Increasing sensitivity of the pituitary to GnRH from early to late anoestrus in the beagle bitch

Abstract

The sensitivity of the pituitary to GnRH in early and late anoestrus and the indirect response of the ovary were investigated in six adult beagle bitches. Plasma concentrations of LH and oestradiol were determined after i.v. injection of graded doses of GnRH (0, 0.01, 0.1, 1, 10 and 100 micrograms kg-1). The responses were measured by the LH and oestradiol concentration profiles over time. The responses of LH and oestradiol were significantly dose dependent (P = 0.002 and P < 0.001, respectively). The responses of LH and oestradiol were significantly higher (P = 0.02 and P = 0.001, respectively) in late anoestrus than in early anoestrus. The responses of LH and the responses of oestradiol were positively correlated (r = 0.97, P = 0.001). It is concluded that during the course of anoestrus in the bitch pituitary sensitivity to GnRH increases while the ovary responds accordingly.

Basal and GnRH-induced secretion of FSH and LH in anestrous versus ovariectomized bitches

“…The measurement of plasma LH and FSH concentrations before and after the administration of a GnRH analogue permits assessment of the capacity of the pituitary to secrete gonadotropins under different reproductive conditions. In agreement with the findings of others, GnRH administration elicited a distinct release of LH [4,[7][8][9][10] and FSH [10]. The course of the GnRH-induced surges in FSH and LH in the anestrous bitches was comparable to what has been reported previously for spontaneous FSH and LH pulses in anestrous bitches [1].…”

“…Plasma LH concentration was measured by a heterologous RIA as described previously [4,21]. Briefly, a rabbit antiserum raised against ovine LH (CSU-204, kindly supplied by G.D. Niswender, Colorado State University, CO, USA), radioiodinated bLH-7981 as prepared for our bovine LH assay [22], and canine pituitary standard LER 1685-1 (a gift from Dr. L.E.…”

“…The extent of GnRH release and subsequently gonadotropin release seems to be dependent upon sensitivity of the hypothalamus to estradiol. During progression of anestrus, there is gradual reestablishment of positive feedback of estradiol on the hypothalamus [2], and GnRH secretion [3] and the responsiveness of the pituitary to GnRH increase [4]. In addition, a large amount of LH is released prior to ovulation, due to this positive feedback elicited by the rapidly rising plasma estradiol level [5].…”

“…Knowledge about the characteristics of gonadotropin secretion after intravenous administration of a GnRH analogue in the anestrous bitch is mainly limited to LH [7], the response of which is dosage-dependent [4,8]. Furthermore, an increasing dosage-dependent response of LH to stimulation with GnRH was found in the bitch during the progression of anestrus [4], but another study revealed no apparent correlation between stage of anestrus and LH response [9]. However, the used GnRH dosage in the latter study was minimal.…”

Effect of deslorelin on the induction of estrus in anestrous bitches

“…Due to this, it is not possible to be sure if the effect of the deslorelin injection was only on the induction of estrus or either on the reduction of interestrus intervals, as previously suggested for the effect of the anestrus stage on the treatment response (Van Haaften et al, 1994;Verstegen et al, 1999). However, in the present study, there is evidence that the deslorelin was able to induce proestrus, estrus and ovulation, when administered in a serial manner, in alternate days, with intramuscular application, in the used dose.…”