ROLE OF STEROIDS IN ANTLER GROWTH OF RED DEER STAGS

A series of six studies were carried out in red deer stags to test hyp otheses concerning the importance of steroid control of velvet antler growth and to investigate mechanisms by which these hormones exert the ir effects. Medroxyprogesterone acetate (MPA) an LH inhibitor administ ered to stags during hard antler caused premature antler casting, redu ced subsequent antler weight and caused a reduction in the LH and test osterone responses to GnRH. In two separate studies blockade of testos terone receptors with cyproterone acetate (CPA) administered to stags, either during early velvet antler growth or during the hard antler st age, significantly reduced LH and testosterone responses to GnRH. In b oth studies antler length, but not weight, was increased by CPA treatm ent. In another study testosterone implants were used to prevent the g radual decline in plasma testosterone levels normally observed during winter. Implants were removed 3 weeks before the anticipated date of a ntler casting. The implants significantly increased plasma testosteron e levels and subsequent antler growth (expressed as a proportional inc rease compared with the previous year) compared with untreated control s. To determine whether the annual cycle of plasma testosterone respon se following GnRH stimulation was due simply to a lack of LH stimulati on, ovine LH was injected on six occasions at defined stages of the an tler cycle to red deer stags and the testosterone response measured. T he testosterone responses were low at antler casting and during velvet antler growth compared with antler cleaning and peak rut. It appears low testosterone levels are due, in part, to a loss of responsiveness by the testes to LH as well as a low level of secretion of LH during t he antler growing season. Finally synthetic ACTH was injected at the s ame defined stages of antler growth as in the previous study to determ ine whether cortisol and adrenal androgen production altered with the stage of the antler cycle. No significant differences were found in th e dehydroepiandrosterone (DHEA) response, but cortisol responses were higher from late velvet antler growth to peak nit, compared with the t imes of antler casting and early velvet growth. Overall it was conclud ed that velvet antler growth can occur without testosterone stimulatio n during the period of velvet growth, but the data reinforce the conce pt that the timing of antler growth is linked to the annual cycle of t estosterone. (C) 1995 Wiley-Liss, Inc.