Evidence for the role of serotonin in the inhibition of specific motor responses.

Two behavioural paradigms were used to test the effects of serotonin depletion on a specific form of response inhibition. Treatment with both p-chlorophenylalanine (p-CPA) at 200 mg/kg and lesions of the medial raphe nucleus impaired the acquisition of a step-off passive-avoidance response. The experimental design allowed the elimination of alternative interpretations in terms of increased sensitivity to shock and increased responsiveness to stimuli. p-CPA also impaired response inhibition during an omission-training schedule. The results of the three studies support a general role of serotonin in withholding specific instrumental (reinforced) motor actions. The results contrast with those of studies supporting a role of noradrenaline in response inhibition. A tentative conclusion supports Konorski's (1967) suggestion for differentiation of various types of response inhibition that are mediated by different neurochemical systems.