Date of Award
Doctor of Philosophy
Zoology and Genetics
Philip G. Haydon
In these studies, the inhibitory effects of a neuropeptide, FMRFamide, on synaptic transmission was examined. A model system of cultured synapses was used to investigate the subcellular mechanisms underlying the action of FMRFamide. In this model system, a unidirectional synapse forms between somata of two identified neurons, B5 and B19. This offers several advantages, the main one being the large size of the synaptic terminals which allows for electrophysiological access to both elements of the synapse as well as control over the intracellular and extracellular environments of the neurons;FMRFamide is endogenous to Helisoma and has widespread effects on molluscan neurons. The magnitude of the action potential-evoked postsynaptic potential was reduced in the presence of FMRFamide. Desynchronized release in response to a long depolarization was first examined to determine the locus of the effects of FMRFamide. The number of postsynaptic currents during the stimulus was reduced which is consistent with a presynaptic locus of action;The effect of FMRFamide of the presynaptic calcium current was examined since calcium is the trigger for neurotransmitter release. Using voltage clamp techniques, the magnitude of the macroscopic calcium current was found to be reversibly reduced by FMRFamide. There is an additional site of action within the presynaptic terminal. Presynaptic neurons were dialyzed with a photosensitive calcium chelator to elevate and clamp the calcium to a constant level to promote desynchronized release. Under these conditions, secretion from the presynaptic cell was reduced in the presence of FMRFamide. The sensitivity of the secretory machinery to calcium was reduced. Thus, FMRFamide has at least two sites of action in the presynaptic terminal to cause an inhibition of transmitter release;The mechanism of the effect of FMRFamide on the calcium current in growth cones as well as somata was examined further. Using guanine nucleotides and toxins, this inhibitory effect was found to be mediated by a pertussis toxin-sensitive G protein. The inhibitory effect of FMRFamide on action potential-evoked release was also found to be mediated by a pertussis toxin-sensitive G protein. Purified, preactivated [alpha]-subunits of G[subscript] o1, G[subscript] o2, and G[subscript] i3 were microinjected into somata to determine which were able to interact with and inhibit the channels. Of the three G proteins tested, only the [alpha]-subunit of G[subscript] o2 was able to reduce the magnitude of the macroscopic calcium current.
Digital Repository @ Iowa State University, http://lib.dr.iastate.edu/
Helen J. Man-Son-Hing
Man-Son-Hing, Helen J., "Modulatory actions of the neuropeptide FMRFamide on synaptic transmission in Helisoma neurons " (1991). Retrospective Theses and Dissertations. 9553.