Temporal relations between plasma concentrations of luteinizing hormone, follicle-stimulating hormone, estradiol-17β, progesterone, prolactin, and α-melanocyte-stimulating hormone during the follicular, ovulatory, and early luteal phase in the bitch

Abstract

Compared with other domestic animals, relatively little is known about the changes in, and temporal relations between, reproductive hormones around the time of ovulation in the domestic bitch. Therefore, plasma concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), estradiol-17β, progesterone, prolactin (PRL), and α-melanocyte-stimulating hormone (α-MSH) were determined one to six times daily from the start of the follicular phase until 5 days after the estimated day of ovulation in six Beagle bitches.

In all bitches, the pre-ovulatory LH surge was accompanied by a pre-ovulatory FSH surge. A pre-ovulatory PRL or α-MSH surge was not observed. The pre-ovulatory FSH and LH surges started concomitantly in four bitches, but in two bitches the FSH surge started 12 h earlier than the LH surge. The FSH surge (110 ± 8 h) lasted significantly longer than the LH surge (36 ± 5 h). In contrast with the pre-ovulatory FSH surge, the pre-ovulatory LH surge was bifurcated in four of six bitches. The mean plasma LH concentrations before (1.9 ± 0.4 μg/L) and after (1.9 ± 0.3 μg/L) the LH surge were similar, but the mean plasma FSH concentration before the FSH surge (1.6 ± 0.3 U/L) was significantly lower than that after the FSH surge (3.1 ± 0.2 U/L). In most bitches the highest plasma estradiol-17β concentration coincided with or followed the start of the pre-ovulatory LH surge. In five of the six bitches the plasma progesterone concentration started to rise just before or concurrently with the start of the LH surge.

In conclusion, the results of this study provide evidence for the differential regulation of the secretion of LH and FSH in the bitch. In addition, the interrelationship of the plasma profiles of estradiol-17β and LH suggests a positive feedback effect of estradiol-17β on LH surge release. The start of the pre-ovulatory LH surge is associated with an increase in the plasma progesterone concentration in this species.

Introduction

The estrous cycle of the domestic dog, a mono-estrous species, is considerably longer than that of most other domestic species. Spontaneous ovulations are followed by a luteal phase that lasts about 75 days in the non-pregnant bitch, and by a non-seasonal anestrus of about 2–10 months [1], [2]. The onset of proestrus is usually characterized by a sanguineous vaginal discharge and it lasts for 3–17 days. Estrus, which is defined as the period of receptivity to mating, has a duration varying from 3 to 21 days. The follicular phase lasts until ovulation, which usually takes place within 3 days after the start of estrus behavior. The occurrence of the pre-ovulatory luteinizing hormone (LH) surge and ovulation cannot be predicted reliably by determining the start of estrus [3].

Unlike most other species, in dogs the duration of the pre-ovulatory LH surge is relatively long, ranging from 1 to 5 days [2], [3], [4]. For example, in cattle the pre-ovulatory LH surge lasts only 8 h [5]. In dogs, ovulation is assumed to occur approximately 2–3 days after the pre-ovulatory LH surge [3], [6], [7]. It is difficult to assess the exact time of ovulation in the bitch with the use of non-invasive techniques, such as ultrasonography, because it is not easy to differentiate between pre-ovulatory follicles and young cavitated corpora lutea [8], [9]. Even more invasive techniques, such as laparoscopy and histological examination of excised ovaries, do not allow exact determination of the time of ovulation [3], [6], [7], [9].

In dogs, follicle-stimulating hormone (FSH) pulses occur concomitantly with LH pulses in all stages of the estrous cycle and in anestrus [10]. The pre-ovulatory LH surge is also associated with a surge in FSH secretion [2], [11]. However, there is little detailed information about the temporal relation between the pre-ovulatory surges of LH and FSH in the bitch.

During the follicular phase, the plasma estradiol-17β concentration increases gradually and peak levels differ considerably between estrous cycles both within and between individual bitches [11], [12]. According to Wildt et al. [13], the pre-ovulatory estradiol-17β surge probably triggers the pre-ovulatory LH surge. In contrast, Concannon et al. [2], [14] have reported that the start of the pre-ovulatory LH surge was associated with a decrease in the plasma estradiol-17β concentration. Onclin et al. [4] reported that the plasma estradiol-17β concentration reached a maximum 24–48 h before the peak of the pre-ovulatory LH surge. The latter findings contrast with observations in other species, such as humans and sheep, in which estradiol is thought to exert a positive feedback effect on the release of the pre-ovulatory LH surge. In these species, the pre-ovulatory LH surge usually starts when the plasma estradiol concentration is high but not yet decreasing [15], [16], [17]. In healthy cyclic women, administration of increasing amounts of estradiol in the mid-follicular phase induces an LH surge [15]. In sheep, estradiol is required for LH surge initiation but not for LH surge maintenance [17].

In the bitch, the pre-ovulatory LH surge is associated with an increase in the plasma progesterone concentration [12]. However, the exact temporal relation between the rise in plasma progesterone concentration relative to the pre-ovulatory LH surge is uncertain. This pre-ovulatory rise in plasma progesterone level is not seen in most other domestic species but has been reported in women [15]. In women, the estrogen-induced LH surge is potentiated by progesterone [18]. In cattle, progesterone is synthesized by ovarian granulosa cells before ovulation but because it remains in the follicular fluid, there is no pre-ovulatory rise in the plasma progesterone concentration [19].

In addition to hypothalamic inhibitory and stimulatory signals [20], gonadal steroids modulate the pituitary secretion of prolactin (PRL) in the bitch [20], [21], [22], [23]. Conversely, PRL may influence the secretion of gonadotropins. In humans and sows, lactational anestrus occurs, which is ascribed to the hyperprolactinemia-induced inhibition of gonadotropin pulsatility [24], [25], [26]. In addition, a pre-ovulatory PRL surge has been reported in other species [27], [28], [29], [30], [31], but its occurrence and relevance in bitches remain to be elucidated.

Differential enzymatic processing of proopiomelanocortin yields several biologically active melanocortins, including adrenocorticotropic hormone and α-melanocyte-stimulating hormone (α-MSH). In rats, melanocortins, acting through the melanocortin 4 receptor, mediate the pre-ovulatory surges of LH and PRL [32], [33], [34]. α-MSH has also been reported to stimulate LH release in women [35]. Furthermore, α-MSH has been detected in follicular fluid of women and the circulating concentration of α-MSH is highest in the late follicular phase [36], [37]. In the bitch, information concerning the plasma α-MSH concentration during the follicular, ovulatory, and early luteal phase is lacking [38].

The aim of this study was to learn more about changes in and temporal relations between plasma concentrations of LH, FSH, estradiol-17β, progesterone, PRL, and α-MSH around the time of ovulation in the bitch.