Age‐related changes in nociception and effect of morphine in the Lou rat

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Abstract
Little is known about how the ageing process affects pain sensitivity and a relevant animal model is therefore required. The effect of age on pain reactivity in animals has been investigated by several experimenters but the results are conflicting. Four groups of male and female Lou/C/Jall rats (4–29 months old) were used for our study. Four pain tests based on evaluation of reflex or more integrated behaviours after a thermal (tail immersion test) or mechanical (paw pressure test and von Frey test) stimulation were used. With mechanical stimulus, a significant decrease in the pain threshold across age was observed, females were more sensitive than males. This increase in nociceptive sensitivity to mechanical stimulation was more pronounced on integrated behaviours (struggle reaction) than on withdrawal reflex. An age‐related increase in sensitivity was found on von Frey test. No effect on the latency of reflex induced by thermal stimulation was observed. In addition, a decrease in the spontaneous motor activity during exploration was observed across ageing; this effect was more marked for the females. The effect of morphine at doses of 1, 3 and 9 mg/kg (s.c.) decreased in intensity across ageing. These data demonstrate the need to use (1) various noxious stimuli because differences were observed in the modification of pain reactivity according to the nature of the stimulus; (2) various pain parameters and particularly integrated behaviours; (3) several age groups. In addition, Lou/C/Jall rat could be a useful model for studying of effect of age on pain.