The midbrain serotonergic system has been implicated in a diverse range of brain functions, including modulation of sensory processing, motor output, respiration and mood. This diversity presumably arises from diffuse projections from a small number of neurons in the raphe nucleus to nearly all regions of the forebrain, and the large number of cellular signaling pathways. Sensory systems offer significant advantages when linking cellular effects of serotonin to systems-level phenomena because of their circumscribed function, better understood circuitry and the easier control over their stimulus-output properties in vivo. In this project, we will conduct innovative experiments to understand how the selective and natural activation of the serotonergic system modulates the function of identified neurons in an ethologically relevant sense, in a genetically-accessible mammalian model, the mouse. We will investigate serotonergic modulation in the olfactory bulb (OB), since it has rich innervation from the raphe nucleus and many serotonin receptors are expressed there. We will use optogenetic and physiological methods to test the hypothesis that serotonergic neurons affect principal neurons in the OB in a rapid and temporally diverse manner using multiple neurotransmitters. To achieve our goals, we will use optogenetic activators to selectively activate serotonergic neurons and trigger natural secretion of neurotransmitters. Then using a suite of methods already established in our group (genetically-encoded calcium indicators, multiphoton microscopy and patch-clamp electrophysiology), we will dissect the cellular mechanisms of serotonergic modulation in principal neurons in the OB. Experiments in this project will be guided by three Aims.
Aim 1 : To determine the effects of dorsal raphe nucleus activation on the activity of different classes of identified neurons in the OB.
Aim 2 : To determine the effects of selective optogenetic activation of raphe neurons on the fast time-scale activity of individual output neurons of the OB.
Aim 3 : To determine the cellular mechanisms underlying the complex effects of serotonergic neuron activation on the OB output neurons. The research proposed here will provide a mechanistic understanding of how natural activation of the serotonergic system alters the functional properties of a sensory circuit. Insighs gained from this study will may help in strategies aimed at correcting dysfunctions involving the serotonergic system.

Public Health Relevance

Serotonin signaling has been implicated in a wide range of physiological processes, and its perturbation is thought to underlie many mental disorders. Our studies will improve our understanding of how the serotonin system affects sensory processing, and may shed light on potential links between mood disorders and processing of sensory information.

National Institute of Health (NIH)
National Institute on Deafness and Other Communication Disorders (NIDCD)
Research Project (R01)
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Neurotransporters, Receptors, and Calcium Signaling Study Section (NTRC)
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Sullivan, Susan L
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Harvard University
Schools of Arts and Sciences
United States
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Reddy, Gautam; Zak, Joseph D; Vergassola, Massimo et al. (2018) Antagonism in olfactory receptor neurons and its implications for the perception of odor mixtures. Elife 7:
Mathis, Alexander; Rokni, Dan; Kapoor, Vikrant et al. (2016) Reading Out Olfactory Receptors: Feedforward Circuits Detect Odors in Mixtures without Demixing. Neuron 91:1110-1123
Kapoor, Vikrant; Provost, Allison C; Agarwal, Prateek et al. (2016) Activation of raphe nuclei triggers rapid and distinct effects on parallel olfactory bulb output channels. Nat Neurosci 19:271-82