A circuit for navigation in Caenorhabditis elegans

Abstract

Caenorhabditis elegans explores its environment by interrupting its forward movement with occasional turns and reversals. Turns and reversals occur at stable frequencies but irregular intervals, producing probabilistic exploratory behaviors. Here we dissect the roles of individual sensory neurons, interneurons, and motor neurons in exploratory behaviors under different conditions. After animals are removed from bacterial food, they initiate a local search behavior consisting of reversals and deep omega-shaped turns triggered by AWC olfactory neurons, ASK gustatory neurons, and AIB interneurons. Over the following 30 min, the animals disperse as reversals and omega turns are suppressed by ASI gustatory neurons and AIY interneurons. Interneurons and motor neurons downstream of AIB and AIY encode specific aspects of reversal and turn frequency, amplitude, and directionality. SMD motor neurons help encode the steep amplitude of omega turns, RIV motor neurons specify the ventral bias of turns that follow a reversal, and SMB motor neurons set the amplitude of sinusoidal movement. Many of these sensory neurons, interneurons, and motor neurons are also implicated in chemotaxis and thermotaxis. Thus, this circuit may represent a common substrate for multiple navigation behaviors.

Fig. 1.

Regulation of reversals and omega turns during feeding, local search, and dispersal. (A) Four different examples of reversals of different lengths and degrees of reorientation. Tracks are visible as indentations in the agar. r1, reversal with a single head swing followed by an ≈40° change in direction; r2, reversal with two head swings and an ≈70° change in direction; R3, reversal with three head swings and an ≈90° change in direction; R4, reversal with four head swings followed by an omega turn, resulting in an ≈170° change in direction. Blue dots indicate position of the animal's head at the start of the reversal. Anterior is up. In R4, D indicates the dorsal side and V indicates the ventral side of the animal during the omega turn. (B) Some omega turns occur in the absence of a reversal, but most occur after a reversal. Most omega turns occur after a reversal of length R3+ (three head swings or greater). n = 285 omega turns. Animals were scored at 1–12 min off food. (C) The longer a reversal, the more likely it is to terminate in an omega turn. n = 249 omega turns. Animals were scored at 1–12 min off food. (D) Tracks of individual animals feeding on food, in the local search period during the first 12 min off food, and during dispersal after 40 min off food. The time intervals shown represent ≈10 min, 5 min, and 30 s, respectively. (E) Frequency of short reversals (r1 and r2), long reversals, and omega turns during 5-min intervals on food and at different intervals off food.

Fig. 2.

Sensory regulation of local search and dispersal behaviors. (A) osm-6 (p811) mutants exhibit dwelling-like behavior (feeding behavior) in the absence of food, with more short reversals and fewer long reversals than controls. (B) Animals with all amphid neurons killed have abnormal local search and dispersal behaviors. (C) osm-3 (p802) mutants exhibit milder defects in the absence of food. (D and E) Killing the AWC (D) or ASK (E) neurons blunts local search behavior, with fewer long reversals and omega bends at 1–6 and 7–12 min. (F) Killing ASI disrupts dispersal behavior. Killing ASI and AWC gives a mixed defect. (G–N) Killing ASH, AFD, AWB, ADL, ASJ, AWA, ASE, or ASG sensory neurons, respectively, has minor effects or no effect on feeding, local search, and dispersal behaviors. (O) tph-1 (mg280) mutants exhibit dwelling-like behavior off food, as indicated by a higher frequency of short reversals. For Figs. 2, 4, and 5, the food column refers to a 5-min interval on food, and the other columns refer to intervals after removal from food. The 1- to 6-min and 7- to 12-min intervals correspond to the local search state, and the 35- to 40-min interval corresponds to dispersal. For descriptions of short reversals or long reversals, see Fig. 1. For each data point, the circle size indicates the frequency of the behavior. Gray circles indicate controls or values not significantly different from controls. Colored circles denote statistical significance: blue, increases from control values; red, decreases from control values. The absence of a symbol indicates a value between 0 and 0.05; a red zero indicates a value in the same range that is statistically different from the control. n.d., not done. Sample sizes and P values are reported in Table 1.

Fig. 3.

A predicted circuit for navigation. (A) Data from serial section reconstructions of electron micrographs (5) were used to assemble a circuit, as described in Supporting Methods. Each of the following neurons represents a bilaterally symmetric left–right pair: AWC, ASI, ASK, AIY, AIZ, AIB, AIA, RIA, RIM, RIB, and RIV. The head and neck motor neurons, SMD, SIA, SMB, and SIB, each have four members that innervate muscle quadrants (see Fig. 5). The interneuron SAA also has four members, a ventral and dorsal member on each side. RMD is a class of six radially arrayed neurons. Red dotted lines indicate connections that were asymmetric in the dorsoventral direction (e.g., seven of eight synapses from AIB to SMD are to the dorsal SMDs). The command interneurons are indicated in green. (B) A schematic showing information flow from sensory neurons to motor neurons.

Fig. 4.

Interneurons regulate pirouette frequency. (A) Killing AIZ reduces reversals during feeding. (B) Killing AIB disrupts local search behavior. (C) Killing AIY disrupts dispersal behavior. (D) Killing RIM increases short reversals during feeding, local search, and dispersal. (E) Killing RIB reduces reversals during local search. (F) Killing RIA slightly decreases reversals during local search and dispersal. (G) AWC/AIY-double-ablated animals resemble AIY-ablated animals. (H) Killing AIY in an osm-6 (p811) background increases reversals and turns. (I) Killing AIB in a ttx-3 (mg280) background reduces reversals and turns.

Fig. 5.

Motor neurons and the AVA command neuron have discrete functions in reversals, omega turns, and sinusoidal movement. (A) Killing AVA eliminates long reversals in local search and diminishes short reversals on food. Killing both RIM and AVA results in fewer reversals than a RIM ablation alone. (B) Killing SMD reduces the frequency of omega turns. Killing either SMD or RMD increases reversal frequency. (C) Killing RIV reduces the frequency of omega turns. (D and E) Killing SIA or SIB does not affect omega turns. (F) Killing SMB increases short reversal frequency. ∼, Because of loopy movement, omega turns could not be accurately scored. (G) Anatomy of head and neck motor neurons and muscles. (Left) Innervation of anterior muscle rows and approximate position of the muscle rows with respect to the pharynx (gray). Muscle groups are symmetric on the left and right sides; in this schematic, only dorsal left and ventral left muscles are shown. SIA and SIB were originally identified as interneurons but also have neuromuscular junctions (D. Hall, personal communication). IL1, URA, RMF, RMG, and RMH motor neurons are not shown. (Right) The four muscle quadrants, dorsal-left (DL), dorsal-right (DR), ventral-left (VL), and ventral-right (VR), and the location of head motor neuron synapses onto the muscles in the nerve ring. (H) Killing SMD reduces the amplitude of turns after a reversal, and killing RIV eliminates the ventral bias of postreversal turns. Turns were scored immediately after a reversal (Fig. 1 A). Green arrows and numbers indicate the average angle by which the direction of movement changed after the turn. A ventral turn was scored as positive (0–180°; blue dots), and a dorsal turn was scored as negative (0–180°; red dots). Black arrows and numbers indicate the average absolute angle by which the direction of movement changed after the turn. For this analysis, both ventral and dorsal turns were scored as positive (0–180°). Sample sizes are 30, 27, 41, and 68 for control (SMD), SMD, control (RIV), and RIV (at least three worms each). t test comparisons indicate statistical significance at P < 0.02 for SMD vs. control and RIV vs. control, comparing either the averages or the averages of the absolute values. (I) Killing SMB results in deeply flexed, loopy sinusoidal movement. Movement tracks are visible on the agar. (J) Neuronal functions in the navigation circuit from sensory input to motor output. ASI may act partly by inhibiting AWC. ASK and AWC may act by inhibiting AIY and stimulating AIB. Omega bends are generated by head motor neurons, and reversals are generated by the command interneurons.