Heterogeneous response of porcine gonadotrope subpopulations to gonadotropin releasing hormone (GnRH) during postnatal development

Melanotrope secretory cycle is regulated by physiological inputs via the hypothalamus

Previously, it has been shown that background color conditions regulate the overall activity of the frog intermediate lobe by varying the proportions of the two subtypes of melanotropes existing in the gland, the highly active or secretory melanotropes and hormone storage melanotropes, depending on melanocyte-stimulating hormone requirements. However, the factors and mechanisms underlying these background-induced changes are still unknown. In the present study, we investigated whether hypothalamic factors known to regulate melanotrope cell function can induce changes in vitro similar to those caused by background adaptation in vivo. We found that the inhibitors apomorphine (a dopamine receptor agonist) and neuropeptide Y decreased the number of active melanotropes and increased simultaneously that of storage melanotropes. On the other hand, the stimulator TRH increased the number of active cells and concomitantly reduced that of storage cells. Inasmuch as none of these treatments modified the apoptotic and proliferation rates in melanotrope cells, it appears that these hypothalamic factors caused actual interconversions of cells from a subpopulation to its counterpart. Taken together, these findings suggest that the hypothalamus would control melanotrope activity not only through short-term regulation of hormone synthesis and release, but also through a long-term regulation of the secretory phenotype of these cells whereby the activity of the intermediate lobe would be adjusted to fulfill the hormonal requirements imposed by background conditions.

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.

Two frog melanotrope cell subpopulations exhibiting distinct biochemical and physiological patterns in basal conditions and under thyrotropin-releasing hormone stimulation

Cell heterogeneity designates the phenomenon by which a particular cell type is composed of morphologically and physiologically distinct cell subpopulations. We have previously isolated two subsets of melanotrope cells in the intermediate lobe of the frog pituitary by means of a separation procedure based on a Percoll density gradient High density (HD) melanotrope cells were found to exhibit a more granulated cytoplasm and a lower secretory rate than low density (LD) cells. In the present study, we have investigated the biochemical and functional characteristics of each melanotrope cell subpopulation by using various approaches, including chromatographic analysis for the measurement of the proportion of acetylated alpha MSH, microfluorimetric measurement of the cytosolic free calcium concentration ([Ca2+]i) and in situ hybridization for quantification of POMC messenger RNA (mRNA). Under basal conditions, LD melanotrope cells showed higher secretory activity, acetylation rate, [Ca2+]i, and POMC mRNA content compared to HD cells. Incubation of the cells with 100 nM TRH for 2 h induced a more pronounced activation of alpha MSH secretion, [Ca2+]i mobilization, and POMC mRNA accumulation in LD than in HD melanotrope cells. Conversely, TRH increased the rate of acetylation of alpha MSH in HD cells, but did not affect acetylation in LD cells. Taken together, these results demonstrate that the frog intermediate lobe is composed of two subsets of endocrine cells with distinct biochemical and functional characteristics. The coexistence of two cell subpopulations in the frog pars intermedia is consistent with the idea of a cell secretory cycle, in which each melanotrope subset represents a specific state of cellular activity.