Calcitonin inhibition of prolactin secretion in lactating rats: Mechanism of action

The effects of intracerebroventricular (10 ng/rat) or intravenous (10 or 40 mu g/15 min/rat) administration of salmon calcitonin (sCT) on the prolacti n (PRL) response to suckling and the activity of tyrosine hydroxylase (TH) were examined in lactating rats. Plasma concentration of PRL increased dram atically in control rats after the onset of the suckling stimulus, while ad ministration of sCT resulted in inhibition of PRL response to suckling. The action of sCT was much more effective with intracerebroventricular adminis tration, which totally blocked PRL release, compared to intravenous adminis tration. The intracerebroventricular administration of sCT increased TH act ivity of tuberoinfundibular dopamine neuron (TIDA) in the stalk-median emin ence, as measured by DOPA accumulation, while completely suppressing the PR L response to suckling. Injection of alpha-methyl-rho-tyrosine (a-MT; 50 mg /kg), an inhibitor of TH and thus dopamine synthesis, increased PRL levels, and suckling caused a further increase in plasma concentrations of PRL, In jection of sCT (intracerebroventricularly) did not inhibit the PRL response to suckling in the presence of a depletion of dopamine. These results sugg est that sCT inhibition of PRL secretion in lactating rats is mediated main ly by TIDA neurons without involvement of other neuroendocrine mechanisms. Copyright (C) 2000 S. Karger AG, Basel.