UC San Diego

Endogenous opioids are neuropeptides, which are a type of specialized neurotransmitter that is poorly understand compared to classical neurotransmitters. It is unknown the time scale of action of neuropeptides, the spread of the peptide, or how one peptide might interact with multiple receptors. Mu and delta opioid receptors (MORs and DORs) are both present on hippocampal interneurons and both bind the opioid peptide enkephalin. It is debated whether or not these two functionally similar receptors interact when co-expressed in the same cells, which would have huge implications on drug design and development. Using electrophysiological assays with novel photoactivateable peptides, we found that DOR has faster onset kinetics and higher ligand potency to enkephalin than MOR, making it the dominant receptor. We found no evidence of functional interactions of MOR and DOR in assays for cross-desensitization and heteromer formation suggesting that the two receptors function independently even though they share signaling pathways. In a separate study, to ask about neuropeptide spread, we tested a novel genetically encoded fluorescent sensor, kLight, based on the kappa opioid receptor (KOR) to detect dynorphin, another opioid peptide. Using simultaneous dynorphin uncaging and imaging of kLight activation, we extracted the apparent diffusion coefficient of dynorphin in brain slices. This coefficient can be used to compare diffusion across multiple conditions and address the question of how far a neuropeptide spreads once it's released. These approaches leverage the latest tools to address the holes in our knowledge of opioid signaling and neuropeptide signaling more broadly.