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A developmental analysis of habituation in C. elegans Gannon, Timothy Neil


C. elegans is particularly well-suited for a developmental analysis of learning and memory. A developmental analysis of learning attempts to establish relationships between the emergence of particular forms of learning and memory, and the emergence of specific neural structures and circuits, and C. elegans’ ontogeny is extremely well-characterized, including the complete developmental lineage of each somatic cell. Previous developmental research found that whereas the adult and young adult worms almost always reversed and swam backwards to tap, the larval worms only reversed about one-half of the time, and accelerated forwards the other half of the time. This response heterogeneity is a serious problem, because accelerations and reversals are two qualitatively different behavioral outcomes that cannot be easily compared. One solution to this problem is to laser ablate the PLM sensory neurons (which results in a worm that only reverses to tap) in one experiment, and ablate the ALM sensory neurons (which results in a worm that only accelerates forward in response to tap) in a second experiment so that in each experiment there is a single, homogeneous response type that can be compared across and within groups. The focus of this thesis was to study the effect of repeated stimulation on accelerations and reversals in ablated worms across development. Habituation training was given to PLM-ablated and ALM-ablated animals at each of the 6 developmental stages (LI, L2, L3, L4, young adult, 4 day old adult), and at each of 2 ISIs (10 second and 60 second) using the tap-withdrawal paradigm. The results showed that habituation and recovery from habituation were both present in C. elegans at all stages, both ISIs, and both response types. There was also surprisingly little systematic variation in the characteristics of habituation and recovery over development, which further suggests that the basic neural machinery underlying this behavioral plasticity is present and functional as early as the LI stage. These experiments represent the first systematic and quantitative investigation of learning and memory in C. elegans over the course of its development.

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