Lesions of pontomesencephalic cholinergic nuclei do not substantially disrupt the reward value of medial forebrain bundle stimulation

Abstract

This study examines the effects of lesioning the pedunculopontine tegmentum (PPTg) and laterodorsal tegmentum (LDTg) on the reward effectiveness of medial forebrain bundle (MFB) stimulation. Although the focus is on the effects of unilateral lesions made ipsilateral to stimulation sites in the hypothalamic and ventral tegmental MFB, the effects of contralateral lesions of both targets are also investigated. Reward effectiveness was assessed using the rate-frequency curve shift paradigm. In nine rats with unilateral PPTg lesions and five rats with unilateral LDTg lesions, the frequency required to maintain half-maximal response rats was generally not changed by more than 0.1 log units relative to prelesion baseline mean. In three rats with contralateral PPTg lesions and four rats with contralateral LDTg lesions, required frequency was also not substantially changed. The results are interpreted in terms of a previously proposed hypothesis regarding the role in MFB self-stimulation of ascending cholinergic input from the pontomesencephalon to ventral tegmental dopaminergic neurons.