Single serotonergic neurons that modulate aggression in Drosophila

Abstract

Monoamine serotonin (5HT) has been linked to aggression for many years across species. However, elaboration of the neurochemical pathways that govern aggression has proven difficult because monoaminergic neurons also regulate other behaviors. There are approximately 100 serotonergic neurons in the Drosophila nervous system, and they influence sleep, circadian rhythms, memory, and courtship. In the Drosophila model of aggression, the acute shut down of the entire serotonergic system yields flies that fight less, whereas induced activation of 5HT neurons promotes aggression. Using intersectional genetics, we restricted the population of 5HT neurons that can be reproducibly manipulated to identify those that modulate aggression. Although similar approaches were used recently to find aggression-modulating dopaminergic and Fru(M)-positive peptidergic neurons, the downstream anatomical targets of the neurons that make up aggression-controlling circuits remain poorly understood. Here, we identified a symmetrical pair of serotonergic PLP neurons that are necessary for the proper escalation of aggression. Silencing these neurons reduced aggression in male flies, and activating them increased aggression in male flies. GFP reconstitution across synaptic partners (GRASP) analyses suggest that 5HT-PLP neurons form contacts with 5HT1A receptor-expressing neurons in two distinct anatomical regions of the brain. Activation of these 5HT1A receptor-expressing neurons, in turn, caused reductions in aggression. Our studies, therefore, suggest that aggression may be held in check, at least in part, by inhibitory input from 5HT1A receptor-bearing neurons, which can be released by activation of the 5HT-PLP neurons.

Figure 1. A single pair of serotonergic PLP neurons enhances aggression

A–D. Serotonergic neurons identified by the enhancer trap (et)-FLP screen. A. Example of a broadly expressed FLP line that targets most of the 5HT neurons in the fly brain. The anti-5HT immunostaining pattern is shown in magenta, the membrane-tethered GFP signal driven by a combination of FLP³⁸³, TRH-Gal4 and UAS>stop>mCD8::GFP is shown in green. The full z-stack frontal projection is shown, scale bar represents 50 μm. B–D. Individual 5HT neurons targeted by the use of different et-FLP lines. The mCD8::GFP signal amplified by anti-CD8 antibody staining is shown in green, the neuropil areas stained by an nc82 (anti-Bruchpilot) antibody are shown in gray, and anti-5HT immunostaining is shown in magenta. Dotted boxes outline the magnified fields shown in the lower panels. The upper panels show full frontal projections, scale bar represents 50 μm. Different frontal z-stacks through either the anterior or posterior areas of the same triple-stained brains were created when required to view the processes or cell bodies shown in the lower panels. B. The FLP⁴¹⁷ line restricts GFP expression to 1–2 bilateral neurons from the PLP cluster (green). These neurons arborize within the ventrolateral protocerebrum (VLP) and send a midline directed process toward the central complex (see Figure 3A for more details). C. The FLP⁵⁵⁰ line restricts GFP expression to 1–2 bilateral neurons from the SE1 cluster (green). These arborize within the suboesophageal ganglion (SOG) and send descending projections to the ventral nerve cord. D. The FLP³⁴² line restricts GFP expression to 1–2 bilateral neurons from the PMP cluster (green) that arborize in the superior medial protocerebrum. For et-FLP lines reproducibility and cell count data see Table S1. E–H. Manipulation of individual 5HT neurons from the PLP cluster targeted by FLP⁴¹⁷ changes aggression. E. Total numbers of lunges performed by pairs of males with TNT-inactivated 5HT-PLP neurons. F. Latency to the first lunge and to the establishment of dominance in flies with TNT-inactivated 5HT-PLP neurons. In E and F both genetic control and experimental flies were reared and fought at constant +25°C conditions. Note, that the reduction in lunge numbers was not due to the increased latency to lunge, because the number of lunges counted for 30 min after the first lunge rather than from the time of landing on the food surface, was also reduced [FLP⁴¹⁷: 74.2 ± 17.5; controls: 134.9 ± 22.0, Mann-Whitney U=54, P=0.043]. G. Total numbers of lunges performed by pairs of males with dTrpA1-activated 5HT-PLP neurons. H. Latency to the first lunge and to the establishment of dominance in flies with dTrpA1-activated 5HT-PLP neurons. In G and H both genetic control and experimental flies were reared at +19°C and transferred to a +27°C experimental room 15 min before the aggression assay. Each dot in panels E and G represents the lunge count for an individual pair of flies. Data are presented as boxplots with a median line. The bottom and top of the box show the 25th and 75th percentile. Latencies in panels F and H are presented as means ± SEM. ** -p<0.01 vs. controls (white bar or white dots), analyzed by nonparametric two-independent-sample Mann-Whitney U-test. See Figure S1C for FLP parental control aggression data.

Figure 2. Manipulation of 5HT-PLP neurons has selective effects on behavior

A. Inactivation of 5HT-PLP neurons produces a mild locomotion deficit. B. Induced activation of 5HT-PLP neurons produced a mild locomotion deficit. C. The inactivation of 5HT-PLP neurons produced sleep deficit measured by average percentage of sleep per 24 h. The data in A–C are presented as means ± SEM; *P < 0.05, **P < 0.01, ***P < 0.001 vs. controls (white bar) analyzed using a nonparametric two-independent-sample Mann–Whitney U test. D. Distribution of sleep during averaged 24-h periods in flies with inactivated 5HT-PLP neurons. Gray line - experimental flies; black line - controls. Data are presented as means ± SEM; **P < 0.01, ***P < 0.001 vs. the corresponding hour data point of the control group, analyzed by an unpaired t-test. The bracket shows the time of the day when aggression assays were performed. E–G. Courtship behavior is unaffected by TNT inactivation of the aggression-modulating PLP (FLP⁴¹⁷) neurons. The data for the courtship vigor index (E) and for the latency to court (F) are presented as means ± SEM. Courtship success is calculated as a percent of males that mated in 10 min of the assay (G). Also see Figure S1B–E for FLP parental control data.

Figure 3. Anatomical characterization of the aggression-modulating 5HT-PLP neurons

A. Arborization patterns of the PLP neurons visualized by membrane-bound CD8::GFP (green) relative to anti-5HT labeled neuropil regions of the brain (magenta). These are displayed in frontal z-projections of an image stack through the ventrolateral protocerebrum and antennal lobes (top left), the ellipsoid body of the central complex and the peduncles of the mushroom bodies (top right), the fan-shaped body of the central complex (bottom left) and a posterior view of the brain where the PLP cell bodies and their axons are located (bottom right). Scale bar represents 50 μm. Short arrows point to cell bodies, long arrows to axons of the PLP neurons. A dotted line outlines the peduncles of the mushroom bodies that are not stained by anti-5HT antibodies. B. Polarity of the serotonergic PLP neurons. (Left) The total arborization field of the PLP neurons visualized using membrane-bound CD8::GFP. (Center) The putative presynaptic terminals of the PLP neurons revealed using the presynaptic marker nsyb::GFP. (Right) The putative dendritic arbors of the PLP neurons visualized by expression of the postsynaptic marker DsCam:GFP. Full z-stack frontal projections are shown, scale bar represents 50 μm.

Figure 4. Putative targets of 5HT-PLP neurons determined using the anatomical and functional analyses

A. A schematic illustration of the combined use of the GRASP and FLP-recombinase techniques to find possible synaptic connections between serotonergic neurons and target neurons that express different subtypes of 5HT receptors. We used the FLP²¹⁰ line that targets most of the serotonergic neurons in the brain to restrict the expression of the spGFP¹¹ part of GFP driven by the TRH-LexA. The other part of GFP, spGFP^1–10, was expressed under control of different 5HT-receptor Gal4 drivers. We found GRASP signals in 3 neuropil regions where arborization of 5HT-PLP neurons was observed: ventrolateral protocerebrum (VLP), around peduncles of the mushroom bodies at the ellipsoid body (EB) focal plane, and near fan-shaped body (FB). The use of FLP⁴¹⁷ line that further restricted the expression of the spGFP¹¹ to aggression-modulating 5HT-PLP neurons yielded no detectable GRASP signals. B–D: Patterns of reconstituted GFP (GRASP signal, green) between most of serotonergic neurons and candidate 5HT receptor neurons in the areas of interest, which are visualized by anti-5HT immunostaining (magenta). The experimental genotypes used were: w¹¹¹⁸; LexAop>stop>spGFP¹¹/FLP²¹⁰; UAS-spGFP^1–10, TRH-LexA/5HT receptor-Gal4. Different frontal z-projections of the image stack were created to view the corresponding neuropils of the same brain. The three neuropil regions (VLP, EB, and FB) were examined for each receptor type, but only regions that showed GRASP signal are shown. The dotted circles outline the peduncles of the mushroom bodies that were not stained by the anti-5HT antibody. White arrows point to areas in which GRASP signal is observed. Scale bar represents 50 μm. For positive and negative GRASP controls see Figure S2, for additional GRASP data see Figure S3. E. dTrpA1-induced activation of 5HT1A receptor neurons decreases the total numbers of lunges. Each dot represents the lunge count for an individual pair of flies. Data are presented as boxplots with a median line. The bottom and top of the box show the 25th and 75th percentile. *- P< 0.05 vs. corresponding control (white dots), analyzed by nonparametric two-independent-sample Mann-Whitney U-test. Latency to the first lunge and to the establishment of dominance in flies with dTrpA1-activated 5HT receptor neurons was not changed. The latencies are presented as means ± SEM.