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 (CB 1 )-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 CB 1 -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 CB 1 -positive terminals expressed vesicular glutamate transporter 3. Reconstruction of somatic boutons revealed that although their volumes are similar, CB 1 -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 CB 1 -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 persomatic innervation of pyramidal neurons, and may help to explain functional differences in their output properties.

An alternative splicing switch shapes neurexin repertoires in principal neurons versus interneurons in the mouse hippocampus

“…The ultrastructure of perisomatic PV + interneurons output synapses formed onto pyramidal cells (identified based on location and ultrastructural characteristics [Takács et al, 2015]) was unaltered in the conditional single and double knock-out mice (Figure 5C–F and Figure 5—figure supplement 1C–F). Moreover, frequency and amplitude of miniature inhibitory postsynaptic currents (mIPSCs) recorded from CA1 neurons in conditional single and double knock-out mice were unchanged (Figure 5G and H and Figure 5—figure supplement 1G and H).…”

“…( C ) Ultrastructure of inhibitory synapses in CA1 stratum pyramidale of littermate controls and Nrxn1/3 ex21ΔPV mice. Presumptive PV + cell perisomatic termini were identified by the presence of large mitochondria in the synapses apposed on the membrane of the soma and by their morphology (Takács et al, 2015). ( D ) Average vesicle numbers per active zone and ( E ) average active zone length in nm.…”

Mild Traumatic Brain Injury Evokes Pyramidal Neuron Axon Initial Segment Plasticity and Diffuse Presynaptic Inhibitory Terminal Loss

“…Importantly, PV+ interneuron DAI manifested shortly after mTBI and occurred primarily in the perisomatic/AIS domain, prior to the initial branch point of their expansive axonal arbor. Since PV+ interneurons target hundreds of postsynaptic pyramidal neurons (Woodruff et al, 2010; Packer and Yuste, 2011; Taniguchi et al, 2013; Scholl et al, 2015; Takács et al, 2015), disconnection prior to the initial branch point results in widespread, diffuse inhibitory terminal loss. In our recent study, widespread PV+ axonal debris fields within 1 day post-mTBI indicated diffuse terminal degeneration.…”

Highly Energized Inhibitory Interneurons are a Central Element for Information Processing in Cortical Networks

“…2,72 Indeed, most basket cells and axo-axonic cells contain considerably higher numbers of mitochondria in dendrites, somata and presynaptic terminals as compared with other interneuron subpopulations and pyramidal cells. 74,[117][118][119][120][121][122][123] This property is highly conserved in various species of mammals. 74,117,124 Moreover, mitochondria of fast-spiking, PV þ interneurons are enriched with proteins such as cytochrome c oxidase (complex IV) and cytochrome c that are crucial for electron transport in the mitochondrial respiratory chain.…”