A Gate-and-Switch Model for Head Orientation Behaviors in Caenorhabditis elegans

Responses to odor removal depend on posture. A, Schematic of position-velocity plots to illustrate oscillatory head movements. B, Position-velocity trajectories from odor removal (circles) to 2 s after (arrowheads). C, Head displacement in the 2 s immediately following odor removal, with the start position normalized to head orientation at the odor switch. D, Relationship between head position at odor removal and the change in head position 2 s later. Linear regression F_(1,124) = 132.3145, p < 0.0001; n = 126.

RIA mediates directional head withdrawal. A, To analyze head withdrawal behaviors, which are mirror-symmetric across the dorsoventral axis, head deflection in either the dorsal (D) or ventral (V) direction is defined as positive. Positive velocities correspond to bending away from the body axis and negative velocities indicate head withdrawal. B, Mean plots of peristimulus head orientation at odor off, odor on, or in constant odor (no switch), binned according to whether the head is unbent (top) or bent in either direction (bottom) at the time of stimulus change (t = 0). Dashed “pre” and “post” boxes indicate time windows used for quantitation in D. C, Comparison of head velocity in response to stimulus changes (or no switch) when the head is bent or unbent. Arrow indicates characteristic head withdrawal in response to odor removal when the head is bent. D, Paired mean pre- and post-switch head velocities (upper panels, open circles and lines are mean ± SD) and estimations of the size of the head velocity change (lower panel, closed circles and lines are mean difference ± 95% confidence interval, along with probability distribution of means, see Materials and Methods). Odor switch (on, off, or constant odor) has no effect on head movements when the head is not bent (left panel, repeated measures ANOVA F_(2,263) = 0.9386, p = 0.3925, n = 100, n = 82, n = 84). Sharp decreases in velocity are seen when odor removal occurs when the head is bent, but not for odor presentation or constant odor (middle panel, repeated measures ANOVA, F_(2,109) = 7.7932, p = 0.0007, n = 26, n = 44, n = 42). In gar-3 mutants and animals expressing tetanus toxin in RIA these sharp decreases in head velocity are absent (right panel, repeated measures ANOVA F_(3,85) = 4.4105, p = 0.0062, n = 42, n = 12, n = 18, n = 17); ***p < 0.001, **p < 0.01 post hoc Student’s t test between odor conditions (on, off, no switch) for wild-type animals; ##p < 0.01, #p < 0.05 post hoc Student’s t test between genetic manipulation (wild type, gar-3, gar-3 rescue, RIA::TeTx) for odor off responses during head bending.

Head orientation gates sensory responses. A, Head position-velocity trajectories color-coded by ∂nr (= nrV – nrD), a measure of normalized mCa²⁺ asymmetry in nrV and nrD. B, Head position-velocity trajectories color-coded by loop calcium signal, n = 126, 40 s each. C, Spontaneous head movements (left) and sCa²⁺ signals in the loop region of the RIA axon (right). Rows are matched and sorted according to head orientation at odor off (t = 10 s). A linear regression of loop response magnitudes and head deflection in either direction was significant (F_(1,124) = 29.98, p < 0.0001, n = 126). D, Mean traces of loop calcium responses 5 s before and after odor removal binned by head orientation at the odor off time point. Shading is SEM. Left to right, n = 14, n = 6, n = 18, n = 22, n = 23, n = 21, n = 13, n = 9. E, Comparison of the magnitude of loop calcium responses to odor removal when the head is bent (head orientation > 0.5 or < –0.5) or unbent (head orientation between –0.5 and 0.5). Pre- and post-measurements are mean loop Ca²⁺ levels in 1s windows just before odor OFF and centered on the peak of the mean response, respectively, shown as paired responses (lines) and mean ± SD (open circles and lines). Estimations of the mean changes are shown below as mean difference ± 95% confidence interval (closed circles and lines) along with probability distributions of the means (see Materials and Methods). In both wild-type (n = 84, n = 42) and gar-3 (n = 20, n = 12) animals, the loop response magnitude is larger when the head is bent (repeated measures ANOVA F_(1,154) = 41.4529, p < 0.0001, ***p < 0.001 post hoc Student’s t test). gar-3 mutants have larger responses under both head orientation conditions, but there is no significant interaction between loop Ca²⁺ responses, head position at odor off, and genotype (repeated measures ANOVA F_(1,154) = 0.2145, p = 0.6439).

Temporal features of sensory responses in RIA axonal compartments. A, Mean nrV, nrD, and loop responses on odor removal. Shading is SEM, dashed lines indicate position of nrV and nrD mean peaks. B, Head orientation predicts the mean lag between nrV and nrD peaks post-odor removal. ANOVA F_(1,124) = 13.47, p = 0.0004. C, Cross-correlation of nrV, nrD, and mCa²⁺ asymmetry (∂nr) and head orientation, showing mCa²⁺ lag with head movements. gar-3 mutants lack mCa²⁺ and do not show head correlations. Re-expression of gar-3 cDNA in RIA partially rescues this relationship. D, nrV and nrD cross-correlations with head velocity for wild-type, gar-3, and RIA-specific gar-3 rescue in a 6-s time window comprising 2-s pre-odor removal and 4-s post-odor removal in relation to head orientation at odor off. When the head was bent, we observed no lag between head withdrawal and peak calcium responses in the nerve ring compartment ipsilateral to the direction of bending. gar-3 mutants show no distinction between nerve ring compartments. Expression of gar-3 cDNA in RIA in gar-3 mutants does not rescue the temporal features of sensory responses in the nerve ring. Analysis groups are the same as Figure 4.