The neuronal connections in the human brain are more complex than anything we can build artificially right now. But researchers are learning more about the principles that underlie the circuitry of the brain, and have now mapped the development of neural circuits that are inhibitory, which dampen signals.
Reporting in Science, researchers utilized thirteen datasets that were created using the cortex of developing mice in several stages: after birth, and as they grew into the equivalent of infants, children, teenagers, and young adults. Using a 'connectomic' method, they mapped the circuitry of neurons in the cerebral cortex's gray matter, where many cerebral synapses are located. The scientists focused on interneurons, which are known to specifically inhibit the activity of other neurons, to track the development of synapses involving these interneurons.
"Surprisingly, different types of interneurons followed very different time courses to establish their favorite synaptic partners. Some were able to innervate their synaptic targets with adult-like preference already in the first investigated circuit stages that correspond to baby brains. This happened immediately when the first chemical synapses were formed in the cortical gray matter. Others showed steep improvements of target choice, which were most likely caused by [the] removal of incorrectly placed synapses," explained the first study author Anjali Gour, a graduate candidate at Max-Planck-Gesellschaft.
In some regions of the brain, development is not only about growing new neurons and synapses, but also about pruning them. This study showed that pruning is crucial to the formation of inhibitory circuits, a surprise to the researchers. They also found that one type of interneuron known as Chandelier neurons innervate their synaptic partner far earlier than was known.
Connectomes are created using brain tissue that is frozen in place, and not living tissue that changes over time. However, measurements could still be taken in the same area in different animals at different time points.
"That we were able to still extract a clear developmental profile from this data illustrates the density of information present in connectomic data," said Gour. "I would not have predicted that we would find such clear circuit patterns in brains that are still under development."
A better understanding of the neuronal network in the brain can help us understand what happens when they become disrupted, or how they may influence the development of brain disorders. Aberrant neuronal circuits are thought to be involved in some psychiatric disorders.
"We hope to be able to map much more precisely the normal and disrupted network formation in cortical circuits for understanding possible alterations in psychiatric disease, and possibly identify the phenotypes of connectopathies," said Max Planck Director Moritz Helmstaedter. Connectopathies is a term for disorders that trace back to problems in how neurons are connected.
Connectomics may one day become as important to diagnostics as genetics, noted the researchers.