Isoflurane inhibits multiple voltage-gated calcium currents in hippocampal pyramidal neurons

The mechanisms by which volatile anesthetics produce general anesthesia are unknown. Voltage-gated calcium currents in central neurons are potential target sites for general anesthetics because they are involved in the regulation of excitability and are essential for synaptic transmission.

Freshly isolated rat hippocampal pyramidal neurons were studied using the whole-cell patch clamp method. Calcium currents were isolated from other voltage-activated currents by blocking sodium and potassium channels. Calcium current subtypes were studied using the specific blockers nitrendipine and omega-conotoxin GVIA.

Isoflurane inhibited multiple voltage-gated calcium currents in hippocampal neurons. Isoflurane inhibited both the high- and low-voltage-activated calcium current in a clinically relevant concentration range, giving half-maximal inhibition of the peak high-voltage-activated current (measured at current maximum) at about 2% gas phase concentration, and the sustained current (measured at the end of an 800-ms depolarization) at about 1%. Isoflurane also accelerated both components of the two-component exponential decay of the high-voltage-activated current. Studies using specific channel blockers showed that the calcium current contained contributions from T, L, N, and other channels, including probably P channels. Isoflurane inhibited all of these in clinically relevant concentrations, although detailed analysis of the effects on the individual channel types was not attempted.

Given the importance of calcium currents in the regulation of excitability in central neurons and the involvement of P and N channels in neurotransmitter release, this effect may represent an important action of volatile anesthetics in producing general anesthesia.