Reproductive stress response in male black molly <i>Poecilia sphenops</i> : Evidence for receptor specific regulation by <scp>GABA</scp>

Gamma-aminobutyric acid (GABA) is a primary inhibitory neurotransmitter involved in several physiological functions, including reproduction in vertebrates. However, its role in stress-induced reproductive modulation in viviparous teleosts remains unclear. This study examined the receptor-specific GABAergic mechanisms influencing gonadal activity in response to stress in the black molly Poecilia sphenops. Four experiments were conducted. In the first experiment, fish were subjected to chronic intermittent stress, whereas in the second experiment, the effect of gabazine (a GABA_A receptor antagonist) and CGP-35348 (a GABA_B receptor antagonist) was assessed in unstressed and stressed fish. In the third and fourth experiments, the effect of GABA_A or GABA_B antagonists and agonists in vitro was examined in testicular tissues and interrenal tissues, respectively. Exposure to stress significantly elevated brain GABA levels and enhanced GABA immunoreactivity in the preoptic-hypothalamic-pituitary regions concomitant with a substantial increase in the whole-body and head kidney cortisol levels, but suppressed testicular spermatogenesis and steroidogenesis. In the second experiment, blockade of GABA_A receptors using gabazine effectively ameliorated stress-induced reductions in the testicular-somatic index, germ cell populations, and testicular 11-ketotestosterone (11-KT) levels, whereas GABA_B receptor blockade via CGP-35348 failed to restore these parameters. In the third experiment, treatment of GABA_A and GABA_B antagonists in vitro significantly increased 11-KT secretion in testicular tissues. Furthermore, selective receptor activation with muscimol (a GABA_A receptor agonist) and baclofen (a GABA_B receptor agonist) significantly enhanced cortisol secretion from the head-kidney, while gabazine and CGP-35348 suppressed cortisol production, indicating direct peripheral GABAergic modulation. These findings demonstrate a dual role for GABA in stress regulation, involving central activation of the GABAergic neuronal system and peripheral stimulation of cortisol secretion. The study provides novel insights into the GABA-mediated suppression of testicular spermatogenesis and steroidogenesis under stress, particularly through GABA_A receptors, and suggests that pharmacological antagonism of GABA_A receptors may help mitigate stress-induced reproductive dysfunction in viviparous teleosts.