Abstract
In order to clarify the effect of age-related change in trigeminal nociception advancing age, phosphorylation of extracellular signal-regulated kinase (ERK) in trigeminal spinal subnucleus caudalis (Vc) neurons was studied in the aged rats following subcutaneous capsaicin injection into the whisker pad and the following results were obtained. Adult (9-12 months) and aged (30-34 months) Fischer male rats were used for the present study. Rats were injected with 10mM capsaicin into the right whisker pad subcutaneously and naloxone was injected intravenously 20 min before capsaicin injection. The mechanical escape threshold was significantly lower in aged rats compared with adult rats. A large number of pERK-LI cells were expressed in the superficial laminae of Vc in adult and aged rats following subcutaneous capsaisin injection into the whisker pad region. The number of pERK-LI cells was largest at about 2.0 mm caudal from the obex and gradually decreased in their numbers in more rostral and caudal sections. The rostro-caudal distribution profile of pERK-LI cells expressed after subcutaneous capsaicin injection into whisker pad was similar in adult and aged rats. The number of pERK-LI cells was slightly larger in aged rats compared with that of adults. The larger number of capsaicin-induced pERK-LI cells was expressed in adult rats than aged rats following intravenous administration of naloxone. The present findings suggest that the descending modulation system was impaired advancing age, resulting in the abnormal pain sensation in aged rats. [J Physiol Sci. 2008;58 Suppl:S124]
