Differential effects of the D1-DA receptor antagonist SCH39166 on positive and negative symptoms of schizophrenia

Phencyclidine in the social interaction test: an animal model of schizophrenia with face and predictive validity

Phencyclidine (PCP) is a hallucinogenic drug that can mimic several aspects of the schizophrenic symptomatology in healthy volunteers. In a series of studies PCP was administered to rats to determine whether it was possible to develop an animal model of the positive and negative symptoms of schizophrenia. The rats were tested in the social interaction test and it was found that PCP dose-dependently induces stereotyped behaviour and social withdrawal, which may correspond to certain aspects of the positive and negative symptoms, respectively. The effects of PCP could be reduced selectively by antipsychotic drug treatment, whereas drugs lacking antipsychotic effects did not alleviate the PCP-induced behaviours. Together these findings indicate that PCP effects in the rat social interaction test may be a model of the positive and negative symptoms of schizophrenia with face and predictive validity and that it may be useful for the evaluation of novel antipsychotic compounds.

Dopamine D1 receptor gene polymorphism and schizophrenia in Japan

We studied the relationship between schizophrenia and the DdeI polymorphism in the 5' untranslated region (5'UTR) of the dopamine D1 receptor (DRD1) gene. This polymorphism is an A (A1 allele) to G (A2 allele) transition in the 5' UTR of exon 2 at bp -48 (A-48G). One hundred forty-eight schizophrenics and 148 control subjects were investigated. No significant differences in genotypic counts and allele frequencies between schizophrenics and controls were found. Although a significant difference between the patients classified as disorganized type and the controls was discovered both in genotypic counts and allele frequencies, neither association proved significant when a Bonferroni correction was used. Moreover, there were no differences in scores of main symptoms of schizophrenia based on the Manchester Scale between patients with A1/A1 genotype and those with A1/A2 genotype. These findings suggest that this gene may not be involved in the pathogenesis of schizophrenia.

The cerebral cortex: a case for a common site of action of antipsychotics

Recent evidence from studies of receptor occupancy and regulation in post-mortem brains of patients with neuropsychiatric disorders and in non-human primates is providing new leads in the ongoing quest to understand the pathophysiology and causes of schizophrenia and to develop more effective methods of treatment. These studies suggest that the cerebral cortex may harbour the elusive common sites of action of antipsychotic medications and indicate that chronic treatment with these drugs differentially regulates both families of dopamine receptors in this structure. Upregulation of the cortical dopamine D2 receptors is accompanied by a downregulation of the D1 sites. Balancing the opposing actions of dopamine D1 and D2 receptor regulation may hold the key to optimal drug therapy and to understanding the pathophysiology of schizophrenia. In this article, Michael Lidow, Graham Williams and Patricia Goldman-Rakic review the evidence supporting the cerebral cortex as a pivotal site for these mechanisms underlying the action of antipsychotics.

High-resolution scatchard analysis shows D1 receptor binding on pyramidal and nonpyramidal neurons

In order to better understand the mechanism of action of atypical antipsychotic drugs (APDs), it is important to clarify how the dopamine system is integrated within local corticolimbic circuits. Toward this end, a high-resolution (HR) Scatchard technique has been used to measure the relative density (Bmax) and affinity (Kd) of D1 receptors on large neurons (> 100 microm2), on small neurons (< 100 microm2), and in neuropil (NPL) of rat medial prefrontal cortex (mPFC) and to determine the laminar distribution of these receptors for each neuronal compartment. Using [3H] SCH23390 as a ligand, all Kd and Bmax values were found to be similar indicating that D1 receptor activity is not preferentially localized to either large or small neuronal subtypes in mPFC. The density of D1 receptor binding in all three compartments was found to be almost twice as great in layers V and VI, as compared to superficial layers II and III. These results suggest that the blockade of D1 receptors associated with some atypical APDs may involve both pyramidal and nonpyramidal neurons in the PFC.

The new antipsychotics, and their potential for early intervention in schizophrenia

Over almost four decades, few fundamentally different antipsychotic drugs evolved to challenge classical neuroleptics as the mainstay of the pharmacotherapy of schizophrenia. However, the recent re-emergence of clozapine, together with the emergence of risperidone, portends an increasing number of new antipsychotics which are now either traversing the stages of regulatory approval or else well-advanced in clinical development. This article first evaluates the significance of clozapine and risperidone; it then reviews some of the new antipsychotics and how they might be classified vis-a-vis potential advantages for patients, outlines putative mechanisms and new therapeutic targets, and considers whether such agents may act on any disease process inherent to schizophrenia. One fundamental issue is the extent to which the new antipsychotics might shift materially the risk benefit balance towards intervention, not just at the earliest possible stage following the onset of psychosis but at a yet earlier, 'prodromal' phase of the disorder where there is a considerably greater likelihood of 'treating' behavioural disturbances that prove not to be the harbingers of psychotic illness.

Behavioral effects of clozapine and dopamine receptor subtypes

The atypical neuroleptic clozapine (CLZ) is an extremely effective antipsychotic that produces relatively few motoric side effects. However, CLZ displays limited antagonism at the dopamine (DA) D2 receptor, the receptor commonly thought to mediate the antipsychotic activity of neuroleptics. The mechanism of action behind the efficacy of CLZ remains to be determined. Miller, Wickens and Beninger [Progr. Neurobiol., 34, 143-184 (1990)] propose a "D1 hypothesis of antipsychotic action" that may explain the antipsychotic effects of CLZ. This hypothesis is built on the interactions between D2, cholinergic and D1 mechanisms in the striatum. These authors assert that although typical neuroleptics block D2 receptors, it is through an indirect action on D1 receptors that their antipsychotic action is manifest. The extra-pyramidal side effects produced by typical neuroleptics are hypothesized to be due to an indirect action on cholinergic receptors. It is argued that the anticholinergic properties of CLZ negate the D2 (motor side effects) action of CLZ, allowing CLZ to diminish psychotic symptoms through a direct action on D1 receptors. Thus, CLZ may function as a D1 receptor antagonist in behavioral paradigms. The current paper reviews and compares the behavioral profile of CLZ to those produced by D2- and D1-selective antagonists with specific reference to unconditioned and conditioned behaviors in order to more fully evaluate the "D1 hypothesis of CLZ action". Although the actions of CLZ remain unique, they do share some striking similarities with D1 receptor antagonists especially in tests of unconditioned behavior, possibly implicating the D1 receptor in the action of this antipsychotic drug.