GABAA and GABAB Receptor Agonists Promote the Hypothalamic-Pituitary-Ovarian Axis in the Postfertilization Model Poecilia sphenops.

Although gamma-aminobutyric acid (GABA) is known to play a role in the regulation of several physiological functions in vertebrates, the role of GABA in regulating the reproductive axis in viviparous teleosts remains opaque. This study examined the effects of GABAA and GABAB receptor agonists, muscimol (MUSC) and baclofen (BAC), on the hypothalamic-pituitary-ovarian (HPO) axis in the black molly Poecilia sphenops. A dose-dependent upregulation of gonadotropin-releasing hormone-I (GnRH-I) mRNA expression in the preoptic hypothalamus was observed in 0.5 and 5 µg MUSC-treated fish compared to experimental controls; however, a significant upregulation of luteinizing hormone-β (LH-β) mRNA expression was observed only at the highest dose. A similar stimulatory effect on GnRH-I mRNA expression was observed in the 5, 10, or 50 µg BAC-treated group compared to experimental controls, but no significant change was observed in LH-β mRNA level. Furthermore, treatment with 0.5 or 5 µg MUSC stimulated the development of all stages of follicles in the ovary except for Stages IV or III, respectively. On the other hand, treatment with 50 µg BAC caused a significant increase in the numbers of Stages I-V follicles. These results were concomitant with a significant increase in some of the embryonic developmental stages and fecundity levels in 0.5 or 5 µg MUSC-treated or 50 µg BAC-treated fish compared to those of experimental controls. Together, these findings suggest that GABAA and GABAB receptor agonists produce a stimulatory effect on the HPO axis in a receptor-specific and concentration-dependent manner for the first time in a viviparous teleost.