Implications for therapy
The effects elicited by dopamine agonists and antagonists are dependent on their selectivity. Selective drugs affect one subtype predominantly and therefore would be expected to have fewer adverse effects than nonselective drugs which have a wider spectrum of activity. A consideration of the D2 subfamily illustrates the potential therapeutic benefits of selective drug development.
The D2 receptor subfamily has been implicated in the positive symptoms of schizophrenia, by the observation that the clinical potency of antipsychotic drugs is related to their affinity for the D2, not D1, receptor subfamily. However, because receptors of the D2 subfamily are found in both limbic and striatal regions, their blockade results respectively in both the desired reduction in psychosis and the unwanted appearance of Parkinson-like adverse effects. Blockade of D2 receptors which inhibit prolactin release results in increased plasma prolactin concentrations.
The recent cloning and identification of the D3 receptor has attracted interest. Its localisation in the limbic areas suggests it may play a role in cognitive and emotional functions and so be an important target for antipsychotic drug therapy. This hypothesis is supported by findings that antipsychotic drugs previously thought to be selective for D2 receptors (raclopride and pimozide), as well as nonselective antipsychotic drugs (flupenthixol and chlorpromazine) and the atypical drug, clozapine, all interact with D3 receptors. If blockade of D3 receptors is involved in antipsychotic effects, then selective D3 antagonists may well provide antipsychotic drug therapy free from motor and hormonal adverse effects. Conversely, the use of dopamine agonists free of D3 activity in Parkinson's disease would be predicted to reduce the incidence of psychosis-like adverse effects.