It has been suggested that some glutamatergic synapses lack functional AMPA receptors.
We used quantitative immunogold localization to determine the number and variability
of synaptic AMPA receptors in the rat hippocampus. Three classes of synapses show
distinct patterns of AMPA receptor content. Mossy fiber synapses on CA3 pyramidal
spines and synapses on GABAergic interneurons are all immunopositive, have less variability,
and contain 4 times as many AMPA receptors as synapses made by Schaffer collaterals
on CA1 pyramidal spines and by commissural/associational (C/A) terminals on CA3 pyramidal
spines. Up to 17% of synapses in the tatter two connections are immunonegative. After
calibrating the immunosignal (1 gold = 2.3 functional receptors) at mossy synapses
of a 17-day-old rat, we estimate that the AMPA receptor content of CIA synapses on
CA3 pyramidal spines ranges from <3 to 140. A similar range is found in adult Schaffer
collateral and C/A synapses.
