Loss of I-A expression and increased excitability in postnatal rat Cajal-Retzius cells

Although an important secretory function of Cajal-Retzius (CR) cells has be en discovered recently, the precise electrical status of these cells among other layer I neurons in particular and in cortical function in general is still unclear. In this paper, early postnatal CR cells from rat neocortex w ere found to express an inactivating K current whose molecular substrate is likely to be the Kv1.4 channel. Both electrophysiological and immunocytoch emical experiments revealed that expression of this A-type current is down- regulated in vivo and virtually disappears by the end of the second postnat al week. At this time, CR cells have become capable of evoked repetitive fi ring, and their action potentials are larger and faster, yet these electric al properties still appear incompatible with a role in cortical network fun ction, as inferred from comparisons with other cortical neurons. Also at th is time, a large proportion of CR cells display spontaneous spiking activit y, which suggests the possibility of additional roles for these cells. We c onclude that the loss of A channels along with an increase in Na channel de nsity shape the changes in excitability of postnatal CR cells, in terms of both the patterns of evoked firing and the emergence of spontaneous spiking .