Cutaneous effects of MSH in the mammal

Melanocyte-stimulating hormone and adaptive behaviour

In order to study the physiological role of alpha-MSH in adaptive behaviour, plasma concentrations of alpha-MSH were measured during acquisition and extinction of active avoidance behaviour and during retention of a passive avoidance response. In addition the effect on behaviour of neutralization of centrally available alpha-MSH by intracerebroventricular administration of antisera to alpha-MSH was investigated. Blood samples were collected either before or immediately after each acquisition session and also before and after the extinction session of an active avoidance response. In addition, blood was collected after retention of a passive avoidance response from rats which had been submitted t different shock intensities during the learning trial, resulting in various passive avoidance groups of animals could be found. Neutralization or reduction of available alpha-MSH in the brain by intracerebroventricular administration of antisera to this neuropeptide results in disturbances in active as well as passive avoidance behaviour. This may indicate that alpha-MSH available in the brain plays a physiological role in brain functions involved in adaptive behaviour and in processes related to learning. Thus, at least in some species, MSH manifests its adaptive function predominantly via the brain.

Nature and control of peptide release from the pars intermedia

Pro-opiocortin, the precursor of ACTH, LPH and gamma-MSH, is biosynthesized in both the cells of the pars intermedia and the corticotrophs of the pars distalis. In the pars distalis its processing does not vary significantly from species to species whereas in the pars intermedia large differences occur. The release of ACTH, beta-LPH and pro-gamma-MSH from the corticotrophs is under common positive control by hypothalamic corticotropin-releasing factor (CRF) and the nature of the peptides remains unchanged when they are secreted. The release of all five pars intermedia peptides that we have measured in vitro appears to be under tonic dopaminergic inhibition. The secreted peptides have also been identified chromatographically. The lack of unequivocal physiological function in the periphery, the diversity of the pars intermedia peptides and this common control mechanism tend to preclude a simple endocrine role for the pars intermedia. The neural effects of MSH and endorphin are well documented and specific neuronal uptake therefore cannot by dismissed. The absence of pars intermedia in the adult human pituitary suggests that such a site of synthesis of these peptides plays a minor role in learning and behaviour in a species (such as Homo sapiens) that has a highly evolved intelligence and may, instead, need to synthesize the peptides only in the brain.