Abstract
The subcellular locations of synapses on pyramidal neurons strongly influences dendritic integration and synaptic plasticity. Despite this, there is little quantitative data on spatial distributions of specific types of synaptic input. Here we use array tomography (AT), a high-resolution optical microscopy method, to examine thalamocortical (TC) input onto layer 5 pyramidal neurons. We first verified the ability of AT to identify synapses using parallel electron microscopic analysis of TC synapses in layer 4. We then use large-scale array tomography (LSAT) to measure TC synapse distribution on L5 pyramidal neurons in a 1.00 × 0.83 × 0.21 mm3 volume of mouse somatosensory cortex. We found that TC synapses primarily target basal dendrites in layer 5, but also make a considerable input to proximal apical dendrites in L4, consistent with previous work. Our analysis further suggests that TC inputs are biased toward certain branches and, within branches, synapses show significant clustering with an excess of TC synapse nearest neighbors within 5–15 μm compared to a random distribution. Thus, we show that AT is a sensitive and quantitative method to map specific types of synaptic input on the dendrites of entire neurons. We anticipate that this technique will be of wide utility for mapping functionally-relevant anatomical connectivity in neural circuits.
Highlights
Cortical pyramidal neurons receive input via thousands of synapses distributed over the entire dendritic tree
array tomography (AT) fluorescence microscopy and electron microscopy (EM) were performed on the same serial sections (Figure 1D1)
The ability to image membranes was diminished under the correlative EM conditions, post-synaptic densities (PSDs) were reliably detected and we consistently found no difference in the number of PSDs detected in L4 barrel cortex compared to traditional EM processing
Summary
Cortical pyramidal neurons receive input via thousands of synapses distributed over the entire dendritic tree. On thalamo-recipient dendrites in primary sensory cortex, thalamocortical input accounts for only a small fraction of the total synapses, yet, functionally, the input is strong (Benshalom and White, 1986; Peters and Payne, 1993; Ahmed et al, 1994; Douglas et al, 1995; Alonso et al, 1996; Stratford et al, 1996; Gil et al, 1999; Beierlein et al, 2002; Bruno and Sakmann, 2006). One prominent yet largely untested hypothesis (Larkum and Nevian, 2008) is that the dendritic location and clustering of the input could be an important factor in determining its strength
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