Influence of Three Different Anesthesia Protocols on Aged Rat Brain: A Resting-State Functional Magnetic Resonance Imaging Study
Resting-state functional magnetic resonance imaging (rs-fMRI) has emerged as a powerful tool for studying brain function, particularly in the context of neurodegenerative diseases such as Alzheimer’s disease (AD) and Parkinson’s disease (PD). This non-invasive imaging technique relies on the blood-oxygen-level-dependent (BOLD) signal to monitor functional responses in the brain. Functional connectivity (FC), a key analysis method in rs-fMRI, examines correlations in low-frequency spontaneous fluctuations of the BOLD signal during the resting state. While rs-fMRI is typically performed on anesthetized animals to minimize motion artifacts and stress, the choice of anesthetic can significantly influence FC patterns. This study investigates the effects of three commonly used anesthesia protocols—isoflurane (ISO), combined ISO + dexmedetomidine (DEX), and α-chloralose (AC)—on FC in the aged rat brain.
The aging process is associated with cognitive decline and changes in brain connectivity, making aged rats an ideal model for studying neurodegenerative diseases. However, the effects of anesthesia on the aged brain remain poorly understood. This study aims to fill this gap by comparing FC patterns in aged rats under three different anesthesia protocols. The findings provide insights into the suitability of these anesthetics for rs-fMRI studies in aged animals and their potential impact on brain network connectivity.
Twenty-four male aged Wistar rats (19 months old) were randomly divided into three groups, each receiving a different anesthesia protocol: ISO (n = 8), ISO + DEX (n = 8), or AC (n = 8). Anesthesia was administered according to established protocols. ISO anesthesia was induced at 3% and maintained at 1–1.3%, while ISO + DEX anesthesia involved induction with 3% ISO followed by an intramuscular injection of DEX (0.015 mg/kg) and continuous infusion (0.03 mg/kg/h). AC anesthesia was administered as a single intravenous dose (60 mg/kg) with additional boluses (30 mg/kg) every hour. Physiological parameters, including heart rate, respiratory rate, and oxygen saturation, were monitored throughout the scanning process to ensure consistency across groups.
Prior to rs-fMRI scanning, all rats underwent behavioral testing using the Morris Water Maze (MWM) to assess spatial working memory and cognitive function. This step ensured that only rats with consistent cognitive baselines were included in the study. The MWM test revealed no significant differences in performance among the three groups, indicating comparable baseline cognition.
Magnetic resonance imaging was performed using a 7.0T Bruker Pharmascan System. Anatomical images were acquired using a fast-spin-echo sequence, while BOLD images were obtained using an EPI-SE-FOVsat sequence. The rs-fMRI data were pre-processed and analyzed using spmratIHEP software based on Statistical Parametric Mapping (SPM12) and the Resting-State fMRI Data Analysis Toolkit (REST). Functional connectivity maps were generated by computing Pearson correlations between the retrosplenial cortex (RSC) seed region and other intracranial voxels. Differences in FC maps between groups were analyzed using one-way analysis of variance (ANOVA) and post-hoc two-sample t-tests.
The study revealed significant differences in FC patterns among the three anesthesia protocols. Compared to ISO + DEX, ISO anesthesia caused increased FC in the posterior brain regions and decreased FC in the middle brain regions of aged rats. Specifically, increased FC was observed in the parietal association cortex, visual cortex, and temporal association cortex, while decreased FC was noted in the hypothalamus and amygdaloid body. Notably, the left temporal association cortex (L-TeA), a component of the rat default mode network (DMN), exhibited increased FC under ISO anesthesia.
In contrast, AC anesthesia resulted in global suppression of FC across the brain, with no observed increases in connectivity. Decreased FC was observed in the hippocampus, thalamus, and retrosplenial cortex, all of which are key components of the rat DMN. The cerebellum, however, showed increased FC under AC anesthesia, potentially as a compensatory response to the global suppression.
The physiological effects of the anesthesia protocols were also examined. ISO + DEX caused significant bradycardia and a lower respiratory rate compared to ISO and AC, consistent with the known effects of DEX. No significant differences in oxygen saturation or body temperature were observed among the groups, ensuring that physiological factors did not confound the rs-fMRI results.
The findings of this study have important implications for the design of rs-fMRI experiments in aged animals. ISO anesthesia, while altering FC in specific brain regions, may be a suitable alternative to ISO + DEX for studies where the L-TeA is not a primary focus. However, the global suppression of FC observed under AC anesthesia suggests that this protocol may not be ideal for rs-fMRI studies in aged rats, particularly those investigating DMN connectivity.
The study also highlights the importance of considering the effects of aging on anesthesia sensitivity. Aged rats may require lower doses of anesthetics due to changes in receptor levels and responsiveness. For example, the reduced efficacy of DEX in aged rats may explain the observed differences in FC patterns compared to previous studies in younger animals.
In conclusion, this study provides a comprehensive comparison of the effects of three anesthesia protocols on FC in the aged rat brain. The results underscore the need for careful selection of anesthesia in rs-fMRI studies, particularly in aged animal models. Future research should explore the mechanisms underlying these anesthesia-induced changes in FC and their implications for the study of neurodegenerative diseases.
doi.org/10.1097/CM9.0000000000001126
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