Differential expression of connexins during neocortical development and neuronal circuit formation.

Gap junctions are membrane channels that mediate the direct passage of ions and molecules between adjacent cells. Recent tracer coupling and optical recording studies have revealed the presence of gap junction-mediated communication between neurons during neocortical development. We have visualized gap junctions in the developing rat cerebral cortex with electron microscopy and studied the pattern of expression and cellular localization of connexins 26, 32, and 43 that take part in their formation. We found that these connexins (Cxs) are expressed differentially during development, and their patterns of expression are correlated with important developmental events such as cell proliferation, migration, and formation of cortical neuronal circuits. Specifically, we observed that the developmental profile of Cx 26 during the first 3 weeks of postnatal life matched closely the development of neuronal coupling, suggesting that coupled neurons use this gap junction protein during circuit formation in the cortex. The subsequent diminution of Cx 26 was mirrored by an increase in Cx 32 immunoreactivity, which became pronounced at the late stages of cortical maturation. In contrast, Cx 43 was localized in the cortex throughout the period of development. Its localization in radial glial fibers closely associated with migrating neurons suggests that this Cx may be involved in neuronal migration.