The Siberian polecat (Mustela eversmanii) is the closest living relative to the endangered black-footed ferret (M. nigripes) and is widely used as an investigational model. In polecats, ferrets and other mammals, gonadotropin-releasing hormone (GnRH) stimulates the release of luteinizing hormone (LH) from the anterior pituitary. LH in turn elicits the release of testosterone, promoting sperm production and reproductive competence. By challenging polecats with GnRH and LH while in peak breeding condition, and again during the nonbreeding (refractory) period, hormonal responses can be characterized to determine if normal hypophyseal-gonadal function has been reached in preparation for breeding.
A reproductive group of genetically defined Siberian polecats was studied. Blood sampling was performed via brief isoflurane/O2 immobilization. After baseline serum sampling, seven male Siberian polecats in peak breeding condition (peak testicular size and firmness) received 200 ng of a GnRH analog (Cystorelin®) intramuscularly. Serum was collected 15, 60, and 90 minutes after challenge, and LH was measured by specific radioimmunoassay. One week later, these polecats were similarly challenged with 1 µg of ovine LH (NICHD, NIH, Bethesda, MD), and again blood samples were collected and assayed for LH. Identical GnRH and LH challenges were performed during the middle of the nonbreeding season (testicles soft, retracted, and greatly reduced). Wilcoxon-sign rank tests were used for statistical analysis at times 0 and 60 minutes after challenge to compare values from the same polecats at breeding and nonbreeding periods.
At peak breeding condition, baseline LH (x±SE in pg/ml) was 702±63, prior to GnRH challenge. LH increased to 2183±166 at 15 minutes, then decreased to 2027±217 at 60 minutes and to 762±165 at 90 minutes after Cystorelin® administration. Baseline LH was 636±174 before LH challenge. LH increased to 2484±302 by 15 minutes, and then dropped to 1965±313 by 60 minutes and to 647±92 at 90 minutes after LH challenge. During the nonbreeding season (refractory period), baseline LH was 672±97 before GnRH challenge, increased to 1310±296 by 15 minutes, then dropped to 762±165 at 60 minutes and 603±108 at 90 minutes. Baseline LH averaged 941±131 before LH challenge, decreased to 710±116 by 15 minutes, to 643±68 by 60 minutes, and to 592±107 by 90 minutes. Baseline LH values during breeding and nonbreeding prior to GnRH (p=0.8125) and LH (p=0.3125) challenge were not significantly different. Siberian polecats responded to GnRH by increasing circulating LH levels during both the breeding and nonbreeding period challenges; however, after 60 minutes during the nonbreeding period, LH was significantly (p=0.0078) lower than values from the same animals just prior to breeding. Likewise, 60 minutes following LH challenge, nonbreeding LH concentrations were significantly (p=0.0312) lower than just prior to breeding.
Although further evaluation of this technique is required, these data suggest that GnRH or LH response testing may provide a quick and simple assessment of reproductive readiness for potential breeders, based on a single LH measurement 60 minutes after IM agonist administration. For example, this noninvasive technique could be of value in species where few outward indications of reproductive fitness are observed, and as a valuable adjunct to other techniques such as semen evaluation. In addition, combined-response testing as evaluated here could help to pinpoint the level of reproductive dysfunction in nonreproductive males.