An increase of neuronal Cav1.3 L-type calcium channels (LTCCs) has been observed in various animal models of epilepsy. However, LTCC inhibitors failed in clinical trials of epileptic treatment. There is compelling evidence that paroxysmal depolarization shifts (PDSs) involve Ca2+ influx through LTCCs. PDSs represent a hallmark of epileptiform activity. In recent years, a probable epileptogenic role for PDSs has been proposed. However, the implication of the two neuronal LTCC isoforms, Cav1.2 and Cav1.3, in PDSs remained unknown. Moreover, Ca2+-dependent nonspecific cation (CAN) channels have also been suspected to contribute to PDSs. Nevertheless, direct experimental support of an important role of CAN channel activation in PDS formation is still lacking.
Primary neuronal networks derived from dissociated hippocampal neurons were generated from mice expressing a dihydropyridine-insensitive Cav1.2 mutant (Cav1.2DHP−/− mice) or from Cav1.3−/− knockout mice. To investigate the role of Cav1.2 and Cav1.3, perforated patch-clamp recordings were made of epileptiform activity, which was elicited using either bicuculline or caffeine. LTCC activity was modulated using the dihydropyridines Bay K 8644 (agonist) and isradipine (antagonist).
Distinct PDS could be elicited upon LTCC potentiation in Cav1.2DHP−/− neurons but not in Cav1.3−/− neurons. In contrast, when bicuculline led to long-lasting, seizure-like discharge events rather than PDS, these were prolonged in Cav1.3−/− neurons but not in Cav1.2DHP−/− neurons. Because only the Cav1.2 isoform is functionally coupled to CAN channels in primary hippocampal networks, PDS formation does not require CAN channel activity.
Our data suggest that the LTCC requirement of PDS relates primarily to Cav1.3 channels rather than to Cav1.2 channels and CAN channels in hippocampal neurons. Hence, Cav1.3 may represent a new therapeutic target for suppression of PDS development. The proposed epileptogenic role of PDSs may allow for a prophylactic rather than the unsuccessful seizure suppressing application of LTCC inhibitors.