Pain fibers contribute to the generation of subcortical somatosensory evoked potentials (SEPs) in rats.

Subcortical somatosensory evoked potentials (SEPs) were obtained by electrical stimulation of the volar surface of the forepaw and were recorded from the skull overlying the contralateral somatosensory area of the cerebral cortex. Three distinct peaks were discernable in the SEPs prior to the first cortical component. Dorsal column transection at C3 level reduced the amplitude of component III by 48.7 +/- 4.7% without affecting the amplitude of components I and II. Morphine given either i.v. (2.5 mg/kg) or intrathalamically (25 microg/kg) almost completely abolished the part of component III that remained after surgical sectioning of the dorsal column of the spinal cord. The effects of morphine were reversed by i.v. naloxone (0.25 mg/kg). These results indicate that both the dorsal column and the anterolateral systems contribute significantly to the generation of component III of subcortical SEPs. Subcortical SEPs may be a useful non-invasive technique for studying the neurophysiological effects of known and experimental analgesics.