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Sensory experience changes the way humans see the world
during early postnatal development. Now, an international team
of researchers, including Tadaharu Tsumoto from the RIKEN
Brain Science Institute in Wako, may have an explanation. They
report in the journal Neuron that visual experience drives the
maturation of inhibitory synapses in the visual cortex during a critica
l period of early postnatal development by activating endocannabinoid
receptors1. “This maturation makes synaptic transmission more
reliable,” explains Tsumoto. These findings may be crucial to
understanding the changes in the way the brain processes
visual information from early postnatal life to puberty.
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The researchers electrically stimulated the visual cortex and
measured the resulting inhibitory currents in the superficial layer
of visual cortex slices from 3-week-old rats, a time during development
soon after the initial opening of the eyes. High-frequency
stimulation of the slices—similar to input that visual cortex neurons
may receive during visual experience—led to a long-lasting drop
in the amount of inhibitory current that the stimulation could elicit.
This is called long-term depression of inhibitory currents (iLTD).
Tsumoto and colleagues could not induce iLTD in the visual cortex
of 5-week-old rats, a stage in development akin to puberty. The
researchers therefore realized that there was a ‘critical period’
for the induction of visual cortex iLTD that occurred after eye opening.
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Interestingly, Tsumoto and colleagues were able to delay the onset of the
critical period by raising the rats in darkness, and even brief exposure
of dark-reared rats to light induced the loss of iLTD in the visual cortex.
The research team realized that visual experience therefore plays an
important role in regulating the timing of the critical period.
Endocannabinoids are signaling molecules that regulate neuronal
activity throughout the nervous system. The researchers showed
that a drug that activates endocannabinoid receptors can induce the
loss of iLTD in the visual cortex in dark-reared animals that would
normally still exhibit iLTD, while a drug that blocks endocannabinoid
signaling could inhibit the loss of iLTD that is normally induced by
light. This suggested to Tsumoto and colleagues that visual experience
drives endocannabinoid signaling to induce iLTD during visual
cortex development.
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The researchers argue that endocannabinoid-mediated iLTD seems
to be important for the maturation of inhibitory synapses that occurs
during development of the visual cortex. Tsumoto suggests that this
maturation could help to “make responses of neurons in the visual
cortex selective to a particular feature of visual stimuli,” such as
orientation of contours or direction of movement.
