Estrogen-dependent effects of norepinephrine on hypothalamic gonadotropin-releasing hormone release in the rabbit

In the rabbit, either coitus or intraventricular administration of norepinephrine (NE) induces gonadotropin release and ovulation. It is hypothesized that ovulation induced with these manipulations involves activation of neuronal pathways that include catecholaminergic and peptidergic neurons. The aim of this study was to examine if perfusion of NE directly through the mediobasal hypothalamus (MBH) stimulates gonadotropin-releasing hormone (GnRH) release from the MBH in ovariectomized (OVEX) and estradiol-treated OVEX does (OVEX/E2). All does were fitted with push-pull (PP) cannulae directed to the MBH and subsequently subjected to PP perfusion at a flow rate of 20 microliters/min for 6 h to measure hypothalamic GnRH release. Five OVEX/E2 and 7 OVEX does received NE that was added to the PP system (intrahypothalamic NE perfusion) at the rate of 2.5 micrograms/min for 2 h during 6 h of PP perfusion. In addition, 6 OVEX/E2 does were given intrahypothalamic perfusion of homovanillic acid (HVA), a metabolite of the adrenergic system, to serve as controls. All PP samples were collected on ice at 10-min intervals, and jugular vein blood samples were obtained at 20-min intervals. The GnRH, luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were measured by specific radioimmunoassays. In OVEX/E2 does, intrahypothalamic perfusion of NE, but not HVA, stimulated a 10-fold increase in peak values of hypothalamic GnRH within 30 min, and a 3-fold increase in peak values of plasma LH within 40 min. Thereafter, both GnRH and LH levels returned to basal values by the end of the NE perfusion.(ABSTRACT TRUNCATED AT 250 WORDS)