Epilepsy-induced decrease of L-type Ca²⁺ channel activity and coordinate regulation of subunit mRNA in single neurons of rat hippocampal 'zipper' slices

L-type voltage-sensitive Ca²⁺ channels (VSCCs) preferentially modulate several neuronal processes that are thought to be important in epileptogenesis, including the slow afterhyperpolarization (AHP), LTP, and trophic factor gene expression. However, little is yet known about the roles of L-type VSCCs in the epileptogenic process. Here, we used cell-attached patch recording techniques and single cell mRNA analyses to study L-type VSCCs in CA1 neurons from partially dissociated (zipper) hippocampal slices from entorhinally-kindled rats. L-type Ca²⁺-channel activity was reduced by > 50% at 1.5-3 months after kindling. Following recording, the same single neurons were extracted and collected for mRNA analysis using a recently developed method that does not amputate major dendritic processes. Therefore, neurons contained essentially full complements of mRNA. For each collected neuron, mRNA contents for the L-type pore-forming α_1D/Ca_v1.3-subunit and for calmodulin were then analyzed by semiquantitative kinetic RT-PCR. L-type α_1D-subunit mRNA was correlated with L-type Ca²⁺-channel activity across single cells, whereas calmodulin mRNA was not. Thus, these results appear to provide the first direct evidence at the single channel and gene expression levels that chronic expression of an identified Ca²⁺-channel type is modulated by epileptiform activity. Moreover, the present data suggest the hypothesis that down regulation of α_1D-gene expression by kindling may contribute to the long-term maintenance of epileptiform activity, possibly through reduced Ca²⁺-dependent AHP and/or altered expression of other relevant genes.