Development of specific chemical and electrical synapses in the longitudinal motor neuron circuit of the leech

摘要

The proper operation of any neural circuit depends on the formation of specific synaptic connections, but there have been few direct physiological investigations of the development of synaptic specificity.;I investigate the formation of specific synapses in the longitudinal motor neuron circuit of the leech. These cells form a well-characterized network of central connections that includes both chemical and electrical synapses. Using electrophysiological techniques, I show that chemical synapses first become active at 55% of embryonic development (ED), while electrical synapses are present as early as 51% ED. During development, the longitudinal motor neurons form inappropriate electrical synapses that are not present in adults. It has been widely hypothesized that the function of such transient electrical synapses is to coordinate neural activity during development, promoting the activity-dependent refinement of chemical synapses; however, I found no evidence that embryonic leech neurons form inappropriate chemical synapses, suggesting that transient electrical synapses have some other function in this system.;To investigate the formation of transient electrical synapses in more detail, I filled longitudinal motor neurons with Neurobiotin. In adult ganglia, each longitudinal motor neuron had a consistent pattern of Neurobiotin staining, indicating that it was reliably coupled to the same cells. Filling the same neurons in embryonic ganglia frequently stained cells that were not part of the adult pattern, confirming that inappropriate coupling occurs during development. Inappropriate coupling was specific to particular cells at particular developmental stages, and some forms of coupling were associated with the ends of growing processes, implicating this phenomenon in dendritic pathfinding.;In addition to my developmental work, I used electrophysiological recordings from longitudinal motor neurons as an assay to study crawling, an adult locomotor behavior. Crawling has previously been thought to require sensory feedback because the intact behavior is strongly modulated by sensory information. I show that sensory feedback is not required because an isolated leech nerve cord can produce a crawling motor pattern similar to the one seen in semi-intact preparations. However, deficiencies in the isolated motor pattern suggest that sensory feedback modulates the isolated central pattern generator to help produce the normal crawling motor pattern.