Regional cerebral blood flow and BOLD responses in conscious and anesthetized rats under basal and hypercapnic conditions: implications for functional MRI studies

Blood oxygenation level-dependent (BOLD) functional MRI of visual stimulation in the rat retina at 11.7 T

Although optically based imaging techniques provide valuable functional and physiological information of the retina, they are mostly limited to the probing of the retinal surface and require an unobstructed light path. MRI, in contrast, could offer physiological and functional data without depth limitation. Blood oxygenation level-dependent functional MRI (BOLD fMRI) of the thin rat retina is, however, challenging because of the need for high spatial resolution, and the potential presence of eye movement and susceptibility artifacts. This study reports a novel application of high-resolution (111 × 111 × 1000 µm(3)) BOLD fMRI of visual stimulation in the anesthetized rat retina at 11.7 T. A high-field MRI scanner was utilized to improve the signal-to-noise ratio, spatial resolution and BOLD sensitivity. Visual stimuli (8 Hz diffuse achromatic light) robustly increased BOLD responses in the retina [5.0 ± 0.8% from activated pixels and 3.1 ± 1.1% from the whole-retina region of interest (mean ± SD), n = 12 trials on six rats, p < 0.05 compared with baseline]. Some activated pixels were detected surrounding the pupil and ciliary muscle because of accommodation reflex to visual stimuli, and were reduced with atropine and phenylephrine eye drops. BOLD fMRI scans without visual stimulations showed no significantly activated pixels (whole-retina BOLD changes were 0.08 ± 0.34%, n = 6 trials on five rats, not statistically different from baseline, p > 0.05). BOLD fMRI of visual stimulation has the potential to provide clinically relevant data to probe hemodynamic neurovascular coupling and dysfunction of the retina with depth resolution.

Baseline CBF, and BOLD, CBF, and CMRO2 fMRI of visual and vibrotactile stimulations in baboons

Neurovascular coupling associated with visual and vibrotactile stimulations in baboons anesthetized sequentially with isoflurane and ketamine was evaluated using multimodal functional magnetic resonance imaging (fMRI) on a clinical 3-Tesla scanner. Basal cerebral blood flow (CBF), and combined blood-oxygenation-level-dependent (BOLD) and CBF fMRI of visual and somatosensory stimulations were measured using pseudo-continuous arterial spin labeling. Changes in stimulus-evoked cerebral metabolic rate of oxygen (CMRO(2)) were estimated using calibrated fMRI. Arterial transit time for vessel, gray matter (GM), and white matter (WM) were 250, 570, and 823 ms, respectively. Gray matter and WM CBF, respectively, were 107.8±7.9 and 47.8±3.8 mL per 100 g per minute under isoflurane, and 108.8±10.3 and 48.7±4.2 mL per 100 g per minute under ketamine (mean±s.e.m., N=8 sessions, five baboons). The GM/WM CBF ratio was not statistically different between the two anesthetics, averaging 2.3±0.1. Hypercapnia evoked global BOLD and CBF increases. Blood-oxygenation-level-dependent, CBF, and CMRO(2) signal changes by visual and vibrotactile stimulations were 0.19% to 0.22%, 18% to 23%, and 4.9% to 6.7%, respectively. The CBF/CMRO(2) ratio was 2.9 to 4.7. Basal CBF and fMRI responses were not statistically different between the two anesthetics. This study establishes a multimodal fMRI protocol to probe clinically relevant functional, physiological and metabolic information in large nonhuman primates.

MRI of retinal and choroidal blood flow with laminar resolution

The retina is nourished by two distinct circulations: the retinal vessels within the inner retina and the choroidal vessels behind the neural retina. The outer nuclear layer and the inner and outer segments of the photoreceptors in between are avascular. The aim of this study was to determine whether arterial spin labeling MRI could provide sufficient resolution to differentiate between quantitative retinal blood flow (rBF) and choroidal blood flow (chBF), and whether this technique is sufficiently sensitive to detect vascular-specific blood flow (BF) changes modulated by anesthetics. Arterial spin labeling MRI was performed at 42 × 42 × 400 µm(3) in the mouse retina at 7 T, and was used to investigate the effects of isoflurane and ketamine/xylazine anesthesia on rBF and chBF. MRI yielded unambiguous differentiation of rBF, chBF and the avascular layer in between. Under isoflurane, chBF was 7.7 ± 2.1 mL/g/min and rBF was 1.3 ± 0.44 mL/g/min (mean ± SD, n = 7, p < 0.01). Under ketamine/xylazine anesthesia in the same animals, chBF was 4.3 ± 1.9 mL/g/min and rBF was 0.88 ± 0.22 mL/g/min (p < 0.01). Under ketamine/xylazine anesthesia, rBF was lower by 29% (P < 0.01) and chBF by 42% (P < 0.01) relative to isoflurane. This study demonstrates, for the first time, the quantitative imaging of rBF and chBF in vivo, providing a new method to study basal values and alterations of rBF and chBF.

MRI of retinal and choroidal blood flow with laminar resolution

The retina is nourished by two distinct circulations: the retinal vessels within the inner retina and the choroidal vessels behind the neural retina. The outer nuclear layer and the inner and outer segments of the photoreceptors in between are avascular. The aim of this study was to determine whether arterial spin labeling MRI could provide sufficient resolution to differentiate between quantitative retinal blood flow (rBF) and choroidal blood flow (chBF), and whether this technique is sufficiently sensitive to detect vascular-specific blood flow (BF) changes modulated by anesthetics. Arterial spin labeling MRI was performed at 42 × 42 × 400 µm(3) in the mouse retina at 7 T, and was used to investigate the effects of isoflurane and ketamine/xylazine anesthesia on rBF and chBF. MRI yielded unambiguous differentiation of rBF, chBF and the avascular layer in between. Under isoflurane, chBF was 7.7 ± 2.1 mL/g/min and rBF was 1.3 ± 0.44 mL/g/min (mean ± SD, n = 7, p < 0.01). Under ketamine/xylazine anesthesia in the same animals, chBF was 4.3 ± 1.9 mL/g/min and rBF was 0.88 ± 0.22 mL/g/min (p < 0.01). Under ketamine/xylazine anesthesia, rBF was lower by 29% (P < 0.01) and chBF by 42% (P < 0.01) relative to isoflurane. This study demonstrates, for the first time, the quantitative imaging of rBF and chBF in vivo, providing a new method to study basal values and alterations of rBF and chBF.

Longitudinal functional magnetic resonance imaging in animal models

Functional magnetic resonance imaging (fMRI) has had an essential role in furthering our understanding of brain physiology and function. fMRI techniques are nowadays widely applied in neuroscience research, as well as in translational and clinical studies. The use of animal models in fMRI studies has been fundamental in helping elucidate the mechanisms of cerebral blood-flow regulation, and in the exploration of basic neuroscience questions, such as the mechanisms of perception, behavior, and cognition. Because animals are inherently non-compliant, most fMRI performed to date have required the use of anesthesia, which interferes with brain function and compromises interpretability and applicability of results to our understanding of human brain function. An alternative approach that eliminates the need for anesthesia involves training the animal to tolerate physical restraint during the data acquisition. In the present chapter, we review these two different approaches to obtaining fMRI data from animal models, with a specific focus on the acquisition of longitudinal data from the same subjects.

Technical and conceptual considerations for performing and interpreting functional MRI studies in awake rats

Functional neuroimaging studies in rodents have the potential to provide insight into neurodevelopmental and psychiatric conditions. The strength of the technique lies in its non-invasive nature that can permit longitudinal functional studies in the same animal over its adult life. The relatively good spatial and temporal resolution and the ever-growing database on the biological and biophysical basis of the blood oxygen level dependent (BOLD) signal make it a unique technique in preclinical neuroscience research. Our laboratory has used imaging to investigate brain activation in awake rats following cocaine administration and during the presentation of lactation-associated sensory stimuli. Factors that deserve attention when planning functional magnetic resonance imaging studies in rats include technical issues, animal physiology and interpretability of the resulting data. The present review discusses the pros and cons of animal imaging with a particular focus on the technical aspects of studies with awake rats. Overall, the benefits of the technique outweigh its limitations and the rapidly evolving methods will open the way for more laboratories to employ the technique in neuroscience research.

Mapping brain networks in awake mice using combined optical neural control and fMRI

Behaviors and brain disorders involve neural circuits that are widely distributed in the brain. The ability to map the functional connectivity of distributed circuits, and to assess how this connectivity evolves over time, will be facilitated by methods for characterizing the network impact of activating a specific subcircuit, cell type, or projection pathway. We describe here an approach using high-resolution blood oxygenation level-dependent (BOLD) functional MRI (fMRI) of the awake mouse brain-to measure the distributed BOLD response evoked by optical activation of a local, defined cell class expressing the light-gated ion channel channelrhodopsin-2 (ChR2). The utility of this opto-fMRI approach was explored by identifying known cortical and subcortical targets of pyramidal cells of the primary somatosensory cortex (SI) and by analyzing how the set of regions recruited by optogenetically driven SI activity differs between the awake and anesthetized states. Results showed positive BOLD responses in a distributed network that included secondary somatosensory cortex (SII), primary motor cortex (MI), caudoputamen (CP), and contralateral SI (c-SI). Measures in awake compared with anesthetized mice (0.7% isoflurane) showed significantly increased BOLD response in the local region (SI) and indirectly stimulated regions (SII, MI, CP, and c-SI), as well as increased BOLD signal temporal correlations between pairs of regions. These collective results suggest opto-fMRI can provide a controlled means for characterizing the distributed network downstream of a defined cell class in the awake brain. Opto-fMRI may find use in examining causal links between defined circuit elements in diverse behaviors and pathologies.

CNS animal fMRI in pain and analgesia

Animal imaging of brain systems offers exciting opportunities to better understand the neurobiology of pain and analgesia. Overall functional studies have lagged behind human studies as a result of technical issues including the use of anesthesia. Now that many of these issues have been overcome including the possibility of imaging awake animals, there are new opportunities to study whole brain systems neurobiology of acute and chronic pain as well as analgesic effects on brain systems de novo (using pharmacological MRI) or testing in animal models of pain. Understanding brain networks in these areas may provide new insights into translational science, and use neural networks as a "language of translation" between preclinical to clinical models. In this review we evaluate the role of functional and anatomical imaging in furthering our understanding in pain and analgesia.

BOLD fMRI of visual and somatosensory-motor stimulations in baboons

Baboon, with its large brain size and extensive cortical folding compared to other non-human primates, serves as a good model for neuroscience research. This study reports the implementation of a baboon model for blood oxygenation level-dependent (BOLD) fMRI studies (1.5 x 1.5 x 4 mm resolution) on a clinical 3T-MRI scanner. BOLD fMRI responses to hypercapnic (5% CO(2)) challenge, 10 Hz flicker visual, and vibrotactile somatosensory-motor stimulations were investigated in baboons anesthetized sequentially with isoflurane and ketamine. Hypercapnia evoked robust BOLD increases. Paralysis was determined to be necessary to achieve reproducible functional activations within and between subjects under our experimental conditions. With optimized anesthetic doses (0.8-1.0% isoflurane or 6-8 mg/kg/h ketamine) and adequate paralysis (vecuronium, 0.2 mg/kg), robust activations were detected in the visual (V), primary (S1) and secondary (S2) somatosensory, primary motor (M cortices), supplementary motor area (SMA), lateral geniculate nucleus (LGN) and thalamus (Th). Data were tabulated for 11 trials under isoflurane and 10 trials under ketamine on 5 baboons. S1, S2, M, and V activations were detected in essentially all trials (90-100% of the time, except 82% for S2 under isoflurane and 70% for M under ketamine). LGN activations were detected 64-70% of the time under both anesthetics. SMA and Th activations were detected 36-45% of the time under isoflurane and 60% of the time under ketamine. BOLD percent changes among different structures were slightly higher under ketamine than isoflurane (0.75% versus 0.58% averaging all structures), but none was statistically different (P>0.05). This baboon model offers an opportunity to non-invasively image brain functions and dysfunctions in large non-human primates.

Mapping resting-state brain networks in conscious animals

In the present study we mapped brain functional connectivity in the conscious rat at the "resting state" based on intrinsic blood-oxygenation-level dependent (BOLD) fluctuations. The conscious condition eliminated potential confounding effects of anesthetic agents on the connectivity between brain regions. Indeed, using correlational analysis we identified multiple cortical and subcortical regions that demonstrated temporally synchronous variation with anatomically well-defined regions that are crucial to cognitive and emotional information processing including the prefrontal cortex (PFC), thalamus and retrosplenial cortex. The functional connectivity maps created were stringently validated by controlling for false positive detection of correlation, the physiologic basis of the signal source, as well as quantitatively evaluating the reproducibility of maps. Taken together, the present study has demonstrated the feasibility of assessing functional connectivity in conscious animals using fMRI and thus provided a convenient and non-invasive tool to systematically investigate the connectional architecture of selected brain networks in multiple animal models.

The effect of different anesthetics on neurovascular coupling

To date, the majority of neurovascular coupling studies focused on the thalamic afferents' activity in layer IV and the corresponding large spiking activity as responsible for functional hyperemia. This paper highlights the role of the secondary and late cortico-cortical transmission in neurovascular coupling. Simultaneous scalp electroencephalography (EEG) and diffuse optical imaging (DOI) measurements were obtained during multiple conditions of event-related electrical forepaw stimulation in 33 male Sprague-Dawley rats divided into 6 groups depending on the maintaining anesthetic - alpha-chloralose, pentobarbital, ketamine-xylazine, fentanyl-droperidol, isoflurane, or propofol. The somatosensory evoked potentials (SEP) were decomposed into four components and the question of which best predicts the hemodynamic responses was investigated. Results of the linear regression analysis show that the hemodynamic response is best correlated with the secondary and late cortico-cortical transmissions and not with the initial thalamic input activity in layer IV. Baseline cerebral blood flow (CBF) interacts with neural activity and influences the evoked hemodynamic responses. Finally, neurovascular coupling appears to be the same across all anesthetics used.

Propofol allows precise quantitative arterial spin labelling functional magnetic resonance imaging in the rat

Functional magnetic resonance imaging (fMRI) techniques highlight cerebral vascular responses which are coupled to changes in neural activation. However, two major difficulties arise when employing these techniques in animal studies. First is the disturbance of cerebral blood flow due to anaesthesia and second is the difficulty of precise reproducible quantitative measurements. These difficulties were surmounted in the current study by using propofol and quantitative arterial spin labelling (QASL) to measure relative cerebral blood volume of labelled water (rCBV(lw),) mean transit time (MTT) and capillary transit time (CTT). The ASL method was applied to measure the haemodynamic response in the primary somatosensory cortex following forepaw stimulation in the rat. Following stimulation an increase in signal intensity and rCBV(lw) was recorded, this was accompanied by a significant decrease in MTT (1.97+/-0.06s to 1.44+/-0.04s) and CTT (1.76+/-0.06s to 1.39+/-0.07s). Two animals were scanned repeatedly on two different experimental days. Stimulation in the first animal was applied to the same forepaw during the initial and repeat scan. In the second animal stimulation was applied to different forepaws on the first and second days. The control and activated ASL signal intensities, rCBVlw on both days were almost identical in both animals. The basal MTT and CTT during the second scan were also very similar to the values obtained during the first scan. The MTT recorded from the animal that underwent stimulation to the same paw during both scanning sessions was very similar on the first and second days. In conclusion, propofol induces little physiological disturbance and holds potential for longitudinal QASL fMRI studies.

Arterial spin labeling demonstrates that focal amygdalar glutamatergic agonist infusion leads to rapid diffuse cerebral activation

OBJECTIVES

To investigate acute effects of intra-amygdalar excitatory amino acid administration on blood flow, relaxation time and apparent diffusion coefficient in rat brain.

MATERIALS AND METHODS

Several days after MR-compatible cannula placement in right basolateral amygdala, anesthetized rats were imaged at 7 T. Relative cerebral blood flow (CBF) was measured before and 60 min after infusion of 10 nmol KA, cAMPA, ATPA, or normal saline using arterial spin labeling. Quantitative T(2) and diffusion-weighted images were acquired. rCBF, T(2) and ADC values were evaluated in bilateral basolateral amygdala, hippocampus, basal ganglia, frontal and parietal regions.

RESULTS

KA led to the highest, and ATPA lowest bilateral rCBF increases. Time courses varied among drugs. T(2) for KA and AMPA was higher while ADC was lower for KA.

CONCLUSIONS

Intra-amygdalar injection of GluR agonists evoked bilateral seizure activity and increased rCBF, greater for KA and AMPA than selective ATPA GluR5 activation.

Diffusion tensor and perfusion MRI of non-human primates

This paper reviews recent non-human primate (NHP) neuroimaging literature using MRI in macaque, baboon and chimpanzee. It describes general challenges and limitations for NHP MRI studies, and reviews recent applications of anatomical, diffusion tensor, cerebral blood flow MRI. Applications to NHP stroke is discussed in some detail.

Effects of hypercapnia, hypocapnia, and hyperoxemia on blood oxygenation level-dependent signal intensity determined by use of susceptibility-weighted magnetic resonance imaging in isoflurane-anesthetized dogs

OBJECTIVE

To assess the effects of alterations in PaCO(2) and PaO(2) on blood oxygenation level-dependent (BOLD) signal intensity determined by use of susceptibility-weighted magnetic resonance imaging in brains of isoflurane-anesthetized dogs.

ANIMALS

6 healthy dogs.

PROCEDURES

In each dog, anesthesia was induced with propofol (6 to 8 mg/kg, IV) and maintained with isoflurane (1.7%) and atracurium (0.2 mg/kg, IV, q 30 min). During 1 magnetic resonance imaging session in each dog, targeted values of PaCO(2) (20, 40, or 80 mm Hg) and PaO(2) (100 or 500 mm Hg) were combined to establish 6 experimental conditions, including a control condition (PaCO(2), 40 mm Hg; PaO(2), 100 mm Hg). Dogs were randomly assigned to different sequences of conditions. Each condition was established for a period of >or= 5 minutes before susceptibility-weighted imaging was performed. Signal intensity was measured in 6 regions of interest in the brain, and data were analyzed by use of an ANCOVA and post hoc Tukey-Kramer adjustments.

RESULTS

Compared with control condition findings, BOLD signal intensity did not differ significantly in any region of interest. However, signal intensities in the thalamus and diencephalic gray matter decreased significantly during both hypocapnic conditions, compared with all other conditions except for the control condition.

CONCLUSIONS AND CLINICAL RELEVANCE

In isoflurane-anesthetized dogs, certain regions of gray matter appeared to have greater cerebrovascular responses to changes in PaCO(2) and PaO(2) than did others. Both PaO(2) and PaCO(2) should be controlled during magnetic resonance imaging procedures that involve BOLD signaling and taken into account when interpreting findings.

A Dynamic Causal Model of the Coupling Between Pulse Stimulation and Neural Activity

We present a dynamic causal model that can explain context-dependent changes in neural responses, in the rat barrel cortex, to an electrical whisker stimulation at different frequencies. Neural responses were measured in terms of local field potentials. These were converted into current source density (CSD) data, and the time series of the CSD sink was extracted to provide a time series response train. The model structure consists of three layers (approximating the responses from the brain stem to the thalamus and then the barrel cortex), and the latter two layers contain nonlinearly coupled modules of linear second-order dynamic systems. The interaction of these modules forms a nonlinear regulatory system that determines the temporal structure of the neural response amplitude for the thalamic and cortical layers.

The model is based on the measured population dynamics of neurons rather than the dynamics of a single neuron and was evaluated against CSD data from experiments with varying stimulation frequency (1–40 Hz), random pulse trains, and awake and anesthetized animals. The model parameters obtained by optimization for different physiological conditions (anesthetized or awake) were significantly different.

Following Friston, Mechelli, Turner, and Price ( 2000 ), this work is part of a formal mathematical system currently being developed (Zheng et al., 2005 ) that links stimulation to the blood oxygen level dependent (BOLD) functional magnetic resonance imaging (fMRI) signal through neural activity and hemodynamic variables. The importance of the model described here is that it can be used to invert the hemodynamic measurements of changes in blood flow to estimate the underlying neural activity.

Magnetic resonance imaging of the retina

This paper reviews recent developments in high-resolution magnetic resonance imaging (MRI) and its application to image anatomy, physiology, and function in the retina of animals. It describes technical issues and solutions in performing retinal MRI, anatomical MRI, blood oxygenation level-dependent functional MRI (fMRI), and blood-flow MRI both of normal retinas and of retinal degeneration. MRI offers unique advantages over existing retinal imaging techniques, including the ability to image multiple layers without depth limitation and to provide multiple clinically relevant data in a single setting. Retinal MRI has the potential to complement existing retinal imaging techniques.

Basal and hypercapnia-altered cerebrovascular perfusion predict mild cognitive impairment in aging rodents

With increasing age, a subset of otherwise healthy individuals undergoes impairments in learning and memory that have been termed mild cognitive impairment (MCI). The enhanced neuronal activity associated with learning and memory requires increased cerebral blood flow (CBF) to specific brain regions. However, the interactions between cerebral blood flow and MCI remain unclear. In this study, we address whether baseline or hypercapnia-induced (increased blood CO(2) levels) changes in CBF are modified with age, and whether these measures are predictive of cognitive status in rodents. Adult and aged rats were evaluated using a hippocampally-dependent task in a water maze. Aged rats were classified as memory-impaired or memory-intact based on performance comparisons with adult rats. Cerebral blood flow was assessed using flow-alternating inversion recovery (FAIR) magnetic resonance imaging (MRI), before and after breathing 10% CO(2). The transition period between CO(2) concentrations was examined with blood oxygen level dependent (BOLD) MRI. Separation of aged animals into memory-intact and impaired categories revealed increased basal perfusion in the dorsal hippocampus of memory-impaired versus memory-intact aged animals. Linear regression revealed that higher hippocampal perfusion was correlated with impaired memory in aged animals, and a logistic regression indicated that hippocampal perfusion predicted spatial memory ability. Several brain regions of aged rats demonstrated an attenuation of the perfusion increase normally observed in adult rats under hypercapnia. Memory-impaired animals were the primary contributor to this effect, as their perfusion response to hypercapnia was significantly reduced compared to adult animals. Aged, memory-intact animals were not significantly different from adults. BOLD MRI demonstrated a reduced response in aged animals to hypercapnia, with impaired animals being the primary contributor to the effect. A logistic regression model based on basal and hypercapnia perfusion correctly predicted cognitive status in 83.3% of animals tested. Our results indicate that age-related changes in vascular reactivity and perfusion are important contributing factors in memory impairment.

Dose-dependent effect of isoflurane on neurovascular coupling in rat cerebral cortex

Neurovascular coupling studies are widely conducted in anesthetized animals using functional magnetic resonance imaging (fMRI). In this study, the dose-dependent effects of isoflurane on neurovascular coupling were examined with concurrent recordings of the local field potential (FP) and cerebral blood flow (CBF) in the rat somatosensory cortex. Electrical forepaw stimulation was used, and consisted of either a single pulse or 10 pulses at various frequencies. We observed that the FP response to single-pulse stimulation remained unaffected across the different levels of isoflurane tested (1.1-2.1%), whereas the CBF response to single-pulse stimulation increased dose-dependently (7 +/- 3% to 17 +/- 4%). The isoflurane dose did not affect the vascular reactivity induced by a hypercapnic challenge. These findings suggest that the action of isoflurane affects the neurovascular mechanisms. For 10-pulse stimulation, the summation of the evoked FP responses monotonically decreased with an increase in the isoflurane dose, possibly due to enhancement of the neural adaptation. In contrast, the dose-dependent effect on the CBF response varied with the stimulus frequency; a dose-dependent decrease in the CBF response was observed for high-frequency stimulation, whereas a dose-dependent increase was observed for low-frequency stimulation. Furthermore, a linear time-invariant model consisting of the single-pulse hemodynamic impulse response convoluted with 10-pulse FP recordings showed that the neurovascular transfer function was altered by the isoflurane dose for high-frequency stimulation. These results indicate that careful and consistent maintenance of the depth of anesthesia is required when comparing fMRI data obtained from different animals or physiological and pharmacological manipulations.

Isoflurane anesthesia is a valuable alternative for alpha-chloralose anesthesia in the forepaw stimulation model in rats

Isoflurane (ISO) can be a valuable alternative for alpha-chloralose (ACL) anesthesia in functional MRI (fMRI) studies. Therefore, we compared the efficacy of the blood oxygen level dependent (BOLD) effect in fMRI studies during ISO and ACL anesthesia sequentially in the same animals. After non-invasive instrumentation for ventilation and monitoring, series of T2* weighted MR images were acquired during forepaw stimulation, first under ISO, then followed by ACL anesthesia. The results demonstrated that ISO and ACL were both suitable to perform this fMRI experiment. The center of activation was at the same stereotactic position for both anesthetics and matched the primary somatosensory cortex (S1). Under the applied conditions, the BOLD response during ISO anesthesia declined in magnitude during the first stimulation period, as compared to ACL. From this study, we conclude that since ISO has several positive properties in comparison to ACL, including fast pharmacokinetics and suitability for repeated measurements, it is a valuable alternative for anesthesia in fMRI studies of rats.

Evolution of the neurochemical profile after transient focal cerebral ischemia in the mouse brain

Evolution of the neurochemical profile consisting of 19 metabolites after 30 mins of middle cerebral artery occlusion was longitudinally assessed at 3, 8 and 24 h in 6 to 8 microL volumes in the striatum using localized 1H-magnetic resonance spectroscopy at 14.1 T. Profound changes were detected as early as 3 h after ischemia, which include elevated lactate levels in the presence of significant glucose concentrations, decreases in glutamate and a transient twofold glutamine increase, likely to be linked to the excitotoxic release of glutamate and conversion into glial glutamine. Interestingly, decreases in N-acetyl-aspartate (NAA), as well as in taurine, exceeded those in neuronal glutamate, suggesting that the putative neuronal marker NAA is rather a sensitive marker of neuronal viability. With further ischemia evolution, additional, more profound concentration decreases were detected, reflecting a disruption of cellular functions. We conclude that early changes in markers of energy metabolism, glutamate excitotoxicity and neuronal viability can be detected with high precision non-invasively in mice after stroke. Such investigations should lead to a better understanding and insight into the sequential early changes in the brain parenchyma after ischemia, which could be used for identifying new targets for neuroprotection.

Peculiar response to methylphenidate in adolescent compared to adult rats: a phMRI study

RATIONALE

Adolescent rodents differ markedly from adults in several neuro-behavioural parameters. Moreover, 'paradoxical' responses to psychostimulants have been reported at this age.

OBJECTIVES

Thus, we investigated the responses of adolescent (post-natal day, PND, 34 to 43) and adult (PND >60) Sprague-Dawley male rats to the psychostimulant drug methylphenidate (MPH). We used pharmacological magnetic resonance imaging (phMRI) performed at 4.7 T under isoflurane anaesthesia. Following anatomical MRI, axial gradient echo images were collected continuously. After baseline recording (32 min), animals received MPH (0 or 4 mg/kg i.p.) and were recorded for further 32 min.

RESULTS

Region-specific changes in the blood-oxygenation level dependent (BOLD) signal were evident as a function of age. As expected, among adults MPH induced an increase of BOLD signal in nucleus accumbens (NAcc) and prefrontal cortex (PFC), with no effects in the hippocampus (Hip). Notably, among adolescents, MPH induced a marked and generalised decrease of BOLD signal, which occurred earlier in NAcc and PFC whilst being delayed in the Hip. Any bias in BOLD responses was excluded by the measurement of physiological parameters.

CONCLUSIONS

The present findings highlight the utility of phMRI in animal models. The peculiar negative BOLD effect found in adolescent rats may be suggestive of a reduced cerebro-vascular feedback and/or an increased MPH-induced neuronal activation. Data are relevant for a better understanding of brain/behavioural regulation during adolescent development. Moreover, a greater understanding of the differences between adult and adolescent drug responses will aid in the development of a more appropriate age-specific treatment strategy.

Cerebral blood flow MRI in mice using the cardiac-spin-labeling technique

Continuous arterial spin labeling MRI with a separate neck labeling coil provides a highly sensitive method to image cerebral blood flow (CBF). In mice, however, this has not been possible because the proximity of the neck coil to the brain uses the neck coil to significantly saturate the brain signal. To overcome this limitation the cardiac spin labeling (CSL) technique is introduced in which the labeling coil is placed at the heart position. To demonstrate its utility, CSL CBF was applied to image quantitative basal CBF and hypercapnia-induced CBF changes. This approach provides a practical means to image CBF with high sensitivity in small animals, compares favorably to existing mouse CBF imaging techniques, and could broaden CBF applications in mice where many brain disease and transgenic models are widely available.

Anesthetic effects on regional CBF, BOLD, and the coupling between task-induced changes in CBF and BOLD: an fMRI study in normal human subjects

Functional MR imaging was performed in sixteen healthy human subjects measuring both regional cerebral blood flow (CBF) and blood oxygen level dependent (BOLD) signal when visual and auditory stimuli were presented to subjects in the presence or absence of anesthesia. During anesthesia, 0.25 mean alveolar concentration (MAC) sevoflurane was administrated. We found that low-dose sevoflurane decreased the task-induced changes in both BOLD and CBF. Within the visual and auditory regions of interest inspected, both baseline CBF and the task-induced changes in CBF decreased significantly during anesthesia. Low-dose sevoflurane significantly altered the task-induced CBF-BOLD coupling; for a unit change of CBF, a larger change in BOLD was observed in the anesthesia condition than in the anesthesia-free condition. Low-dose sevoflurane was also found to have significant impact on the spatial nonuniformity of the task-induced coupling. The alteration of task-induced CBF-BOLD coupling by low-dose sevoflurane introduces ambiguity to the direct interpretation of functional MRI (fMRI) data based on only one of the indirect measures-CBF or BOLD. Our observations also indicate that the manipulation of the brain with an anesthetic agent complicates the model-based quantitative interpretation of fMRI data, in which the relative task-induced changes in oxidative metabolism are calculated by means of a calibrated model given the relative changes in the indirect vascular measures, usually CBF and BOLD.

Imaging the neural circuitry and chemical control of aggressive motivation

BACKGROUND

With the advent of functional magnetic resonance imaging (fMRI) in awake animals it is possible to resolve patterns of neuronal activity across the entire brain with high spatial and temporal resolution. Synchronized changes in neuronal activity across multiple brain areas can be viewed as functional neuroanatomical circuits coordinating the thoughts, memories and emotions for particular behaviors. To this end, fMRI in conscious rats combined with 3D computational analysis was used to identifying the putative distributed neural circuit involved in aggressive motivation and how this circuit is affected by drugs that block aggressive behavior.

RESULTS

To trigger aggressive motivation, male rats were presented with their female cage mate plus a novel male intruder in the bore of the magnet during image acquisition. As expected, brain areas previously identified as critical in the organization and expression of aggressive behavior were activated, e.g., lateral hypothalamus, medial basal amygdala. Unexpected was the intense activation of the forebrain cortex and anterior thalamic nuclei. Oral administration of a selective vasopressin V1a receptor antagonist SRX251 or the selective serotonin reuptake inhibitor fluoxetine, drugs that block aggressive behavior, both caused a general suppression of the distributed neural circuit involved in aggressive motivation. However, the effect of SRX251, but not fluoxetine, was specific to aggression as brain activation in response to a novel sexually receptive female was unaffected.

CONCLUSION

The putative neural circuit of aggressive motivation identified with fMRI includes neural substrates contributing to emotional expression (i.e. cortical and medial amygdala, BNST, lateral hypothalamus), emotional experience (i.e. hippocampus, forebrain cortex, anterior cingulate, retrosplenial cortex) and the anterior thalamic nuclei that bridge the motor and cognitive components of aggressive responding. Drugs that block vasopressin neurotransmission or enhance serotonin activity suppress activity in this putative neural circuit of aggressive motivation, particularly the anterior thalamic nuclei.

Layer-specific anatomical, physiological and functional MRI of the retina

Most retinal imaging has been performed using optical techniques. This paper reviews alternative retinal imaging methods based on MRI performed with spatial resolution sufficient to resolve multiple well-defined retinal layers. The development of these MRI technologies to study retinal anatomy, physiology (blood flow, blood volume, and oxygenation) and function, and their applications to the study of normal retinas, retinal degeneration and diabetic retinopathy in animal models are discussed. Although the spatiotemporal resolution of MRI is poorer than that of optical imaging techniques, it is unhampered by media opacity and can thus image all retinal and pararetinal structures, and has the potential to provide multiple unique clinically relevant data in a single setting and could thus complement existing retinal imaging techniques. In turn, the highly structured retina with well-defined layers is an excellent model for advancing emerging high-resolution anatomical, physiological and functional MRI technologies.

Layer-specific anatomical, physiological and functional MRI of the retina

Most retinal imaging has been performed using optical techniques. This paper reviews alternative retinal imaging methods based on MRI performed with spatial resolution sufficient to resolve multiple well-defined retinal layers. The development of these MRI technologies to study retinal anatomy, physiology (blood flow, blood volume, and oxygenation) and function, and their applications to the study of normal retinas, retinal degeneration and diabetic retinopathy in animal models are discussed. Although the spatiotemporal resolution of MRI is poorer than that of optical imaging techniques, it is unhampered by media opacity and can thus image all retinal and pararetinal structures, and has the potential to provide multiple unique clinically relevant data in a single setting and could thus complement existing retinal imaging techniques. In turn, the highly structured retina with well-defined layers is an excellent model for advancing emerging high-resolution anatomical, physiological and functional MRI technologies.

The influence of moderate hypercapnia on neural activity in the anesthetized nonhuman primate

Hypercapnia is often used as vasodilatory challenge in clinical applications and basic research. In functional magnetic resonance imaging (fMRI), elevated CO(2) is applied to derive stimulus-induced changes in the cerebral rate of oxygen consumption (CMRO(2)) by measuring cerebral blood flow and blood-oxygenation-level-dependent (BOLD) signal. Such methods, however, assume that hypercapnia has no direct effect on CMRO(2). In this study, we used combined intracortical recordings and fMRI in the visual cortex of anesthetized macaque monkeys to show that spontaneous neuronal activity is in fact significantly reduced by moderate hypercapnia. As expected, measurement of cerebral blood volume using an exogenous contrast agent and of BOLD signal showed that both are increased during hypercapnia. In contrast to this, spontaneous fluctuations of local field potentials in the beta and gamma frequency range as well as multiunit activity are reduced by approximately 15% during inhalation of 6% CO(2) (pCO(2) = 56 mmHg). A strong tendency toward a reduction of neuronal activity was also found at CO(2) inhalation of 3% (pCO(2) = 45 mmHg). This suggests that CMRO(2) might be reduced during hypercapnia and caution must be exercised when hypercapnia is applied to calibrate the BOLD signal.

Blood flow magnetic resonance imaging of retinal degeneration

PURPOSE

This study aims to investigate quantitative basal blood flow as well as hypercapnia- and hyperoxia-induced blood flow changes in the retinas of the Royal College of Surgeons (RCS) rats with spontaneous retinal degeneration, and to compare with those of normal rat retinas.

METHODS

Experiments were performed on male RCS rats at post-natal days P90 (n=4) and P220 (n=5), and on age-matched controls at P90 (n=7) and P220 (n=6). Hyperoxic (100% O(2)) and hypercapnic (5% CO(2), 21% O(2), balance N(2)) challenges were used to modulate blood flow. Quantitative baseline blood flow, and hypercapnia- and hyperoxia-induced blood flow changes in the retinas were imaged using continuous arterial spin labeling MRI at 90 x 90 x 1500 microm.

RESULTS

In the normal rat retinas, basal blood flow of the whole-retina was 5.5 mL/gram per min, significantly higher than those reported in the brain (approximately 1 mL/gram per min). Hyperoxia decreased blood flow due to vasoconstriction and hypercapnia increased blood flow due to vasodilation in the normal retinas. In the RCS rat retinas, basal blood flow was diminished significantly (P<0.05). Interestingly, absolute hyperoxia- and hypercapnia-induced blood flow changes in the RCS retinas were not statistically different from those in the normal retinas (P>0.05). However, blood flow percent changes in RCS retinas were significantly larger than in normal retinas due to lower basal blood flow in the RCS retinas.

CONCLUSIONS

Retinal degeneration markedly reduces basal blood flow but does not appear to impair vascular reactivity. These data also suggest caution when interpreting relative stimulus-evoked functional MRI changes in diseased states where basal parameters are significantly perturbed. Quantitative blood flow MRI may serve as a valuable tool to study the retina without depth limitation.

Experimental comparison of four FAIR arterial spin labeling techniques for quantification of mouse cerebral blood flow at 4.7 T

Pulsed arterial spin labeling (ASL) is an attractive and robust method for quantification of rodent cerebral blood flow (CBF) in particular, although there is a need for sensitivity optimization. Look-Locker flow-sensitive alternating inversion recovery (FAIR) echo planar imaging (EPI) (LLFAIREPI) was expected to be a likely candidate for assessing sensitivity, although it has not yet been applied to rodents. In this study, the performance of two FAIR techniques and two Look-Locker FAIR techniques were compared in mouse brain at 4.7 T. FAIR-EPI (single inversion time, FAIREPI-1TI), FAIR-EPI (eight inversion times, FAIREPI-8TI), LLFAIREPI and Look-Locker FAIR gradient echo (LLFAIRGE) sequences were implemented with equal spatial resolution and equal FAIR preparation modules. Measurements were carried out sequentially on the brain in 10 healthy mice, and quantitative CBF maps were obtained after different acquisition times up to 23 min. All methods gave similar group variability in CBF. Especially at shorter acquisition times, LLFAIREPI gave lower relative variations in CBF within selected brain regions than the other techniques at the same acquisition time. The Look-Locker techniques, however, overestimated CBF compared with classical FAIR-EPI, which was attributed to bulk flow in arterioles and T(2) effects. The image quality with LLFAIREPI was less reproducible within the group. Both FAIREPI-1TI and LLFAIREPI appear to be good candidates for serial rapid measurement of CBF, but LLFAIREPI has the additional advantage that apparent T(1) can be measured simultaneously with CBF.

Comparison of cerebral pharmacokinetics of buprenorphine and norbuprenorphine in anin vivosheep model

The pharmacokinetics and time course of blood-brain equilibration of buprenorphine (BUP) and norbuprenorphine (norBUP) in sheep were characterized. Sheep were administered 0.04 mg kg(-1) BUP or 0.6 mg kg(-1) norBUP as 4-min i.v. infusions. The cerebral kinetics were inferred from arterio-sagittal sinus concentration gradients and changes in cerebral blood flow. These data were fitted to physiologically based pharmacokinetic models. BUP cerebral kinetics were best described by a membrane-limited model with a large equilibration delay (half-life of 20 min) between brain and blood due to intermediate permeability (47 ml min(-1)) and a large cerebral distribution volume (595 ml). Significant limitation in permeability (6 ml min(-1)) characterized the cerebral kinetics of norBUP with a cerebral distribution volume (157 ml) giving a blood-brain equilibration half-life (21 min) similar to that for BUP. The logD of BUP and norBUP were 3.93 +/- 0.08 and 1.18 +/- 0.04 (mean +/- SD), respectively. Both compounds revealed slow cerebral equilibration with variations in degree of permeability and distribution volume reflecting the difference in lipophilicity. It is possible that norBUP contributes to the central effects seen after chronic BUP administration as this study demonstrated its entry into the brain.

Longitudinal MRI studies in the isoflurane-anesthetized rat: long-term effects of a short hypoxic episode on regulation of cerebral blood flow as assessed by pulsed arterial spin labelling

MRI is a powerful tool for measuring cerebral blood flow (CBF) longitudinally. However, most animal studies require anesthesia, potentially interfering with normal physiology. Isoflurane anesthesia was used here to study CBF regulation during repetitive scanning in rats. MR perfusion images were acquired using FAIR (flow-sensitive alternating inversion recovery) arterial spin labeling, and absolute CBF was calculated. CBF changes in response to a hypoxic (12% O2) and hypercapnic (5% CO2) gas stimulus were monitored. Hypercapnia led to a robust increase in CBF compared with baseline (195.5+/-21.5 vs 123.6+/-17.9 ml/100 g/min), and hypoxia caused a smaller non-significant increase in mean CBF values (145.4+/-13.4 ml/100 g/min). Strikingly, when measurements were repeated 5 days later, CBF was dramatically reduced in hypoxia (93.2+/-8.1 ml/100 g/min) compared with the first imaging session. Without application of the hypoxic and hypercapnic gases during the first MRI, baseline CBF and CBF changes in response to hypoxia at the second MRI were similar to naive rats. Blood gas analyses revealed a slight reduction in arterial oxygenation during the second period of anesthesia compared with the first. These findings indicate that, in isoflurane-anesthetized rats, even a short hypoxic episode can have long-lasting effects on cerebrovascular regulation.

Spinal cord blood flow measurement by arterial spin labeling

The assessment of spinal cord (SC) hemodynamics, and especially SC blood flow (SCBF), plays a key role in the pathophysiological description and understanding of many SC diseases such as ischemia, or spinal cord injury. SCBF has been previously measured in animals with invasive techniques such as autoradiography or labeled microspheres; no MR technique, however, has been proposed so far. The possibility of quantitatively measuring SCBF in mice using MRI was investigated using a presaturated FAIR (flow-sensitive alternating inversion recovery) arterial spin labeling (ASL) technique. SCBF measurements were performed at the cervical level of the mouse as well as on the brain so as to use cerebral blood flow (CBF) values as internal references. With a spatial resolution of 133 x 133 microm(2) for the SCBF maps, absolute regional perfusion values could be measured within the different structures of the SC (gray matter, white matter, and cerebrospinal fluid area). Similar perfusion values were found in SC gray matter (330+/-90 mL/100g/min) and in brain (295+/-22 mL/100g/min for thalamus). This result, in agreement with SCBF/CBF measurements performed with non-MR techniques, opens new perspectives for noninvasive longitudinal and in vivo animal studies. Application to human experiments may also be possible.

Differential recovery of behavioral status and brain function assessed with functional magnetic resonance imaging after mild traumatic brain injury in the rat

OBJECTIVE

The relationship between cerebral integrity, recovery of brain function, and neurologic status after mild traumatic brain injury is incompletely characterized.

DESIGN

Prospective and randomized study in rodents.

SETTING

University laboratory.

SUBJECTS

Male Wistar rats (290-310 g).

INTERVENTIONS

In rats, quantitative diffusion weighted imaging (DWI), perfusion weighted imaging (PWI), T2-weighted imaging (T2WI), and functional magnetic resonance imaging (fMRI) were performed up to 21 days after weight-induced, closed-head, mild traumatic brain injury (MTBI, n = 6) or sham operation (n = 6). Pixel-by-pixel analysis and region of interest analysis were used to evaluate structural (apparent diffusion coefficient [ADC] and basal cerebral blood flow [bCBF]) and functional magnetic resonance signal changes within the brain, respectively. Quantitative fMRI signal changes were correlated with behavioral measures.

MEASUREMENTS AND MAIN RESULTS

Despite normal appearing DWI and T2WI findings following MTBI, persistent hypoperfusion developed that was not associated with cytotoxic edema. In contrast, the ADC was significantly increased by approximately 5% at 1 and 7 days post-MTBI. Post-MTBI fMRI responses to hypercapnia and forepaw stimulation were significantly impaired and showed a differential recovery rate between and within investigated region of interests. Significant dysfunction in forepaw placement test persisted up to day 1 and correlated significantly with fMRI signal changes in the primary somatosensory and motor cortices.

CONCLUSIONS

MTBI produced distinct changes on multimodal MRI and behavioral variables acutely and chronically. Following MTBI, fMRI and ADC-bCBF pixel-by-pixel analysis identified subtle structural and functional alterations in the brain that appeared completely normal on conventional DWI and T2WI after concussion injury. The former techniques may therefore provide great potential for understanding mild traumatic brain injury, identifying mechanisms underlying recovery, and investigating specific interventions to enhance functional outcome.

Glial regulation of the cerebral microvasculature

The brain is a heterogeneous organ with regionally varied and constantly changing energetic needs. Blood vessels in the brain are equipped with control mechanisms that match oxygen and glucose delivery through blood flow with the local metabolic demands that are imposed by neural activity. However, the cellular bases of this mechanism have remained elusive. A major advance has been the demonstration that astrocytes, cells with extensive contacts with both synapses and cerebral blood vessels, participate in the increases in flow evoked by synaptic activity. Their organization in nonoverlapping spatial domains indicates that they are uniquely positioned to shape the spatial distribution of the vascular responses that are evoked by neural activity. Astrocytic calcium is an important determinant of microvascular function and may regulate flow independently of synaptic activity. The involvement of astrocytes in neurovascular coupling has broad implications for the interpretation of functional imaging signals and for the understanding of brain diseases that are associated with neurovascular dysfunction.

Alpha-chloralose is a suitable anesthetic for chronic focal cerebral ischemia studies in the rat: a comparative study

alpha-Chloralose is widely used as an anesthetic in studies of the cerebrovasculature because it provides robust metabolic and hemodynamic responses to functional stimulation. However, there have been no controlled studies of focal ischemia in the rat under alpha-chloralose anesthesia. Artificially ventilated rats were prepared using 1.2-1.5% isoflurane anesthesia for filament occlusion of the right middle cerebral artery (MCA), and anesthesia was either switched to alpha-chloralose (60 mg/kg bolus, 30 mg/kg/h; n=10) or was maintained on 1% isoflurane (n=10). Following temporary MCA occlusion EEG was monitored from a screw electrode and changes in cerebral blood flow (rCBF) measured with a laser Doppler probe placed over the ischemic cortex. This study shows that alpha-chloralose is a safe anesthetic for ischemia studies and provides excellent survival. Compared with isoflurane, the cortical and total infarct volumes are larger in the alpha-chloralose-anesthetized animals, while the functional outcome at 72 h is similar. The total duration of peri-infarct flow transients (PIFTs) is also significantly longer in alpha-chloralose-anesthetized animals. The average amplitude of the flow transients showed a good correlation with the extent of edema in all animals as did the total duration of non-convulsive seizures (NCS) in the alpha-chloralose-anesthetized animals.

Blood-flow magnetic resonance imaging of the retina

This study describes a novel MRI application to image basal blood flow, physiologically induced blood-flow changes, and the effects of isoflurane concentration on blood flow in the retina. Continuous arterial-spin-labeling technique with a separate neck coil for spin labeling was used to image blood flow of the rat retina at 90 x 90 x 1500-microm resolution. The average blood flow of the whole retina was 6.3+/-1.0 ml/g/min under 1% isoflurane, consistent with the high blood flow in the retina reported using other techniques. Blood flow is relatively constant along the length of the retina, except it dipped slightly around the optic nerve head and dropped significantly at the distal edges where the retina terminates. Hyperoxia (100% O(2)) decreased blood flow 25+/-6% relative to baseline (air) due to vasoconstriction. Hypercapnia (5% CO(2)+21% O(2)) increased blood flow 16+/-6% due to vasodilation. Increasing isoflurane (a potent vasodilator) concentration to 1.5% increased blood flow to 9.3+/-2.7 ml/g/min. Blood-flow signals were confirmed to be genuine by repeating measurements after the animals were sacrificed in the MRI scanner. This study demonstrates a proof of concept that quantitative blood flow of the retina can be measured using MRI without depth limitation. Blood-flow MRI has the potential to provide unique insights into retinal physiology, serve as an early biomarker for some retinal diseases, and could complement optically based imaging techniques.

Spectacular shrinking deficit: insights from multimodal magnetic resonance imaging after embolic middle cerebral artery occlusion in Sprague-Dawley rats

Almost no data is available on the serial changes in the brain after spectacular shrinking deficit (SSD) that may help understand this relatively rare clinical phenomenon. Quantitative diffusion-(DWI), perfusion-(PWI), T(1)-(T1WI), T(2)-weighted (T2WI), and functional magnetic resonance imaging (fMRI) were performed before, during, and up to 7 days after embolic middle cerebral artery occlusion (eMCAO) in male Sprague-Dawley rats (n=9). Region of interest (ROI) analysis was used to evaluate structural and functional MR signal changes within three ROIs defined by the apparent diffusion coefficient (ADC), cerebral blood flow (CBF) signatures, and final tissue viability. DWI, PWI, and T2WI lesion volumes were calculated using previously established viability thresholds and final infarct volumes ascertained with 2,3,5-triphenyltetrazolium chloride (TTC) staining. Serial MRI demonstrated spontaneous reperfusion of initially hypoperfused MCA regions accompanied by substantial reduction of initial ADC and CBF lesions and gradual recovery of neurological outcome. Recovery rates of CBF/ADC abnormalities differed among ROIs. Functional magnetic resonance imaging showed persistent tissue dysfunction after the recovery of the CBF/ADC lesions. This study may facilitate our understanding of the pathophysiological mechanisms by which early, spontaneous reperfusion affects tissue fate and neurological function.

BOLD study of stimulation-induced neural activity and resting-state connectivity in medetomidine-sedated rat

Functional magnetic resonance imaging (fMRI) in anesthetized-animals is critical in studying the mechanisms of fMRI and investigating animal models of various diseases. Medetomidine was recently introduced for independent anesthesia for longitudinal (survival) fMRI studies in rats. Since stimulation-induced fMRI signal is anesthesia-dependent and its characteristics in rats under medetomidine are not fully elucidated, the blood oxygenation level dependent (BOLD) fMRI response to electrical forepaw stimulation under medetomidine was systematically investigated at 9.4 T. Robust activations in contralateral primary somatosensory cortex (SI) and thalamus were observed and peaked at the stimulus frequency of 9 Hz. The response in SI saturates at the stimulus strength of 4 mA while that in thalamus monotonically increases. In addition to fMRI data acquired with the forepaw stimulation, data were also acquired during the resting-state to investigate the synchronization of low frequency fluctuations (LFF) in the BOLD signal (<0.08 Hz) in different brain regions. LFF during resting-state have been observed to be synchronized between functionally related brain regions in human subjects while its origin is not fully understood. LFF have not been extensively studied or widely reported in anesthetized-animals. In our data, synchronized LFF of BOLD signals are found in clustered, bilaterally symmetric regions, including SI and caudate-putamen and the magnitude of the LFF is approximately 1.5%, comparable to the stimulation-induced BOLD signals. Similar to resting-state data reported in human subjects, LFF in rats under medetomidine likely reflect functional connectivity of these brain regions.

Differential effects of the D- and L- isomers of amphetamine on pharmacological MRI BOLD contrast in the rat

RATIONALE

The D - and L-amphetamine sulphate isomers are used in the formulation of Adderall XR(R), which is effective in the treatment of attention-deficit hyperactivity disorder (ADHD). The effects of these isomers on brain activity has not been examined using neuroimaging.

OBJECTIVES

This study determines the pharmacological magnetic resonance imaging blood-oxygenation-level-dependent (BOLD) response in rat brain regions after administration of each isomer.

MATERIALS AND METHODS

Rats were individually placed into a 2.35 T Bruker magnet for 60 min to achieve basal recording of variation in signal intensity. Either saline (n = 9), D-amphetamine sulphate (2 mg/kg, i.p.; n = 9) or L: -amphetamine sulphate (4 mg/kg, i.p.; n = 9) were administered, and recording continued for a further 90 min. Data were analysed for BOLD effects using statistical parametric maps. Blood pressure, blood gases and respiratory rate were monitored during scanning.

RESULTS

The isomers show overlapping effects on the BOLD responses in areas including nucleus accumbens, medial entorhinal cortex, colliculi, field CA1 of hippocampus and thalamic nuclei. The L-isomer produced greater global changes in the positive BOLD response than the D-isomer, including the somatosensory and motor cortices and frontal brain regions such as the orbitofrontal cortices, prelimbic and infralimbic cortex which were not observed with the D-isomer.

CONCLUSIONS

The amphetamine isomers produce different BOLD responses in brain areas related to cognition, pleasure, pain processing and motor control probably because of variations on brain amine systems such as dopamine and noradrenaline. The isomers may, therefore, have distinct actions on brain regions affected in ADHD patients.

Quantitative regional cerebral blood flow MRI of animal model of attention-deficit/hyperactivity disorder

The spontaneously hypertensive rat (SHR) has been widely used as an animal model for attention-deficit/hyperactivity disorder (AD/HD), a developmental disorder that affects 3-5% of school-age children. Quantitative high-resolution (180 x 180 x 1500 microm) perfusion magnetic resonance imaging was performed to evaluate regional CBF in AD/HD rats (SHR, n=7) and control Wistar Kyoto rats (WKY, n=9) in the frontal cortex, motor cortex, sensory cortex, corpus callosum, hippocampus, thalamus, globus pallidus, caudoputamen and whole brain. The accuracy of repeated cerebral blood flow (CBF) measurements within animals in these brain regions ranged from 3% to 10% (7 repeated measures) and across animals ranged from 15% to 18% (n=7 rats), respectively, indicating highly accurate and reproducible CBF measurements. Regional CBF of the SHR were statistically different from those of the WKY rats in all structures analyzed (P<0.05) except for the caudate putamen (P=0.09) and the globus pallidus (P=0.12). Whole brain CBF of the SHR (1.5+/-0.2 ml/g/min, mean+/-S.D.) was approximately 25% higher than that of the WKY rats (1.2+/-0.2 ml/g/min), likely due to the hypertensive nature of the AD/HD rat model. Following normalization to eliminate global CBF differences, CBF in the medial prefrontal cortex, a structure thought to be the equivalent of the human dorsolateral prefrontal cortex and widely implicated in AD/HD, was found to be higher in SHR compared to WKY rats (P<0.05). The only other structure that was found to be statistically different after normalization is the corpus callosum (P<0.05). Since resting cerebral blood flow is intricately coupled to resting neural activity, these results suggest that there was abnormal resting neural activity in the medial prefrontal cortex and the corpus callosum between the control and AD/HD animals, consistent with the hyperactivity, impulsivity, inattention, and other AD/HD-like behaviors in this animal model.

Theoretical and experimental optimization of laser speckle contrast imaging for high specificity to brain microcirculation

The functional spatial resolution in most of hemodynamics-based functional neuroimaging techniques is limited by the fineness of hemodynamic control with the active vascular beds likely at submillimeter resolution. This study was designed to visualize changes of cerebral blood flow (CBF) at submillimeter spatial scale on the prolonged isoflurane-anesthetized rats model by using laser speckle contrast imaging (LSCI) technique. Recently, this old method has attracted an increasing interest in studies of brain activities under normal and pathophysiologic conditions. However, some paramount assumptions behind this imaging technique have been kept ignored in this field since 1981 firstly proposed by Fercher and Briers. Most recently, these assumptions are claimed as serious mistakes that made LSCI fail to reproducibly and correctly measure blood flow speed. In our study, these issues are also re-examined theoretically and re-evaluated experimentally based on the results from the classical carbon dioxide challenge model. The detailed distribution of CBF responses to the stimulation induced by different levels of carbon dioxide pressure was obtained with tens of micron spatial resolution. The relative CBF images over the exposed cortical area acquired by LSCI were also compared with laser-Doppler measurements. Our results show that these assumptions would not produce any significant errors on investigating changes of blood flow and also achieve high specificity to assess cerebral microcirculation, as would facilitate its broad application in functional imaging field.

A Description of the Preterm Fetal Sheep Systemic and Central Responses to Maternal General Anesthesia

BACKGROUND

The second trimester is recommended as the optimal time to conduct a surgical procedure on pregnant patients, even though the fetal responses to anesthesia at this age are not known. Here we assessed the responses of preterm fetal sheep to a standard anesthetic regimen of midazolam, thiopental, and isoflurane.

METHODS

Variables were monitored in previously instrumented preterm pregnant sheep before, during, and after 4 h of general anesthesia. Isoflurane produced moderate fetal hypotension and bradycardia, whereas extubation was accompanied by increases in fetal heart rate and mean arterial blood pressure.

RESULTS

We observed an initial increase in fetal Sao2 followed by a gradual decline to baseline. Within the fetal brain, oxygenated hemoglobin changed by <10% (nonsignificant) and deoxygenated hemoglobin and total hemoglobin varied by <5%. Overall, although O2 levels within the preterm fetal brain were not independently enhanced by isoflurane (as occurs in the older fetus and in the adult), they did remain constant even as fetal mean arterial pressure decreased by more than 20%. By extension, we failed to identify changes in cerebral oxygenation that could be construed as injurious.

CONCLUSION

Any adverse preterm fetal response to maternal surgery should not be attributed solely to the actions of general anesthesia upon the fetus.

Haemodynamic and neural responses to hypercapnia in the awake rat

The relationship between localized changes in brain activity and metabolism, and the blood oxygenation level-dependent (BOLD) signal used in functional magnetic resonance imaging studies is not fully understood. One source of complexity is that stimulus-elicited changes in the BOLD signal arise both from changes in oxygen consumption due to increases in activity and purely 'haemodynamic' changes such as increases in cerebral blood flow. It is well established that robust cortical haemodynamic changes can be elicited by increasing the concentration of inspired CO(2) (inducing hypercapnia) and it is widely believed that these haemodynamic changes occur without significant effects upon neural activity or cortical metabolism. Hypercapnia is therefore commonly used as a calibration condition in functional magnetic resonance imaging studies to enable estimation of oxidative metabolism from subsequent stimulus-evoked functional magnetic resonance imaging BOLD signal changes. However, there is little research that has investigated in detail the effects of hypercapnia upon all components of the haemodynamic response (changes in cerebral blood flow, volume and oxygenation) in addition to recording neural activity. In awake animals, we used optical and electrophysiological techniques to measure cortical haemodynamic and field potential responses to hypercapnia (60 s, 5% CO(2)). The main findings are that firstly, in the awake rat, the temporal structure of the haemodynamic response to hypercapnia differs from that reported previously in anaesthetized preparations in that the response is more rapid. Secondly, there is evidence that hypercapnia alters ongoing neural activity in awake rats by inducing periods of cortical desynchronization and this may be associated with changes in oxidative metabolism.

Cerebral blood flow and BOLD fMRI responses to hypoxia in awake and anesthetized rats

This study investigated the functional MRI responses to graded hypoxia in awake/restrained and anesthetized animals by measuring cerebral blood flow (CBF) and blood oxygenation (BOLD) changes and estimating changes in cerebral metabolic rate of oxygen (CMRO2). Hypoxia in isoflurane anesthetized rats reduced blood pressure but did not change heart rate and respiration rate. In contrast, hypoxia in awake animals showed compensatory responses by sustaining blood pressure, increasing heart rate and respiration rate. Basal CBF was higher under isoflurane anesthesia than awake state because isoflurane is a vasodilator. Graded hypoxia decreased BOLD signals. Surprisingly, hypoxia also decreased CBF likely because hypoxia induced hypocapnia. Hypoxia-induced CBF and BOLD decreases were smaller in awake, relative to anesthetized, rats at low pO2, but similar at high pO2. CBF leveled off with decreasing hypoxia-induced pCO2 in awake rats, but monotonically decreased in anesthetized rats. CMRO2 estimated using a biophysical BOLD model did not change under mild hypoxia but was reduced under severe hypoxia relative to baseline. These results showed that isoflurane attenuated autonomic responses to hypoxia, hypoxia-induced hypocapnia dominated CBF changes, tissues in awake conditions appeared better oxygenated, and severe hypoxia reduced oxygen metabolism. This study underscored the marked differences in BOLD and CBF MRI responses to hypoxia in vivo between awake and anesthetized conditions and has implications for functional MRI studies of hypoxia in anesthetized animal models.

Confounding effects of volatile anesthesia on CBV assessment in rodent forebrain following ethanol challenge

PURPOSE

To compare and contrast the pattern and characteristics of the cerebral blood volume (CBV) response to ethanol (EtOH) in rats under awake and anesthetized conditions.

MATERIALS AND METHODS

Acute EtOH (0.75 g/kg) challenge-induced CBV changes were measured using a contrast-enhanced functional MRI CBV method in 15 male Sprague Dawley rats under three experimental conditions: 1.0% to 1.2% isoflurane (N = 5); 0.8% halothane (N = 5); and awake with no anesthetic (N = 5). Physiological parameters were collected from bench settings in nine rats from the above different conditions. Four parameters: 1) area under the curve (AUC%); 2) the maximum signal change (Max%); 3) EtOH absorption rate (alpha(2)); and 4) EtOH elimination rate (alpha(1)) were employed to compare EtOH-induced MRI signals between the awake and anesthetized groups.

RESULTS

Both awake and anesthetized animals responded with an increase in CBV to EtOH challenge. However, the presence of anesthesia promoted a significant preferential flow to subcortical areas not seen in the awake condition.

CONCLUSION

Unclear mechanisms of anesthesia add a layer of uncertainty to the already complex interpretation of EtOH's influence on neuronal activity, cellular metabolism, and hemodynamic coupling.

Quantification of rodent cerebral blood flow (CBF) in normal and high flow states using pulsed arterial spin labeling magnetic resonance imaging

PURPOSE

To implement a pulsed arterial spin labeling (ASL) technique in rats that accounts for cerebral blood flow (CBF) quantification errors due to arterial transit times (dt)-the time that tagged blood takes to reach the imaging slice-and outflow of the tag.

MATERIALS AND METHODS

Wistar rats were subjected to air or 5% CO(2), and flow-sensitive alternating inversion-recovery (FAIR) perfusion images were acquired. For CBF calculation, we applied the double-subtraction strategy (Buxton et al., Magn Reson Med 1998;40:383-396), in which data collected at two inversion times (TIs) are combined.

RESULTS

The ASL signal fell off more rapidly than expected from TI = one second onward, due to outflow effects. Inversion times for CBF calculation were therefore chosen to be larger than the longest transit times, but short enough to avoid systematic errors caused by outflow of tagged blood. Using our method, we observed a marked regional variability in CBF and dt, and a region dependent response to hypercapnia.

CONCLUSION

Even when flow is accelerated, CBF can be accurately determined using pulsed ASL, as long as dt and outflow of the tag are accounted for.