PARENT SESSION
REPRODUCTIVE BEHAVIOR
Laurel
7:30 AM-10:00 AM

(404) ESTROGEN REQUIREMENTS FOR ENABLING LHRH FOS ACTIVATION FOLLOWING ADMINISTRATION OF PROGESTERONE.

Schulterbrandt, T.¹, Legan, S.², Le, W.W.¹, Hoffman, G.¹, ¹ Department of Antomy and Neurobiology, Baltimore, Maryland² Department of Physiology, Lexington, Kentucky.

ABSTRACT- It is well known that treatment with estrogen (E) is essential for sensitizing the hypothalamic pituitary gonadal axis. Earlier studies indicated that the normal paradigm for E priming stimulates progesterone receptors (PR) thereby activating LHRH neurons and releasing LH even in the absence of exogenous progesterone (P). While the degree of LHRH/LH stimulation with this paradigm is less that that observed when P is administered, we desired to maximize the difference between activation with and without P. We therefore sought a paradigm for E priming that produces sensitization with minimal LHRH/LH activation in the absence of exogenous P. Female adult Sprague Dawley rats were ovariectomized and 1 week later administered sc E, 2-5 g, on the first day, and 2, 5 or 50 g on the second. Half of the rats then received 5 mg P sc 24 h after the second E injection. At 1 hr before lights out (animals treated with P) or at the time of lights out (animals treated only with E) rats were anesthetized, perfused with saline followed by 4% paraformaldehyde + 2.5% acrolein, and the brains were processed for localization of Fos and LHRH. Reduction of E dosage to 2 g/day (for two days) was insufficient to evoke consistent negative feedback in ovariectomized rats, and failed to produce any Fos activation in LHRH neurons. Increasing the E dose to 5 g on the first day did produce negative feedback and a second 5 g dose produced only a low degree of LHRH Fos activation easily distinguished from the activation that followed 5 mg P on day 3 (which increased Fos activation to approximate that normally seen with 5 g:50 g E priming+P). Thus two 5 g doses of E appears sufficient for enabling P to enhance stimulation of the LHRH system.

KEY WORDS: LHRH, LH Surge, gonadal steroids, positive feedback