Basal testosterone concentrations after the application of a slow-release GnRH agonist implant are associated with a loss of response to buserelin, a short-term GnRH agonist, in the tom cat
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Abstract
Slow-release GnRH agonist implants are considered an effective, reversible alternative
to surgical castration in male tom cats. Individual differences exist regarding the
onset of efficacy and might be delayed in some animals. Single measurements of testosterone
(T) might result in basal concentrations also in intact male cats. Consequently, GnRH
stimulation tests are performed to measure T increase in intact animals and to differentiate
castrated from intact male cats. In this study, five tom cats were treated with a
4.7-mg deslorelin implant and GnRH stimulation tests using buserelin were performed
before treatment and at 4-week intervals afterward until Week 20. After the last test
in Week 20 all animals were castrated. Four of five animals had basal T after 4 weeks
and-in contrast to pretreatment-application of buserelin did not result in any further
T increase. In one animal, T was low after implant insertion, but not basal; however,
a GnRH stimulation test induced a slight increase of T in Week 8 and 16 only and no
response in Weeks 4, 12, and 20. Testicular volume was significantly decreased and
penile spines disappeared in all cats. Testicular histology showed mixed atrophy,
but also fully elongated spermatids in three of five male cats making infertility
questionable. Because of the loss of the stimulatory effect of short-term GnRH application
(buserelin), it can be assumed that long-term GnRH agonists also act by some mechanisms
of downregulation of pituitary GnRH receptors in the tom cat.