Alone in a crowd: Effect of a nonfunctional lateral line on expression of the social hormone parathyroid hormone 2.

Parathyroid hormone 2 (Pth2) is a vertebrate-specific neuropeptide whose thalamic expression is upregulated by social contact with conspecifics. However, social interactions fail to stimulate pth2 expression in isolated zebrafish whose lateral line hair cells have been chemically ablated. These results suggest that modulation of pth2 by social context is acutely dependent on mechanosensory information from the lateral line. However, it is unclear how a congenital loss of lateral line function influences the ability of zebrafish to interpret their social environment. In this study, we measure pth2 levels in zebrafish mutants lacking hair cell function in either the lateral line only, or in both the inner ear and lateral line. Socially-raised lateral line mutants express lower levels of pth2 relative to wild type siblings, but there is no further reduction when all sensory hair cells are nonfunctional. However, social isolation of hair cell mutants causes a further reduction in pth2 expression, pointing to additional unidentified sensory cues that influence pth2 production. Lastly, we report that social context modulates fluorescent transgenes driven by the pth2 promoter. Altogether, these data suggest that lateral line mutants experience a form of isolation, even when raised in a social environment. SUMMARY STATEMENTExpression of the pro-social neuropeptide pth2 is downregulated in larval zebrafish with a congenital loss of lateral line function. Thus, even in social environments, fish with compromised lateral lines may experience a form of isolation.