Autonomous circuitry for substrate exploration in freely moving Drosophila larvae

Abstract

Background: Many organisms, from bacteria to human hunter-gatherers, use specialized random walk strategies to explore their environment. Such behaviors are an efficient stratagem for sampling the environment and usually consist of an alternation between straight runs and turns that redirect these runs. Drosophila larvae execute an exploratory routine of this kind that consists of sequences of straight crawls, pauses, turns, and redirected crawls. Central pattern generating networks underlying rhythmic movements are distributed along the anteroposterior axis of the nervous system. The way in which the operation of these networks is incorporated into extended behavioral routines such as substrate exploration has not yet been explored. In particular, the part played by the brain in dictating the sequence of movements required is unknown.

Results: We report the use of a genetic method to block synaptic activity acutely in the brain and subesophageal ganglia (SOG) of larvae during active exploratory behavior. We show that the brain and SOG are not required for the normal performance of an exploratory routine. Alternation between crawls and turns is an intrinsic property of the abdominal and/or thoracic networks. The brain modifies this autonomous routine during goal-directed movements such as those of chemotaxis. Nonetheless, light avoidance behavior can be mediated in the absence of brain activity solely by the sensorimotor system of the abdomen and thorax.

Conclusions: The sequence of movements for substrate exploration is an autonomous capacity of the thoracic and abdominal nervous system. The brain modulates this exploratory routine in response to environmental cues.

Figure 1. Driver Lines to Manipulate the Activity of Distinct Regions of the Nervous System

(A–C) Expression patterns of the driver lines in the CNS of early third-instar larvae were assessed with UAS-mCD8-GFP and immunostaining with anti-GFP and anti-SCR antibodies. SCR labels the most posterior (labial) segment of the subesophageal ganglion. (A) The panneural driver line elav-Gal4. (B) The nerve cord (NC) driver line tsh-GAL4. (C) A driver line for the brain lobes, SOG, and descending neurons, called BL-Gal4, was generated by the combination of elav-Gal4 with two Gal80 repressor lines, one expressed in the NC, tsh-GAL80, and one in sensory neurons, cha3.3-GAL80. GFP expression is restricted to neurons located in the brain lobes and SOG. Descending fibers are labeled (arrow). A few scattered motorneurons in abdominal segments as well as a group of neurons in abdominal segment 9 (A9) are labeled (arrow head). (D) GFP expression in a whole larva. Upper panel shows that most sensory neurons are excluded from the BL expression pattern. A group of neurons in A9 send processes to the anal plate (arrow head). Lower panel shows that a few anterior sensory neurons are seen in the BL expression pattern. These include a pair of multidendritic neurons (arrow head) and the Bolwig organs (arrow). (E–G) Double staining with FasII to assess the positioning of the descending fibers. (E) Projection of whole CNS. (F) Inset from e showing descending fibers (arrows) and a few motoneurons (arrow heads). (G) Transverse view of inset from (F). The upper panel shows the FasII positive tracts, marked with a dashed line, as well as BL descending fibers. The lower panel shows that the BL neurons descend in the DL, DM, and VM tracts. (H–J) Images showing boundaries of the driver lines. (H) Upper and lower panels show BL > st-RFP; RFP expression is confined to the brain lobes and SOG. Anti-SCR marks posterior boundary of expression. (I) elav > st-RFP. Expression levels in the brain were similar in the panneural and BL lines. (J) NC > st-RFP. Expression is confined to the region posterior to the SOG marked by anti-SCR. An average of 9 elav-Gal4, 18 BL-Gal4, and 9 NC-Gal4 animals was analyzed for each panel.

Figure 2. Inhibiting Synaptic Activity in the Brain Lobes and SOG Does Not Affect the Propagation of Peristaltic Contraction Waves

(A) Description of a peristaltic wave. Video frames and diagrams showing a ventral view of a WT larva, OrR, at different stages during a peristaltic wave. The arrow highlights the segments contracting as the wave progress along the abdominal segments A8/9 to A1. (B–E) Early third-instar larvae expressing shibire^ts were transferred to 36°C to block synaptic transmission and their behavior was evaluated. An average of 26 animals was tested for each one of the genotypes. (B) Average number of forward waves/min (±SEM). (C) Average number of backward waves/min (±SEM). (D) Average duration of forward waves in seconds (±SEM). (E) Average duration of backward waves (±SEM). See also Movies S1, S2, S3, and S4. (F and G) The peristaltic crawling of larvae expressing eNpHR evaluated under blue or green light. Green light selectively activates halorhodopsin and leads to membrane hyperpolarization. (F) Average number of forward waves/min (±SEM). (G) Average duration of forward waves (±SEM). A 10 s recovery phase was allowed before resuming the analysis at the end of the green light pulse. One elav > eNpHR animal executed a few forward waves under green light. It is probable that this animal was slightly bigger and that this affected the light penetration. Eight BL > eNpHR and 11 elav > eNpHR animals were evaluated. A one-way ANOVA with a Bonferroni post hoc test was performed to compare the treatments. **p < 0.01; NS means nonsignificant. See also Figure S1 and Movies S5 and S6. BL refers to the driver directed to the brain lobes and SOG; NC, the driver directed to the nerve cord.

Figure 3. The Brain Lobes and SOG Are Not Required to Sustain and Initiate Peristaltic Waves

(A) Larvae were pinned down loosely and filleted without damage to anterior and posterior segments. The CNS was exposed and the number of peristaltic waves was quantified by observing the muscle field. (B) Average number of forward and backward peristaltic waves (±SEM) when the CNS was intact (first 5 min) and after the ablation of the brain and SOG (next 25 min). (C) Time to reinitiate peristalsis after ablation of brain lobes and SOG. The transverse line represents the average time to resume peristalsis. (D) Representative immunostaining against HRP and SCR to evaluate the precision of the surgery. The lack of SCR staining indicates that the SOG was removed. Twenty individuals were analyzed. A one-way ANOVA with Bonferroni post hoc test comparing the data for 5, 10, and 15 min was performed. An ANOVA for repeated measures was employed for 15 to 30 min. **p < 0.01.

Figure 4. Blocking Synaptic Transmission in the BL Does Not Alter the Number and Angle of Pause Turns

(A) Description of a pause turn. Video frames and diagram showing a ventral view of a WT larva, OrR, at different stages during a pause turn. A unilateral wave of muscle contraction moves backward from A1 to A4 on the left hand side. Segments A5 to A9 remain relaxed on both sides of the animal. See also Figure S2. (B–D) Crawling behavior was recorded on video and analyzed. (B) Representative crawling patterns depicted by perimeter stacks from a 2 min crawling episode at 1 frame/s. An arrow head marks the beginning of each track, while asterisks mark pause turn events. (C) Scatter plot and average number of pause turns/min (±SEM). Blocking activity in the BL did not affect the number of turns as confirmed by a Kruskal-Wallis and a Dunn's Multiple Comparison test. An average of 46 animals was tested. **p < 0.01; ***p < 0.001. (D) Polar histogram of turning angles frequency. The frequency distribution of turning angles (α) see (A) for both controls and the BL > shi^ts was nonsignificantly different. A Watson's U² test for nonparametric two-samples test was performed: shi^ts /+ versus BL > shi^ts U²_0.05,197,137 = 0.12; BL/+ versus BL > shi^ts U²_0.05,269,137 = 0.06. (E) Scatterplot including average number of pause turns/min (±SEM). Hyperpolarizing the BL domain with Kir2.1 does not affect the number of turns. A one-way ANOVA with a Bonferroni post hoc test was used for comparison between genotypes, *p < 0.05.

Figure 5. Blocking Synaptic Transmission in the Brain and Peripheral Nervous System Does Not Preclude the Alternation between Peristalsis and Turns

(A) Representative confocal images revealing the pattern of expression of BL+sens driver line with UAS-mCD8-GFP. Left shows the immunostaining against GFP highlights strong expression in the sensory terminals in the nerve cord. Right shows that the peripheral nervous system can be observed in the whole animal. (B) Average number of forward peristaltic waves/min (±SEM). (C) Average number of backward peristaltic waves/min (±SEM). (D) Average duration of forward peristaltic waves in s (±SEM). (E) Average duration of backward peristaltic waves in s (±SEM). A one-way ANOVA with Bonferonni post hoc test was performed to compare the different treatments in (B)–(E). (F) Average number (±SEM) of pause turns performed per min. A Kruskal-Wallis test was performed followed by a Dunn's multiple comparison. * means p < 0.05; **p < 0.01; and ***p < 0.001. An average of 26 animals was tested for each of the genotypes.

Figure 6. Chemotaxis-Dependent Change in Frequency of Pause Turns Is Prevented when Synaptic Activity Is Inhibited in the BL

(A) The BL driver line is expressed in the olfactory lobe as labeled with anti-GFP. (B) Representative tracks of larvae crawling under green light in an arena with a central olfac-tory cue. The asterisk indicates the beginning of the track. (C) Percentage of success. The percentage of larvae that reached the central area of the plate defined by a circle with a 3 cm diameter was quantified. A Fisher's exact test was performed to evaluate differences between the genotypes. (D) The number of pause turns/min was quantified in concentric areas of different diameters. Thirteen animals of each control group and 17 of the experimental were tested. A Kruskal-Wallis test with Dunn's multiple comparison was performed to evaluate the significance between the different diameters in each of the treatments. a’ means p < 0.05, a” p < 0.001 comparing with < 3cm; b means p < 0.01 and b’ p < 0.05 comparing with 3–5 cm. NS, nonsignificant.

Figure 7. The Photoavoidance Response Does Not Require the Brain

(A) The ability of larvae to escape from a spot of intense blue light in 5 s was assessed. Immunostaining showing that photoreceptors are ablated in BL > GMR-hid; shi^ts third-instar larvae. Arrows indicate Bolwig's nerve and developing adult photoreceptor projection (APP), labeled with anti-chaoptin, in controls and residual staining in BL > GMR-hid; shi^ts. (B) Percentage of animals avoiding the blue light (±SEM). There were no significant differences between genotypes as tested with a Kruskal-Wallis test followed by a Dunn's multiple comparison. An average of 43 animals was tested for each genotype.