| Chronic pain is a major clinical problem. Target-derived growth factors have been implicated in the initiation and maintenance of persistent pain states. Artemin, a member of the glial cell line-derived neurotrophic factor (GDNF) family, binds to its GPI-anchored receptor, GFRα3, and initates intracellular signaling via the tyrosine kinase, Ret. Expression of the GFRα3 receptor is largely restricted to the peripheral nervous system and is found in a subpopulation of nociceptive sensory neurons of the dorsal root and trigeminal ganglia (DRG & TG) that coexpress the Ret and TrkA receptor tyrosine kinases and the thermosensitive channel TRPV1. To investigate the role of artemin in regulating nociceptor properties and function, we isolated transgenic mice that overexpress artemin in keratinized tissues (ART-OE). Expression of artemin increased DRG neuron number, confirming the survival promoting effects of artemin. In addition, ART-OE mice had increased mRNA encoding GFRα3, TrkA, TRPV1 and the putative noxious cold and mustard oil detecting channel, TRPA1. Immunolabeling showed that nearly all GFRα3-positive neurons expressed TRPV1 and most of these neurons were also TRPA1-positive. Somas of GFRα3/TRPV1-positive neurons in the ART-OE mice were hypertrophied and there was increased staining for these proteins in the periphery. Interestingly, increases in TRPV1 and TRPA1 mRNA were more robust in TG than DRG. Because of these differential effects, lingual afferents innervating the heavily keratinized tongue were also examined. Retrogradely-labeled lingual afferents from ART-OE tongues showed an increased percentage of GFRα3- and TRPV1-positive neurons. Behavior analysis showed that these anatomical changes were correlated with increased sensitivity to noxious heat, noxious cold, capsaicin and mustard oil applied to the hindpaw, as well as oral sensitivity to capsaicin and mustard oil placed in the drinking water of these mice. Functional analysis of dissociated sensory neurons using calcium imaging showed hypersensitivity to capsaicin and mustard oil in trigeminal neurons isolated from ART-OE mice, and even greater sensitivity in the lingual subpopulation. Taken together, these results indicate that artemin promotes the survival and modulates functional properties of a select population of TRPV1- and TRPA1-positive nociceptors critical for the detection of noxious thermal and chemical stimuli in both cutaneous and lingual systems. |
