephalin mRNA and decreases D1 dopamine receptor mRNA and mu and delta opioid receptors in rat striatum

The consequences of irreversibly-inhibiting D2 dopaminergic receptors on the expression of D1 and D2 dopamine receptor mRNAs and proenkephalin mRNA and on the levels of mu- and delta-opioid receptors in rat striatum were studied following single or repeated administration of the irreversibly-acting D2 dopamine receptor antagonist, fluphenazine-N-mustard (FNM). The density of dopamine and opioid receptors was determined by receptor autoradiography and the levels of the mRNA for the D1 and D2 dopamine receptors and proenkephalin were measured by in situ hybridization histochemistry. Repeated treatment of rats with FNM for 6 days produced more than 80% inhibition of D2 dopamine receptors but less than 25% inhibition of D1 dopamine receptors. Repeated treatment with FNM also resulted in statistically significant increases in D2 dopamine receptor mRNA but decreases in D1 dopamine receptor mRNA. In contrast, acute treatment with FNM for 3 h had no significant effects on D1 or D2 dopamine receptor mRNAs in striatum. An examination of the effects of FNM on the opioid system showed that repeated treatment with FNM for 6 days produced more than a 2-fold increase in the expression of proenkephalin mRNA in striatum. This was accompanied by significant decreases in mu- and delta-opioid receptors in striatum, mainly by reducing the size of the patch compartment of striatum. Acute treatment with FNM for 3 h produced small increases in proenkephalin mRNA and mu-opioid receptors in striatum but had no significant effects on delta-opioid receptors. These results suggest that persistent inhibition of D2 dopamine receptors differentially regulates the expression of D1 and D2 dopamine receptor mRNA in striatum, and that the magnitude, duration and interval of inhibiting dopaminergic transmission may be important factors in regulating dopamine receptor mRNA expression. These results also suggest that D2 dopamine antagonists indirectly down-regulate opioid receptors by increasing the expression of proenkephalin mRNA, thereby increasing enkephalin which, in turn, decreases opioid receptors in striatum.

Effects of dose and interdose interval on locomotor sensitization to the dopamine agonist quinpirole

To assess whether the interval between injections affects the course of locomotor sensitization to quinpirole, groups of rats were injected every 2, 4, or 8 days with quinpirole (0, 0.025, 0.25, 0.5, and 2.5 mg/kg; n = 222) and their locomotor activity monitored after each injection for a total of 10 tests. Results indicate that the number of drug injections, rather than the interval between them, predominantly controls the development of locomotor sensitization to quinpirole. It is suggested that this may reflect a rapid induction but slow decay time for a response-enhancing factor stimulated by each injection of quinpirole, and that the effects of this putative factor are cumulative but saturable.

Single preexposure to fluphenazine produces persisting behavioral sensitization accompanied by tolerance to fluphenazine-induced striatal dopamine overflow in rats

Single, previous exposure to a neuroleptic has been shown to produce long-lasting changes in various measures of behavior and neurochemistry upon subsequent drug exposure. The present study examined the effects of a single preexposure to fluphenazine (0.3 or 1.0 mg/kg) or vehicle on the effects of subsequent fluphenazine administration 15 or 30 days later. Intracranial microdialysis was used to assess changes in striatal extracellular dopamine concentrations. Animals were tested for catalepsy response on a horizontal bar test while concurrently collecting dialysis samples. Previous fluphenazine exposure produced a profound tolerance to the effects of subsequent fluphenazine at day 15 or day 30 on increasing extracellular dopamine levels. In addition, animals that had received fluphenazine on the first trial showed significant sensitization to the cataleptic effects of fluphenazine at both time points. Pretreatment with vehicle did not result in tolerance to dopamine overflow and there was only minimal evidence of cataleptic sensitization to a subsequent fluphenazine challenge. Although the tolerance to dopamine overflow may only indirectly relate to behavioral sensitization, these results support the hypothesis that significant behavioral and neurochemical alterations persist for prolonged time periods following single neuroleptic exposure.

Electrical kindling of rats treated discontinuously or continuously with haloperidol

Intermittent as opposed to continuous treatment of rats with haloperidol resulted in a long-lasting potentiation of oral activity. To examine if this behavioural sensitization to discontinuous neuroleptic treatment facilitates seizure development in electrical kindling, rats treated either intermittently or continuously with haloperidol for 15 weeks were kindled in the nucleus amygdala. Development of kindled seizures was significantly faster in the intermittently treated group (P < 0.01) than in controls or continuously treated rats. Furthermore, discontinuously treated animals displayed electroencephalographic afterdischarges in the substantia nigra from the beginning of treatment. The findings of cross-sensitivity between electrical amygdala kindling and pharmacological sensitization and of early appearance of epileptiform nigral activity have implications for the pathogenesis of both conditions. We suggest that depressed gamma-aminobutyric acid activity in substantia nigra could be a common mechanism.

Persistent vacuous chewing in rats following neuroleptic treatment: relationship to dopaminergic and cholinergic function

In order to relate the effects of pharmacological intervention to neuroleptic induced increases in oral activity rats were treated continuously (7 mg/kg per week) or discontinuously (7 mg/kg per week or 2 mg/kg per week) with haloperidol for 6 months. Only the two intermittently treated groups developed persisting increases in vacuous chewing movements (VCM) following drug withdrawal. Opposed to control animals and continuously treated rats, the discontinuously treated groups demonstrated significant elevation in mouth movements following stimulation with the dopamine (DA) D1 receptor agonist SK&F 38393 (23 mg/kg), whereas they did not response to an acute challenge with the selective DA D1 receptor antagonist NNC-756 (0.1 mg/kg). The DA D2 receptor antagonist raclopride (1 mg/kg) provoked a general fall in VCM; however, this was only significant in rats treated intermittently with haloperidol 7 mg/kg per week and in control rats. Intermittent neuroleptic treatment also increased apomorphine-induced stereotypy. The effect of challenge with the anticholinergic drug scopolamine (0.25 mg/kg) was not related to oral activity; furthermore, the finding of severe agitation in rats tested with the latter drug points to caution in the interpretation of rating of rats treated with anticholinergics. These results support that intermittent ingestion of neuroleptic drugs lead to long-lasting increases in VCM. They also suggest a relation of persisting elevated oral activity to supersensitivity to DA receptor agonists, as opposed to subsensitivity to D1 receptor antagonists.

Interactions between chronic haloperidol treatment and cocaine in rats: an animal model of intermittent cocaine use in neuroleptic treated populations

This experiment investigated the possibility that rats maintained on chronic haloperidol treatment would show increased behavioral responsiveness to cocaine, similar to that observed in human stimulant abusers who are chronically treated with neuroleptics. Thus, the effects on locomotion and stereotyped behavior of intermittent injections of cocaine were investigated in female rats receiving chronic haloperidol treatment. Daily injections of haloperidol (0.2 mg/kg, IP) or vehicle were administered for 6, 12 or 18 days prior to the start of testing with cocaine and were then continued throughout cocaine testing. All rats received four doses of cocaine (0.0, 3.0, 7.5, or 15.0 mg/kg, IP) in random order with an intervening vehicle day between successive drug days. The four dose sequence of cocaine was repeated a total of four times. Initial cocaine administration produced dose dependent increases in locomotion and stereotyped behavior. When the sequence of cocaine doses was repeated, differences among treatment groups emerged. Groups treated with haloperidol exhibited heightened locomotion in response to cocaine and with repeated injections, showed a higher rate of behavioral sensitization than control animals. These differences in the behavioral response to cocaine were maintained for at least 2 months following termination of daily haloperidol treatment. In order to examine the mechanisms underlying this heightened responsiveness to cocaine, apomorphine-induced locomotion (dose range, 0-250 micrograms/kg, SC) was determined. Regardless of dose, rats treated with haloperidol showed different temporal patterns of locomotion in response to apomorphine suggesting that the increased response to cocaine was related to changes in dopaminergic receptor sensitivity.

Subcortical dopamine and serotonin turnover during acute and subchronic administration of typical and atypical neuroleptics

The effects of acute (1 day) and subchronic (28 days) treatment with three atypical antipsychotic drugs [clozapine, (+/-)-sulpiride and (-)-3-PPP] on dopamine and serotonin turnover in both the nucleus accumbens (NA) and corpus striatum (CS) of rodents was compared to haloperidol and saline treatment. The equivalent doses of all drugs were determined based upon their ability to compete in vivo for 3H-spiperone binding in the NA and CS. All three atypical drugs, compared to haloperidol, produced preferential elevations of dopamine turnover in the NA. Further, the development of tolerance of this effect was more apparent for the three atypical drugs than for haloperidol. Surprisingly, all three atypical drugs, but not haloperidol, produced changes in serotonin turnover, despite the fact that (+/-)-sulpiride and (-)-3-PPP have no known direct effects on brain serotonin systems. All three atypical drugs produced acute increases in serotonin turnover in both the NA and CS, followed by later diseases.

Comparison of chronic intermittent haloperidol and raclopride effects on striatal dopamine release and synaptic ultrastructure in rats

The effects of chronic intermittent administration (7 months) of two neuroleptics, haloperidol (HAL) and raclopride (RAC), were compared using several different measures. Both drugs were administered weekly by subcutaneous injection at 7.0 mg/kg. Both neuroleptics consistently produced catalepsy throughout the treatment period, although HAL was generally more cataleptogenic than RAC. Assessment of dopamine (DA) release in the caudate putamen (CPu), through the use of in vivo microdialysis, showed that chronic HAL or RAC administration caused a prolonged decrease of DA release in response to a low dose of the DA D2 agonist quinpirole (0.03 mg/kg, sc). Injection of the muscarinic agonist pilocarpine (1.0 mg/kg, IP) did not have any significant within-group effects, although both neuroleptic treatment groups showed decreased DA release when compared to controls. Ultrastructural analysis of the dorsolateral CPu showed that both HAL and RAC treatment resulted in a significant increase in the number of perforated synapses, which contain a discontinuous density along the postsynaptic membrane. These results demonstrate that two different DA D2 receptor antagonists produce a similar effect on DA function and ultrastructural changes within the CPu following chronic, intermittent treatment.

Quantification of SCH 39166, a novel selective D1 dopamine receptor antagonist, in rat brain and blood

A gas chromatographic method for measuring concentrations of a novel D1 antagonist SCH 39166 [(-)-trans-6,7,7a,8,9-13b-hexahydro-3-chloro-2-hydroxy-N-methyl-5- H-benzo[d]naphto(2,1-6)azepine] in rat brain and plasma was developed. The method was applied to descriptive pharmacokinetics of two subcutaneous doses of SCH 39166 (0.25 mg/kg and 2.5 mg/kg). For comparison, concentrations of the "prototype" D1 antagonist SCH 23390 (0.25 mg/kg, SC) [R-(+)-chloro-2,3,4,5-tetrahydro-3-methyl-5-phenyl-1-H-3-benzazepine] were also measured in plasma and brain. SCH 23390 (0.25 mg/kg, SC) had a very short elimination half-life of about 30 min in plasma, and disappeared in a slightly slower manner from striatum and cortex. SCH 39166 (0.25 and 2.5 mg/kg, SC), however, had a longer elimination half-life of about 1.5-2.5 h in plasma and brain. Interestingly, the 2.5 mg/kg dose of SCH 39166 produced only two-to five-fold increases in maximum concentrations in plasma and brain compared to the 0.25 mg/kg dose. The reason for this is not clear. The ability of these two doses of SCH 39166 to induce catalepsy in the bar test was also evaluated. It was found that SCH 39166 in these two doses, unlike SCH 23390, was not cataleptic. In conclusion, these pharmacokinetic features of SCH 39166 in the rat should be useful when designing experiments with this novel selective D1 antagonist. Furthermore, the longer elimination half-life of SCH 39166 makes it a more useful probe in pharmacodynamic comparisons of D1 receptor antagonists and classical as well as atypical neuroleptics.

Sensitization to haloperidol-induced suppression of milk intake: effect of interdose interval

The purpose of this study was to determine the effect of manipulating the interdose interval (IDI) on the suppression of milk intake induced by haloperidol (HAL). Groups of rats were given chronic injections of either HAL (0.625 mg/kg) or saline at IDIs of 1, 2, 7, or 14 days. Dose-response curves were determined at the conclusion of the chronic phase. The results indicated that injections of HAL given at IDIs of 1 or 2 days produced neither tolerance nor sensitization, whereas injections given at intervals of 7 or 14 days produced sensitization. Sensitization was also observed in the control groups, perhaps as a result of the intermittent schedule of HAL injections given during the dose-response tests. Sensitization to HAL was not accompanied by changes in sensitivity to amphetamine. The results of this experiment are consistent with those of other studies in showing that the behavioral effects of neuroleptics are strongly influenced by the schedule of injections. In addition, evidence is presented that sensitization to HAL-induced hypophagia is contingent on behavioral experience under the drug.

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.

Effects of acute and chronic systemic administration of some typical antipsychotic drugs on turnover of dopamine and potassium ion-induced release of dopamine in the striatum of the rat in vivo

Chronoamperometry was used to measure the dose dependency of antipsychotic drug (spiperone, haloperidol and chlorpromazine)-induced increases in depolarization-stimulated release of dopamine in the striatum of the rat in vivo. The dose-response curves were found to be at least biphasic (small doses increased release and large doses inhibited release) and different in shape from dose-response curves for increases in the turnover of dopamine, suggesting that the two processes may not be related. The threshold dose to induce increased depolarization-stimulated release of dopamine correlated with the values in the literature for doses required to block stereotype induced by apomorphine and amphetamine and doses sufficient to cause a maximal increase in release correlated with the doses required for the induction of catalepsy. In addition, the ratio of the doses required to obtain half-maximal and maximal increases in stimulated release of dopamine, matched values in the literature of the ratio of doses required to ameliorate psychotic symptoms, whereas the doses required to reverse the increase induced by antipsychotic drugs did not. Chronic administration of haloperidol resulted in alterations in the dose-response curves for both the release and turnover of DA. Chronic administration caused a reversal of the effect of acute administration in the small dose range (inhibition as opposed to enhancement of release), a decrease in the maximum magnitude of release of dopamine obtained and an overall shift to the right of the dose-response curve. Chronic administration of haloperidol decreased turnover, relative to animals receiving the drug acutely and no shift in the position of the dose-response curve was observed.