The neurotransmitters involved in the inhibition of luteinizing hormone (LH) release induced by electrochemical stimulation (anodic d.c, 100 µA/30 s) of the medial raphe nucleus (MRn) were studied. Stimulation applied at noon on the day of proestrus blocked the preovulatory surge of LH and ovulation. This effect was prevented by pretreating the animals (15 min before stimulation) with the 5-HT antagonist, methysergide (3.5 mg/kg, i.p.) The inhibition of LH release induced by stimulation of the MRn was also suppressed by the injection of the γ-aminobutyric acid (GABA) antagonists, picrotoxin (0.8 mg/kg, i.p.) and bicuculline (6 mg/kg, i.p.). Injection of 5-HT(15 µg) into the third ventricle on the day of proestrus mimicked the effect of MRn stimulation, a response which was prevented by methysergide, picrotoxin or bicuculline. An intraventricular injection of GABA (10 µg) also inhibited the preovulatory surge of LH and ovulation, but whereas the administration of bicuculline prevented the effect of GABA, that of methysergide failed to produce any change. It is concluded that stimulation of the MRn inhibits the proestrous surge of LH by activating a serotonergic pathway and that the effect is mediated by GABAergic neurons.

Abstract. The effect of stimulation or lesions of either the dorsal or the median raphe nucleus on the proestrous surge of LH and on ovulation was studied in rats kept under constant illumination. Electrochemical stimulation (anodic DC of 100 μA during 30 sec) was applied at noon on the day of proestrus through chronically implanted electrodes. Lesions of the raphe nuclei were made by passing a cathodic current of 1 mA for 20 sec through nichrome electrodes stereotaxically implanted. Blood samples from freely behaving rats were obtained hourly through cannulae inserted into the jugular vein. Rats under constant light from diestrous day 1 or diestrous day 2 showed a delay in the onset of the LH surge of the next proestrus. Stimulation applied into the median raphe nucleus inhibited proestrous LH release and blocked ovulation, whereas stimulation of the dorsal raphe nucleus resulted in enhanced LH release in rats under constant light from diestrous day 2 but not from diestrous day 1. In turn, in rats bearing lesions in the dorsal raphe nucleus LH surges were decreased and ovulation was blocked, but rats with lesions in the median raphe nucleus exhibited enhanced LH release. It is concluded that the dorsal and the median raphe nuclei exert opposite effects on the proestrous surge of LH and on ovulation.

Intraventricular injection of 5-hydroxytryptamine (5-HT) into female rats at 11:00 h on the day of proestrus inhibited the preovulatory surge of luteinizing hormone (LH) and ovulation. A similar response was observed after the activation of the serotonergic system by stimulation of the median raphe nucleus. A diurnal rhythm of these responses was observed. In rats acclimated to a 14-h:10-h light:dark cycle the potency of 5-HT to inhibit the LH surge and ovulation was 2.06 and 2.3 times greater, respectively, when injected at 11:00 h than at 13:00 h. Also stimulation of the median raphe nucleus at 11:00 h was significantly more effective in inhibiting these parameters than stimulation at 13:00 h. Similarly, the ability of gamma-amino-butyric acid (GABA) to inhibit the preovulatory LH surge and ovulation was greater in rats injected in the morning than in the afternoon. The results of this study indicate that during proestrus the sensitivity of 5-HT and GABA to induce inhibition of preovulatory LH release and ovulation shows daily variations with maximal effect before the critical period.

Taleisnik S, Haymal B, Caligaris L. Intraventricular injection of agents that enhance cyclic adenosine monophosphate formation leads to inhibition of proestrous luteinizing hormone surge in rats. Acta Endocrinol 1993:129:273-8. The effect of increasing hypothalamic levels of 3\m=' \,5\m=' \-cyclicadenosine monophosphate (cAMP) on the preovulatory surge of luteinizing hormone (LH) and ovulation was studied in cycling rats. Animals bearing chronically implanted guiding cannulae into the third ventricle were injected with agents known to enhance the cellular levels of cAMP. Hourly blood samples from the unanesthetized, unrestrained rats were obtained between 11.00 and 1 7.00 h through a plastic cannula inserted into the jugular vein. Intraventricular injections of serotonin (7.5 mg/ml; 2 \g=m\l ) in the morning of proestrous blocked the preovulatory surge of LH and ovulation. This effect was assigned to an increased neuronal level of cAMP because it was prevented by a serum anti-cAMP. Third-ventricle injections of 2 \ g=m\ l of forskolin (0.5 mmol/l), guanosine 5\m='\-0-(3-thiotriphosphate)(2 mmol/l) or dibutyryl-cAMP (1 mmol/l) at 11.00 h on the day of proestrus mimicked the inhibitory effect of serotonin on the proestrous release of LH. It is suggested that serotonin inhibits LH surge by acting directly on LH-releasing hormone neurons and/or on neurons that provide inputs to these neurons involving cAMP as a second messenger. Neurons releasing \g=g\-aminobutyricacid (GABA) may serve as interneurons sensitive to serotonin, as well as to cAMP, inasmuch as the inhibitory effect of forskolin on the release of LH was partially blocked by the GABA antagonists, picrotoxin and bicuculline.Regulation of gonadotropin secretion depends directly on the secretion of gonadotropin-releasing hormone (GnRH). The neurons that elaborate and secrete this hormone form a continuum from the telencephalic diagonal band of Broca and the diencephalic areas of the periventricular, preoptic and anterior hypothalamic areas. These neurons constitute a final common path¬ way of which the most prominent projection is to the median eminence (1). Inputs to the GnRH neurons play a fundamental role in controlling the secretion of the products of these neurons. There is evidence for synaptic associations between GnRH neurons and nerve endings containing dopamine /Miydroxylase (2), tyrosine hydroxylase (2-4), y-aminobutyric acid decarboxylase (2, 5), neurotensin and substance P (6, 7), serotonin (8), opioids (9, 10) and neuropeptide Y (11). In addition, a variety of neurotransmitters have been demonstrated to influence the secretion of luteinizing hormone (LH) (12).The generation of cyclic adenosine 3',5'-monophosphate (cAMP) is an obligatory step for the action of many neurotransmitters after their binding to specific recep¬ tors on the surface of the target cells. The accumulation of cAMP in GnRH neurons may therefore be a possible process in the action of neurotransmitters affecting LH secretion.The aim of this study was to explore whether changes...