Oxytocin increases nociceptive thresholds in a long-term perspective in female and male rats.

Oxytocin (0.1 and 1.0 mg/kg s.c.) given to male rats during 5 days, increased tail-flick latency when measured 1 (P < 0.05) and 7 days (0.1 mg/kg, P < 0.05; 1.0 mg/kg, P < 0.01) after the last injection. The effect was gone 2 weeks after the end of the treatment. If an additional injection of oxytocin was given 10 days after a previous 5 day treatment period, the significant difference persisted after 3 weeks (P < 0.05). Tail-flick latency was significantly delayed also in oxytocin-treated females when measured 1 week after the treatment period (P < 0.05). Naloxone, but not an oxytocin antagonist, temporarily antagonised the oxytocin induced delay in withdrawal latency. This indicates that oxytocin may act by increasing the activity of opioid mechanisms.