Comparison of touch- and laser heat-evoked cortical field potentials in conscious rats

Field potentials and multiunit activities from chronically implanted cortic al electrodes were used to study tactile and nociceptive information proces sing from the tail of the rat. Fourteen stainless steel screws implanted in the skull were used as electrodes to record field potentials in different cortical areas. Electrical, mechanical, and laser pulses were applied to th e tail to induce evoked cortical field potentials. Evoked responses were co mpared before and after sodium pentobarbital anesthesia (50 mg/kg, i.p.). I n both electrical- and mechanical-evoked potential (EEP and MEP) studies, t wo major peaks were found in the conscious animal. The polarity of the late component was modified after pentobarbital anesthesia. In the laser-evoked potential (LEP) study, two distinct negative peaks were found. Both peaks were very sensitive to anesthesia. Following quantitative analysis, our dat a suggest that the first positive peak of EEP and MEP corresponded to the a ctivation of the A beta fiber, the second negative peak of MEP and the firs t peak of LEP corresponded to A delta fiber activation, while the second pe ak of LEP corresponded to C fiber activation. The absolute magnitudes of al l cortical components were positively related to the intensity of the stimu lation. From spatial mapping analysis, a localized concentric source of fie ld potential was observed in the primary somatosensory cortex (SI) only aft er activation of the A beta fiber. Larger responsive cortical areas were fo und in response to A delta and C fiber activation. In an intracortical reco rding experiment, both tactile and nociceptive stimulation evoked heightene d unit activity changes at latencies corresponding to respective field pote ntials. We conclude that different cortical areas are involved in the proce ssing of A and C fiber afferent inputs, and barbiturate anesthesia modifies their processing. (C) 1999 Elsevier Science B.V. All rights reserved.