Reversal of morphine-induced catalepsy by naloxone microinjections into brain regions with high opiate receptor binding: a preliminary report.

The relationship between opiate binding density and morphine-induced catalepsy was estimated via dose-response analysis of the brain sites in which naloxone microinjections reversed the catalepsy induced by intraperitoneal morphine. One-hundred forty-one experimentally naive male Long-Evans rats were implanted with chemical microinjection guide cannulae aimed for various high-to-moderate binding density areas within caudate nucleus, central gray matter, thalamus, hypothalamus, amygdala, and frontal cortex as well as low density sites in pyriform cortex and various fiber tracts. Overall, 48 out of 91 animals microinjected with naloxone in brain sites having high-to-moderate density of opiate binding showed reversal of the cataleptic response. Dose-response effects were found in all 6 high-to-moderate density sites: ranging from 85% reversals at 100 mcg naloxone over all sites to 34% reversals at 0.01 mcg naloxone. There were no reversals out of 38 naloxone microinjection in brain sites having a low density of opiate binding and no reversals out of 18 saline microinjections in either high-to-moderate or low opiate binding density loci. These results suggest a role for limbic and basal ganglia portions of the opiate system in a motor aspect of narcotic action. We speculate that these loci may also play a role in the motor expression of the response to the analgesic and euphoric actions of morphine to supplement actions mediated through periventricular structures.