PHYSIOLOGICAL-PROPERTIES OF THE MAUTHNER SYSTEM IN THE ADULT ZEBRAFISH

We investigated the morphological and electrophysiological properties of the Mauthner (M-) cell and its networks in the adult zebrafish (Dan io rerio) in comparison with those in the goldfish (Carassius auratus) . The zebrafish M-cell has an axon cap, a high resistivity structure w hich surrounds the initial segment of the M-axon, and accounts for an unusual amplification of the fields generated within and around it. Se cond, extra-and intracellular recordings were performed with microelec trodes. The resting potential was similar to-80 mV with an input resis tance of similar to 0.42 M Omega. The M-cell extracellular field was l arge (10-20 mV), close to the axon hillock, and the latency of antidro mic spikes short (similar to 0.4 milliseconds), confirming a high cond uction velocity in the M-axon. The extrinsic hyperpolarizing potential (EHP), which signals firing of presynaptic cells and collateral inhib ition, was markedly lower at frequencies of spinal stimulation > simil ar to 5/second, suggesting an organization of the recurrent collateral network similar to that in the goldfish. Inhibitory postsynaptic pote ntials (IPSPs) were highly voltage-dependent; their decay time constan t was increased by depolarizations. The presynaptic neurons which are numerous could be identified by their passive hyperpolarizing potentia l (PHP) produced by the M-spike current. Auditory responses, mediated via mixed synapses (electrical and chemical), had short delays and hen ce are well suited to trigger the escape reaction. The similarities of their properties indicate that the wealth of information generated ov er decades in the goldfish can be extrapolated to the zebrafish. J. Co mp. Neurol. 395:493-509, 1998. (C) 1998 Wiley-Liss, Inc.