[GABA initiates the acrosome reaction and fertilizing ability in human sperm]

The present study was undertaken to investigate whether GABA induced the acrosome reaction (AR) and fertilizing ability, as well as its possible mode of action in human spermatozoa. Spermatozoa from fifteen health fertile men isolated by the swim-up technique were preincubated in a modified BWW with 0.35% BSA for 1-11 h under 5% CO2 in 95% air at 37 degrees C. Aliquots of spermatozoa were collected at 0, 1, 3, 5, 7, 9 and 11 h of incubation for evaluation of the AR by chlortetracycline (CTC) staining. The sperm penetration assay (SPA) was carried out by using the zona-free hamster oocyte test. Intracellular calcium ([Ca2+]i) concentrations were determined by means of fluorescent probe Frua-2/AM. GABA at 1.25 mumol/L significantly induced the AR in human spermatozoa preincubated for 3 h, with a maximal response in preincubated for 9 h, and the effect changed in a concentration-dependent manner. The maximal stimulatory effect was observed with 1.25 mumol/L GABA, and the AR then decreased markedly with further increase of GABA concentration to 10 mumol/L. Exposure of preincubated spermatozoa to GABA in combination with progesterone resulted in a higher proportion of the AR as compared with that obtained with each agonist applied alone. In addition, GABA prompted a rapid increase in interacellular [Ca2+]i. Furthermore, the AR induced by GABA was prevented by inclusion of 1 mmol/L EGTA or 100 mumol/L La3+. Also, GABA enhanced significantly the ability of spermatozoa penetrating zona-free hamster oocytes and the index of fertilization. These results indicate that GABA may be involved in the modulation of the AR and the fertilization process in capacitated human spermatozoa through a calcium mediated mechanism, thus opening up possibilities for studies of signal transductions through activation of the GABAA receptor present on the sperm surface.