Functional properties of the high-affinity TRPV1 (VR1) vanilloid receptor antagonist (4-hydroxy-5-iodo-3-methoxyphenylacetate ester) iodo-resiniferatoxin.
We have synthesized iodinated resiniferatoxin bearing a 4-hydroxy-5-iodo-3-methoxyphenylacetate ester (I-RTX) and have characterized its activity on rat and human TRPV1 (VR1) receptors, as well as in behavioral assays of nociception. In whole cell patch-clamp recordings from transfected cells the functional activity of I-RTX was determined. Currents activated by capsaicin exhibited characteristic outward rectification and were antagonized by capsazepine and I-RTX. On rat TRPV1 the affinity of I-RTX was 800-fold higher than that of capsazepine (IC50 = 0.7 and 562 nM, respectively) and 10-fold higher on rat versus human receptors (IC50 = 0.7 and 5.4 nM, respectively). The same difference was observed when comparing the inhibition of [3H]RTX binding to rat and human TRPV1 membranes for both RTX and I-RTX. Additional pharmacological differences were revealed using protons as the stimulus. Under these conditions capsazepine only partly blocked currents through rat TRPV1 receptors (by 70 to 80% block), yet was a full antagonist on human receptors. In contrast, I-RTX completely blocked proton-induced currents in both species and that activated by noxious heat. I-RTX also blocked capsaicin-induced firing of C-fibers in a rat in vitro skin-nerve assay. Despite this activity and the high affinity of I-RTX for rat TRPV1, only capsazepine proved to be an effective antagonist of capsaicin-induced paw flinching in rats. Thus, although I-RTX has limited utility for in vivo behavioral studies it is a high-affinity TRPV1 receptor antagonist that will be useful to characterize the functional properties of cloned and native vanilloid receptor subtypes in vitro.
Seabrook
Guy R
GR
The Neuroscience Research Centre, Merck Sharp and Dohme, Harlow, Essex, United Kingdom. seabrook@merck.com
Sutton
Kathy G
KG
Jarolimek
Wolfgang
W
Hollingworth
Gregory J
GJ
Teague
Simon
S
Webb
Janine
J
Clark
Natalie
N
Boyce
Susan
S
Kerby
Julie
J
Ali
Zahid
Z
Chou
Margaret
M
Middleton
Richard
R
Kaczorowski
Gregory
G
Jones
A Brian
AB
D000200:Action Potentials / Q000187:drug effects* / Q000502:physiology; D000818:Animals; D016466:CHO Cells; D002186:Cannabinoids / Q000037:antagonists & inhibitors / Q000235:genetics; D002211:Capsaicin / Q000378:metabolism* / Q000494:pharmacology; D006224:Cricetinae; D004224:Diterpenes / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D006801:Humans; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D051381:Rats; D017207:Rats, Sprague-Dawley; D011955:Receptors, Drug / Q000037:antagonists & inhibitors* / Q000172:deficiency* / Q000235:genetics
D002186:Cannabinoids; D004224:Diterpenes; D011955:Receptors, Drug; C468130:iodoresiniferatoxin; D002211:Capsaicin