Physiology of the Canine Anoestrus and Methods for Manipulation of Its Length

Abstract

Progression from early to late anoestrus is characterized by the appearance of a larger number of gonadotrophin-releasing hormone (GnRH) pulses with a higher amplitude, an increase in the sensitivity of the pituitary to GnRH, an increase in ovarian responsiveness to gonadotrophins, and an increase in basal plasma follicle-stimulating hormone (FSH) concentration. A period of increased luteinizing hormone (LH) pulsatility has been observed shortly before the onset of pro-oestrus. Apart from these changes in the hypothalamus-pituitary-ovary axis, the initiation of a new follicular phase in the bitch is also stimulated by dopaminergic influences other than the accompanying plasma prolactin decrease. Metergoline, a drug which in a low dosage lowers the plasma prolactin concentration via a serotonin-antagonistic pathway, does not shorten the anoestrus; while bromocriptine, in a dosage insufficient to cause a decrease in the plasma prolactin concentration, does prematurely induce a follicular phase. These observations indicate that it is not the decrease in the plasma prolactin concentration, but another dopamine-agonistic influence that plays a crucial role in the transition to a new follicular phase. The dopamine-agonist induced oestrus is associated with a rapid rise in the basal plasma FSH concentration, similar to what is observed during the physiological late anoestrus. Administration of GnRH, eCG and oestrogens may also be used to induce oestrus but with variable results. Oestrus can be prevented surgically or medically, for which purpose progestagens are the most important drugs. The mechanism is still unclear, although it has been demonstrated that with continuing medroxyprogesterone acetate (MPA) treatment the FSH response to GnRH stimulation decreases and changes occur in the pulsatile release of the gonadotrophins. In general, LH pulses coincide with a FSH pulse, but during MPA treatment, LH pulses were observed while there was such a small increase in FSH that it was not recognized as significant FSH pulse.

Endocrinologic Control of Normal Canine Ovarian Function

“…In contrast, basal LH remains at low levels between the infrequent high‐amplitude pulses. Low‐amplitude FSH pulses increase values to only 20–100% over baseline, while high‐amplitude LH pulses increase values to 300–1000% above baseline (de Gier et al 2008). The chronic suppression of LH basal concentrations and LH pulse frequency likely involves oestrogen negative feedback at both the hypothalamus and pituitary.…”

“…During the last 20 days before proestrus, mean daily LH concentrations are increased for 5–10 days owing to an increased LH pulse rate likely occurring in all cycles (Concannon et al. 1990;de Gier et al 2008). It may not appear as increased mean LH in a group of bitches when data are aligned to the next LH surge or oestrus (Olson et al 1982) owing to the variability in proestrus onset and length.…”

Anoestrus Treated with Bromocriptine 2.5 mg (Parlodel®) in Atlas Shepherd Bitch: A Case-Control Study

“…The use of certain antiprolactinics during pseudo-lactation decreases the inter-estral interval of the treated bitches [1,8,9]. The use of dopaminergic agonists in anticipation of estrus (less than 7 months after the last heat) is very disappointing [10].…”

“…The central dopaminergic effect of antiprolactinics that seems to act favorably during this heat-inducing effect. Their efficacy in the onset of estrus depends on dose, duration of treatment [1,8,9].…”

“…On the other hand, we did not observe cases of hair discoloration related to the use of bromocriptine, in contrast to cabergoline, which induces transient skin discoloration in 25% of bitches that receive this molecule for 14 to 45 days [1,9].…”