Heterogeneous distribution and upregulation of mu, delta and kappa opioid receptors in the amygdala.

Levels of mu, delta and kappa opioid receptors in 4 subnuclei of the rat amygdala were determined by quantitative autoradiography following chronic treatment with naloxone or saline. A different distribution of each receptor subtype was observed, with mu binding greatest in the lateral nucleus (La), delta greatest in the basolateral (Bl), and kappa greatest in the medial (Me). Levels of all 3 receptors were very low in the central nucleus. Receptor upregulation following chronic naloxone treatment was also anatomically heterogeneous. Increases in mu receptors were statistically significant in the Me, Bl and La, while increases in delta and kappa receptors were significant only in the Bl.