Current-clamp experiments on primary hippocampal neurons shed light on the role of L-type voltage-gated calcium channels in depolarization shifts

Abstract

we showed that L-type voltage-gated calcium channels (LTCCs) are capable of augmenting brief neuronal depolarizations. Hence, we were interested in the question whether LTCCs contribute to neuronal depolarization shifts (DS). Methods: 1 mM caffeine was applied as an epileptogenic agent, and LTCC activity was modulated by Bay K8644 (BayK) and isradipine, respectively. Results: In contrast to earlier studies on hippocampal slices [2], caffeine alone failed to induce DS in all but one out of 12 neurons tested. However, when BayK (3 µM) was co-administered, DS were readily evoked in about 60% of 21 neurons tested. DS were abolished by subsequent addition of isradipine (3 µM). Since oxidative stress has been shown to augment LTCC currents, we tested hydrogen peroxide in our DS assay, again employing caffeine coadministration. Within 5 min of exposure to 3 mM H2O2, DS appeared in a subpopulation of neurons. Testing both BayK and H2O2 on the same neurons we found that H2O2 evoked DS only in those neurons where BayK showed a clear DS-inducing effect. In those cells, where BayK failed to cause DS, H2O2 also had no effect. Conclusions: Our data suggest that potentiation of LTCCs promotes the formation of depolarization shifts. Oxidative stress appears to be a pathogenic initiator of DS, and this activity may require LTCCs. Since LTCC augmentation is being considered as a pathological mechanism in neurodegenerative diseases, our data point to the possibility that the initiation of IIS may be a precipitating factor and that LTCC inhibition may provide a means to counteract neuropathogenic mechanisms.