Synchronization of neural activity is a promising mechanism of memory information processing in networks of columns

Comparison of spontaneous brain activity revealed by regional homogeneity in AQP4-IgG neuromyelitis optica-optic neuritis versus MOG-IgG optic neuritis patients: a resting-state functional MRI study

OBJECTIVE

Many previous studies have demonstrated that neuromyelitis optica (NMO) patients have abnormalities of brain anatomy and function. However, differences in spontaneous brain activity between myelin oligodendrocyte glycoprotein (MOG)-IgG ON and aquaporin 4(AQP4)-neuromyelitis optica-optic neuritis (ON) remain unknown. In the current study, we investigated the brain neural homogeneity in MOG-IgG ON versus AQP4-IgG NMO-ON subjects by regional homogeneity (ReHo) method using magnetic resonance imaging (MRI).

PATIENTS AND METHODS

A total of 32 NMO-ON and ON subjects (21 with AQP4-IgG+NMO-ON and 11 with MOG-IgG+ON) and 34 healthy controls (HCs) closely matched for age were recruited, and scans were performed for all subjects. A one-way analysis of variance (ANOVA) was performed to determine the regions in which the ReHo was different across the three groups. NMO-ON and ON subjects were distinguished from HCs by a receiver operating characteristic (ROC) curve. The relationship between the mean ReHo in many brain regions and clinical features in NMO subjects was calculated by Pearson correlation analysis.

RESULTS

Compared with HCs, MOG-IgG+ON subjects had significantly decreased ReHo values in the posterior lobe of the left cerebellum and increased ReHo values in the left inferior frontal gyrus, right prefrontal gyrus, and left precentral/postcentral gyrus. AQP4-IgG+NMO-ON subjects showed higher ReHo values in the left inferior frontal gyrus and right middle temporal/occipital gyrus. Compared with MOG-IgG+ON subjects, AQP4-IgG+NMO-ON subjects had lower ReHo values in the posterior lobe of the right cerebellum. AQP4-Ig+NMO-ON subjects showed higher ReHo values in the left precentral/postcentral gyrus and right superior temporal gyrus.

CONCLUSION

AQP4-IgG+NMO-ON and MOG-IgG+ON subjects showed abnormal synchronized neuronal activity in many brain regions, which is consistent with deficits in visual, motor, and cognitive function. Furthermore, different patterns of synchronized neuronal activity occurred in the AQP4-IgG+NMO-ON and MOG-IgG+ON.

Neurotrophic and neuroprotective actions of an enhancer of ganglioside biosynthesis

To address the role of brain gangliosides in synaptic plasticity, the synthetic ceramide analog, 1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) was used to manipulate the biosynthesis of gangliosides in cultured cortical neurons. Spontaneous synchronized oscillatory activity of intracellular Ca(2+) between the neurons, which represents synapse formation, was suppressed by the depletion of endogenous gangliosides by d-threo-PDMP, an inhibitor of glucosylceramide synthase. On the other hand, the enantiomer of inhibitor, l-threo-PDMP, could elevate cellular levels of gangliosides by upregulating several glycosyltransferases responsible for ganglioside biosynthesis. This review presents our findings on the neurotrophic actions of l-threo-PDMP in vitro and in vivo. We found that l-PDMP could upregulate neurite outgrowth, and functional synapse formation through activating GM3, GD3, and GQ1b synthases. Simultaneously, the activity of p42 mitogen-activated protein kinase was also facilitated by l-PDMP. To evaluate the efficacy of this drug on long term memory, rats were trained for 2 weeks using an 8-arm radial maze task, and then forebrain ischemia was induced by four-vessel occlusion. Repeated treatment of l-PDMP starting 24h after the ischemia, improved the deficit of the well-learned spatial memory and prevented the ischemia-induced apoptosis in hippocampus, demonstrating the potential therapeutic use of the ceramide analog for treatment of neurodegenerative disorders.

Up-regulation of ganglioside biosynthesis, functional synapse formation, and memory retention by a synthetic ceramide analog (L-PDMP)

To address the role of brain gangliosides in synaptic activity, the ceramide analogs, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (D-PDMP) and its enantiomer, L-PDMP, were used to inhibit and stimulate ganglioside biosynthesis in cultured cortical neurons. Prolonged treatment with both PDMP isomers exhibited opposite effects on functional synapse formation measured by spontaneous synchronized oscillatory activity of intracellular Ca2+ between the neurons: suppression by D-PDMP and facilitation by L-PDMP. Up-regulation of synaptic activity by L-PDMP could be correlated with the slow but robust stimulation of ganglioside biosynthesis through activating GM3, GD3 and GQ1b synthases. In a similar time course, the activity of p42 mitogen-activated protein kinase was also enhanced by L-PDMP. To evaluate the efficacy of this drug in long-term memory, rats were trained for 2 weeks using an 8-arm radial maze task, and then forebrain ischemia was induced by 4-vessel occlusion. Treatment with L-PDMP starting 24 hours after the transient ischemia ameliorated the deficit of a well-learned spatial memory, demonstrating the potential therapeutic intervention of the ceramide analog for neurodegenerative disorders.

Bulbocortical interplay in olfactory information processing via synchronous oscillations

Emergence of synchronous oscillatory activity is an inherent feature of the olfactory systems of insects, mollusks and mammals. A class of simple computational models of the mammalian olfactory system consisting of olfactory bulb and olfactory cortex is constructed to explore possible roles of the related neural circuitry in olfactory information processing via synchronous oscillations. In the models, the bulbar neural circuitry is represented by a chain of oscillators and that of cortex is analogous to an associative memory network with horizontal synaptic connections. The models incorporate the backprojection from cortical units to the bulbar oscillators in particular ways. They exhibit rapid and robust synchronous oscillations in the presence of odorant stimuli, while they show either nonoscillatory states or propagating waves in the absence of stimuli, depending on the values of model parameters. In both models, the backprojection is shown to enhance the establishment of large-scale synchrony. The results suggest that the modulation of neural activity through centrifugal inputs may play an important role at the early stage of cortical information processing.

Memory Recall by Quasi-Fixed-Point Attractors in Oscillator Neural Networks

It is shown that approximate fixed-point attractors rather than synchronized oscillations can be employed by a wide class of neural networks of oscillators to achieve an associative memory recall. This computational ability of oscillator neural networks is ensured by the fact that reduced dynamic equations for phase variables in general involve two terms that can be respectively responsible for the emergence of synchronization and cessation of oscillations. Thus the cessation occurs in memory retrieval if the corresponding term dominates in the dynamic equations. A bottomless feature of the energy function for such a system makes the retrieval states quasi-fixed points, which admit continual rotating motion to a small portion of oscillators, when an extensive number of memory patterns are embedded. An approximate theory based on the self-consistent signal-to-noise analysis enables one to study the equilibrium properties of the neural network of phase variables with the quasi-fixed-point attractors. As far as the memory retrieval by the quasi-fixed points is concerned, the equilibrium properties including the storage capacity of oscillator neural networks are proved to be similar to those of the Hopfield type neural networks.