It has been previously demonstrated that thyrotropin-releasing hormone (TRH) stimulates in vitro the release of alpha-melanocyte-stimulating hormone (alpha-MSH) in frog. In the present study, the effects of various neuropeptides on spontaneous and/or TRH-induced alpha-MSH secretion were investigated, using a well-defined perifusion system technique. Vasoactive intestinal peptide, (VIP) a neurohormone which stimulates TRH target cells in mammals, was totally devoid of effect on frog melanotrophs although VIP-like material could be detected in neurointermediate lobe extracts. Somatostatin-like immunoreactive material was found in high concentrations in the frog neurointermediate lobe complex, but synthetic somatostatin (from 10(-10) to 10(-6) M) did not modify the spontaneous release of alpha-MSH. At doses of 10(-8) and 10(-6) M, synthetic somatostatin did not modify TRH-induced alpha-MSH secretion. Morphine (10(-5) M) and opioid peptides (10(-10) to 10(-6) M) had no effect on spontaneous alpha-MSH secretion. In addition, methionine enkephalin (10(-5) M) did not modify the stimulatory effect of TRH on alpha-MSH secretion. From these results we conclude that, among the neuropeptides which modulate prolactin secretion in mammals, only TRH is involved in alpha-MSH secretion in the frog.
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