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In order to evaluate the relationship between locus coeruleus (LC) spontaneous activity and LC activity in response to sensory stimuli, the effects of carbachol on both types of LC neuronal activity were investigated in anesthetized rats. Carbachol increased LC spontaneous discharge rate and altered the pattern of LC discharge in response to sciatic nerve stimulation, while saline had no effect. Moreover, the magnitudes of increases in LC spontaneous discharge rate produced by carbachol were highly correlated (r = 0.79) with a measure of simultaneous effects on LC sensory responses. These results suggest that drugs which increase LC spontaneous activity will also disrupt the pattern of LC discharge to sensory stimuli.