Miniscope Recording Calcium Signals at Hippocampus of Mice Navigating an Odor Plume

My research aims to understand how the brain processes olfactory and spatial information. And we're trying to understand the role of hippocampal CA1 neurons in odor-plume navigation. Currently, the technology of freely-moving recordings of neurons with miniature microscopes has been used to advance research in the field.

We found that's possible to decode the trajectory of the mouse navigating odor-plume based on neurons and calcium signals in dorsal CA1. This technique combines the advantages of mini scope technology for recording GCaMP calcium signals with the well-established row of CA1 hippocampus spatial navigation. to understand better how neural circuits drive complex behaviors, We will investigate how odor-plume navigation is impaired in a mouse model of Alzheimer's disease with abnormal CA1 hippocampus function.

To begin, construct a chamber with two acrylic walls, and acrylic ceiling, and a wide-expanded polyvinyl chloride floor. The other two unique walls at the front and rear must facilitate airflow. Position four sets of odor sources paired with water delivery spouts 10 centimeters apart along the X-axis.

Install a fast digital camera above the arena to monitor animal behavior. Use a custom python code to manage the odor arena hardware, and the software integrates the camera and all hardware for setting up the experiment. Set up the digital camera to export a clock signal when recording video frames for post-hoc synchronization with the mini scope.

Place a fast response miniature photoionization detector, or PID, close to the odor source, and position another one 10 centimeters farther away. Change the gain switch on the front panel of the PID controller to the X five position. Then change the pump switch on the front panel of the PID controller to the high position.

Check the light-emitting diode or LED status light on the front panel of the controller to ensure the sensor output shows zero voltage in the absence of odorants. Switch the potentiometer offset to zero, the voltage output in the absence of odorants, and turn on the odor valve in the odor arena. Measure the delay in detecting the odor plume with the PID at each location after opening the valve.

To begin, set up the chamber, camera, and the photoionization detector, or PID sensors, for the experiment. For training the mouse, prompt it to move to the back of the arena. After the mouse reaches the back, manually deliver odor and water in a random lane, and let the mouse locate the source and drink the water.

Once the mouse learns to initiate trials, switch to automated software for odor delivery. In the two-lane odor navigation task, randomly select one of two odor ports to deliver odor. And reward the mouse with water when it reaches the correct water spout Head fix the mouse and place the mini scope on top of the base plate using a micro manipulator.

Tighten the set screw to secure the mini scope. Adjust the electrowetting lens to find the optimal focal plane, ensuring the largest number of cells with the highest fluorescence intensity. To obtain optimal dynamic range, use dorsal CA1 and tie one GCaMP six-F mice, setting the mini scope power to around 30%at a 30 hertz acquisition rate.

Release the mouse inside the odor arena with the mini scope attached to the base plate. Start acquisition with the interface board to record the transistor-transistor logic, or TTL output of the digital camera, and the mini scope for synchronization. Begin recording the mini scope and behavioral movies, and turn on the automated software for the two-spout odor navigation task.

Then synchronize the odor arena metadata, recorded digital camera frames, and mini scope frames using the MATLAB code synchronize_files_jove.m. Using norm correction, perform motion correction of the synchronized mini scope frames. Identify the regions of interest with time varying delta F by F zero signals using extract.

Use behavior ensemble and neural trajectory observatory to visualize the behavior and regions of interest of each separate trial. The PID response increased significantly upon release of the odor plume, indicating the timing of the odor delivery. Multiple calcium transients were observed in the mouse's dorsal CA1 during odor navigation, correlating with odor and water-reward events.

Calcium responses were associated with different stages of the navigation task, including trial start, decision making, and returning. The spatial trajectory of the mouse was decoded from the calcium signals, revealing the dorsal CA1's role in mapping odor and spatial information.