Summary
A quantitative analysis has been made of the distribution of presynaptic profiles containing round (or spheroidal) and flattened (or ellipsoidal) synaptic vesicles in the apical and basal dendritic zones and in the layer of pyramidal cell somata of fields CA1 and CA3 of the hippocampus, and in the molecular and granular layers of the dentate gyrus of the rat and cat.
In the apical and basal dendritic zones of fields CA1 and CA3 the overwhelming majority of the synapses are of the asymmetrical variety, the axon terminals ending principally upon dendritic spines, and to a lesser extent upon the shafts and secondary or tertiary branches of the dendrites. Between 1 and 8% of the axon terminals in these zones contained flattened vesicles: all of these formed symmetrical contacts upon medium-sized or large dendritic shafts. In the molecular layer of the dentate gyrus a slightly higher percentage of flattened vesicle containing profiles was observed (∼10%); again these formed symmetrical contacts upon dendritic shafts. In the stratum pyramidale of the hippocampal fields and the stratum granulosum of the dentate gyrus of the rat, flattened vesicle containing synapses are two or three times more numerous than those with spheroidal vesicles. In the cat hippocampus the axosomatic synapses are about equally distributed between those containing round, and those with flattened vesicles.
The finding that at the focus of post-synaptic inhibition, at the level of the pyramidal cell somata, the majority of the axon terminals contains flattened synaptic vesicles, whereas in the region of termination of the extrinsic, commissural and long association pathways (all of which are excitatory) virtually all the synapses contain round vesicles, strongly supports the view that endings containing flattened vesicles mediate post-synaptic inhibition in the hippocampal formation.
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Supported in part by Grant EY-00599 from the National Eye Institute.
We should like to thank Mr. Paul Myers and Mr. Milburn W. Rhoades for their technical assistance, and Mrs. Doris Stevenson for secretarial help.
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Gottlieb, D.I., Cowan, W.M. On the distribution of axonal terminals containing spheroidal and flattened synaptic vesicles in the hippocampus and dentate gyrus of the rat and cat. Z. Zellforsch 129, 413–429 (1972). https://doi.org/10.1007/BF00307297
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DOI: https://doi.org/10.1007/BF00307297