title: Quantitative ultrastructural analysis of basket and axo-axonic cell terminals in the mouse hippocampus.
abstract: Three functionally different populations of perisomatic interneurons establish GABAergic synapses on hippocampal pyramidal cells: parvalbumin (PV)-containing basket cells, type 1 cannabinoid receptor (CB1)-positive basket cells both of which target somata, and PV-positive axo-axonic cells that innervate axon initial segments. Using electron microscopic reconstructions, we estimated that a pyramidal cell body receives synapses from about 60 and 140 synaptic terminals in the CA1 and CA3 area, respectively. About 60 % of these terminals were PV positive, whereas 35-40 % of them were CB1 positive. Only about 1 % (CA1) and 4 % (CA3) of the somatic boutons were negative for both markers. Using fluorescent labeling, we showed that most of the CB1-positive terminals expressed vesicular glutamate transporter 3. Reconstruction of somatic boutons revealed that although their volumes are similar, CB1-positive boutons are more flat and the total volume of their mitochondria was smaller than that of PV-positive boutons. Both types of boutons contain dense-core vesicles and frequently formed multiple release sites on their targets and innervated an additional soma or dendrite as well. PV-positive boutons possessed small, macular synapses; whereas the total synaptic area of CB1-positive boutons was larger and formed multiple irregular-shaped synapses. Axo-axonic boutons were smaller than somatic boutons, had only one synapse and their ultrastructural parameters were closer to those of PV-positive somatic boutons. Our results represent the first quantitative measurement-using a highly reliable method-of the contribution of different cell types to the perisomatic innervation of pyramidal neurons, and may help to explain functional differences in their output properties.
authors.affiliation: Laboratory of Cerebral Cortex Research, Department of Cellular and Network Neurobiology, Institute of Experimental Medicine, Hungarian Academy of Sciences, 1083, Budapest, Hungary.
authors.forename: Virág T
authors.initials: VT
authors.lastname: Takács
authors.forename: András
authors.initials: A
authors.lastname: Szőnyi
authors.forename: Tamás F
authors.initials: TF
authors.lastname: Freund
authors.forename: Gábor
authors.initials: G
authors.lastname: Nyiri
authors.forename: Attila I
authors.initials: AI
authors.lastname: Gulyás
mesh_terms: D026922:Amino Acid Transport Systems, Acidic; D000818:Animals; D006624:Hippocampus / Q000378:metabolism / Q000648:ultrastructure*; D007395:Interneurons / Q000378:metabolism / Q000648:ultrastructure*; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D008928:Mitochondria / Q000648:ultrastructure; D010320:Parvalbumins / Q000032:analysis; D017729:Presynaptic Terminals / Q000378:metabolism / Q000648:ultrastructure*; D017966:Pyramidal Cells / Q000378:metabolism / Q000648:ultrastructure*; D043884:Receptor, Cannabinoid, CB1 / Q000032:analysis
chemical_list: D026922:Amino Acid Transport Systems, Acidic; D010320:Parvalbumins; D043884:Receptor, Cannabinoid, CB1; C504941:Slc17a8 protein, mouse