Date of Award


Degree Name

Biological Sciences


College of Science

Type of Degree


Document Type


First Advisor

Brian L. Antonsen

Second Advisor

Elmer M. Price

Third Advisor

Eric R. Blough


The lateral giant (LG) neural circuit is a model system used to study the function of individual neurons. The LG circuit is part of a tail flip escape reflex that is a defensive behavior for crayfish. This thesis begins by addressing the effects of dopamine (DA), a neurotransmitter involved in normal and abnormal behaviors of most animals. Here it is shown that dopamine decreases the excitability of the LG neuron, a trigger for the escape reflex. An electrophysiology protocol was used to mimic sensory input to the LG neuron. Stimulating a sensory nerve in the last ganglion with an electrode evoked an excitatory post synaptic potential (EPSP) in the LG similar to a real tactile stimulus. Exogenous dopamine was applied to the mostly intact nervous system at different concentrations and different bath temperatures while EPSPs were recorded from inside the LG’s axon. Dopamine consistently caused EPSPs to decrease 10% in bath temperatures near 20°C. Below 17°C the depressive effects of dopamine were more variable. The second part of this thesis addresses the effects of dopamine and serotonin (5- HT) on the LG. The influence of serotonin on behavior and physiology has been shown to overlap with that of dopamine. Both amines are present in the terminal ganglion near the LG, and both are expected to reach the LG through classical synapses, volume transmission, and through the hemolymph. Therefore the effects of dopamine and serotonin on excitability of the LG were tested. Together the amines increased the amplitude of EPSPs beyond threshold to evoke action potentials in the LG. However, when dopamine was applied long before serotonin, EPSP amplitude did not increase. These data indicate that there is a temporal component to the summation of modulatory effects in neurons. Cross talk between the two systems should be considered to gain a better understanding of diseases associated with either amine. Monoamine receptors and second messengers that mediate the effects of dopamine and serotonin are highly conserved.


Neural circuitry.

Crayfish - Behavior.