Isoflurane inhibits nicotinic acetylcholine receptor-mediated 22Na+ influx and muscarinic receptor-evoked cyclic GMP production in cultured bovine adrenal medullary cells

Inhalation anaesthetic isoflurane inhibits the muscarinic cation current and carbachol-induced gastrointestinal smooth muscle contractions

Gastrointestinal tract motility may be demoted significantly after surgery operations at least in part due to anaesthetic agents, but there is no comprehensive explanation of the molecular mechanism(s) of such adverse effects. Anesthetics are known to interact with various receptors and ion channels including several subtypes of transient receptor potential (TRP) channels. Two members of the canonical subfamily of TRP channels (TRPC), TRPC4 and TRPC6 are Ca²⁺-permeable cation channels involved in visceral smooth muscle contractility induced by acetylcholine, the primary excitatory neurotransmitter in the gut. In the present study, we aimed to study the effect of anesthetics on muscarinic receptor-mediated excitation and contraction of intestinal smooth muscle. Here we show that muscarinic cation current (mI_CAT) mediated by TRPC4 and TRPC6 channels in mouse ileal myocytes was strongly inhibited by isoflurane (0.5mM), one of the most commonly used inhalation anesthetics. Carbachol-activated mI_CAT was reduced by 63 ± 11% (n = 5), while GTPγS-induced (to bypass muscarinic receptors) current was inhibited by 44 ± 9% (n = 6). Furthermore, carbachol-induced ileum and colon contractions were inhibited by isoflurane by about 30%. We discuss the main sites of isoflurane action, which appear to be G-proteins and muscarinic receptors, rather than TRPC4/6 channels. These results contribute to our better understanding of the signalling pathways affected by inhalation anesthetics, which may cause ileus, and thus may be important for the development of novel treatment strategies during postoperative recovery.

The recent progress in research on effects of anesthetics and analgesics on G protein-coupled receptors

The exact mechanisms of action behind anesthetics and analgesics are still unclear. Much attention was focused on ion channels in the central nervous system as targets for anesthetics and analgesics in the 1980s. During the 1990s, major advances were made in our understanding of the physiology and pharmacology of G protein coupled receptor (GPCR) signaling. Thus, several lines of studies have shown that G protein coupled receptors (GPCRs) are one of the targets for anesthetics and analgesics and especially, that some of them inhibit the functions of GPCRs, i.e,, muscarinic receptors and substance P receptors. However, these studies had been focused on only G(q) coupled receptors. There has been little work on G(s)- and G(i)-coupled receptors. In the last decade, a new assay system, using chimera G(i/o)-coupled receptor fused to Gq(i5), has been established and the effects of anesthetics and analgesics on the function of G(i)-coupled receptors is now more easily studied. This review highlights the recent progress of the studies regarding the effects of anesthetics and analgesics on GPCRs.

Dexmedetomidine inhibits muscarinic type 3 receptors expressed in Xenopus oocytes and muscarine-induced intracellular Ca2+ elevation in cultured rat dorsal root ganglia cells

Dexmedetomidine, an alpha(2)-adrenoceptor agonist, has been approved for clinical use, although the mechanism of dexmedetomidine action has not been fully elucidated. Several studies have shown that G protein-coupled receptors (GPCRs) are recognized as targets for anesthetics and analgesics. Therefore, it is of interest to determine whether dexmedetomidine affects the function of GPCRs other than the alpha(2)-adrenoceptor. We examined the effects of dexmedetomidine on M(1), M(3), 5-HT(2C), substance P, and orexin 1 receptors in Xenopus oocytes expressing individual receptors. In addition, we investigated the effects of dexmedetomidine on muscarinic receptor-mediated changes in [Ca(2+)](i) in the dorsal root ganglia (DRG) of 3-week-old Wister rats. Dexmedetomidine did not affect the 5-HT(2C)-, or substance P-induced Cl(-) currents and had little inhibition on the orexin A-induced current in oocytes expressing the respective receptors. The compound also had little effect on the acetylcholine (ACh, 1 microM)-induced Ca(2+)-activated Cl(-) currents in Xenopus oocytes expressing M(1) receptors. In contrast, dexmedetomidine inhibited the ACh-induced currents in Xenopus oocytes expressing M(3) receptors; 1 nM, 10 nM, 100 nM, and 1 microM dexmedetomidine reduced the current to 66.5 +/- 4.8, 51.3 +/- 12, 34.6 +/- 11, and 26.8 +/- 6.4% of the control value, respectively (EC(50) = 3.5 +/- 0.7 nM). Dexmedetomidine reduced the ACh-induced Cl(-) currents after treatment with the selective protein kinase C inhibitor GF109203X. Moreover, the compound inhibited the muscarinic receptor-mediated increases in [Ca(2+)](i) in cultured DRG cells in a concentration-dependent manner. Dexmedetomidine inhibits the function of M(3) receptors, in addition to its agonistic effects on alpha(2)-adrenoceptors, which provides further insight into the pharmacological properties of dexmedetomidine.

Pharmacological aspects of the effects of tramadol on G-protein coupled receptors

Tramadol is an analgesic that is used worldwide, but its mechanisms of action have not been elucidated. It has been speculated that tramadol acts primarily through the activation of micro-opioid receptors and the inhibition of monoamine reuptake. The majority of studies to date have focused on ion channels in the central nervous system as targets of anesthetics and analgesics. During the past decade, major advances have been made in our understanding of the physiology and pharmacology of G-protein coupled receptor (GPCR) signaling. Several studies have shown that GPCRs and ion channels are targets for analgesics and anesthetics. In particular, tramadol has been shown to affect GPCRs, including muscarinic acetylcholine receptors and 5-hydroxytryptamine receptors. Here, the effects of tramadol on monoamine transporters, GPCRs, and ion channels are presented, and recent research on the pharmacology of tramadol is discussed.

Excitatory and inhibitory actions of isoflurane in bovine chromaffin cells

Isoflurane, a halogenated volatile anesthetic, is thought to produce anesthesia by depressing CNS function. Many anesthetics, including isoflurane, are thought to modulate and/or directly activate GABA(A) receptors. Chromaffin cells are known to express functional GABA(A) receptors. We previously showed that activation of the GABA(A) receptors, with specific agonists, leads to cellular excitation resulting from the depolarized anion equilibrium potential. In this study, our goal was to determine whether isoflurane mimicked this response and to explore the functional consequences of this activation. Furthermore, we sought to study the actions of isoflurane on nicotinic acetylcholine receptors (nAChRs) as they mediate the "sympathetic drive" in these cells. For these studies the Ca(2+)-indicator dye fura-2 was used to assay [Ca(2+)](i). Amperometric measurements were used to assay catecholamine release. We show that bovine adrenal chromaffin cells were excited by isoflurane at clinically relevant concentrations. Isoflurane directly activated GABA(A) receptors found in chromaffin cells, which depolarized the cells and elevated [Ca(2+)](i). Application of isoflurane directly to the chromaffin cells elicited catecholamine secretion from these cells. At the same time, isoflurane suppressed activation of nAChRs, which presumably blocks "sympathetic drive" to the chromaffin cells. These latter results may help explain why isoflurane produces the hypotension observed clinically.

The effects of the tramadol metabolite O-desmethyl tramadol on muscarinic receptor-induced responses in Xenopus oocytes expressing cloned M1 or M3 receptors

O-desmethyl tramadol is one of the main metabolites of tramadol. It has been widely used clinically and has analgesic activity. Muscarinic receptors are involved in neuronal functions in the brain and autonomic nervous system, and much attention has been paid to these receptors as targets for analgesic drugs in the central nervous system. We have reported that tramadol inhibits the function of type-1 muscarinic (M(1)) receptors and type-3 muscarinic (M(3)) receptors, suggesting that muscarinic receptors are sites of action of tramadol. However, the effects of O-desmethyl tramadol on muscarinic receptor functions have not been studied in detail. In this study, we investigated the effects of O-desmethyl tramadol on M(1) and M(3) receptors, using the Xenopus oocyte expression system. O-desmethyl tramadol (0.1-100 microM) inhibited acetylcholine (ACh)-induced currents in oocytes expressing the M(1) receptors (half-maximal inhibitory concentration [IC(50)] = 2 +/- 0.6 microM), whereas it did not suppress ACh-induced currents in oocytes expressing the M(3) receptor. Although GF109203X, a protein kinase C inhibitor, increased the ACh-induced current, it had little effect on the inhibition of ACh-induced currents by O-desmethyl tramadol in oocytes expressing M(1) receptors. The inhibitory effect of O-desmethyl tramadol on M(1) receptor was overcome when the concentration of ACh was increased (K(D) with O-desmethyl tramadol = 0.3 microM). O-desmethyl tramadol inhibited the specific binding of [(3)H]quinuclidinyl benzilate ([(3)H]QNB) to the oocytes expressed M(1) receptors (IC(50) = 10.1 +/- 0.1 microM), whereas it did not suppress the specific binding of [(3)H]QNB to the oocytes expressed M(3) receptors. Based on these results, O-desmethyl tramadol inhibits functions of M(1) receptors but has little effect on those of M(3) receptors. This study demonstrates the molecular action of O-desmethyl tramadol on the receptors and may help to explain its neural function.

The effects of the neurosteroids: pregnenolone, progesterone and dehydroepiandrosterone on muscarinic receptor-induced responses in Xenopus oocytes expressing M1 and M3 receptors

The neurosteroids pregnenolone, progesterone, and dehydroepiandrosterone (DHEA) occur naturally in the nervous system. They act on neural tissues, participate in neuronal signaling, and are reported to alter neuronal excitability via nongenomic mechanisms. Muscarinic receptors have important roles in neuronal functions in the brain and autonomic nervous system. In this study, we investigated the effects of pregnenolone, progesterone, and DHEA on M(1) and M(3) muscarinic receptors using the Xenopus oocyte expression system. Pregnenolone and progesterone inhibited the acetylcholine (ACh)-mediated responses of M(1) and M(3) receptors expressed in Xenopus oocytes, whereas DHEA did not. The half-maximal inhibitory concentrations (IC(50)) for pregnenolone inhibition of M(1) receptor- and M(3) receptor-mediated currents were 11.4 and 6.0 microM respectively; the IC(50) values for progesterone inhibition of M(1) receptor- and M(3) receptor-mediated currents were 2.5 and 3.0 microM respectively. The selective protein kinase C (PKC) inhibitor GF109203X had little effect on the pregnenolone or progesterone inhibition of the ACh-induced currents in Xenopus oocytes expressing M(1) or M(3) receptors. The inhibitory effects of pregnenolone and progesterone were overcome at higher concentrations of ACh. Pregnenolone and progesterone inhibited the [(3)H]quinuclidinyl benzilate (QNB) binding to M(1) and M(3) receptor expressed in Xenopus oocytes, and Scatchard plot analysis of [(3)H]QNB binding revealed that pregnenolone and progesterone altered the K(d) value and the B(max), indicating noncompetitive inhibition. In conclusion, pregnenolone and progesterone inhibited M(1) and M(3) receptor functions noncompetitively by the mechanism independent of PKC and by interfering with ACh binding to the receptors.

The inhibitory effects of alphaxalone on M1 and M3 muscarinic receptors expressed in Xenopus oocytes

Alphaxalone is a neurosteroid anesthetic, but its mechanisms of action are not completely understood. Muscarinic receptors are involved in a variety of neuronal functions in the brain and autonomic nervous system, and much attention has been paid to them as targets of anesthetics. In this study, we investigated the effects of alphaxalone on M(1) and M(3) muscarinic receptors using the Xenopus oocyte expression system. Alphaxalone inhibited acetylcholine-induced currents in oocytes expressing M(1) receptors at clinically relevant concentrations. Alphaxalone also suppressed acetylcholine-induced currents in oocytes expressing M(3) receptors. The half-maximal inhibitory concentration values for the inhibition of M(1)- and M(3)-mediated currents were 1.8 +/- 0.6 micro M and 5.3 +/- 1.0 micro M, respectively. GF109203X, a selective protein kinase C inhibitor, had little effect on the inhibition of acetylcholine-induced currents by alphaxalone in oocytes expressing these receptors. Alphaxalone inhibited the specific binding of [(3)H]quinuclidinyl benzilate to oocytes expressing M(1) or M(3) receptors. These findings suggest that alphaxalone at clinically relevant concentrations inhibits the function of M(1) and M(3) receptors through a protein kinase C-independent mechanism by interfering with the [(3)H]quinuclidinyl benzilate binding sites on the receptors.

IMPLICATIONS

Alphaxalone, a neurosteroid anesthetic, inhibited the function of muscarinic M(1) and M(3) receptors and the specific binding of [(3)H]quinuclidinyl benzilate ([(3)H]QNB) to oocytes expressing these receptors. These findings suggest that alphaxalone inhibits these receptors by interfering with the QNB binding sites.

The inhibitory effects of tramadol on muscarinic receptor-induced responses in Xenopus oocytes expressing cloned M(3) receptors

Tramadol is a widely used analgesic, but its mechanism of action is not completely understood. Muscarinic receptors are involved in neuronal function in the brain and autonomic nervous system, and much attention has been paid to these receptors as targets of analgesic drugs in the central nervous system. In this study, we investigated the effects of tramadol on type-3 muscarinic (M(3)) receptors using the Xenopus oocyte expression system. Tramadol (10 nM-100 micro M) inhibited acetylcholine-induced currents in oocytes expressing M(3) receptor. Although GF109203X, a protein kinase C inhibitor, increased the basal current, it had little effect on the inhibition of acetylcholine-induced currents by tramadol. Moreover, tramadol inhibited the specific binding sites of [(3)H]quinuclidinyl benzilate. These findings suggest that tramadol at clinically relevant concentrations inhibits M(3) function via quinuclidinyl benzilate-binding sites. This may explain the modulation of neuronal function and the anticholinergic effects of tramadol.

IMPLICATIONS

Muscarinic receptors are involved in neuronal function and are targets of analgesic drugs. We here report that tramadol inhibits type-3 muscarinic receptors function via quinuclidinyl benzilate-binding sites at clinically relevant concentrations. These findings may explain the modulation of neuronal function and the anticholinergic effects of tramadol.

A neurosteroid anesthetic, alphaxalone, inhibits nicotinic acetylcholine receptors in cultured bovine adrenal chromaffin cells

Several lines of evidence suggest that nicotinic acetylcholine receptors (nAChRs) are a target of general anesthetics. Alphaxalone (5alpha-pregnan-3alpha-ol-11, 20-dion) is a neurosteroid, which was used clinically for anesthesia, but its effects on the function of nAChRs have not been well investigated. We examined the effects of alphaxalone on nAChRs in cultured bovine adrenal chromaffin cells. We studied the effects of alphaxalone on nicotine-induced increases in the cytosolic Ca(2+) concentration ([Ca(2+)](i)) and on membrane currents using Ca(2+)-imaging and whole-cell patch-clamp techniques, respectively, in these cells. We also examined the effects of alphaxalone on gamma-aminobutyric acid A receptors in the same cells and compared them with the effects on nAChRs. Alphaxalone (0.1-100 micro M) inhibited nicotine-induced [Ca(2+)](i) increases in a concentration-dependent manner. Alphaxalone inhibited high K(+)-induced [Ca(2+)](i) increases, but the inhibition was observed only at 100 micro M. In voltage-clamp experiments using negative holding potentials, alphaxalone (0.1-100 micro M) itself induced inward currents, which were abolished by the gamma-aminobutyric acid A receptor antagonist picrotoxin. Alphaxalone also inhibited nicotine-induced inward currents, and the inhibition was unaffected by picrotoxin. We conclude that alphaxalone, at anesthetic concentrations, inhibits nAChRs in adrenal chromaffin cells. Alphaxalone may affect the sympathetic and other nervous systems via inhibition of nAChRs.

IMPLICATIONS

Alphaxalone inhibits the function of nAChRs at clinically relevant concentrations in adrenal chromaffin cells. Thus, the present findings may provide some information for understanding the anesthetic mechanism of alphaxalone.

Inhibitory effects of tramadol on nicotinic acetylcholine receptors in adrenal chromaffin cells and in Xenopus oocytes expressing alpha 7 receptors

1. Tramadol has been used clinically as an analgesic; however, the mechanism of its analgesic effects is still unknown. 2. We used bovine adrenal chromaffin cells to investigate effects of tramadol on catecholamine secretion, nicotine-induced cytosolic Ca(2+) concentration ([Ca(2+)](i)) increases and membrane current changes. We also investigated effects of tramadol on alpha7 nicotinic acetylcholine receptors (AChRs) expressed in Xenopus oocytes. 3. Tramadol concentration-dependently suppressed carbachol-induced catecholamine secretion to 60% and 27% of the control at the concentration of 10 and 100 microM, respectively, whereas it had little effect on veratridine- or high K(+)-induced catecholamine secretion. 4. Tramadol also suppressed nicotine-induced ([Ca(2+)](i)) increases in a concentration-dependent manner. Tramadol inhibited nicotine-induced inward currents, and the inhibition was unaffected by the opioid receptor antagonist naloxone. 5. Tramadol inhibited nicotinic currents carried by alpha7 receptors expressed in Xenopus oocytes. 6. Tramadol inhibited both alpha-bungarotoxin-sensitive and -insensitive nicotinic currents in bovine adrenal chromaffin cells. 7. In conclusion, tramadol inhibits catecholamine secretion partly by inhibiting nicotinic AChR functions in a naloxone-insensitive manner and alpha7 receptors are one of those inhibited by tramadol.