Cortical δ-Opioid Receptors Potentiate K⁺ Homeostasis During Anoxia and Oxygen–Glucose Deprivation

Abstract

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 d-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-Ala 2 , D-Leu 5 ]-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.

Oxygen and Glucose Deprivation-Induced Changes in Astrocyte Membrane Potential and Their Underlying Mechanisms in Acute Rat Hippocampal Slices

“…Early in vivo cerebral ischemia studies showed that [K + ] o could reach as high as 50 to 80 mmol/L (Somjen, 1979), and this [K + ] o increase was not linear with time but could be subdivided into three phases (Hansen, 1985). In a recent study using acute cortical slices (Chao et al, 2006) The mammalian brain can tolerate a lack of blood supply for no more than 10 mins, which is followed by an irreversible functional neuronal damage (Nedergaard and Dirnagl, 2005). In a hippocampal pyramidal neuron study, a similar time period of 8 mins was determined for reaching OGD-induced anoxic depolarization, an indication of fuel exhaustion causing ion gradient breakdown (Allen et al, 2005).…”

“…Coincident with the observation of a [K + ] o undershoot in the post-OGD reperfusion phase (Mü ller and Somjen, 2000;Chao et al, 2006), astrocytic V m hyperpolarized transiently on OGD withdrawal (Figure 2). Ischemia-like insults enhance cultured astrocyte Na + /K + -ATPase activity (Stanimirovic et al, 1997) suggesting a possibility that this could be a responsible mechanism for the post-OGD astrocytic V m hyperpolarization.…”

“…Removal of glucose and oxygen from the perfusion solution for acutely prepared brain slices is a commonly used in vitro ischemia model (Martin et al, 1994;Allen et al, 2005;Chao et al, 2006). In this study, we applied glucose and oxygen-deficient artificial cerebrospinal fluid (aCSF) (oxygen and glucose deprivation (OGD) solution) to acute rat hippocampus slices and examined the OGD-induced astrocyte V m changes by patch-clamp recording.…”

Endomorphins and morphine limit anoxia–reoxygenation-induced brain mitochondrial dysfunction in the mouse

“…DADLE blocked PC12 cell death caused by serum deprivation in a naltrexone-sensitive manner (Borlongan et al, 2004;Su, 2000). The activation of DORs protects the cortex against anoxia-and OGD-induced alterations of potassium homeostasis (Chao et al, 2007).…”

δ-Opioid receptor antagonism induces NMDA receptor-dependent excitotoxicity in anoxic turtle cortex

“…DORs are a class of inhibitory G protein (G i )-coupled receptor whose activation is neuroprotective against hypoxic or ischemic insults in mammalian neurons (Chao et al, 2007b;Zhang et al, 2002;Zhang et al, 2000). Recently, DORs have also been linked to HPC-mediated neuroprotective mechanisms.…”