Developing a Rat Model of Brainstem Coma: Initial MRI and MRA Investigations of Basilar Artery Occlusion
Patricia Pais Roldán1, Brian Edlow2, and Xin Yu1

1Translational Neuroimaging and Neural Control Research Group, High Field Magnetic Resonance Department, Max Planck Institute for Biological Cybernetics, Tübingen, Germany, 2Department of Neurology, Massachusetts General Hospital, Harvard Medical School, Boston, MA, United States

Synopsis

The ascending reticular activating system (ARAS) of the brainstem mediates arousal, which is an essential component of consciousness. In order to infarct the ARAS and induce an unarousable state in rats, we performed a two-point basilar artery (BA) occlusion. We used high resolution MRI to map the neuroanatomical distribution of the resulting brainstem infarction and MRA to map the penetrating branches of the BA. BA occlusion reproducibly caused medial-ventral brainstem infarction but did not create an unarousable state, suggesting that a larger region of ARAS infarction will be needed to create a rat model of brainstem coma.

Target audience

Scientists/radiologists who study brainstem injuries and cerebrovascular diseases.

Introduction/significance

Bilateral damage to the rostral brainstem causes coma by disrupting the ascending pathways of the reticular activating system1,2. Despite the relevance of coma-causing brainstem lesions to multiple human diseases, an animal model of coma due to brainstem injury is lacking. Here, we attempt to induce a comatose state in a rat by producing a focal brainstem infarction. We occluded the basilar artery (BA), which is the primary source of blood supply to the rostral brainstem, at two points3, sparing the anterior inferior cerebellar artery (AICA). We first mapped the neuroanatomical distribution of the resulting brainstem infarction. Using a high resolution MRA method, we then mapped the major branches of the BA. The two-point BA occlusion reproducibly produced infarction of medial and ventral brainstem, but the rats were not comatose. This result provides a foundation to further modify the BA occlusion strategy to more precisely target the arousal pathways of the reticular activating system and induce a comatose state in rats.

Methods

For BA occlusion, the anesthetized animals were secured dorsally and the cervical skin was shaved and cleaned with 10% povidine iodine. After dissecting the sternohyoid and omohyoid muscles, the base of the occipital cranium was exposed. A high speed drill was used to thin the exposed basilar bone and the dura was opened widely. The BA and several branches could be identified (Fig. 2 A) and two points along the BA were permanently occluded with 6-0 silk suture (Fig. 2 B). An MRI scanner with a 14T magnet (Agilent, with a Bruker Biospin AVIII console system) equipped with a 12 cm gradient set, capable of providing 100 G/cm with a rise time of 150 ms was used for the characterization of the brain of healthy and BA-occluded rats. A FLASH sequence was used to visualize vasculature in the brain (MR Angiography), setting an Echo Time (TE) of 1,867 ms and a Repetition Time (TR) of 20 ms with flip angle at 50°, which allows highlighting the blood vessels (Fig.1 and Fig.2 B). For T2-weighted images that allow detection of infarcted areas (visualized as bright zones), a RARE sequence was used, with TE = 35.88 and TR = 2000 ms (Fig. 2 C-F).

Results

Nerve tissue in brainstem is supported by a capillary network emerging from two vertebral arteries, which join to form the BA. Small branches of the BA penetrate deeper into the brainstem, which was detected with the high resolution MRA sequence (Fig 1). Large arteries travelling over the ventral brainstem are readily observable through craniotomy in the ventral skull in vivo (Fig. 2 A), and a picture of the whole network can be realized with short-TR MR sequences (MRA, Fig. 1 and Fig. 2 B). BA occlusion above the level of AICA (Fig. 2 B) resulted in vanishing of the corresponding BA segment when visualized with MRA (data not shown). Disruption of blood flow within the BA triggered the emergence of infarcted areas in the medial-ventral brainstem, which could be detected with T2-weighted MRI (Fig. 2 C and E, compared to Fig. 2 D and F). Nevertheless, BA-occluded rats did not enter a comatose state, and thus a large-scale constriction affecting AICA (besides BA) to occlude blood to the dorsal brainstem might be used in future studies intended to induce an unarousable state.

Conclusions

High resolution MRA was used to characterize the main vessels that supply blood to brainstem, showing the BA and numerous branches that penetrate deeper through the medulla, pons and part of the midbrain. Occlusion of BA resulted in local blood flow disruption confirmed by MRA and subsequent infarcts to the brainstem observed in T2-weighted images, setting the base for the development of a model for local injury to brainstem in the rat.

Acknowledgements

We thank Dr. Yaohui Tang for his technical support on establishing the two-point BA occlusion model.

References

1. Parvizi, J. & Damasio, A. R. Neuroanatomical correlates of brainstem coma. Brain. 2003; 126: 1524-1536.

2. Laureys, S., Owen, A. M. & Schiff, N. D. Brain function in coma, vegetative state, and related disorders. Lancet Neurol. 2004;3: 537-546.

3. Wojak, J. C., DeCrescito, V. & Young, W. Basilar artery occlusion in rats. Stroke. 1991; 22: 247-252.

Figures

Fig. 1. High resolution MRA of a normal rat brain. A. Sagittal view of the brain vasculature (maximal projection of medial slices). BA and its branches can be easily visualized on the ventral part of the brainstem. B. MRA three dimensional reconstruction of the BA and its branches.

Fig. 2. A. Vasculature of the ventral brainstem under bright field microscopy. B. MRA of a control rat, showing the points of BA occlusion (white arrows). C-D. T2-w coronal images of a BA-occluded and control rat. E-F. T2-w sagittal images of a BA-occluded and a control rat.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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