Synaptic signaling in the human hippocampal CA3 network

The hippocampal CA3 network is the largest autoassociative network in the mammalian brain. CA3 pyramidal neurons are connected to each other through recurrent synapses endowed with Hebbian synaptic plasticity, forming an almost infinitely large synaptic matrix for storage and retrieval of information. In humans, the number of CA3 pyramidal neurons is 1.7 million, more than an order of magnitude higher than in mice (110,000). How this huge difference in cell numbers impacts on synaptic function and connectivity remains unclear.

To address this question, we applied cutting-edge electrophysiology and superresolution-expansion microscopy to slices from the human hippocampus. Recordings were focused on tissue from MRI-negative patients (“nonsclerotic”), in which cellular organization was largely preserved. Neuronal density in the CA3 region in MRI-negative patients was similar to that of postmortem tissue, further suggesting that the circuit may approach the physiological situation. We first characterized the CA3 network by multicellular patch clamp-based circuit mapping, and found that the recurrent CA3–CA3 synapse was characterized by high reliability and sparse connectivity. The average connectivity was 1.27%, substantially lower than in mice. To investigate the determinants of connectivity, we analyzed the number of spines in CA3 pyramidal neurons dedicated to the recurrent collateral inputs using superresolution-expansion microscopy. In humans, CA3 pyramidal cells had 16,100 spines, only moderately higher than in mice (13,200). Thus, our results indicate that cell number is the main determinant of connectivity, and suggest an inverse scaling rule between connectivity and cell number across species.

We next characterized the mossy fiber input by paired recordings between mossy fiber terminals and CA3 pyramidal neurons. Presynaptic mossy fiber terminals were stimulated in the non-invasive bouton-attached configuration. We found that stimulation of single stimuli in presynaptic terminals reliably triggered spiking in postsynaptic CA3 pyramidal neurons, demonstrating that the human mossy fiber synapse acts as a “detonator”. To assess connectivity, we estimated the number of large terminals impinging on the apical dendrites of CA3 pyramidal neurons using superresolution-expansion microscopy. In humans, single CA3 pyramidal neurons received inputs from 280 mossy fiber terminals, ~fivefold more than in mice (~50). Although the number of inputs per cell is higher, the total granule cell-to-CA3 pyramidal neuron connectivity is lower in humans. While the higher number of mossy fiber inputs per cell may enhance combinatorial coding, the sparse connectivity may facilitate pattern separation.

Taken together, our results show that both CA3–CA3 recurrent collateral synapses and mossy fiber synapses on CA3 pyramidal neurons follow complex evolutionary scaling rules, which may enhance the computational power of the human brain.