Dose–Response Studies for Pituitary and Testicular Function in Male Dogs Treated with the GnRH Superagonist, Deslorelin A Junaidi 1,2 , PE Williamson 1 , GB Martin 2 , MA Blackberry 2 , JM Cummins 1 and TE Trigg 3 1 Division of Veterinary and Biomedical Sciences, Murdoch University, Murdoch; 2 School of Animal Biology M092, The University of Western Australia, Crawley, WA; 3 Peptech Animal Health Pty Limited, Macquarie Park, NSW, Australia Contents We tested the effect of dose of GnRH superagonist on pituitary and testicular function in a study with four groups of four male dogs. The Controls received blank implants and the other three groups received implants containing 3, 6 or 12 mg deslorelin (D-Trp 6 -Pro 9 -des-Gly 10 -GnRH ethylamide). In all deslorelin-treated groups, there was initially an acute increase in plasma concentrations of LH and testosterone, followed by declines such that both hormones became unde- tectable after approximately 12 days. There was a dose– response in some of these early aspects of the hormone profiles. With respect to long-term effects of treatment, the 12-mg dose had significantly greater effects than the smaller doses for the duration of minimum testicular volume [366 ± 77, mean ± - SEM (3 mg), 472 ± 74 (6 mg), and 634 ± 59 (12 mg) days], absence of ejaculate [416 ± 88 (3 mg), 476 ± 83 (6 mg), and 644 ± 67 (12 mg) days], undetectable plasma concentrations of LH and testosterone [367 ± 64 (3 mg), 419 ± 72 (6 mg), and 607 ± 69 (12 mg) days], the delay until complete recovery of LH and testosterone secretion [394 ± 65 (3 mg), 484 ± 72 (6 mg) and 668 ± 47 (12 mg) days], and the delay until testes had regrown to normal volume [408 ± 77 (3 mg), 514 ± 74 (6 mg), 676 ± 59 (12 mg) days]. The time taken to restore full ejaculates was also longest for the 12-mg dose: 716 ± 67 (12 mg) days vs 440 ± 66 (3 mg) and 538 ± 83 (6 mg) days after implantation. There was no correlation between delay to recovery of normal ejaculate quality and body mass. We conclude that the dose–response relationship with deslorelin implants is not expressed with respect to the degree of suppression of reproduction, but on the maximum duration of suppression and thus to delay until recovery. Introduction In the search for fertility control for domestic animals, such as dogs, there is a focus on treatments that are safe, less invasive than gonadectomy and, for breeding stock, completely reversible. Contraceptives for dogs must induce infertility by reducing the output of fertile gametes and, if feasible, reduce the expression of libido and secondary sexual characters, whilst having negligi- ble side-effects. For males, in particular, very few options are currently available that satisfy all of these criteria. We have been testing a superagonist of gonadotro- phin-releasing hormone (GnRH) that markedly reduces the secretion of gonadotrophins by down-regulating GnRH receptors on the pituitary gonadotrophs (Loumaye and Catt 1983). Chronic treatment with superagonist allows gonadal activity to subside so there is a fall in the production of sex steroids and gametes and, after withdrawal, all of the effects seem to be reversed. Several superagonists of GnRH have been tested, with varying degrees of success, in males from a wide range of species, including the rat, Hawaiian monk seal, boar, stallion, ram and dog (Rivier et al. 1979; Sandow et al. 1980; Fraser 1982; Vickery et al. 1984, 1985; Lincoln et al. 1986; Boyle et al. 1991; Atkinson et al. 1993; Paramo et al. 1993; Xue et al. 1994; Inaba et al. 1996). In our hands, deslorelin (D-Trp 6 -Pro 9 -des-Gly 10 -GnRH ethylamide) has been successful in male dogs, with a 6 mg slow-release implant reducing the production of testos- terone and spermatozoa for up to a year (Trigg et al. 2001; Junaidi et al. 2003). The effect was completely reversible (Junaidi et al. 2003, Junaidi et al. 2007a). By studying the responses to endocrine challenges, we have shown that deslorelin desensitizes the pituitary gonado- trophs to GnRH and also leads to a desensitization of the Leydig cells to LH (Junaidi et al. 2007b). In the present study, we addressed one of the major questions that remain – does the degree of suppression, the duration of suppression, or the reversibility of suppression of the reproductive axis depend on the dose of deslorelin? Our hypothesis was that, for implants of the same physical dimensions, the dose of deslorelin would affect only the duration of the effect. Materials and Methods All procedures complied with the National Health and Medical Research Council of Australia Code of Animal of Practice and were approved by the Animal Ethics and Experimentation Committee of Murdoch University. Animals and treatments We used mature male dogs (age 2–5 years, body mass 15–22 kg) of mixed breeding. They were housed indoors at night, allowed outdoors every day for 2–6 h in large, shaded, sandy runs, had access to water ad libitum. Each dog was fed daily with approximately 600 g biscuits (Pedigree PAL Ò , Uncle Ben’s of Australia, Wodonga, Vic., Australia) and three times per week with approx- imately 400 g canned meat (Pedigree PAL Ò , Uncle Ben’s of Australia). Dogs were given a complete physical examination, including palpation of the testes and at least three semen evaluations. We selected 16 dogs that produced ejaculates with at least 80% sperm motility and 80% morphologically normal sperm cells were chosen. The pre-treatment characteristics of the ejaculates of the selected animals are summarized in Table 1 and were subsequently used to define ‘normal Reprod Dom Anim 44, 725–734 (2009); doi: 10.1111/j.1439-0531.2008.01060.x ISSN 0936-6768 Ó 2008 The Authors. Journal compilation Ó 2008 Blackwell Verlag