Abstract
To understand characteristics of pain system in the elderly, we investigated the nocifensive behavior to heat stimulation and activity of nociceptive neurons in the lumbar spinal dorsal horn of aged and young rats. The paw withdrawal latency was significantly shorter and paw-licking occurrence was lower in aged rats than those of young rats. The responses of nociceptive neurons to noxious thermal stimulation as well as the spontaneous firing rate were significantly higher in the aged as compared to young rats. Furthermore, the size of high threshold receptive field area of wide dynamic range neurons was larger and that of low threshold area was smaller in aged rats than in young rats. The increased nociceptive neuronal activity in aged rats may cause shorter paw withdrawal latencies to a noxious heat stimulus. Reversible local anesthetic block of descending pathways resulted in a dramatic increase in neuronal activity in young rats but had little effect in aged rats. There was also a significant loss of serotoninergic and noradrenergic fibers in the spinal dorsal horn. These results demonstrate an age-related plasticity in spinal nociceptive processing that is related to impairment of descending modulating pathways.
