The induction of oestrus in female domestic animals is used for several purposes. Regarding its use bitches, the main aims of oestrus induction are the establishment programmed mating, interruption of anoestrus longer than the normal and acquirement of high-quality oocytes for scientific purposes.

The bitch presents only one oestrus during the oestral or reproductive cycle. Therefore, bitches are classified as seasonal monoestric animals. The oestral cycle is divided in four phases in the canine specie: proestrus, oestrus, diestrus and a prolonged anoestrus, where progesterone and estrogen are usually within the basal levels. The anoestrus is an intermediary phase between two oestral cycles. Studies performed with bitches in anoestrus revealed that the production of gonadotropins do not decrease, leading to the hypothesis that the ovaries could present decreased responsiveness to these hormones. Prolactin is one of responsible for the ovarian low responsiveness. It is likely that prolactin negatively influences the hypothalamic-pituitary-ovarian axis. Dopamine is a neurotransmitter which is considered as a Prolactin Inhibitor Factor (PIF). Regarding this function, a large amount of agonists of the PIF receptor have been widely employed in the induction of oestrus in bitches. Nowadays, the most common dopamine agonists in use are:

 Bromocriptine: D₂-type dopaminergic agonist that also presents activity over GABA (γ-aminobutyric acid) receptors, serotoninergic or adrenergic. Among the main complains of its use, the absence of veterinary formulations and lower specificity when compared to dopaminergic agonists are highlighted. Vomiting, inappetence, anorexia and depression related to the lack of specificity to dopaminergic receptors may occur.

 Cabergoline: slow-releasing selective dopaminergic agonist. There are veterinary formulations of cabergoline available in some countries. Due to its specificity to the dopaminergic receptors, cabergoline produces mild or even irrelevant collateral effects.

 Metergoline: serotoninergic antagonist that present dopaminergic antagonist properties if serum concentrations reach more than 0.3 mg/kg. veterinary formulations of metergoline can be found. Patients may present nervousness, depression and/or anorexia due to its poor specificity.

The efficacy of the treatment on the reduction of the anoestrus time and return to the reproductive cycle directly depends on the period of the beginning of the treatment when dopamine agonists are used. The drugs should be administered for a longer period than the intermediary phase of the anoestrus right after the end of the oestral cycle.

The use of gonadotropins aims to mimic the integration that naturally occurs in the pituitary-ovarian axis during the proestrus, which leads to follicular development. The gonadotropins that are most used on the induction of oestrus in female canines are: FSH, eCG, swine LH, hCG and hMG. Protocols with associations of high-dose eCG and hCG (44 UI) or prolonged treatment with eCG (i.e., > 7 days), usually do not show good results on the pregnancy rate. However, protocols with 29 UI/kg/day for 5 days of eCG and single dose of 500 UI of hCG have been showing better efficiency. The protocols with FSH and LH alone are less efficient than protocols with ECG. Despite that fact, better results may be achieved if FSH and LH are associated to diethylstilbestrol (DES, 5mg, for 7 days consecutively) associated to 5 mg of LH given on the fifth day after the beginning of the proestrus.

Another oestrus induction method is based on the exogenous administration of GnRH or its agonists, which promotes the releasing of endogen gonadotropins, follicular growth and beginning of the oestral cycle. This oestrus induction method is being accomplished recently in some countries with pulsatile administration of GnRH, every 90 minutes for 6 to 12 days. An infusion pump is required in order to obtain adequate pulsatile plasmatic levels of GnRH, which can be economically unviable. Other alternatives are SC injections of de 1ng/kg every 8 hours, for 3 days consecutively. The fertility rates obtained by the use of protocols of induction of the oestrus with GnRH or its agonists ranges from 37 to 85%. Recently, a biodegradable subdermal implant containing slow-releasing GnRH agonist, for use in bitches, was released in Europe. The use of this slow-releasing device has been showing excellent results concerning the return to oestral cycle and fertility rates.

Prostaglandins administered during the diestrus can be used successfully for the shortening of the interoestrus intervals, by means of induction of the luteolysis. The treatment can decrease in 80 days the return to oestral cycle.

References

1.  Stornelli MC, Gimenez F, Tittarelli CM, Sanignone CA, De La Sota RL, Stornelli MA. Inducción de ciclos estrales em la perra: actualización bibliográfica. Analecta Veterinaria, v. 26, n. 2, p. 39-45, 2006.

2.  Ribeiro APC, Vicente WRR, Santos IW, Pires EA, Alves AE, Apparício M. Gonadotrofinas na indução do estro em cadelas. Ver. Bras. Saúde prod. Na., v.8, n.4, p. 324-334, 2007.

3.  Gobello C. Aplicaciones clínicas de los agonistas dopaminérgicos em reproducción canina. Analecta Veterinária, v. 20, n. 2, p. 31-38, 2000.