Capsaicin produces
thermal allodynia in animals and humans by acting as an agonist at
vanilloid receptor subtype 1 [VR1; also known as transient receptor potential vanilloid type 1 (TRPV1)]. VR1 receptors are widely distributed in the periphery (e.g., on primary afferent neurons). These studies examined the ability of
loperamide (0.1-1 mg/kg s.c.; a micro-
opioid agonist that is peripherally selective after systemic administration), in preventing and reversing
thermal allodynia caused by topical
capsaicin (0.004 M) in rhesus monkeys, within a tail withdrawal assay (n = 4; 38 degrees C and 42 degrees C; normally non-noxious thermal stimuli). The effects of
loperamide were compared with those of the centrally penetrating micro-agonist,
fentanyl (0.0032-0.032 mg/kg s.c.). We also characterized the allodynic effects of the endogenous VR1 agonist ("endovanilloid"),
N-oleoyldopamine (OLDA; 0.0013-0.004 M). In this model,
loperamide and
fentanyl produced dose-dependent prevention of
capsaicin-induced
allodynia, whereas only
fentanyl produced robust reversal of ongoing
allodynia. Antagonism experiments with
naltrexone (0.1 mg/kg s.c.) or its analog,
methylnaltrexone (0.32 mg/kg s.c.), which does not readily cross the blood-brain barrier, suggest that the antiallodynic effects of
loperamide and
fentanyl were predominantly mediated by peripherally and centrally located micro-receptors, respectively.
Loperamide and
fentanyl (1 mg/kg and 0.032 mg/kg, respectively) also prevented OLDA (0.004 M)-induced
allodynia. Up to the largest dose studied,
loperamide was devoid of thermal antinociceptive effects at 48 degrees C (a noxious thermal stimulus, in the absence of
capsaicin). By contrast,
fentanyl (0.01-0.032 mg/kg) caused dose-dependent antinociception in this sensitive thermal antinociceptive assay (a presumed centrally mediated effect). These studies show that
loperamide, acting as a peripherally selective micro-agonist after systemic administration, can prevent
capsaicin-induced
thermal allodynia in primates in vivo, in the absence of thermal antinociceptive effects.