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CENTRAL GOALS AND QUESTIONS

A fundamental problem in the study of neuroethology is understanding how sensory information, acquired from the environment, is coded by higher-level structures in the central nervous system (CNS), leading to some new or modified behavioral act. Understanding how behavioral acts and rituals are expressed and modulated is my central interest. Specifically, why does an animal at a specific time perform one behavior as opposed to another? I have been using crustaceans primarily the American lobster, Homarus americanus, as an animal to model. This animal is intrinsically aggressive and can be manipulated to fight in close quarters in experimental tanks within a closely monitored laboratory environment.

The goal of my studies is to reveal how animals exchange and integrate sensory information during agonistic encounters; how this information is incorporated and integrated into the CNS, how that information is relayed to individual neural circuits; and what change in behavior can be documented. To understand these processes, I focus on three major areas: 1) identifying single specific behaviors that can be measured and quantified; 2) defining constraints on the plasticity of the systems involved in these behaviors, such as the properties of peripheral and central elements in ultimately changing and modifying the behaviors; 3) determining the role, if any, of neurotransmitters, neurohormones, and steroids in modulating these behaviors. To begin to address these issues, I have developed a combination of behavioral and physiological techniques that I have applied to the lobster system. The results of my inquiries provide comparative data that, together with other known systems, can enhance our understanding of the evolution of plasticity of behaviors.

DISSERTATION RESEARCH

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My dissertation research was designed to investigate neuromuscular transmission of junctional potentials of the phasic flexor muscles over the molt cycle to determine whether changes in these responses might contribute to the changing escape response. Experiments involved measuring: 1) postjunctional potentials over the molt cycle, using both juvenile and adult animals; 2) regional muscle differences of excitatory junction potential (EJP) parameters within each molt stage; and 3) EJP and inhibitory junction potential (IJP) characteristics over the molt cycle in the presence of specific neurotransmitter blockers. The results demonstrated for the first time that neuromuscular transmission in both juvenile and adult lobsters is altered over the course of the molt cycle due to presynaptic plasticity. In the juvenile escape behavior, all lobsters, regardless of molt stage, escaped when presented with a stimulus; soft-shelled lobsters (stages A and B) consistently outperformed hard-shelled lobsters (stages C and D) in all parameters of escape. In juvenile neuromuscular transmission of the phasic flexor muscles that drive the escape behavior, distinct differences in EJPs were found among the four molt stages. EJPs of hard-shelled lobsters, failed at high stimulation frequencies, while soft-shelled lobsters continued to produce EJPs. In the adult escape behavior, only 20% of all soft-shelled lobsters escaped compared to 60% for hard-shelled lobsters. Frequency of tail-flips for soft-shelled lobsters was also lower than hard-shelled lobsters. In contrast, electrophysiology experiments on adult hard-shelled lobsters did not show any depression in size of EJPs at high stimulation frequencies, while EJPs of soft-shelled lobsters were depressed or failed altogether at high frequencies.

Therefore, experiments showed that synaptic plasticity in the phasic flexor neuromuscular system exists over a molt cycle and that changes occur over the age of the animal (within a single molt stage). These changes result from increased altered presynaptic transmitter release (since both membrane potentials and resistances did not affect EJP parameters). The changes in the EJPs correlated to the observed behavioral differences in the escape response behavior. It appeared that peripheral modulation at least partly explains the behavioral differences observed. Questions now arise as to how much the CNS contributes to this modulation. Is there modulation of sensory receptor information across age and molt stages?

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MASTER'S RESEARCH

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My master's thesis focused on a single specific behavior-the escape response in the American lobster, Homarus americanus. Through videotape analysis I found that dramatic differences exist in the escape behavior over the molt cycle-as the lobster makes its transformation from a hard- to a soft-shelled state. I found that soft-shelled juvenile lobsters were much more likely to respond to a threat with tail-flipping, whereas hard-shelled, juvenile premolt lobsters were more likely to respond with an aggressive display, such as the meral spread, rather than escape swimming. Premolt lobsters also had a quick, forceful initial power swim, followed by subsequent swims that rapidly decreased in velocity, acceleration, force, and work. As a consequence of the greater force of their initial power swim and subsequent swims, premolt lobsters do more work during an escape, but travel less distance, than do their soft-shelled counterparts. I suspected that these behavioral differences in escape behavior might be reflected in differences in neuromuscular physiology of the abdominal phasic flexor muscles, which are "modulated" throughout the molt cycle.