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. 2020 Dec 14;9:e62576. doi: 10.7554/eLife.62576

Figure 26. Feedback from MBONs to DANs.

Input from MBONs onto DANs occurs in three distinct circuit motifs which are diagrammed. MBON connections to DANs from the same compartment are depicted as a yellow arrow, while MBON connections to DANs from a different compartment are depicted as a purple arrow. DANs downstream of these MBONs are depicted as dashed yellow arrows. (A) In the simplest case, the MBON and DAN form a reciprocal loop where the MBON innervates DAN(s) that send their axons back to the same compartment where the MBON’s dendrites arborize. Below the diagram of this circuit motif, each of the instances we observed with this arrangement is presented; inputs onto PAM and PPL1 DANs are presented separately. The color of the MBON name indicates its neurotransmitter, as indicated. (B) In this case, MBON(s) synapse onto DAN(s) that send their axons to MB compartment(s) different from the compartment where the MBON’s dendrites reside, providing a mechanism for cross-compartment communication. It is common for the DAN population of a compartment to receive input from several MBONs. (C) This motif combines the previous two motifs, with DANs receiving input from both MBON(s) of same and different compartments. (D) The table summarizes DAN populations that participate in the three different feedback motifs.

Figure 26.

Figure 26—figure supplement 1. Axo-axonal connections from MBON05, MBON06 and MBON11 onto DANs inside MB lobes.

Figure 26—figure supplement 1.

These three MBONs send axonal processes to other MB compartments (Aso et al., 2014a) where they make synapses onto other MBONs (see Figure 22 and Figure 22—figure supplement 1). As summarized here, they also make axo-axonal synapses onto DANs in the same compartments where they contact other MBONs. In some other compartments, they make a smaller number of synapses onto DANs. For example, MBON06 (β1>α) and MBON11 (γ1pedc>α/β) make a total of 279 synapses onto the axons of two PPL1 and six PAM DANs in the α lobe and 53 synapses onto six PAMs in the β′2, β2, and β1 compartments. Similary, MBON05 (γ4>γ1γ2) makes a total of 156 synapses onto one PPL1 and five PAM DANs in the γ1 and γ2 compartments and a total of 10 synapes onto two PAM DANs in other compartments. Presynapses of MBON11 in the β′2 compartment were also observed in the FAFB EM volume by Felsenberg et al., 2018. It is worth noting that these connections represent a much smaller fraction of the total inputs that DANs receive within the MB lobes when compared with percentage of inputs provided by the MBON feedback that goes to DAN dendrites shown in Figure 26. (A) MBON05 and MBON06 feedforward to the axons of PAMs from the same compartments. (B) Feedforward connections from MBON06 and MBON11 to PPL1 and PAM DANs in other compartments. The connections in the β′2, β2, and γ5 compartments are weaker than those in the α-lobe. (C) The PPL101 (γ1-pedc) DAN receives feedback from both MBON11 (same compartment) and MBON05 (other compartment).
Figure 26—figure supplement 2. MBON-to-DAN feedback mediated by an interneuron.

Figure 26—figure supplement 2.

MBONs also make indirect connections to DANs in which the MBON connects to an interneuron which then connects to a DAN. This matrix shows the effective connection strength of such MBON-to-DAN feedback loops and is meant to complement the data presented in Figure 26, which describes direct connections between MBONs and DANs. Effective connection strength is a measure that takes into account both the strength of the MBON’s connection to the interneuron and that of the interneuron to the DAN. Effective connection strength is computed by multiplying the matrix of DAN inputs with the matrix of MBON outputs (both matrices are normalized so that the inputs to each neuron sum to 1). This connection strength is averaged across all DANs of the indicated type. Highlighted boxes indicate interactions between MBONs and DANs that share a compartment, and the box color indicates the neurotransmitter or predicted neurotransmitter of the MBON. MBONs are ordered by their proportion of self-feedback, as defined and shown in Figure 26—figure supplement 3.
Figure 26—figure supplement 3. Distribution of DAN feedback mediated by an interneuron.

Figure 26—figure supplement 3.

This plot shows the ratio of feedback to the same compartment (self-feedback) to feedback to other compartments (other-feedback) for each MBON type. Circles are color-coded to indicate the MBON neurotransmitter. The plot reveals a robust pattern of glutamatergic MBONs indirectly modulating their associated DANs. The plot is meant to complement the data presented in Figure 26, which describes direct connections between MBONs and DANs. The strength of indirect self-feedback is defined as the average effective connection strength mediated by one interneuron between a given MBON and the DANs that innervate the same compartment. The strength of average other-feedback is defined as the average effective connection strength between the MBON and all other DANs, again mediated by one interneuron. The value plotted on the y-axis is the ratio of these two quantities. That is, MBONs with higher ratios have a larger fraction of the interneuron-mediated feedback they initiate directed back to their own compartment, rather than to other compartments.