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Abstract
BackgroundWe investigated the influence of morphine and ketamine or clonidine in mice on the expression of genes that may mediate pronociceptive opioid effects.Material and MethodsC57BL/6 mice received morphine injections thrice daily using increasing doses (5-20mg∙kg-1) for 3 days (sub-acute, n=6) or 14 days (chronic, n=6) and additionally either s-ketamine (5mg∙kg-1, n=6) or clonidine (0.1mg∙kg-1, n=6). Tail flick test and the assessment of the mechanical withdrawal threshold of the hindpaw was performed during and 4 days after cessation of opioid treatment. Upon completion of the behavioural testing the mRNA-concentration of the NMDA receptor (NMDAR1) and β-arrestin 2 (Arrb2) were measured by PCR.ResultsChronic opioid treatment resulted in a delay of the tail flick latency with a rapid on- and offset. Simultaneously the mice developed a static mechanical hyperalgesia with a delayed onset that that outlasted the morphine treatment. Sub-acute morphine administration resulted in a decrease of NMDAR1 and Arrb2 whereas during longer opioid treatment the expression NMDAR1 and Arrb2 mRNA increased again to baseline values. Coadministration of s-ketamine or clonidine resulted in a reversal of the mechanical hyperalgesia and inhibited the normalization of NMDAR1 mRNA expression but had no effect on the expression of Arrb2 mRNA.ConclusionIn the model of chronic morphine therapy the antinociceptive effects of morphine are represented by the thermal analgesia while the proniceptive effects are represented by the mechanical hyperalgesia. The results indicate that the regulation of the expression of NMDAR1 and Arrb2 may be associated to the development of OIH in mice.PerspectiveThe results indicate that co-administration of clonidine or ketamine may influence the underlying mechanisms of OIH.
Highlights
Opioids are a cornerstone of the treatment of moderate to severe pain, including acute pain, cancer-related pain, and chronic non-cancer pain [1]
Common concerns regarding the use of opioids are the potential for detrimental side effects, physical dependence and addiction, accumulating evidence suggests that opioids may cause another problem, often referred to as opioid-induced hyperalgesia (OIH) [2]
Possible factors in the development OIH may be the regulation of the expression of the NMDAreceptor as one main focus of pronociceptive effects of opioids and of the scaffolding protein β-arrestin 2 that is involved in the regulation of μ-opioid receptor internalization and re-expression [14]. The goals of this project were: 1) to establish a model of OIH in mice that reflects the clinical setting of chronic morphine therapy and to study the effects of opioid therapy on mechanical hyperalgesia and thermal analgesia; 2) to observe the effect of clonidine and ketamine in this model. 3) to measure the expression levels of β-arrestin 2 and NMDA receptor subunit 1 in the brain to determine further mechanisms of OIH
Summary
Opioids are a cornerstone of the treatment of moderate to severe pain, including acute pain, cancer-related pain, and chronic non-cancer pain [1]. We investigated the influence of morphine and ketamine or clonidine in mice on the expression of genes that may mediate pronociceptive opioid effects. The mice developed a static mechanical hyperalgesia with a delayed onset that that outlasted the morphine treatment. Sub-acute morphine administration resulted in a decrease of NMDAR1 and Arrb whereas during longer opioid treatment the expression NMDAR1 and Arrb mRNA increased again to baseline values. Coadministration of s-ketamine or clonidine resulted in a reversal of the mechanical hyperalgesia and inhibited the normalization of NMDAR1 mRNA expression but had no effect on the expression of Arrb mRNA. The results indicate that the regulation of the expression of NMDAR1 and Arrb may be associated to the development of OIH in mice.
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