Abstract
Reactivity to traumatic stress varies between individuals and only a minority of those exposed to trauma develops stress-induced psychopathologies. Currently extensive effort is made to unravel the specific mechanisms predisposing to vulnerability vs. resilience to stress. We investigated in rats the role of β-endorphin metabolism in vulnerability to acute traumatic stress. Responders (showing extreme anxiety; n=7) and resilient non-responders (not differing from the non-stressed individuals; n=8) to traumatic foot-shock stress were compared for their blood levels of stress hormones as well as brain levels and activity of two opioid-degrading enzymes. β-endorphin is a substrate to insulin degrading enzyme, which also degrades insulin. Therefore, the effects of insulin application on behavioral and hormonal responses and on β-endorphin degradation were tested. Pre- and post-stress levels of serum corticosterone, and post-stress plasma β-endorphin concentration differentiated between the responders and the non-responders. In brain, responders showed enhanced degradation rates of β-endorphin, assessed by Liquid Chromatography–Tandem Mass Spectrometry (LC-MS/MS), in hippocampal and amygdalar slices as compared to non-responders. Application of insulin to the amygdala, prior to exposure to traumatic stress, reduced post-stress anxiety and serum corticosterone levels only in the responders. In parallel, amygdalar β-endorphin degradation rate was also reduced by insulin. These results suggest that slowing down β-endorphin degradation rate may constitute an integral part of the normal stress-response, upon a failure of which an extreme anxiety develops. Modulation of opioid degradation may thus present a potential novel target for interference with extreme anxiety.
Introduction
In a population exposed to severe stressors about 20–30% will develop post-traumatic stress disorder (PTSD), suggesting an individual conductance of stress-response mechanisms (Yehuda and LeDoux, 2007). Many physiological systems have been studied in an attempt to delineate factors predisposing certain individuals to vulnerability vs. resilience to trauma. For example, the magnitude of serum surge of cortisol (in rodents-corticosterone; CORT), which is suggested as one of the major indexes of stress response intensity, has been shown to differentiate between responders and non-responders to trauma (Charmandari et al., 2005). Another key-factor of stress response is the endogenous opioids system (EOS), extensively implicated in analgesia, reward and anxiety (Bodnar, 2009), which reciprocally interacts with other components of stress response, such as CORT surge (Blalock and Smith, 1985, Charmandari et al., 2005, Bodnar, 2009 and Wittmann et al., 2009). The major representative of the endorphin subclass of this system is β-endorphin, a 31 amino-acid peptide (Bodnar, 2009). Studies in animals and in humans demonstrated blood and cerebrospinal fluid levels of β-endorphin as differentiating between the vulnerable and resilient individuals at rest or in stress-related situations (Hoffman et al., 1989, Darko et al., 1992, Hamner and Hitri, 1992, Kocijan-Hercigonja et al., 1996, Baker et al., 1997, Fontana et al., 1997 and Grisel et al., 2008).
One of the factors determining the functioning of EOS is the activity of proteases that degrade endogenous opioids. β-endorphin is a substrate for a soluble enzyme Insulin Degrading Enzyme (IDE) (Safavi et al., 1996 and Reed et al., 2008). Hormones of HPA-axis, specifically CORT, influence levels and activity of these proteases (Jaskowski et al., 1989, Safavi et al., 1996, Kulstad et al., 2005, Reed et al., 2008 and Osmanovic et al., 2010) and it was suggested that slowing down their activity may reduce anxiety (Jutkiewicz, 2007). Despite ample evidence on EOS involvement in stress response, its specific functional modifications that underlie the individual reactivity to traumatic stress and the role of the enzymes that degrade endogenous opioids are still unclear. We hypothesized that functioning of opioid-degrading enzymes in traumatic stress contributes to the vulnerability/resilience of the individual.
The present study investigated in the rat the degradation rate of β-endorphin in dorsal hippocampus and the basolateral amygdala (BLA) in individual response to acute traumatic stress. These two brain structures are extensively implicated in stress and anxiety and contain high concentrations of both opioid receptors and IDE (Bremner, 2006 and Bodnar, 2009). Our previous results showed these regions to be modified by the behavioral stressors used in this study (Kavushansky and Richter-Levin, 2006 and Kavushansky et al., 2009). Insulin was applied to the amygdala in an attempt to interfere with β-endorphin degradation, as both are substrates to the same protease. Insulin effects on behavioral and hormonal responses, as well as on degradation rate of β-endorphin were studied.
