During the first half of the project, we worked on technologies to control neurons with light (termed "optogenetics") at a wide optical spectrum and at different temporal scales. In parallel, we worked on improved optical methods to investigate synaptic transmission. Using fast imaging at single synapses together with patch-clamp electrophysiology and biophysical modeling, we measured novel, genetically encoded sensors of fast synaptic transmission. In another collaborative effort, we developed a new, genetically encoded optical sensor termed SynTagMA that enables identification of thousands of active synapses at the same time. We demonstrated its applicability in vivo as tool to map active synapses in the living mouse brain.
Moreover, we investigated the long-term consequences of synaptic plasticity on the stability of single synapses during the following days. This work demonstrates that strengthening synapses reduces their risk for elimination from the circuit. We further showed that sequential synaptic potentiation and depression interact over days to influence synapse stability, with the last plasticity event fully determining synaptic survival probability. The long-term consequence is that initial functional adaptations of synapses, seen as potentiation or depression of synaptic pathways are transformed into changes in circuit wiring, which, from a macroscopic point-of-view, have the same effect on synaptic pathways.
During the third funding period, we published a novel optogenetic tool termed BiPOLES (Bidirectional Pair of Opsins for Light-induced Excitation and Silencing), for bidirectional control of neuronal activity. To better understand brain function and dysfunction, including neurological disorders, one must be able to manipulate nerve cells with high precision and specificity in the living organism. In principle, optogenetic manipulations allow the activation and inhibition of the same population of neurons to test their sufficiency and necessity for a particular brain function. However, existing optogenetic tools do not easily allow such bidirectional manipulations with light. BiPOLES overcomes these limits and enables multiple new applications including (1) potent excitation and inhibition of the same neurons, (2) exclusive dual-color excitation of two distinct neuronal populations, and (3) precise optical tuning of the membrane voltage. Due to its applicability in numerous model systems - such as worms, flies, mice and ferrets - BiPOLES fills an important gap in the optogenetic toolbox and might become the tool of choice to address a number of yet inaccessible problems in neuroscience.
Finally, we investigated how different anesthetics alter synaptic connections, neuronal activity and memories. Understanding how different anesthetics affect the brain, particularly the hippocampus, is important for both clinicians with human patients and experimental scientists who work with animals. We recorded brain activity from the hippocampus while mice were anesthetized using one of three common combinations of general anesthetics: isoflurane, ketamine/xylazine (Keta/Xyl), and medetomidine/midazolam/fentanyl (MMF). We found a number of differences in how the specific anesthetics affected the brain, synaptic connections and memory stabilization. Knowing these varying effects on the hippocampus and memory formation should be useful for doctors or experimenters when considering which method to use.
