A multimodality investigation of cerebral hemodynamics and autoregulation in pharmacological MRI

Abstract

Pharmacological MRI (phMRI) methods have been widely applied to assess the central hemodynamic response to pharmacological intervention as a surrogate for changes in the underlying neuronal activity. However, many psychoactive drugs can also affect cardiovascular parameters, including arterial blood pressure (BP). Abrupt changes in BP or the anesthetic agents used in preclinical phMRI may impair cerebral blood flow (CBF) autoregulation mechanisms, potentially introducing confounds in the phMRI response. Moreover, relative cerebral blood volume (rCBV), often measured in small-animal phMRI studies, may be sensitive to BP changes even in the presence of intact autoregulation. We applied laser Doppler flowmetry and MRI to measure changes in CBF and microvascular CBV induced by increasing doses of intravenous norepinephrine (NE) challenge in the halothane-anesthetized rat. NE is a potent vasopressor that does not cross the blood–brain barrier and mimics the rapid BP changes typically observed with acute drug challenges. We found that CBF autoregulation was maintained over a BP range of 60–120 mmHg. Under these conditions, no significant central rCBV responses were observed, suggesting that microvascular rCBV changes in response to abrupt changes in perfusion pressure are negligible within the autoregulatory range. Larger BP responses were accompanied by significant changes in both CBV and CBF that might confound the interpretation of phMRI results.

Introduction

Pharmacological MRI (phMRI) methods can be applied to assess the effects of acute drug challenge on cerebral hemodynamics as a surrogate for changes in the underlying neuronal activity. This approach has been widely applied to study central effects of drugs on the central nervous system (CNS) in humans and animal models [1], [2]. However, many of these drugs can also induce significant peripheral effects, including severe alterations of cardiovascular parameters. Under physiological conditions, mechanisms of autoregulation keep cerebral blood flow (CBF) relatively constant in the presence of changes in mean arterial blood pressure (MABP). However, general anesthetics, widely used in preclinical phMRI studies to avoid head motion and to better control animal physiology, may affect the central vasoadaptive response to peripheral MABP changes, thus making it difficult to predict the influence of systemic vasopressive effects on cerebral hemodynamics. Moreover, large and rapid changes in MABP may cause a breakdown in the autoregulatory mechanisms that control brain microcirculation, thus introducing potential confounds in the interpretation of phMRI data.

While blood-oxygen-level-dependent (BOLD) signals are most often measured in humans, relative cerebral blood volume (rCBV) has been widely used in phMRI studies in small laboratory animals due to the increased sensitivity afforded by rCBV measurements with intravascular contrast agents over BOLD [3]. However, dilation and constriction of cerebral blood vessels are thought to modulate vascular resistance in order to maintain CBF relatively constant in the presence of changes in perfusion pressure [4]. As a consequence, CBV might be sensitive to MABP changes even in the presence of intact autoregulation.

Several attempts to correlate the magnitude of systemic MABP changes with the central hemodynamic responses in the rodent brain have been published. Zaharchuk et al. [5] did not observe significant CBF, CBV or BOLD changes as MABP was gradually decreased (−1 mmHg/min) by continuous arterial blood withdrawal over the range of maximally effective autoregulation in the halothane-anesthetized rat. However, in a more recent study, Kalisch et al. [6] argued that slow and gradual decreases in MABP may not be representative of the abrupt changes typically observed in pharmacological MR experiments. Indeed, the same authors reported a significant correlation between BOLD signal time courses and MABP changes following rapid arterial blood withdrawal–reinfusion under three different anesthetic regimes in the rat (isoflurane, halothane and propofol). However, the use of the blood withdrawal–reinfusion method presents potential drawbacks such as the need to account for hemodilution, the risk of hemorrhagic shock-like complications and the lack of a stable and normotensive prestimulus MABP baseline.

Other investigators have measured the BOLD signal changes produced by pharmacologically evoked MABP alterations. Tuor et al. [7] and, more recently, Wang et al. [8] reported significant correlations between BOLD signal and the MABP changes induced by norepinephrine (NE), a non-brain-penetrant vasopressor, in rats anesthetized with α-chloralose. Luo et al. [9] observed significant fMRI responses under urethane anesthesia following acute challenge with cocaine but not with cocaine methiodide, a non-brain-penetrant cocaine analogue, at doses that increased MABP up to 180 mmHg, thus suggesting that potentially confounding peripheral effects were negligible in that specific protocol. However, the BOLD response may result from changes in several metabolic and hemodynamic parameters whose contributions cannot be easily disentangled, and these conclusions cannot be extended to phMRI methods based on rCBV measurements.

Here, we have applied laser Doppler flowmetry (LDF) and MRI to measure changes in CBF and microvascular CBV induced by increasing doses of an intravenous NE challenge in the halothane-anesthetized rat. The rCBV protocol employed has been used by us as well as by other groups to map the central hemodynamic response to a number of neuroactive compounds, including amphetamine [10], [11], [12], [13], cocaine [14], [15], apomorphine [16], [17] and nicotine [18], [19]. Following Tuor's approach, we explored increasing doses of NE in order to correlate the magnitude of the cardiovascular response with the corresponding changes in CBV, to assess the potentially confounding effects of MABP changes on CBV-based phMRI data. The use of a pharmacological vasopressor has the advantage of circumventing the limitations of the blood withdrawal and reinfusion method while reproducing the abrupt MABP changes that are typically observed upon drug injection. By measuring LDF changes, we were able to assess the integrity of CBF autoregulatory mechanisms in our model. The independent measurement of CBF and CBV enabled us to investigate the interplay of these two parameters in the range of effective vasoadaptive response and under autoregulation breakdown.