Jeannette A. Vizuete

Desflurane Selectively Suppresses Long-latency Cortical Neuronal Response to Flash in the Rat

Background:The effect of inhalational anesthetics on sensory-evoked unit activity in the cerebral cortex has been controversial. Desflurane has desirable properties for in vivo neurophysiologic studies, but its effect on cortical neuronal activity and neuronal responsiveness is not known. The authors studied the effect of desflurane on resting and visual evoked unit activity in rat visual cortex in vivo. Methods:Desflurane was administered to adult albino rats at steady-state concentrations at 2%, 4%, 6%, and 8%. Flashes from a light emitting diode were delivered to the left eye at 5-s intervals. Extracellular unit activity within the right visual cortex was recorded using a 49-electrode array. Individual units were identified using principal components analysis. Results:At 2% desflurane, 578 active units were found. Of these, 75% increased their firing rate in response to flash. Most responses contained early (0–100 ms) and late (150–1000 ms) components. With increasing desflurane concentration, the number of units active at baseline decreased (–13%), the number of early-responding units increased (+31%), and number of late-responding units decreased (–15%). Simultaneously, baseline firing rate decreased (–77%), the early response was unchanged, and the late response decreased (–60%). Conclusions:The results indicate that visual cortex neurons remain responsive to flash stimulation under desflurane anesthesia, but the long-latency component of their response is attenuated in a concentration-dependent manner. Suppression of the long-latency response may be related to a loss of corticocortical feedback and loss of consciousness.

Differential effects of isoflurane on high-frequency and low-frequency γ oscillations in the cerebral cortex and hippocampus in freely moving rats.

Background:Cortical &ggr; oscillations are thought to play a role in conscious cognitive functions. Suppression of 40-Hz &ggr; activity was implicated in the loss of consciousness during general anesthesia. However, several experimental studies found that &ggr; oscillations were preserved in anesthesia. The authors investigated the concentration-dependent effect of isoflurane on spontaneous &ggr; oscillations in two frequency bands and three distinct brain regions in the rat. Methods:Adult Sprague-Dawley rats were chronically implanted with epidural and coaxial depth electrodes to record cortical field potentials in frontal cortex, visual cortex, and hippocampus in waking and at steady-state isoflurane concentrations of 0.4, 0.8, and 1.2%. The &ggr; power was calculated for the frequency bands 30–50 and 70–140 Hz. Temporal variation and interregional synchrony of &ggr; activity were analyzed using wavelet transform. Loss of consciousness was indexed by the loss of righting reflex. Results:Rats lost their righting reflex at 0.8 ± 0.1% isoflurane. High-frequency &ggr; power was decreased by isoflurane in a concentration-dependent manner (P < 0.001, 50% decrease at 0.8% isoflurane) in all brain regions. Low-frequency &ggr; power was unaffected by isoflurane. The duration and interregional synchrony of high-frequency &ggr; bursts was also reduced (P l < 0.001, 40% decrease at 0.8% isoflurane). Conclusions:Distinction between high- and low-frequency &ggr; bands is important when evaluating the effect of general anesthetics on brain electrical activity. Spontaneous 40-Hz &ggr; power does not indicate the state of consciousness. The attenuation and interregional desynchronization of high-frequency &ggr; oscillations appear to correlate with the loss of consciousness.

Multiphasic modification of intrinsic functional connectivity of the rat brain during increasing levels of propofol

The dose-dependent effects of anesthetics on brain functional connectivity are incompletely understood. Resting-state functional magnetic resonance imaging (rsfMRI) is widely used to assess the functional connectivity in humans and animals. Propofol is an anesthetic agent with desirable characteristics for functional neuroimaging in animals but its dose-dependent effects on rsfMRI functional connectivity have not been determined. Here we tested the hypothesis that brain functional connectivity undergoes specific changes in distinct neural networks at anesthetic depths associated with loss of consciousness. We acquired spontaneous blood oxygen level-dependent (BOLD) signals simultaneously with electroencephalographic (EEG) signals from rats under steady-state, intravenously administered propofol at increasing doses from light sedation to deep anesthesia (20, 40, 60, 80, and 100 mg/kg/h IV). Power spectra and burst suppression ratio were calculated from the EEG to verify anesthetic depth. Functional connectivity was determined from the whole brain correlation of BOLD data in regions of interest followed by a segmentation of the correlation maps into anatomically defined regional connectivity. We found that propofol produced multiphasic, dose dependent changes in functional connectivity of various cortical and subcortical networks. Cluster analysis predicted segregation of connectivity into two cortical and two subcortical clusters. In one cortical cluster (somatosensory and parietal), the early reduction in connectivity was followed by transient reversal; in the other cluster (sensory, motor and cingulate/retrosplenial), this rebound was absent. The connectivity of the subcortical cluster (brainstem, hippocampal and caudate) was strongly reduced, whereas that of another (hypothalamus, medial thalamus and n. basalis) did not. Subcortical connectivity increased again in deep anesthesia associated with EEG burst suppression. Regional correlation analysis confirmed the breakdown of connectivity within and between specific cortical and subcortical networks with deepening propofol anesthesia. Cortical connectivity was suppressed before subcortical connectivity at a critical propofol dose associated with loss of consciousness.

Norepinephrine infusion into nucleus basalis elicits microarousal in desflurane-anesthetized rats

Background: The nucleus basalis of Meynert of the basal forebrain has been implicated in the regulation of the state of consciousness across normal sleep-wake cycles. Its role in the modulation of general anesthesia was investigated. Methods: Rats were chronically implanted with bilateral infusion cannulae in the nucleus basalis of Meynert and epidural electrodes to record the electroencephalogram in frontal and visual cortices. Animals were anesthetized with desflurane at a concentration required for the loss of righting reflex (4.6 ± 0.5%). Norepinephrine (17.8 nmol) or artificial cerebrospinal fluid was infused at 0.2 &mgr;l/min (1 &mgr;l total). Behavioral response to infusion was measured by scoring the orofacial, limb, and head movements, and postural changes. Results: Behavioral responses were higher after norepinephrine (2.1 ± 1) than artificial cerebrospinal fluid (0.63 ± 0.8) infusion (P < 0.01, Student t test). Responses were brief (1–2 min), repetitive, and more frequent after norepinephrine infusion (P < 0.0001, chi-square test). Electroencephalogram delta power decreased after norepinephrine in frontal (70 ± 7%) but not in visual cortex (P < 0.05, Student t test). Simultaneously, electroencephalogram cross-approximate entropy between frontal and visual cortices increased from 3.17 ± 0.56 to 3.85 ± 0.29 after norepinephrine infusion (P < 0.01, Student t test). Behavioral activation was predictable by the decrease in frontal delta power (logistic regression, P < 0.05). Conclusions: Norepinephrine infusion into the nucleus basalis of Meynert can modulate anesthetic depth presumably by ascending activation of the cortex. The transient nature of the responses suggests a similarity with microarousals normally observed during natural sleep, and may imply a mechanism for transient awareness under light anesthesia.

The electrocortical effects of enflurane: experiment and theory.

BACKGROUND: High concentrations of enflurane will induce a characteristic electroencephalogram pattern consisting of periods of suppression alternating with large short paroxysmal epileptiform discharges (PEDs). In this study, we compared a theoretical computer model of this activity with real local field potential (LFP) data obtained from anesthetized rats. METHODS: After implantation of a high-density 8 × 8 electrode array in the visual cortex, the patterns of LFP and multiunit spike activity were recorded in rats during 0.5, 1.0, 1.5, and 2.0 minimum alveolar anesthetic concentration (MAC) enflurane anesthesia. These recordings were compared with computer simulations from a mean field model of neocortical dynamics. The neuronal effect of increasing enflurane concentration was simulated by prolonging the decay time constant of the inhibitory postsynaptic potential (IPSP). The amplitude of the excitatory postsynaptic potential (EPSP) was modulated, inverse to the neocortical firing rate. RESULTS: In the anesthetized rats, increasing enflurane concentrations consistently caused the appearance of suppression pattern (>1.5 MAC) in the LFP recordings. The mean rate of multiunit spike activity decreased from 2.54/s (0.5 MAC) to 0.19/s (2.0 MAC). At high MAC, the majority of the multiunit action potential events became synchronous with the PED. In the theoretical model, prolongation of the IPSP decay time and activity-dependent EPSP modulation resulted in output that was similar in morphology to that obtained from the experimental data. The propensity for rhythmic seizure-like activity in the model could be determined by analysis of the eigenvalues of the equations. CONCLUSION: It is possible to use a mean field theory of neocortical dynamics to replicate the PED pattern observed in LFPs in rats under enflurane anesthesia. This pattern requires a combination of a moderately increased total area under the IPSP, prolonged IPSP decay time, and also activity-dependent modulation of EPSP amplitude.

Monosynaptic functional connectivity in cerebral cortex during wakefulness and under graded levels of anesthesia

The balance between excitation and inhibition is considered to be of significant importance for neural computation and cognitive function. Excitatory and inhibitory functional connectivity in intact cortical neuronal networks in wakefulness and graded levels of anesthesia has not been systematically investigated. We compared monosynaptic excitatory and inhibitory spike transmission probabilities using pairwise cross-correlogram (CCG) analysis. Spikes were measured at 64 sites in the visual cortex of rats with chronically implanted microelectrode arrays during wakefulness and three levels of anesthesia produced by desflurane. Anesthesia decreased the number of active units, the number of functional connections, and the strength of excitatory connections. Connection probability (number of connections per number of active unit pairs) was unaffected until the deepest anesthesia level, at which a significant increase in the excitatory to inhibitory ratio of connection probabilities was observed. The results suggest that the excitatory–inhibitory balance is altered at an anesthetic depth associated with unconsciousness.

Critical Changes in Cortical Neuronal Interactions in Anesthetized and Awake Rats

Background:Neuronal interactions are fundamental for information processing, cognition, and consciousness. Anesthetics reduce spontaneous cortical activity; however, neuronal reactivity to sensory stimuli is often preserved or augmented. How sensory stimulus–related neuronal interactions change under anesthesia has not been elucidated. In this study, the authors investigated the visual stimulus–related cortical neuronal interactions during stepwise emergence from desflurane anesthesia. Methods:Parallel spike trains were recorded with 64-contact extracellular microelectrode arrays from the primary visual cortex of chronically instrumented, unrestrained rats (N = 6) at 8, 6, 4, and 2% desflurane anesthesia and wakefulness. Light flashes were delivered to the retina by transcranial illumination at 5- to 15-s randomized intervals. Information theoretical indices, integration and interaction complexity, were calculated from the probability distribution of coincident spike patterns and used to quantify neuronal interactions before and after flash stimulation. Results:Integration and complexity showed significant negative associations with desflurane concentration (N = 60). Flash stimulation increased integration and complexity at all anesthetic levels (N = 60); the effect on complexity was reduced in wakefulness. During stepwise withdrawal of desflurane, the largest increase in integration (74%) and poststimulus complexity (35%) occurred before reaching 4% desflurane concentration—a level associated with the recovery of consciousness according to the rats’ righting reflex. Conclusions:Neuronal interactions in the cerebral cortex are augmented during emergence from anesthesia. Visual flash stimuli enhance neuronal interactions in both wakefulness and anesthesia; the increase in interaction complexity is attenuated as poststimulus complexity reaches plateau. The critical changes in cortical neuronal interactions occur during transition to consciousness.

Brainstem stimulation augments information integration in the cerebral cortex of desflurane-anesthetized rats

States of consciousness have been associated with information integration in the brain as modulated by anesthesia and the ascending arousal system. The present study was designed to test the hypothesis that electrical stimulation of the oral part of the pontine reticular nucleus (PnO) can augment information integration in the cerebral cortex of anesthetized rats. Extracellular unit activity and local field potentials were recorded in freely moving animals from parietal association (PtA) and secondary visual (V2) cortices via chronically implanted microwire arrays at three levels of anesthesia produced by desflurane: 3.5, 4.5, and 6.0% (where 4.5% corresponds to that critical for the loss of consciousness). Information integration was characterized by integration (multiinformation) and interaction entropy, estimated from the statistical distribution of coincident spike patterns. PnO stimulation elicited electrocortical activation as indicated by the reductions in δ- and θ-band powers at the intermediate level of anesthesia. PnO stimulation augmented integration from 1.13 ± 0.03 to 6.12 ± 1.98 × 103 bits and interaction entropy from 0.44 ± 0.11 to 2.18 ± 0.72 × 103 bits; these changes were most consistent in the PtA at all desflurane concentrations. Stimulation of the retina with discrete light flashes after PnO stimulation elicited an additional 166 ± 25 and 92 ± 12% increase in interaction entropy in V2 during light and intermediate levels. The results suggest that the PnO may modulate spontaneous ongoing and sensory stimulus-related cortical information integration under anesthesia.

Repertoire of mesoscopic cortical activity is not reduced during anesthesia.

Consciousness has been linked to the repertoire of brain states at various spatiotemporal scales. Anesthesia is thought to modify consciousness by altering information integration in cortical and thalamocortical circuits. At a mesoscopic scale, neuronal populations in the cortex form synchronized ensembles whose characteristics are presumably state-dependent but this has not been rigorously tested. In this study, spontaneous neuronal activity was recorded with 64-contact microelectrode arrays in primary visual cortex of chronically instrumented, unrestrained rats under stepwise decreasing levels of desflurane anesthesia (8%, 6%, 4%, and 2% inhaled concentrations) and wakefulness (0% concentration). Negative phases of the local field potentials formed compact, spatially contiguous activity patterns (CAPs) that were not due to chance. The number of CAPs was 120% higher in wakefulness and deep anesthesia associated with burst-suppression than at intermediate levels of consciousness. The frequency distribution of CAP sizes followed a power-law with slope -1.5 in relatively deep anesthesia (8-6%) but deviated from that at the lighter levels. Temporal variance and entropy of CAP sizes were lowest in wakefulness (76% and 24% lower at 0% than at 8% desflurane, respectively) but changed little during recovery of consciousness. CAPs categorized by K-means clustering were conserved at all anesthesia levels and wakefulness, although their proportion changed in a state-dependent manner. These observations yield new knowledge about the dynamic landscape of ongoing population activity in sensory cortex at graded levels of anesthesia. The repertoire of population activity and self-organized criticality at the mesoscopic scale do not appear to contribute to anesthetic suppression of consciousness, which may instead depend on large-scale effects, more subtle dynamic properties, or changes outside of primary sensory cortex.

Strain differences in cortical electroencephalogram associated with isoflurane-induced loss of consciousness

Introduction:Previously observed increased sensitivity to noxious stimulation in the Dahl salt-sensitive rat strain (SS/JrHsdMcwi, abbreviated as SS) compared to Brown Norway rats (BN/NhsdMcwi abbreviated as BN) is mediated by genes on a single chromosome. The current study used behavioral and electrocortical data to determine if differences also exist between SS and BN rats in loss of consciousness. Methods:Behavioral responses, including loss of righting, (a putative index of consciousness) and concurrent electroencephalogram recordings, in 12 SS and BN rats were measured during isoflurane at inhaled concentrations of 0, 0.3, 0.6, 0.8, 1.0 and 1.2%. Results:In SS compared to BN rats, the mean ± SEM EC50 for righting was significantly less (0.65 ± 0.01% vs. 0.74 ± 0.02% inhaled isoflurane) and delta fraction in parietal electroencephalogram was enhanced 50–100% at all isoflurane levels during emergence. The frequency decay constant of an exponential fit of the parietal electroencephalogram spectrum graphed as a function of isoflurane level was three times less steep (mean ± SEM slope −57 ± 13 vs. −191 ± 38) and lower at each level of isoflurane in SS versus BN rats (i.e., shifted toward low frequency activity). Electroencephalogram differences between strains were larger during emergence than induction. Conclusions:Sensitivity is higher in SS compared to BN rats leading to unconsciousness at lower levels of isoflurane. This supports using additional strains in this animal model to study the genetic basis for differences in anesthetic action on mechanisms of consciousness. Moreover, induction and emergence appear to involve distinct pathways.