Corpus callosum and cerebral laterality in a modular brain model

Sex differences in microstructural alterations in the corpus callosum tracts in drug-naïve children with ADHD

Widespread alterations in the corpus callosum (CC) microstructure and organization have been found in children with attention-deficit/hyperactivity disorder (ADHD); however, few studies have investigated the diffusion characteristics and volume of transcallosal fiber tracts defined by specific cortical projections in ADHD, which is important for identifying distinct functional interhemispheric connection abnormalities. In the current study, an automated fiber-tract quantification (AFQ) approach based on diffusion tensor imaging identified seven CC tracts according to their cortical projections and estimated diffusion parameters and volume among 76 drug-naïve ADHD patients (53 boys and 23 girls) and 37 typically developing children (TDC) (20 boys and 17 girls) matched for age, IQ, and handedness. We found significantly lower fractional anisotropy (FA) in the occipital and superior parietal tracts and higher mean diffusivity (MD) in the posterior, superior parietal and anterior frontal tracts in children with ADHD compared with TDC. In addition, lower FA and higher radial diffusivity (RD) in the occipital callosal tract were significantly associated with higher hyperactivity and impulsivity performance in ADHD. In addition, sex-by-diagnosis interactions were observed in the occipital, posterior and superior parietal tracts. Girls with ADHD showed decreased FA and volume in the occipital tract, which were significantly associated with increased impulsivity performance and poor response control, and increased MD in the posterior and superior parietal callosal tracts, which were significantly associated with increased inattention performance, whereas boys with ADHD merely showed decreased volume in the frontal tract. Our results elucidated that sex-specific alterations in the CC tracts potentially underlie ADHD symptomatology and further suggested a differential contribution of abnormalities in different CC tracts to impulsivity and inattention among girls with ADHD.

Associations of Unilateral Whisker and Olfactory Signals Induce Synapse Formation and Memory Cell Recruitment in Bilateral Barrel Cortices: Cellular Mechanism for Unilateral Training Toward Bilateral Memory

Somatosensory signals and operative skills learned by unilateral limbs can be retrieved bilaterally. In terms of cellular mechanism underlying this unilateral learning toward bilateral memory, we hypothesized that associative memory cells in bilateral cortices and synapse innervations between them were produced. In the examination of this hypothesis, we have observed that paired unilateral whisker and odor stimulations led to odorant-induced whisker motions in bilateral sides, which were attenuated by inhibiting the activity of barrel cortices. In the mice that showed bilateral cross-modal responses, the neurons in both sides of barrel cortices became to encode this new odor signal alongside the innate whisker signal. Axon projections and synapse formations from the barrel cortex, which was co-activated with the piriform cortex, toward its contralateral barrel cortex (CBC) were upregulated. Glutamatergic synaptic transmission in bilateral barrel cortices was upregulated and GABAergic synaptic transmission was downregulated. The associative activations of the sensory cortices facilitate new axon projection, glutamatergic synapse formation and GABAergic synapse downregulation, which drive the neurons to be recruited as associative memory cells in the bilateral cortices. Our data reveal the productions of associative memory cells and synapse innervations in bilateral sensory cortices for unilateral training toward bilateral memory.

Associations between corpus callosum size and ADHD symptoms in older adults: The PATH through life study

Neuroimaging studies of attention-deficit/hyperactivity disorder (ADHD) have revealed deviations of the corpus callosum in children and adolescents. However, little is known about the link between callosal morphology and symptoms of inattention or hyperactivity in adulthood, especially later in life. Here, we investigated in a large population-based sample of 280 adults (150 males, 130 females) in their late sixties and early seventies whether ADHD symptoms correlate with callosal thickness. In addition, we tested for significant sex interactions, which were followed by correlation analyses stratified by sex. Within males, there were significant negative correlations with respect to inattention and hyperactivity in various callosal regions, including the anterior third, anterior and posterior midbody, isthmus, and splenium. A thinner corpus callosum may be associated with fewer fibers or less myelination of fibers. Thus, the observed negative correlations suggest impaired inter-hemispheric communication channels necessary to sustain motor control and attention, which may contribute to symptoms of hyperactivity, impulsivity and/or inattention. Interestingly, within females, callosal thickness was positively related to hyperactivity in a small area within the rostral body, suggesting a sexually dimorphic neurobiology of ADHD symptoms. Altogether, the present results may reflect a lasting relationship between callosal morphology and ADHD symptoms throughout life.

Physiological functions of the small GTPase Arf6 in the nervous system

The small GTPase ADP-ribosylation factor 6 (Arf6) plays important roles in membrane dynamics-based neuronal cell events such as neurite outgrowth and spine formation. However, physiological functions of Arf6 in the nervous system at whole animal level have not yet been explored. We have recently generated conditional knockout mice lacking Arf6 in neurons or oligodendrocytes of central nervous system (CNS) or both cell lineages, and analyzed them. We found that ablation of Arf6 gene from neurons, but not from oligodendrocytes, caused the defect in axon myelination at the fimbria of hippocampus (Fim) and corpus callosum (CC). We also found that migration of oligodendrocyte precursor cells (OPCs) from the subventricular zone to the Fim and CC in mice lacking Arf6 in neurons was impaired. Finally, it was found that secretion of fibroblast growth factor-2 (FGF-2), a guidance factor for OPC migration, from hippocampi lacking Arf6 was impaired. Collectively, these findings demonstrate that Arf6 in neurons of the CNS plays an important role in OPC migration by regulating secretion of FGF-2 from neurons, thereby contributing to the axon myelination. Here, we discuss our current understanding of physiological functions of Arf6 in the nervous system.

Decreased callosal thickness in attention-deficit/hyperactivity disorder

BACKGROUND

Neuroimaging studies of attention-deficit/hyperactivity disorder (ADHD) have revealed structural abnormalities in the brains of affected individuals. One of the most replicated alterations is a significantly smaller corpus callosum (CC), for which conflicting reports exist with respect to the affected callosal segments.

METHODS

We applied novel surface-based geometrical modeling methods to establish the presence, direction, and exact location of callosal alterations in ADHD at high spatial resolution. For this purpose, we calculated the thickness of the CC at 100 equidistant midsagittal points in an age-matched male sample of 19 individuals with ADHD and 19 typically developing control subjects.

RESULTS

In close agreement with many prior observations, the CC was shown to be significantly thinner in ADHD subjects in anterior and, particularly, posterior callosal sections. Covarying for intelligence did not significantly alter the observed ADHD effects. However, group differences were no longer present in anterior sections when covarying for brain volume and after excluding ADHD subjects comorbid for oppositional defiant disorder.

CONCLUSIONS

Decreased callosal thickness may be associated with fewer fibers or a decrease in the myelination of fibers connecting the parietal and prefrontal cortices. This might affect interhemispheric communication channels that are necessary to sustain attention or motor control, thus contributing to symptoms of hyperactivity and impulsivity, or inattention, observed in ADHD. Future studies are necessary to determine whether callosal abnormalities reflect maturational delays or persist into adulthood.

Rapid-eye-movement sleep involves the memory-conversion circuits in a brain model

People can remember the content of a dream in rapid eye movement (REM) sleep but cannot do so in slow-wave sleep. According to a brain model, memory is stored in encoding synapses as presynaptic axonal 'on-off' patterns and modulating synapses help encoding synapses convert short-term memory into long-term memory. These lead to the hypothesis that REM sleep involves modulating synapses of the memory-conversion circuits including the anterior nuclei and dorsomedial nuclei of the thalamus. Cortical neurons get more rest in slow-wave sleep than in REM sleep. The locus coeruleus, raphe nuclei, and tuberomammillary nuclei get more rest during REM sleep when these nuclei cease to fire. The paralyses of peripheral muscles during REM sleep and cataplexy, and cessation of chorea, athetosis, hemiballismus, and parkinsonism tremor during sleep may result from spinal cord inhibition by the gigantocellular nuclei and raphe nuclei at the reticular formation. Sleep and wake relate to the light-dark cycle on the Earth. Were the light-dark cycle 50 hours a day, the human circadian clock might be around 50 hours. With increasing use of artificial light to keep people awake at night, it may affect the circadian rhythm and firing rate of neurons, the presynaptic axonal 'on-off' patterns as content of consciousness, and the mood.