HYPERPOLARIZING GABA-A TRANSMISSION IS NECESSARY FOR POTENTIATION OF AMPA RESPONSES IN TELENCEPHALON OF DOMESTIC CHICKS, AN ANIMAL MODEL FOR ASSESSMENT OF ASD RISK CHEMICALS

Filial imprinting is impaired if exposed to ASD risk chemicals during embryonic stage, and systemic application of bumetanide (a blocker of Cl-intruder NKCC1) rescues the formation of social preferences in neonatal chicks. Accordingly, (i) whole-cell recording of the telencephalic neurons responsible for imprinting (IMM, intermediate medial mesopallium) revealed significant effects on GABA-AR mediated transmission. After embryonic exposure to valproic acid (VPA) or imidacloprid (IMI, a neonicotinoid insecticide), the reversal potential of GABA-A current in hatchlings was significantly depolarized than those in controls, up around the resting membrane potential. (ii) In these VPA/IMI exposed slices, extracellular field potential showed a hyper-excitation via AMPA-R mediated transmission, and low-frequency tetanic stimulation (5Hz, 60sec) failed to induce its potentiation. (iii) Bath-applied bumetanide acutely rescued the potentiation in VPA slices, whereas VU0463271 (a blocker of neuron-specific KCC2 Cl-extruder) blocked the potentiation in control slices. However, (iv) qPCR/RNA-seq analyses failed to detect significant VPA effects on SLC12A2 (NKCC1) and SLC12A5 (KCC2). Significant effect was also not found on the gene expression of PRKCB/WNK2 after VPA/IMI exposure, whereas effective blockade of GABA shift was found by systemic application of staurosporine/WNK463 (blockers for PKC/WNK). When repetitively activated, GABA could transiently act as a depolarizing agent via a rapid mass inflow of chloride ion, thereby assisting potentiation of the concurrent AMPA response. It remains to be elucidated how the embryonic VPA/IMI exposure led to the neonatal depolarizing shift of GABA-A transmission without changes of expression in relevant genes; lasting post-transcriptional/translational modifications might be considered.