There aren't really very useful selective ligands developed for the sigma receptors, making it difficult to isolate characterization of their functions.
Cell death, Anxiety, psychosis and depression or the prevention of them seem to be among possible effects mediated by agonism or antagonism of sigma receptors - sometimes apparently paradoxically.
What is interesting is that sigma1 agonism is thought to have neuroprotective effects via several ways: vs. oxidative and nitric oxide stress and calcium homeostatic imbalance
http://www.sciencedirect.com/science/article/pii/S1347861314000267