Psychophysical Studies of Itch

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Chemical activation of nociceptive neurons may contribute to chronic itch and pain in many disorders of the nervous system. During the last funding period, we determined that cowhage produces itch by a non¿ histaminergic mechanism, and we identified the pruritic agent in cowhage to be a cysteine protease. Endogenous proteases may contribute to clinical pruritus and pain. Using, among other stimuli, a new method of punctate application of a chemical that has been loaded into a heat-inactivated spicule of cowhage, we will psychophysically measure itch, pain and itchy skin (enhanced itch or pain) in human volunteers. Our aims are the following: (1) We will determine whether itch, pain, and cutaneous dysesthesias (enhanced itch or pain) from punctate application of mechanical or chemical pruritic stimuli such as an endogenous cysteine protease, are mediated by capsaicin sensitive primary afferent neurons; (2) We will test whether co-activation of the histaminergic and non-histaminergic pathways leads to the prolongation or enhancement of pruritic and nociceptive sensations and dysesthesias; (3) We will study the inhibitory effects of scratching, heating and algogenic chemicals on itch, pain and itchy skin while teasing apart the separate roles of stimulus modality (type of neuron activated), locus and intensity. (4) We will us a pressure block of conduction in myelinated nerve fibers to exam the separate roles of nociceptors with myelinated vs. non¿ myelinated axons in chemically and mechanically evoked itch, nociceptive sensations and dysesthesias. The outcomes of these studies will increase understanding of how itch and pain and dysesthetic states can co¿ exist as in certain clinical disorders; and how one or another sensory quality or dysesthetic state can be enhanced or inhibited. These psychophysical studies in human will provide a set of sensory conditions that must be accounted for by the responses of candidate pruriceptive and nociceptive-specific sensory neurons in the peripheral and central nervous system. RELEVANCE (See instructions): Psychophysical measurements of experimentally produced itch, pain and itchy skin in humans will further our understanding of the sensory conditions contributing to clinical pruritus and point to their underlying neural mechanisms. These mechanisms provide targets for future pharmacological therapies designed to relieve itch and pain in humans.