Influence of low and high doses of naloxone on tonic pain and motor activity in rats.

Naloxone has been reported to affect pain and locomotor activity differently depending on the dose. The objective of the present investigation was to study the effects of low and high (6 micrograms and 3 mg/kg, s.c.) doses of naloxone (Nx) on formalin-induced pain (tonic pain) and spontaneous motor activity and any correlation between them. The experiments were conducted on adult male Wistar rats. Tonic pain and spontaneous motor activity were recorded by the formalin test and video monitoring respectively. An increase in spontaneous motor activity (locomotion, movements and distance) was observed following formalin injection as compared to basal activity (P < 0.05). Low dose of Nx reduced the pain intensity and also the spontaneous motor activity during the later phase (after 15 min of formalin injection) (P < 0.05). High dose of Nx on the other hand increased the pain intensity but still reduced motor activity (P < 0.05). Both doses of Nx initially produced hyperalgesia (5 min peak). The bidirectional effects of Nx on formalin pain were dissociated from the spontaneous motor behavior of rats. A direct correlation could not be established between pain intensity and spontaneous motor activity.