Area-specific synapse structure in branched axons reveals a subcellular level of complexity in thalamocortical networks

Abstract

37Thalamocortical Posterior nucleus (Po) axons innervating the somatosensory (S1) and 38 motor (MC) vibrissal cortices are key links in the brain neuronal network that allows 39 rodents to explore the environment whisking with their motile vibrissae. Here, using 40 high-end 3D electron microscopy, we demonstrate massive differences between MC vs. 41 S1 Po synapses in a) bouton and active zone size; b) neurotransmitter vesicle pool size; 42 c) mitochondria distribution near synapses; and d) proportion of non-spinous dendrite 43contacts. These differences are as large, or bigger, than those between Po and 44 ventroposterior thalamic nucleus synapses in S1. Moreover, using single-axon 45 transfection labeling, we show that the structure of boutons in the MC vs. S1 branches 46 of individual Po axons is different. These structural differences parallel striking, 47recently-discovered divergences in functional efficacy and plasticity between S1 and 48 MC Po synapses, and overall reveal a new, subcellular level of thalamocortical circuit 49 complexity, unaccounted for in current models. 50 51 Introduction 52 53 Rodents explore their environment by rhythmically "whisking" with their motile facial 54 vibrissae. Time-dependent correlations between motor commands and vibrissal follicle 55 mechanoreceptor signals are used for inferring object position and texture. Such 56 computations are carried out in multilevel, closed-loop neuronal networks 57 encompassing the brainstem, thalamus and neocortex (reviewed in 1 ). 58Two thalamocortical pathways lay at the core of these loops: Ventral Posteromedial 59 thalamic nucleus (VPM) axons arborizing focally on L4 in the vibrissal "barrel" domain 60 of the primary somatosensory cortex (S1); and Posterior thalamic nucleus (Po) axons 61 arborizing mainly in L5a but also L1 of S1 2; 3 . Importantly, Po axons may target, in 62 addition, the motor cortex (MC) middle layers 3, 4, 5 (L5-L3). The VPM axons relay 63 time-locked mechanoreceptive single-whisker trigeminal signals, crucial for computing 64 object location. In turn, Po axons convey information mainly about timing/amplitude 65 differences between ongoing cortical outputs and multi-whisker sensory signals, which 66 may be important for computing whisker position 1, 6, 7 . Po cells are primarily driven by 67 heavy and highly effective L5 cortico-thalamic inputs, while ascending trigeminal 68 inputs to Po are sparser and largely modulatory in character 7, 8 . 69Activation of S1-L4 VPM synapses elicits large currents in cortical neurons 9; 10 and 70 drives their firing with low failure rates 11, 12 . VPM synapses are driven only by 71 ionotropic receptors, depress rapidly upon repetitive stimulation 9 and, after an early 72 postnatal period, show considerable resistance to sensory experience-dependent changes 73 13 . In contrast, synaptic potentials evoked in S1 by Po axons show slower rise and decay 74 times, and elicit smaller currents 10, 14 . The Po S1 synapses involve both ionotropic and 75 metabotropic glutamate conductances, sho...