On the stimulatory nature of the control of MSH secretion in ducks

Though birds lack the pars intermedia of the hypophysis, their pituitary glands do secrete MSH. This hormone and ACTH are elaborated in special cells of the pars distalis called corticomelanotrops. The present study was designed to ascertain whether the release of MSH in aves is under either stimulatory or inhibitory hypothalamic control. Extracts of median eminence were injected in ducks and plasma MSH was observed to rise after the injections: on the other hand, when pituitaries were ectopically grafted, significant changes in the levels of circulating MSH were not detected. Twenty days after grafting, the transplants were extirpated and incubated in media containing median eminence extracts. The extracts stimulated the release of MSH not only from grafts but also from pieces of normotopic glands. The grafts showed cells which contained ACTH but not MSH; however, they contained small amounts of MSH, detectable by RIA. The administration of ergocryptine brought about the inhibition of MSH secretion in vivo, and it is suggested that this drug acted on hypothalamic structures rather than directly on the corticomelanotrops. On the basis of the preceding results, it is concluded for the first time that in ducks the release of MSH has stimulatory control from the hypothalamus, contrarily to that occurring in almost all the animals so far investigated.