Manganese-enhanced MRI of layer-specific activity in the visual cortex from awake and free-moving rats

Abstract

Cortical responses to visual stimulation have been studied extensively in the rodent, but often require post-stimulation ex vivo examination of the tissue. Here, we test the hypothesis that visual stimulusdependent cortical activity from awake and free-moving rats can be encoded following systemically administered MnCl 2 , and activity subsequently readout using manganese-enhanced MRI (MEMRI), a technique that can be performed without sacrificing the animal. Unanaesthetized Sprague-Dawley rats, with or without systemic injection of MnCl 2 , were maintained for eight hours in either a visually stimulating environment or darkness. To identify vision-dependent changes in cortical activity, animals were anesthetized and cortices were examined by 3D RARE MEMRI. Mean signal intensities in sub-cortical regions (e.g., superior colliculus and the lateral geniculate), and cortical regions (primary and accessory visual cortices) were compared. Cortex linearization was performed to aid in layer-specific signal intensity comparisons. Manganese administration alone globally increased signal intensity in the brain (P < 0.0001). In visually stimulated and unstimulated rats, layer-specific analysis revealed that stimulated rats had on average significantly (P < 0.05) higher signal intensities in layers IV and V of the primary visual cortex, as well as in deeper portions of the superficial superior colliculus, relative to dark adapted rats. Such differences went undetected without layer-specific analysis. We demonstrate, for the first time, the feasibility of layer-specific stimulus-dependant noninvasive MEMRI readout after encoding activity in awake and free moving rats. Future MEMRI studies are envisioned that measure the effects on cortical activity of sensory stimulation, as well as normal development, disease, plasticity, and therapy in longitudinal studies.Visual processing, an important function of the central nervous system, starts in the retina and continues in several sub-cortical regions and the visual cortex. In the rat brain, visual information is first processed at the level of the lateral geniculate nucleus, which then provides thalamic input to visual cortical layers IV and deep layer III, (Paxinos, 1985). Pathways to other brain regions implicated in vision also originate in the primary visual cortex, including those to the superior colliculus (from layer V) and the accessory visual cortex (from layers II through VI).Brain activity from awake and free-moving animals is assessed using either immunocytochemical staining techniques (Montero and Jian, 1995) or encoding activity based on the accumulation of an injected metabolic marker like 2-14 C deoxyglucose (Cooper and

Mapping of functional brain activity in freely behaving rats during voluntary running using manganese-enhanced MRI: Implication for longitudinal studies

“…According to a recent study, such differential distribution of manganese in the brain can be explained by preferential manganese transport via cerebrospinal fluid (CSF) and thus a higher manganese uptake in the brain structures adjacent to the ventricles (Bock et al, 2008a). Manganese-enhanced tissue contrast in cortical regions has been also reported in animals by several investigators, yet using much higher manganese concentrations than used in the present study (Bissig and Berkowitz, 2009;Bock et al, 2009;Silva et al, 2008). In the present study, a trans-synaptic manganese transport along the activated neural pathways likely contributed to the cortical labeling.…”

“…In combination with the basic principles of MEMRI, i.e. the paramagnetic nature of manganese, and its neural activity-dependent increases within the cells, AIM-MRI has been now used for different applications including functional mapping of sensory pathways in awake and unrestrained animals (Bissig and Berkowitz, 2009;Chuang et al, 2009;Yu et al, 2005). In our study, we implanted rats with manganese-loaded osmotic pumps and gave the animals a free access to a running wheel for 6 days.…”

“…Manganese-enhanced MRI (MEMRI) with systemically injected MnCl 2 is a promising method that has been recently applied for functional brain imaging in non-restrained rodents (Berkowitz et al, 2006;Bissig and Berkowitz, 2009;Yu et al, 2005). MEMRI is -in general -predominantly used in animals due to a number of distinct features of manganese.…”

“…Most importantly, because manganese's transfer is through voltage-gated channels and at relatively low rates, the accumulation of the substance inside cells as well as in the surrounding extracellular space is proportional to neural activity (Lin and Koretsky, 1997). The combination of these properties with the fact that the divalent Mn 2+ is paramagnetic and reduces the longitudinal relaxation times (T1) of water protons, thereby resulting in enhanced signal intensity on T1-weighted MR images at the locations where Mn 2+ accumulates (Silva et al, 2004), instigated the developing of many MEMRI applications including visualization of functional maps in freely moving animals (Bissig and Berkowitz, 2009;Yu et al, 2005).…”

“…Manganese doses that have been used for brain activity mapping in awake and behaving rodents (e.g. 66 mg/kg for rats and 64 mg/kg for mice) have been considered as non-toxic because they produced no or minimal side effects detected in rodent general behavior (Berkowitz et al, 2006;Berkowitz et al, 2007;Bissig and Berkowitz, 2009;Yu et al, 2005). Berkowitz and colleagues reported no retinal toxic effects after a single systemic injection of MnCl 2 assessed by number of histological, physiological and electrophysiological indices (Berkowitz et al, 2006;Berkowitz et al, 2007).…”