FACILITATED ACQUISITION OF A TEMPORAL DISCRIMINATION FOLLOWING DESTRUCTION OF THE ASCENDING 5-HYDROXYTRYPTAMINERGIC PATHWAYS

This experiment examined the effect of destroying the 5-hydroxytryptam inergic (5HTergic) pathways on the acquisition and performance of disc rimination between two brief time intervals. Rats that had received in jections of 5,7-dihydroxytryptamine into the dorsal and median raphe n uclei, and sham-lesioned control rats were trained in a series of disc rete trials to press lever A following a 200-ms presentation of a ligh t stimulus and lever B following an 800-ms presentation of the same st imulus. Both groups gradually acquired accurate performance, attaining 80% - 85% accuracy by the end of 40 sessions. The lesioned group lear nt the task significantly faster than the control group. When stable p erformance had been attained, ''probe'' trials were introduced in whic h the light was presented for intermediate durations. Both groups show ed sigmoid functions relating percent choice of lever B to log stimulu s duration. The bisection point (duration corresponding to 50% choice of lever B) did not differ significantly between the two groups; howev er, the Weber fraction was significantly smaller in the lesioned group than in the control group. The levels of 5HT and 5-hydroxy-indole-ace tic acid were markedly reduced in the brains of the lesioned rats, but the levels of noradrenaline and dopamine were not altered. The result s indicate that destruction of the 5HTergic pathways facilitates acqui sition of a temporal discrimination. The lack of an effect of the lesi on on the bisection point contrasts with our previous finding using lo nger stimulus durations; it is suggested that different behavioural pr ocesses may underlie millisecond-range and second-range temporal discr imination, and that these may be differently affected by 5HT depletion .