Expression of the TRPM8-immunoreactivity in dorsal root ganglion neurons innervating the rat urinary bladder

The neurochemical phenotypes of the transient receptor potential melastatin-8 (TRPM8)-immunoreactive afferent neurons innervating the rat urinary bladder were examined by using a highly sensitive tyramide signal amplification method, combined with wheat-germ agglutinin-horseradish peroxidase (WGA-HRP) retrograde tracing. TRPM8-immunoreactivity was detected in a small proportion of the WGA-HRP-labeled bladder afferent neurons in the dorsal root ganglia of the Th13-L1 (1.14%) and the L6-S1 (1.27%), and these neurons were small in size (<600 μm2). The 82.6 ± 3.8% of the TRPM8-immunoreactive bladder afferent neurons and 80.9 ± 1.5% of the total population of the TRPM8-immunoreactive afferent neurons in the observed dorsal root ganglia expressed NF200. On the other hand, the proportions of the co-expression of TRPM8 and nociceptive markers such as calcitonin gene-related peptide (CGRP), transient receptor potential vanilloid-1 (TRPV1), and isolectin B4 (IB4) in the bladder afferent neurons (81.5 ± 5.2% for CGRP, 36.1 ± 4.0% for TRPV1, and 15.8 ± 5.5% for IB4) were higher in comparison to those in the total population of the TRPM8-immunoreactive afferent neurons (21.9 ± 2.4% for CGRP, 16.6 ± 1.7% for TRPV1, and 5.4 ± 0.5% for IB4), although no significant difference existed for IB4. Our results suggest that the TRPM8-expressing bladder afferents should be classified as Aδ-fibers and C-fibers, while some of these afferents may be involved in nociceptive sensations.