Previous studies have shown that systemic administration of cocaine transiently alters stimulus-evoked responses of ventral posteromedial (VPM) thalamic neurons. Results from these single-unit electrophysiological studies revealed that cocaine was equally likely to augment or attenuate the magnitude of sensory evoked responses following threshold level stimulation of peripheral receptive fields. In an attempt to clarify the impact of cocaine administration on sensory signal processing, we examined the drug's effects on responses of individual neurons and ensembles of VPM thalamic neurons to sensory stimuli, and performance of subjects in a sensory detection behavioural task. Extracellular responses of single (n = 1 cell/rat) or multiple VPM thalamic neurons (n = 10-40 cells/rat) were monitored before and after cumulative doses of cocaine (0.25-2.0 mg/kg i.v.). Neuronal responses were characterized by assessing the response profile to a range of peri-threshold-level deflections of the optimal whisker on the contralateral face. Drug effects on stimulus-response curves, determined from quantitative analysis of spike train data, indicated that whereas cocaine elicits variable effects at the single cell level, the stimulus-evoked response of the recorded population was likely to increase following lower (0.25-1.0 mg/kg i.v.) doses of cocaine. Furthermore, cocaine preferentially enhanced responses to smaller magnitude deflections of vibrissa, altering the response profile from a mode that accurately conveyed stimulus strength to one that increased detection at the expense of discrimination. Finally, a similar pattern emerged in a behavioural paradigm involving rats trained to detect variable amplitude whisker pad stimulation, suggesting a common action of cocaine that may contribute to the drug's addictive properties.
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