Synaptic activity of the AFD neuron in Caenorhabditis elegans correlates with thermotactic memory

Thermotactic behavior in Caenorhabditis elegans is sensitive to both a worm's ambient temperature (T(amb)) and its memory of the temperature of its cultivation (T(cult)). The AFD neuron is part of a neural circuit that underlies thermotactic behavior. By monitoring the fluorescence of pH-sensitive green fluorescent protein localized to synaptic vesicles, we measured the rate of the synaptic release of AFD in worms cultivated at temperatures between 15 and 25 degrees C, and subjected to fixed, ambient temperatures in the same range. We found that the rate of AFD synaptic release is high if either T(amb) > T(cult) or T(amb) < T(cult), but AFD synaptic release is low if T(amb) congruent with T(cult). This suggests that AFD encodes a direct comparison between T(amb) and T(cult).