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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
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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.
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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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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.