Melanophore Responses and Intermediatelobe Activity in the EelAnguilla anguillaafter Injection of 6-OH-dopamine

[The intermediate lobe of the pituitary, model of neuroendocrine communication]

The intermediate lobe of the pituitary is composed of a homogeneous population of endocrine cells, the melanotrophs, which secrete several bioactive peptides including alpha-melanocyte-stimulating hormone (alpha-MSH) and beta-endorphin. In contrast to most endocrine glands which are richly vascularized, the intermediate lobe of the pituitary contains very few blood vessels; in some species, the pars intermedia is virtually totally avascular. In contrast, pituitary melanotrophs are richly supplied by nerve fibers originating from the hypothalamus. The pars intermedia thus appears as a pure model of neuroendocrine communication, i.e. it is an archetype of the mode of transducing interface between the central nervous system and endocrine effectors. In mammalian species, different types of nerve terminals containing dopamine, norepinephrine, gamma-aminobutyric acid (GABA) and serotonin have been identified. In lower vertebrates, particularly in fish and amphibians, the pars intermedia is also innervated by peptidergic fibers which are though to take part in regulation of the secretory activity of the melanotroph. In these animals, the pars intermedia is regarded as a major center of neuroendocrine integration and an exceptional model to investigate the process of communication between the brain and the endocrine glands. The purpose of the present review is to summarize our current knowledge on the synthesis, processing and release of peptide hormones from pars intermedia cells and to survey the multiple regulatory mechanisms which are involved in the control of the activity of pituitary melanotrophs. Proopiomelanocortin, a multifunctional precursor. Pituitary melanotrophs synthetise a major precursor protein called proopiomelanocortin (POMC) which generates through proteolytic cleavage several biologically active peptides including adrenocorticotropic hormone (ACTH), endorphins and MSHs. In lower vertebrates, alpha-MSH is generally considered as the major hormone secreted by melanotrophs, in that it is involved in the process of skin colour adaptation. The post-translational processing of POMC, which yields to the mature hormones released by melanotrophs, includes a number of steps: glycosylation, phosphorylation, tissue-specific proteolytic cleavage, amidation and acetylation. Some of these posttranslational modifications can be regulated by neuroendocrine factors. For instance, in frogs, it has been shown that dopamine inhibits acetylation of alpha-MSH and thus reduces the secretion of the biologically active form of the peptide. The intermediate lobe of the pituitary: a model of neuroendocrine integration. In most vertebrate species, the intermediate lobe of the pituitary is innervated by catecholamine-containing fibers. In particular, the presence of dopaminergic nerve fibers has been observed in the pars intermedia of mammals and poikilotherms.(ABSTRACT TRUNCATED AT 400 WORDS)

Naltrexone induces melanocyte-stimulating hormone release and degenerative changes in pituitary innervation in amphibians

Adult Rana pipiens (common frog) were treated with the long-acting opiate antagonist drug naltrexone hydrochloride to study potential effects of an opiate-receptor block on release of pro-opiomelanocortin (POMC) peptides from the pituitary intermediate lobe. The melanophore response of the animals was used to assess relative release of melanocyte-stimulating hormone (MSH). Animals were adapted to a light background, then injected with 20 or 50 mg/kg naltrexone for up to 3.5 days. All drug-treated animals showed increased release of MSH while on the light background while controls remained light-adapted. Ultrastructure of the pars intermedia of naltrexone-treated frogs revealed nerve fibers containing myelin figures, swelling of neurovesicles, and abnormal membrane components indicative of degenerative changes. The observations suggest that naltrexone has a damaging effect on pituitary innervation, and that MSH release may be the result of loss of neuroregulation from the normally inhibitory innervation. The findings are in agreement with our earlier studies in rats, which also demonstrated degenerative changes induced by naltrexone in intermediate lobe innervation. The long-acting antagonist may therefore have a "neurotoxic" effect after multiple injections or chronic use.

6-Hydroxydopamine induces degenerative changes in innervation of the rat pituitary gland

Administration of 6-hydroxydopamine to adult male rats by peripheral injections induces degenerative changes in nerve terminals innervating cells of the pituitary intermediate lobe. Additional animals were treated with 5-hydroxydopamine, which produced images of nerve profiles containing vesicles with electron-dense centers, indicative of uptake of the false catecholamine (CA) neurotransmitter. Endocrine cells showed cytologic evidence of activation of synthetic and secretory compartments. The ultrastructural observations suggest that innervation to opiomelanocortin cells is sensitive to a CA-specific neurotoxin and strengthens the hypothesis that CA-containing fibers play a role in intermediate lobe neuroregulation.

Facilitation and inhibition of plasma cortisol following electrical stimulation of the goldfish forebrain

To examine the role of forebrain structures in the control of ACTH release, electrical stimulation was performed in the brains of continuously respirated , immobilized goldfish in which endogenous ACTH release was suppressed with dexamethasone. Three active areas of the forebrain were identified. Electrical stimulation for 5 min resulted in elevations of plasma cortisol 15 min following stimulation of the nucleus preopticus (NPO) or of the nucleus tenia (NT) but a decrease in plasma cortisol following stimulation of the nucleus lateral tuberis (NLT). The results of this study demonstrate that the NPO and NT facilitate, whereas nerve fibers originating in or passing through the NLT inhibit, ACTH release in the goldfish.

Effect of pimozide on the cytology of the eel pituitary. II. MSH-secreting cells

Pimozide, a specific blocker of dopaminergic receptors, was injected for 4 or 9 days in freshwater (FW) eels or eels acclimated to sea water (SW) for 10 or 30 days. The daily dose was 100 or 200 microgram/100 g. Melanophore index values increase in FW and in 1 month-SW injected eels. All the treated fish react by a total or subtotal degranulation of the lead-hematoxylin positive cells in the pars intermedia. These cells were previously identified as alpha-MSH-secreting cells. The MSH cell nuclear area is significantly increased, nucleoli are larger and the endoplasmic reticulum more developed. The intensity of the response is similar in FW and SW eels, but it does not increase with the higher dose. The rapid release of pituitary alpha-MSH is also visualized by immunofluorescence and immunoenzymologic techniques. No effect on the second cell type of the pars intermedia (PAS-positive cell) is detected. The amount of neurosecretory material is often reduced in the neurohypophysis. These results suggest that the hypothalamic inhibitory control of MSH release and synthesis is mediated through dopaminergic fibers in the eel, but other factors cannot be ignored in this regulation.