EFFECT OF LESIONS OF THE ASCENDING 5-HYDROXYTRYPTAMINERGIC PATHWAYS ON TIMING BEHAVIOR INVESTIGATED WITH THE FIXED-INTERVAL PEAK PROCEDURE

Twelve rats received injections of 5,7-dihydroxytryptamine into the do rsal and median raphe nuclei; 12 rats received sham lesions. The rats were then trained for 60 sessions under a discrete-trials fixed-interv al schedule (peak procedure). In half the trials, a reinforcer became available 40 s after trial onset, and the trial was terminated upon re inforcer delivery; the remaining trials were 120 s in duration, and re inforcement did not occur in these trials. Performance during the 120- s trials was characterized by increasing response rate during the firs t 40 s of the trial, declining response rate between 40 s and 80 s, an d a secondary increase in response rate during the final 40 s of the t rial. The lesioned group showed a broader ''spread'' of the response r ate function than the control group (time between attainment of 70% of the peak response rate and subsequent decline of response rate below this level); however, the peak response rate and the time from trial o nset until attainment of the peak response rate did not differ signifi cantly between the groups; the spread/peak-time ratio was significantl y greater in the lesioned group than in the control group. The levels of 5-hydroxytryptamine (5HT) and 5-hydroxyindoleacetic acid in the par ietal cortex, hippocampus, amygdala, nucleus accumbens and hypothalamu s were markedly reduced in the lesioned group, but the levels of norad renaline and dopamine were not significantly affected by the lesion. T he results confirm the involvement of 5HTergic function in timing beha viour.