KCC2 Expression Promotes the Termination of Cortical Interneuron Migration in a Voltage-Sensitive Calcium-Dependent Manner

The molecular mechanisms controlling the termination of cortical interneuron migration are unknown. Here we demonstrate that prior to synaptogenesis, migrating interneurons change their responsiveness to ambient GABA from a motogenic to a stop signal. We found that during migration into the cortex, ambient GABA and glutamate initially stimulate the motility of interneurons through both GABAA and AMPA/NMDA receptor activation. Once in the cortex, up-regulation of the potassium-chloride co-transporter KCC2 is both necessary and sufficient to reduce interneuron motility through its ability to reduce membrane potential upon GABAA receptor activation which decrease the frequency of spontaneous intracellular calcium transients initiated by L-type Voltage-Sensitive Calcium Channels (VSCC) activation. Our results suggest a novel mechanism whereby migrating interneurons determine the relative density of surrounding interneurons and principal cells through their ability to sense the combined extracellular levels of ambient glutamate and GABA once GABAA receptor activation becomes hyperpolarizing