Scientists in the NIEHS Synaptic and Developmental Plasticity Group, led by Serena Dudek, Ph.D., have discovered a novel mechanism by which certain memories may be formed in the brain. Their report, published Jan. 25 in the journal Nature Communications, provides the first evidence of a specific mechanism for encoding social and contextual information in a part of the brain known as the CA2 region of the hippocampus.
Pioneering studies on hippocampal area CA2
Scientists in Dudek’s group were among the first to study area CA2. They suspected that neurons in the area are involved in developing memories pertaining to contextual information — place, time, experience —and social information, so they studied these neurons in active animals.
Georgia Alexander, Ph.D., and Shannon Farris, Ph.D., are co-lead authors on the study. According to Farris, it is widely accepted that the hippocampus is a critical region for encoding contextual information, but the role of the CA2 region had been completely unknown until recently. “Our findings support the notion that CA2 plays a role in processing novel and social information,” the authors wrote.
Dudek further described the insights. “We discovered that the CA2 neurons can rapidly update this information by actually revising their existing spatial representations,” she said.
The researchers also looked at the targets of the CA2 neurons. “Our study is the first to assess the function of CA2’s direct target, CA1 deep neurons, during social processing,” they wrote.
A two-pronged approach
To identify mechanisms in the CA2 region that could be associated with social memory, Alexander and Farris worked together on an extensive multi-year study. They used state-of-the-art methods to examine immediate early gene mapping, which measures neuronal activity over large populations of neurons.
The scientists also used in vivo single neuron recording, which involved placing electrical probes in CA2 or the adjacent CA1 region of the brains of adult male rats. Using these complementary approaches, they found that CA2 neurons were likely doing something subtler than simply increasing their firing rates to encode the information.
Alexander decided to look deeper into the neuronal activity. She found certain changes in the activity of CA2 neurons, in response to social or novel updates to the environment.
“Neuronal activity in the CA2 region may link social and spatial information,” Farris said, “providing a mechanism for learning social information, or the who that relates to the what, when, and where.”
The team joined efforts with the laboratory of Laura Colgin, Ph.D., at the University of Texas at Austin, where they performed similar studies using a different strain of rat. “They obtained strikingly similar results,” the authors wrote.
Citation: Alexander GM, Farris S, Pirone JR, Zheng C, Colgin LL, Dudek SM 2016. Social and novel contexts modify hippocampal CA2 representations of space. Nat Commun 7:10300.
(Robert Brown, Ph.D., is an Intramural Research and Training Award fellow in the NIEHS Cell Biology Group.)