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  • Title: Evidence that noradrenergic activity does not mediate the inhibition of luteinizing hormone secretion caused by vasoactive intestinal peptide.
    Author: Alexander MJ, Clifton DK, Matsumoto AM, Steiner RA.
    Journal: Brain Res; 1988 Apr 05; 445(2):247-53. PubMed ID: 3130956.
    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.
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