Social feeding in Caenorhabditis elegans is induced by neurons that detect aversive stimuli

Abstract

Natural Caenorhabditis elegans isolates exhibit either social or solitary feeding on bacteria. We show here that social feeding is induced by nociceptive neurons that detect adverse or stressful conditions. Ablation of the nociceptive neurons ASH and ADL transforms social animals into solitary feeders. Social feeding is probably due to the sensation of noxious chemicals by ASH and ADL neurons; it requires the genes ocr-2 and osm-9, which encode TRP-related transduction channels, and odr-4 and odr-8, which are required to localize sensory chemoreceptors to cilia. Other sensory neurons may suppress social feeding, as social feeding in ocr-2 and odr-4 mutants is restored by mutations in osm-3, a gene required for the development of 26 ciliated sensory neurons. Our data suggest a model for regulation of social feeding by opposing sensory inputs: aversive inputs to nociceptive neurons promote social feeding, whereas antagonistic inputs from neurons that express osm-3 inhibit aggregation.

Figure 1

Aggregation of npr-1 mutant animals requires food and is enhanced by increased population density and daf-7 TGF-β mutations. a, Time courses for aggregation of 20, 40, 60 or 80 npr-1(ad609) animals, or of 80 N2 animals, as indicated. Populations of 20, 40 and 60 N2 animals show less aggregation than 80 N2 animals (data not shown). b, Well-fed social feeders do not aggregate without food. Population density in these assays was about 26 animals per cm². The npr-1 allele is ad609; CB4856 is a wild social strain bearing the npr-1 215F allele. c, Aggregation behaviour with 80 adult animals per assay. ky13 is a predicted null allele of npr-1; g320 is the natural npr-1 215F allele. The daf-7(e1372) allele is a point mutation that acts like a strong loss-of-function allele. daf-7(e1372) enhances the aggregation of both npr-1(ky13) (P < 0.02) and npr-1(g320) animals (P < 0.01). d, Average number of animals in a group for the assays in c. Only animals in groups were scored to get the average group size. daf-7(e1372); npr-1(ky13) animals form significantly larger groups than npr-1(ky13) animals (P < 0.05). e, f, Per cent aggregation and average group size in assays involving a population of 40 adult animals. daf-7(e1372); npr-1(ky13) animals aggregate more strongly and form larger groups than npr-1(ky13) animals (P < 0.001 for both behaviours). In all panels n ≥ 5 assays.

Figure 2

Social feeding of npr-1 is lost in ocr-2, osm-9, odr-4 or odr-8 mutant backgrounds. a, b, cat-4(e1141), mec-3(e1338), osm-3(p802), odr-1(n1936) odr-7(ky4) and ttx-3(ks5) mutations do not disrupt aggregation or bordering of npr-1(ad609) animals. The odr-1 odr-7 npr-1 and ttx-3 npr-1 animals also bear the lon-2(e678) X mutation. c, d, Mutations in ocr-2, osm-9, odr-4 and odr-8 strongly reduce aggregation and bordering of npr-1 animals (P < 0.001). e, f, Individual animals of each genotype were tested for their ability to join groups of marked npr-1 animals, or to border in the presence of npr-1 animals (see Supplementary Information). For a–d, n ≥ 9 population assays; for e, f, n ≥ 30 individual animals, each tested 2–3 times.

Figure 3

Mutations in ocr-2, osm-9, odr-4 and odr-8 restore the ability of npr-1 mutant animals to slow down upon encountering food. n ≥ 25 animals each recorded for 4 min.

Figure 4

The nociceptive neurons ASH and ADL are required for social feeding behaviour. a, Expression of ocr-2 in the ASH or ADL neurons restores aggregation behaviour to ocr-2; npr-1 mutant animals (P < 0.003 compared to ocr-2; npr-1), whereas expression in the ADF or AWA neurons does not. odr-3 is expressed in ASH, AWA, AWB, AWC and ADF neurons. b, odr-4 expression in ADL restores social behaviour to odr-4; npr-1 mutant animals (P < 0.001 compared to odr-4; npr-1), whereas expression in ADF does not. Expression of odr-4 in AWA, AWB, ASI and ASK neurons also fails to restore aggregation behaviour to odr-4; npr-1 animals (data not shown). c, Ablation of both pairs of ADL and ASH neurons suppresses aggregation (P < 0.001 compared to sham). Ablation of the ASH neuron pair, the ADL neuron pair, or the ASH neuron pair and 1 ADL neuron, does not suppress aggregation.

Figure 5

Disruption of osm-3 kinesin restores social feeding to mutants defective in nociceptive neuron function. a, b, Aggregation (a) and bordering (b) behaviours were scored in animals of the indicated genotype. osm-3 ocr-2; npr-1 and odr-4; osm-3; npr-1 animals aggregate and border strongly to similar levels as npr-1 and osm-3; npr-1 animals (P > 0.05). n ≥ 10 assays. c, A model for social feeding in C. elegans. The ASH and ADL nociceptive neurons are proposed to respond to aversive stimuli from food to promote social feeding. This function requires the putative OCR-2/OSM-9 ion channel. The ODR-4 protein may act in ADL to localize seven transmembrane domain chemoreceptors that respond to noxious stimuli. In the absence of ASH and ADL activity, an unidentified neuron (XXX) represses social feeding, perhaps in response to a different set of food stimuli. The photograph shows social feeding of a group of >30 npr-1 mutant animals on a lawn of E. coli.