Presence of calcitonin-like peptide in rat milk: possible physiological role in regulation of neonatal prolactin secretion.

Previous results have shown that salmon calcitonin (sCT), a peptide in rat brain and pituitary gland, inhibits basal and TRH-stimulated PRL release and reduces PRL mRNA levels in isolated anterior pituitary cells of adult rats in culture. Rat milk contains a variety of neuropeptides and hormones, some of which are absorbed in bioactive form to exert endocrine influences in the developing offspring. The present studies were undertaken to investigate whether a CT-like peptide is present in rat milk. Circulating PRL levels in neonatal rats are low, and there is an abrupt increase in the basal secretion of this hormone at weaning. A second objective was to examine whether CT plays a role in the regulation of PRL secretion in neonatal animals. A sensitive and specific RIA for sCT was developed and used to assay rat milk on various days of lactation for sCT-like immunoreactivity. sCT-like activity was present in the water-soluble (infranatant) fraction of milk throughout lactation in concentrations as high as 1589 pg/ml. There were no statistically significant differences in immunoreactive levels of the peptide in milk samples from different days of lactation. sCT-like immunoreactivity in rat milk infranatant coeluted with synthetic sCT on reverse phase HPLC, and these HPLC fractions inhibited basal PRL release when added to cultures of anterior pituitary cells. This inhibition of PRL release by the sCT-immunoreactive HPLC fractions was comparable to that exerted by equivalent concentrations of synthetic sCT. Newborn rats were injected sc with 10 microliters normal rabbit serum or anti-sCT serum from the day of birth until postpartum day 10. The rats were killed on day 11, and their sera were analyzed for PRL. Anti-sCT-injected rats showed a significant increase in serum PRL levels compared to those in untreated or normal serum-treated rats. These results demonstrate that a CT-like peptide, which is a potent inhibitor of PRL release, is present in rat milk throughout lactation, and that passive immunization with a highly specific anti-sCT serum leads to an increase in serum PRL levels in neonatal rats. CT, possibly of milk origin, may be a physiologically relevant PRL-inhibiting factor during the neonatal period.