Graduate School of Biomedical Sciences, Program in Neuroscience
Dissertations, UMMS; Synapses; Synaptic Transmission; Neuronal Plasticity; Octopamine; Drosophila; Neuromuscular Junction
Life Sciences | Medicine and Health Sciences | Neuroscience and Neurobiology
Synaptic plasticity—the ability of a synapse to change—is fundamental to basic brain function and behavioral adaptation. Studying the mechanisms of synaptic plasticity benefits our understanding of the formation of neuronal connections and circuitry, which has great implications in the field of learning and memory and the studies of numerous human diseases.
The Drosophila larval neuromuscular junction (NMJ) system is a powerful system for studying synaptic plasticity. The NMJ consists of at least two different types of motorneurons innervating the body wall muscles. Type I motorneurons controls muscle contraction using glutamate as the neurotransmitter, while type II are modulatory neurons that contain octopamine. Octopamine is a potent modulator of behavior in invertebrates. Nevertheless, its function at the synapse is poorly understood.
In my thesis research, I investigated the role of octopamine in synaptic plasticity using the Drosophila NMJ system. Preliminary observations indicate that increased larval locomotion during starvation results in an increase of filopodia-like structures at type II terminals. These structures, which we termed as “synaptopods” in our previous studies, contain synaptic proteins and can mature into type II synapses. I demonstrated that this outgrowth of type II terminals is dependent on activity and octopamine. Mutations and genetic manipulations affecting the production of octopamine decrease synaptopods, whereas increase of type II activity or exogenous application of octopamine increase synaptopods. Interestingly, I found that the type II octopaminergic neurons have an absolute dependence on activity for their innervation of the muscles. Blocking activity in these neurons throughout development results in no type II synapses at the NMJ, whereas blocking activity after the formation of synapses results in gradual degradation of type II terminals.
Next, I examined the autoregulatory mechanism underlying the octopamine-induced synaptic growth in octopaminergic neurons. I discovered that this positive-feedback mechanism depends on an octopamine autoreceptor, Octß2R. This receptor in turn activates a cAMP- and CREB-dependent pathway that is required in the octopamine-induction of synaptopods. Furthermore, I demonstrated that this octopaminergic autoregulatory mechanism is necessary for the larva to properly increase its locomotor activity during starvation.
Thirdly, I investigated the possibility that type II innervation might regulate type I synaptic growth through octopamine. We found that ablation, blocking of type II activity, or the absence of octopamine results in reduced type I outgrowth, and this paracrine signaling is mediated by Octß2R which is also present in type I motorneurons.
Lastly, the function of another octopamine receptor, Octß1R, was examined. In contrast to Octß2R, Octß1R is inhibitory to synaptic growth. I demonstrated that the inhibitory effect of this receptor is likely accomplished through the inhibitory G-protein Goα. Similar to Octß2R, Octß1R also regulates the synaptic growth of both type I and type II motorneurons in a cell-autonomous manner. The inhibitory function of this receptor potentially breaks the positive feedback loop mediated by Octß2R, allowing the animal to reset its neurons when the environment is favorable.
In summary, the research presented in this thesis has unraveled both autoregulatory and paracrine mechanisms in which octopamine modulates synaptic and behavior plasticity through excitatory and inhibitory receptors.
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Koon, Alex C., "Autoregulatory and Paracrine Control of Synaptic and Behavioral Plasticity by Dual Modes of Octopaminergic Signaling: A Dissertation" (2011). University of Massachusetts Medical School. GSBS Dissertations and Theses. Paper 572.