GnRH agonists and antagonists for non-surgical contraception of cats and dogs
As with certain immunological approaches to fertility control, gonadotropin releasing hormone (GnRH) agonists and antagonists also target this “master hormone” that controls reproductive processes in both males and females and across mammalian species. GnRH agonists and antagonists work very differently, however, from GnRH immunocontraceptive vaccines.
An effective GnRH agonist will mimic the effect of native GnRH (produced naturally by a dog’s body), binding to and causing down-regulation (suppressed response) of the GnRH receptors in the pituitary gland. GnRH must be “seen” through these receptors in the pituitary cells in order to have a hormonal effect; consequently, the continuous administration of a GnRH agonist suppresses the effect of GnRH. This, in turn, suppresses both fertility and hormonal-driven sexual behaviors. In order for a GnRH agonist to have effect, it must be consistently administered. For cats and dogs, the most feasible mechanism is a long-lasting implant, or depot application.
Suprelorin® (deslorelin acetate) is a GnRH agonist approved and available in multiple countries for minimum 6- or 12-month contraception of male dogs, depending on implant size. Suprelorin has also been studied in female dogs, as well as in cats of both sexes. It has, moreover, been used for over 15 years to prevent fertility in diverse mammalian species, including wild canids and felids, across more than 100 zoos as part of a long-running study. Males and females have both been treated with the product.
The lack of permanence of GnRH agonists may be viewed as a strength or weakness, depending on the context. A more definitive disadvantage of a GnRH agonist is that while it will ultimately suppress fertility for many months, it may cause an initial temporary flare in follicle-stimulating hormone (FSH) and luteinizing hormone (LH). In females, this may induce estrus.
GnRH antagonists prevent fertility by blocking GnRH receptors on the pituitary cells; they do not elicit the initial flare sometimes seen with GnRH agonists. While this is nearly unequivocally an advantage, other factors, such as cost, could make them less attractive than their GnRH agonist counterparts. Overall, there has been limited research to date regarding GnRH antagonists in cats and dogs, particularly relative to GnRH agonists.
For more information on immunological approaches to fertility control for dogs and cats: