Primate neuronal connections are sparse as compared to mouse

Abstract

Detailing how primate and mouse neurons differ is critical to understanding how brains evolve and for translating findings from mice into humans. However, scarce data about neuronal connections, particularly in primates, makes such comparisons difficult. We use large volume electron microscopy to address this gap, reconstructing 11,415 synapses onto morphologically equivalent adult primate and mouse neurons in Layer 2/3 of primary visual cortex (V1). Neurons from both species distribute in cortex at similar densities, have similar somatic volumes, and dendritic branching patterns, but primate excitatory and inhibitory neurons receive ∼4 times fewer excitatory and inhibitory connections than equivalent mouse neurons. The reduction in excitatory inputs is larger, resulting in a 2-fold decreased ratio of excitatory to inhibitory inputs in primate neurons. Finally, despite reductions in inhibitory synapse number, neighboring excitatory primate neurons are co-innervated by inhibitory axons at rates and proportions similar to mouse neurons. We present a first glimpse of large-scale connectivity comparisons across mouse and primate brains which underscore the need for future analyses since such differences likely represent a substantial part of how mouse and primate brains differ.