MORPHOFUNCTIONAL EVIDENCE FOR MATURE SYNAPTIC CONTACTS ON THE MAUTHNER CELL OF 52-HOUR-OLD ZEBRAFISH LARVAE

In a previous study, miniature inhibitory synaptic events recorded in the Mauthner cell of the 52-hour-old zebrafish larvae (Brachydanio rer io) were found to be mainly glycinergic. Their amplitude distribution was not Gaussian and it was proposed that their large amplitude variat ion might reflect the activation of immature synapses. However, ultras tructural studies of the synaptic contacts over the M-cell soma of 52 h larvae described here, revealed that numerous synaptic contacts on t his neuron are already mature at this developmental stage and that mos t of them already contain a single active zone. As in the adult goldfi sh, immunohistochemistry indicates the presence of both glycine- and G ABA-immunoreactive boutons which establish synaptic contacts. We also found that, in addition to the predominant glycinergic postsynaptic in hibitory currents, some postsynaptic currents are also GABAergic since they are specifically inhibited by bicuculline (20 mu M). GABAergic m iniature events (time to peak close to 0.8 ms and decay time-constant close to 4-5 ms) were only detected in the presence of 11.5 mM [KCl](o ). Their amplitude distributions were well fitted by one, or at most t wo, Gaussian curves. Outside-out recordings showed one class of GABA r eceptors with a main conductance state of 23 pS. This indicates that t he smallest GABAergic miniature inhibitory synaptic events correspond to the opening of 14-20 chloride channels. Pre- and postsynaptic facto rs which contribute to the predominance of glycinergic synaptic curren ts over GABAergic ones in untreated preparations and to the striking d ifferences between their frequencies and their respective amplitude di stribution histograms are discussed with reference to the morphologica l characteristics of the mature synaptic endings impinging on this sti ll developing neuron. (C) 1997 IBRO. Published by Elsevier Science Ltd .