Specific behavioral and cellular adaptations induced by chronic morphine are reduced by dietary omega-3 polyunsaturated fatty acids

Abstract

Opiates, one of the oldest known drugs, are the benchmark for treating pain. Regular opioid exposure also induces euphoria making these compounds addictive and often misused, as shown by the current epidemic of opioid abuse and overdose mortalities. In addition to the effect of opioids on their cognate receptors and signaling cascades, these compounds also induce multiple adaptations at cellular and behavioral levels. As omega-3 polyunsaturated fatty acids (n-3 PUFAs) play a ubiquitous role in behavioral and cellular processes, we proposed that supplemental n-3 PUFAs, enriched in docosahexanoic acid (DHA), could offset these adaptations following chronic opioid exposure. We used an 8 week regimen of n-3 PUFA supplementation followed by 8 days of morphine in the presence of this diet. We first assessed the effect of morphine in different behavioral measures and found that morphine increased anxiety and reduced wheel-running behavior. These effects were reduced by dietary n-3 PUFAs without affecting morphine-induced analgesia or hyperlocomotion, known effects of this opiate acting at mu opioid receptors. At the cellular level we found that morphine reduced striatal DHA content and that this was reversed by supplemental n-3 PUFAs. Chronic morphine also increased glutamatergic plasticity and the proportion of Grin2B-NMDARs in striatal projection neurons. This effect was similarly reversed by supplemental n-3 PUFAs. Gene analysis showed that supplemental PUFAs offset the effect of morphine on genes found in neurons of the dopamine receptor 2 (D2)-enriched indirect pathway but not of genes found in dopamine receptor 1(D1)-enriched direct-pathway neurons. Analysis of the D2 striatal connectome by a retrogradely transported pseudorabies virus showed that n-3 PUFA supplementation reversed the effect of chronic morphine on the innervation of D2 neurons by the dorsomedial prefontal and piriform cortices. Together these changes outline specific behavioral and cellular effects of morphine that can be reduced or reversed by dietary n-3 PUFAs.

Dietary Supplementation with Omega-3 Polyunsaturated Fatty Acids Reduces Opioid-Seeking Behaviors and Alters the Gut Microbiome

“…We have previously shown that a PUFA-enriched diet alters the behavioral and cellular adaptations to non-contingent chronic morphine exposure [43]. In this study, we examined how this same dietary regimen altered contingent opioid use.…”

“…Mice (C57Bl6/J Jax stock # 000664), 6-8 weeks of age at the start of the experiment, were randomly assigned to a control lab chow diet containing 0.5% DHA (Control) alone or supplemented with 2.5% DHA, 1.1% EPA and 0.75% other omega-3 PUFAs, (Nordic Naturals, Watsonville, CA) for 8 weeks. As we have previously shown a relative lack of effect of this DHA supplementation protocol on DHA enrichment in specific brain regions in female C57Bl6/J mice [43], only male C57Bl6/J mice were used for these experiments. Where possible, experimenters were blind to treatment and diet for both behavioral testing and data analyses.…”

“…We have previously shown that a DHA-enriched diet reduces the anxiety state of mice following chronic morphine [43]. Here we used the well-established light-dark test to repeatedly assess the state of anxiety [48] [55] (Figure 2G).…”

“…We have previously shown that chronic, noncontingent, morphine induces diverse adaptations of the glutamatergic system in the striatum, a hub of the reward-centered mesolimbic circuitry [43]. Many of these cellular and behavioral adaptations were reduced or offset by a PUFA-enriched diet.…”

“…We achieved this by examining the gut microbiome during multiple phases of the opioid exposure model. Lastly, we examined the state of the microglia in the striatum, a site we had previously studied [43], at the completion of the trial to determine if there was evidence of altered microglial activation due to the opioid exposure or DHA supplementation.…”