There is a wide body of evidence to suggest the existence of at least three distinct opioid receptor types in the CNS, referred to as~, 6, and K. This paper reviews some of the findings that have led to this conclusion and the anatomical distributions of these sites in the rat brain. Their relation to the opioid peptides and some of the proposed functions mediated by these receptor systems are also discussed.

Largely because of their clinical significance and recent technological advances, the opioid systems are among the best understood peptide systems in the CNS. While it is difficult to pinpoint precisely what has caused this interest, it is clear that the key elements included the discovery of opioid peptides in the CNS 1, the description of their genes, mRNAs and precursors z-4, the characterization of their receptors 5, and the finding that these pepfides are released following certain forms of stimulation s. Since it is well beyond the scope of this paper to review this field comprehensively, we have chosen to concentrate our efforts on the anatomical distribution of the opioid receptors in the CNS and their relationships with the opioid pepfide systems. Having presented this anatomical information, we will then discuss some of the functions thought to be mediated by these multiple receptors.