Ascending propriospinal modulation of thoracic sympathetic preganglionic neurons during lumbar locomotor activity

Although the autonomic sympathetic system is activated in parallel with locomotion, the underlying neural mechanisms mediating this coordination are not completely understood. Descending exercise or "central command" signals from hypothalamic and brainstem regions are thought to activate thoracic spinal sympathetic neurons in parallel with descending locomotor commands. In turn, subsets of thoracic sympathetic preganglionic neurons (SPNs) increase activity in a constellation of tissues and organs that provide homeostatic and metabolic support during movement and exercise. It is known that ascending drive from lumbar locomotor networks is mediated in part via propriospinal neurons that can also activate and coordinate autonomic systems. However, the extent to which this ascending drive is distributed to SPNs within thoracic regions is unknown. To investigate this, we applied neurochemicals to elicit whole-cord or lumbar-evoked locomotor activity in an in vitro spinal cord preparation, simultaneously recording lumbar ventral root (VR) activity and changes in normalized calcium fluorescence (Ca-RI) of pre-labelled SPNs in thoracic segments. Using whole-bath drug application SPN responses appeared unimodal, such that SPN Ca-RI was increased in rostral (T4-FT7) compared to caudal (T8-T11) segments during tonic activity. During rhythmic activity in either whole or split-bath configuration, and during tonic activity in split-bath configuration, SPN responses appeared trimodal, such that SPN Ca-RI was increased in mid-thoracic segments (T6-7) and reduced at more rostral (T4-5) and caudal (T8-9) levels. In both approaches, the greatest increases in SPNs Ca-RI during rhythmic activity were at T6-7, and most decreased at caudal segments (T8-T11). Together, these findings reveal a strong ascending lumbar to thoracic integrating communication pathway, which may represent a key feature of spinal neural network function normally. Such communication pathways should be further investigated for targeted autonomic function(s) activation and therapeutic benefit after spinal cord injury.