Neurons in the lateral part of the lumbar spinal cord show distinct novel axon trajectories and are excited by short propriospinal ascending inputs

Abstract

Order of Authors Secondary Information:The role of spinal dorsal horn propriospinal connections in nociceptive processing is not yet established. Recently described, rostrocaudally oriented axon collaterals of lamina I projection and local-circuit neurons (PNs and LCNs) running in the dorsolateral funiculus (DLF) may serve as the anatomical substrate for intersegmental processing. Putative targets of these axons include lateral dendrites of superficial dorsal horn neurons, including PNs, and also neurons in the lateral spinal nucleus (LSN) that are thought to be important integrator units receiving, among others, visceral sensory information. Here we used an intact spinal cord preparation to study intersegmental connections within the lateral part of the superficial dorsal horn. We detected brief monosynaptic and prolonged polysynaptic excitation of lamina I and LSN neurons when stimulating individual dorsal horn neurons located caudally, even in neighbouring spinal cord segments. These connections, however, were infrequent. We also revealed that some projection neurons outside the dorsal grey matter and in the LSN have distinct, previously undescribed course of their projection axon. Our findings indicate that axon collaterals of lamina I PNs and LCNs in the DLF rarely form functional connections with other lamina I and LSN neurons and that the majority of their targets are on other elements of the dorsal horn. The unique axon trajectories of neurons in the dorsolateral aspect of the spinal cord, including the LSN do not fit our present understanding of midline axon guidance and suggest that their function and development differ from the neurons inside lamina I. These findings emphasize the importance of understanding the connectivity matrix of the superficial dorsal horn in order to decipher spinal sensory information processing. Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation Answers to Reviewer 1First of all, the authors would like to thank Reviewer 1 for his constructive comments and suggestions that helped us improving the manuscript. Below we give detailed answers to all major and minor concerns raised. Major issues 1)We completely agree with the reviewer that the statistical analysis would have helped to make our conclusions on morphometric parameters stronger. The reason why we decided to talk about the tendencies only without statistical tests was the low number of the reconstructed neurons in the groups. However, since we observed and wanted to demonstrate the difference between neurons within and outside lamina I (in the adjacent lateral white matter), in this revised version we pooled projection neurons (PNs) and localcircuit neurons (LCNs) from our previous reports to form a group of cells in lamina I called "L-I" whereas neurons lateral to the edge of the dorsal horn and in the LSN are now referred to as "OUT/LSN". Statistical analyses were performed between these two groups and showed significant differences in the morphometric parameters investigated, with the...

Characterisation of lamina I anterolateral system neurons that express Cre in a Phox2a-Cre mouse line

“…The membranous expression of the YFP-channelrhodopsin fusion protein in the Phox2a::Cre;Rosa26 LSL-ChR2-YFP mice allowed clear visualisation of dendritic spines, and also of Homer puncta in both dendritic spines and shafts. There have apparently been few reports of dendritic spines on lamina I projection neurons, although Antal et al 41 examined 5 lamina I projection neurons and found a mean spine density of 5/100 μm, which is consistent with the present result. We found that all of the cells possessed dendritic spines, although the mean density (6.3 spines/100 μm of dendrite) was considerably www.nature.com/scientificreports/ lower than that on some other neurons in the superficial dorsal horn, since we have reported values of 17-19 spines/100 μm for excitatory interneurons with substance P or that express green fluorescent protein (GFP) in the transgenic GRP::eGFP line 42 .…”

The Pathways and Processes Underlying Spinal Transmission of Low Back Pain: Observations From Dorsal Root Ganglion Stimulation Treatment

“…For example, when the diaphragm is irritated by air in the peritoneum, or by inflammation of the gallbladder, the patient complains of shoulder pain (C3‐4‐5), or with angina, left arm pain (T1). The same phenomenon occurs when neurons within lamina I receive input from lower lumbar nerves and integrate this information with the muscles and joints (63, 96). Somatic and visceral C‐fibers also converge onto the same neurons in the lamina II; therefore, C‐fibers of different origin can inhibit each other (32).…”

“…In some cases, C‐fiber pain may ascend up to as many as six segments in LT (46, 48, 56, 59–62). This is the anatomical substrate for intersegmental processing (63). Once in the DH, the nociceptors synapse in Rexed laminae I and II of the gray matter, the posteromarginal nucleus and the substantia gelatinosa (30, 46–48), and from there they synapse on second‐order neurons.…”

“…Somatic and visceral C‐fibers also converge onto the same neurons in the lamina II; therefore, C‐fibers of different origin can inhibit each other (32). Deep somatic structures converge with cutaneous input (63, 158) and inhibit each other. For example, intercostal afferent stimulation inhibits visceral afferents from the heart and esophagus (159).…”

Developmental origin of long‐range neurons in the superficial dorsal spinal cord

“…). In support of this, morphological analysis of DLF neurons by single‐cell labeling demonstrated the presence of several subtypes of long‐range neurons with distinct axonal trajectories (Antal et al ., ). Only a few cells in the DLF were EdU‐positive ones in mice treated with EdU later than E10.5 (Fig.…”