Dopamine receptor alteration in schizophrenia: Neuroendocrine evidence

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Abstract
Growth hormone (hGH) responses to centrally acting dopamine agonists were used as indices of CNS dopaminergic function in order to test hypotheses implicating dopaminergic alteration in the etiopathology of schizophrenia. Apomorphine, a direct acting dopamine receptor agonist, and L-Dopa, an indirect agonist dependent upon presynaptic conversion to dopamine for its action, both elicited elevations in plasma hGH in most young male schizophrenic-and control-subjects. A highly significant difference was seen between the distribution of hGH responses to apomorphine for schizophrenics and that for controls. Unsually high hGH response to apomorphine was seen in schizophrenics who subsequently failed to respond to neuroleptic therapy; intermediate hGH response was seen in controls; and low hGH response was seen in subsequent neuroleptic responders; differences in hGH response were statistically significant for all intergroup comparisons. No such differences were seen in response to L-Dopa. An inverse relationship was seen between responses of individuals to L-Dopa and apomorphine. The findings suggest that the variability of hGH response to apomorphine is a reflection of dopamine receptor sensitivity, and that this variability may be an index of non-endocrine related dopaminergic sensitivity. They are consistent with hypotheses relating schizophrenia to alteration in dopamine receptors, although the type of receptor and the direction of alteration may be complex.