Toll-like receptor 7: Expression patterns within mammalian Trigeminal and Dorsal Root Ganglia and its selected facial innervation target, the tongue

Abstract

Toll-like receptors (TLRs) are known to be expressed in the mammalian nervous system where they play an important role in the hosts’ immune defence against bacterial and viral pathogens. Recent studies have looked into the expression of a wide range of TLRs in various neuronal sub-populations of the central and peripheral nervous systems. However, the co-expression of TLRs within the sub-populations of neurons as well as the expression of TLRs in the terminals of various innervation targets are yet to be understood. In this study, Immunohistochemistry was used to quantify the expression of TLR7 with known neurochemical markers in two sets of tissue samples from the cephalic nervous system of naïve rats, namely, the Trigeminal Ganglia (TG) and one of its innervation targets, the Tongue. The patterns of TLR7 expression within the TG were compared to that of the Dorsal Root Ganglia (DRG) of the same naïve rat, which showed no significant difference. Although very little co-localization was seen between TLR7 and its co-stained neurochemical markers, NF200 and IB4, more co-localization was seen with IB4 than with NF200. On the other hand, the peripheral nerves innervating the tongue showed unique structures that allowed the identification of the origin of the nerve bundles, in addition to revealing the identity of each axon within each nerve bundle. These results indicate that TLR7 may play a distinct role in the response to pathogens within the tongue, which provides a basis for further studies on the specificity of TLR7 in the peripheral nerves.