, 2001). Intra-LC administration of a CRF antagonist during the stress prevented the stress-induced excitation and revealed a greater post-stress inhibition that is naloxone-sensitive (Valentino and Wehby, 1988a and Curtis et al., AZD6738 mouse 2001). Additionally, LC administration of naloxone alone increased the time taken for LC excitation
to recover to pre-stress levels. This study suggested that opioid inhibition was important in recovery of LC activity from this physiological stressor. Together these findings support a model whereby acute stressors engage both CRF and opioid inputs to the LC (Fig. 2A). CRF is the predominant afferent and shifts LC discharge to a high tonic mode that favors
increased arousal, scanning attention and behavioral flexibility, effects that would be adaptive coping responses to an acute threat. At the same time endogenous opioid afferents that have opposing actions are engaged. These function to restrain the CRF excitation and to promote recovery after stressor termination. These CRF/opioid interactions adjust the activity and reactivity of LC neurons so that level of arousal IPI-145 order and processing of sensory stimuli are optimized to facilitate adaptive behavioral responses to stressors. The protective effects of opioids are apparent in the many studies documenting that morphine administration shortly after a single traumatic event reduces the incidence of PTSD (Bryant et al., 2009 and Holbrook et al., 2010). During acute stress MOR regulation of the LC serves as an adaptive counterbalance that curbs the excitatory effects of CRF and protects against the consequences of a hyperactive
brain norepinephrine system. However, tipping the balance in favor of a MOR influence incurs alternative costs (Fig. 2B). Like the CRF response to stress, the opposing opioid response must be limited. The persistence of an opioid influence can produce enduring modifications in neural circuits that result in opioid tolerance and dependence. Indeed, this may be an underlying basis for the association between stress and substance abuse. A bias toward opioid regulation of the LC was recently demonstrated to occur with repeated Megestrol Acetate social stress, which diminishes CRF function and enhances MOR function in the LC (Chaijale et al., 2013). Unlike acute stressors, repeated social stress decreased LC neuronal discharge rate by 48 h after the last stress and this inhibition was naloxone-sensitive indicating that MOR receptors were occupied. Analysis of CRF1 and MOR protein levels and receptor trafficking in the LC demonstrated that this paradoxical stress-induced inhibition is due to both a loss of CRF-elicited excitation as a result of CRF1 internalization and to increased opioid release and MOR signalling (Chaijale et al., 2013).