Final Report Summary - SERRACO (Modulation of cortical activity by median raphe neuronal assemblies with identified behavioural effects)
The median raphe region (MRR) of the mammalian brainstem modulates the forebrain circuitries (including the septum and the hippocampus) by serotonin and/or glutamate transmission. We used a variety of techniques including optogenetics, in vivo and in vitro electrophysiology and pharmacogenetics and neuroanatomy to better understand this dual neuro-transmission system. We discovered that although the MRR is well known for its serotonergic cells and their role in the brain, the fast, glutamatergic effect of the MRR seems even more important, which were clearly supported by our behavioural, physiological, pharmacological and anatomical results. We revealed important roles of MRR in learned and innate fear as well as in learning, influenced by glutamatergic transmission, the neural mechanisms of which we thoroughly investigated. We studied the behavioural roles of the median raphe in conditioned fear, the social interaction test of anxiety, and operant learning. To better understand how amygdala - that is also targeted by MRR - controls emotional states, we elucidated their neuron types and their connectivity. We characterized, for the first time, the neurochemical features of transmitter release from an identified subpopulation of serotonergic neurons in mouse MRR. Optogenetic activation of MRR was correlated with enhanced locomotor activity of mice and local release of serotonin and glutamate. Similarly, optical stimulation increased serotonin and glutamate release in hippocampal and MRR slices. The majority of serotonin release from MRR, but not from hippocampal terminals, is mediated by glutamate. The frequency- dependence and cell type-dependent modulation of hippocampal serotonin release by serotonin and purine receptors was also revealed, which may have important implications in the mode of action of existing and future drugs used to treat psychiatric disorders. Furthermore, we characterized the hippocampus-coupled activity pattern of the different neuron types of MRR. The switch to high resolution behavioural monitoring resulted in significant discoveries about the electrophysiological correlates of certain exploratory behaviours. We demonstrated layer-specific responses and a shift in the dominant hippocampal oscillation during MRR stimulation in exploring animals. Hence, the MRR-hippocampal connection may modulate information encoding in hippocampus in an input pathways selective manner. In parallel, we studied the feedback regulation of subcortical modulation by the hippocampus. We have uncovered that the hippocampus exerts inhibitory gain control over the medial septum, a key station in hippocampal feedback loops. We took advantage of these experiments as testbeds for optimizing the combination of recording and optogenetic manipulation. We revealed how the hippocampus, that is essential in memory formation, generates diverse healthy and pathological activity patterns. We found that sharp-waves are the default activity patterns of the hippocampus and identified similarities and differences in the generation mechanisms of healthy and pathologic activities. We also studied how the activation of the MRR-hippocampal or the septal cholinergic pathway modulates hippocampal activity and surprisingly we found that these cholinergic cells are also GABAergic and inhibitory. Our findings were strongly supported by our anatomical investigations showing that the ascending MRR projections are mainly glutamatergic in the mouse forebrain. We described the cellular architecture and transmitter phenotypes of neurons of the mouse MRR and found that most projecting neurons are glutamatergic and act thorough glutamate receptors. Interestingly, while investigating MRR, which does not send GABAergic fibers to the hippocampus, we found that the nucleus incertus that is directly adjacent to MRR sends a strong, behaviourally relevant GABAergic input to the hippocampus.