In vivo autoradiography: visualization of stress-induced changes in opiate receptor occupancy in the rat brain

A method of in vivo autoradiography was utilized which allows the visualization of local changes in opiate receptor occupation in the intact rat brain. The method is based on the exclusion of [3H]diprenorphine binding in areas in which the release of endogenous opiate peptides is increased by behavioral manipulation. The technique lends itself to the use of film autoradiography, allowing the mapping of relative levels of functional receptor occupancy throughout the whole brain. Prolonged intermittent footshock and forced swims in cold water (two stress-inducing manipulations which are known to release endogenous opiates) were found to cause highly significant decreases in specific high-affinity [3H]diprenorphine binding, as measured by liquid scintillation counting. These changes were unaccompanied by corresponding changes in non-specific binding and were not related to local changes in blood flow. A prolonged non-stressful swim in warm water caused no changes in [3H]diprenorphine binding. The use of tritium-sensitive film autoradiography allowed the resolution of these decreases to the level of individual nuclei. Differences in specific binding were found to be greatest in the periaqueductal gray, the reticular formation, and in midline-intralaminar thalamic nuclei, all of which have been implicated in the modulation of pain sensation.