Multiple synaptic and membrane sites of anesthetic action in the CA1 region of rat hippocampal slices

BACKGROUND: Anesthesia is produced by a depression of central nervous system function, however, the sites and mechanisms of action underlying this depression remain poorly defined. The present study compared and contrasted effects produced by five general anesthetics on synaptic circuitry in the CA1 region of hippocampal slices. RESULTS: At clinically relevant and equi-effective concentrations, presynaptic and postsynaptic anesthetic actions were evident at glutamate-mediated excitatory synapses and at GABA-mediated inhibitory synapses. In addition, depressant effects on membrane excitability were observed for CA1 neuron discharge in response to direct current depolarization. Combined actions at several of these sites contributed to CA1 circuit depression, but the relative degree of effect at each site was different for each anesthetic studied. For example, most of propofol's depressant effect (> 70 %) was reversed with a GABA antagonist, but only a minor portion of isoflurane's depression was reversed (< 20 %). Differences were also apparent on glutamate synapses-pentobarbital depressed transmission by > 50 %, but thiopental by only < 25 %. CONCLUSIONS: These results, in as much as they may be relevant to anesthesia, indicate that general anesthetics act at several discrete sites, supporting a multi-site, agent specific theory for anesthetic actions. No single effect site (e.g. GABA synapses) or mechanism of action (e.g. depressed membrane excitability) could account for all of the effects produced for any anesthetic studied

Do Complexity Measures of Frontal EEG Distinguish Loss of Consciousness in Geriatric Patients Under Anesthesia?

While geriatric patients have a high likelihood of requiring anesthesia, they carry an increased risk for adverse cognitive outcomes from its use. Previous work suggests this could be mitigated by better intraoperative monitoring using indexes defined by several processed electroencephalogram (EEG) measures. Unfortunately, inconsistencies between patients and anesthetic agents in current analysis techniques have limited the adoption of EEG as standard of care. In attempts to identify new analyses that discriminate clinically-relevant anesthesia timepoints, we tested 1/f frequency scaling as well as measures of complexity from nonlinear dynamics. Specifically, we tested whether analyses that characterize time-delayed embeddings, correlation dimension (CD), phase-space geometric analysis, and multiscale entropy (MSE) capture loss-of-consciousness changes in EEG activity. We performed these analyses on EEG activity collected from a traditionally hard-to-monitor patient population: geriatric patients on beta-adrenergic blockade who were anesthetized using a combination of fentanyl and propofol. We compared these analyses to traditional frequency-derived measures to test how well they discriminated EEG states before and after loss of response to verbal stimuli. We found spectral changes similar to those reported previously during loss of response. We also found significant changes in 1/f frequency scaling. Additionally, we found that our phase-space geometric characterization of time-delayed embeddings showed significant differences before and after loss of response, as did measures of MSE. Our results suggest that our new spectral and complexity measures are capable of capturing subtle differences in EEG activity with anesthesia administration-differences which future work may reveal to improve geriatric patient monitoring

Electronic energy migration in Microtubules

The repeating arrangement of tubulin dimers confers great mechanical strength to microtubules, which are used as scaffolds for intracellular macromolecular transport in cells and exploited in biohybrid devices. The crystalline order in a microtubule, with lattice constants short enough to allow energy transfer between amino acid chromophores, is similar to synthetic structures designed for light harvesting. After photoexcitation, can these amino acid chromophores transfer excitation energy along the microtubule like a natural or artificial light-harvesting system? Here, we use tryptophan autofluorescence lifetimes to probe energy hopping between aromatic residues in tubulin and microtubules. By studying how the quencher concentration alters tryptophan autofluorescence lifetimes, we demonstrate that electronic energy can diffuse over 6.6 nm in microtubules. We discover that while diffusion lengths are influenced by tubulin polymerization state (free tubulin versus tubulin in the microtubule lattice), they are not significantly altered by the average number of protofilaments (13 versus 14). We also demonstrate that the presence of the anesthetics etomidate and isoflurane reduce exciton diffusion. Energy transport as explained by conventional Förster theory (accommodating for interactions between tryptophan and tyrosine residues) does not sufficiently explain our observations. Our studies indicate that microtubules are, unexpectedly, effective light harvesters

Slow GABAA mediated synaptic transmission in rat visual cortex

<p>Abstract</p> <p>Background</p> <p>Previous reports of inhibition in the neocortex suggest that inhibition is mediated predominantly through GABA_Areceptors exhibiting fast kinetics. Within the hippocampus, it has been shown that GABA_Aresponses can take the form of either fast or slow response kinetics. Our findings indicate, for the first time, that the neocortex displays synaptic responses with slow GABA_Areceptor mediated inhibitory postsynaptic currents (IPSCs). These IPSCs are kinetically and pharmacologically similar to responses found in the hippocampus, although the anatomical specificity of evoked responses is unique from hippocampus. Spontaneous slow GABA_AIPSCs were recorded from both pyramidal and inhibitory neurons in rat visual cortex.</p> <p>Results</p> <p>GABA_Aslow IPSCs were significantly different from fast responses with respect to rise times and decay time constants, but not amplitudes. Spontaneously occurring GABA_Aslow IPSCs were nearly 100 times less frequent than fast sIPSCs and both were completely abolished by the chloride channel blocker, picrotoxin. The GABA_Asubunit-specific antagonist, furosemide, depressed spontaneous and evoked GABA_Afast IPSCs, but not slow GABA_A-mediated IPSCs. Anatomical specificity was evident using minimal stimulation: IPSCs with slow kinetics were evoked predominantly through stimulation of layer 1/2 apical dendritic zones of layer 4 pyramidal neurons and across their basal dendrites, while GABA_Afast IPSCs were evoked through stimulation throughout the dendritic arborization. Many evoked IPSCs were also composed of a combination of fast and slow IPSC components.</p> <p>Conclusion</p> <p>GABA_Aslow IPSCs displayed durations that were approximately 4 fold longer than typical GABA_Afast IPSCs, but shorter than GABA_B-mediated inhibition. The anatomical and pharmacological specificity of evoked slow IPSCs suggests a unique origin of synaptic input. Incorporating GABA_Aslow IPSCs into computational models of cortical function will help improve our understanding of cortical information processing.</p

Case Reports1. A Late Presentation of Loeys-Dietz Syndrome: Beware of TGFβ Receptor Mutations in Benign Joint Hypermobility

Background: Thoracic aortic aneurysms (TAA) and dissections are not uncommon causes of sudden death in young adults. Loeys-Dietz syndrome (LDS) is a rare, recently described, autosomal dominant, connective tissue disease characterized by aggressive arterial aneurysms, resulting from mutations in the transforming growth factor beta (TGFβ) receptor genes TGFBR1 and TGFBR2. Mean age at death is 26.1 years, most often due to aortic dissection. We report an unusually late presentation of LDS, diagnosed following elective surgery in a female with a long history of joint hypermobility. Methods: A 51-year-old Caucasian lady complained of chest pain and headache following a dural leak from spinal anaesthesia for an elective ankle arthroscopy. CT scan and echocardiography demonstrated a dilated aortic root and significant aortic regurgitation. MRA demonstrated aortic tortuosity, an infrarenal aortic aneurysm and aneurysms in the left renal and right internal mammary arteries. She underwent aortic root repair and aortic valve replacement. She had a background of long-standing joint pains secondary to hypermobility, easy bruising, unusual fracture susceptibility and mild bronchiectasis. She had one healthy child age 32, after which she suffered a uterine prolapse. Examination revealed mild Marfanoid features. Uvula, skin and ophthalmological examination was normal. Results: Fibrillin-1 testing for Marfan syndrome (MFS) was negative. Detection of a c.1270G > C (p.Gly424Arg) TGFBR2 mutation confirmed the diagnosis of LDS. Losartan was started for vascular protection. Conclusions: LDS is a severe inherited vasculopathy that usually presents in childhood. It is characterized by aortic root dilatation and ascending aneurysms. There is a higher risk of aortic dissection compared with MFS. Clinical features overlap with MFS and Ehlers Danlos syndrome Type IV, but differentiating dysmorphogenic features include ocular hypertelorism, bifid uvula and cleft palate. Echocardiography and MRA or CT scanning from head to pelvis is recommended to establish the extent of vascular involvement. Management involves early surgical intervention, including early valve-sparing aortic root replacement, genetic counselling and close monitoring in pregnancy. Despite being caused by loss of function mutations in either TGFβ receptor, paradoxical activation of TGFβ signalling is seen, suggesting that TGFβ antagonism may confer disease modifying effects similar to those observed in MFS. TGFβ antagonism can be achieved with angiotensin antagonists, such as Losartan, which is able to delay aortic aneurysm development in preclinical models and in patients with MFS. Our case emphasizes the importance of timely recognition of vasculopathy syndromes in patients with hypermobility and the need for early surgical intervention. It also highlights their heterogeneity and the potential for late presentation. Disclosures: The authors have declared no conflicts of interes

The selective effects produced by anaesthetics on a single isolated neuron

Bibliography: p. 90-102

Isoflurane depresses hippocampal CA1 glutamate nerve terminals without inhibiting fiber volleys-4

<p><b>Copyright information:</b></p><p>Taken from "Isoflurane depresses hippocampal CA1 glutamate nerve terminals without inhibiting fiber volleys"</p><p>BMC Neuroscience 2006;7():5-5.</p><p>Published online 12 Jan 2006</p><p>PMCID:PMC1369004.</p><p>Copyright © 2006 Winegar and MacIver; licensee BioMed Central Ltd.</p>epresentative whole-cell perforated patch in the pyramidal cell body layer of the hippocampal CA1 region. 100 ms voltage pulses to -30 mV were applied at 10 s intervals to elicit spikes (V= -70 mV). Currents in green show four spikes recorded after a 15 min exposure to 60 nM TTX. At right are overlays of spike currents recorded before and after a 10 min exposure to 350 μM isoflurane (1 MAC). The blue traces were recorded after exposure to isoflurane

Isoflurane depresses hippocampal CA1 glutamate nerve terminals without inhibiting fiber volleys-2

<p><b>Copyright information:</b></p><p>Taken from "Isoflurane depresses hippocampal CA1 glutamate nerve terminals without inhibiting fiber volleys"</p><p>BMC Neuroscience 2006;7():5-5.</p><p>Published online 12 Jan 2006</p><p>PMCID:PMC1369004.</p><p>Copyright © 2006 Winegar and MacIver; licensee BioMed Central Ltd.</p>SP depression under each condition were almost identical. The actions of these treatments are shown for EPSP depression (), FV-EPSP curve slopes (I/O slopes; ) and paired-pulse facilitation (PPF; ). Each bar represents the mean ± standard deviation for at least five experiments from different slice preparations. Note that each treatment substantially depressed EPSPs but only isoflurane and low Caproduced a significant change in the I/O slope compared to TTX (p < 0.001 in both cases, ANOVA). PPF with isoflurane and low Cawas also significantly greater than with TTX (p < 0.001 and p < 0.001 respectively, ANOVA)

Isoflurane depresses hippocampal CA1 glutamate nerve terminals without inhibiting fiber volleys-0

<p><b>Copyright information:</b></p><p>Taken from "Isoflurane depresses hippocampal CA1 glutamate nerve terminals without inhibiting fiber volleys"</p><p>BMC Neuroscience 2006;7():5-5.</p><p>Published online 12 Jan 2006</p><p>PMCID:PMC1369004.</p><p>Copyright © 2006 Winegar and MacIver; licensee BioMed Central Ltd.</p> measurement: a minimum-to-baseline (MTB) algorithm was used to measure FV and EPSP amplitudes (dashed lines); a (slope) algorithm was used to measure the initial slope of FVs (1) and EPSPs (2). , () comparison of slope and amplitude measures of FVs as a function of stimulus voltage. Open circles are FV amplitudes measured by MTB (r= 0.99; n = 60). Green circles are FV slope measurements that were scaled to match the amplitude data (r= 0.85; n = 60). Each data set was fit by linear regression (solid lines). , () comparison of slope and amplitude measures of EPSPs as a function of stimulus voltage. Open circles are EPSP amplitudes (r= 0.97; n = 30). Blue circles are EPSP slope measurements that were scaled to match the amplitude data (r= 0.83; n = 30). Solid lines represent fits by linear regression. , () an experiment showing changes in FV amplitudes (gray circles) and scaled slopes (green triangles) as stimulus voltage was increased and decreased. EPSP amplitudes (open circles) are plotted together with scaled EPSP slopes (blue triangles). Although the stimulus voltage was changed over a 3–6 V range there was no change in facilitation (solid black line). , Isoflurane depressed EPSPs (white and gray circles) and increased facilitation (black diamonds). , Low external Ca(0.8 mM) depressed EPSPs and increased facilitation. , 60 nM TTX depressed EPSPs while facilitation remained unchanged. Note that changes in the stimulus voltage in all three experiments (appearing as ramps in EPSP amplitudes) did not affect facilitation