Scientists reveal the dynamic change process of hippocampus cells caused by spatial location

Click on the blue word to pay attention to when we explore a new environment, animals will form spatial memories.
These memories will be updated with the renewal of experience and will be reactivated after being exposed to the same environment again.

Position cells in the hippocampus encode spatial memory information.
These neuron units that rely on spatial position-selective firing are called place fields (PFs).
Almost all hippocampal subregions (CA1, CA3, DG) have PFs during navigation.

CA1 pyramidal neurons quickly become PFs during the exploration of new environments, but it has always been a mystery whether CA3, which is the main input of CA1, has similar rapid dynamics.

On May 20, 2021, the Mark EJ Sheffield research team of the University of Chicago used two-photon microscopy in vivo imaging to monitor the dynamic changes (formation, disappearance, and stability) of PFs in the hippocampus CA1 and CA3 regions, and found that the CA1 brain region has more transient, Unstable PFs.
Mice are trained in VR virtual technology environment.
Researchers use VR virtual technology to place the mice in an unfamiliar and familiar environment.
On the first day, the mice are in the familiar environment for 10 minutes, and then they enter the unfamiliar environment A.
20 minutes; the next day, after the mice adapt to the familiar environment for 10 minutes, they then enter the unfamiliar environment B for 20 minutes.

The two-photon microscope observes the dynamic changes of the cells in the hippocampus brain area.
After the calcium ion indicator GCaMP6f is injected into the CA1 and CA3 brain areas, the dynamic changes of the neurons in the mice in the above environmental test are dynamically observed under the two-photon microscope.

The results found that about 30% of the pyramidal neurons in the CA1 area quickly converted to PFs under the stimulation of the new environment, while the proportion in the CA3 area was only 9%.

In addition, PFs in CA1 area appeared earlier than CA3.

It was further found that the PFs in the CA1 area were not stable in the new environment, and about half quickly degenerated into the original pyramidal neurons, while the PFs in the CA3 area were more stable than the CA1 area, and less than one-third transformed into pyramidal neurons.

Through the analysis of the neuron changes during the whole process of the above behavior, it is found that the conversion speed of CA1 neurons from PFs to pyramidal neurons in the new environment gradually decreases, and this conversion remains constant in the familiar environment.

The extraction process of spatial memory is considered to be the reactivation of stable PFs.

The researchers exposed the mice to the A environment and the B environment in turn.
During the transition from the A environment to the B environment, the mice seemed to be more familiar with the B environment.

In the above process, the proportion of stable PFs in CA3 area is higher than that in CA1.

When exposed to the previous environment again, the appearance of stable PFs appeared earlier than the appearance of newly formed PFs.

This time difference is more obvious in the CA3 brain area.

In general, this paper finds the different characteristics of pyramidal neurons in the hippocampus CA1 and CA3 regions of the hippocampus transforming to PFs in the process of spatial environment transformation.
CA1 is more able to adapt to new environmental changes, while CA3 is more familiar with environmental information.

It reveals the mechanism of the hippocampus encoding different types of spatial memory information.

[References] 1.
https://doi.
org/10.
1038/s41467-021-23260-3, the pictures in the text are all from the references