Matthias Ringkamp - US grants

During the previous grant period, we found that topical, intradermal application of cowhage spicules induces itch by non-histaminergic mechanisms. Electrophysiological recordings from unmyelinated (C-fiber) and myelinated (A-fiber) afferents in peripheral nerves revealed that histamine and cowhage activate different peripheral pathways. For C-fibers, mechano-insensitive afferents (MIAs) were activated by histamine but not cowhage, whereas mechanosensitive nociceptive afferents (MSAs) were responsive to both, but the responses to cowhage were substantially greater. Similarly, histamine but not cowhage activated A-fiber MIAs, whereas A-fiber MSAs responded to cowhage or histamine or both. Again, the responses to cowhage were significantly greater than the responses to histamine. These findings indicate that the sensation of itch can be mediated through multiple afferent pathways. The proposed electrophysiological experiments will help to determine the relative roles of these different classes of afferents in signaling the different sensations associated with the pruritic stimuli. Specifically, we propose: 1) To determine whether pruriceptive and/or nociceptive selective classes of MIAs exist by investigating the responsiveness of A- and C-fiber MIAs to pruritic and algesic stimuli. 2) To investigate a potential peripheral mechanism for scratch induced itch relief or itch relieve by application of thermal stimuli. We will determine if application of such stimuli leads to a decrement in the response to pruritic stimuli of A-fiber and C-fiber MSAs. Preliminary results reveal significant fatigue in the responses of C-MSAs to cowhage after mechanical stimulation. 3) To investigate the differential chemosensitivity of nociceptive and pruriceptive small-diameter afferents using a novel application technique, i.e. heat-inactivated, cowhage-spicules shown in project 1 to cause itch and pain when loaded with chemicals including capsaicin, histamine and non-histaminergic pruritic agents such as cysteine proteases. We postulate that different populations of small myelinated and unmyelinated MSAs mediate the sensation of itch and pain produced by these stimuli. The results from these studies will enhance our understanding of the peripheral neuronal apparatus underlying the sensation of itch and pain. RELEVANCE (See Instructions): Itch that is resistant to treatment, for example with antihistamines, is a major clinical problem. The proposed studies will increase our understanding about how the sensation of itch is mediated in the nerve fibers of the skin. The findings of these studies may potentially lead to the development of better strategies for the treatment of itch.

? DESCRIPTION (provided by applicant): Itch accompanies many neurological, dermatological and systemic diseases and leads to suffering and loss in the quality of life. The peripheral neuronal mechanisms underlying this sensation in human are still poorly understood. The overall goal of our proposal is to determine how acute itch and the prolonged itch from a pruritic disease are encoded in the discharges of cutaneous nociceptors. We propose electrophysiological experiments in non-human primate to characterize the responses to pruritic chemical stimuli in subtypes of peripheral nociceptive nerve fibers and complementary, correlative psychophysical studies in human to determine the role of these different neuronal populations in itch sensation. We will test different models of itch coding, modulate neuronal activity in nociceptive afferents, and investigate the effect of such modulation on itch sensation. We will investigate how a single class of nociceptors can signal both strong chemical pain from capsaicin and itch from histamine, and define, for the first time, the discharge associated with allodynia vs. alloknesis and other dysesthesias. In a clinically relevant model of prolonged itch, namely the SADBE model of allergic contact dermatitis, we will investigate the neuronal mechanisms of spontaneous and enhanced itch to chemical stimuli using psychophysical studies in humans and electrophysiological recordings from nociceptive afferents in non-human primates. By studying both acute and prolonged itch, we will be able to identify, for the first tim, changes that occur in the nervous system in a clinically relevant itch condition. A better understanding of how nociceptors mediate the sensation of itch in normal and pruritic skin will increase the possibility of developing and evaluating novel, peripherally acting drugs that reduce the abnormal nociceptor function underlying chronic itch. RELEVANCE: Although itch is a significant clinical problem affecting millions of people worldwide, the neuronal mechanisms of this sensation in human are not well understood. Using a combination of psychophysical studies in human and electrophysiological studies in non-human primate, we will investigate how itch is encoded in the neuronal activity of different types of peripheral nerve fibers in normal skin and in inflamed and itchy skin produced by allergic contact dermatitis. The outcomes will lead to a better understanding of how acute itch in normal skin and prolonged itch from a clinically relevant pruritic disorder are encoded by nerve fibers and thereby provide potential neuronal targets for future development of peripherally acting anti-pruritic drugs.

? DESCRIPTION (provided by applicant): HIV-induced sensory neuropathy (HIV-SN) is a major global health problem as 35 million people are infected with HIV and a considerable percentage of these will develop neuropathy. HIV-SN poses a major burden on the quality of life of affected patients as many suffer from neuropathic pain that is resistant to antiretroviral therapy as well a other drugs commonly used to treat neuropathic pain. Despite the pressing need for novel therapeutics, the development of new treatment strategies has been hampered by our lack of understanding of the mechanisms underlying HIV-induced neuropathy. Using an SIV/macaque model of HIV-neuropathy that closely recapitulates HIV-induced damage, we propose to investigate the underlying cause of HIV-SN by 1) developing tools that allow the use of corneal confocal microscopy to non-invasively track the development of peripheral neuropathy and 2) measuring levels of SIV-induced inflammatory mediators in plasma and dorsal root ganglia. In this macaque model, we will also record neuronal activity from unmyelinated nociceptive afferents in peripheral nerves to investigate whether such afferents develop signs of peripheral sensitization after infection as sensitization could contribute to spontaneous pain and hyperalgesia seen in patients with HIV-SN. In addition to peripheral nociceptive nerve fibers, nociceptive neurons in DRG may be damaged by inflammatory mediators produced locally in DRG or present in the circulation. Using the patch-clamp electrophysiology technique, we will investigate whether nociceptive DRG neurons develop hyperexcitability during SIV infection and whether inflammatory mediator-sensitive voltage-gated sodium channel isoforms play a role. By correlating the findings obtained with the different techniques, we will obtain a better understanding of the mechanisms underlying HIV-sensory neuropathy to develop new strategies for treatment.