Chronic itch, like chronic pain, is clinically recognized as a major cause of suffering and loss in the quality of life. Although itch accompanies many neurological, dermatological and systemic diseases, it has received less attention than pain in studies of underlying peripheral neural mechanisms. There have been two general approaches to studying the physiological properties of dorsal root ganglion neurons that respond to chemical stimuli that evoke itch in humans or itch-associated behavior in animals. The first is to electrophysiologically record action potentials evoked in these neurons in response to a pruritic chemical applied to their cutaneous receptive fields. The second approach is to examine the cellular molecular mechanisms of chemical sensory transduction using the cell body (soma), dissociated and placed in culture, as a model of the peripheral terminal endings. In a test of this model, we will combine these two approaches and compare the response properties of the soma and the receptive field in the intact, visualized, nociceptive DRG neuron, in vivo. We will use genetic-molecular fluorescent markers to identify the soma of a particular type of DRG neuron in the anesthetized mouse. Action potentials will be recorded extracellularly from the soma in response to pruritic and algesic stimuli delivered to its cutaneous receptive field. Then pruritic chemicals that activated the receptive field will be topically applied to the soma and alterations in somal membrane excitability or intracellular calcium will be measured. Similar measurements will be obtained, for comparison, from acutely dissociated somas with the same fluorescent markers. If successful, the outcome will point to new methods of studying the underlying cellular mechanisms of how itch mediating chemicals are sensed in peripheral neurons, an important precursor to the development of drugs that will act specifically to block pathological itch in humans.
The experiments will test the feasibility of a new physiological preparation that will allow cellular-molecular studies of chemically evoked itch to be carried out in functionally identified peripheral sensory neurons, in vivo. The approach is to use genetic-molecular markers to identify specific subtypes of sensory neurons and use electrophysiological recording and calcium imaging methods both in vivo and also in vitro to determine their functional properties. The outcome will point to new methods of studying the underlying cellular mechanisms of how itch mediating chemicals are sensed in peripheral neurons, an important precursor to the development of drugs that will act specifically to block pathological itch in humans.
