Isoflurane prevents transitions to tonic and burst firing modes in thalamic neurons

General anesthesia alters CNS and astrocyte expression of activity-dependent and activity-independent genes

General anesthesia represents a common clinical intervention and yet can result in long-term adverse CNS effects particularly in the elderly or dementia patients. Suppression of cortical activity is a key feature of the anesthetic-induced unconscious state, with activity being a well-described regulator of pathways important for brain health. However, the extent to which the effects of anesthesia go beyond simple suppression of neuronal activity is incompletely understood. We found that general anesthesia lowered cortical expression of genes induced by physiological activity in vivo, and recapitulated additional patterns of gene regulation induced by total blockade of firing activity in vitro, including repression of neuroprotective genes and induction of pro-apoptotic genes. However, the influence of anesthesia extended beyond that which could be accounted for by activity modulation, including the induction of non activity-regulated genes associated with inflammation and cell death. We next focused on astrocytes, important integrators of both neuronal activity and inflammatory signaling. General anesthesia triggered gene expression changes consistent with astrocytes being in a low-activity environment, but additionally caused induction of a reactive profile, with transcriptional changes enriched in those triggered by stroke, neuroinflammation, and Aß/tau pathology. Thus, while the effects of general anesthesia on cortical gene expression are consistent with the strong repression of brain activity, further deleterious effects are apparent including a reactive astrocyte profile.

Characterization of Active Electrode Yield for Intracortical Arrays: Awake versus Anesthesia

Intracortical microelectrode arrays are used for recording neural signals at single-unit resolution and are promising tools for studying brain function and developing neuroprosthetics. Research is being done to increase the chronic performance and reliability of these probes, which tend to decrease or fail within several months of implantation. Although recording paradigms vary, studies focused on assessing the reliability and performance of these devices often perform recordings under anesthesia. However, anesthetics—such as isoflurane—are known to alter neural activity and electrophysiologic function. Therefore, we compared the neural recording performance under anesthesia (2% isoflurane) followed by awake conditions for probes implanted in the motor cortex of both male and female Sprague-Dawley rats. While the single-unit spike rate was significantly higher by almost 600% under awake compared to anesthetized conditions, we found no difference in the active electrode yield between the two conditions two weeks after surgery. Additionally, the signal-to-noise ratio was greater under anesthesia due to the noise levels being nearly 50% greater in awake recordings, even though there was a 14% increase in the peak-to-peak voltage of distinguished single units when awake. We observe that these findings are similar for chronic time points as well. Our observations indicate that either anesthetized or awake recordings are acceptable for studies assessing the chronic reliability and performance of intracortical microelectrode arrays.

Monochromatic Blue Light Activates Suprachiasmatic Nucleus Neuronal Activity and Promotes Arousal in Mice Under Sevoflurane Anesthesia

Background: Monochromatic blue light (MBL), with a wavelength between 400–490 nm, can regulate non-image-forming (NIF) functions of light in the central nervous system. The suprachiasmatic nucleus (SCN) in the brain is involved in the arousal-promoting response to blue light in mice. Animal and human studies showed that the responsiveness of the brain to visual stimuli is partly preserved under general anesthesia. Therefore, this study aimed to investigate whether MBL promotes arousal from sevoflurane anesthesia via activation of the SCN in mice.

Methods: The induction and emergence time of sevoflurane anesthesia under MBL (460 nm and 800 lux) exposure was measured. Cortical electroencephalograms (EEGs) were recorded and the burst-suppression ratio (BSR) was calculated under MBL during sevoflurane anesthesia. The EEGs and local field potential (LFP) recordings with or without locally electrolytic ablated bilateral SCN were used to further explore the role of SCN in the arousal-promoting effect of MBL under sevoflurane anesthesia. Immunofluorescent staining of c-Fos was conducted to reveal the possible downstream mechanism of SCN activation.

Results: Unlike the lack of effect on the induction time, MBL shortened the emergence time and the EEG recordings showed cortical arousal during the recovery period. MBL resulted in a significant decrease in BSR and a marked increase in EEG power at all frequency bands except for the spindle band during 2.5% sevoflurane anesthesia. MBL exposure under sevoflurane anesthesia enhances the neuronal activity of the SCN. These responses to MBL were abolished in SCN lesioned (SCNx) mice. MBL evoked a high level of c-Fos expression in the prefrontal cortex (PFC) and lateral hypothalamus (LH) compared to polychromatic white light (PWL) under sevoflurane anesthesia, while it exerted no effect on c-Fos expression in the ventrolateral preoptic area (VLPO) and locus coeruleus (LC) c-Fos expression.

Conclusions: MBL promotes behavioral and electroencephalographic arousal from sevoflurane anesthesia via the activation of the SCN and its associated downstream wake-related nuclei. The clinical implications of this study warrant further study.

Hypersensitivity of the anesthesia-induced comatose brain

Increasing levels of anesthesia are thought to produce a progressive loss of brain responsiveness to external stimuli. Here, we present the first report of a state window within anesthesia-induced coma, usually associated with an EEG pattern of burst suppression, during which brain excitability is dramatically increased so that even subliminal stimuli elicit bursts of whole-brain activity. We investigated this phenomenon in vivo using intracellular recordings of both neurons and glia, as well as extracellular calcium and EEG recordings. The results indicate that the bursting activity elicited with mechanical microstimulations, but also with auditory and visual stimuli, is dependent on complex mechanisms, including modulation of excitatory (NMDA) components, gap junction transmission, as well as the extracellular calcium concentration. The occurrence of bursting events is associated with a postburst refractory period that underlies the genesis of the alternating burst-suppression pattern. These findings raise the issue of what burst spontaneity during anesthesia-induced coma means and opens new venues for the handling of comatose patients.

Prolonged sedation with propofol in the rat does not result in sleep deprivation

The use of propofol provides sedation without prolonging emergence in patients in the Intensive Care Unit. When prolonged, however, continuous sedation may overlap with naturally occurring sleep periods and potentially increase the risk of sleep deprivation. We modified an established rat model of sleep to determine whether prolonged, continuous sedation results in sleep deprivation. Rats were continuously sedated for a 12-h period overlapping completely with their normal sleep phase. Electroencephalogram (EEG) and movement data were collected before and after the sedation period. Rats were evaluated for EEG and movement evidence of sleep deprivation after sedation. When compared with baseline, the time spent in rapid eye movement (REM) and non-REM sleep was decreased during the first 4 h after sedation. The duration of non-REM sleep bouts was not altered. Power in the delta band (0.5-4 Hz) during non-REM sleep was diminished during the first 2 h only. Movements were reduced during the first hour after emergence from sedation only. In summary, no EEG or behavioral evidence of sleep deprivation was observed on emergence from sedation. These results imply that sedation is associated with a restorative process reversing the natural accumulation of sleep need that occurs during wakefulness.

IMPLICATIONS

Prolonged sedation in the Intensive Care Unit may alter the restorative effects of naturally occurring sleep. We sedated rats during their sleep phase to determine whether sedation interferes with sleep. Upon emergence, no evidence of sleep deprivation was observed. Sedation may thus be associated with a restorative effect similar to sleep.

Is anesthesia caused by potentiation of synaptic or intrinsic inhibition? Recent insights into the mechanisms of volatile anesthetics

Volatile anesthetics modulate synaptic (GABAA receptor-mediated) and intrinsic (K+ channel-controlled) neuronal inhibition. GABAA receptor activity is enhanced, leading to increased charge transfer and prolonged synaptic inhibition, and members of the two pore domain family of potassium channels are activated, leading to neuronal hyperpolarization and reduced excitability. These effects may underlie different components of the complex anesthetic state.

Isoflurane induces dose-dependent changes of thalamic somatosensory information transfer

In spite of several reports about suppressive effects of volatile anesthetics on somatosensation, their neuronal mechanisms are largely unknown. The present study investigates somatosensory impulse transmission at the thalamic level in rats under varied concentrations of isoflurane by recordings of neuronal responses to mechanical stimulation of the body surface. Single-unit recordings of thalamo-cortical relay neurons (TCNs, third order neurons; n=28) and presumed trigemino-thalamic fibers (TTFs, second order neurons; n=7) were performed in the ventral posteromedial nucleus. Functional response characteristics were quantified following defined tactile stimulation (trapezoidal or vibratory deflection of sinus hairs or fur) applied to the neuronal receptive fields. End-tidal isoflurane concentration was increased in steps of 0.2% between 0.6% (baseline) and 2.0%. The response activity in all TCNs studied was suppressed in a dose-dependent manner (2.0% isoflurane decreased responses to 3. 5+/-1.1% of baseline; mean+/-S.E.M.); the response activity in TTFs was much less affected (decrease to 55.0+/-8.2%). Suppression of ongoing activity, however, was similar for both, TCNs and TTFs. Furthermore, in TCNs, the response characteristics changed with increasing isoflurane between 1.0% and 1.8%: tonic and sustained responses were converted to phasic on-responses. In contrast, the tonic and sustained response characteristics of TTFs were preserved even at higher isoflurane concentrations. The results indicate that isoflurane attenuates the output of somatosensory signals in the specific nucleus of the rat's thalamus, while its input is only marginally affected. The observed changes of thalamic neuronal response characteristics, at least in part, may cause the loss in sensory discrimination observed during general anesthesia.

Mechanism of anesthesia revealed by shunting actions of isoflurane on thalamocortical neurons

By using thalamic brain slices from juvenile rats and the whole cell recording technique, we determined the effects of aqueous applications of the anesthetic isoflurane (IFL) on tonic and burst firing activities of ventrobasal relay neurons. At concentrations equivalent to those used for in vivo anesthesia, IFL induced a hyperpolarization and increased membrane conductance in a reversible and concentration-dependent manner (ionic mechanism detailed in companion paper). The increased conductance short-circuited the effectiveness of depolarizing pulses and was the main cause for inhibition of tonic firing of action potentials. Despite the IFL-induced hyperpolarization, which theoretically should have promoted bursting, the shunt blocked the low-threshold Ca2+ spike (LTS) and associated burst firing of action potentials as well as the high-threshold Ca2+ spike (HTS). Increasing the amplitude of either the depolarizing test pulse or hyperpolarizing prepulse or increasing the duration of the hyperpolarizing prepulse partially reversed the blockade of the LTS burst. In voltage-clamp experiments on the T-type Ca2+ current, which produces the LTS, IFL decreased the spatial distribution of imposed voltages and hence impaired the activation of spatially distant T channels. Although IFL may have increased a dendritic leak conductance or decreased dendritic Ca2+ currents, the somatic shunt appeared to block initiation of the LTS and HTS as well as their electrotonic propogation to the axon hillock. In summary, IFL hyperpolarized thalamocortical neurons and shunted voltage-dependent Na+ and Ca2+ currents. Considering the importance of the thalamus in relaying different sensory modalities (i.e., somatosensation, audition, and vision) and motor information as well as the corticothalamocortical loops in mediating consciousness, the shunted firing activities of thalamocortical neurons would be instrumental for the production of anesthesia in vivo.

Volatile anesthetic sensitivity of T-type calcium currents in various cell types

We evaluated the effects of volatile anesthetics on T-type calcium current (ICa,T) present in four different cell types using the whole cell version of the patch clamp technique. In dorsal root ganglion neurons and in two neuroendocrine cells--adrenal glomerulosa cells (AG) and thyroid C-cells--ICa,T was reversibly decreased by volatile anesthetics at clinically relevant concentrations, with isoflurane and enflurane being more potent that halothane. In AG cells, the most sensitive cell type tested, ICa,T was reduced 47%+/-4% (n = 6) by isoflurane (0.7 mM) and 56%+/-2% (n = 5) by enflurane (1.2 mM), but by only 24%+/-1% (n = 5; P < 0.05) by halothane (0.7 mM). Isoflurane caused a significant increase in the rate of deactivation of ICa,T in AG cells. In ventricular myocytes, however, ICa,T was much less sensitive to both isoflurane and halothane. The differential sensitivity of ICa,T in various cell types to the anesthetics may reflect differences in the channels expressed in these tissues or differences in the cellular intermediates involved in anesthetic action. Depression of ICa,T in neuronal cells may contribute to anesthetic action through decreases in cellular excitability.

IMPLICATIONS

Using the patch clamp technique, we showed that T-type calcium channels, which promote cellular excitability, are inhibited by volatile anesthetics in neuronal and neuroendocrine cells, but not in ventricular myocytes. Inhibition of neuronal T-type channels may contribute to the mechanism of action of volatile anesthetics.

Analgesic and sedative concentrations of lignocaine shunt tonic and burst firing in thalamocortical neurones

The effects of lignocaine [lidocaine] HCl (0.6 microM(-1) mM) on the membrane electrical properties and action potential firing of neurones of the ventral posterolateral (VPL) nucleus of the thalamus were investigated using whole cell recording techniques in rat brain slices in vitro. Bath application of lignocaine reversibly decreased the input resistance (Ri) of VPL neurones. This effect was observed at low, clinically sedative and analgesic concentrations (i.e., maximal amplitude at 10 microM) whereas higher concentrations (300 microM(-1) mM) had no effect on Ri. Lignocaine (10-100 microM) depolarized VPL neurones up to 14 mV in a reversible manner. Consistent with a decreased Ri, low concentrations of lignocaine shunted the current required for spike generation in the tonic pattern. Lignocaine increased the threshold amplitude of current required for firing and decreased the tonic firing frequency, without concomitant elevation of the voltage threshold for firing or reduction in the maximal rate of rise (dV/dt(max)) of spikes. Low concentrations of lignocaine shunted low threshold spike (LTS) burst firing evoked either from hyperpolarized potentials or as rebound bursts on depolarization from prepulse-conditioned potentials. Higher concentrations of lignocaine (300 microM - 1 mM), not associated with a decrease in Ri, elevated the voltage threshold for firing and reduced the dV/dt(max) of spikes in a concentration-dependent fashion. In conclusion, low concentrations of lignocaine shunted tonic and burst firing in VPL neurones by decreasing Ri, a mechanism not previously described for local anaesthetics in the CNS. We suggest that a decreased resistance in thalamocortical neurones contributes to the sedative, analgesic, and anaesthetic properties of systemic lignocaine in vivo.

Enhanced isoflurane suppression of excitatory synaptic transmission in the aged rat hippocampus

1. The effects of the volatile anaesthetic, isoflurane, were investigated on evoked dendritic field excitatory postsynaptic potentials (f.e.p.s.p.) and antidromic and orthodromic population spikes recorded extracellularly in the CA1 cell layer region in the in vitro hippocampal slice taken from young mature (2-3 months) and old (24-27 months) Fisher 344 rats. 2. Isoflurane depressed the f.e.p.s.ps and the orthodromically-evoked population spikes in both old and young hippocampi. However, the magnitude of the anaesthetic-induced depression was greater in slices taken from old rats compared to those taken from young rats during the application of different isoflurane concentrations (0.5-5%). 3. In the presence of the GABA(A) antagonist, bicuculline methiodide (15 microM), isoflurane suppressed the f.e.p.s.ps to the same extent as was observed in the absence of the GABA(A) antagonist. 4. Orthodromically evoked population spikes were suppressed by isoflurane in a manner quantitatively similar to the suppression of the f.e.p.s.ps. However, antidromic population spikes and presynaptic volleys evoked in young and old slices were resistant to anaesthetic action. In addition, paired pulse facilitation ratio of the evoked dendritic f.e.p.s.ps was not affected in both young and old slices during the application of isoflurane. 5. When slices were exposed to low Ca2+/high Mg2+ solution, isoflurane (1 and 3%) depressed the f.e.p.s.ps in aged slices to the same extent as in young slices. 6. The augmented anaesthetic depression of f.e.p.s.ps in old compared to young hippocampi in the absence and presence of bicuculline, and the lack of anaesthetic effects on antidromic population spikes and presynaptic volleys in old and young slices, suggest that the increased sensitivity of anaesthetic actions in old hippocampi is due to age-induced attenuation of synaptic excitation rather than potentiation of synaptic inhibition. Furthermore, elimination of the increased sensitivity of old slices to anaesthetic actions when the slices were perfused with low Ca2+/high Mg2+ medium, which presumably would decrease intracellular [Ca2+], suggests that the enhanced anaesthetic effects in aged neurones might be related to increased intraneuronal [Ca2+] in the synaptic terminal.

Modulation of bursts and high-threshold calcium spikes in neurons of rat auditory thalamus

Neurons in the ventral partition of the medial geniculate body are able to fire high-threshold Ca2+-spikes. The neurons normally discharge such spikes on low-threshold Ca2+-spikes after the action potentials of a burst. We studied membrane mechanisms that regulate the discharge of high-threshold Ca2+-spikes, using whole-cell recording techniques in a slice preparation of rat thalamus. A subthreshold (persistent) Na+-conductance amplified depolarizing inputs, enhancing membrane excitability in the tonic firing mode and amplifying the low-threshold Ca2+-spike in the burst firing mode. Application of tetrodotoxin blocked the amplification and high-threshold Ca2+-spike firing. A slowly inactivating K+ conductance, sensitive to blockade with 4-aminopyridine (50-100 microM), but not tetraethylammonium (2-10 mM), appeared to suppress excitability and high-threshold Ca2+-spike firing. Application of 4-aminopyridine increased the low-threshold Ca2+-spike and the number of action potentials in the burst, and led to a conversion of the superimposed high-threshold Ca2+-spike into a plateau potential. Application of the Ca2+-channel blocker Cd2+ (50 microM), reduced or eliminated this plateau potential. The tetrodotoxin sensitive, persistent Na+-conductance also sustained plateau potentials, triggered after 4-aminopyridine application on depolarization by current pulses. Our results suggest that high-threshold Ca2+-spike firing, and a short-term influx of Ca2+, are regulated by a balance of voltage-dependent conductances. Normally, a slowly inactivating A-type K+-conductance may reduce high-threshold Ca2+-spike firing and shorten high-threshold Ca2+-spike duration. A persistent Na+-conductance promotes coupling of the low-threshold Ca2+-spike to a high-threshold Ca2+-spike. Thus, the activation of both voltage-dependent conductances would affect Ca2+ influx into ventral medial geniculate neurons. This would alter the quality of the different signals transmitted in the thalamocortical system during wakefulness, sleep and pathological states.

Isoflurane attenuates resonant responses of auditory thalamic neurons

In thalamocortical neurons, sensory signals are transformed differently during various states of consciousness. We investigated the effects of a general anesthetic, isoflurane, on the frequency responses of neurons in the ventral medial geniculate body, the primary nucleus of the auditory thalamus. Using slice preparations, whole cell current-clamp recording techniques, and frequency-domain analyses with oscillatory inputs, we observed a resonance in the hyperpolarized voltage range, implying a frequency preference near 1 Hz in the subthreshold frequency responses of medial geniculate neurons. As in other thalamocortical neurons, an interaction of a T-type Ca2+ current with passive membrane properties generates the resonant responses. The frequency preference shapes the input-output signal transformation, coupling oscillatory inputs at preferred frequencies to firing. Thus resonance may contribute to the rhythmic synchronization of the output to the cortex. In a concentration range of 0.5-3%, isoflurane application reversibly decreased the resonant responses of medial geniculate neurons. Throughout the subthreshold voltage range, it reduced impedance at frequencies < 10 Hz. At depolarized potentials near -60 mV, isoflurane reduced the low-pass filter selectivity of the neuron membrane. At rest near -70 mV or at hyperpolarized potentials, isoflurane had a greater effect on resonance (centered at approximately 1 Hz), reducing the peak impedance more than the magnitudes at other frequencies. At concentrations of > or = 2%, isoflurane completely blocked the resonance peak, thereby imposing low-pass characteristics of poor quality throughout the subthreshold voltage range. Application of isoflurane reversibly increased membrane conductance and the current threshold for firing evoked by depolarizing pulses from potentials between -60 and -90 mV. The neurons discharged in a tonic pattern on depolarization from about -60 mV and in a phasic (burst) mode from potentials negative to about -70 mV. An increase in current amplitude compensated the suppression of tonic firing much more readily than that of the burst firing on a low-threshold Ca2+ spike. Although a reduction in T-type Ca2+ channel activation may occur during isoflurane application, the depression of resonance is consistent with an interaction of a greatly increased leak conductance with the low-threshold Ca2+ current and the membrane capacitance. In the intact animal, this would tend to disrupt synchronized neural oscillations and the transfer of auditory information.

Isoflurane inhibits calcium currents in neocortical neurons

We used voltage-clamp techniques to assess the effects of isoflurane anaesthesia in slices of sensorimotor cortex of guinea pigs and neonatal rats. Isoflurane (0.5-4%) depressed inward Ca(2+)-currents evoked by depolarizing commands from -50 mV. With additional blockade of K(+)-currents by internal Cs+, an early component of the sustained inward currents was a high voltage-activated current whereas the delayed component was an unclamped Ca(2+)-current; this was consistent with a simple compartmental model. Isoflurane decreased the magnitude of the high voltage-activated current.