Regulation of melanophore stimulating hormone (MSH) release.

Melanin pigmentation of the vertebrate integument is regulated by melanophore-stimulating hormone (MSH) which originates from the pars intermedia of the pituitary gland (1). Early experiments established that MSH release, as for other pituitary secretions, was under control by the hypothalamus (2). Similar to prolactin release (3), MSH secretion is under an inhibitory control by the hypothalamus (4). Hypothalamic lesions (5, 6) or ectopic transplantation (7) of the pituitary leads to hypertrophy of the pars intermedia and elevated levels of circulating MSH (8). Both a direct neuronal (adrenergic) inhibition (9) and a hypothalamic MSH release-inhibiting factor (MRIF, 10) may possibly regulate pars intermedia function. It is not yet clear whether there is a functional relationship between these two suggested mechanisms of pars intermedia regulation or whether one or both of these inhibitory mechanisms is operative in each of the vertebrate classes (1 ). Whatever the nature of the hormone(s) or neurotransmitter(s) controlling MSH secretion, these first messengers (12) probably interact directly with the pars intermedia cells. An understanding of their mechanisms of action, as for other chemical messengers, is clearly an important goal of the physiologist interested in the control of melanin pigmentation. An attempt will be made here to discuss and evaluate thexperimental data relating to the factors and mechanisms said to be involved in pars intermedia cell regulation.