Gating of Social Behavior by Inhibitory Input from Hippocampal CA1 to Retrosplenial Agranular Cortex

Background

Social behavior is a fundamental requirement for mammalian survival and reproduction, requiring the perception of sensory information, processing of social relevance, and further integration in the prefrontal cortex. Neuropsychiatric disorders, such as Autism Spectrum Disorder (ASD), are closely associated with abnormal social behavior, severely affecting an individual’s quality of life. Recent studies have found that ketamine treatment increases neuronal activity in the retrosplenial cortex (RSC) and reduces social behavior in mice. In the MECP2 gene overexpression ASD animal model, abnormal upregulation of neuronal activity in RSC and enhanced functional connectivity between RSC and other brain regions were observed. Furthermore, our research group and Li et al. found significant changes in excitatory and inhibitory synaptic transmission in RSC in different genetic mouse models of ASD. These studies collectively suggest that RSC may play a role in regulating social behavior.

Research Overview

This study was conducted by Yuhan Shi, Jingjing Yan, Xiaohong Xu, Zilong Qiu, and others from Shanghai Jiao Tong University School of Medicine, Fudan University School of Life Sciences, and the Chinese Academy of Sciences Shanghai Branch. The article was published in “Neuroscience Bulletin” in 2024.

Research Objectives and Methods

This study aims to explore the direct role of the retrosplenial agranular cortex (RSA) in social behavior and further investigate whether the inhibitory input between RSA and the hippocampal CA1 region affects social behavior, especially in ASD models.

Animal Models and Experimental Design

The experiments used 5-6 week old male and female mice, including C57BL/6J, PV-Cre, and VGAT-Cre strains, as well as MEF2C-heterozygous mice from Qi Zhang’s laboratory at Zhejiang University. All experimental procedures were approved by the ethics committee.

cfos Immunostaining

The experiment verified RSA neuronal activity during social contact through cfos immunostaining. Mice were isolated for 12 hours before introducing a stranger mouse for interaction, compared with mice introduced to a new object. Immunostaining revealed significantly increased cfos expression in RSA neurons of mice interacting with stranger mice.

In Vivo Optogenetic Stimulation

To precisely control mouse RSA neuronal activity, the experiment used adeno-associated virus (AAV) expressing CHR2 in in vivo optogenetic stimulation, activating neurons with blue light and recording neuronal activity changes during social interaction.

Calcium Imaging

The experiment injected AAV expressing GCamp6s into mouse RSA and recorded calcium signals through optical fibers. This method was used to record real-time calcium dynamics of RSA neurons during conspecific contact.

Electrophysiological Recording

Using standard acute brain slice electrophysiology techniques, the experiment performed patch-clamp recordings on neurons in the RSA layer to verify inhibitory input from CA1.

Social Behavior Tests

The experiments included home cage tests and the classic three-chamber test to assess mouse social interaction behavior. Behavioral data were analyzed through video recording and analysis software (Ethovision XT).

Experimental Results

Up-Down Phases of RSA Neuronal Activity

Initially, researchers found through cfos immunostaining that RSA neurons were activated when interacting with stranger mice. However, calcium imaging results showed that calcium signals in RSA neurons rapidly decreased after social contact. This finding suggests that rapid inhibition of RSA neurons is crucial for normal social behavior.

They further used optogenetic techniques to inhibit or activate neurons in the RSA region and found that continuous activation of RSA neurons significantly reduced mouse social interaction, while inhibiting RSA neurons immediately after contact increased social interaction time. Additionally, results from the classic three-chamber test showed that continuous activation of RSA neurons led to loss of sociability and social novelty preference in mice.

PV-positive Neurons in Hippocampal CA1 Project to RSA

Through retrograde tracing and immunostaining techniques, the study found that PV-positive neurons in the hippocampal CA1 region project to RSA. Acute brain slice electrophysiological recordings confirmed functional synaptic connections between these PV-positive neurons and RSA neurons. Furthermore, immunostaining results showed that PV-positive neurons were activated during social interaction, which was further supported by calcium imaging results.

Importance of CA1-PV-RSA Inhibitory Input for Social Behavior

To verify the impact of this inhibitory input on social behavior, researchers used chemogenetic methods by locally injecting CNO (clozapine-N-oxide) into RSA to activate AAV-hM4D expressing neurons. Results showed that blocking the inhibitory input from CA1 PV-positive neurons to RSA significantly impaired mouse social behavior.

Intervention Effects in Autism Models

In MEF2C+/− mice, the loss of the MEF2C gene leads to social behavior deficits. Through optogenetic techniques, researchers found that activating the CA1-PV-RSA pathway immediately after social contact could significantly improve social behavior deficits in MEF2C (+/-) mice.

Conclusions and Significance

This study reveals the role of RSA in regulating social behavior and proposes the hypothesis that PV-positive neurons in the hippocampal CA1 region filter non-social information by inhibiting RSA, providing a new perspective for understanding the neural mechanisms of social behavior. Additionally, this study offers potential targets for intervention in neuropsychiatric disorders such as autism.

By precisely regulating neural circuit activity, this research provides new possibilities for improving social behavior in autism patients. This discovery not only helps understand the neural basis of social behavior but also provides scientific basis for developing new treatment methods in the future.

Highlights

1. First revelation that PV-positive neurons in the CA1 region project to RSA and regulate social behavior by filtering out non-social information.
2. Precise regulation of this neural circuit through optogenetic and chemogenetic techniques, thereby verifying its function in social behavior regulation.
3. Activation of this inhibitory pathway significantly improved social behavior deficits in autism mouse models, providing new ideas for future autism treatment.