Stimulation of somatostatin release from median eminence tissue incubated in vitro by taurine and related amino acids

To determine if the putative amino acid transmitter taurine could affect the release of somatostatin (SRIF) from median eminence fragments, median eminences were incubated in vitro in the presence of various concentrations of taurine and its precursors, hypotaurine, cysteamine, cysteic acid, and L-cysteine sulfinic acid. Taurine was effective in releasing SRIF at doses of 50 microM and higher. Surprisingly, cysteic acid was the most potent stimulant of SRIF release, with a minimal effective dose of 0.1 microM. Cysteamine was effective at a dose of 50 microM, hypotaurine was effective only at 200 microM, and cysteine sulfinic acid was ineffective in stimulating SRIF release at any of the doses tested. To determine if the effect of taurine was mediated by the dopaminergic system, the dopamine receptor blocker pimozide (1 microM) was added to the medium containing either taurine or cysteic acid. Pimozide did not prevent the stimulatory effect of the amino acids. In conclusion, these investigations suggest that taurine and related amino acids may be involved in SRIF release by a nondopaminergic pathway. In view of the high concentrations of taurine in the hypothalamus it must be considered a putative transmitter releasing SRIF.

Neuropeptide Y affects secretion of luteinizing hormone and growth hormone in ovariectomized rats

Neuropeptide Y (NPY) has recently been localized in the rat hypothalamus. We have evaluated the effects of NPY on hypothalamic and pituitary function by injecting NPY into the third ventricle in vivo and by examining its action on perifused pituitary cells in vitro. Injections of NPY into the third ventricle of conscious ovariectomized rats led to a dramatic and highly significant reduction in plasma luteinizing hormone (LH) relative to pretreatment levels in these animals or to those of controls injected with physiological saline. Significant inhibition was obtained with doses ranging from 0.02 to 5.0 micrograms (4.7-1175 pmol) of NPY. These inhibitory effects on LH release were dose dependent and lasted for at least 120 min after injection of 5.0 micrograms of NPY. Intraventricular injection of NPY also significantly decreased plasma growth hormone; however, the threshold dose was 2.0 micrograms (470 pmol), a dose 100-fold greater than the lowest dose that inhibited LH release. Plasma follicle-stimulating hormone was unaffected by injection of NPY. NPY (10(-6) and 10(-7) M) stimulated secretion of LH, growth hormone, and follicle-stimulating hormone from perifused anterior pituitary cells loaded in a Bio-Gel P-2 column. These results indicate that NPY acts on structures adjacent to the third ventricle to inhibit the secretion of LH and growth hormone but not follicle-stimulating hormone, whereas it can directly stimulate the secretion of all three hormones from the cells of the anterior pituitary in vitro. Since NPY has been found in the hypothalamus and median eminence, it is quite likely that it plays a physiologically significant role at both hypothalamic and pituitary sites: influencing secretion of pituitary hormones.

The effects of gastric inhibitory polypeptide (GIP) on the release of anterior pituitary hormones

Several members of the secretin family of hormones have been demonstrated to alter anterior pituitary hormone secretion. Here we report the action of gastric inhibitory polypeptide (GIP) on gonadotropin and somatotropin release. Intraventricular injection of 1 microgram (0.2 nmole) GIP (2.5 microliters) produced a significant decrease in plasma FSH at 30 (p less than 0.02) and 60 min after its injection (p less than 0.01). The FSH-lowering effect of a higher dose of 5 micrograms (1 nmole) of GIP was already developed at 15 min (p less than 0.01) and was prolonged until the end of the experiment (60 min, p less than 0.05). No change in plasma LH was detected at any time during the experimental period. If 5 micrograms of estradiol-benzoate were given SC 48 hr prior to experiment, the initial values of FSH and LH were markedly decreased. In these animals GIP failed to influence plasma FSH and LH. When dispersed anterior pituitary cells from OVX rats were cultured overnight and incubated in vitro with GIP, the peptide was found to induce both FSH and LH release. Highly significant release occurred with the lowest dose tested of 10(-7) M and there was a dose-response effect for both hormones. The slope of the dose-response curve was similar for both FSH and LH release. GIP was less potent than LHRH which produced a greater stimulation of both FSH and LH release at a dose of 10(-9) M than did 10(-7) M GIP. The two peptides had an additive effect on the release of both FSH and LH.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification of a non-dopaminergic and non-GABAergic prolactin release-inhibiting factor (PIF) in sheep stalk-median eminence

Prolactin release-inhibiting factor (PIF) extracted from 1200 sheep stalk-median eminences was purified by gel filtration on a Sephadex G-25 column (4.5 X 150 cm). PIF activity was determined by measuring the inhibition of prolactin release from dispersed anterior pituitary cells of adult male or estrogen-primed, ovariectomized rats. Using this system, PIF was detected in tube fractions 122-127 (volume = 20 ml/tube). These fractions also contained LHRH and somatostatin; however, these peptides had no prolactin-inhibiting activity in the quantities present. No dopamine or gamma-aminobutyric acid (GABA) was detected in the active fractions by radioenzymatic assay and fluorophotoenzymatic assay, respectively. Furthermore, receptor blockers for dopamine or GABA did not interfere with the PIF activity. These findings indicate that the PIF activity cannot be attributed to either dopamine or GABA, both of which are known to inhibit prolactin release, and provide evidence for the presence of a non-dopaminergic and non-GABAergic PIF within the hypothalamus.

Pancreatic polypeptides affect luteinizing and growth hormone secretion in rats

We have examined the effects of third cerebroventricular (3V) injections of avian and bovine pancreatic polypeptide (APP and BPP) and the C-terminal hexapeptide amide of human PP (CHPP) on the secretion of anterior pituitary hormones in conscious ovariectomized rats. Injection of APP (2.0 micrograms; 472 pmoles) or BPP (5.0 micrograms; 1191 pmoles) decreased plasma levels of luteinizing hormone (LH) when compared to pre-injection levels in these animals or to saline-injected controls. The lower dose of BPP (0.5 micrograms; 119 pmoles) decreased plasma LH versus pre-injection levels and control animals, however, these effects diminished at later times. Plasma growth hormone (GH) also decreased following 3V injections of APP (2.0 micrograms) or BPP (5.0 micrograms). The lower dose of BPP (0.5 microgram) initially inhibited GH release, however, this effect was rapidly reversed and GH levels were significantly greater than those in controls at 60 and 120 min. Injections of BPP or APP did not alter prolactin (PRL) or thyroid stimulating hormone (TSH) secretion. Administration of 2.0 micrograms and 0.2 microgram of CHPP (2488 and 249 pmoles) produced no significant effects on plasma LH, GH, PRL or TSH. APP and BPP had no consistent effects on hormone secretion from dispersed anterior pituitary cells. The results indicate that APP and BPP exert potent central effects which inhibit LH and GH release from the pituitary gland.

Effect of destruction of the dorsal anterior hypothalamus on follicle-stimulating hormone secretion in the rat

The role of the paraventricular nucleus-dorsal anterior hypothalamus (PVN-DAHA) in the control of anterior pituitary gland secretion of FSH and LH in castrated male and female rats was examined. Bilateral radiofrequency lesions of the PVN-DAHA in chronically ovariectomized (OVX) rats lowered plasma FSH levels by 33% (P less than 0.005) compared to values in unoperated and sham-operated control rats; plasma LH concentrations were unaltered. RIA of median eminence (ME) LHRH concentrations in these animals revealed no differences among the three experimental groups. Other categories of diencephalic destruction did not result in this pattern of selectively reduced FSH release. Bilateral radiofrequency destruction of the PVN-DAHA also attenuated by 50% (P less than 0.025 to P less than 0.005) the progesterone-induced surge of FSH in estrogen-primed OVX rats. Progesterone-induced LH release was unaffected by PVN-DAHA lesions. Other lesion categories failed to show the same result. Bilateral ablation of the PVN-DAHA in male rats resulted in a selective diminution of the postcastration rise of plasma FSH beginning 48 h postcastration (P less than 0.05 to P less than 0.005) and persisting for 14 days (P less than 0.005) after orchidectomy, thus revealing the time course and permanence of this procedure on plasma FSH levels. The postcastration rise of plasma LH levels was not affected by PVN-DAHA lesions. The concentration of ME LHRH was the same among orchidectomized male rats whether they bore PVN-DAHA lesions, sham lesions, or no lesions. In summary, destruction of the PVN-DAHA was found to reduce significantly the elevation of plasma FSH, but not LH, in the OVX rat and the estrogen-progesterone-stimulated OVX rat. PVN-DAHA lesions also attenuated the postcastration rise of FSH, but not that of LH, in the male. The failure of lesions of the PVN-DAHA to alter ME LHRH concentrations in the face of decreased FSH release does not prove that LHRH release is totally unaffected by this procedure. This finding is, however, consistent with the concept that diminished FSH secretion could be the result of a deficiency of a hypothalamic releasing factor (FSH-releasing factor?) other than that of the LHRH decapeptide.

Physiological role of alpha-melanocyte-stimulating hormone in modulating the secretion of prolactin and luteinizing hormone in the female rat

Long-term ovariectomized (OVX) rats were injected in the third cerebral ventricle with 5 microliter of the globulin fraction of an antiserum raised against alpha-melanocyte-stimulating hormone (alpha-MSH) or an equal volume of the globulin fraction of normal rabbit serum (NRS). Immunoneutralization of brain alpha-MSH produced an increase in the area under the secretion curve of prolactin (Prl), the amplitude of Prl pulses, and mean plasma Prl (P less than 0.01). In animals that had received two injections of NRS or anti-MSH and were subjected to a 2-min ether stress, Prl levels significantly increased within 5 minutes in the NRS-injected rats, whereas Prl levels in the antiserum-injected rats did not increase any further from the initially high baseline levels. The administration of antibodies against alpha-MSH produced a small increase (P less than 0.05) in the area under the secretion of luteinizing hormone (LH) and mean plasma LH; however, the number of LH pulses was unaffected. We conclude that endogenous alpha-MSH of central origin is a physiological neuromodulator of release of Prl and LH in the OVX rat and is involved in the stress-induced release of Prl.

Hypothalamic control of prolactin secretion during the perinatal period in the rat

The development of hypothalamic control of PRL secretion in the late prenatal and early postnatal periods in the rat was studied by employing a static system for incubation of pituitaries. PRL was detected in the incubation medium after incubating fetal pituitaries as early as day 18. A gradual increase in the amount of hormone released into the medium occurred during development, with the greatest change occurring between days 20 and 21 prenatally. A progressive increase in the pituitary content of PRL occurred during development. The percentage of PRL released from the gland was higher pre- (19-39%) than postnatally (17%). Hypothalamic extracts from fetuses and neonates stimulated the secretion of PRL when coincubated with pituitaries from animals of the same age. However, extracts from 1-day-old neonates inhibited PRL secretion from adult male hemipituitaries. Extracts prepared from adult male hypothalami stimulated PRL secretion from neonatal pituitaries from rats at 1 day of age, as did cerebral cortical extracts from adults, but not 1-day-old, rats. TRH significantly stimulated PRL secretion in vitro from pituitaries of donors as early as day 19 of the fetal period. The response of the pituitaries to this peptide diminished by day 8 postnatally. Immunoneutralization of hypothalamic TRH significantly decreased but did not abolish the PRL-releasing activity of hypothalamic extracts, whereas this procedure had no effect on the GH-releasing activity of the extracts. The dopamine receptor agonist apomorphine (10(-5) M) inhibited PRL secretion in vitro on day 19 of the fetal period and postnatally starting on day 1. The response to apomorphine increased with advancing age. The results suggest that a combination of factors contribute to maintaining high circulating PRL levels during the late fetal and early neonatal periods. These include high rates of PRL release by the pituitary and a relative insensitivity of the pituitary to dopamine in the face of high sensitivity to PRL-releasing factors such as TRH.

Changes in hypothalamic and pituitary content of immunoreactive alpha-melanocyte-stimulating hormone during the gestational and postpartum periods in the rat

alpha-Melanocyte-stimulating hormone (alpha-MSH) was measured in the mediobasal hypothalamus (MH), median eminence (ME), preoptic-suprachiasmatic area (POA-SCN), anterior (AL), and posterior lobes (PL) of the pituitary gland during the gestational and postpartum periods in the rat. The content of alpha-MSH in the MH and POA-SCN compared to estrous levels was lower during the later days of gestation and decreased further in the MH during lactation in association with the elevated plasma prolactin (Prl). Distinct increases in the ME content of alpha-MSH compared to estrous levels occurred on Days 8 and 12 of the gestational period and Day 14 of the postpartum period. A significant increase in PL content of alpha-MSH compared to Days 5-11 and 17-20 occurred on Day 4 of gestation, and no significant changes were detected in the AP concentration of alpha-MSH throughout the period studied. In vitro, PLs and ALs from females on Day 4 of gestation secreted more alpha-MSH into the incubation medium than tissues from animals on Day 20. These results suggest that alpha-MSH of both brain and pituitary origin may play a role in mediating some of the physiological changes which occur during pregnancy and lactation.

Intrahypothalamic action of corticotrophin-releasing factor (CRF) to inhibit growth hormone and LH release in the rat

The effects of intravenous or intraventricular injection of synthetic ovine corticotrophin-releasing factor (oCRF) on plasma levels of anterior pituitary hormones were studied in conscious, ovariectomized (OVX) female rats and compared with the actions of the peptide on dispersed anterior pituitary cells from OVX female rats incubated in the presence of CRF. Third ventricular injection of oCRF in freely moving rats caused a significant increase in plasma levels of ACTH in a dose-related manner with a minimal effective dose of less than 0.5 micrograms (0.1 nmol). The effect was observable at 5 min after injection and persisted for the 60 min duration of the experiment. In contrast, growth hormone levels were significantly depressed within 15 min with a minimal effective intraventricular dose of 0.5 micrograms. The suppression persisted for the duration of the experiment but there was no additional effect of the higher dose of 5 micrograms. Plasma LH levels were also lowered by the highest dose of 5 micrograms (1.0 nmol) of oCRF, with the first significant lowering at 30 min. Lower doses had no effect on plasma LH. Plasma TSH levels were not significantly altered. Control injections of the 0.9% NaCl diluent were without effect on the levels of any of the hormones. Intravenous injection of similar doses of oCRF had no effect on plasma levels of GH or LH. The ACTH-releasing action of the oCRF preparation was confirmed by in vitro incubation of the peptide with dispersed anterior pituitary cells for 2 h. A dose-related release of ACTH occurred in doses ranging from 0.1-10 nM, but there were no effects on the release of the other anterior pituitary hormones. The results suggest that oCRF may act within the hypothalamus to suppress the release of GH and to a lesser extent LH. The stimulation of ACTH release following intraventricular CRF is presumably related to its uptake by portal blood vessels with delivery to the pituitary and stimulation of the corticotrophs.

Testosterone control of brain and anterior pituitary beta-adrenergic receptors

Orchidectomy of adult albino rats resulted in a quick, (approximately 70%) increase in the density of beta-adrenergic receptors in the anterior pituitary gland within the first day. There was a concurrent rapid increase in plasma levels of pituitary gonadotropins. The beta-receptor density continued to increase slowly for at least 16 days after castration, but it could be lowered significantly to the levels of sham-operated animals by treatment with testosterone (3 mg/kg/day) beginning on the fourth day after castration and continuing for 4 days. This treatment also completely reversed the elevation in plasma levels of luteinizing hormone (LH), and significantly reduced the circulating follicle-stimulating hormone (FSH) levels. Prolactin levels were not significantly altered by the treatments used in these studies. Most of the beta-adrenergic receptors induced by orchidectomy in the anterior pituitary were shown, using a beta 1-selective antagonist, practolol, or a beta 2-selective antagonist, IPS-339, to be of the beta 2-subtype. The density of the beta-adrenergic receptors in the cerebral cortex also increased significantly (10-24%) after castration, and returned to the levels of sham-operated animals following treatment with testosterone. No significant change in the density of the beta-adrenergic receptors in the hypothalamus resulted from either castration or testosterone replacement.

Differential role of the opioid mu and delta receptors in the activation of prolactin (PRL) and growth hormone (GH) secretion by morphine in the male rat

Administration of naloxazone (50 mg/kg i.v.), an irreversible, selective and long acting antagonist of the mu 1 subclass of the opioid receptors, strongly reduced stimulation of PRL secretion by morphine (5.0 mg/kg i.v.) injected 24 hours later into conscious, unrestrained rats. In contrast, the effect of morphine on PRL release was unimpaired in rats treated 24 hours beforehand with either the reversible opioid antagonist naloxone (50 mg/kg i.v.), or the vehicle for naloxazone. A complete suppression of the PRL response to morphine (3.0 mg/kg i.v.) was observed in animals given intraventricular (IVT) injection of beta- funaltrexamine (beta-FNA, 2.5 micrograms), another selective, irreversible and long acting antagonist of the mu receptors, 24 hours beforehand. Neither naloxazone nor beta-FNA had any effect on the activation of GH secretion by morphine, which, however, was conspicuously reduced by ICI 154, 129, a preferential delta receptor antagonist, injected IVT (50 micrograms) 5 minutes before morphine. ICI 154, 129 had no effect on the PRL response to morphine. It is concluded that the PRL stimulating effect of morphine is mediated by the mu receptors, whereas activation of GH probably involves the delta sites.

A possible role of prostacyclin to stimulate prolactin and growth hormone release by hypothalamic and pituitary actions, respectively

Prostacyclin (PGI2) (1-5 micrograms in 3 microliters 0.05 M Tris/HCl buffer, pH 7.5) and its stable metabolite, 6-oxo-PGF1 alpha, were microinjected into the third ventricle of ovariectomized rats, and plasma FSH, GH, PRL, and TSH levels were measured by RIA. Control animals received 3 microliters buffer. Injection of 5 micrograms PGI2 dramatically elevated plasma PRL values (4- to 5-fold) at 5 and 15 min, whereas the same dose of 6-oxo-PGF1 alpha produced a significant but smaller (2-fold) stimulatory effect. A delayed increase (1.5-fold) in plasma GH occurred after intraventricular PGI2 at 30 and 60 min. 6-Oxo-PGF1 alpha failed to alter GH levels. There were no alterations in plasma FSH and TSH after intraventricular injection of PGI2. Dispersed, overnight cultured cells from anterior pituitaries of ovariectomized rats were tested with 10(-4)-10(-7) M PGI2 and its metabolite. After 15 min of incubation, 3 X 10(-5) PGI2 produced a highly significant elevation in GH release (P less than 0.001), whereas there was no alteration in PRL levels. Only pharmacological doses of 6-oxo-PGF1 alpha (10(-4) M) stimulated GH release. There was no alteration in PRL release by the cultured cells even in the presence of 10(-4) PGI2. These results suggest that PGI2 stimulates PRL release by a hypothalamic action either to increase the release of PRL-releasing factor, or to decrease release of PRL-inhibiting factor, or by both mechanisms. The delayed stimulatory effect of PGI2 on the release of GH may be exerted via an effect on the anterior lobe itself, since PGI2 was effective in stimulating GH release by the incubated pituitary cells.

Partial characterization of immunoreactive substance P in the rat pituitary gland

Two distinct carboxy-terminus-directed anti-substance P (SP) sera (R-1C and R-6G) were used to characterize immunoreactive SP (I-SP) in acetic acid extracts of anterior pituitary (AP) and posterior pituitary (PP) glands of adult male rats. The tissue concentrations of I-SP measured by R-1C and R-6G were comparable. The contents of I-SP were 600-1150 pg/AP and 25-52 pg/PP. I-luteinizing hormone releasing hormone and I-somatostatin (I-SOM) were undetectable in AP extracts, but PP extracts contained the equivalents of 325-785 pg I-SOM/gland. Serial dilutions of AP and PP extracts produced displacement curves with both SP antisera that were parallel to the respective synthetic SP standard and hypothalamic extract displacement curves. Gel filtrations of AP and PP extracts on a Sephadex G-25 column produced I-SP peaks eluting in the same fractions as synthetic SP and hypothalamic I-SP. However, the AP I-SP profile also revealed a side peak migrating between the void volume and the major I-SP peak. Neither immunoreactive species in the AP extract were eliminated when eluted with 6.0 M guanidine HCl, a strong denaturing agent. In vitro incubation of paired anterior hemipituitaries for 30 min in the presence of a 56 mM K+ concentration resulted in a significant (p less than .0001), 25-fold increase in the release of I-SP into the incubation medium above the mean control value. Radiofrequency lesions placed in the median eminence-arcuate region of male rats caused a significant (p less than .001) reduction of I-SP in both the AP and PP. These reductions were inversely related to the plasma prolactin values. The elevation in plasma prolactin was taken as an index of completeness of lesions. We conclude that: 1) the rat pituitary contains I-SP as assessed by its immunologic and chromatographic behavior, 2) K+ depolarization is a potent stimulator of the release of AP I-SP in vitro, 3) the ME-arcuate region is important for the maintenance of pituitary I-SP levels in the rat.

Presence and possible site of action of secretin in the rat pituitary and hypothalamus

Secretin-like immunoreactivity was detected in extracts of several rat brain structures by radioimmunoassay, most notably in the pituitary, hypothalamus, pineal and septum. Its localization to these structures suggested that it might play a role in neuroendocrine events similar to its structural homolog vasoactive intestinal peptide. Dose-related stimulations (MED, 10(-7) M) of prolactin (PRL) release were observed after incubation of synthetic secretin with dispersed, cultured pituitary cells from male and ovariectomized (OVX) female rats. In OVX females, i.v. infusion of a high dose of secretin (10 micrograms) resulted in a significant elevation of PRL levels. Doses of secretin as low as 0.1 micrograms when administered into the third cerebroventricle were capable of significantly inhibiting PRL release in both males and OVX females, suggesting an ultrashort-loop, negative feedback of secretin. Secretin can now be added to the growing list of putative PRL-releasing agents.

The effect and possible mode of action of alpha-melanocyte-stimulating hormone on gonadotropin release in the ovariectomized rat: an in vivo and in vitro analysis

This study was designed to examine the possible effects of alpha MSH on gonadotropin release. Injection of alpha MSH into the third ventricle of the brain and sampling at 5, 10, 15, 30, and 60 min postinjection produced a significant lowering of plasma LH, but not of FSH, in conscious ovariectomized (OVX) rats. The minimum effective dose was 1 microgram. This procedure did not affect plasma levels of LH or FSH in estradiol benzoate-primed, OVX rats. Multiple blood sampling every 10 min before and after intraventricular injection of alpha MSH (2 micrograms) produced a significant reduction in the area under the secretion curve of LH. Intravenous injection of alpha MSH had little effect; however, a slight lowering of plasma LH occurred in OVX rats after a 50-micrograms dose. The effect of alpha MSH on LH release was blocked by pretreatment of the animals with alpha-methyl-paratyrosine, an inhibitor of catecholamine synthesis, as well as by pretreatment with an iv injection of spiroperidol, a dopamine receptor blocker. The peptide failed to alter basal or K+-stimulated LHRH release from median eminence fragments of OVX rats incubated in vitro. Alpha MSH had no effect on LHRH-induced LH release in vivo and failed to alter the release of FSH and LH from hemipituitaries of OVX rats or dispersed cells from OVX, estradiol benzoate-primed rats in vitro. It is concluded that alpha MSH exerts an inhibitory effect on LH release by an action on the hypothalamus, probably via activation of the tuberoinfundibular dopaminergic system.

Recent studies on the role of brain peptides in control of anterior pituitary hormone secretion

Recent work in our laboratory on the role of peptides to influence release of pituitary hormones by direct action on the gland and also some of the interactions of these peptides at the hypothalamic level to alter release of pituitary hormones will be reviewed. Considerable evidence from hypothalamic stimulation and lesion studies suggests the existence of a separate FSH-releasing factor (FSHRF). We have been able to purify a bioactive FSHRF which appears to be distinct from LHRH. Consequently, we believe that a distinct FSHRF will ultimately be isolated. With regard to prolactin, it is now clear that it is under dual control by both prolactin-inhibiting (PI) and prolactin-releasing factors (PRF). Although dopamine acts as a PIF, our recent fractionation studies indicate the existence of a peptidic PIF in hypothalamic extracts which can be separated from dopamine and GABA. The peptidic PIF is eluted from Sephadex in the same position originally described by us a number of years ago. Thus, inhibitory control is probably mediated by a combination of factors which would include dopamine, possibly GABA and a peptidic PIF. A number of peptides have been shown to have PRF activity which include TRF and also VIP. In recent studies, we have shown a prolactin-releasing action of oxytocin on male hemipituitaries or dispersed pituitary cells. Furthermore, high doses of oxytocin given intravenously released prolactin in male rats. There is a correlation between estrogen-induced prolactin release and an increase in plasma oxytocin and a correlation between suckling-induced oxytocin and prolactin release. These results suggest that oxytocin may be an important PRF.(ABSTRACT TRUNCATED AT 250 WORDS)

Gamma aminobutyric acid (GABA), a modulator of anterior pituitary hormone secretion by hypothalamic and pituitary action

We have evaluated the role of GABA in the control of anterior pituitary (AP) hormone secretion by injecting it into the third ventricle of ovariectomized, ovariectomized-steroid primed and male rats. Specificity of the effects was determined by injecting the GABA blocker, bicuculline. The action of GABA directly on the pituitary was evaluated in vitro. The results indicate that intraventricular GABA can stimulate LH, growth hormone (GH) and, at high doses, prolactin (Prl) release, whereas low doses inhibit Prl and al doses inhibit TSH release. All of these actions are blocked by bicuculline. Intraventricular GABA administration is followed by an elevation of hypothalamic norepinephrine (NE) and median eminence dopamine (DA) levels and AP DA levels, which indicates that the compound stimulates both NE and DA release. The actions on GH and LH appear to proceed independently of DA, since the DA receptor blocker, pimozide, did not interfere with these effects, whereas the action to elevate Prl and to lower TSH was blocked by DA receptor blockade. Anterior pituitary hormone release by AP's incubated with GABA in vitro was unaltered except for an inhibition of Prl release by very high GABA doses, which could be blocked by bicuculline. Intravenous injection of bicuculline to assess the physiological significance of GABA in control of AP hormone secretion revealed no effect on FSH but a delayed rise in LH, an initial rise in Prl, followed by a fall, a tendency for GH values to rise and dramatic fall in TSH levels. These results suggest the possibility that GABA plays a physiological role in the control of AP hormone secretion, mainly via a hypothalamic action.

Motilin: a novel growth hormone releasing agent

Motilin, a gastrointestinal peptide recently detected in the rat brain, was capable of stimulating growth hormone (GH) release from dispersed anterior pituitary cells in a dose-related fashion. In initial experiments, the minimum effective concentration was 10(-7) M and the effect was specific for just GH. Subsequent experiments demonstrated that concentrations of synthetic motilin as low as 10(-9) M could significantly stimulate GH release. Only large IV doses (100 micrograms) of motilin significantly elevated circulating GH levels in vivo. However, administration of antiserum to porcine motilin (100 microliters, IV) significantly depressed plasma GH levels, suggesting a physiologic role for median eminence and hypothalamic motilin in the control of GH secretion. Furthermore, infusion of motilin into the third ventricle of conscious rats resulted in a significant depression of GH levels, suggesting an ultrashort loop feedback action of motilin on the release of motilin itself or somatostatin. In light of motilin's only minor structural similarity to human pancreatic tumor GH-releasing factor (GRF) and the ability of passive immunoneutralization of motilin to lower GH, this 22-amino acid peptide must now be considered a physiologic GRF.

Pre- and postnatal developmental changes in hypothalamic and pituitary content of alpha-melanocyte-stimulating hormone and somatostatin in the rat

The developmental pattern of hypothalamic and pituitary somatostatin and alpha-melanocyte-stimulating hormone (alpha-MSH) was studied in the rat by radioimmunoassay (RIA). Prenatally hypothalamic somatostatin was low and the first increase in content occurred on day 5 of the postnatal period. Pituitary somatostatin was undetectable prenatally and was first detected on day 6 postnatally. Pituitary somatostatin showed the same development pattern as hypothalamic somatostatin. Whole pituitaries were analyzed from the fetuses and neonates. Analysis of anterior and posterior lobes from adults revealed that all the somatostatin was in the posterior lobe. Therefore, it is believed that the somatostatin found in the glands of the younger animals was in the neural lobe. alpha-MSH was detected in the fetal hypothalamus as early as day 16, with a marked increase occurring on day 19 prenatally and a further increase on day 10 postnatally. Pituitary alpha-MSH content, readily detectable on the first day examined, i.e. day 16 of the fetal period, increased throughout development. The effect of alpha-MSH on the release of growth hormone in vitro from pituitaries of rats of various ages was tested. alpha-MSH had no effect on growth hormone release.

Evidence for an FSH-releasing factor in the posterior portion of the rat median eminence

To test the hypothesis that an FSH-releasing factor might be contained within the posterior portion of the median eminence (ME), the anterior half of the ME (aME) and the posterior half of the ME (pME) were removed separately from the brains of adult male rats and extracted in 0.2 N acetic acid. LH and FSH-releasing activities of the extracts were measured in vitro by incubating 8 hemipituitaries from adult male rats for 6 h at a dose of 5 tissue equivalents and determining the radioimmunoassayable LH and FSH released into the medium. LH release induced by the aME extracts was significantly greater than that induced by the pME in both experiments, whereas there were no differences in FSH release between aME and pME extracts. A significant dose-related increase in FSH release was noted in this system when 1 and 2 ng of synthetic LHRH were tested which indicates that the assay was sensitive to different amounts of LHRH with regard to FSH-releasing action. The content of immunoreactive LHRH in the extracts was almost twice as high in the aME as in the pME. Therefore, the results indicate that the pME has greater FSH-releasing activity than can be accounted for by its content of LHRH. The additional FSH-releasing activity is presumably due to an FSH-releasing factor distinct from LHRH.

Effect of beta-adrenergic drugs on LH, FSH, and growth hormone (GH) secretion in conscious, ovariectomized rats

The effects of third ventricular (3V) injection of the beta-adrenergic antagonist, propranolol (PROPR), a selective beta 1-antagonist, metoprolol (MET), a selective beta 2-antagonist, IPS 339, and a beta-adrenergic agonist (-) isoproterenol (ISOPR), on plasma concentrations of luteinizing hormone (LH), follicle stimulating hormone (FSH), and growth hormone (GH) were studied in conscious, ovariectomized (OVX) rats. Samples were removed from unrestrained rats which had been previously implanted with atrial and 3V cannulae, and plasma hormone levels were determined by radioimmunoassay (RIA). Intraventricular injection of PROPR (30 micrograms), MET (40 micrograms), or IPS 339 (20 micrograms) induced a gradual elevation in plasma GH concentrations, whereas ISOPR (30 micrograms) reduced plasma GH. ISOPR (30 micrograms) brought about a decrease in plasma LH concentrations, but PROPR, MET and IPS 339 had no effect on LH levels. PROPR (30 micrograms) increased plasma FSH concentrations, but there was no significant effect of MET, IPS 339 or ISOPR on FSH secretion. The results indicate that the beta-adrenergic system can inhibit the release of GH, LH, and FSH. This system appears to have a tonic inhibitory effect on GH and FSH but not LH release in the OVX rat.

Prolactin-releasing activity of porcine intestinal peptide (PHI-27)

Porcine intestinal peptide (PHI), a twenty-seven amino acid peptide isolated from porcine gut extracts, is a close structural homolog of the secretin family hormones. The structural and biological similarities of PHI to vasoactive intestinal peptide (VIP) together with its presence in the rat hypothalamus suggested a possible role for the peptide in the control of prolactin (PRL) secretion. PHI induced significant, dose-related stimulations of PRL release from cultured, dispersed rat pituitary cells in vitro. The minimum effective dose is 10(-7) molar, compared to 10(-9) molar for VIP. No interactive effect with thyrotropin-releasing hormone was observed; however, PHI partially overcame the dopamine inhibition of PRL release.

Interaction of atropine with naloxone to alter anterior pituitary hormone secretion

The effects of third ventricular injection of naloxone and atropine, alone and together, on plasma concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin (PRL), growth hormone (GH), and thyroid-stimulating hormone (TSH) were studied in conscious, ovariectomized (OVX) rats. Samples were removed from unrestrained rats that had been previously implanted with an atrial cannula, and plasma hormone levels were determined by radioimmunoassay. Intraventricular injection of naloxone (30 micrograms) induced increases in plasma LH and GH concentrations, decreased plasma PRL levels, but did not alter plasma TSH. When atropine (100 micrograms) was injected intraventricularly, there was a decrease in plasma LH and an increase in PRL levels but no alterations in the levels of the other hormones. The action of naloxone on PRL and GH release was eliminated by simultaneous administration of atropine, but there was only a slight reduction in its effect on LH. The results indicate that central cholinergic neurons may play an important role, via muscarinic type receptors, in mediation of the effects of naloxone and presumably opiates on PRL and GH release, whereas the effects on LH appear to proceed independently of cholinergic neurons.

Effect of unilateral orchidectomy on plasma FSH concentration: evidence for a direct neural connection between testes and CNS

The effect of unilateral orchidectomy on plasma FSH, LH and LHRH content in the right and left half of the median eminence-arcuate nucleus region was examined in adult rats. Following right-sided orchidectomy, plasma FSH began to rise but a significant increase was not observed until 1 week after the operation. No significant changes were observed in LH values. LHRH content in the ipsilateral median eminence-arcuate region showed a significant increase only when the median eminence was removed together with the arcuate nucleus, but not when it was removed separately, which suggests that the increase in LHRH content was attributable to an increased LHRH content of the ipsilateral arcuate nucleus. Radiofrequency lesions placed unilaterally in the same side of the dorsal anterior hypothalamic area as the hemiorchidectomy partially blocked the selective FSH release following unilateral orchidectomy, while contralateral or sham lesions had no effect. These results suggest that the gonad has a direct neural connection with the hypothalamus, which is involved in selective FSH release following hemiorchidectomy.