Relationships among LH, FSH and prolactin secretion, storage and response to secretagogue and hypothalamic GNRH content in ovariectomized pony mares administered testosterone, dihydrotestosterone, estradiol, progesterone, dexamethasone or follicular fluid

Abstract

Thirty-five ovariectomized pony mares were used to study the relationships among luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL) concentrations in blood (secretion), in pituitary (storage) and in blood after secretagogue administration, as well as the content of gonadotropin releasing hormone (GnRH) in hypothalamic areas, under various conditions of steroidal and nonsteroidal treatment. Five mares each were treated daily for 21 d with vegetable shortening (controls), testosterone (T; 150 μg/kg of body weight, BW), dihydrotestosterone (DHT; 150 μg/kg BW), estradiol (E 2; 35 μg/kg BW), progesterone (P 4; 500 μg/kg BW), dexamethasone (DEX; 125 μg/kg BW) or charcoal-stripped equine follicular fluid (FF; 10 ml). Secretagogue injections (GnRH and thyrotropin releasing hormone, TRH, at 1 and 4 μg/kg of BW, respectively) were given one d prior to treatment and again after 15 d of treatment. Relative to controls, treatment with T, DHT and DEX reduced (P <. 05) LH secretion, storage and response to exogenous GnRH, whereas treatment with E 2 increased (P < .05) these same characteristics. Treatment with P 4 reduced (P < .05) only LH secretion. Treatment with T, DHT, E 2 and DEX reduced (P < .05) FSH secretion, whereas treatment with P 4 increased (P < .05) it and FF had no effect (P > .1). All treatments increased (P < .05) FSH storage, whereas only treatment with T and DHT increased (P < .05) the FSH response to exogenous GnRH. Other than a brief increase (P < .05) in PRL secretion in mares treated with E 2, secretion of PRL did not differ (P > .1) among groups. Only treatment with E 2 increased (P < .01) PRL storage, yet treatment with T or DHT (but not E 2) increased (P < .05) the PRL response to exogenous TRH. Content of GnRH in the body and pre-optic area of the hypothalamus was not affected (P > .1) by treatment, whereas treatment with T, E 2 and DEX increased (P < .1) GnRH content in the median eminence. For LH, secretion, storage and response to exogenous GnRH were all highly correlated (r ≥ .77; P < .01). For FSH, only storage and response to exogenous GnRH were related (r = .62; P < .01). PRL characteristics were not significantly related to one another. Moreover, the amount of GnRH in the median eminence was not related (P > .1) to any LH or FSH characteristic.