In ancient times, physicians had a limited number of
therapies to provide
pain relief. Not surprisingly,
plant extracts applied topically often served as the primary
analgesic plan. With the discovery of the
capsaicin receptor (
transient receptor potential cation channel, subfamily V, member 1 [TRPV1]), the search for "new"
analgesics has returned to compounds used by physicians thousands of years ago. One such compound,
capsaicin, couples the paradoxical action of nociceptor activation (
burning pain) with subsequent
analgesia following repeat or high-dose application. Investigating this "paradoxical" action of
capsaicin has revealed several overlapping and complementary mechanisms to achieve
analgesia including receptor desensitization, nociceptor dysfunction,
neuropeptide depletion, and nerve terminal destruction. Moreover, the realization that TRPV1 is both sensitized and activated by endogenous products of
inflammation, including
bradykinin, H+,
adenosine triphosphate,
fatty acid derivatives,
nerve growth factor, and trypsins, has renewed interest in TRPV1 as an important site of
analgesia. Building on this foundation, a new series of preclinical and clinical studies targeting TRPV1 has been reported. These include trials using brief exposure to high-dose topical
capsaicin in conjunction with prior application of a
local anesthetic. Clinical use of
resiniferatoxin, another ancient but potent TRPV1 agonist, is also being explored as a
therapy for
refractory pain. The development of orally administered high-affinity TRPV1 antagonists holds promise for pioneering a new generation of
analgesics capable of blocking painful sensations at the site of
inflammation and tissue injury. With the isolation of other members of the TRP channel family such as
TRP cation channel, subfamily A, member 1, additional opportunities are emerging in the development of safe and effective
analgesics.