Effects of thiopental and its optical isomers on nicotinic acetylcholine receptors.

BACKGROUND: With the exception of gamma-aminobutyric acidA (GABAA) receptors, the major molecular targets underlying the anesthetizing actions of thiopental have yet to be established. Neuronal nicotinic acetylcholine receptors (nAChRs) are closely related to GABAA receptors and hence might also be major targets. If so, they might be expected to be substantially inhibited by surgical concentrations (EC50 = 25 micrometer) of thiopental and to display the same stereoselectivity as does general anesthesia. METHODS: Neuronal alpha4beta2, neuronal alpha7 and muscle alphabetagammadelta nAChRs were expressed in Xenopus oocytes. Peak acetylcholine-activated currents were measured at -70 mV using the two-electrode voltage clamp technique. Racemic thiopental and its two optical isomers were applied with and without preincubation and at high and low concentrations of acetylcholine. RESULTS: Inhibition of all three nAChRs was enhanced by preincubation with thiopental, a protocol that mimics the pharmacologic situation in vivo. Using this protocol, inhibition was further enhanced by high concentrations of acetylcholine, with IC50 = 18 +/- 2, 34 +/- 4, and 20 +/- 2 micrometer (mean +/- SEM) thiopental for the neuronal alpha4beta2, neuronal alpha7 and muscle alphabetagammadelta nAChRs, respectively, with Hill coefficients near unity. Neither the neuronal alpha7 nor the muscle alphabetagammadelta nAChR differentiated between the optical isomers of thiopental. However, R(+)-thiopental was significantly more effective than the S(-) isomer at inhibiting the neuronal alpha4beta2 nAChR; interestingly, this is diametrically opposite to their stereoselectivity for general anesthesia. CONCLUSIONS: Both central neuronal and peripheral muscle nAChRs can be substantially inhibited by thiopental at surgical EC50 concentrations but with either no stereoselectivity or one opposite to that for general anesthesia. Thus, nAChRs are probably not crucial targets for producing thiopental anesthesia, although nAChRs may play a part in the side effects produced by this agent.

Differential sensitivities of mammalian neuronal and muscle nicotinic acetylcholine receptors to general anesthetics.

BACKGROUND: Nicotinic acetylcholine receptors (nAChRs) are members of a superfamily of fast neurotransmitter-gated receptor channels that includes the gamma-aminobutyric acidA (GABAA), glycine and serotonin type 3 (5-HT3) receptors. Most previous work on the interactions of general anesthetics with nAChRs has involved the muscle-type receptor. The authors investigate the effects of general anesthetics on defined mammalian neuronal and muscle nAChRs expressed in Xenopus oocytes. METHODS: Complementary deoxyribonucleic acid (cDNA) or messenger ribonucleic acid (mRNA) encoding for various neuronal or muscle nAChR subunits was injected into Xenopus oocytes, and the resulting ACh-activated currents were studied using the two-electrode voltage-clamp technique. The effects of halothane, isoflurane, sevoflurane, and propofol on the peak acetylcholine-induced currents were investigated, and concentration-response curves were constructed. RESULTS: The neuronal nAChRs were found to be much more sensitive to general anesthetics than were the muscle nAChRs, with IC50 concentrations being 10- to 35-fold less for the neuronal receptors. For the inhalational general anesthetics, the IC50 concentrations were considerably less than the free aqueous concentrations that cause general anesthesia in mammals. In addition, qualitative (dependence on acetylcholine concentration) and quantitative (steepness of concentration-response curves) differences in the anesthetic interactions between the neuronal and muscle nAChRs suggest that different mechanisms of inhibition may be involved. CONCLUSIONS: Although there is considerable uncertainty about the physiologic roles that neuronal nAChRs play in the central nervous system, their extraordinary sensitivity to general anesthetics, particularly the inhalational agents, suggests they may mediate some of the effects of general anesthetics at surgical, or even subanesthetic, concentrations.

Effects of inhalational general anaesthetics on native glycine receptors in rat medullary neurones and recombinant glycine receptors in Xenopus oocytes.

1. Glycine responses were studied under voltage clamp in Xenopus oocytes injected with cDNA encoding mammalian glycine receptor subunits and in rat medullary neurones. Bath application of glycine gave strychnine-sensitive currents which reversed close to the expected equilibrium potentials for chloride ions. The peak currents for the receptors expressed in oocytes fitted a Hill equation with EC50 = 215 +/- 5 microM and Hill coefficient nH = 1.70 +/- 0.05 (means +/- s.e. means). The peak currents from the receptors in medullary neurones fitted a Hill equation with EC50 = 30 +/- 1 microM and Hill coefficient nH = 1.76 +/- 0.08. The current-voltage relationship for the receptors expressed in oocytes showed strong outward rectification (with Vrev = -21 +/- 2 mV), while that for the glycine responses from the medullary neurones in symmetrical Cl- was linear (with Vrev = 3.2 +/- 0.6 mV). 2. Inhalational general anaesthetics, at concentrations close to their human minimum alveolar concentrations (MACs), potentiated responses to low concentrations of glycine. The potentiation observed with the recombinant receptors (between 60-22%) was approximately twice that found with the medullary neurones (between 40-80%). For both the recombinant receptors and the receptors in medullary neurones, the degree of potentiation increased in the order of methoxyflurane approximately sevoflurane < halothane approximately isoflurane approximately enflurane. There was no significant difference between the potentiations observed for the two optical isomers of isoflurane. 3. For both the recombinant and native receptors, isoflurane potentiated the currents in a dose-dependent manner at low concentrations of glycine, although at high glycine concentrations the anaesthetic had no significant effect on the glycine-activated responses. The major effect of isoflurane was to cause a parallel leftward shift in the glycine concentration-response curves. The glycine EC50 concentration for the recombinant receptors decreased from a control value of 215 +/- 5 microM to 84 +/- 7 microM glycine at 610 microM isoflurane, while that for the medullary neurones decreased from a control value of 30 +/- 1 microM to 18 +/- 2 microM glycine at the same concentration of isoflurane. The potentiation was independent of membrane potential. 4. Isoflurane also potentiated responses to taurine, a partial agonist at the glycine receptor. This was observed for receptors expressed in oocytes at both low and saturating concentrations of taurine. The EC50 concentration decreased from a control value of 1.6 +/- 0.2 to 0.9 +/- 0.1 mM taurine in the presence of 305 microM isoflurane, while the maximum response to taurine increased from 47 +/- 2 to 59 +/- 2% of the maximum response to glycine. 5. Glycine receptors, like other members of the fast ligand-gated receptor superfamily, are sensitive to clinically relevant concentrations of inhalational general anaesthetics. Effects at these receptors may, therefore, play some role in the maintenance of the anaesthetic state.

Detached, purified nerve terminals from skate electric organ for biochemical and physiological studies.

Electric organs of skate (Raja species) dissociate to form populations of individual electrocytes when incubated in saline solutions containing collagenase. The rate of dissociation was highly temperature dependent, with an apparent Q10 of > 6 in the range of 6 degrees-26 degrees C. The number of electrocytes per organ was relatively constant and independent of electric organ size, whereas mean cell diameters increased with organ size. The activities of two cholinergic marker enzymes, choline acetyltransferase (ChAT) and acetylcholinesterase (AChE), in extracts of whole fresh organs were much less than those reported for the electric ray Torpedo, suggesting a lower volume of terminals in the organ. Electrocytes prepared from collagenase-treated organs had good resting potentials and generated postsynaptic evoked potentials. Spontaneous and electrode pressure-evoked miniature endplate potentials (MEPPs) were readily recorded from isolated electrocytes. Incubation periods of more than 4 days in collagenase at 6 degrees C produced electrocytes with good resting potentials and very low MEPP frequencies, indicating denervation. Detachment of terminals and decreased MEPP frequencies were concurrent. The time course of denervation was followed with the appearance of ChAT and AChE activities in a small particulate fraction derived from washed electrocytes. Peak activities of both enzymes were seen at 4 days of incubation at 16 degrees C, but after 20 h at 16 degrees C. Electrocytes from 4-day, 6 degrees C incubations showed detached, mitochondria-rich nerve terminals and dissociated Schwann cells. In unfixed preparations examined with Nomarski optics, isolated nerve terminals were recognized and distinguished from nucleated Schwann cells. Electron micrographs show that isolated terminals were similar to attached terminals just before they dissociated. The MEPP frequencies and evoked potentials were normal at terminals just before dissociation. We conclude that the transmitter release process was normal in detached terminals and in terminals free of Schwann cells.

The interaction of trichloroethanol with murine recombinant 5-HT3 receptors.

1. The effects of ethanol, chloral hydrate and trichloroethanol upon the 5-HT3 receptor have been investigated by use of electrophysiological techniques applied to recombinant 5-HT3 receptor subunits (5-HT3R-A or 5-HT3R-As) expressed in Xenopus laevis oocytes. Additionally, the influence of trichloroethanol upon the specific binding of [3H]-granisetron to membrane preparations of HEK 293 cells stably transfected with the murine 5-HT3R-As subunit and 5-HT3 receptors endogenous to NG 108-15 cell membranes was assessed. 2. Ethanol (30-300 mM), chloral hydrate (1-30 mM) and trichloroethanol (0.3-10 mM), produced a reversible, concentration-dependent, enhancement of 5-HT-mediated currents recorded from oocytes expressing either the 5-HT3R-A, or the 5-HT3R-As subunit. 3. Trichloroethanol (5 mM) produced a parallel leftward shift of the 5-HT concentration-response curve, reducing the EC50 for 5-HT from 1 +/- 0.04 microM (n = 4) to 0.5 +/- 0.01 microM (n = 4) for oocytes expressing the 5-HT3R-A. A similar shift, from 2.1 +/- 0.05 microM (n = 11) to 1.3 +/- 0.1 microM (n = 4), was observed in oocytes expressing the 5-HT3R-As subunit. Trichloroethanol (5 mM) had little or no effect upon the maximum current produced by 5-HT for either recombinant receptor. 4. Trichloroethanol (5 mM) similarly reduced the EC50 for 2-methyl-5-HT from 13 +/- 0.4 microM (n = 4) to 4.6 +/- 0.2 microM (n = 4) and from 15 +/- 2 microM (n = 4) to 5 +/- 0.4 microM (n = 4) for oocytes expressing the 5-HT3R-A and 5-HT3R-As subunit respectively. Additionally, trichloroethanol (5 mM) produced a clear enhancement of the maximal current to 2-methyl-5-HT (expressed as a percentage of the maximal current to 5-HT) from 63 +/- 0.7% (n = 4) to 101 +/- 1.6% (n = 4) and from 9 +/- 0.2% (n = 4) to 74 +/- 2% (n = 4) for oocytes expressing the 5-HT3R-A and 5-HT3R-As subunit respectively. 5. Trichloroethanol (2.5 mM) had no effect upon the Kd, or Bmax, of specific [3H]-granisetron binding to membrane homogenates of NG 108-15 cells or HEK 293 cells. Similarly, competition for [3H]-granisetron binding by the 5-HT3 receptor antagonists ondansetron and tropisetron was unaffected. However, competition for [3H]-granisetron binding by the 5-HT3 receptor agonists, 5-HT, 2-methyl-5-HT and phenylbiguanide was enhanced by trichloroethanol (2.5 mM). 6 Unexpectedly, the competition for [3H]-granisetron binding by the 5-HT3 receptor antagonist,quipazine, was enhanced by 2.5 mM trichloroethanol. Quipazine (1 nM-0.3 microM) antagonized 5-HT evoked currents recorded from oocytes expressing the 5-HT3R-A subunit with an IC50 of 18 +/- 3 nM(n = 4). Additionally, quipazine (30 nM-0.3 microM) produced a small inward current which was greatly enhanced by 5 mM trichloroethanol and antagonized by 100 nM ondansetron. Collectively, these observations suggest that quipazine may act as a partial agonist.7. The demonstration that a recombinant homo-oligomeric receptor, expressed in a foreign membrane,retains a sensitivity to alcohols, together with the sequencing of alcohol-insensitive 5-HT3 receptor subunits, may lead to a better definition of the alcohol binding site(s).

Pharmacological characterization of the apparent splice variants of the murine 5-HT3 R-A subunit expressed in Xenopus laevis oocytes.

The actions of 5-hydroxytryptamine3 (5-HT3) receptor agonists and antagonists have been determined on the recombinant murine 5-HT3 R-A and an apparent splice variant of this subunit, termed 5-HT3 R-AS. When expressed in Xenopus laevis oocytes, both forms of the subunit functioned as a homo-oligomeric complex and exhibited inward current responses to bath applied 5-HT. Analysis of the 5-HT concentration-response curve obtained with either homo-oligomer gave Hill coefficients greater than two, suggesting positive co-operativity within the receptor complex. The rank order of potency of a range of 5-HT3 receptor agonists [m-chlorophenylbiguanide > 5-HT > 2-methyl-5-HT (2-Me-5-HT) > or = phenylbiguanide] was identical for both subunits. Indeed, with the exception of 2-Me-5-HT, for the agonists tested there was little difference across the subunits in either their potency, or the maximal current response that they elicited relative to 5-HT. Although 2-Me-5-HT exhibited a similar potency for both subunits, the maximal response evoked by this agonist at the 5-HT3 R-AS subunit was much reduced when compared to the 5-HT3 R-A subunit. The 5-HT-induced current mediated by either form of the subunit was inhibited by the 5-HT3 receptor selective antagonists BRL 46470, granisetron and ondansetron and the non-selective antagonists (+)-tubocurarine, metoclopramide and cocaine in a reversible and concentration-dependent manner. These antagonists did not discriminate between the subunits and their potencies were similar to those reported previously for 5-HT3 receptors native to murine neuronal cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning and functional expression of an apparent splice variant of the murine 5-HT3 receptor A subunit.

The polymerase chain reaction has been employed to isolate a cDNA encoding a functional 5-HT3 receptor subunit from the murine neuroblastoma cell line N1E-115. Overall, the amino acid sequence predicted from this clone demonstrates a 98% homology with the 5-HT3 receptor A subunit cloned from NCB-20 hybridoma cells. A deletion of 6 amino acid residues located within the putative large intracellular loop, which may result from alternative splicing, represents the principal difference between the two clones. Upon expression in Xenopus oocytes, the homo-oligomeric receptor displayed pharmacological properties which define it as a functional 5-HT3 receptor.