Morphine and ACTH1-24: correlative behavioral excitations following micro-injections in rat periaqueductal gray

Rats implanted with bilateral cannulas in the periaqueductal gray exhibited similar behavioral excitations following microinjections of morphine sulphate and ACTH1-24. Injections were more effective when the sites were located within rather than below the periaqueductal gray. Analgesia was observed following morphine but not ACTH microinjection. These results confirm that morphine exerts a dual action, inhibitory (i.e. analgesic) and excitatory, with ACTH mimicking only the latter action.