Isoflurane induces dose-dependent changes of thalamic somatosensory information transfer

In spite of several reports about suppressive effects of volatile anesthetics on somatosensation, their neuronal mechanisms are largely unknown. The present study investigates somatosensory impulse transmission at the thalamic level in rats under varied concentrations of isoflurane by recordings of neuronal responses to mechanical stimulation of the body surface. Single-unit recordings of thalamo-cortical relay neurons (TCNs, third order neurons; n=28) and presumed trigemino-thalamic fibers (TTFs, second order neurons; n=7) were performed in the ventral posteromedial nucleus. Functional response characteristics were quantified following defined tactile stimulation (trapezoidal or vibratory deflection of sinus hairs or fur) applied to the neuronal receptive fields. End-tidal isoflurane concentration was increased in steps of 0.2% between 0.6% (baseline) and 2.0%. The response activity in all TCNs studied was suppressed in a dose-dependent manner (2.0% isoflurane decreased responses to 3. 5+/-1.1% of baseline; mean+/-S.E.M.); the response activity in TTFs was much less affected (decrease to 55.0+/-8.2%). Suppression of ongoing activity, however, was similar for both, TCNs and TTFs. Furthermore, in TCNs, the response characteristics changed with increasing isoflurane between 1.0% and 1.8%: tonic and sustained responses were converted to phasic on-responses. In contrast, the tonic and sustained response characteristics of TTFs were preserved even at higher isoflurane concentrations. The results indicate that isoflurane attenuates the output of somatosensory signals in the specific nucleus of the rat's thalamus, while its input is only marginally affected. The observed changes of thalamic neuronal response characteristics, at least in part, may cause the loss in sensory discrimination observed during general anesthesia.