Absence of T-type calcium currents in ground squirrel photoreceptors

Abstract

Purpose : Electrophysiological and molecular studies on mammalian cones identify a major role for L-type Ca²⁺ channels in transmitter release. A recent study, however, suggests that mammalian cone terminals also contain T-type currents. Unlike L-type currents whose voltage activation range and sustained behavior is well-matched to that of the cone light response, T-currents activate transiently during depolarization from hyperpolarized voltages that are at the nadir of the light response range. If present together, the two Ca²⁺ currents could potentially signal different features of the light response. Here, we use voltage clamp recordings to test whether T-type currents are a ubiquitous feature of mammalian cone terminals.

Methods : Experiments were performed on adult Ictidomys tridecemlineatus of either sex. For recordings, the eyecup was divided along the dorsal to ventral axis into superior, middle, and inferior parts. To block transporter currents, proton feedback, K⁺ currents, and I_h, TBOA (325 µM) or TFD-TBOA (2 µM), HEPES (10 mM), TEA-Cl (20 mM), and ZD7288 (50 µM) were added to the external medium. TEA-Cl (20 mM) and BAPTA (10 mM) were added to the internal medium to block K⁺ and Ca²⁺-dependent currents. Cone voltage was maintained at -85 mV and either ramped to +25 mV at a rate of 0.1 mV/ms or stepped to a series of more depolarized voltages for 60 ms. The T-channel blocker ML218 (5 µM) and the L-channel blocker isradipine (ISR, 2 µM) were applied directly to the cone terminal via puffer pipettes.

Results : During voltage ramps, ML218 produced a small leftward shift in the activation curve of an L-type current (ΔV₅₀ = -1.2±0.9 mV [±SD], p=0.0294) without a significant change in the current peak (-7.0±20.8%, n=6 cones). When L-type currents were blocked by first applying ISR, the addition of ML218 was accompanied by only a small continuation of the L-current block (n=17 cones). Voltage steps to -55 and -45 mV exclusively produced sustained currents that were unaffected by ML218 (n=4 cones). Similar responses were obtained in all 3 retinal regions. Voltage ramps in On and Off bipolar cells revealed prominent T-type currents that were blocked by ML218 under the same experimental conditions.

Conclusions : Based on voltage-dependent activation and pharmacological properties, adult ground squirrel cones lack T-type Ca²⁺ currents.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.