Neuropeptides regulate many physiological functions, including neuromodulatory actions, gene transcription and emotional responses.^1-3 They also have been implicated in neuroadaptation, influencing neuronal plasticity and memory formation. However, the neuropeptide-mediated pathways underpinning memory formation have not been elucidated.^1,3,4 Complex expression patterns of their respective receptors in various brain regions, in combination with a lack of receptor-subtype selectivity, their long half-life in the cerebral spinal fluid and the resulting diffusion distances throughout the brain make precise in vivo studies difficult.^5,6

To address these issues, we are developing molecular probes to simulate endogenous neuronal secretion for investigating signalling pathways, with a focus on the role of neuropeptides in emotional memory formation. Using organic chemistry, we synthesised a selection of photolabile protecting groups (photocages) that follow different cleavage mechanisms and wavelengths ranging from UV light to the visible spectrum (365-527 nm). We incorporated these photocages into the neuropeptides oxytocin (OT) and vasopressin (VP) using solid and solution phase synthesis strategies and characterised them pharmacologically at their respective G protein-coupled receptors (OTR, V_1aR, V_1bR) in cellular functional assays.

The photocaged neuropeptides had a >100-1000-fold reduced activity compared to the endogenous ligands, validating our design strategy. Removal of the photocages (uncaging) was carried out via irradiation with UV light and confirmed by LC-MS and confocal microscopy. Incubation of cells with the photocaged neuropeptides did not generate a cellular response, but uncaging with a laser at the probes’ specific uncaging wavelengths yielded the desired physiological response, including cellular contractions and neuropeptide-mediated receptor internalisation.