Dongman Chao

Current Research on Opioid Receptor Function

The use of opioid analgesics has a long history in clinical settings, although the comprehensive action of opioid receptors is still less understood. Nonetheless, recent studies have generated fresh insights into opioid receptor-mediated functions and their underlying mechanisms. Three major opioid receptors (μ-opioid receptor, MOR; δ-opioid receptor, DOR; and κ-opioid receptor, KOR) have been cloned in many species. Each opioid receptor is functionally sub-classified into several pharmacological subtypes, although, specific gene corresponding each of these receptor subtypes is still unidentified as only a single gene has been isolated for each opioid receptor. In addition to pain modulation and addiction, opioid receptors are widely involved in various physiological and pathophysiological activities, including the regulation of membrane ionic homeostasis, cell proliferation, emotional response, epileptic seizures, immune function, feeding, obesity, respiratory and cardiovascular control as well as some neurodegenerative disorders. In some species, they play an essential role in hibernation. One of the most exciting findings of the past decade is the opioid-receptor, especially DOR, mediated neuroprotection and cardioprotection. The upregulation of DOR expression and DOR activation increase the neuronal tolerance to hypoxic/ischemic stress. The DOR signal triggers (depending on stress duration and severity) different mechanisms at multiple levels to preserve neuronal survival, including the stabilization of homeostasis and increased pro-survival signaling (e.g., PKC-ERK-Bcl 2) and antioxidative capacity. In the heart, PKC and KATP channels are involved in the opioid receptor-mediated cardioprotection. The DOR-mediated neuroprotection and cardioprotection have the potential to significantly alter the clinical pharmacology in terms of prevention and treatment of life-threatening conditions like stroke and myocardial infarction. The main purpose of this article is to review the recent work done on opioids and their receptor functions. It shall provide an informative reference for better understanding the opioid system and further elucidation of the opioid receptor function from a physiological and pharmacological point of view.

δ‐, but not µ‐, opioid receptor stabilizes K+ homeostasis by reducing Ca2+ influx in the cortex during acute hypoxia

Past work has shown that δ‐opioid receptor (DOR) activation by [D‐Ala2,D‐Leu5]‐enkephalin (DADLE) attenuated the disruption of K+ homeostasis induced by hypoxia or oxygen‐glucose deprivation (OGD) in the cortex, while naltrindole, a DOR antagonist blocked this effect, suggesting that DOR activity stabilizes K+ homeostasis in the cortex during hypoxic/ischemic stress. However, several important issues remain unclear regarding this new observation, especially the difference between DOR and other opioid receptors in the stabilization of K+ homeostasis and the underlying mechanism. In this study, we asked whether DOR is different from µ‐opioid receptors (MOR) in stabilizing K+ homeostasis and which membrane channel(s) is critically involved in the DOR effect. The main findings are that (1) similar to DADLE (10 µM), H‐Dmt‐Tic‐NH‐CH (CH2COOH)‐Bid (1–10 µM), a more specific and potent DOR agonist significantly attenuated anoxic K+ derangement in cortical slice; (2) [D‐Ala2, N‐Me‐Phe4, glycinol5]‐enkephalin (DAGO; 10 µM), a MOR agonist, did not produce any appreciable change in anoxic disruption of K+ homeostasis; (3) absence of Ca2+ greatly attenuated anoxic K+ derangement; (4) inhibition of Ca2+‐activated K+ (BK) channels with paxilline (10 µM) reduced anoxic K+ derangement; (5) DADLE (10 µM) could not further reduce anoxic K+ derangement in the Ca2+‐free perfused slices or in the presence of paxilline; and (6) glybenclamide (20 µM), a KATP channel blocker, decreased anoxia‐induced K+ derangement, but DADLE (10 µM) could further attenuate anoxic K+ derangement in the glybenclamide‐perfused slices. These data suggest that DOR, but not MOR, activation is protective against anoxic K+ derangement in the cortex, at least partially via an inhibition of hypoxia‐induced increase in Ca2+ entry‐BK channel activity. J. Cell. Physiol. 212: 60–67, 2007.

Cortical δ-opioid receptors potentiate K+ homeostasis during anoxia and oxygen-glucose deprivation

Central neurons are extremely vulnerable to hypoxic/ischemic insult, which is a major cause of neurologic morbidity and mortality as a consequence of neuronal dysfunction and death. Our recent work has shown that δ-opioid receptor (DOR) is neuroprotective against hypoxic and excitotoxic stress, although the underlying mechanisms remain unclear. Because hypoxia/ischemia disrupts ionic homeostasis with an increase in extracellular K+, which plays a role in neuronal death, we asked whether DOR activation preserves K+ homeostasis during hypoxic/ischemic stress. To test this hypothesis, extracellular recordings with K+-sensitive microelectrodes were performed in mouse cortical slices under anoxia or oxygen–glucose deprivation (OGD). The main findings in this study are that (1) DOR activation with [D-Ala2, D-Leu5]-enkephalinamide attenuated the anoxia- and OGD-induced increase in extracellular K+ and decrease in DC potential in cortical slices; (2) DOR inhibition with naltrindole, a DOR antagonist, completely abolished the DOR-mediated prevention of increase in extracellular K+ and decrease in DC potential; (3) inhibition of protein kinase A (PKA) with N-(2-[p-bromocinnamylamino]-ethyl)-5-isoquinolinesulfonamide dihydrochloride had no effect on the DOR protection; and (4) inhibition of protein kinase C (PKC) with chelerythrine chloride reduced the DOR protection, whereas the PKC activator (phorbol 12-myristate 13-acetate) mimicked the effect of DOR activation on K+ homeostasis. These data suggest that activation of DOR protects the cortex against anoxia- or ODG-induced derangement of potassium homeostasis, and this protection occurs via a PKC-dependent and PKA-independent pathway. We conclude that an important aspect of DOR-mediated neuroprotection is its early action against derangement of K+ homeostasis during anoxia or ischemia.

Activation of DOR Attenuates Anoxic K+ Derangement via Inhibition of Na+ Entry in Mouse Cortex

We have recently found that in the mouse cortex, activation of delta-opioid receptor (DOR) attenuates the disruption of K(+) homeostasis induced by hypoxia or oxygen-glucose deprivation. This novel observation suggests that DOR may protect neurons from hypoxic/ischemic insults via the regulation of K(+) homeostasis because the disruption of K(+) homeostasis plays a critical role in neuronal injury under hypoxic/ischemic stress. The present study was performed to explore the ionic mechanism underlying the DOR-induced neuroprotection. Because anoxia causes Na(+) influx and thus stimulates K(+) leakage, we investigated whether DOR protects the cortex from anoxic K(+) derangement by targeting the Na(+)-based K(+) leakage. By using K(+)-sensitive microelectrodes in mouse cortical slices, we showed that 1) lowering Na(+) concentration and substituting with impermeable N-methyl-D-glucamine caused a concentration-dependent attenuation of anoxic K(+) derangement; 2) lowering Na(+) concentration by substituting with permeable Li(+) tended to potentiate the anoxic K(+) derangement; and 3) the DOR-induced protection against the anoxic K(+) responses was largely abolished by low-Na(+) perfusion irrespective of the substituted cation. We conclude that external Na(+) concentration greatly influences anoxic K(+) derangement and that DOR activation likely attenuates anoxic K(+) derangement induced by the Na(+)-activated mechanisms in the cortex.

Na+ mechanism of δ-opioid receptor induced protection from anoxic K+ leakage in the cortex

Abstract.Activation of δ-opioid receptors (DOR) attenuates anoxic K+ leakage and protects cortical neurons from anoxic insults by inhibiting Na+ influx. It is unknown, however, which pathway(s) that mediates the Na+ influx is the target of DOR signal. In the present work, we found that, in the cortex, (1) DOR protection was largely dependent on the inhibition of anoxic Na+ influxes mediated by voltage-gated Na+ channels; (2) DOR activation inhibited Na+ influx mediated by ionotropic glutamate N-methyl-D-aspartate (NMDA) receptors, but not that by non-NMDA receptors, although both played a role in anoxic K+ derangement; and (3) DOR activation had little effect on Na+/Ca2+ exchanger-based response to anoxia. We conclude that DOR activation attenuates anoxic K+ derangement by restricting Na+ influx mediated by Na+ channels and NMDA receptors, and that non-NMDA receptors and Na+/Ca2+ exchangers, although involved in anoxic K+ derangement in certain degrees, are less likely the targets of DOR signal.

δ-Opioid receptors protect from anoxic disruption of Na+ homeostasis via Na+ channel regulation

Hypoxic/ischemic disruption of ionic homeostasis is a critical trigger of neuronal injury/death in the brain. There is, however, no promising strategy against such pathophysiologic change to protect the brain from hypoxic/ischemic injury. Here, we present a novel finding that activation of δ-opioid receptors (DOR) reduced anoxic Na+ influx in the mouse cortex, which was completely blocked by DOR antagonism with naltrindole. Furthermore, we co-expressed DOR and Na+ channels in Xenopus oocytes and showed that DOR expression and activation indeed play an inhibitory role in Na+ channel regulation by decreasing the amplitude of sodium currents and increasing activation threshold of Na+ channels. Our results suggest that DOR protects from anoxic disruption of Na+ homeostasis via Na+ channel regulation. These data may potentially have significant impacts on understanding the intrinsic mechanism of neuronal responses to stress and provide clues for better solutions of hypoxic/ischemic encephalopathy, and for the exploration of acupuncture mechanism since acupuncture activates opioid system.

DOR activation inhibits anoxic/ischemic Na+ influx through Na+ channels via PKC mechanisms in the cortex.

Activation of delta-opioid receptors (DOR) is neuroprotective against hypoxic/ischemic injury in the cortex, which is at least partially related to its action against hypoxic/ischemic disruption of ionic homeostasis that triggers neuronal injury. Na(+) influx through TTX-sensitive voltage-gated Na(+) channels may be a main mechanism for hypoxia-induced disruption of K(+) homeostasis, with DOR activation attenuating the disruption of ionic homeostasis by targeting voltage-gated Na(+) channels. In the present study we examined the role of DOR in the regulation of Na(+) influx in anoxia and simulated ischemia (oxygen-glucose deprivation) as well as the effect of DOR activation on the Na(+) influx induced by a Na(+) channel opener without anoxic/ischemic stress and explored a potential PKC mechanism underlying the DOR action. We directly measured extracellular Na(+) activity in mouse cortical slices with Na(+) selective electrodes and found that (1) anoxia-induced Na(+) influx occurred mainly through TTX-sensitive Na(+) channels; (2) DOR activation inhibited the anoxia/ischemia-induced Na(+) influx; (3) veratridine, a Na(+) channel opener, enhanced the anoxia-induced Na(+) influx; this could be attenuated by DOR activation; (4) DOR activation did not reduce the anoxia-induced Na(+) influx in the presence of chelerythrine, a broad-spectrum PKC blocker; and (5) DOR effects were blocked by PKCβII peptide inhibitor, and PKCθ pseudosubstrate inhibitor, respectively. We conclude that DOR activation inhibits anoxia-induced Na(+) influx through Na(+) channels via PKC (especially PKCβII and PKCθ isoforms) dependent mechanisms in the cortex.

Effect of δ-Opioid Receptor Activation on BDNF-TrkB vs. TNF-α in the Mouse Cortex Exposed to Prolonged Hypoxia

We investigated whether δ-opioid receptor (DOR)-induced neuroprotection involves the brain-derived neurotrophic factor (BDNF) pathway. We studied the effect of DOR activation on the expression of BDNF and other proteins in the cortex of C57BL/6 mice exposed to hypoxia (10% of oxygen) for 1–10 days. The results showed that: (1) 1-day hypoxia had no appreciable effect on BDNF expression, while 3- and 10-day hypoxia progressively decreased BDNF expression, resulting in 37.3% reduction (p < 0.05) after 10-day exposure; (2) DOR activation with UFP-512 (1 mg/kg, i.p., daily) partially reversed the hypoxia-induced reduction of BDNF expression in the 3- or 10-day exposed cortex; (3) DOR activation partially reversed the hypoxia-induced reduction in functional TrkB (140-kDa) and attenuated hypoxia-induced increase in truncated TrkB (90-kDa) in the 3- or 10-day hypoxic cortex; and (4) prolonged hypoxia (10 days) significantly increased TNF-α level and decreased CD11b expression in the cortex, which was completely reversed following DOR activation; and (5) there was no significant change in pCREB and pATF-1 levels in the hypoxic cortex. We conclude that prolonged hypoxia down-regulates BDNF-TrkB signaling leading to an increase in TNF-α in the cortex, while DOR activation up-regulates BDNF-TrkB signaling thereby decreasing TNF-α levels in the hypoxic cortex.

δ-Opioid receptors up-regulate excitatory amino acid transporters in mouse astrocytes.

Astrocytic excitatory amino acid transporters (EAATs) regulate extracellular glutamate concentrations and play a role in preventing neuroexcitotoxicity. As the δ‐opioid receptor (DOP receptor) is neuroprotective against excitotoxic injury, we determined whether DOP receptor activation up‐regulates EAAT expression and function.

Animal behavioral assessments in current research of Parkinson's disease.

Parkinsons disease (PD) is traditionally classified as a movement disorder because patients mainly complain about motor symptoms. Recently, non-motor symptoms of PD have been recognized by clinicians and scientists as early signs of PD, and they are detrimental factors in the quality of life in advanced PD patients. It is crucial to comprehensively understand the essence of behavioral assessments, from the simplest measurement of certain symptoms to complex neuropsychological tasks. We have recently reviewed behavioral assessments in PD research with animal models (Asakawa et al., 2016). As a companion volume, this article will systematically review the behavioral assessments of motor and non-motor PD symptoms of human patients in current research. The major aims of this article are: (1) promoting a comparative understanding of various behavioral assessments in terms of the principle and measuring indexes; (2) addressing the major strengths and weaknesses of these behavioral assessments for a better selection of tasks/tests in order to avoid biased conclusions due to inappropriate assessments; and (3) presenting new concepts regarding the development of wearable devices and mobile internet in future assessments. In conclusion we emphasize the importance of improving the assessments for non-motor symptoms because of their complex and unique mechanisms in human PD brains.