This project aims to determine, for the first time, the functional characteristics of a recently discovered pathway which has been proposed to be a specific pain pathway. Subnucleus caudalis (SNC) is the major site in the trigeminal sensory complex involved in processing pain information arising in the orofacial region and in relaying it to thalamus. The main thalamic projection sites of SNC are the ventroposteromedial nucleus (VPM) and the more medially located intralaminar nuclei. It is believed that the sensory-discriminative aspects of pain are processed in VPM and the motivational-affective components in medial regions of thalamus. However, few studies have investigated the role of the medial thalamus in pain processing in spite of its potential relevance to clinical pain states. Recent anatomical studies have described a hitherto unknown pathway originating in the marginal layer of SNC, relaying in nucleus submedius (Sm) in medial thalamus and terminating in the ventrolateral orbital (VLO) cortex. Indirect evidence strongly suggests that this pathway is involved specifically in processing and relay of pain signals. No physiological studies have addressed this potentially very important pathway. The proposed electrophysiological experiments will study the functional properties of this pathway. Microelectrodes will be placed in VLO cortex, Sm and SNC of anesthetized cats and used to antidromically stimulate SNC neurons which project to Sm and Sm neurons which project to VLO cortex. The functional characteristics of these SNC and Sm projection neurons will be determined by applying various nociceptive and nonnociceptive mechanical, thermal and electrical stimuli to the orofacial region including the tooth pulp. The morphology of the SNC neurons and their location within SNC will be determined by intracellular injection of horseradish peroxidase. The effect of stimulating descending pathways originating in the cortex, periaqueductal gray and raphe magnus on the responses of SNC neurons will be investigated. The results of these studies will contribute to our understanding of the mechanisms involved in dental pain and other painful conditions of the orofacial region and may possibly lead to improved clinical methods for the control of pain.
