Gaba-stimulated chloride uptake during avian CNS development: modulation by neurosteroids

In the present report we studied the GABA-stimulated ³⁶Cl⁻ uptake during chick optic lobe development in order to establish the ontogenetic profile of the functional GABA_A receptor complex. A concentration-dependent stimulation of ³⁶Cl⁻ influx by GABA was demonstrated, starting at developmental stages as early as 10 days of incubation. The maximal GABA-induced ³⁶Cl⁻ uptake changed significantly during ontogeny with highest values near hatching. However, GABA potency to stimulate ion influx remained unchanged. We also examined the effect of two neurosteroids, allopregnanolone and epipregnanolone, on GABA-stimulated ³⁶Cl⁻ influx at three developmental stages (embryonic day 14, post-hatching day 1 and adult stage). Both steroids enhanced ion uptake in a concentration-dependent manner, exerting greater stimulatory effects at early developmental stages. Allopregnanolone displayed EC₅₀ values lower than epipregnanolone at all three time points and was also more potent at post-hatching stages. Analysis of the GABA concentration-effect curve disclosed that both steroid decreased EC₅₀ values for GABA stimulation while E_max levels were unaffected. In conclusion, our results showed an early appearance of the GABA-associated chloride channel together with the ability of neurosteroids to modulate GABA-gating of such channel.