Tolerance develops to the antiallodynic effects of the peripherally acting opioid loperamide hydrochloride in nerve-injured rats

Shao Qiu He, Fei Yang, Federico M. Perez, Qian Xu, Ronen Shechter, Yong Kwan Cheong, Alene F. Carteret, Xinzhong Dong, Sarah M. Sweitzer, Srinivasa N. Raja, Yun Guan

Research output: Contribution to journalArticlepeer-review

14 Scopus citations


Peripherally acting opioids are potentially attractive drugs for the clinical management of certain chronic pain states due to the lack of centrally mediated adverse effects. However, it remains unclear whether tolerance develops to peripheral opioid analgesic effects under neuropathic pain conditions. We subjected rats to L5 spinal nerve ligation (SNL) and examined the analgesic effects of repetitive systemic and local administration of loperamide hydrochloride, a peripherally acting opioid agonist. We found that the inhibition of mechanical hypersensitivity, an important manifestation of neuropathic pain, by systemic loperamide (1.5 mg/kg subcutaneously) decreased after repetitive drug treatment (tolerance-inducing dose: 0.75 to 6.0 mg/kg subcutaneously). Similarly, repeated intraplantar injection of loperamide (150 μg/50 μL intraplantarly) and D-Ala2-MePhe4- Glyol5 enkephalin (300 μg/50 μL), a highly selective mu-opioid receptor (MOR) agonist, also resulted in decreased inhibition of mechanical hypersensitivity. Pretreatment with naltrexone hydrochloride (5 mg/kg intraperitoneally) and MK-801 (0.2 mg/kg intraperitoneally) attenuated systemic loperamide tolerance. Western blot analysis showed that repetitive systemic administration of morphine (3 mg/kg subcutaneously), but not loperamide (3 mg/kg subcutaneously) or saline, significantly increased MOR phosphorylation in the spinal cord of SNL rats. In cultured rat dorsal root ganglion neurons, loperamide dose-dependently inhibited KCl-induced increases in [Ca 2+]i. However, this drug effect significantly decreased in cells pretreated with loperamide (3 μM, 72 hours). Intriguingly, in loperamide-tolerant cells, the delta-opioid receptor antagonist naltrindole restored loperamide's inhibition of KCl-elicited [Ca2+]i increase. Our findings indicate that animals with neuropathic pain may develop acute tolerance to the antiallodynic effects of peripherally acting opioids after repetitive systemic and local drug administration.

Original languageEnglish (US)
Pages (from-to)2477-2486
Number of pages10
Issue number11
StatePublished - Nov 2013


  • Nerve injury
  • Neuropathic pain
  • Peripheral opioid receptor
  • Rats
  • Tolerance

ASJC Scopus subject areas

  • Neurology
  • Clinical Neurology
  • Anesthesiology and Pain Medicine


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