Lesions of midline midbrain structures leave medial forebrain bundle self-stimulation intact

Previous work with psychophysically-based collision methods and pharmacological manipulation suggests a role in medial forebrain bundle (MFB) self-stimulation for neurons lying along the midline between the cerebral hemispheres, in the mid- and/or hindbrain. Also, recently-proposed models of the anatomical substrate for medial forebrain bundle stimulation reward suggest that at least part of the directly-activated axons of this substrate arise from mid- and/or hindbrain somata, bifurcate, and send bilateral projections to the MFB of each hemisphere. Branches of these axons are thought to cross the midline at some point near the ventral tegmental area. This study examines the effects on MFB stimulation reward of lesioning midbrain structures that lie along the midline between hemispheres. In 13 rats, lesions of the median raphe, the decussation of the superior cerebellar peduncle, or the interpeduncular nucleus were all ineffective in altering the stimulation frequency required to maintain half-maximal levels of operant responding for stimulation reward. These results are discussed in terms of implications for recent models of the anatomical substrate for brain stimulation reward.