Morphine injected into the habenula and dorsal posteromedial thalamus produces analgesia in the formalin test.

Microinjection of morphine into the area of the habenula and dorsal posteromedial thalamus (H-PMT) produces analgesia for tonic pain as measured by the formalin test in the rat. Control injections of morphine into sites near the H-PMT result in less or no reduction in pain, indicating that the analgesia observed is probably due to a site of action within the H-PMT rather than at surrounding neural structures. The analgesia is fully developed by the first time of testing, 10-16 min following the microinjection, and is completely reversible by naloxone, an opiate antagonist. The analgesia recorded is most likely due to morphine's action on the habenula, parafascicular or paraventricular nucleus of the thalamus, or a combination of these structures.