EXCITATORY GABA RESPONSES IN EMBRYONIC AND NEONATAL CORTICAL SLICES DEMONSTRATED BY GRAMICIDIN PERFORATED-PATCH RECORDINGS AND CALCIUM IMAGING

Gramicidin perforated-patch-clamp recordings in brain slices were used to obtain an accurate assessment of the developmental change in the G ABA(A) receptor reversal potential (E(GABAA)) in embryonic and early p ostnatal rat neocortical cells including neuroepithelial precursor cel ls. cortical plate neurons, and postnatal neocortical neurons. Our res ults demonstrate that there is a progressive negative shift in E(GABAA ), with the most positive values found in the youngest cortical precur sor cells. At the early stages of neocortical development, E(GABAA) is determined by the chloride (Cl-) gradient, and the internal chloride concentration ([Cl-](i)) decreases with development. E(GABAA) is posit ive to the resting potential, indicating that GABA serves to depolariz e developing neocortical cells. Consistent with this conclusion, GABA( A) receptor activation with muscimol was found to increase the interna l calcium concentration ([Ca2-](i)) in both embryonic and early postna tal neocortical cells through the activation of voltage-gated calcium channels (VGCCs). Postnatal cells exhibit spontaneous postsynaptic syn aptic currents, which are eliminated by bicuculline methiodide (BMI) b ut not glutamate receptor antagonists and reverse at the Cl- equilibri um potential. Likewise, brief spontaneous increases in [Ca2+](i), sens itive to BMI and TTX, are observed at the same ages, suggesting that e ndogenous synaptic GABA(A) receptor activation can depolarize cells an d activate VGCCs, These results suggest that GABA(A) receptor-mediated depolarization may influence early neocortical developmental events, including neurogenesis and synaptogenesis, through the activation of C a2--dependent signal transduction pathways.