Ionic requirements for melanophore-stimulating hormone (MSH) release

Nature and control of peptide release from the pars intermedia

Pro-opiocortin, the precursor of ACTH, LPH and gamma-MSH, is biosynthesized in both the cells of the pars intermedia and the corticotrophs of the pars distalis. In the pars distalis its processing does not vary significantly from species to species whereas in the pars intermedia large differences occur. The release of ACTH, beta-LPH and pro-gamma-MSH from the corticotrophs is under common positive control by hypothalamic corticotropin-releasing factor (CRF) and the nature of the peptides remains unchanged when they are secreted. The release of all five pars intermedia peptides that we have measured in vitro appears to be under tonic dopaminergic inhibition. The secreted peptides have also been identified chromatographically. The lack of unequivocal physiological function in the periphery, the diversity of the pars intermedia peptides and this common control mechanism tend to preclude a simple endocrine role for the pars intermedia. The neural effects of MSH and endorphin are well documented and specific neuronal uptake therefore cannot by dismissed. The absence of pars intermedia in the adult human pituitary suggests that such a site of synthesis of these peptides plays a minor role in learning and behaviour in a species (such as Homo sapiens) that has a highly evolved intelligence and may, instead, need to synthesize the peptides only in the brain.

Functions of alpha-melanotropin and other opiomelanocortin peptides in labour, intrauterine growth and brain development

In a number of animals and in humans, factors from the fetal hypothalamus function in intrauterine growth, in labour and in brain development. Peptides of the opiomelanocortin family are produced by the pituitary, brain and placenta and are probably involved in these developmental processes. In the rat, alpha-MSH stimulates fetal growth, protein synthesis, wound healing and liver regeneration and it reduces periosteal bone resorption. In chick embryos, alpha-MSH restores the corticosteroid-induced growth retardation. Thus alpha-MSH seems to possess general trophic properties. The fetal brain in humans is involved in timing the moment of birth. This process is probably mediated by peptides of the opiomelanocortin family as suggested from observations in anencephaly and other congenital brain anomalies and from the influence of corticosteroids or ACTH on labour. The high percentage of premature deliveries in heroin addicts is worth examining endocrinologically, in this respect. The exact nature of the peptides and mechanisms involved in labour is not yet known. Some peptides of the opiomelanocortin family induce an acceleration of brain development. Neonatal treatment of rats with alpha-MSH alters their later behaviour while ACTH fragments accelerate the onset of eye-opening. Opiates and methadone inhibit brain development, and neonatal administration of beta-endorphin or naloxone causes permanent insensibility to temperature stimuli. The interrelated nature of the fetal pituitary, brain and placenta does not, at present, allow us to pin down which of these structures is primarily involved in the regulation of intrauterine growth, labour and brain development.

Characterization of functional GABAergic synapses formed between rat hypothalamic neurons and pituitary intermediate lobe cells in coculture: Ca2+ dependence of spontaneous IPSCs

Rat hypothalamic neurons and endocrine cells from the intermediate lobe of the pituitary were grown in dissociated coculture. Neurons positively stained with an antibody against glutamate decarboxylase established apparent contacts with the alpha-melanocyte-stimulating hormone-positive endocrine cells. These sites of contact were intensely labeled with an antibody against the synaptic protein synapsin I and displayed ultrastructural features characteristic of synapses. Using patch-clamp recordings, we have demonstrated that these contacts correspond to functional GABAergic synapses. The synaptic currents were blocked reversibly by bicuculline (5 microM) and SR95531 (5 microM), two competitive antagonists of the GABAA receptor. At a holding potential of -60 mV, spontaneously occurring IPSCs (s-IPSCs) had small amplitudes (10-100 pA), whereas electrically evoked IPSCs (ee-IPSCs) had amplitudes up to 1 nA. The rise times of both types of IPSCs were fast ( < or = 1 msec), and their decaying phases were fitted in most cases with a single exponential function (time constant 50 msec). The amplitude distribution of s-IPSCs did not reveal clear, equally spaced peaks and was little affected by tetrodotoxin, suggesting that most s-IPSCs were miniature IPSCs. Reduction of extracellular calcium concentration to 0.3 mM induced a marked decrease in s-IPSC frequency and revealed a single amplitude peak at 10 pA, suggesting that a single quantum of GABA activates 8-10 GABAA channels. Thus, our preparation might be an interesting model to study different aspects of synapse formation between a central neuron and its target as well as the fundamental mechanisms of synaptic transmission at central synapses.

The secretion of alpha-MSH from xenopus melanotropes involves calcium influx through omega-conotoxin-sensitive voltage-operated calcium channels

The secretory activity of endocrine cells largely depends on the concentration of free cytosolic calcium. We have studied the mechanisms that are involved in supplying the calcium necessary for the secretion of alpha-melanophore-stimulating hormone (alpha-MSH) from melanotrope cells in the pituitary intermediate lobe of the amphibian Xenopus laevis. Using whole-cell voltage clamp, high-voltage activated calcium currents were observed, with a peak current between 0 and +20 mV. Two types of Ca(2+)-currents appeared, depending on the experimental setup. An inactivating current, which was observed after a 10 msec depolarizing prepulse, resembled currents through N-type channels as it was clearly inhibited by 1 microM omega-conotoxin. The second type was a non-inactivating current, which was blocked up to 50% by 1 microM nifedipine, indicating its L-type nature. Only a small component of this inactivating current could be blocked by omega-conotoxin. No evidence was found for the presence of transient, low-voltage activated currents. The spontaneous secretion of alpha-MSH from superfused neurointermediate lobes was dependent on extracellular calcium, as low calcium conditions (10(-4)-10(-8) M) rapidly inhibited this process. Under these conditions, secretion was not affected by depolarizing concentrations of potassium chloride. The calcium ionophore A23187 increased secretion under low calcium conditions, but had no effect on spontaneous alpha-MSH release. These results suggest that spontaneous alpha-MSH release depends on influx of calcium through voltage-operated calcium channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Secretory morphology of the intermediate lobe of the rat pituitary incubated in vitro

The ultrastructure of the incubated intermediate lobe of the rat pituitary and the morphological effect of isoproterenol stimulation on its cells were studied under in vitro conditions. The general structure of isolated neurointermediate lobes maintained for 2-3 h in vitro was well preserved, and the presence of intact nerve terminals establishing synaptic contacts with the glandular cells of the intermediate lobe was confirmed. Removal of the intermediate lobe from central inhibition leads to increased hormonal secretion, which was reflected by large Golgi areas and the appearance of secretory images. However, no obvious degranulation or peripheral migration of the secretory granules after 2-3 h in vitro was seen. The secretory granules varied in electron density; totally electron-lucent granules were regularly observed and exocytotic phenomena were shown. In addition, more extensive invaginations suggesting secretion by compound exocytosis were seen. A three-fold increase in the beta-endorphin secretion during a 4-min stimulation with 10(-6) M isoproterenol did not induce any morphometrically detectable changes in the incubated cells. This indicates that only a minor fraction of the total granule content is mobilized during an acute increase in secretory activity.

The effects of nitrous oxide on the secretory activity of pro-opiomelanocortin peptides from basal hypothalamic cells attached to cytodex beads in a superfusion in vitro system

Dispersed cells from adult rat basal hypothalami, attached to Cytodex-3 microcarrier beads, were placed in a column and superfused with aerated high glucose media or media enriched with variable concentrations of nitrous oxide with oxygen. beta-Endorphin and alpha-MSH content was measured in the effluent collected during superfusion and demonstrated a near constant baseline release. Nitrous oxide, 60% (P less than 0.025) and 80% (P less than 0.02), caused significant increases in release of beta-endorphin. Potassium chloride (50 mM) caused a significant increase in release (P less than 0.007) of beta-endorphin whereas saline and 30% nitrous oxide did not. Neither nitrous oxide-enriched media nor potassium chloride had any statistically significant effect on alpha-MSH release. The increase in beta-endorphin secretory activity during exposure to nitrous oxide demonstrates that nitrous oxide may have a stimulatory effect on central pro-opiomelanocortin neurons.

Stimulation of calcium-dependent release of labelled protein from pulse-labelled mouse pituitary intermediate lobe tissue

1. The effect of K depolarization on the release of labelled protein from pulse-labelled intermediate lobe tissue of mouse pituitaries has been studied.2. Depolarization briefly stimulated Ca-dependent release of labelled protein. Co (2.5 mM) and verapamil (0.08 mM) reversibly blocked stimulation. Efflux of (45)Ca was also briefly stimulated by depolarization, but not in the presence of Co or verapamil, nor in the absence of Ca.3. The response to maintained depolarization quickly inactivated. Inactivation probably resulted from Ca-channel inactivation rather than exhaustion of available labelled protein, since responses different in magnitude but of constant time course could be obtained by depolarization in different concentrations of Ca. In addition, much more labelled protein could be released by exposure to Ba.4. Depolarization did not cause a response if the Ca concentration was 0.4 mM or less; neither did inactivation occur in these conditions, and a response occurred as soon as the Ca concentration was raised. Two separate responses could be generated by a stepwise increase in the Ca concentration during maintained depolarization. The magnitude of the two responses together was similar to the magnitude of one response at the higher Ca concentration.5. Recovery from inactivation was complete after about 20 min in normal K. However, recovery also occurred during maintained depolarization provided the Ca concentration was reduced sufficiently. Recovery was complete at 0.4 mM-Ca or less, and at higher concentrations the extent of recovery depended on the Ca concentration selected.6. It is concluded that depolarization opened potential-dependent Ca channels permitting Ca entry and causing the release of labelled products. The response was brief probably because the Ca channels were inactivated. Since the channels were not inactivated by depolarization in low Ca, inactivation may result from Ca entry.

Calcium component to action potentials in rat pars intermedia cells

1. The ionic dependence of the action potential of rat pars intermedia cells was investigated by using intracellular recording techniques. 2. In the presence of tetrodotoxin (TTX, 5 x 10(-6)M), the action potentials evoked by passing depolarizing current through the recording electrode were abolished, confirming that they are mainly dependent on Na; however, when tetraethylammonium (TEA, 10 mM) was added to the TTX-containing solution the imposed depolarizations triggered all-or-none regenerative potentials indicative of involvement of another ion. 3. There TTX-insensitive regenerative potentials persisted when the cells were perifused with Na-free solution but were severely reduced or abolished by Ca-free solution. This suggests that the ion producing these potentials is Ca. 4. These Ca action potentials were suppressed by Ni, Co and Mn in concentrations that did not suppress the "Na spikes' recorded in the absence of TTX and TEA. 5. Sr and Ba could substitute for Ca in maintaining the action potentials recorded in the presence of TTX. These ions also prolonged the duration of these action potentials. 6. The demonstration of a Ca component to the predominantly Na-dependent action potentials of pars intermedia cells heightens the possibility that these action potentials participate in the regulation of secretion.

Release of immunoreactive alpha-MSH by synaptosome-enriched fractions of homogenates of hypothalami

Immunoreactive alpha-melanocyte-stimulating hormone (alpha-MSH) was found to be concentrated in a synaptosome-enriched fraction prepared by differential centrifugation of rat hypothalamic homogenates. The release of the hormone from this preparation was investigated. After incubation, the synaptosomes were isolated by ultrafiltration and alpha-MSH in the ultrafiltrate was determined by radioimmunoassay. Particle-bound alpha-MSH, recovered by extraction with acid ethanol, and alpha-MSH released from the synaptosome preparation, were immunologically similar to synthetic alpha-MSH and had an accompanying melanotropic activity. Less than 10% of the particle-bound alpha-MSH was released during incubation in 0.32 M sucrose. However, in the presence of 2 mM Ca2+, alpha-MSH release increased with increasing concentrations (30-150 mM) of K+. The stimulatory effect of 60 mM K+ was complete within 2 min and was potentiated by increasing Ca2+ concentrations over the range of 0 to 2 mM. K+-induced release of alpha-MSH was independent of temperature from 1 to 30 degrees C, and neither glucose (10 mM) nor dopamine (10(-10)-10(-2) M) had any effect on the release of the peptide. It is concluded that a synaptosome-enriched fraction from the hypothalamus contains a releasable pool of immunoreactive alpha-MSH that is mobilized by depolarizing concentrations of K+ in a Ca2+-dependent manner.

Action potentials in gland cells of rat pituitary pars intermedia: inhibition by dopamine, an inhibitor of MSH secretion

1. Cells were dissociated from rat pituitary pars intermedia, maintained in culture, and subjected to electrophysiological study.2. Recorded membrane potentials varied widely (range - 18 to about - 80 mV). They were relatively insensitive to changes in external Na but were rapidly and reversibly lowered by excess K.3. Action potentials were elicited by passing current through intracellular recording electrodes. They were reversibly blocked by tetrodotoxin (TTX, 2 x 10(-6)M) or by removal of Na from the recording solution and thus appeared to be Na spikes. Cells yielding action potentials in response to depolarization had relatively high membrane potentials (about - 65 mV) which may be representative of the true resting membrane potential of pars intermedia cells.4. Spontaneous action potentials were recorded extracellularly from nearly all the many isolated pars intermedia cells studied with a microsuction electrode. Their amplitude was reduced by TTX (0.1-2 x 10(-6)M). Electron microscopic examination of cells producing action potentials showed them to be hormone-containing parenchymal (gland) cells.5. Dopamine (10(-6)M), a presumed physiological inhibitor of secretion of melanocyte stimulating hormone (MSH) from pars intermedia cells, decreased the frequency of spontaneous action potentials but not their amplitude. Similar effects were seen with noradrenaline (10(-6)M), another inhibitor of MSH secretion, whereas isoprenaline and 5-hydroxytryptamine (5-HT), which do not inhibit MSH secretion, had no effect.6. Action potentials may be involved in stimulus-secretion coupling in pars intermedia cells.

Pituitary-testicular axis in patients on lithium therapy

Plasma follicle-stimulating hormone (FSH), luteinizing hormone (LH), and testosterone levels were studied in 10 patients undergoing lithium therapy. Testosterone levels were below the normal range in 7 of the 10 patients. FSH levels were high in two patients and LH levels were high in one. The effects of lithium on the pituitary-testicular axis are discussed.

Biogenic amines and control of melanophore stimulating hormone release

Release of melanophore stimulating hormone (MSH) from the vertebrate pars intermedia is under inhibitory control by the hypothalamus. Removal of the rat pituitary or the neurointermediate lobe of the frog (Rana pipiens) to in vitro incubation medium results in rapid uninhibited release of MSH. This secretion is inhibited by norepinephrine, epinephrine, phenylephrine, and dopamine, and the inhibition is antagonized by alpha-adrenergic receptor blocking agents. Isoproterenol stimulation of MSH secretion from isolated glands is blocked by pro-pranolol, a beta-adrenergic receptor antagonist. These results implicate dopaminergic or classical alpha-adrenergic receptors (or both) in inhibition of MSH release by catecholamines, and implicate beta-adrenergic receptors in stimulation of MSH release by the bioamines.

Melanophore stimulating hormone: release inhibition by ring structures of neurohypophysial hormones

Tocinamide and tocinoic acid, ring structures of oxytocin, are potent inhibitors of the release of melanophore stimulating hormone from the rat and hamster pituitary in vitro. Tocinamide is effective at concentrations as low as 10-(14)M on the mammalian pituitary. These peptides do not affect release of the hormone on the frog (Rana pipiens) pars intermedia, but they do inhibit release in the bullfrog (Rana catesbeiana) and the toad (Bufo marinus). The specificity of the peptides on inhibition of the hormone is demonstrated by the fact that oxytocin, lysine vasopressin, and pressinoic acid and pressinamide (ring structures of the vasopressins) do not show such inhibitory activity. Hypothalamic extracts of either the frog (Rana pipiens) or the rat inhibit release of the hormone from pituitaries of either species. The inhibitory effects of tocinamide and tocinoic acid, like that of hypothalamic extracts, are reversible.