When mice are sleep-deprived, there is an increase in activity in inhibitory neurons in the hippocampus
U-M scientists Sara Aton and James Delorme found when mice are sleep-deprived, there is an increase in activity in inhibitory neurons in the hippocampus, an area of the brain essential for navigation, as well as for processing and storing new memories. “Because these neurons limit activity in their neighbours, this physiological response makes it impossible to muster normal neuronal activity in the hippocampal structure,” said Aton, an associate professor in the U-M Department of Molecular, Cellular and Developmental Biology and a member of the U-M Center for RNA Biomedicine executive committee. The research is published in the Proceedings of the National Academy of Sciences.
Aton and Delorme, formerly a U-M neuroscience graduate student, studied the possible links between changes in neurons’ activity after learning and changes in their protein translation.
First, Delorme investigated the interaction between sleeping and waking, hippocampal neuron activity, and activity-driven phosphorylation of S6, a component of ribosomes. This phosphorylation event is thought to affect which mRNAs that are being translated into protein as neurons become more active. To do this, Delorme gave mice a fear stimulus. When mice were allowed to freely sleep following the stimulus, he saw that S6 phosphorylation increased in a part of the hippocampus called the dentate gyrus, the first region where memories begin to form. But when the mice were deprived of sleep, Delorme found that phosphorylation decreased throughout the hippocampus. This disrupted the mice’s memories that otherwise would have been formed in response to the fear stimulus.
Delorme’s next question was whether this reduction in activity driven S6 phosphorylation affected all neurons similarly after sleep loss. Using bioinformatics, he compared the abundance of mRNAs associated with phosphorylated S6-containing ribosomes. He also examined mRNA profiles under conditions of prior sleep or no sleep. Delorme then collaborated with U-M Advanced Genomics core for the RNA sequencing. He observed that, after sleep deprivation, there was a significant increase in abundance of a type of RNA transcripts known to be present specifically in interneurons that express the neuropeptide somatostatin as well as the inhibitory neurotransmitter GABA. This relative increase suggested that greater activity of S6 phosphorylation in the hippocampus acts as a gate that slows down their firing. When they mimicked this inhibitory gating mechanism in freely sleeping mice, they were able to disrupt hippocampal activity and memory consolidation. In contrast, suppressing the activity of somatostatin-expressing interneurons after learning increased activity among dentate gyrus neurons and was beneficial to memory consolidation.
Source: University of Michigan news release