Limbic circuits and monoamine receptors: Dissecting the effects of antipsy chotics from disease processes

Abstract

There is considerable evidence for the involvement of brain dopaminergic and serotonergic systems in schizophrenia pathology. However, post-mortem studies have been limited by difficulties in separating the effects of chronic exposure to antipsychotics from that of the disease process. Our recent studies directly explored this by comparing groups that were free from antipsychotic treatment for up to a year prior to death and that were maintained on antipsychotics. We have used this approach to identify that there are prominent effects of both disease and of antipsychotic treatment. There appears to be a high association for schizophrenics between elevations of D3 receptors in target regions of the mesolimbic dopamine (DA) system and elevated numbers of 5-HT 1A receptors in prefrontal cortex (PFc). Antipsychotic treatment was correlated with a reduction of D3 receptors in the ventral striatum and its output structures. It also led to a reduction in the number of 5-HT 2 receptors in some regions of the PFc without modifying the concentration of 5-HT 1A receptors. The limbic loop interconnecting the PFc and ventral striatum may be the site of antipsychotic regulation of certain symptoms in schizophrenia, particularly anhedonia and depression. The positive symptoms of schizophrenia are more likely to be associated with disturbances in the temporal lobe. However, dopaminergic systems in the temporal lobe have historically been thought to be underdeveloped compared to that in the basal ganglia and unlikely to be the target of antipsychotics. Our studies of the expression of the DA D2 receptor in the temporal lobe has shown a complex organization in the perirhinal and temporal cortices that is disrupted in schizophrenia. The disturbances, which might be of neurodevelopmental origin and are unrelated to antipsychotic treatment, include altered laminar distribution of the D2 receptor and modified modular organization of D2 receptors in the superior temporal gyrus. We hypothesize that modified expression of D2 receptors in these regions play a key role in the genesis of hallucinations. Treatment with antipsychotics leading to D2 receptor blockade in temporal cortex may reduce the presence of positive symptoms.