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

Seasonal plasticity of the pituitary pars intermedia of the one-humped camel (Camelus dromedarius)

The pituitary pars intermedia of Camelus dromedarius is well developed and completely surrounds the pars nervosa. Two major groups of cells are present: endocrine (ec) and glial-like cells (glc). The ec group is composed of three morphologically distinct cell types. Type I, or polyhedral light cells (LC-I) and type II, or polyhedral dark cells (DC-II), have secretory granules of heterogeneous electron density whose size ranges from 170 to 300nm. Type III cells are elongated with homogeneous electron-dense secretory granules of 80-200nm. The glc make up an organized network, form follicles in the centrolobular zones and are positive for vimentin and S-100β immunolabelling. The nerve fibres penetrating the lobe are numerous, and can be classified into two types according to the membrane bound vesicles found in their endings (ne). Ultrastructural quantitative analysis revealed significant variations in PI elements between winter and summer seasons (F=8.014, p=0.006). DC-II cells characterized by developed biosynthetic machinery and a large pool of secretory granules storage are increased with the ne in winter. However, LC-I cells showing frequent cytoplasmic degranulation are predominant with glc in summer. Thus, important cellular remodelling occurs in the dromedary PI that may depend upon, or perhaps anticipate, external living conditions.

Angiogenesis in the intermediate lobe of the pituitary gland alters its structure and function

The pars distalis (PD) and the pars intermedia (PI) have the same embryonic origin, but their morphological and functional characteristics diverge during development. The PD is highly vascularized, whereas the highly innervated PI is essentially non-vascularized. Based on our previous finding that vascular endothelial growth factor-A (VEGF-A) is involved in vascularization of the rat PD, attempt was made to generate transgenic Xenopus expressing VEGF-A specifically in the melanotrope cells of the PI as a model system for studying the significance of vascularization or avascularization for the functional differentiation of the pituitary. The PI of the transgenic frogs, examined after metamorphosis, were distinctly vascularized but poorly innervated. The experimentally induced vascularization in the PI resulted in a marked increase in tissue volume and a decrease in the expression of both alpha-melanophore-stimulating hormone (α-MSH) and prohormone convertase 2, a cleavage enzyme essential for generating α-MSH. The transgenic animals had low plasma α-MSH concentrations and displayed incomplete adaptation to a black background. To our knowledge, this is the first report indicating that experimentally induced angiogenesis in the PI may bring about functional as well as structural alterations in this tissue.

Revisiting the regulated secretory pathway: from frogs to human

The regulated secretory pathway is a hallmark of endocrine and neuroendocrine cells. This process comprises different sequential steps, including ER-associated protein synthesis, ER-to-Golgi protein transport, Golgi-associated posttranslational modification, sorting and packing of secretory proteins into carrier granules, cytoskeleton-based granule transport towards the plasma membrane and tethering, docking and fusion of granules with specialized releasing zones in the plasma membrane. Each one of these steps is tightly regulated by a large number of factors that function in a spatially and temporarily coordinated fashion. During the past three decades, much effort has been devoted to characterize the precise role of the yet-known proteins participating in the different steps of this process and to identify new regulatory factors in order to obtain a unifying picture of the secretory pathway. In spite of this and given the enormous complexity of the process, certain steps are not fully understood yet and many players remain to be identified. In this review, we offer a summary of the current knowledge on the main molecular mechanisms that govern and ensure the correct release of secretory proteins. In addition, we have integrated the advance on the field made possible by studies carried out in non-mammalian vertebrates, which, although not very numerous, have substantially contributed to acquire a mechanistic understanding of the regulated secretory pathway.

Alpha-melanocyte-stimulating hormone is decreased in plasma of patients with acute brain injury

The neuropeptide alpha-melanocyte-stimulating hormone (alpha-MSH) is a proopiomelanocortin derivative that has potent anti-inflammatory influences within the brain. The aim of the present research was to determine if there are changes in blood concentrations of this peptide in patients with acute traumatic brain injury (TBI) or subarachnoid hemorrhage (SAH). Concomitantly, we recorded clinical parameters and measured blood concentrations of the proinflammatory cytokine tumor necrosis factor-alpha (TNF-alpha). Twenty-three patients were enrolled in this study--18 had TBI and five SAH. Blood samples for determination of alpha-MSH and TNF-alpha were collected daily from day 1 to day 4 after injury. Baseline concentration of plasma alpha-MSH in patients with acute brain injury of either traumatic or vascular origin was significantly lower than in controls. Patients with TBI or SAH had similar alpha-MSH concentrations and the peptide remained consistently low over four post-injury days. Circulating TNF-alpha on day one was measurable in all patients and there was a negative correlation between plasma TNF-alpha and alpha-MSH. Alpha-MSH was measured again after the acute phase in eight patients. The peptide was substantially increased in all subjects except for two who had an unfavorable outcome. From the well-known protective anti-inflammatory influences of alpha-MSH in the host, reduction in this circulating peptide may have detrimental consequences in brain injury. The data raise the possibility that restoration of normal circulating alpha-MSH through administration of the peptide could be beneficial in patients with brain injury.

Release of false transmitter serotonin from the dopaminergic nerve terminals of the rat pituitary intermediate lobe

Rat pituitary intermediate lobe contains two types of serotonin-immunoreactive nerve terminals. Most of them are dopaminergic, in which serotonin acts as a false transmitter, while the rest are true serotoninergic nerves. In the present study, release of the false transmitter serotonin from the dopaminergic nerve terminals was studied by loading the neurons in vivo with serotonin precursor L-tryptophan and MAO inhibitor pargyline, which results in accumulation of false transmitter serotonin. Subsequently pituitary neurointermediate lobe complexes were incubated in the presence of various agents. Potassium induced dramatic release of serotonin. This release was Ca(2+)-dependent, as demonstrated by an inhibition by Mg2+, and transporter-independent, since it was unaffected by GBR 12909 (a dopamine transport inhibitor). Tyramine and sodium nitroprusside, a nitric oxide donor, caused slight to remarkable release of serotonin. This release was inhibited by GBR 12909, suggesting that it was transporter-dependent. Presynaptic stimulation with apomorphine or haloperidol, dopamine receptor agonist or antagonist, respectively, or isoproterenol, agonist of the beta-adrenergic receptor, did not significantly release serotonin. Thus, it seems that presynaptic receptors per se cannot induce release of significant amounts of serotonin from the IL dopaminergic fibers. Our results suggest that false transmitter serotonin in the IL dopaminergic nerve terminals is released primarily by the classical exocytotic release mechanism, but may also be partly released by the transporter-dependent, non-exocytotic release.

Pharmacological characterization of serotonin synthesis and uptake suggest a false transmitter role for serotonin in the pituitary intermediate lobe

A subpopulation of nerve fibers in the rat pituitary intermediate lobe (IL) have been shown to exhibit colocalization of serotonin (5-HT-IR) and tyrosine hydroxylase immunoreactivities and they are sensitive to neurotoxins specific to catecholamine neurons. This study was set out to examine the uptake and synthesis mechanisms of serotonin in these fibers. We developed an in vitro technique in which the neurointermediate lobe explants were incubated (14 and 48 h) in the presence of various drugs and serotonin was subsequently visualized by immunohistochemistry. Control incubation in the presence of serotonin (10(-6) M) resulted in a rich plexus of 5-HT-IR fibers in both posterior and intermediate lobes. Fluoxetine and citalopram (10(-6) M and 10(-5) M), inhibitors of 5-HT transporter, did not affect 5-HT-IR in the IL fibers, unless they were used in concentrations high enough (10(-4) M and 10(-3) M) to block unspecifically a number of monoamine transporters. The same applied for desipramine (10(-5)-10(-7) M), an inhibitor of the noradrenaline transporter. However, cocaine (10(-5)-10(-6) M) blocked serotonin uptake into these terminals, suggesting that serotonin uptake occurs through a dopamine transporter. Incubation of the IL in presence of L-tryptophan (10(-4) M) did not result in 5-HT-IR in the IL fibers showing colocalization of 5-HT-IR and tyrosine hydroxylase, which suggests that these fibers do not synthesize serotonin. The present results suggest that serotonin is taken up into the IL terminals by a dopamine transporter and is not synthesized in them, at least in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Benzodiazepine suppression of corticotropin-releasing factor (CRF)-induced beta-endorphin release from rat neurointermediate pituitary

Dopamine and gamma-aminobutyric acid (GABA) inhibit POMC peptide release from the pituitary intermediate lobe, via interaction with D2 or GABA-A/benzodiazepine receptors. Here, we examined the effects of an antianxiety triazolobenzodiazepine, adinazolam, on corticotropin-releasing factor (CRF)-stimulated POMC peptide secretion from the rat neurointermediate pituitary. Neurointermediate lobes (NILS) were incubated with CRF (10(-7) M), then adinazolam (10(-8) or (10(-9) M) was added, with CRF remaining in the medium. Aliquots were removed at 15-min intervals and frozen for radioimmunoassay of beta-endorphin. Adinazolam alone did not significantly affect secretion as compared to controls or CRF alone. Adinazolam incubated with CRF led to significant inhibition of beta-endorphin secretion, as compared to CRF alone. In addition, adinazolam was as effective as dopamine or the CRF antagonist, alpha-helical CRF, in preventing CRF-induced beta-endorphin release. Adinazolam appears to act directly on the pituitary to suppress hormone release induced by a stress-related hypothalamic peptide.