Differential control of high-voltage activated Ca2+ current components by a Ca2+-dependent inactivation mechanism in thalamic relay neurons

Ca²⁺-dependent inactivation of Ca²⁺ channels represents a feedback mechanism to limit the influx of Ca²⁺ into cells. Since large Ca²⁺ transients are present in thalamocortical relay neurons and Ca²⁺-dependent mechanisms play a pivotal role for thalamic physiology, the existence of this inactivation mechanism and the involvement of different Ca²⁺ 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 Ca²⁺ current. The blocker-resistant current constituted about 12±3% of the total Ca²⁺ current. The degree of Ca²⁺ 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 Ca²⁺-dependent inactivation is involved in the fine tuning of Ca²⁺ entry into relay neurons mediated by L- and Q-type channels locally operated by Ca²⁺ beneath the plasma membrane.