Abstract
The distribution of glycine receptors on the plasmalemma of the teleost Mauthner cell was examined with light and electron microscopy using monoclonal antibodies raised against the glycine receptors of the rat. These immunoglobulins recognize a peptide of 93 kDa associated with the intracellular side of the receptor complex. With indirect immunofluorescence, labeling appeared as small clusters over the whole surface of the neuron. This distribution indicates that, unlike other neurons, the Mauthner cell can express glycine receptors at the dendritic and at the somatic level as well. Vestibular or reticular neurons lying in the proximity of the Mauthner cell were also stained. At the ultrastructural level, the immunoenzymatic reaction product, localized in front of the presynaptic active zones, slightly overextends the limits of the synaptic complex. This observation may account for vesicular exocytosis at the border of the presynaptic grid. More generally, inside and outside the axon-cap, stained postsynaptic membranes were apposed to terminals identified as unmyelinated club endings and some small vesicle boutons, respectively. Each of them could establish classical flat synapses or spinelike contacts, the latter being described here for the first time. Patches of membrane remained unstained, however, particularly along the lateral and ventral dendrites of the cell; they corresponded to the contact sites of excitatory afferents. In order to enhance the penetration of immunoreactive substances, a particular fixation procedure was used; its effect on the shape of the synaptic vesicles is described. Our results support the notion that glycine is responsible for both somatic and remote dendritic Cl- -dependent inhibition of the Mauthner cell. The first one is produced by the so-called PHP exhibiting neurons, which correspond to cells of recurrent collateral network and vestibulovestibular pathway. In contrast, cells giving rise to the more distal inhibitory synapses on both dendrites are not yet characterized.
