NeuroImage
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Following injury and inflammation, pain to light stroking (dynamic mechanical allodynia) might develop at the damaged site (primary area) or in adjacent normal tissue (secondary area). Using fMRI we mapped changes in the spinal trigeminal nucleus (spV), and supraspinal brainstem nuclei following heat/capsaicin-induced primary and secondary dynamic mechanical allodynia in the human trigeminal system. The role of these structures in dynamic mechanical allodynia has not been clarified yet in humans. ⋯ The vlPAG showed decreased activity that inversely correlated with pain ratings during primary allodynia, i.e. the more deactivated the vlPAG the higher the pain intensity (p<0.05, Pearson's correlation). Primary and secondary dynamic mechanical allodynia were also characterized by significant differences involving distinct supraspinal structures mainly involved in pain modulation and including the rostroventromedial medulla, pons reticular formation, dorsolateral PAG, all more active during primary versus secondary allodynia, and the medial reticular formation of the caudal medulla that was more active during secondary versus primary allodynia. These results indicate that the pain modulatory system is involved to a different extent during primary versus secondary mechanical allodynia.
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Cold allodynia, meaning that innocuous cold stimuli become painful, is a characteristic, but enigmatic feature of neuropathic pain. Here, we used functional magnetic resonance imaging (fMRI) and investigated brain activations underlying menthol-induced cold allodynia. 12 healthy volunteers were investigated using a block-design fMRI approach. Firstly, brain activity was measured during application of innocuous cold stimuli (at 5 degrees C above cold pain threshold) and noxious cold stimuli (at 5 degrees C below cold pain threshold) to normal skin of the forearm using a peltier- driven thermostimulator. ⋯ However, comparing cold allodynia and equally intense cold pain conditions, we found significantly increased activations in bilateral dorsolateral prefrontal cortices (DLPFC) and the brainstem (ipsilateral parabrachial nucleus) during cold allodynia. Furthermore, in contrast maps cold allodynia contributed significantly more to activations of the bilateral anterior insula, whereas the contribution to activation of the contralateral posterior insula was equal. It is concluded that cold allodynia activates a network similar to that of normal cold pain but additionally recruits bilateral DLPFC and the midbrain, suggesting that these brain areas are involved in central nociceptive sensitisation processes.
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White matter (WM) fractional anisotropy (FA) is thought to be related to WM integrity and decline in FA is often used as an index of decreasing WM health. However, the relationship of FA to other structural indices of cerebral health has not been well studied. We hypothesized that the decline in WM health will be associated with changes in several other indices of cerebral health. ⋯ Intra-hemispheric correlations between FA and other measures of cerebral health had generally greater effect sizes than inter-hemispheric correction, with correlation between left FA and left GM thickness being the most significant (r=0.6, p<0.01). Regional analysis of FA values showed that late-myelinating fiber tracts of the genu of corpus callosum had higher association with other cerebral health indices. These data are consistent with the hypothesis that late-myelinating regions of the brain bear the brunt of age-related degenerative changes.
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High resolution ex vivo diffusion tensor imaging (DTI) studies of neural tissues can improve our understanding of brain structure. In these studies we can modify the tissue relaxation properties of the fixed tissues to better suite the scanner hardware. We investigated the use of Gd-DTPA contrast agent to provide the optimum signal-to-noise (SNR) ratio in 3D DTI scans of formalin fixed nonhuman primate brains at 4.7 T. ⋯ Studies of the temperature dependence of diffusion in these tissues suggested that a 30 degrees increase in sample temperature may yield an improvement of up to 55% in SNR-efficiency for a given diffusion weighting. Our Gd soaking regimen appeared to have no detrimental effect on standard histology of the fixed brain sections. Our methods yield both high SNR and spatial resolution DTI data in fixed primate brains, allowing us to perform high resolution tractography which will facilitate the process of 'validation' of DTI fiber tracts against traditional measures of brain fiber architecture.