Effects of Angiotensin II on LHRH Release, as Measured by in vivo Microdialysis of the Anterior Pituitary Gland of Conscious Female Rats

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Abstract
The present experiments assessed the effects of central administration of angiotensin II (Ang II) on mean levels of luteinizing hormone-releasing hormone (LHRH) in the extracellular fluid of the anterior pituitary gland, monitored by in vivo microdialysis. Ovariectomized rats were tested under two conditions: (1) nonhormone-treated where Ang II infusion inhibits luteinizing hormone (LH) release, and (2) ovarian hormone-treated where Ang II stimulates LH secretion. Animals were ovariectomized and chronic guide cannulae were implanted, one into the lateral cerebral ventricle for infusion of Ang II and one directed toward the anterior pituitary gland for the insertion of the microdialysis probe. Approximately 1 week later, the dialysis probe was inserted and cemented into place. The length of the dialysis probe transected the pituitary gland from its dorsal to ventral aspects. Dialysis samples were collected at 15-min intervals. Levels of LHRH were continuously monitored in nonhormone-treated animals, prior to and during intracerebroventricular (i.c.v.) infusion of Ang II. The dialysis probe was removed at the end of the experiment. One week later, the same animals were treated with estrogen and progesterone and dialysis of the anterior pituitary gland was performed 3 days later using a protocol identical to the first dialysis sampling session. A separate group of animals was tested to confirm the effects of lateral ventricle infusion of this dose of Ang II on LH release. There were no detectable values of LHRH in dialysis samples from non-hormone-treated animals. However, following treatment of the animals with estrogen and progesterone, LHRH was detectable in 62% of the basal samples and in 83% of the samples collected during the Ang II infusion. Under these latter conditions, there was a significant increase in mean LHRH levels during the first 30 min of the i.c.v. infusion, compared to the mean of the basal values. During the second 30 min of the infusion, LHRH levels were similar to basal levels. In a separate group of animals, infusion of the same dose of Ang II stimulated both water intake and LH release. These data demonstrate that, in rats treated with estrogen and progesterone, i.c.v. infusion of Ang II stimulates the release of LHRH from the median eminence, which, in turn, acts at the anterior pituitary gland to mediate the effects of Ang II on LH secretion.

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