Reassessment of opioid binding sites in the rat brain.

Opioid binding sites have been characterized pharmacologically in membranes from different areas of the rat brain. Delta, mu and sites belonging to the kappa family (K1, K2, K3) have been detected. Delta sites were more abundant in cortex and striatum, mu sites in striatum and hypothalamus, while kappa binding site concentration was higher in deeper enkephalic structures (brainstem, cerebellum, hypothalamus) and the pituitary gland. A distinct distribution of each subtype of the kappa site was found: kappa 1 sites were higher in the spinal cord, kappa 2 sites in the brainstem and kappa 3 sites in cerebellum. The distribution of delta and kappa sites in the central nervous system was correlated with the distribution of proenkephalin-A derived peptides and precursors, suggesting that these peptides could be their endogenous ligands.