Similar GABAA receptor subunit composition in somatic and axon initial segment synapses of hippocampal pyramidal cells.

Katalin Kerti-Szigeti,Zoltan Nusser

doi:10.7554/elife.18426

Abstract

Hippocampal pyramidal cells (PCs) express many GABAAR subunit types and receive GABAergic inputs from distinct interneurons. Previous experiments revealed input-specific differences in α1 and α2 subunit densities in perisomatic synapses, suggesting distinct IPSC decay kinetics. However, IPSC decays evoked by axo-axonic, parvalbumin- or cholecystokinin-expressing basket cells were found to be similar. Using replica immunogold labeling, here we show that all CA1 PC somatic and AIS synapses contain the α1, α2, β1, β2, β3 and γ2 subunits. In CA3 PCs, 90% of the perisomatic synapses are immunopositive for the α1 subunit and all synapses are positive for the remaining five subunits. Somatic synapses form unimodal distributions based on their immunoreactivity for these subunits. The α2 subunit densities in somatic synapses facing Cav2.1 (i.e. parvalbumin) or Cav2.2 (cholecystokinin) positive presynaptic active zones are comparable. We conclude that perisomatic synapses made by three distinct interneuron types have similar GABAA receptor subunit content.

Highlights

A salient feature of cortical microcircuits is the presence of distinct types of GABA-releasing interneurons (INs; Freund and Buzsaki, 1996; Somogyi and Klausberger, 2005; Ascoli et al, 2008; Krook-Magnuson et al, 2012; DeFelipe et al, 2013)
The basic principle of sulphatedigested freeze-fracture replica immunolabeling technique (SDS-FRL) is that following random fracturing of frozen tissue, the fracturing plane often crosses the lipid bilayer of plasma membranes with transmembrane proteins remaining either in the protoplasmic (P-face) or extracellular (E-face) half of the membrane (Fujimoto, 1995; Rash et al, 1998; Masugi-Tokita and Shigemoto, 2007)
We found a strong positive correlation between P- and E-face gold particle numbers (Figure 1F), demonstrating that GABAA receptors (GABAARs) fracture to both faces of the plasma membrane and the synapse-to-synapse variability in gold particle number is not the Neuroscience consequence of synapse-to-synapse variability in fracturing to E- and protoplasmic face (P-face)

Summary

Introduction

A salient feature of cortical microcircuits is the presence of distinct types of GABA-releasing interneurons (INs; Freund and Buzsaki, 1996; Somogyi and Klausberger, 2005; Ascoli et al, 2008; Krook-Magnuson et al, 2012; DeFelipe et al, 2013). Hippocampal PV+ and CCK+ basket cells receive similar excitatory inputs and provide their output to the same subcellular domains of PCs (somata and proximal dendrites), but fire in different phases of theta oscillations, and only PV+ cells are active during sharp wave-associated ripples (Klausberger et al, 2005). To achieve this, they must utilize different molecular machinery for integrating their synaptic inputs, generating their action potential outputs and releasing GABA. An elegant example of their molecular specialization is that the release of GABA from the axon terminals of CCK+ basket cells is

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Conclusion