Six lighthorse stallions with previous sexual experience were used to determine the short-term effects of sexual stimulation (SS; 5 min exposure to an estrous mare), SS plus ejaculation (SSE), and no stimulation (control) on serum concentrations of LH, FSH, testosterone, cortisol and prolactin. Stallions received one treatment per day on d 1, 4 and 7. Treatments were assigned such that each stallion 1) received each treatment once and 2) experienced a unique sequence of treatments. Neither SS nor SSE had any consistent effects on LH or FSH concentrations. Testosterone concentrations during control bleedings increased (P less than .05) with time. This increase was suppressed (P less than .05) by both SS and SSE. Cortisol concentrations increased (P less than .05) immediately after SS and SSE. Cortisol concentrations also tended to increase during the control bleedings, but only in stallions that previously had been exposed to SS or SSE. Prolactin concentrations increased (P less than .05) immediately after SS and SSE and tended to rise during control bleedings in stallions previously exposed to SS or SSE. We conclude that 1) prolactin and cortisol were secreted rapidly in response to SS and SSE, 2) the rise in cortisol concentrations likely suppressed testosterone secretion within the next hour, and 3) stallions appeared to associate the distant sounds of other stallions with their own previous exposure to SS and SSE, resulting in a cortisol response (and perhaps a prolactin response) even in the absence of direct stimulation.

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Six pony geldings were actively immunized against GnRH conjugated to bovine serum albumin (BSA) to study 1) the relative dependency of LH and FSH storage, secretion and response to GnRH analog on GnRH bioavailability and 2) the effects of reduced GnRH bioavailability on GnRH storage in the hypothalamus. Five geldings were immunized against BSA. Geldings were immunized in December and 4, 8, 14, 20, 26 and 32 wk later. Ponies immunized against GnRH had increased (P less than .01) GnRH binding in plasma within 6 wk. By June, plasma concentrations of LH and FSH in ponies immunized against GnRH had decreased (P less than .02) by 86 and 59%, respectively, relative to ponies immunized against BSA. The LH response to an injection of GnRH analog, which did not bind to anti-GnRH antibodies, was reduced (P less than .005) by 90% in ponies immunized against GnRH relative to ponies immunized against BSA. In contrast, the FSH response to GnRH analog was similar (P greater than .1) for both groups. Immunization against GnRH reduced (P less than .05) weight of the anterior pituitary (AP) by 31%, LH content in AP by 91% and FSH content in AP by 55% relative to ponies immunized against BSA. There was no effect of GnRH immunization on prolactin characteristics or on GnRH concentrations in the median eminence, preoptic area or body of the hypothalamus.(ABSTRACT TRUNCATED AT 250 WORDS)

In this experiment we have identified and partially characterized the immunosuppressive activity of preimplantation horse conceptus-conditioned medium (HCCM). Horse conceptuses were nonsurgically flushed from mares at Days 9-10 (n = 6), 15-16 (n = 3), and 25-26 (n = 3). After incubating the conceptuses for 24 h in RPMI-1640 supplemented with 15% fetal calf serum (FCS) and 1% penicillin/streptomycin, HCCM was obtained from cultures and tested for immunosuppressive activity in lymphocyte proliferation assays. Peripheral blood lymphocytes obtained from randomly selected mares were stimulated with mitogens (pokeweed mitogen [PWM], concanavalin A [Con A], and phytohemagglutinin [PHA]) in cultures supplemented with 0%, 25%, or 50% HCCM. HCCM from all cultures suppressed lymphocyte proliferation induced by all three mitogens (p less than 0.001). After being subjected to various treatments (heating, freeze-thawing, and nitrocellulose filtration), HCCM maintained its full biological suppressor activity. Amicon microconcentrators with 10,000 and 30,000 molecular weight (MW) exclusion filter membranes were used to fractionate HCCM by molecular weight. The suppressor factor was found to be in the greater than 30,000 MW fraction. HCCM was further tested interspecifically on donkey and goat lymphocytes stimulated with PWM. HCCM did suppress proliferation of interspecific lymphocytes (p less than 0.01); however, the suppressive capacity of HCCM in caprine lymphocyte cultures was less (p less than 0.05) than that observed in equine cultures. These data support the hypothesis that the horse conceptus produces an immunoregulatory factor. This factor is extremely stabile and appears to exhibit some degree of species-specificity. The production and immunosuppressive effectiveness of such a factor may play an important role in maintaining the fetal allograft throughout gestation.

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