Magnetic resonance imagingChanges in MRI signal intensity during hypercapnic challenge under conscious and anesthetized conditions
Introduction
Head movement in the field of view or disturbances in magnetic field homogeneity caused by respiration, swallowing and muscle contractions in the face and neck are the major sources of motion artifact in fMRI [1], [2], [3]. While motion artifact is problematic in human imaging studies, it is a major limitation in animal imaging. Animals, unlike humans, must be physically restrained to minimize motion artifacts during an imaging session. Consequently, a majority of animal imaging studies have been done under anesthetized conditions. Unfortunately, anesthesia precludes the investigation of cognitive processes that are critical to the study of the brain and, in addition, anesthesia alters both the cellular and cardiovascular components contributing to the fMRI signal.
Functional MRI is a technique sensitive to the oxygenation status of hemoglobin and therefore produces images that reflect changes in cerebral blood flow and volume. The general anesthetics commonly used in animal fMRI studies depress CNS metabolic activity causing a reduction in basal cerebral blood flow [4], [5] and BOLD signal intensity [6]. Even an anesthetic known to preserve neuronal function, α-chloralose, reduces blood flow and metabolism in certain brain regions [7].
Recently, technologies and methods have been developed for imaging conscious rats [6], [8], [9], [10] and monkeys [12], [13], [14], [15], [16]. Studies comparing conscious vs. anesthetized conditions in the same animal during a single imaging session report a greater BOLD signal change in the conscious condition as compared to the anesthetized state [8], [17]. The enhanced BOLD signal change in conscious animals is most likely due to elevation in cerebral blood flow to areas of activation. Data obtained from laser-Doppler studies show large increases in cerebral blood flow in conscious animals as compared to the anesthetized condition [5]. Enhanced cerebral blood flow in the conscious condition may reflect an increase in cerebrovascular reactivity that is normally depressed with anesthetics [18]. The effect of anesthesia on cerebrovascular reactivity and cerebral blood flow can be studied with MRI and hypercapnic challenge.
Cerebral arterial smooth muscle is very sensitive to the partial pressure of CO2 in the blood. In the presence of carbonic anhydrase, elevated CO2 is rapidly hydrated to form carbonic acid and its dissociation products, bicarbonate and hydrogen ions. The local acidic environment enhances the vasodilatory effects of adenosine [19] and increases potassium ion conductance across vascular smooth muscle [20] resulting in a passive dilation of blood vessels, decreased resistance and increased blood flow [21]. Indeed, the BOLD effect is mediated in part by metabolic changes in pH affecting cerebrovascular reactivity. Since hypercapnic challenge does not alter metabolic oxygen consumption [22] the change in T2* MRI signal caused by enhanced cerebral blood flow is directly related to the change in PaCO2 [23]. Mapping changes in T2* MRI signal in response to hypercapnic challenge is a simple, robust method for assessing cerebrovascular reactivity in functional imaging studies [24].
In the present study, animals were challenged with gas mixtures of 5% and 10% CO2 while conscious and anesthetized during a single imaging session. Cerebrovascular reactivity was much greater in the conscious condition as measured by dose-dependent changes in MRI signal to cortical and subcortical brain regions. While baseline signal-to-noise was higher in the conscious condition, the contrast-to-noise ratio of the hypercapnic challenge was equal to and greater than that of the anesthetized state. These findings indicate the enhanced T2* signal observed in conscious animals during hypercapnic challenge is due to increased cereborvascular reactivity.
Section snippets
Animal preparation
Male Sprague-Dawley rats weighing 200–300 gm were obtained from Charles River Laboratories (Charles River, MA). Animals were housed in pairs, maintained on 12:12 light:dark cycle (lights on at 9:00 h) and provided food and water ad libitum. All animals were acquired and cared for in accordance with the guidelines published in the Guide for the Care and Use of Laboratory Animals (National Institutes of Health Publications No. 85-23, Revised 1985).
On the day of imaging, animals (n = 6) were
Physiological measures
Table 1 summarizes the physiological data collected prior to and during the imaging trials for each of the four conditions. There was a significant decrease in blood pH for both conscious and anesthetized conditions during inhalation of CO2 corresponding to a concentration-dependent increase in PaCO2. There was also a significant concentration- dependent, linear increase in the rate of respiration in the anesthetized condition but not the conscious conditions in response to hypercapnia.
Discussion
The present study clearly demonstrates a difference in cerebrovascular reactivity to hypercapnic challenge in conscious and isoflurane anesthetized rats. The enhanced T2* MRI signal in the conscious condition was concentration-dependent, robust and rapid in onset. While there was a significant increase in T2* MRI signal response to hypercapnia in the anesthetized condition, there was no clear dose-dependence to this response. The depressed responsiveness of cerebral vascular smooth muscle to
Conclusion
These data confirm the feasibility of fMRI studies in fully conscious animals. The increased responsiveness to carbon dioxide seen in conscious animals suggest that the cerebrovasculature of these animals is more sensitive to the factors that contribute to BOLD signal during neuronal activity. The benefits of studying conscious animals; increased BOLD signal, increased contrast to noise, higher orders of sensory input and cognition etc, seem to encourage the further use of conscious animals in
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