Effect of genetic deletion of nociceptin/orphanin FQ receptors on spatial and associative memory in rats.

Rationale: A growing body of evidence suggests that the N/OFQ-NOP receptor system modulates learning and memory in rodents, with NOP receptor agonists impairing memory and antagonists reversing this effect. Moreover, previous studies on transgenic mice showed that genetic deletion of the NOP receptor enhances spatial and aversive memory.

Objectives: To further investigate the role of the NOP receptor system in learning and memory, we used a rat model to assess how genetic deletion of NOP receptors affects spatial and associative memory, comparing it with a wild-type (WT) control group.

Methods: Male Wistar Han and NOP rats were tested for spatial memory using the classical Morris Water Maze (MWM) test. A modified MWM was then used to assess cued learning and associative memory. Conditioned place aversion (CPA) further evaluated associative memory. Anxiety-like behavior and motor skills were tested using the Elevated Plus Maze (EPM), Open Field, and Rotarod tests.

Results: NOP rats displayed impaired acquisition in the spatial and cued MWM tasks but performed comparably to controls in CPA and spatial MWM retrieval. Notably, NOP rats exhibited an anxiogenic-like phenotype in the MWM, EPM, and OF tests, without showing any motor impairment.

Conclusions: Deletion of NOP receptors impairs spatial and associative memory acquisition in the MWM, but this is likely not due to a direct cognitive deficit. Instead, our data suggest that these impairments depend on the anxiogenic-like phenotype observed in NOP rats. These findings highlight the complex interplay between anxiety and memory processes in the context of NOP receptor signaling.