Background: We investigated the influence of morphine and ketamine or clonidine in mice on the expression of genes that may mediate pronociceptive opioid effects.
Material and methods: C57BL/6 mice received morphine injections thrice daily using increasing doses (5-20 mg∙kg(-1)) for 3 days (sub-acute, n=6) or 14 days (chronic, n=6) and additionally either s-ketamine (5 mg∙kg(-1), n=6) or clonidine (0.1 mg∙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.
Results: Chronic 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.
Conclusion: In 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.
Perspective: The results indicate that co-administration of clonidine or ketamine may influence the underlying mechanisms of OIH.