Luteinizing Hormone Releasing Hormone (LHRH) in Pituitary Stalk Blood of Rhesus Monkeys: Relationship to Level of LH Release1

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Abstract
The quantitative relationship between LHRH and LH [luteinizing hormone] secretion was investigated in the rhesus monkey [Macaca mulatta]. Pituitary stalk blood was collected using a trans-sphenoidal surgical approach and analyzed for LHRH by radioimmunoassay. Stalk blood was collected from 3 groups of monkeys: 5 animals in the mid-follicular phase of the menstrual cycle exhibiting low peripheral plasma LH levels, 7 animals undergoing preovulatory-like LH surges one of which occurred spontaneously and 6 of which were induced by estradiol benzoate treatment, and 4 ovariectomized animals exhibiting tonically elevated LH levels. LHRH was detected in the stalk plasma of all animals (> 10 pg/ml) but not in peripheral plasma. The correlation coefficient for peripheral plasma LH levels and stalk plasma LHRH levels was 0.66 (P < 0.01) when all animals were considered and 0.79 (P < 0.01) when only the follicular phase and preovulatory animals were considered. Moreover, LHRH levels in stalk plasma were significantly elevated (P < 0.05) during preovulatory-like LH surges (104 .+-. 34 pg/ml; .hivin.x .+-. SE) as compared with the follicular phase (23 .+-. 5 pg/ml). LHRH concentrations in the 4 ovariectomized animals were heterogeneous: high levels (100 and 226 pg/ml) were observed in 2 monkeys ovariectomized 18 mo. previously and low levels (19 and 32 pg/ml) were found in 2 monkeys ovariectomized 1/2 or 3 mo. previously. These findings suggest that the stimulatory effects of estrogen on LH secretion are accomplished, at least in part, by evoking an increase in hypothalamic LHRH secretion. The heterogeneous results in ovariectomized animals do not allow a conclusion about where the inhibitory effects of estrogens on LH secretion are exerted.