Investigating the role of the amygdala orexin receptor 1 in memory acquisition and extinction in a rat model of PTSD.

Understanding the mechanisms underlying memory is essential for the treatment of post-traumatic stress disorder (PTSD). Orexin, as a lateral hypothalamic (LH) neuropeptide, interferes with the stages of memory, primarily through the orexin receptor1 (OrxR). The aim of this study was to evaluate the effects of amygdala OrxR in the acquisition and extinction processes of PTSD modeled in animals. In three experiments, rats were divided into three groups: control (Naïve), shock (receiving a foot shock), and PTSD (experiencing Single prolonged stress (SPS) method). The first experiment aimed to evaluate LH activity in PTSD modeled rats. The second and third experiments aimed to evaluate the effects of OrxR in the acquisition and extinction of fear memory in PTSD modeled animals. SB334867 (SB) or its solvent was microinjected into the amygdala and the rats were subjected to conditioning thereafter. In the second group, we used a single injection after conditioning. In the third group, we used three consecutive injections (one after each memory test). Some behaviors and OrxR expression were evaluated. The freezing response was significantly longer in the PTSD group than on the control. Similarly, anxiety and sensitized fear were also intensified. CFos expression levels in LH was higher in the PTSD group. Inhibition of OrxR in the amygdala significantly decreased memory acquisition, diminished anxiety, and decreased the sensitized fear in the SB group. Applying SB to the amygdala after each fear memory test significantly decreased freezing. Expression of OrxR was significantly higher following fear conditioning. These results indicate a likely involvement of the orexin and amygdalar OrxR in memory acquisition and in extinction of PTSD.