Inflammation and macrophage infiltration exacerbate adult incision response by early life injury.

BACKGROUND

Neonatal hindpaw incision can evoke long-lasting changes in nociceptive processing following repeat injury in adulthood. Studies have focused on the effects and mechanisms in the spinal cord and brain, however changes in inflammation and macrophages in the periphery, especially at the site of early life injury, remain poorly defined. In this paper, we investigated the role of macrophages in the injured tissue in pain hypersensitivity caused by repeat hindpaw incisions and primed by neonatal injury.

METHODS

Hindpaw incision was performed in anesthetized adult rats. Among them, some had neonatal hindpaw incisions on postnatal day 3. To assess the role of inflammatory response in the priming of adult incision pain, the rats were treated with clodronate liposome, a macrophage depletion agent, and ketorolac tromethamine, the commonly used anti-inflammatory drug following surgery. Their mechanical pain sensitivity was measured via von Frey filaments. Inflammation induced by hindpaw incision was evaluated via Enzyme-linked Immunosorbent Assay, H&E, and immunofluorescence staining. The phenotypes of macrophages were examined by analyzing their surface markers by flow cytometry.

RESULTS

Mechanical pain hypersensitivity caused by the hindpaw incision in the adult rats was enhanced by previous neonatal injury, which also significantly increased microglial activation in the spinal dorsal horn, aggravation of inflammation, and infiltration of both M1 and M2 macrophages in damaged hindpaw tissue after the repeat incision in the adult rats on POD 5. Intraperitoneal injection of clodronate liposome alleviates nociceptive and inflammatory responses in neonatal injured rats. Intramuscular injection of ketorolac tromethamine decreased mechanical hyperalgesia and inflammatory responses primed by prior neonatal injury.

CONCLUSIONS

Neonatal tissue injury exacerbated mechanical hypersensitivity, infiltration, and activation of macrophages evoked by repeat hindpaw incision in adulthood.