N-METHYL-D-ASPARTATE LESIONS OF THE PEDUNCULOPONTINE NUCLEUS BLOCK ACQUISITION AND IMPAIR MAINTENANCE OF RESPONDING REINFORCED WITH BRAIN-STIMULATION

Excitotoxin lesions of the pedunculopontine tegmental nucleus have bee n found to block the acquisition of a conditioned place preference ind uced by morphine or amphetamine, and it has been suggested that such l esions may attenuate the primary reinforcing effects of these drugs an d, possibly, other reinforcers. The present study examined the effects of pedunculopontine lesions on the reinforcing effects of brain stimu lation N-methyl-D-aspartate-induced lesions of the pedunculopontine nu cleus prevented spontaneous acquisition of lever pressing for brain st imulation reinforcement during five daily 1 h sessions of training. Th e effective lesions damaged the retrorubral fields in addition to the pedunculopontine tegmental nucleus. N-methyl-D-aspartate (25 or 50 nmo l) lesions of the retrorubral fields did not block acquisition of self -stimulation, however, controls reached their maximum rate of respondi ng in the first session, responding of rats with retrorubral field les ions gradually increased over five days. When excitotoxin-induced lesi ons of the pedunculopontine nucleus were made after acquisition of sel f-stimulation, lesioned rats continued to respond to brain stimulation , but at a lower rate than controls. The results show that pedunculopo ntine lesions interfere with the learning and expression of a response reinforced by brain stimulation just as they block learning motivated by drugs and natural rewards. They also suggest that collateral damag e to the retrorubral fields may contribute to the effects of pedunculo pontine lesions on reinforced behaviour. These data support the view t hat the pedunculopontine tegmental nucleus is involved in the process by which reinforcers control purposive behaviour.