ASAH1 and major depressive disorder: Whereas specific social fear was accompanied by an increased activity of ceramidases in the ventral hippocampus and ventral mesencephalon and by relatively small changes in sphingolipid levels in the dorsal hippocampus, social fear with comorbid depression was accompanied by changes in the activity of sphingomyelinases and acid ceramidase, as well as in sphingolipid levels and sphingolipid ratios in most of the investigated brain regions.