BackgroundIncreased cortical excitability has been hypothesized to play a critical role in various neurological disorders, such as restless legs syndrome, epilepsy and migraine. Particularly for migraine, local hyperexcitability has been reported. Levels of regional excitatory and inhibitory neurotransmitters are related to cortical excitability and hence may play a role in the origin of the disease. Consequently, a mismatch of the excitatory-inhibitory neurotransmitter network might contribute to local hyperexcitability and the onset of migraine attacks. In this study we sought to assess local levels of glutamate / glutamine (GLX) and gamma-aminobutyric acid (GABA) in the occipital cortex and right thalamus of migraineurs and healthy subjects.MethodsWe measured interictally local biochemical concentrations in the occipital lobe and the right thalamus in patients with migraine (without aura) and healthy controls (HCs) using proton magnetic resonance spectroscopy at 3 T. GLX levels were acquired using PRESS and GABA levels using the GABA-sensitive editing sequence MEGA-PRESS. Regional GLX and GABA levels were compared between groups.ResultsStatistical analyses revealed significantly increased GLX levels in both the primary occipital cortex and thalamus. However, we found no group differences in GABA levels for these two regions. Correlation analyses within the migraine group revealed no significant correlations between pain intensity and levels of GLX or GABA in either of the two brain regions.ConclusionsFurther research is needed to investigate the role of GABA/GLX ratios in greater depth and to measure changes in neurotransmitter levels over time, i.e. during migraine attacks and interictally.

The relationship between glutamate and γ-aminobutyric acid (GABA) levels in the living human brain and pain sensitivity is unknown. Combined glutamine/glutamate (Glx), as well as GABA levels can be measured in vivo with single-voxel proton magnetic resonance spectroscopy. In this cross-sectional study, we aimed at determining whether Glx and/or GABA levels in pain-related brain regions are associated with individual differences in pain sensitivity. Experimental heat, cold, and mechanical pain thresholds were obtained from 39 healthy, drug-free individuals (25 men) according to the quantitative sensory testing protocol and summarized into 1 composite measure of pain sensitivity. The Glx levels were measured using point-resolved spectroscopy at 3 T, within a network of pain-associated brain regions comprising the insula, the anterior cingulate cortex, the mid-cingulate cortex, the dorsolateral prefrontal cortex, and the thalamus. GABA levels were measured using GABA-edited spectroscopy (Mescher-Garwood point-resolved spectroscopy) within the insula, the anterior cingulate cortex, and the mid-cingulate cortex. Glx and/or GABA levels correlated positively across all brain regions. Gender, weekly alcohol consumption, and depressive symptoms were significantly associated with Glx and/or GABA levels. A linear regression analysis including all these factors indicated that Glx levels pooled across pain-related brain regions were positively associated with pain sensitivity, whereas no appreciable relationship with GABA was found. In sum, we show that the levels of the excitatory neurotransmitter glutamate and its precursor glutamine across pain-related brain regions are positively correlated with individual pain sensitivity. Future studies will have to determine whether our findings also apply to clinical populations.

The importance of terrestrial arthropods has been documented in temperate stream ecosystems, but little is known about the magnitude of these inputs in tropical streams. Terrestrial arthropods falling from the canopy of tropical forests may be an important subsidy to tropical stream food webs and could also represent an important flux of nitrogen (N) and phosphorus (P) in nutrient-poor headwater streams. We quantified input rates of terrestrial insects in eight streams draining lowland tropical wet forest in Costa Rica. In two focal headwater streams, we also measured capture efficiency by the fish assemblage and quantified terrestrially derived N-and P-excretion relative to stream nutrient uptake rates. Average input rates of terrestrial insects ranged from 5 to 41 mg dry mass/m 2 /d, exceeding previous measurements of aquatic invertebrate secondary production in these study streams, and were relatively consistent year-round, in contrast to values reported in temperate streams. Terrestrial insects accounted for half of the diet of the dominant fish species, Priapicthys annectens. Although terrestrially derived fish excretion was found to be a small flux relative to measured nutrient uptake rates in the focal streams, the efficient capture and processing of terrestrial arthropods by fish made these nutrients available to the local stream ecosystem. This aquatic-terrestrial linkage is likely being decoupled by deforestation in many tropical regions, with largely unknown but potentially important ecological consequences.Abstract in Spanish is available in the online version of this article.

Transcutaneous spinal direct current stimulation (tsDCS) is a noninvasive method that can modulate spinal reflexes, sensory afferent conduction, and even pain perception. Although neurophysiological evidence suggests that tsDCS alters somatosensory and nociceptive afferent conduction to the cortex, its supraspinal effects have not yet been investigated by using functional imaging to investigate tsDCS-induced alterations in intrinsic functional connectivity (FC). Therefore, we hypothesize that tsDCS-induced changes in neurophysiological measures might also be reflected in spontaneous brain activity. We investigated tsDCS-induced changes in somatosensory cortical connectivity by using seed-to-voxel-based analyses from the bilateral primary somatosensory cortex (S1) and the thalamus in a double-blind, crossover study design. Resting state FC was measured by using blood oxygenation level-dependent, functional magnetic resonance imaging (3T Philips) before and after anodal, cathodal, and sham tsDCS (20 min, 2.5 mA, active electrode centered over T11 spinous process, reference electrode over left shoulder blade) in a double-blind, crossover study of 20 healthy men (24 ± 0.7 years). As compared with sham, anodal tsDCS resulted in a decreased connectivity between the S1 and the ipsilateral posterior insula for both left and right hemispheres. Anodal tsDCS also resulted in decreased thalamic connectivity with the anterior cingulate cortex, and increased connectivity between S1 and the thalamus. Cathodal tsDCS showed increased FC between the right thalamus and both left and right posterior insulae, and decreased connectivity between the S1 seeds and the occipital cortex. Our results provide evidence of supraspinal effects of tsDCS and suggest that tsDCS may provide a noninvasive intervention that is able to target cortical sensory networks.