Ultramicronized N-palmitoylethanolamine contributes to morphine efficacy against neuropathic pain: implication of mast cells and glia.

In the current management of neuropathic pain, in addition to antidepressants and anticonvulsants, the use of opioids is wide, despite their related and well-known issues. N-palmitoylethanolamine (PEA), a natural fatty-acid ethanolamide whose anti-inflammatory, neuroprotective, immune-modulating and anti-hyperalgesic activities are known, represents a promising candidate to modulate and/or potentiate the action of opioids. This study was designed to evaluate if the preemptive and morphine concomitant administration of ultramicronized PEA, according to fixed or increasing doses of both compounds, delays the onset of morphine tolerance and improves its analgesic efficacy in the chronic constriction injury (CCI) model of neuropathic pain in rats. Behavioral experiments showed that the preemptive and co-administration of ultramicronized PEA significantly decreased the effective dose of morphine and delayed the onset of morphine tolerance. The activation of spinal microglia and astrocytes, commonly occurring both on opioid treatment and neuropathic pain, was investigated through GFAP and Iba-1 immunofluorescence. Both biomarkers were found to be increased in CCI untreated or morphine treated animals in a PEA-sensitive manner. The increased density of endoneural mast cells within the sciatic nerve of morphine- treated and untreated CCI rats was significantly reduced by ultramicronized PEA. The decrease of mast cell degranulation, evaluated in terms of reduced plasma levels of histamine and N-methylhistamine metabolite, was mainly observed at intermediate-high doses of ultramicronized PEA, with or without morphine. Overall, these results show that the administration of ultramicronized PEA in CCI rats according to the study design fully fulfilled the hypotheses of this study.