Natural products as probes of the pain pathway

Nociception is the process whereby a subset of somatosensory nerve fibers (called nociceptors) detect noxious stimuli and transmit this information to the spinal cord and brain, ultimately producing a percept of discomfort or pain. Nociceptors are faced with the complex task of detecting disparate environmental and endogenous signals of both a physical and chemical nature; these include temperature, pressure, irritants, pruritogens, and inflammatory agents. Consequently, nociceptor activation elicits acute pain as well as injury-evoked pain hypersensitivity and can contribute to so-called ‘maladaptive’ processes underlying persistent pain syndromes. Our goal is to understand how nociceptors detect and integrate these signals in response to changing environmental or physiological conditions. Natural products from plants or venomous creatures have served as tremendously valuable tools for deciphering cellular and molecular mechanisms contributing to nociception and pain. Notable examples include the use of natural analgesics, such as morphine (from the opium poppy) and salicylate (from willow bark) to discover opioid receptors and cyclooxgenases, respectively. Other important examples include the use of natural irritants, such as capsaicin (from chili peppers) and menthol (from mint leaves) to identify ion channels that detect heat and cold, respectively. Indeed, each of these natural product receptors represents a validated or potential target for pharmacological management of acute or chronic pain. This proposal is aimed at elucidating molecules, cells, and mechanisms that contribute to nociception in the context of acute (protective) or pathological (chronic) pain states. We shall continue to exploit the vast chemical ‘space’ of natural product pharmacology to identify and characterize ion channels and sensory neuron subtypes that contribute to distinct nociceptive modalities. At the most reductionist level, we will use biophysical, biochemical, and pharmacological tools to elucidate structural mechanisms underlying ion channel function, including stimulus detection, drug binding, and gating. Here, the main emphasis will be on members of the TRP channel family that are targeted by natural pungent agents and play major roles in thermo- or chemo-nociception. At a more integrative level, we will use molecular and pharmacological probes to characterize distinct nociceptor subtypes, whose functionalities will be examined in mice using genetic, anatomical, and in vivo imaging methods. As one such example, we are interested in characterizing a population of presumptive Aδ nociceptors and asking how they contribute to mechanical pain. Together, these studies will provide important mechanistic insights for the development of novel analgesic therapies.