Studies have shown that topical administration of exogenous opioid drugs impairs wound healing by inhibiting the peripheral release of neuropeptides, thereby inhibiting neurogenic inflammation. This delay is immediate and peaks during the first days of wound closure. This study examined the effects of topical morphine treatment in a cutaneous wound healing model in the rat.
Full-thickness 4mm diameter wounds were placed on the periscapular region of rats that subsequently received twice-daily topical applications of IntraSite Gel (Smith+Nephew, Hull, United Kingdom) alone or gel infused with 5 mM morphine sulfate on days 0–3 or 4–10 post-wounding or throughout the time course. Wound tissue was taken on days 1, 3, 5, 8, and 18 post-wounding and immunostained for myofibroblast and macrophage markers or stained with hematoxylin and eosin.
Delays in wound closure observed during morphine application on days 0–3 post-wounding mimicked those seen in wounds treated with morphine throughout the entire healing process. However, no significant delays in closure were seen in wounds treated with morphine beginning on day 4 post-wounding. Treatment of wounds with morphine significantly reduced the number of myofibroblasts and macrophages in the closing wound. Additionally, morphine application resulted in decreases in skin thickness and an increase in residual scar tissue in healed skin.
These findings demonstrate the time-dependent and persistent nature of the detrimental effects of topical morphine on cutaneous wound healing. The data identify specific limitations that could be ameliorated to optimize topical opioid administration as an analgesic therapeutic strategy in the treatment of painful cutaneous wounds.