Effect of central 5-hydroxytryptamine depletion on tolerance of delay of reinforcement: Evidence from performance in a discrete-trials 'time-left' procedure

This experiment examined the effect of destruction of the ascending 5- hydroxytryptaminergic (5HTergic) pathways on performance on a new discrete- trials version of the 'time-left' procedure. Rats received either injections of 5,7-dihydroxytryptamine into the dorsal and median raphe nuclei or sham lesions. They were trained in a discrete trials schedule in which reinforcers were provided for responding on either of two levers, A and B. At a random time point, t s after the start of each trial, the two levers were inserted into the operant chamber; a response on A resulted in the delivery of one food pellet after d(A) s, whereas a response on B resulted in the delivery of two pellets after 84-t s. The value of d(A) was varied between 1 and 12 s in different phases of the experiment. Both groups showed an increasing tendency to respond on lever B as a function of time within the trial. Logistic functions were fitted to the data from each group, and a value of the 'indifference point' (T₅₀: the time within the trial at which proportional choice of B attained a value of 50%) was derived for each rat. For each value of d(A), the values of T₅₀ were significantly greater in the lesioned rats than in the control rats, reflecting a rightward shift of the logistic function in the lesioned group. The levels of 5HT and 5-hydroxyindoleacetic acid were reduced in the brains of the lesioned rats, but the levels of noradrenaline and dopamine were not significantly altered. The results provide further evidence for the involvement of the ascending 5HTergic pathways in the control of operant behaviour by delayed positive reinforcers.