Evidence for a possible dopaminergic control of pituitary alpha-MSH during ontogenesis in mice.

This study was aimed at determining the alpha-MSH and ACTH contents of the neurointermediate lobe (NIL) of the mouse hypophysis during ontogenesis, as well as the ability of the gland, incubated in a perfusion system, to respond to dopamine (DA) and high potassium (K+). We showed that: (1) the NIL content of alpha-MSH exhibited a biphasic pattern of evolution, characterized by a dramatic increase appearing between postnatal days 3 and 5. By contrast, NIL ACTH content followed a completely different pattern of evolution; (2) both DA and high K+ reversibly inhibited MSH release from superfused NIL, the latter effect being more pronounced with the use of pituitaries from 7-day-old than from 1-day-old mice; (3) the inhibitory influence of high K+ was impaired by haloperidol pretreatment. From these data, as well as from morphological observations, it appears that functional DA receptors seem to be present on melanotrophs of the developing hypophysis in mice and that the increase in alpha-MSH in IL cells observed shortly after birth may result from the onset of inhibitory influence exerted by dopaminergic innervation on hormonal secretion. This view, however, does not preclude possible effects of other hypothalamic releasing and/or inhibiting factors.