Table 4.

Neurotoxic effects of TiO₂ NPs.

Compound	Exposure model	Experimental design	Effect	Reference
TiO2 NPs	Mice	Intratracheal instillations once per week for 4 weeks 13.2 mg/kg	Inflammatory cell aggregation and neuron necrosis. Ti level in the brain 3 days after a single instillation was upregulated by 100%.	[131]
	Wistar rats, male	Intratracheal 0.1, 1.0, 10.0 mg/kg	Ti accumulation in the brain and dose-dependent injury. TiO2 NPs with diameter of 200 nm did not cause significant alterations in the brain.	[132]
	BBB model based on rat primary endothelial cells (BECs) and astrocytes	Acute exposure: 24 h, 0–500 μg/ml Chronic exposure: 5 days, 0–100 μg/ml	Reduced expression of P-gp, claudin 5, caveolin-1, and caveolin-2 associated with BBB integrity.	[133]
	Fisher F344 rats, male	Intravenous single dose 1 mg/kg	Upregulation of tight junction proteins, modulation of P-gp mRNA expression and persistent brain inflammation markers: IL-1β, IP-10, GFAP and CXCL1. No Ti accumulation in the brain after 24	[134]
	Mice	Intranasal 90 days 2.5, 5.0, 10 mg/kg	Ti accumulation in the brain. Oxidative stress, high levels of lipid, protein, and DNA peroxidation, overproliferation of glial cells, tissue necrosis, hippocampal cell apoptosis. Microarray showed significant alterations of 249 genes expression.	[135]
	Mice, female	Intranasal instillation every other day for 2, 10, 20, 30 days 500 μg	Ti accumulation in hippocampus after 30 days of rutile exposure. Irregular arrangement and loss of neurons, morphological changes and oxidative damage in hippocampus. Increased TNF-α and IL-1β levels.	[136]
	Mice, female	Intranasal instillation every other day for 2, 10, 20, 30 days 500 μg	Imbalance of monoaminergic neurotransmitters, increased NE and 5-HT, while levels of DA, DOPAC, HVA and 5-HIAA were decreased.	[137]
	Wistar rats, male	Intragastrical 60 days 50, 100, 200 mg/kg	Downregulated AChE activity. Increased plasmatic and brain IL-6. Increased GFAP expression.	[138]
	Zebrafish embryos	96 hpf 0.1, 1, 10 μg/ml	Hatching time was decreased, with increase in malformation rate. Accumulation in brain with ROS and cell death in hypothalamus. Alterations in behavior and PD-related genes.	[139]
	Caenorhabditis elegans	24 h 7.7, 38.5 μg/ml	GC–MS-based metabolomics perturbations mainly occurred in TCA cycle, glyoxalate, tricarboxylate, inositol phosphate, Gly, Ser, Thr, Gln, and Glu metabolism.	[140]
	Caenorhabditis elegans	96 h under dark or light conditions 1–100 mg/l	Light exposure induced the production of ROS and increased toxicity from a median effect concentration of more than 100 mg/l to 53 mg/l.	[141]
	D384 glial cell line and SH-SY5Y human neuroblastoma	24 h 15, 31 μg/ml,	Concentration- and time-dependent alterations of mitochondrial function, cell membrane damage, inhibition of cell proliferation. Effects dependent on TiO2 size. Neuronal cells were more sensitive than glial cells.	[142]
	U373 human glial cells and C6 rat glial cells	24–96 h 2.5–40 μg/cm2	DNA fragmentation assessed in U373 cells, but not in C6 cells. Morphological changes associated with depolymerization of F-actin, apoptotic cell death.	[143]
	U373 human glial cells and C6 rat glial cells	2–24 h 20 μg/cm2	Increased expression of antioxidant enzymes: GPx, CAT, SOD2, lipid peroxidation and mitochondrial depolarization.	[144]
	PC12 rat pheochromocytoma	6–48 h 1–100 μg/ml for	Apoptosis prevented by a ROS scavenger, N-MPG.	[145]
	Co-culture of PC12 cells with primary rat microglia	24–48 h 0.25–0.5 mg/ml	Supernatant from TiO2 NPs treated microglia caused significant cytotoxicity in PC12 cells.	[146]
	PC12 cell line	24 h 1–125 μg/ml	Decreased cell viability, mitochondrial impairment and decreased DA levels.	[139]
	BV2 microgial cells	6, 18 h 2.5–120 ppm	Release of ROS, mitochondrial hyperpolarization	[147]
	BV2 microgial cells, N27 neurons, primary cultures of rat striatum	2, 6, 24, 48 h 2.5–120 ppm	Microglia generated ROS damages neurons in complex primary cultures. No cytotoxicity in isolated N27 neurons	[148]

Abbreviations: 5-HIAA: 5-hydroxyindole; 5-HT: 5-hydroxytriptamine; AchE: acetylcholine estarese; BBB: blood-brain barrier; CAT: catalase; CXCL1: chemokine C-X-C motif ligand 1; DA: dopamine; DOPAC: 3,4-dihydrophenylacetic acid; GC–MS: gas chromatography mass spectrometry; GFAP: glial fibrillary acidic protein; Gly: glycine; Gln: glutamine: Glu: glutamate; GPx: glutathione peroxidase; HVA: homovanillic acid; hpf: hours post fertilization; IL-1β: interleukin-1β; IL-6: interleukin-6; IP-10: interferon gamma-induced protein 10; NE: norepinephrine; N-MPG: N-(2-mercaptopropionyl)glycine; NPs: nanoparticles; PD: Parkinson’s disease; P-gp: P-glycoprotein; ROS: reactive oxygen species; Ser: serine; SOD2: superoxide dismutase 2; TCA: tricarboxylic acid cycle; TNF-α: tumor necrosis factor α; Thr: threonine; Ti: titanium; TiO₂: titanium dioxide.