Seeing is Believing: Chasing Sevoflurane Vapor Trails.

PURPOSE: Sevoflurane is an inhalational general anesthetic that has been used recently to treat chronic, painful lesions, reportedly supporting analgesia and wound healing. The potential for repeated exposure to off-gassed sevoflurane vapor, especially outside the air-conditioned operating theatre environment, is of some concern. DESIGN: This paper explores the qualitative and quantitative pathing of off-gassed sevoflurane from a topically applied liquid source. METHODS: Using a small, unventilated test-box (total volume 0.5 m3) with infra-red imaging and gas-analysing, we investigated the spatial distribution of sevoflurane vapor following complete vaporization of a 20 mL liquid sample. Utilizing the infra-red absorption of sevoflurane, it was possible to visualize (as an apparent reduction in temperature) the streaming path of the sevoflurane vapor. Sevoflurane levels (%) in the test-box were measured using an infra-red gas analyzer. FINDINGS: In keeping with its higher density than air, sevoflurane vapor was seen to "waterfall" from the liquid source and accumulate in the bottom of the test-box. Sevoflurane vapor concentration was minimal above the liquid source. When extrapolated to a larger (unventilated) room, we estimate that the sevoflurane concentration would be less than 10 ppm one centimetre above the liquid pool. With vacuum extraction, these levels would be even lower. CONCLUSIONS: Due to sevoflurane's tendency to accumulate on the floor, it is concluded that topical application of liquid sevoflurane posses virtually no risk to off-gas exposure in unventilated spaces.

Determination of Krogh Coefficient for Oxygen Consumption Measurement from Thin Slices of Rodent Cortical Tissue Using a Fick's Law Model of Diffusion.

To investigate the impact of experimental interventions on living biological tissue, ex vivo rodent brain slices are often used as a more controllable alternative to a live animal model. However, for meaningful results, the biological sample must be known to be healthy and viable. One of the gold-standard approaches to identifying tissue viability status is to measure the rate of tissue oxygen consumption under specific controlled conditions. Here, we work with thin (400 µm) slices of mouse cortical brain tissue which are sustained by a steady flow of oxygenated artificial cerebralspinal fluid (aCSF) at room temperature. To quantify tissue oxygen consumption (Q), we measure oxygen partial pressure (pO2) as a function of probe depth. The curvature of the obtained parabolic (or parabola-like) pO2 profiles can be used to extract Q, providing one knows the Krogh coefficient Kt, for the tissue. The oxygen trends are well described by a Fick's law diffusion-consumption model developed by Ivanova and Simeonov, and expressed in terms of ratio (Q/K), being the rate of oxygen consumption in tissue divided by the Krogh coefficient (oxygen diffusivity × oxygen solubility) for tissue. If the fluid immediately adjacent to the tissue can be assumed to be stationary (i.e., nonflowing), one may invoke conservation of oxygen flux K·(∂P/∂x) across the interface to deduce (Kt/Kf), the ratio of Krogh coefficients for tissue and fluid. Using published interpolation formulas for the effect of salt content and temperature on oxygen diffusivity and solubility for pure water, we estimate Kf, the Krogh coefficient for aCSF, and hence deduce the Kt coefficient for tissue. We distinguish experimental uncertainty from natural biological variability by using pairs of repeated profiles at the same tissue location. We report a dimensionless Krogh ratio (Kt/Kf)=0.562±0.088 (mean ± SD), corresponding to a Krogh coefficient Kt=(1.29±0.21)×10-14 mol/(m·s·Pa) for mouse cortical tissue at room temperature, but acknowledge the experimental limitation of being unable to verify that the fluid boundary layer is truly stationary. We compare our results with those reported in the literature, and comment on the challenges and ambiguities caused by the extensive use of 'biologically convenient' non-SI units for tissue Krogh coefficient.

Application of a benchtop colorimetric method for quantification of blood propofol levels.

Quantification of plasma propofol (2,6-diisopropylphenol) in the context of clinical anaesthesia is challenging because of the need for offline blood sample processing using specialised laboratory equipment and techniques. In this study we sought to refine a simple procedure using solid phase extraction and colorimetric analysis into a benchtop protocol for accurate blood propofol measurement. The colorimetric method based on the reaction of phenols (e.g. propofol) with Gibbs reagent was first tested in 10% methanol samples (n = 50) containing 0.5-6.0 µg/mL propofol. Subsequently, whole blood samples (n = 15) were spiked to known propofol concentrations and processed using reverse phase solid phase extraction (SPE) and colorimetric analysis. The standard deviation of the difference between known and measured propofol concentrations in the methanol samples was 0.11 µg/mL, with limits of agreement of - 0.21 to 0.22 µg/mL. For the blood-processed samples, the standard deviation of the difference between known and measured propofol concentrations was 0.09 µg/mL, with limits of agreement - 0.18 to 0.17 µg/mL. Quantification of plasma propofol with an error of less than 0.2 µg/mL is achievable with a simple and inexpensive benchtop method.

A Metabolic Mechanism for Anaesthetic Suppression of Cortical Synaptic Function in Mouse Brain Slices-A Pilot Investigation.

Regulation of synaptically located ionotropic receptors is thought to be the main mechanism by which anaesthetics cause unconsciousness. An alternative explanation, which has received much less attention, is that of primary anaesthetic disruption of brain metabolism via suppression of mitochondrial proteins. In this pilot study in mouse cortical slices, we investigated the effect of disrupting cellular metabolism on tissue oxygen handling and cortical population seizure-like event (SLE) activity, using the mitochondrial complex I inhibitor rotenone, and compared this to the effects of the general anaesthetics sevoflurane, propofol and ketamine. Rotenone caused an increase in tissue oxygen (98 mmHg to 157 mmHg (p < 0.01)) before any measurable change in SLE activity. Thereafter, tissue oxygen continued to increase and was accompanied by a significant and prolonged reduction in SLE root mean square (RMS) activity (baseline RMS of 1.7 to 0.7 µV, p < 0.001) and SLE frequency (baseline 4.2 to 0.4 events/min, p = 0.001). This temporal sequence of effects was replicated by all three anaesthetic drugs. In conclusion, anaesthetics with differing synaptic receptor mechanisms all effect changes in tissue oxygen handling and cortical network activity, consistent with a common inhibitory effect on mitochondrial function. The temporal sequence suggests that the observed synaptic depression-as seen in anaesthesia-may be secondary to a reduction in cellular metabolic capacity.

Structure-Activity Relationships for the Anaesthetic and Analgaesic Properties of Aromatic Ring-Substituted Ketamine Esters.

A series of benzene ring substituted ketamine N-alkyl esters were prepared from the corresponding substituted norketamines. Few of the latter have been reported since they have not been generally accessible via known routes. We report a new general route to many of these norketamines via the Neber (oxime to α-aminoketone) rearrangement of readily available substituted 2-phenycyclohexanones. We explored the use of the substituents Cl, Me, OMe, CF3, and OCF3, with a wide range of lipophilic and electronic properties, at all available benzene ring positions. The 2- and 3-substituted compounds were generally more active than 4-substituted compounds. The most generally acceptable substituent was Cl, while the powerful electron-withdrawing substituents CF3 and OCF3 provided fewer effective analogues.

Transcriptional changes in response to ketamine ester-analogs SN 35210 and SN 35563 in the rat brain.

BACKGROUND: Ketamine ester analogs, SN 35210 and SN 35563, demonstrate different pharmacological profiles to ketamine in animal models. Both confer hypnosis with predictably rapid offset yet, paradoxically, SN35563 induces a prolonged anti-nociceptive state. To explore underlying mechanisms, broad transcriptome changes were measured and compared across four relevant target regions of the rat brain. RESULTS: SN 35563 produced large-scale alteration of gene expression in the Basolateral Amygdala (BLA) and Paraventricular Nucleus of the Thalamus (PVT), in excess of 10x that induced by ketamine and SN 35210. A smaller and quantitatively similar number of gene changes were observed in the Insula (INS) and Nucleus Accumbens (ACB) for all three agents. In the BLA and PVT, SN 35563 caused enrichment for gene pathways related to the function and structure of glutamatergic synapses in respect to: release of neurotransmitter, configuration of postsynaptic AMPA receptors, and the underlying cytoskeletal scaffolding and alignment. CONCLUSION: The analgesic ketamine ester analog SN 35563 induces profound large-scale changes in gene expression in key pain-related brain regions reflecting its unique prolonged pharmacodynamic profile.

Understanding the Effects of General Anesthetics on Cortical Network Activity Using Ex Vivo Preparations.

General anesthetics have been used to ablate consciousness during surgery for more than 150 yr. Despite significant advances in our understanding of their molecular-level pharmacologic effects, comparatively little is known about how anesthetics alter brain dynamics to cause unconsciousness. Consequently, while anesthesia practice is now routine and safe, there are many vagaries that remain unexplained. In this paper, the authors review the evidence that cortical network activity is particularly sensitive to general anesthetics, and suggest that disruption to communication in, and/or among, cortical brain regions is a common mechanism of anesthesia that ultimately produces loss of consciousness. The authors review data from acute brain slices and organotypic cultures showing that anesthetics with differing molecular mechanisms of action share in common the ability to impair neurophysiologic communication. While many questions remain, together, ex vivo and in vivo investigations suggest that a unified understanding of both clinical anesthesia and the neural basis of consciousness is attainable.

Ketamine esters and amides as short-acting anaesthetics: Structure-activity relationships for the side-chain.

N-Aliphatic ester analogues of the non-opioid ketamine (1) retain effective anaesthetic/analgesic properties while minimising ketamine's psychomimetic side-effects. We show that the anaesthetic/analgesic properties of these ester analogues depend critically on the length (from 2 to 4 carbons), polarity and steric cross-section of the aliphatic linker chain. More stable amide and ethylsulfone analogues generally showed weaker anaesthetic/analgesic activity. There was no correlation between the anaesthetic/analgesic properties of the compounds and their binding affinities for the N-methyl-d-aspartate (NMDA) receptor.

Transient electroencephalographic alpha power loss during maintenance of general anaesthesia.

BACKGROUND: EEG activity in the extended alpha frequency range (7-17 Hz) during maintenance of general anaesthesia is primarily determined by effect-site concentrations of the hypnotic and analgesic drugs used. Intermittent alpha loss during surgery, unexplained by changes in anaesthetic or opioid concentrations, could represent arousal of the cortex as a result of increased surgical stimulation. METHODS: A generalised linear model was fitted to alpha power recorded from patients undergoing general anaesthesia from induction until waking using three explanatory variables: age-adjusted volatile anaesthetic effect-site concentration, and estimated effect-site propofol and opioid concentrations. Model residuals were decomposed into uncorrelated white noise and a fluctuating auto-correlated trend. Deviations of this local trend were classified as 'unexpected alpha dropout events'. To investigate whether these alpha dropouts might be explained by the effect of noxious stimulation, we related their occurrence to whether a patient was undergoing surgery involving the body cavity or not. RESULTS: Alpha power dropouts occurred in 73 of the 237 patients included in the final analysis (31%, median amplitude of -3.5 dB, duration=103 s). They showed a bimodal or broadly skewed distribution, being more probable soon after initial incision (32%), dropping to around 10% at 1 h, and then again increasing to >30% in operations lasting >3 h. Multivariate analysis showed that alpha dropouts were significantly associated with body cavity surgery (P=0.003) and with longer operations (P<0.001). CONCLUSIONS: A loss of alpha power, unexplained by changes in anaesthetic or opioid concentrations, is suggestive of thalamocortical depolarisation induced by body cavity noxious stimuli, and could provide a measure of nociception during surgery.

Ketamine Action in the In Vitro Cortical Slice Is Mitigated by Potassium Channel Blockade.

BACKGROUND: Ketamine is a general anesthetic thought to act by antagonizing N-methyl-D-aspartate receptors. However, ketamine acts on multiple channels, many of which are potential targets-including hyperpolarization-activated cyclic nucleotide-gated and potassium channels. In this study we tested the hypothesis that potassium leak channels contribute to the anesthetic action of ketamine. METHODS: Adult mouse cortical slices (400 µm) were exposed to no-magnesium artificial cerebrospinal fluid to generate seizure-like event activity. The reduction in seizure-like event frequency after exposure to ketamine (n = 14) was quantified as a signature of anesthetic effect. Pharmacologic manipulation of hyperpolarization-activated cyclic nucleotide-gated and potassium channels using ZD7288 (n = 11), cesium chloride (n = 10), barium chloride (n = 10), low-potassium (1.5 mM) artificial cerebrospinal fluid (n = 10), and urethane (n = 7) were investigated. RESULTS: Ketamine reduced the frequency of seizure-like events (mean [SD], -62 [22]%, P < 0.0001). Selective hyperpolarization-activated cyclic nucleotide-gated channel block with ZD7288 did not significantly alter the potency of ketamine to inhibit seizure-like event activity. The inhibition of seizure-like event frequency by ketamine was fully antagonized by the potassium channel blockers cesium chloride and barium chloride (8 [26]% and 39 [58%] increase, respectively, P < 0.0001, compared to ketamine control) and was facilitated by the potassium leak channel opener urethane (-93 [8]%, P = 0.002 compared to ketamine control) and low potassium artificial cerebrospinal fluid (-86 [11]%, P = 0.004 compared to ketamine control). CONCLUSIONS: The results of this study show that mechanisms additional to hyperpolarization-activated cyclic nucleotide-gated channel block are likely to explain the anesthetic action of ketamine and suggest facilitatory action at two-pore potassium leak channels.

Electromyographic activation reveals cortical and sub-cortical dissociation during emergence from general anesthesia.

During emergence from anesthesia patients regain their muscle tone (EMG). In a typical population of surgical patients the actual volatile gas anesthetic concentrations in the brain (CeMAC) at which EMG activation occurs remains unknown, as is whether EMG activation at higher CeMACs is correlated with subsequent severe pain, or with cortical activation. Electroencephalographic (EEG) and EMG activity was recorded from the forehead of 273 patients emerging from general anesthesia following surgery. We determined CeMAC at time of EMG activation and at return of consciousness. Pain was assessed immediately after return of consciousness using an 11 point numerical rating scale. The onset of EMG activation during emergence was associated with neither discernible muscle movement nor with the presence of exogenous stimulation in half the patients. EMG activation could be modelled as two distinct processes; termed high- and low-CeMAC (occurring higher or lower than 0.07 CeMAC). Low-CeMAC activation was typically associated with simultaneous EMG activation and consciousness, and the presence of a laryngeal mask. In contrast, high-CeMAC EMG activation occurred independently of return of consciousness, and was not associated with severe post-operative pain, but was more common in the presence of an endotracheal tube. Patients emerging from general anesthesia with an endotracheal tube in place are more likely to have an EMG activation at higher CeMAC concentrations. These activations are not associated with subsequent high-pain, nor with cortical arousal, as evidenced by continuing delta waves in the EEG. Conversely, patients emerging from general anesthesia with a laryngeal mask demonstrate marked neural inertia-EMG activation occurs at a low CeMAC, and is closely temporally associated with return of consciousness.

A comparison of different synchronization measures in electroencephalogram during propofol anesthesia.

Electroencephalogram (EEG) synchronization is becoming an essential tool to describe neurophysiological mechanisms of communication between brain regions under general anesthesia. Different synchronization measures have their own properties to reflect the changes of EEG activities during different anesthetic states. However, the performance characteristics and the relations of different synchronization measures in evaluating synchronization changes during propofol-induced anesthesia are not fully elucidated. Two-channel EEG data from seven volunteers who had undergone a brief standardized propofol anesthesia were then adopted to calculate eight synchronization indexes. We computed the prediction probability (P K ) of synchronization indexes with Bispectral Index (BIS) and propofol effect-site concentration (C eff ) to quantify the ability of the indexes to predict BIS and C eff . Also, box plots and coefficient of variation were used to reflect the different synchronization changes and their robustness to noise in awake, unconscious and recovery states, and the Pearson correlation coefficient (R) was used for assessing the relationship among synchronization measures, BIS and C eff . Permutation cross mutual information (PCMI) and determinism (DET) could predict BIS and follow C eff better than nonlinear interdependence (NI), mutual information based on kernel estimation (KerMI) and cross correlation. Wavelet transform coherence (WTC) in α and ß frequency bands followed BIS and C eff better than that in other frequency bands. There was a significant decrease in unconscious state and a significant increase in recovery state for PCMI and NI, while the trends were opposite for KerMI, DET and WTC. Phase synchronization based on phase locking value (PSPLV) in δ, θ, α and γ1 frequency bands dropped significantly in unconscious state, whereas it had no significant synchronization in recovery state. Moreover, PCMI, NI, DET correlated closely with each other and they had a better robustness to noise and higher correlation with BIS and C eff than other synchronization indexes. Propofol caused EEG synchronization changes during the anesthetic period. Different synchronization measures had individual properties in evaluating synchronization changes in different anesthetic states, which might be related to various forms of neural activities and neurophysiological mechanisms under general anesthesia.

Multi-scale sample entropy of electroencephalography during sevoflurane anesthesia.

The electroencephalogram (EEG) has been widely applied in the assessment of depth of anesthesia (DoA). Recent research has found that multi-scale EEG analysis describes brain dynamics better than traditional non-linear methods. In this study, we have adopted a modified sample entropy (MSpEn) method to analyze anesthetic EEG series as a measure of DoA. EEG data from a previous study consisting of 19 adult subjects undergoing sevoflurane anesthesia were used in the present investigation. In addition to the modified sample entropy method, the well-established EEG indices approximate entropy (ApEn), response entropy (RE), and state entropy (SE) were also computed for comparison. Pharmacokinetic/pharmacodynamic modeling and prediction probability (P k ) were used to assess and compare the performance of the four methods for tracking anesthetic concentration. The influence of the number of scales on MSpEn was also investigated using a linear regression model. MSpEn correlated closely with anesthetic effect. The P k (0.83 ± 0.05, mean ± SD) and the coefficient of determination R (2) (0.87 ± 0.21) for the relationship between MSpEn and sevoflurane effect site concentration were highest for MSpEn (P k : RE = 0.73 ± 0.08, SE = 0.72 ± 0.07, ApEn = 0.81 ± 0.04; R (2): RE = 0.75 ± 0.08, SE = 0.64 ± 0.09, ApEn = 0.81 ± 0.10). Scales 1, 3 and 5 tended to make the greatest contribution to MSpEn. For this data set, the MSpEn is superior to the ApEn, the RE and the SE for tracking drug concentration change during sevoflurane anesthesia. It is suggested that the MSpEn may be further studied for application in clinical monitoring of DoA.

Preliminary pharmacokinetics and patient experience of jet-injected dexmedetomidine in healthy adults.

Jet injection is a drug delivery system without a needle. A compressed liquid drug formulation pierces the skin, depositing the drug into the subcutaneous or intramuscular tissues. We investigated the pharmacokinetics and patient experience of dexmedetomidine administered using jet injection in six healthy adult study participants. This needleless jet injection device was used to administer dexmedetomidine 0.5 µg/kg to the subcutaneous tissues overlying the deltoid muscle. Serum concentrations of dexmedetomidine were assayed at approximately 5 minutes, 15 minutes, 30 minutes, 1 hour and 4 hours after administration. Pharmacokinetic interrogation of concentration time profiles estimated an absorption half time for jet-injected dexmedetomidine of 21 minutes (coefficient of variation 69.4%) with a relative bioavailability assumed unity. In our samples the measured median peak (range) concentration was 0.164 µg/l (0.011-0.325 µg/l), observed in the sample taken at a median (range) of 13.5 minutes (11-30 minutes). The Richmond agitation sedation scale was used to assess the sedative effect, and scored 0 (alert and calm) or -1 (drowsy) in all participants. Five of the six participants stated they would prefer jet injection to needle injection in the future and one had no preference. The findings suggest that the use of a larger dose (>2 µg/kg) would be required to achieve the clinically relevant target concentration of 1 µg/l necessary to achieve deeper sedation (Richmond agitation sedation scale ≤3).

Quantitative investigation into methods for evaluating neocortical slice viability.

BACKGROUND: In cortical and hippocampal brain slice experiments, the viability of processed tissue is usually judged by the amplitude of extracellularly-recorded seizure-like event (SLE) activity. Surprisingly, the suitability of this approach for evaluating slice quality has not been objectively studied. Furthermore, a method for gauging the viability of quiescent tissue, in which SLE activity is intentionally suppressed, has not been documented. In this study we undertook to address both of these matters using the zero-magnesium SLE model in neocortical slices. METHODS: Using zero-magnesium SLE activity as the output parameter, we investigated: 1) changes in the pattern (amplitude, frequency and length) of SLE activity as slice health either deteriorated; or was compromised by altering the preparation methodology and; 2) in quiescent tissue, whether the triggering of high frequency field activity following electrode insertion predicted subsequent development of SLE activity - and hence slice viability. RESULTS: SLE amplitude was the single most important variable correlating with slice viability, with a value less than 50 µV indicative of tissue unlikely to be able to sustain population activity for more than 30-60 minutes. In quiescent slices, an increase in high frequency field activity immediately after electrode insertion predicted the development of SLE activity in 100% of cases. Furthermore, the magnitude of the increase in spectral power correlated with the amplitude of succeeding SLE activity (R2 40.9%, p < 0.0001). CONCLUSION: In conclusion, the findings confirm that the amplitude of population activity is a suitable field potential parameter for judging brain slice viability - and can be applied independent of the mechanism of tissue activation.

Effects of volatile anesthetic agents on cerebral cortical synchronization in sheep.

BACKGROUND: The exact neurophysiological mechanisms of anesthetic-induced unconsciousness are not yet fully elucidated. The cortical information integration theory hypothesizes that loss of consciousness during general anesthesia is associated with breakdown of long-distance cortical connectivity across multiple brain regions. However, what is the effect of anesthetics on neural activities at a smaller spatial scale? METHODS: The authors analyzed a set of previously published eight-channel electrocorticogram data, obtained from a 14-mm-long linear array of electrodes in eight adult merino sheep during general anesthesia induced by sevoflurane, desflurane, isoflurane, and enflurane. The S-estimator was applied to the bi-channel coherence matrix to construct an overall index called the SI, which is the entropy of the eigenvalues of the cortical coherence for each pair of channels within the multichannel electrocorticographic dataset. RESULTS: The SI values increased ~30-50% from the waking to the burst-suppression states, and returned to baseline during recovery. The anesthetic-induced increase in synchrony was most marked in the α (8-13 Hz) and ß (13-30 Hz) frequency bands (P<0.05). Using prediction probability (PK) analysis, we found a significant correlation between the increase in spatial synchrony (as estimated by the SI at various frequency bands) and anesthetic-induced cortical depression (as estimated by the approximate entropy). CONCLUSIONS: The results suggest that it is feasible to use the SI to measure cortical synchrony, and over a local spatial scale of 2-14 mm, synchrony increased during general anesthesia.

Structure-activity relationships for ketamine esters as short-acting anaesthetics.

A series of aliphatic esters of the non-opioid anaesthetic/analgesic ketamine were prepared and their properties as shorter-acting analogues of ketamine itself were explored in an infused rat model, measuring the time after infusion to recover from both the anaesthetic (righting reflex) and analgesic (response to stimulus) effects. The potency of the esters as sedatives was not significantly related to chain length, but Me, Et and i-Pr esters were the more dose potent (up to twofold less than ketamine), whereas n-Pr esters were less potent (from 2- to 6-fold less than ketamine). For the Me, Et and i-Pr esters recovery from anaesthesia was 10-15-fold faster than from ketamine itself, and for the n-Pr esters it was 20-25-fold faster than from ketamine. A new dimethylamino ketamine derivative (homoketamine) had ketamine-like sedative effects but was slightly less potent than, but ester analogues of homoketamine had very weak sedative effects.

Tissue oxygen partial pressure as a viability metric for ex vivo brain tissue slices.

BACKGROUND: Despite the prevalent use of the ex vivo brain slice preparation in neurophysiology research, a reliable method for judging tissue viability - and thus suitability of a slice for inclusion in an experiment - is lacking. The utility of indirect electrophysiological measures of tissue health is model-specific and needs to be used cautiously. In this study, we verify a more direct test of slice viability, based on tissue oxygen consumption rate. NEW METHOD: We hypothesised that the minimum intra-slice partial pressure of oxygen (pO2min) would correlate with tissue oxygen consumption rate, providing an accessible method for reliably assessing tissue viability status. Using mouse brain cortex slices, we measured tissue oxygen consumption rate using a Fick's law diffusion-consumption model applied to full intra-tissue pO2 profiles and compared this to pO2min and 2,3,5-triphenol tetrazolium chloride (TTC) viability staining. RESULTS: Tissue pO2min correlated strongly with oxygen consumption rate in both neurophysiological active and quiescent tissue (in "no-magnesium" and "normal" artificial cerebrospinal fluid, respectively) (R2 =49.7% and 42.1%, respectively). Both correlated with TTC viability stain. Oxygen consumption rate was positively related to the frequency of seizure-like event activity in no-magnesium artificial cerebrospinal fluid (R2 = 44.8%). COMPARISON WITH EXISTING METHODS: While measurement of tissue oxygen levels and oxygen consumption is not new, intra-tissue pO2min is a novel approach to assess brain slice viability. CONCLUSION: The results confirm that tissue oxygen minimum pO2min is a robust metric for estimating tissue viability status - the lower the pO2min, the healthier the tissue.

A systematic exploration of local network state space in neocortical mouse brain slices.

The ex vivo cortical slice is an extremely versatile preparation, but its utility ultimately depends on understanding its limitations and functional constraints. A question for experimentalists new to the field of cortical slice electrophysiology might be - what are the different network dynamical states available to a cortical slice as a function of excitatory drive? The purpose of this study is to provide a coherent answer to this question, within the context of extracellularly recorded population field potentials. Cortical slices (400 µm) were prepared from adult male or female C57 mice. Evoked responses were recorded within cortical layer III/IV using extracellularly positioned metal electrodes. In the first part of the study, slice excitatory drive was increased by reducing the concentration of magnesium ions in the artificial cerebrospinal fluid - and the evoked responses categorized during the transition. In the second part, each of the identified functional states were explored in greater detail with tissue perfusion conditions and excitatory drive optimised for the requisite response state. As expected, rodent cortical slices did not generate spontaneous, persistent EEG-like field potential activity. However, distinct response states (spontaneous and evoked) characterized by intermittent population bursts could be differentiated as a function of excitatory drive. Each state reflected different modes of neocortical activation: "monosynaptic" responses were brief, non-propagating activations, reflecting an inhibited cortex with sensory processing blocked; polysynaptic and epileptiform activity propagated intra-cortically, the latter reflecting a hyperactivated, hypersynchronous "seizing" cortex. Polysynaptic activity most closely resembled sensory "up states" associated with intracortical sensory processing. Understanding the functional distinction between the different cortical slice response states is the starting point for designing experiments that maximise the utility of this ex vivo model. The results and descriptions in this study should help slice experimentalists less experienced in the nuances of cortical slice neurophysiology to make informed choices about how to tailor the parameters of the model to suit the specific aims of their research.