CCK administration ameliorates memory encoding impairment of NR2A-KO transgenic mice by improving synaptic plasticity

Studies have shown that NMDA receptors (NMDARs) can mediate multiple forms of synaptic plasticity, including induction of long-term enhancement (LTP) or long- term inhibition (LTD) ^[1] . The NR2A subunit has a strong correlation with LTP production and can in turn induce learning-related behaviors. For example, NR2A knockout (NR2A-/-) mice exhibit some significant memory-deficit behaviors ^[2] . Therefore, it can be used as a valuable model for exploring the pathways and mechanisms involved in memory encoding and may also inform the treatment of memory disorders. The cholecystokinin (CCK) neuronal ensemble has been shown to be involved in memory imprinting function in multiple brain regions ^[3–6] . However, it remains unclear whether pharmacological modulation of CCK can improve memory- related deficits in NR2A knockout animals. This study utilizes a novel object recognition task and a context-dependent fear learning paradigm to evaluate the role of CCK in memory generalization in NR2A-KO mice. Our results show that NR2A- KO mice exhibit significantly impaired discrimination, learning, and fear conditioning. Notably, CCK manipulation enhanced the associative memory generalization and discrimination abilities of these animals. Similarly, we performed electrophysiological studies, including in vitro multichannel recordings, to examine the effects of CCK on LTP-induction in the cortex and hippocampus. Notably, CCK restored LTP in the cortex and hippocampus in NR2A-KO mice. In addition, monitoring of calcium (Ca2+) activity during memory recall using fiber photometry in the auditory cortex (AC) showed a significant enhancement of calcium signal after CCK administration. To test our hypothesis that the deletion of NR2A may affect the release of CCK, we used a specific CCKBR sensor system combined with optogenetic viruses to detect the release of CCK after high-frequency stimulation of the CCK axon terminus projected by LEC to AC, but the release of CCK was blocked by the application of NR2A antagonists. These results suggest that CCK not only compensates for the loss of LTP in NR2A-KO mice, but also improves the corresponding memory-deficit- like behavior in animals. Overall, our findings identify CCK as a promising target for the treatment of memory deficits and elucidate its role in memory encoding mechanisms.