Effect of apomorphine on schizophrenic symptoms

Alternative Therapy of Psychosis: Potential Phytochemicals and Drug Targets in the Management of Schizophrenia

Schizophrenia is a chronic mental and behavioral disorder characterized by clusters of symptoms including hallucinations, delusions, disorganized thoughts and social withdrawal. It is mainly contributed by defects in dopamine, glutamate, cholinergic and serotonergic pathways, genetic and environmental factors, prenatal infections, oxidative stress, immune system activation and inflammation. Management of schizophrenia is usually carried out with typical and atypical antipsychotics, but it yields modest benefits with a diversity of side effects. Therefore, the current study was designed to determine the phytochemicals as new drug candidates for treatment and management of schizophrenia. These phytochemicals alter and affect neurotransmission, cell signaling pathways, endocannabinoid receptors, neuro-inflammation, activation of immune system and status of oxidative stress. Phytochemicals exhibiting anti-schizophrenic activity are mostly flavonoids, polyphenols, alkaloids, terpenoids, terpenes, polypropanoids, lactones and glycosides. However, well-designed clinical trials are consequently required to investigate potential protective effect and therapeutic benefits of these phytochemicals against schizophrenia.

The Pharmacology of Visual Hallucinations in Synucleinopathies

Visual hallucinations (VH) are commonly found in the course of synucleinopathies like Parkinson's disease and dementia with Lewy bodies. The incidence of VH in these conditions is so high that the absence of VH in the course of the disease should raise questions about the diagnosis. VH may take the form of early and simple phenomena or appear with late and complex presentations that include hallucinatory production and delusions. VH are an unmet treatment need. The review analyzes the past and recent hypotheses that are related to the underlying mechanisms of VH and then discusses their pharmacological modulation. Recent models for VH have been centered on the role played by the decoupling of the default mode network (DMN) when is released from the control of the fronto-parietal and salience networks. According to the proposed model, the process results in the perception of priors that are stored in the unconscious memory and the uncontrolled emergence of intrinsic narrative produced by the DMN. This DMN activity is triggered by the altered functioning of the thalamus and involves the dysregulated activity of the brain neurotransmitters. Historically, dopamine has been indicated as a major driver for the production of VH in synucleinopathies. In that context, nigrostriatal dysfunctions have been associated with the VH onset. The efficacy of antipsychotic compounds in VH treatment has further supported the notion of major involvement of dopamine in the production of the hallucinatory phenomena. However, more recent studies and growing evidence are also pointing toward an important role played by serotonergic and cholinergic dysfunctions. In that respect, in vivo and post-mortem studies have now proved that serotonergic impairment is often an early event in synucleinopathies. The prominent cholinergic impairment in DLB is also well established. Finally, glutamatergic and gamma aminobutyric acid (GABA)ergic modulations and changes in the overall balance between excitatory and inhibitory signaling are also contributing factors. The review provides an extensive overview of the pharmacology of VH and offers an up to date analysis of treatment options.

Parkinson's disease, visual hallucinations and apomorphine: A review of the available evidence

BACKGROUND

Visual hallucinations (VH) occur in the clinical course of Parkinson's disease (PD) and are predictive for PD dementia. The genesis of VH is related to impaired bottom-up and/or top-down visual processing which can be linked to cholinergic dysfunction and mono-amine imbalance. The risk of developing VH with oral dopamine agonists seems to increase with advancing disease, while in contrast some clinical studies suggest that apomorphine does not worsen VH, or might even improve VH.

METHODS

The aim of this study is to review the current evidence of apomorphine and its effects on VH in PD patients.

RESULTS

Apomorphine is well-tolerated in PD patients with VH, also in long-term follow-up studies. Apomorphine is also suggested to have the potential to alleviate VH. Some data suggest that the positive effect of apomorphine on VH is related to its piperidine moiety, part of many anti-psychotics. Irrespective this piperidine moiety, apomorphine has a high D1-like receptor affinity, and acts as a serotonin 5-HT2A receptor antagonist, which might explain the potential anti-hallucinogenic properties as well.

CONCLUSION

The anecdotal evidence suggesting that apomorphine has a relatively low proclivity to induce VH in PD may be due to its capacity to reduce serotonergic activity in particular. Therefore apomorphine is still an option to consider in fluctuating PD patients with VH, if they are treated properly with respect to their cholinergic deficits and existing VH.

Third generation antipsychotic drugs: partial agonism or receptor functional selectivity?

Functional selectivity is the term that describes drugs that cause markedly different signaling through a single receptor (e.g., full agonist at one pathway and antagonist at a second). It has been widely recognized recently that this phenomenon impacts the understanding of mechanism of action of some drugs, and has relevance to drug discovery. One of the clinical areas where this mechanism has particular importance is in the treatment of schizophrenia. Antipsychotic drugs have been grouped according to both pattern of clinical action and mechanism of action. The original antipsychotic drugs such as chlorpromazine and haloperidol have been called typical or first generation. They cause both antipsychotic actions and many side effects (extrapyramidal and endocrine) that are ascribed to their high affinity dopamine D(2) receptor antagonism. Drugs such as clozapine, olanzapine, risperidone and others were then developed that avoided the neurological side effects (atypical or second generation antipsychotics). These compounds are divided mechanistically into those that are high affinity D(2) and 5-HT(2A) antagonists, and those that also bind with modest affinity to D(2), 5-HT(2A), and many other neuroreceptors. There is one approved third generation drug, aripiprazole, whose actions have been ascribed alternately to either D(2) partial agonism or D(2) functional selectivity. Although partial agonism has been the more widely accepted mechanism, the available data are inconsistent with this mechanism. Conversely, the D(2) functional selectivity hypothesis can accommodate all current data for aripiprazole, and also impacts on discovery compounds that are not pure D(2) antagonists.

GPCR functional selectivity has therapeutic impact

Many in vitro data show that some ligands can cause the differential activation of signaling pathways mediated by a single receptor (termed 'functional selectivity'). It remains unclear, however, whether functionally selective properties are meaningful in vivo. Data obtained with experimental compounds that are functionally selective at the dopamine D2L receptor in vitro suggest that these properties might predict atypical behavioral actions. Moreover, the antipsychotic drug aripiprazole is commonly thought to be a D2 partial agonist, but data clearly show that aripiprazole is functionally selective in vitro. It is proposed that the effects of aripiprazole in animal models and humans can be reconciled only with its functionally selective D2 properties, not its partial D2 agonism. Together, these data provide support for the hypothesis that compounds with functionally selective properties in vitro are likely to have novel actions in vivo, opening doors to new avenues of drug discovery.

Amphetamines for schizophrenia

BACKGROUND

It is estimated that between 10% and 65% of people with schizophrenia use illicit drugs such as amphetamines. This group have an increased rate of hospitalisation, homelessness, unemployment and suicide compared with those with schizophrenia who do not abuse drugs.

OBJECTIVES

To evaluate the effects of amphetamines for people with schizophrenia in terms of clinically meaningful outcomes, cognitive functioning and physiological tests.

SEARCH STRATEGY

We searched the Cochrane Schizophrenia Group's Register (February 2002).

SELECTION CRITERIA

We included all randomised controlled trials investigating the effects of amphetamines on people with schizophrenia, compared with a placebo intervention.

DATA COLLECTION AND ANALYSIS

Working independently, we selected and critically appraised studies, extracted data and analysed on an intention-to-treat basis. Where possible and appropriate we calculated risk ratios (RR) and their 95% confidence intervals (CI), with the number needed to treat (NNT). For continuous data we calculated Weighted Mean Differences (WMD).

MAIN RESULTS

We included four short studies with a total of 83 participants. Data were few and poorly reported. The results indicated a reduction of negative symptoms for people allocated to amphetamines (n = 16, 1 RCT, WMD -3 CI -5.02 to -0.98). No such effect was found for positive symptom change (n = 16, 1 RCT, WMD 0 CI -4.46 to 4.46). Compared with placebo, amphetamines significantly increased metabolism in the left and right cerebellum (n = 23, 1 RCT, WMD 0.12 CI 0.06 to 0.18; n = 23 1 RCT, WMD 0.12 CI 0.06 to 0.18) and left striatum (n = 23, 1 RCT, WMD 0.14 CI 0.00 to 0.28) and also significantly decreased metabolism in the left dorsolateral prefrontal cortex (n = 23, 1 RCT, WMD -0.09 CI -0.17 to -0.01).

REVIEWERS' CONCLUSIONS

Understandably amphetamines are rarely formally evaluated in randomised studies and therefore unpublished work in this area is likely to exist. Addition of more studies may clarify reasons why people with schizophrenia persist in taking these harmful stimulants.

Antipsychotic properties of the partial dopamine agonist (-)-3-(3-hydroxyphenyl)-N-n-propylpiperidine(preclamol) in schizophrenia

BACKGROUND

In an ongoing effort to characterize the clinical pharmacologic profile of the partial dopamine agonist (-)-3-(3-hydroxyphenyl)-N-n-propylpiperidine [(-)-3PPP], we administered it to drug-free schizophrenic patients in two consecutive studies.

METHODS

In a preliminary dose-finding study, 9 patients were treated using a 6-week placebo-controlled crossover design. Then, to properly demonstrate the antipsychotic effect, we carried out an early efficacy study; here 10 patients received (-)-3PPP, 300 mg B.I.D., in a 1-week placebo-controlled crossover study.

RESULTS

Dose-Finding Study: (-)-3PPP showed apparent antipsychotic effect in repeated dosing, with 300 mg B.I.D. being the most effective dose for antipsychotic action; however, the apparent antipsychotic action was not sustained for longer than 1 week, presumably because of desensitization of the receptor by the agonist. Early Efficacy Study: Positive symptoms as measured by the Psychosis Change Scale decreased in 1 week by 30% with (-)-3PPP compared to placebo, and negative symptoms measured with the Brief Psychiatric Rating Scale Withdrawal subscale decreased by 28% with the drug. In both studies, (-)-3PPP lacked any evidence of motor side effects.

CONCLUSIONS

These data show that psychotic symptoms decrease with (-)-3PPP and suggest that the treatment of schizophrenia with partial dopamine agonist is a promising strategy. Future attention will be directed toward testing techniques to diminish the tachyphylaxis to allow an ongoing therapeutic effect.

Dopamine agonists in schizophrenia: a review

Although it is unlikely that the different types of course and severity of schizophrenia are caused by one neurochemical abnormality alone, indirect pharmacological evidence still suggests a relative excess of dopaminergic activity being implicated in the pathogenesis of most of the schizophrenic symptoms, e.g. positive symptomatology. Synthesis and release of dopamine as well as firing rates of dopaminergic neurons are controlled by stimulation of autoreceptors via a negative feedback regulation. Investigations on therapeutic effects of autoreceptor-nonselective dopamine agonists in schizophrenia have yielded inconsistent results. Dopamine autoreceptor agonists like pramipexole, roxindole, talipexole and OPC-4392 as well as partial agonists like terguride and SDZ HDC 912 have been tested in positive schizophrenic symptomatology in order to reduce the postulated excess of central dopaminergic activity. However, administration of autoreceptor-'selective' agonists did not result in a significant improvement of positive symptoms. In predominantly negative schizophrenic symptomatology, a dopamine deficit rather than an excess has been hypothesized. Consequently, a nonselective dopamine agonistic action could be effective in some negative symptoms. Current evidence from several open and one placebo-controlled clinical trial suggests that some dopamine autoreceptor agonists like pramipexole, roxindole and talipexole may produce a mild improvement of symptoms like affective flattening, depressed mood, alogia and avolition. Nevertheless, these findings do not yet allow a reliable judgement and remain to be clarified by further double-blind placebo-controlled studies over a sufficient treatment duration.

Methylphenidate increases thought disorder in recent onset schizophrenics, but not in normal controls

The effects of apomorphine and methylphenidate on thought disorder, as measured by the Thought Disorder Index, in schizophrenic patients and in normal controls were evaluated. Methylphenidate, but not apomorphine, increased thought disorder in patients. Neither drug significantly increased thought disorder in controls.

Dopamine autoreceptor agonists in the treatment of schizophrenic disorders

1. Synthesis and release of dopamine as well as firing rates of dopaminergic neurons are controlled by stimulation of autoreceptors via a negative feedback regulation. Investigations on therapeutic effects of autoreceptor-nonselective dopamine agonists in schizophrenia have yielded inconsistent results. 2. With respect to the dopamine hypothesis of schizophrenia, dopamine autoreceptor agonists have been tested in positive schizophrenic symptomatology in order to reduce the postulated excess of central dopaminergic activity. However, administration of selective dopamine autoreceptor agonists like talipexole or roxindole did not result in a significant improvement of psychopathological symptoms. 3. In negative schizophrenic symptomatology, a dopamine deficit rather than an excess has been hypothesized. Current evidence from open clinical trials suggests that dopamine autoreceptor agonists may produce a minor to moderate improvement of symptoms like affective flattening, depressed mood, alogia and avolition.

Do autoreceptors mediate dopamine agonist--induced yawning and suppression of exploration? A critical review

The hypothesis that stimulation of dopamine autoreceptors is the mechanism by which dopamine agonists induce yawning and suppression of exploration is critically examined. It is shown that the relation between reduced extracellular dopamine levels, assessed by microdialysis, and behavioural effects of dopamine agonists, a dopamine synthesis inhibitor and a granule storage blocker is highly inconsistent. The time-course and duration of the behavioural effects of dopamine agonists differ from the reduction of extracellular dopamine. Amphetamine cotreatment is shown to increase dopamine levels, while yawning and suppression of exploration can still be induced. The data strongly indicate that autoreceptors are not the mediators of these behavioural effects. It is proposed that postsynaptic receptors mediate dopamine agonist induced yawning and suppression of exploration. Evidence is also presented showing that yawning and suppression of exploration are not functionally equivalent.

The potential antipsychotic activity of the partial dopamine receptor agonist (+)N-0437

The (+) enantiomer of the very potent and selective dopamine D-2 agonist, 2-(N-propyl-N-2-thienylethylamino)-5-hydroxytetralin (N-0437), displays partial agonistic activity at dopamine D-2 receptors. In this study (+)N-0437 was investigated for its antagonistic activity at postsynaptic DA receptors in four behavioural tests which are commonly used to evaluate potential neuroleptic activity, i.e. d-amphetamine-induced stereotypy, passive avoidance responding, intracranial self-stimulation behaviour, and catalepsy. (+)N-0437 (25-50 mumol/kg) was active in the first three models, but did not cause catalepsy. Haloperidol, which was used as a reference compound for classical DA antagonists, showed clear activity in all four models at low doses (0.5-1.0 mumol/kg). (-)N-0437, a full D-2 agonist, displayed no activity in these behavioural models. These results suggest that (+)N-0437 could be used to examine the hypothesis that the use of partial agonists could provide a new treatment for schizophrenia.

Effects of repeated administration of low doses of apomorphine in three behavioural models in the rat

A low dose of the dopamine (DA) receptor agonist apomorphine (APO 0.05 mg/kg) was given repetitively and the effects were tested in three different behavioural models: reduction of spontaneous locomotion, induction of yawning and decrease in water intake in water-deprived animals. The APO-induced suppression of exploration and decrease in water intake were not affected by a previous injection of APO given 1 or 3 hours before the test dose of APO. There was a small, but significant, decrease in the induction of yawning by a previous dose of APO given 1 hour or 30 min before the test dose. However, pretreatment with APO 3 hours before the test dose did not diminish the yawning response. It is suggested that the dopaminergic mechanisms mediating APO induced yawning are different from those mediating decrease in water intake and suppression of exploration. The results are also discussed in relation to the proposed efficiency of low doses of DA agonists in the treatment of various neurological and psychiatric disorders.

Apomorphine: clinical studies on erectile impotence and yawning

1. The erectile response to the short-acting dopamine (DA) receptor agonist, apomorphine (Apo) HCl (0.25, 0.5, 0.75 and 1.0 mg sc), and placebo was evaluated in 28 impotent patients and penile circumference monitored using a mercury strain gauge and strip chart recording. 2. A full erection (increment in penile circumference greater than 2 cm and lasting at least one minute) occurred in 17 patients with Apo; no erection developed after placebo. An erection occurred in 6/8 patients with impaired glucose tolerance, 2/6 patients with diabetes mellitus and in both patients on lithium. 3. Nine patients who responded to Apo were treated in an open trial with bromocriptine; 6 reported improvement in potency. 4. Impairment in DA function may play a role in idiopathic impotence and in impotence associated with impaired glucose tolerance and diabetes mellitus. 5. An erectile response to Apo may predict therapeutic response to bromocriptine or other long acting dopaminergic agents. 6. Lithium, which inhibits DA-sensitive adenylate cyclase, does not prevent Apo-induced erections. This provides further support indicating that Apo induces erections by an effect on D2 receptors. 7. The yawning response to placebo and four doses of Apo HC1 (3.5, 5.0, 7.0, and 10.5 ug/kg sc) was evaluated in five normal men using a polygraphic technique. The yawning response was also assessed in normal young (less than 30 yrs; N = 16) and elderly (greater than 60 yrs; N = 12) volunteers. 8. Under experimental conditions of study, placebo induced spontaneous yawning. This was antagonized by 3.5 and 5.0 ug/kg Apo HC1 but increased by 7.0 ug/kg Apo HC1. These observations are compatible with the view that Apo HC1 in doses of 3.5-5.0 ug/kg stimulates presynaptic DA receptors whereas 7.0 ug/kg stimulates postsynaptic DA receptors. 9. Spontaneous and Apo-induced yawning were significantly decreased in the elderly which suggests that D2 receptor function declines with normal aging.

On the selectivity and specificity of the antagonism of apomorphine-induced suppression of exploration by sulpiride

Ten behavioural variables were recorded by means of an automatic holeboard apparatus. The behaviour of rats placed for the first time in the apparatus was recorded for 10 min. The suppression of this exploratory behaviour by the dopamine agonist apomorphine (0.01-0.1 mg/kg) was shown to be reversible in a surmountable fashion by the dopamine antagonist sulpiride (2 and 4 mg/kg). Suppression of exploration induced by clonidine (0.05-0.2 mg/kg) or diazepam (2 mg/kg) was not antagonised by sulpiride (10 and 50 mg/kg, respectively). The partial dopamine D1-agonist SKF 38393 (2-20 mg/kg) also suppressed exploration but neither sulpiride (20 mg/kg) nor the D1-antagonist SCH 23390 (0.02 mg/kg) could antagonise this effect. The data show that dopamine agonist induced suppression of exploration display pharmacological characteristics of a receptor-mediated response and the data support our previous suggestion that these receptors may be pharmacologically distinct from other dopamine D2-receptors.

Dopamine agonists at repeated "autoreceptor-selective" doses: effects upon the sensitivity of A10 dopamine autoreceptors

Previous reports have established the ability of dopamine (DA) agonists to stimulate inhibitory DA autoreceptors at doses which minimally stimulate postsynaptic DA receptors, suggesting that hyperactive DA transmission may be controlled clinically by treatment with DA agonists. Little is known, however, about the possible loss of autoreceptor sensitivity that may occur after repeated treatment with low doses of DA agonists. Extracellular single cell recording and microiontophoretic techniques were used to measure the sensitivity of impulse-regulating DA autoreceptors on A10 DA cells in the ventral tegmental area (VTA) of chloral hydrate-anesthetized rats pretreated for seven days with repeated subcutaneous (s.c.) doses of the DA agonist apomorphine (APO). The ability of intravenous (i.v.) administration of the potent D2 DA agonist quinpirole (QUIN) to inhibit the firing of A10 cells was not attenuated in rats pretreated with repeated low doses (2 x 50 micrograms/kg/day, s.c.) of APO for 7 days, although higher doses (2 x 250 or 500 micrograms/kg/day) did cause subsensitive responses to QUIN. In rats pretreated with repeated low doses of APO, microiontophoretic application of DA on A10 cells revealed somewhat subsensitive responses. However, ibotenic acid lesions of postsynaptic cells in the nucleus accumbens (NAc) prior to initiation of APO treatment (2 x 50 micrograms/kg/day) did not alter the response of A10 cells to systemic QUIN, contradicting the possibility that the feedback projection from the NAc to the VTA was compensating for autoreceptor down-regulation during systemic challenge with QUIN. In contrast, administration of the irreversible DA antagonist EEDQ (2 mg/kg, i.p.) to control and APO-treated rats (2 x 50 micrograms/kg/day) 24 hr prior to recording did reveal a difference in A10 cell sensitivity to systemic QUIN and to microiontophoretic DA between the two groups, suggesting that "spare" DA autoreceptors may have concealed the down-regulation of autoreceptors induced by repeated low doses of APO. Challenge of A10 DA cells with the partial DA autoreceptor agonist (-)-3-(3-hydroxyphenyl)-N-n-propylpiperidine [(-)3-PPP], for which an autoreceptor reserve should not exist, produced slightly attenuated responses in APO-treated rats (2 x 50 micrograms/kg/day). These findings provide evidence for the existence of spare somatodendritic DA autoreceptors on A10 DA cells with respect to potent DA agonists, suggesting that repeated administration of "autoreceptor-selective" doses of DA agonists may not result in a diminished inhibition of DA neuronal activity.

Yawning and suppression of exploration in amphetamine-treated rats, incompatibility with the autoreceptor hypothesis

The hypothesis that yawning and suppression of exploration, induced by low doses of dopamine agonists, are mediated by stimulation of dopamine autoreceptors was tested by studying the influence of amphetamine on these behavioural models and on extracellular levels of dopamine sampled by microdialysis. Behaviour was measured in a holeboard apparatus. A low dose of amphetamine (0.2 mg/kg) caused slight activation of habituated rats. The same dose of amphetamine completely counteracted the decrease in extracellular dopamine levels caused by pergolide (0.02 mg/kg) and, instead, elevated the dopamine levels to 300% above baseline. The same or higher doses of amphetamine (0.5-1.0 mg/kg) did not completely antagonise suppression of exploration or yawning induced by apomorphine (0.05 mg/kg) or pergolide (0.02 mg/kg). The results suggest that both yawning and suppression of exploration induced by low doses of dopamine agonists are not mediated by dopamine autoreceptors, since these behaviours could be elicited when the extracellular levels of dopamine were elevated above baseline. The alternative hypothesis that these behaviours are mediated by sensitive post-synaptic receptors is suggested. It was also found that combined treatment with SCH 23390 (0.05 mg/kg) and amphetamine (2 mg/kg) induced yawning, which further supports the new hypothesis.

Longitudinal growth hormone studies in schizophrenia

The growth hormone (GH) response to apomorphine hydrochloride (APO) was examined monthly in 12 schizophrenic patients on drug holiday for up to 22 months and compared with age- and sex-matched controls. There was more variability in the response of patients than controls on the first trial and on several subsequent challenges. Patients' and controls' GH responses to an APO challenge did not distinguish them from each other on the first trial. However, longitudinal data revealed that in a subgroup of five schizophrenic patients and five controls, studied for 12 consecutive trials, the GH response averaged over the 12 trials was significantly lower in the patients than in the controls. Moreover, when schizophrenics' responses on each successive trial were compared, responses decreased over time, but were significantly different from controls only in the later trials. Three of the patients were followed for more than 12 trials, and their GH responsivity increased in the later trials. GH response to APO was significantly correlated with positive symptom scores in three patients but not in four others. There was a trend toward an association between the occurrence of relapse and GH increment induced by APO. A significant association between change in body weight and change in GH response to APO was discovered, suggesting that a changing body weight may contribute to the variability in subjects' response to APO.

Apomorphine in the evaluation of dopaminergic function in man

1. Apomorphine (Apo), a short acting dopamine (DA) receptor agonist, stimulates growth hormone (GH) secretion, decreases prolactin secretion, induces yawning, penile erections and other physiological effects in man. An effect on behavior, movement disorders and alcoholism has also been described. 2. Apo-mediated responses are used to evaluate DA function in psychiatric and neurological disorders. Many of the studies in schizophrenia using the GH response to Apo as an index of central DA function are difficult to interpret because of failure to control for key variables. 3. The GH response to Apo is a useful system to evaluate the effects of various drugs including peptides which may not cross the blood brain barrier on DA function in man. 4. Apo is a potent sedative. Specific antimanic, antischizophrenic, and anticraving effects in alcoholics have not been convincingly demonstrated. Side effects of Apo and failure to use active placebo make double-blind studies difficult. 5. Apo improves parkinsonian symptoms and certain forms of reflex epilepsy but beneficial effects in other involuntary movement disorders requires further documentation. 6. Apo may be a useful agent to evaluate DA function in impotent patients and predict a therapeutic response to long-acting dopaminergic agents. 7. Impairment of DA function may play a role in diabetic impotence. 8. The development of a simple polygraphic method to monitor the yawning response to Apo may facilitate clinical studies on the basic physiology of yawning in man and the use of the yawning response as a measure of central DA function in schizophrenia and other clinical disorders. 9. The use of Apo with 18F-fluorodeoxyglucose positron emission tomography to examine regional DA function in man opens up a promising area of research. 10. Though long-acting orally active aporphine DA agonists and antagonists have been developed the problem of tolerance may limit their therapeutic potential.

Dopamine receptors and the dopamine hypothesis of schizophrenia

The discovery of neuroleptic drugs in 1952 provided a new strategy for seeking a biological basis of schizophrenia. This entailed a search for a primary site of neuroleptic action. The Parkinsonian effects caused by neuroleptics suggested that dopamine transmission may be disrupted by these drugs. In 1963 it was proposed that neuroleptics blocked "monoamine receptors" or impeded the release of monoamine metabolites. The neuroleptic concentration in plasma water or cerebrospinal fluid was of the order of 2 nM for haloperidol in clinical therapy. A systematic research was made between 1963 and 1974 for a primary site of neuroleptic action which would be sensitive to 2 nM haloperidol and stereoselective for (+)-butaclamol. Direct evidence that neuroleptics selectively blocked dopamine receptors occurred in 1974 with the finding that nanomolar concentrations of these drugs stereoselectively inhibited the binding of [3H]-dopamine or [3H]-haloperidol. These binding sites, now termed D2 dopamine receptors (which inhibit adenylate cyclase), are blocked by neuroleptics in direct relation to the antipsychotic potencies of the neuroleptics. No such correlation exists for D1 receptors (which stimulate adenylate cyclase). Based on the fact that dopamine-mimetic drugs elicited hallucinations, and that neuroleptics caused rigidity, Van Rossum in 1966 had suggested a hypothesis that dopamine pathways may be overactive in schizophrenia. The D2-selective blockade by all neuroleptics (except the monoamine-depleting reserpine) provided strong support for the dopamine hypothesis. Further support now comes from postmortem data and in vivo positron tomographic data, both of which indicate that the density of D2 receptors are elevated in the schizophrenic brain. The postmortem data indicate a bimodal pattern with half the schizophrenics having striatal D2 densities of 14 pmol/g (control is 13 pmol/g) and the other half having 26 pmol/g. Current positron tomographic data indicate D2 densities of 14 pmol/g in control subjects, but values of 34 pmol/g in drug-naive schizophrenics. Future tests of the dopamine hypothesis of schizophrenia may entail an examination of the amino acid composition and genes for D2 receptors in schizophrenic tissue, an examination of the ability of the D2 receptor to become phosphorylated and to desensitize into the low-affinity state, and an examination of the interaction of D2 receptors with D1 receptors or other neurotransmitters.

On the mode of action of six putative dopamine receptor agonists on suppression of exploratory behaviour in rats

The effects of six putative dopamine receptor agonists on exploratory behaviour in rats were assessed: pergolide, (+)- and (-)-3-PPP, bromocriptine, mesulergine and CQ 32-084. Behaviour was automatically recorded in a holeboard apparatus and the data were analysed by the novel multivariate statistical method of partial least squares. All six substances suppressed exploratory behaviour at low doses. Pergolide and (+)-3-PPP-induced stereo-typed behaviour at higher doses. The suppression of exploration induced by pergolide was completely antagonised by sulpiride, partly antagonised by metoclopramide and weakly affected by haloperidol pretreatment. The effects of a low dose of (+)-3-PPP, bromocriptine or CQ 32-084, but not (-)-3-PPP or mesulergine, were antagonised by sulpiride. These findings support the hypotheses that pergolide, (+)-3-PPP, bromocriptine and CQ 32-084 inhibit exploration via stimulation of dopamine receptors. The present data do not substantiate the hypothesis that the suppression of exploration induced by (-)-3-PPP is mediated by stimulation of dopamine autoreceptors. A detailed analysis of the dose curves for pergolide and (+)-3-PPP indicates that the latter compound may have effects in addition to those of a dopamine receptor agonist.

Effects of neuroleptic drugs on the inhibition of exploratory behaviour induced by a low dose of apomorphine: implications for the identity of dopamine receptors

Apomorphine in low doses inhibits spontaneous exploratory behaviour in rats. This effect is commonly referred to as an expression of selective stimulation of dopaminergic autoreceptors. The aim of the present study was to investigate the influence of neuroleptic drugs with different pharmacological profiles on this apomorphine induced inhibition of exploration using techniques for detailed recording of behaviour and multivariate statistical analysis of the results. By comparison with dose response analyses of apomorphine it was possible to determine whether a neuroleptic specifically antagonised the apomorphine effect or if the pattern of behaviour was qualitatively changed in some way. Apomorphine (0.05 mg/kg) was tested against cis-flupenthixol (0.01-0.5 mg/kg), haloperidol (0.01-0.1 mg/kg), metoclopramide (0.2-5 mg-kg), sulpiride (0.5-50 mg/kg) and SCH 23390 (0.005-0.05 mg/kg). Metoclopramide and haloperidol had weak antagonising effects against apomorphine while cis-flupenthixol and SCH 23390 was completely inefficient in this respect. The multivariate analysis indicated that the effects of haloperidol was restricted to only some aspects of the behavioural effects of apomorphine. Only sulpiride did selectively and dose-dependently antagonise the apomorphine induced behavioural suppression. The data provide evidence for a functional subdivision of dopamine receptors at the behavioural level.

Effects of intravenous and subcutaneous administration of apomorphine on the clinical symptoms of chronic schizophrenics

The effects of apomorphine, a stimulant of dopamine autoreceptors, were studied in 12 chronic schizophrenics on neuroleptic treatment; both subcutaneous and intravenous administration were used. Apomorphine has been reported to have therapeutic effects in previous studies but, we were not able to confirm any significant and specific differences in psychotic symptoms or tardive dyskinesia scores with apomorphine administration, compared with placebo. These results do not support the importance of dopamine autoreceptors in the regulation of schizophrenic and dyskinetic symptoms in chronic neuroleptic-treated patients.

An electrophysiological analysis of the actions of the 3-PPP enantiomers on the nigrostriatal dopamine system

Extracellular single unit recording and microiontophoretic studies were carried out in chloral hydrate-anesthetized gallamine-paralyzed rats to investigate the actions of the enantiomers of the dopamine (DA) analogue 3-(3-hydroxyphenyl)-N-n-propylpiperidine, 3-PPP, on the nigrostriatal DA system. Intravenously administered (+)- or (-)-3-PPP consistently inhibited nigral DA neuronal activity; these actions were readily antagonized by haloperidol but were not affected by a pretreatment of reserpine plus alpha-methyltyrosine. In contrast to (+)-3-PPP, the (-)-enantiomer produced only partial inhibition of the majority of cells studied and was also capable of partially reversing the inhibitory action of apomorphine. A prior hemitransection of the brain did not alter the inhibitory action of either enantiomer. Whereas iontophoretically ejected (+)-3-PPP consistently reduced DA cell firing rate, similarly applied (-)-3-PPP reduced the activity of only some DA cells, while the majority were not influenced. In addition, iontophoresis of (-)-3-PPP could reduce the inhibitory effect of similarly applied DA or (+)-3-PPP. The (+)-enantiomer reduced caudate neuronal activity both after intravenous administration and iontophoresis. Intravenously administered (-)-3-PPP failed to influence or increased the activity of these neurons and reversed the inhibitory action of apomorphine. However, iontophoretically ejected drug reduced caudate cell activity and did not influence the inhibitory action of DA. The activity of non-DA zona reticulata neurons was inconsistently influenced by the 3-PPP enantiomers. It is concluded that (+)-3-PPP is a directly acting DA agonist, stimulating both DA autoreceptors and postsynaptic DA receptors. In contrast, (-)-3-PPP appears to be a partial agonist at nigral DA autoreceptors, whereas the action of the drug at putative postsynaptic DA receptors in the caudate remains to clarified.

Dopamine-receptor agonists: mechanisms underlying autoreceptor selectivity. I. Review of the evidence

The behavioural, biochemical, neuroendocrinological and electrophysiological actions of the enantiomers of the dopamine (DA) analogue 3-(3-hydroxyphenyl)-N-n-propylpiperidine, 3-PPP, are extensively reviewed. (+)-3-PPP acts in a fashion similar to classical direct-acting DA agonists, stimulating both DA autoreceptors and postsynaptic DA receptors, although in some situations the drug appears to exhibit partial agonist activity. (-)-3-PPP exerts a variety of actions in different pharmacological models. Either agonistic, antagonistic or both agonistic and antagonistic activity are observed depending on the anatomical location of the relevant DA receptors and the experimental conditions. The actions of transdihydrolisuride (TDHL) and the trans-fused 7-OH-1,2,3,4,4a,5,6,10b-octahydrobenzo(f)quinoline (HW 165) are also discussed. These agents possess a similar spectrum of action to (-)-3-PPP suggesting a new generation of DA agonists which exhibit variable intrinsic activity at different DA receptors. Finally, evidence is presented indicating that the 3-PPP enantiomers display selectivity for DA receptors.

Continuous low-level apomorphine administration induces motor abnormalities and hallucinogen-like behaviors

Continuous low-level (0.825 mg/kg/h for 20 h) administration of AP through SC in-dwelling silicone reservoirs in the rat induced behavioral and biochemical changes that were similar to those induced by low levels (0.1 mg/kg) of acutely administered AP (decreased behavioral activity and decreased dopamine metabolism in the striatum). With longer periods of continuous AP exposure (40 h or more) the activity-depressing effects of low-level AP diminished. Concurrently a novel behavioral syndrome emerged characterized by limb flicks, body shakes, sudden orienting responses, and motor abnormalities, such as tremors of the jaw muscles, chewing movements, prominent tongue extensions, and body 'tics'. This behavioral syndrome became more apparent following cessation of drug treatment. These novel behavioral changes, which were accompanied by increased behavioral responsiveness to acutely administered AP and amphetamine, were correlated with increased levels of dopamine, homovanillic acid, and 3,4-dihydroxyphenylacetic acid in the striatum but not the nucleus accumbens. This novel behavioral syndrome appears to reflect a rebound increase in dopaminergic mechanisms in striatum following their chronic suppression by low levels of AP.

Effects of apomorphine and haloperidol on "spontaneous" stereotyped licking behaviour in the Cebus monkey

Three recently arrived drug naive Cebus apella monkeys with "spontaneous" stereotyped oral movements were treated with apomorphine and haloperidol using a wide dose range. Low doses of apomorphine (0.05-0.1 mg/kg) suppressed the oral stereotypies without affecting normal behaviour such as grooming and scratching. Higher doses of apomorphine (0.25-1.0 mg/kg) and haloperidol (0.01-0.1 mg/kg) also decreased or abolished the oral stereotypies, but induced generalized stereotypies (apomorphine) or dystonia/parkinsonism (haloperidol), suppressing normal behaviour. The findings indicate that dopamine is involved in these presumably stress-induced (not drug-induced) stereotypies.

Clinical effects of apomorphine in schizophrenia

There have been reports that low doses of the dopamine-agonist apomorphine, which may inhibit dopamine neurotransmission, are of therapeutic benefit in schizophrenia. We conducted a placebo-controlled study of acute and chronic schizophrenics in which videotaped interviews were blindly rated. No specific therapeutic effect was demonstrated for apomorphine other than a reduction in anxiety in acute schizophrenics. Furthermore, there was no difference in the frequency of side-effects of apomorphine between schizophrenic patients and controls, and no specific effect of apomorphine on blink-rates. These findings emphasize the importance of placebo-controlled studies in schizophrenia research.

Dopamine and schizophrenia: an analysis of the theory

The postulated relationship of dopamine to schizophrenia ranks among the most important contemporary theories pertinent to the biological bases of behavior. However, as an examination of the relevant research literature makes clear, the theory has not yet been convincingly validated. This lack of validation is due, in part, to a failure to address the following questions: Is dopamine hyperactivity an etiological and/or a symptom factor in schizophrenia; do laboratory measures used to test the theory truly parallel the relevant clinical phenomena; is attenuated dopamine activity a necessary and/or sufficient condition for remission of schizophrenic symptoms? Analysis of these questions not only provides a departure point for examining the theory, but sets the stage for a reformulation of the theory itself.

Assessment of dopamine autoreceptor agonist properties of apomorphine, (+)-3-PPP and (-)-3-PPP by recording of yawning behaviour in rats

Yawning behaviour in rats was studied by direct observation. Apomorphine dose dependently induced yawning: 0.05 mg/kg was most effective, 0.2 mg/kg induced locomotor and sniffing behaviour but less yawning. Sulpinide (2 and 10 mg/kg) dose dependently blocked the apomorphine (0.05 mg/kg)-induced yawning. (+)-3-PPP (1-10 mg/kg) induced yawning in a manner similar to that of apomorphine but (-)-3-PPP (1-10 mg/kg) did so only weakly. Yawning induced by (+)-3-PPP was blocked by sulpiride 10 mg/kg. It is concluded that (+)-3-PPP but not (-)-3-PPP is at least as effective as apomorphine to induce yawning in rats, indicating that (+)-3-PPP, but not (-)-3-PPP, is a pure agonist on dopamine autoreceptors.

Growth hormone responses to apomorphine HCl in schizophrenic patients on drug holidays and at relapse

In nine schizophrenic patients on drug holiday, growth hormone (GH) response to apomorphine HCl 0.75 mg (APO) was compared with normal control data. Patients were tested at two month intervals for up to 14 months. Seven patients relapsed, and of these, five had exaggerated GH responses to APO at that time. Of these five, three had exaggerated GH responses to APO prior to clinical deterioration. Further work is required to determine whether this test can be a useful predictor of relapse.

Prolactin studies in normals: implications for clinical research

Dopaminergic (DA) transmission is a major regulator of pituitary prolactin (PRL) secretion. Strategies to assess abnormalities of DA regulation in mental illness have thus included comparisons of patients' and normals' serum PRL levels before and after the administration of DA agonists and antagonists. These clinical research strategies suffer from a number of shortcomings. There is a wide interindividual variability of normal basal PRL levels, and intraindividual variability has been little studied. Large interindividual variability of PRL responses to DA antagonist challenges has also been observed in normals and reported to be strongly correlated to variation in serum levels of the challenge drug. Assessment of DA agonist challenges is hampered by the fact that low basal levels of serum PRL make suppression difficult to measure; a further problem is the confounding effect of nausea when these drugs are given in high doses. In this study of normals, individual basal serum PRL levels were found to be stable over a mean period of 10 months, with interindividual variance vastly greater than intraindividual variance. Thus, state alterations in mental illness may best be studied using a longitudinal design for measurements of PRL levels in patients, thereby avoiding confounding interindividual variability. Moreover, it appears that alterations of PRL levels between groups or within patients, even though within the normal range, may have individual physiological significance. A study of the PRL responses to haloperidol (hal) and hal + apomorphine (apo) challenges in normals revealed a strong correlation despite a highly significant 51% reduction in PRL response with the addition of apo. Because this correlation is dependent upon a normal or limited range of DA regulation, the study of these two responses in abnormal populations may be more revealing of DA abnormalities than the study of PRL responses to single DA agonist or antagonist challenges.

Influence of gamma-butyrolactone on behavior due to dopaminergic drugs

Gamma-butyrolactone (GBL) is well known to reduce dopamine neuronal impulse flow, but the effects of the drug on dopamine-dependent behavioral states is not well studied. Qualitative and quantitative data are presented, detailing the influence of GBL on stereotypy when this drug is given after administration of the indirectly-acting dopamine mimetics amphetamine and methylphenidate, and the direct dopamine agonist apomorphine. GBL reversed the stereotypy produced by all three drugs, given at moderate doses producing comparable intensities of stereotypy. GBL unexpectedly proved effective in reversing stereotypy after higher-dose apomorphine while producing a rather weak influence on that of higher-dose amphetamine and methylphenidate. The results are discussed in terms of the dopamine impulse flow, and possible GABA-potentiating, effects of GBL, and the sedative effects of apomorphine.

Bromocriptine therapy in chronic schizophrenia: effects on symptomatology, sleep patterns, and prolactin response to stimulation

Ten chronic schizophrenic patients were given bromocriptine in doses increasing from 1.25 to 5 mg over 6 days (the low-dose therapy) and then up to 40 mg over 15 days (the high-dose therapy). Psychopathological status was assessed using the Brief Psychiatric Rating Scale, twice daily the first 6 days, and every 2 days thereafter. The prolactin (PRL) response to haloperidol stimulation (1 mg i.v.) was measured in five cases before and 3 days after the end of high-dose therapy, and in one patient before and 3 days after the end of low-dose therapy. Electroencephalographic sleep studies were carried out before therapy and every 2 nights during low-dose therapy in five patients, and in two cases during high-dose therapy. Bromocriptine therapy modified neither clinical symptomatology nor sleep patterns. The PRL response to haloperidol after therapy was markedly lower than that before therapy in the five patients treated with high doses, and markedly higher in the single patient tested who was treated only with low-dose therapy.

Comparison of the effects of chronic haloperidol treatment on A9 and A10 dopamine neurons in the rat

The effects of chronic haloperidol (CHAL) treatment on A9 and A10 dopamine (DA) neurons were compared using extracellular single cell recording techniques. CHAL caused a time-dependent reduction in the number of spontaneously active A9 and A10 DA cells and induced an irregular firing pattern in many of the DA cells that remained active. Both of these effects occurred earlier and to a greater extent in A10 than in A9. Intravenous injection of the DA agonist apomorphine reversed both the reduction of active DA neurons and the irregular discharge pattern, suggesting that both effects were due to the process of depolarization inactivation. Lesions of the nucleus accumbens (NAc) produced by ibotenic acid prevented the development of depolarization inactivation of A10 DA neurons, indicating that this process is mediated primarily by NAc-A10 feedback pathways. The results suggest that the slow development of depolarization inactivation of DA cells produced by CHAL may contribute to the delayed onset of the clinical effects of long-term treatment with antipsychotic drugs.

Effect of low-dose bromocriptine in treatment of psychosis: the dopamine autoreceptor-stimulation strategy

Bromocriptine (0.5-6.0 mg/day) was administered to seven unmedicated chronic schizophrenic and two schizoaffective patients. Transient slight improvement was noted in four patients and marked improvement in one other. Clinical improvement was associated with nausea and drowsiness. These doses of bromocriptine stimulated serum growth hormone and inhibited serum prolactin levels in some subjects. These results suggest that bromocriptine may stimulate dopamine autoreceptors and, through this mechanism, attenuate symptoms in a small proportion of psychiatric patients.

Low-dose apomorphine reduces serum homovanillic acid concentrations in schizophrenic patients

This study was carried out to evaluate the postulated dopaminergic auto-receptor regulatory effect in man of low-dose apomorphine. Behavior and serum homovanillic acid concentrations following low-dose apomorphine were investigated. Five medicated chronic schizophrenic patients had serum homovanillic acid concentrations measured by mass fragmentography before and after 0.005 mg/kg of apomorphine or saline placebo. Results demonstrate significant reductions in serum homovanillic acid concentrations in all five subjects following apomorphine as compared with placebo. These findings present direct evidence of a specific dopamine autoreceptor effect of low-dose apomorphine in schizophrenic patients.

Dopamine autoreceptor stimulation: clinical significance

Recent studies of the effects of low doses of dopamine agonists, designed to stimulate dopamine autoreceptors and hence diminish the synthesis and release of dopamine, were based on a series of basic research studies which demonstrated the existence of autoreceptors on dopamine neurones of the nigrostriatal, mesolimbic and mesocortical dopaminergic neurones. Evidence for autoreceptors on the tuberoinfundibular dopamine neurones which participate in the regulation of prolactin and growth hormone secretion is lacking. Some recent reports have questioned the existence of dopamine autoreceptors on the mesolimbic and nigrostriatal dopamine neurones. Specificity of various dopamine agonists and antagonists for the dopamine autoreceptor will be reviewed. The sedative, anxiolytic, antipsychotic, antidyskinetic and neuroendocrine effects of low dose dopamine agonists in man will be described. Low dose apomorphine, N-propylapomorphine and bromocriptine have been reported to have antipsychotic effects in the major psychoses, to diminish tardive dyskinesia and to enhance extrapyramidal insufficiency. A unique depressive state which developed in a small proportion of psychiatric patients after low dose apomorphine will be described. Further evidence for the lack of dopamine autoreceptors on the tuberoinfundibular dopamine neurones in man will be presented. Strategies for further study of the dopamine autoreceptor concept in man will be discussed.

Apomorphine: bioavailability and effect on stereotyped cage climbing in mice

Plasma levels fo apomorphine and its conjugates were studied following intravenous, intraperitoneal, and oral administrations to mice. Following hydrolysis, apomorphine and its conjugates were assayed by high-performance liquid chromatography. The absolute bioavailability of apomorphine was 4%. A significant first-pass effect due to extensive conjugation in the liver was postulated based on calculated bioavailabilities and comparison of plasma levels of apomorphine and its conjugates following oral, intraperitoneal, and intravenous administrations. Apomorphine-induced stereotypical cage climbing in mice was investigated following administration of apomorphine by the three routes. Analysis of time-course data obtained from the cage-climbing experiments indicated an absolute bioavailability of 16% for apomorphine.