Serotonergic Modulation of Inhibitory Input to Lateral Horn Affects Odor-Mediated Attraction in D. melanogaster

To survive, animals must eat, mate, and avoid danger by sensing external stimuli, processing sensory information into perception, and eliciting the appropriate behavioral responses in accordance with their physiological states. Unlike learned responses, innate responses are genetically inherited and do not require the animal to have a previous encounter with the stimulus to elicit an action. However, innate responses can be flexible based on the physiological state of the animal. This flexibility is achieved through constant alterations in network dynamics which ensures that the animal\u27s responses are most effective. This alteration occurs to some extent by modulatory neurons/nuclei at multiple processing stages of a neural network and mediated via several modulatory receptor subtypes in the network. In this study, I take advantage of the well-characterized olfactory system of Drosophila to explore connectivity between a single pair of modulatory serotonergic neurons, the CSD neurons, and a population of ventral projection neurons, vPNs. Both types of neurons span two different olfactory processing stages and vPNs are vital for eliciting attraction to odors that have innately important values. I further, investigate the degree to which innate responses can be modulated by dissecting the role of one serotonergic receptor, 5-HT1A, expressed by vPNs in innate attraction and the behavioral consequences of reducing its expression in vPNs. I found that CSD neurons synapse upon the dendrites and the axons of vPNs in the antennal lobe and lateral horn, respectively, to potentially modulate the activities of vPNs in both regions simultaneously. I also found that reducing the expression of the 5-HT1A receptor in vPNs selectively increases flies\u27 attraction to the innately attractive odor apple cider vinegar, but neither to other food odors with different biological values, nor to aversive odors such as benzaldehyde. This study contributes to our understanding of the simultaneous modulation of interconnected networks by a single pair of modulatory neurons as well as the consequences of neuron class-specific expression of a single modulatory receptor. The outcome knowledge is critical for better understanding of neural coding processes in sensory systems and the extent to which innate behaviors are modulated