Differential control of high-voltage activated Ca 2+ current components by a Ca 2+ -dependent inactivation mechanism in thalamic relay neurons

Ca 2+ -dependent inactivation of Ca 2+ channels represents a feedback mechanism to limit the influx of Ca 2+ into cells. Since large Ca 2+ transients are present in thalamocortical relay neurons and Ca 2+ -dependent mechanisms play a pivotal role for thalamic physiology, the existence of this inactivation mechanism and the involvement of different Ca 2+ channel subtypes was investigated. The use of subtype-specific antibodies revealed the expression of α1A–α1E channel proteins on the cell body and proximal dendrites of acutely isolated cells from the rat dorsolateral geniculate nucleus (dLGN). In addition, subtype-specific channel blocking agents were used in whole cell patch clamp experiments: nifedipine (1–5 μM; L-type) blocked 35 ± 3%, ω-conotoxin GVIA (1 μM; N-type) blocked 27 ± 8%, and ω-conotoxin MVIIC (4 μM; P/Q-type) blocked 33 ± 5% of the total HVA Ca 2+ current. The blocker-resistant current constituted about 12 ± 3% of the total Ca 2+ current. The degree of Ca 2+ current inactivation was assessed by using a two-pulse protocol. Under control conditions the post-pulse I/V was U-shaped with 35 ± 4% of the current undergoing inactivation. Inclusion of BAPTA to the internal pipette solution reduced the degree of inactivation to 15 ± 1%. When L- and P/Q-type current was blocked, the degree of inactivation was lowered to 20 ± 2 and 27 ± 3%, respectively. In the presence of ω-agatoxin TK (35 ± 6%) and ω-conotoxin GVIA (32 ± 1%) there was no change in inactivation. These data suggest that Ca 2+ -dependent inactivation is involved in the fine tuning of Ca 2+ entry into relay neurons mediated by L- and Q-type channels locally operated by Ca 2+ beneath the plasma membrane.