Excitatory neurons sculpt and refine maps of the
external world throughout development and experience, while inhibitory
neurons form maps that become broader with maturation. Networks of inhibitory brain cells or
neurons develop through a mechanism opposite to the one followed by
excitatory networks, discovered scientists.
adds a new piece to the puzzle of how the brain organizes and processes
information. Knowing how the normal brain works is an important step
toward understanding the nature of neurological conditions and opens the
possibility of finding treatments in the future. The results appear in
‘Networks of inhibitory brain cells or neurons develop through a mechanism opposite to the one followed by excitatory networks.’
"The brain represents the external world as specific maps of
activity created by networks of neurons," said senior author Dr.
Benjamin Arenkiel, associate professor of molecular and human genetics
and of neuroscience at Baylor College of Medicine, who studies neural
maps in the olfactory system of the laboratory mouse. "Most of these
maps have been studied in the excitatory circuits of the brain because
excitatory neurons in the cortex outnumber inhibitory neurons."
The studies of excitatory maps have revealed that they begin as a
diffuse and overlapping network of cells. "With time," said Arenkiel,
"experience sculpts this diffuse pattern of activity into better defined
areas, such that individual mouse whiskers, for instance, are
represented by discrete segments of the brain cortex. This progression
from a diffuse to a refined pattern occurs in many areas of the brain."
In addition to excitatory networks, the brain has inhibitory
networks that also respond to external stimuli and regulate the activity
of neural networks. How the inhibitory networks develop, however, has
remained a mystery.
In this study, Arenkiel and colleagues studied the development of
maps of inhibitory neurons in the olfactory system of the mouse.
Studying inhibitory brain networks of the mouse sense of smell
"Unlike sight, hearing or other senses, the sense of smell in the
mouse detects discrete scents from a large array of molecules," said
Arenkiel, who is also a McNair Scholar at Baylor.
Mice can detect a vast number of scents thanks in part to a
complex network of inhibitory neurons. Inhibitory neurons are the most
abundant type of cells in the mouse brain area dedicated to process
scent. To support this network, newly born inhibitory neurons are
continually added and integrated into the circuits.
Arenkiel and colleagues followed the paths of these newly added
neurons in time to determine how inhibitory circuits develop. First,
they genetically labeled the cells so they would glow when the neurons
were active. Then, they offered individual scents to the mice and
visually recorded through a microscope the areas or networks of the
brain that glowed for each scent the live, anesthetized animal smelled.
The scientists repeated the experiment several times to determine how
the networks changed as the animal learned to identify each scent.
The scientists expected that inhibitory networks would mature in a
way similar to that of excitatory networks. That is, the more the
animal experienced a scent, the better defined the networks of activity
would become. Surprisingly, the scientists discovered that the
inhibitory brain circuits of the mouse sense of smell develop in a
manner opposite to the excitatory circuits. Instead of becoming narrowly
defined areas, the inhibitory circuits become broader. Thanks to this
new finding scientists now better understand how the brain organizes and
Arenkiel and colleagues think that the inhibitory networks work
hand-in-hand with the excitatory networks. They propose that the
interaction between excitatory and inhibitory networks could be compared
to a network of roads (excitatory networks) whose traffic is regulated
by a network of traffic lights (inhibitory networks). The scientists
suggest that the formation of useful neural maps depends on inhibitory
networks driving the refinement of excitatory networks, and that this
new information will be essential towards developing new approaches for
repairing brain tissue.