These studies tested the interrelated hypotheses that the ovarian hormones produce their positive feedback effects on luteinizing hormone (LH) secretion through activation of noradrenergic and adrenergic systems in specific hypothalamic regions. Furthermore, the ovarian hormones may alter the activity of opioid neuropeptide and Gamma-Aminobutyric Acid (GABA) systems to produce these alterations in catecholamine transmission and gonadotropin secretion. Radioimmunoassays were utilized to determine plasma LH and median eminence LHRH, and hypothalamic catecholamine concentrations were measured by radioenzymatic assay. The first two studies tested whether epinephrine (EPI) synthesis inhibition blocks the accumulation of median eminence LHRH that precedes the ovarian hormone-induced LH surge and also to test whether the stimulatory ovarian hormone regimen enhances the activity of hypothalamic EPI systems. Ovariectomized rats were primed with estradiol (EB), followed 2 days later by progesterone (Prog.). Animals were treated before Prog, administration with saline, one of the EPI synthesis inhibitors SKF 64139 or LY 78335, or the norepinephrine (NE) synthesis inhibitor, FLA-63. The catecholamine synthesis inhibitors blocked or delayed the LH surge. FLA-63 completely prevented the accumulation of LHRH in the median eminence that preceded the rise in LH release. However, selective reduction in EPI levels with SKF 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 EB plus Prog, to ovariectomized rats increased the alpha-methyltyrosine (aMT) induced depletion of EPI in the medial basal hypothalamus (MBH). The depletion of NE after synthesis inhibition was enhanced in both the MBH and preoptic-anterior hypothalamus (POA). Experiments 3 and 4 examined a possible mechanism underlying these ovarian hormone effects on LH release and catecholamine activity. These studies tested whether the opiate antagonist, naloxone, which increases LH release, enhances the activity of NE and EPI neurons in the hypothalamus, and also tested whether morphine, an opiate agonist which decreases LH release, depresses the activity of hypothalamic NE and EPI activity. Administration of naloxone to EB-primed rats increased LH release and potentiated the depletion of NE in the POA and MBH, and enhanced the decline of EPI and dopamine (DA) in the MBH, suggesting increased catecholamine activity in these regions. Administration of the opiate agonist, morphine, to rats pretreated with EB and Prog., decreased LH and decreased the depletion of the catecholamines in the POA and MBH, suggesting reduced activity. In most cases, naloxone antagonized the inhibitory effect of morphine. Experiments 3, 6, and 7 examined the involvement of (GABA) systems in the positive feedback effects of EB and Prog, on LHRH and LH release. These studies tested 1) the effects of GABAergic drugs on the LH surge induced by EB and Prog., 2) whether GABA agonists reduce NE and EPI activity in the hypothalamus, and 3) whether a GABA agonist prevents the accumulation of median eminence LHRH induced by EB and Prog. Ovariectomized rats received the stimulatory EB plus Prog, treatment. Simultaneously with Prog., rats received either saline, the barbiturate, phenobarbital, the GABAg agonist, baclofen, the GABA^ agonist, muscimol, or either the GABA^ antagonist, bicuculline, or the putative GABAg antagonist, 5-aminovalerate. Additional experiments tested the effects of the GABA drugs on LH release in ovariectomized, hormonally untreated rats and in response to exogenous LHRH. The LH surge induced by EB+Prog. was blocked by treatment with either baclofen, muscimol, or phenobarbital. Bicuculline was ineffective in preventing the effect of baclofen and phonobarbital but partially prevented the effect of muscimol. Neither baclofen nor muscimol significantly affected LH release in hormonally untreated, ovariectomized rats or in rats receiving LHRH administration. In the results of Experiment 6, in EB plus Prog.-treated rats, baclofen and muscimol significantly reduced the concentrations of EPI and NE in the POA and MBH and prevented their decline after administration of otMT, suggesting decreased catecholamine transmission. In Experiment 7, rats were primed with the ovarian hormones and received, concurrently with Prog., either saline, or baclofen. The GABAg agonist, baclofen, blocked the LH surge and selectively increased LHRH concentrations. Experiment 8 tested 1) whether baclofen reverses the enhancement of LH release and catecholamine activity produced by naloxone, and 2) whether the opiate antagonist, nalmefene, prevents the blockade of the LH surge produced by baclofen. In the first study of Experiment 8, naloxone increased LH release and enhanced catecholamine activity in EB-primed rats. Baclofen was unable to reverse these effects. In the second study, baclofen administration to EB plus P treated rats blocked the LH surge and concomitant administration of nalmefene was unable to prevent this effect of baclofen. These results suggest that: 1) 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, 2) the ovarian hormones may produce their positive feedback effects on LH secretion by removing an inhibitory GABA or opioid neuropeptide influence on catecholamine transmission, allowing NE and EPI to stimulate LHRH, and subsequently, LH release, and 3) these modulatory actions of GABA and opiates may represent effects of two parallel, yet independent hypothalamic systems which regulate catecholamine neurotransmission and subsequently LH secretion.
Implication of D2-like dopaminergic receptors in the median eminence during the establishment of long-day inhibition of LH secretion in the ewe
