Modulation of Synaptic Plasticity in Basolateral Amygdala can Mediate Anxiety-like and Depressive-like Behaviors in Mice

Abstract

Depression and anxiety are worldwide mental issues, causing millions of suicides every year. Current antidepressants and anxiolytics fail to produce long-term treatment effects. Some studies showed around 29% to 46% of patients displayed partial responses or even no response to current treatments. It is urgent to find more effective antidepressants and anxiolytics. In this dissertation, I show evidence that either blocking excitatory plasticity or inducing inhibitory plasticity in BLA can reduce anxiety-like and depressive-like behaviors in mice.

CCK has been proved to be critical in long-term potentiation (LTP) induction which is the major form of synaptic plasticity and molecular basis of memory formation. BLA is regarded as emotion hub, which is responsible for emotional memory encoding, consolidation, and retrieval. Stress-induced dysfunction of synaptic plasticity in BLA has been widely reported to contribute to depression and anxiety. Therefore, we hypothesize that blocking the formation of aversive memory in BLA may produce antidepressant and anxiolytic effects.

We first showed CCK and CCKBR signaling are necessary in LTP induction in BLA. Then, we found CCKBR neurons in BLA are hyperactive after acute stress, suggesting CCKBR signaling in BLA was activated when mice were stressed. It indicates that CCKBR signaling in BLA is involved in processing stress information. We further infused CCKBR antagonists (YM022) into BLA to block the excitatory plasticity and found it can prevent mice from depression and anxiety induced by chronic social defeat stress (CSDS).

We next wondered where the CCKBR neurons in BLA receive CCK from. A previous study found the circuit EC^CCK → LA mediates the formation of trace fear memory and LA is a part of BLA, we therefore hypothesize that CCK released from EC CCK terminals in BLA may be necessary to depression and anxiety development. We used the anterograde tracing strategy by injecting Cre-dependent virus into EC of CCK-Cre mice and we found abundant CCK terminals in BLA. We also used the retrograde tracing technique to confirm that CCK neurons from several brain regions send projections to BLA, including medial prefrontal cortex, entorhinal cortex (EC), auditory cortex, hippocampus, bed nucleus of the stria terminalis, lateral septal nucleus and ectorhinal cortex. We further used fiber photometry to confirm high frequency laser stimulation (HFLS) of EC^CCK terminals in BLA can cause CCK release. Then, we adopted the CRISPR-Cas9 method to delete the CCK gene in EC and we found after HFLS of EC^CCK terminals in BLA, mice susceptibility was significantly reduced, as shown by less antidepressant-like and anxiety-like behaviors than mice with the intact circuit-specific CCK gene. These results suggest a novel circuit EC^CCK → BLA^CCKBR regulates depression and anxiety.

Many studies reported the coexistence of impaired fear extinction and anxiety, and the behavioral training of fear memory extinction has been applied in clinical studies for anxiety disorders. It suggests a possibility that facilitating the extinction of aversive memory may produce anxiolytic effects. Previous studies have shown that both inhibitory transmission and inhibitory plasticity contribute to fear extinction. Our lab recently identified a novel CCK receptor (GPR173) which localized at CCKGABA synapses and mediated the inhibitory long-term potentiation (iLTP) in neocortex. As CCKGABA neurons are one of the major inhibitory neurons in BLA, we assume that CCK-IN-mediated GABAergic transmission and plasticity in BLA may regulate anxiety-like behavior in mice.

To examine the assumption above, we first confirmed CCK interneurons (CCK-INs) sent abundant projections to local glutamatergic neurons, indicating the CCK-INs may modulate the activities of local glutamatergic neurons. And then, we used chemo-genetics to activate the CCK-INs neurons in BLA and found it can significantly suppress electrical stimulation induced local neuronal activities and reduce acute stress induced anxiety-like behaviors in mice. Besides, we adopted high frequency laser stimulation (HFLS) of CCK-INs in BLA and found it can induce iLTP which can be blocked by GPR173 antagonists. Also, we found low frequence laser stimulation (LFLS) failed to induce iLTP but extra CCK8s application can rescue it. These results suggest iLTP in BLA is CCK and GPR173 dependent. Moreover, we found that inducing iLTP by HFLS of CCK-INs in BLA can reduce stress-induced anxiety-like behaviors in mice.

In conclusion, this dissertation shows evidence that blocking the excitatory synaptic plasticity or inducing inhibitory synaptic plasticity in BLA produce antidepressant or anxiolytic effects, respectively. In other words, preventing the formation of aversive memories and facilitating the extinction of aversive memories could be two effective strategies to reduce depressive and anxious phenotypes. Future work may combine these two therapies to see whether it could have better antidepressant or anxiolytic effects.

Date of Award	5 Sept 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Jufang HE (Supervisor)