ABSTRACT
Ovariohysterectomy in the rat led to the induction of abdominal postures and referred mechanical allodynia in the hind paws. The latter was differentiated into static and dynamic subtypes. The abdominal postures were present up to 4-5 h, whilst the two types of allodynia lasted for at least 2 days. A single administration of morphine 30 min before surgery dose-dependently (0.1-3 mg/kg, s.c.) blocked the development of abdominal postures and the two types of mechanical allodynia. The highest dose of morphine almost completely blocked these responses. The duration of action of 3 mg/kg morphine was short and similar (1.5-2 h) when administered either before or after surgery. However, multiple administrations of morphine (0.5 h before, and 0.5 and 2 h after surgery) blocked the development of abdominal postures and both allodynias for up to 2 days. In contrast, administration of three doses of morphine (3 mg/kg) in a similar dosing regime but starting 24 h after surgery, only blocked the two types of allodynia for 4 h. These data indicate the importance of blocking the induction phase of surgical pain and support the concept of pre-emptive analgesia. It is suggested that the ovariohysterectomy model should prove to be useful for studying mechanisms and designing novel therapeutic strategies for the treatment of post-operative pain.
