Systemic naloxone does not affect pain-related behaviour in the formalin test in rat

The effects of systemic naloxone on pain behaviour in rats were examined in modified formalin test. Dilute formalin was subcutaneously injected in the plantar surface of the hindpaw, after which pain was rated according to behavioural criteria. Fifteen minutes after formalin injection, naloxone in various doses (10 micrograms/kg, 100 micrograms/kg, 300 micrograms/kg, 600 micrograms/kg, 800 micrograms/kg, 2000 micrograms/kg) or saline was intraperitoneally injected. Pain behaviour in naloxone injected and saline injected rats was not different. The findings are discussed in relation to previous reports about naloxone effects on different types of pain.