Figure 2. PDM-DN is a descending neuron that receives olfactory input.

(A) Anatomical characterization of the PDM-DN neuron using light microscopy. (Ai) Anatomy of the PDM-DN neuron revealed by maximum projection of confocal sections. The soma of the descending neuron (PDM-DN) is located in the brain lobes (BL, arrow). Arrowheads indicate large axonal varicosities in the subesophageal zone (SEZ) and thoracic regions. The axon of the descending neuron extends posteriorly to the fourth abdominal segment (dashed arrow). Cross section of the third thoracic segment is shown in the inset. Arrow in the inset indicates the dorso-medial location of the PDM-DN axon. D: dorsal, V: ventral. VNC: ventral nerve cord, A1-A9 abdominal segments 1–9 of the VNC. (Aii) 3D reconstruction of the PDM-DN neuron (green) and larval neuropil (magenta). Antibody staining against CsChrimson::mVenus expressed in PDM-DN neuron reveals that its soma is located in the posterior-dorsal-medial region of the brain lobes (arrows). The following brain regions are demarcated by dashed lines: antennal lobe (AL), tip of the ventral lobe (vl) of the mushroom body, spur and the mushroom body calyx. (Aiii) Dendritic arborizations of the PDM-DN neuron (green) are concentrated around the mushroom body peduncle (pd) and its surroundings. (Aiv) Use of the MCFO technique (Nern et al., 2015) to label a single PDM-DN neuron. The dendritic tree and the axon of the PDM-DN neuron are contralateral to its soma (arrow). Arrowheads indicate the large axonal varicosities in the anterior part of the PDM-DN axon. Green: PDM-DN neuron; magenta: neuropile. (B) EM reconstruction of the postsynaptic partners of PDM-DN. Anterior view of the Or42a and Or42b PNs (yellow and light pink); lateral-horn local interneurons (LH-LNs) upstream of PDM-DN (dark and light purple) and PDM-DN neuron (black). Antennal lobe (AL), lateral horn (LH) and mushroom body calyx (cx) are demarcated by dashed lines. Note that the PDM-DN neuron dendrites overlap with LH-LNs exclusively in the lateral horn region. (C) Reconstruction of the upstream partners of the PDM-DN neuron reveals that PDM-DN receives olfactory input via the lateral horn. The Or42a and Or42b OSNs are the main sensory neurons that are responsible for the attraction toward ETB, which is the odor used in the loss-of-function screen (Figure 1). PDM-DN receives input from the Or42a and Or42b projection neurons (PNs) via two local interneurons located in the lateral horn (LH-LN1 and LH-LN2), which form a feedforward motif (left inset). The signs of the interactions of the feedforward motif are unknown. Abbreviations used in the anatomical maps: A: anterior, D: dorsal, P: posterior, V: ventral.

Figure 2—figure supplement 1. EM reconstruction of the PDM-DN neuron and its main upstream partners.

(A) Comparison of the EM-reconstructed PDM-DN to light microscopy (LSM). Representation of the input (cyan) and output (red) regions of the PDM-DN neuron. (B) EM reconstruction of the pair of LH-LN1 neurons. Both the left and the right LH-LN1 neurons are shown. The synapses between LH-LN1 and PDM-DN are shown in green. Each LH-LN1 neuron projects onto both the left and the right PDM. (C) EM reconstruction of the left and the right LH-LN2 neurons. The synapses between LH-LN2 and PDM-DN are shown in green. The LH-LN2 neurons give input to both the left and right PDM-DN neurons at a distinct, more proximal dendritic region compared to the LH-LN1 pair. Abbreviations used in the anatomical maps: A: anterior, D: dorsal, P: posterior, V: ventral, M: medial, L: lateral.

Figure 2—figure supplement 2. A subset of olfactory sensory neurons target PDM-DN through LH-LN1.

(A) EM reconstruction of uniglomerular PNs (uPNs, gray) and LH-LN1 neuronal pair (light purple). Synapses between uPNs and LH-LN1 are shown in cyan (post). Antennal lobe (AL), lateral horn (LH) and mushroom body calyx (cx) are demarcated by dashed lines. (B–C): LH-LN1 receives input from multiple OSNs via uniglomerular (uPNs, gray) and multiglomerular olfactory PNs (mPNs, blue). (B) Both left and right LH-LN1 neurons receive large fractions of their inputs from olfactory uPNs and mPNs. (C) Number of synapses between all uPNs and mPNs presynaptic to LH-LN1. Or42a and Or42b uPNs are two of the strongest presynaptic partners of LH-LN1 (highlighted in red are the fractions of synapses from the Or42a and Or42b PNs). None of the mPNs that are presynaptic to LH-LN1 receive significant input from Or42a and Or42b OSNs (Berck et al., 2016).