Acid Sphingomyelinase Is a Modulator of Contextual Fear

Abstract

Acid sphingomyelinase (ASM) regulates a variety of physiological processes and plays an important role in emotional behavior. The role of ASM in fear-related behavior has not been investigated so far. Using transgenic mice overexpressing ASM (ASMtg) and ASM deficient mice, we studied whether ASM regulates fear learning and expression of cued and contextual fear in a classical fear conditioning paradigm, a model used to investigate specific attributes of post-traumatic stress disorder (PTSD). We show that ASM does not affect fear learning as both ASMtg and ASM deficient mice display unaltered fear conditioning when compared to wild-type littermates. However, ASM regulates the expression of contextual fear in a sex-specific manner. While ASM overexpression enhances the expression of contextual fear in both male and female mice, ASM deficiency reduces the expression of contextual fear specifically in male mice. The expression of cued fear, however, is not regulated by ASM as ASMtg and ASM deficient mice display similar tone-elicited freezing levels. This study shows that ASM modulates the expression of contextual fear but not of cued fear in a sex-specific manner and adds a novel piece of information regarding the involvement of ASM in hippocampal-dependent aversive memory.

Keywords: ASM; PTSD; contextual fear; cued fear; fear conditioning; fear extinction; fear learning; knock-out mice; transgenic mice.

Figure 1

Schematic representation of the fear conditioning procedure. During fear conditioning on day 1, mice were placed in the conditioning chamber (context A) and, after a 5-min adaptation period, were exposed to five CS–US pairings. The conditioned stimulus (CS) was an 80 dB, 8 kHz, 30 s sine-wave tone, which co-terminated with a mild electric foot shock (unconditioned stimulus, US; 0.7 mA, pulsed current, 2 s). During cued fear extinction training on day 2, mice were placed in a different box (context B) and were exposed to 20 CS presentations to assess the cue-induced fear response. During cued fear extinction retention on day 3, mice were placed again in context B and were exposed to five CS presentations. During contextual fear measurement on day 4, mice were placed in the conditioning chamber (context A) and freezing was assessed as an indicator of contextual fear expression for 5 min.

Figure 2

Acid sphingomyelinase (ASM) overexpression increases the expression of contextual fear in both male and female mice. (a,e) On day 1, the ASM transgenic (ASMtg) mice and wild-type littermates (WT) were fear conditioned in context A; (b,f) on day 2, the expression of cued fear (i.e., tone-elicited freezing) was assessed in context B; (c,g) on day 3, the retention of cued fear was assessed in context B; (d,h) on day 4, the expression of contextual fear was assessed in context A. Data represent the mean time of CS-elicited freezing ± SEM, and numbers in parentheses indicate group sizes. * p < 0.05 compared with WT mice.

Figure 3

Acid sphingomyelinase (ASM) deficiency decreases the expression of contextual fear in male mice. (a,e) On day 1, the homozygous (ASM−/−) and heterozygous (ASM+/−) ASM deficient mice and wild-type littermates (ASM+/+) were fear conditioned in context A; (b,f) on day 2, the expression of cued fear (i.e., tone-elicited freezing) was assessed in context B; (c,g) on day 3, the retention of cued fear was assessed in context B; (d,h) on day 4, the expression of contextual fear was assessed in context A. Data represent the mean time of CS-elicited freezing ± SEM, and numbers in parentheses indicate group sizes. * p < 0.05 compared with ASM+/+ mice.