Involvement of central corticotropin-releasing factor (CRF) in suckling-induced inhibition of luteinizing hormone secretion in lactating rats.

The lack of ovulation and the inhibition of reproductive functions observed in many species during lactation is closely related to the intensity of the suckling stimulus. However, the mechanisms by which suckling inhibits hypothalamic GnRH and pituitary LH secretion in rats are still unclear. Since we recently demonstrated that suckling is a persistent stimulus to the adrenococortical system of the rat, we tested the hypothesis that suckling-induced activation of central CRF release may mediate the associated inhibition of GnRH secretion. Lactating females were ovariectomized (OVX) on day 2 of lactation, and equipped with icv guide cannula on day 2 and indwelling jugular catheters on day 5 before testing on day 7. Lactating females were separated from their pups for 24 h prior to the suckling test with the following pretreatments: 1) icv injection with artificial CSF (aCSF) or a specific CRF antagonist, alpha-helical CRF (9-41), (25 micrograms/rat, CRF-AX) 15 min prior to pup reunion or 2) iv injection of normal sheep serum (NSS) or CRF antiserum (CRF-AB) 4 h prior to pup reunion. Plasma ACTH, LH and PRL concentrations were determined prior to and at various intervals after pup reunion. After 3 h of suckling, LH and PRL responses to a bolus injection of GnRH (10 ng/rat) were measured; a bolus injection of Angiotensin II (AII, 5 micrograms/rat) was administered after 4 h to test for ACTH responses. Non-lactating females injected with GnRH and AII were used as controls.(ABSTRACT TRUNCATED AT 250 WORDS)