Effects of aging on habituation in the nematode Caenorhabditis elegans.

The effects of aging on spontaneous locomotor behavior and habituation in a mechanosensory reflex were examined in the nematode Caenorhabditis elegans. Worms were tested at 4 days (at the peak of egg laying), at 7 days (when egg laying ends) and at 12 days post-hatching. Both spontaneous and reflexive movements were smaller in older worms than in younger worms. In addition the magnitude of these movements was related to life span; the shorter an animal's life span, the smaller its reversal movements while still young. Worms at all ages expressed habituation and dishabituation at a 10 s interstimulus interval (ISI); thus even aged worms were capable of non-associative learning. However, older worms showed greater habituation than did 4-day-old worms to stimuli delivered at a 60 s ISI. There was also an age-related change in the recovery from habituation. At days 4 and 7, worms had recovered from habituation by 30 min after training; However, responses of day 12 worms were still significantly smaller than baseline at 30 min after training. Further behavioral tests with normal and mutant worms may help elucidate the nature of the age-related changes in the learning and memory processes of C. elegans and the genetic mechanisms which underlie them.