This study was carried out in order to clarify the relationship between neuropathicpain and histopathological alternation in the inferior alveolar nerve (IAN) and trigeminal subnucleus caudalis (CauV) in the rat. The procedures of the present experiments were approved by the Animal Care and Use Committee of the Kyusyu Dental College, Japan. Forty-eight male Sprague-Dawley rats, weighing 262g on average, were used. All animals were equally divided into three groups: Chronic IAN compression group, sham operation group and nontreated group. In the IAN compression group, each animal was anesthetized with chloral hydrate (400 mg/kg, i. p.) and a 10μl of medical silicon (Wacker Chemie) was injected into the right mandibular canal through a 24-gauge plastic catheter for applying a sustained compression to the IAN bundle. This operation caused hyperalgesia on the postoperative 4th day. The sham operation group received the insertion of the plastic catheter, but not the injection of silicon. Face withdrawal latency for the noxious heating was measured preoperatively, and on 4, 8, 12, 16, 20, 24 and 28 days postoperatively. Light microscopic observation was also performed on day 21 for the IAN, and on 4, 7, 14, and 21 days for the CauV. The results obtained were summarized as follows: 1) The hyperalgesia due to chronic IAN compression continued throughout the postoperative 4th to 28th days. 2) Chronic IAN compression caused destruction of endoneurium and myelin sheath in the IAN. 3) In the CauV, degenerating neuronal cell bodies and myelin sheaths were found on the postoperative 4th day. The cytoplasms of the cell bodies were deeply stained with both hematoxylin and eosin. Nucleoli of these cells were not observed clearly. Axons underwent atrophy and myelin sheaths were ruptured. 4) The glia cells increased in number from the postoperative 7th day in the degenerating sites in the CauV. In conclusion, the present study showed that neuropathic pain and morphologic changes appear simultaneously in the inferior alveolar nerve and trigeminal subnucleus caudalis in the rat.
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