PT  - JOURNAL ARTICLE
AU  - Eduardo R. Butelman
AU  - Todd J. Harris
AU  - Mary Jeanne Kreek
TI  - Antiallodynic Effects of Loperamide and Fentanyl against Topical Capsaicin-Induced Allodynia in Unanesthetized Primates
AID  - 10.1124/jpet.104.068411
DP  - 2004 Oct 01
TA  - Journal of Pharmacology and Experimental Therapeutics
PG  - 155--163
VI  - 311
IP  - 1
4099  - http://jpet.aspetjournals.org/content/311/1/155.short
4100  - http://jpet.aspetjournals.org/content/311/1/155.full
SO  - J Pharmacol Exp Ther2004 Oct 01; 311
AB  - 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 μ-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°C and 42°C; normally non-noxious thermal stimuli). The effects of loperamide were compared with those of the centrally penetrating μ-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 μ-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°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 μ-agonist after systemic administration, can prevent capsaicin-induced thermal allodynia in primates in vivo, in the absence of thermal antinociceptive effects. The American Society for Pharmacology and Experimental Therapeutics