Orexins (hypocretins) contribute to fear and avoidance in rats exposed to a single episode of footshocks

Abstract

Orexins (hypocretins) are peptides that have been shown to regulate behavioral arousal and wakefulness. Recent evidence indicates that orexin neurons are activated by stress and that orexins play a role in anxiety. The present paper describes a series of experiments that examined whether orexins are involved in the anxiety that resulted from exposing rats to an acute episode of footshocks (5 × 2 s of 1.5 mA shocks). We found that prepro-orexin (ppOX) mRNA was elevated in rats at 6 and 14 days after exposure to footshock and that ppOX mRNA levels were correlated with fear at 14 days post-shock. Systemic injections of the non-selective dual orexin receptor antagonist TCS-1102 (10 and 20 mg/kg, i.p.) were found to decrease fear and anxiety in rats 14 days after exposure to footshock. We also found that rats that exhibited a high level of immobility to a novel tone the day after the footshock episode (high responders, HR) showed significantly elevated levels of ppOX mRNA at 14 days post-shock compared to control rats. Furthermore, TCS-1102 (10 mg/kg, i.p.) was found to have anxiolytic effects that were specific for HR when tested in the elevated T-maze. This study provides evidence linking the orexin system to the anxiety produced by exposure of rats to a single episode of footshocks. It also provides preclinical evidence in support of the use of orexin antagonists for the treatment of anxiety in response to an acute episode of stress.

Orexins and fear: implications for the treatment of anxiety disorders

“…Mice lacking OX1R showed impaired freezing responses and reduced expression of zif268 (an IEG that is considered a marker of neuronal activation) in the lateral AMY in both cued and contextual fear-conditioning paradigms [33]. In agreement, the dual orexin antagonist almorexant reduced fear-potentiated startle responses [34], and increased prepro-orexin mRNA levels were positively correlated with immobility time after footshock exposure in rats [35]. Notably, restoration of OX1R expression in noradrenergic neurons of the locus coeruleus by using an adeno-associated virus vector normalized cued fear behavior and increased lateral AMY activity after cued testing in OX1R-knockout mice [33].…”

Blocking of orexin receptors in the paraventricular nucleus of the thalamus has no effect on the expression of conditioned fear in rats

“…The results show that microinjections of a DORA in the PVT did not attenuate the fear expressed following exposure of rats to cued and contextual fear conditioning paradigms. A lack of an effect on contextual fear was surprising considering that the protocol used was the same as the one used to show that systemic injections of the same DORA reduced contextual fear (Chen et al, 2014b). However, the DORA had an anxiolytic effect when the contextual fear conditioned rats were tested in a social interaction test.…”

“…The choice to use more intense footshocks for the contextual conditioning experiment was based on previous evidence that exposure of 1.5 mA footshocks produces an orexin-mediated fear and anxiety (Chen et al, 2014b). For example, exposure of rats to footshocks of this intensity has been shown to produce long-lasting increases in prepro-orexin mRNA as well as contextual fear and anxiety that are attenuated by systemic injections of a DORA (Chen et al, 2014b). Rats were assigned to drug treatment groups as described for Experiment 1.…”

“…For instance, systemic injections of the OX1R antagonist SB334867 blocked the behavioral and autonomic responses elicited in a rat model of panic (Johnson et al, 2010). In other studies, acute systemic injections of the dual orexin receptor antagonist (DORA) TCS-1102 reduced contextual fear as well as the defensive and avoidance behaviors in rats previously exposed to footshocks (Chen et al, 2014b). In other studies, chronic oral administration of another DORA called almorexant reduced anxiety after footshock exposure (Viviani et al, 2015) while an acute oral treatment with almorexant attenuated fear in a fear-potentiated startle paradigm (Steiner et al, 2012).…”

“…The purpose of the first experiment was to determine if orexin receptors in the PVT contribute to fear to an auditory tone produced by a conditioning protocol similar to the one used to show an involvement of the PVT in fear (Padilla-Coreano et al, 2012; Li et al, 2014; Do-Monte et al, 2015). The second experiment was done to examine the contribution of orexin receptors in the PVT to the fear associated with the shock chamber using a contextual fear protocol known to cause the activation of the orexin system (Chen et al, 2014a,b). Consequently, the effect of blocking orexin receptors in the PVT on the anxiety that resulted from the contextual conditioning procedure was used to determine the effectiveness of orexin antagonist used in the present experiment.…”