Abstract: Simple Summary: Gonadotropin-releasing hormone (GnRH) agonists bind to GnRH receptors in the anterior pituitary causing an initial release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) followed by a desensibilization of the GnRH receptors. On the first days of treatment, this results in an increase in testosterone secretion in males, while in females, it may induce an estrous cycle. This is commonly referred to "flare-up effect" and its duration depends on the active substance and the dose. After this initial stimulation, pituitary GnRH receptors are downregulated with a consequent decrease in ovarian and testicular activity. In tomcats, researchers found that testosterone levels rapidly decline 20 days after the insertion of a 4.7 mg deslorelin implant, remaining at basal levels (below 0.1 ng/mL) for a prolonged period of time. However, there is a high individual variability in the response to treatment, and the duration of efficacy varies between 6 and 24 months. As a consequence of basal testosterone levels, testicular volume decreases, and sexual behavior and penile spines disappear. All of these effects are completely reversible once the implant is not active any longer or following implant removal. Deslorelin implants have been used to produce a reversible sterilization in several species. In cats, the prolonged duration (12–15 months in tomcats and 18–22 months in queen) is often too much for cat breeders who request early implant removal. The interval between implant removal and resumption of reproductive function in cats has never been investigated. Eighteen tomcats received a 4.7 mg deslorelin implant placed in the periumbilical area and surgically removed during all seasons of the year after 3, 6, or 9 months (n = 6, 6, and 6 cats, respectively). Following implant removal, all cats received a clinical exam every two weeks, including testicular ultrasonographic measurement, observation of penile spikes, and blood collection for serum testosterone assay. Restoration of serum testosterone secretion occurred after 23 ± 6, 23 ± 6, and 22 ± 7 days in the 3-, 6-, and 9-month groups, respectively. Restoration of testicular function was confirmed by histology in 13/15 cats undergoing orchiectomy at the end of the study while the owners of the remaining two cats opted to maintain their animals intact. Removal of a 4.7 mg deslorelin implant after 3, 6, or 9 months is followed by resumption of serum testosterone secretion after about 3 weeks independent of age or season. [ABSTRACT FROM AUTHOR]
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