IPOACUSIA NEUROSENSORIALE E DANNO ISCHEMICO. MESSA A PUNTO DI UN MODELLO ANIMALE PER VALUTARNE GLI EFFETTI VASCOLARI E OSSIDATIVI.

Hearing impairment is an increasingly common disease. In Italy deaf people are about seven million, including half a million adults with disabling hearing loss and over one thousand births per year with congenital deafness. This causes difficulty in integration in society for adults and prevents the language acquisition for children (Fekete, 1999). As hearing loss has high social costs, the expectation for the development of new treatments is extensive and diseases leading to hearing damage are increasingly studied from clinic and base research. Hearing loss (HL) can have genetic causes, can be associated with aging or exposure to noise or ototoxic substances, and the aetiology can be attributed to vascular injury, trauma, tumours, infections or autoimmune response. All of these factors could be ascribed to alterations in cochlear microcirculation resulting in hypoxia. This condition can damage cochlear hair cells and neurons possibly leading to HL. Hypoxia and ischemia can then be identified as possible factors contributing to the aetiology of deafness, but they have not been experimentally studied yet. The purpose of this work is to develop animal models of ischemia and infarction suitable for the study of cochlear vascular damage, and to characterize them with electrophysiology and gene/protein expression analyses. For this reason it was decided to monitor the effects of ischemia in thrombosis mimicked by complete temporary carotid occlusion, and in stroke mimicked by incomplete permanent left coronary artery. In particular this study focused on the analysis of: organ of Corti and spiral ganglion structures, coagulation, oxidative stress and apoptosis. A further aim was to compare these models with other models of ototoxic damage, such as noise and cisplatin. These models are both characterized for electrophysiology, oxidative stress and apoptosis, but the possible involvement of vascular damage has not been investigated yet. This comparison helped us to characterize the new models of vascular injury in the oxidation and apoptosis expression patterns. In our models, both infarction and ischemia cause a small but significant hearing loss, localized at the cochlear apex. Furthermore, there is a slight induction of the coagulation cascade, both in procoagulant and anticoagulant part, and an activation of JNK, that may lead to cell survival. In addition, only in the carotid ischemia the cuticular plate of outer hair cells is damaged. Even noise and cisplatin cause vascular damage, but while in noise-treated animals the coagulation genes show only an mRNA expression increase, after cisplatin administration an mRNA and protein increase of the tissue factor is detected, which leads to the coagulation cascade activation. In the ischemic models we did not detect any apoptosis activation, while in the other models we noticed opposite reactions: in noise there is an increased transcription of the anti-apoptotic genes that leads to cell survival, while cisplatin activates pro-apoptotic factors. The activation of apoptosis is documented in literature and is confirmed in both conditions by OHC loss detected in histological sections, which leads to a more severe deafness than in the ischemia models. In conclusion, the two models of ischemia developed are suitable for the study of cochlear vascular damage, as they produce a slight hearing loss and give modifications in coagulative, oxidative and apoptotic factors gene expression. Furthermore, the comparison with two other widely used models allowed us to specify the pathways involved. We can therefore say that all types of damage taken into consideration act on the inner ear with vascular damage and oxidative mechanisms.