ResearchAnxiety-and depressive-like responses and c-fos activity in preproenkephalin k n ockout mice : Oversensitivity hypothesis of enkephalin deficit-induced posttraumatic stress disorder

The present study used the preproenkephalin knockout (ppENK) mice to test whether the endogenous enkephalins deficit could facilitate the anxietyand depressive-like symptoms of posttraumatic stress disorder (PTSD). On Day 1, sixteen wildtype (WT) and sixteen ppENK male mice were given a 3 mA or no footshock treatment for 10 seconds in the footshock apparatus, respectively. On Days 2, 7, and 13, all mice were given situational reminders for 1 min per trial, and the freezing response was assessed. On Day 14, all mice were tested in the open field test, elevated plus maze, light/dark avoidance test, and forced swim test. Two hours after the last test, brain tissues were stained to examine c-fos expression in specific brain areas. The present results showed that the conditioned freezing response was significant for different genotypes (ppENK vs WT). The conditioned freezing effect of the ppENK mice was stronger than those of the WT mice. On Day 14, the ppENK mice showed more anxietyand depressive-like responses than WT mice. The magnitude of Fos immunolabeling was also significantly greater in the primary motor cortex, bed nucleus of the stria terminalis-lateral division, bed nucleus of the stria terminalis-supracapsular division, paraventricular hypothalamic nucleus-lateral magnocellular part, central nucleus of the amygdala, and basolateral nucleus of the amygdala in ppENK mice compared with WT mice. In summary, animals with an endogenous deficit in enkephalins might be more sensitive to PTSD-like aversive stimuli and elicit stronger anxiety and depressive PTSD symptoms, suggesting an oversensitivity hypothesis of enkephalin deficit-induced PTSD. Background Posttraumatic stress disorder (PTSD) shows a variety of symptoms including the exaggerated fear, helplessness, and horror after patients suffer from an extremely stressful traumatic event (an unconditioned stimulus [US]) [1]. For example, the reexperiencing of symptoms of an earlier traumatic event includes panic attack, phobic avoidance of situations that resemble the traumatic event, and psychic numbing [2,3]. Additional symptoms comprise autonomic hyperarousal responses and fear sensitization, such as exaggerated startle responses, hypervigilance, insomnia, irritability, and impaired concentration [4]. In addition to the traumatic event US inducing PTSDlike responses, the environmental stimulus (conditioned stimulus [CS]) associated with the traumatic event US is also able to elicit PTSD-like avoidance fear responses [5]. Accordingly, an animal model of PTSD has been developed in which individuals are repeatedly exposed to situational reminders that have been previously associated with a traumatic stress US to elicit the fear response [6,7]. The PTSD-like symptoms have been shown to be governed by specific neurotransmitters [8,9]. For example, a recent report has demonstrated that the releasing concentrations of serotonin, norepinephrine, and dopamine in the hippocampus and frontal cortex would be enhanced, and the plasma corticosterone levels in the hypothalamic-pituitary-adrenal axis were increased after acute stress exposure [10]. Moreover, a recent study dem* Correspondence: acwhuang@gmail.com 4 Department of Psychology, Fo Guang University, Yi-Lan 26247, Taiwan,