Effect of epidural analgesia with ambulation on labor duration

BACKGROUND

Ambulatory epidural analgesia (AEA) is a popular choice for labor analgesia because ambulation reportedly increases maternal comfort, increases the intensity of uterine contractions, avoids inferior vena cava compression, facilitates fetal head descent, and relaxes the pelvic musculature, all of which can shorten labor. However, the preponderance of evidence suggests that ambulation during labor is not associated with these benefits. The purpose of this study is to determine whether ambulation with AEA decreases labor duration from the time of epidural insertion to complete cervical dilatation.

METHODS

In this prospective, randomized study, 160 nulliparous women with AFA were randomly assigned to one of two groups: AEA with ambulation and AEA without ambulation. AEA blocks were initiated with 15-20 ml ropivacaine (0.07%) plus 100 microg fentanyl, followed by a continuous infusion of 0.07% ropivacaine plus 2 microg/ml fentanyl at 15-20 ml/h. Maternal measured variables included ambulation time, time from epidural insertion to complete dilatation, stage II duration, pain Visual Analogue Scale scores, and mode of delivery. APGAR scores were recorded at 1 and 5 min. Results are expressed as mean +/- SD or median and analyzed using the t test, chi-square, or the Mann-Whitney test at P < or = 0.05.

RESULTS

The ambulatory group walked 25.0 +/- 23.3 min, sat upright 40.3 +/- 29.7 min, or both. Time from epidural insertion to complete dilatation was 240.9 +/- 146.1 min in the ambulatory group and 211.9 +/- 133.9 min in the nonambulatory group (P = 0.206).

CONCLUSION

Ambulatory epidural analgesia with walking or sitting does not shorten labor duration from the time of epidural insertion to complete cervical dilatation.

Dose-dependent enhancement of in vivo GABA(A)-benzodiazepine receptor binding by isoflurane

BACKGROUND

Abundant in vitro and animal model data suggest the postsynaptic gamma-aminobutyric acid receptor type A (GABA(A)-R) is an important target for volatile general anesthetics, but the relevance of these models is untested in humans. Because benzodiazepines have also been shown to act via a specific GABA(A)-R site, they provide sensitive probes for the GABA(A)-R. Availability of the 11C-labeled benzodiazepine ligand, flumazenil, allowed us to quantitatively test in humans whether the volatile anesthetic isoflurane affects GABA(A)-Rs in vivo in a dose-dependent manner.

METHODS

11C-flumazenil positron emission tomography scans were obtained in 12 healthy subjects while awake (control condition) and anesthetized with either 1.0 or 1.5 minimum alveolar concentration isoflurane (n = 7 and 5, respectively; isoflurane conditions). Regions of interest included areas of high, intermediate, and low GABA(A)-benzodiazepine site density. For each subject and experimental condition, the binding of 11C-flumazenil, expressed as distribution volume (which linearly correlates to maximal binding site density and apparent ligand affinity), was obtained by curve fitting using a two-compartment model.

RESULTS

The ratio of distribution volume increased significantly in each examined region during the isoflurane conditions compared with control conditions (P < 0.01, one-tailed t test). Furthermore, the increases in ratio of distribution volume during the 1.5-minimum alveolar concentration isoflurane condition were significantly greater than those measured during 1.0 minimum alveolar concentration isoflurane inhalation (P < 0.002, one-tailed t test).

CONCLUSIONS

Isoflurane exposure appeared to enhance receptor-specific 11C-flumazenil binding in a dose-dependent manner. The results suggest the possibility that a conformational change of the GABA(A)-R is involved in the mechanism of action of isoflurane in the living human brain.

Effects of normal alcohols and isoflurane on lipid headgroup dynamics in nicotinic acetylcholine receptor-rich lipid vesicles

The trend of evidence suggests that general anesthetics act directly on proteins in the neural membrane. However, the fact that the functions of nicotinic acetylcholine receptor (sodium permeability, desensitization rate) are modulated by the composition of the membrane in which it is reconstituted has been thought to be a result of the variation of interactions between acetylcholine receptor and membrane. In this study, protein-lipid interaction at the level of the lipid headgroup was investigated using electron paramagnetic resonance (EPR) and headgroup spin label. Lipid headgroup mobility was evaluated with rotational correlation time from the EPR spectrum. Protein-lipid interaction at headgroup depth was demonstrated from the motionally restricted component of the spectrum. Rotational correlation time increased to 13 ns from 7 ns due to protein-lipid interaction. The effect of anesthetic (ethanol, 1-hexanol, and isoflurane) on protein-lipid interaction was investigated, and the correlation time was 13 ns. It is concluded that the anesthetics used in this study did not alter protein-lipid interaction at the level of the lipid headgroup, so far as observed by rotational correlation time, without excluding the possibility that anesthetics that perturb protein-lipid interactions modulate receptor functions via this mechanism.

Halothane and desflurane requirements in alcohol-tolerant and -nontolerant rats

On the basis of data implicating GABAA receptors in the effects of volatile general anaesthetics, we hypothesized that alcohol-, barbiturate-, and benzodiazepine-sensitive alcohol-nontolerant (ANT) rats would also be more sensitive than alcohol-tolerant (AT) rats to two clinical general anaesthetics with differing potencies, halothane and desflurane. The obtunding effect of halothane and desflurane on mature ANT (n = 17) and AT (n = 16) rats was assessed by the loss-of-righting reflex endpoint. ANT rats were significantly (P < 0.0001) more sensitive to the obtunding effects of both halothane and desflurane (ED50 = 0.45 +/- 0.03% atm for ANT vs 0.95 +/- 0.04% atm for AT and 2.16 +/- 0.17 vs 3.69 +/- 0.13% atm, respectively). The immobilization effect of halothane and desflurane was assessed with the tail clamp/withdrawal endpoint. ANT rats were more sensitive to the effects of halothane (ED50 = 1.10 +/- 0.08% atm for ANT vs 1.72 +/- 0.09% atm for AT; P < 0.0001) but not desflurane (ED50 = 6.25 +/- 0.25% atm for ANT vs 5.85 +/- 0.21% atm for AT). The data presented support the hypothesis that volatile anaesthetics interact with specific neuronal proteins (possibly GABAA receptors) and agree with recent hypotheses that different elements of the anaesthetic state are produced by separate sites or mechanisms.

Mice lacking the long splice variant of the gamma 2 subunit of the GABA(A) receptor are more sensitive to benzodiazepines

The gamma 2 subunit is required for benzodiazepine modulation of the GABA(A) receptor. Alternate splicing of precursor GABA(A) gamma 2 mRNA results in two splice variants, a short (gamma 2S) and a long (gamma 2L) variant. We investigated the roles of these splice variants in benzodiazepine pharmacology using mice lacking genes for the gamma 2L splice variant. Sleep time responses to midazolam and zolpidem were 20 and 18% greater, respectively, in null allele mice compared with wild-type mice, while responses to nonbenzodiazepine agents such as etomidate and pentobarbital were unchanged. Although the GABA(A) receptor number was not altered in null allele mice, there was a corresponding increase in affinity of brain membranes for benzodiazepine agonists (midazolam, diazepam, and zolpidem), while affinity for benzodiazepine inverse agonists (beta CCM and Ro15-4513) was decreased. These changes were not observed in inbred mice of the parental strains (C57BL/6J and 129/SvJ) used to create the genetically altered mice, indicating that differences between gamma 2L null allele and wild-type mice were unlikely to be simply due to cosegregation of linked alleles. Absence of the gamma 2L splice variant increases the affinity of receptors for benzodiazepine agonists, and is associated with a modest increase in behavioral sensitivity to benzodiazepine agonists. Lack of the gamma 2L subunits may shift the GABA(A) receptor from an inverse agonist-preferring toward an agonist-preferring configuration.

Sensory thresholds and the antinociceptive effects of GABA receptor agonists in mice lacking the beta3 subunit of the GABA(A) receptor

A line of mice was recently created in which the gabrb3 gene, which encodes the beta3 subunit of the GABA(A) receptor, was inactivated by gene-targeting. The existence of mice with a significantly reduced population of GABA(A) receptors in the CNS enabled an investigation of the role of GABA and GABA(A) receptors in nociception. The present study examined the sensory thresholds of these mice, as well as the antinociceptive effects of subcutaneously or intrathecally administered GABA(A) and GABA(B) receptor agonists. Homozygous null (beta3-/-) mice displayed enhanced responsiveness to low-intensity thermal stimuli in the tail-flick and hot-plate test compared to C57BL/6J and 129/SvJ progenitor strain mice, and their wild-type (beta3+/+) and heterozygous (beta3+/-) littermates. The beta3-/- mice also exhibited enhanced responsiveness to innocuous tactile stimuli compared to C57BL/6J, 129/SvJ and to their beta3+/+ littermates as assessed by von Frey filaments. The presence of thermal hyperalgesia and tactile allodynia in beta3-/- mice is consistent with a loss of inhibition mediated by presynaptic and postsynaptic GABA(A) receptors in the spinal cord. As expected, subcutaneous administration of the GABA(A) receptor agonist 4,5,6,7-tetrahydroisoxazolo-(5,4-c)pyridin-3-ol did not produce antinociception in beta3-/- mice, whereas it produced a dose-dependent increase in hot-plate latency in C57BL/6J, 129/SvJ, beta3+/+ and beta3+/- mice. However, the antinociceptive effect of the GABA(B) receptor agonist baclofen in the tail-flick and hot-plate tests was also reduced in beta3-/- mice compared to the progenitor strains, beta3+/+ or beta3+/- mice after either subcutaneous or intrathecal administration. This finding was unexpected and suggests that a reduction in GABA(A) receptors can affect the production of antinociception by other analgesic drugs as well.

Attenuated sensitivity to neuroactive steroids in gamma-aminobutyrate type A receptor delta subunit knockout mice

gamma-Aminobutyric acid (GABA) type A receptors mediate fast inhibitory synaptic transmission and have been implicated in responses to sedative/hypnotic agents (including neuroactive steroids), anxiety, and learning and memory. Using gene targeting technology, we generated a strain of mice deficient in the delta subunit of the GABA type A receptors. In vivo testing of various behavioral responses revealed a strikingly selective attenuation of responses to neuroactive steroids, but not to other modulatory drugs. Electrophysiological recordings from hippocampal slices revealed a significantly faster miniature inhibitory postsynaptic current decay time in null mice, with no change in miniature inhibitory postsynaptic current amplitude or frequency. Learning and memory assessed with fear conditioning were normal. These results begin to illuminate the novel contributions of the delta subunit to GABA pharmacology and sedative/hypnotic responses and behavior and provide insights into the physiology of neurosteroids.

Platelet membrane fluidity individuals at risk for Alzheimer's disease: a comparison of results from fluorescence spectroscopy and electron spin resonance spectroscopy

RATIONALE

Previous fluorescence studies employing 1,6-diphenyl-1,3,5-hexatriene (DPH) have revealed an increase in the fluidity of platelet membranes from individuals with Alzheimer's disease (AD) and their first-degree relatives. This biophysical alteration has been reported to be relatively specific for the hydrocarbon core of platelet membranes, where DPH preferentially localizes; this effect is not reflected by the fluorescent reporter triethylamino-DPH, which labels membranes at the lipid-aqueous interface.

OBJECTIVE

The goal of this study was to explore the validity and reproducibility of these findings using an independent biophysical technique, electron spin resonance (ESR) spectroscopy.

METHODS

Platelet membranes prepared from first-degree relatives of patients with AD were labeled with DPH, or the spin-labeled fatty acid probes 5-doxylstearate (5-DS) and 12-doxylstearate (12-DS). These spin labeled probes provide an index of structural order at the respective depths of their nitroxide moieties in the membrane. The resulting preparations were examined by fluorescence and ESR spectroscopy.

RESULTS

Increased platelet membrane fluidity (PMF), as determined by the fluorescence anisotropy of DPH, was associated with only a modest reduction in the order parameter derived for 5-DS labeled membranes. In contrast, the mean order parameters derived from the paired samples labeled with 12-DS differed substantially from each other, and revealed decreased order (increased fluidity) in the hydrocarbon 12-C region where DPH preferentially localizes.

CONCLUSIONS

These results provide an independent validation of the biophysical alterations of platelet membranes that are manifested by a subgroup of patients with AD and their first-degree relatives.

Pharmacologic and behavioral responses of inbred C57BL/6J and strain 129/SvJ mouse lines

Gene-targeting technology is creating an explosion in the number of animals available with single gene mutations that affect the function of the central nervous system. Most gene-targeted mice are produced on a mixed genetic background of C57BL/6J and substrains of Strain 129. Understanding the behavioral characteristics and responses to various drugs of these parental strains is vital to interpreting data from gene-targeted mice. We directly compared C57BL/6J and Strain 129/SvJ mouse lines on several behavioral paradigms and in response to several hypnotic and anesthetic drugs. Compared to Strain 129/SvJ mice, C57BL/6J animals are more sensitive to the hypnotic effects of midazolam, zolpidem, and propofol, less sensitive to etomidate and ethanol, and do not differ in sensitivity to Ro15-4513 or pentobarbital. These strains do not differ in their sensitivity to the motor ataxic effects of the volatile anesthetics enflurane or halothane. However, Strain 129/SvJs are more sensitive to the immobilizing effects of halothane but not enflurane. Motor coordination differs initially, but with repeated testing strain differences are no longer apparent. Strain 129/SvJ mice are more anxious on the elevated plus maze and open-field activity assays. Thus, these mouse strains harbor polymorphisms that influence some, but not all, traits of interest to behavioral neuroscientists.

Normal electrophysiological and behavioral responses to ethanol in mice lacking the long splice variant of the gamma2 subunit of the gamma-aminobutyrate type A receptor

The gamma subunit of the gamma-aminobutyric acid type A receptor (GABA(A)-R) is essential for bestowing both normal single channel conductance and sensitivity to benzodiazepines on native GABA(A)-Rs. The long splice variant of the gamma2 subunit (gamma2L) has been postulated to be essential in mediating the modulatory actions of ethanol at the GABA(A)-R. In order to evaluate this hypothesis, gene targeting was used to delete the 24bp exon which distinguishes gamma2L from the short splice variant (gamma2S). Mice homozygous for this exon deletion (gamma2L-/-) are viable and indistinguishable from wild-type (gamma2L+/+) mice. No gamma2L mRNA was detected in these mice, nor could gamma2L-containing GABA(A)-R protein be detected by specific antibodies. Radioligand binding studies showed the total amount of gamma2 subunit protein to be not significantly changed, suggesting that gamma2S replaces gamma2L in the brains of the knockout animals. Electrophysiological recordings from dorsal root ganglion neurons revealed a normal complement of functional receptors. There was no difference in the potentiation of GABA currents by ethanol (20-200 mM) observed in neurons from gamma2L+/+ or gamma2L-/- mice. Several behavioral effects of ethanol, such as sleep time, anxiolysis, acute functional tolerance, chronic withdrawal hyperexcitability and hyperlocomotor activity were also unaffected by genotype. It is concluded that gamma2L is not required for ethanol's modulatory action at the GABA(A)-R or whole animal behavioral effects.

Genetic dissection of the molecular target(s) of anesthetics with the gene knockout approach in mice

Techniques have recently been developed that enable the creation of mice that harbor specific, predetermined genetic changes. These 'gene knockout mice', which contain a single genetic modification that is determined by the investigator, can subsequently be analyzed with tests that span the molecular, cellular, and behavioral levels. Application of such a multi-level approach to mechanisms of drug action should ultimately allow general anesthetic responses to be properly attributed to a molecular site.

Alcohol and anesthetic mechanisms in genetically engineered mice

Genetically engineered animals (e.g., transgenics, gene knockouts, gene knockins) are being utilized with increasing frequency to investigate the mechanisms of action of alcohol and anesthetics. By creating and analyzing animals that harbor precise, preplanned changes in candidate genes, researchers are rapidly making progress toward uncovering how these drugs exert their effects on the central nervous system to bring about their behavioral effects. Since these sedative / hypnotic agents are likely to exert their effects by altering neurotransmission, the majority of investigations to date have focused on neurotransmitter receptors and modulators of neurotransmission such as kinases.

Anesthesia sensitivity in mice that lack the beta3 subunit of the gamma-aminobutyric acid type A receptor

BACKGROUND

The mammalian gamma-aminobutyric acid type A (GABA(A)) receptor, a likely target of anesthetic action, exhibits remarkable subunit heterogeneity. In vitro expression studies suggest that there is subunit specificity to anesthetic responses at the GABA(A) receptor. The authors tested whether genetically engineered mice that lack the beta3 subunit of the GABA(A) receptor differed in their sensitivities to several general anesthetic agents.

METHODS

Median effective concentrations for loss-of-righting reflex and tail clamp/withdrawal for enflurane and halothane were determined in mice with and without the beta3 gene and gene product. Sleep time was measured after intraperitoneal injection of pentobarbital, ethanol, etomidate, and midazolam.

RESULTS

Null allele mice (beta3 -/-) did not differ from wild-type mice (beta3 +/+) in the obtunding response to enflurane and halothane but were significantly more resistant to enflurane (null allele half-effect concentrations [EC50] of 2.59 +/- 0.10 vs. wild-type EC50 of 2.06 +/- 0.12 atm %, P < 0.001) and halothane (null allele EC50 of 1.73 +/- 0.04 vs. wild-type EC50 of 1.59 +/- 0.05 atm %, P = 0.01) as determined by tail clamp response. Wild-type and null allele mice exhibited divergent responses to other sedative agents active at the GABA(A) receptor. No differences were noted in sleep times after administration of pentobarbital and ethanol, but null allele mice were more resistant to etomidate (null allele EC50 of 17.8 +/- 1.9 min vs. wild-type EC50 of 26.2 +/- 2.4 min, P < 0.02) and midazolam (null allele EC50 of 14.2 +/- 7.8 min vs. wild-type EC50 of 41.3 +/- 10.4 min, P < 0.05).

CONCLUSIONS

The beta3 subunit of the GABA(A) receptor appears to be important in the mediation of the immobilizing (tail clamp) but not obtunding (loss-of-righting reflex) effects of the volatile anesthetic agents enflurane and halothane. These data support the hypotheses that separate components of the anesthetic state are mediated via different central nervous system loci; that the GABA(A) receptor is a likely target for the immobilizing response to volatile anesthetic agents; and that the beta3 subunit plays a direct or indirect role in the mediation of this response. Absence of the beta3 subunit appears to attenuate the obtunding effect of midazolam and etomidate but appears not to alter the obtunding effect of pentobarbital, enflurane, and halothane, suggesting that these anesthetic agents produce hypnosis by different specific molecular mechanisms.

Haemodynamic effects of rocuronium bromide in adult cardiac surgical patients

PURPOSE

To measure the haemodynamic effects of rocuronium in adults undergoing cardiac surgery with cardiopulmonary bypass (CPB).

METHODS

Twenty patients undergoing elective cardiac surgical procedures with moderate hypothermic nonpulsatile bypass participated in this prospective, observational study. After anaesthetic induction, recovery from succinylcholine, and achievement of baseline haemodynamic stability, patients received 0.6 mg.kg-1 rocuronium as an initial rapid intravenous bolus. Maintenance dosing of 0.2 mg.kg-1 was continued for the remainder of the procedure. Haemodynamic measurements (heart rate, systemic arterial systolic, diastolic, and mean arterial pressure, pulmonary arterial systolic, diastolic, and mean pressure, pulmonary capillary wedge pressure, central venous pressure, and thermodilution cardiac output measurements) were obtained for the first five minutes after rocuronium administration, and subjects were observed for histamine-related symptoms.

RESULTS

Central venous pressure decreased from baseline at two and five minutes after the rocuronium bolus, and mean pulmonary artery pressure decreased at five minutes. No changes were observed in heart rate, mean systemic arterial pressure, pulmonary capillary wedge pressure, cardiac index, stroke volume, systemic vascular resistance, or pulmonary vascular resistance, nor did any patient manifest any other histamine-related symptoms.

CONCLUSION

The haemodynamic profile for a 0.6 mg.kg-1 bolus of rocuronium is acceptable for patients with cardiovascular disease.

Cerebellar gamma-aminobutyric acid type A receptors: pharmacological subtypes revealed by mutant mouse lines

The vast molecular heterogeneity of brain gamma-aminobutyric acid type A (GABAA) receptors forms the basis for receptor subtyping. Using autoradiographic techniques, we established the characteristics of cerebellar granule cell GABAA receptors by comparing wild-type mice with those with a targeted disruption of the alpha6 subunit gene. Cerebellar granule cells of alpha6(-/-) animals have severe deficits in high affinity [3H]muscimol and [3H]SR 95531 binding to GABA sites, in agonist-insensitive [3H]Ro 15-4513 binding to benzodiazepine sites, and in furosemide-induced increases in tert-[35S]butylbicyclophosphorothionate binding to picrotoxin-sensitive convulsant sites. These observations agree with the known specific properties of these sites on recombinant alpha6beta2/3gamma2 receptors. In the presence of GABA concentrations that fail to activate alpha1 subunit-containing receptors, methyl-6,7-dimethoxy-4-ethyl-beta-carboline (30 microM), allopregnanolone (100 nM), and Zn2+ (10 microM) are less efficacious in altering tert-[35S]butylbicyclophosphorothionate binding in the granule cell layer of the alpha6(-/-) than alpha6(+/+) animals. These data concur with the deficiency of the cerebellar alpha6 and delta subunit-containing receptors in the alpha6(-/-) animals and could also account for the decreased affinity of [3H]muscimol binding to alpha6(-/-) cerebellar membranes. Predicted additional alterations in the cerebellar receptors of the mutant mice may explain a surplus of methyl-6,7-dimethoxy-4-ethyl-beta-carboline-insensitive receptors in the alpha6(-/-) granule cell layer and an increased diazepam-sensitivity in the molecular layer. These changes may be adaptive consequences of altered GABAA receptor subunit expression patterns in response to the loss of two subunits (alpha and delta) from granule cells.

Mice devoid of gamma-aminobutyrate type A receptor beta3 subunit have epilepsy, cleft palate, and hypersensitive behavior

gamma-Aminobutyric acid type A receptors (GABA(A)-Rs) mediate the bulk of rapid inhibitory synaptic transmission in the central nervous system. The beta3 subunit is an essential component of the GABA(A)-R in many brain regions, especially during development, and is implicated in several pathophysiologic processes. We examined mice harboring a beta3 gene inactivated by gene targeting. GABA(A)-R density is approximately halved in brain of beta3-deficient mice, and GABA(A)-R function is severely impaired. Most beta3-deficient mice die as neonates; some neonatal mortality, but not all, is accompanied by cleft palate. beta3-deficient mice that survive are runted until weaning but achieve normal body size by adulthood, although with reduced life span. These mice are fertile but mothers fail to nurture offspring. Brain morphology is grossly normal, but a number of behaviors are abnormal, consistent with the widespread location of the beta3 subunit. The mice are very hyperactive and hyperresponsive to human contact and other sensory stimuli, and often run continuously in tight circles. When held by the tail, they hold all paws in like a ball, which is frequently a sign of neurological impairment. They have difficulty swimming, walking on grids, and fall off platforms and rotarods, although they do not have a jerky gait. beta3-deficient mice display frequent myoclonus and occasional epileptic seizures, documented by electroencephalographic recording. Hyperactivity, lack of coordination, and seizures are consistent with reduced presynaptic inhibition in spinal cord and impaired inhibition in higher cortical centers and/or pleiotropic developmental defects.

Gene knockout of the alpha6 subunit of the gamma-aminobutyric acid type A receptor: lack of effect on responses to ethanol, pentobarbital, and general anesthetics

The alpha6 subunit of the gamma-aminobutyric acid type A receptor (GABA(A)-R) has been implicated in mediating the intoxicating effects of ethanol and the motor ataxic effects of general anesthetics. To test this hypothesis, we used gene targeting in embryonic stem cells to create mice lacking a functional alpha6 gene. Homozygous mice are viable and fertile and have grossly normal cerebellar cytoarchitecture. Northern blot and reverse transcriptase-polymerase chain reaction analyses demonstrated that the targeting event disrupted production of functional alpha6 mRNA. Autoradiography of histological sections of adult brains demonstrated that diazepam-insensitive binding of [3H]Ro15-4513 to the cerebellar granule cell layer of wild-type mice was completely absent in homozygous mice. Cerebellar GABA(A)-R density was unchanged in the mutant mice; however, the apparent affinity for muscimol was markedly reduced. Sleep time response to injection of ethanol after pretreatment with vehicle or Ro15-4513 did not differ between genotypes. Sleep time response to injection of pentobarbital and loss of righting reflex and response to tail clamp stimulus in mice anesthetized with volatile anesthetics also did not differ between genotypes. Thus, the alpha6 subunit of the GABA(A)-R is not required for normal development, viability, and fertility and does not seem to be a critical or unique component of the neuronal pathway mediating the hypnotic effect of ethanol and its antagonism by Ro15-4513 in mice. Similarly, the alpha6 subunit does not seem to be involved in the behavioral responses to general anesthetics or pentobarbital.

In vivo imaging of nitrous oxide-induced changes in cerebral activation during noxious heat stimuli

BACKGROUND

Although previous studies have provided some insight into the pharmacologic aspects of nitrous oxide analgesia, the neural circuits mediating its antinociceptive effect remain relatively unexplored. Position emission tomography was used in nine volunteers to identify the loci of nitrous oxide-modulated cerebral responses to a peripheral noxious stimulus.

METHODS

Nitrous oxide-pain interactions were studied by comparing regional cerebral blood flow responses to a 48 degrees C tonic heat stimulus, applied to each volunteer's left forearm, during room air inhalation with those obtained while 20% nitrous oxide was administered. Two cerebral blood flow scans were obtained with the 15O-water technique during each condition. Locations of specific regional activation related to pain, and nitrous oxide, were identified using the statistical parametric mapping method, with a significance level of P < 0.01. Pain was rated by visual analog scale and the values were compared using Wilcoxon rank sum analysis.

RESULTS

Pain produced cerebral activation in the contralateral thalamus, anterior cingulate, and supplementary motor area. Adding nitrous oxide during pain stimulation abolished activation in these areas but was associated with activation in the contralateral infralimbic and orbitofrontal cortices. In parallel, mean visual analog scale scores decreased significantly from 67 +/- 4 (SEM) to 54 +/- 5 (P < 0.05).

CONCLUSIONS

Nitrous oxide, at 20% concentration, appears to modulate pain processing in the brain's medial pain system, and also activates the infralimbic and orbitofrontal cortices. The potential contribution of the affected brain areas to nitrous oxide analgesia is discussed.

Regional brain activity changes associated with fentanyl analgesia elucidated by positron emission tomography

Recent positron emission tomography (PET) studies have demonstrated areas of pain processing in the human brain. Given the inhibitory effects of opioids on neuronal activity, we predicted that fentanyl's analgesic effects would be associated with suppression of pain-evoked responses in these distinct brain areas. To test this, PET was used to measure cerebral blood flow responses, as reflections of regional neuronal activity, to painful and nonpainful thermal stimuli both in the absence and presence of fentanyl in humans. During each PET scan in nine healthy volunteers a tonic heat source was placed against the subject's left forearm, delivering a preset temperature of either 40 degrees C (nonpainful) or 47-48 degrees C (painful). Subjects underwent eight blood flow studies, each consisting of 50 mCi [15O]water injection and a PET scan. The first four studies were performed during placebo administration in the stimulus sequence: nonpainful, painful, painful, nonpainful. This sequence was then repeated during intravenous (i.v.) administration of fentanyl 1.5 micrograms/kg [corrected]. Significant differences in regional cerebral blood flow (rCBF) between the placebo and the fentanyl conditions during nonpainful and painful stimuli were identified using statistical parametric mapping. It was found that pain increased rCBF in the anterior cingulate, ipsilateral thalamus, prefrontal cortex, and contralateral supplementary motor area. Fentanyl increased rCBF in the anterior cingulate and contralateral motor cortices, and decreased rCBF in the thalamus (bilaterally) and posterior cingulate during both stimuli. During combined pain stimulation and fentanyl administration, fentanyl significantly augmented pain-related rCBF increases in the supplementary motor area and prefrontal cortex. This activation pattern was associated with decreased pain perception, as measured on a visual analog scale. In contrast to our hypothesis, these data indicate that fentanyl analgesia involves augmentation of pain-evoked cerebral responses in certain areas, as well as both activation and inhibition in other brain regions unresponsive to pain stimulation alone.

In vivo imaging of human limbic responses to nitrous oxide inhalation

Human behavioral studies have shown that nitrous oxide, in subanesthetic concentrations, impairs psychomotor function, cognitive performance, and learning and memory processes. However, the cerebral mechanisms of such effects remain unknown. Positron emission tomography (PET) was used to map the brain areas associated with nitrous oxide effects. Regional cerebral blood flow (rCBF) was measured in eight volunteers, during room air (control) or 20% nitrous oxide (nitrous oxide) inhalation using 15(O)-water, to reflect regional neuronal activity. To control for the possibility that 20% nitrous oxide uncoupled cerebral blood flow and metabolism, in four of the subjects, regional cerebral metabolic rate (rCMR) was also measured using 18F-deoxyglucose during the two experimental conditions. Results of rCBF and rCMR scans were compared between conditions using the statistical parametric mapping method, and areas of nitrous oxide-related activation or deactivation were identified at a significance level of 0.005. Percent changes in rCBF scan pixels from these activated or deactivated areas were then compared with those of stereotactically corresponding rCMR scan pixels with t statistics (P < 0.05 was defined as a significant difference). It was found that cerebral blood flow and metabolism were not uncoupled by 20% nitrous oxide, since percent changes in rCBF and rCMR, detected during nitrous oxide inhalation, did not differ significantly from each other (P < 0.05). Nitrous oxide inhalation was associated with significant activation in the anterior cingulate cortex, a limbic area known to mediate psychomotor and cognitive processes. Deactivation was found in the posterior cingulate, hippocampus, parahippocampal gyrus, and visual association cortices in both hemispheres; the former two regions are known to mediate learning and memory. These areas identified by PET in vivo may provide the neuroanatomical basis for the behavioral responses associated with subanesthetic nitrous oxide inhalation.

Human brain activity response to fentanyl imaged by positron emission tomography

Positron emission tomography (PET) is a noninvasive imaging technique that can be used to observe drug actions on human brain in vivo. We used 15O-water PET scanning in six volunteers to examine the effects on regional cerebral activity as reflected by regional cerebral blood flow (rCBF) of a small intravenous bolus of fentanyl. rCBF was compared between scans obtained after fentanyl or a placebo using three separate statistical criteria including a pixel-by-pixel t statistic; significance was stringently defined at P values < 0.01. Anatomic locations of regional cerebral activity changes were verified by aligning rCBF PET scans with cranial magnetic resonance images using mathematical coregistration. Fentanyl administration was associated with significant increases in rCBF consistent with regional neuronal activation in both cingulate and orbitofrontal and medial prefrontal cortices, as well as caudate nuclei. These areas are responsive to nociceptive stimuli and are involved in avoidance learning, reward and addiction, visceromotor control, maintenance of attention, and pain-related affective behavior. Significant decreases were noted in both frontal and temporal areas and the cerebellum, a distribution far less extensive than that of opiate receptors in general. These data indicate that fentanyl's effects are highly localized and specifically affect cerebral regions associated with a range of pain-related behaviors.

Isoflurane's enhancement of chloride flux through rat brain gamma-aminobutyric acid type A receptors is stereoselective

BACKGROUND

Recent evidence is consistent with the view that volatile anesthetics interact directly with excitable membrane-bound channel proteins. If these agents interact directly with chiral centers in the neuronal cell membrane, then their effects should be stereoselective. Using rat brain membranes enriched in gamma-aminobutyric acid type A (GABAA) receptors, we investigated the hypothesis that the permeability response of this well-characterized central nervous system channel protein to stereoisomers of isoflurane is stereoselective.

METHODS

Rat brain synaptic microvesicles were prepared by differential centrifugation. Agonist-stimulated 36Cl- flux through membrane-bound GABAA receptors was assayed in the presence of (+)- and (-)-isoflurane and compared with control conditions.

RESULTS

Both isomers increased the potency and efficacy of GABA; however, (+)-isoflurane was significantly more potent and efficacious than the (-)-isomer. For example, the (+)-isomer (140 microM) reduced the median effective concentration of GABA from 12.7 +/- 1.0 to 5.4 +/- 0.5 microM, whereas the (-)-isomer reduced it to 9.6 +/- 1.0 microM (P < 0.001). The (+)-isomer also was 1.6 times as potent as the (-)-isomer in augmenting 5 microM GABA-gated flux (79 +/- 11 vs. 130 +/- 17 microM, respectively; P = 0.01). In addition, the (+)-isomer produced significantly greater maximal enhancement of flux (9.4 +/- 0.4 vs. 7.0 +/- 0.3 nmol.mg-1.3 s-1; P < 0.001).

CONCLUSIONS

Isoflurane's effects on GABA-gated chloride flux were stereoselective. This result supports direct interaction with a stereoselective site, possibly the GABAA channel protein itself, rather than a nonspecific perturbation of the surrounding membrane lipid. In addition, these findings, from a functional assay using mammalian brain, agree with recent observations in invertebrate ion channels and mammalian neuronal cell cultures.

Staurosporine, a protein kinase inhibitor, increases the intoxicating potencies of ethanol and other n-alkanols in Rana pipiens tadpoles

Central nervous system protein kinases are the intracellular effectors for many of the signal transduction pathways essential to neurotransmission. Although the in vitro activity of at least one of these important enzymes, protein kinase C, is diminished by therapeutic concentrations of ethanol and other central depressants, the relationship of this effect to intoxication in vivo is not known. If intoxication by ethanol involves central protein kinase inhibition, then other inhibitors of these enzymes should enhance ethanol's intoxicating potency. To test this hypothesis, we compared the median effective concentrations of ethanol and two other n-alkanols for loss-of-righting reflex in Rana pipiens tadpoles pretreated with staurosporine and in untreated controls. Alkanol concentrations were confirmed by gas chromatography and staurosporine concentrations by ultraviolet absorbance spectrophotometry. Results obtained with 650 animals demonstrate that pretreatment with staurosporine concentrations in the nanomolar range significantly decrease the median effective concentration for ethanol (56% of control; p < 0.001), butanol (38% of control; p < 0.001), and octanol (59% of control; p < 0.001). This finding supports that central protein kinase inhibition may be involved in the acute intoxicating effects of ethanol and other n-alkanols.

Halothane sensitivity in replicate mouse lines selected for diazepam sensitivity or resistance

We have previously shown that mice selected for sensitivity to diazepam are also more sensitive to halothane, and that halothane augments the gamma-aminobutyric acid (GABA)-mediated chloride flux response in brain tissue from diazepam-sensitive (DS) mice to a greater degree than in diazepam-resistant (DR) mice. These findings suggest that the GABAA receptor is an important site of halothane action. To confirm this correlation, halothane requirement was determined in two independently developed replicate lines of DS and DR mice. Association of the traits of diazepam and halothane sensitivity in replicate lines of DS mice diminishes the probability that the original finding was due to a false-positive correlation, and instead suggests that it results from the common action of genes controlling diazepam sensitivity. Halothane median effective concentration (EC50) was determined by using the end-point of loss of righting reflex in two replicate lines of mice selected for diazepam sensitivity (resistant mice = diazepam high performance-1 and -2 [DHP-1 and DHP-2], sensitive mice = diazepam low performance-1 and -2 [DLP-1 and DLP-2]). DLP-1 and DLP-2 mice were sensitive to halothane, whereas DHP-1 and DHP-2 mice were resistant to halothane. Halothane EC50 in the DLP-1 and DHP-1 mice was 0.86 +/- 0.01 (SE) and 1.10 +/- 0.04 atm%, respectively (P < 0.0001), and that in the DLP-2 and DHP-2 mice was 0.88 +/- 0.01 and 0.97 +/- 0.02 atm%, respectively (P < 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Does general anesthetic-induced desensitization of the Torpedo acetylcholine receptor correlate with lipid disordering?

We have tested the hypothesis that general anesthetics stabilize the desensitized state of the nicotinic acetylcholine receptor by disordering its surrounding lipids. Acetylcholine receptor-rich postsynaptic membranes from the electroplaques of Torpedo were used in this study to obtain the highest possible receptor specific activity in native membranes. We examined 18 general anesthetics, including six inhalation agents, eight 1-alcohols, the enantiomers of 2-octanol, and two intravenous general anesthetics (pentobarbital and ethylcarbamate). The degree of desensitization after preincubation with the general anesthetics was determined by brief exposure to [3H]acetylcholine, making use of the facts that desensitized receptors have much higher affinity than do those in the resting state and that interconversion between the states is slow. All of the general anesthetics desensitized the receptor within minutes, exhibiting steep concentration-response curves with Hill coefficients generally within the range of 2-4. At the highest general anesthetic concentrations, almost all receptors were desensitized. The concentrations that desensitized half of the resting state receptors varied by > 3000-fold. The 2-octanol enantiomers were without stereoselectivity. Membrane order was examined in parallel by using spin-labeled fatty acids doped into the native membranes. The spin label 5-doxylpalmitate reported from the most ordered part of the bilayer near the aqueous interface, whereas 12-doxylstearate reported from the less ordered region nearer the center of the bilayer. The spin label deeper in the membranes was 3 times more sensitive to a given anesthetic than was the other probe. At both depths in the membrane general anesthetics decreased lipid order linearly with increasing concentration. The range of disordering potencies (change in order parameter induced by a unit concentration of general anesthetic in the aqueous phase) was 5333 for 5-doxylpalmitate and 7143 for 12-doxylstearate, but the range of disordering compared at equally desensitizing concentrations was reduced by 875- and 1430-fold, respectively. The average degrees of disordering at concentrations that desensitized half of the resting state receptors were 1.5% and 4.4%, respectively. It is unlikely that changes in membrane order parameter per se cause desensitization, because the associated changes in order parameter can be reproduced by changes in cholesterol content or temperature that do not cause desensitization. We conclude that, although there is a strong association between anesthetic-induced membrane disordering and desensitization, more detailed tests of a mechanistic nature will be necessary to elucidate the mechanisms underlying the Meyer-Overton-type behavior we have observed.

Staurosporine, a protein kinase C inhibitor, decreases the general anesthetic requirement in Rana pipiens tadpoles

Protein kinase C, the intracellular effector for the inositol phosphate-mediated signal transduction pathway, plays a key role in neurotransmission in the central nervous system. Although the in vitro activity of protein kinase C is inhibited by therapeutic concentrations of volatile anesthetics, the relation of this effect to in vivo obtundation has not been established. If obtundation by volatile anesthetics involves protein kinase C inhibition, then an inhibitor of this enzyme should decrease the anesthetic requirement. To test this hypothesis, we compared the EC50S of halothane and diethylether for loss of the righting reflex in Rana pipiens tadpoles pretreated with staurosporine and in untreated controls. Anesthetic concentrations were confirmed by gas chromatography and staurosporine concentrations by ultraviolet absorbance spectrophotometry. Results obtained in more than 1000 animals indicated that pretreatment with staurosporine concentrations in the nanomolar range significantly decreased the EC50 for both halothane (68% of control; P < 0.035) and diethylether (41% of control; P < 0.001). This finding implies that protein kinase C inhibition may play a role in general anesthetic-induced obtundation.

Volatile anesthetic requirements differ in mice selectively bred for sensitivity or resistance to diazepam: implications for the site of anesthesia

One approach to elucidating the general anesthetic target has used genetic selection procedures, wherein animals are bred for sensitivity or resistance to general anesthetics and correlations are sought with a specific neuronal structural or functional defect. For example, murine strains have been developed that are either sensitive or resistant to the obtunding effects of diazepam, as assessed by their ability to maintain balance on a rotating rod. The present study explored whether diazepam-sensitive (DS) and diazepam-resistant (DR) mice might also be similarly divergent in the obtunding response to general anesthetics, by testing the requirements for halothane and enflurane in these strains. Using a carousel enclosed in a chamber, the end-point of loss-of-righting reflex was defined. For both anesthetics, the DS groups had a lower median effective dose (ED50, %atm) than did the DR group, and the reductions paralleled diazepam susceptibility. For example, with halothane, the ED50 for the DS group was 0.72 +/- 0.022 (SE); the ED50 for the DR group was 0.87 +/- 0.030 (P < 0.0001). Similar results were obtained with enflurane. Such findings associate an inbred difference in response to diazepam with altered volatile anesthetic requirement, suggesting that these two phenotypes are mediated by a common underlying mechanism.

Halothane's effects on GABA-gated chloride flux in mice selectively bred for sensitivity or resistance to diazepam

The DS (diazepam-sensitive) and DR (diazepam-resistant) lines of mice, selected on the basis of their ataxic response to diazepam, also diverge in the physiologic response of their brain gamma-aminobutyric acidA (GABAA) receptors to benzodiazepines, as indicated by augmentation of GABA-mediated chloride flux. Cross-sensitivity and -resistance to other sedatives known to interact with the GABAA-receptor have also been demonstrated in DS and DR mice. Based on the finding that these mice also show cross-sensitivity and -resistance to obtundation by halothane, we predicted that their GABAA-receptors would also exhibit a differential response to halothane as assayed by an in vitro 36Cl- influx assay using purified brain microvesicles. Consistent with this prediction, therapeutic concentrations of halothane enhanced 1 mumol/l GABA-gated flux with significantly greater potency in DS than in DR mice (halothane EC50 336 +/- 64 mumol/l (S.E.M.) vs. 605 +/- 110 mumol/l, respectively, P = 0.03), but there was no difference in maximal flux enhancement between the two lines (DS 4.7 +/- 0.4 nmol.mg-1 x 3 s-1, vs. DR 4.7 +/- 0.5 nmol.mg-1 x 3 s-1). Halothane (500 mumol/l) also shifted the entire GABA concentration-flux relationship significantly to the left, decreasing the EC50 for GABA in both the DS and DR lines. Importantly, the shift in the GABA concentration-flux response in the presence of halothane was more pronounced in the DS mice (GABA EC50 1.8 +/- 0.4 mumol/l vs. 14.7 +/- 0.9 mumol/l without halothane) than in the DR mice (GABA EC50 4.7 +/- 0.6 mumol/l vs. 14.7 +/- 0.9 mumol/l without halothane).(ABSTRACT TRUNCATED AT 250 WORDS)

Ligand-dependent effects of ethanol and diethylether at brain benzodiazepine receptors

The GABAA receptor chloride channel complex interacts with various categories of sedatives, including the benzodiazepines, and possibly ethanol and volatile general anesthetics. Thus, specific binding of tritiated derivatives of a benzodiazepine antagonist, flumazenil, and an agonist, flunitrazepam, to rat brain membrane fragments was monitored at equilibrium in the presence and absence of anesthetizing concentrations of ethanol and diethylether. Ethanol produced a concentration-dependent inhibition of [3H]flumazenil binding, which was not reversed by the GABAA receptor competitive antagonist bicuculline, but had no effect on [3H]flunitrazepam binding. Both ethanol and diethylether decreased the affinity of the benzodiazepine site for [3H]flumazenil. These data indicate that ethanol and diethylether have GABA-independent effects at the benzodiazepine sites of the GABAA receptor. These findings are inconsistent with a two-state functional model of the benzodiazepine site and, instead, support a model containing a specific, antagonist-favored conformation.

Nonanesthetic alcohols dissolve in synaptic membranes without perturbing their lipids

While many theories of general anesthesia postulate a lipid site of action, there has been no adequate explanation for the lack of anesthetic potency of the highly hydrophobic primary alkanols with more than 12 carbons (the cut-off). Some work suggests that these nonanesthetic alcohols do not dissolve in membranes. Other work contradicts this and suggests that an anesthetic site on a protein provides a better explanation. Here we show that both the anesthetic dodecanol and the nonanesthetic tetradecanol are taken up equally well into the tissues of animals and into isolated postsynaptic membranes. When a group of Rana pipiens tadpoles were treated with dodecanol, half were anesthetized by 4.7 microM (free aqueos concentration), and the corresponding concentration in the tissues was found to be 0.4 mmol per kg wet weight. Prolonged exposure (92 hr) to tetradecanol produced even higher tissue concentrations (0.7 mmol per kg wet weight), yet no anesthetic effects were observed. Furthermore, general anesthetics are thought to act on postsynaptic membranes but both alkanols partitioned into postsynaptic membranes from Torpedo electroplaques. The spin label, 12-doxyl stearate, was incorporated into these membranes. The lipid order parameter it reported was decreased by the anesthetic alcohols (octanol, decanol, and dodecanol), whereas the nonanesthetic alcohols either did not change it significantly (tetradecanol) or actually increased it (hexadecanol and octadecanol). Thus, although lipid solubility is unable to account for the pharmacology of the cut-off in potency of the long-chain alcohols, lipid perturbations provide an accurate description.

Anaesthetic potencies of primary alkanols: implications for the molecular dimensions of the anaesthetic site

1. We have redetermined the anaesthetic potencies (EC50S) for a series of primary alkanols, to resolve uncertainties about the molecular dimensions of the anaesthetic site resulting from the use of data from different laboratories. 2. For each alkanol, concentration-response relationships for loss of righting reflex (LRR) were plotted for over one hundred tadpoles, and the median effective concentrations determined. Aqueous concentrations present during potency assays were determined independently, and for alkanols with chain length greater than nonanol, correction was made for depletion from the aqueous phase. 3. The EC50S were found to decrease logarithmically with increasing number of carbon atoms in the hydrocarbon chain of the alkanol (CN), such that, on average, each additional methylene group was associated with an approximately four fold increase in potency. 4. The relationship between log EC50 and CN was best described by the quadratic equation, log EC50 = 0.022 (+/- 0.0038) CN2 + 0.76 (+/- 0.051) CN + 3.7 (+/- 0.14) (r2 = 0.9951). 5. A previously described correlation between the apparent changes in the free energy of binding of an additional methylene group both to luciferase and to the sites for LRR in tadpoles was not confirmed. 6. A cut-off in potency beyond dodecanol was established in experiments where tadpoles were maintained in supersaturated solutions of tridecanol for 20 h without demonstrable LRR. 7. These findings indicate that the soluble enzyme firefly luciferase does not adequately model the anaesthetic site. Specifically, there are discrepancies in the position of cut-off, and the apparent changes in the free energy of binding, per methylene group, of an alkanol to luciferase do not parallel that for tadpoles.

Is agonist self-inhibition at the nicotinic acetylcholine receptor a nonspecific action?

Agonist concentration-response relationships at nicotinic postsynaptic receptors were established by measuring 86Rb+ efflux from acetylcholine receptor rich native Torpedo membrane vesicles under three different conditions: integrated net ion efflux (in 10 s) from untreated vesicles, integrated net efflux from vesicles in which most acetylcholine sites were irreversibly blocked with alpha-bungarotoxin, and initial rates of efflux (5-100 ms) from vesicles that were partially blocked with alpha-bungarotoxin. Exposure to acetylcholine, carbamylcholine, suberyldicholine, phenyltrimethylammonium, or (-)-nicotine over 10(8)-fold concentration ranges results in bell-shaped ion flux response curves due to stimulation of acetylcholine receptor channel opening at low concentrations and inhibition of channel function at 60-2000 times higher concentrations. Concentrations of agonists that inhibit their own maximum 86Rb+ efflux by 50% (KB values) are 110, 211, 3.0, 39, and 8.9 mM, respectively, for the agonists listed above. For acetylcholine and carbamylcholine, KB values determined from both 10-s and 15-ms efflux measurements are the same, indicating that the rate of agonist-induced desensitization increases to maximum at concentrations lower than those causing self-inhibition. For all partial and full agonists studied, Hill coefficients for self-inhibition are close to 1.0. Concentrations of agonists up to 8 times KB did not change the order parameter reported by a spin-labeled fatty acid incorporated in Torpedo membranes. We conclude that agonist self-inhibition cannot be attributed to a general nonspecific membrane perturbation. Instead, these results are consistent with a saturable site of action either at the lipid-protein interface or on the acetylcholine receptor protein itself.

Lipid-dependent differential effects of stereoisomers of anesthetic alcohols

The cis- and trans-alkenols are equally potent general anesthetics but, respectively, lower and raise the gel-to-liquid crystalline phase transition temperature of saturated phosphatidylcholines (Pringle, M.J. and Miller, K.W. (1978) Biochem. Biophys. Res. Commun. 85, 1191-1198). Here we show that although this differential effect is somewhat reduced when a double bond is introduced into the sn-2 position of phosphatidylcholine, it is abolished when the ethanolamine head group is substituted for the choline head group in dimyristoyl lipids at neutral pH. At high pH, however, dimyristoylphosphatidylethanolamine assumes a negative charge, and its phase transition temperature drops to a value close to that for the corresponding phosphatidylcholine. Under these conditions the differential effect of the alkenol isomers is restored; the cis-alkenol lowers, while the trans-alkenol raises, the phase transition temperature of deprotonated dimyristoylphosphatidylethanolamine. Thus, the differential effects of cis- and trans-alkenols on the gel-to-liquid crystalline phase transition are dependent on the physical chemical characteristics of the polar region of the perturbed lipid species, but only weakly on that of the acyl region.

Two pools of cholesterol in acetylcholine receptor-rich membranes from Torpedo

The acetylcholine receptor (AChR)-containing electroplax membranes from Torpedo californica have a relatively high cholesterol content. Reconstitution studies suggest that this cholesterol may be important in preserving or modulating the function of the acetylcholine receptor-channel complex. We have manipulated cholesterol levels in intact Torpedo AChR-rich membrane fragments using small, unilamellar phosphatidylcholine liposomes. Conditions have been established that allow further subfractionation of sucrose gradient purified Torpedo electroplax membranes into AChR-rich and ATPase-rich populations and that, at the same time, achieve cholesterol depletion without phospholipid back exchange or fusion. The incubation of membranes with excess liposomes could only achieve about a 50% reduction in the molar ratio of cholesterol to phospholipid. In no case was the number of cholesterol molecules per AChR oligomer reduced below 36. The remaining cholesterol could not be depleted either by longer incubations or by multiple, sequential depletions. Cholesterol depletion was accompanied by a significant increase in bulk membrane fluidity as measured by electron spin resonance spectroscopy, but the equilibrium binding parameters of acetylcholine to its receptor were unaltered. This suggests strongly that there exist two pools of cholesterol in the AChR-rich Torpedo electroplax membrane: an easily depleted fraction that influences bulk fluidity, and a tightly-bound fraction perhaps surrounding the AChR oligomer.

Anesthetic potencies of secondary alcohol enantiomers

The Meyer-Overton rule has been interpreted to mean that general anesthetics act at a nonpolar site, either in a lipid bilayer or a protein. Optical isomers, also called enantiomers, are pairs of compounds with the same molecular formula and functional groups, but which differ in the arrangement of the groups around an "asymmetric" carbon atom and in the direction they rotate plane-polarized light. By definition, enantiomers that are anesthetics can distinguish between stereoselective and nonselective sites of anesthetic action. We used such enantiomers to determine whether anesthetics are stereoselective in their actions on animals by measuring the potencies of a homologous series of secondary aliphatic alcohols from 2-butanol through 2-octanol in tadpoles, using reversible loss of righting reflex as the endpoint. None of the isomeric pairs exhibited significant differences in potency. Anesthetic potency increased logarithmically with the number of carbon atoms in the hydrocarbon chain of the alcohol. The ED50 +/- SE (mM) for the (+) and (-) forms of the alcohols, respectively, were as follows: 2-butanol 17 +/- 1.2, 17 +/- 1.1; 2-pentanol 4.7 +/- 0.28, 4.8 +/- 0.27; 2-hexanol 1.33 +/- 0.068, 1.42 +/- 0.079; 2-heptanol 0.32 +/- 0.011, 0.33 +/- 0.020; and 2-octanol 0.063 +/- 0.0042, 0.061 +/- 0.0032. These data demonstrate a lack of stereoselectivity in the interactions between the anesthetic secondary alcohols and their site of action in animals.

Actions of general anesthetics on acetylcholine receptor-rich membranes from Torpedo californica

The molecular mechanisms by which general anesthetics act on postsynaptic membranes can only be worked out in a highly purified, homogeneous system. The nicotinic acetylcholine receptor-rich membranes from the electric tissue of Torpedo californica are currently the only postsynaptic membranes that fulfill this condition. Is this peripheral synapse acted on with a pharmacologic specificity similar to that for general anesthesia, and how much less sensitive is it to anesthetic action than the unknown central site? To answer these questions, the authors studied the effects of 13 anesthetic compounds, including volatile general anesthetics, alkanols, and urethane, on the equilibrium binding of 3H-acetylcholine to these nicotinic receptors. As the anesthetic concentration was raised, all the agents first increased acetylcholine binding steeply and then, with few exceptions, decreased it again at higher concentrations. Anesthetics increased acetylcholine binding by decreasing acetylcholine's dissociation constant without changing the Hill coefficient or the number of sites. To a first approximation, the relative ability of these agents to increase 3H-acetylcholine binding parallels that of anesthesia in vivo as predicted by the Meyer-Overton lipid solubility rule. On average, they produced half maximal increases in acetylcholine binding (EC50) at about four times the concentration that causes loss of righting reflex in one-half of a group of animals (ED50). However, a few agents deviated from this relationship. They were the agents with greatest general anesthetic potency in both the volatile anesthetic series (thiomethoxyflurane) and the normal alcohol series (octanol), and required up to 17 times their ED50s to achieve a half effect on acetylcholine binding. Although the concentrations required were high, these effects were reversible.(ABSTRACT TRUNCATED AT 250 WORDS)