title: Evidence that noradrenergic activity does not mediate the inhibition of luteinizing hormone secretion caused by vasoactive intestinal peptide.
abstract: We have recently shown that intracerebroventricular (i.c.v.) infusion of vasoactive intestinal peptide (VIP) in ovariectomized (OVX) rats causes a pronounced inhibition of pulsatile luteinizing hormone (LH) secretion. Since there is increasing evidence for an interaction between VIP- and norepinephrine-secreting systems in the brain, we tested the hypothesis that disruption of central noradrenergic activity would attenuate VIP-induced suppression of pulsatile LH release. Seven days prior to blood sampling, we administered either 6-hydroxydopamine (30 micrograms) or vehicle into the third cerebral ventricle of long-term OVX rats. This treatment reduced hypothalamic norepinephrine concentrations to 36% of control values. On the day of blood sampling, serial 300-microliter blood samples were taken from conscious, unrestrained animals at 5-min intervals for a total of 4 h. Each sample was replaced with an equal volume of a blood replacement mixture. Immediately following a control bleeding period of 2 h, VIP was administered i.c.v. to each animal for 2 h (at a dose of 1.8 nmol/h). The suppression of pulsatile LH secretion caused by VIP infusion was virtually identical in the 6-hydroxydopamine- and vehicle-pretreated groups. These results indicate that a substantial depletion of hypothalamic norepinephrine does not attenuate VIP-induced suppression of pulsatile LH secretion. This provides evidence that centrally administered VIP inhibits pulsatile LH secretion by a mechanism independent of noradrenergic transmission.
mesh_terms: D000818:Animals; D002552:Cerebral Ventricles / Q000187:drug effects / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D005260:Female; D006892:Hydroxydopamines / Q000494:pharmacology*; D007276:Injections, Intraventricular; D007986:Luteinizing Hormone / Q000097:blood / Q000378:metabolism*; D009638:Norepinephrine / Q000502:physiology*; D010052:Ovariectomy; D016627:Oxidopamine; D051381:Rats; D014660:Vasoactive Intestinal Peptide / Q000008:administration & dosage / Q000494:pharmacology*
chemical_list: D006892:Hydroxydopamines; D014660:Vasoactive Intestinal Peptide; D016627:Oxidopamine; D007986:Luteinizing Hormone; D009638:Norepinephrine