Kristen Coates

Ketamine and its preservative, benzethonium chloride, both inhibit human recombinant α7 and α4β2 neuronal nicotinic acetylcholine receptors in Xenopus oocytes

Ketamine is a dissociative anaesthetic that is formulated as Ketalar, which contains the preservative benzethonium chloride (BCl). We have studied the effects of pure racemic ketamine, the preservative BCl and the Ketalar mixture on human neuronal nicotinic acetylcholine receptors (nAChRs) composed of the α7 subunit or α4 and β2 subunits expressed in Xenopus laevis oocytes. Ketamine inhibited responses to 1 mM acetylcholine (ACh) in both the human α7 and α4β2 nAChRs, with IC50 values of 20 and 50 μM respectively. Inhibition of the α7 nAChRs occurred within a clinically relevant concentration range, while inhibition of the α4β2 nAChR was observed only at higher concentrations. The Ketalar formulation inhibited nAChR function more effectively than was expected given its ketamine concentration. The surprising increased inhibitory potency of Ketalar compared with pure ketamine appeared to be due to the activity of BCl, which inhibited both α7 (IC50 value of 122 nM) and α4β2 (IC50 value of 49 nM) nAChRs at concentrations present in the clinical formulation of Ketalar. Ketamine is a noncompetitive inhibitor at both the α7 and α4β2 nAChR. In contrast, BCl causes a parallel shift in the ACh dose‐response curve at the α7 nAChR suggesting competitive inhibition. Ketamine causes both voltage‐dependent and use‐dependent inhibition, only in the α4β2 nAChR. Since α7 nAChRs are likely to be inhibited during clinical use of Ketalar, the actions of ketamine and BCl on this receptor subtype may play a role in the profound analgesia, amnesia, immobility and/or autonomic modulation produced by this anaesthetic.

Lipoprotein-associated phospholipase A2 activity and risk of recurrent stroke.

Background: Mass levels of lipoprotein-associated phospholipase A2 (Lp-PLA2), a leukocyte-derived enzyme involved in the metabolism of low-density lipoprotein to pro-inflammatory mediators, are associated with prognosis after stroke. Lp-PLA2 mass correlates only moderately with levels of Lp-PLA2 activity. The relationship of Lp-PLA2 activity to risk of stroke recurrence is unknown. We hypothesized that Lp-PLA2 activity levels would predict risk of recurrence. Methods: In the population-based Northern Manhattan Stroke Study, first ischemic stroke patients ≥40 years were followed for recurrent stroke. Levels of Lp-PLA2 activity were assessed in 467 patients, and categorized by quartile. Cox proportional hazard models were used to calculate hazard ratios (HR) and 95% confidence intervals (95% CI) for risk of recurrent stroke associated with marker quartiles after adjusting for demographics, vascular risk factors, and high-sensitivity C-reactive protein (hsCRP). Results: Mean age was 68.9 ± 12.7 years; 54.6% were women; 53.3% Hispanic, 27.2% black, and 17.8% white. Median follow-up was 4.0 years, and there were 80 recurrent strokes. Compared to the lowest quartile of Lp-PLA2 activity, those in the highest had an increased risk of recurrent stroke (adjusted HR 2.54, 95% CI 1.01–6.39). Conclusion: Stroke patients withLp-PLA2 activity levels in the highest quartile, compared to those in the lowest quartile, had an increased risk of recurrence after first ischemic stroke. Further studies are warranted to determine whether this biomarker has clinical utility in determining high-risk populations of stroke survivors, and whether anti-inflammatory strategies that reduce levels of activity of Lp-PLA2 reduce the risk of stroke recurrence.

Alterations in plasma complement levels after human ischemic stroke.

OBJECTIVE Stroke is a leading cause of morbidity and mortality in the United States. Recent animal studies have implicated the complement system in cerebral ischemia/reperfusion injury and suggest that complement inhibition may improve stroke outcomes. To assess the applicability of these findings to humans, we evaluated the characteristics and time course of human complement activation after stroke. METHODS We compared peripheral blood levels of complement factor 3a (C3a), 5a (C5a), and sC5b-9 drawn from 15 patients on poststroke Days 1, 2, 3, 7, 14, 21, and 28 to age-, race/ethnicity-, and sex-matched controls from the same population. Statistical analysis was performed using unpaired Mann-Whitney nonparametric tests with Bonferroni correction. All data is presented as the mean +/- standard deviation. RESULTS Mean C3a concentrations showed significant early elevations in stroke patients relative to matched controls (controls: 1080 +/- 189 ng/ml; Day 1: 1609 +/- 422 ng/ml, P = 0.0008; Day 3: 1520 +/- 317 ng/ml, P = 0.0005; Day 7: 1526 +/- 386 ng/ml, P = 0.001). C3a was also significantly elevated on Day 28 (1448 +/- 386 ng/ml, P = 0.004). Before poststroke Day 7, mean C5a levels did not differ significantly from controls. However, beginning on Day 7 and continuing through Day 14, there were significant elevations in C5a (controls: 3.33 +/- 2.1 ng/ml; day 7: 6.86 +/- 3.5 ng/ml, P = 0.005; Day 14: 7.65 +/- 4.6 ng/ml, P = 0.004). Mean sC5b-9 concentrations showed early depressions that reached significance on Days 1 and 2 (controls: 275.6 +/- 107 ng/ml; Day 1: 167.0 +/- 108 ng/ml, P = 0.006; Day 2: 156.3 +/- 80.0 ng/ml, P = 0.005) and did not differ significantly from controls at any other time point. CONCLUSION C3a is acutely elevated after human ischemic stroke, C5a shows delayed elevations 7 to 14 days after cerebral ischemia, and sC5b-9 is acutely depressed after stroke. Together, these data confirm complement activation after stroke and suggest that this activation is a heterogeneous process, with varying responses for different components.

Isoflurane Hyperalgesia Is Modulated by Nicotinic Inhibition

Background The inhaled anesthetic isoflurane inhibits neuronal nicotinic acetylcholine receptors (nAChRs) at concentrations lower than those used for anesthesia. Isoflurane produces biphasic nociceptive responses, with both hyperalgesia and analgesia within this concentration range. Because nicotinic agonists act as analgesics, the authors hypothesized that inhibition of nicotinic transmission by isoflurane causes hyperalgesia. Methods The authors studied female mice at 6–8 weeks of age. They measured hind paw withdrawal latency at isoflurane concentrations from 0 to 0.98 vol% after the animals had received a nicotinic agonist (nicotine), a nicotinic antagonist (mecamylamine or chlorisondamine), or saline intraperitoneally. In addition, the authors tested the interactions between mecamylamine and isoflurane and nicotine and isoflurane in heterologously expressed &agr;4&bgr;2 nAChRs. Results Female mice had significant hyperalgesia from isoflurane. Nicotine administration prevented isoflurane-induced hyperalgesia without altering the antinociception produced by higher isoflurane concentrations. Mecamylamine treatment caused a biphasic nociceptive response similar to that caused by isoflurane. Mecamylamine and isoflurane had an additive effect, both at heterologously expressed &agr;4&bgr;2 nAChRs and on the production of hyperalgesia in vivo. Mecamylamine thus potentiated hyperalgesia but did not affect analgesia. Conclusions Since hyperalgesia occurs in vivo at isoflurane doses that antagonize nAChRs in vitro, is prevented by a nicotinic agonist, and is mimicked and potentiated by nicotinic antagonists, the authors conclude that isoflurane inhibition of nAChRs activation is involved in the pathway that causes hyperalgesia. At subanesthetic doses, isoflurane can either enhance pain responses (produce hyperalgesia) or be analgesic (antinociceptive). In rats, low volatile anesthetic concentrations (0.1–0.2 minimum alveolar concentration [MAC]) elicit hyperalgesia, while 0.4–0.6 MAC elicits antinociception.

Heteromeric nicotinic inhibition by isoflurane does not mediate MAC or loss of righting reflex.

Background Neuronal nicotinic acetylcholine receptors (nAChRs) have been implicated in the mechanism of action of isoflurane as they are inhibited at subanesthetic concentrations. Despite clear evidence for nicotinic inhibition at relevant isoflurane concentrations, it is unclear what behavioral result ensues, if any. Methods The authors have modeled two behaviors common to all general anesthetics, immobility and hypnosis, as minimum alveolar concentration that prevents movement in response to a supramaximal stimulus (MAC) and loss of righting reflex (LORR). They have tested the ability of nicotinic pharmacologic modulators and congenital absence of most heteromeric nAChRs to affect concentration of isoflurane required for these behaviors. Results Neither mecamylamine, 5 mg/kg, nor chlorisondamine, 10 mg/kg, affected isoflurane MAC. Nicotine caused a small decrease in MAC. None of the above agents had any effect on the concentration of isoflurane required for LORR. Mice genetically engineered to lack the &bgr;2 nicotinic gene product were not different in MAC or LORR from controls. Conclusions Nicotinic antagonists do not cause MAC or LORR. Inhibition of nicotinic acetylcholine receptors by isoflurane is not likely related to its ability to provide immobility and hypnosis in a surgical setting. This is perhaps not surprising as the inhibition of nAChRs in vitro is complete at an isoflurane concentration equal to one half of MAC. Nicotinic inhibition may, however, be involved in anesthetic behaviors such as amnesia and analgesia, which occur at lower anesthetic concentrations.

Levels of acute phase proteins remain stable after ischemic stroke

BackgroundInflammation and inflammatory biomarkers play an important role in atherosclerosis and cardiovascular disease. Little information is available, however, on time course of serum markers of inflammation after stroke.MethodsFirst ischemic stroke patients ≥40 years old had levels of high-sensitivity C-reactive protein (hsCRP), serum amyloid A (SAA), and fibrinogen measured in plasma samples drawn at 1, 2, 3, 7, 14, 21 and 28 days after stroke. Levels were log-transformed as needed, and parametric and non-parametric statistical tests were used to test for evidence of a trend in levels over time. Levels of hsCRP and SAA were also compared with levels in a comparable population of stroke-free participants.ResultsMean age of participants with repeated measures (n = 21) was 65.6 ± 11.6 years, and 13 (61.9%) were men, and 15 (71.4%) were Hispanic. Approximately 75% of patients (n = 15) had mild strokes (NIH Stroke Scale score 0–5). There was no evidence of a time trend in levels of hsCRP, SAA, or fibrinogen for any of the markers during the 28 days of follow-up. Mean log(hsCRP) was 1.67 ± 1.07 mg/L (median hsCRP 6.48 mg/L) among stroke participants and 1.00 ± 1.18 mg/L (median 2.82 mg/L) in a group of 1176 randomly selected stroke-free participants from the same community (p = 0.0252).ConclusionLevels of hsCRP are higher in stroke patients than in stroke-free subjects. Levels of inflammatory biomarkers associated with atherosclerosis, including hsCRP, appear to be stable for at least 28 days after first ischemic stroke.

Thiopental is a Competitive Inhibitor at the Human α7 Nicotinic Acetylcholine Receptor

The nicotinic acetylcholine receptors (nAChRs) in the central nervous system may be a potential target for the anesthetic effects of thiopental. We evaluated the mechanism of action of thiopental on the human &agr;7 nAChR by using 2-electrode voltage clamp methodology. Concentration response curves for agonist were prepared in the presence of 25–250 &mgr;M of thiopental. Inhibition by the S- and R-thiopental enantiomers was compared with inhibition by racemic thiopental. We found that thiopental acts as a competitive inhibitor at the human &agr;7 nAChR. Inhibition is independent of membrane potential and the Ki(apparent) is 13 &mgr;M of thiopental. The clinical 50% effective concentration for thiopental in humans is 25 &mgr;M. Thus, with a Ki(apparent) of 13 &mgr;M, inhibition of the human &agr;7 nAChR is within a clinically relevant range. The S- and R-enantiomers of thiopental cause inhibition indistinguishable from the inhibition caused by racemic thiopental. This discordance makes it unlikely that the &agr;7 nAChR plays a role in loss of righting reflex induced by thiopental in mice, although nicotinic inhibition by thiopental may mediate other anesthetic effects and side effects.

Droperidol Inhibits GABAA and Neuronal Nicotinic Receptor Activation

Background Droperidol is used in neuroleptanesthesia and as an antiemetic. Although its antiemetic effect is thought to be caused by dopaminergic inhibition, the mechanism of droperidols anesthetic action is unknown. Because &ggr;-aminobutyric acid type A (GABAA) and neuronal nicotinic acetylcholine receptors (nAChRs) have been implicated as putative targets of other general anesthetic drugs, the authors tested the ability of droperidol to modulate these receptors. Methods &ggr;-Aminobutyric acid type A &agr;1&bgr;1&ggr;2 receptor, &agr;7 and &agr;4&bgr;2 nAChRs were expressed in Xenopus oocytes and studied with two-electrode voltage clamp recording. The authors tested the ability of droperidol at concentrations from 1 nm to 100 &mgr;m to modulate activation of these receptors by their native agonists. Results Droperidol inhibited the GABA response by a maximum of 24.7 ± 3.0%. The IC50 for inhibition was 12.6 ± 0.47 nm droperidol. At high concentrations, droperidol (100 &mgr;m) activates the GABAA receptor in the absence of GABA. Inhibition of the GABA response is significantly greater at hyperpolarized membrane potentials. The activation of the &agr;7 nAChR is also inhibited by droperidol, with an IC50 of 5.8 ± 0.53 &mgr;m. The Hill coefficient is 0.95 ± 0.1. Inhibition is noncompetitive, and membrane voltage dependence is insignificant. Conclusions Droperidol inhibits activation of both the GABAA &agr;1&bgr;1&ggr;2 and &agr;7 nAChR. The submaximal GABA inhibition occurs within a concentration range such that it might be responsible for the anxiety, dysphoria, and restlessness that limit the clinical utility of high-dose droperidol anesthesia. Inhibition of the &agr;7 nAChR might be responsible for the anesthetic action of droperidol.

Sensitivity of the α7 nicotinic acetylcholine receptor to isoflurane may depend on receptor inactivation

In previous studies, we demonstrated that nicotinic acetylcholine receptors (nAChRs) composed of the &agr;7 subunit are unaffected by the co-application of isoflurane with agonists at concentrations up to 640 &mgr;M (two times the minimum alveolar anesthetic concentration). Modulation of &agr;7-nAChR activity by isoflurane might have important behavioral ramifications because these receptors are expressed diffusely in the central and peripheral nervous systems and play pre- and postsynaptic roles in synaptic transmission. Here we have demonstrated that under some potentially physiologically relevant circumstances, the activation of &agr;7 nAChRs may be inhibited by clinically relevant concentrations of isoflurane. We evaluated isoflurane inhibition of &agr;7 nAChRs from chicks and humans expressed in Xenopus oocytes using two-electrode voltage clamp methodology. We determined the influence of time of preperfusion of isoflurane, agonist concentration, and membrane potential on inhibition by isoflurane. Both activation by a large concentration of agonist and isoflurane preperfusion increased inhibition. The half-maximal inhibitory concentration for isoflurane inhibition of chick &agr;7 nAChR with isoflurane preperfusion and activation by 100 &mgr;M of acetylcholine was 938 ± 26, and when activated by 1 mM of acetylcholine, it was 408 ± 51 &mgr;M. The increase in inhibition with isoflurane preexposure and large agonist concentration raises the possibility that isoflurane interacts preferentially with a closed or closed-desensitized state of the channel.