Hippocampal pyramidal cells (PCs) express many GABAAR subunit types and receive GABAergic
inputs from distinct interneurons. Previous experiments revealed input-specific differences
in alpha1 and alpha2 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 alpha1, alpha2, beta1, beta2,
beta3 and gamma2 subunits. In CA3 PCs, 90% of the perisomatic synapses are immunopositive
for the alpha1 subunit and all synapses are positive for the remaining five subunits.
Somatic synapses form unimodal distributions based on their immunoreactivity for these
subunits. The alpha2 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.
