It has been long known that the part of brain essential to social memory and interaction is hippocampus, pair of horse shaped structures present in temporal lobes. This was illustrated in 1953 by the case of Henry Molaison whose hippocampus was removed by surgeons in order to cure the severe epilepsy he suffered from, after the removal of hippocampus he was unable to form social memories and thus unable to remember people.
Recently, study at Columbia University Medical Center (CUMC) showed that the different parts of hippocampus played different roles regarding social behaviors, for example – a region known as ‘dentate gyrus‘ is essential for distinguishing between similar enviroments, another part called CA3 is important for formation of partial cues and CA1 is critical for all kinds of memories. This research illustrated the function of CA2, a small region of hippocampus, the exact function of which scientists previously had not known.
Some studies, however had shown CA2 to be involved in social memory as this area has a high level of expression for receptors for vasopressin, a hormone important for sexual motivation, social attachment and other social behaviors. The function of CA2 has been shown by the research team of of senior author Steven A. Siegelbaum, PhD, professor of neuroscience and pharmacology, chair of the Department of Neuroscience, a member of the Mortimer B. Zuckerman Mind Brain Behavior Institute and Kavli Institute for Brain Science, and a Howard Hughes Medical Institute Investigator.
The team created transgenic mice in which CA2 neurons had been inhibited. After inhibition of neurons, the mice were given a series of behavioral tests. The mice performed normal on all tests except the tests concerning social memories. Normally mice are naturally curious about a mouse hey have never met and thus spend more time investigating the unfamiliar mouse but the mice with inactivated CA2 region showed no preference for any mouse, whether previously known or the new one, indicating a lack of social memory.
To confirm the role of CA2 in social memory, two other tests were given. In one the mice showed normal curiosity about the objects they had not seen before proving the CA2 only plays role on Memories involving the member of the same species and the transgenic mice didn’t lack complete interest in novelty. In the other test, the mice showed no difference than normal mice in interest of detecting the odors of familiar and non familiar mice. This test was to conform that the sense of olfaction had nothing to do with the inhibition of CA2 neurons (Olfaction in mice is important for social interaction).
Previously, scientists have observed that lesions in hippocampus are associated with impaired social memory both in humans and rodents. Not only that but decreased number of CA2 inhibitory neurons are found in individuals with schizophrenia and bipolar disorder and altered vasopressin signaling in autism.
These finding will prove helpful in understanding and finding cures for several neuropsychiatric disorders associated with altered social behaviors.
[Image Credit: CUMC]