Evidence for an extended duration of GABA-mediated excitation in the developing male versus female hippocampus

Gamma-aminobutyric acid (GABA) is as an excitatory neurotransmitter during brain development. Activation of GABA(A) receptors in neonatal rat hippocampus results in chloride efflux and membrane depolarization sufficient to open voltage sensitive calcium channels. As development progresses, there is a decline in the magnitude of calcium influx subsequent to GABA(A) receptor activation and the number of cells that respond to GABA with excitation. By the second postnatal week in the rat, GABA action in the hippocampus is predominantly inhibitory. The functional consequences and endogenous regulation of developmental GABA-mediated excitation remains under-explored. Hippocampal neurons in the newborn male and female rat respond to GABA(A) receptor activation with increased intracellular calcium and are susceptible to GABA-mediated damage -- both being indicative of the excitatory nature of GABA. In the present study we observed that by postnatal day 7, only males are susceptible to GABA(A) agonist-induced damage and respond to GABA(A) agonist administration with elevated levels of intracellular calcium in cultured hippocampal neurons. By postnatal day 14, GABA(A) agonist administration was without effect on intracellular calcium in both males and females. The age-related sex difference in the impact of GABA(A) receptor activation correlates with a sex difference in chloride co-transporter expression. Males have elevated protein levels of pNKCC1 on PN0 and PN7, with no sex difference by PN14. In contrast, females displayed elevated levels of KCC2 on PN7. This converging evidence infers that sex affects the duration of GABA(A) receptor-mediated excitation during normal hippocampal development, and provides a mechanism by which the effect is mediated.