RETINAL GANGLION NEURONS EXPRESS A TOXIN-RESISTANT DEVELOPMENTALLY-REGULATED NOVEL TYPE OF HIGH-VOLTAGE-ACTIVATED CALCIUM-CHANNEL

1. High-voltage-activated Ca2+ currents [I-Ca(HVA)] were studied in im munolabeled mouse retinal ganglion neurons (RGNs) to elucidate channel -specific components and their developmental changes in vitro. 2. Neur ons were dissociated at postnatal day 5. RGNs were selected for electr ophysiological measurements by vital labeling with an antibody against Thy-1.2. I-Ca(HVA) were recorded with patch electrodes in the whole c ell configuration at a holding voltage (V-h) of -90 mV. 3. A total of 111 neurons was studied. On average, 13% of I-Ca(HVA) was reversibly b locked by 10 mu M nifedipine, similar to 30% of the compound current d isplayed an irreversible block by 2.5 mu M omega-conotoxin (omega-CTX) GVIA. The remainder current was resistant to both drugs, suggesting t hat the total I-Ca(HVA) was a mixture of at least three different comp onents. 4. Developmental analysis revealed a significant increase of t he omega-CTX-GVIA/nifedipine-resistant component of I-Ca(HVA) (31% at day in vitro (DIV) 0-2, 70% at DIV 18-26) mainly at the expense of the omega-CTX-GVIA-sensitive current. No significant change was found in the nifedipine-sensitive component of I-Ca(HVA). 5. To characterize th e Ca2+ current component that was resistant to both omega-CTX-GVIA and nifedipine at V-h -90 mV, three tests were performed. The P channel a ntagonist omega-agatoxin IVA (omega-Aga-IVA, 200 nM) completely failed to block I-Ca(HVA) in mouse RGNs. The novel Ca2+ channel blocker omeg a-CTX-MIIC (5 mu M) decreased the I-Ca(HVA) remaining after omega-CTX- GVIA treatment by only similar to 10%. At V-h-50 mV, nifedipine inhibi ted the Ca2+ current remaining after omega-CTX-GVIA application only b y 40%, leaving a significant portion of I-Ca(HVA) uneffected. 6. Our r esults suggest that 1) in postnatal mouse RGNs a major portion of I-Ca (HVA) is resistant to omega-CTX-GVIA, nifedipine, omega-Aga-IVA, and o mega-CTX-MIIC, thus representing a yet unidentified non-N, non-L, and non-P type Ca2+ current. 2) This current seems to be developmentally r egulated. Its increasing relative density is consistent with a role in retinal neuronal maturation.