Region selective up-regulation of μ-, δ- and κ-opioid receptors but not opioid receptor-like 1 receptors in the brains of enkephalin and dynorphin knockout mice

The role of endogenous opioid peptides and receptors has recently been investigated using knockout mice. Although the affinities of opioid peptides for opioid receptors has been known for many years there is still some uncertainty over which receptor is the endogenous target for each peptide. To address this issue we have studied using quantitative autoradiography the levels of all four opioid receptor subtypes (μ, δ, κ and opioid receptor-like 1 [ORL1]) in brains sectioned from enkephalin and dynorphin knockouts, as well as from double knockouts. Because receptor up-regulation has been observed when its cognate ligand-peptide is genetically ablated, regional changes in receptor binding in knockout mice may reflect areas where the peptide ligand is tonically active at its receptor or played a role in receptor regulation. In addition, the study aimed to correlate previously observed behaviour in these animals with receptor modulation. Marked region-specific up-regulation of the μ, δ, and κ opioid receptors but not ORL1 receptors was observed in proenkephalin and prodynorphin knockouts. In proenkephalin knockouts this was most pronounced for the μ- and δ-receptor and in prodynorphin knockouts for the κ-receptor. Combinatorial double knockouts did not show any changes in addition to those observed in single knockouts. The largest changes were observed in limbic regions and our results suggest that proenkephalin peptides are tonically active at μ and δ-receptors predominantly in these areas. Prodynorphin peptides appear to regulate mostly the κ-receptor but they are also modulators of μ- and δ-receptors.