Actions of endogenous vasopressin and oxytocin on anterior pituitary hormone secretion

Neuroimmune regulation in immunocompetence, acute illness, and healing

Adaptive immunocompetence is maintained by growth hormone (GH), prolactin (PRL), and vasopressin (VP). Innate or natural immunocompetence depends on cytokines, hormones (especially of the hypothalamus-pituitary-adrenal axis), and catecholamines. The acute phase response (APR, or acute febrile illness) is an emergency defense reaction whereby the adaptive, T cell-dependent, immune reactions are suppressed and the innate immune function is dramatically amplified. Infection and various forms of injury induce APR. Cytokines [interleukin (IL)-1beta, tumor necrosis factor-alpha, and IL-6] stimulate corticotropin-releasing hormone (CRH) and VP secretion and cause a "sympathetic outflow." Colony-stimulating factors activate leukocytes. CRH is a powerful activator of the pituitary adrenocortical axis and elevates glucocorticoid (GC) levels. Cytokines, GCs, and catecholamines play fundamental roles in the amplification of natural immune defense mechanisms. VP supports the APR at this stage. However, VP remains active and is elevated for a longer period than is CRH. VP, but not CRH, is elevated during chronic inflammatory diseases. VP controls adaptive immune function and stimulates adrenocorticotropic hormone (ACTH) and PRL secretion. PRL maintains the function of the thymus and of the T cell-dependent adaptive immune system. The ACTH-adrenal axis stimulates natural immunity and of suppressor/regulatory T cells, which suppress the adaptive immune system. VP also has a direct effect on lymphoid cells, the significance of which remains to be elucidated. It is suggested that VP regulates the process of recovery from acute illness.

Atrial natriuretic peptide mediates oxytocin secretion induced by osmotic stimulus

Atrial natriuretic peptide (ANP), first discovered in the heart, has been also detected in various brain regions involved in the control of cardiovascular function and water and sodium balance. The anteroventral region of the third ventricle (AV3V) and the subfornical organ (SFO) have ANP-immunoreactive projections towards the paraventricular (PVN) and supraoptic (SON) nuclei of the hypothalamus. Extracellular fluid (ECF) hyperosmolality stimulates the secretion of oxytocin (OT) which induces ANP release by the atrium. On the other hand, passive immunoneutralization of ANP reduces OT secretion in response to ECF hypertonicity. Previous studies have shown the co-localization of ANP and OT in PVN and SON neurons and in the periventricular region, as well as the presence of ANPergic and oxytocinergic neurons in the median eminence. The aim of the present study was to investigate the OT and ANP content in the SON and PVN of the hypothalamus and in the posterior pituitary (PP) after an osmotic stimulus that induces OT secretion. The results showed that intracerebroventricular microinjection of normal rabbit serum (NRS) or of ANP antiserum followed or not by an intraperitoneal injection of isotonic saline did not alter OT secretion or OT content in the PVN, SON, and PP; passive ANP immunoneutralization reduced the basal content of ANP in the PVN, SON, and PP of animals in a situation of isotonicity; the ANP antiserum inhibited the increase of OT secretion and content of OT and ANP in the PVN, SON and PP induced by the osmotic stimulus. Thus, the increase in plasma OT and oxytocinergic neurons of the hypothalamus-posterior pituitary system in response to hypertonicity depends on the action of endogenous ANP, i.e., ECF hypertonicity must activate ANPergic neurons which directly or indirectly stimulate OT release.

Oxytocin secretion induced by osmotic stimulation in rats during the estrous cycle and after ovariectomy and hormone replacement therapy

Hyperosmolality is a potent stimulus for the secretion of oxytocin. Oxytocinergic neurons are modulated by estrogen and oxytocin secretion in rats varies according to the phase of the estrous cycle, with higher activity during proestrus. We investigated the oxytocin secretion induced by an osmotic stimulus (0.5 M NaCl) in female rats. Plasma oxytocin and the oxytocin contents in the neurohypophysis and the paraventricular and supraoptic nuclei were determined during the morning (8-9 h) and afternoon (17-18 h) of the estrous cycle and after ovariectomy followed or not by hormone replacement. Plasma oxytocin peaked in control animals during proestrus. Oxytocin content decreased in the paraventricular and supraoptic nuclei during proestrus and estrus compared to diestrus and increased in the neurohypophysis during proestrus morning. No significant difference was observed in the oxytocin content of the neurohypophysis, nuclei or plasma between ovariectomized animals and ovariectomized animals treated with estrogen or estrogen plus progesterone. Therefore, any ovarian factor other than estrogen or progesterone seems to play a direct or indirect role in the increase in oxytocin secretion. The osmotic stimulus caused an increase in plasma oxytocin throughout the estrous cycle. A reduction in oxytocin content during diestrus and an increase during proestrus were observed in the paraventricular nuclei. In ovariectomized animals, the treatment with estrogen potentiated the response of oxytocin to the osmotic stimulus, with the response being even stronger in the case of estrogen plus progesterone. In conclusion, the ovarian steroids estrogen plus progesterone could modulate the osmoreceptor mechanisms related to oxytocin secretion.

Central actions of the nonpeptide growth hormone secretagogue GHS-25

Growth hormone secretagogues (GHSs) increase the activity of hypothalamic arcuate nucleus neurons thought to be involved in controlling the release of growth hormone (GH). The GHS receptor is also found in hypothalamic regions not associated with the release of GH, suggesting that GHSs may influence other hypothalamic systems. This study utilized double-labeling immunocytochemical techniques to examine the hypothalamic actions of a novel nonpeptide GHS, GHS-25. In common with other GHSs, GHS-25 induced significant amounts of Fos immunoreactivity in the arcuate nucleus of conscious male rats. However, unlike other GHSs, GHS-25 also induced Fos immunoreactivity in the supraoptic nucleus. Double labeling revealed that approx 66% of supraoptic nucleus cells that were Fos positive after the administration of GHS-25 were also immunoreactive for oxytocin. Thus, in addition to its actions on the GH axis, GHS-25 may influence the release of neurohypophyseal hormone.

Prolactin and corticosterone secretion in response to acute stress after paraventricular nucleus lesion by ibotenic acid

The cellular organization of the paraventricular nucleus (PVN) is complex and eight distinct regions have been identified by Nissl staining. Three consist of magnocellular neurons and five of parvocellular neurons. Ibotenic acid, a glutamate analogue, is a toxin with neuroexcitatory properties which acts on N-methyl-D-aspartate and metabotropic receptors. Depending on the dose used, ibotenic acid causes extensive damage of parvocellular neurons of the paraventricular nucleus but preserves magnocellular neurons and passage fibers, in contrast to electrolytic lesions, which causes diffuse and nonspecific destruction. We studied the prolactin (PRL) and corticosterone secretion in response to acute stress induced by exposure to the ether, 3 weeks after selective neurotoxic lesion of parvocellular neurons of the paraventricular nucleus by microinjection of ibotenic acid. There was no significant difference in the basal levels of PRL and corticosterone between control and lesioned animals. The plasma PRL increased in the sham and lesioned groups after stress of similar manner. However, the increase in plasma corticosterone in response to stress was significantly higher in lesioned animals. In conclusion, the selective lesion of parvocellular neurons of the PVN did not change basal or stress induced PRL secretion but it caused hypersensitivity of the hypothalamus-pituitary-adrenal axis 3 weeks later, probably by corticotropin releasing hormone (CRH) from hypothalamic areas others than parvocellular neurons of the PVN; hypersensitivity of corticotropes to the secretagogues others than CRH; or hyperresponsiveness of AVP receptors in the adenohypophysis. Furthermore, we cannot rule out a putative inhibitory factor of the hypothalamus-pituitary axis produced by parvocellular neurons of the PVN. This factor modulator of corticotropin secretion could be absent after recuperation of the response of the hypothalamus-pituitary axis to the stress.

Catecholaminergic neurotransmission: flagship of all neurobiology

The paper describes, with focus on the first half of the century, the roles played by study of the sympathoadrenal system for developing modern neurobiology. Adrenaline isolated from extracts of adrenal medulla was the first intercellular messenger to be chemically identified and synthesized. Similarities between effects of adrenaline and sympathetic nerve stimulation led to the first concrete proposal of chemical neurotransmission. That effluent from a sympathetically or parasympathetically stimulated frog heart induced acceleration or slowing of an unstimulated recipient heart was the first conclusive proof of chemical neurotransmission. Acetylcholine (in parasympathetic or somatomotor) and noradrenaline (in sympathetic nerves) are the first identified mammalian neurotransmitters. The existence of a'receptive substance for adrenaline' represents the first proposal that target cells recognize and react to the released transmitter. Deviations for the '-ergic concept, in which sympathetic and parasympathetic nerves are termed 'adrenergic' and 'cholinergic', led to discovery of 'non-adrenergic, non-cholinergic' nerves and a range of other transmitters. That some effects of e.g. sympathetic nerve stimulation are not blocked by any noradrenaline antagonist led to the recognition that some nerves utilize more than one transmitter. Noradrenaline in sympathetic nerves was the first neurotransmitter to be visualized in the light microscope. catecholamines in adrenal medulla and sympathetic nerves were the first messengers to be shown to be stored in vesicles.

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.

Hypothalamic oxytocin in the developing ovine fetus: interaction with pituitary-adrenocortical function

Oxytocin (OT) stimulates corticotroph function in adult sheep, however, there is little information on OT synthesis and its potential involvement in hypothalamo-pituitary-adrenal (HPA) function in the fetus. The objectives of this study were to examine developmental changes in hypothalamic OT synthesis and to investigate the actions of OT on fetal corticotroph function. Hypothalami were removed at various stages of pre- and post-natal development. OT mRNA levels were measured using in situ hybridization. For in vitro studies, fetal pituitaries were removed on days 129 and 138 of gestation. Anterior pituitary cells were dispersed and cells were treated with different concentrations and combinations of OT, corticotrophin-releasing hormone (CRH), vasopressin (AVP) and cortisol. OT mRNA was present in the paraventricular nucleus (PVN) and supraoptic nucleus (SON) by day 60 of gestation, and levels significantly increased at term. OT mRNA was present in parvocellular and magnocellular fields of the PVN. In vitro, OT stimulated adrenocorticotropin (ACTH) output in a dose-dependent fashion, but had no effect on cellular pro-opiomelanocortin (POMC) mRNA levels. There was no significant difference in corticotroph responsiveness to secretagogues between cells harvested at gestation day 129 or gestation day 138. Simultaneous exposure to CRH and OT stimulated increases in ACTH output that were significantly greater than for OT or CRH alone. However, no similar synergistic interaction existed between OT and AVP. Cortisol attenuated OT-stimulated ACTH output. In conclusion, hypothalamic OT mRNA increases at term and OT can stimulate ACTH output from fetal corticotrophs. Together, these data indicate that OT may be involved in the regulation of ACTH secretion in fetal sheep in late gestation.

Changes in oxytocin content in rat brain during morphine withdrawal

In this study the modification in the oxytocin content in different hypothalamic nuclei during morphine withdrawal was analysed. Male rats were implanted with placebo (naïve) or morphine (tolerant/dependent) pellets for 7 days. On day 7, groups of rats received an acute injection of saline s.c. (control) or naloxone (1 mg/kg s.c.) and were decapitated 30 min later. After administration of naloxone to tolerant rats (withdrawal) an increase in the oxytocin content in the paraventricular nucleus (PVN) and median eminence (ME) was found. No changes were found in the arcuate nucleus (AN) and supraoptic nucleus (SON). Present data demonstrate that administration of naloxone to tolerant rats alters the brain oxytocin system, which suggests that this peptide might contribute to the behavioural, emotional and neuroendocrine response to opioid.

Arginine vasopressin (AVP) depletion in neurons of the suprachiasmatic nuclei affects the AVP content of the paraventricular neurons and stimulates adrenocorticotrophic hormone release

Arginine vasopressin (AVP) produced in the hypothalamic suprachiasmatic nuclei (SCN) plays a role in establishing neuroendocrine rhythms and, in particular, in regulating the corticotrope axis rhythm. It has recently been shown that AVP from SCN inhibits corticosteroid release. In order to investigate the influence of suprachiasmatic AVP on the different peptidergic systems through the hypothalamus, SCN neurons containing AVP were functionally lesioned by using toxins associated with a cytotoxic monoclonal antibody (MAb) raised against AVP. Six days later, the AVP contents and AVP mRNA were measured in different hypothalamic and extrahypothalamic sites. Adrenocorticotrophic hormone (ACTH) concentration was also measured in plasma. Microinjection of the AVP-MAb/toxin mixture into SCN brought about a significant decrease in the AVP expression in SCN. This is demonstrated by the decrease in the AVP immunoreactive content (24%, P < 0.01) and the decrease of AVP hybridized mRNA (33%, P < 0.01). This points to the efficiency of the microinjection in decreasing the production of AVP in the injection area. Modifications of the AVP contents in the two subdivisions of the hypothalamic paraventricular nucleus (PVN) were also observed. AVP contents decreased in the parvocellular subdivision (pPVN); this is coherent with the AVP depletion in SCN since pPVN is the major site of the SCN hypothalamic efferences. AVP content and AVP mRNA increased in the magnocellular subdivision (mPVN); this also confirms the difference in AVP synthesis regulation according to the PVN subdivisions. The microinjection did not modify AVP expression in supraoptic nuclei or oxytocin (OT) immunoreactive content in the main hypothalamic OT containing sites. Plasma ACTH values were double (P < 0.02) the values measured under non-specific IgG treatment 10 hr after lights on. This probably resulted from the stimulation of the hypothalamo-pituitary-adrenal system since corticotrophin-releasing hormone (CRH) mRNA increased simultaneously by 24% (P < 0.05) in the PVN and the immunoreactive CRH content of the median eminence significantly decreased (26%, P < 0.05). Overall, our data confirm that AVP produced in the SCN inhibits the CRH-adrenocorticotrope axis in normal conditions, probably because of SCN projections of AVP neurons on the PVN.

Changes in hypothalamic oxytocin levels during morphine tolerance

The role of hypothalamic oxytocin neurons in the hypothalamus-pituitary-adrenal (HPA) axis adaptation during opioid tolerance has not been explored. In this study the modification of oxytocin levels in different hypothalamic nuclei was determined after acute or chronic morphine exposure. Male rats were implanted with placebo (naïve) or morphine (tolerant) pellets for 7 days. On day 8, groups of rats received an acute injection of either saline i.p. or morphine (30 mg/kg i.p.) and were sacrificed 30 min later. In morphine-tolerant rats, there was a decrease in the oxytocin content in the median eminence (ME) and in the supraoptic nucleus (SO) after acute injection of saline or morphine. No modifications were seen in the paraventricular nucleus (PVN). The present study demonstrates that chronic morphine administration alters the brain oxytocin system, which suggests that this peptide might contribute to the behavioural, emotional and neuroendocrine responses to opioids.

Evidence for a dual function of oxytocin in the control of growth hormone secretion in rats

The aim of the present study was to investigate the role of oxytocin (Oxy) in the control of growth hormone (GH) release. Oxy was administered subcutaneously (s.c.) or intracerebroventricularly (i.c.v.) to male rats. The animals were decapitated and trunk blood was collected at 30 and 120 min after Oxy administration. GH levels were analyzed by radioimmunoassay. Oxy (100 microg, s.c) increased plasma levels of GH significantly 30 min after administration. Oxy (2 ng, i.c.v.) caused a significant rise of GH after 120 min. This effect was completely abolished by previous administration of the Oxy antagonist 1-deamino-2-D-Tyr-(OEt)-4-Thr-8-Orn-oxytocin. When 5 microg of Oxy were given i.c.v. or 1 mg s.c., an inhibition of GH secretion was seen after 120 min. This effect was also abolished by the Oxy antagonist. Thus Oxy may influence GH in opposite directions depending on the doses given.

Oxytocin and vasopressin mRNA expression in rat hypothalamus following kainic acid-induced seizures

In this study, the regulation of hypothalamic oxytocin and vasopressin messenger RNA expression following the induction of seizures was investigated by in situ hybridization. Following kainic acid-induced seizures, a significant increase in oxytocin messenger RNA in the paraventricular nucleus was demonstrated at 1.5 h, one and two weeks; its level decreased at three weeks and was significantly increased again at four weeks; at eight weeks the messenger RNA level still remained higher than that of controls. Vasopressin messenger RNA in the paraventricular nucleus was increased significantly only at 1.5 h following induction of seizures. The oxytocin messenger RNA level in the supraoptic nucleus was also increased early at 1.5 h and later at four weeks following seizures; however, these increases did not last as long as those in the paraventricular nucleus. Vasopressin messenger RNA in the supraoptic nucleus was also increased after the initial seizures; however, its messenger RNA level vacillated up and down throughout the post-seizure times studied. The earliest significant increase of vasopressin messenger RNA was at one week after seizures, and there was a late significant increase of vasopressin messenger RNA at three weeks after seizures. The present study demonstrates that following kainic acid-induced seizures both, the oxytocin and vasopressin messenger RNA expressions, were up-regulated and these up-regulations were long-term events. The increase of oxytocin messenger RNA in the paraventricular nucleus was more persistent than the others. The pattern of messenger RNA up-regulation was different for oxytocin and vasopressin, and different in the paraventricular nucleus and supraoptic nucleus. These different patterns of messenger RNA elevations suggest that the different components of the rat hypothalamus were regulated differentially by kainic acid-induced seizures.

Effects of oxytocin on the function and morphology of the rat adrenal cortex: in vitro and in vivo investigations

The effects of oxytocin (OX) on the function and morphology of the rat adrenal cortex were studied in vivo and in vitro. OX exerted a potent stimulatory action on basal, but not 10(-8) M ACTH-stimulated corticosterone (B) secretion of dispersed rat inner (zona fasciculata and zona reticularis) adrenocortical cells (maximal effective concentration: 10(-9) M); in contrast, at higher concentrations (10(-7)/10(-6) M) OX inhibited maximally ACTH-stimulated B output. A single subcutaneous (s.c.) injection of 1.2 nmol/100 g body weight OX resulted in a long-lasting (up to 12 h) rise in plasma B concentration (PBC). The prolonged administration of OX (daily s.c. injections of 0.6 or 1.2 nmol/100 g for 10 days) caused a marked lowering in the adrenal weight and volume of all adrenocortical zones, that in turn was due to a decrease in the number of their parenchymal cells; however, the average volume of inner adrenocortical cells was significantly increased. Basal PBC was lowered, but its response to ether stress was unchanged in comparison with control rats. Prolonged OX treatment did not change B secretion by adrenal slices, but it markedly raised that of dispersed inner adrenocortical cells. Our present findings clearly show that the effects of OX on the adrenal cortex depend on the experimental model employed (in vitro versus in vivo) and the duration of treatment (acute versus chronic). Taken together they allow us to conclude that OX exerts an acute direct stimulatory effect on the rat adrenal cortex, and a chronic inhibitory one, that at least in part could be due to the interference of OX with the mechanism(s) of intracellular transduction of the ACTH secretagogue signal.

Altered ACTH and corticosterone responses to interleukin-1 beta in male rats exposed to an alcohol diet: possible role of vasopressin and testosterone

We have previously shown that female rats exposed to an alcohol (ethanol, E) diet exhibited a blunted ACTH response to systemically administered interleukin-1 beta (IL-1 beta). Because of the presence of gender differences in the activity of the hypothalamic-pituitary-adrenal (HPA) axis, and of the possible role played by sex steroids in modulating the inhibitory influence of E in females, we studied the ability of a 10-day E diet to alter ACTH and corticosterone secretion of intact or castrated male rats injected with IL-1 beta or endotoxin, a releaser of endogenous cytokines. Pituitary responsiveness to secretagogues that mediate the endocrine effects of IL-1 beta, namely corticotropin-releasing factor (CRF) and vasopressin (VP), was also investigated. The ACTH responses of animals fed ad libitum (C group) or pair-fed (PF group) to the intravenous administration of IL-1 beta or endotoxin were not statistically different (p > 0.05); therefore, results from these two groups were combined in the initial experiments. Subsequent experiments only used E and C animals. When compared with this latter group, intact E males showed a significant (p < 0.01) decrease in ACTH levels measured 30 and 60 min after the intravenous injection of IL-1 beta or endotoxin. In contrast, E rats released as much corticosterone as C rats in response to IL-1 beta, but significantly (p < 0.05) more following administration of endotoxin (lipopolysaccharide). The stimulatory effect of VP on ACTH release was also measurably blunted by alcohol, whereas that of CRF was not. In none of these experiments were any significant differences observed between C and PF rats.(ABSTRACT TRUNCATED AT 250 WORDS)