Migraine is a collection of neurological symptoms that include recurring, intense, throbbing headache pain that can last from 4 to 72 hours and affects millions of people. The discovery of the triptan drugs was a major breakthrough for the treatment of debilitating pain for many migraine sufferers. Unfortunately, repeated or prolonged use of triptans and other analgesic drugs (e.g. opioids, ergotamines) often results in production of a serious, disabling adverse effect known as medication overuse headache (MOH). The mechanisms that underlie development of MOH are not well-understood, but it appears that prolonged treatment with antimigraine medications, like triptans, promote pro-nociceptive changes in primary peripheral pain-sensing neurons (nociceptors) of the trigeminal ganglion (TG). Such neuroadaptive changes in peripheral nociceptors are thought to contribute to the induction and maintenance of additional modifications in the CNS that lead to long-term increases in pain sensation and MOH. However, little is known of the effects of prolonged application of antimigraine drugs on peripheral nociceptor function. Triptans, such as the prototypical antimigraine drug sumatriptan, are agonists for 5-hydroxytryptamine1B (5-HT1B), 5-HT1D and 5-HT1F receptor subtypes. Recent advances in our understanding of drug-receptor interactions and desensitization suggest a possible mechanism for triptan-induced MOH. It is known that receptors can regulate the activity of several cellular signal transduction pathways and studies demonstrate that desensitization (reduced function) is not uniform across all signaling pathways. Some pathways desensitize more, or less, than others. Furthermore, prolonged agonist treatment can cause a receptor to couple to a different and opposing signaling pathway (signal switching). We hypothesize that following prolonged triptan use for migraine treatment, 5-HT1 receptor-mediated signaling in TG nociceptors differentially desensitizes such that inhibitory pathways responsible for analgesia are less active (desensitized), but other excitatory cellular signaling pathways that lead to activation/sensitization of pain-sensing neurons and pain sensation (headache) are more efficiently activated. Differential desensitization and/or signal switching may also underlie effects of prolonged treatment with other antimigraine drugs (e.g. opioids, ergotamines) that result in MOH. In this exploratory R21 application we propose to test the hypothesis that prolonged sumatriptan treatment of rats and mice induces differential desensitization/signal switching of inhibitory (G?i) versus excitatory (G?s, extracellular signal-regulated kinase, and/or nitric oxide) signaling coupled to 5-HT1B/1D/1F receptors in trigeminal nociceptors. Results of these experiments will provide the first information about changes in nociceptor receptor function that contribute to the induction and/or maintenance of MOH following triptan treatment for migraine. This information will aid in development of improved drugs, devoid of the propensity to induce MOH, for the effective treatment of migraine.