Activation of gamma-aminobutyric acid B-receptors abolishes naloxone-stimulated luteinizing hormone release

Recent evidence suggests that the neurotransmitter gamma-aminobutyric acid (GABA) plays an important role in the control of gonadotropin secretion. The present study was conducted to identify the effect and site of action of different GABAergic drugs on LH secretion in vivo and to characterize the precise roles of different GABA receptors in these actions. Three different GABAergic drugs were used: muscimol and baclofen, which act at the level of the GABA A- and GABA B-receptors, respectively, and aminooxyacetic acid (AOAA), which increases the GABA content in the brain. The effects of these drugs were investigated in situations of enhanced LH secretion due to administration of naloxone or LHRH. In an initial experiment, adult male rats were treated ip with AOAA, followed by naloxone. AOAA treatment decreased basal LH levels and prevented naloxone-stimulated LH release. PRL levels were decreased by either AOAA or naloxone; however, the combination of these two drugs did not induce an additional or synergistic effect on the decreased PRL levels. In subsequent experiments, freely moving rats bearing Silastic cannulae in the right jugular vein received AOAA, muscimol, or baclofen a few minutes before either naloxone or LHRH administration. Baclofen and AOAA completely suppressed the naloxone-stimulated LH increase. Muscimol did not prevent the effect of naloxone. None of the three GABAergic drugs affected LH release in rats receiving LHRH. The results of these in vivo experiments suggest that the GABAergic system exerts primarily an inhibitory effect on gonadotropin secretion which is mediated at a central level, since pituitary responsiveness to LHRH is not affected by GABAergic drug treatment. GABA B-receptors are responsible for the inhibitory action of GABA.     

Role of central serotonin systems in the stimulatory effects of ovarian hormones and naloxone on luteinizing hormone release in female rats

The present experiments tested whether serotonergic neurons that innervate discrete areas of the hypothalamus are involved in the stimulation of LH release by ovarian hormones or the opiate antagonist naloxone in female rats. Ovariectomized rats received oil vehicle, estradiol benzoate (EB) alone, or EB followed by progesterone (P) 2 days later. Serotonin (5-HT) activity was assessed from the accumulation observed after the administration of the monoamine oxidase inhibitor pargyline. Two days after EB treatment, LH concentrations were reduced in the morning and rose by later afternoon. Administration of P to EB-primed rats stimulated a LH surge. This latter treatment also enhanced pargyline-induced 5-HT accumulation, suggesting increased 5-HT activity, in the medial preoptic nucleus and interstitial nucleus of the stria terminalis, but reduced 5-HT accumulation, suggesting decreased 5-HT activity, in the ventromedial nucleus. 5-HT activity was unaffected after EB alone, either in the morning or afternoon, or by the administration of naloxone to EB-primed rats. Specific depletion of 5-HT in the medial preoptic/stria terminalis area, achieved by microinjection of the neurotoxic indoleamine 5,7-dihydroxytryptamine resulted in a blockade of the LH surge induced by EB plus P. The present results suggest that central 5-HT neurons innervating the preoptic area are involved in the LH surge induced by progesterone, but not in the increases in LH occurring after treatment with estradiol alone or after blockade of opiate receptors.     

Interactions of delta 9-tetrahydrocannabinol (THC) with hypothalamic neurotransmitters controlling luteinizing hormone and prolactin release

The effects of delta 9-tetrahydrocannabinol (THC) on hypothalamic norepinephrine (NE) and dopamine (DA) turnover and hypothalamic serotonin (5-HT), 5-hydroxyindole acetic acid (5-HIAA) and LHRH content preceding and during a progesterone- (P) induced LH and prolactin (PRL) surge were investigated in ovariectomized estrogen-primed rats. THC had no effect on basal LH levels, but it inhibited basal PRL levels and blocked the surges of both LH and PRL. The turnover of NE, as estimated by measuring NE depletion after inhibition of tyrosine hydroxylase with alpha-methyl tyrosine (250 mg/kg), in both the anterior (AH) and medial basal hypothalamus (MBH) was significantly inhibited by THC. THC did not significantly affect AH or MBH DA or 5-HT content nor MBH-DA-turnover. Hypothalamic LHRH levels were significantly elevated 4 h after THC administration as compared to the vehicle-injected controls, but pituitary response to exogenous LHRH was not affected. These data suggest that THC inhibits the steroid-induced positive feedback release of LH by reducing NE metabolism and the release of hypothalamic LHRH. Although the mechanism for the inhibition of PRL release by THC is not clear from these experiments, it does not appear that alterations in DA turnover are a contributing factor.     

Stimulation with estrogen and progesterone of luteinizing hormone (LH)-releasing hormone release from perifused adult female rat hypothalami: correlation with the LH surge

The effects of estradiol benzoate (EB) and progesterone (P) in vivo and P in vitro on LHRH release from perifused preoptic area-medial basal hypothalamus (POA-MBH) tissue fragments were assessed. In the first series of experiments, ovariectomized female rats were given either 10 or 30 micrograms EB on day 0, followed by either oil or 5 mg P at 1000 h on day 2. Rats were killed at 2-h intervals after P or oil injection, and the POA-MBH from these rats were incubated in a perifusion system. LHRH release from the POA-MBH of rats primed with either 10 or 30 micrograms EB displayed similar patterns; from a basal rate at 1200 and 1400 h, LHRH secretion rose to significantly higher levels at 1600 and 1800 h. This increase in LHRH release in vitro was evident from the POA-MBH obtained just before and during the afternoon LH rise in vivo. On the other hand, the LHRH secretion pattern in vitro was different in two ways when rats were treated additionally with P. First, LHRH output was significantly higher at 1200 and 1400 h than that in rats treated with 10 or 30 micrograms EB only; this pattern was maintained later at 1600 and 1800 h from the POA-MBH of rats primed with 30 micrograms EB. Second, within the EB/P-treated (EBP) group, significant elevations in the LHRH output occurred from the POA-MBH obtained at 1800 h when rats were in the midst of the LH surge. In the other series of experiments, the POA-MBH of the EB-primed rats (30 micrograms/rat) were perifused with P (10 ng/ml) or vehicle for 6 h. The mean LHRH release during the 6-h interval was significantly higher after P perifusion; the bulk of this rise was evident at 4 h. These studies show that in vitro, the LHRH output of the POA-MBH from the EB- or EBP-treated rats varies significantly during the day and that the POA-MBH of EB-treated rats secretes large amounts of LHRH in vitro, which correlate well with the afternoon LH rise. In addition, treatment with P in vivo changes the in vitro pattern of LHRH output from the POA-MBH of these EB-primed rats, whereas superfusion of P in vitro induces only a marginal stimulation of LHRH release.     

Restraining action of GABA on estradiol-induced LH surge in the rat: GABA activity in brain nuclei and effects of GABA mimetics in the medial preoptic nucleus.

The relationship between GABA dynamics and LH release was studied on day 2 after subcutaneous estrogen implant in short-term ovariectomized rats. GABA accumulation, used as an index of GABA turnover, was determined in the medial preoptic nucleus (MPN), medial (MS) and lateral (LS) septal nuclei, median eminence-mediobasal hypothalamus (MBH) and locus ceruleus (LC). Measurements of GABA were performed at two different times of day (11.00 and 15.00 h), 3 h after intraperitoneal administration of gamma-vinyl-GABA (GVG), an irreversible inhibitor of GABA transaminase. Either morning or afternoon ovariectomized rats (OVX) showed a significant increase in GABA accumulation after GVG treatment in all the areas studied. Estrogen-treated OVX rats showed in the morning a lower GABA accumulation in the MPN, MBH and LC, and GABA levels remained unchanged in the LS and MS. In the afternoon, the MPN and LS showed a lower rate of GABA accumulation whereas in the MBH and LC the GABA increase was not observed. In contrast the MS showed a rate of GABA accumulation similar as in the OVX rats. Local administration in the MPN of 20 micrograms GVG, or GABA-A receptor stimulation by muscimol (50 ng), prior to the increase in plasma LH levels, prevented the occurrence of the estradiol-induced LH surge. The effect of muscimol was reversed by bicuculline (30 ng), a GABA-A receptor antagonist. Bicuculline in low doses lacked effect by itself. In conclusion, these results strongly suggest that a decreased GABAergic activity in MPN, MBH and LC precedes the estradiol-evoked LH surges in ovariectomized rats. Moreover, that in septal nuclei, a low GABAergic activity takes place well before the occurrence of plasma LH increase.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of infusing gamma-aminobutyric acid receptor agonists and antagonists into the medial preoptic area and ventromedial hypothalamus on prolactin secretion in male sheep

We investigated the effects of gamma-aminobutyric acid (GABA) agonists muscimol and baclofen (GABA(A) and GABA(B) agonists, respectively) and antagonists bicuculline methiodide (BMI, GABA(A) antagonist) or 2-hydroxysaclofen (SAC) and CGP 55845A (GABA(B) antagonists) on prolactin (PRL) secretion in castrated rams. The drugs were applied by microdialysis into either the medial preoptic area (mPOA) or ventromedial hypothalamus (VMH). Dialysis of baclofen into the mPOA significantly increased mean PRL (p < 0.05), whereas SAC caused a small, but significant decrease (p < 0.01). Dialysis of either muscimol or BMI into the mPOA had no effect on prolactin. In the VMH, baclofen significantly increased (p < 0.01) mean PRL but SAC and CGP 55845A were ineffective, whereas dialysis of either muscimol or BMI increased mean prolactin (p < 0.01). These results show that infusion into the mPOA of drugs that affect GABA(B) receptor alter PRL release, whereas infusion of a GABA(A) agonists and antagonist was without effect on PRL release. In contrast, infusion of both GABA(A) and GABA(B) agonists and a GABA(A) antagonist into the VMH altered PRL secretion. This suggest that GABAergic neurons in both regions participate in regulating PRL secretion, but by different receptor systems.     

Effects of ovarian hormones on the concentrations of immunoreactive neuropeptide Y in discrete brain regions of the female rat: correlation with serum luteinizing hormone (LH) and median eminence LH-releasing hormone

Recent pharmacological studies have suggested a role for neuropeptide Y (NPY) in control of LH secretion. The present study examined the effects of estradiol benzoate (EB) given alone or in combination with progesterone (P), on the concentrations of immunoreactive NPY in microdissected nuclei of the rat brain, in association with changes in LHRH concentrations and LH release. Forty-eight hours after the administration of EB to ovariectomized rats, serum LH was significantly reduced. Concentrations of NPY, measured by a sensitive and specific RIA, were also reduced in the median eminence, arcuate nucleus, and interstitial nucleus of the stria terminalis. As expected, administration of P to these EB-primed rats induced a sequential rise and fall of LHRH in the median eminence, followed by a marked LH surge in the afternoon. Interestingly, NPY concentrations in the median eminence also increased and then decreased after P, with a time course similar to that shown by LHRH. In the arcuate nucleus and interstitial nucleus of the stria terminalis, P treatment did not affect NPY concentrations further. Sequential EB plus P treatment significantly reduced NPY levels in the medial preoptic nucleus compared to oil vehicle controls. Levels of immunoreactive NPY in the ventromedial nucleus and periventricular nucleus were largely unaffected by ovarian hormone treatments. These results indicate that ovarian steroids, which modulate LH secretion, affect NPY concentrations in those specific areas of the brain which are known to be innervated by the LHRH neurons. These observations support the hypothesis that NPY may participate in the neural regulation of LHRH and LH secretion.     

Progesterone enhances the surge of luteinizing hormone by increasing the activation of luteinizing hormone-releasing hormone neurons

The ability of progesterone (P) to enhance the surge of LH in the rat is well documented, but whether its primary site of action is on the pituitary or brain is unclear. To determine whether P can alter the activation of LHRH neurons, 1) intact female rats were treated with the P antagonist RU486 (5 mg) at 1230 h on proestrus and killed at specified times during the afternoon and evening for comparison of plasma LH levels and cFos expression in LHRH neurons with untreated proestrous rats. RU486 treatment greatly reduced both the magnitude of the LH surge and the degree of cFos induction (numbers of cells expressing cFos and intensity of cFos staining) in LHRH neurons during proestrus. 2) Ovariectomized (OVX) rats were primed with estradiol benzoate (EB, 1 microgram) and then were treated with EB alone (50 microgram) or EB plus P (5 mg). Treatment with EB without P resulted in significantly lower peak LH levels and a reduced cFos response in LHRH neurons than the EB-P treated rats. These data suggest that the actions of P eventuate in an enhanced activation of LHRH neurons that may be responsible for the increased magnitude of the LH surge.     

Neuropeptide Y potentiates luteinizing hormone (LH)-releasing hormone-induced LH secretion only under conditions leading to preovulatory LH surges.

We recently demonstrated that neuropeptide Y (NPY) potentiates the ability of pulsatile LHRH infusions to restore LH surges in pentobarbital (PB)-blocked, proestrous rats. In the present study we determined if specific endocrine conditions are necessary for the expression of these direct pituitary effects of NPY. Facilitatory actions of NPY were examined in the absence of gonadal feedback [ovariectomy (OVX)], in the presence of negative gonadal feedback (metestrus), after estrogen priming of the pituitary gland [OVX plus 30 micrograms estradiol benzoate (EB) 2 days before experiments], and after treatments which evoke preovulatory-like LH surges (OVX plus EB and 5 mg progesterone or P the morning of experiments). Rats received jugular catheter implants the day before experiments. On the day of experiments, hourly blood samples were taken from 1100-2100 h. At 1330 h, rats received injections of PB to block endogenous LHRH release, or saline. Every 30 min from 1400-1800 h, PB-treated rats received iv pulses of LHRH (15 ng/pulse) or saline, along with concurrent pulses of NPY (1 or 5 micrograms/pulse) or saline. Plasma samples were analyzed by LH RIA. In all cases, pulsatile administration of 15 ng LHRH resulted in plasma LH levels that were significantly elevated above saline-treated, PB-blocked controls. Only in the case of EB+P-treated rats did coadministration of 5 micrograms NPY along with LHRH significantly enhance LHRH-stimulated LH secretion (P < 0.001). NPY had no effect on LHRH-stimulated LH secretion in OVX, OVX + EB-treated, or metestrous rats. Pulsatile administration of either dose of NPY alone did not stimulate LH release in any of the four groups examined. These results demonstrate that the facilitatory effects of NPY on LHRH-stimulated LH secretion can be manifest only under the endocrine conditions required to produce full, preovulatory-like LH surges, i.e. after estrogen and P treatment.     

gamma-Aminobutyric acid-opioid interactions in the regulation of gonadotropin secretion in the immature female rat.

Previous studies in the male rat have demonstrated that GABA acting via GABAB receptors can abolish naloxone-induced LH secretion. The purpose of the present study was to determine if an analogous situation exists in the female rat. Naloxone administered to ovariectomized immature rats had no effect on LH release. In contrast, naloxone potently stimulated LH release in estrogen-primed ovariectomized immature rats. Elevation of endogenous brain levels of GABA by administering amino-oxyacetic acid, an inhibitor of GABA catabolism, prevented the naloxone-stimulated release of LH. This effect appeared to be both GABAA and GABAB receptor mediated, since exogenous administration of either muscimol (GABAA agonist) or baclofen (GABAB agonist) prevented the naloxone-induced release of LH. Neither GABA agonist had any effect on LHRH-stimulated LH release in vivo, suggesting that their effect was specific and achieved at the level of the CNS. In contrast to its inhibitory effect on naloxone-stimulated LH and FSH release, muscimol increased basal LH and FSH release in vivo and from hemipituitaries incubated in vitro, while having no effect on LHRH release from mediobasal hypothalamic and preoptic area fragments in vitro. Thus, under conditions of basal LH release, activation of GABAA receptors in the anterior pituitary can actually lead to enhanced LH secretion. Finally, naloxone-stimulated LH release was found to be inhibited by the alpha 1- and alpha 2-adrenergic blockers, prazosin and yohimbine, suggesting that naloxone-stimulated LH release is mediated via catecholamine neurotransmission involving alpha 1- and alpha 2-adrenergic receptor activation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Progesterone-induced changes in hypothalamic luteinizing hormone-releasing hormone and catecholamines: differential effects of pentobarbital

The effects of pentobarbital (Pnt) treatment on the progesterone (P)-induced afternoon increase in the medial basal hypothalamic (MBH) LHRH and serum LH and FSH levels in ovariectomized estradiol benzoate-primed rats were studied. Pnt injection before P blocked the afternoon rise in serum gonadotropins but failed to alter the increase in the MBH LHRH levels. Moreover, when Pnt was injected 150 min after P, the MBH LHRH content continued to rise to levels 25-37% above those seen in control rats. Analyses of LHRH concentrations in discrete hypothalamic nuclei revealed that the Pnt-induced accumulation was confined mainly to the median eminence, with a small increase in the suprachiasmatic nuclei region. P administration increased the MBH norepinephrine activity and concurrently decreased dopamine activity. Pnt was ineffective in suppressing the MBH LHRH response in these rats, but drastically reduced norepinephrine and accelerated dopamine turnovers in the MBH. These studies show 1) no definitive cause and effect relationship of the increments in MBH LHRH either with LH release (or LHRH release) or with changes in hypothalamic catecholamines induced by P treatment, and 2) that the striking rise in the MBH LHRH levels in estradiol benzoate-primed rats may represent formation of new immunoreactive LHRH predominantly in the median eminence region.     

Interleukin-1 inhibits the ovarian steroid-induced luteinizing hormone surge and release of hypothalamic luteinizing hormone-releasing hormone in rats

Interleukin-1 (IL-1), a polypeptide cytokine secreted by activated macrophages, has been postulated as a chemical messenger between the immune and endocrine systems. IL-1-immunopositive neurons and fibers have been visualized in the human and rat hypothalamus, and IL-1 receptors are present in the rat brain. We have examined the effects of human recombinant IL-1 (alpha- and beta-subtypes) on LH release in vivo and hypothalamic LHRH release in vitro. Ovariectomized rats were primed with estradiol benzoate, and progesterone was injected 48 h later to elicit a LH surge in the afternoon. IL-1 alpha and IL-1 beta were injected either intracerebroventricularly (icv) via a preimplanted cannula in the third ventricle of the brain or iv. Systemic injection of IL-1 alpha or IL-1 beta (58.8 pmol at 1300 and 1500 h) failed to influence the afternoon LH surge seen in saline-injected control rats. However, IL-1 beta (1.76 pmol) administered icv at 1300 and 1500 h or a single icv injection at 1300 h blocked the progesterone-induced LH surge. Similar icv injections of IL-1 alpha also significantly suppressed the afternoon LH surge compared to that in saline-injected control rats. However, IL-1 alpha was relatively less effective than the beta-subtype, since the LH surge was detected in some rats. To ascertain whether suppression of the LH surge was due to inhibition of LHRH release, the medial basal hypothalamus-preoptic area of estradiol benzoate-progesterone-treated ovariectomized rats was incubated with and without IL-1. Both IL-1 alpha and IL-1 beta, at concentrations of 0.1 nM and higher, significantly suppressed LHRH release in vitro from the medial basal hypothalamus-preoptic area. In contrast, IL-1 (10 nM) was completely ineffective in suppressing LHRH release from the microdissected median eminence. These results demonstrated an overall inhibitory effect of icv IL-1 on the LHRH-LH axis and suggest that suppression of the steroid-induced LH surge by IL-1 may primarily be due to inhibition of LHRH release at hypothalamic sites located within the blood-brain barrier.     

Role of gamma-aminobutyric acid in the zona incerta in the control of luteinizing hormone release and ovulation

gamma-Aminobutyric acid (GABA) neurones are present in the zona incerta (ZI) where other systems have been shown to influence gonadotrophin release. This report investigates the effect of GABA agents in the ZI on ovulation and luteinizing hormone (LH) release. In intact females under Saffan anaesthesia, bilateral stereotactic injections into the ZI of two GABA transaminase inhibitors [amino(oxy)acetic acid and gamma-acetylene GABA] on the morning of pro-estrus or two GABA agonists on the afternoon of pro-estrus inhibited ovulation. The selective GABA B agonist baclofen was effective at 0.05 nM; muscimol, a mixed GABA A and B agonist, was 50-fold less potent, while the selective GABA A agonist isoguvacine had no effect at 500 nM. Administration of baclofen at 0.05 and 5 nM into the ZI significantly reduced plasma LH concentration in untreated ovariectomized rats and also prevented the rise in LH normally induced in ovariectomised rats primed with 5 micrograms oestradiol benzoate (OB) plus 0.5 mg progesterone. In ovariectomised rats primed with 5 micrograms OB alone, administration of the selective GABA A antagonist bicuculline (200 and 260 pg/side) had no effect on plasma LH, while the GABA B antagonist phaclofen (10 pg/side) stimulated a rise in plasma LH, 40 and 60 min after injection. These results indicate that GABA activity in the ZI exerts an inhibitory effect on LH release and ovulation and this is preferentially exerted via GABA B receptors.     

Effects of opiate antagonists on serotonin turnover and on luteinizing hormone and prolactin secretion in estrogen- or morphine-treated rats

Previous findings indicate that estradiol benzoate (EB) acutely activates serotonergic projections in the medial preoptic area that are stimulatory to prolactin (PRL) secretion. Because opioid agonists also stimulate both PRL release and serotonin (5-HT) turnover, the present experiments tested whether endogenous opioid neurons may mediate estrogen feedback effects. In experiment 1, ovariectomized rats received either 50 micrograms EB or oil vehicle, and either saline or a long-acting opiate receptor blocker, nalmetrene (10 mg/kg), simultaneously, 3 h prior to decapitation. The pargyline method was used to determine 5-HT turnover; hence members of each treatment group received either saline or pargyline (75 mg/kg i.p.) 30 min prior to decapitation. Plasma PRL and luteinizing hormone (LH) concentrations were determined by radioimmunoassays, and 5-HT concentrations from microdissected, individual brain nuclei were measured by liquid chromatography with electrochemical detection. Nalmetrene blocked both the acute elevation of PRL and the increase in 5-HT turnover in the medial preoptic nucleus and ventromedial nucleus induced by EB. Nalmetrene alone also enhanced LH release and 5-HT turnover in the bed nucleus of the stria terminalis, and these effects were prevented by EB. A second study tested whether the opioid agonist, morphine, mimics estrogen effects on PRL and preoptic 5-HT turnover. Animals received saline, morphine (10 mg/kg i.p.) and/or naloxone (5 mg/kg i.p.) 30 min prior to decapitation. Simultaneously, half in each group received pargyline as above.(ABSTRACT TRUNCATED AT 250 WORDS)     

A decline in endogenous opioid influence during the steroid-induced hypersecretion of luteinizing hormone in the rat

Studies were undertaken to evaluate the influence of endogenous opioid peptides (EOP) on the LH hypersecretion induced in ovariectomized rats by estradiol benzoate (EB) or EB plus progesterone (EBP). Naloxone (0.1-15.0 mg/kg) was injected before (1200 h) and during (1400 and 1530 h) the LH surge induced by EBP treatment and during the LH surge after EB treatment (1600 h). The opiate antagonist readily stimulated LH secretion before the LH surge in EBP-treated rats at 1200 h and during the LH surge in EB-treated rats at 1600 h, but was much less effective during the LH hypersecretion induced by EBP treatment at 1400 and 1530 h. This decline in the LH secretory response to naloxone during the EBP-induced LH surge was not due to changes in the response of pituitary to LHRH. These studies indicate that during the period of LH hypersecretion induced by the sequential administration of EB plus P, the influence of EOP neuronal systems on LH secretion is diminished. Thus, EOP neurons may play a role in the timing and magnitude of the LH surge in EBP-treated rats.     

Effects of the GABA-A receptor agonist and antagonist on the in vitro release of hypothalamic catecholamines: apparent parallelism between these effects and the LHRH secretion in adult male rats.

GABA (gamma-aminobutyric acid) has a well-known inhibitory effect on the luteinizing hormone-releasing hormone (LHRH) secretion. In order to evaluate the contribution of the catecholaminergic neurotransmitters on the inhibitory effect produced by GABA on the LHRH release, we measured in adult male rats the in vitro hypothalamic output of LHRH, epinephrine (E), norepinephrine (NE) and dopamine (DA); after the administration of, either muscimol 1 microM (GABA-A agonist), and/or 1 microM bicuculline (GABA-A antagonist). The following results were obtained: muscimol inhibited LHRH secretion, and this effect was accompanied by a decrease of NE, E and DA output. The opposite effects were observed after the addition of bicuculline, i.e, stimulation of LHRH, NE, E and DA release. In conclusion, our results show that, in the adult male rats, GABA has an inhibitory effect on the in vitro release of LHRH, acting on the GABA-A receptor. This effect on LHRH secretion might be exerted directly, or indirectly, by altering the release of either NE,E, and/or DA.     

Evidence that norepinephrine and epinephrine systems mediate the stimulatory effects of ovarian hormones on luteinizing hormone and luteinizing hormone-releasing hormone

Results from previous investigations have suggested an important role for central epinephrine (EPI) systems in mediating the stimulatory effects of ovarian hormones on LH release in ovariectomized female rats. The purpose of these experiments was 1) to test whether selective inhibition of EPI synthesis blocks the sequential accumulation and decline of LHRH concentrations in the median eminence that precedes the ovarian hormone-induced LH surge and 2) to test whether the stimulatory ovarian hormone regimen enhances the activity of EPI systems in the hypothalamus. Ovariectomized rats were treated with estradiol, followed 2 days later by progesterone. Animals were treated before progesterone administration with saline, one of the EPI synthesis inhibitors [SK&F 64139 (2,3-dichloro-tetrahydroisoquinoline HCl) or LY 78335 (dichloro-alpha-methylbenzylamine)], or the dopamine-beta-hydroxylase inhibitor FLA-63 (bis-4-methyl-1-homopiperazinyl thiocarbonyl disulfide), which inhibits NE and EPI synthesis. The catecholamine synthesis inhibitors blocked or delayed the afternoon LH surge. FLA-63 completely prevented the accumulation of LHRH in the median eminence that preceded the rise in LH release. However, selective EPI synthesis inhibition with SK&F 64139 only partially prevented this increase in LHRH. A second EPI synthesis inhibitor, LY 78335, delayed both the LH surge and the rise in LHRH. In a second experiment, the administration of estradiol and progesterone to ovariectomized rats increased the alpha-methyltyrosine-induced depletion of hypothalamic EPI, suggesting increased activity in this system during the LH surge. Further experiments localized this effect to the medial basal hypothalamus. The depletion of both NE and EPI after synthesis inhibition was also enhanced during an earlier period, approximating the time of LHRH accumulation. These results suggest that the ovarian hormones activate both NE and EPI systems to stimulate the early afternoon rise of LHRH in the median eminence and to induce the subsequent LH surge.     

Different gamma-aminobutyric acid receptor subtypes are involved in the regulation of opiate-dependent and independent luteinizing hormone-releasing hormone secretion

The neurotransmitter gamma-aminobutyric acid (GABA) appears to be involved in the control of gonadotropin secretion. These studies were conducted 1) to evaluate the effect of GABAergic drugs on in vitro LHRH secretion and 2) to characterize the role of different types of GABA receptors (the GABA-A and GABA-B subtypes) in these actions. Arcuate nuclei-median eminence fragments were incubated in vitro, and the release of LHRH, prostaglandin E2 (PGE2), arginine vasopressin, and oxytocin was measured by RIA. Both GABA and muscimol at different concentrations induced an increase in LHRH release, but did not affect the release of arginine vasopressin and oxytocin. This stimulatory effect was blocked by the specific GABA antagonist bicuculline, suggesting the involvement of GABA-A type receptors. Muscimol-stimulated LHRH release was not affected by the presence of phentolamine, suggesting that the stimulatory effect of GABA-A receptors on LHRH release is not mediated by interactions with the noradrenergic system. PGE2 has been shown to be a potent secretagogue of LHRH from the median eminence in vitro, and in this model the stimulatory effect of PGE2 was enhanced by muscimol. Baclofen, a specific GABA-B type receptor agonist, had no effect on basal LHRH release, but completely suppressed naloxone-stimulated LHRH and PGE2 secretion. The inhibitory effect of baclofen was blocked by the presence of 5-aminovalerate, a drug that has been shown to block the inhibitory effect of baclofen on NE release from noradrenergic terminals. This suggests the possibility that GABA-B receptors interacting with noradrenergic terminals may be responsible for the inhibitory effect of baclofen on naloxone stimulation. This study uncovered both stimulatory and inhibitory effects of GABA on LHRH release after activation of GABA-A or GABA-B receptors, respectively. Further, the data show possible relationships among the GABAergic, endogenous opiate peptide, and noradrenergic systems in the control of LHRH release from the hypothalamus.     

Hypothalamic-pituitary interactions during the periovulatory secretion of follicle-stimulating hormone in the rat

We investigated the importance of anterior afferents to the medial basal hypothalamus (MBH) on the increases in plasma FSH during the periovulatory period in the 4-day cyclic rat. We served the anterior connections to the MBH either at 1200 h on proestrus (before the time of onset of the normal spontaneous LH surge in plasma and the associated first phase of FSH release) or near the end of the LH surge and first phase of FSH release at 2000 h on proestrus (before the onset of the second or selective phase of FSH release). Analyses of FSH and LH in blood collected through indwelling atrial catheters or from the trunk after decapitation showed that anterior deafferentation of the MBH at 1200 h on proestrus blocked the proestrous LH surge, the elevations in plasma FSH during proestrus and estrus, and ovulation. In contrast, when brain surgery was delayed until 2000 h on proestrus, the second phase of FSH release and ovulation occurred. In rats with retrochiasmatic transections made at 1200 h, a constant rate iv infusion of LHRH from 1500-1800 h on proestrus restored the LH surge, both phases of increased plasma FSH, and ovulation. The results suggest that 1) the prevolutory LH surge and the first phase of FSH release are dependent on rostral afferents to the MBH which result in hypothalamic LHRH release and 2) the role of rostral afferents to the MBH in the second phase of FSH release is solely to result in hypothalamic LHRH release during proestrus.     

Parallel declines in Fos activation of the medial anteroventral periventricular nucleus and LHRH neurons in middle-aged rats

The middle-age decline in reproductive function is manifested by reduced LHRH release, resulting in a decreased magnitude and delay of onset of the LH surge. Earlier studies suggested that the reductions in LHRH neural activation in middle-aged rats resulted from deficits in the afferent drive to the LHRH neurons. One critical afferent to the LHRH neurons lies in the anteroventral periventricular preoptic area (AVPv) nucleus. The neurons of the medial AVPv are synchronously activated to express Fos with LHRH neurons at the time of an LH surge in young adult animals. The present study examined whether, in middle age, reductions in the activation of AVPv neurons accompany the reduction in Fos activation in LHRH neurons. Young (3- to 4-month-old) and middle-aged (10- to 12-month-old) spontaneously cycling and ovariectomized steroid-replaced rats were killed during peak and early descending phase of the LH surge, and their brains were examined for Fos in LHRH and AVPv neurons. Young animals had a characteristic increase in Fos expression in both LHRH and AVPv neurons. In middle-aged rats, the proportion of LHRH neurons expressing Fos at the time of an LH surge was reduced by approximately 50%, irrespective of whether surges were spontaneous or induced by exogenous steroids. A similar reduction in the number of Fos+ cells (by approximately 50%) was noted in the medial AVPv. Linear regression analysis of the relationship between the extent of Fos activation in LHRH and AVPv neurons revealed a strong positive correlation (r(2) = 0.66; P < 0.01), suggesting that changes in the AVPv's drive to LHRH neurons underlie the decrease in LHRH activity in middle age. A second series of experiments examined whether decreased input from the AVPv could account for reduced Fos activation in LHRH neurons seen in middle-aged animals. When the medial AVPv was lesioned, LHRH neurons failed to express Fos on the side ipsilateral to the lesion. Animals with lesioned medial AVPv also had significantly lower LH values than animals with an intact medial AVPv. Taken together, these data suggest that a principal deficit in middle-aged rats is the ability of the medial AVPv to stimulate LHRH neurons.