Synapse-to-neuron ratio is inversely related to neuronal density in mature neuronal cultures

Synapse formation is a fundamental process in neurons that occurs throughout development, maturity, and aging. Although these stages contain disparate and fluctuating numbers of mature neurons, tactics employed by neuronal networks to modulate synapse number as a function of neuronal density are not well understood. The goal of this study was to utilize an in vitro model to assess the influence of cell density and neuronal maturity on synapse number and distribution. Specifically, cerebral cortical neurons were plated in planar culture at densities ranging from 10 to 5000 neurons/mm2, and synapse number and distribution were evaluated via immunocytochemistry over 21 days in vitro (DIV). High-resolution confocal microscopy revealed an elaborate three-dimensional distribution of neurites and synapses across the heights of high-density neuronal networks by 21 DIV, which were up to 18 μm thick, demonstrating the complex degree of spatial interactions even in planar high-density cultures. At 7 DIV, the mean number of synapses per neuron was less than 5, and this did not vary as a function of neuronal density. However, by 21 DIV, the number of synapses per neuron had jumped 30- to 80-fold, and the synapse-to-neuron ratio was greatest at lower neuronal densities (< 500 neurons/mm2; mean approximately 400 synapses/neuron) compared to mid and higher neuronal densities (500–4500 neurons/mm2; mean of approximately 150 synapses/neuron) (p < 0.05). These results suggest a relationship between neuronal density and synapse number that may have implications in the neurobiology of developing neuronal networks as well as processes of cell death and regeneration.

Research highlights
► Mature neuronal networks possess a complex 3-D distribution of neurites and synapses.
► Synapses per neuron increased up to 80-fold in mature vs immature cultures.
► A nonlinear, inverse relationship exists between synapse-to-neuron ratio and mature neuronal density.
► Paradoxically, mature neurons respond to fewer synaptic partners by making more synapses.