Epidemiological studies revealed that vitamin D deficiency w

Epidemiological studies revealed that vitamin D deficiency was associated with a wide range of neuropsychiatric disorders and neurodegenerative diseases, in particular, Alzheimer's and Parkinson's disease (Annweiler et al., 2013; Knekt et al., 2010), dementia and cognitive function impairment (Balion et al., 2012), depression (Anglin et al., 2013), schizophrenia or psychosis (Belvederi Murri et al., 2013; Valipour et al., 2014), and autism (Eyles, 2010; Kočovská et al., 2017; Staal, 2016; Vinkhuyzen et al., 2018). Vitamin D deficiency can also aggravate brain disorders and worsen recovery from brain stressors (Groves et al., 2014). Glutamate and γ-aminobutyric L-Phenylephrine (GABA) are major neurotransmitters of excitation and inhibition in the central nervous system. Impairment of their transport characteristics underlies neuronal dysfunction and pathogenesis of a wide variety of neurological disorders. The normal physiological extracellular concentrations of glutamate and GABA between exocytotic events are kept at low levels through permanent transporter-mediated turnover of the neurotransmitters across the plasma membrane (Borisova, 2018, 2016; Borisova et al., 2016; Borisova and Borysov, 2016). The neurotransmitter transporters terminate synaptic neurotransmission providing neurotransmitter reuptake into the cytosol. Glutamate and GABA transporters belong to different transporter families, i.e., glutamate transporters are in the SLC1 family, whereas GABA transporters − SLC6 family. 1,25(OH)2D3 could protect dopaminergic neurons against glutamate excitotoxicity in rat mesecephalic culture (Ibi et al., 2001), and in turn this fact indicates that it can also be protective against seizures (Harms et al., 2011). Vitamin D deficiency significantly reduced the levels of glutamic acid decarboxylase, the key enzymes in GABAergic interneurons, and glutamate and glutamine in mouse brain tissue (Groves et al., 2013). Puberty rodents subjected to a vitamin D deplete diet revealed 25OHD3 deficiency in 4–6 weeks (Cui et al., 2015). Notably, puberty onset is between the 28th and 45th postnatal days in rodents and between 8 and 15 years in humans; adolescence lasts until ∼ the 56th postnatal day in rats (Kilb, 2012). The aims of this study were to analyse key glutamate and GABA transport characteristics in presynaptic nerve terminals (synaptosomes) isolated from the cortex of puberty rats underwent nutritional vitamin D3 deficiency. The latter was accompanied with a reduction of 25OHD3 in the serum, a biomarker of vitamin D3, and the disturbances of metabolic processes in bone tissue that correlated with osteoporosis manifestation (Labudzynskyi et al., 2015). Here, we analysed the effects of vitamin D3 deficiency on: (a) the initial rates of L-[14C]glutamate and [3H]GABA uptake by nerve terminals mediated by the plasma membrane Na+-dependent transporters; (b) expression of glutamate and GABA transporters, EAAC-1 and GAT-3 types, respectively; (c) the ambient levels; (d) Ca2+-dependent exocytotic release and (i) Ca2+-independent transporter-mediated release of L-[14C]glutamate and [3H]GABA from nerve terminals.
Methods and materials
Results
Discussion Here, we found that vitamin D3 deficiency decreased L-[14C]glutamate and [3Н]GABA uptake (Fig. 1, Fig. 4A) and expression of GABA transporter GAT-3 and glutamate transporter EAAT-1 (a downward tendency) (Fig. 1, Fig. 4B,C). The lesser amount of the transporters expressed in the plasma membrane of nerve terminals is expected to transfer the lesser amount of the neurotransmitter molecules through the plasma membrane. In this context, a decrease in transporter expression can underlie weakened initial rates of L-[14C]glutamate and [3Н]GABA uptake and accumulation of the neurotransmitters by nerve terminals under conditions of vitamin D3 deficiency. Despite a decreased uptake of the neurotransmitters, vitamin D3 deficiency did not increase the extracellular level of L-[14C]glutamate and [3Н]GABA in nerve terminals (Fig. 2, Fig. 5). So, even reduced number of the transporters expressed in the plasma membrane of nerve terminals is enough to resist permanent tonic neurotransmitter release and support the low ambient levels of L-[14C]glutamate and [3Н]GABA in preparations of nerve terminals between episodes of exocytosis. Importantly, we did not find severe exitotoxicty in our experiments because the ambient level of glutamate was insignificantly elevated in vitamin D3 deficiency. However, the upward tendency in this level shown in the study is expected to be more significant and lead to exitotoxic consequences during more prolonged exposure to vitamin D3 deficiency. Our data on the ambient levels of the neurotransmitters are in accordance with data of (Tenenhouse et al., 1991), who showed that glutamate concentration in different brain regions was not altered in vitamin D3 deficient rats, whereas GABA concentration increased. Vitamin D deficiency studies using adult mice have identified significantly reduced glutamic acid decarboxylase levels that is a key enzyme in GABAergic interneurons and decreased levels of glutamate and glutamine in brain tissue (Groves et al., 2013).