Morphine analgesia in the formalin test: evidence for forebrain and midbrain sites of action.

A mapping study was performed to determine where in the rat brain morphine acts to produce analgesia in the formalin test, which is an animal model of prolonged pain associated with tissue injury. A single dose (5 nmol) of morphine was bilaterally microinjected into a wide range of brain areas throughout the midbrain and forebrain. Strong analgesia was elicited from the posterior hypothalamic area, the periaqueductal gray and ventral tegmental area. Other sites from which analgesia was elicited were the nucleus accumbens and a few sites in the retrorubral field and caudate-putamen. Analgesia from the periaqueductal gray or nucleus accumbens was accompanied by decreased locomotor activity and catalepsy, whereas analgesia from the posterior hypothalamic area or ventral tegmentum was accompanied by a noticeable increase in locomotor activity and rearing. Morphine into various thalamic nuclei had no effect. These results indicate that the primary sites of action of morphine in the formalin test are probably the posterior hypothalamic area and periaqueductal gray, with an additional contribution from regions innervated by tegmental dopamine cells.