Analysis of autofeedback mechanisms in the secretion of pro-opiomelanocortin-derived peptides by melanotrope cells of Xenopus laevis

Comparison of long-term perifused pars intermedia of Anolis carolinensis, Rana pipiens and Hyla chrysoscelis: their responses to dopamine

Pituitary pars nervosa-pars intermedia of Anolis carolinensis, Rana pipiens and Hyla crysoscelis were perifused with synthetic medium 199 for up to 35 h. The pre- and post-perifused tissues were examined by electron microscopy. No neuronal endings were found in Anolis tissue, but both Rana and Hyla had occasional synaptic end bulbs, which remained visible in the post-perifused tissue, although the synaptic vesicles appeared to cluster in the center of the end bulbs. Exposure to dopamine HCl from 10(-8) to 10(-5) M had little effect on Anolis pituitary but inhibited Rana and Hyla pituitaries from releasing skin-darkening substances. The skin-darkening substances, presumably derivatives of the proopiomelanocortin molecule, were assayed on Anolis skin. No dose-dependent responses to dopamine were seen at the concentrations used. We saw the possibility of a short-loop feedback.

Background adaptation by Xenopus laevis: a model for studying neuronal information processing in the pituitary pars intermedia

This review is concerned with recent literature on the neural control of the pituitary pars intermedia of the amphibian Xenopus laevis. This aquatic toad adapts skin colour to the light intensity of its environment, by releasing the proopiomelanocortin (POMC)-derived peptide alpha-MSH (alpha-melanophore-stimulating hormone) from melanotrope cells. The activity of these cells is controlled by brain centers of which the hypothalamic suprachiasmatic and magnocellular nuclei, respectively, inhibit and stimulate both biosynthesis and release of alpha-MSH. The suprachiasmatic nucleus secretes dopamine, GABA, and NPY from synaptic terminals on the melanotropes. The structure of the synapses depends on the adaptation state of the animal. The inhibitory transmitters act via cAMP. Under inhibition conditions, melanotropes actively export cAMP, which might have a first messenger action. The magnocellular nucleus produces CRH and TRH. CRH, acting via cAMP, and TRH stimulate POMC-biosynthesis and POMC-peptide release. ACh is produced by the melanotrope cell and acts in an autoexcitatory feedback on melanotrope M1 muscarinic receptors to activate secretory activity. POMC-peptide secretion is driven by oscillations of the [Ca2+]i, which are initiated by receptor-mediated stimulation of Ca2+ influx via N-type calcium channels. The hypothalamic neurotransmitters and ACh control Ca2+ oscillatory activity. The structural and functional aspects of the various neural and endocrine steps in the regulation of skin colour adaptation by Xenopus reveal a high degree of plasticity, enabling the animal to respond optimally to the external demands for physiological adaptation.

Biphasic effect of MCH on alpha-MSH release from the tilapia (Oreochromis mossambicus) pituitary

The effect of melanin-concentrating hormone (MCH) on the release of alpha-melanocyte stimulating hormone (alpha-MSH) from the tilapia pituitary gland was studied in vitro. In a superfusion set up, 10 nM to 1 microM synthetic salmon MCH caused a concentration-dependent inhibition of alpha-MSH release from tilapia neurointermediate lobes (NILs). Immunoneutralization of MCH in tilapia NILs further indicated that endogenous MCH has an inhibitory effect on the melanotropes. The release of monoacetylated alpha-MSH release was more strongly inhibited by MCH than that of des-, and diacetylated alpha-MSH, indicating that MCH modulates the secretory signal of the melanotropes in a quantitative and qualitative manner. A high concentration of MCH (10 microM) substantially increased the release of alpha-MSH. Further evidence in support of a stimulatory action of high concentrations of MCH was provided by the observation that the MCH analogue MCH(2-17) at 10 and 35 microM enhanced alpha-MSH release as well. Therefore, we conclude that the response of pituitary melanotropes to MCH is biphasic, as was reported previously for the effects of MCH on other targets in fish and mammals. Under physiological conditions the inhibitory action of MCH on fish melanotropes most likely dominates.