Metoclopramide: antipsychotic efficacy of a drug lacking potency in receptor models

Prophylactic Administration of Diphenhydramine to Reduce Neuroleptic Side Effects in the Acute Care Setting: A Systematic Review and Meta-Analysis

BACKGROUND

Neuroleptics are commonly prescribed drugs to treat acute conditions (e.g., migraines) in the emergency department, but can cause serious adverse effects. Using diphenhydramine to prevent these adverse effects is very common but remains controversial.

OBJECTIVE

We performed a systematic review to determine whether prophylactic administration of diphenhydramine reduces the incidence of neuroleptic adverse effects in patients with acute conditions.

METHODS

Medline, Embase, Cochrane, PsycInfo, and Web of Science were searched for randomized controlled trials evaluating any neuroleptic with diphenhydramine vs. the same neuroleptic with any inactive agent. Primary outcome was incidence of any extrapyramidal adverse effect. Secondary outcomes were akathisia, rescue medication, subjective restlessness, neuroleptic malignant syndrome, and sedation. Independent reviewers scanned identified citations, extracted data, and assessed risk of bias. Meta-analysis was performed using random effect models.

RESULTS

Of 1566 identified citations, nine studies (n = 1648 patients) met eligibility criteria. Four studies were specifically designed to compare the incidence of neuroleptic adverse effects with and without co-administration of diphenhydramine. Four studies were at high risk of bias. In primary analysis, diphenhydramine had no effect on the incidence of extrapyramidal symptoms (7 studies, n = 1393, risk ratio [RR] 0.75; 95% confidence interval [CI] 0.44-1.31) or akathisia (5 studies, n = 1094; RR 0.78; 95% CI 0.33-1.82) or any of the secondary outcomes. In subgroup analysis, diphenhydramine was associated with a significant decrease in extrapyramidal adverse effects compared with placebo (4 studies, n = 705; RR 0.61; 95% CI 0.41-0.90). Dosage analysis yielded no further information.

CONCLUSIONS

When compared with placebo, diphenhydramine was associated with a significant reduction of extrapyramidal adverse effects. Overall quality of evidence is low. Further studies are warranted.

Mechanisms of action of clozapine in the treatment of neuroleptic-resistant and neuroleptic-intolerant schizophrenia

The mechanisms of action which account for the effectiveness of clozapine as a pharmacotherapy for the treatment of neuroleptic non-responders and neuroleptic intolerant schizophrenic subjects remain elusive. We review recent data concerning the actions of clozapine in laboratory animals, and discuss the likely sites of action of clozapine and the receptors through which clozapine acts. We suggest that actions at dopamine D2 receptors in the caudate nucleus and putamen underlie the extrapyramidal side effects of conventional neuroleptics. In contrast, we propose that clozapine acts in the prefrontal cortex, specifically targeting an as yet unidentified DA receptor of the D2 family, to exert therapeutic actions in neuroleptic non-responders. We suggest that the ability of clozapine to augment extracellular dopamine levels in the prefrontal cortex may represent a key mechanism contributing to the therapeutic effects of this drug, and suggest some alternative approaches which might be expected to result in effects similar to those of clozapine.

Effects on resting cerebral blood flow and functional connectivity induced by metoclopramide: a perfusion MRI study in healthy volunteers

BACKGROUND AND PURPOSE The substituted benzamide, metoclopramide, is a dopamine receptor antagonist and is widely prescribed in the symptomatic treatment of nausea and vomiting, although it can cause adverse motor and non-motor side effects. The effects of metoclopramide on brain metabolism have not been investigated to date. EXPERIMENTAL APPROACH To determine the effects of metoclopramide on brain function, cerebral perfusion changes after a single oral dose were assessed in healthy volunteers using magnetic resonance imaging (MRI) techniques. Arterial spin labelling (ASL) perfusion MRI was used to measure cerebral blood flow before and after metoclopramide. Blood haemodynamics in the vertebral and internal carotid arteries were evaluated using phase-contrast MRI. KEY RESULTS Metoclopramide altered haemodynamics in the carotid arteries and the cerebral perfusion. Perfusion increased bilaterally in the putamen, consistent with antagonism of dopamine D(2) receptors by metoclopramide and possibly related to its motor side effects. In contrast, reduced perfusion was observed in the insular cortices and anterior temporal lobes. In addition, functional connectivity between the insular cortex and the dorsolateral prefrontal cortex was decreased. These cortical changes affecting neural circuits between high-order association areas may underlie certain neuropsychiatric conditions occasionally reported after metoclopramide administration. CONCLUSIONS AND IMPLICATIONS The present results show the sensitivity of ASL to detect small changes in regional blood flow, closely related to brain function, after a single pharmacological challenge, highlighting the potential of this technique for human pharmacological studies.

Cell membrane lytic action of metoclopramide and its relation to tardive dyskinesia

The long-term use of many antipsychotic medications carries a risk of tardive dyskinesia in a small proportion of patients. Although metoclopramide is an antipsychotic at high doses, this drug is more commonly used at low daily doses to accelerate stomach movement of food. Because prolonged use of metoclopramide has also been associated with tardive dyskinesia, this drug is convenient to study to examine the possible basis of tardive dyskinesia. Previous work proposed that antipsychotics accumulated in the melanin granules of the human substantia nigra, ultimately building up to high concentrations that could disrupt cell membranes of nigral neurons. While previous work demonstrated the accumulation of metoclopramide in postmortem human nigral tissue, it remained to be tested whether high concentrations of metoclopramide would actually disrupt cell membranes. Therefore, the present work examined whether metoclopramide could disrupt cell membranes, using human erythrocytes directly exposed to various concentrations of metoclopramide in vitro. It was found that metoclopramide caused disruption of the red cells starting at a threshold of 1 mM, which would result in ~280 μmoles of metoclopramide per kilogram of dry red cell membranes. However, the nonspecific adsorption of metoclopramide to human substantia nigra is ~23 μmol/kg of dry solids (measured at the clinical spinal fluid concentration of metoclopramide). Therefore, the membrane-lytic concentration of metoclopramide is only about 12 times higher than that after a single exposure of the drug to the nigral tissue. Hence, metoclopramide accumulation in the substantia nigra over a matter of months may lead to nigral neuron damage.

Antipsychotic drug binding in the substantia nigra: an examination of high metoclopramide binding in the brains of normal, Alzheimer's disease, Huntington's disease, and Multiple Sclerosis patients, and its relation to tardive dyskinesia

This project was done in order to determine why the annual incidence of metoclopramide-associated tardive dyskinesia is much higher than that for the commonly used antipsychotics. To test the hypothesis that metoclopramide tardive dyskinesia may be associated with high concentrations of metoclopramide in the substantia nigra under clinical conditions, the nonspecific binding of tritiated antipsychotics to the dissected melaninized regions of postmortem human substantia nigra was measured. The nonspecific binding at 1 nM [³H]ligand was 7.3, 4.2, 2.6, 0.91 and 0.66 fmoles/mg for [³H]haloperidol, [³H]clozapine, [³H]raclopride, [³H]metoclopramide, and [³H]olanzapine, respectively. After adjusting these values for the known free concentrations of these drugs in plasma or spinal fluid, the amounts that would be bound under clinical conditions would be 231, 113, 15, 11, and 3.4 fmoles/mg for metoclopramide, clozapine, raclopride, haloperidol, and olanzapine, respectively. Using rat striatum as baseline to define antipsychotic binding to nonnigral tissue, the excess amount of binding to the Alzheimer nigral tissue under clinical conditions would be 209, 19, 0, 3.4 and 0.8 fmole/mg for metoclopramide, clozapine, raclopride, haloperidol, and olanzapine, respectively, with a similar pattern for nigral tissues from Huntington and Multiple Sclerosis patients. The high accumulation of metoclopramide is sufficiently high to cause nigral nerve cell membrane damage by metoclopramide's detergent-like action, possibly explaining metoclopramide's toxic ability to elicit early tardive dyskinesia. In addition, the nonspecific binding of metoclopramide was much higher in Alzheimer-diseased substantia nigra, consistent with the fact that older individuals are relatively more vulnerable to metoclopramide tardive dyskinesia.

Severe akathisia as a side effect of metoclopramide

Case description A case of severe metoclopramide-induced akathisia in a breast cancer patient being treated with chemotherapy is presented, eventually culminating in hospital admission. In retrospect, this adverse effect was not recognized for several weeks because the prescription had not been properly recorded in the chart, the patient initially denied using the drug, and extensive psychological adjustment difficulties were also present. Conclusion Movement disorders as an adverse effect of metoclopramide have been described on a regular basis over the past decades. Case reports such as this confirm there is under-recognition of adverse effects and emphasize the need to take a comprehensive medication history and recognize well known side effects of medications such as metoclopramide.

Effect of Lipophilicity of Substituted Benzamides on the Dopaminergic Effects in Vivo and in Vitro

The antidopaminergic potencies of some new substituted benzamides related to remoxipride have been studied in the rat using in vitro (3H-spiperone binding) and in vivo techniques (antagonism of apomorphine induced stereotypies). The lipophilicities of the different benzamides were calculated by the addition of the lipophilic contribution (pi value) of the added substituent. It was found that the in vivo potencies of the compounds were not direct proportional to the in vitro potencies. The discrepancies in the potencies in vitro and in vivo (expressed as the ratio) are correlated to the calculated lipophilicities. Certain substituents in the benzene nucleus affect this in vitro--in vivo ratio more than could be expected from the change in lipophilicity caused by the substituent. The type of side chain is also of great importance. The reasons for the discrepancies between in vitro and in vivo antidopaminergic potencies of the studied substituted benzamides are discussed.

Anesthetic management for cesarean delivery in a woman with Gilles de la Tourette’s syndrome

Gilles de la Tourette's syndrome is a chronic neuropsychiatric disorder with an early childhood onset featuring mainly motor and vocal tics. We present the anesthetic management for cesarean delivery of a 21-year-old pregnant woman with Tourette's syndrome. She had shrugging of the shoulders and sudden, jerky, repetitive, irregular movements of the hands. General anesthesia was given for cesarean delivery. A live male infant weighing 3130 g was delivered. Her perioperative course was uneventful. No complication was observed. The patient and baby were discharged on the 4th postoperative day. It was decided to prescribe haloperidol 5 mg per day after lactation. Anesthesiologists should remember that there are special considerations when managing anesthesia in patients with Tourette's syndrome. The motor tics may lead to technical difficulty in performing regional anesthesia and surgery. Therefore general anesthesia may be appropriate in order to prevent agitation or involuntary movements. If patient movement and agitation can be controlled by sedation, regional block may be attempted. Drugs such as metoclopramide, ondansetron, midazolam and opioids may be used safely for anesthesia in Tourette's syndrome.

A randomized, double-blind, placebo-controlled trial of metoclopramide for the treatment of Tourette's disorder

OBJECTIVE

The pattern of dopamine antagonism by metoclopramide suggests benefits in the treatment of tic disorders. The purpose of this study was to examine the efficacy and safety of metoclopramide in the treatment of children and adolescents with tic disorders.

METHOD

Twenty-seven medication-free patients (age 11.9 +/- 2.7 years) with Tourette's disorder or a chronic tic disorder participated in an 8-week double-blind, randomized, placebo-controlled trial of metoclopramide. Metoclopramide was started at 5 mg daily and titrated as needed to a maximum dose of 40 mg daily. Tics were rated every 2 weeks, and adverse effects, including weight, cardiac, and laboratory measures, were monitored.

RESULTS

After 8 weeks of treatment, subjects receiving metoclopramide showed a 39% reduction in their total tic score on the Yale Global Tic Severity Scale, while subjects receiving placebo showed only a 13% reduction in tic severity (p = .001). Metoclopramide was well tolerated with no significant laboratory or cardiac changes noted other than an increase in serum prolactin.

CONCLUSIONS

The results of this small controlled study suggest that metoclopramide is an effective and well-tolerated treatment for children and adolescents with tic disorders. Further trials are needed to confirm its efficacy and safety in pediatric patients and adults.

Use of the "inverse neuroleptic" metoclopramide in Tourette syndrome: an open case series

Neuroleptics are generally highly effective in suppressing tics, but their many adverse effects limit their usefulness. Animal studies have shown that, compared with both typical and atypical neuroleptics, metoclopramide has effects that are regionally circumscribed to rat motor striatum. Based on this observation and two prior case reports, metoclopramide was openly prescribed and individually titrated to diminish tics in 10 patients with Tourette syndrome. All patients improved on the Yale Global Tic Severity Scale by an average of 55%. Although we did not observe frank extrapyramidal symptoms, including tardive dyskinesia, these data are not sufficient to support clinical recommendations because of many limitations, including the absence of systematic ratings of nontic abnormal movements. However, controlled clinical studies and additional basic investigations of metoclopramide are warranted.

Mechanisms of action of atypical antipsychotic drugs: a critical analysis

Various criteria used to define atypical antipsychotic drugs include: 1) decrease, or absence, of the capacity to cause acute extrapyramidal motor side effects (acute EPSE) and tardive dyskinesia (TD); 2) increased therapeutic efficacy reflected by improvement in positive, negative, or cognitive symptoms; 3) and a decrease, or absence, of the capacity to increase prolactin levels. The pharmacologic basis of atypical antipsychotic drug activity has been the target of intensive study since the significance of clozapine was first appreciated. Three notions have been utilized conceptually to explain the distinction between atypical versus typical antipsychotic drugs: 1) dose-response separation between particular pharmacologic functions; 2) anatomic specificity of particular pharmacologic activities; 3) neurotransmitter receptor interactions and pharmacodynamics. These conceptual bases are not mutually exclusive, and the demonstration of limbic versus extrapyramidal motor functional selectivity is apparent within each arbitrary theoretical base. This review discusses salient distinctions predominantly between prototypic atypical and typical antipsychotic drugs such as clozapine and haloperidol, respectively. In addition, areas of common function between atypical and typical antipsychotic drug action may also be crucial to our identification of pathophysiological foci of the different dimensions of schizophrenia, including positive symptoms, negative symptoms, and neurocognitive deficits.

Prefrontal cortical dopamine systems and the elaboration of functional corticostriatal circuits: implications for schizophrenia and Parkinson's disease

The dopaminergic innervation of the prefrontal cortex is able to transsynaptically regulate the activity of subcortical dopamine innervations. Disruption of the prefrontal cortical DA innervation results in the enhanced biochemical responsiveness of the dopamine innervation of the nucleus accumbens. We present recent data indicating that distinct prefrontal cortical dopamine innervations can be functionally dissociated on the basis of responsiveness to stress. The ventral striatal projection target (nucleus accumbens shell) of the prefrontal cortical region that is stress sensitive is also responsive to stress. In this manner interconnected cortico-striato-pallido-mesencephalic loops can be defined on the basis of the biochemical responsive of local dopamine systems to stress and on the basis of responsiveness to antipsychotic drugs. These data suggest the functional derangement of a distinct corticofugal loops in schizophrenia and in certain aspects of Parkinson's disease.

Treatment of schizophrenia: a clinical and preclinical evaluation of neuroleptic drugs

Forty years after the first clinical report on the effectiveness of chlorpromazine in psychiatric patients, neuroleptic drugs are still the most widely used drugs in the treatment of schizophrenia. Indeed, there are no other drugs which have proven to be as effective in the treatment of this severe psychiatric disorder. Yet, there are still many unresolved problems relating to neuroleptic drugs. The present review gives a comprehensive overview of our knowledge (and our lack of knowledge) with respect to the clinical and preclinical effects of neuroleptic drugs and tries to integrate this knowledge in order to identify the neuronal mechanisms underlying the therapeutic and side effects of neuroleptic drugs.

Effects of antipsychotic drugs on latent inhibition: sensitivity and specificity of an animal behavioral model of clinical drug action

Latent inhibition (LI) of a conditioned emotional response (CER) has been proposed as a quantitative measure of selective attention. We have assessed the parallels of the pharmacology of LI in rats with the clinical pharmacology of schizophrenia. Drug and vehicle treated rats were divided into groups and preexposed 20 times to cage illumination as a CS, or not preexposed. All groups were conditioned with 2 CS-footshock pairings. The following day CER, as measured by interruption of drinking in response to CS presentation, was recorded. LI was observed as a decreased CER in preexposed relative to non-preexposed animals. LI was enhanced by haloperidol 0.3 mg/kg after 7 or 14 daily treatments, but not after a single acute dose. Haloperidol doses of 0.3 and 0.03 mg/kg enhanced LI, while doses of 0.003 and 3.0 mg/kg had no effect. Haloperidol enhancement of LI was unaffected by the coadministration of the anticholinergic agent trihexyphenidyl. Enhancement of LI is exhibited by the antipsychotic drugs fluphenazine, chlorpromazine, thiothixene, thioridazine, mesoridazine, and metoclopramide but not clozapine. The non-antipsychotic drugs pentobarbital, imipramine, chlordiazepoxide, trihexyphenidyl, and promethazine failed to enhance LI. LI exhibits striking parallels to the clinical pharmacology of schizophrenia.

Metoclopramide-associated tardive dyskinesia in hemodialysis patients with diabetes mellitus. Two case reports

Metoclopramide, a drug used almost exclusively for medical indications, is a dopamine (D-2) receptor blocker and has been reported to cause extrapyramidal side effects. We present two case reports of hemodialysis patients who were treated with metoclopramide for diabetic gastroparesis. Within 12 months of beginning treatment, both patients developed persistent tardive dyskinesia. These cases highlight the fact that some patients who benefit from metoclopramide may also have a relatively high risk of developing persistent tardive dyskinesia. The consultation-liaison psychiatrist can play an important role in the education of the medical staff regarding metoclopramide-induced tardive dyskinesia.

Differential behavioural interactions between the dopamine D-1 antagonist SCH 23390 and the dopamine D-2 antagonists metoclopramide and sulpiride in nonhabituated mice

This study investigated the effects of the selective D-1 antagonist SCH 23390, when administered alone and in combination with a typical (metoclopramide) or atypical neuroleptic (sulpiride), on species-typical behaviours in nonhabituated mice. When tested singly, all three compounds caused a progressive dose-dependent inhibition of locomotion, rearing and grooming, though their potencies varied widely. Mixtures of a threshold dose of 0.01 mg/kg SCH 23390 and metoclopramide (0.05-1.25 mg/kg) interacted synergistically to promote hypomotility and to decrease rearing, but did not affect grooming. By contrast, combinations of 0.01 mg/kg SCH 23390 and sulpiride (2-10 mg/kg) resulted in a marked potentiation of grooming, but only additive reductions in horizontal and vertical movements, consistent with sulpiride and metoclopramide occluding different populations of D-2 receptors. The results show that blockade of D-1 receptors and certain populations of D-2 receptors can interact positively to modify animal motor behaviour, and add a fresh perspective to the concept that these two types of dopamine receptor normally work interdependently to control movements of the body.

Blockade of acquisition of one-way conditioned avoidance responding by haloperidol and metoclopramide but not by thioridazine or clozapine: implications for screening new antipsychotic drugs

The classical neuroleptic drugs haloperidol and pimozide have a strong disruptive effect on the acquisition of conditioned avoidance responding (CAR), yet have relatively little impact on the performance of previously acquired responses. Separate experiments compared the effects of haloperidol, two atypical neuroleptics, thioridazine and clozapine, and a substituted benzamide, metoclopramide, on one-way avoidance by rats. Thioridazine (10-50 mg/kg) and clozapine (1.25-10.0 mg/kg) disrupted both acquisition and performance of CAR. In contrast, haloperidol (0.075-0.150 mg/kg) and metoclopramide (5.0-7.5 mg/kg) completely blocked the acquisition of CAR, yet initially produced only a slight disruption in the performance of a previously acquired response. The ineffectiveness of the atypical neuroleptics in producing a complete disruption of acquisition of CAR may be due to the anticholinergic properties of these drugs. Alternatively, the differences between metoclopramide and the atypical neuroleptics may be due to a preferential effect of metoclopramide on striatal or amygdaloid dopamine neurotransmission. These results suggest that caution should be exercised in using CAR as an animal model for assessing the antipsychotic potential of new pharmacological agents.

Neuroleptic-induced reduction of quipazine-elicited head-twitches in rats: possible involvement of striatal dopaminergic supersensitivity

Rats were treated with a single large dose of various neuroleptic compounds and, 5-7 days after, they were assayed for either behavioral sensitivity to apomorphine (hypermotility and stereotyped behavior) or head-twitch response to the mixed serotonin-dopamine agonist quipazine. The animals withdrawn from chlorpromazine, fluphenazine, haloperidol, metoclopramide and the D1 selective blocker SCH 23390, showed enhanced hypermotility and/or stereotyped responses to apomorphine and reduced head-twitch response to quipazine. The rats withdrawn from thioridazine, (-)-sulpiride and sultopride responded to apomorphine only with enhanced hypermotility while their response to quipazine was either unchanged or even increased. The results are discussed in terms of dopaminergic brain areas and/or receptor subtypes involved in the modulation of the head-twitch response to quipazine. We concluded that an enhancement of dopaminergic tone at the striatal level could be related to the reduced head-twitch response to quipazine.

The inhibitory effect of metoclopramide on plasma cholinesterase activity

The in vitro effect of metoclopramide on plasma cholinesterase (PCHE) activity was studied to investigate a mechanism for metoclopramide-induced prolongation of succinylcholine action. The mean PCHE of the control samples was 0.86 +/- 0.02 unit.ml-1. PCHE activity in the presence of metoclopramide, at concentrations of 0.05, 0.10, 0.50, 1.0, 2.5 and 5.0 micrograms.ml-1, was reduced to 0.78 +/- 0.02, 0.69 +/- 0.04, 0.50 +/- 0.03, 0.39 +/- 0.02, 0.24 +/- 0.01 and 0.15 +/- 0.01 unit.ml-1, respectively. Our data demonstrated that PCHE activity was significantly depressed by metoclopramide at all concentrations studied (p less than 0.001). Our data also show that the concentration of metoclopramide required to inhibit 50 per cent of PCHE activity (I50) was 0.8 micrograms.ml-1 (2.4 x 10(-6) M). We recommend caution when succinylcholine and or ester type local anaesthetics are administered to patients who are also receiving metoclopramide, especially in high doses.

Actions of dopamine antagonists on stimulated striatal and limbic dopamine release: an in vivo voltammetric study

1. Fast cyclic voltammetry at carbon fibre microelectrodes was used to study the effects of several dopamine antagonists upon stimulated dopamine release in the rat striatum and nucleus accumbens. 2. In both nuclei, stimulated dopamine release was increased by D2-receptor-selective and mixed D1/D2-receptor antagonists. The D1-selective antagonist SCH 23390 had no effect. 3. Striatal and limbic dopamine release were elevated by cis- but not trans-flupenthixol. 4. The 'atypical' neuroleptics (clozapine and thioridazine) did not cause a selective elevation of dopamine release in the limbic terminal region, whereas the non-antipsychotic drug metoclopramide increased dopamine release more in striatum than nucleus accumbens. 5. We conclude from this study that striatal and limbic dopamine release are under the control of a stereoselective dopamine D2-autoreceptor on the nerve terminal and that atypical neuroleptics do not show a limbic-selective effect at this receptor after acute administration.

Chronic treatment with classical and atypical antipsychotic drugs differentially decreases dopamine release in striatum and nucleus accumbens in vivo

In vivo electrochemical techniques were employed to demonstrate that repeated treatment with classical antipsychotic drugs reduced basal dopamine (DA) release in the striatum and nucleus accumbens, whereas repeated treatment with atypical antipsychotics decreased DA release only in accumbens. Administration of apomorphine temporarily reversed these decreases to values comparable to those measured in vehicle-treated controls. These results suggest that the delayed onset of antipsychotic efficacy and extrapyramidal side effects involve a decrease in DA release in mesolimbic and nigrostriatal DA terminal fields, respectively. The results further suggest that induction of depolarization block in DA neurons may be the mechanism underlying these effects.

Metoclopramide potentiates d-amphetamine-induced hypermotility and stereotypy in rat

The substituted benzamide metoclopramide has been reported to block the behavioral effects of dopamine agonists, whereas its congener sulpiride potentiates these effects. We injected metoclopramide 2.0, 4.0, or 8.0 mg/kg PO into rats 2 hr before d-amphetamine 1.5 mg/kg IP and measured locomotion for 3 hr. We injected metoclopramide 8.0 mg/kg PO into rats 2 hr before d-amphetamine 1.5, 3.0, or 6.0 mg/kg IP and measured stereotypy for 3 hr. Metoclopramide potentiated the effects of all doses of d-amphetamine on both measures; peak effects occurred in the second or third hr after d-amphetamine injection. Metoclopramide alone tended to reduce behavior. The results suggest that metoclopramide is qualitatively similar to sulpiride in its interaction with d-amphetamine, and that metoclopramide's mechanism of action is not a simple dopaminergic antagonism. Clinicians are advised that metoclopramide, which is presently extensively for gastrointestinal and other disorders, may interact adversely with drugs that affect dopaminergic function.

Chronic autoreceptor blockade and neuroleptic-induced dopamine receptor hypersensitivity

Metoclopramide and sulpiride, two benzamide compounds, are equally potent in terms of their ability to block postsynaptic D2 dopamine receptors. However these compounds show a marked divergence in their ability to block dopamine autoreceptors, as metoclopramide is 20-25-fold more potent than sulpiride in blocking these receptors. When injected twice daily for 16 days, metoclopramide at a dose of 10 mg/kg/day will result in the development a postsynaptic dopamine receptor hypersensitivity (i.e., increased behavioral response to apomorphine upon cessation of the chronic treatment). An equivalent dose and treatment schedule with sulpiride has no apparent effect on dopamine receptor sensitivity. Because of the divergent pre- and postsynaptic potency of these two drugs it was possible to construct an autoreceptor "dose response curve" by varying the amount of these two drugs injected. Combinations of these two drugs were chosen so that the level or amount of postsynaptic dopamine receptor blockade was held constant while the amount of dopamine autoreceptor blockade was gradually increased. The results of this autoreceptor blockade "dose response curve" indicated that chronic autoreceptor blockade was involved in the increased dopamine receptor sensitivity that develops upon withdrawal from the neuroleptic drugs. These results suggest that the blockade of dopamine autoreceptors, and perhaps the resulting increase in autoreceptor sensitivity, is an integral component of the neuroleptic-induced dopamine receptor hypersensitivity.

Typical and atypical antipsychotic occupancy of D2 and S2 receptors: an autoradiographic analysis in rat brain

In thin sections of rat brain, [3H]spiperone binds to D2 sites in the basal ganglia (caudate-putamen, nucleus accumbens, olfactory tubercle) and S2 sites in the claustrum and motor cortex. The in vitro displacement of [3H]spiperone from these regions was quantified autoradiographically with the "atypical" neuroleptics clozapine and thioridazine, which ameliorate psychosis, a "typical" neuroleptic, haloperidol, which also induces extrapyramidal side effects, or with metoclopramide, which induces extrapyramidal side effects but is an ineffective antipsychotic. Whereas metoclopramide was equipotent at D2 sites, haloperidol was less potent and clozapine and thioridazine more potent by 2- to 3-fold at competing for D2 sites in the nucleus accumbens or olfactory tubercle than in the caudate-putamen. As measured autoradiographically or with tissue homogenates, clozapine, thioridazine, and five other atypical neuroleptics were 4- to 800-times more potent at competing for S2 sites in the frontal cortex than for D2 sites in the basal ganglia. A preference of atypical antipsychotics for D2 receptors in the nucleus accumbens and olfactory tubercle and for the S2 receptor may explain the relative lack of extrapyramidal side effects produced by these compounds.

Effects of remoxipride and some related new substituted salicylamides on rat brain receptors

A number of potential neuroleptic drugs of the substituted benzamide type have been compared with some reference neuroleptic drugs regarding their affinities for rat brain receptors using in vitro receptor binding techniques. The effects on dopamine-stimulated adenylate cyclase were also investigated. All 6-methoxysalicylamides were very potent inhibitors of the dopamine-D2 receptor and possessed much less affinity for other receptors as well as for the dopamine-stimulated adenylate cyclase. In contrast, the classical neuroleptic drugs were not selective dopamine-D2 receptor blockers. Several of these neuroleptics were thus potent inhibitors of alpha 1-receptors, 5-HT2 receptors, muscarinic receptors and histamine-H1 receptors. Sulpiride was the most selective dopamine-D2 receptor blocker of the reference compounds with a selectivity ratio of 65. The 6-methoxysalicylamides had selectivity ratios ranging from 18 to 167.

Antagonism of the amphetamine cue by both classical and atypical antipsychotic drugs

Rats were trained to discriminate the stimulus properties of 1 mg/kg of d-amphetamine sulphate (AMPH) from saline in a two-lever task in which correct responding was reinforced with water under a fixed ratio (FR 32) schedule. Classical antipsychotic drugs from different chemical classes were all able to block the AMPH cue. Doses (mg/kg) inhibiting the cueing effect to 50% (ID50) were 0.035 (haloperidol), 0.04 (spiroperidol), 0.09 (cis(Z)-flupenthixol), 0.12 (trifluperazine), 0.15 (perphenazine), 0.92 (chlorpromazine) and 1.40 (pimozide). The AMPH cue was also antagonized by antipsychotic drugs that are considered atypical due to their relative lack of activity in conventional animal models or inability to produce extrapyramidal symptoms in the clinic. The following ID50 values were obtained: 0.88 (molindone), 1.22 (clozapine), 5.48 (metoclopramide), 15.4 (thioridazine) and 52.8 [-)-sulpiride). In addition, the AMPH cue was blocked by the D-1 selective dopamine (DA) antagonist, SCH 23390 (ID50 = 0.014 mg/kg). The abilities of these drugs to block the AMPH cue were unrelated to the drugs' effect upon the rate of responding. For example, some drugs (e.g. haloperidol, spiroperidol and SCH 23390) blocked the AMPH cue completely without any effect on the response rate. Furthermore, the non-antipsychotic phenothiazine, promethazine (2.5-12.5 mg/kg) failed to affect the AMPH cue although the drug strongly suppressed the response rate. However, the potent DA agonists, apomorphine (0.05-0.33 mg/kg) and lisuride (0.02-0.08 mg/kg), and the DA and norepinephrine agonist, DPI (0.4 and 0.8 mg/kg), did not mimic the AMPH cue or did so only partially. These results suggest that the 1 mg/kg AMPH cue depends on (DA) systems other than those involved in the stereotyped motor behavior commonly produced by high doses of AMPH or DA agonists. Low-dose AMPH discrimination may thus serve as a new model for studying antipsychotic drug action.

Opposite effects of sulpiride and metoclopramide on amphetamine-induced stereotypy

The effects of the atypical neuroleptic sulpiride (0-20 mg/kg s.c.) and the classical neuroleptic metoclopramide (0-4 mg/kg s.c.) on behaviours produced by D-amphetamine (0-5 mg/kg i.p.) were measured in a time-sampling observational paradigm in rats. Sulpiride had one clear dose-dependent effect: it enhanced amphetamine-induced stereotyped behaviours (repetitive head movements, sniffing down and some gnawing). In contrast, metoclopramide dose-dependently decreased amphetamine-induced stereotypy, locomotion, rearing, and sniffing up, and concurrently antagonized the suppression of lying down produced by amphetamine. Sulpiride's facilitatory effects on amphetamine-induced stereotypy follow a pattern previously observed for two other atypical neuroleptics: clozapine and thioridazine. This may be a common effect of atypical neuroleptics. Since these neuroleptics are antipsychotic, amphetamine-induced stereotypy appears to be a poor animal model for human psychoses. It is suggested that sulpiride's effects may be mediated through a preferential presynaptic versus postsynaptic action on dopamine neurons in the nigrostriatal bundle.

Discrimination of neuroleptics by means of their interaction with amfonelic acid: an attempt to characterize the test

The non-amphetamine stimulant amfonelic acid (AFA) is known to enhance the effects of haloperidol, trifluoperazine and spiperone but not those of the atypical neuroleptics clozapine, thioridazine, and sulpiride, on the striatal levels of 3,4-dihydroxyphenylacetic acid. Consequently, the interaction between neuroleptics and AFA has been proposed as a test to discriminate typical and atypical neuroleptics. In this study, these findings were confirmed. Essentially the same results were obtained when the levels of homovanillic acid were measured. However, striatal dopamine levels were decreased similarly by combinations of AFA with typical and atypical neuroleptics. A comparison of the results with 17 neuroleptics with their reported clinical liability to cause extrapyramidal symptoms supported the idea that the test may discriminate drugs with a better ratio of therapeutic vs side effects. A series of antidepressants and alpha-noradrenergic antagonists possessing antidopaminergic properties was found to perform like atypical neuroleptics, i.e. their effects on dopamine metabolites were not enhanced by AFA.

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.

Atypical neuroleptics increase self-administration of cocaine: an evaluation of a behavioural screen for antipsychotic activity

Several drugs have been shown to exert antipsychotic effects, yet they display an atypical profile with respect to standard neuroleptic drug screens. Low doses of traditional neuroleptics are known to increase self-administration of psychomotor stimulants; we sought to determine whether these atypical drugs would cause a comparable effect. Sulpiride, metoclopramide and thioridazine produced a dose-dependent increase in cocaine intake similar to that found for chlorpromazine, haloperidol, pimozide and flupenthixol. This effect was found to correlate (r = 0.94) with daily clinical dose. Clozapine, however, produced a dose-dependent decrease in cocaine intake. The advantages and disadvantages of using this measure as a screening procedure for neuroleptic drugs are discussed.

Differential effects of classical and atypical antipsychotic drugs on A9 and A10 dopamine neurons

Prolonged treatment with classical antipsychotic drugs decreased the number of spontaneously active dopamine neurons in both the substantia nigra (A9) and the ventral tegmental area (A10) of the rat brain. In contrast, treatment with atypical antipsychotic drugs selectively decreased the number of A10 dopamine neurons. Related drugs lacking antipsychotic efficacy failed to decrease dopamine activity. These findings suggest that the inability of atypical antipsychotic drugs to decrease A9 dopamine neuronal activity may be related to their lower potential for causing tardive dyskinesia and that the inactivation of A10 neurons may be involved in the delayed onset of therapeutic effects during treatment.

Neuroleptic inhibition of 6,7-ADTN-induced hyperactivity after injection into the nucleus accumbens. Specificity and comparison with other models

The dopamine agonist 6,7-ADTN induced locomotor stimulation after injection into the nucleus accumbens of rats. This hyperactivity was stereospecifically antagonized by peripheral injection of typical and atypical neuroleptics, except sulpiride, regardless of structural class. The potency of 14 neuroleptics correlated significantly to that in other neuroleptic in vivo models in rats, i.e. cataleptogenic and amphetamine-antagonistic (stereotypy, circling) activity. Sulpiride was inactive after peripheral injection, whereas intraacumbens sulpiride antagonized 6,7-ADTN-induced hyperactivity with a higher potency than that found for locally injected cis-(Z)-flupentixol, haloperidol and clebopride. Blockade of alpha 1-adrenergic receptors by systemic prazosin or locally injected phentolamine also antagonized 6,7-ADTN, whereas antagonists of alpha 2-adrenergic, beta-adrenergic, muscarinic and serotoninergic receptors were without effect. Furthermore, the cis-(Z)-flupentixol and haloperidol-induced 6,7-ADTN blockade was only slightly or not reversed by additional antimuscarinic (scopolamine) or antiserotonergic (methysergide) treatment. It is concluded that antagonism of 6,7-ADTN-induced hyperactivity is a relevant model for detecting neuroleptic activity with minimal influence of anticholinergic and antiserotonergic activity, but that the dopamine receptor within the nucleus accumbens responsible for the locomotor stimulation is similar to that involved in other behavioural effects of neuroleptics. It is suggested that an apparent preferential limbic action of some neuroleptics (e.g. clozapine and chlorprothixene) is more likely to be explainable by their simultaneous blocking effect on dopaminergic, alpha-adrenergic and muscarinic receptors.

Metoclopramide. An updated review of its pharmacological properties and clinical use

Since previously reviewed in the Journal (Vol. 12, No. 2), metoclopramide has been confirmed as an effective drug in treating and preventing various types of vomiting and as a useful agent in oesophageal reflux disease, gastroparesis, dyspepsia, and in a variety of functional gastrointestinal disorders. Of considerable importance is the recent evidence of its efficacy when administered intravenously in high dosages in preventing severe vomiting associated with cisplatin. Good results have been achieved in patients not previously treated with cisplatin, but further studies are needed to determine its level of efficacy in patients who have experienced severe vomiting during earlier courses of cytotoxic therapy. Side effects consisting of mild sedation, diarrhoea and reversible extrapyramidal reactions have occurred, but are tolerated by many patients.

Spontaneous orofacial dyskinesia and dopaminergic function in rats after 6 months of neuroleptic treatment

A syndrome of spontaneous orofacial dyskinesia was identified in groups of rats treated for 6 months with a wide range of neuroleptic drugs. Phenothiazines, thioxanthenes, and substituted benzamides were particularly likely to induce the syndrome. It was observed in the presence of a functional blockade of dopamine receptors and endured for at least 2.5 months after drug withdrawal. There was no relation between the syndrome and changes in striatal dopamine receptors, as indexed by the binding of tritiated spiperone and tritiated cis(Z)-flupenthixol. The syndrome parallels several of the features of clinical tardive dyskinesia, whose pathophysiology thus may not involve changes in the characteristics of striatal dopamine receptors.

Comparison of inhibitory effects of substituted benzamides and classical neuroleptics on operant behavior in rats and squirrel monkeys

The operant behavioral effects of the substituted benzamides metoclopramide, sultopride and sulpiride were compared with those of chlorpromazine and haloperidol using lateral hypothalamic self-stimulation of the brain in rats and Sidman avoidance responding in rats and squirrel monkeys. All the drugs inhibited both phenomena dose-dependently in rats. The order of potency was: haloperidol greater than chlorpromazine greater than metoclopramide greater than sultopride greater than sulpiride. Sulpiride only produced moderate inhibition even at 128 mg/kg s.c. All drugs also suppressed Sidman avoidance in squirrel monkeys, which were far more sensitive than rats to the inhibitory effects of all three benzamides. In monkeys, sulpiride 8 mg/kg i.m. apparently suppressed Sidman avoidance with a delayed onset and a prolonged duration, which contrasted with the rapid onset and short duration of metoclopramide action. Thus, long-term observation of the behavioral effects of benzamides in squirrel monkeys may provide more precise information on the clinical antipsychotic efficacy of substituted benzamides.

Differential effects of sulpiride and metoclopramide on brain. Homovanillic acid levels and shuttle box avoidance after systemic and intracerebral administration

Sulpiride (SUL) is more efficacious in antipsychotic activity than metoclopramide (MET), although both appear to possess almost equal affinities for dopamine (DA) receptors. We studied the effects of SUL and MET on the brain homovanillic acid (HVA) level and shuttle box avoidance to clarify their differential modes of action on the blockade of DA receptors. SUL was several times less potent than MET, which was about 50 times less potent than the control drug haloperidol (HAL), for increasing HVA at 4 h after s.c. administration. However, SUL was about 50 times more potent than MET and about equipotent to HAL, at 2 h after drug administration into the lateral ventricle. The potency of SUL by intraventricular administration was 20-fold or more than that by intracisternal administration. HAL and MET did not show such a discrepancy. The HVA level reached the maximum 1 h after intraventricular administration of MET or HAL, while s.c., intracisternal or intraventricular administration of SUL caused a gradual increase in the HVA level with the maximum being reached after 6-8 h. In shuttle box avoidance, MET was about 25 times more potent than SUL by intraperitoneal administration but about 200 times less potent than SUL, which was about 10 times more potent than HAL, by intraventricular administration. These results suggest that SUL causes a potent, long-lasting blockade of DA receptors, while MET has only a weak, short-lasting action. These findings may be related to the differences in their clinical efficacy.

Septo-hippocampal system: target for substituted benzamides

Effects of the substituted benzamide, and of oxiperomide on DA receptors and on Da-related behaviors were the object of the study. The benzamides were practically inactive on DA-sensitive adenylate cyclase and on in vitro [3H] spiperone binding (in the absence of sodium ions). However, all of them inhibited in vivo [3H] spiperone binding in various rat brain regions; this in vivo effect was especially apparent in hippocampus and septum. In frontal cortex, the benzamides (with the exception of metoclopramide) produced only a partial inhibition of [3H] spiperone binding. Such inhibition also occurred in the striatum with sulpiride and tiapride. The results suggest that substituted benzamides are DA antagonists due to their ability to inhibit in vivo [3H] spiperone binding. The lack of agreement between in vivo and in vitro tests is also discussed.

Differential affinities of molindone, metoclopramide and domperidone for classes of [3H]spiroperidol binding sites in rat striatum: evidence for pharmacologically distinct classes of receptors

Rat striatum contains two populations of dopaminergic [3H]spiroperidol binding sites. The two populations are similar in their affinities for chlorpromazine and dopamine. Only one population, that with a somewhat higher affinity for spiroperidol itself, exhibits high affinity for the selective D2 antagonists molindone, metoclopramide and domperidone. Hence, this population may represent D2 receptor sites. The other larger population may represent either a separate class of receptor sites or a different form of D2 receptor sites.

Differential reversal of various dopamine antagonists by anticholinergics in Sidman avoidance: possible relationship to adrenergic blockade

Various dopamine receptor antagonists have divergent clinical and neurochemical properties. The relative ability of anticholinergics (benztropine and scopolamine) to reverse these drugs was assessed in squirrel monkeys and rats performing a Sidman avoidance task. In monkeys, benztropine markedly attenuated the effects of oxiperomide, metoclopramide, halopemide, tiapride and mezilamine as well as haloperidol. Chlorpromazine and fluphenazine were antagonized to a moderate extent; thioridazine and perlapine were not antagonized; and clozapine was actually potentiated by benztropine. In the rat, benztropine antagonized haloperidol strongly but reversed fluphenazine, thioridazine or clozapine only weakly or not at all. The overall effects of scopolamine in both species were similar to those of benztropine. The dopamine receptor antagonists that were most completely reversed by benztropine were found to inhibit 3H-spiroperidol more strongly than 3H-WB-4101 binding in calf caudate, while the reverse was true for drugs that were antagonized only moderately or not at all by benztropine. These results support a previous suggestion that anticholinergic reversal is less marked against dopamine antagonists with alpha-adrenergic blocking properties. Benztropine reversal of experimental dopamine receptor antagonists in the squirrel monkey Sidman avoidance test may contribute to their preclinical characterization.