Frequency coding ability of the somatosensory thalamocortical system and its modulation by anesthesia depth

The purposes of the present study were to characterize and compare the mid-tail cortical and thalamic somatosensory evoked potentials (SEPs), and to examine how the depth of the barbiturate anesthesia affected them. After the tail representative locations of sacrococcygeal dorsal root (S2 or S3), thalamus (ventroposterior lateral nucleus, VPL) and primary somatosensory cortex (SI) were set up for recording, the rats were infused serially with diluted sodium pentobarbital solution beginning from light (5 to 10 mg/kg/hr) to deep (30 to 40 mg/kg/hr) and then stop infusion (recovery). The effects of anesthetic depth on SEPs were examined of dorsal root, thalamic and cortical field potentials evoked by mid-tail stimulation of various stimulation intensities (100 microA to 2mA, step 100 pA, at 2 Hz) and frequencies (0.5 to 11 Hz, step 0.5 to 1 Hz, at 3T). The depth of anesthesia did not affect the strength-response curves of the SEPs. In contrast, the depth of anesthesia differentially influenced the frequency following capabilities of different recording sites. Under light anesthesia, thalamic SEP was only significantly affected with stimulation frequencies higher than 8 Hz, whereas cortical SEP was significantly affected with 2 Hz or higher. Under deep anesthesia, thalamic SEP evoked by low frequency tail stimulation was not significantly changed. In contrast, cortical SEP was affected much strongly so that under 1 Hz stimulation, a significant difference could be observed. We concluded, therefore, that thalamus was only partially responsible for the limited frequency following capability of the SI, and that the main effect of pentobarbital was on the cortical level. From the data obtained, an exponentially decaying curve could be observed for the cortical SEP under different stimulation frequencies. The decay constant showed a 50% change with a change in anesthesia depth. We propose that the decay constant could be used as a sensitive index for the monitoring of anesthetic depth.