https://www.biorxiv.org/content/10.1101/2022.03.26.485911v1.full.pdf neuropeptides are hard to measure, because they live in CSF, and it's hard to get CSF samples in mice. these people came up with fluorescent biosensors that use the neuropeptide-sensitive GCPRs in target cells as sensors, detecting binding at nanomolar concentration. you can even detect them in real time in vivo in mice, with fiber photometry (you run a fiber into the mouse's brain).