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 neighboring 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.This work was supported by FEDER funds through the Operational Competitiveness Programme-COMPETE and by national funds through FCT-Fundacao para a Ciencia e a Tecnologia under the project FCOMP-01-0124-FEDER-029632 (PTDC/NEU-SCC/0347/2012 to BS), the Hungarian Academy of Sciences (MTA-TKI 242 to MA), the Janos Bolyai Research Scholarship of the Hungarian Academy of Sciences (PSz), the Hungarian Brain Research Program (KTIA_NAP_13-2-2014-0005 to PSz and KTIA_NAP_13-1-2013-0001 to MA) and TAMOP-4.2.4.A/2-11/1-2012-0001 'National Excellence Program' supported by the Sate of Hungary and the European Union, co-financed by the European Social Fund (ZsA). The authors are grateful to Raquel Pinho for her excellent help with the histological processing and reconstruction
