Opioid Receptors and G Proteins

University of Michigan Medical School, Ann Arbor, Michigan, U.S.A. 

The delta opioid receptor is a member of the large family of seven trans-membrane-spanning G protein-coupled receptors (GPCRs), as discussed extensively in Chapter 2. Delta opioid receptors modulate many intracellular effectors through their activation of GTP-binding proteins (G proteins), including adenylyl cyclase, K+ channels, Ca2+ channels, the MAP kinase cascade, phospholipase C, and intracellular Ca2+ release [1] (see Chap. 5). 

Agonist occupancy of GPCRs, such as the delta opioid receptor, leads to physiological effects through interactions with heterotrimeric G proteins. Such G proteins consist of a Ga subunit and its Gpy dimeric partner. There are four major families of Ga proteins with different profiles of effector interaction 1) Gas, which activate adenylyl cyclase 2) Gai/o, so-called inhibitory G proteins named for their ability to inhibit adenylyl cyclase, but interact with many effectors 3) Gaq/11, which activate phospholipase C- 3 (PLC- 3) and 4) Gal2/13, which may regulate small GTP-binding proteins. Delta opioid receptors, like mu and kappa opioid receptors, couple to mem- 

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There are several steps from agonist binding to effector response via G protein activation  

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There is considerable evidence that opioid receptors are coupled to G-proteins and produce their effects through these proteins (see Refs. 96, 97 for reviews of opioid receptors and G-proteins). The structure of cloned opioid receptors is consistent with their belonging to this receptor superfamily (see below). G-pro-teins are heterotrimers, consisting of a, jS, and y subunits, which bind guanine nucleotides to their a-subunit and catalyze the hydrolysis of GTP to GDP. G-proteins mediate the interac-... 

Agonist-dependent desensitization of the kappa opioid receptor by G protein receptor kinase and beta-arrestin. J Biol Chem 1999 274 233802-233807. 

Fan GH, Zhao J, Wu YL, Lou LG, ZhangZ, Jing Q, Ma L, Pei G (1998) N-Methyl-D-aspartate attenuates opioid receptor-mediated G protein activation and this process involves protein kinase C. Mol Pharmacol 53 684-690... 

As described in Chapter 4, regulatory G proteins act as an intermediate link between receptor activation and the intracellular effector mechanism that ultimately causes a change in cellular activity. In the case of opioid receptors, these G proteins interact with three primary cellular effectors calcium channels, potassium channels, and the adenyl cyclase enzyme.27 At the presynaptic terminal, stimulation of opioid receptors activates G proteins that in turn inhibit the opening of calcium channels on the nerve membrane.65 Decreased calcium entry into the presynaptic terminal causes decreased neurotransmitter release because calcium influx mediates transmitter release at a chemical synapse. At the postsynaptic neuron, opioid receptors are linked via G proteins to potassium channels, and... 

Opioid receptors are G-protein-coupled pre- and post-synaptic receptors, with seven transmembrane domains (TM), to which opioids bind. 

Hydrocodone and oxycodone are widely prescribed opioid analgesics that are agonists at the mu, kappa, and delta receptors in the central nervous system. The cellular changes that occur with agonism at the opioid receptors are still under investigation. However, it is known that mu opioid receptors are G protein-coupled receptors that decrease intracellular levels of cAMP. This decrease in intracellular cAMP inhibits the release of critical neurotransmitters and hormones including substance P, acetylcholine, GABA, somatostatin, and other substances that activate or sensitize nociceptors. Inhibition of neurotransmitter release causes a subsequent decrease in the perceived level of pain by the patient. Also, activation of opioid receptors modifies specific calcium channels at the surface of cells, which hyperpolarizes and decreases excitability of neurons. 

The mu, delta and kappa opioid receptors are coupled to G° and G proteins and the inhibitory actions of the opioids occur from the closing of calcium channels (in the case of the K receptor) and the opening of potassium channels (for /i, d and ORL-1). These actions result in either reductions in transmitter release or depression of neuronal excitability depending on the pre- or postsynaptic location of the receptors. Excitatory effects can also occur via indirect mechanisms such as disinhibition, which have been reported in the substantia gelatinosa and the hippocampus. Flere, the activation of opioid receptors on GABA neurons results in removal of GABA-mediated inhibition and so leads to facilitation. 

In vivo studies employing tests of acute pain unequivocally showed the involvement of Katp, Kv1.1 and Ca2+-activated K+ channels in supraspinal, spinal and peripheral analgesia produced by different classes of analgesics. Furthermore, two-pore-domain and G-protein gated inward rectifier (Kir3.x) K+ channels are affected by volatile anesthetics. The latter also contribute to p- and K-opioid receptor-mediated analgesia (Ikeda et al., 2000). 

A second strategy used cloning by homology. Although this approach undoubtedly identified opioid receptor clones, it did not reveal the identity of opioid receptors and claims were retrospective. The approach was based on the assumption that the opioid receptors would be G protein-coupled receptors, and as indicated above, earlier pharmacological evidence strongly... 

Got subunits of the Gi/o type of G proteins can be ADP-ribosylated in the presence of pertussis toxin at Cys351, four amino acids from the C-terminus. Petussis toxin sensitivity is the major method of identifying a role for Gai/o proteins in GPCR-mediated signaling. This treatment prevents receptor-mediated G-protein activation and thus exchange of GTP for GDP and so blocks signaling by Ga and G(3y. There are numerous examples of the use of this technique to identify coupling of the delta opioid receptor [e.g., 2,3,41,42,73,77]. One Ga protein in this class, Gaz, lacks the Cys residue that is the site for pertussis toxin action and so is insensitive to pertussis toxin treatment [see 17 for review]. 

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