Trigeminal nociceptors express TLR-4 and CD14: a mechanism for pain due to infection

Abstract

Although certain bacterial species appear to be risk factors for pain due to odontogenic infections, comparatively little is known about the potential mechanisms mediating this effect. In this study, we tested the hypothesis that trigeminal nociceptive neurons express the TLR4 or CD14 receptors, thus enabling sensory neurons to detect and respond to tissue levels of bacterial substances such as lipopolysaccharide (LPS). Immunohistochemical analyses of human and rat trigeminal neurons demonstrated that a capsaicin-sensitive subclass of nociceptors (defined by expression of TRPV1, a capsaicin receptor) expresses both TLR4 and CD14. Moreover, human dental pulp collected from patients with caries lesions demonstrated co-localization of TLR4 and CD14, with markers of peripheral sensory neurons. Collectively, these studies indicate that the capsaicin-sensitive subclass of trigeminal nociceptors expresses TLR4 and CD14. These results indicate that pain due to bacterial infections may result, in part, from direct activation of nociceptors by bacterial products such as LPS.

Figure 1

Evaluation of the expression patterns of TLR4 and CD14 in trigeminal sensory neurons. White arrows depict examples of neurons expressing both markers for each row of three images, and yellow arrows depict examples of neurons that express one but not both markers. Human trigeminal neurons were evaluated for co-localization of TLR4 (Panels A,D), CD14 (Panel J), with a marker for the capsaicin-sensitive subclass of nociceptors (TRPV1, Panels B,C for TLR4 and Panels K,L for CD14), or a marker of myelinated sensory neurons (N52, Panels E,F). Rat trigeminal neurons were evaluated for co-localization of TLR4 with TRPV1 (Panels G-I) and CD14 with TRPV1 (Panels M-O). The addition of blocking peptide or the deletion of primary or secondary antisera produced a complete loss of signal in both the human and rat tissues. Scale bar is 100 μm for Panels A-F and J-L, and 200 μm for Panels G-I and M-O.

Figure 2

Evaluation of the expression patterns of TLR4 and CD14 in human coronal dental pulp. Normal control dental pulp was examined for expression of TLR4 (Panel A), CD14 (Panel B), and a marker of myelinated neurons (N52, Panel C). Inflamed dental pulp (defined as teeth with a response to thermal stimulation but(AQ) deep caries lesions) had two populations of TLR4 and CD14, with TLR4 (Panel D) or CD14 (Panel G) observed in round or stellate cells or in fibers coursing through coronal pulp. At least some of these fibers expressed N52 (TLR4, Panels E,F; CD14, Panels H,I). Exposure of inflamed pulp to only the secondary antibodies produced low background signal (Panels J,K). Scale bar is 100 μm.