关键词: 高唤醒, 5-羟色胺, 色氨酸羟化酶-2, 听觉惊吓反射

Abstract:

Severe stress is one of the major external triggers of emotion-related mental disorders such as post-traumatic stress disorder (PTSD). Stress-induced sustained hyperarousal state is not only a core symptom but also a contributor to other symptoms such as sleep disturbance and negative mood. Serotonin, or 5-hydroxytryptamine (5-HT), is a monoamine neurotransmitter that regulates emotional response. In addition, the 5-HT system is the target for pharmacological treatment such as selective-serotonin re-uptake inhibitors (SSRIs) for major depressive disorder, PTSD, and other emotional disorders. However, it remains unknown whether serotonin is involved in the hyperarousal state caused by severe stress, as well as the mechanism by which genetic polymorphism in serotonin regulation contributes to the vulnerability of stress-related psychiatric disorders. Tryptophan-hydroxylase-2 (Tph2) is a serotonin synthesizing enzyme that converts tryptophan to 5-hydroxytryptophan in the brain. A genetic deficiency in the expression of Tph2 may lead to a lower level of serotonin in the brain. The present study focused on the role of serotonin in the development of stress-induced hyperarousal, investigating the behavioral effect of Tph2 gene-deficiency after severe stress in a mice model.
Mice lacking Tph2 (Tph2-/-) in the brain have a vitally low level of serotonin and a bad health condition, so we used heterozygous Tph2-deficient mice (Tph2+/-) which have been shown to have a mild low level of serotonin in the brain. We measured the auditory startle reflex as an indicator of arousal level at different time points after predator-exposure stress or footshock stress in both male and female Tph2+/- and wild-type mice. The predator-exposure stress was to exposure a mouse to a cat for 5 minutes with a trained experimenter protecting the mouse from direct attack from the cat. The footshock stress was to exposure a mouse to a series of footshock (1.5 mA × 5s × 5, inter-shock interval 60 ~ 120 s) in a shock chamber. Then we measured the auditory startle reflex at 1-, 2-, 10-, and 18-day post-stress. For each startle test session, a total of 30 white noise stimuli were presented to the mice in a sound-isolated chamber (90 dB, 100 dB, 110 dB, ten stimuli for each level).
The results showed that the Tph2+/- male mice had a higher level of startle than the non-stressed group at 1, 2, and 10 days after footshock stress, indicating a sustained hyperarousal. However, wild-type male mice only had an increased startle response on the day after the footshock stress. For mice with predator exposure stress, both Tph2+/- male mice and wild-type male mice showed an increased startle response on the first day after the predator stress, but then returned to the same level as the non-stressed mice. We also observed a sex difference in mice’s startle response that the female mice had a lower level of startle amplitude than that of male mice at baseline test before stress. In addition, female mice with different genotypes showed minor differences in their startle response at different time points after both types of stress.
The results of the study indicate that the Tph2 genotype interacts with stress types in the regulation of long-term hyperarousal after severe stress events. Our results also provide preclinical evidence that individuals with Tph2 gene deficiency may be more vulnerable to stress-induced hyperarousal and highlight the potential of targeting the serotonin system for post-traumatic intervention.

Key words: hyperarousal, 5-hydroxytryptamine, tryptophan-hydroxylase-2, auditory startle reflex