Infusion of Neuropeptide Y into the Stalk-Median Eminence Stimulatesin VivoRelease of Luteinizing Hormone-Releasing Hormone in Gonadectomized Rhesus Monkeys*

Abstract

Studies in the rat and rabbit indicate that facilitatory effects of neuropeptide Y (NPY) as well as norepinephrine (NE) on LH and LHRH release are dependent on the presence of the ovarian steroid estrogen. However, we have previously found the NE and an alpha-1-adrenergic agonist are both stimulatory to pulsatile LHRH release in ovariectomized rhesus monkeys. In the present experiment the effects of NPY on LHRH release were examined in conscious monkeys using a push-pull perfusion method. Twelve gonadectomized monkeys (8 females and 4 males) were used. Perfusate samples from the stalk-median eminence (S-ME) were obtained through a push-pull cannula at 10-min intervals for 12 h, and the amount of LHRH in samples were determined with RIA. NPY dissolved in a modified Krebs-Ringer phosphate buffer solution at concentrations of 10(-8), 10(-7), 10(-6), and 10(-5) M was directly infused into the S-ME through the push cannula for 10 min at 90-min intervals. Vehicle was infused as a control. Since sex differences in LHRH response to NPY were not present, data from males and females were combined for analysis. NPY infusion into the S-ME stimulated LHRH release in a dose-dependent manner (P less than 0.001). The peak LHRH responses (mean +/- SEM) to NPY at different concentrations were: 10(-8) M = 2.1 +/- 0.4 pg/ml; 10(-7) M = 2.6 +/- 0.5 pg/ml; 10(-6) M = 6.5 +/- 1.1 pg/ml; 10(-5) M = 15.1 +/- 2.9 pg/ml, whereas to vehicle 0.37 +/- 0.17 pg/ml. All NPY doses tested were significantly effective as compared to vehicle (P less than 0.01). The LHRH response to 10(-6) M was greater (P less than 0.01) than that of 10(-8) M or 10(-7) M, and the response to 10(-5) M was greater (P less than 0.01) than that of all lower doses. The results indicate that NPY infusion into the S-ME elicits the release of LHRH in vivo in a dose-dependent manner in the monkey. The data further suggest that LHRH neurons and/or neuroterminals in the monkey are responsive to NPY stimulation in the absence of gonadal steroids. It is concluded that in addition to NE, NPY is an important regulator of pulsatile LHRH release in the nonhuman primate.