Delta opioid receptors (DOPs) play an important role in depression and other mood disorders. However, little is known about the underlying physiological mechanisms. The hypothalamic-pituitary-adrenal axis, adult hippocampal neurogenesis, and neuroinflammation are regarded as key pathophysiological factors in depression. In this study, we investigated the influence of DOP activation on those factors in a valid animal model of depression, chronic vicarious social defeat stress (cVSDS) mice. cVSDS mice (male C57BL/6J mice) were produced following a 10-day exposure to witness of social defeat stress, and each evaluation was performed more than 28 days after the stress period. Repeated administrations to cVSDS mice with a selective DOP agonist, KNT-127, both during (10 days) and after (28 days) the stress period respectively improved their decreased social interaction behaviors and increased serum corticosterone levels. When administered during the stress period, KNT-127 suppressed decreases in the hippocampal newborn neuron survival rate in cVSDS mice. Moreover, in both administration paradigms, KNT-127 reduced the number of Iba-1- and CD11b-positive cells in the subgranular zone and the granule cell layer of the hippocampal dentate gyrus, indicating a suppression of cVSDS-induced microglial overactivation. These results suggest that KNT-127 acts over the hypothalamic-pituitary-adrenal axis and regulates neurogenesis and neuroinflammation resulting in anti-stress effects, and the antidepressant-like effects of the DOP agonist are implicated in the suppression of the neuroinflammation. This study presents a new finding on the effects of repeated DOP activations on the pathophysiological states of depression.
Keywords: Antidepressant; Delta opioid receptor; Hippocampus; Neurogenesis; Neuroinflammation; Vicarious social defeat stress.
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