Recurrent connections between CA2 pyramidal cells

Abstract

Recurrent excitatory synapses are shown theoretically to play roles in memory storage and associative learning, and such recurrent synapses 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 located preferentially in the superficial layer and proximal part (CA2b) of the CA2 region. These results indicate that recurrent excitatory circuits are dense in the CA2 region as well as in the CA3 region.