Neuroscientists at the University of Massachusetts at Amherst examining genetically identified neurons in the forebrain of a songbird found a remarkable landscape of physiology, auditory coding, and network roles that mirrored those of the mammalian brain.
The research, published May 13 in Current biology, provides a better understanding of the fundamental functioning of complex brain circuits. This suggests that ancient types of cells in the pallium – the outer regions of the brain that include the cortex – most likely retained features over millions of years that are the building blocks of advanced cognition in birds and mammals. .
“As neuroscientists, we understand that birds can do sophisticated things and that they have sophisticated circuitry to do those things,” says behavioral neuroscientist Luke Remage-Healey, associate professor of psychological and brain sciences and lead author of the article.
For the first time, the team of neuroscientists, including lead author Jeremy Spool, who worked as a National Institutes of Health (NIH) postdoctoral fellow in the Remage-Healey lab, used viral optogenetics to define the molecular identities of excitatory and inhibitory cell types in zebra finches (Taeniopygia guttata) and relate them to their physiological properties.
“In the songbird community, we have long had the intuition that when we record the electrical signatures of these two types of cells, we say – ‘this is a putative excitatory neuron, this is a putative inhibitory neuron.’ We now know that these characteristics are based on molecular truth, ”explains Remage-Healey. “Without being able to identify the types of cells with these viruses, we would not be able to learn how the characteristics of the cells and the network resemble those of mammals, because the brain architectures are so different.”
The research team used viruses from a collection curated by co-author Yoko Yazaki-Sugiyama of the Okinawa Institute of Science and Technology in Japan to conduct viral optogenetic experiments in the brain. With optogenetics, the team used flashes of light to manipulate one type of cell independently of the other. The team targeted excitatory and inhibitory neurons (using CaMKII ™ and GAD1 promoters, respectively) in the auditory pallium of the zebra finch to test predictions based on mammalian pallium.
“There is so much work on the physiology of these different cell types in the mammalian cortex that we were able to line up a series of predictions with what characteristics birds may or may not have,” says Spool.
The CaMKII? and the songbird populations of GAD1 were distinct “in exactly the proportions you’d expect from mammalian brains,” Spool explains. Once the cell-type populations were isolated, the researchers then systematically examined whether each population would match the physiology of their mammalian counterparts.
“As we went along, these cell populations acted over and over again as if they were basically from the mammalian cortex in a number of physiological ways,” says Spool.
Remage-Healey adds: “The correspondence between the cortex in mammals and what we draw with the cell types identified molecularly in birds is quite striking.”
In birds and mammals, these neurons are believed to support advanced cognitive functions, such as memory, individual recognition, and associative learning, Spool explains.
Remage-Healey says the research, supported by NIH grants, is helping to delineate “the basic inner workings of the brain.” Knowing about nuts and bolts lays the foundation necessary to develop breakthroughs that could lead to neurological interventions for brain disorders.
“It can help us understand what brain diversity is by unpacking these circuits and the ways they can go wrong,” says Remage-Healey.