Adenylyl cyclase inhibition

Table 3). For example, arabinose and xylose differ from ribose only in the orientation of the 2 - and 3 -OH groups yet exhibit markedly different potencies. Whereas 9-(tetrahydrofuryl)-Ade ( SQ 22,536) and 9-(cyclopentyl)-Ade are without hydroxyl groups and are less potent, they offer metabolic and biochemical stability useful for many types of studies. It is, however, the removal of two of the hydroxyl groups, that elicits the largest improvement in inhibitory potency, in particular the 2, 5 -dideoxy- modification (Table 3). With these improvements in potency, these cell permeable compounds, in particular 2, 5 -dd-Ado, have become useful research tools and have been used to inhibit adenylyl cyclases and to lower cAMP levels and alter function in numerous studies in isolated cells or intact tissues. 

Table 4), but inhibit adenylyl cyclase by conforma-tionally distinct mechanisms (cf. Fig. 6) by binding within the catalytic cleft in unique structures (Fig. 7). 

Functionally, the Dl-like receptors (Dl, D5) are coupled to the G protein Gas and thus can stimulate adenylyl cyclase. The D2-like receptors (D2, D3, and D4) couple to pertussis toxin sensitive G proteins (Gai/0), and consequently inhibit adenylyl cyclase activity. While the Dl-like receptors almost exclusively signal through Gas-mediated activation of adenylyl cyclase, the D2-like receptors have been reported to modulate the activity of a plethora of signaling molecules and pathways. Many of these actions are mediated through the G(3y subunit. Some of these molecules and pathways include the calcium channels, potassium channels, sodium-hydrogen exchanger, arachidonic acid release, and mitogen-activated protein kinase pathways. 

P-site ligands inhibit adenylyl cyclases by a noncompetitive, dead-end- (post-transition-state) mechanism (cf. Fig. 6). Typically this is observed when reactions are conducted with Mn2+ or Mg2+ on forskolin- or hormone-activated adenylyl cyclases. However, under- some circumstances, uncompetitive inhibition has been noted. This is typically observed with enzyme that has been stably activated with GTPyS, with Mg2+ as cation. That this is the mechanism of P-site inhibition was most clearly demonstrated with expressed chimeric adenylyl cyclase studied by the reverse reaction. Under these conditions, inhibition by 2 -d-3 -AMP was competitive with cAMP. That is, the P-site is not a site per se, but rather an enzyme configuration and these ligands bind to the post-transition-state configuration from which product has left, but before the enzyme cycles to accept new substrate. Consequently, as post-transition-state inhibitors, P-site ligands are remarkably potent and specific inhibitors of adenylyl cyclases and have been used in many studies of tissue and cell function to suppress cAMP formation. 

G —inhibits adenylyl cyclase (inactivated by Pertussis toxin)... 

Dopamine Di Dz, D4 D3 Gs Go, Gi G, Stimulates adenylyl cyclase raising cAMP Inhibits adenylyl cyclase, Ca +/K+ channels Phospholipase-C to IPsto [Ca +]i regulation... 

Glutamate-metabotropic mGluRl, 3, 5 mGluR2, 4, 6, 7 G, Go, Gi Phospholipase-C to IP3 to [Ca +]i regulation Inhibits adenylyl cyclase, Ca /K channels... 

The precise role of melatonin in sleep and waking is uncertain but it seems to act as a go-between for the light and biological cycles and evidence suggests that it has a reciprocal relationship with the SCN (Fig. 22.3). Its actions are mediated by (MLi) receptors which are found predominantly in the SCN as well as thalamic nuclei and the anterior pituitary. These are G protein-coupled receptors, with seven transmembrane domains, that inhibit adenylyl cyclase. Their activation by melatonin, or an MLi agonist such as 2-iodomelatonin, restores the impaired circadian cycle in aged rats. 

In addition to their well-known action to inhibit adenylyl cyclase activity, opiates inhibit Ca++ conductance in neurons by modulating Ca++ channel activity [42]. Inhibition of Ca++ influx is a major mechanism by which opiates inhibit neurotrans-... 

Mutagenesis studies have shown that morphine and sufentanil bind differently to the jj, receptor [83, 85]. Mutation of an aspartic acid at residue 114 of the // receptor to an asparagine resulted in a mutant that did not bind morphine and morphine was ineffective in inhibiting adenylyl cyclase via that receptor. In contrast, sufentanil bound to the mutant and wild-type receptors equally well and it effectively inhibited cAMP accumulation via the mutant receptor. These findings demonstrate that morphine and sufentanil have different requirements for binding to the // receptor. By binding differentially, these two agonists may induce the ft receptor to interact with different G proteins to induce distinct cellular effects. 

Prather PL, Loh HH, Law PW. Interaction of d-opioid receptors with multiple G proteins A non-relationship between agonist potency to inhibit adenylyl cyclase and to activate G proteins. Mol Pharmacol 1994 45 997-1003. 

The selectivity in muscarinic receptor coupling is not, however, absolute. Overexpression of receptors or of particular G proteins supports interactions that may differ from those described above. For example, M2 receptors expressed in Chinese hamster ovary cells not only inhibit adenylyl cyclase but also can stimulate phosphoinositide hydrolysis through a pertussis-toxin-sensitive G protein [52] this is not seen, however, when M2 receptors are expressed in Y1 cells. These findings indicate that caution must be exercised in interpreting data obtained when receptors are expressed, often at high levels, in cells in which they normally do not function. 

The P-site of adenylyl cyclase inhibits cyclic AMP accumulation. Since P, and P2 receptors are located on the cell surface, they bind purines or pyrimidines in the extracellular space. There also is an adenosine binding site located intracellularly on the enzyme adenylyl cyclase (see Ch. 21). This is referred to as the P-site of adenylyl cyclase. Binding of adenosine and other purines, notably 3 AMP, 2 deoxy-3 -ATP and 2, 5 -dideoxyadenosine to this site, inhibits adenylyl cyclase activity [8]. The P-site of adenylyl cyclase and other intracellular purine binding sites are not classified as purinergic receptors. 

A1 adenosine receptors are inhibitory in the central nervous system. A receptors were originally characterized on the basis of their ability to inhibit adenylyl cyclase in adipose tissue. A number of other G-protein-mediated effectors of A receptors have subsequently been discovered these include activation of K+ channels, extensively characterized in striatal neurons [13], and inhibition of Ca2+ channels, extensively characterized in dorsal root ganglion cells [14]. Activation of A receptors has been shown to produce a species-dependent stimulation or inhibition of the phosphatidylinositol pathway in cerebral cortex. In other tissues, activation of A receptors results in synergistic activation of the phosphatidylinositol pathway in concert with Ca2+-mobilizing hormones or neurotransmitters [15]. The effectors of A adenosine receptors and other purinergic receptor subtypes are summarized in Table 17-2. 

G proteins regulate intracellular concentrations of second messengers. G proteins control intracellular cAMP concentrations by mediating the ability of neurotransmitters to activate or inhibit adenylyl cyclase. The mechanism by which neurotransmitters stimulate adenylyl cyclase is well known. Activation of those neurotransmitter receptors that couple to Gs results in the generation of free G(IS subunits, which bind to and thus directly activate adenylyl cyclase. In addition, free Py-subunit complexes activate certain subtypes of adenylyl cyclase (see Ch. 21). A similar mechanism appears to be the case for G(IO f, a type of G protein structurally related to G that is enriched in olfactory epithelium and striatum (Ch. 50). 

The mechanism by which neurotransmitters inhibit adenylyl cyclase and decrease neuronal levels of cAMP has been more difficult to delineate. By analogy with the action of Gs, it was proposed originally that activation of neurotransmitter receptors that couple to G, results in the generation of free Gai subunits, which bind to and,... 

In contrast, pertussis toxin catalyzes the ADP-ribosyl-ation of a specific cysteine residue in Gai) G(m and Gal [1]. Only a subunits bound to their Py subunits can undergo this modification. Pertussis-toxin-mediated ADP-ribosylation inactivates these a subunits such that they cannot exchange GTP for GDP in response to receptor activation (Fig. 19-1B). By this mechanism, pertussis toxin blocks the ability of neurotransmitters to inhibit adenylyl cyclase or to influence the gating of K+ and Ca2+ channels in target neurons. However, since G is not a substrate for pertussis toxin, the toxin may not be able to block neurotransmitter-mediated inhibition of adenylyl cyclase in all cases. The Gq and Gn 16 types of G protein a subunit are not known to undergo ADP-ribosylation. 

Opioid receptors generally mediate neuronal inhibition. They couple to G or G0> and produce inhibition of Ca2+ channels and opening of K+ channels. They also inhibit adenylyl cyclase. Through this and other downstream signaling pathways, opioid receptors modulate... 

Dopamine Binds to dopamine receptors D1 and D5, activating adenylyl cyclase binds to receptors D2, D3, and D4, inhibiting adenylyl cyclase. 

One of the best-characterized effectors and second messenger systems is the cAMP cascade that can be either activated or inhibited by neurotransmit-ter/neuropeptide receptors, including those implicated in anxiety/stress such as CRE Receptors that activate cAMP synthesis couple with the stimulatory G protein, Gsa, and those that inhibit this second messenger couple with the inhibitory G protein, Gia, and these either stimulate or inhibit adenylyl cyclase, the effector enzyme responsible for synthesis of cAMP (Duman and Nestler 1999). There are at least nine different forms of adenylyl cyclase that have been identified by molecular cloning, each with a unique distribution in the brain. The different types of adenylyl cyclase are activated by Gsa as well as the diterpene forskolin, but are differentially regulated by Gia, the Py subunits, Ca, and by phosphorylation. This provides for fine control of adenylyl cyclase enzyme activity and regulation by other effector pathways. 

Activation of M2 and M4 receptors inhibits adenylyl cyclase, and activation of M2 receptors opens potassium channels. The opening of potassium channels hy-perpolarizes the membrane potential and decreases the excitability of cells in the sinoatrial (SA) and atrioventricular (A-V) nodes in the heart. The inhibition of adenylyl cyclase decreases cellular cyclic adenosine... 

The nature of the second messenger response to a given neurotransmitter depends on the subtype of receptor to which it binds and the G protein to which the receptor is coupled. Three of the most commonly utilized G proteins include G, which stimulates adenylyl cyclase to produce cyclic AMP (cAMP) Gj, which inhibits adenylyl cyclase, resulting in lower intracellullar levels of cAMP and Gq, which activates phospholipase C to produce the second messengers IP3 and DAG. In general, these activities refer to the function of the a subunit however, it should be pointed out that the py complex has its own set of activities (on adenylyl cyclase, phospholipase C, channels, mitogen-activated protein kinase [MAPK]) that are just now becoming better clarified. 

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