Developmental age influences the effect of epidural dexmedetomidine on inflammatory hyperalgesia in rat pups

Background: Epidural alpha(2)-adrenergic agonists produce analgesic effects in children and adults, but efficacy and safety have not been established in neonates and infants. The aim of this study was to determine the effect of epidural dexmedetomidine on sensory processing, reversal of inflammatory hyperalgesia, and sedation during early development in rats. Methods: In rat pups aged 3, 10, and 21 postnatal days, mechanical withdrawal thresholds of the hind limbs were measured at baseline and after unilateral inflammation due to carrageenan. The effect of epidural dexmedetomidine on withdrawal thresholds was measured for 90 min after injection, and dose-response curves were constructed for each age group. The duration of the righting reflex was measured to assess sedation. The effects of epidural and systemic administration of dexmedetomidine were compared. Results: At all ages, carrageenan-induced hyperalgesia was reversed by doses of epidural dexmedetomidine that did not affect the threshold of the contralateral paw or prolong the righting reflex. Higher doses of epidural dexmedetomidine affected baseline nociception in the contralateral paw and produced sedation but had no effect when given systemically. Reversal of hyperalgesia and sedation were produced by lower doses of epidural dexmedetomidine in the youngest pups. Conclusions. Spinally mediated selective reversal of inflammatory hyperalgesia by epidural dexmedetomidine can be achieved at all ages; relatively lower doses are effective in early life, but the therapeutic window is narrow. These data have implications for the use and dosing of epidural alpha(2) agonists in neonates and infants.