(4). Calmodulin (CaM) seems to be involved in this process through two CaM binding sites located one at the C-terminal (5) and the other at the ankyrin repeat domain in the N-terminal end of the protein (6, 7). Deletion of these sites relieves TRPV1 desensitization and tachyphylaxis. Other factors that can modulate the activity of TRPV1 are changes of pH or temperature, inflammatory mediators, and phosphorylation (8). These modulators may be involved in sensitization to pain. A large part of TRPV1 naturally expressed in DRG neurons locates in endomembranes rather than in the plasma membrane (9 -12), and a similar situation has been reported for other TRP channels. The function of these endomembrane channels is not known, although it has been speculated that they may represent a reserve pool that could be rapidly mobilized to the plasma membrane when required (13). In this direction, it has been reported that activation of protein kinase C (14), nerve growth factor-induced phosphorylation via Src kinase (15) or coupling with phosphoinositide 3-kinase (16) promotes insertion of ER-located TRPV1 channels (TRPV1 ER ) into the plasma membrane (TRPV1 PM ). This mobilization could be the basis of inflammatory sensitization and hyperalgesia.On the other hand, previous work has shown that TRPV1 ER channels are functional (9 -12, 17) and that its activation leads to alterations of ER and mitochondria, followed by cell death (9, 18). Cell death due to ER stress following ER Ca 2ϩ emptying by TRPV1 ER stimulation has also been documented in human lung cells (19). Transfection of HEK293T cells with TRPV1 reproduces the neuronal model with expression of functional TRPV1 ER and TRPV1 PM channels (9,16,18,20,21).In all of the previous studies, the effects of TRPV1 on [Ca 2ϩ ] ER were inferred from the changes of the cytosolic Ca 2ϩ concentration ([Ca 2ϩ ] C ). We can now monitor directly [Ca 2ϩ ] ER in living cells using ER-targeted aequorins (22-24). Here we have studied in detail the release of Ca 2ϩ from ER induced by activation of TRPV1 in DRG neurons and in HEK293T cells expressing TRPV1 channels.