When a worm gets hungry, it does not simply turn down its heat alarm. Starvation changes the alarm’s neural code—and flips several signals from promoting escape to suppressing it. To survive, animals must detect and avoid noxious stimuli, including harmful chemicals, toxic gases, and extreme temperatures.

These avoidance behaviors are often categorized as nociceptive, are conserved across phyla, and are essential for minimizing damage and guiding appropriate behavioral responses. C. elegans produces robust avoidance behaviors in response to noxious temperatures above twenty-six degrees Celsius or to fast-rising thermal stimuli even below twenty-six degrees Celsius.

When a forward-moving worm receives acute heat on its head or whole body, it initiates backward locomotion, called a reversal event; tail heating instead accelerates forward locomotion. AWC neurons are polymodal sensory neurons that respond to olfactory cues and temperature in a context-dependent manner.

ASI sensory neurons respond to temperature changes in the innocuous range, but their implication in noxious-heat responses was not known. Neuromodulators, including neuropeptides and monoamines, play a central role in reconfiguring behavioral responses to sensory inputs based on context.

A reduction of nociceptive responsiveness during prolonged starvation could help worms travel across harsher environments to find new food sources. The impact of prolonged starvation and neuromodulation on context-dependent thermonociceptive behavior was largely unknown.

The study investigates how feeding state modulates heat avoidance and reports that prolonged starvation suppresses thermonociceptive responses because of nutrient availability rather than external chemosensory cues. To assess how feeding state modulates thermonociceptive behavior, responses were compared across different durations of food deprivation.

Fed animals showed high sensitivity to noxious heat, but they also had an elevated baseline of spontaneous reversals, limiting their usefulness as a control group. One hour off food reduced spontaneous reversals and mildly attenuated responses to low-intensity heat, while stronger responses remained comparable to fed animals.

Prolonged food deprivation caused a striking progressive reduction in thermonociceptive responses, and after six hours of starvation the responses approached baseline spontaneous reversal rates. After six hours of starvation, thermonociceptive responses approached baseline spontaneous reversal rates, indicating robust inhibition of nociceptive behavior.

Similar starvation experiments were conducted in the presence of food odor to distinguish the absence of nutrients from the absence of chemosensory cues. The reduction in thermonociceptive response persisted with food odor, indicating that the effect was driven by the internal starvation state rather than external olfactory input.

In panel B, the six-hour curve remains near the bottom across the heating-power range, while the one-hour curve rises toward roughly eighty percent reversing at the highest stimulus. Panel C shows the same separation over time: the four-hundred-watt response falls from about eighty percent at the start to below twenty percent after six hours.

To identify neural substrates of thermonociceptive behavior and its modulation by starvation, candidate thermosensory neurons were genetically ablated. Ablation of AFD, the canonical thermosensory neuron, had no significant effect under either one hour or six hours of food deprivation.

Removing AWC nearly abolished heat-evoked reversal behavior across stimulus intensities and timepoints, confirming its essential role in the thermonociceptive response. In AWC-ablated animals, response levels were unaffected by starvation, suggesting that AWC was also required for expressing starvation-dependent plasticity.

ASI ablation had almost no effect during early food deprivation but almost entirely suppressed the starvation-induced reduction in heat-evoked reversals after prolonged starvation. Under these experimental conditions, AWC was the primary driver of heat-evoked reversals during early food deprivation, while ASI selectively mediated their suppression during prolonged starvation.

In panel A, the wild-type curves separate sharply at higher heating powers, reaching roughly eighty percent versus twenty percent at four hundred watts, with significance markers at two hundred through four hundred watts. In panel D, the four-hundred-watt bars show the same genotype-specific pattern, while the ablated groups have closely overlapping early-deprivation and starvation bars.

AWC neurons are glutamatergic and express several neuropeptides, including FLP-6, INS-22, and NLP-5. eat-4 mutants lacking the vesicular glutamate transporter EAT-4 showed strong impairments at high stimulus intensities from three hundred to four hundred watts, while flp-6 mutants showed a marked deficit across two hundred to four hundred watts.

Impairments in ins-22 and nlp-5 mutants were less pronounced and more selective, affecting the two-hundred-to-three-hundred-watt range. Different communication molecules were therefore used over different thermal ranges to mediate heat-evoked reversals. Cell-specific rescue experiments tested whether AWC neurons were relevant sources of glutamate and FLP-6 neuropeptides promoting heat-evoked reversals after one hour off food.

Restoring eat-4 expression in either AWCON or AWCOFF alone was sufficient to recover heat-evoked responses to near-wild-type levels. The AWCOFF eat-4 transgene also enhanced reversals at one hundred watts, and spontaneous reversal rates were restored only by AWCOFF eat-4.

For FLP-6, only AWCOFF flp-6, rather than AWCON flp-6, rescued heat-evoked behavior, suggesting asymmetry in neuropeptidergic signaling. In panels B and D, the rescue curves separate most clearly at the higher heating powers: the AWCOFF rescue traces approach the wild-type reference, while the AWCON FLP-six trace remains close to its mutant curve.

Panel E summarizes this visually, assigning glutamate from both AWC subtypes to heat-evoked reversals, but linking FLP-six specifically to AWCOFF. To investigate starvation effects at the cellular level, heat-evoked calcium dynamics were recorded in AWCON and AWCOFF neurons using YC2.3 cameleon-expressing transgenic lines.

A microfluidic setup delivered defined thermal stimuli from a twenty-degree-Celsius baseline, reaching twenty-two, twenty-four, twenty-six, and twenty-eight degrees Celsius in sequence. Individual traces were classified as calcium up, variable, or calcium down according to their patterns after the twenty-four, twenty-six, and twenty-eight-degree stimuli.

After one hour off food, most animals showed stereotyped, intensity-dependent calcium increases: forty-five of fifty-six AWCOFF traces and thirty-seven of forty-nine AWCON traces. After prolonged starvation, most traces became variable or showed consistent calcium down-regulation: seventeen of twenty-three AWCOFF traces and fourteen of seventeen AWCON traces.

The altered average trace was largely caused by redistribution among response types, while the dynamics of individual calcium-up and calcium-down traces remained similar in each condition. Under early food deprivation, AWC neurons responded consistently to heat; under prolonged starvation, they entered a heterogeneous and less predictable response mode.

Starvation therefore did not simply attenuate AWC calcium responses; it fundamentally altered their dynamics from a largely deterministic excitation profile to a heterogeneous regime with decreased and variable responses. Because ASI neurons were required for starvation-dependent thermonociceptive plasticity and had been shown to signal feeding state, the study tested whether ASI mediated starvation’s impact on AWC activity patterns.

AWCON and AWCOFF activity were recorded in ASI-ablated animals, which lacked starvation-evoked plasticity. After six hours off food, starvation produced no noticeable effect in ASI-ablated animals, neither for AWCON nor for AWCOFF. The starvation-dependent shift in AWC activity from a deterministic to a stochastic pattern was therefore mediated by the neuromodulatory neurons ASI.

In both upper plots, the black and red average traces nearly overlap across all four temperature steps, with peaks occurring at similar times and amplitudes. The heat maps below likewise show closely matched orange response bands in the early-deprivation and prolonged-starvation rows, rather than a visibly different spatial pattern.

The study tested whether starvation-dependent plasticity was preserved in eat-4 and flp-6 mutant backgrounds. In flp-6 mutants, heat-evoked responses were reduced after early food deprivation, but a significant further decline occurred after prolonged starvation.

These results indicate that starvation-dependent plasticity can operate independently of FLP-6. Because ASI is required for starvation-evoked plasticity but does not release classical neurotransmitters, the study tested whether ASI function was mediated by neuropeptidergic signaling.

Mutants for ins-6, ins-32, and nlp-18 showed reduced responsiveness after early food deprivation but abnormally elevated responses after prolonged starvation. ASI-specific expression of ins-32 significantly restored reversal response in early food-deprived animals and significantly reduced heat-evoked reversal response under starvation.

ASI-specific expression of nlp-18 partially restored the response decrease after six hours of starvation, although it did not restore normal responsiveness after one hour off food. The results indicate that ASI is a functionally relevant source of INS-32 and NLP-18 for promoting starvation-evoked thermonociceptive inhibition, while contributions from other sources remain possible.

Starvation-dependent thermonociceptive plasticity was orchestrated by multiple opposing signaling pathways in which neuropeptide and glutamate signals from different neurons produced context-dependent, bidirectional effects. In panel A, the wild-type curves separate at higher heating powers: the prolonged-starvation trace is near forty percent at four hundred watts, while early deprivation reaches about eighty percent.

In panel D, AWCOFF rescue rises to roughly eighty percent at three hundred and four hundred watts, compared with about fifty to sixty percent for the non-transgenic eat-4 mutant. Starvation modulated thermonociceptive behavior by engaging distinct neurons, altering neural response dynamics, and mobilizing opposing neuromodulatory pathways.

Progressive attenuation of heat-evoked reversals under prolonged food deprivation depended on an internal-state shift that was not rescued by food-associated olfactory cues. AWC and ASI formed two key regulatory nodes integrating thermal and nutritional cues into an adaptive behavioral output.

In the left panel, the AWC traces are drawn as repeated upward calcium responses, with green arrows from AWC, ASI, and other sources converging on heat-evoked reversals. In the right panel, the traces include upward and downward deflections, while several pathways terminate in red inhibitory bars; dashed green arrows leave only selected routes active.

AWC neurons used glutamatergic and neuropeptidergic signals to encode heat intensity: glutamate through EAT-4 primarily drove stronger stimuli, while FLP-6 acted across a broader thermal range. Prolonged starvation transformed heat-evoked AWC calcium responses from a stereotyped, intensity-dependent activation pattern into a heterogeneous, less predictable regime.

This shift was a qualitative reorganization of response states, including frequent calcium down-regulation or variable traces that could contain both calcium up- and down-regulation during one stimulus train. Under prolonged starvation, dampened and variable AWC responses may bias the animal away from escape behaviors and prioritize food-seeking over threat avoidance.

ASI neurons were dispensable for baseline thermonociceptive responses but essential for starvation-induced behavioral plasticity. ASI ablation abolished the starvation-dependent attenuation of heat-evoked reversals and prevented the stochastic shift in AWC calcium dynamics.

These results suggest that ASI gates AWC activity modulation according to internal nutritional state. Starvation most likely uses different circuitry to modulate noxious-heat-evoked reversal than to modulate thermotaxis. The balance between starvation-induced suppression and preservation of thermonociceptive behavior appears to be controlled by signaling molecules whose effects are bidirectional and context-dependent.

During early food deprivation, INS-32 from ASI, NLP-18 from an unidentified source, and glutamatergic signals from AWC neurons promote thermonociceptive behaviors. During prolonged starvation, the same signaling molecules can produce the opposite effect and suppress thermonociceptive behavior.

The findings suggest a regulatory architecture in which bidirectional neuromodulatory signals integrate metabolic context with sensory drive to determine behavioral priorities. Rather than globally silencing the thermonociceptive pathway, starvation may reconfigure it through neuronal state shifts, circuit-level inhibition, and peptide-based tuning.

Under prolonged food deprivation, protective escape may come at the cost of missed opportunities for locating food. A heightened aversive response could cause excessive avoidance of mildly threatening environments that may nevertheless contain nutritional resources.

By reducing the sensitivity or reliability of nociceptive pathways, particularly through modulating AWC output, C. elegans may adopt a more exploratory behavioral mode and tolerate greater risk to increase the chances of finding food. This trade-off between protection and exploration mirrors state-dependent shifts observed in other species and suggests a conserved strategy in which metabolic needs reshape threat evaluation and behavioral responses.

Prolonged starvation reconfigures heat avoidance through AWC and ASI neurons: AWC activity becomes variable, while glutamate and neuropeptides can switch from promoting reversals to suppressing them. The result is a circuit tuned to internal nutritional state.