Neurons that process sensory information such as touch and
vision are arranged in precise, well-characterised maps that are crucial for
translating perception into understanding.
A study published by Cell Press in the journal
Developmental Cell reveals that the actual act of birth in mice causes a
reduction in a brain chemical called serotonin in the newborn mice, triggering
sensory maps to form.
The findings shed
light on the key role of a dramatic environmental event in the development of
neural circuits and reveal that birth itself is one of the triggers that
prepare the newborn for survival outside the womb.
"Our results clearly demonstrate that birth has active
roles in brain formation and maturation," says senior study author Hiroshi
Kawasaki of Kanazawa University in Japan. "We found that birth regulates
neuronal circuit formation not only in the somatosensory system but also in the
visual system. Therefore, it seems reasonable to speculate that birth actually
plays a wider role in various brain regions."
Crucial role of serotonin
Mammals ranging from mice to humans have brain maps that
represent various types of sensory information. In a region of the rodent brain
known as the barrel cortex, neurons that process tactile information from
whiskers are arranged in a map corresponding to the spatial pattern of whiskers
on the snout, with neighbouring columns of neurons responding to stimulation of
Although previous studies have shown that the
neurotransmitter serotonin influences the development of sensory maps, its
specific role during normal development has not been clear until now.
In this new study, Kawasaki and his team find that the birth
of mouse pups leads to a drop in serotonin levels in the newborn's brain,
triggering the formation of neural circuits in the barrel cortex and in the
lateral geniculate nucleus (LGN), a brain region that processes visual
When mice were treated with drugs that either induced
preterm birth or decreased serotonin signalling, neural circuits in the barrel
cortex as well as in the LGN formed more quickly. Conversely, neural circuits
in the barrel cortex failed to form when the mice were treated with a drug that
increased serotonin signalling, suggesting that a reduction in levels of this
neurotransmitter is crucial for sensory map formation.
Because serotonin also plays a key role in mental disorders,
it is possible that abnormalities in birth processes and the effects on
subsequent serotonin signalling and brain development could increase the risk of
psychiatric diseases. "Uncovering the entire picture of the downstream
signalling pathways of birth may lead to the development of new therapeutic
methods to control the risk of psychiatric diseases induced by abnormal
birth," Kawasaki says.