Researchers have identified a control mechanism in the brain that processes sensory and emotive information that humans experience as "disappointment".
"The idea that some people see the world as 'a glass half empty' has a chemical basis in the brain," said Roberto Malinow, a professor at University of California, San Diego School of Medicine.
"What we have found is a process that may dampen the brain's sensitivity to negative life events," he added.
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As people struggling with depression are believed to register negative experiences more strongly than others, the findings have implications for understanding not just why some people have a brain chemistry that predisposes them to depression, but also how to treat it.
In experiments with rodents, researchers discovered that neurons feeding into a small region above the thalamus known as the lateral habenula (LHb) secrete both a common excitatory neurotransmitter, glutamate, and its opposite, the inhibitory neurotransmitter GABA.
Excitatory neurotransmitters promote neuronal firing while inhibitory ones suppress it, and although glutamate and GABA are among two of the most common neurotransmitters in the mammalian brain, neurons are usually specialists, producing one but not both kinds of chemical messengers.
"Our study is one of the first to rigorously document that inhibition can co-exist with excitation in a brain pathway," said lead author Steven Shabel.
"In our case, that pathway is believed to signal disappointment," he added.
The discovery was reported online in the journal Science.