Abstract
The retrograde transsynaptic tracer pseudorabies virus (PRV) has been widely used as a marker for synaptic connectivity in the spinal cord. Notably, the PRV-152 construct expresses enhanced green fluorescent protein (EGFP). We recently reported a significant attenuation of PRV-152 labeling of the intermediolateral cell column (IML) and celiac ganglia after complete T4 spinal cord transection versus sham injury in rats at 96 h after PRV-152 inoculation of the left kidney. Here we found a significant increase in noxious colorectal distention (CRD)-evoked c-Fos expression in spinal cords of injured versus sham rats without PRV infection. In order to assess whether enhancing neuronal activity in spinalized rats might increase PRV-152 labeling, we subjected awake spinalized rats to 1.5 h of intermittent noxious CRD either: (1) just prior to inoculation, or (2) 96 h after inoculation (n = 3/group). Equal numbers of spinalized rats in both groups received PRV-152 inoculations without CRD (non-stimulated; n = 3/group). At 96 h post-inoculation fixed spinal cords and left celiac ganglionic tissues were assessed for the distribution and quantification of EGFP-labeled cells. The injured cohort that received CRD just prior to PRV injection showed a significant reduction in EGFP-labeled cells in both the IML and left celiac ganglion compared to non-stimulated injured rats. In contrast, the injured cohort that received CRD 96 h after PRV-152 inoculation showed no differences in EGFP-labeled cell numbers in the IML or celiac ganglia versus non-stimulated injured rats. Interestingly, microglia near c-Fos-positive cells after acute CRD appeared more reactive compared to non-stimulated spinalized rats, and activated microglial cells markedly reduce viral transduction and progression following PRV inoculation of the CNS. Hence our results imply that increased CRD-induced c-Fos expression in the injured paradigm, prior to but not after PRV injection, further attenuates PRV-152 uptake, perhaps through changes in neuronal activity and/or innate neuro-immune responses.