Ca 1.3 channels play a crucial role in the formation of paroxysmal depolarization shifts in cultured hippocampal neurons.

OBJECTIVE

An increase of neuronal Ca 1.3 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 Ca 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, Ca 1.2 and Ca 1.3, in PDSs remained unknown. Moreover, Ca -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.

METHODS

Primary neuronal networks derived from dissociated hippocampal neurons were generated from mice expressing a dihydropyridine-insensitive Ca 1.2 mutant (Ca 1.2DHP mice) or from Ca 1.3 knockout mice. To investigate the role of Ca 1.2 and Ca 1.3, 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).

RESULTS

Distinct PDS could be elicited upon LTCC potentiation in Ca 1.2DHP neurons but not in Ca 1.3 neurons. In contrast, when bicuculline led to long-lasting, seizure-like discharge events rather than PDS, these were prolonged in Ca 1.3 neurons but not in Ca 1.2DHP neurons. Because only the Ca 1.2 isoform is functionally coupled to CAN channels in primary hippocampal networks, PDS formation does not require CAN channel activity.

SIGNIFICANCE

Our data suggest that the LTCC requirement of PDS relates primarily to Ca 1.3 channels rather than to Ca 1.2 channels and CAN channels in hippocampal neurons. Hence, Ca 1.3 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.