Quantitative ultrastructural analysis of basket and axo-axonic cell terminals in the mouse hippocampus

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.