The effects of agents that modify muscarinic tone upon behavior controlled by an avoidance schedule that employs signaled unavoidable shock

Abstract

Six male squirrel monkeys were subjected to a Sidman nondiscriminated avoidance schedule that superimposed conditioned stimuli (CS)—unavoidable shock pairings upon the ongoing avoidance behavior. Four of the six animals demonstrated facilitated avoidance response rates during the CS, while two demonstrated suppressed rates during the CS. Scopolamine hydrobromide (0.06, 0.125, 0.50, 1.0 mg/kg IM) reversed these patterns, causing the facilitators to suppress and vice versa. Scopolamine hydrobromide (0.50, 1.0 mg/kg) also led to a reliable reduction in overall response rate and an increase in the number of shocks received. Neither scopolamine methylbromide (0.50, 1.0 mg/kg), a peripheral acting anticholinergic, nor pilocarpine nitrate (1.25, 2.50 mg/kg), a muscarinic stimulant, produced any reliable effects upon avoidance behavior. These results were interpreted to indicate that the central acting antimuscarinic agent, scopolamine hydrobromide, may be able to reduce level of fear motivation either directly by interfering with cholinergic systems that mediate fear or by producing decrements in cognitive (i.e., memory) processes that are needed for animals to fully appreciate the aversive qualities of the CS.