SYSTEMICALLY ADMINISTERED COCAINE ALTERS STIMULUS-EVOKED RESPONSES OFTHALAMIC SOMATOSENSORY NEURONS TO PERITHRESHOLD VIBRISSAE STIMULATION

Previous studies have shown that systemically administered cocaine can transiently alter responses of primary somatosensory cortical neurons to threshold level stimulation of peripheral receptive fields. The go al of the present investigation was 2-fold: (1) characterize the effec ts of systemic cocaine on stimulus-evoked responses of the ventral pos terior medial (VPM) thalamic neurons which relay somatosensory informa tion to the cortex and (2) determine the time course and magnitude of changes in monoamine levels within the somatosensory thalamus followin g systemic administration of cocaine. Extracellularly recorded respons es of single VPM thalamic neurons to whisker stimulation were monitore d before and after cocaine administration in halothane anaesthetized r ats. Each cell was first characterized by assessing its response profi le to a range of perithreshold level deflections of the optimal whiske r on the contralateral face. Drug effects on stimulus-response curves, response magnitude and latency were determined from quantitative anal ysis of spike train data. The results indicate that cocaine elicits a predictable augmentation or attenuation of the sensory response magnit ude, with the direction of the change inversely related to the initial magnitude of the stimulus-evoked discharge. In addition, cocaine cons istently reduced the response time of somatosensory thalamic neurons t o peripheral receptive field stimulation. At the same dose and over th e same time period, cocaine also produced marked elevation of norepine phrine and serotonin levels within the ventrobasal thalamus, as determ ined by in vivo microdialysis. These results suggest that cocaine-indu ced increases in norepinephrine and serotonin are responsible for drug -related modulation of the transfer of sensory signals through primary thalamocortical relay circuits. (C) 1998 Elsevier Science B.V. All ri ghts reserved.