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 (Orx1R). The aim of this study was to evaluate the effects of amygdala Orx1R 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 Orx1R 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 Orx1R 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 Orx1R 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 Orx1R was significantly higher following fear conditioning. These results indicate a likely involvement of the orexin and amygdalar Orx1R in memory acquisition and in extinction of PTSD.

Salehabadi S ，Abrari K ，Elahdadi Salmani M ，Nasiri M ，Lashkarbolouki T ... -  《-》

Intra-accumbal orexin-1 receptor inhibition prevents the anxiolytic-like effect of ethanol and leads to increases in orexin-A content and receptor expression.

Alcohol use is frequently associated with mood disorders. Similarly, individuals suffering from these disorders have a higher risk of developing alcoholism. Several reports have implicated orexin signaling in different behaviors related to alcohol consumption, whereas antagonists block these actions. However, the involvement of orexin-1-receptor (Orx1R) in ethanol-induced anxiolysis remains relatively unexplored. The purpose of this study was to investigate whether intra-accumbal inhibition of Orx1R blocks the anxiolytic-like effect of ethanol and to determine if ethanol administration modifies orexin-A content and Orx1R expression in the nucleus accumbens (NAc). The elevated-plus-maze test (EPM-test) was used to measure anxiety; orexin-A content and Orx1R expression were determined by enzyme-immunoassay and western blot, respectively. The results showed that the pretreatment with a selective antagonist of Orx1R, SB-334867 (SB, 3 μg/side), prevents the anxiolytic-like behavior induced by acute ethanol (2.5 g/kg). SB-334867 per se had no effect on anxiety levels. Pretreatment with SB-334867 followed by ethanol (SB + Et) increased orexin-A content and Orx1R levels in the NAc in comparison to the groups that only received ethanol (V + Et) or SB-334867 (SB + S). Ethanol treatment significantly augmented Orx1R expression but not the peptide content. The increase in orexin-A observed in SB + Et animals could be due in part to the inhibition of Orx1R, since SB-334867 prevents the binding of orexin-A to the receptor. This increase in orexin-A may, in turn, induce an up-regulation of receptor. Other possible explanations were discussed. In general, these findings suggest that orexin-A contributes largely to expression of ethanol-induced anxiolytic-like effect through the signaling of Orx1R in the NAc.

Morales-Mulia M 《-》

The hypocretin/orexin system mediates the extinction of fear memories.

Flores Á ，Valls-Comamala V ，Costa G ，Saravia R ，Maldonado R ，Berrendero F ... -  《-》

Involvement of orexin-A in the regulation of neuronal activity and emotional behaviors in central amygdala in rats.

The amygdala is a complex structure involved in the regulation of emotional behaviors including fear and anxiety. The central amygdala is the main output of the amygdala and plays an important role in emotional processing. Recent studies indicate that orexin, a kind of neuropeptides responsible for maintaining wakefulness, is also associated with emotion-related behaviors, such as depression- and anxiety-like behaviors. Central amygdala receives orexinergic fibers originating from the lateral hypothalamus and expresses OX1 receptors in rats. To test the electrophysiological and behavioral effects of orexins in the central amygdala, single unit in vivo extracellular recordings, open field and elevated plus maze tests were performed in rats. Micro-pressure administration of orexin-A (0.01 mmol/L) increased the firing rate in 18 out of the 31 central amygdala neurons, while the other 13 neurons were not excited by orexin-A. The excitatory effects of orexin-A on central amygdala neurons were mainly mediated by OX1 receptors rather than OX2 receptors. Orexin-B (0.01 mmol/L) did not change the firing activity in all recorded central amygdala neurons. Selectively blocking OX1 receptors by SB-334867 (0.01 mmol/L) significantly decreased the spontaneous firing rate in 14 out of the 33 central amygdala neurons, leaving the remaining 19 neurons were not affected. However, blocking OX2 receptors by TCS-OX2-29 (0.01 mmol/L) did not change the firing activity. Finally, both open field test and elevated plus maze test showed that bilateral microinjection of orexin-A into the central amygdala induced significantly anxiolytic-like behaviors. The specific OX1 receptor antagonist tended to produce opposite effects although there was no statistical difference. The present electrophysiological and behavioral studies suggested that orexin-A participates in anxiety-like behaviors by modulating the spontaneous firing activity of central amygdala neurons.

Pan YP ，Liu C ，Liu MF ，Wang Y ，Bian K ，Xue Y ，Chen L ... -  《-》

Orexin/hypocretin receptor 1 signaling mediates Pavlovian cue-food conditioning and extinction.

Learned food cues can drive feeding in the absence of hunger, and orexin/hypocretin signaling is necessary for this type of overeating. The current study examined whether orexin also mediates cue-food learning during the acquisition and extinction of these associations. In Experiment 1, rats underwent two sessions of Pavlovian appetitive conditioning, consisting of tone-food presentations. Prior to each session, rats received either the orexin 1 receptor antagonist SB-334867 (SB) or vehicle systemically. SB treatment did not affect conditioned responses during the first conditioning session, measured as food cup behavior during the tone and latency to approach the food cup after the tone onset, compared to the vehicle group. During the second conditioning session, SB treatment attenuated learning. All groups that received SB, prior to either the first or second conditioning session, displayed significantly less food cup behavior and had longer latencies to approach the food cup after tone onset compared to the vehicle group. These findings suggest orexin signaling at the 1 receptor mediates the consolidation and recall of cue-food acquisition. In Experiment 2, another group of rats underwent tone-food conditioning sessions (drug free), followed by two extinction sessions under either SB or vehicle treatment. Similar to Experiment 1, SB did not affect conditioned responses during the first session. During the second extinction session, the group that received SB prior to the first extinction session, but vehicle prior to the second, expressed conditioned food cup responses longer after tone offset, when the pellets were previously delivered during conditioning, and maintained shorter latencies to approach the food cup compared to the other groups. The persistence of these conditioned behaviors indicates impairment in extinction consolidation due to SB treatment during the first extinction session. Together, these results demonstrate an important role for orexin signaling during Pavlovian appetitive conditioning and extinction.

Keefer SE ，Cole S ，Petrovich GD 《-》