Volume electron microscopy of the distribution of synapses in the neuropil of the juvenile rat somatosensory cortex

J R Rodriguez,A Merchan-Perez,J Defelipe,A Santuy

doi:10.1007/s00429-017-1470-7

Abstract

Knowing the proportions of asymmetric (excitatory) and symmetric (inhibitory) synapses in the neuropil is critical for understanding the design of cortical circuits. We used focused ion beam milling and scanning electron microscopy (FIB/SEM) to obtain stacks of serial sections from the six layers of the juvenile rat (postnatal day 14) somatosensory cortex (hindlimb representation). We segmented in three-dimensions 6184 synaptic junctions and determined whether they were established on dendritic spines or dendritic shafts. Of all these synapses, 87–94% were asymmetric and 6–13% were symmetric. Asymmetric synapses were preferentially located on dendritic spines in all layers (80–91%) while symmetric synapses were mainly located on dendritic shafts (62–86%). Furthermore, we found that less than 6% of the dendritic spines establish more than one synapse. The vast majority of axospinous synapses were established on the spine head. Synapses on the spine neck were scarce, although they were more common when the dendritic spine established multiple synapses. This study provides a new large quantitative dataset that may contribute not only to the knowledge of the ultrastructure of the cortex, but also towards defining the connectivity patterns through all cortical layers.

Highlights

The cerebral cortex displays a highly complex synaptic organization but there are general rules that can be applied—albeit with a certain degree of variation—to all the cortical areas and species examined (DeFelipe 2011)
We have studied the proportions of asymmetric synapses (AS) and SS on spines and dendritic shafts in the neuropil of all cortical layers of the somatosensory cortex of Wistar rats on postnatal day 14 (P14)
We discarded 1383 (18.28%) because the structures onto which synapses were established were truncated by the margins of the stack so it was not possible to identify with certainty if these postsynaptic structures were dendritic spines or dendritic shafts

Summary

Introduction

The cerebral cortex displays a highly complex synaptic organization but there are general rules that can be applied—albeit with a certain degree of variation—to all the cortical areas and species examined (DeFelipe 2011). There are two main morphological types of synapses that can be identified based on morphological criteria—asymmetric and symmetric synapses (Colonnier 1968; Gray 1959). This distinction is important because, in general, asymmetric synapses (AS) are excitatory (glutamatergic) and symmetric synapses (SS) are inhibitory (GABAergic) (Ascoli et al 2008). It has been described that terminals that establish AS and SS can synthesize neurotransmitters other than glutamate and GABA, respectively, such as acetylcholine, serotonin, noradrenaline or dopamine, a large proportion of these axonal systems are nonsynaptic (Beaulieu and Somogyi 1990; DeFelipe and Jones 1988; Descarries and Mechawar 2000). It has been reported that some spines lack synapses, while other spines can have multiple synapses (Feldman 1984; Harris et al 1992; Harris and Kater 1994; Jones and Powell 1969; Popov et al 2005)

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