Chronic Supplementation Of Magnesium Restores Visual Plasticity In Adult Mice

Ocular dominance (OD) plasticity, which refers to an OD shift of neurons in the visual cortex following the alteration of visual experience, reflects the plastic ability of the cerebral cortex. This capability peaks during the critical period of early postnatal development, and diminished gradually thereafter. The attenuation of plasticity consolidates neural circuitry, but limits the potential treatment for numerous brain diseases. Therefore, how to elevate plasticity in adulthood is a hotspot issue of recent scientific research. Magnesium is an abundant element of organisms, involves in various physiological process, such as enzyme reaction, energy metabolism. Chronic elevation of brain magnesium concentration effectively increases the synaptic plasticity in the hippocampus and prefrontal cortex of rodents(Slutsky et al.2010, Abumaria et al.2011). However, the effect of magnesium supplementation on visual plasticity in adult mice is still unclear. Recently, the balance between cortical excitation and inhibition (E/I balance) is proposed to be the functional barrier of plasticity. Resetting the balance with decreased cortical inhibition is believed to be the prerequisite in rescuing visual plasticity in adulthood. Here, the contribution of cortical inhibition in restoring plasticity will be further investigated. Furthermore, the developmental switch of NMDA receptor composition from NR2B to NR2A dominance is concurrent with the termination of the critical period, while the contribution of those two subunits in elevation of plasticity is unknown.In this study, we used single-unit recording and chronic recording of visually evoked potentials to examine the effect of magnesium supplementation on OD plasticity. Meanwhile, we performed whole-cell recording in layer 2/3 pyramidal neurons in the visual cortex, and investigated the effect of development and magnesium supplementation on basal synaptic transmission. The contribution of NR2A and NR2B in the rescued plasticity was differentiated by local infusion of NR2A and NR2B antagonists using osmotic minipump. We also explored the underlying mechanism of restoration of visual plasticity in adults with Western blot, immunohistochemistry, Golgi staining and in vivo electrophysiology. Moreover, using behavioral test and in vivo electrophysiology, we examined visual functions in adult amblyopic mice. We found:a) Short-period (4 days) of monocular deprivation did not affect the contralateral eye dominance in the visual cortex of normal adult mice. However, following one month of magnesium supplementation, the OD was dramatically shifted to the nondeprived ipsilateral eye. The chronic recording of visually evoked potentials further suggested that this shift was mediated by a decrease in cortical response to the deprived eye, implying a juvenile-like OD plasticity is rescued in adult mice following magnesium supplementation.b)By analyzing the properties of miniature excitatory and inhibitory postsynaptic currents, we found that relative to juvenile mice within the critical period, the excitatory synaptic transmission was lower and the inhibitory synaptic transmission was higher in adult mice, suggesting a gradually reduction of cortical excitation and inhibition ratio during development. However, the developmental alternation in synaptic properties could be reversed by magnesium supplementation. Meanwhile, the density of dendritic spine on layer 2/3 pyramidal neurons in the visual cortex increased following magnesium supplementation, indicating the additional formation of functional excitatory synapses, which may provide structural basis for the rescued plasticity.c) Magnesium supplementation elevated the protein expression of NR2A and NR2B subunit of N-methyl-D-aspartic acid receptors (NMDARs) in the visual cortex of adult mice. The rescued OD plasticity could be fully blocked by local infusion of NR2B antagonist Ro 25-6981 within the period of monocular deprivation, but not by the infusion of NR2A antagonist PPPA or TCN 201. To further exclude the possibility that NR2B subunits merely participated in the expression but not the restoration of plasticity, we simultaneously administered magnesium and the NR2B antagonist Ro 25-6981 for 2 weeks, followed by the removal of both prior to the onset of MD. Compared to mice with only magnesium treatment, the OD shift was significantly reduced by the concurrent infusion of Ro 25-6981. Therefore, those results suggest that NR2B is important to the restoration of visual plasticity in adulthood.d)The protein expression of several inhibition-related factors, such as glutamate decarboxylase 65/67, brain-derived neurotrophic factor and perineuronal nets was unaltered following magnesium treatment, indicate that cortical inhibition may not represent the predominant target for magnesium treatment. However, the plasticity could be blocked by elevation of cortical inhibition with intracerebroventricular injection of GABAA receptor agonist diazepam.e)The plasticity rescued by decreasing cortical inhibition with GABA synthesis inhibitor MPA was barely affected by Ro 25-6981, indicated that magnesium supplementation rescues plasticity with a pathway independent on the reduced cortical inhibition.f) The combination of chronic magnesium supplementation and 2 weeks of reverse suturing could induced juvenile-like cortical responses, which further promoted the recovery of impaired visual acuity and OD in the visual cortex of amblyopic mice.Collectively, we demonstrated that magnesium treatment is effective in elevating NR2B-dependent OD plasticity in the primary visual cortex of adult mice, without dramatic reduction in cortical inhibition in the visual cortex, indicating a decrease in cortical inhibition is not the prerequisite to rescuing plasticity. Moreover, we further replenish the functional mechanism of plasticity, and prove the importance of E/I balance in maintaining the stability of neural circuits in a new perspective. We also found that the plasticity restored via the decrease in cortical inhibition is independent on the activation of NR2B, indicating two distinct pathways of the rescued plasticity: (1) the plasticity restored by decreased inhibition is independent on NR2B, (2) the pathway required the participation of NR2B may be induced by the elevation of glutamatergic excitation. Moreover, magnesium treatment facilitates the rejuvenation of neuronal and synaptic responses in the visual cortex of adult mice, which is benefit to the recovery of visual functions in amblyopic mice. Therefore, our results not only facilitates the understanding of the maturation of visual system, but also provide a new avenue to the clinical therapy of several brain diseases, such as amblyopia, stroke and aphasia.