Targeting Nociceptin/Orphanin FQ receptor to rescue cognitive symptoms in a mouse neuroendocrine model of chronic stress

Chronic stress causes cognitive deficits, such as impairments in episodic-like hippocampus-dependent memory. Stress regulates an opioid-related neuropeptide named Nociceptin/Orphanin FQ (N/OFQ), the ligand of the G protein-coupled receptor NOP. Since this peptide has deleterious effects on memory, we hypothesized that the N/OFQ system could be a mediator of the negative effects of stress on memory. Chronic stress was mimicked by chronic exposure to corticosterone (CORT). The NOP receptor was either acutely blocked using selective antagonists, or knocked-down specifically in the hippocampus using genetic tools. Long-term memory was assessed in the object recognition (OR) and object location (OL) paradigms. Acute injection of NOP antagonists before learning had a negative impact on memory in naive mice whereas it restored memory performances in the chronic stress model. This rescue was associated with a normalization of neuronal cell activity in the CA3 part of the hippocampus. Chronic CORT induced an upregulation of the N/OFQ precursor in the hippocampus. Knock-down of the NOP receptor in the CA3/Dentate Gyrus region prevented memory deficits in the CORT model. These data demonstrate that blocking the N/OFQ system can be beneficial for long-term memory in a neuroendocrine model of chronic stress. We therefore suggest that NOP antagonists could be useful for the treatment of memory deficits in stress-related disorders.

Acute single non-sedative doses of NOP receptor agonists affect acquisition of object location memory but repeated high doses do not induce long-lasting deficits

The Nociceptin/Orphanin FQ (N/OFQ) system has been shown to modulate various aspects of long-term memory. It is therefore important to study the effects on memory impairment by nociceptin receptor (NOP) agonists under preclinical development. In the present study, we investigated the effect of systemic injection of two small molecule selective NOP agonists, AT-202 and AT-524, in the object location memory task in male and female mice. Since high doses of NOP agonists have been shown to induce sedation, we first determined the sedative doses for the two compounds and found them to be higher in female than in male mice. We then observed that sub-sedative doses of NOP agonists administered before learning, induced memory impairment during a test session performed 24 h later. Again, female mice were less sensitive to the amnesic effects than males. On the contrary, in male mice, NOP agonists did not produce amnesia when they were injected after learning, suggesting that they do not affect the consolidation of object location memory. Finally, repeated administration of high doses of NOP agonists over 7 days did not impair long-term spatial memory. Together, our data show for the first time that NOP receptor agonists impair the acquisition of object location memory with sex-dependent potency but do not affect memory consolidation, and that repeated stimulation of the receptor does not compromise long-term episodic-like spatial memory.