EFFECT OF LESIONS OF THE ASCENDING 5-HYDROXYTRYPTAMINERGIC PATHWAYS ON TIMING BEHAVIOR INVESTIGATED WITH AN INTERVAL BISECTION TASK

Seventeen rats received injections of 5,7-dihydroxytryptamine into the dorsal and median raphe nuclei: 12 rats received sham injections. The rats were trained in a series of discrete trials to press lever A fol lowing a 2-s presentation of a light stimulus and to press lever B fol lowing an 8-s presentation of the same stimulus. Both groups learnt th e task rapidly and maintained >90% accuracy throughout the experiment. When stable performance had been attained, ''probe'' trials were intr oduced in which the light was presented for intermediate durations. Bo th groups showed sigmoid functions relating percent choice of lever B to log stimulus duration. The bisection point (duration corresponding to 50% choice of lever B) was significantly shorter in the lesioned gr oup than in the control group. There was no significant difference bet ween the slopes of the psycho-physical functions or the Weber fraction s derived for the two groups. The levels of 5-hydroxytryptamine (5HT) and 5-hydroxyindoleacetic acid in the parietal cortex, hippocampus, am ygdala, nucleus accumbens and hypothalamus were markedly reduced in th e lesioned group, but the levels of noradrenaline and dopamine were no t significantly affected by the lesion. The results confirm the involv ement of 5HTergic function in timing behaviour, but suggest that destr uction of these pathways does not disrupt the capacity for temporal di scrimination.