A caged DAMGO for selective photoactivation of endogenous mu opioid receptors

Photoactivatable drugs and peptides are valuable tools that can drive quantitative studies into endogenous receptor signaling in relatively intact tissue preparations with high spatiotemporal resolution. We previously developed photoactivatable or caged variants of the opioid neuropeptides [Leu5]-enkephalin and dynorphin A (1-8), but these reagents activate multiple opioid receptors simultaneously upon photolysis. To achieve selective engagement of mu opioid receptors (MORs), we developed a photoactivatable derivative of the MOR-selective peptide agonist [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO). Starting from commercially available DAMGO, we appended a negatively charged carboxynitroveratryl (CNV) caging group to the N-terminal tyrosine side chain (Y) to produce CNV-Y-DAMGO. Dose-response curves in a heterologous functional assay revealed that CNV-Y-DAMGO is ~1,000-fold less potent than DAMGO at MORs. In acute hippocampal brain slices, CNV-Y-DAMGO photoactivation with ultraviolet light resulted in rapid, transient suppression of inhibitory synaptic transmission, and this was blocked by the MOR-selective antagonist CTAP. These results validate CNV-Y-DAMGO as a photopharmacological reagent for manipulating endogenous MOR signaling in brain tissue with light.