Stimulation-produced analgesia under repeated morphine treatment in rats

Components of complex emotional reaction to nociceptive stimulation as well as antinociceptive effect of periaqueductal gray matter (PAG) electrical stimulation were determined in rats. Animals were treated with morphine hydrochloride or saline for 10 days. Morphine analgesic effect during subchronic dosage (50 mg/kg a day) was gradually decreased. The same was true for stimulation-produced analgesia (SPA). Naloxone (2 mg/kg) exerted a partially antagonistic effect in relation to SPA in saline-treated and failed to abolish SPA in morphine-treated rats. It is concluded that the opiate component of the antinociceptive system is of importance for the tolerance development to morphine-like drugs.