ROLE OF ANTERIOR PRETECTAL NUCLEUS IN SOMATOSENSORY CORTICAL DESCENDING MODULATION OF JAW-OPENING REFLEX IN RATS

Both the somatosensory cerebral cortex and anterior pretectal nucleus (APT) have been shown to produce descending modulation of trigeminal (V) and spinal somatosensory neurone and reflex activities. Since the APT receives a direct projection from the somatosensory cortex, experiments were performed to compare the effects of APT and somatosensory cortex stimulation on the jaw-opening reflex (JOR) and to examine the possible involvement of APT in corticofugal modulation. Conditioning stimulation of the ipsilateral or contralateral somatosensory cortex in chloralose-urethane anaesthetized rats induced inhibition of the JOR elicited by test stimulation of the maxillary skin or tooth pulp, at conditioning-test intervals between 30 and 200 ms. A similar time course of inhibition of the JOR was noted with APT conditioning stimulation. Local unilateral injection of 2% lidocaine (0.3-0.6 μl) into APT could partially and reversibly reduce the ipsilateral cortically evoked inhibition of the JOR. Ibotenic acid (5 μg, 0.5 μl)-induced unilateral lesions of APT and its adjoining structures also greatly reduced the ipsilateral cortically induced inhibition of the JOR. Histological reconstruction of APT lesion sites and data analysis indicated that this reduction in the corticofugal inhibition was proportionally and significantly related to the extent of damage to APT, but not to its adjoining structures. These findings collectively suggest that the cortically-induced inhibition of the JOR is at least partly mediated by a relay in the APT. © 1990.