Migratory interneurons express functional glycine receptors during early development of the cerebral cortex

The strychnine-sensitive glycine receptor (GlyR) is a member of the ligand-gated ion channel superfamily. In the adult, the GlyR is known to mediate fast inhibitory neurotransmission in the spinal cord and in the brainstem. The GlyR has also been described in the embryonic cortex from embryonic day 19 (E19) where it could participate in developmental processes, but its presence at earlier stages has not been documented. Since other neurotransmitter systems, i.e. GABA and its receptors, are known to be present during corticogenesis, we wondered if this could also be the case for glycine and its GlyR. In this study, we analyze the presence and physiological relevance of GlyR in the early development of the cortex using in vitro and ex vivo cultures of slices, patch-clamp, two photon microscopy for time lapse and for calcium imaging, immunocytochemistry and western-blot. Electrophysiological experiments confirmed the presence of GlyR mediated currents in migrating interneurons during early stages of development (E13-E15). Using in vitro labeling of interneurons we have described the pharmacological properties of glycinergic currents present in interneurons born at E13. The concentration-response curve showed an EC50 of 69 ± 12 micro M for glycine. All these currents were fully blocked by strychnine with an IC50 of 0.10 ± 0.02 micro M. Picrotoxinin and picrotin also blocked these currents, but with different potency, remaining 20 % of the current when 10 micro M of picrotin was used. Similar glycinergic currents were also observed in ex vivo preparations from Dlx5/6-Cre EGFP transgenic animals, where it was clear that GlyR expressing cells are a subpopulation of migratory interneurons. Consequently, immunostainings directed against the alpha 2 subunit of GlyR showed that 29 ± 2 % of cortical migrating interneurons, which were mainly born in the medial ganglionic eminence (MGE) at E13, express GlyR. All this evidences shows that GlyR appears earlier than ever described during cortex development and it is composed, mainly, by alpha 2 homomeric channels. It also shows that GlyR is not homogenously expressed and it is only present in a subpopulation of migrating interneurons born at a defined space-temporal window during brain development. In search for the physiological function of GlyR, two photon time lapse analysis for cell migration and calcium imaging was performed on ex vivo slices. All these studies were complemented by gain and loss of function experiments. As it has been previously described for the GABA system, we show here that GlyR play a role acting on the migratory behavior of interneurons and its effects are linked to modulation of calcium dynamic and the activity of calcium downstream targets. More in detail molecular mechanisms were analyzed by western-blot