Examining the Roles of Octopamine and Proctolin as Co-Transmitters in Drosophila melanogaster
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The nervous system is a highly complex and intricate system that interacts with and controls nearly all the other body systems. The basic functions of nerve cells are conserved across most species and are very similar between vertebrates and invertebrates. Chemical transmitters (neurotransmitters) facilitate communication between nerve cells and their targets. The effects of these signals can be modified by co-transmitters that are released from neurons in conjunction with neurotransmitters, and by neuromodulators that are released as hormones. This thesis examines the effect of two neuromodulators on neuromuscular junctions of the fruit fly, Drosophila melanogaster. Two modulators, proctolin and octopamine, have been identified in motor nerve terminals and are thought to be released as co-transmitters to modify the effects of glutamate, the neurotransmitter that depolarizes muscle cells and triggers contraction. The neuropeptide proctolin (Arg-Tyr-Leu-Pro-Thr) was found to increase the amplitude of body wall muscle contractions elicited by glutamate in the absence of nerve stimulation. Thus, proctolin appears to enhance contractions by acting postsynaptically. Previous work reported that increasing neural activity lowers the threshold and EC50 for proctolin’s ability to enhance nerve-evoked contractions by two orders of magnitude. To determine whether such activity-dependence is caused by increased release of glutamate, effects of varying glutamate concentrations on the effectiveness of proctolin are examined here. The threshold for proctolin to increase body wall contractions decreased from 100 nM to 10 nM when glutamate concentration increased from 5 mM to 7 mM, but the threshold increased again to 100 nM for glutamate concentrations of 10-20 nM. Thus, although the effectiveness of proctolin shows some dependence on glutamate concentration, alterations in glutamate levels do not appear to account entirely for the more substantial and more consistent changes in proctolin threshold that occur with increasing neural activity, reported elsewhere. Since octopamine in known to be present in motor neurons innervating most of the body wall muscles of 3rd instar larvae, it was hypothesized that stimulating the motor neurons should release octopamine together with glutamate, and that increasing motor neuron activity should increase the release of both octopamine and glutamate. This hypothesis led to the prediction that an octopamine antagonist, phentolamine, should reduce the amplitude of nerve-evoked contractions, and that the antagonist should be more effective when the motor neurons are stimulated at higher frequencies. Phentolamine, however, did not alter the amplitude of body wall muscle contractions elicited by stimulating the motor axons using impulse bursts with intraburst stimulus frequencies of 5, 32 and 50 Hz. Surprisingly, exogenously applied octopamine did enhance the amplitude of nerve-evoked contractions, and, this effect was antagonized by phentolamine when contractions were elicited by impulse bursts with frequencies of 5 and 50 Hz. At a concentration of 1x10-6 M, octopamine did not induce contractions or alter the amplitude of glutamate-evoked contractions. These results do not support the hypothesis that endogenous octopamine is released onto muscle fibers as a co- transmitter to augment contraction amplitude. One possible explanation for these findings is the octopamine may be released at higher concentrations at neuromuscular synapses, and the effects of octopamine on nerve-evoked contractions are mediated presynaptically, by increasing transmitter release. Overall, the results of this thesis indicate that both octopamine and proctolin modulate muscle contractions in an activity-dependent manner; the level of external nerve-stimulus or exogenous glutamate concentration alter the effectiveness of the contransmitters. However, further work is needed to elucidate the mechanisms of such activity-dependence.