Anesthesia with Dexmedetomidine and Low-dose Isoflurane Increases Solute Transport via the Glymphatic Pathway in Rat Brain When Compared with High-dose Isoflurane

Abstract

Background: The glymphatic pathway transports cerebrospinal fluid through the brain, thereby facilitating waste removal. A unique aspect of this pathway is that its function depends on the state of consciousness of the brain and is associated with norepinephrine activity. A current view is that all anesthetics will increase glymphatic transport by inducing unconsciousness. This view implies that the effect of anesthetics on glymphatic transport should be independent of their mechanism of action, as long as they induce unconsciousness. We tested this hypothesis by comparing the supplementary effect of dexmedetomidine, which lowers norepinephrine, with isoflurane only, which does not.

Methods: Female rats were anesthetized with either isoflurane (N = 8) or dexmedetomidine plus low-dose isoflurane (N = 8). Physiologic parameters were recorded continuously. Glymphatic transport was quantified by contrast-enhanced magnetic resonance imaging. Cerebrospinal fluid and gray and white matter volumes were quantified from T1 maps, and blood vessel diameters were extracted from time-of-flight magnetic resonance angiograms. Electroencephalograms were recorded in separate groups of rats.

Results: Glymphatic transport was enhanced by 32% in rats anesthetized with dexmedetomidine plus low-dose isoflurane when compared with isoflurane. In the hippocampus, glymphatic clearance was sixfold more efficient during dexmedetomidine plus low-dose isoflurane anesthesia when compared with isoflurane. The respiratory and blood gas status was comparable in rats anesthetized with the two different anesthesia regimens. In the dexmedetomidine plus low-dose isoflurane rats, spindle oscillations (9 to 15 Hz) could be observed but not in isoflurane anesthetized rats.

Conclusions: We propose that anesthetics affect the glymphatic pathway transport not simply by inducing unconsciousness but also by additional mechanisms, one of which is the repression of norepinephrine release.

Fig. 1. Dexmedetomidine and low-dose isoflurane (DEXM-I) enhances glymphatic transport of Gd-DTPA in comparison to isoflurane

A: Average time signal curves (TSC) of Gd-DTPA induced signal changes from whole brain of rats anesthetized with DEXM-I (blue, filled circles) and isoflurane (red, filled circles). The TSCs were extracted from the dynamic contrast-enhanced T1-weighted MRIs and represent glymphatic transport of brain parenchyma (excluding cerebrospinal fluid compartment). There is overall more whole brain uptake (as measured by % signal change from baseline) of Gd-DTPA in the DEXM-I anesthetized rats when compared to isoflurane (see result section for quantitative details). The data are expressed as mean ± SD. B and C: 3D volume rendered color-coded maps of Gd-DTPA induced signal changes 2hrs after administration of contrast into the CSF from a rat anesthetized with DEXM-I (B) and isoflurane (C). The red and blue colors represent high and low contrast uptake, respectively. Over the first 2 hrs of contrast circulation more contrast had penetrated the brain in the DEXM-I rat (B) when compared to isoflurane (C). Scale bar: 3 mm.

Fig. 2. Glymphatic transport in the hippocampus of rats anesthetized with dexmedetomidine + isoflurane (DEXM=I) and isoflurane

A shows average scatter plots of hippocampal time signal curves (TSC) from rats anesthetized with DEXM-I (blue) in comparison to rats anesthetized with pure isoflurane (red). The hippocampal TSC data can be divided into three time period based on the kinetics: t₁ = 0–12.3 min presents the period of delay from the time of contrast infusion into the cerebrospinal fluid until the contrast reaches the hippocampus region; t₂ = 12.3–61.5 min presents the period of glymphatic contrast transport where influx is greater than clearance; and t₃ = 61.5–160 min present the period where glymphatic contrast clearance is dominating. The linear regression parameters of the average hippocampal clearance profiles from the two groups (DEXM-I, blue; and isoflurane, red) are shown in B. Data are presented at mean ± SD. ISO = isoflurane.

Fig. 3. Physiological parameters during MRI scans

Respiratory rate and heart rate were recorded non-invasively during measurement of glymphatic transport with MRI. The respiratory rate for isoflurane and DEXM-I anesthesia groups were similar over the 2–3 hr experimental period (A). The heart rate was significantly lower in DEXM-I rats compared to isoflurane anesthetized rats (B). The data are expressed as mean ± SD. ISO = isoflurane.

Fig. 4. MR angiograms from rat anesthetized with isoflurane versus dexmedetomidine + isoflurane (DEXM-I)

A, B shows typical 3D volume rendered MR angiograms (maximum intensity projections) displaying the major vascular landmarks including the vessel segments of interest highlighted in red. SSS=superior sagittal sinus; SS=straight sinus, ICA=internal carotid artery; EJV=external jugular vein. C–F show 2D MR angiograms overlaid on the corresponding T1 weighted MRIs. Isoflurane produced greater vasodilation in the SS when compared to DEXM-I (compare C with 3D). In contrast, no differences in average vessel diameter were observed at the level of the ICA (compare Figs. E, F). Scale bar: 2 mm.

Fig. 5. MR contrast (Gd-DTPA) efflux via the olfactory nerves

A: Sagittal T1-weighted MRI at the level of the olfactory bulb and endoturbinates. The field of view includes the olfactory portion of the nasal cavity (Olf=olfactory bulb). Scale bar: 3 mm. The dashed line indicates the cross-sectional area for the nasal cavity shown in B (anatomy displayed) and C (color coded MR contrast map representing Gd-DTOA after 50 min of cerebrospinal fluid circulation overlaid on the anatomical map). As can be observed in C, the distribution pattern of contrast follows the surface curvature of the endoturbinates. D: Average time signal curves (TSCs) acquired from the endoturbinates of rats anesthetized with isoflurane (red) vs dexmedetomidine + isoflurane (DEXM-I, Blue). Quantitative analysis show that the time-to-peak and peak amplitude of Gd-DTPA induced signal changes are significantly higher in isoflurane compared to DEXM-I anesthetized rats. The data are presented as mean ± SD. The data were extracted from the contrast-enhanced MRIs via an anatomical mask of the olfactory portion of the nasal cavity.

Fig. 6. Spectrograms and time domain electroencephalogram (EEG) signatures of isoflurane and dexmedetomidine + isoflurane (DEXM-I)

A. 2D Spectrogram of EEG from a rat receiving isoflurane. Burst suppression in the spectrogram is shows as periods of blue (isoelectric activity) interspersed with periods of red-yellow (delta and theta oscillations). The horizontal red line shows the principal period of burst suppression. B. Ten-second electroencephalogram trace from A showing burst events (intermittent high amplitude activity) and suppression events (isoelectric activity). C. 2D EEG spectrogram from a rat receiving DEXM-I. The spectrogram shows spindles (intermittent red period 9–15Hz) and slow-delta oscillations. D. Ten-second electroencephalogram trace from the period in C emphasizing spindles (red lines). ISO = isoflurane.