New structural features of synapses in the anteroventral cochlear nucleus prepared by direct freezing and freeze‐substitution

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Abstract
The rostral anteroventral cochlear nucleus (AVCN) of the chinchilla provides a preparation in which neuronal cell bodies and synapses in the mammalian central nervous system can be examined after direct freezing and freeze‐substitution of rapidly excised brain stem slices. Cell bodies and synapses in the freeze‐substituted AVCN differed from those in perfusion‐fixed AVCN in several interesting respects. Despite of these differences, four types of synaptic terminal were distinguished in freeze‐substituted AVCN and correlated with the four well‐known types of perfusion‐fixed terminal. Since the transmitter at each of the four types of terminal has been tentatively identified, the structure of synaptic vesicles and junctions in the freeze‐substituted terminals could be related to transmitter type. Synaptic vesicles were uniformly round, but their diameters, deployment, and related cytoskeletal elements near the synaptic junction differed in each chemical type of synapse; the synapses thought to be cholinergic, for instance, had only a few vesicles clustered at their presynaptic junctions while the rest of the vesicles were separated from the junction by a network of fine filaments. Two types of filamentous components, short vertical projections from the postsynaptic membrane and thin filaments protruding from these projections, comprised the basic structure of the postsynaptic specialization, but their sizes and distribution differed at each chemical type of terminal. For instance, the postsynaptic specialization at the glycine terminal was distinguished by numerous thin filaments which curved sideways to run parallel to the plasmalemma. Thus, freeze‐substitution gives new information about structural differences between chemically different types of synapses, which may reflect differences in their transmitter storage, release, and reception. In addition, the AVCN preparation is of general interest in making cell bodies in the mammalian central nervous system available to the various structural and analytical techniques which depend on direct, rapid freezing.