Gamma-aminobutyric acid (GABA) regulates several physiological, neuroendocrine, and behavioural functions, yet the specific role of its receptors in the hypothalamic-pituitary-testis (HPT) axis remains unclear in teleosts. We investigated the effects of muscimol (MUSC, a GABAA receptor agonist) and baclofen (BAC, a GABAB receptor agonist) on the HPT axis in the male black molly Poecilia sphenops. Administration of 0.05 μg MUSC on alternate days for 21 days upregulated gonadotropin-releasing hormone-I (GnRH-I) mRNA levels in the preoptic-hypothalamus, whereas they were downregulated in 0.5 or 5 μg MUSC-treated groups. The GnRH-I mRNA levels were unaltered following treatment with 5 or 10 μg BAC, but they were upregulated in 50 μg BAC-treated fish. The luteinizing hormone-β (LH-β) mRNA levels were unaltered following MUSC or BAC treatment across all doses. The numbers of all germ cell types were significantly decreased in 5 μg MUSC-treated fish concomitant with significantly lower levels of testicular 11-ketotestosterone (11-KT). On the other hand, treatment with 50 μg BAC caused a significant increase in the majority of the germ cells and testicular levels of 11-KT compared to controls. Furthermore, in vitro 11-KT levels in the testis were significantly increased in MUSC + human chorionic gonadotropin (hCG)-treated fish compared to those of the MUSC-alone group; however, a similar treatment of BAC + hCG did not attenuate the inhibitory effect of BAC-alone treatment. These findings suggest that GABAA and GABAB receptor agonists produce differential effects on the HPT axis of the black molly. In particular, MUSC suppresses the GnRH-I mRNA expression and spermatogenesis process at higher doses, whereas BAC promotes these processes at high concentration.
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