Elsevier

NeuroImage

Volume 86, 1 February 2014, Pages 417-424
NeuroImage

Detection of functional connectivity in the resting mouse brain

https://doi.org/10.1016/j.neuroimage.2013.10.025Get rights and content

Highlights

  • Bilateral functional connectivity across mouse brain detected at low dose of medetomidine.

  • Connectivity depressed at higher medetomidine dose compared to rats.

  • Consistent with rats, somatosensory activation in mouse unchanged across medetomidine doses.

  • Medetomidine sedation enables longitudinal functional connectivity imaging in mouse models.

Abstract

Resting-state functional connectivity, manifested as spontaneous synchronous activity in the brain, has been detected by functional MRI (fMRI) across species such as humans, monkeys, and rats. Yet, most networks, especially the classical bilateral connectivity between hemispheres, have not been reliably found in the mouse brain. This could be due to anesthetic effects on neural activity and difficulty in maintaining proper physiology and neurovascular coupling in anesthetized mouse. For example, α2 adrenoceptor agonist, medetomidine, is a sedative for longitudinal mouse fMRI. However, the higher dosage needed compared to rats may suppress the functional synchrony and lead to unilateral connectivity. In this study, we investigated the influence of medetomidine dosage on neural activation and resting-state networks in mouse brain. We show that mouse can be stabilized with dosage as low as 0.1 mg/kg/h. The stimulation-induced somatosensory activation was unchanged when medetomidine was increased from 0.1 to 6 and 10 folds. Especially, robust bilateral connectivity can be observed in the primary, secondary somatosensory and visual cortices, as well as the hippocampus, caudate putamen, and thalamus at low dose of medetomidine. Significant suppression of inter-hemispheric correlation was seen in the thalamus, where the receptor density is high, under 0.6 mg/kg/h, and in all regions except the caudate, where the receptor density is low, under 1.0 mg/kg/h. Furthermore, in mice whose activation was weaker or took longer time to detect, the bilateral connectivity was lower. This demonstrates that, with proper sedation and conservation of neurovascular coupling, similar bilateral networks like other species can be detected in the mouse brain.

Introduction

Resting-state functional connectivity MRI has emerged as a method for mapping intrinsic brain networks. These networks are based on coherent brain activities that are mostly detected by the blood oxygenation level dependent (BOLD) (Biswal et al., 1995) or perfusion (Chuang et al., 2008) functional MRI (fMRI). Similar and consistent brain networks, especially the bilateral connectivity in sensory and motor related areas, have been identified across species, from humans, monkeys to rats (Fox and Richle, 2007) (Hutchison et al., 2010a, Vincent et al., 2007). As many transgenic models of diseases are available in mouse, it is enticing to be able to apply the same technique in the mouse brain. Recently, resting-state networks have been reported in mouse using fMRI (Guilfoyle et al., 2013, Jonckers et al., 2011) and optical imaging (Bero et al., 2012) (White et al., 2011). However, despite the structural and anatomical similarities between the mouse and the rat brain, only unilateral connectivity in major cortical and subcortical areas was detected by fMRI (Guilfoyle et al., 2013, Jonckers et al., 2011).

Functional MRI studies in the mouse brain have been challenging due to various technical and physiological issues. It suffers from lower detection sensitivity and more severe susceptibility-induced image distortions and signal losses from larger air-tissue interface in the smaller brain. Strategies such as using cryo-probe (Baltes et al., 2011) or susceptibility matching material (Adamczak et al., 2010) have been explored to overcome these challenges. Since resting-state BOLD signal is only a fraction of the activated signal, the lack of bilateral connectivity in the mouse brain may be due to limited sensitivity.

Besides the technical issues, the need for proper maintenance of physiological conditions to preserve neurovascular coupling is more critical and largely dependent on the choice of anesthetics. So far, a few anesthesias have been demonstrated to allow robust BOLD activation to be measured in the mouse brain. Somatosensory BOLD activations using forepaw or hindpaw stimulation has been reported in isoflurane anesthetized mice with either free-breathing (Nair and Duong, 2004) or mechanical ventilation (Baltes et al., 2011), and in medetomidine sedated mice (Adamczak et al., 2010). In addition, anesthetics have been shown to have different impact on the resting-state networks. For example, we have shown that medetomidine can disrupt synchrony in the brain at high dose (Nasrallah et al., 2012). The lack of bilateral connectivity in the mouse brain may be due to the medetomidine dosage used (Jonckers et al., 2011). Another study using high isoflurane level of 1.5% reported functional connectivity in areas related to the default mode network, but no other networks (Guilfoyle et al., 2013). Since isoflurane could cause bursting activity and neural suppression (Liu et al., 2011) and has variable effects on neurovascular coupling (Masamoto et al., 2009), it is crucial to ensure that proper anesthetic level is used and proper neurovascular relationship is maintained.

In this study, we evaluated the influence of medetomidine dosage on neural activation and resting-state functional connectivity in the mouse brain using BOLD fMRI. Similar to what was found in the rat, somatosensory activation was not affected by medetomidine and bilateral connectivity can be detected in all the major brain areas at low dose of medetomidine. The dosage dependent suppression of bilateral connectivity was seen in the very high dose, which indicates different pharmacodynamics in mice compared to rats.

Section snippets

Experimental design

To investigate the effect of medetomidine on functional connectivity in mice, resting state BOLD was assessed at two time points — 30 and 120 min after bolus injection of medetomidine – and under three dosages of medetomidine sedation – 0.1, 0.6, or 1 mg/kg/h (Fig. 1a). After 2 h of experiment, all mice were very lethargic and therefore atipamezole, an α2 adrenoceptor antagonist, was required for reversal of the effects. For such reason, the dosage effects were investigated in separate sets of

Physiological measurements

To assess the physiological change, SpO2, HR, RR, and rectal temperature were recorded under constant infusion of 0.1, 0.6, and 1.0 mg/kg/h medetomidine on the bench. The RR quickly stabilized after 15 min likely due to the start of continuous infusion (Fig. 1b). The HR started high and gradually decreased. It took slightly longer time to stabilize and reached plateau between 30 to 60 min after the bolus injection. All physiological parameters were stable in the period between 40 and 120 min after

Discussion

The advances in resting state functional connectivity imaging have provided valuable insight into how brain networks are disrupted in diseases. Applying similar technique in mouse models of diseases can help to determine the underlying pathophysiological changes and to evaluate potential treatments. However, resting-state fMRI in the mouse brain has been challenging due to low detection sensitivity and large physiological variations. Here we demonstrated that robust functional activation can be

Conclusion

In this work, we demonstrated the existence of bilateral functional connectivity in major areas of the resting mouse brain. Therefore the resting-state functional connectivity seems to be a well preserved phenomenon in mammal. The potential to detect functional networks in the mouse with a simple and longitudinal sedative protocol will enable broader application of fMRI in the investigation of a wide variety of transgenic models available to further understand disease progression, therapy and

Acknowledgments

We would like to thank Mr. Krzysztof Pyka for his technical input. The work was supported by the Intramural Research program of the Biomedical Sciences Institutes, Agency for Science, Technology and Research (A*STAR), Singapore.

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