C. elegans can associate cultivation temperature with feeding state and modify behavioral outputs in response to environmental changes. After the animals were grown under well-fed condition, they migrate to cultivation temperature on a temperature gradient. We found that animals that were conditioned to thermotax to cultivation temperature become to avoid that temperature after starved under uncrowded condition for only 20 minutes (for 25C-grown animals) or for 2.5 hours (for 17C-grown animals). We also found that recovery from the starved state occurs very quickly: after animals that had been conditioned to avoid cultivation temperature were re-cultivated with food only for 10 minutes, they are fully capable of migrating to cultivation temperature (for both 25C- and 17C-grown animals). Like in the case for egg-laying, pharyngeal pumping and locomotion activity, exogenous serotonin and octopamine were found to mimic well-fed and starved state, respectively, in thermotaxis. To further address the importance of serotonin signaling in thermotaxis, we assayed thermotaxis of serotonin-defective mutants (
cat-1 ,
cat-4 ,
bas-1 and
tph-1 ). Surprisingly, these mutants showed almost normal thermotactic responses in any aspects. We noticed however that
cat-1 mutants recover from the starved state 10 minutes more slowly than the other mutants and wild type animals. This delay in
cat-1 mutants implicates the involvement of synaptic transmission in thermotaxis, since CAT-1, vesicular monoamine transporter, is thought to be required for neurotransmitter release at synaptic terminal. We also showed that killing NSM serotonergic neurons in the pharynx did not affect thermotaxis. Altogether, these results led us to propose two models. Although exogenous serotonin can substitute food signal in thermotaxis, other in vivo component might be important for food-temperature association. Alternatively, endogenous serotonin that could be still synthesized even at low level in serotonin-defective mutants sufficiently works as food signal in thermotaxis. To identify genes and neurons required for associating temperature memory with feeding state, mutants were sought that can respond to feeding states normally, but are defective in starvation-induced temperature avoidance. By screening about 5,300 genomes for animals that constitutively migrated to cultivation temperatures despite starvation experience, we obtained several mutants designated aho (abnormal hunger orientation). These aho mutants responded normally to food in locomotion assay, indicating that they can feel feeding states properly. The dominant
aho-1(
nj5) mutation maps to chromosome I and leads to abnormal defecation as well as pharyngeal pumping that is reflected in EPG (T. Niacaris and L. Avery, pers. comm.), although
aho-1(
nj5) mutants exhibited normal chemotaxis to Na + ion and odorants, and normal olfactory adaptation. The recessive aho
(nj15) mutation maps to the center of chromosome I, and aho
(nj15) mutants showed normal chemotaxis to Na + ion and odorants. We suggest that aho mutants are the first example of novel thermotaxis-defective mutants, in which modulation of thermotaxis by the feeding state is specifically impaired. This also implies that the process for modulatory aspect of thermotaxis is separable from the process for primary thermosensation and temperature memory formation. The
hen-1 mutant defective in integration of aversive and attractive stimuli, and chemotaxis-based learning paradigm (see abstract by Ishihara et al.) also showed Aho phenotype. It is thus likely that mutants generally defective in learning are included among our aho mutants. We are currently testing this possibility. We are grateful to G. Ruvkun, Y. Shibata and S. Takagi for serotonin-deficient mutants; J. Sze for
tph-1::GFP construct; T. Niacaris and L. Avery for EPG recording; S. Wicks for SNP data.