• Suellen M Walker, Keri K Tochiki, and Maria Fitzgerald.
• Portex Unit, Pain Research, UCL Institute of Child Health and Great Ormond St. Hospital, 6th Floor Cardiac Wing, London, UK. suellen.walker@ich.ucl.ac.uk
• Pain. 2009 Dec 15;147(1-3):99-106.

AbstractPain in early life can enhance the response to subsequent injury, but effects are influenced by both the nature and timing of neonatal injury. Using plantar hindpaw incision, we investigated how postnatal age influences the response to repeat surgical injury two weeks later. The degree and time course of behavioural changes in mechanical withdrawal threshold were measured, and injury-related hyperalgesia was further quantified by flexion reflex electromyographic responses to suprathreshold mechanical stimuli 24 h following incision. Plantar hindpaw incision produces acute mechanical hyperalgesia in neonatal and adult rats, but incision in neonatal pups has an additional effect on the response to subsequent injury. With initial incision at postnatal day (P) 3 or 6, the degree of hyperalgesia following repeat incision 2 weeks later was greater than in animals having a single incision at the same age. At older ages (initial incision at P10, P21 or P40) responses did not differ in repeat and single incision groups. To test the role of primary afferent activity, levobupivacaine sciatic block was performed prior to P6 plantar incision, and controls received saline or subcutaneous levobupivacaine. Repeat peri-operative, but not a single pre-operative sciatic block, prevented the enhanced response to repeat incision two weeks later. Our results show that the first postnatal week represents a critical period when incision increases hyperalgesia following repeat surgery two weeks later, and effects are initiated by peripheral afferent activity. This has potential therapeutic implications for the type and duration of peri-operative analgesia used for neonatal surgery.

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