Differential effects of intracerebrally microinjected enkephalin analogs on centrally versus peripherally induced pain, and evidence for a facial versus lower body analgesic effect

[d-Ala²-Met-Enkephalin and [d-Ala²]-Met-Enkephalinamide were microinjected (10–30 μg) into the midbrain ventrolateral central gray of rats. The opiate analogs produced profound analgesia in left and right facial areas, and on the hot plate test. The tail-flick test showed significant analgesia, but in a significantly smaller amount than that obtained with noxious face heating. All effects were blocked by naloxone pre-treatment. The drugs had no effect on thresholds for defense responses to high (200 Hz) and low (20 Hz) frequency aversive stimulation in midbrain areas associated with pain perception.