Effects of analgesic midbrain stimulation on reflex withdrawal and thermal escape in the rat

Strong analgesia produced by mesencephalic electrical brain stimulation in rats significantly increased escape latencies in two-way escape from a floor heated by hydraulic circulation. Reflective reactions to pinching and needling were abolished or greatly diminished. Individual differences in the strengths of analgesia, as assessed by instrumental and reflex indicants, were highly correlated. Induction of analgesia was demonstrated in the absence of positive reinforcement effects produced by brain stimulation. The findings are discussed in relation to possible neural pain suppression systems with critical components situated in the midbrain.