Intrinsic vasoactive intestinal polypeptide (VIP)-containing neurons in the baroreceptor nucleus of the solitary tract in rat.

Vasoactive intestinal polypeptide (VIP)-immunoreactive cell bodies, dendrites and nerve terminals in the nucleus of the solitary tract have been identified with electron microscopic immunocytochemistry in rats. The relatively high concentration of VIP in this nucleus, which corresponds to the primary baroreceptor center, was depleted only insignificantly by uni- or bilateral transection of the solitary tract, which eliminates peripheral neuronal (via glossopharyngeal and vagal nerves) input to the nucleus. Both immunocytochemical and biochemical data suggest that the majority of VIP in the nucleus of the solitary tract is present in intrinsic neurons.

Release of vasoactive intestinal peptide from rat brain slices by various depolarizing agents.

The release of vasoactive intestinal peptide (VIP) from rat brain cortical and amygdala slices was studied by using various depolarizing agents such as potassium (K+), veratridine (VER) and batrachotoxin (BTX). The basal release of VIP observed is of the same order of magnitude for both structures and represents less than 0.1% of the tissue content per minute measured by a specific radioimmunoassay. Maximal stimulation obtained with 56 mMK+, 50 microM VER and 1 microM BTX corresponds to a mean 3-fold increase above the basal release of VIP in both cortex and amygdala. When the incubation medium did not contain any calcium, the action of potassium on the release of VIP was suppressed. When tetrodotoxin (1 microM) was added to the incubation medium, the veratridine- and batrachotoxin-induced release of VIP was inhibited whereas K+-induced release was unaffected. These results support the hypothesis that VIP can be a neurotransmitter in the central nervous system.

Rapid glucocorticoid inhibition of vasoactive intestinal peptide-induced cyclic AMP accumulation and prolactin release in rat pituitary cells in culture.

Vasoactive intestinal peptide (VIP) stimulates both adenosine 3',5'-cyclic monophosphate (cAMP) accumulation and prolactin release in normal rat pituitary cells in culture. cAMP accumulation is significant (P less than 0.01) at VIP concentrations as low as 1 nM and reaches a maximum with 0.1 microM. Addition of dexamethasone as early as 15 min before VIP inhibits VIP stimulation of both cAMP production and PRL secretion. The rapid inhibition is dose-dependent: it appears at doses as low as 0.01 pM and is complete at 1 pM dexamethasone. Increasing concentrations of dexamethasone induce a noncompetitive type of inhibition, as shown by the decrease in Vmax with no change in the apparent Km for VIP. Cycloheximide (1 mM) counteracts the inhibitory effect of dexamethasone on VIP-induced cAMP production, which suggests the involvement of a rapid protein synthesis mechanism. Ru-26988, a specific glucocorticoid devoid of any mineralocorticoid activity and which does not bind to intracellular transcortin-like component, also produces an inhibition of VIP-induced cAMP accumulation. Corticosterone also inhibits VIP-induced cAMP production but at concentrations higher than those of dexamethasone. In contrast, aldosterone, progesterone, estradiol, and testosterone have no effect. These results demonstrate that, in normal rat pituitary cells in culture, glucocorticoids at physiological concentrations rapidly inhibit the cAMP production and prolactin release induced by VIP by acting through specific glucocorticoid receptors.

Gonadotrophin release by gonadotrophs incubated with gonadotrophin-releasing hormone is independent of intracellular cAMP accumulation.

Rat pituitary cells were dispersed with trypsin and separated by sedimentation at unit gravity. The distributions of prolactin (Prl), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were determined, and two enriched cell populations (mammotrophs and gonadotrophs) were subsequently cultured. During a 4 h incubation, gonadotrophin-releasing hormone (GnRH) stimulated the release of LH and of FSH by both the unfractionated population and the enriched gonadotrophs; the magnitude of this stimulation increased with the length of the pre-culture periods, and the amount of LH released into the medium correlated strongly with the amount of FSH, whatever the length of the pre-culture period. The cellular cAMP content was also enhanced during the 4 h incubation, but no correlation was found between the hormone release and the cAMP accumulation. Furthermore, during the first 30 min of incubation with GnRH there was no increase of cellular cAMP, whatever cell population used. We conclude that the gonadotrophin release was independent of the cAMP accumulation observed in pituitary cells several hours after stimulation by GnRH; consequently, the late increase in the nucleotide is suggested to be a non-specific secondary process.

Effect of vasoactive intestinal peptide (VIP) on the release of adenohypophyseal hormones from purified cells obtained by unit gravity sedimentation. Inhibition by dexamethasone of vip-induced prolactin release.

The effect of vasoactive intestinal peptide (VIP) on the release of prolactin (PRL), gonadotropins (LH and FSH), growth hormone (GH) and corticotropin (ACTH) was studied using purified rat anterior pituitary cells obtained by means of velocity sedimentation at unit gravity. VIP, at concentrations ranging from 10(-10) to 10(-7) M, stimulated PRL secretion in a dose-dependent manner with an ED50 of 2 nM and a maximal response of 530% of control values. In contrast, similar concentrations of VIP did not affect the release of either LH, FSH, GH or ACTH from the corresponding enriched cell populations. Addition of dexamethasone (10(-9) M) to both preincubation and incubation medium of PRL cells completely inhibited VIP-induced PRL release. The present results further support the hypothesis that VIP is of physiological importance in the control of PRL secretion and demonstrate that corticosteroids can modify the responsiveness of PRL cells to VIP.

Effect of steroids on vasoactive intestinal peptide in discrete brain regions and peripheral tissues.

The effect of castration and adrenalectomy was studied on vasoactive intestinal peptide (VIP) concentrations in various brain and peripheral structures of male rats. Castration has no effect on any of the structures studied, whereas 4 weeks adrenalectomy (ADX) decreases VIP concentrations in the hippocampus and increases them in the adenohypophysis, two structures known to contain large amounts of glucocorticoid receptors. Corticosterone administration restores VIP levels to control values only in the dorsal hippocampus. In contrast, dexamethasone counteracts both the decrease and the increase of VIP concentrations obtained after ADX in the hippocampus and in the adenohypophysis respectively. The present results suggest that corticosteroids can be involved in the regulation of VIP actions both in the brain and in the adenohypophysis.

Stimulation of in vitro prolactin release by vasoactive intestinal peptide.

VIP stimulated prolactin secretion from incubated rat hemipituitaries. Under the same conditions, the secretion of GH, LH, FSH was not affected. The stimulation of prolactin was dose-dependent, with an apparent affinity of VIP of 10.9 +/- 3.1 nM and a maximal stimulation of 57.7 +/- 4.2%. Secretin, a structurally related peptide, was also active at higher concentrations whereas another partial analogue, glucagon, was ineffective. The effect of VIP was not blocked by alpha-flupentixol, a potent dopaminergic antagonist, at concentrations which antagonized the dopamine inhibition of prolactin secretion. Stimulation by VIP and TRH was additive. Neither Met-enkephalin nor naloxone interfered with the response to VIP. It thus seems that specific VIP receptors are present on pituitary prolactin cells. VIP, present in the mediobasal hypothalamus and detected in the hypothalamo-hypophyseal portal blood therefore is a good candidate as a physiological PRF.

[Dexamethasone regulation of the prolactin release induced by vasoactive intestinal peptide (VIP)]. / Régulation par la dexaméthasone de la libération de prolactine induite par le peptide vasoactif intestinal (VIP).

The regulation of prolactin (PRL) secretion induced by the vasoactive intestinal peptide (VIP) has been studied on purified male Rat Pituitary PRL cells obtained by means of velocity sedimentation at unit gravity. Dexamethasone (10(-9) M) in the incubation medium inhibits the stimulatory effect of VIP (10(-7) M) on PRL release. This result suggests that glucocorticoids play an important role in the regulation of VIP effects at the effects at the pituitary level.

Interaction of the anti-estrogen CI-628 and estradiol on plasma LH and hypothalamic LH-RH in the female rat.

The effects of the anti-estrogen Ci-628 have been tested on both plasma LH and hypothalamic LH-RH content in ovariectomized or ovariectomized, estradiol (E2)-implanted rats, in order to correlate those parameters with the [3H]E2 retention in the hypothalamus and the pituitary. Incresing doses of CI-628 induced a dose-dependent inhibition of [3H]E2 retention in both cytosolic and nuclear fractions of the pituitary. In contrast, hypothalamic retention of [3H]E2 is only inhibited significantly with higher doses of CI-628 (2.4 and 24 mg/kg). In ovariectomized rats, only a high dose of CI-628 (24 mg/kg) is able to decrease elevated LH levels observed following castration. In the presence of E2, CI-628 has both estrogenic and anti-estrogenic properties on LH secretion. CI-628 acts at the pituitary level to decrease the tissue sensitivity to LH-RH, but has no effect on mediobasal hypothalamic (MBH) LH-RH content.

Kinetic characteristics of LH and FSH responses to LHRH in incubated pituitaries from ovariectomized or ovariectomized, estrogen-implanted rats.

Incubated pituitary halves from ovariectomized, estrogen-implanted female rats were shown to be much more sensitive to LHRH than pituitaries from castrated, nontreated animals. LHRH in a concentration of 1,885 pg/ml increased the release of LH and FSH from 7.3 +/- 0.9 and 0.91 +/- 0.13 ng/h/hemipituitary respectively to 21.4 +/- 1.9 and 1.97 +/- 0.18 ng/h in animals implanted with the steroid. In contrast, 5,000 pg/ml of LHRH increased LH secretion from 3.4 +/- 0.3 to 8.4 +/- 0.4 ng/h in ovariectomized, nontreated animals. In pituitaries from both steroid and nontreated animals a highly significant dose response for LH and FSH secretion to the actual concentration of LHRH measured in each incubation tube by radioimmunoassay was observed. When expressed as percent of the corresponding control release, maximal stimulation of LH and FSH was comparable. Pituitaries from implanted animals provided a very sensitive bioassay for LHRH, in which amounts of the peptide lower than 100 pg/ml were detected. The apparent responsiveness to LHRH of pituitaries from estradiol-treated rats was found to be over 20 times greater than that of pituitaries from nontreated castrates.

Vasoactive intestinal peptide inhibits release of somatostatin from hypothalamus in vitro.

The effect of vasoactive intestinal peptide (VIP) was studied on the release of somatostatin (SRIF) from slices of several regions of the rat brain in vitro. VIP induced a dose-dependent inhibition of SRIF release from mediobasal hypothalamic slices but did not interfere with SRIF release from preoptic area, amygdala or cortex. VIP inhibition had an apparent affinity: Kd = 6.8 +/- 3.9 x 10(-11) M. Secretin had a similar effect but at 600-fold higher concentrations (Kd secretin = 4.2 +/- 0.6 x 10(-8) M). Gucagon was ineffective in concentrations ranging from 10(-10) M to 10(-7) M. The data are consistent with a role of VIP in the hypothalamic control of growth hormone secretion.

Unilateral ovariectomy-induced luteinizing hormone-releasing hormone content changes in the two halves of the mediobasal hypothalamus.

The luteinizing hormone-releasing hormone (LHRH) content of the two halves of the mediobasal hypothalamus (MBH) has been studied two weeks after unilateral or bilateral ovariectomy (OVX) in the rat. In control animals, the LHRH content of the right-side MBH was higher than that of the left side. Right-side OVX induced a significant increase in LHRH content on the right-side MBH while left-side OVX was followed by elevated LHRH content on the left-side MBH. In contrast, bilateral OVX induced a significant decrease on the right-side MBH. The observation that unilateral OVX induced increased LHRH content in the ipsilateral half of the MBH to OVX, supports the view that a neural pathway might connect the ovary and the MBH.

Effect of neutransmitters on in vitro release of luteinizing-hormone-releasing hormone from the mediobasal hypothalamus of male rats.

Effects of neurotransmitters on in vitro release of lutein-hormone-releasing hormone (LHRH) from fragments of hypothalamic tissue were investigated. Neither acetylcholine (ACh) or histamine (HA) at concentrations of 10(-7) and 10(-5) M, or GABA at 10(-7) to 5 x 10(-5) M affected LHRH release from mediobasal hypothalamic (MBH) fragments or the organum vasculosum of the laminae terminalis (OVLT). In contrast, serotonin (10(-7) and 10(-5) M) significantly inhibited the release of LHRH from the MBH but not from the OVLT. This response to serotonin (5-HT) is no longer observed in presence of methiothepin, a 5-HT receptor antagonist. The present results confirm in vivo data suggesting that the inhibitory effect of serotonin on the release of LHRH from median eminence involves a direct action on neurosecretory nerve-endings.