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The melanophore-stimulating hormone (MSH) cells of the amphibian pars intermedia secrete the peptide a-melanophore-stimulating hormone (a-MSH), which induces pigment dispersion in dermal melanophores. The purpose of the present study was to determine (1) whether prolonged activation of the secretory activity of the pars intermedia is associated with hypertrophy, hyperplasia, or both and (2) whether the MSH cells function as a homo geneous or heterogeneous population in meeting the physiological demand for MSH. The demand for MSH was manipulated by adapting animals for at least 3 weeks to white, two shades of grey, or black backgrounds. Morphometric analysis showed that the intermediate lobe volume was positively correlated with the degree of pigment dispersion in the mela nophores. The number of MSH cells per lobe was not affected by the degree of pigment dispersion. Therefore, we conclude that enlargement of the tissue associated with MSH cell activation involves hypertrophy rather than hyperplasia. Ultrastructural examination indi cated that the majority of MSH cells in black-adapted animals are biosynthetically active, whereas the cells of white-adapted animals are relatively inactive and show granule storage. The pars intermedia of grey-adapted toads contained both active and inactive cells, indi cating that MSH cells respond as a heterogeneous cell population in meeting the endocrine demand imposed by background. © 1990 Academic Press, inc.In general, activation of endocrine cells may involve hypertrophy, hyperplasia, or both. Furthermore, two patterns of cell re sponses can be distinguished during activa tion: either all cells become activated or only subpopulations of cells become acti vated. An example of the former are the oxytocin-producing neuroendocrine cells that respond as a homogeneous population to the suckling stimulus (Poulain and Wakerley, 1982). An example of the latter are the vasopressin-producing neuroendocrine cells that respond as a heterogeneous pop ulation to osmotic stress (Poulain and Wakerley, 1982). Morphometric and biochemi cal evidence has been advanced that the rat gonadotropes and lactotropes (Lucque et al., 1986) as well as the p cells of the pancreatic islets (Schuit et al., 1988) and thyroid follicular cells in mice (Gerber et al., 1987) form heterogeneous populations with regard to their secretory response to demand for hormone.In the present investigation we studied the response of amphibian melanophorestimulating hormone (MSH)-producing cells to a physiological stimulus. Two major questions were addressed. First, does the activation of the pars intermedia involve hypertrophy, hyperplasia, or both? Second, do the endocrine cells of this tissue function as a homogeneous or as a heterogeneous cell population in response to a demand for MSH? The MSH cell of Xenopus laevis provides a good model to study these ques tions because the pars intermedia activity can be easily manipulated. The MSH cells of animals placed on a black background release a-MSH, which stimulates pigment ...

This study considers the possible involvement of the tripeptide TRH (thyrotropin releasing hormone) in the physiological regulation of melanophore stimulating hormone (MSH) secretion from the pars intermedia of the toad, Xenopus laevis. TRH was shown to stimulate release of MSH from superfused neurointermediate lobes obtained from white-background adapted animals, but had no effect on secretion from lobes of black-background adapted animals. Immunohistochemical analysis revealed a rich TRH-containing neuronal network terminating in the neural lobe of the Xenopus pituitary. Plasma levels of TRH, determined with a specific radioimmunoassay, proved to be extremely high and no significant difference in this level could be found between white- and black-adapted animals. Plasma TRH probably originates from the skin, and our results show that its concentration is within the effective concentration range established for this peptide in stimulating MSH release from the pars intermedia. Therefore, while both our superfusion and immunohistochemical results argue favourably for a function of TRH in the regulation of MSH secretion, we conclude that, in any regulatory role, it would likely have to function within the pars intermedia at concentrations exceeding the high plasma values. While TRH could be involved in short-term activation of the secretory process in white-background adapted animals or in animals undergoing the initial stages of black background adaptation, our results indicate that this peptide may have no function in the maintenance of secretion from the pars intermedia of animals fully adapted to black background.

Neuropeptide Y (NPY) is a 36-amino acid peptide first isolated and characterized from porcine brain extracts. A number of immunocytochemical investigations have been conducted to determine the localization of NPY-containing neurons in various animal species including both vertebrates and invertebrates. These studies have established the widespread distribution of NPY in the brain and in sympathetic neurons. In the rat brain, a high density of immunoreactive cell bodies and fibers is observed in the cortex, caudate putamen and hippocampus. In the diencephalon, NPY-containing perikarya are mainly located in the arcuate nucleus of the hypothalamus; numerous fibers innervate the paraventricular and suprachiasmatic nuclei of the hypothalamus, as well as the paraventricular nucleus of the thalamus and the periaqueductal gray. At the electron microscope level, using the pre- and post-embedding immunoperoxidase techniques, NPY-like immunoreactivity has been observed in neuronal cell body dendrites and axonal processes. In nerve terminals of the hypothalamus, the product of the immunoreaction is associated with large dense core vesicles. In lower vertebrates, including amphibians and fish, neurons originating from the diencephalic (or telencephalic) region innervate the intermediate lobe of the pituitary where a dense network of immunoreactive fibers has been detected. At the ultrastructural level, positive endings have been observed in direct contact with pituitary melanotrophs of frog and dogfish. These anatomical data suggest that NPY can act both as a neurotransmitter (or neuromodulator) and as a hypophysiotropic neurohormone. In the rat a few NPY-containing fibers are found in the internal zone of the median eminence and high concentrations of NPY-like immunoreactivity are detected in the hypothalamo-hypophyseal portal blood, suggesting that NPY may affect anterior pituitary hormone secretion. Intrajugular injection of NPY causes a marked inhibition of LH release but does not significantly affect other pituitary hormones. Passive immunoneutralization of endogenous NPY by specific NPY antibodies induces stimulation of LH release in female rats, suggesting that NPY could affect LH secretion at the pituitary level. However, NPY has no effect on LH release from cultured pituitary cells or hemipituitaries. In addition, autoradiographic studies show that sites for 125I-labeled Bolton-Hunter NPY or 125I-labeled PYY (2 specific ligands of NPY receptors) are not present in the adenohypophysis, while moderate concentrations of these binding sites are found in the neural lobe of the pituitary. It thus appears that the inhibitory effect of NPY on LH secretion must be mediated at the hypothalamic level.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunocytochemical studies showed the presence of a fiber system containing a CRF-like peptide in the median eminence and in the neural lobe of the pituitary gland of Xenopus laevis. During in vitro superfusion of neurointermediate lobe tissue, CRF, sauvagine and urotensin I induced a rapid and dose-dependent stimulation of secretion of MSH and endorphin. Tissue of white-background adapted animals displayed a remarkably higher sensitivity to CRF and sauvagine than tissue from animals that were adapted to a black background. During superfusion of isolated melanotrope cells in suspension, it was shown that CRF and sauvagine exerted their effect directly on the melanotrope cell. We therefore conclude that there is morphological and biochemical evidence to consider a CRF-like peptide as a physiological MSH-releasing factor.