In order to assess the effects of scopolamine on the learning of a conditioned suppression (“fear”) response, four groups of rats were trained and tested under drug or saline, according to a 2×2 factorial design. Conditioned suppression was virtually absent in the group trained under scopolamine and tested under saline, but was well developed and equally strong in the other three groups. This result was attributed to dissociation of learning and a drug-induced impairment of learning, which act in concert only in the Scopolamine-Saline subgroup. The effects of each factor in isolation could be demonstrated in the Saline-Scopolamine and Scopolamine-Scopolamine subgroups if the sensitivity of the test were increased by the use of low shock intensity and low hunger motivation. Rats trained under scopolamine methyl nitrate or chlorpromazine exhibited normal suppression when tested without drug. The negative results with methyl scopolamine suggest that scopolamine impairs the acquisition of conditioned suppression by a blockade of central cholinergic synapses. The negative results with chlorpromazine suggest that adrenergic synapses play a less important role than cholinergic ones in fear conditioning.
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