Serotonin (5-HT) neurons located in the raphe nuclei modulate a wide range of behaviors by means of an expansive innervation pattern. In turn, the raphe receives a vast array of synaptic inputs, and a remaining challenge lies in understanding how these individual inputs are organized, processed, and modulated in this nucleus to contribute ultimately to the core coding features of 5-HT neurons. The details of the long-range, top-down control exerted by the medial prefrontal cortex (mPFC) in the dorsal raphe nucleus (DRN) are of particular interest, in part, because of its purported role in stress processing and mood regulation. Here, we found that the mPFC provides a direct monosynaptic, glutamatergic drive to both DRN 5-HT and GABA neurons and that this architecture was conducive to a robust feed-forward inhibition. Remarkably, activation of cannabinoid (CB) receptors differentially modulated the mPFC inputs onto these cell types in the DRN, in effect regulating the synaptic excitatory/inhibitory balance governing the excitability of 5-HT neurons. Thus, the CB system dynamically reconfigures the processing features of the DRN, a mood-related circuit believed to provide a concerted and distributed regulation of the excitability of large ensembles of brain networks.optogenetics | synapse | glutamate receptors | anxiety | depression T he phylogenetically old neurotransmitter serotonin (5-HT) is known to regulate a remarkably wide range of brain functions and behaviors, such as mood, aggression, reward, perception, levels of arousal, and decision making (1-5). The 5-HT-containing neurons are localized in a collection of small brainstem nuclei collectively known as the raphe nuclei, with the largest being the dorsal raphe nucleus (DRN). These neurons send extensive and widespread axonal projections throughout the forebrain (6). As such, the firing of a relatively small number of 5-HT neurons is powerfully poised to regulate the excitability of large ensembles of neural networks distributed across virtually the entire brain. However, little is known about how the synaptic networks impinging onto the DRN regulate the excitability of its constituent cells to generate the core feature selectivity of 5-HT neurons and, ultimately, to regulate the dynamic output of 5-HT to the brain. A significant step forward in this direction has recently been achieved by a number of anatomical studies that provide a comprehensive list of the brain regions projecting to the DRN, as well as detailed cartography of the connectivity patterns of these inputs to local cellular elements within the DRN subnetwork (i.e., 5-HT neurons, local GABAergic neurons) (7-9). The top-down control exerted by the phylogenetically recent medial prefrontal cortex (mPFC) over the DRN is of particular importance, in part, because of the purported role of this pathway in regulating several aspects of motivation and stress processing, and in mediating antidepressant-like effects (1, 10, 11). The mechanistic details of this long-range connectivity are elusive, howe...

No abstract