Comparative ultrastructural features of excitatory synapses in the visual and frontal cortices of the adult mouse and monkey
That the dendritic architecture of pyramidal neurons varies depending on cortical area has been known since the time of Cajal (Conel, 1941; Ramón y Cajal, 1894). Recent studies have used high resolution anatomical and electrophysiological methods to further elaborate on the structural and functional properties of individual pyramidal neurons across distinct cortical areas (Amatrudo et al., 2012; DeFelipe, Alonso‐Nanclares, & Arellano, 2002; Elston, 2000; Medalla & Luebke, 2015). In the rhesus monkey, layer 3 (L3) pyramidal neurons in the lateral prefrontal cortex (LPFC) and V1 are strikingly different in terms of both their morphological and electrophysiological properties. Structurally, the dendritic arbors of L3 pyramidal neurons in LPFC are 3–4x larger, more complex and contain ∼16x higher number of dendritic spines than those of V1 (Amatrudo et al., 2012; Elston, 2000; Gilman, Medalla, & Luebke, 2016). Functionally, LPFC neurons are less excitable and have significantly more frequent excitatory synaptic events with larger amplitude and longer decay time, compared to V1 neurons (Medalla & Luebke, 2015). To determine whether the differences in synaptic properties could be explained by differences in the excitatory synapses themselves, Medalla and Luebke (2015) assessed the ultrastructural properties of excitatory synapses in the layers 2–3 (L2–3) neuropil. Interestingly, both presynaptic boutons and postsynaptic densities of axospinous synapses were significantly larger in LPFC compared to V1. Further, there was a higher proportion of large perforated synapses in LPFC than in V1. These findings of much larger synapses in LPFC (together with the much higher density of spines) is consistent with the idea that significantly larger and more frequent synaptic currents are likely due to more numerous, larger and more powerful synapses in LPFC compared to V1.
By contrast to the highly distinctive characteristics of L3 pyramidal neurons in the monkey visual versus lateral prefrontal cortices, neurons in analogous areas of the mouse are remarkably similar both functionally and structurally (DeFelipe, 2011; DeFelipe et al.