Adaptive food-leaving in wild-type animals. (A) Food-leaving probability increases over time as animals deplete their food source. Food-leaving probability is calculated as the number of worms leaving per minute divided by the total number of worms on the food at start of that minute, averaged over 15 min (3). Error bars show SD. (B) Single frames from 1-, 6-, and 12-h movies and schematic drawings. The arrows point at animals outside of food or at the outer edge of the food border. (C) Speed on food and at the border, and the proportion of animals encountering the border that leave food increased over time (D). Error bars denote SEM (C and D). ***P < 0.005, **P < 0.01, *P < 0.05. n = 6 or more per strain.

Adaptive food-leaving is promoted by ASE, BAG, and the body cavity neurons AQR, PQR, and URX and suppressed by ADF neurons. (A) The tax-4 and tax-2 mutants show lower food-leaving than wild-type animals, whereas osm-9 and ocr-2 mutants show higher food-leaving. (B) Increased food-leaving of tax-2(p694) animals can be rescued by expressing tax-2 cDNA in AFD (pgcy-8), BAG (pflp-17), ASE (an ASE-specific pflp-6 fragment), and AQR, PQR, and URX (pgcy-32). (C) The ttx-1 mutants behave similarly to wild-type animals with respect to food-leaving. In contrast, animals in which BAG neurons are ablated using a pgcy-33::egl-1 transgene, as well as che-1 mutants defective for ASE function, and animals genetically ablated for AQR, PQR, and URX neurons using egl-1 expression from the gcy-36 promoter stayed strongly on food. (D) After 15 min of airflow at 21% O₂, wild-type animals and che-1;ttx-1 pgcy-33::egl-1 animals (which are defective in the CO₂ sensing neurons ASE, AFD, and BAG) were subjected to a shift to 3% CO₂ (21% O₂). This shift elicited a substantial increase in food-leaving after about 10 min that was partially suppressed in che-1;ttx-1 pgcy-33::egl-1 animals. (E) High food-leaving in ocr-2 mutants can be partially rescued by expressing ocr-2 cDNA in ADF neurons. (F) The egl-4 loss-of-function mutants show higher food-leaving than wild-type. Wild-type levels of food-leaving can be restored by expressing egl-4 from the tax-4 promoter. Expressing egl-4 from the gcy-32 promoter, which drives expression specifically in AQR, PQR, and URX neurons, or the odr-3 promoter, which drives expression in AWA, AWB, AWC, ASH, and ADF neurons, results in partial rescue. Error bars denote SEM. ***P < 0.005, **P < 0.01, *P < 0.05. n = 6 or more per genotype.

Neuronal insulin and TGF-β signaling from the ASI neurons promote food-leaving. (A) The daf-2 and age-1 mutants exhibit reduced food-leaving compared with wild-type animals. The daf-16 mutants exhibit higher food-leaving. daf-2 suppresses the high food-leaving phenotype of daf-16 mutants. (B) Expression of daf-16 in all neurons from the unc-119 promoter restores wild-type food-leaving. Expression of daf-16 in muscle from the myo-3 promoter shows partial rescue. Intestinal expression of daf-16 from the ges-1 promoter does not rescue the food-leaving phenotype of daf-16 mutants. (C) age-1 expression from the unc-119 promoter, but not the osm-6 promoter, restores wild-type food-leaving to age-1 mutants. (D) The daf-7 mutants exhibit reduced food-leaving. Wild-type food-leaving can be restored by expressing daf-7 in the ASI neurons, using the gpa-4 promoter. Error bars denote SEM. ***P < 0.005, *P < 0.05. n = 6 or more per strain.

The neuropeptide receptor NPR-1 functions in ASE, ASH, and RMG neurons to inhibit food-leaving and high ambient O₂ or ectopic activation of AQR, PQR, and URX induces high food-leaving. (A) The npr-1 mutants show higher food-leaving than wild-type animals. gcy-35 partially suppresses npr-1 food-leaving. Expression of gcy-35 from the gcy-32 promoter in gcy-35; npr-1 mutants shows that gcy-35 acts in AQR, PQR, and URX to suppress the high food-leaving of npr-1 mutants. Neither a flp-18; flp-21 double-mutant nor mutants in their additional receptors npr-4 and npr-5 (50) showed higher food-leaving than wild-type. (B) The npr-1 mutants show increased speed and border-arrival rate, which is suppressed in gcy-35;npr-1 mutants. (C) Wild-type food-leaving can be restored in npr-1 mutants by expressing npr-1 specifically in ASH (and ASI) from the sra-6 promoter, and in RMG using Cre-mediated recombination (51). Full rescue is also achieved from the ncs-1 promoter, driving expression in RMG and 10 other neurons. (D) Expression of npr-1 in both ASEL and ASER partially rescue food-leaving in npr-1 mutants. (E and F) Food-leaving is context-dependent. (E) Wild-type and npr-1 mutants were recorded at 21% oxygen and then switched to 11% oxygen in a Perspex chamber. Food-leaving of npr-1 mutants was reduced at low oxygen tension, when the AQR, PQR, and URX neurons are less active (15, 16). (F) Ectopic stimulation of AQR, PQR, and URX neurons in npr-1 lite-1 mutants by channelrhodopsin activation is sufficient to induce high food-leaving in animals kept at 11% oxygen. Animals were filmed 3 min before and after light activation. The orange bars denote the addition of the necessary cofactor, retinal. Error bars denote SEM. ***P < 0.005, **P < 0.01, *P < 0.05. n = 6 or more.

Model for regulation of adaptive food-leaving behavior. The BAG, ASE, and AQR, PQR, URX neurons promote food-leaving via TAX-2/TAX-4. Food-leaving is also promoted by the ASI neurons via DAF-7 and by neuronal insulin signaling. High CO₂ and O₂ promote food-leaving. Food-leaving is suppressed by the ADF neuron via OCR-2, the AFD neuron and by NPR-1 neuropeptide signaling in ASE, ASH, and RMG neurons.