Effects on Plasma Luteinizing Hormone and Follicle-Stimulating Hormone of Varying the Frequency and Amplitude of Gonadotropin-Releasing Hormone Pulses in Ovariectomized Ewes with Hypothalamo-Pituitary Disconnection

Abstract

The effects on luteinizing hormone (LH) and follicle-stimulating hormone (FSH) secretion of various regimens of pulsatile gonadotropin-releasing hormone (GnRH) replacement were examined in ovariectomized (OVX) ewes after hypothalamo-pituitary disconnection (HPD). Hourly pulses of 500 ng GnRH restored gonadotropin secretion in OVX-HPD sheep. Replacement beginning 2 days after HPD gave consistent responses of LH and FSH within a week. Replacement beginning 61–96 days after HPD caused more gradual re-establishment of LH and FSH secretion with LH responses appearing immediately and FSH responses appearing 2 weeks later. When hourly GnRH pulses were increased in amplitudes from 250 to 500 ng the plasma LH baseline, peak values and pulse amplitudes were increased. There was no significant change in plasma FSH levels over 10 pulses at the higher dose. Decreases in GnRH pulse frequency led to increases in LH pulse amplitude and decreases in plasma LH baseline. In contrast, immediately after a change from a 2-hourly to an hourly mode, an increase in LH baseline occurred without an immediate reduction in LH pulse amplitude. Mean plasma FSH concentrations increased when the frequency was reduced from hourly to 2-hourly or 4-hourly. However, a change from 4-hourly to hourly pulses did not reduce FSH values within 7 days. It is concluded that changes in the pattern of LH secretion observed during the ovine estrous cycle could be accounted for, in part, by changes in GnRH pulse frequency.

Season influences FSH concentration in ovariectomized Ile-de-France ewes

“…In sexually quiescent Soay rams, exogenous GnRH pulses at one pulse every 3-4 h increased the FSH concentrations (Lincoln & Short, 1980). In other studies in sheep, a reduction in the frequency of exogenous GnRH pulses from one every hour to one every 2 or 4 h also increased FSH concentrations (Clarke et al, 1984). Similarly, in female rhesus monkeys, exogenous GnRH pulses every 2 h resulted in higher FSH concentrations than did pulses every hour or every 3 h (Pohl et al, 1983).…”

“…LH is coupled with pulsatile GnRH release from the hypothalamus (Levine et al, 1982;Clarke & Cummins, 1982) and exogenous GnRH pulses influence FSH concentrations in ewes (Clarke et al, 1984). Therefore, direct effects of photoperiod on the pulse generator would be expected to influence both the frequency of LH pulses and the plasma concentrations of FSH.…”

Changes in LH pulse frequency and amplitude in intact mares during the transition into the breeding season

“…Although the results of several studies suggest that pituitary gland responsiveness is reduced by high-frequency GnRH stimulation (Wildt et al, 1981;Clarke et al, 1984) (Oxender et al, 1977;Freedman et al, 1979). The mechanisms underlying this modification of the amplitude of the periovulatory LH rise are unknown although it is unlikely, as suggested earlier (Freedman et al, 1979), that this seasonal alteration is directly modulated by daylength.…”

“…Heretofore, such a relationship has not been reported for the mare and the mechanisms underlying this phenomenon remain to be elucidated. Studies with ovariectomized ewes, in which communication between the hypothalamus and pituitary gland has been interrupted (Clarke et al, 1984), or in rhesus monkeys bearing lesions of the arcuate nucleus (Wildt et al, 1981), indicate that high-frequency exogenous GnRH stimulation is associated with low LH pulse amplitude and vice versa. Furthermore, there is an upper limit to the ability of the pituitary gland to respond to pulsatile GnRH administration, at least in the rhesus monkey (Wildt et al, 1981 (Fitzgerald et al, 1983) and in intact mares during the follicular phase of the oestrous cycle when ovarian negative feedback effects on the hypothalamus-pituitary system are absent or minimal (Ginther, 1979).…”

5-HT1 and 5-HT2 receptor activation reduces N-methyl-D-aspartate (NMDA)-stimulated LH secretion in prepubertal male and female rats

“…Alternatively, it should be considered that the selective release of both gonadotropins is related to the frequency and amplitude of LHRH pulses. Pituitary stimulation with low-frequency pulses of LHRH selectively stimulates FSH secretion (32,33); it is possible that activation of 5-HT 1A , 5-HT 2 and 5-HT 3 receptors selectively reduced the frequency of LHRH pulses.…”