Identified Serotonergic Modulatory Neurons Have Heterogeneous Synaptic Connectivity within the Olfactory System of Drosophila

Abstract

Modulatory neurons project widely throughout the brain, dynamically altering network processing based on an animal's physiological state. The connectivity of individual modulatory neurons can be complex, as they often receive input from a variety of sources and are diverse in their physiology, structure, and gene expression profiles. To establish basic principles about the connectivity of individual modulatory neurons, we examined a pair of identified neurons, the "contralaterally projecting, serotonin-immunoreactive deutocerebral neurons" (CSDns), within the olfactory system of Specifically, we determined the neuronal classes providing synaptic input to the CSDns within the antennal lobe (AL), an olfactory network targeted by the CSDns, and the degree to which CSDn active zones are uniformly distributed across the AL. Using anatomical techniques, we found that the CSDns received glomerulus-specific input from olfactory receptor neurons (ORNs) and projection neurons (PNs), and networkwide input from local interneurons (LNs). Furthermore, we quantified the number of CSDn active zones in each glomerulus and found that CSDn output is not uniform, but rather heterogeneous, across glomeruli and stereotyped from animal to animal. Finally, we demonstrate that the CSDns synapse broadly onto LNs and PNs throughout the AL but do not synapse upon ORNs. Our results demonstrate that modulatory neurons do not necessarily provide purely top-down input but rather receive neuron class-specific input from the networks that they target, and that even a two cell modulatory network has highly heterogeneous, yet stereotyped, pattern of connectivity. Modulatory neurons often project broadly throughout the brain to alter processing based on physiological state. However, the connectivity of individual modulatory neurons to their target networks is not well understood, as modulatory neuron populations are heterogeneous in their physiology, morphology, and gene expression. In this study, we use a pair of identified serotonergic neurons within the olfactory system as a model to establish a framework for modulatory neuron connectivity. We demonstrate that individual modulatory neurons can integrate neuron class-specific input from their target network, which is often nonreciprocal. Additionally, modulatory neuron output can be stereotyped, yet nonuniform, across network regions. Our results provide new insight into the synaptic relationships that underlie network function of modulatory neurons.

Regulation of modulatory cell activity across olfactory structures in Drosophila melanogaster

“…These results are consistent with previous studies demonstrating that odors generally inhibit the CSDns during electrophysiological somatic recordings ( Fig. 1E) (30), which is likely due to prominent input from GABAergic local interneurons (LN) (25,30).…”

“…Our compartmental modeling shows that AL and LH processes of the CSDn are electrotonically connected, but that voltage preferentially passes from the AL to the LH. CSDn inhibition scales with increasing odor strength due to robust GABAergic LN input in the AL (25,30), and this inhibition shunts LH responses proportionally.…”

“…Interestingly, the CSDn has numerous release sites in the AL (25,30) and CSDn derived 5-HT directly inhibits several classes of neurons in the AL (30). But how are CSDn release sites activated in the AL?…”

“…The modulation of cVA-evoked behavioral responses is especially interesting because the CSDns are thought to participate in top-down modulation and to have their effects mainly in the first olfactory relay, the antennal lobe (AL) (24,28,29). However, CSDn processes avoid the cVAsensitive DA1 glomerulus (25,30) and DA1 projection neuron (PN) odor responses are not modulated with strong CSDn stimulation (30). Finally, whole-cell recordings show strong inhibition of the CSDn during stimulation with cVA (30).…”

Complete connectomic reconstruction of olfactory projection neurons in the fly brain

“…For this, we chose multiple cross sections through the base of each tract and reconstructed all neurons within it to identification. For completeness, we note the serotonergic CSD neuron which we excluded from our list because it does not represent a projection neuron in the classical sense due to its unpolarised nature [78,79].…”