The neuroprotective effect of TUDCA treatment in our model of experimental retinal detachment was correlated with inhibition of caspases 2, 3 and 9 and a decrease in TUNEL- positive cells. Daily TUDCA administration reduced the number of TUNEL positive cells by about 50% and reduced the loss of ONL thickness by a similar amount initially but the effect becomes less pronounced on day 5. This could be the result of the existence of alternative mechanisms of cell loss in retinal detachment as was recently described by our group. It has been shown by others and us that TUNEL staining is not restricted to apoptotic cells but encompasses necrotic cells as well. It is possible that TUDCA is not effective in blocking both apoptotic and necrotic pathways that are activated upon RD and Usaramine thus the protection offered by it may be limited. Inflammation is thought to play a significant role in retinal detachment mediated photoreceptor cell loss. Several studies have shown upregulation of inflammatory cytokines such as TNF-a and MCP1 and have demonstrated increased infiltration of macrophages. However, we showed that TUDCA did not affect the inflammatory cytokine production measured in total retina homogenates and did not alter the inflammatory cell infiltration after retinal detachment. Consistent with our findings, a study on hepatocytes showed that TUDCA did not affect the levels of the TNF-a released from Kuppfer cells isolated from the liver while another study found that TUDCA protected hepatocytes from TNF-a induced death. In a study of Oleuropein isolated biliary epithelial cells TUDCA did not alter the levels of IL-6 or MCP1 secretion. These data suggest that the cytoprotective action of TUDCA may be mediated through a direct effect on damaged cells rather than through regulation of inflammatory processes. Endoplasmic Reticulum stress has been shown to be a feature in various neurodegenerative disorders, as well as in retinal detachment. Persistent ER stress leads to pro-apoptotic molecule induction such as growth arrest DNA damage-inducible gene 153 also known as C/EBP homologous protein. In a study of isolated pancreatic acini it was shown that TUDCA decreased ER stress and CHOP expression. Similar to the previous study of ER stress in RD we found that RD lead to increased levels of CHOP but in contrast to the pancreatic acini study TUDCA did not significantly alter its expression. In line with this finding, TUDCA did not decrease Caspase 11 induction, a downstream effector of CHOP. The related drug UDCA has been shown to inhibit changes in mitochondrial transmembrane potential and ROS generation in isolated mitochondria from the liver of adult rats. Oxidative stress is a factor playing a critical role in photoreceptor death after RD and we have shown that ROS reduction is associated with neuroprotective effect on photoreceptors after RD. Administration of TUDCA led to almost complete abolishment of the increase in protein carbonyl content, a measure of ROS levels. Hence, a combination of the inhibition of caspases and decrease in the ROS levels could be partially responsible for the neuroprotective mechanism of TUDCA in the RD model. The exact mechanism TUDCA protects photoreceptor cells remains unknown and its effects may be direct or indirect. Given the limited effect on inflammatory cytokines and infiltrating leukocytes, it seems that the inflammatory cell may not be a major target of TUDCA -at least not in this model. Thus, it seems that a direct effect on photoreceptor cells is more likely. This may also partially explain why the delayed administration of TUDCA lead to reduced efficiency since bioavailability of TUDCA to the outer retina is expected to be impaired after photoreceptor separation from the tissues supplying them with nutrients.