Age-related changes in the pattern of luteinizing hormone release induced in female rats by male rat urine

The release of luteinizing hormone (LH) in young (3-4 months) and aged (24-25 months) ovariectomized female Sprague-Dawley rats, s.c. implanted with a 17 beta-estradiol benzoate silastic capsule, was studied in the presence and absence of stimulation by exposure to male rat urine. After taking an initial blood sample at 12:00 (reference sample), either urine, collected from young adult male rats, or distilled water was poured into the female's cage. Blood samples were then collected hourly up to 18:00 via a catheterized jugular cannula. The concentration of LH in the plasma was measured by RIA. The basal plasma LH level in young control rats was found to increase significantly at 16:00 compared with the 12:00 reference sample while no statistically significant change in plasma LH concentration occurred in old controls over the same period. Male rat urine caused a significantly earlier (at 15:00) and prolonged (from 15:00 to 18:00) elevation of plasma LH in young rats compared with young controls. In contrast, exposure of old female rats to male rat urine resulted in no marked change in plasma LH levels. These results suggest that both basal LH release and the response to pheromonal stimulation by male rat urine may be modified with increasing age in female rats.