Chemically induced hibernation (QIH) in mice eradicates more than half of their synaptic connections without impairing memory retrieval upon arousal, according to research from Tanaka’s laboratory. Scientists implanted tetrodes into the hippocampus of freely moving mice to record individual neurons firing, discovering that brain activity drops by about 70 percent once hibernation set in.
How Synaptic Purge Affects Hippocampal Memory Storage
During a 48-hour period of chemically induced hibernation, brain tissues imaged with serial block-face scanning electron microscopy showed that the process eradicated more than half of the synapses in the subject animals. According to Tanaka, this massive synaptic loss should theoretically erase most stored memories if memory traces reside purely in the efficacy of individual synapses.
However, behavioral testing revealed no such impairment. Before entering hibernation, the mice underwent training on two standard memory tasks: contextual fear conditioning, where animals learn to associate a box with a mild electric shock, and a plus-maze task requiring navigation to a reward. Following hibernation and subsequent arousal, the subjects performed on these tasks just as well as control mice that did not hibernate.
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Place cells—hippocampal neurons that fire when an animal occupies a specific location—still fired in the exact same locations after arousal, proving that memory traces remained stable despite the structural purge of synapses.
Neural Activity Recordings Confirm Intact Place Cell Function
The behavioral findings were backed up by direct brain activity recordings. Researchers utilized a decoder that read out population activity from the hippocampus, allowing them to reconstruct where the mouse was located just as accurately after hibernation as before.
To confirm that these specific tasks relied on the hippocampus, the research team created a lesion in the region after training, which successfully caused the memories to disappear. This control step verified that the preserved memories observed after QIH were indeed dependent on the hippocampal networks that underwent the 70 percent drop in activity and subsequent synaptic reduction.
Frequently Asked Questions
What is chemically induced hibernation (QIH)?
Chemically induced hibernation is an experimental state that can be switched on and off at will, causing test subjects to enter a hibernation-like state for a set duration—such as 48 hours in Tanaka’s experiments—before waking up.
Does losing synapses during hibernation destroy memory?
According to Tanaka’s team, despite the eradication of more than half of the synapses during QIH, subjects showed no memory impairment and performed just as well on spatial and contextual tasks as non-hibernating controls.
How did researchers measure brain activity during QIH?
Scientists implanted tetrodes, which are bundles of fine electrodes, into the hippocampus of freely moving mice to record individual neuron firing rates and track place cell activity.
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