Characterization of voltage-gated Ca²⁺ conductances in layer 5 neocortical pyramidal neurons from rats

Neuronal voltage-gated Ca²⁺ channels are involved in electrical signalling and in converting these signals into cytoplasmic calcium changes. One important function of voltage-gated Ca²⁺ channels is generating regenerative dendritic Ca²⁺ spikes. However, the Ca²⁺ dependent mechanisms used to create these spikes are only partially understood. To start investigating this mechanism, we set out to kinetically and pharmacologically identify the sub-types of somatic voltage-gated Ca²⁺ channels in pyramidal neurons from layer 5 of rat somatosensory cortex, using the nucleated configuration of the patch-clamp technique. The activation kinetics of the total Ba²⁺ current revealed conductance activation only at medium and high voltages suggesting that T-type calcium channels were not present in the patches. Steady-state inactivation protocols in combination with pharmacology revealed the expression of R-type channels. Furthermore, pharmacological experiments identified 5 voltage-gated Ca²⁺ channel sub-types - L-, N-, R- and P/Q-type. Finally, the activation of the Ca²⁺ conductances was examined using physiologically derived voltage-clamp protocols including a calcium spike protocol and a mock back-propagating action potential (mBPAP) protocol. These experiments enable us to suggest the possible contribution of the five Ca²⁺ channel sub-types to Ca²⁺ current flow during activation under physiological conditions.