Morphofunctional evidence for mature synaptic contacts on the Mauthner cell of 52-hour-old zebrafish larvae

• Published in: Neuroscience Vol. 80; no. 1; pp. 133 - 145
• Main Authors: Triller, A, Rostaing, P, Korn, H, Legendre, P
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01.09.1997; Elsevier
• Subjects: Animals; Axons - ultrastructure; Biological and medical sciences; Brachydanio rerio; Brain - ultrastructure; Central nervous system; electron microcopy; Electrophysiology; Freshwater; Fundamental and applied biological sciences. Psychology; GABA A receptors; glycine receptors; Immunohistochemistry; inhibitory synapses; Microscopy, Electron; miniature synaptic events; patch-clamp; Patch-Clamp Techniques; Synapses - physiology; Synapses - ultrastructure; Time Factors; Vertebrates: nervous system and sense organs; Zebrafish - physiology; inhibitory synapses; glycine receptors; patch-clamp; HEPES, N-2-hydroxyethylpiperazine- N′-2-ethanesulphonic acid; miniature synaptic events; M-cell, Mauthner cell; EGTA, ethyleneglycolbis(aminoethylether)tetra-acetate; electron microcopy; IR, immunoreactive; PDP, presynaptic dense projection; TTX, tetrodotoxin; PBS, phosphate-buffered saline; GABA A receptors; ttp, time-to-peak; EPTA, ethanolic phosphotungstic acid; mIPSC, miniature inhibitory postsynaptic current; AC, axon-cap; Vertebrata; Brachydanio rerio; Larva; Mauthner neuron; Pisces; Glycine receptor; Electrophysiology; Patch clamp method; Gabaergic receptor A
• ISSN: 0306-4522; 1873-7544
• DOI: 10.1016/S0306-4522(97)00092-4

In a previous study, miniature inhibitory synaptic events recorded in the Mauthner cell of the 52-hour-old zebrafish larvae ( Brachydanio rerio) were found to be mainly glycinergic. Their amplitude distribution was not Gaussian and it was proposed that their large amplitude variation might reflect the activation of immature synapses. However, ultrastructural studies of the synaptic contacts over the M-cell soma of 52 h larvae described here, revealed that numerous synaptic contacts on this neuron are already mature at this developmental stage and that most of them already contain a single active zone. As in the adult goldfish, immunohistochemistry indicates the presence of both glycine- and GABA-immunoreactive boutons which establish synaptic contacts. We also found that, in addition to the predominant glycinergic postsynaptic inhibitory currents, some postsynaptic currents are also GABAergic since they are specifically inhibited by bicuculline (20 μM). GABAergic miniature 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 two, Gaussian curves. Outside-out recordings showed one class of GABA receptors with a main conductance state of 23 pS. This indicates that the smallest GABAergic miniature inhibitory synaptic events correspond to the opening of 14–20 chloride channels Pre- and postsynaptic factors which contribute to the predominance of glycinergic synaptic currents over GABAergic ones in untreated preparations and to the striking differences between their frequencies and their respective amplitude distribution histograms are discussed with reference to the morphological characteristics of the mature synaptic endings impinging on this still developing neuron.