Recurrent connections between CA2 pyramidal cells.

Recurrent excitatory synapses have theoretically been shown to play roles in memory storage and associative learning and are well described to occur in the CA3 region of the hippocampus. Here, we report that the CA2 region also contains recurrent excitatory monosynaptic couplings. Using dual whole-cell patch-clamp recordings from CA2 pyramidal cells in mouse hippocampal slices under differential interference contrast microscopic controls, we evaluated monosynaptic excitatory connections. Unitary excitatory postsynaptic potentials occurred in 1.4% of 502 cell pairs. These connected pairs were preferentially located in the superficial layer and proximal part (CA2b) of the CA2 region. These results indicate that recurrent excitatory circuits are denser in the CA2 region than in the CA1 region, as well as in the CA3 region.