Abstract
It has previously been shown that nitric oxide (NO) synthase is involved in the development of opioid tolerance. The aim of the present work was to study the effect of NO on μ-opioid receptor (MOR) desensitization. Furthermore, we explored the possible role of reactive oxygen species (ROS) in this effect. Single-unit extracellular and whole-cell patch-clamp recordings were performed on locus coeruleus (LC) neurons from rat brain slices. Perfusion with high concentrations of Met5-enkephalin (ME) caused a concentration-related reduction of opioid effect, reflecting the induction of homologous MOR desensitization. The NO donors sodium nitroprusside and diethylamine NONOate markedly enhanced the ME-induced MOR desensitization, although the acute effect of ME on K+ conductance was not affected by sodium nitroprusside. Continuous perfusion with the antioxidants melatonin, trolox, 21-[4-(2,6-di-1-pyrrolidinyl-4-pyrrimidinyl)-1-piperazinyl]-pregna-1,4,9(11)-triene-3,20-dione(Z)-2-butenedioate (U74389G), and diethyldithiocarbamate prevented the effect of sodium nitroprusside on MOR desensitization, but they did not themselves alter the desensitization. Like sodium nitroprusside, the ROS-generating molecule H2O2 enhanced MOR desensitization induced by ME. However, α2-adrenoceptor desensitization induced by noradrenaline was not modified by H2O2, suggesting a selective action of ROS on MOR. Our results suggest that elevated levels of NO, which may be reached in pathological processes, enhance homologous desensitization of MOR in the LC, probably through a mechanism involving ROS generation.
Footnotes
This work was supported by the Ministerio de Ciencia e Innovación [Grant SAF2008-03612]; the Ministerio de Salud y Consumo [Grants RTA G03/005; PI05/0513]; and the University of the Basque Country [Grant GIU07/46]. J.P.'s research group takes part in a network unit supported by the University of the Basque Country [Grant UFI 11/35]. J.L. received a predoctoral fellowship from the Ministerio de Ciencia e Tecnología, and M.T.S. received a predoctoral fellowship from the Basque Government.
These data are a part of dissertation work: Llorente J (2007) Neuropharmacological mechanisms underlying opioid tolerance in the rat brain. Ph.D. thesis. University of the Basque Country, Bizkaia, Spain.
Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
ABBREVIATIONS:
- MOR
- μ-opioid receptor
- aCSF
- artificial cerebrospinal fluid
- ANOVA
- analysis of variance
- DEA/NO
- diethylamine NONOate
- DMS
- least significant differences
- GIRK
- inwardly rectifying K+ channel
- LC
- locus coeruleus
- ME
- Met5-enkephalin
- NA
- noradrenaline
- NO
- nitric oxide
- NOS
- NO synthase
- PKC
- protein kinase C
- ROS
- reactive oxygen species
- SNP
- sodium nitroprusside
- DDC
- sodium diethyldithiocarbamate
- U74389G
- 21-[4-(2,6-di-1-pyrrolidinyl-4-pyrrimidinyl)-1-piperazinyl]-pregna-1,4,9(11)-triene-3,20-dione(Z)-2-butenedioate.
- Received March 10, 2012.
- Accepted May 14, 2012.
- Copyright © 2012 by The American Society for Pharmacology and Experimental Therapeutics
JPET articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|