Melanotrope cell plasticity: a key mechanism for the physiological adaptation to background color changes

The intermediate lobe of the pituitary secretes the melanotropic hormone alpha-MSH, which in amphibians plays a crucial role in skin color adaptation. It has been previously demonstrated that, in the frog Rana ridibunda, the intermediate lobe is composed of two distinct subpopulations of melanotrope cells that can be separated in vitro by using Percoll density gradients. These two melanotrope cell subsets, referred to as high-density (HD) and low-density (LD) cells, differ in their ultrastructural characteristics as well as in their biosynthetic and secretory activity. However, the specific, physiological role of the heterogeneity displayed by melanotrope cells remains elusive. In the present study, we investigated the effects of background color adaptation on melanotrope cell subpopulations. We found that adaptation of frogs to dark or white environment did not modify either the overall number of cells per intermediate lobe or the apoptotic and proliferation rates of melanotrope cells. On the other hand, adaptation of the animals to a white background significantly increased the proportion of hormone-storage HD cells and caused a concomitant decrease in that of LD cells (which exhibit higher levels of alpha-MSH release and POMC messenger RNA than HD cells). Conversely, after black-background adaptation the proportion of LD cells was markedly increased, suggesting that interconversion of HD cells to LD cells occurs during physiological activation of the intermediate lobe. In addition, black-background adaptation also enhanced alpha-MSH release by both cell subpopulations and increased inositol phosphate production in LD cells. These data indicate that, in frog, the proportions of the two melanotrope cell subsets undergo marked modifications during skin color adaptation, likely reflecting the occurrence of a secretory cell cycle whose dynamics are highly correlated to the hormonal demand imposed by the environment.