The thalamus is the major gateway for the flow of sensory information from the periphery to the cortex and the disruption of thalamocortical connectivity may be an essential common feature of the hypnotic effects of many general anesthetic (GAs). Furthermore, recent studies have identified important roles of the central medial nucleus (CeM) and ventrobasal (VB) nucleus of the thalamus in control of arousal and natural sleep. Although the role of thalamic T-type calcium channels (T-channels) in natural sleep is reasonably well established, the role of these channels in anesthesia remains poorly understood. In addition, in the past cycle of this grant we established that exposure of rat pups to clinically-relevant GAs causes increased T- current densities in the reticular thalamic nucleus (nRT), which in turn contributes to lasting hyperexcitabil- ity and pathological oscillations in thalamocortical networks in vitro and in vivo. These changes in neuronal function were reversed by selective antagonism of T-channels. We postulate that the use of pharmacological inhibitors that target T-channels may facilitate GA-induced loss of consciousness and hence may reduce usage of potent volatile GAs needed for surgery. The overall objective of this application, is to test the central hypothesis that the CaV3.1 isoform of T-channels in the CeM and VB thalamic nuclei is important for anesthetic-induced hypnosis. To test this hypothesis, we will use in vitro patch-clamp recordings from acute brain slices and recombinant cells in vitro and electroencephalographic (EEG) recordings in vivo from the CeM and VB nuclei, selective pharmacological T-channel inhibitors, and mouse genetics with global and region-specific silencing of CaV3.1 channels in the thalamus to pursue the following specific aims:
Aim 1 : To determine the biophysical and molecular mechanism of recombinant and native CaV3.1 channel inhibition in vitro by two common GAs, isoflurane (ISO) and sevoflurane (SEVO), and their effect on neu- ronal excitability of CeM. We will also use new approach with photoaffinity ligands of ISO (AZI-ISO) and SEVO (AZI-SEVO) to identify specific molecular binding sites on CaV3.1 channels for volatile GAs.
Aim 2 : To determine whether CaV3.1 channels in the CeM contribute to the alterations in thalamocortical network function during sedation/hypnosis with volatile GAs and administration of the selective T-channel inhibitor TTA-P2, as assessed by in vivo EEG recordings.
Aim 3 : To determine if region-specific silencing of CaV3.1 channels in the CeM and VB thalamus will have differential effects on sedation/hypnosis and thalamocortical oscillations in vivo induced by ISO and SEVO. The proposed work is innovative in that new mechanisms of anesthetic-induced loss of consciousness will be characterized. It is medically significant because it describes the importance of drugs that target voltage- gated calcium channels for potential development of safer practices in clinical anesthesia.
This proposed research will study how inhibition of voltage-gated calcium currents in thalamo- cortical circuitry contributes to the spectrum of clinical effects exerted by different categories of general anesthetics. The findings may be important for understanding cellular mechanisms of anesthesia such as loss of consciousness and potential development of novel, safer anesthetics.
