EFFECTS OF LESIONS OF THE ASCENDING 5-HYDROXYTRYPTAMINERGIC PATHWAYS ON CHOICE BETWEEN DELAYED REINFORCERS

The possible involvement of the ascending 5-hydroxytryptaminergic (5HT ergic) pathways in determining the effectiveness of delayed positive r einforcers was investigated using Mazur's (1984) adjusting-delay parad igm. Fourteen rats received injections of 5,7-dihydroxytryptamine into the dorsal and median raphe nuclei; 12 rats received sham lesions. Th e rats made repeated choices in a two-lever operant conditioning chamb er between a smaller reinforcer delivered after a 2-s delay and a larg er reinforcer delivered after a variable delay, the length of which wa s determined by the subject's previous choices. When the two reinforce rs consisted of one and two food pellets, the ''indifference point'' ( the delay to the larger reinforcer that rendered the two reinforcers e qually effective) was shorter in the lesioned group than in the contro l group. Increasing the sizes of the reinforcers to three and six pell ets reduced the indifference point in both groups and abolished the be tween-group difference. The levels of 5HT and 5-hydroxyindoleacetic ac id (5HIAA) in the parietal cortex, hippocampus, amygdala, nucleus accu mbens and hypothalamus were greatly reduced in the lesioned group, but the levels of noradrenaline and dopamine were not significantly affec ted. The results are consistent with the suggestion that the 5HTergic pathways play a role in maintaining the effectiveness of delayed reinf orcers.