NOREPINEPHRINE INHIBITION OF PULSATILE LH-RELEASE - RECEPTOR SPECIFICITY

Infusion of norepinephrine (NE) into the third ventricle of ovariectomized (OVX) adult rats inhibits pulsatile luteinizing hormone (LH)secretion. This study examines the effects of pretreatment with specific .alpha.- and .beta.-adrenergic blockers on the subsequent NE-induced suppression of LH release. Unanesthetized long-term OVX rats were injected ip or iv with phenoxybenzamine (.alpha.-adrenergic receptor blocker), propranolol (.beta.-adrenergic blocker), or an equal volume of saline (as controls). Approximately 1-1.5 h later, the animals were given an intraventricular (ivt) administration of NE. Infusion of acidified saline into the third ventricle of OVX rats failed to affect the pulsatile pattern of LH release characteristic of these animals. After ivt of NE, the control rats showed a significant decrease in mean blood LH level of ca. 30%. Pretreatment of OVX rats with propranolol either ivt or ip failed to affect pulsatile LH release. Subsequent ivt infusion of NE either at 1 h or 15 min after propranolol treatment also caused a 28-30% decrease in the mean LH levels and suppressed pulsatile discharge of LH, an effect that lasted for .apprx. 45-50 min. In sharp contrast to the saline and propranolol-treated groups, rats pretreated with phenoxybenzamine had sporadic or no LH pulses during the 70-min postphenoxybenzamine period and had significantly lower mean LH levels when compared with control animals (P < 0.05). Moreover, a drop in mean blood LH levels after NE infusion was not observed in the rats pretreated with phenoxybenzamine. Pretreatment of OVX rats with either prazosin (.alpha.1-adrenergic blocker) or piperoxan (.alpha.2-adrenergic antagonist) also prevented the NE-induced inhibition of LH release. These data further support the notion that hypothalamic .alpha.-adrenoceptors may be involved in both stimulation and inhibition of LH release, perhaps reflecting innervation of different excitatory and inhibitory neuronal populations.