Inhibition of intracranial self-stimulation in brain stem-transected cats--a proposed mechanism of aversive effects produced by brain stimulation.

Effects of intracranial self-stimulation of central 'punishment areas' were studied on an operant conditioning of vertical eye movements in the midpontine pretrigeminal cats as well as in the encéphale isolé cats. In 36 pretrigeminal cats, the ventromedial hypothalamus (VMH), basal amygdaloid nuclei (AMY), dorsal central gray (CG) of the midbrain and the thalamic nuclei such as the ventralis posteromedialis (VPM) and ventralis posterolateralis (VPL) were tested. No suppression of eye movements indicating a passive avoidance conditioning from stimulation of these 'punishment areas' was obtained in 92 electrode tip sites. In 49 encéphale isolé cats, stimulation of the VPM associated with contraction of the facial muscles, demonstrated a marked passive avoidance effect on the eye movements. After blocking both the trigeminal (5N) and facial nerves (7N), VPM stimulation no longer produced an increase of facial EMG activity and the suppressive effect on eye movements was abolished. Extracranial blockade of 7N alone, which induced facial muscle paralysis also showed similar effects. Bilateral blockade of cranial nerves from acoustic (8N) to hypoglossal (12N) nerves had no significant effect on the avoidance conditioning. The mass neural activity recorded from the 5N showed a marked increase of discharge by VPM stimulation which was reduced significantly after 7N blockade. These results may suggest a possibility that punishing effects of brain stimulation depend on feedback from the periphery (muscles, blood vessels and visceral organs), whereas reward effects essentially depend on neural circuitry confined within the forebrain above the rostral pons.