Behavioral and Neurochemical Effects of Nociceptin/Orphanin FQ Receptor Activation in the Social Defeat Protocol

The nociceptin/orphanin FQ receptor (NOP receptor) has wide expression in the nervous system and is involved in neurotransmitter release. However, the role of the NOPR in depression is not widely recognized. This study aims to evaluate behavioral and biochemical effects of the NOPR agonist Ro 65-6570 in mice submitted to social defeat protocol. The open-field test, social interaction test, and tail suspension test were applied to evaluate depressive behavior in male Swiss mice. Blood and brain tissue samples were obtained to evaluate the oxidative stress. The NOP agonist, Ro 65-6570 (1 mg/kg), or the social defeat stress reduced exploration rate in the open-field test. The social defeat stress and/or the NOP agonist also increased immobility time in the tail suspension test and the grooming time, as well as reduced the social interaction on the last day of social defeat protocol. Seven days after the end of the protocol, only the drug alone was able to affect the animals' interaction. Additionally, the NOP agonist increased the concentration of carbonyl groups (CGs) in hippocampus and malondialdehyde in serum. The stress of social defeat and the NOP agonist, together, increased malondialdehyde in animals' serum and prefrontal cortex, as well as increased the CGs concentration in the prefrontal cortex. These findings indicate a chronic depressive effect induced by the NOPR activation, sometimes regardless of the social defeat stress. We suggest that the NOPR signaling can activate pathways involved in cellular oxidative stress, contributing to the depression pathology.