When navigating at temperatures above their cultivation temperature, worms move down spatial thermal gradients. However, at or near their cultivation temperature, worms track isotherms. We have shown that while the mechanism for isothermal tracking works both in the presence and absence of food, migration down gradients is suppressed in the presence of food. Therefore, food appears to specifically inhibit only one of the two thermotactic behavioral modes. Since the positions of isothermal tracks reflect the memory of cultivation temperature, this provides us with a simple assay to measure thermal memory. In this assay, we impose a linear thermal gradient from 15C-25C on a plate containing a population of worms on a bacterial lawn, calculate the trajectories of crawling worms using video microscopy and particle tracking, and score the positions of isothermal tracks. We have used this memory assay to determine the rates and characteristics of learning in normal and mutant worms. N2 worms originally cultivated at 15C and shifted as adults to 25C will adjust the locations of their isothermal tracks to reflect the new temperature in ~3 hours. However, animals originally cultivated at 25C and shifted to 15C require ~4-6 hours to 'learn' the new temperature. Using enriched populations of AFD neurons and microarrays, we identified the diacylglycerol kinase gene
dgk-3 which is expressed strongly in the AFD neuron and more weakly in additional sensory neuron types.
dgk-3 mutants exhibit wild-type behaviors on spatial and temporal thermal gradients. Interestingly, while
dgk-3 mutant adults shifted from 25C to 15C learn the new temperature at the same rate as wild-type animals (~4-6 hours), they require ~10-12 hours to learn the new temperature when shifted from 15C to 25C. These data suggest that the pathways required for resetting thermal memory upon temperature upshifts and downshifts may be distinct, and that
dgk-3 might play a specific role in memory formation or resetting upon upshifts. We are exploring the cellular basis of the memory formation and investigating the role of DGK-3 mediated pathways in thermotactic behavioral plasticity.