A DPDPE-induced enhancement of inward rectifier potassium current via opioid receptor in neuroblastomaxglioma NG108-15 cells

A novel opioid receptor-mediated enhancement of GABAA receptor function induced by stress in ventral tegmental area neurons

Opioid receptors are G-protein-coupled receptors (GPCRs) that modulate synaptic function. Depending upon their nervous system site of action, opioid receptor agonists alter food consumption, pain perception, responses to stress, and drug reward. Opioid receptors signal primarily via G(i/o)-proteins that modulate ion channels to directly inhibit neurons or decrease neurotransmitter release from nerve terminals. Here we report that following stress, activating δ opioid receptors (DORs) on midbrain ventral tegmental area (VTA) neurons causes a novel synaptic effect: the augmentation of GABA(A) receptor (GABA(A)R)-mediated inhibitory postsynaptic currents. Most neurons showing this augmentation were identified as dopaminergic. In addition, in both stressed and unstressed animals, DOR activation decreases GABA(A)R currents in some VTA neurons. Surprisingly, both augmentation and inhibition were also observed when we bypassed the presynaptic terminal by iontophoretically applying GABA, indicating that postsynaptic mechanisms are responsible for both effects. Using a variety of blockers we determined that the augmentation is probably due to insertion of GABA(A)Rs into the synapse by a mechanism that is G-protein independent and mediated by activation of Akt via PI3K. GABA(A)Rs are inserted into the extra-synaptic plasma membrane before trafficking to the synapse, a mechanism consistent with our observation that the DOR-mediated increase in GABA(A)R signalling occurs significantly earlier in iontophoretically applied than in electrically evoked synaptic GABA. This G-protein-independent signalling pathway is not only a novel mechanism of opioid receptor-mediated inhibition, but it also represents the first reported link between activation of a GPCR and insertion of GABA(A)Rs into the plasma membrane.

Ionic storm in hypoxic/ischemic stress: can opioid receptors subside it?

Neurons in the mammalian central nervous system are extremely vulnerable to oxygen deprivation and blood supply insufficiency. Indeed, hypoxic/ischemic stress triggers multiple pathophysiological changes in the brain, forming the basis of hypoxic/ischemic encephalopathy. One of the initial and crucial events induced by hypoxia/ischemia is the disruption of ionic homeostasis characterized by enhanced K(+) efflux and Na(+)-, Ca(2+)- and Cl(-)-influx, which causes neuronal injury or even death. Recent data from our laboratory and those of others have shown that activation of opioid receptors, particularly delta-opioid receptors (DOR), is neuroprotective against hypoxic/ischemic insult. This protective mechanism may be one of the key factors that determine neuronal survival under hypoxic/ischemic condition. An important aspect of the DOR-mediated neuroprotection is its action against hypoxic/ischemic disruption of ionic homeostasis. Specially, DOR signal inhibits Na(+) influx through the membrane and reduces the increase in intracellular Ca(2+), thus decreasing the excessive leakage of intracellular K(+). Such protection is dependent on a PKC-dependent and PKA-independent signaling pathway. Furthermore, our novel exploration shows that DOR attenuates hypoxic/ischemic disruption of ionic homeostasis through the inhibitory regulation of Na(+) channels. In this review, we will first update current information regarding the process and features of hypoxic/ischemic disruption of ionic homeostasis and then discuss the opioid-mediated regulation of ionic homeostasis, especially in hypoxic/ischemic condition, and the underlying mechanisms.

Endogenous opiates: 2000

This paper is the twenty-third installment of the annual review of research concerning the opiate system. It summarizes papers published during 2000 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunological responses.