Schedule dependent changes in operant responding after lesions of the dorsal tegmental noradrenergic projection

The present experiments examined the role of the dorsal tegmental noradrenergic bundle (DTNB) in operant responding for food under various schedules of reinforcement. This catecholaminergic neuronal system originates in the nucleus locus coeruleus and has diffuse projections to hippocampus and cerebral cortex. Bilateral stereotaxic injections of 6-hydroxydopamine into the DTNB of rats reduced hippocampal-cortical noradrenaline to less than 5% of control levels. Animals with these lesions acquired a continuously reinforced (CRF) bar-press response at the same rate as controls. Compared to controls rats with DTNB lesions responded at significantly lower rates on a variable interval 30 (VI 30) schedule. Extinction after VI 30 responding did not differ significantly between control and lesioned animals. In another experiment no significant difference was observed between DTNB lesioned and control groups on the rate of responding on a fixed ratio 30 (FR 30) schedule. The results are discussed with reference to previous reports indicating changes in operant responding after intraventricular injections of 6-hydroxydopamine. The data failed to support the hypothesis that the DTNB is critically involved in learning and memory.