Fine structure of the secretory activity of the pars intermedia of Rana pipiens

Fine structure and cytochemistry of the mammalian pars intermedia

Various cytophysiological aspects of the pars intermedia of the pituitary are discussed. Cells containing melanocyte-stimulating hormone (MSH) have been studied under normal and experimental conditions. They react to variations in ionic equilibrium, but without any clear correlation with natraemia and osmotic blood pressure. The MSH-cell stimulation in hypernatraemic mice, which is not inhibited by bromocriptine, seems more specific than the stimulation in hyponatraemic mice, which is blocked by bromocriptine. The existence of a corticotropic cell system has been clearly demonstrated in the mouse (where it is particularly obvious), in the rat and in the cat but its significance is not clear. Although very poorly vascularized, the pars intermedia is rapidly invaded by tracer protein (horseradish peroxidase) injected either intravenously or intracerebroventricularly. The hypophysial cleft rapidly stores the tracer which can be resorbed by macrophagic epithelial cells lying free in the colloid contained in the cleft. Horseradish peroxidase lingers in the pars intermedia but is rapidly eliminated from the other hypophysial lobes after intraventricular (third ventricle) injection. Diffuse innervation of the pars intermedia applies to both glandular (MSH and ACTH) and non-glandular (epithelial and stellate) cells. While aminergic innervation of the pars intermedia is obvious, cholinergic innervation has not been demonstrated ultrastructurally or histochemically. Peptidergic fibres only occasionally penetrate marginal areas of the pars intermedia and seldom establish synaptic contacts with glandular cells. A specific relationship might exist between the pars intermedia and oxytocin fibres in view of the marginal distribution of the latter in the neural lobe. Numerous stellate cells of the pars intermedia react intensely with antiserum to gliofibrillar acid protein, indicating their astrocyte nature, which reinforces the idea of an analogy between the folliculo-stellate system of the hypophysis and the glial cells.

Endogenous production of nitric oxide and effects of nitric oxide and superoxide on melanotrope functioning in the pituitary pars intermedia of Xenopus laevis

Previous studies have focused on the immunohistochemical detection of a nitric oxide (NO)-cyclic 3',5'-monophosphate (cGMP) pathway in the brain and pituitary of the aquatic toad Xenopus laevis. We here investigate the endogenous production and possible involvement of NO signaling in the regulation of melanotrope cell activity in the pituitary pars intermedia of this amphibian. Using immunohistochemical staining of cultured cells with a polyclonal antiserum against inducible NO synthase (iNOS), immunoreactivity was observed both in melanotropes and in stellate-shaped cells. Part of these stellate-shaped cells is characterized as folliculo-stellate cells by their capacity of beta-Ala-Lys-N(epsilon)-AMCA uptake. Using chemiluminescence detection we demonstrate the presence of NO and reaction products like nitrite (NO(-)(2)) or peroxynitrite (ONOO(-)) in the incubation medium of cultured melanotropes. Bacterial lipopolysaccharide (LPS) stimulates the generation of NO and reaction products, the effect of which was blocked by S-methyl-l-thiocitrulline hydrochloride, a potent general NOS inhibitor. With [(3)H]lysine incorporation and a superfusion technique, it is shown that peptide release from melanotropes is stimulated by administration of superoxide dismutase (SOD), which was added to the superfusion medium to prevent scavenging of NO by superoxide anions. Pretreating the cells with the general NOS inhibitor l-nitroarginine methyl ester for 48 h attenuated the SOD-induced stimulation, but did not affect the stimulation by sodium nitroprusside (SNP) or 3-morpholinylsydnoneimine chloride (SIN-1), whereas hemoglobin blocked the combined effect of SOD plus NO donors. The soluble guanylate cyclase inhibitor 1H-[1,2, 4]oxadiazolo[4,3a]-quinoxaline-1-one did not inhibit but even significantly potentiated the effect of NO donors on peptide release without affecting the SOD-induced stimulation of peptide release. In addition to the previously described neuronal NOS (nNOS) immunoreactivity in nerve fibers in the pars intermedia of Xenopus, the present data reveal iNOS and nNOS as potential sources of endogenous NO production in cultured cells of the pars intermedia. Our study shows that also in nonmammalian vertebrates endogenous NO production may be physiologically relevant under conditions where protection against oxidative damage is needed. The endocrine cells of the pars intermedia themselves, as well as the folliculo-stellate cells, under such conditions may dispose of a protective mechanism against oxidative stress. The sensitivity of the endogenous NO production to LPS suggests that NO may also play a role during systemic inflammation.

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

The secretion of most pituitary hormones is under the control of feedback mechanisms. The feedback control of alpha-melanophore-stimulating hormone (alpha-MSH) from melanotrope cells is controversial. The possible existence of an autofeedback exerted by alpha-MSH or other POMC-derived peptides on melanotrope cells of the amphibian Xenopus laevis has been investigated. alpha-MSH or its potent agonist 4-norleucine,7-D-phenylalanine-alpha-MSH has no effect on the release of radiolabeled POMC-derived peptides or immunoreactive beta-endorphin from superfused neurointermediate pituitary lobes. Melanin concentrating hormone, previously reported to have an alpha-MSH-like effect on melanophores, did not affect alpha-MSH secretion. Neurointermediate lobe superfusate, which contains a mixture of POMC-derived peptides, failed to affect the secretory activity of melanotropes. It is concluded that in X. laevis the secretory activity of melanotropes is not under the control of short-term autofeedback mechanisms involving alpha-MSH or other POMC-derived peptides.

Effect of background color and low temperature on skin color and circulating alpha-MSH in two species of leopard frog

Circulating levels of alpha-melanocyte stimulating hormone (alpha-MSH) in two species of leopard frog, Rana pipiens and R. chiricahuensis, were measured by radioimmunoassay to reveal the correlation between skin color change induced by background color and by low temperature. High levels of alpha-MSH were found in both species of frog on a black background, but R. chiricahuensis had eight times higher levels than R. pipiens, R. chiricahuensis also exhibited the ability to darken its ventral surface, whereas the ventral surface of R. pipiens remained white. Neither skin color nor plasma alpha-MSH of R. pipiens was affected by cold. Low temperature did, however, darken dorsal and ventral skin of R. chiricahuensis in vivo, which corresponded to increased levels of plasma alpha-MSH. Dorsal and ventral skin of R. chiricahuensis, in vitro, darken in a dose-dependent manner to alpha-MSH, but not to cold.

Changes in formaldehyde-induced fluorescence of the hypothalamus and pars intermedia in the frog, Rana temporaria, following background adaptation

Adaptation of the frog, Rana temporaria, to a white background for 12 hr has resulted in an intense formaldehyde-induced fluorescence (FIF) in the neurons of the preoptic recess organ (PRO), paraventricular organ (PVO), nucleus infundibularis dorsalis (NID) and their basal processes permitting visualization of the PRO- and PVO-hypophysial tracts that extend into the median eminence (ME) and pars intermedia (PI); the FIF is reduced in all the structures by 3 days. In frogs adapted to a black background, for 12 hr and 3 days, there was a general reduction in the FIF of the PRO neurons and PRO-hypophysial tract. By 12 hr black background adaptation, the PVO/NID neurons and only their adjacent basal processes show FIF which was sharply reduced by 3 days, making the PVO-hypophysial tract undetectable. In the PI fibers the fluorescence was more intense in black-adapted frogs than in white-adapted ones at both the intervals studied. The simultaneous changes in the FIF of the hypothalamic nuclei, tracts and PI suggest that the PRO and PVO/NID neurons participate in PI control through release of neurotransmitter(s) at the axonal ends.

Secretion of melanophore-stimulating hormone (MSH) in long-term cultures of pituitary neurointermediate lobes

Neurointermediate lobes from pituitaries of the frog, Rana berlandieri forreri (Rana pipiens, sensu lato), were maintained in organ culture in media with and without serum for up to six months. The cultured tissues were examined periodically by light microscopy and transmission electron microscopy and by bioassay of the melanophore-stimulating hormone (MSH) secreted and present in the culture media. Light-microscopic observations revealed a high degree of preservation of the pars intermedia at four weeks with isolated areas of some glands maintaining histological integrity for the entire six months. Similarly, at the ultrastructural level the cells appeared morphologically intact and to be actively synthesizing and secreting hormone. Bioassays showed the glands to be continuously secreting MSH; however, larger yields of hormone were obtained in media lacking serum. No significant ultrastructural differences between cells grown in the presence or absence of serum were detected. The difference in concentration of MSH between the two groups therefore apparently results from enzymatic degradation of the hormone by the serum. Organ culture of the vertebrate neurointermediate lobe may provide a unique method for the production of large quantities of MSH and for the study of other melanotropic and opiate peptides as they may be synthesized and secreted by the pars intermedia.

Effects of reserpine administration on the fine structure of the rat pars intermedia

Hormone release from the pars intermedia is under inhibitory control of the hypothalamus. Control may be mediated via dopaminergic fibers which directly contact secretory cells. Administration of reserpine in vivo to adult male rats for four consecutive days results in major alternations in pars intermedia cytology. Cells show expanded areas and whorls of rough endoplasmic reticulum, as well as extensive Golgi zones with numerous dense granules. Some nerve fibers exhibit alterations in vesicle content, while others retain a more normal appearance. Freeze-fracture of glands from reserpine-treated animals provides evidence for exocytosis of granules, although such phenomena are not observed in thin sections. The ultrastructural findings suggest that reserpine alters the content of local inhibitory neurotransmitters in the pars intermedia, leading to unrestrained hormone release, followed by renewed granule synthesis.

Distribution of 3H-alpha-MSH in rat brain

Radioactive synthetic alpha-MSH, prepared by selective tritiation of its dibromo-L-tyrosine-derivative, was injected into the carotid artery of intact, hypophysectomized or pinealectomized rats. More radioactivity was found in the occipital cortex, cerebellum, and pons-medulla than in most other brain parts of rats decapitated 15 sec and, to a less significant extent, 30 min after injection. Part of this radioactivity behaved like alpha-MSH in several identification procedures. After administration of 3H-tyrosine, the least radioactivity was consistently found in the pons-medulla. Large amounts of radioactivity after 3H-alpha-MSH were found in the pituitary and, particularly in the hypophysectomized rats, in the pineal. The half-time disappearance of the first component of radioactivity from the blood of each group of rats injected with tritiated-alpha-MSH was no more than 2.5 minutes, a time considerably shorter than the behavioral and EEG effects of this pituitary hormone on visual attention.