Altered Thalamocortical Functional Connectivity by ...

Original Paper Pharmacology 2011;88:322–326 DOI: 10.1159/000334168

Received: September 26, 2011 Accepted after revision: October 3, 2011 Published online: November 23, 2011

Altered Thalamocortical Functional Connectivity by Propofol Anesthesia in Rats Ye Tu a Tian Yu a Xiao-Yun Fu a Peng Xie a Sv Lu b Xiao-Qi Huang b Qi-Yong Gong b a

Department of Anesthesiology, Zunyi Medical College, Zunyi, and b HMRRC, Department of Radiology, Center for Diagnostic Imaging, West China Hospital, Sichuan University, Chengdu, China

Key Words Propofol ⴢ Functional magnetic resonance imaging ⴢ Functional connectivity ⴢ Blood oxygen level-dependent

in the absence of external stimulation. However, our experiment suggests that fcMRI can be used to investigate brain networks that exhibit correlated fluctuations. Copyright © 2011 S. Karger AG, Basel

Abstract Anesthesia, a state of profound central nervous system suppression, involves a sequence of events that is still not well understood. In the present study, we examined the action of propofol (a sedative-hypnotic drug commonly used as anesthetic) on thalamocortical functional connectivity in rats by using functional connectivity magnetic resonance imaging (fcMRI) with a 3.0-tesla MR scanner. Intraperitoneal injections of propofol (80 or 160 mg/kg) were administered to Sprague-Dawley rats. Synchronized low-frequency fluctuations (LFF) of blood oxygen level-dependent (BOLD) signals were found between the thalamic and somatosensory cortices (S1/S2) after administration of 80 mg/kg propofol. However, after application of 160 mg/kg propofol, synchronized LFF of BOLD signals disappeared. These observations indicate that thalamocortical connectivity may play an important role in propofol anesthesia. We also observed that regionally specific long-range correlations of spontaneous low-frequency physiological fluctuations in BOLD signals may be present across somatosensory networks of the brain

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Introduction

It is clear that functional magnetic resonance imaging (fMRI) is an in vivo imaging technique for detecting cerebral hemodynamic changes in response to alterations in neural activity with relatively high spatial and temporal resolution in humans and laboratory animals. In pharmacology studies, functional connectivity (fc)MRI may offer an additional valuable means of investigating the sites of drug action and, thereby, of visualizing the effects of therapeutic interventions on the system level. Several recent studies have traditionally focused on taskdependent changes in neural activity in different brain areas [1, 2]. It is known that fMRI techniques highlight cerebral vascular responses which are coupled to the functions of neurons. For this reason, stimulus-free or resting-state fcMRI has become increasingly popular. Biswal et al. [3] reported that slow oscillatory blood oxygen level-dependent (BOLD) signal changes were first Tian Yu Department of Anesthesiology, Zunyi Medical College 201 Dalian Road Zunyi City, Guizhou 563000 (China) Tel. +86 13 985 238 598, E-Mail zmcyutian @ hotmail.com

identified using the fMRI methodology, which provides evidence that the fluctuations in blood oxygenation reflect the functional connectivity of the brain. In addition, some previous studies alerted us to the possibility that these low-frequency fluctuations (LFF; 0.01 Hz ! f ! 0.08 Hz) and spontaneous neural activity may be independent of neural activation under physiological conditions in the brain [4, 5]. It was reported that anesthetic-induced unconsciousness may result from disruption of functional interactions within neural networks [1, 6]. However, the relationship between depth of anesthesia and changes in functional connectivity is not clear. Propofol, a sedative-hypnotic drug, has been widely used in anesthesia and intensive care units. To determine whether thalamocortical functional connectivity was influenced by different depths of anesthesia, we examined temporal correlations of spontaneous fluctuations between brain regions in Sprague-Dawley (SD) rats anesthetized with different concentrations of propofol by using the fcMRI technique.

tion. Blood gas values were maintained within the physiological range (30 mm Hg ! PCO2 ! 50 mm Hg; PO2 1100 mm Hg). Image Analysis All data were preprocessed and analyzed by using SPM2 (www.fil.ion.ucl.ac.uk/spm). Images were corrected for movement by using least-square minimization without higher-order corrections for spin history, and were normalized to the rat template. The images were then resampled every 2 mm by using sinc interpolation, and smoothed with an 8-mm gaussian kernel to decrease spatial noise. Statistical analysis was performed on individual and group data by using the general linear model and gaussian random field theory as implemented in SPM2. Group analyses were performed by using a random effects model that estimates the error variance for each condition of interest across subjects, rather than across scans, and therefore provides a stronger generalization to the population from which data are acquired. Statistical maps were superimposed on the normalized high-resolution rat template [7]. Region of Interest Regions of interest (ROI) corresponding to selected structures were delineated freehand using a 3-D volumetric reconstruction of the Paxinos and Watson rat brain atlas with MarsBar, coregistered with the rat brain template [8]. In view of the importance of the thalamus in anesthesia, we selected the thalamus as the ROI, consisting of 9 (3 ! 3) voxels.

Animals and Method The animal research protocols were reviewed and consented to by the Sichuan University animal care committee, in accordance with the Principles of Laboratory Animal Care (NIH publication 86-23). The animals had free access to standard rat chow and tap water, and were housed in groups of 5 in solid-bottom cages. Room temperature (20–22 ° C), relative humidity (45–65%) and dark-light cycles were controlled. Thirty SD rats (250–350 g) were randomly divided into two groups, a light-anesthesia group (n = 15) and a deep-anesthesia group (n = 15). In each group, the rats were treated with different concentrations of propofol (80 or 160 mg/kg) via bolus intraperitoneal injection in different groups. The duration of propofol injection was 90–100 s. Three minutes after the loss of the righting reflex, the rats’ heads were fixed with four glass fiber bars on a nonmagnetic stereotaxic apparatus. Body temperature was maintained at 37–38 ° C by circulating water.

Image Acquisition Images were acquired by a 3-tesla MR system (Achieva; Philips, The Netherlands), using a 4-channel phased-array rat head coil. Functional and anatomical images of each rat were acquired using a single-shot gradient-echo echo-planar imaging sequence (TR/TE: 2,000/27.5 ms; slice thickness: 1 mm; slice gap: 0 mm; matrix: 96 ! 96; flip angle: 85°; FOV: 50 ! 50 mm; total: 135 volumes) and a gradient-echo pulse sequence (TR/TE: 2,500/240 ms; slice thickness: 0.5 mm; slice gap: 0 mm; matrix: 224 ! 224; flip angle: 90°; FOV: 50 ! 50 mm), respectively. The total functional image acquisition time was 4 min and 55 s. Additionally, ambient lights were turned off during image acquisition to prevent background interference. The PaCO2 and PaO2 measurements were performed prior to and at the end of the fMRI time series acquisi-

Thalamocortical Functional Connectivity with Propofol

fcMRI Analyses To perform the fcMRI analyses, time series from the restingstate scan were extracted for the rat-specific ROI in the right thalamus by averaging the time series of all voxels in the ROI. Before averaging voxel data, scaling and filtering steps were performed across all brain voxels as follows. To minimize the effect of global drift, voxel intensities were scaled by dividing the value for each time point by the mean value for the whole-brain image at that time point. Then the scaled waveform of each brain voxel was filtered by using a bandpass filter (0.01 Hz ! f ! 0.08 Hz) to reduce the effect of low-frequency drift and high-frequency noise [5]. The resulting time series, representing the average intensity (after scaling and filtering) of all voxels in the ROI, was then used as a covariate of interest in a whole-brain linear regression statistical parametric analysis. Contrast images corresponding to this regressor were determined individually for each rat and entered into a second-level random effects analysis (height and extent thresholds of p ! 0.001) to determine the brain areas that showed significant functional connectivity across subjects. We used a less conservative statistical threshold of p ! 0.01 for this whole-brain analysis. Finally, functional connectivity locations in the right thalamus for each group were displayed by superimposing them on the rat brain template.

Results

Synchronized LFF of BOLD Signals in Light Propofol Anesthesia In the present study, no statistical difference between pre- and postdata acquisition of PaCO2 and PaO2 values Pharmacology 2011;88:322–326

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Color version available online

Color version available online

Thalamus

Thalamus

15

10 S1 Thalamus

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Thalamus 5

5 S2 0

Fig. 1. Section images of the low-dosage propofol anesthesia group (left panel) from functional analysis showed functional connectivity between the thalamus and ipsilateral primary somatosensory cortex (S1) and contralateral secondary somatosensory cortex (S2; p ! 0.05).

was found among the experimental groups. Blood gas values were maintained within physiological range before and after data acquisition (30 mm Hg ! PCO2 ! 50 mm Hg; PO2 1100 mm Hg). Synchronized LFF of BOLD signals were observed between the right thalamus and the cortex in light propofol anesthesia. Within-group analysis showed that functional connectivity existed between the right thalamus and the ipsilateral primary somatosensory cortex (S1; p ! 0.001) as well as the contralateral second somatosensory cortex (S2; p ! 0.01) in light propofol anesthesia (fig. 1). Synchronized LFF of BOLD Signals Were Inhibited in Deep Propofol Anesthesia To examine whether synchronized LFF of BOLD signals can be influenced by high-concentration propofol, 160 mg/kg propofol was applied. As a result, we found that synchronized LFF of BOLD signals were markedly inhibited between thalamic and other cerebral areas. This observation suggests that functional connectivity was absent between thalamic and other cerebral areas in deep propofol anesthesia (p ! 0.01). Incidentally, we noted that the similar pattern of functional connectivity with different concentrations was consistent for all rats (fig. 2). 324

Pharmacology 2011;88:322–326

0

Fig. 2. Section images of the high-dose propofol anesthesia group

from functional analysis showed functional connectivity between the thalamus and the cortex was absent (p ! 0.05).

Discussion

Previous functional neuroimaging studies of anesthesia have reported global and regionally specific suppressive effects of various anesthetic agents on human brain activity [9, 10]. It seems that these specific suppressive effects are important mechanistic features of anesthesia. Propofol, a sedative-hypnotic drug, has been widely used in anesthesia and intensive care units. In the present study, we also found that the deep anesthesia state (160 mg/kg propofol) was more potent than light anesthesia (80 mg/kg propofol) at suppressing thalamocortical functional connectivity in the brain, which largely confirmed that the thalamus and the cortex are important sites for propofol action in the central nervous system. It is known that the uncoupling of the regional cerebral blood oxygen level from neural activity might be associated with decreased functional connectivity during the resting state [4, 11]. Our data also indicated that different depths of anesthesia are directly associated with the capacity of BOLD signals to integrate information, which implies that the general anesthetic propofol is associated with a decrease in neuronal activity and sensitivity. In agreement with our suggestion, positron emission tomography imaging also showed global dose-related Tu /Yu /Fu /Xie /Lu /Huang /Gong

blood flow as well as regional decreases in the thalamus, basal forebrain, posterior cingulate and occipital cortices after propofol administration in human volunteers [12]. The dynamic changes in neuronal excitability and sensitivity may reflect the brain’s ability to integrate information between the thalamus and cortex [13]. Thus, our results open up the possibility that propofol inhibits neuronal excitability in the thalamus and cortex, which contributes to the impairment of sensory information transfer in the thalamocortical network. On the other hand, synchronized LFF of BOLD signals were observed between the right thalamus and the cortex in light propofol anesthesia, which implies that the anesthetic may not necessarily require all neurons to be inactivated. The thalamus serves as a gate that regulates the flow of sensory input to the neocortex. Under anesthesia, thalamic spontaneous activity is mainly driven by feedback connections coming from cortical neurons [14]. In this study, we observed that the regional cerebral blood oxygen level decreased after a high dose of propofol. In addition, the reduction in resting-state functional connectivity has also been observed in humans with other anesthetics such as sevoflurane (an inhaled anesthetic) [15]. However, in contrast to propofol and sevoflurane, midazolam (benzodiazepines) and medetomidine (␣2-receptor agonist) have been shown to enhance resting-state BOLD fluctuations in children [16, 17]. Despite these contradictory results, we can learn that vascular action may be required in the alteration of functional connectivity after the administration of anesthetics. The findings may be important for us to understand the mechanisms of anesthesia, such as loss of consciousness and potentially damaging consequences of general anesthetics on developing mammalian brains. Physiological fluctuation stems from autoregulation of the cerebral vasculature in response to arterial blood pressure changes which can be triggered by anesthesia, or in response to arterial carbon dioxide changes due to respiration and CO2 inhalation [18, 19]. This type of BOLD

References

Thalamocortical Functional Connectivity with Propofol

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