Morphine alleviates pain after heart cryonjury in zebrafish without impeding regeneration

Nociceptive response belongs to a basic animal behavior facilitating adaptability and survival upon external or internal stimuli. Fish, similarly to higher vertebrates, also possess nociceptive machinery. Current protocols involving procedures performed on adult zebrafish including heart cryoinjury do not, however, take into account the adverse effects including pain that may potentially arise from these methodologies. Here, we assess the effect of two analgesics, lidocaine and morphine, followed after the heart cryoinjury in zebrafish. Monitoring swimming behaviour together with histology and gene expression analysis at the single cell level using scRNA sequencing and RNAscope fluorescent in situ hybridization technology, we show morphine, but not lidocaine, significantly improves animal welfare 6 hours post-cryoinjury, without impairing heart regeneration process. Altogether, morphine should be considered as the analgesic of choice to reduce post-surgical pain in adult zebrafish. Overall design: Single cell RNAseq of morphine treated and untreated zebrafish heart samples at 3, 7 and 15 days post injury