Formation of appropriate synapses in the nervous system of the medicinal leech

• Main Author: Todd, Krista L
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2009
• Subjects: Neurosciences
• ISBN: 1109319215; 9781109319217

The ability of complex organisms to sense their environment, make decisions, and behave appropriately relies upon the synaptic connections within their nervous systems. Developmentally, neurons reliably form predictable connections onto other neurons. My work examined how neurons choose their specific synaptic partners. We chose the leech to study the developmental formation of synapses based on its simple nervous system, identified neurons, characterized neuronal development, and known behavioral circuitry. Chapter 2 demonstrates that electrical synaptic connections are required prior to chemical synaptogenesis to permit the development of appropriate chemical synapses between neurons. We used RNA interference in single embryonic neurons to transiently decrease the mRNA of innexin-1, a component of electrical synapses in the leech CNS. Pressure-mechanosensory (P) cells treated in this manner failed to form a normal chemical synaptic connection onto a known post-synaptic partner, the Anterior Pagoda (AP) cell. Each of the four P cells in a midbody ganglion transduces sensory information from a quadrant of the leech’s body wall and through chemical synapses with 24 interneurons mediates an avoidance behavior called a local bend. Disrupting the expression of electrical synapses in a single P ablated the behavioral response to stimuli within that P cell’s receptive field. Treated P cells transduced sensory input, but failed to transmit that information to postsynaptic neurons. Chapter 3 describes a novel technique to study motor neuron development in leech embryos. Somata of most leech motor neurons lie on the dorsal surface of each ganglion, facing the interior of the animal. Thus, following the development of motor neurons over time requires that embryos must be dissected, embedded in agarose, and maintained in organotypic culture. This technique allowed us to keep embryos healthy many days longer than previously reported, revealing to us that the RNA interference method of the previous chapter was effective in motor neurons, as well as in sensory neurons. Chapter 4, reports how transplanting peripheral target tissues affected the morphology and central synaptic connections of developing motor neurons. After transplanting pieces of body wall either ectopically or eutopically, we determined how the presence of this altered peripheral tissue affected neuronal development.