DESTRUCTION OF CENTRAL NORADRENERGIC NEURONS WITH DSP4 IMPAIRS THE ACQUISITION OF TEMPORAL DISCRIMINATION BUT DOES NOT AFFECT MEMORY FOR DURATION IN A DELAYED CONDITIONAL DISCRIMINATION TASK

This experiment examined the effect of destroying central noradrenergi c neurones using the selective neurotoxin N-(2-chloroethyl)-n-ethyl-2- bromobenzylamine (DSP4) on the acquisition of a temporal discriminatio n and on memory for duration, using a delayed conditional discriminati on task. In phase I, rats that had received systemic treatment with DS P4 and vehicle-treated control rats were trained in a series of discre te trials to press lever A following a 2-s presentation of a light sti mulus, and lever B following an 8-s presentation of the same stimulus. Following stimulus offset, a response on a panel placed midway betwee n the two levers was required to initiate lever presentation; a single response on either lever resulted in withdrawal of both levers and, i n the case of a ''correct'' response. reinforcer delivery. Both groups acquired accurate discrimination. achieving 90% correct choices withi n 50 sessions; the DSP4-treated group acquired accurate performance mo re slowly than the control group. In phase II, delays were interposed between stimulus offset and lever presentation in 50% of the trials. I n the absence of a delay, discriminative accuracy was lower in the DSP 4-treated group than in the control group. Accuracy declined as a func tion of post-stimulus delay in both groups; both groups showed a delay -dependent bias towards responding on lever A (''choose-short'' bias). Neither of these effects differed significantly between the two group s. The concentrations of noradrenaline in the parietal cortex and hipp ocampus were reduced by 90% and 89% in the DSP4-treated group, compare d to the levels in the control group, but the levels of dopamine, 5-hy droxytryptamine and 5-hydroxyindoleacetic acid did not differ signific antly between the groups. The results confirm the deleterious effect o f DSP4 on the acquisition of temporal discrimination, but do not provi de evidence for a role of the noradrenergic innervation of the hippoca mpus and neocortex in temporal working memory.