Rabies anterograde monosynaptic tracing reveals organization of spinal sensory circuits

Abstract

Somatosensory neurons detect vital information about the environment and internal status of the body, such as temperature, touch, itch and proprioception. The circuit mechanisms controlling the coding of somatosensory information and the generation of appropriate behavioral responses are not clear yet. In order to address this issue, it is important to define the precise connectivity patterns between primary sensory afferents dedicated to the detection of different stimuli and recipient neurons in the central nervous system. In this study we used a rabies tracing approach for mapping spinal circuits receiving sensory input from distinct, genetically defined, modalities. We analyzed the anatomical organization of spinal circuits involved in coding of thermal and mechanical stimuli and showed that somatosensory information from distinct modalities is relayed to partially overlapping ensembles of interneurons displaying stereotyped laminar organization, thus highlighting the importance of positional features and population coding for the processing and integration of somatosensory information.

Spinal premotor interneurons controlling antagonistic muscles are spatially intermingled

“…Surprisingly, despite our attempts at replication, we did not observe segregation of flexor and extensor premotor circuits. However, the contribution of anterograde transsynaptic spread to the spinal cord through the sensory route was clearly detected in AAV-G experiments (but, as expected, not in AAV-flex-oG experiments), reflecting the contribution of muscle-innervating sensory afferents that have post-synaptic targets predominantly located in the dorsal and medial aspects of the spinal cord ( Zampieri et al, 2014 ; Pimpinella and Zampieri, 2021 ). Notably, this observation does not explain why we failed to replicate segregation of flexor and extensor premotor circuits, as we detected overlapping distributions both in the presence (AAV-G) or absence (AAV-flex-oG) of sensory contributions.…”

Adult spinal Dmrt3 neurons receive direct somatosensory inputs from ipsi‐ and contralateral primary afferents and from brainstem motor nuclei

“…However, our characterization of traced DRG cells suggests that contralateral inputs to Dmrt3 neurons are also predominantly proprioceptive. Previous descriptions of contralateral proprioceptive primary afferents are scarce in the literature; however, neurons targeted by proprioceptive neurons have been found in the contralateral ventral horn (Pimpinella & Zampieri, 2021 ). Another possibility is that Dmrt3 interneurons, in addition to midline decussating axons, also extend commissural dendrites, as it has been found in medial motor column motor neurons (Goetz et al., 2015 ).…”