Progesterone and luteinizing hormone secretion following stress-induced interruption of constant estrus in aged rats.

Concentrations of progesterone and luteinizing hormone (LH) were measured during the first diestrus and subsequent proestrus after constant estrous was interrupted by restraint stress. Stress stimulated an acute increase in serum progesterone. Although stress resulted in changes in vaginal cytology characteristic of normal estrous cycles, stress-treated constant estrous (CE) rats had no midcycle (diestrous day 1) elevation of progesterone and markedly lower LH and progesterone during the afternoon of proestrous than age-matched normally cycling controls. The majority of the treated rats resumed CE without completing a cycle. In a separate experiment, progesterone injection during proestrous resulted in increased LH secretion in aging rats with normal estrous cycles. These data indicate that decreased proestrous progesterone secretion may contribute to the decrease in neuro-endocrine stimulation of proestrous gonadotropin secretion in the aged rat and may be associated with the initiation of CE.