Friday, October 31, 2008

Circadian clock may be critical to learning and memory

Circadian clock may be critical to learning and memory



Working with Siberian hamsters, biologist Norman Ruby has shown that having a functioning circadian system is critical to the hamsters' ability to remember what they have learned. Without it, he said, "They can't remember anything." ...

The change in learning retention appears to hinge on the amount of a neurochemical called GABA, which acts to inhibit brain activity. All mammal brains function according to the balance between neurochemicals that excite the brain and those that calm it. The circadian clock controls the daily cycle of sleep and wakefulness by inhibiting different parts of the brain by releasing GABA.

But if the hippocampus—the part of the brain where memories are stored—is overly inhibited, then the circuits responsible for memory storage don't function properly. "Those circuits need to be excited to strengthen and encode the memories at a molecular level," Ruby said.

"What I thought was happening was that our animals were having chronically high levels of GABA because they had lost their circadian rhythm," Ruby said. "So instead of rhythmic GABA, it is just constant GABA output."

To test that idea, Ruby and his colleagues gave the circadian-deficient hamsters a GABA antagonist called pentylenetetrazole, or PTZ, which blocks GABA from binding to synapses, thereby allowing the synapses to continue firing and keeping the brain in a more excited state. It worked. The learning-impaired hamsters caught up with their intact peers to exhibit the same level of learning retention.

...
The finding is even more striking when you consider that when a hamster loses its circadian system, it gets even more sleep than usual.

"What our data are showing is that these animals still performed terribly on a simple learning task, even though they're getting loads of sleep," Ruby said. "What this says is that the circadian system really is necessary for something that is deeply important: learning."