Differential effects of dopamine on two frog melanotrope cell subpopulations

The frog intermediate lobe consists of a single endocrine cell type, the me lanotrope cells, which are under the tonic inhibitory control of dopamine. Separation of dispersed pars intermedia cells in a Percoll density gradient has revealed the existence of two melanotrope cell subpopulations, referre d to as high-density (HD) and low-density (LD) cells. The aim of the presen t study was to investigate the effects of dopamine on each of these melanot rope cell subsets. Increasing doses of dopamine, ranging from 10(-9)-10(-6) M, inhibited the release of alpha-melanocyte-stimulating hormone (alpha-MS H) in LD (but not in HD) melanotrope cells. In addition, dopamine provoked a significant reduction of the rate of acetylation of alpha-MSH in LD cells but not in HD cells. Similarly, dopamine significantly decreased the accum ulation of POMC messenger RNA in LD cells, whereas it did not affect POMC g ene expression in the HD melanotrope subset. On the other hand, microfluori metric studies revealed that dopamine induced a significant reduction of KC l-stimulated cytosolic free calcium concentration in both LD and HD cells. The present study provides additional evidence for functional heterogeneity of melanotrope cells in the frog pars intermedia. Because dopamine plays a pivotal role in the regulation of alpha-MSH secretion, these data suggest the involvement of cell heterogeneity in the physiological process of backg round color adaptation in amphibians.