Changes in luteinizing hormone pulse frequency and prolactin levels in bitches in response to estrus induction by cabergoline-its cases where it is delayed to induce estrus

Abstract

The effect of estrus induction by cabergoline on gonadotropin and steroid hormone responses was examined in anestrous bitches. Eleven beagles were used in the study; seven were included in the estrus induction group and four were included in the spontaneous estrus group. Cabergoline was orally administered to the estrus induction group at 5 µg/kg once daily for four weeks, or until hemorrhagic discharge was detected. The inter-estrus interval in the estrus induction group was significantly shorter than the previous estrus interval. Bitches that showed proestrus within four weeks of treatment showed increased luteinizing hormone (LH) pulse frequency and, subsequently, increased estradiol (E₂) levels. Prolactin (PRL) levels declined promptly after treatment, except in one bitch that did not show proestrus during the cabergoline treatment period. There was a significant correlation between the time to proestrus induction and the reduction in PRL levels. A positive correlation was found between the LH levels two weeks after cabergoline administration and PRL reduction. This study demonstrates that an abrupt reduction in PRL is likely to be important for initiation of estrus in bitches. A reduction in PRL indirectly leads to an increase in LH pulse frequency, which regulates follicular development in bitches. However, if the period from the end of the previous estrus to the cabergoline treatment is short, it may take some time to show proestrus without increasing E₂ levels, even if the LH level increases after cabergoline administration.

Keywords: bitch; cabergoline; estrus induction; luteinizing hormone pulse; prolactin.

Fig. 1.

Endocrine changes in the spontaneous estrus and estrus induction groups. Mean ± standard error of the mean serum concentrations of follicle-stimulating hormone (FSH) (A), luteinizing hormone (LH) (B), prolactin (PRL) (C) estradiol (E₂) (D), and progesterone (P₄) (E) in the spontaneous estrus (n=4; filled circle with solid line) and estrus induction groups (n=7; open circle with dotted line) using cabergoline. Error bar not shown when smaller than symbol size. *: Significantly different between the two groups (P<0.01).

Fig. 2.

Changes in the follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin (PRL), estradiol (E₂), and progesterone (P₄) levels after the start of cabergoline treatment in the estrus induction group. Arrows indicate the start of proestrus.

Fig. 3.

Correlation between the period from the end of previous estrus to the start of cabergoline treatment, and the period between treatment to onset of proestrus in bitches (n=7).