Voltage-dependent Calcium Channels in the Rat Retina: Involvement in NMDA-stimulated Influx of Calcium

Abstract

Rises in intracellular Ca2+induced by activation of glutamate receptors are of ultimate importance for neuronal excitability and pathophysiological processes. In the present study, we aimed to elucidate the types of voltage-dependent Ca2+channels involved in the NMDA-stimulated influx of Ca2+into the isolated rat retina by using selective blockers. Additionally, the number of binding sites for radioligands labelling L- ([3H]nitrendipine), N- ([125I]ω-conotoxin MVIIA) and P/Q-type ([125I]ω-conotoxin MVIIC) Ca2+channels was assessed in the rat retina and, for further comparison, in the rat cortex. Incubation of isolated rat retinas with 100μM NMDA produced a three-fold increase in the influx of45Ca2+that was completely blunted by MK-801, a NMDA receptor antagonist, and partially attenuated (approximately 20%) by tetrodotoxin, a Na+channel blocker. The L-type Ca2+channel blocker nifedipine reduced NMDA-stimulated Ca2+influx in a dose-related fashion, with a maximum reduction of approximately 50%. Similar effects were observed with verapamil and diltiazem. Blockers of N- and P/Q-type Ca2+channels had no significant effect on the influx of Ca2+evoked by NMDA. Co2+, a non-specific Ca2+channel blocker, caused an inhibition of NMDA-stimulated Ca2+influx similar to that of nifedipine. Therefore, of all voltage-dependent Ca2+channels, L-type channels appear to make the greatest contribution (up to 50%) to the NMDA-stimulated influx of Ca2+into the isolated rat retina. This finding contrasts with evidence obtained in brain neurones supporting a role for L-, N- and P/Q-type channels in NMDA-evoked Ca2+signals. A comparison of the number of radioligand binding sites associated with L-, N- or P/Q-type Ca2+channels in the rat cortex and retina revealed that such a difference cannot be ascribed to a distinct expression pattern of these channels in both tissues, although some variations were found. Interestingly, a different affinity of [3H]nitrendipine for L-type Ca2+channels in the rat retina and cortex was observed which may reflect the expression of different classes of L-type channels in these tissues. The ability of L-type Ca2+channel blockers to attenuate NMDA-stimulated Ca2+influx may underlie their neuroprotective effects in the retina.