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Five lighthorse mares were actively immunized against gonadotropin releasing hormone (GnRH) to determine the relative importance of this hypothalamic hormone in the secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Five mares immunized against the conjugation protein served as controls. Mares were initially immunized in November and received secondary immunizations 4 wk later, and then at 6-wk intervals until ovariectomy in June. All mares immunized against GnRH exhibited an increase (p<0.01) in the binding of tritiated GnRH by plasma, an indication that antibodies against this hormone had been elicited. Concentrations of LH, FSH and progesterone in weekly blood samples were lower (p<0.05) in GnRH-immunized mares than in controls after approximately 4 mo of immunization. However, the LH concentrations were affected to a greater degree than were FSH concentrations. All five control mares exhibited normal cycles of estrus and diestrus in spring, whereas no GnRH-immunized mare exhibited cyclic displays of estrus up to ovariectomy. All mares were injected intravenously with a GnRH analog (which cross-reacted < 0.1% with the anti-GnRH antibodies) in May, after all control mares had displayed normal estrous cycles, to characterize the response of LH and FSH in these mares; two days later, the mares were injected with GnRH. The LH response to the analog, which was assessed by net area under the curve, was lower (p<0.01) by approximately 99% in mares immunized against GnRH than in control mares. In contrast, the FSH response to the analog was similar for both groups. The FSH response to GnRH in mares immunized against GnRH was 85% lower than that in control mares (p<0.05), an indication that the antibodies against GnRH did prevent much of the injected GnRH from reaching the pituitary. At ovariectomy, combined ovarian weights of GnRH-immunized mares were 57% lower than those of control mares (p<0.01); numbers offollicles and corpora lutea were also lower (p<0.O1) than those of controls. It was concluded that LHand to some degree FSH secretion, and probably LHproduction, in the mare are dependent upon the bioavailability of GnRH. However, the lack of effect of GnRH-immunization on FSH response to the analog indicates that FSH stores in the pituitary (and perhaps FSH production) are relatively independent of GnRH bioavailability.

Anestrous lighthorse mares were treated in December with dihydrotestosterone (DHT; 150 μg/kg of body weight), progesterone (P; 164 μg/kg), both DHT and P (DHT+P), testosterone (T; 150 μg/kg), or vehicle (n = 4/group). Daily blood sampling was started on Day 1, and on Day 4 all mares were administered a pretreatment injection of gonadotropin-releasing hormone (GnRH) and were bled frequently to characterize the responses of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) concentrations. Treatment injections were given on Day 4 and then daily through Day 17. On Day 18, all mares were again administered GnRH and were bled frequently. Treatment of mares with DHT, P, or T increased (p<0.01) plasma concentrations of these steroids to approximately 1.5 ng/ml during the last 10 days of treatment. There was no effect (p>0.10) of treatment on LH or FSH concentrations in daily blood samples. Relative to the pretreatment GnRH injection, mares treated with T or DHT+P secreted approximately 65% more (p<0.01) FSH in response to the post-treatment GnRH injection; FSH response to the second GnRH injection was not altered (p>0.10) in control mares or in DHT- or P-treated mares. There was no effect of any steroid treatment on LH secretion after administration of GnRH (p>0.10). Averaged over all mares, approximately 94 times more FSH than LH was secreted in response to injection of GnRH. We conclude that: 1) anestrous mares respond to T treatment in a manner similar to intact cyclic mares and ovariectomized mares with regard to FSH secretion; 2) there is no effect on FSH of DHT or P treatment alone in anestrous mares; but 3) there is an interaction between DHT and P treatment on FSH secretion after exogenous GnRH.

Preantral follicles of cyclic hamsters were isolated on proestrus, estrus and diestrus I, incubated for 3 h in 1 ml TC-199 containing 1 μg ovine luteinizing hormone (LH) (NIH-S22), and the concentrations of progesterone (P), androstenedione (A) and estradiol (E2) determined by radioimmunoassay. At 0900–1000 h on proestrus (pre-LH surge) preantral follicles produced 2.4 ± 0.3 ng A/follicle per 3 h, < 100 pg E2/follicle and < 250 pg P/follicle. At the peak of the LH surge (1500–1600 h) preantral follicles produced 1.8 ± 0.2 ng P and 1.9 ± 0.1 A and < 100 pg E2/follicle. After the LH surge (1900–2000 h proestrus and 0900–1000 h estrus) preantral follicles were unable to produce A and E2but produced 4.0 ± 1.0 and 5.0 ± 1.1 ng P/follicle, respectively. By 1500–1600 h estrus, the follicles produced 8.1 ± 3.1 ng P/follicle but synthesized A (1.6 ± 0.2 ng/follicle) and E2(362 ± 98 pg/follicle). On diestrus 1 (0900–1000 h), the large preantral-early antral follicles produced 1.9 ± 0.3 ng A, 2.4 ± 0.4 ng E2and 0.7 ± 0.2 ng P/follicle. Thus, there was a shift in steroidogenesis by preantral follicles from A to P coincident with the LH surge; then, a shift from P to A to E2after the LH surge. The LH/follicle-stimulating hormone (FSH) surges were blocked by administration of 6.5 mg phenobarbital (PB)/100 g BW at 1300 h proestrus. On Day 1 of delay (0900–1000 h) these follicles produced large quantities of A (2.2 ± 0.2 ng/follicle) and small amounts of E2(273 ± 27 pg/follicle) but not P (< 250 pg/follicle). PB treatment with LH or LH/FSH induced the secretion of P; but not FSH + PB. LU + PB and/or FSH + PB induced A/E2secretion by the follicles. Incubation of granulosa cells and theca at 0900 h and 1500 h estrus (when A production reappeared) indicated that theca was the major site of P and A synthesis in the preantral follicles. These results indicate that the LH surge enhances P production and inhibits A production in theca of preantral follicles and thus possibly regulates the onset of A/E2synthesis during follicular development.