Isoflurane can indirectly depress lumbar dorsal horn activity in the goat via action within the brain

We have examined the response of lumbar dorsal horn cells to a noxious mech anical stimulus during differential delivery of isoflurane to the brain and spinal cord of goats. We hypothesized that isoflurane, acting in the brain , would depress dorsal horn neuronal responses to a noxious mechanical stim ulus applied to the hindlimb. Eight goats were anaesthetized with isofluran e and neck dissections performed which allowed cranial bypass. Lumbar lamin ectomies were performed to allow measurements of single-unit dorsal horn ne uronal activity. Isoflurane 1.3% was administered before bypass, and during differential delivery it was administered at each of the following head/to rso combinations: 1.3%/1.3%, 0.8%/1.3%, 0.3%/1.3%, 1.3%/0.8%, 0.8%/0.8% and 0.3%/0.8%. When the torso isoflurane concentration was 1.3%, decreasing cr anial isoflurane from 1.3% to 0.3% did nor significantly affect dorsal horn responses (from mean 325 (SD 262) to 379 (412) impulses min(-1); P>0.05). However, when torso isoflurane was 0.8%, decreasing cranial isoflurane from 1.3% to 0.3% increased mean evoked dorsal horn activity by 42% (388 (359) to 551 (452) impulses min(-1); P<0.05). These data suggest that the major e ffect of isoflurane on dorsal horn responses to noxious stimuli is direct, but there is an indirect effect occurring via descending projections from s upraspinal regions.