Morphological changes in different populations of bladder afferent neurons detected by herpes simplex virus (HSV) vectors with cell-type-specific promoters in mice with spinal cord injury

•A new HSV vector method is useful to identify bladder afferent subpopulations.•CGRP-expressing bladder afferent neurons increase their size and number after SCI.•TRPV1-expressing neurons increase the number, but decrease the size after SCI.•A-fiber bladder neurons may switch their phenotype to express CGRP after SCI.•SCI-induced changes in A-fiber afferents may be different between mice and rats.

Functional and morphological changes in C-fiber bladder afferent pathways are reportedly involved in neurogenic detrusor overactivity (NDO) after spinal cord injury (SCI). This study examined the morphological changes in different populations of bladder afferent neurons after SCI using replication-defective herpes simplex virus (HSV) vectors encoding the mCherry reporter driven by neuronal cell-type-specific promoters. Spinal intact (SI) and SCI mice were injected into the bladder wall with HSV mCherry vectors driven by the cytomegalovirus (CMV) promoter, CGRP promoter, TRPV1 promoter or neurofilament 200 (NF200) promoter. Two weeks after vector inoculation into the bladder wall, L1 and L6 dorsal root ganglia (DRG) were removed bilaterally for immunofluorescent staining using anti-mCherry antibody. The number of CMV promoter vector-labeled neurons was not altered after SCI. The number of CGRP and TRPV1 promoter vector-labeled neurons was significantly increased whereas the number of NF200 vector-labeled neurons was decreased in L6 DRG after SCI. The median size of CGRP promoter-labeled C-fiber neurons was increased from 247.0 in SI mice to 271.3 μm2 in SCI mice whereas the median cell size of TRPV1 promoter vector-labeled neurons was decreased from 245.2 in SI mice to 216.5 μm2 in SCI mice. CGRP and TRPV1 mRNA levels of laser-captured bladder afferent neurons labeled with Fast Blue were significantly increased in SCI mice compared to SI mice. Thus, using a novel HSV vector-mediated neuronal labeling technique, we found that SCI induces expansion of the CGRP- and TRPV1-expressing C-fiber cell population, which could contribute to C-fiber afferent hyperexcitability and NDO after SCI.