Sexual stimulation induces rapid secretion of cortisol and prolactin (PRL) in stallions. Experiment 1 was designated to determine whether stallions associated location and(or) procedure with previous sexual stimulation in that location. After a control period on d 1, four stallions were exposed to an estrous mare for 5 min on d 2. On d 3, 4, 5, and 6, the same procedure was followed with no mare present. Concentrations of PRL and cortisol increased (P less than .05) after mare exposure on d 2 but did not vary (P greater than .05) on d 1, 3, 4, 5, or 6. In Exp. 2, six stallions were used to determine the short-term effects of 1) sexual stimulation, 2) acute physical exercise, 3) restraint via a twitch (twitching), 4) epinephrine administration, and 5) no stimulation on plasma concentrations of PRL and cortisol. Stallions received one treatment per day separated by 2 d of no treatment. Concentrations of cortisol increased (P less than .05) within 10 min after sexual stimulation, exercise, twitching, and epinephrine administration but not during control bleedings. Concentrations of PRL increased (P less than .05) immediately after sexual stimulation, exercise, and twitching but not after epinephrine administration or during control bleeding. In Exp. 3, the same five treatments were administered to six geldings. Concentrations of cortisol increased (P less than .05) after epinephrine administration, exercise, and twitching but not after sexual stimulation or during control bleedings. Concentrations of PRL increased (P less than .05) after exercise and sexual stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)
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.
Short-term patterns of growth hormone (GH) secretion and factors affecting it were studied in mares and stallions. In Exp. 1, hourly blood samples were collected from three mares and three stallions in summer and winter. Although GH concentrations varied in a pulsatile manner in all horses, there was no effect of sex or season (P greater than .1) on plasma GH concentrations and no indication of a diurnal pattern of GH secretion. In Exp. 2, 10-min blood samples were drawn for 8 h from 12 mares; after 6 h, porcine GH-releasing hormone (GHRH) was administered i.v. at 0, 45, 90, or 180 micrograms/mare (three mares per dose). Pulsatile secretion of GH occurred in all mares and averaged 2.4 +/- .3 peaks/6 h; amplitudes were variable and ranged from 2.6 to 74.4 ng/mL. Eight of nine mares responded within 20 min to GHRH injection, but there was no difference (P greater than .1) among the three doses tested. In Exp. 3, plasma GH concentrations in stallions increased (P less than .05) 8- to 10-fold after 5 min of acute physical exercise or exposure to an estrual mare. Restraint via a twitch (5 min) and epinephrine administration (3 mg i.v.) also increased (P less than .05) plasma GH concentrations by approximately fourfold. In Exp. 4 and 5, administration of either .4, 2, or 10 mg of thyrotropin-releasing hormone (TRH) or 100 or 500 mg of sulpiride (a dopamine receptor antagonist) increased (P less than .01) plasma prolactin concentrations but had no effect (P greater than .1) on GH concentrations during the same period of time.(ABSTRACT TRUNCATED AT 250 WORDS)
Ten lighthorse stallions were used to determine 1) whether prolactin (PRL) and cortisol responses previously observed after acute exercise in summer would occur in winter when PRL secretion is normally low, 2) whether subsequent treatment with a dopamine receptor antagonist, sulpiride, for 14 d would increase PRL secretion and response to thyrotropin-releasing hormone (TRH) and exercise, and 3) whether secretion of LH, FSH, and cortisol would be affected by sulpiride treatment. On January 11, blood samples were drawn from all stallions before and after a 5-min period of strenuous running. On January 12, blood samples were drawn before and after an i.v. injection of GnRH plus TRH. From January 13 through 26, five stallions were injected s.c. daily with 500 mg of sulpiride; the remaining five stallions received vehicle. The exercise and secretagogue regimens were repeated on January 27 and 28, respectively. Before sulpiride injection, concentrations of both cortisol and PRL increased (P less than .05) 40 to 80% in response to exercise; concentrations of LH and FSH also increased (P less than .05) approximately 5 to 10%. Sulpiride treatment resulted in (P less than .05) a six- to eightfold increase in daily PRL secretion. The PRL response to TRH increased (P less than .05) fourfold in stallions treated with sulpiride but was unchanged in control stallions. Sulpiride treatment did not affect (P greater than .05) the LH or FSH response to exogenous GnRH.(ABSTRACT TRUNCATED AT 250 WORDS)
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)
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