Trigeminal pain is a powerful noxious stimulus which engages the central nervous system in a multiplicity of ways with sensory, motor, and motivational-emotional consequences. Specifically, dental and temporomandibular joint (TMJ) pain represent a very common and stressful experience whose underlying neuronal mechanisms are known only in part; in particular, the effects of dental pain on structures outside the trigeminal projection system are poorly understood. Older anatomical and behavioral studies suggested an involvement of the superior colliculus in central pain mechanisms and some recent neurophysiological results support this claim. The deeper strata of the superior colliculus receive a massive projection from the trigeminal brainstem complex and, in turn, project heavily upon the intralaminar thalamus, a major component of central pain systems. Dental pain may constitute a significant input to the superior colliculus and subsequently to the medial thalamus. It is the primary aim of the present study to determine the response patterns of deep tectal neurons responding to tooth pulp stimulation and relate these responses to the tectothalamic projection terminating in the intralaminar thalamus in the anesthetized rat. A secondary aim is to analyze the nociceptive input to the medial thalamus and superior colliculus from the brainstem trigeminal nuclei. In addition to electrical stimulation, chemical stimulation of the tooth pulp with bradykinin will be studied because of the powerful excitation of C fevers by this nonapeptide. Tooth pulp and TMJ responsive neurons will be classified as wide dynamic range or nociceptive specific and by means of antidromic stimulation, nociceptive neurons projecting to the intralaminar thalamus and superior colliculus will be identified. An anatomical analysis of the stained, frozen sections will be made and successful recording sites will be plotted to reveal their distribution in the superior colliculus and trigeminal nuclei. These results should significantly extend our knowledge of central connections of trigeminal pain mechanisms, especially pain of orofacial origin, and aid in the future treatment of trigeminal neuralgias and TMJ dysfunction and management.
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