POSTEMBRYONIC DEVELOPMENT OF LEG MOTOR CIRCUITS

The proper functioning of the adult nervous system requires not
only that neurons develop and interconnect properly during the
embryonic period, but also that they retain some degree of
plasticity in postembryonic life. As the nervous system matures,
existing neurons may be modified to participate in new behavior.
The primary objectives of this proposal are to understand the
extent to which existing neurons can be modified structurally and
functionally during postembryonic life, how these modifications
are related to behavioral change, and how this plasticity is
induced and regulated. Answers to these important questions
require a simple model system in which it is possible to identify
individual neurons and follow them as they are modified
postembryonically. The experiments in this proposal concentrate
upon the neural reorganization that accompanies insect
metamorphosis, specifically upon the circuitry controlling the
larval and adult legs of the moth, Manduca sexta. Although many
of the same neurons are retained, the function of the legs is quite
different in two stages. These retained neurons must delete
synaptic connections important for larval behavior and form new
interconnections appropriate for driving the adult legs. In this
simple model system, intracellular recording techniques will be
used to identify individual motorneurons, sensory neurons, and
interneurons and to inject persistent marker molecules which will
be used to follow these cells through metamorphosis. Specific
aims are to determine which neurons are modified structurally
and functionally, and to relate these modifications to changes in
behavior. Furthermore, just as steroid hormones are known to
influence the development of vertebrate nervous systems, steroid
and steroid-like hormones are important regulators of insect
metamorphosis. Precise hormonal manipulations, including the
introduction of steroids into individual neurons, will be used in the
future to pursue the mechanisms responsible for the induction of
neural plasticity. These experiments will result in a better
understanding of the cellular mechanisms underlying the
development of normal neural function, and provide a model
system which may be useful in the future for investigating the
molecular mechanisms involved in neuronal differentiation.