This study employed bilateral, intraventricular injections of 6-hydroxydopamine (6-HDA) to examine the effects of monoamine depletion on Pavlovian conditioning of the rabbit's nictitating membrane response. 6-HDA produced dose-dependent and highly correlated decreases in the rate of acquisition of conditioned responses and in the telencephalic content of 5-HT, DA, and NE. At the highest dose of 6-HDA (1340 μg), 5-HT, DA, and NE were reduced by 42, 48, and 89, respectively, and the number of trials required to achieve criterion acquisition was increased by 123. Control experiments established that the highest dose of 6-HDA: 1) had no effect on the unconditioned nictitating membrane reflex; 2) had no effect on the threshold of the conditioned stimulus for eliciting conditioned responses; and 3) produced only a small, less than 5, decrease in nonassociative responding. It was concluded that decreases in 5-HT, DA, and NE can impair associative learning without altering sensory or motor function
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