Abstract
Changes in pain response and the analgesic efficacy of morphine and clonidine during senescence were studied in three groups of rats with different ages (54.4 ± 1.2, 162.2 ± 3.7, and 327.8 ± 1.7 days, mean ± se), using the hot plate assay as the nociceptive test. We found a progressive decrease in the hot plate latency, and a gradual reduction in the analgesic potency of morphine and clonidine in aging rats. We speculate that such changes were related to a decline in the content of catecholamines and acetylcholine, as well as a decrease in concentrations of dopamine, acetylcholine, and opiate receptors in the aging brain. These results have implications for the role of the medullary nucleus reticularis gigantocellularis as a common neural substrate for pain suppression and analgesia promoted by morphine and clonidine.
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