Background & Aims
Aldehyde dehydrogenase-2 (ALDH2) plays a key role in controlling toxic aldehydes, such as 4-hydroxinonenal (4-HNE), involved in inflammatory and neuropathic pain models. An ALDH2 genetic variant, ALDH2*2, is present in 540 million people, mostly East Asians, who have impaired ALDH2 enzymatic activity1. Genomic studies revealed that opioid-dependence in Asians is higher than in population controls2. Our group previously showed that ALDH2*2 mice, which mimics the East Asian phenotype, are more sensitive to opioid-induced tolerance. However, whether this genetic variant contributes to opioid-induced hypernoniception (OIH) and peripheral neuroinflammation is still unclear. Here, we investigated whether ALDH2 enzyme modulates OIH and opioid-induced neuroinflammation in the dorsal root ganglia (DRG) after long opiate exposure in both wild-type (WT) and ALDH2*2 transgenic mice. Further, we determined whether Alda-1, a small molecule activator of ALDH2, reverses this effect.
Methods
ALDH2*2 mice were generated as previously described3. Male mice C57BL/6J WT and ALDH2*2 (10-12 weeks old) received morphine once daily (10 mg/kg) for 10 days. A subset of these mice received the ALDH2 activator Alda-1 (16 mg/kg/day) for the duration of morphine treatments. To measure OIH, the nociceptive threshold was evaluated daily before morphine administration, using von Frey filaments. Western blot analyses were performed in dorsal root ganglia (DRGs) to evaluate the levels of 4-HNE and neuronal activation through FosB and ATF-3 expression. Immunohistochemistry was used to evaluate TRPV1, CGRP and barrestin2 expressions in DRGs. Data were analyzed by two-way ANOVA with post-hoc Bonferroni correction. Statistical significance was indicated by p<0.05.
Results
Behavior experiments showed that morphine (10 mg/Kg) treatment induced OIH in WT mice and this effect was detected earlier in ALDH2*2 (day 5 vs day 3, respectively). Alda-1 completely blocked this effect, when compared to vehicle-treated controls. Biochemical assays showed that morphine increases the 4-HNE protein adducts in the DRGs (45%±1 in both genotypes), when evaluated 5 days after morphine injection. Alda-1 treatment prevents this effect in ALDH2*2 mice. In parallel to increased hypernociception and 4-HNE levels, ALDH2*2 exhibited higher neuronal activation when compared to WT animals, characterized by increased expression in FosB (WT: 30%±2 vs ALDH2*2: 90%±1) and ATF-3 (WT: 7%±1 and ALDH2*2: 90%±1). Moreover, chronic morphine increases TRPV1 and CGRP expressions in WT animais (10%±1), and this increase is higher in ALDH2*2 mice (15%±3), when compared WT control group.
Conclusions
Our findings indicate that ALDH2 regulates chronic morphine-induced side effects, such as OIH, by controlling morphine-induced aldehydic load and neuroinflammation, which leads to neuronal activation in the peripheral nerve system. Alda-1 prevents OIH and neuroinflammation by reducing 4-HNE. Therefore, ALDH2 activators may be potential therapeutic molecules to control morphine adverse effects. This proposed molecular mechanism may be a way to describe differences between populations in the onset of opioid-induced side effects. Therefore, understanding the molecular mechanisms involved in this condition can contribute to the development of an individualized pharmacotherapy with potential non-opioid analgesics.
References
1 Annu Rev Pharmacol Toxicol. 2015; 55:107-27, 2 Drug Alcohol Depend. 2012;120:220–24, 3 Sci Transl Med 6:251ra118, 2014
Presenting Author
Beatriz Stein Neto
Poster Authors
Topics
- Treatment/Management: Pharmacology: Opioid