Vitamin C

Ascorbate (ascorbic acid, AA), a ubiquitous water-soluble antioxidant and a cofactor for several enzymes, can inhibit the generation of ROS, directly scavenge ROS/RNS, and repair other oxidized scavengers [129]. ROS generation is limited by ascorbate through the inhibition of NOX and nNOS. It also helps in the regeneration of alpha-tocopherol from alpha-tocopheroxyl radical and repair of glutathione. The highest concentration of ascorbate is expressed in the brain and is involved in CNS homeostasis. Endogenous ascorbate exists in two biological forms, the deprotonated ascorbate anion and dehydroascorbate (DHA), which is the product of the two-electron reversible oxidation of ascorbate [129]. The mechanism of ascorbate uptake involves absorption of dietary ascorbate in the intestine by sodium-dependent transporter-1 (SVCT-1) and dissolution in the blood. Ascorbate enters the CNS by slow transport from the plasma to the cerebrospinal fluid across the choroid plexus epithelium. Ascorbate can easily enter the brain through the glucose transporter (GLUT1) when a considerable amount of DHA is present in the blood. Ascorbate or DHA in the cerebrospinal fluid enters the neuron via sodium-dependent transporter-2 (SVCT-2) or GLUT1 transporters [130,131]. Once they enter the neuron, DHA can be reduced to ascorbate or released by GLUT1. Ascorbate produces ascorbate free radicals as one electron donor, which is reduced back to ascorbate within the cells by NADH- and NADPH-dependent reductase. Glial cells obtain ascorbate by reduction of DHA through GLUT1, and ascorbate uptake does not involve SVCT-2, which is different from neurons [131]. DHA goes through GLUT1 slowly to enter the choroid plexus and astrocytes [132–134]. The rapid entry of ascorbate is driven by SVCT-2 through the blood-brain-barrier (BBB) to neurons, and ascorbate modulates SVCT-2 translocation to the plasma membrane, ensuring optimal ascorbate uptake in the neurons [135,136] (Figure 3). The oxidizing and free radical scavenging activity of ascorbate

inside the cell is not limited to the aqueous phase, but also includes protection of membranes and other hydrophobic compartments through interaction with vitamin E [137]. Aging decreases the extracellular concentration and the uptake of ascorbate in the brain, which is consistent with an increase in oxidative stress. The neurobiological role of ascorbate in the brain is seen in neuromodulation and neuroprotection [129]. In the role of a neuromodulator, ascorbate release is involved in the uptake or clearance of glutamate from the synapse after its release from the axon terminals. Specifically, L-glutamate promotes ascorbate release as a consequence of removing glutamate from the synapse [138]. Although ascorbate does not act as a classical neurotransmitter, extracellular ascorbate may influence neurotransmission. In particular, ascorbate directly modulates neural excitability through inhibition of T-type Ca2<sup>+</sup> channels [139], participates in the reduction of extracellular oxidants, which effects the redox status of catecholamines [140], and influences the release of biogenic amines in striatum [141] and pituitary neuropeptides [142]. Another neuroprotective role of ascorbate is the attenuation of neurotoxicity, which results from the scavenging activity [143]. Ascorbate inhibits the oxidative stress triggered by various neurotoxins and protects against ethanol-induced apoptotic neurodegeneration in prenatal rat hippocampal neurons [144]. Oxidative stress in stroke, hypoxia, ischemia, and seizure activity leads to massive glutamate release and subsequent excitotoxicity, a result of over-activation of glutamate receptors [145]. Therefore, ascorbate can protect against glutamate-induced excitotoxicity and neurodegeneration.

Many researchers have reported that neurodegeneration can be reversed or lessened by ascorbate treatment [146–148]. From the viewpoint of the effect of aging, the extracellular concentration and uptake of ascorbate is decreased in the brain, which is consistent with an increase in oxidative stress [149]. Moreover, excess ascorbate intake or deficiency may influence brain aging [150]. Thus, careful maintenance of ascorbate levels in the brain may be important during the life span. There have been extensive reviews on the role of ascorbate in the brain with a focus on neurodegenerative disease [130,131]. Even though ascorbate has a complicated interaction with the neurotransmitter system, ascorbate is considered relevant for use as an antioxidant therapy because neurons are sensitive to ascorbate deficiency and excess oxidant stress. In our previous work [151], we studied the protective effects of AA and DHA on KA-induced oxidative stress using organotypic hippocampal slice cultures. After 12 h of KA treatment, significant delayed neuronal death was detected in the CA3, but not in the CA1 region. Pretreatment with intermediate doses of AA and DHA significantly prevented cell death and reduced ROS levels, as well as mitochondrial dysfunction in the CA3 region. However, pretreatment with high doses of AA or DHA was not effective. Attempting to elevate the brain ascorbate by the systemic administration of high doses of ascorbate is very difficult. The level of AA in the extracellular fluid of the striatum was decreased in a transgenic mouse model of HD; hence, restoring striatal extracellular AA levels with high doses of ascorbate improved behavior [152,153].
