Development and Biological Characterization of Fluorescent Dynorphins for the Visualization of Kappa Opioid Receptors.

The kappa opioid receptor (KOR) controls a wide variety of biological processes, including pain and reward responses. However, its precise spatiotemporal location remains poorly understood due to limited selective molecular tools. We addressed this problem by developing dynorphin-based fluorescent KOR tracers that are selective for KOR over its closely related mu- and delta-family members (MOR, DOR). Our lead tracer is highly selective for KOR (>360-fold over MOR and DOR) and can effectively visualize KORs in the somatodendritic compartment of cultured primary cortical neurons, as well as in superficial layer neurons of the spinal cord dorsal horn. Furthermore, it blocked the KOR agonist U50,488-induced reduction in the excitatory drive of spinal cord neurons. The developed tracer can also be immobilized with paraformaldehyde and used for post hoc imaging, rendering it a highly valuable tool for mapping KOR expression in physiological and pathophysiological contexts.