A morphine-like factor in mammalian brain: analgesic activity in rats.

A partially purified morphine-like peptide 'enkephalin' (PPE) extract from bovine brain elicited pronounced apparent analgesia after injection into the periaqueductal gray matter of rat brain. This analgesia was reversed by the opiate antagonist naloxone in a dose-dependent fashion. Analgesia was more rapid in onset and much shorter in duration after PPE than after morphine administration. Analgesia was elicited only by those ion exchange column fractions of PPE that competed potently for opiate receptor binding. No analgesia could be detected when PPE or morphine injections were administered at a site 2 mm lateral to the periaqueductal gray matter. The potencies of synthetic methionine- and leucine-enkephalin in eliciting analgesia were less than 1% of those of partially purified enkephalin extracts when doses of equivalent ability to compete for opiate receptor binding were compared.