Temporal and spatial characterization of nicotinic acetylcholine receptor expression in Lepeophtheirus salmonis

Nicotinic acetylcholine receptors (nAChRs) are key mediators of synaptic transmission in invertebrates and major targets of pesticide action. In early life stages of marine Copepoda, such as the parasitic salmon louse (Lepeophtheirus salmonis), the expression dynamics and tissue localization of nAChRs remain poorly characterized. Here, we investigate the expression of nAChR-related transcripts across the life cycle of L. salmonis, and spatial distribution of expression in the early copepodid life stage. Using RNA sequencing of pooled specimens from eleven groups (nauplius I to adults), we identify statistically significant differences in expression patterns. Early life stages appear to have higher expression levels of all nAChR subunits compared to mobile life stages. Comparison with synapsin indicates that these patterns largely correlate with neural tissue abundance. However, specific subunits exhibit distinct stage- and sex-specific expression profiles, suggesting temporal specialization of nicotinic cholinergic signalling. RNA-FISH localization further reveals receptor-specific spatial patterns in the central ganglia, antennae, and near ocular structures in copepodids. Together, these results demonstrate that nAChR expression in L. salmonis is governed by a combination of neural tissue abundance and subunit-specific regulatory dynamics, providing a foundation for understanding nAChR function across life stages.