Simultaneous Measurement of Luteinizing Hormone (LH)-Releasing Hormone, LH, and Follicle-Stimulating Hormone Release in Intact and Short-Term Castrate Rats*

Abstract

The temporal relationship between LHRH release and gonadotropin secretion as well as the effects of castration on LHRH release were investigated in conscious, freely moving male rats. LHRH release was measured in hypothalamic/median eminence perfusates, while levels of pituitary gonadotropins (LH, FSH) were determined in sequential blood samples obtained via atrial catheters. Twenty-four to 26 h before experiments, rats underwent sham surgery or castration. LHRH release in push-pull perfusates from both groups was pulsatile, and nearly all identified LH pulses (83.3%) were temporally associated with LHRH pulses. Of the fewer irregular FSH pulses that were observed, only 43.7% were temporally associated with LHRH pulses. Mean LHRH pulse amplitude and mean LHRH levels were not different in intact and castrate animals. The frequency of LHRH pulses was moderately increased in castrate rats (1.30 pulses/h) compared to that in intact animals (0.83 pulses/h), and this acceleration was accompanied by a significant increase in LH pulse frequency, pulse amplitude, and mean level. It was also noted that the number of silent LHRH pulses (those not associated with LH pulses) was dramatically reduced in castrate animals. Characteristics of gonadotropin release (pulse frequency, pulse amplitude, and mean level) were not significantly different in animals undergoing push-pull perfusion/bleeding procedures from those in rats not receiving push-pull cannula implants. We conclude from these studies that 1) LH pulses show a high concordance with LHRH pulses, providing evidence that the LHRH pulse generator operates as the neural determinant of LH pulses in male rats, 2) FSH secretion is not associated with LHRH release in an obvious and consistent manner, suggesting that LHRH/FSH relationships are not easily discerned in these animals or that a FSH-releasing factor distinct from the LHRH decapeptide may regulate FSH secretion, 3) a modest increase in LHRH pulse frequency occurs 24-30 h after castration, and 4) silent LHRH pulses occur with much greater regularity in intact than in castrate rats. The latter two observations suggest that both hypothalamic and intrapituitary sequelae of castration may be critically important in the development of postcastration increases in LH secretion and the negative feedback of gonadal steroids.

Microdialysis Measurement of In Vivo Gonadotropin-Releasing Hormone Secretion in the Goat.

“…The results were consistent with those reported in rats [7], monkeys [19] and sheep [2,3] in which GnRH secretion was examined either by the push-pull method or pituitary portal blood sampling. It is therefore suggested that the microdialysis technique could be an alternative means to the previously developed approaches.…”

“…In vivo GnRH secretion has been examined by either pituitary portal blood sampling [1][2][3] or the push-pull cannulae method [4][5][6][7]. However, these methods have practical disadvantages arising mainly from technical difficulties which have hindered them from being widely adopted.…”

Hypophyseal Portal Blood Collection for Analysis of GnRH Neurosecretion in Shiba Goats.

“…However, the pulsatile pattern of GnRH neurosecretion can not be detected in peripheral blood because of a rapid turnover and dilution of GnRH in the peripheral circulation. The temporal relationship between GnRH pulses and LH pulses has been documented so far in the monkey [15,16], sheep [2][3][4]17], rabbit [18], rat [19,20] and horse [21]. In the present study, we adapted the portal blood sampling technique, which had been originally developed for sheep, for use in goats and confirmed that GnRH pulses were associated with LH pulses in all the ovariectomized goats examined.…”

Effects of oestradiol on LH, FSH and prolactin in ovariectomized ewes

“…This may account in part for continued inhibition of FSH by oestradiol, even during a surge of LH which is presumed to be caused by increasing concentrations of GnRH (Clarke & Cummins, 1982; Levine & Duffy, 1988).…”