The T-type calcium channel isoform Cav3.1 is a target for the hypnotic effect of the anaesthetic neurosteroid (3ß,5ß,17ß)-3-hydroxyandrostane-17-carbonitrile.

BACKGROUND: The mechanisms underlying the role of T-type calcium channels (T-channels) in thalamocortical excitability and oscillations in vivo during neurosteroid-induced hypnosis are largely unknown. METHODS: We used patch-clamp electrophysiological recordings from acute brain slices ex vivo, recordings of local field potentials (LFPs) from the central medial thalamic nucleus in vivo, and wild-type (WT) and Cav3.1 knock-out mice to investigate the molecular mechanisms of hypnosis induced by the neurosteroid analogue (3ß,5ß,17ß)-3-hydroxyandrostane-17-carbonitrile (3ß-OH). RESULTS: Patch-clamp recordings showed that 3ß-OH inhibited isolated T-currents but had no effect on phasic or tonic γ-aminobutyric acid A currents. Also in acute brain slices, 3ß-OH inhibited the spike firing mode more profoundly in WT than in Cav3.1 knockout mice. Furthermore, 3ß-OH significantly hyperpolarised neurones, reduced the amplitudes of low threshold spikes, and diminished rebound burst firing only in WT mice. We found that 80 mg kg-1 i.p. injections of 3ß-OH induced hypnosis in >60% of WT mice but failed to induce hypnosis in the majority of mutant mice. A subhypnotic dose of 3ß-OH (20 mg kg-1 i.p.) accelerated induction of hypnosis by isoflurane only in WT mice, but had similar effects on the maintenance of isoflurane-induced hypnosis in both WT and Cav3.1 knockout mice. In vivo recordings of LFPs showed that a hypnotic dose of 3ß-OH increased δ, θ, α, and ß oscillations in WT mice in comparison with Cav3.1 knock-out mice. CONCLUSIONS: The Cav3.1 T-channel isoform is critical for diminished thalamocortical excitability and oscillations that underlie neurosteroid-induced hypnosis.

Global genetic deletion of CaV3.3 channels facilitates anaesthetic induction and enhances isoflurane-sparing effects of T-type calcium channel blockers.

We previously documented that the CaV3.3 isoform of T-type calcium channels (T-channels) is inhibited by clinically relevant concentrations of volatile anaesthetics, including isoflurane. However, little is understood about the functional role of CaV3.3 channels in anaesthetic-induced hypnosis and underlying neuronal oscillations. To address this issue, we used CaV3.3 knock-out (KO) mice and a panselective T-channel blocker 3,5-dichloro-N-[1-(2,2-dimethyltetrahydro-pyran-4-ylmethyl)-4-fluoro-piperidin-4-ylmethyl]-benzamide (TTA-P2). We found that mutant mice injected with the vehicle showed faster induction of hypnosis than wild-type (WT) mice, while the percent isoflurane at which hypnosis and immobility occurred was not different between two genotypes. Furthermore, we found that TTA-P2 facilitated isoflurane induction of hypnosis in the CaV3.3 KO mice more robustly than in the WT mice. Isoflurane-induced hypnosis following injections of TTA-P2 was accompanied with more prominent delta and theta EEG oscillations in the mutant mice, and reached burst-suppression pattern earlier when compared to the WT mice. Our findings point to a relatively specific value of CaV3.3 channels in anaesthetic induced hypnosis. Furthermore, we propose that T-channel blockers may be further explored as a valuable adjunct to reducing the usage of potent volatile anaesthetics, thereby improving their safety.