The long-term objective of this research project is to discover how a nerve cell can be induced to change its anatomy, its function and its molecular components when its growing terminals encounter a potential target. Such inductive interactions help to produce the remarkable precision in the function of nerve cell networks that is established during embryogenesis and re-established after nervous system lesions. The studies in this project focus on the development of particular identified neurons, the Retzius (Rz) cells, in the medicinal leech. This animal was chosen because its nervous system is simple and accessible, and because individual nerve cells can be identified before and after induction takes place. One type of target tissue, the embryonic reproductive tissue, induces changes in several feature of the Rz cells, including their branching pattern, their sensory inputs, and the types of receptors in their cell membranes that respond to the neurotransmitter acetylcholine (ACh). The current proposal is to investigate the cellular and molecular mechanisms responsible for these induced changes, primarily those which control the choice of ACh receptors. Individual Rz cells will be placed into culture with a variety of potential target tissues, in order to determine which tissues affect the type of ACh receptor expressed by Rz cells. If the cultured cells retain their developmental capabilities, reproductive tissue will selectively induce the expression of an ACh receptor whose activation inhibits rather than excites Rz cells. Using this in vitro assay, several features about the inducing substance will be determined: whether it is secreted from the target cells or is bound to them; whether it is present throughout development or only transiently; and whether it is a protein similar to growth factors in other animals. The mechanisms underlying the Rz cell responses to the inducer will also be investigated, by determining: whether known second messengers (i.e., cAMP, cGMP, Ca2+, IP3, CAG and receptor kinases) are involved; what is the contribution of axonal transport; and whether electrical activity in the Rz cells is necessary. Also to be determined is whether the expression of the hyperpolarizing ACh receptor is controlled by activating its gene (transcription), by producing its protein subunit (translation), or by modifying its subunit post-translationally. It is clear that cell-cell interactions determine many of the structural and functional features of nerve cells, and that disruption of these interactions leads to a variety of pathological states. The proposed studies, by determining the molecular nature of one inductive interaction in a relatively simple nervous system, should help to understand how normal-- and abnormal--function is brought about.