Mu-opioid receptor and delta-opioid receptor differentially regulate microglial inflammatory response to control proopiomelanocortin neuronal apoptosis in the hypothalamus: effects of neonatal alcohol

Abstract

BackgroundOpioid receptors are known to control neurotransmission of various peptidergic neurons, but their potential role in regulation of microglia and neuronal cell communications is unknown. We investigated the role of mu-opioid receptors (MOR) and delta-opioid receptors (DOR) on microglia in the regulation of apoptosis in proopiomelanocortin (POMC) neurons induced by neonatal ethanol in the hypothalamus.MethodsNeonatal rat pups were fed a milk formula containing ethanol or control diets between postnatal days 2–6. Some of the alcohol-fed rats additionally received pretreatment of a microglia activation blocker minocycline. Two hours after the last feeding, some of the pups were sacrificed and processed for histochemical detection of microglial cell functions or confocal microscopy for detection of cellular physical interaction or used for gene and protein expression analysis. The rest of the pups were dissected for microglia separation by differential gradient centrifugation and characterization by measuring production of various activation markers and cytokines. In addition, primary cultures of microglial cells were prepared using hypothalamic tissues of neonatal rats and used for determination of cytokine production/secretion and apoptotic activity of neurons.ResultsIn the hypothalamus, neonatal alcohol feeding elevated cytokine receptor levels, increased the number of microglial cells with amoeboid-type circularity, enhanced POMC and microglial cell physical interaction, and decreased POMC cell numbers. Minocycline reversed these cellular effects of alcohol. Alcohol feeding also increased levels of microglia MOR protein and pro-inflammatory signaling molecules in the hypothalamus, and MOR receptor antagonist naltrexone prevented these effects of alcohol. In primary cultures of hypothalamic microglia, both MOR agonist [D-Ala 2, N-MePhe 4, Gly-ol]-enkephalin (DAMGO) and ethanol increased microglial cellular levels and secretion of pro-inflammatory cell signaling proteins. However, a DOR agonist [D-Pen2,5]enkephalin (DPDPE) increased microglial secretion of anti-inflammatory cytokines and suppressed ethanol’s ability to increase microglial production of inflammatory signaling proteins and secretion of pro-inflammatory cytokines. In addition, MOR-activated inflammation promoted while DOR-suppressed inflammation inhibited the apoptotic effect of ethanol on POMC neurons.ConclusionsThese results suggest that ethanol’s neurotoxic action on POMC neurons results from MOR-activated neuroinflammatory signaling. Additionally, these results identify a protective effect of a DOR agonist against the pro-inflammatory and neurotoxic action of ethanol.Electronic supplementary materialThe online version of this article (doi:10.1186/s12974-017-0844-3) contains supplementary material, which is available to authorized users.

A Novel Mu-Delta Opioid Agonist Demonstrates Enhanced Efficacy With Reduced Tolerance and Dependence in Mouse Neuropathic Pain Models

“…Lastly, the anti-inflammatory effect we observed in the spinal cord (Figure 7) is consistent with the observed role for the DOR in reducing inflammation; this is further supported by our finding that naltrindole blocked the anti-inflammatory effect of SRI-22141 (Figure 8). 2,41 This effect is consistent with our hypothesis that MOR-DOR dual agonists could act in part through an anti-inflammatory mechanism. Neuropathic pain states,…”

“…49,50 Relatedly, the DOR has been shown to have a strong anti-inflammatory role in neurons and neuronal tissue. 2,41 These concepts together suggest that MOR-DOR dual agonists could have enhanced efficacy with reduced side effects in pain states with an inflammatory component due to the anti-inflammatory effect mediated by the DOR. This hypothesis has not been tested in the literature, although 2 MOR-DOR dual agonists did show high efficacy with acute administration in inflammatory and neuropathic pain.…”

Impact of ambient temperature on inflammation-induced encephalopathy in endotoxemic mice—role of phosphoinositide 3-kinase gamma

“…Iba1 is a protein that acts to modulate membrane-ruffling changes during microglial activation [108]. Therefore, Iba1 appears to be a suitable indicator for microglial indexing and has been extensively used to identify, count, or gage activation of microglia within the CNS [109][110][111][112][113]. Furthermore, while the morphometric analysis of microglia for indexing the microglial activation status was the first approach characterizing different microglial response states [114][115][116], it remains also an appropriate measure for microglial activation, in particular if the expression level of several molecular biomarkers, such as Iba1, is mostly increasing with microglial activation [14].…”

Opposite Roles of δ- and μ-Opioid Receptors in BACE1 Regulation and Alzheimer’s Injury

“…In their work, a relatively non-specific opioid ligand, DADLE, was used as a DOR agonist in non-neuronal cell line (HEK293T cells). Since DADLE may bind to MOR and MOR may have a different role in certain neural functions (Lutz and Kieffer, 2013;Xia, 2015;Shrivastava et al, 2017;Wang et al, 2018) and since nonneuronal cells may react to AD stress in a different way, it is important to adopt more specific methodologies in neuron-like cells to yield more reliable conclusions in terms of the role of DOR in the pathology of AD.…”