Anatomical and functional characterization of serotonergic neurocircuitry

Abstract

The serotonin system arises from a small collection of brainstem structures called the raphe nuclei and innervates much of the central nervous system. A relatively small number of serotonin neurons modulate a wide variety of functions, from the basic such as homeostatic regulation of sleep and feeding to more complex roles in mood, motivation and emotion. Furthermore, serotonin signaling is implicated in the etiology and treatment of major depression, and is linked to numerous other mood and neuropsychiatric disorders. However, it has been difficult to assign a defined functional role to serotonin, possibly due to the heterogeneity of the neural population, including molecular, neurochemical, electrophysiological diversity and especially its broad connectivity. The aims of this thesis are to examine the neuroanatomical circuits involving the major serotonergic nuclei, the possible heterogeneity of anatomical wiring including inputs and outputs of the serotonergic population, and connectivity with the basal ganglia structures of the brain.

In paper I, we characterize and quantify inputs to serotonergic neurons of the dorsal and median raphe nuclei on a whole-brain scale. We reconstruct whole-brain connectivity patterns by utilizing a genetic strategy for retrograde transsyaptic tracing of direct inputs to genetically-defined serotonergic neurons to uncover previously unidentified or disputed circuits, and provide functional confirmations of direct connections from forebrain regions to serotonergic cells. We characterize functional inputs from the prefrontal cortex, lateral habenula and basal ganglia. In paper II, we determine whether dorsal raphe serotonergic heterogeneity can be characterized by the input-output circuitry of the population. We describe the whole-brain presynaptic inputs to dorsal raphe serotonergic subpopulations that project to either the prefrontal cortex or striatum and contrast these findings with input-output circuitry of other cell types that interact with serotonergic neurons, including midbrain dopaminergic and dorsal raphe GABAegic populations. In paper III, we dissociate the functional connectivity between the different neuronal types of the striatum, a basal ganglia structure that receives serotonergic inputs and also projects to the dorsal raphe. We show that parvalbumin-expressing fast spiking interneurons in the striatum provide direct inhibition onto the projecting medium spiny neurons, which regulate the output to downstream basal ganglia structures, while avoiding other types of neighboring interneurons.

In summary, the work of this thesis provides a further step in untangling the heterogeneity of the serotonergic system, by targeting genetically-defined serotonergic neurons of the raphe nuclei and their afferent and efferent connectivity. Ultimately, knowledge of connectivity between genetically defined neuron types will be essential for modeling and understanding circuit function in health and disease.