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Involvement of Dorsal Column Nucleus Neurons in Nociceptive Transmission in Aged Rats

¹Department of Physiology, School of Dentistry, Nihon University, Tokyo, ²Division of Functional Morphology, Dental Research Center, Nihon University School of Dentistry, Tokyo, ³Division of Applied System Neuroscience, Advanced Medical Research Center, Nihon University Graduate School of Medical Science, Tokyo, ⁴Department of Dental Anesthesiology, Osaka University, Graduate School, Faculty of Dentistry, Osaka, ⁵Department of Histology, Kanagawa Dental College, Kanagawa, ⁶Division of Oral and Maxillofacial Biology, Institute for Oral Science, Matsumoto Dental University, Nagano, and ⁷Motor and Autonomic Nervous System Integration Research Group, Tokyo Metropolitan Institute of Gerontology, Tokyo, Japan; and ⁸Department of Biomedical Sciences, University of Maryland Dental School, Baltimore, Maryland

Submitted 8 March 2005; accepted in final form 9 August 2005

To clarify the functional role of the dorsal column nucleus (DCN) in nociception in rats with advancing age, single neuronal activity and substance P–like immunoreactivity (SP-LI) of the gracile nucleus (GN) were studied in aged rats (29 to 34 mo old) and adult rats (9 to 12 mo old). A total of 122 neurons [aged: 34 wide-dynamic-range (WDR), two nociceptive-specific (NS), and 32 low-threshold mechanical (LTM) neurons; adult: 22 WDR and 32 LTM neurons] were recorded from GN. For WDR neurons, the latency to antidromic activation of the ventral posterior lateral nucleus of the thalamus showed no difference between the aged and adult rats. Sciatic nerve stimulation with C-fiber intensity induced responses of GN with significantly longer latency in aged rats than in adults, whereas there was no difference in the response latency to A-fiber intensity stimulation. Background activity and afterdischarges were significantly higher in the aged rats than those in the adult rats. Responses to noxious mechanical and thermal stimuli were significantly greater in the aged rats during application of graded stimuli. There were no significant differences in responses to nonnoxious mechanical stimulus, mechanical response threshold, and the size of the receptive fields between neurons in the aged and adult rats. The area occupied by SP-LI fibers in the GN and the size of SP-LI dorsal root ganglia neurons were significantly larger in aged rats than in adults. The present findings suggest that the hyperexcitability of GN neurons could be involved in abnormal noxious pain sensations with advancing age.