We plan to continue our investigation of the causes and functional consequences of dendritic regression in proleg motoneurons (MNs) in the tobacco hornworm, Manduca sexta, with these objectives: (1) to characterize anatomical and electrophysical changes in the monosynaptic connections between planta hair afferent neurons and the proleg retractor MNs during the larval-pupal transformation. This is the time during which the MNs undergo their hormonally-mediated regression and the proleg withdrawal reflex disappears. These studies will involve conventional neuronal staining techniques in combination with intracellular neuronal recordings to determine the number and strength of afferent-to-MN synapses. Possible pre- and postsynaptic causes for the weakening of the afferent-to-MN reflex will be sought. (2) To test possible causal relationships between afferent input to the MNs and MN structure, both during larval development and during the larval-pupal transformation. These experiments will involve surgical ablation of prolegs during larval life, and hormonal treatments to produce heterochronic mosaics with mixed larval and pupal attributes. Anatomical and electrophysiological studies will examine the possible causal roles of afferent synaptic input in the attainment of the larval morphology of the MN, and in the subsequent regression of the larval arbor during pupal development. (3) The final objective is to expand these studies to include interneurons (INs). Excitatory and inhibitory INs which provide monosynaptic input to the proleg MNs will be sought, and the status of these synapses during the larval-pupal period will be followed anatomically and electrophysiologically.
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