Adenylate cyclase opioid receptors

Opiates iateract with three principal classes of opioid GPCRs )J.-selective for the endorphiQS,5-selective for enkephalins, and K-selective for dynorphias (51). AU. three receptors have been cloned. Each inhibits adenylate cyclase, can activate potassium channels, and inhibit A/-type calcium channels. The classical opiates, morphine and its antagonists naloxone (144) and naltrexone (145), have moderate selectivity for the. -receptor. Pharmacological evidence suggests that there are two subtypes of the. -receptor and three subtypes each of the 5- and K-receptor. An s-opiate receptor may also exist. 

Since they are linked to G-proteins, opioid receptors affect intracellular Ca and protein phosphorylation. Another principal biochemical effect of opiates is the inhibition of adenylate cyclase (AC), which decreases cAMP production. 

Opioids induce an inhibitory effect on gastrointestinal motility and fluid secretion (Kromer, 1990). The effect is peripherally and centrally mediated. The peripheral component is related to p- and K-receptors in intestinal organs, which are densely equipped with opioid receptors. They are located at parasympathic ganglia and inhibit the release of acetylcholine, which stimulates the contraction of smooth muscles. Inhibition of the intestinal fluid secretion is mediated via inhibition of adenylate cyclase. The intestinal effects of opioids extend to all parts of the gut and results in inhibition of stomach emptying and inhibition of secretion and motility of duodenum, jejunum, colon and rectum. 

In the spinal cord, a2-agonists act on receptors located on the terminals of primary afferent fibers in the dorsal horn substantia gelatinosa to reduce nociceptive transmission by inhibiting the release of glutamate and substance P (Collin et al., 1994 Hamalainen and Pertovaara, 1995) (see Fig. 2). These receptors appear to be primarily of the a2A subtype which is negatively coupled to adenylate cyclase (Lakhlani et al., 1997 see Millan, 1999 but see Sawamura et al., 2000, and references therein for a discussion of the possible involvement of other a2-receptor subtypes in antinociception). Like activation of p-opioid receptors, the activation of a2-receptors increases the potassium conductance of the cells bearing these receptors, thus reducing cellular excitability. 

Opioid receptors. Direct binding of highly radioactive opiates has permitted localization of specific opiate receptors of several types.863-866 The three major types (p, 8, k) are all 7-helix receptors coupled to adenylate cyclase, K+ and Ca2+ channels, and the MAP kinase cascade.866 The p receptors bind morphine most tightly.867 8673 These receptors are found in various cortical and subcortical regions of the brain. Most narcotics are polycyclic in nature and share the grouping indicated in Fig. 30-30. However, the flexible molecule methadone binds to the same receptors.868 Among antagonists that block the euphoric effects of opiates the most effective is naloxone (Fig. 30-30). 

Figure 3. Proposed model for the action of opiates and opioid peptides on glyco-syltransferase activity. It is postulated that receptors for both opiates and neurotransmitters are linked to adenylate cyclase by a guanylnucleotide regulatory protein (GNRP). The presence of opiates or opioid peptides inactivates the cyclase, which normally activates a protein kinase-glycosyltransferase system, thereby initiating...

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... 

Fig. 8. Known and potential interactions of the ACTH/adenylate cyclase/cyclic AMP-dependent protein kinase system in the adrenocortical cell with other hormones and intracellular messengers. Epinephrine activates adenylate cyclase in the adrenocortical cell [33]. Adrenocortical cells have receptors for several hormones which may activate G, including angiotensin II [34], acetylcholine [35], and endogenous opioid peptides [36]. Angiotensin II, acetylcholine and vasopressin [37-39] have all been demonstrated to activate the breakdown of PIP2 in adrenocortical cells and to stimulate steroidogenesis 5-hydroxytrypt-amine is also a known steroidogenic agent [40]. Probable receptor subtypes involved are indicated (/3 M (muscarinic) 5-HT, and V,). This is not a comprehensive diagramming of all stimuli or all possible interactions. Modified from Ref. 7.

Opioid receptors are seven transmembrane G-protein coupled inhibitory receptors (Bockaert, 1991), although an association to excitatory G-proteins has also been reported (Varga et al., 2003). Intracellular signaling by these receptors involves the inhibition of adenylate cyclase with a subsequent decrease in cAMP levels (Surprenant et al., 1990), regulation of intracellular calcium levels, modulation of potassium channels (Williams et al., 2001), and control of MAP-kinase and ERK activity (Eitan et al., 2003 Varga et al., 2003). 

At micromolar concentrations opioids cause an increase in the cell membrane threshold, shortened action potentials, and inhibition of neurotransmitter release. At nanomolar concentrations opioid agonists are excitatory and prolong the action potential via the stimulatory G proteins, which act on the adenylate cyclase/cAMP system and on protein kinase A-dependent ion channels. Tolerance is proposed to be the result of an increase in the association of opioid receptors to stimulatory G proteins, to an activation of A-methyl-o-aspartate receptors via protein kinase C, and calmodulin-dependent increases in cytosolic calcium, resulting in cellular hyperexcitability. 

Of the effector systems that have been implicated in the transduction mechanisms for opioid receptors, the best studied is opioid inhibition of adenylyl cyclase (see Refs. 69, 97-99 for reviews). Thus binding of an agonist to opioid receptors inhibits the activity of adenylyl cyclase and decreases intracellular cAMP in a number of different tissues. Pertussis toxin sensitivity of opioid inhibition of adenylyl cyclase has been demonstrated in many systems, indicating the involvement of either Gi or Go in the transduction mechanism. Agonist activation of all three types of cloned opioid receptors to inhibit adenyl cyclase has been demonstrated (see Ref 100 and references cited therein). There is also some evidence that (M and 8 opioid receptors can stimulate adenylyl cyclase in certain tissues (see Refs. 69, 97 for reviews). There are conflicting reports on whether k opioid receptors stimulate or inhibit phosphatidylinositol turnover in some tissues (see Ref 100) 8 and fx receptors, however, do not appear to be coupled to phosphatidylinositol turnover in neuroblastoma cell lines NG108-15 and SK-N-SH (101). 

Endogenous opioid receptors have been identified, cloned, and sequenced. They are members of the trimeric G-protein-binding superfamily, which are coupled to signal transduction via adenyl cyclase, and to Ca and ion-channel transport. Three major receptor subtypes are known (mu, kappa, and delta). Mu (p) receptor activation results in sedation, euphoria (via dopamine release), analgesia, respiratory depression, and GI dysmoflity. Kappa (k) receptors mediate spinal analgesia, miosis (via acetylcholine... 

Fig. 24.3. Schematic representation of a 5 enkephalinergic nerve terminal. (1) Pro-opioid proteins (proenkephalin A) are synthesized in the cell nucleus. (2) Pro-opioid proteins undergo microtubular transport to the nerve terminal. (3) Active endogenous opioids (E) are cleaved from the pro-opioid proteins by the action of processing proteases. (4) The active peptides (E) are taken up and stored in presynaptic vesicles. (5) The peptides are released when the presynaptic neuron fires. (6) The endogenous opioid peptides bind to postsynaptic receptors and activate second messenger systems. (7) For all opioid receptors, the second messenger effect is primarily mediated by a G-inhibitory (Gj/o) protein complex, which promotes the inactivation of adenylate cyclase (AC), a decrease in intracellular cyclic-adenosine-3, 5 -monophosphate (cAMP),...

The signal transduction mechanism for p, 5, and k receptors is through Gi/o proteins. Activation of opioid receptors is linked through the G protein to an inhibition of adenylate cyclase activity. The... 

Tolerance to and withdrawal from the opioids is explained by the cellular adaptation that occurs on repeated activation of p opioid receptors (60). When an agonist binds to the p receptor, Gi/o second messenger proteins are activated, and inhibition of adenylate cyclase occurs. Continual activation of the receptors results in an upregulation of adenylate cyclase to compensate for the decrease in cellular concentrations of cAMP. In addition, cellular mechanisms are activated that result in a decrease in the synthesis of Gi/o protein subunits and an internalization of the p... 

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