In their experiment, the researchers examined VIP (vasoactive intestinal polypeptide-expressing) cells, a type of CA1 neuron, in the hippocampus. VIP cell activity was first monitored while mice ran on a treadmill with various familiar and new stimuli. Then, the researchers hid a water reward at a specific location along the treadmill path. Elevated VIP cell activity was observed both while the mice were solely running the treadmill and when they were seeking the water reward. Normally, in the brain, CA1 excitatory neurons are kept off by a group of inhibitory neurons. But when VIP cell activity rises, the group of inhibitory neurons are shut off, allowing CA1 excitatory neurons to switch on. This activates the CA1 memory circuit and subsequently allows the mice to learn. Based on these results, the researchers classified VIP neurons as “disinhibitory” neurons.
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