PROJECT SUMMARY Chronic pain is a global healthcare problem that reduces quality of life and increases medical expenditures. Inadequate pain control with available treatments indicates a need to understand mechanisms underlying path- ogenesis to develop novel treatment strategies for chronic pain. The GHC1 gene (Gch1 in rodents) encodes GTP cyclohydrolase 1 (GTPCH1), the rate-limiting enzyme in tetrahydrobiopterin (BH4) synthesis, which is an essential cofactor for catecholamine (dopamine, norepinephrine, and epinephrine), serotonin, and nitric oxide production. Alterations in Gch1 gene expression are implicated in the development and maintenance of inflam- matory and neuropathic pain in rodents, and genetic mutations in the GCH1 gene are also linked with pain perception in humans. Evidence regarding the role of Gch1 in chronic pain has so far come from animal models of sciatic nerve injury-induced neuropathic pain, cancer, or visceral pain, with studies focused on the role of Gch1 in the peripheral nervous system. No study to date has investigated the role of Gch1 in a chronic orofacial neuropathic pain model, or in reward and brain regions involved in pain modulation and reward. The hypothesis is that orofacial neuropathic pain increases the expression of Gch1 in trigeminal ganglia and central pain- and reward-related pathways, and that inhibition of GTPCH1 via the GTP cyclohydrolase inhibitor 2,4-diamino-6-hydroxypyrimidine (DAHP) will alleviate nerve injury-induced pain responses in parallel with alter- ing Gch1 gene expression and monoamine turnover in the brain and periphery. Given legitimate concerns about the misuse potential of novel analgesics, the motivating and rewarding effects of DAHP will also be evaluated in naïve and chronic pain conditions. The working hypothesis is that DAHP acquires rewarding properties only in a chronic pain state. These questions will be addressed using an operant orofacial pain assay, a chronic con- striction injury model of the infraorbital nerve to induce orofacial neuropathic pain, targeted molecular methods, and place conditioning behavior in rats. Data generated from this application are expected to provide avenues toward non-addictive, safe treatment strategies for the management of chronic pain via modulation of GTPCH1.