OUTFLOW FROM THE NUCLEUS-ACCUMBENS TO THE PEDUNCULOPONTINE TEGMENTAL NUCLEUS - A DISSOCIATION BETWEEN LOCOMOTOR-ACTIVITY AND THE ACQUISITION OF RESPONDING FOR CONDITIONED REINFORCEMENT STIMULATED BY D-AMPHETAMINE

Output of neuronal information from the nucleus accumbens to the ventr al pallidum is known to be a critical pathway in the expression of loc omotion and incentive-related behaviour. Some signals from this struct ure are relayed forward through the dorsomedial nucleus of the thalamu s to the medial prefrontal cortex, but the other major pathway from th is site is a descending innervation to the pedunculopontine tegmental nucleus. Information carried by these descending neurons has been link ed with both the output of locomotor activity and incentive-related in formation. Previous studies carried out in this laboratory have shown no changes in locomotor activity-either spontaneous or in response to systemic administration of d-amphetamine or apomorphine-in rats with e xcitotoxic lesions of the pedunculopontine tegmental nucleus. The pres ent experiments compare the effects of ibotenate lesions of this nucle us in tests of locomotor activity or the acquisition of responding wit h conditioned reinforcement, following injections of d-amphetamine dir ectly into the nucleus accumbens. In general agreement with previous r esults, ibotenate lesions of the pedunculopontine tegmental nucleus di d not alter locomotion stimulated directly from the nucleus accumbens. However, comparable lesions in a group of trained rats produced an ar ray of deficits in the conditioned reinforcement paradigm. Most notabl y, these rats directed their attention almost entirely towards pressin g the levers (practically ignoring the food-hopper panel), but did not appear to be able to discriminate between them, while controls focuse d almost all their efforts on pressing the reinforcing lever (virtuall y ignoring the non-reinforcing lever) and the food-hopper panel. These results indicate that pedunculopontine tegmental nucleus lesions disr upt an element of reward-related responding, but do not affect the pro duction of locomotor activity. This highlights the unlikely existence of specific ''locomotion-inducing'' centres in the mesencephalon and i mplicates the pedunculopontine tegmental nucleus in the formation of s timulus-reward associations. These data are discussed with respect to a role for the pedunculopontine tegmental nucleus in response selectio n.