Strengthening during Waking Hours

The human brain expends up to 80 percent of its energy on synaptic activity, constantly adding and strengthening connections in response to all kinds of stimulation, explains study author Chiara Cirelli, associate professor of psychiatry. Given that each of the millions of neurons in the human brain contains thousands of synapses, this energy expenditure "is huge and can't be sustained." "We need an off-line period, when we are not exposed to the environment, to take synapses down," Cirelli say. "We believe that's why humans and all living organisms sleep.“

To test the theory, researchers conducted both molecular and electro-physiological studies in rats to evaluate synaptic potentiation, or strengthening, and depression, or weakening, following sleeping and waking times. In one set of experiments, they looked at brain slices to measure the number of specific receptors, or binding sites, that had moved to synapses. "Recent research has shown that as synaptic activity increases, more of these glutamatergic receptors enter the synapse and make it bigger and stronger," explains Cirelli.

The University of Wisconsin School of Medicine and Public Health group was surprised to find that rats had an almost 50 percent receptor increase after a period of wakefulness compared to rats that had been asleep.

In a second molecular experiment, the scientists examined how many of the receptors underwent phosphorylation, another indicator of synaptic potentiation. They found phosphorylation levels were much higher during waking than sleeping. The results were the same when they measured other enzymes that are typically active during synaptic potentiation.

"Taken together, these molecular and electro-physiological measures fit nicely with the idea that our brain circuits get progressively stronger during wakefulness and that sleep helps to recalibrate them to a sustainable baseline," says Cirelli.

The most popular notion these days, says Cirelli, is that during sleep synapses are hard at work replaying the information acquired during the previous waking hours, consolidating that information by becoming even stronger.; Source: University of Wisconsin-Madison