Chronic itch affects up to 15% of the population but is poorly controlled with currently available treatments. Pruriceptors are small diameter nerve fibers that innervate the skin and transduce itch. While several histamine-dependent and histamine-independent itch pathways have been discovered, little is known about how chronic itch develops. Studies of patients with atopic dermatitis and psoriasis indicate that increased epidermal innervation and increased levels of neurotrophic factors may be involved. Most pruriceptors express Ret, the receptor for GDNF-family ligands (GFLs), but no studies have investigated the role of GFL-Ret signaling in pruriceptor function. Our preliminary data indicate that Ret+ epidermal fibers are increased in a model of dry skin itch. Furthermore, we have found that pretreatment with artemin (a GFL) significantly potentiates scratching behavior and neuronal responses to the pruritogen chloroquine. Based on these findings, we hypothesize that GFLs modulate pruriceptor structure and function.
In Aim 1 of this proposal we will determine if GFLs can induce itch or modulate pruritogen-induced scratching. We will also determine if chronic treatment with GFLs induces epidermal hyperinnervation and whether these changes correlate with increases in scratching behavior.
In Aim 2, we will utilize RetEGFP/+ reporter mice and retrograde labeling to determine the direct effects of artemin on pruritogen receptor expression and function by recording calcium responses in dissociated trigeminal ganglion neurons.
In Aim 3, we will use tamoxifen-inducible sensory neuron-specific Ret conditional knock-out mice (Ret-cKOs) to determine if endogenous Ret signaling is important for normal pruriceptor expression and function. We will also determine if Ret expression is necessary for GFL-induced effects on itch. The work generated by this proposal will ultimately determine whether GFLs are potential targets for the treatment of chronic pruritus.
Chronic pruritus is a debilitating symptom that significantly affects the quality of life of patients with a variety of conditions, but remains poorly controlled with currently available medications. A better understanding of how neurotrophic factors may sensitize or modulate pruriceptor function can provide targets for the development of novel treatments with better efficacy.