DOES THE EFFECT OF CENTRAL 5-HYDROXYTRYPTAMINE DEPLETION ON TIMING DEPEND ON MOTIVATIONAL CHANGE

In a previous experiment we found 'that destruction of the ascending 5 -hydroxytryptaminergic (5HTergic) pathways by microinjection of 5,7-di hydroxytryptamine into the dorsal and median raphe nuclei resulted in impaired acquisition of temporal differentiation under an interrespons e-time-greater-than-15-s (IRT>15 s) schedule of sucrose reinforcement. This paper reports three experiments, the results of which bear on th e interpretation of that finding. In Experiment 1, 32 rats were traine d for 120 sessions under the IRT > 15 s schedule; then 16 received les ions of the 5HTergic pathways and 16 received sham lesions. Comparison s of the IRT frequency distributions of the two groups showed that the lesion produced a significant reduction of the mean IRT and an increa se in the dispersion of IRTs, as expressed by the coefficient of varia tion. Obtained reinforcement rates were significantly reduced in the l esioned group, but response rates were not significantly altered. Leve ls of 5HT and 5-hydroxyindoleacetic acid were markedly reduced in all forebrain areas examined, without significant change in noradrenaline and dopamine levels. The results indicate that destruction of the 5HTe rgic pathways disrupts performance as well as acquisition of temporal differentiation. Experiments 2 and 3 examined whether changes in depri vation level and reinforcer magnitude, which are known to affect reinf orcer value, would influence temporal differentiation in a similar fas hion to destruction of the 5HTergic pathways. In experiment 2, 20 rats were trained under the IRT > 15 s schedule while maintained at 80% or 90% of free-feeding body weight; the more severe deprivation conditio n was associated with a longer mean IRT and a lower coefficient of var iation. In experiment 3, 16 rats were trained under the IRT > 15 s sch edule using 100 mul or 20 mul of a 0.6 M sucrose solution as the reinf orcer; indices of temporal differentiation did not differ between the two conditions. These results indicate that the deleterious effect of destruction of the 5HTergic pathways upon timing behaviour is unlikely to be secondary to the motivation enhancing effect of the lesion.