Reproductive neuroendocrine function in the mare as reflected in the intercavernous sinus during ovulatory, anovulatory, and transitional seasons
We hypothesized that marked reductions in secretion of luteinizing hormone (LH) during transitional and anovulatory periods can be accounted for by similar reductions in hypothalamic gonadotropin-releasing hormone (GnRH) secretion. Catheters were inserted surgically into the intercavernous sinus (ICS) of seven non-pregnant mares via the superficial facial vein during the ovulatory season (August 12-23), fall transition (November 15-30), the anovulatory season (January 19 - February 1) and spring transition (March 24 - May 12). Catheter placement was confirmed and standardized in each mare by lateral radiography. Ovarian status was monitored throughout the study by transrectal ultrasonography and serum concentrations of progesterone. During the breeding season, ICS blood samples were collected at 5-min intervals for 8 h when the dominant follicle reached approximately 35 mm and estrous behavior was observed. All mares ovulated within 5 d after sampling, except one mare who ovulated < 24 h before sampling. During the fall, mares were anovulatory (n = 5) or had a final ovulation within 5 d following intensive sampling (n = 2). Winter anovulation sampling was performed when all mares were anovulatory. During spring transition, each mare was sampled just before the second ovulation of the season. Similar to the ovulatory season, mares were sampled when the dominant, preovulatory follicle reached approximately 35 mm and estrous behavior was observed. Mean concentrations of LH were markedly higher (P < 0.01) during the breeding season than during all other seasons. Lower mean concentrations of LH in the fall transition, winter anovulation and spring transition sampling periods occurred coincident with a similar reduction (P < 0.01) in amplitude of LH pulses. Unexpectedly, neither the frequency (pulse/8 h) of LH pulses, frequency and amplitude of GnRH pulses, nor mean concentrations of GnRH differed among seasons. In addition, there were no differences observed due to season in mean concentrations of FSH or amplitude of FSH pulses. However, a small but significant (P < 0.05) reduction in the frequency of FSH pulses was observed during fall transition compared to all other seasons. In summary, contrary to accepted dogma, these results indicate that the photoperiodic initiation of seasonal anovulation in the mare is mediated at the level of the anterior pituitary, and appears to occur through a dampening of gonadotroph responsiveness to an unchanging pattern and magnitude of GnRH secretion.