In the present study we have examined changes in whole-brain neural networks in anesthetized rats. Our results suggested that functional neural networks were reorganized from the awake to anesthetized state, and this reorganization was governed by the same topological principles. One remarkable finding was that although the connectivity strength was on average decreased in the anesthetized condition, long-distance connections were not preferentially reduced. To our knowledge, this is the first study to examine the reconfiguration of the architecture of large-scale resting-state neural networks in the anesthetized state by directly comparing topological features and connectivity strength between the awake and anesthetized brains in animals.
Perhaps the most important finding of the present study is the preservation of global topological characteristics of the whole-brain neural network in the anesthetized state. Among all global network metrics calculated, only global clustering coefficients showed a marginal but statistically insignificant decrease (p=0.18). Mean shortest path length was even slightly shorter in the anesthetized condition, suggesting the overall information integration capacity was not impaired in the anesthetized rat brain. Likewise, small-worldness and modularity did not show any changes between the two states, again indicating a similar level of modular organization. Numerous human rsfMRI studies have showed altered topological features of the global network (e.g. global clustering coefficient) in various neurological and psychiatric diseases (Bassett and Bullmore, 2009
), implying that the architecture of the brain network might be sensitive to pathological disruptions. Given the profound impact of anesthesia on brain functions, it is striking that the anesthetized brain was able to maintain intact global organization. However, this result was indeed consistent with a previous human EEG study, in which global scale-free organization was found to be preserved across consciousness, anesthesia and recovery states (Lee et al., 2010
). Therefore, this conclusion is very likely not limited to the specific spatial and temporal scales of the rsfMRI technique. An important implication of this finding is that, unlike disrupted global networks in pathological conditions, the brain is able to maintain intact topological structures under pharmacologically induced unconsciousness. This property might be related to the ability of the brain to quickly recover from the unconscious state to the conscious state once the anesthetic is discontinued. It may also suggest that the governing principles of intrinsic brain organization might be fundamental characteristics of the healthy brain.
Despite similar global network topology, local neural networks were considerably reorganized in the anesthetized rat brain. For instance, local clustering coefficients of the nucleus accumbens and septal nuclei were significantly reduced by anesthesia, suggesting those regions were less connected to their neighboring regions in theanesthetized condition. Interestingly, these two regions were reported to enhance anesthetic effects when they were pharmacologically inactivated (Ma et al., 2002
; Ma and Leung, 2006
). In addition, a rat study reported reduced glutamate and aspartate levels in the nucleus accumbens during sleep (Lena et al., 2005
). These results and the findings in the present study collectively underscore the importance of the nucleus accumbens and septal nuclei in anesthetic-induced unconsciousness. Furthermore, several thalamic nuclei showed a significant reduction in betweeness centrality, indicating reduced information relay in the thalamus in the anesthetized rat brain. Consistent with the report by Boveroux et. al. (Boveroux et al., 2010
), we also observed a preferential reduction in high-level thalamo-cortical connectivity relative to low-level thalamo-cortical connectivity under anesthesia (). Taken together, these findings well agree with the extensive literature regarding the role of thalamus in anesthesia and (un)consciousness (Nallasamy and Tsao, 2011
). Moreover, detailed community structure considerably differed even at a similar global modularity (Q) value. Consistent with our previous study (Liang et al., 2011
), modules in the awake brain were more likely to contain both cortical and sub-cortical regions, whereas modules in theanesthetized brain tend to include only cortical or only subcortical regions, implying compromised communications between the cortex and subcortex. Taken together, these results clearly suggested that although the global organizational principles were not changed at the anesthetized state, the brain networks are locally reorganized to support new patterns of information integration among neuronal groups.
It has been repeatedly reported that anesthesia can change FC strength between brain regions (Peltier et al., 2005
; Boveroux et al., 2010
; Martuzzi et al., 2010
; Stamatakis et al., 2010
). For instance, our previous study reported decreased anticorrelated FC between the infralimbic cortex and amygdala in anesthetized rodents (Liang et al., 2012
). Additionally, Liu and colleagues found that FC decreased as the anesthetic depth increased (Liu et al., 2011
). Consistent with these results, in the present study we found that the connectivity strength was on average weaker in the anesthetized condition. When individually comparing the corresponding functional connections between theawake and anesthetized states, most significantly changed connections were weaker in connectivity strength at the anesthetized condition, and these connections were spatially distributed throughout cortical and subcortical areas (). Therefore, our data indicated that the effect of anesthesia was widespread across the whole brain. However, it has to be noted that anesthesia did not uniformly affect all brain regions and functional connections. In fact, the basal ganglia area including the striatum and pallidum showed the largest decrease in FC strength. By contrast, a number of functional connections showed increased connectivity strength in the anesthetized state particularly in hippocampus, hypothalamus and amygdala. These brain regions and connections are relatively less studied regarding their roles in anesthesia. Interestingly, all these regions are part of the limbic system which generally subserves the functions of emotion, memory and homeostatic regulation. Therefore, it can be hypothesized that anesthesia, or perhaps unconsciousness in a more general case, can lead to hyper-synchrony in this part of the limbic system.
Another interesting aspect of connectional strength is its relation with the physical distance of the functional connection. It has been suggested that the disruption of long-distance functional connections, in particular fronto-parietal connections, contributes to unconsciousness (Laureys and Schiff, 2011
). However, in the present study we observed that long-distance functional connections were not particularly diminished at the anesthetic-induced unconscious state, rather, the short-distance connections showed obvious reductions (). This result suggests that the disruption of long-distance connectivity is not necessarily a general mechanism of unconsciousness. However, it does not exclude the possibility that certain long-distance connections might play a key role in maintaining consciousness. Further studies are necessary to identify these potentially vital long-distance connections.
There are several methodological limitations in the present study. First, different levels of motion can affect network metrics as well as the connectional strength (Power et al., 2012
; Satterthwaite et al., 2012
; Van Dijk et al., 2012
). This issue was particularly troublesome when the awake condition had higher motion level than the anesthetized condition. However, a stringent motion control was applied in our study to address this problem. Scans with head displacement more than 0.25mm (i.e. half voxel size) were discarded, and all scans were motion corrected and motion parameters were regressed out. It should be noted that even with the rigorous control of motion, the influence of motion on FC may still persist (Power et al., 2012
; Satterthwaite et al., 2012
; Van Dijk et al., 2012
). To further examine this issue, global network metrics were recalculated from a subset of data with the smallest motion at the awake condition (movement<0.125mm). The motion level in this sub-dataset did not significantly differ from the anesthetized condition (p values >0.1). Results were in excellent agreement with those calculated from the whole dataset. Also, a very similar relationship between connectional strength and physical distance was obtained in this subset of data. Therefore, it is unlikely that different levels of motion can account for the changes between the two conditions observed in the present study. Second, the anesthetic agent used (i.e. isoflurane) is a vasodilator. The vasodilatory effect might have significant effects on the fMRI signal. However, Liu and colleagues (Liu et al., 2011
) reported a strong neurovascular coupling in isoflurane-anesthetized rats, suggesting resting-state FC measured by rsfMRI in isoflurane anesthetized rats was mostly of neural origin. Third, only one type of anesthetic agent was used at one dosage in the present study. Whether these results can be generalized to other anesthetic agents and/or different dosages needs to be confirmed.
The explicit neural mechanism underlying anesthetic-induced unconsciousness is likely to be extremely complex and manifests at various spatial and temporal scales. Here our results show that the integrity of the whole-brain network can be conserved in a wide physiologic range from awake to anesthetized states while local neural networks can flexibly adapt in new conditions. They also illustrate that the governing principles of intrinsic brain organization might represent fundamental characteristics of the healthy brain. With the unique spatial and temporal scale provided by rsfMRI, this study has opened a new avenue for investigating the neural mechanism underlying anesthetic-induced unconsciousness. Considering that all unconscious states share many the same endpoints in brain functions such as amnesia, analgesia, immobility and attenuation of autonomic responses to noxious stimulation (Paul G. Barash, 2009
), our results may help to decipher other unconscious states such as coma.