Biphasic effect of GABA on rat sperm acrosome reaction: Involvement of GABA_a and GABA_b receptors

The functional relationship between GABA_A and GABA_B receptors in regulating acrosome reaction (AR) of rat spermatozoa was demonstrated by studying the differential effects of a GABA_B agonist and an antagonist on the process. AR rates were determined using the chlortetracycline staining assay. The induction of AR in rat sperm by GABA was found to be a biphasic phenomenon; i.e., AR rates increased with increasing GABA concentrations up to <5 μM and at higher concentrations of the neurotransmitter (>5 μM), there was a reduction in the AR rates. This biphasic phenomenon is apparently due to the differential interaction of the neurotransmitter with GABA receptor subtypes in a dose-dependent manner; i.e., GABA_A receptors (stimulatory) are primarily activated at low concentration of GABA, while GABA_B receptors (inhibitory) become activated at higher concentrations. This hypothesis is supported by the present findings that treatment with saclofen, a GABA_B receptor antagonist, did not influence the AR rates effected by GABA at low concentrations; while the AR rates were maintained at the maximum level at higher concentrations of GABA, resulting in the elimination of the biphasic phenomenon. Baclofen, a GABA_B receptor agonist, blocks the AR activating action of GABA at both low and high concentrations. It would appear that the induction of AR in rat sperm by GABA is regulated by the proportionality of activated GABA_A and GABA_B receptors acting as a yin-yang control.