| Summary: | 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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