NEUROENDOCRINE CONTROL OF EPISODIC RELEASE OF LH FOLLOWING OVARIECTOMY

DENNIS DEAN RASMUSSEN, Purdue University

Abstract

Effects of acute and repetitive stress on episodic secretion of LH were studied in ovariectomized sheep sampled via indwelling jugular vein cannulae at 5 min intervals. Six ewes which had been acclimated to the sampling procedure on three previous days were sampled for 2 h and then were stressed by isolation in a small confinement chamber. This stress decreased the average LH level as well as the amplitude of secretory fluctuations. Eleven previously unsampled ewes were transferred directly into the confinement chamber and blood samples were collected for 4 h. This procedure was repeated on the following two days. The average LH level, amplitude of LH secretory episodes, and rhythmicity of LH peaks were low on day 1, but they increased to a maximum on day 3. In summary, these results indicate that stress of confinement in a novel environment inhibits episodic secretion of LH in ovariectomized ewes. Habituation of this stress response occurs with repeated exposure to the same stressful stimulus. Simultaneous collections of external carotid arterial and jugular venous blood samples were utilized to characterize the time course and secretory dynamics of pulsatile LH secretion in ovariectomized sheep. A continuous blood sampling system permitted collection of paired arterial and venous samples over 20 sec intervals throughout 45-60 min sampling periods. Pulsatile LH release was detected in 6 of 9 animals and was characterized by a brief period (2-6 min) when venous LH concentrations were much higher than arterial concentrations, followed by a period when arterial LH levels were greater than venous levels, indicating that LH was removed from the aterial circulation by tissues of the head. Administration of threshold dosages of synthetic gonadotropin-releasing hormone through the arterial cannula of halothane anesthetized ewes elicited in 3 of 10 cases a brief pulse of LH release whose arteriovenous profile was similar to those of spontaneous LH discharges. Therefore, individual episodes of spontaneous LH discharge in ovariectomized sheep are very brief and probably the results of correspondingly brief pulses of LH-releasing hormone from the hypothalamus. Profiles of circulating LH peaks are also modified by concentration-dependent diffusion of blood plasma LH into and out of non-circulating tissues. Techniques were developed for electronic recording of chronic multiple-unit activity from multiple brain sites in conscious sheep. Consistent relationships to LH release were not observed. The timing of the episodic secretion of LH was investigated with respect to an ultradian rest-activity cycle. Ovariectomized sheep were enclosed in a chamber where physical activity was monitored and blood for LH analysis was collected at 5 min intervals. Concurrent profiles of motor activity and plasma LH during 5 h sampling trials were each analyzed for rhythmicity by power spectral analysis. Occurrences of rhythmicity for LH and for activity were correlated, and the periods for the LH rhythms and activity rhythms were highly correlated within individual trials. Cross-spectral analysis indicated that plasma LH tended to be elevated during times of either greatest or least motor activity. These results suggest a relationship between rhythmic secretion of LH and the rest-activity cycle which may be based on a mechanism of independent entrainment to a common timing rhythm. Pharmacologic agents were utilized to manipulate endogenous brain serotonin of ovariectomized rats. Chronic inhibition of brain serotonergic mechanisms did not affect LH secretion, but acute increases in extraneuronal serotonin decrease the amplitude of episodes of LH secretion.

Degree

Ph.D.

Subject Area

Physiology

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