Functionally opposed Orx1 and Orx2 systems suggest the potential for orexins to powerfully influence the onset and maintenance of affective disorders such as anxiety and depression. Recent exciting results from our laboratory suggest that activation of Orexin (Orx) type 1 and 2 receptors (Orx1 and Orx2) in the basolateral amygdala (BLA) are functionally opposed, and may differentially trigger or inhibit anxious and depressive behavior. Interplay between Orx1 and Orx2receptors on the excitatory and inhibitory systems of the BLA makes them potential targets for therapeutic treatments of affective disorders. As anxiogenic and anxiolytic properties are both encoded into the circuitry associated with the BLA, what remains is the discovery of the trigger that initiates the change between them. While several limbic and cortical regions may be involved in the onset of affective disorders, the research in this proposal is focused on the BLA, because it is necessary for learning related to fearful and aversive events. The excitatory (glutamatergic) circuit in which th BLA receives input from limbic and cortical structures, and projects to the principle output of the amygdala, (medial Central Amygdala; mCeA), produces learning and expression of anxious or fearful behavior. Modified by inhibitory GABAergic neurons of the BLA, intercalated cells (ITC) and the lateral CeA (lcCeA and lCeA) to reduce fearful responses or produce anxiolytic effects. In the interplay between these excitatory and inhibitory systems Orx activity in the BLA has the potential to enhance or diminish anxious and depressive behaviors. We propose that orexinergic functions are organized in specific local BLA neurocircuitry so as to produce opposing actions and functional responses via the two Orx receptor subtypes. To facilitate the discovery of the trigger that initiates the change between the orexin receptor subtypes we have designed experiments that utilize a model limited or chronic social defeat to produce threshold or dramatic changes in social anxiety, general anxiety, despair, and anhedonia. To test the effects of Orx1 and Orx2 systems on these behaviors, we employ two approaches: pharmacological and shRNA knockdown of gene expression in mice. Antagonists of Orx1 and Orx2 receptors, Orx1 stimulation via OrxA + Orx2 antagonist, and an Orx2 agonist are applied bilaterally to the BLA, examining dose response, social defeat and a battery of tests for anxious and depressive behavior: Social preference (social anxiety, depressive behavior), Elevated plus maze and open field (general anxiety), sucrose preference test (anhedonia, depression) and forced swim (despair, depression). To demonstrate the role of endogenous orexin in these behaviors, AAV vectors for shRNA will be used to bilaterally knockdown gene expression of Orx1 and Orx2 receptors in the BLA, followed by social defeat and the battery of behavioral tests. Using these techniques (without social defeat) we have demonstrated that shRNA KD of the BLA Orx2 receptor significantly enhances anxious behavior.
Anxiety and depression are psychological disorders that commonly occur together, and affect more than 40 million Americans (nearly 20% of the population). While monoamine transmitter systems, stress hormones, and neurotrophins explain much of the underlying mechanics of the symptoms of anxiety and/or depression once they have been expressed, no theory of affective disorder origin or cause adequately explains how they are acquired in normally behaving individuals. Orexins are small proteins found in the brain that may provide the triggering mechanisms that induce or end the symptoms of affective disorders by means of two receptor subtypes with opposing functions within the basolateral amygdala.
