Effects of apomorphine on blood levels of homovanillic acid, growth hormone and prolactin in medicated schizophrenics and healthy control subjects.

Two doses of apomorphine (0.005 mg/kg as a subcutaneous injection and 0.015 mg/kg as a 90 min i.v. infusion), and corresponding placebo treatments, were administered to 11 chronic medicated schizophrenic patients and to 8 healthy control subjects. The purpose of the study was to asses the usefulness of drug-induced alterations in the concentration of homovanillic acid (HVA) in plasma as indicators of dopamine autoreceptor sensitivity in the central nervous system. Growth hormone and prolactin in serum were also measured and used as indicators of postsynaptic dopaminergic drug effects. In the control subjects, i.v. apomorphine increased growth hormone in serum from 1.8 +/- 0.2 to 28.3 +/- 4.6 ng/ml and reduced prolactin by 57 +/- 7%. In the patients, apomorphine caused only weak neuroendocrine effects. HVA in plasma was not affected by apomorphine in either group of subjects. The results for growth hormone and prolactin indicate that postsynaptic dopamine receptors in the tubero-infundibular system are antagonized to a considerable degree also during chronic treatment with neuroleptics. The lack of effect of apomorphine on HVA levels suggests that HVA in plasma is not a sensitive indicator of the inhibition of dopamine release caused by small doses of apomorphine and mediated through dopamine auto-receptors. Supersensitivity of this class of receptors could not be demonstrated in our patients, which contrasts with some earlier results.