Nicotinic acetylcholine receptor agonists: a potential new class of analgesics

Current analgesics, such as opioids and nonsteroidal anti-inflammatory drugs (NSAIDs), are largely refinements of approaches available for more than 100 years and have critical liabilities and limitations. A number of new molecular targets for analgesia have been proposed in recent years, including the neuronal nicotinic acetylcholine receptor (nAChR). Agonists at neuronal nAChRs have antinociceptive effects in a variety of preclinical pain models. Moreover, nicotine can decrease experimentally-induced pain in humans without disrupting normal tactile sensation. These data from both experimental animals and humans suggest that compounds targeting neuronal nAChRs may represent a new class of analgesic agents. In this paper, we provide brief overviews of the physiology of pain, the animal models used to assess potential analgesics preclinically, and the biology of nAChRs. We then provide a review of preclinical data on the antinociceptive effects of a variety of neuronal nAChR agonists and a discussion of potential mechanisms, including evidence that antinociception is mediated by activation of brainstem nuclei with descending inhibitory inputs to the spinal cord. An evaluation of the clinical potential of this approach must also consider potential side effects. Undesirable side effects of nicotine are well known, but as we will discuss in detail, these effects are not produced by all neuronal nAChR agonists and the existence of neuronal nAChR subtypes may provide a basis for separating therapeutic effects from toxicities.