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The aim of the present study was to determine whether analogues of gonadotrophin-releasing hormone (GnRH) could be used to both induce an acute testosterone response and suppress anterior pituitary function in male koalas, and induce a luteal phase in female koalas. Experiment 1 characterised the steroidogenic response of male koalas to administration of 30 ?g (4.3 ?g kg?1) natural-sequence GnRH. Intra-muscular injection of natural-sequence GnRH induced the release of LH and testosterone with peak concentrations at 30 min (3.7 � 1.9 ng mL?1) and 2 h (5.4 � 0.5 ng mL?1), respectively. In Experiment 2, a single injection of the GnRH antagonist acyline (100 ?g (14.3 ?g kg?1) or 500 ?g (71.4 ?g kg?1)) did not influence the testosterone response to subsequent injections of natural-sequence GnRH. In Experiment 3, 4 ?g (~0.67 ?g kg?1) of the GnRH agonist buserelin induced a luteal phase in five female koalas based on a LH surge, secretion of progestogen, and a normal-length oestrous cycle. The findings have shown that (1) natural-sequence GnRH can be used to test gonadotroph cell function and determine the testosterone-secreting capacity of male koalas, (2) the GnRH antagonist, acyline, at the dose rates used, does not suppress the pituitary-testis axis in male koalas, and (3) the GnRH agonist, buserelin, induces a normal luteal phase in female koalas. |
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