Dissociated changes of somatosensory evoked low-frequency scalp responses and 600 Hz bursts after single-dose administration of lorazepam

electrical stimulation of upper limb nerves allows one to record two types of scalp responses, that is conventional low-frequency somatosensory evoked potentials (SEPs), and bursts of high-frequency (about 600 Hz) wavelets. to further clarify the functional meaning of both types of responses, we investigated whether changes of GABAergic drive could cause significant modifications of conventional as well as high-frequency SEPs. We recorded median nerve SEPs from six healthy volunteers before and after a single oral administration of lorazepam. In order to explain scalp SEP distribution before and after lorazepam administration, we performed the brain electrical source analysis of raw data. after lorazepam administration, conventional scalp SEPs showed a significant amplitude decrease of all cortical components including the primary N20/P20 response, while the subcortical P14 response remained substantially unchanged. similarly, dipolar analysis showed a significant strength decrease of all cortical dipoles, whereas the strength of both subcortical dipoles (possibly located at the level of the brainstem and thalamus, respectively) remained unchanged. by contrast, no significant changes of high-frequency SEPs were induced by drug intake. therefore, our findings suggest that the inhibitory effect induced by lorazepam mainly affects intracortical circuitry. tonic increase of the inhibitory drive, possibly mediated by GABAA receptors, can account for the reduced activity of first order deep spiny neurons generating the primary N20/P20. conversely, intrinsic firing properties of the cell population generating high-frequency SEP responses are unaffected by the increase of GABAergic drive. this finding lends further substance to the hypothesis that conventional and high-frequency SEPs are generated by different cell populations.