Artificial hibernation reduced synapse numbers in mice by 52.5%, but memories and clusters of connections within their neural traces were preserved
Artificial hibernation reduced synapse numbers in mice by 52.5%, but memories and clusters of connections within their neural traces were preserved
On August 13, Science published a paper on an artificially induced hibernation-like state in mice. Synapse density in the hippocampus fell by 52.5%, yet the animals retained memories of familiar places and events. Compact clusters of connections within the neural traces of those memories were also preserved.
Long-term memory has often been linked to the strength of individual synapses, the sites where neurons make contact. However, synapses in the hippocampus are continually formed and lost. The authors needed a controlled way to reorganize the network so that they could determine which features of its structure would survive a sharp loss of connections. A team from the Okinawa Institute of Science and Technology (OIST), together with its collaborators, induced controlled body cooling and metabolic slowing in mice for 48 hours.
In the hippocampus, a brain region involved in spatial and contextual memory, neuronal activity fell by approximately 70%, while synapse density fell by 52.5%. After waking, the mice distinguished the chamber where they had previously received an electric shock from a neutral chamber, found a learned target in a maze, and retained their neural maps of space. After rewarming, 82.1% of spines, small neuronal protrusions where synapses are usually located, reappeared at the same sites from which they had disappeared. Random coincidence would have produced 16.8%.
For comparison, the researchers used a different protocol: prolonged anesthesia combined with pharmacological disruption of actin, a protein that helps cells change shape. Neuronal activity and synapse numbers also declined under this protocol, but after recovery, the animals had poorer memory of the context in which they had received the shock. This comparison separates the extent of synapse loss from the question of which connections remain in the network and how the network subsequently recovers.
The researchers then labeled synapses between neurons belonging to the same engram, a group of cells and connections active during a specific memory. During artificial hibernation, individual contacts within the engram often disappeared, but their spatial clusters remained. Under anesthesia with actin disruption, the clusters declined together with memory. Electron microscopy showed that, in some surviving clusters, a single axon terminal connected to several spines at once.
In this mouse model, preservation of clusters of connections within engrams coincided with preservation of memory. The authors therefore consider these clusters a possible stable structural trace of a memory.
A discussion of cryonics as a way to transmit personal memory into the future centered on the question of which brain structures must be preserved for future methods to recover them. For biostasis, an attempt to preserve an organism at an extremely low temperature in anticipation of future medical treatment, this experiment makes the task more specific: researchers need to identify which relationships within a neural network must survive its reorganization for memory to be reconstructed from them.