click to enlarge Figure. fMRI visualization of integrated neural pathways (with activated areas highlighted in red) from the brains of awake rats for three different treatments utilizing a three dimensional brain atlas. (Source: Ekam Imaging Inc.)

A major limitation of current non-clinical imaging—especially relating to the brain—is that animals must be anesthetized.¹ This creates a significant gap in the ability to provide serial data in diseases affecting the brain and for new and existing drugs focused on the central nervous system (CNS). With the advent of functional magnetic resonance imaging (fMRI) in awake animals, it is now possible to resolve changes in or patterns of neuronal activity across the entire brain with exquisite spatial and temporal resolution by evaluating a change in blood oxygen level dependent (BOLD) measurements.² Synchronized changes in neuronal activity across different regions of the brain can be viewed as functional neuroanatomical circuits coordinating the thoughts, memories, and emotions for particular behaviors.

Awake animal fMRI is applicable to a wide range of study types ranging from CNS safety to neurodegeneration and psychotherapy.³ fMRI studies have been conducted in conscious rats employing an integrated imaging system and analysis technology that combines RF coil systems/electronics, automated data analysis, and visualization of activation patterns using 3D segmented brain atlases.²

The figure illustrates the putative distributed neural circuit involved in aggressive motivation and how this circuit is affected by drugs that block aggressive behavior. To trigger an aggressive motivation response, conscious animals were subjected to a series of provocation paradigms.⁴ As hypothesized, brain areas previously identified as critical in the organization and expression of aggressive behavior were activated. Unexpected was the intense activation of the forebrain cortex and anterior thalamic nuclei. A difference in the BOLD response in treatment groups demonstrated that the putative neural circuit of aggressive motivation identified with fMRI includes neural substrates contributing to emotional expression, emotional experience, and the anterior thalamic nuclei that bridge the motor and cognitive circuits. Such studies are only possible in conscious animals.

Awake animal imaging is further demonstrated by utilizing a microsphere model of rotenone-induced Parkinson’s Disease (PD) that recapitulates disease progression without the debilitating sickness and CNS variability often associated with rotenone treatment.⁵ Studies employing fMRI and BOLD measurements can be used to monitor neurodegeneration by a loss of dopamine neurotransmission following amphetamine treatment to assess release of dopamine in the dorsal striatum. Since imaging studies are conducted in awake, non-terminal studies, behavioral components can easily be added to document loss of motor function.

References
1. King JA, et al. Procedure for minimizing stress for fMRI studies in conscious rats. J Neurosci Methods. 2005; 148(2):154.
2. Ferris CF, et al. Functional Magnetic Resonance Imaging in Conscious Animals: A New Tool in Behavioural Neuroscience Research. J Neuroendocrinology. 2006; 18(5):307.
3. Icard KM, et al. Differential recovery of multimodal MRI and behavior after transient focal cerebral ischemia in rats. J Cereb Blood Flow Metab. 2006; 26(11):1451.
4. Ferris CF, et al. Imaging the neural circuitry and chemical control of aggressive motivation. BMC Neuroscience. 2008; 9:111 doi:10.1186/1471-2202-9-111
5. Marella M, et al. Protection by the NDI1 gene against neurodegeneration in a rotenone rat model of Parkinson’s disease. PLoS One. 2008;3(1):e1433.

This article was published in Drug Discovery & Development magazine: Vol. 13, No. 2, March 2010, p. 20.