Identification of MSH release-inhibiting elements in the neurointermediate lobe of the rat

Neurointermediate lobes of rats comprise elements which, when excited in vitro, bring about an inhibition of the release of melanocyte stimulating hormone (MSH). Superfusion of neurointermediate lobes of intact donor rats with medium containing 45 mM K+ induced a stimulation of the release of oxytocin, arginine-vasopressin and dopamine (DA) and inhibited the release of MSH. Fluorescence histochemical observations and the results of release studies indicate that electrothermic lesions in the mediobasal hypothalamus induced a more rapid degeneration of dopaminergic than of peptidergic terminals in the neurointermediate lobe. Dopaminergic nerve terminals and the stimulated release of DA had vanished completely on the second day after these lesions, which coincided with the disappearance of K+-induced inhibition of MSH release. Frontal hypothalamic deafferentations resulted in disappearance of peptidergic nerve terminals as evidenced by the development of diabetes insipidus and the strong decline of depolarization-induced release of oxytocin and vasopressin from neurointermediate lobes in vitro. In contrast, the dopaminergic plexus was left intact, as was the K+-induced stimulation of DA release and inhibition of MSH release. We conclude that the K+-induced inhibition of MSH release is mediated by DA rather than by neuropeptides from terminals in the neurointermediate lobe. The results are in agreement with the proposed MSH release-inhibiting role of the dopaminergic tuberoinfundibular neurones.