Effect of oxytocin pretreatment on the development of morphine tolerance and dependence in rats

•Repeated oxytocin is suggested to increase α-2 adrenergic and opioidergic activity.•Oxytocin increases nociceptive threshold in the hot plate 24 h after the last injection.•Oxytocin pretreatment facilitates tolerance to morphine-induced analgesia.•Oxytocin pretreatment causes less severe naloxone-induced morphine withdrawal.•Oxytocin pretreatment increases nociceptive and body temperature responses to clonidine.