In pursuit of neurological therapies, the opioid system, specifically delta opioid receptors and delta opioid peptides, demonstrates promising therapeutic potential for stroke, Parkinson’s disease, and other degenerative neurological conditions. Recent studies offer strong evidence in support of the therapeutic use of delta opioid receptors, and provide insights into the underlying mechanisms of action. Delta opioid receptors have been shown to confer protective effects by mediating ionic homeostasis and activating endogenous neuroprotective pathways. Additionally, delta opioid agonists such as (D-Ala 2, D-Leu 5) enkephalin (DADLE) have been shown to decrease apoptosis and promote neuronal survival. In its entirety, the delta opioid system represents a promising target for neural therapies.
The opioid system is composed of various opioid peptides and their corresponding receptors. Opioids are a group of inhibitory neurotransmitters that are involved in a variety of functions including pain regulation and respiratory rate control. The pharmacologic effects exhibited by various opioid peptides are mediated by opioid receptors, with each receptor recognizing a unique group of opioid and non-opioid ligands. The family of classical Gi-protein coupled receptors which inhibit adenylyl cyclase, is divided into three primary subgroups: μ- (MOR), κ- (KOR), and δ-opioid receptors (DOR). Endogenous opioid peptides including the endorphins, dynorphins, and enkephalins, associate with the MOR, KOR, or DOR respectively. Opioid receptors elicit diverse pharmacologic effects depending on their opioid classification. These receptors are present throughout the central and peripheral nervous systems, as well as various peripheral organs including the heart, lungs, liver, and gastrointestinal tract. Accumulating evidence suggests that the opioid system may confer protection against degenerative neurological diseases characterized by oxygen-, blood-, and energy depleting states. A study conducted by Mayfield and colleagues demonstrated extended survival during hypoxia when animals were pretreated with an opioid receptor agonist. Additionally, it was shown that opioid-induced protection could be inhibited by DOR antagonists, but not MOR and KOR antagonists. These data suggest that the opioid system is involved in neuroprotection against hypoxic and ischemic events, and is likely mediated primarily by DOR and delta opioid peptides.
Accumulating evidence has demonstrated that DOR activation in response to hypoxic/ischemic stress, may afford neuroprotective effects. In the late 1980s, Xia and colleagues observed that the turtle brain has a higher density of DOR than the rat brain. Furthermore, the turtle brain demonstrates a higher tolerance to hypoxic/ischemic conditions than the rat brain. That increased DOR density in the brain lends resistance against hypoxic/ischemic injury implicates the close participation of this opioid receptor in neuroprotection. Indeed, the potential relationship between these two phenomena becomes of interest in our quest to understand the role of DOR in neuroprotection. To investigate this proposed connection between DOR activation and neuroprotection, researchers devised an experimental paradigm of neuroexcitotoxicity by adding glutamate to cultured cortical neurons. Neurons which had been exposed to 100 μmol/L glutamate for 4 h daily over a period of 8–10 days showed significant neuronal injury. However, activation of the DOR, via administration of (D-Ala 2, D-Leu 5) enkephalin (DADLE), decreased glutamate-induced injury by almost half. Additionally, activation of MOR and KOR did not elicit any significant protective effects, suggesting that the DORs, not MORs or KORs, are responsible for the observed neuroprotective effects.
Recent studies have also implicated DORs in ischemia. Following middle cerebral artery occlusion (MCAO) in mice, delta binding sites decreased prior to reductions in κ- or μ-binding sites, concomitant with infarct core extension. These in vitro and in vivo studies provide evidence that the increased DOR activation is key to suppressing glutamate-induced neurotoxicity and hypoxic/ischemic injury. These observed DOR expression patterns in the brain are likely coupled with DOR signaling pathways, altogether affording neuroprotection against injurious brain insults. Although the underlying signaling mechanism of DORs in neuroprotection is not fully known, recent scientific advances have been made in our understanding of these downstream pathways accompanying DOR expression patterns. Here, we discussed two major DOR-mediated mechanisms, namely Na+ channel and protein kinase signaling pathways. Acute hypoxic/ischemic stress causes an immediate loss of ionic homeostasis characterized by an efflux of K+, and an influx of Cl−, Na+, and Ca+2. This increased efflux of K+ is typical of hypoxia/ischemia and can cause neuronal injury and death. Numerous studies have demonstrated that activation of the DOR reduces K+ leakage following ischemia thereby decreasing consequent neuronal death. Furthermore, increased expression of DOR has been shown to inhibit the function of voltage-gated Na+ channels and thus directly decreasing the influx Na+ and indirectly suppressing the efflux of K+. This inhibition of Na+ influx and the resulting neuroprotective effects of DOR were blocked following exposure to low Na+ perfusion, Na+ channel blocker TTX, and NMDA receptor channel blocker MK 801. Together these data suggest that inhibition of Na+ influx is a pivotal underlying mechanism for DOR neuroprotection against the disruption of ionic homeostasis associated with hypoxic/ischemic insult.
(A) A DOR agonist such as DADLE is administered via the middle cerebral artery; (B) The DOR agonist binds to and activates DOR in the brain, inhibiting the influx of Na+ and activating the PKC/ERK pathway; (C) Activation of DOR results in decreased neuronal injury and death following an ischemic event.
DOR may also elicit neuroprotective effects by mediating endogenous protein kinase signaling pathways. Activation of DORs has been shown to prevent cell death by blocking the phosphorylation of p38 via stimulation of protein kinase C (PKC) and mitogen-activated protein kinase (MAPK)-ERK1/2. Indeed, treatment with a PKC inhibitor has been shown to diminish DOR mediated neuroprotection against ischemia/hypoxia. Using a hypoxic preconditioning (HPC) approach, DOR-mediated neuroprotection in HPC neurons were found to solicit PKC activities, in that DOR-transduced signals enhance pERK-Bcl 2 activity, but suppress those of pp38-cytochrome c, displaying a “yin-yang” pattern. These results highlight DOR as an oxygen-sensitive protein, which enhances the intracellular activity of the G protein-PKC-pERK-Bcl 2 pathway and suppresses the pp38 and cytochrome c death signals. This PKC-dependent pathway has also been implicated in DOR attenuation of K+ efflux and maintenance of ionic homeostasis. Similarly, DOR as an oxygen-sensitive protein has been extended in HPC-mediated retinoprotection following intraocular pressure (IOP) elevation. HPC attenuated neuronal loss and apoptosis in the IOP retina via upregulation of hypoxia-inducible factor-1alpha (HIF-1alpha) that induces the expression of DOR, and subsequently activates extracellular signal-regulated kinase resulting in anti-oxidative protection of the IOP retina. In essence, Peng et al. demonstrated that an up-regulation of HIF-1alpha occurs following hypoxic preconditioning. This upregulation of HIF-1alpha increases the expression of DOR which induced neuroprotection via the ERK signaling pathway. Additional support of an ERK-mediated neuroprotection by DOR reveals that these DOR-induced neuroprotective effects are inhibited by treatment with an ERK inhibitor. In addition to neuroprotection, DOR via the PKC signaling pathway, has also been implicated in neurogenesis in that the stimulation of DOR by a selective DOR agonist promoted neural differentiation of multipotent neural stem cells, which was inhibited by treatment with a PKC inhibitor. In parallel, this PKC-mediated neuroprotection via DOR activation has been seen with DADLE treatment in the severe hypoxic state of oxygen-glucose deprivation (OGD) model. DADLE increased phosphorylation of ERK and prevented OGD-induced p38 phosphorylation, but no appreciable effect on phosphorylation of JNK, further highlighting that DOR-DADLE neuroprotection may be due to the dynamic balance between the activation of ERK and the p38. Altogether these data demonstrate the pivotal role of DOR mediation of PKC and ERK signaling pathways for neuroprotection. Nonetheless, further studies are warranted to elucidate DOR-mediated neuroprotection via MAPK (p38 or ERK) and PKC signaling, especially on the involvement of genes that are either upregulated or downregulated, which should guide treatment strategies designed to highly regulate DOR expression patterns. Furthermore, additional studies are also needed to better understand why and how neuroprotective signals through DOR differ from MOR and KOR, given the fact that PKC pathways are also activated by MOR and KOR ligands.
Hibernation is a unique natural model that allows animals to survive typically detrimental oxygen-, blood-, and energy-deprived conditions. For this reason, hibernation has been a particular interest for many researchers in search of potential neural therapies for disease states with similar conditions. A search for further understanding of the molecular components involved in hibernation led to the discovery that plasma from thirteen-lined ground squirrels could induce hibernation when injected into summer active ground squirrels. The hibernation inducing trigger (HIT) was identifie
