The Effect Of Propofol Anesthesia On Thalamocortical Sensory Processing In Rat

Background:The intravenous anesthetic propofol is known as a positive modulator of GABAA receptors at the molecular level in the thalamocortical circuit and even globally, but the effects of propofol on sensory processing in thalamocortical systems at the system level remain an open issue. In this study, we investigated the changes of sensory processing in and between ventral posteromedial nucleus (VPM) and somatosensory cortex (S1) of rats during different depths of propofol anesthesia.Methods:In eight rats, two extracellular tungsten electrodes were slowly inserted into VPM and S1 for recording spontaneous local field potentials (sLFPs), whiskers stimulation-evoked somatosensory evoked potentials (SEPs), and stimulation-associated multiple unit activities (MUA) simultaneously at increasing levels of propofol anesthesia. Whisker stimuli were executed by deflections of the optimal whisker on the contralateral face and presented every 6s for 1000s after 300s of spontaneous activity at each anesthesia depth. The duration of stimuli was 200ms. Anesthesia levels were modulated by the infusion rate of propofol administrated from tail vein. At each depth of anesthesia, spontaneous LFPs was band-pass-filtered toδ,θ,α,β, andγfrequency bands; SEPs and peri-stimulus time histograms (PSTHs) were extracted and averaged from responses to whisker stimuli.Results:1. All powers of spontaneous activity in VPM decreased with the increasing levels of propofol anesthesia, whereas only a and P powers decreased significantly in S1.2. Propofol increased onset latency of the SEPs in a dose-dependent manner, but had no effect on the onset latency of thalamic SEPs.3. Propofol augmented amplitude of cortical stimulation-evoked early response (0-100ms) and the time delay, between peaks of early and late response (100-1000ms), but reduced the amplitude of stimulation-evoked late response in a dose-dependent manner.4. Thalamocortical synchronization of neuronal activity during whisker stimulation at deep level of anesthesia was significantly higher than that at light level of anesthesia.5. Spontaneous spindles of 2-6s in duration and 7-14Hz in frequency presented approximately every 2-5s in both VPM and S1 during all depths of propofol anesthesia. Whisker stimulation could induce 2-6s long spindles after the termination of the early response in both regions at all levels of anesthesia.Conclusions:These findings suggest:1. Thalamocortical circuits preserve a certain degree of sensory responsiveness even in deep propofol anesthesia.2. The enhanced amplitude of cortical stimulation-evoked early response and synchrony between VPM and S1 with the increasing depths of anesthesia may represent that the number of discriminable firing patterns available to thalamocortical circuit shrinks, which may finally induce a loss of information capacity and loss of consciousness.