Table 2.

Metabolomic studies on animal models of hearing loss.

Study/Country	Animal Model	Hearing Loss	Samples	Metabolomic Analysis	Key Findings
Ji et al., 2019 USA [49]	Mice	Noise induced	Inner ear tissues	LC-MS/MS	A total of 40 metabolites exhibited differential responses to noise exposure. Among these, 25 metabolites were up-regulated, encompassing nucleotides, cofactors, carbohydrates, and glutamate, while 15 metabolites were down-regulated, primarily comprising amino acids. Moreover, the impact of noise on the inner ear metabolome in mice was found to be contingent on both the intensity and duration of exposure.
He et al., 2017 New Zealand China [48]	Rats	Noise induced	Brain tissue	GC-MS	88 metabolites were identified from the analysis of 12 different brain regions. The levels of 17 metabolites were found to be significantly affected by acoustic trauma in at least one of the analyzed brain areas. These were mostly involving amino acid metabolism, i.e., alanine, aspartate, glutamate, arginine, proline, and purine metabolic pathways, as well as urea cycle and oxidative reactions.
Pirttilä et al., 2019 USA [45]	Guinea pigs	Noise induced	Perilymph	HILIC-UHPLC-Q-TOF–MS	Guinea pigs exposed to noise and treated with Hydrogen gas (H2) showed reduced hearing loss compared to those exposed to noise alone. Metabolomic analysis confirmed the otoprotective role of H2 by revealing a closer similarity in the perilymphatic metabolome of H2-treated animals exposed to noise to that of control and H2 only groups, compared to the noise only group. Additionally, the noise only group exhibited higher levels of various acyl carnitines, while lower levels of osmoprotectans stachydrine and homostachydrine were observed.
Fujita et al., 2015 Japan [46]	Guinea pigs	Noise induced	Inner ear fluid, Plasma	GC-MS	Higher levels for 6 metabolites (ascorbic acid, fructose, galactosamine, inositol, pyruvate + oxaloacetic acid and meso-erythritol) and lower levels for 9 (phosphate, valine, glycine, glycerol, ornithine, glucose, citric acid + isocitric acid, mannose and trans-4-hydroxy-l-proline) were observed in the inner ear fluid than in plasma. The levels of 10 metabolites (3-hydroxy-butyrate, glycerol, fumaric acid, galactosamine, pyruvate + oxaloacetic acid, phosphate, meso-erythritol, citric acid + isocitric acid, mannose and inositol), changed significantly in the inner ear fluids as a consequence of noise exposure.
Pierre et al., 2017 Sweden [47]	Guinea pigs	Chemical induced	Serum	LC-MS	Some animals received sodium thiosulfate (STS), a potential otoprotectant, 30 min before cisplatin administration, while another group received sodium chloride. Non-significant differences were observed between the two groups. On the other hand, 4 metabolite changes showed significant correlation with high-frequency hearing loss, but only in the group receiving NaCl. These metabolites were N-acetylneuraminic acid, L-acetylcarnitine, ceramides, and cysteinylserine. Notably, a higher increase in the level of metabolite change was associated with a lower hearing threshold shift.
Miao et al., 2022 China [50]	Mice	Noise induced	Cochlea tissue	GC-MS	Exposure to noise caused alterations in the levels of 17 metabolites, of which only 3 (spermidine, 3-hydroxybutyric acid and orotic acid) had increased levels. In addition, 9 metabolic pathways, namely β-alanine metabolism, pyrimidine metabolism, cAMP signaling pathway, butanoate metabolism, ketone body synthesis and degradation, arginine and proline metabolism, GABAergic synapse, insulin secretion, and purine metabolism, were significantly affected by noise exposure.
Boullaud et al., 2022 [51]	Sheep	Noise induced	Perilymph	LC-MS	Following noise exposure, certain metabolites demonstrated an increasing trend, including urocanate, oleate, 5-Oxo-L-Proline, N-Acetyl-Glucose, N-Acetylneuraminate, L-Tyrosine, Trigonelline, Leukotriene-B4, 5,6-Dihydrouracil, and 3-Ureidopropionate. Conversely, a decreasing trend was observed for deoxycarnitine, L-Carnitine, N-Acetyl-L-Leucine, S-(5′-Adenosyl)-L-Homocysteine, and epinephrine. In addition, 5 metabolic pathways were found to be involved: phenylalanine/tyrosine/tryptophan metabolism, β-alanine metabolism, pantothenate and CoA biosynthesis, pyrimidine metabolism, and amino and nucleotide sugar metabolism.