Tight neurovascular coupling in the spinal cord during nociceptive stimulation in intact and spinal rats

Functional magnetic resonance imaging (fMRI) is based on neurovascular coupling, which allows inferring neuronal activity from hemodynamic changes. Spinal fMRI has been used to examine pain processes, although spinal neurovascular coupling has never been investigated. In addition, fluctuations in mean arterial pressure (MAP) occur during nociceptive stimulation and this may affect neurovascular coupling. The objective of this study was to examine neurovascular coupling in the rat spinal cord during nociceptive stimulation while MAP was manipulated by cervical spinal transection, which prevents nociception-related MAP increases. Six male Wistar rats were anesthetized with isoflurane (1.2-1.5%). Local field potentials (LFP) and spinal cord blood flow (SCBF) were recorded concurrently in the lumbar enlargement, where activity was evoked by electrical stimulation of the sciatic nerve. In intact conditions, stimulation of graded intensity produced proportional changes in SCBF and LFP that were paralleled by similar changes in MAP. However, spinal transection almost abolished MAP changes (p<0.001), while SCBF and LFP responses were not significantly affected (p>0.3) and remained similarly coupled before and after spinal transection. This indicates that spinal hemodynamic changes reflect neuronal activity even when large fluctuations in MAP occur. This contrasts with results from previous studies on cerebral neurovascular coupling and suggests that spinal autoregulation might allow better adaptation to sudden MAP changes than cerebral autoregulation. Although assessment of the coupling between spinal neuronal activity and BOLD signal remains to be investigated, this study supports the use of spinal fMRI, based on the tight coupling between SCBF and LFP.