Absence of acquired tolerance to neuroleptics in schizophrenic patients

Antipsychotic drugs rapidly induce dopamine neuron depolarization block in a developmental rat model of schizophrenia

Repeated administration of antipsychotic drugs to normal rats has been shown to induce a state of dopamine neuron inactivation known as depolarization block, which correlates with the ability of the drugs to exhibit antipsychotic efficacy and extrapyramidal side effects in schizophrenia patients. Nonetheless, in normal rats depolarization block requires weeks of antipsychotic drug administration, whereas schizophrenia patients exhibit initial effects soon after initiating antipsychotic drug treatment. We now report that, in a developmental disruption rat model of schizophrenia [methyl-azoxymethanol acetate (20 mg/kg, i.p.) injected into G17 pregnant female rats, with offspring tested as adults], the extant hyperdopaminergic state combines with the excitatory actions of a first- (haloperidol; 0.6 mg/kg, i.p.) and a second- (sertindole; 2.5 mg/kg, i.p.) generation antipsychotic drug to rapidly induce depolarization block in ventral tegmental area dopamine neurons. Acute injection of either antipsychotic drug induced an immediate reduction in the number of spontaneously active dopamine neurons (cells per electrode track; termed population activity). Repeated administration of either antipsychotic drug for 1, 3, 7, 15, and 21 d continued to reduce dopamine neuron population activity. Both acute and repeated effects on population activity were reversed by acute apomorphine injections, which is consistent with the reversal of dopamine neuron depolarization block. Although this action may account for the effects of D2 antagonist drugs on alleviating psychosis and the lack of development of tolerance in humans, the drugs appear to do so by inducing an offsetting deficit rather than attacking the primary pathology present in schizophrenia.

Dopamine-cell depolarization block as a model for the therapeutic actions of antipsychotic drugs

Antipsychotic drugs used in the treatment of schizophrenia have in common the property of being dopamine-receptor antagonists. However, the rapid timecourse of receptor blockade produced upon drug administration does not correlate with the emergence of clinical actions, which typically require weeks of treatment to become manifest. Studies in rats have shown that repeated antipsychotic drug treatment results in a delayed inactivation of dopamine-neuron firing in the midbrain due to depolarization block. Furthermore, the therapeutic efficacy of antipsychotic drugs in humans correlates with their ability to induce depolarization block of mesolimbic dopamine neurons, whereas their potential to produce extrapyramidal side effects correlates with their propensity for inducing depolarization block in the nigrostriatal dopamine system. Therefore, dopamine-cell depolarization block is an effective model for evaluating antipsychotic drug efficacy, and provides a potential mechanism to account for their therapeutic impact on a dysregulated dopamine system.

Loss of striatal cholinergic neurons as a basis for tardive and L-dopa-induced dyskinesias, neuroleptic-induced supersensitivity psychosis and refractory schizophrenia

In the first section of this paper several aspects of tardive dyskinesia (TD) (clinical, epidemiological, pharmacological) are reviewed. We propose that this syndrome is not the consequence of dopamine receptor proliferation, but results from damage or degeneration of striatal cholinergic interneurons. We suggest that this cellular damage is caused by prolonged overactivation of these neurons, which occurs when they are released from dopaminergic inhibition following neuroleptic administration. Overactivity of central cholinergic systems during akinetic and motor retarded depression could be a contributory cause. The predisposition to L-DOPA-induced peak-dose dyskinesia in Parkinson's disease may depend on the same type of striatal neuronal loss. In the second part of the paper, the subject of supersensitivity psychosis and drug-resistant schizophrenia is reviewed. These two syndromes, are commonly associated with TD, have similar predisposing factors and pharmacology to TD, and are potentially persistent. We suggest that these conditions also result from degeneration of cholinergic striatal interneurons following chronic neuroleptic administration. The efficacy of clozapine for such treatment-refractory psychoses is explained in terms of its blockade of D-1 dopamine receptors. Other drugs effective against refractory psychoses (e.g. risperidone) are predicted to reduce activation at D-1 receptors.

Severe cases of neuroleptic-induced supersensitivity psychosis. Diagnostic criteria for the disorder and its treatment

Tardive dyskinesia is thought to result from neostriatal dopaminergic receptor supersensitivity induced by chronic treatment with neuroleptics. Similarly, receptor supersensitivity occurring in other dopaminergic regions of the brain could result in the development of supersensitivity psychosis. As with tardive dyskinesia, severe forms of the disorder are rare. Ten such cases are described whose main characteristic is that psychotic symptoms can no longer be masked by increased dosages of neuroleptics. Diagnostic criteria for the disorder are proposed, and treatment with antiepileptic medication is described.

Patterns of neuroleptic usage in continuously hospitalized chronic schizophrenic patients: evidence for development of drug tolerance

A retrospective review of neuroleptic dosages over a five-year period in 19 chronic schizophrenic patients revealed that the majority (85 percent) had either consistent yearly increases or decreases in dosage, although clinical status remained unchanged. The authors present differences in adverse effects between the two groups and discuss the implications of their findings for the concepts of neuroleptic drug tolerance and supersensitivity psychosis.