The impact of general anesthesia on memory consolidation, hippocampal network activity and spine dynamics and the application of genetically-encoded tools for mapping neuronal and synaptic activity in vivo

Abstract

Abstract Learning and memory are important to brain functions that are essential for the survival of animals and humans. Amnesia refers to memory loss caused by brain diseases, injuries, or pharmacological interventions such as the use of general anesthetics. In this thesis, I presented two projects related to the hippocampus, a brain region central to memory formation and consolidation. In the first project, I investigated the effects of three different general anesthetics (GAs), isoflurane (Iso), medetomidine/midazolam/fentanyl (MMF), and ketamine/xylazine (Keta/Xyl) on memory consolidation, hippocampal network activity and spine dynamic. First, using Morris Water Maze, I found that anesthesia with MMF or Keta/Xyl, but not Iso, impaired memory consolidation. Using in vivo calcium imaging with the genetically encoded indicator GCaMP6f, I imaged the same field of view of CA1 of each individual mouse under different anesthetics to compare the population dynamics and recovery timeline for each condition. I found that different GAs induced highly distinct network states. Iso induced 0.1 Hz oscillations, Keta/Xyl had the strongest reduction in calcium dynamics, and MMF strongly impaired population dynamics for many hours. Furthermore, I found different long-term effects on dendritic spine dynamics, with Keta/Xyl having the strongest effect, by imaging the formation and elimination of dendritic spines under each condition. Finally, to compare general anesthesia with sleep, I performed calcium imaging under NREM and REM sleep states and found that sleep modifies CA1 activity to a much lower degree than anesthesia. Taken together, this study revealed distinct effects of Iso, MMF and Keta/Xyl on network activity and structural spine dynamics in the hippocampal CA1 of adult mice. In the second part of this thesis, I tested a photoconvertible fluorescent probe, postSynTagMA, in vivo by mapping active neurons and synapses during behavior. SynTagMA is developed by my colleagues in order to rapidly and stably label synapses with high calcium levels. After injecteing hSyn-postSynTagMA or mDlx-postSynTagMA in dorsal CA1 of the mouse hippocampus, I found that there was a small percentage of photoconverted CA1 neuronal nuclei or photoconverted synapses after illumination with 405 nm light under anesthesia. Then, I made the behavioral application to investigate active neuronal populations under goal-directed navigation with 405 nm light illumination triggered by the reward. In sum, these results show that postSynTagMA photoconversion could selectively map behaviorally relevant neurons and synapses with high calcium transients in behaving mice. Taken together, the findings in this thesis 1) demonstrate that different anesthetics, despite inducing similar physiological states, differ significantly in their effects on synaptic stability, hippocampal network activity and memory consolidation; and 2) validate that postSynTagMA photoconversion selectively maps behaviorally relevant neurons and synapses with high calcium transients in behaving mice.