Defining neurochemical properties and functions of TG primary sensory neurons

Objective: The trigeminal ganglion (TG) is a complex sensory structure and multiple lines of evidence suggest that significant differences exist in anatomical, neurochemical and physiological properties between it and its equivalent structure in the somatosensory system, the dorsal root ganglion (DRG).  This is likely to be a reflection, first on the unique areas of tissue innervation of the TG and second, on the unusual responses to injury which give rise to distinct pain symptoms such as toothache, migraine and temporomandibular joint disorders.  In an attempt to address this disparity in knowledge, we have carried out an in-depth study investigating neurochemical populations and cell size distributions of sensory neurons within the rat TG and DRG.  In an attempt to define functional phenotype of neurons within the TG we have placed emphasis upon the combinatorial co-expression of neurochemicals, transducer proteins and cytokine receptor components on TG sensory neurons.

Method: Indirect dual immunofluorescence to analyze co-expression of receptor components for NGF (TrkA), TNFα (p55) and IL-6 (gp130), and thermo-transducers TRPV1 and TRPM8 within neurochemically defined sub-populations.  Immunocytochemistry to examine activation status of STAT3 in TG neuronal cells following exposure to IL-6, quantified using Western blot.  CGRP-release EIA and electrophysiological analysis to explore mechanisms by which activation of gp130 by IL-6 might lead to sensitization of TRPV1.

Result: Our data show significant differences in proportions of neurochemical populations, cell size distributions and patterns of co-expression of receptor components and transducers between TG and DRG.  Moreover, we have described novel populations of TG neurons displaying unusual phenotype.  In addition, we show that the action of IL-6 on TG neuronal cells results in atypical response mechanisms.

Conclusion: Our findings support the emerging picture of a complex combinatorial pattern of co-expression of sensory neurochemicals, transducers and receptor components that help to define TG neuronal modality and function.