Regional brain blood flow in the conscious gerbil

Cerebral perfusion mapping during retrieval of spatial memory in rats

Studies of brain functional activation during spatial navigation using electrophysiology and immediate-early gene responses have typically targeted a limited number of brain regions. Our study provides the first whole brain analysis of cerebral activation during retrieval of spatial memory in the freely-moving rat. Rats (LEARNERS) were trained in the Barnes maze, an allocentric spatial navigation task, while CONTROLS received passive exposure. After 19 days, functional brain mapping was performed during recall by bolus intravenous injection of [¹⁴C]-iodoantipyrine using a novel subcutaneous minipump triggered by remote activation. Regional cerebral blood flow (rCBF)-related tissue radioactivity was analyzed by statistical parametric mapping from autoradiographic images of the three-dimensionally reconstructed brains. Functional connectivity was examined between regions of the spatial navigation circuit through interregional correlation analysis. Significant rCBF increases were noted in LEARNERS compared to CONTROLS broadly across the spatial navigation circuit, including the hippocampus (anterior dorsal CA1, posterior ventral CA1-3), subiculum, thalamus, striatum, medial septum, cerebral cortex, with decreases noted in the mammillary nucleus, amygdala and insula. LEARNERS showed a significantly greater positive correlation of rCBF of the ventral hippocampus with retrosplenial, lateral orbital, parietal and primary visual cortex, and a significantly more negative correlation with the mammillary nucleus, amygdala, posterior entorhinal cortex, and anterior thalamic nucleus. The complex sensory component of the spatial navigation task was underscored by broad activation across visual, somatosensory, olfactory, auditory and vestibular circuits which was enhanced in LEARNERS. Brain mapping facilitated by an implantable minipump represents a powerful tool for evaluation of mammalian behaviors dependent on locomotion.

Recruitment of prefrontal-striatal circuit in response to skilled motor challenge

A variety of physical fitness regimens have been shown to improve cognition, including executive function, yet our understanding of which parameters of motor training are important in optimizing outcomes remains limited. We used functional brain mapping to compare the ability of two motor challenges to acutely recruit the prefrontal-striatal circuit. The two motor tasks - walking in a complex running wheel with irregularly spaced rungs or walking in a running wheel with a smooth internal surface - differed only in the extent of skill required for their execution. Cerebral perfusion was mapped in rats by intravenous injection of [C]-iodoantipyrine during walking in either a motorized complex wheel or in a simple wheel. Regional cerebral blood flow (rCBF) was quantified by whole-brain autoradiography and analyzed in three-dimensional reconstructed brains by statistical parametric mapping and seed-based functional connectivity. Skilled or simple walking compared with rest, increased rCBF in regions of the motor circuit, somatosensory and visual cortex, as well as the hippocampus. Significantly greater rCBF increases were noted during skilled walking than for simple walking. Skilled walking, unlike simple walking or the resting condition, was associated with a significant positive functional connectivity in the prefrontal-striatal circuit (prelimbic cortex-dorsomedial striatum) and greater negative functional connectivity in the prefrontal-hippocampal circuit. Our findings suggest that the level of skill of a motor training task determines the extent of functional recruitment of the prefrontal-corticostriatal circuit, with implications for a new approach in neurorehabilitation that uses circuit-specific neuroplasticity to improve motor and cognitive functions.

Effects of water avoidance stress on peripheral and central responses during bladder filling in the rat: A multidisciplinary approach to the study of urologic chronic pelvic pain syndrome (MAPP) research network study

Stress plays a role in the exacerbation and possibly the development of functional lower urinary tract disorders. Chronic water avoidance stress (WAS) in rodents is a model with high construct and face validity to bladder hypersensitive syndromes, such as interstitial cystitis/bladder pain syndrome (IC/BPS), characterized by urinary frequency and bladder hyperalgesia and heightened stress responsiveness. Given the overlap of the brain circuits involved in stress, anxiety, and micturition, we evaluated the effects chronic stress has on bladder function, as well as its effects on regional brain activation during bladder filling. Female Wistar-Kyoto rats were exposed to WAS (10 days) or sham paradigms. One day thereafter, cystometrograms were obtained during titrated bladder dilation, with visceromotor responses (VMR) recorded simultaneously. Cerebral perfusion was assessed during passive bladder distension (20-cmH₂O) following intravenous administration of [¹⁴C]-iodoantipyrine. Regional cerebral blood flow was quantified by autoradiography and analyzed in 3-dimensionally reconstructed brains with statistical parametric mapping. WAS animals compared to controls demonstrated a decreased pressure threshold and visceromotor threshold triggering the voiding phase. At 20-cmH₂O, VMR was significantly greater in WAS animals compared to controls. WAS animals showed greater activation in cortical regions of the central micturition circuit, including the posterior cingulate, anterior retrosplenial, somatosensory, posterior insula, orbital, and anterior secondary (“supplementary”) motor cortices, as well as in the thalamus, anterior hypothalamus, parabrachial and Barrington nuclei, and striatum. Seed analysis showed increased functional connectivity of WAS compared to control animals of the posterior cingulate cortex to the pontine parabrachial nucleus; of the Barrington nucleus to the anterior dorsal midline and ventrobasilar thalamus and somatosensory and retrosplenial cortices; and of the posterior insula to anterior secondary motor cortex. Our findings show a visceral hypersensitivity during bladder filling in WAS animals, as well as increased engagement of portions of the micturition circuit responsive to urgency, viscerosensory perception and its relay to motor regions coordinating imminent bladder contraction. Results are consistent with recent findings in patients with interstitial cystitis, suggesting that WAS may serve as an animal model to elucidate the mechanisms leading to viscerosensitive brain phenotypes in humans with IC/BPS.

Exercise training reinstates cortico-cortical sensorimotor functional connectivity following striatal lesioning: development and application of a subregional-level analytic toolbox for perfusion autoradiographs of the rat brain

Current rodent connectome projects are revealing brain structural connectivity with unprecedented resolution and completeness. How subregional structural connectivity relates to subregional functional interactions is an emerging research topic. We describe a method for standardized, mesoscopic-level data sampling from autoradiographic coronal sections of the rat brain, and for correlation-based analysis and intuitive display of cortico-cortical functional connectivity (FC) on a flattened cortical map. A graphic user interface "Cx-2D" allows for the display of significant correlations of individual regions-of-interest, as well as graph theoretical metrics across the cortex. Cx-2D was tested on an autoradiographic data set of cerebral blood flow (CBF) of rats that had undergone bilateral striatal lesions, followed by 4 weeks of aerobic exercise training or no exercise. Effects of lesioning and exercise on cortico-cortical FC were examined during a locomotor challenge in this rat model of Parkinsonism. Subregional FC analysis revealed a rich functional reorganization of the brain in response to lesioning and exercise that was not apparent in a standard analysis focused on CBF of isolated brain regions. Lesioned rats showed diminished degree centrality of lateral primary motor cortex, as well as neighboring somatosensory cortex-changes that were substantially reversed in lesioned rats following exercise training. Seed analysis revealed that exercise increased positive correlations in motor and somatosensory cortex, with little effect in non-sensorimotor regions such as visual, auditory, and piriform cortex. The current analysis revealed that exercise partially reinstated sensorimotor FC lost following dopaminergic deafferentation. Cx-2D allows for standardized data sampling from images of brain slices, as well as analysis and display of cortico-cortical FC in the rat cerebral cortex with potential applications in a variety of autoradiographic and histologic studies.

Functional connectivity-based parcellation and connectome of cortical midline structures in the mouse: a perfusion autoradiography study

Rodent cortical midline structures (CMS) are involved in emotional, cognitive and attentional processes. Tract tracing has revealed complex patterns of structural connectivity demonstrating connectivity-based integration and segregation for the prelimbic, cingulate area 1, retrosplenial dysgranular cortices dorsally, and infralimbic, cingulate area 2, and retrosplenial granular cortices ventrally. Understanding of CMS functional connectivity (FC) remains more limited. Here we present the first subregion-level FC analysis of the mouse CMS, and assess whether fear results in state-dependent FC changes analogous to what has been reported in humans. Brain mapping using [¹⁴C]-iodoantipyrine was performed in mice during auditory-cued fear conditioned recall and in controls. Regional cerebral blood flow (CBF) was analyzed in 3-D images reconstructed from brain autoradiographs. Regions-of-interest were selected along the CMS anterior-posterior and dorsal-ventral axes. In controls, pairwise correlation and graph theoretical analyses showed strong FC within each CMS structure, strong FC along the dorsal-ventral axis, with segregation of anterior from posterior structures. Seed correlation showed FC of anterior regions to limbic/paralimbic areas, and FC of posterior regions to sensory areas–findings consistent with functional segregation noted in humans. Fear recall increased FC between the cingulate and retrosplenial cortices, but decreased FC between dorsal and ventral structures. In agreement with reports in humans, fear recall broadened FC of anterior structures to the amygdala and to somatosensory areas, suggesting integration and processing of both limbic and sensory information. Organizational principles learned from animal models at the mesoscopic level (brain regions and pathways) will not only critically inform future work at the microscopic (single neurons and synapses) level, but also have translational value to advance our understanding of human brain architecture.

Remote brain network changes after unilateral cortical impact injury and their modulation by acetylcholinesterase inhibition

We explored whether cerebral cortical impact injury (CCI) effects extend beyond direct lesion sites to affect remote brain networks, and whether acetylcholinesterase (AChE) inhibition elicits discrete changes in functional activation of motor circuits following CCI. Adult male rats underwent unilateral motor-sensory CCI or sham injury. Physostigmine (AChE inhibitor) or saline were administered subcutaneously continuously via implanted minipumps (1.6 micromoles/kg/day) for 3 weeks, followed by cerebral perfusion mapping during treadmill walking using [(14)C]-iodoantipyrine. Quantitative autoradiographs were analyzed by statistical parametric mapping and functional connectivity (FC) analysis. CCI resulted in functional deficits in the ipsilesional basal ganglia, with increased activation contralesionally. Recruitment was also observed, especially contralesionally, of the red nucleus, superior colliculus, pedunculopontine tegmental nucleus, thalamus (ventrolateral n., central medial n.), cerebellum, and sensory cortex. FC decreased significantly within ipsi- and contralesional motor circuits and between hemispheres, but increased between midline cerebellum and select regions of the basal ganglia within each hemisphere. Physostigmine significantly increased functional brain activation in the cerebellar thalamocortical pathway (midline cerebellum→ventrolateral thalamus→motor cortex), subthalamic nucleus/zona incerta, and red nucleus and bilateral sensory cortex. In conclusion, CCI resulted in increased functional recruitment of contralesional motor cortex and bilateral subcortical motor regions, as well as recruitment of the cerebellar-thalamocortical circuit and contralesional sensory cortex. This phenomenon, augmented by physostigmine, may partially compensate motor deficits. FC decreased inter-hemispherically and in negative, but not positive, intra-hemispherical FC, and it was not affected by physostigmine. Circuit-based approaches into functional brain reorganization may inform future behavioral or molecular strategies to augment targeted neurorehabilitation.

Functional reorganization of motor and limbic circuits after exercise training in a rat model of bilateral parkinsonism

Exercise training is widely used for neurorehabilitation of Parkinson's disease (PD). However, little is known about the functional reorganization of the injured brain after long-term aerobic exercise. We examined the effects of 4 weeks of forced running wheel exercise in a rat model of dopaminergic deafferentation (bilateral, dorsal striatal 6-hydroxydopamine lesions). One week after training, cerebral perfusion was mapped during treadmill walking or at rest using [(14)C]-iodoantipyrine autoradiography. Regional cerebral blood flow-related tissue radioactivity (rCBF) was analyzed in three-dimensionally reconstructed brains by statistical parametric mapping. In non-exercised rats, lesions resulted in persistent motor deficits. Compared to sham-lesioned rats, lesioned rats showed altered functional brain activation during walking, including: 1. hypoactivation of the striatum and motor cortex; 2. hyperactivation of non-lesioned areas in the basal ganglia-thalamocortical circuit; 3. functional recruitment of the red nucleus, superior colliculus and somatosensory cortex; 4. hyperactivation of the ventrolateral thalamus, cerebellar vermis and deep nuclei, suggesting recruitment of the cerebellar-thalamocortical circuit; 5. hyperactivation of limbic areas (amygdala, hippocampus, ventral striatum, septum, raphe, insula). These findings show remarkable similarities to imaging findings reported in PD patients. Exercise progressively improved motor deficits in lesioned rats, while increasing activation in dorsal striatum and rostral secondary motor cortex, attenuating a hyperemia of the zona incerta and eliciting a functional reorganization of regions participating in the cerebellar-thalamocortical circuit. Both lesions and exercise increased activation in mesolimbic areas (amygdala, hippocampus, ventral striatum, laterodorsal tegmental n., ventral pallidum), as well as in related paralimbic regions (septum, raphe, insula). Exercise, but not lesioning, resulted in decreases in rCBF in the medial prefrontal cortex (cingulate, prelimbic, infralimbic). Our results in this PD rat model uniquely highlight the breadth of functional reorganizations in motor and limbic circuits following lesion and long-term, aerobic exercise, and provide a framework for understanding the neural substrates underlying exercise-based neurorehabilitation.

Antenatal maternal stress alters functional brain responses in adult offspring during conditioned fear

Antenatal maternal stress has been shown in rodent models and in humans to result in altered behavioral and neuroendocrine responses, yet little is known about its effects on functional brain activation. Pregnant female rats received a daily foot-shock stress or sham-stress two days after testing plug-positive and continuing for the duration of their pregnancy. Adult male offspring (age 14 weeks) with and without prior maternal stress (MS) were exposed to an auditory fear conditioning (CF) paradigm. Cerebral blood flow (CBF) was assessed during recall of the tone cue in the nonsedated, nontethered animal using the ((14))C-iodoantipyrine method, in which the tracer was administered intravenously by remote activation of an implantable minipump. Regional CBF distribution was examined by autoradiography and analyzed by statistical parametric mapping in the three-dimensionally reconstructed brains. Presence of fear memory was confirmed by behavioral immobility ("freezing"). Corticosterone plasma levels during the CF paradigm were measured by ELISA in a separate group of rats. Antenatal MS exposure altered functional brain responses to the fear conditioned cue in adult offspring. Rats with prior MS exposure compared to those without demonstrated heightened fear responsivity, exaggerated and prolonged corticosterone release, increased functional cerebral activation of limbic/paralimbic regions (amygdala, ventral hippocampus, insula, ventral striatum, and nucleus accumbens), the locus coeruleus, and white matter, and deactivation of medial prefrontal cortical regions. Dysregulation of corticolimbic circuits may represent risk factors in the future development of anxiety disorders and associated alterations in emotional regulation.

Anxiolytic-like effect of pregabalin on unconditioned fear in the rat: an autoradiographic brain perfusion mapping and functional connectivity study

Clinical and preclinical evidence suggests anxiolytic-like efficacy of pregabalin (PGB, Lyrica). However, its mechanism of action remains under investigation. The current study applied [(14)C]-iodoantipyrine cerebral blood flow (CBF) mapping to examine the effect of PGB on neural substrates underlying unconditioned fear in a rat model of footshock-induced fear. Regional CBF (rCBF) was analyzed by statistical parametric mapping. Functional connectivity and graph theoretical analysis were used to investigate how footshock and PGB affect brain activation at the network level. Pregabalin significantly attenuated footshock-induced ultrasonic vocalization, but showed no significant effect on freezing behavior. Footshock compared to no-shock controls elicited significant increases in rCBF in limbic/paralimbic regions implicated in the processing of unconditioned fear and ultrasonic vocalization, including the amygdala, hypothalamus, lateral septum, dorsal periaqueductal gray, the anterior insular (aINS) and medial prefrontal cortex (mPFC). The activation pattern was similar in vehicle- and PGB-treated subjects, with PGB significantly attenuating activation in the amygdala, hypothalamus, and aINS. The vehicle/no-shock group showed strong, positive intra-structural correlations within the cortex, hypothalamus, amygdala, thalamus, and brainstem. The cortex was negatively correlated with the hypothalamus and brainstem. Footshock reduced the total number of significant correlations, but induced greater intra-cortical connectivity of the aINS and mPFC, and new positive correlations between the hypothalamus and amygdala. In no-shock controls, PGB significantly reduced the positive intra-structural correlations within the cortex and amygdala, as well as the negative cortico-subcortical correlations. Following footshocks, PGB disrupted both the network recruitment of aINS and mPFC, and the positive hypothalamic-amygdaloid correlations. Our findings suggest that PGB may exert anxiolytic effect by attenuating cortico-cortical and cortico-subcortical communication and inhibiting network recruitment of the aINS, mPFC, amygdala, and hypothalamus following a fear-inducing stimulus. Functional brain mapping in rodents may provide new endpoints for preclinical evaluation of anxiolytic drug candidates with potentially improved translational power compared to behavioral measurements alone.

Mapping functional brain activation using [14C]-iodoantipyrine in male serotonin transporter knockout mice

Background

Serotonin transporter knockout mice have been a powerful tool in understanding the role played by the serotonin transporter in modulating physiological function and behavior. However, little work has examined brain function in this mouse model. We tested the hypothesis that male knockout mice show exaggerated limbic activation during exposure to an emotional stressor, similar to human subjects with genetically reduced transcription of the serotonin transporter.

Methodology/Principal Findings

Functional brain mapping using [¹⁴C]-iodoantipyrine was performed during recall of a fear conditioned tone. Regional cerebral blood flow was analyzed by statistical parametric mapping from autoradiographs of the three-dimensionally reconstructed brains. During recall, knockout mice compared to wild-type mice showed increased freezing, increased regional cerebral blood flow of the amygdala, insula, and barrel field somatosensory cortex, decreased regional cerebral blood flow of the ventral hippocampus, and conditioning-dependent alterations in regional cerebral blood flow in the medial prefrontal cortex (prelimbic, infralimbic, and cingulate). Anxiety tests relying on sensorimotor exploration showed a small (open field) or paradoxical effect (marble burying) of loss of the serotonin transporter on anxiety behavior, which may reflect known abnormalities in the knockout animal's sensory system. Experiments evaluating whisker function showed that knockout mice displayed impaired whisker sensation in the spontaneous gap crossing task and appetitive gap cross training.

Conclusions

This study is the first to demonstrate altered functional activation in the serotonin transporter knockout mice of critical nodes of the fear conditioning circuit. Alterations in whisker sensation and functional activation of barrel field somatosensory cortex extend earlier reports of barrel field abnormalities, which may confound behavioral measures relying on sensorimotor exploration.

Determination of the minimum alveolar concentration for halothane, isoflurane and sevoflurane in the gerbil

The present work determined the minimum alveolar concentrations (MAC) of halothane, isoflurane and sevoflurane in adult female gerbils (n=24). Animals were placed in a chamber for anaesthetic induction before performing tracheal intubation. The tracheal tube was connected to a non-rebreathing circuit with minimal dead space. Body temperature, blood pressure, heart and respiratory rates and end-tidal volatile anaesthetic levels were measured continuously. MAC was determined using a standard noxious stimulus (tail-clamp). All experiments were performed at the same time of the day, body temperature was maintained constant and blood-gas analysis was performed to confirm that values were within normal limits. The mean+/-SD MAC values were 1.06+/-0.11% halothane (n=8), 1.55+/-0.08% isoflurane (n=8) and 2.90+/-0.12% sevoflurane (n=7). Cardiovascular parameters at 1xMAC did not differ significantly among anaesthetics but the respiratory rate was significantly higher in the halothane group than in the isoflurane and sevoflurane groups. The SpO2 values recorded throughout anaesthesia and the pH and partial oxygen pressure values determined at the end of the study did not differ among the studied anaesthetics at 1xMAC. These data suggest that the MAC for halogenated inhalant anaesthetics in gerbils is lower than the average MAC values obtained in rats and mice.

An oral administration of cilostazol before focal ischemia reduces the infarct volume with delayed cerebral blood flow increase in rats

We studied the acute brain protection provided by an antiplatelet agent, cilostazol, in rat experimental focal ischemia model. We administered 30 mg/kg of cilostazol or vehicle orally 2 hours before transient middle cerebral artery (MCA) occlusion (MCAO) by the intraluminal thread method. We measured the absolute cerebral blood flow (CBF) 2 hours after cilostazol administration, the regional CBF (rCBF) of MCA territory during MCAO, and neurologic deficits and the infarct volume at 22 hours after reperfusion. We found that cilostazol did not increase absolute CBF just before MCAO. rCBF in the MCA territory was reduced to the same degree in both groups up to 60 minutes post-MCAO. A significant increase of rCBF was observed in cilostazol-treated rats at 90 minutes and maintained until reperfusion compared with the controls (P = .031 and P = .047). The average neurologic score and the infarct volume, determined by 2,3,5-triphenyltetrazolium chloride monohydrate staining, were significantly lower in cilostazol-treated rats (P = .010). The single oral administration of cilostazol before transient ischemia in healthy adult rats induced a delayed penumbral CBF increase and resulted in a significant reduction of stroke damage.

Brain maps on the go: functional imaging during motor challenge in animals

Brain mapping in the freely moving animal is useful for studying motor circuits, not only because it avoids the potential confound of sedation or restraints, but because activated brain states may serve to accentuate differences that only manifest partially while a subject is in the resting state. Perfusion or metabolic mapping using autoradiography allows one to examine changes in brain function at the circuit level across the entire brain with a spatial resolution (approximately 100 micro) appropriate for the rat or mouse brain, and a temporal resolution (seconds-minutes) sufficient for capturing acute brain changes. Here we summarize the application of these methods to the functional brain mapping of behaviors involving locomotion of small animals, methods for the three-dimensional reconstruction of the brain from autoradiographic sections, voxel based analysis of the whole brain, and generation of maps of the flattened rat cortex. Application of these methods in animal models promises utility in improving our understanding of motor function in the normal brain, and of the effects of neuropathology and treatment interventions such as exercise have on the reorganization of motor circuits.

Reorganization of functional brain maps after exercise training: Importance of cerebellar-thalamic-cortical pathway

Exercise training (ET) causes functional and morphologic changes in normal and injured brain. While studies have examined effects of short-term (same day) training on functional brain activation, less work has evaluated effects of long-term training, in particular treadmill running. An improved understanding is relevant as changes in neural reorganization typically require days to weeks, and treadmill training is a component of many neurorehabilitation programs. Adult, male rats (n=10) trained to run for 40 min/day, 5 days/week on a Rotarod treadmill at 11.5 cm/s, while control animals (n=10) walked for 1 min/day at 1.2 cm/s. Six weeks later, [(14)C]-iodoantipyrine was injected intravenously during treadmill walking. Regional cerebral blood flow-related tissue radioactivity was quantified by autoradiography and analyzed in the three-dimensionally reconstructed brain by statistical parametric mapping. Exercised compared to nonexercised rats demonstrated increased influence of the cerebellar-thalamic-cortical (CbTC) circuit, with relative increases in perfusion in deep cerebellar nuclei (medial, interposed, lateral), thalamus (ventrolateral, midline, intralaminar), and paravermis, but with decreases in the vermis. In the basal ganglia-thalamic-cortical circuit, significant decreases were noted in sensorimotor cortex and striatum, with associated increases in the globus pallidus. Additional significant changes were noted in the ventral pallidum, superior colliculus, dentate gyrus (increases), and red nucleus (decreases). Following ET, the new dynamic equilibrium of the brain is characterized by increases in the efficiency of neural processing (sensorimotor cortex, striatum, vermis) and an increased influence of the CbTC circuit. Cerebral regions demonstrating changes in neural activation may point to alternate circuits, which may be mobilized during neurorehabilitation.

Changes in brain functional activation during resting and locomotor states after unilateral nigrostriatal damage in rats

To evaluate functional neuronal compensation after partial damage to the nigrostriatal system, we lesioned rats unilaterally in the striatum with 6-hydroxydopamine. Five weeks later, cerebral perfusion was mapped at rest or during treadmill walking using [(14)C]-iodoantipyrine. Regional CBF-related tissue radioactivity (CBF-TR) was quantified by autoradiography and analyzed by statistical parametric mapping and region-of- interest analysis. Lesions were confirmed by tyrosine hydroxylase immunohistochemistry and changes in rotational locomotor activity. Functional compensations were bilateral and differed at rest and during treadmill walking. Consistent with the classic view of striatopallidal connections, CBF-TR of lesioned compared to sham-lesioned rats increased in the ipsilateral subthalamic nucleus (STN) and internal globus pallidus, and decreased in the striatum and external globus pallidus. Contrary to the classic view, CBF-TR increased in the ipsilateral ventral lateral, ventral anterior thalamus and motor cortex, as well as in the central medial thalamus, midline cerebellum, and contralateral STN. During walking, perfusion decreased in lesioned compared to sham-lesioned rats across the ipsilateral striato-pallidal-thalamic-cortical motor circuit. Compensatory increases were seen bilaterally in the ventromedial thalamus and red nucleus, in the contralateral STN, anterior substantia nigra, subiculum, motor cortex, and in midline cerebellum. Enhanced recruitment of associative sensory areas was noted cortically and subcortically. Future models of compensatory changes after nigrostriatal damage need to address the effects of increased neural activity by residual dopaminergic neurons, interhemispheric interactions and differences between resting and locomotor states. Identification of sites at which functional compensation occurs may define useful future targets for neurorehabilitative or neurorestorative interventions in Parkinson's disease.

Rapid nongenomic actions of thyroid hormone

The binding of thyroid hormone to the thyroid hormone receptor (TR) mediates important physiological effects. However, the transcriptional effects of TR mediated by the thyroid response element (TRE) cannot explain many actions of thyroid hormone. We postulate that TR can initiate rapid, non-TRE-mediated effects in the cardiovascular system through cross-coupling to the phosphatidylinositol 3-kinase (PI3-kinase)/protein kinase Akt pathway. In vascular endothelial cells, the predominant TR isoform is TRalpha1. Treatment of endothelial cells with L-3,5,3'-triiodothyronine (T3) increased the association of TRalpha1 with the p85alpha subunit of PI3-kinase, leading to the phosphorylation and activation of Akt and endothelial nitric oxide synthase (eNOS). The activation of Akt and eNOS by T3 was abolished by the PI3-kinase inhibitors, LY294002 and wortmannin, but not by the transcriptional inhibitor, actinomycin D. To determine the physiological relevance of this PI3-kinase/Akt pathway, we administered T3 to mice undergoing transient focal cerebral ischemia. Compared with vehicle, a single bolus infusion of T3 rapidly increased Akt activity in the brain, decreased mean blood pressure, reduced cerebral infarct volume, and improved neurological deficit score. These neuroprotective effects of T3 were greatly attenuated or absent in eNOS-/- and TRalpha1-/-beta-/- mice and were completely abolished in WT mice pretreated with LY294002 or a T3 antagonist, NH-3. These findings indicate that the activation of PI3-kinase/Akt pathways can mediate some of the rapid, non-TRE effects of TR and suggest that the activation of Akt and eNOS contributes to some of the acute vasodilatory and neuroprotective effects of thyroid hormone.

The effects of cerebral ischemia on the rat choroid plexus

Although the blood-brain barrier effects of cerebral ischemia have been extensively examined, less attention has focused on ischemia-induced damage to the choroid plexuses that form the blood-cerebrospinal fluid (CSF) barrier (BSCFB). This study examined the rat lateral ventricle choroid plexuses (LVCP) in three ischemic models, bilateral common carotid artery occlusion (2VO)+hypotension with or without reperfusion and permanent middle cerebral artery (MCA) occlusion with or without a tandem common carotid artery occlusion. Blood flow was assessed using [(14)C]-N-isopropyl-p-iodoamphetamine, and LVCP injury by tissue edema, alterations in [(14)C]glutamine transport and BSCFB disruption (measured with [(3)H]inulin). 2VO+hypotension caused an 87% reduction in LVCP blood flow (P<0.01) and a progressive reduction in LVCP glutamine transport. In contrast to cortex, there was no LVCP hyperemia or delayed hypoperfusion on reperfusion, but there was marked BSCFB disruption. After 30 mins of 2VO+hypotension with 6 h of reperfusion, the [(3)H]inulin entry into CSF was increased threefold (P<0.05). Blood-CSF barrier rather than blood-brain barrier disruption appeared to be the main cause of enhanced [(3)H]inulin entry into hippocampus. Middle cerebral artery occlusion with and without a tandem common carotid artery occlusion only caused 53% and 38% reductions in LVCP blood flow but induced LVCP edema. Results suggest that the LVCP is selectively vulnerable to ischemic injury in terms of the absolute blood flows or, for the MCA occlusion models, the % reductions in flows required to induce injury. BCSFB disruption early after ischemia may enhance the movement of compounds from blood to areas close to the ventricular system and participate in delayed neuronal death.

Mapping cerebral blood flow changes during auditory-cued conditioned fear in the nontethered, nonrestrained rat

Conditioned fear (CF) is one of the most frequently used behavioral paradigms; however, little work has mapped changes in cerebral perfusion during CF in the rat-the species which has dominated CF research. Adult rats carrying an implanted minipump were exposed to a tone (controls, n = 8) or a tone conditioned in association with footshocks (CS group, n = 9). During reexposure to the tone 24 h later, animals were injected intravenously by remote activation with [14C]-iodoantipyrine using the pump. Significant group differences in regional CBF-related tissue radioactivity (CBF-TR) were determined by region-of-interest analysis of brain autoradiographs, as well as in the reconstructed, three-dimensional brain by statistical parametric mapping (SPM). CS animals demonstrated significantly greater, fear-enhanced increases in CBF-TR in auditory cortex than controls. The lateral amygdala was activated, whereas the basolateral/basomedial and central amygdala were deactivated. In the hippocampus and medial prefrontal cortex, CBF-TR increased significantly ventrally but not dorsally. Significant activations were noted in medial striatum and the thalamic midline and intralaminar nuclei. However, the ventrolateral/dorsolateral striatum and its afferents from motor and somatosensory cortex were deactivated, consistent with the behavioral immobility seen during CF. Significant activations were also noted in the lateral septum, periaqueductal gray, and deep mesencephalic nucleus/tegmental tract. Our results show that auditory stimuli endowed with aversive properties through conditioning result in significant redistribution of cerebral perfusion. SPM is a useful tool in the brain mapping of complex rodent behaviors, in particular the changes in activation patterns in limbic, thalamic, motor, and cortical circuits during CF.

Statistical parametric mapping applied to an autoradiographic study of cerebral activation during treadmill walking in rats

Autoradiographs are conventionally analyzed by a region-of-interest (ROI) analysis. However, definition of ROIs on an image set is labor intensive, is subject to potential inter-rater bias, and is not well suited for anatomically variable structures that may not consistently correspond to specific ROIs. Most importantly, the ROI method is poorly suited for whole-brain analysis, where one wishes to detect all activations resulting from an experimental paradigm. A system developed for analysis of imaging data in humans, Statistical Parametric Mapping (SPM), avoids some of these limitations but has not previously been adapted as a tool for the analysis of autoradiographs. Here, we describe the application of SPM to an autoradiographic data set mapping cerebral activation in rats during treadmill walking. We studied freely moving, non-tethered rats that received injections of the cerebral blood flow tracer [14C]-iodoantipyrine, while they were performing a treadmill task (n = 7) or during a quiescent control condition (n = 6). Results obtained with SPM were compared to those previously reported using a standard ROI-based method of analysis [J. Cereb. Blood Flow Metab. 23(2003) 925]. The SPM method confirmed most areas detected as significant using the ROI approach. However, in the subcortex, SPM detected additional significant regions that, because of their irregular structures, fell short of statistical significance when analyzed by ROI. The SPM approach offers the ability to perform a semi-automated whole-brain analysis, and coupled with autoradiography, provides an effective means to globally localize functional activity in small animals.

Evidence for a central pathway in the cerebrovascular effects of spinal cord stimulation

OBJECTIVE

Cervical spinal cord stimulation (SCS) augments cerebral blood flow (CBF) in a number of animal models. The mechanisms underlying the cerebrovascular effects of SCS are not yet well delineated. In this study, we analyzed two alternative pathways in CBF alterations induced by SCS in rats, one involving direct modulation of sympathetic outflow and the other through central vasomotor influence.

METHODS

Resection of the superior cervical ganglion (SCG), SCS alone, or SCS after SCG removal was performed in adult male Sprague-Dawley rats. CBF was measured with (14)C-inosine monophosphate radiotracer studies. In another set of experiments, SCS was performed after spinalization at the cervicomedullary junction or after laminectomy alone.

RESULTS

Baseline CBF in the SCG removal group was 71 +/- 8 ml/100 g/min, similar to controls. SCS alone significantly increased blood flow to 100 +/- 10 ml/100 g/min (P < 0.05). Animals that underwent SCS after SCG removal demonstrated a similar robust augmentation in CBF. SCS-induced changes in CBF were completely attenuated by spinalization.

CONCLUSION

The profound effects of spinal cord transection on SCS-induced CBF augmentation, together with the lack of effect of surgical sympathectomy, suggest that the mechanisms underlying the effects of SCS involve central influences rather than cervical sympathetic outflow. These findings suggest a possible role for brainstem vasomotor centers in the cerebrovascular effects of SCS.

Laser speckle flowmetry for the study of cerebrovascular physiology in normal and ischemic mouse cortex

Laser speckle flowmetry (LSF) is useful to assess noninvasively two-dimensional cerebral blood flow (CBF) with high temporal and spatial resolution. The authors show that LSF can image the spatiotemporal dynamics of CBF changes in mice through an intact skull. When measured by LSF, peak CBF increases during whisker stimulation closely correlated with simultaneous laser-Doppler flowmetry (LDF) measurements, and were greater within the branches of the middle cerebral artery supplying barrel cortex than within barrel cortex capillary bed itself. When LSF was used to study the response to inhaled CO2 (5%), the flow increase was similar to the response reported using LDF. For the upper and lower limits of autoregulation, mean arterial pressure values were 110 and 40 mm Hg, respectively. They also show a linear relationship between absolute resting CBF, as determined by [C]iodoamphetamine technique, and 1/tau(c) values obtained using LSF, and used 1/tau(c) values to compare resting CBF between different animals. Finally, the authors studied CBF changes after distal middle cerebral artery ligation, and developed a model to investigate the spatial distribution and hemodynamics of moderate to severely ischemic cortex. In summary, LSF has distinct advantages over LDF for CBF monitoring because of high spatial resolution.

Iodoamphetamine (IMP) uptake in the brain is increased after experimental cerebral venous hypertension in the rat

Cerebral venous hypertension (VH) is a pathological condition associated with arteriovenous malformation, and is an important determinant factor for clinical symptom and outcome. Using a rat whole brain VH model created by a carotid-jugular fistula and contralateral jugular vein ligation we measured superior sagittal sinus pressure (SSSP) and cerebral blood flow (CBF) by the 14C-iodoamphetamine (IMP) indicator fractionation technique under isoflurane anesthesia with controlled ventilation. SSSP was significantly elevated in the fistula group (15 +/- 1 mmHg) (mean +/- SEM) compared to the vessel occlusion (5 +/- 1) and sham operated groups (5 +/- 1) (P < 0.05). Mean absolute CBF in the fistula (103 +/- 3 ml/ min/100 g) and vessel occlusion (90 +/- 4) groups were elevated compared to the sham group (77 +/- 3) (P < 0.05). Physiological parameters (mean blood pressure, heart rate, and blood gas analysis) before measurement were not different. In this VH model we found increases in SSSP and IMP uptake in the brain. We assume the mechanism of IMP uptake increase in VH animal may be slow circulation with a possible change of first pass extraction of IMP.

Intracerebral hirudin injection attenuates ischemic damage and neurologic deficits without altering local cerebral blood flow

There has been considerable interest in the use of thrombin inhibitors to reduce the occurrence of stroke or to potentiate tissue plasminogen activator-induced reperfusion. However, there is growing evidence that thrombin may also have extravascular effects that influence ischemic brain injury. Male Sprague-Dawley rats were subjected to either 90 minutes of temporary middle cerebral artery (MCA) occlusion or sham operation to examine thrombin and protease activated receptor-1 (PAR-1) expression. In another set of rats, the MCA was occluded for 90 minutes and 10 U of hirudin or the same volume of vehicle was injected into the caudate followed by reperfusion for up to 28 days, to test the effects of local thrombin inhibition on ischemic damage, neurologic outcome and cerebral blood flow (CBF). Thrombin immunoreactivity was increased in the ischemic caudate at 4 and 24 hours, whereas PAR-1 expression was unchanged. Hirudin reduced infarct volume in the caudate at 24 hours (79 +/- 41 vs. 115 +/- 20 mm3, P < 0.05) and resulted in a larger residual tissue volume in the caudate at 28 days (17.6 +/- 3.9 vs. 11.8 +/- 6.3 mm3, P < 0.05). Hirudin treatment also had a beneficial effect on body weight and ameliorated neurologic deficits tested by forelimb placing and forelimb use asymmetry during 28 days survival. These beneficial effects of hirudin were not associated with improved regional CBF during reperfusion. These results suggest that, in addition to their effects on coagulation and circulation, thrombin inhibitors also have direct neuroprotective properties and may be considered in stroke therapy.

Functional brain mapping in freely moving rats during treadmill walking

A dilemma in functional neuroimaging is that immobilization of the subject, necessary to avoid movement artifact, extinguishes all but the simplest behaviors. Recently, we developed an implantable microbolus infusion pump (MIP) that allows bolus injection of radiotracers by remote activation in freely moving, nontethered animals. The MIP is examined as a tool for brain mapping in rats during a locomotor task. Cerebral blood flow-related tissue radioactivity (CBF-TR) was measured using [14C]-iodoantipyrine with an indicator-fractionation method, followed by autoradiography. Rats exposed to walking on a treadmill, compared to quiescent controls, showed increases in CBF-TR in motor circuits (primary motor cortex, dorsolateral striatum, ventrolateral thalamus, midline cerebellum, copula pyramis, paramedian lobule), in primary somatosensory cortex mapping the forelimbs, hindlimbs and trunk, as well as in secondary visual cortex. These results support the use of implantable pumps as adjunct tools for functional neuroimaging of behaviors that cannot be elicited in restrained or tethered animals.

Spinal cord stimulation reducing infarct volume in a model of focal cerebral ischemia in rats

OBJECT

The authors previously showed that spinal cord stimulation (SCS) increases cerebral blood flow in rats, indicating that this technique may be useful in the treatment of focal cerebral ischemia. In the present study, the neuroprotective potential of SCS in the setting of middle cerebral artery occlusion (MCAO) was investigated.

METHODS

The authors induced permanent, focal cerebral ischemia by using either suture-induced occlusion or direct division of the MCA in Sprague-Dawley rats. Electrical stimulation of the cervical spinal cord was performed during cerebral ischemia. Cerebral blood flow was assessed using both laser Doppler flowmetry (LDF) and quantitative radiotracer analysis. Stroke volumes were analyzed after 6 hours of ischemia. Spinal cord stimulation resulted in a 52.7 +/- 13.3% increase in LDF values (nine animals). Following MCAO, LDF values decreased by 64.1 +/- 3.6% from baseline values (10 animals). Spinal cord stimulation subsequently increased LDF values to 30.9 +/- 13.5% below original baseline values. These findings were corroborated using radiotracer studies. Spinal cord stimulation in the setting of transcranial MCAO significantly reduced stroke volumes as well (from 203 +/- 33 mm3 [control] to 32 +/- 8 mm3 [MCAO plus SCS], seven animals in each group, p < 0.001). Similarly, after suture-induced MCAO, SCS reduced stroke volumes (from 307 +/- 29 mm3 [control] to 78 +/- 22 mm3 [MCAO plus SCS], 10 animals in each group, p < 0.001).

CONCLUSIONS

A strategy of performing SCS for the prevention of critical ischemia is feasible and may have the potential for the treatment and prevention of stroke.

Ischemic cell death: dynamics of delayed secondary energy failure during reperfusion following focal ischemia

Reperfusion injury is believed to contribute to the pathophysiology of ischemic cell death, but the precipitating factors have yet to be completely elucidated. The goal of this study was to examine if reflow-induced secondary energy failure is a component in the events that lead to cell death following increasing periods of middle cerebral artery (MCA) occlusion in Wistar rats. Discrete sections within the MCA distribution were dissected and analyzed for high-energy phosphates and glucose. Regional cerebral blood flow was determined by [14C]-iodoantipyrine technique in representative groups. The levels of ATP + P-creatine were initially depressed at the end of the focal ischemia and the concentrations in the penumbra were unchanged for up to 8 h after 2 h of ischemia which contrasts with response in the ischemic core, striatum, and penumbra where the HEP generally recovered to values near those of control only to decrease with increasing periods of reflow. The possibility of a rebound ischemia in secondary energy failure (SEF) was precluded by regional CBF values and concentrations of glucose that were significantly higher than the threshold for an ischemic effect. The depletion of cellular energy stores following SEF strongly indicates that the evolution of infarct during reflow results from loss of ATP and its synthesis.

Neurotrophin-3 promotes cell death induced in cerebral ischemia, oxygen-glucose deprivation, and oxidative stress: possible involvement of oxygen free radicals

To explore the role of neurotrophin-3 (NT-3) during cerebral ischemia, NT-3-deficient brains were subjected to transient focal ischemia. Conditional mutant brains produced undetectable amounts of NT-3 mRNA, whereas the expression of the neurotrophin, BDNF, the NT-3 receptor, TrkC, and the nonselective, low-affinity neurotrophin receptor p75NTR, were comparable to wild-type. Baseline absolute blood flow, vascular and neuroanatomical features, as well as physiological measurements were also indistinguishable from wild-type. Interestingly, the absence of NT-3 led to a significantly decreased infarct volume 23 h after middle cerebral artery occlusion. Consistent with this, the addition of NT-3 to primary cortical cell cultures exacerbated neuronal death caused by oxygen-glucose deprivation. Coincubation with the oxygen free radical chelator, trolox, diminished potentiation of neuronal death. NT-3 also enhanced neuronal cell death and the production of reactive oxygen species caused by oxidative damage inducing agents. We conclude that endogenous NT-3 enhanced neuronal injury during acute stroke, possible by increasing oxygen-radical mediated cell death.

Inducible cyclooxygenase-2 expression after experimental intracerebral hemorrhage

Cyclooxygenase-2 (COX-2) is an inducible isoform of cyclooxygenase, which catalyzes the conversion of arachidonic acid to prostaglandins and thromboxane. Recent evidence suggests it has a pathological role in cerebral insults, but its involvement in intracerebral hemorrhage (ICH) is unknown. The present study investigates the temporal and anatomic distribution of COX-2 as well as the effect of the selective COX-2 inhibitor NS-398 on brain edema formation and cerebral blood flow in a rat model of ICH. Immunohistochemistry for COX-2 was performed in control rats and 6 h, as well as 1, 3, 7 and 10 days after the injection of 100 microl autologous blood into the right basal ganglia. Double-labeling immunohistochemistry was used to determine the type of COX-2 immunoreactive microvascular-associated cells. Western blot analysis was used to quantify COX-2 protein. The effect of NS-398 on brain water content, ion concentration and cerebral blood flow were assessed 24 h after ICH. The results demonstrated that COX-2 protein was expressed in control brain tissue and induced significantly in the ipsilateral hemisphere at 6 h, as well as 1 and 3 days after ICH. Increased staining of COX-2 in neurons was observed around the blood clot with a peak at 6 h. COX-2 was induced in endothelial cells, perivascular cells as well as infiltrating leukocytes 1 day after ICH. Brain water and ion contents and cerebral blood flow were unaffected by NS-398 administration. Thus, although COX-2 expression was increased in the ipsilateral hemisphere after an autologous blood injection, its products do not appear to be major regulators of blood flow or edema formation following ICH.

Attenuation of ischemic brain edema and cerebrovascular injury after ischemic preconditioning in the rat

Ischemic preconditioning (IPC) induces neuroprotection to subsequent severe ischemia, but its effect on the cerebrovasculature has not been studied extensively. This study evaluated the effects of IPC on brain edema formation and endothelial cell damage that follows subsequent permanent focal cerebral ischemia in the rat. Transient (15 minute) middle cerebral artery occlusion (MCAO) was used for IPC. Three days after IPC or a sham operation, permanent MCAO was induced. Twenty-four hours after permanent MCAO, neurologic deficit, infarction volume, and water and ion content were evaluated. Six hours post-ischemia, blood-brain barrier (BBB) permeability was examined using [3H]-inulin. Water, ion contents, and BBB permeability were assessed in three zones (core, intermediate, and outer) depending on their relation to the MCA territory. Heat shock protein 70 (HSP70) was also examined as a potential marker of vascular injury. The model of IPC significantly reduced brain infarction and neurologic deficit. Compared with a sham operation, IPC also significantly attenuated brain edema formation in the intermediate (sham and IPC water contents: 5.99+/-0.65 vs. 4.99+/-0.81 g/g dry weight; P < 0.01) and outer zones (5.02+/-0.48 vs. 4.37+/-0.42 g/g dry weight; P < 0.01) of the ipsilateral hemisphere but not in the core zone. Blood-brain barrier disruption assessed by [3H]-inulin was significantly attenuated in the IPC group and the number of blood vessels that displayed HSP70 immunoreactivity was also reduced. Thus, IPC significantly attenuates ischemic brain edema formation, BBB disruption, and, as assessed by HSP70, vascular injury. Understanding the mechanisms involved in IPC may provide insight into methods for preserving cerebrovascular function during ischemia.

Increased susceptibility to ischemia-induced brain damage in transgenic mice overexpressing a dominant negative form of SHP2

Cell culture studies have established SH2 domain-containing protein tyrosine phosphatase-2 (SHP2) as an important factor in growth factor and cytokine-activated signaling pathways. However, the significance of SHP2 in the mammalian central nervous system (CNS) is not known since early embryonic lethality occurs in shp2 null mice. To bypass this embryonic lethality, transgenic animals containing a catalytically inactive mutant of SHP2 (SHP2-CS) under the control of a nestin intron II/thymidine kinase minimal promoter were generated. In the developing CNS of these animals, although high-level transgene expression was detected in the neuroepithelium, there was no obvious abnormality in progenitor cell proliferation or migration. In the adult brain, high-level transgene expression was detected in the subventricular zone, rostral migratory stream, dentate gyrus of hippocampus, and cerebellum. Because SHP2 function is likely important in cell survival pathways, we used a focal cerebral ischemia model to examined whether SHP2 is important during CNS injury. Ischemia-induced damage and neuronal death was found to be significantly greater in nestin-SHP2-CS mice than in wild-type littermates. These findings indicate that SHP2 is a required factor in signaling pathway(s) important for neuronal survival.

Acute upregulation of blood-brain barrier glucose transporter activity in seizures

Brain extraction of (18)F-labeled 2-fluoro-2-deoxy-D-glucose (FDG) was significantly higher in pentylene tetrazole (PTZ)-treated rats (32 +/- 4%) than controls (25 +/- 4%). The FDG permeability-surface area product (PS) was also significantly higher with PTZ treatment (0.36 +/- 0.05 ml. min(-1). g(-1)) than in controls (0.20 +/- 0.06 ml. min(-1). g(-1)). Cerebral blood flow rates were also elevated by 50% in seizures. The internal carotid artery perfusion technique indicated mean [(14)C]glucose clearance (and extraction) was increased with PTZ treatment, and seizures increased the PS by 37 +/- 16% (P < 0.05) in cortical regions. Because kinetic analyses suggested the glucose transporter half-saturation constant (K(m)) was unchanged by PTZ, we derived estimates of 1) treated and 2) control maximal transporter velocities (V(max)) and 3) a single K(m). In cortex, the glucose transporter V(max) was 42 +/- 11% higher (P < 0.05) in PTZ-treated animals (2.46 +/- 0.34 micromol. min(-1). g(-1)) than in control animals (1.74 +/- 0.26 micromol. min(-1). g(-1)), and the K(m) = 9.5 +/- 1.6 mM. Blood-brain barrier (BBB) V(max) was 31 +/- 10% greater (P < 0.05) in PTZ-treated (2.36 +/- 0. 30 micromol. min(-1). g(-1)) than control subcortex (1.80 +/- 0.25 micromol. min(-1). g(-1)). We conclude acute upregulation of BBB glucose transport occurs within 3 min of an initial seizure. Transporter V(max) and BBB glucose permeability increase by 30-40%.

Endothelial nitric oxide synthase-dependent cerebral blood flow augmentation by L-arginine after chronic statin treatment

Nitric oxide, a product of nitric oxide synthase activity, relaxes vascular smooth muscle and elevates brain blood flow. We evaluated the importance of eNOS to cerebral blood flow augmentation after L-arginine infusion and increases in flow after eNOS upregulation in SV-129 mice. Blood flow was measured by laser-Doppler flowmetry before and after L-arginine infusion (450 mg/kg during a 15-minute period) or measured by 14C-iodoamphetamine indicator fractionation or 14C-iodoantipyrine tissue equilibration techniques. rCBF increased by 26% (laser Doppler flowmetry) after L-arginine infusion but did not change in mutant mice deficient in eNOS expression. After eNOS upregulation by chronic simvastatin treatment (2 mg/kg subcutaneously, daily for 14 days), L-arginine amplified and sustained the hyperemia (38%) and increased absolute brain blood flow from 86 +/- 7 to 119 +/- 10 mL/100 g per minute. Furthermore, pretreatment with simvastatin enhanced blood flow within ischemic brain tissue after middle cerebral artery occlusion. Together, these findings suggest that eNOS activity is critical for blood flow augmentation during acute L-arginine infusion, and chronic eNOS upregulation combined with L-arginine administration provides a novel strategy to elevate cerebral blood flow in the normal and ischemic brain.

Implementation of quantitative FAIR perfusion imaging with a short repetition time in time-course studies

Flow-sensitive alternating inversion recovery (FAIR) is a pulsed arterial spin labeling magnetic resonance imaging method for perfusion quantification. In its standard implementation for quantification with full longitudinal relaxation between acquisitions, its use in time-course investigations of rapidly changing flow values is limited. The time efficiency can be improved by decreasing the repetition time but quantification becomes problematic. This situation is further complicated if a whole-body radiofrequency transmit coil is not used since fresh blood spins will flow in from outside the coil. To alleviate these problems, the use of global pre-saturation is proposed. The resulting expression for the flow signal depends on the relationship between the imaging parameters and the coil inflow time and can be significantly simplified under certain combinations of these parameters. With this implementation of FAIR, quantitative flow maps of gerbil brains were obtained with a 3 minute time resolution in a study of the effects of reperfusion. The pre-occlusion flow measurements were in good agreement with values obtained by the standard FAIR implementation and by other techniques, but the low values following occlusion were underestimated due to the increased transit times.

Blood-brain barrier glutamine transport during normoglycemic and hyperglycemic focal cerebral ischemia

This study examines the effects of middle cerebral artery (MCA) occlusion in the rat on blood to brain glutamine transport, a potential marker of early endothelial cell dysfunction. It also examines whether the effects of ischemia on glutamine transport are exacerbated by hyperglycemia. In pentobarbital-anesthetized rats, 4 hours of MCA occlusion resulted in a marked decline in the influx rate constant for [14C]L-glutamine from 16.1+/-1.2 microL.g(-1).min(-1) in the contralateral hemisphere to 7.3+/-2.5 microL.g(-1).min(-1) in the ischemic core (P < 0.001). This reduction was even greater in xylazine-ketamine-anesthetized rats in which the influx decreased to 2.6+/-1.1 microL.g(-1) min(-1). This greater reduction appears related to the hyperglycemia induced by xylazine-ketamine anesthesia. Glucose injection in pentobarbital-anesthetized rats also resulted in a greater decline in [14C]L-glutamine influx in the ischemic core but had no effect on the contralateral tissue. The effects of hyperglycemia on glutamine transport in the ischemic tissue were associated with a decline in plasma volume, which may reflect either endothelial cell swelling or plugging of the microvasculature. The reduction in glutamine transport during ischemia was progressive, but even as early as 1 hour, there was a 60% and 40% decline in influx in hyperglycemic and normoglycemic rats, respectively. The fall in [14C]L-glutamine influx may reflect a dissipation of the endothelial cell [Na+] gradient. A decline in this gradient would affect many blood-brain barrier transporters with potentially deleterious effects on the ischemic brain.

Contribution of hypercapnia and trigeminal stimulation to cerebrovascular dilation during simulated diving

We investigated the relative contribution of humoral (carbon dioxide) and neural (trigeminal stimulation) inputs in the cerebrovasodilatory response to simulated diving in the rat. The cerebral hemodynamic profile of rats was determined using the brain blood flow tracer N-[14C]isopropyl-p-iodoamphetamine. During a simulated dive response, cerebral vascular resistance (CVR) decreased 63.1%, resulting in a 1.5-fold increase in cerebral blood flow (CBF). To investigate the contribution of hypercapnia to the decrease in CVR during simulated diving, we measured CBF during simulated diving in rats with preexisting hypocapnia. To investigate the contribution of trigeminal input, we measured CBF during periods of trigeminal stimulation alone with continued ventilation. Preexisting hypocapnia abolished the cerebrovasodilatory response to simulated diving. Trigeminal stimulation alone did not produce a significant increase in CBF from control values in any brain region, suggesting that trigeminal input does not contribute to the cerebrovascular response to simulated diving in rats. These results suggest that the cerebrovasodilatory response observed during diving in small mammals is driven primarily by progressive hypercapnia associated with asphyxia.

Reduced ischemic brain injury in interleukin-1 beta converting enzyme-deficient mice

A variety of recent studies suggest a role for both inflammatory cytokines such as interleukin-1 beta (IL-1 beta), and apoptosis in ischemic brain injury. Because IL-1 beta converting enzyme (ICE) is required for the conversion of proIL-1 beta to its biologically active form, and has homology with proteins that regulate apoptosis in invertebrates, we studied the effect of cerebral ischemia on brain injury in mutant mice deficient in the ICE gene (ICE knockout [KO] mice). Focal cerebral ischemia, produced by occlusion of the middle cerebral artery, resulted in brain edema (increased water and sodium content) at 4 hours and a histologically defined brain lesion at 24 hours. Both of these markers of brain injury were significantly reduced in the ICE KO mice as compared to wild-type C57BL/6 mice. Regional cerebral blood flow, determined using the flow tracer, N-isopropyl [methyl 1,3-(14)C] p-iodoamphetamine (14C-IMP), was similar in the two strains of mice, indicating that the reduced brain injury in the KO mice was not a result of a lesser degree of ischemia. These data show that ICE contributes to the development of ischemic brain damage, and that it plays a role at an early time in the pathologic process. Although the mechanism of this effect is uncertain, our results suggest that pharmacologic inhibition of ICE may be a useful treatment for stroke.

Mechanisms of edema formation after intracerebral hemorrhage: effects of thrombin on cerebral blood flow, blood-brain barrier permeability, and cell survival in a rat model

Recently, the authors showed that thrombin contributes to the formation of brain edema following intracerebral hemorrhage. The current study examines whether the action of thrombin is due to an effect on cerebral blood flow (CBF), vasoreactivity, blood-brain barrier (BBB) function, or cell viability. In vivo solutions of thrombin were infused stereotactically into the right basal ganglia of rats. The animals were sacrificed 24 hours later; CBF and BBB permeability were measured. The actions of thrombin on vasoreactivity were examined in vitro by superfusing thrombin on cortical brain slices while monitoring microvessel diameter with videomicroscopy. In separate experiments C6 glioma cells were exposed to various concentrations of thrombin, and lactate dehydrogenase release, a marker of cell death, was measured. The results indicate that thrombin induces BBB disruption as well as death of parenchymal cells, whereas CBF and vasoreactivity are not altered. The authors conclude that cell toxicity and BBB disruption by thrombin are triggering mechanisms for the edema formation that follows intracerebral hemorrhage.

Mechanisms of edema formation after intracerebral hemorrhage: effects of thrombin on cerebral blood flow, blood-brain barrier permeability, and cell survival in a rat model

Recently, the authors showed that thrombin contributes to the formation of brain edema following intracerebral hemorrhage. The current study examines whether the action of thrombin is due to an effect on cerebral blood flow (CBF), vasoreactivity, blood-brain barrier (BBB) function, or cell viability. In vivo solutions of thrombin were infused stereotactically into the right basal ganglia of rats. The animals were sacrificed 24 hours later; CBF and BBB permeability were measured. The actions of thrombin on vasoreactivity were examined in vitro by superfusing thrombin on cortical brain slices while monitoring microvessel diameter with videomicroscopy. In separate experiments C6 glioma cells were exposed to various concentrations of thrombin and lactate dehydrogenase release, a marker of cell death, was measured. The results indicate that thrombin induces BBB disruption as well as death of parenchymal cells, whereas CBF and vasoreactivity are not altered. The authors conclude that cell toxicity and BBB disruption by thrombin are triggering mechanisms for the edema formation that follows intracerebral hemorrhage.

Induced response to hypercapnia in the two-compartment total cerebral blood volume: influence on brain vascular reserve and flow efficiency

This study was undertaken to investigate the mechanisms of CBF increase as induced by hypercapnia. It was achieved in anesthetized rats by determining total cerebral blood volume (TCBV), parenchymal blood (CBV), plasma (CPV), erythrocyte (CEV) volumes and cerebral hematocrit (CHct) as well as CBF at about 40, 60, and 80 mm Hg PaCO2. TCBV was measured by a noninvasive blood dilution method using [99mTc]pertechnetate. CBV, CPV, and CEV were measured on isolated brain by 125I-serum albumin and 51Cr-erythrocytes. CBF was measured by both [131I/14C]iodoantipyrine and 57Co-microsphere extractions. The extraparenchymal blood volume (ECBV) was evaluated by subtracting CBV from TCBV. Under normocapnia, ECBV was 2.8 times larger than CBV. Under moderate hypercapnia, ECBV increased by 44%, CBV was not modified, and CBF increased by 52%. These results demonstrate that the main site of vasodilation is located in the extraparenchymal vasculature, which thus acts as a vascular reserve. By contrast, under severe hypercapnia, ECBV remained unchanged, whereas CBV then increased by 17%; CBF simultaneously showed an additional augmentation of either 52 or 309% when diffusible tracer or microspheres were used. This important increase in CBF cannot be explained either by capillary recruitment of closed capillaries or by active diameter lengthening of already open capillaries. The concomitant and great increase in capillary blood velocity was also shown to reduce cerebral flow efficiency, a situation consistent with a "luxury perfusion."

Developmental modulation of blood-brain-barrier glucose transport in the rabbit

Blood-brain barrier (BBB) glucose transport rates were measured using the intracarotid injection method in newborn, 14-day-old suckling, 28-day-old weanling and adult rabbits, and compared with membrane transporter density. Light microscope immunochemistry confirmed the presence of the GLUT1 glucose transporter isoform in these rabbits. Quantitative electron microscopic immunogold analyses of GLUT1-immunoreactive sites per micrometer of capillary membrane indicated GLUT1 density increased with age, and correlated with in vivo measurements of Vmax. Maximal transport velocities (Vmax) of glucose transfer (an indicator of the activity and relative number of transporter proteins) increased significantly (P = 0.05) with age: in neonates Vmax = 0.61 mumol.min-1.g-1, in sucklings Vmax = 0.68 mumol.min-1.g-1, in weanlings Vmax = 0.88 mumol.min-1.g-1, and in adults Vmax = 1.01 mumol.min-1 g-1. Cerebral blood flow (CBF) rates, increased with age from 0.19 and 0.26 ml.min-1.g-1 in neonates and sucklings to 0.51 (weanlings) and 0.70 (adults) ml.min-1.g-1. Non-linear regression analyses indicated the half-saturation constant (Km) for glucose transport ranged from 13 mM in adult rabbits to 19 mM in 14-day-old sucklings: differences in Km were not significant. Age-related changes in the Permeability-Surface Area product (PS +/- S.E.) of both water and glucose were also seen. At all ages studied, the diffusion component (Kd) of glucose uptake was not distinguishable from zero. We conclude developmental up-regulation of the rabbit BBB glucose transporter is characterized by no changes in transporter affinity, and provide the first demonstration of increased membrane transporter proteins correlating with an age-related increase (65%) in glucose transporter maximal velocity.

Experimental intracerebral hemorrhage: relationship between brain edema, blood flow, and blood-brain barrier permeability in rats

There have been few investigations of brain edema formation after intracerebral hemorrhage (ICH), despite the fact that mass effect and edema are important clinical complications. The present study was designed to investigate the time course for the formation and resolution of brain edema and to determine how changes in cerebral blood flow (CBF) and blood-brain barrier (BBB) permeability are temporally related to edema formation following ICH. Anesthetized adult rats received a sterile injection of 100 microliters of autologous blood into the caudate nucleus. Water and ion contents were measured immediately, at 4 and 12 hours, and daily to Day 7 (10 time points, six rats at each time) after experimental ICH. The water content of the ipsilateral basal ganglia increased progressively (p < 0.002) over the first 24 hours, then remained constant until after Day 5, when the edema began to resolve. Edema was most severe in the tissue immediately surrounding the hemorrhage; however, it was also present in the ipsilateral cortex, the contralateral cortex, and the basal ganglia. Measurements of local CBF (using [14C]-iodoantipyrine) and BBB permeability (using [3H]-alpha-aminoisobutyric acid) were obtained in separate groups of six to eight rats at various time intervals between 1 and 48 hours after ICH. Cerebral blood flow was reduced to 50% of control at 1 hour, returned to control values by 4 hours, but then decreased to less than 50% of control between 24 and 48 hours after ICH. The BBB permeability increased significantly prior to the occurrence of significant edema in the tissue surrounding the clot. However, BBB permeability in the more distant structures remained normal despite the development of edema. These results demonstrate a time course for the formation and resolution of brain edema following ICH similar to that observed during focal ischemia. Brain edema forms in the immediate vicinity of the clot as a result of both BBB disruption and the local generation of osmotically active substances and then spreads to adjacent structures. While local ischemia, due to the mass effect of the hemorrhage, may play a role in producing cytotoxic and vasogenic edema, the release of toxic substances from the clot should also be considered. Since edema is nearly maximal by 24 hours after ICH, therapy directed at reducing edema formation must be instituted within the 1st day.

Imaging ischemic tissue at risk of infarction during stroke

Autoradiograms obtained after middle cerebral artery occlusion (MCAO) in spontaneously hypertensive rats show that the 99mTc complex of a 2-nitroimidazole-derivatized propylene amine oxime (BMS-181321) is selectively retained in acutely ischemic brain before disruption of the blood-brain barrier (BBB), but not in the ischemic infarct. BMS-181321 is therefore a marker of ischemic tissue at risk of infarction and its uptake, unlike that of x-ray and magnetic resonance contrast agents, does not require disruption of the BBB. In keeping with this conclusion, we have found that the single-pass cerebral extraction fraction of BMS-181321 is 0.67 at normal rat whole-brain blood flow. Sequential single-photon emission computed tomographic images obtained from cats after MCAO show that the initial distribution of BMS-181321 approximates regional CBF and that selective retention subsequently produces a positive image within the ischemic territory. BMS-181321 is the first Tc complex able to indicate not only ischemia, but also ischemic tissue at risk of infarction. Use of this novel Tc complex to monitor biochemical events during ischemia may contribute to the clinical management of acute stroke.

Measurement of regional cerebral blood flow with positron emission tomography: a comparison of [15O]water to [11C]butanol with distributed-parameter and compartmental models

To further our understanding of the best way to measure regional CBF with positron emission tomography (PET), we directly compared two candidate tracers ([15O]water and [11C]butanol, administered intravenously) and two popular implementations of the one-compartment (1C) model: the autoradiographic implementation representing a single PET measurement of tissue radioactivity over 1 min and a dynamic implementation representing a sequence of measurements of tissue radioactivity over 200 s. We also examined the feasibility of implementing a more realistic, and thus more complex, distributed-parameter (DP) model by assigning fixed values for all of its parameters other than CBF and tracer volume of distribution (Vd), a requirement imposed by the low temporal resolution and statistical quality of PET data. The studies were performed in three normal adult human subjects during paired rest and visual stimulation. In each subject seven regions of interest (ROIs) were selected, one of which was the primary visual cortex. The corresponding ROI were anatomically equivalent in the three subjects. Regional CBF, Vd, tracer arrival delay, and dispersion were estimated for the dynamic data curves. A total of 252 parameter sets were estimated. With [11C]butanol both implementations of the 1C model provided similar results (r = 0.97). Flows estimated using the 1C models were lower (p < 0.01) with [15O]water than with [11C]butanol. In comparison with the 1C model, the constrained version of the DP used in these studies performed inadequately, overestimating high flow and underestimating low flow with both tracers, possibly as the result of the necessity of assigning fixed values for all of its parameters other than CBF and Vd.

Microwave fixation for the determination of cerebral blood volume in rats

The cerebral blood volume (CBV) is sensitive to changing hydrostatic pressures. Thus, measurement methods that rely on removing tissue from unfixed brain may lead to underestimates of the CBV due to the loss of blood from the tissue. In situ fixation of tissue before removal may offer improved accuracy. We employed a triple-label method to measure simultaneously whole brain CBF and CBV in halothane-anesthetized Sprague-Dawley rats, which were then killed either by focused microwave irradiation (approximately 8 kW of incident power x 770 ms) or by decapitation. CBF was measured with [3H]nicotine while the CBV was determined as the sum of the cerebral red cell volume (CRCV--measured with 99mTc-labeled red cells) and the cerebral plasma volume (CPV--measured with [14C]dextran). Animals were studied during hypocarbic (PaCO2 approximately 25 mm Hg), normocarbic, or hypercarbic (PaCO2 approximately 70 mm Hg) conditions. Added studies were performed to verify that the microwave irradiation scheme used was capable of fixing previously administered tracers in place, and also halting the entry of tracer given after irradiation. Results indicate that the method of killing had no effects on CBF measurements, as assessed either by absolute values during normocarbia or responsiveness to changing PaCO2. However, all three volume measurements made using nondiffusible tracers (CRCV, CPV, and CBV) were significantly lower in animals killed by decapitation. Furthermore, CO2 responsiveness for all three variables (as assessed by the slope of the PaCO2/volume) was not evident in decapitated animals. We conclude that in situ fixation offers significant advantages when examining the cerebral distribution space of nondiffusible tracers.

Thromboxane receptor antagonism and synthase inhibition in cerebral ischemia

Thromboxane A2 (TXA2) is a proaggregatory vasoconstrictor that may suppress regional cerebral blood flow (rCBF) during postischemic hypoperfusion. This study was undertaken to determine if rCBF could be elevated by postischemic treatment with a TXA2 receptor antagonist, SQ29,548, given alone or in combination with 1-benzylimidazole (1-BI), a thromboxane synthase inhibitor. Wistar rats were subjected to 30 min of reversible forebrain ischemia and treated with SQ29,548 or an SQ29,548/1-BI combination during 60 min of reperfusion. Cerebral TXB2, the stable metabolite of TXA2, was 1.33 +/- 0.91 ng mg brain protein-1 in animals treated with SQ29,548 and exposed to ischemia, compared to 1.15 +/- 0.32 in ischemic controls (p = NS). Administration of SQ29,548/20 mg kg-1 1-BI reduced cerebral TXB2 to 0.20 +/- 0.25 (p < or = 0.01). Regional CBF was depressed significantly in ischemic controls compared to sham-ischemic animals (p < or = 0.01 in all regions except for p < or = 0.05 in diencephalon) and was not altered by treatment with SQ29,548. Rats given the SQ29,548/1-BI combination showed an overall increase in rCBF that did not reach statistical significance when compared to ischemic controls. However, rCBF in hippocampus and diencephalon of animals given the drug combination was significantly greater than in rats treated with SQ29,548 alone (p < or = 0.05).

Blood-brain barrier transfer of L-Trp and alpha-MTrp in Li-treated rats

Blood-brain barrier (BBB) transport for L-Trp and alpha-methyl-L-tryptophan was evaluated in Li-treated rats. Five different brain areas as well as left to right differences were examined. No left to right difference in the PS product was observed. Lithium treatment had a significant effect on the plasma concentration of Val, Leu and Ile but no effect on plasma total or free Trp. The ratio of plasma Trp to the sum of Leu, Val, Ile, Phe, Met and Tyr is increased in the Li-treated rats but not significantly. However, the ratio of Trp/(Val+Leu+Ile) is significantly increased in the Li-treated rats. The Km apparent (Kmapp) for the BBB Trp transport is significantly decreased (affinity of the carrier for Trp is increased) in the Li-treated rats. A decrease in the Kmapp is one of the possible factors responsible for an increase in the brain Trp concentration and subsequent increase in the brain serotonin synthesis in Li-treated rats.

Threshold relationship between cerebral blood flow, glucose utilization, and energy metabolites during development of stroke in gerbils

Focal brain ischemia was produced in halothane-anesthetized Mongolian gerbils by occluding the right common and the left external carotid artery. Ninety minutes after vascular occlusion the following regional hemodynamic and metabolic parameters were evaluated in adjacent cryostat sections taken from seven different coronal planes of each brain: cerebral blood flow (CBF), glucose utilization (CMRG), and the tissue content of ATP and glucose. NADH fluorescence was recorded from the surface of the cryostat block. In addition, tissue slices were taken from each brain to determine the rate of phosphorylation of 2-deoxyglucose in ischemic and nonischemic regions. Depending on the density of ischemia, the following metabolic disturbances were observed. At CBF values below 35 ml x 100 g-1 x min-1 CMRG increased and at values below 25 ml x 100 g-1 x min-1 it declined sharply. Glucose content declined when CBF was below 35 ml x 100 g-1 x min-1 and ATP fell at CBF below 20 ml x 100 g-1 x min-1. At 10 ml x 100 g-1 x min-1 ATP was completely depleted. NADH fluorescence was found elevated at flow rates that caused an increase of glucose utilization and was maximal when CBF stopped. The ischemic thresholds for the initial increase in CMRG and the complete depletion of ATP content represent the metabolic equivalent of the penumbra zone and provide a basis for the evaluation of therapeutic procedures for the treatment of stroke.

Evaluation of experimental early acute cerebral ischemia before the development of edema: use of dynamic, contrast-enhanced and diffusion-weighted MR scanning

The ability of dynamic, contrast-enhanced, magnetic susceptibility-weighted scanning to delineate early experimental acute cerebral infarction was compared with that of heavily T2-weighted and diffusion-weighted spin echo scanning. Spontaneously hypertensive rats, which had undergone right middle cerebral artery occlusion, were studied from 15 min to 3 h post ligation on a 1.5-T clinical whole-body imager. In contrast to the diffusion- and T2-weighted spin echo scans, the dynamic, contrast-enhanced technique clearly and consistently delineated the nonperfused regions as early as 15 min post ligation.