We have evaluated the capacitating effect of gamma-aminobutyric acid (
GABA) in ram spermatozoa in vitro, in a chemically defined medium, by
means of the chlortetracycline (CTC) binding assay. Semen from adult A
ustralian Merino rams was collected in an artificial vagina; spermatoz
oa were washed once in modified Biggers, Whitten, and Wittingham mediu
m (m-BWW), without BSA or serum, and incubated in m-BWW alone or in m-
BWW containing GABA, GABA agonists, or antagonists for 2 h at 38.5 deg
rees C under 5% CO2 in air. Samples were taken for assessment of CTC b
inding pattern or were further incubated for 15 min in the presence of
5 mu M calcium ionophore A23187. Acrosomal exocytosis was evaluated b
y Pisum sativum agglutinin binding, Addition of GABA to the incubation
medium resulted in a concentration-dependent increase in the percenta
ge of CTC forms II and III, corresponding to mid-capacitated and capac
itated spermatozoa, respectively. The effect was marginally significan
t at 1 mu M and maximal at 20 mu M. The action of 20 mu M GABA was mim
icked by the GABA(A)-receptor agonist, muscimol, but not by the GABA(B
)-receptor agonist, baclofen, and completely blocked by the GABA(A)-re
ceptor antagonists, bicuculline and picrotoxin, which lacked effect pe
r se. In a separate set of experiments, incubation of spermatozoa with
GABA at a concentration of 1 mu M, which was insufficient to stimulat
e sperm capacitation, together with the neuroactive steroid allopregna
nolone (1 mu M) provoked a capacitating effect similar to that achieve
d by 20 mu M GABA alone. These results show that GABA has a capacitati
ng action on ram spermatozoa through a GABA(A) receptor-mediated mecha
nism.