ZEBRAFISH TOUCH-INSENSITIVE MUTANTS REVEAL AN ESSENTIAL ROLE FOR THE DEVELOPMENTAL REGULATION OF SODIUM CURRENT

Developmental changes in neuronal connectivity and membrane properties underlie the stage-specific appearance of embryonic behaviors. The be havioral response of embryonic zebrafish to tactile stimulation first appears at 27 hr postfertilization. Because the touch response require s the activation of mechanosensory Rohon-Beard neurons, we have used w hole-cell recordings in semi-intact preparations to characterize Rohon -Beard cell electrical membrane properties in several touch-insensitiv e mutants and then to correlate the development of excitability in the se cells with changes in wildtype behavior. Electrophysiological analy sis of mechanosensory neurons of touch-insensitive zebrafish mutants i ndicates that in three mutant lines that have been examined the sodium current amplitudes are reduced, and action potentials either have dim inished overshoots or are not generated. In macho mutants the action p otential never overshoots, and the sodium current remains small; allig ator and steifftier show similar but weaker effects. The effects are s pecific to sodium channel function; resting membrane potentials are un affected, and outward currents of normal amplitude are present. Develo pmental analysis of sodium current expression in mechanosensory neuron s of wild-type embryos indicates that, during the transition from a to uch-insensitive to a touch-sensitive embryo, action potentials acquire larger overshoots and briefer durations as both sodium and potassium currents increase in amplitude. However, in macho touch-insensitive mu tants, developmental changes in action potential overshoot and sodium current are absent despite the normal regulation of action potential d uration and potassium current. Thus, the maturation of a voltage-depen dent sodium current promotes a behavioral response to touch. A study o f these mutants will allow insight into the genes controlling the matu ration of the affected sodium current.