Comparison of alpha-chloralose, medetomidine and isoflurane anesthesia for functional connectivity mapping in the rat

Abstract

Functional connectivity measures based upon low-frequency blood-oxygenation-level-dependent functional magnetic resonance imaging (BOLD fMRI) signal fluctuations have become a widely used tool for investigating spontaneous brain activity in humans. Still unknown, however, is the precise relationship between neural activity, the hemodynamic response and fluctuations in the MRI signal. Recent work from several groups had shown that correlated low-frequency fluctuations in the BOLD signal can be detected in the anesthetized rat - a first step toward elucidating this relationship. Building on this preliminary work, through this study, we demonstrate that functional connectivity observed in the rat depends strongly on the type of anesthesia used. Power spectra of spontaneous fluctuations and the cross-correlation-based connectivity maps from rats anesthetized with alpha-chloralose, medetomidine or isoflurane are presented using a high-temporal-resolution imaging sequence that ensures minimal contamination from physiological noise. The results show less localized correlation in rats anesthetized with isoflurane as compared with rats anesthetized with alpha-chloralose or medetomidine. These experiments highlight the utility of using different types of anesthesia to explore the fundamental physiological relationships of the BOLD signal and suggest that the mechanisms contributing to functional connectivity involve a complicated relationship between changes in neural activity, neurovascular coupling and vascular reactivity.

Fig. 1

Map of activation acquired from one rat during forepaw stimulation after the medetomidine scan and prior to the isoflurane scan. Cross-correlation values were thresholded at 0.2 and overlaid on the first EPI image of the series. Normal activation is seen in the SI contralateral to the stimulation. The box indicates the 2×2 ROI, including the highest cross-correlation value obtained during activation, which was used as the seed for functional connectivity analysis.

Fig. 2

Spatial distribution of the temporal variance in the respiratory (left) and cardiac (right) bands in one rat. Respiratory and cardiac components exhibit localized areas of high contribution on the surface and at the base of the brain.

Fig. 3

Cross-correlation maps for a seed region in the SI thresholded at a minimum of 0.2 and overlaid on the first EPI image of the series (top figures in both rows) and power spectral density for the time course from the seed region for frequencies between 0 and 0.5 Hz (bottom figures). The top row was acquired with medetomidine anesthesia; the bottom, with isoflurane. Each column contains data from a single rat. It is clear that the extent of correlation with the seed region increases when the anesthesia is switched to isoflurane. Power spectra under both anesthetics exhibit high power in the very low frequencies, sometimes with a clear peak at 0.1 Hz. For comparison, a cross-correlation map and power spectral density plot from a rat anesthetized with α-chloralose are shown at the far right.

Fig. 4

Normalized histograms of the distribution of cross-correlation coefficients for rats anesthetized with α-chloralose (blue), medetomidine (red) or isoflurane (green). The distribution for α-chloralose is similar in shape to that for medetomidine but shifted slightly. The peak for both distributions is near zero. In contrast, the distribution for isoflurane is broader and the peak is shifted toward positive values.

Fig. 5

Cross-correlation maps created using seeds in the SI (left column), SII (center) and CP (right) in rats anesthetized with α-chloralose (top row), medetomidine (center) or isoflurane (bottom). Lower correlation but greater localization is present in the medetomidine scans than in the isoflurane or α-chloralose scans. Cortical localization is greater in α-chloralose scans than in isoflurane scans.

Fig. 6

Summary of average correlation values for seed areas in ipsilateral SI, SII or CP and ROIs in contralateral SI, ipsilateral SII, contralateral SII, ipsilateral CP and contralateral CP measured for animals under isoflurane, medetomidine or α-chloralose. Lower internetwork correlation is observed in the medetomidine scans, with paired t tests between medetomidine and isoflurane conditions obtained from the same animal indicating significant differences (P<.05) for correlation values between iSI and iSII and between iSII and iCP.