EFFECT OF CENTRAL 5-HYDROXYTRYPTAMINE DEPLETION ON PERFORMANCE IN THETIME LEFT PROCEDURE - FURTHER EVIDENCE FOR A ROLE OF THE 5-HYDROXYTRYPTAMINERGIC PATHWAYS IN BEHAVIORAL SWITCHING

This experiment examined the effect of destruction of the ascending 5- hydroxytryptaminergic (5HTergic) pathways on performance in a free-ope rant timing schedule: the ''time-left'' procedure. Rats received eithe r injections of 5,7-dihydroxytryptamine into the dorsal and median rap he nuclei or sham lesions. They were trained in a discrete trials sche dule in which reinforcers were provided for responding on either of tw o levers, A and B. At a random time point, t s after the start of each trial, a response on A resulted in the delivery of one food pellet af ter d(A) s, whereas a response on B resulted in the delivery of two pe llets after 60-t s. The value of d(A) was varied between 1 and 8 s in different phases of the experiment. Both groups showed decreasing resp onse rates on lever A and increasing response rates on lever B as a fu nction of time within the trial. An index of timing (T-75: the time wi thin the trial at which relative response rate on B attained a value o f 75%) was systematically related to the value of d(A), but did not di ffer significantly between lesioned and control rats. However the lesi oned group showed significantly higher rates of switching bi:tween res ponse alternatives than the sham-lesioned group at all values of d(A). The levels of 5HT and 5-hydroxyindoleacetic acid were reduced in the brains of the lesioned rats, but the levels of noradrenaline and dopam ine were not significantly altered. The results provide further eviden ce that the ascending 5HTergic pathways may contribute to the inhibito ry regulation of switching between behavioural states.