Effect of central 5-hydroxytryptamine depletion on performance in the freeoperant psychophysical procedure: facilitation of switching, but no effecton temporal differentiation of responding

This experiment examined the effect of destroying the ascending 5-hydroxytr yptaminergic (5-HTergic) pathways on timing and switching behaviour in the free-operant psychophysical procedure. Rats received injections of 5,7-dihy droxytryptamine into the dorsal and median raphe nuclei or sham lesions. Th ey were trained to press levers for sucrose reinforcement; sessions consist ed of fifty 50-s trials in which reinforcers were available on a variable-i nterval 30-s schedule. In the first 25 s,of each trial, reinforcement was o nly available for responses on lever A: in the Last 25 s, it was available only for responses on lever B. In phase 1 (70 sessions) repetitive switchin g between the levers was prevented by withdrawal of lever A after the first response on lever B in each trial, in phase 3 (40 sessions) this constrain t on switching was removed; in phase 3 (40 sessions) the constraint was rei nstated. Data were collected from probe trials (four per session) in which no reinforcers were delivered, during the last ten sessions of each phase. In all phases, both groups showed declining response rates on lever A and i ncreasing response rates on lever B as a function of time from the onset of the trial. Response rate on lever BI expressed as percentage of overall re sponse rate, could be described by a tao-parameter logistic function. Remov al of the constraint on switching reduced the slope of the function without changing the indifference point (time corresponding to 50% responding on l ever B). The parameters of the timing function did not differ between the g roups in any of the phases. However, the lesioned group showed a greater en hancement of switching rate during phase 2 than the control group. The leve ls of 5-HT and 5-hydroxyindoleacetic acid were reduced in the brains of the lesioned rats, but the levels of noradrenaline and dopamine wen not altere d. The results provide further evidence for the involvement of the ascendin g 5-HTergic pathways in switching between response alternatives, but cast d oubt on our previous suggestion that the effects of 5-HT depletion on tempo ral differentiation of behaviour are mediated by facilitated switching.