Abstract
<Objectives> The aim of present study is to elucidate the underlying neuronal mechanism of the change in trigeminal nociceptive transmission with advancing age. <Methods> The adult (8-12 months old) and aged (30-12 months old) rats were injected with 10 mM capsaicin into the right whisker pat under adequate anesthesia. In addition, other adult and aged rats were injected capsaicin into the right whisker pat at 20 min after the naloxone administration (1.2 mg/kg, i.v.) in order to study the involvement of descending modulation system on trigeminal nociception in aged rats. They were perfused at 5 min after capsaicin injection. The whole brain was removed, and then 30 μm thick serial sections were made. We analyzed the change in pERK-LI expression in the medulla and the upper spinal cord of adult and aged rats. <Results> A large number of pERK-LI cells were expressed in the superficial laminae of the ipsilateral trigeminal subnucleus caudalis (Vc) and C1 of the spinal cord in each rats. A few pERK-LI cells were observed in the paratrigeminal nucleus bilaterally in the adult and aged rats. The expression pattern of pERK-LI cells in Vc/C1 was not difference between adult and aged rats. After the naloxone administration, the number of pERK LI cells in the ipsilateral Vc/C1 in the adult rats was significantly larger than that of aged rats. <Conclusions> These findings suggest that the advancing age may lead to the dysfunction of descending pain modulation system as well as ascending system, resulting in the abnormal pain sensation in aged rats. [J Physiol Sci. 2006;56 Suppl:S181]
