Der Yow Chen

Deciphering laminar-specific neural inputs with line-scanning fMRI

Using a line-scanning method during functional magnetic resonance imaging (fMRI), we obtained high temporal (50-ms) and spatial (50-μm) resolution information along the cortical thickness and showed that the laminar position of fMRI onset coincides with distinct neural inputs in rat somatosensory and motor cortices. This laminar-specific fMRI onset allowed us to identify the neural inputs underlying ipsilateral fMRI activation in the barrel cortex due to peripheral denervation-induced plasticity.

Thalamocortical Inputs Show Post-Critical Period Plasticity

Experience-dependent plasticity in the adult brain has clinical potential for functional rehabilitation following central and peripheral nerve injuries. Here, plasticity induced by unilateral infraorbital (IO) nerve resection in 4-week-old rats was mapped using MRI and synaptic mechanisms were elucidated by slice electrophysiology. Functional MRI demonstrates a cortical potentiation compared to thalamus 2 weeks after IO nerve resection. Tracing thalamocortical (TC) projections with manganese-enhanced MRI revealed circuit changes in the spared layer 4 (L4) barrel cortex. Brain slice electrophysiology revealed TC input strengthening onto L4 stellate cells due to an increase in postsynaptic strength and the number of functional synapses. This work shows that the TC input is a site for robust plasticity after the end of the previously defined critical period for this input. Thus, TC inputs may represent a major site for adult plasticity in contrast to the consensus that adult plasticity mainly occurs at cortico-cortical connections.

The Use of Silica Coated MnO Nanoparticles to Control MRI Relaxivity in Response to Specific Physiological Changes

MnO nanoparticles have been tested to engineer a delayed increase in MRI T(1) relaxivity caused by cellular uptake via endocytosis into acidic compartments. Various coatings on core-shell structured MnO nanoparticles were tested for those that had the lowest T(1) relaxivity at pH 7.4, a pH where MnO does not dissolve into Mn(2+) ions. The rate of dissolution and release of Mn(2+) of the different coated MnO particles as well as changes in T(1) relaxivity were measured at pH 5, a pH routinely obtained in the endosomal-lysosomal pathway. Of a number of coatings, silica coated MnO (MnO@SiO(2)) had the lowest relaxivity at pH 7.4 (0.29 mm(-1) sec(-1)). About one third of the MnO dissolved within 20 min and the T(1) relaxivity increased to that of free Mn(2+) (6.10 mm(-1) sec(-1)) after three days at pH 5. MRI of MnO@SiO(2) particles injected into the rat brain showed time-dependent signal changes consistent with the in vitro rates. Thalamocortical tract-tracing could be observed due to the released Mn(2+). Intravenous infusion of MnO@SiO(2) particles showed little enhancement in any tissue except gallbladder. The gallbladder enhancement was interpreted to be due to endocytosis by liver cells and excretion of Mn(2+) ions into the gallbladder. The MnO@SiO(2) core-shell nanoparticles show the best potential for delaying the release of MRI contrast until endocytosis into low pH compartments activate MRI contrast. The delayed enhancement may have benefits for targeting MRI contrast to specific cells and surface receptors that are known to be recycled by endocytosis.

3D mapping of somatotopic reorganization with small animal functional MRI

There are few in vivo noninvasive methods to study neuroplasticity in animal brains. Functional MRI (fMRI) has been developed for animal brain mapping, but few fMRI studies have analyzed functional alteration due to plasticity in animal models. One major limitation is that fMRI maps are characterized by statistical parametric mapping making the apparent boundary dependent on the statistical threshold used. Here, we developed a method to characterize the location of center-of-mass in fMRI maps that is shown not to be sensitive to statistical threshold. Utilizing centers-of-mass as anchor points to fit the spatial distribution of the BOLD response enabled quantitative group analysis of altered boundaries of functional somatosensory maps. This approach was used to study cortical reorganization in the rat primary somatosensory cortex (S1) after sensory deprivation to the barrel cortex by follicle ablation (F.A.). FMRI demonstrated an enlarged nose S1 representation in the 3D somatotopic functional maps. This result clearly demonstrates that fMRI enables the spatial mapping of functional changes that can characterize multiple regions of S1 cortex and still be sensitive to changes due to plasticity.

Infusion of lidocaine into the dorsal hippocampus before or after the shock training phase impaired conditioned freezing in a two-phase training task of contextual fear conditioning.

Learning in a contextual fear conditioning task involves forming a context representation and associating it with a shock. The dorsal hippocampus (DH) is implicated in representing the context, but whether it also has a role in associating the context and shock is unclear. To address this issue, male Wistar rats were trained on the task by a two-phase training paradigm, in which rats learned the context representation on day 1 and then reactivated it to associate with the shock on day 2; conditioned freezing was tested on day 3. Lidocaine was infused into the DH at various times in each of the two training sessions. Results showed that intra-DH infusion of lidocaine shortly before or after the context training session on day 1 impaired conditioned freezing, attesting to the DH involvement in context representation. Intra-DH infusion of lidocaine shortly before or after the shock training session on day 2 also impaired conditioned freezing. This deficit was reproduced by infusing lidocaine or APV (alpha-amino-5-phosphonovaleric acid) into the DH after activation of the context memory but before shock administration. The deficit was not due to drug-induced state-dependency, decreased shock sensitivity or reconsolidation failure of the contextual memory. These results suggest that in contextual fear conditioning integrity of the DH is required for memory processing of not only context representation but also context-shock association.

Functional tracing of medial nociceptive pathways using activity-dependent manganese-enhanced MRI

&NA; Manganese ion (Mn2+) was used as a paramagnetic contrast agent in T1‐weighted magnetic resonance imaging (MRI) images. They enter neural cells though voltage‐gated calcium channels and are activity‐dependently transported along axons and across synapses. The aim of the present study was to investigate the nociceptive medial thalamus projection in rats by activity‐dependent manganese‐enhanced magnetic resonance imaging (MEMRI). Rats under urethane and &agr;‐chloralose anesthesia were microinjected with manganese chloride (MnCl2, 120 mmol/L, iontophoretically with a 5‐&mgr;A current for 15 min) into the right medial thalamus. Innocuous (at a 50‐&mgr;A intensity for 0.2 ms) or noxious (at a 5‐mA intensity for 2 ms) electrical stimuli were applied through a pair of needles in the left forepaw pads once every 6 s for 5 h. Enhanced transport of Mn2+ were found in the anterior cingulate cortex, midcingulate cortex, retrosplenial cortex, ventral medial caudate‐putamen, nucleus accumbens, and amygdala in the noxious‐stimulated group. Enhancements in the anterior cingulate cortex, midcingulate cortex, ventral medial caudate‐putamen, nucleus accumbens, and amygdala, but not the retrosplenial cortex, were attenuated by an intraperitoneal injection of morphine (5 mg/kg and 1 mg/kg/h, intraperitoneal). These results indicate that a combination of MEMRI with activity‐induced manganese‐dependent contrast is useful for delineating functional connections in the pain pathway. Noxious stimulation induced enhancement of manganese ion transportation from medial thalamus to cingulate cortex and medial striatum, but not motor cortex. A combination of manganese‐enhanced magnetic resonance imaging with activity‐dependent contrast is useful for delineating functional connections of the medial pain pathway.

Wireless Amplified Nuclear MR Detector (WAND) for High-Spatial-Resolution MR Imaging of Internal Organs: Preclinical Demonstration in a Rodent Model

PURPOSE To assess the feasibility of imaging deep-lying internal organs at high spatial resolution by imaging kidney glomeruli in a rodent model with use of a newly developed, wireless amplified nuclear magnetic resonance (MR) detector. MATERIALS AND METHODS This study was approved by the Animal Care and Use Committee at the National Institutes of Health/National Institute of Neurologic Disorder and Stroke. As a preclinical demonstration of this new detection technology, five different millimeter-scale wireless amplified nuclear MR detectors configured as double frequency resonators were chronically implanted on the medial surface of the kidney in five Sprague-Dawley rats for MR imaging at 11.7 T. Among these rats, two were administered gadopentetate dimeglumine to visualize renal tubules on T1-weighted gradient-refocused echo (GRE) images, two were administered cationized ferritin to visualize glomeruli on T2*-weighted GRE images, and the remaining rat was administered both gadopentetate dimeglumine and cationized ferritin to visualize the interleaved pattern of renal tubules and glomeruli. The image intensity in each pixel was compared with the local tissue signal intensity average to identify regions of hyper- or hypointensity. RESULTS T1-weighted images with 70-μm in-plane resolution and 200-μm section thickness were obtained within 3.2 minutes to image renal tubules, and T2*-weighted images of the same resolution were obtained within 5.8 minutes to image the glomeruli. Hyperintensity from gadopentetate dimeglumine enabled visualization of renal tubules, and hypointensity from cationic ferritin enabled visualization of the glomeruli. CONCLUSION High-spatial-resolution images have been obtained to observe kidney microstructures in vivo with a wireless amplified nuclear MR detector.

Post-training infusion of glutamate into the bed nucleus of the stria terminalis enhanced inhibitory avoidance memory: An effect involving norepinephrine

This study examined an interaction between glutamate and norepinephrine in the bed nucleus of the stria terminalis (BNST) in modulating affective memory formation. Male Wistar rats with indwelling cannulae in the BNST were trained on a one-trial step-through inhibitory avoidance task and received pre- or post-training intra-BNST infusion of glutamate, norepinephrine or their antagonists. Results of the 1-day test indicated that post-training intra-BNST infusion of DL-2-amino-5-phosphonovaleric acid (APV) impaired retention in a dose- and time-dependent manner, while infusion of glutamate had an opposite effect. Co-infusion of 0.2microg glutamate and 0.02microg norepinephrine resulted in marked retention enhancement by summating non-apparent effects of the two drugs given at a sub-enhancing dose. The amnesic effect of 5.0microg APV was ameliorated by 0.02microg norepinephrine, while the memory enhancing effect of 1.0microg glutamate was attenuated by 5.0microg propranolol. These findings suggest that training on an inhibitory avoidance task may alter glutamate neurotransmission, which by activating NMDA receptors releases norepinephrine to modulate memory formation via beta adrenoceptors in the BNST.

The capacity constraint in the prefrontal and parietal regions for coordinating dual arithmetic tasks

Using a dual-serial-arithmetic paradigm, we examined whether a capacity limitation constrains the neural activation that underlies dual-task performance. Six conditions were run in the experiment (the baseline, single-addition, single-subtraction, dual-addition, dual-subtraction, and the dual-operation condition). In the baseline condition, participants were asked to remember the initial pair of numbers and ignore subsequent stimuli. In the single-addition and single-subtraction conditions, participants had to calculate a running total over a series of stimuli. In the dual-addition and dual-subtraction conditions, they had to do two arithmetic tasks involving the same operand (e.g., + 2 and + 7, - 3 and - 5). Participants performed one addition and one subtraction task (e.g., + 2 and - 7, - 3 and + 5) in the dual-operation condition. The functional magnetic resonance imaging results showed strict left prefrontal and parietal regions in the single-addition condition and bilateral activation in the single-subtraction condition. Greater activation in the prefrontal and parietal regions was observed in both the dual-operation condition and the dual-addition condition in comparison to the single-addition condition. No greater activation was observed in either the dual-operation condition or dual-subtraction condition in comparison to the single-subtraction condition. These results suggest a constraint imposed by a limit in capacity for the neural activity subserving dual-task performance when one of the tasks places high resource demands on the executive network.

Development of NTU standard Chinese brain template: Morphologic and functional comparison with MNI template using magnetic resonance imaging

Background and Purpose: The brain structure mismatch between western and eastern people may lead to an inappropriate interpretation of neurocognitive studies. To minimize this interracial misinterpretation, we developed the National Taiwan University Chinese Brain Template (NTU-CBT). Methods: 102 (M/F = 3D55/47) healthy Chinese subjects were recruited and received 3T MR brain scans. The template development processes were based on the construction process of Montreal Neurological Institute (MNI) template. Further pilot functional magnetic resonance imaging (fMRI) studies with blocked design visual stimulation and foot tapping task were performed on 3 volunteers and applied to both MNI template and NTU-CBT for analyses. Results: 7 subjects were excluded due to motion artifacts. The average brain size of 95 (M/F = 3D50/45) subjects was 16.0 cm in length, 13.9 cm in width and 11.3 cm in height, which was 88.9%, 97.9% and 84.3% of the size of MNI template, respectively. Maximum dimensional differences came from the height of superior brain and the length of posterior brain. The average activation voxel volume of the fMRI studies applying to NTU-CBT was 80.7% of that to MNI template in visual stimulation, and 72.8% in foot tapping task. Noticeable mismatches were noted between interpolating original data to NTU-CBT and MNI template. Conclusions: Morphologic differences between MNI template and NTU-CBT do lead to spatial mismatch in functional studies, especially at cortical regions of superior and posterior brain. With the development of NTU-CBT, we look forward to more accurate interpretation in neurocognitive studies for Chinese subjects.