Lesions and inactivation implicate dorsolateral hindbrain in MFB self-stimulation

Two experiments explored the role of the motor nucleus of the trigeminal nerve (Mo5) and surrounding area in the rewarding effects of medial forebrain bundle (MFB) stimulation. In the first, eight rats received serial bilateral lesions of the target region. The reward value of MFB stimulation was assessed at 200, 400, and 800 microA using the rate-frequency curve shift paradigm. In five rats, no lesions affecting the motor nucleus or its surrounding area affected the frequency required to maintain half-maximal response rate at any current. One rat with a relatively ventrally placed lesion showed substantial enhancement of stimulation reward value at two currents, while two rats with lesions affecting the area around the descending fibers of the superior cerebellar peduncle (scp) showed substantial increases in required frequency. In the second experiment, six rats received uni- and bilateral injections of lidocaine to temporarily inactivate the target area. Two rats with injections centered near the descending fibers of the scp showed substantial increases in required frequency, as great as 0.30 log(10) units. Two rats with injections slightly rostral to these showed little change in required frequency. Two rats with injections in the ventral cerebellum, just lateral to the fastigial nucleus, showed increases in required frequency, particularly following injections contralateral to the MFB stimulation site. These data are interpreted to imply a role for the area around the lateral pole of the scp, perhaps including axons arising from the cerebellum, in MFB stimulation reward.