Intermediate zone cells express calcium-permeable AMPA receptors and establish close contact with growing axons

Recent studies have shown that cells in the intermediate zone (IZ) of the e mbryonic neocortex originate in the basal telencephalon and migrate tangent ially in the cortical wall (Anderson et al., 1997; Tamamaki et al., 1997; W ichterle et al., 1999). We had previously observed growing cortical axons c losely apposed to calbindin-positive, tangentially oriented cells in the IZ (Metin and Godement, 1996), and it has been shown that neurites in the IZ express a glutamate transporter (Furuta et al., 1997). To test if glutamate released by corticofugal growth cones could influence the tangential IZ ce lls, we characterized the glutamate receptors expressed by IZ cells using p atch-clamp techniques, histochemical labeling, and immunostaining on slices of embryonic mice forebrain. We show that tangential IZ cells express inwa rdly rectifying kainate responses, but not NMDA responses, and accumulate c obalt after AMPA receptor activation. We conclude that IZ cells express cal cium-permeable AMPA receptors. This property correlates with our observatio n that the GluR2 subunit is not expressed in the IZ. AMPA receptors are act ivated by a millimolar concentration of glutamate. To know whether this hig h level of glutamate could occur at the surface of IZ cells, we examined co ntacts made by corticofugal growth cones and calbindin-positive IZ cells us ing electron microscopy. We show vesicle-containing neurites tightly appose d to calbindin-positive IZ cells over remarkably long length. This suggests that glutamate released by growing corticofugal axons could reach high con centrations close to AMPA receptors of tangential IZ cells and efficiently activate them to control the intracellular calcium in embryonic IZ cells.