In order to understand dental pain we need better information (1) about the normal distribution, cytochemistry, ultrastructure and pulpal interactions of the different types of sensory nerves in teeth; and (2) about the mechanisms that drive their responses to injury in the peripheral tissue, in the trigeminal ganglion and in central pain pathways. Nociceptive fibers are now known to have a set of dynamic interactions with healthy tissue, and a very different set of functions after injury that cause pain and contribute to peripheral neuropathies, hyperexcitation of the dorsal horn, inflammation and wound healing. The proposed research is designed to learn more about the normal functions and injury reactions of dental pain fibers. We will use immunocytochemistry, quantitative morphometry biochemical assays, in situ hybridization, and electron microscopy to study three hypotheses related to normal dental innervation: (1) that normal booth function (chewing) of adult rat molars causes changes in neuropeptide immunoreactivity in dental nerves compared to a resting condition; (2) that preterminal regions of dental and periodontal nerves in rats have a focal concentration of glial fibrillary acidic protein, a newly discovered component of peripheral nerves; (3) that pulpal production of nerve growth factor (NGF) continues in old rat molars in regions near surviving dentinal innervation and primary odontoblasts. We will also study the effects of tooth injury on the expression of NGF receptors, neuropeptides, glial fibrillary acidic protein and Fos protein in injured rat molars, trigeminal ganglia and brainstem. We hypothesize that the duration and intensity of the ganglionic changes will depend on the severity of tooth injury, and that Fos protein in the CNS will continue as long as the ganglionic changes occur. We further hypothesize that pulpal inflammation and elevated NGF precede the ganglionic reactions, and that those reactions return to normal as pulpal tissue heals. Graded injuries will be made to produce specific types of pulpal and neural reactions, injury size and duration, and pulpal/dentinal healing. We will analyze the temporal relationships among reactions in teeth, trigeminal ganglia and brainstem at 1,3,7 and 14 days after injury in order to document the condition of the injured tissue when pathologic reactions are found in ganglion and CNS. The long range goal is to improve clinical treatment for pain by understanding the dynamic structure and cytochemistry of sensory nerve fibers in normal teeth and to determine the mechanisms that drive pathologic responses in nociceptive fibers, sensory ganglia and central pain pathways.
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