Dopamine receptors and schizophrenia: contribution of molecular genetics and clinical neuropsychology.
Family, twin and adoption studies suggest that genetic factors play an important role in the aetiology of schizophrenia. The mode of inheritance, however, is complex and non-Mendelian. Although the aetiology of schizophrenia is unknown, it has been hypothesized that the necessary conditions for developing the disease are environmental stress and a vulnerability to psychosis. The implication of dopamine receptors to schizophrenia has been greatly studied. Several linkage and association studies have been performed in an attempt to establish the involvement of dopamine receptors in schizophrenia. However, although no conclusive evidence of linkage or association to any gene has been established, some results, suggestive of linkage for chromosomes 6, 22 and 13, await confirmation from other studies. Concerning association studies, it is also of interest that some studies support an association between schizophrenia and homozygosity at D(3). More work in larger samples is required before conclusive linkage hypothesis or association to a dopamine receptor may be established. Schizophrenic patients have been shown to have significant deficits in a wide range of cognitive processes, including memory, attention, reasoning ability and language. Since cognitive deficits are significant symptoms of schizophrenia which require effective treatment, their assessment in schizophrenic patients and during clinical trials of new potential antipsychotics is highlighted. Cognitive impairment in schizophrenia impedes psychosocial performance and is therefore an especially relevant target variable in the development of new therapeutic approaches. It is most prominent in tasks involving attention, memory and executive functions which are thought to reflect involvement of prefrontal and left-temporal brain areas. Semantic networks in schizophrenic patients with a younger age of onset are observed to be more disorganized and differ significantly to those of control subjects. The need to use broader approaches such as neuropsychological-related measures to identify pertinent phenotypes in non-affected subjects carrying vulnerability genes is also emphasized. Since dopamine receptors are the primary targets in the treatment of schizophrenia, improved therapy may be obtained by drugs that selectively target a particular subtype of dopamine receptor. In the development of novel antipsychotics, D(3) and D(4) receptors have received much attention and this is partly related to the fact that these receptors have a high abundance in brain areas associated with cognitive and emotional functions, such as parts of the limbic system and cortex. Recent studies suggest that atypical neuroleptics may significantly improve the cognitive deficits observed in schizophrenic patients and that atypical neuroleptics such as risperidone appear to improve memory and alertness suggesting that further clinical studies are needed to determine the precise influence of antipsychotics on the cognitive system of schizophrenic patients. Such studies could lead to useful insights as to the potential advantages of the newer antipsychotics which appear to have a sparing or beneficial effect on various components of cognitive function. However, the observation that cortical D(2) receptors are important sites of action for antipsychotics, that the cerebral cortex may harbour the common sites of actions of antipsychotics and that the balancing of the opposing actions of D(1) and D(2) receptor regulation may be an appropriate drug treatment suggests that the adjustment of D(1) receptor levels in the cortex may become an important goal of future antipsychotic generation. Such antipsychotics will be able to treat the positive, negative and cognitive deficits of schizophrenia.
Emilien
Gérard
G
Department of Pharmacology, Université Catholique de Louvain, Cliniques Universitaires Saint Luc, B-1200 Brussels, Belgium.
Maloteaux
Jean-Marie
JM
Geurts
Muriel
M
Owen
Michael J.
MJ