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 critical 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.
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.
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.
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.
The corresponding author for this highlight is based at the Laboratory for Cortical Circuit Plasticity, RIKEN Brain Science Institute
Jiang, B., Huang, S., de Pasquale, R., Millman, D., Lee, H.-K., Tsumoto, T. & Kirkwood, A. The maturation of GABAergic transmission in visual cortex requires endocannabinoid-mediated LTD of inhibitory inputs during a critical period. Neuron 66, 248–259 (2010).
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