DeMaegd ML. 2021. Physiological consequences of neuromodulation and the cellular properties that underlie them. Dissertation for Doctor of Philosophy (PhD), School of Biological Sciences, Illinois State University.
https://doi.org/10.30707/ETD2021.20210719070603173187.87
Abstract
Neuronal activity is a product of more than the underlying neuronal
connections. Modulatory influences like changes in the animal’s
environment, the animals physiological state, or the release of
neuromodulators can dramatically alter neuronal activity. Modulatory
influences can be beneficial for the animal because they are a source of
neuronal and behavioral plasticity, and they can provide neuronal
circuits with the robustness needed to continue to function in new
conditions, states, or tasks.However, malfunctions of the modulatory
system can disrupt neuronal activity and lead to pathologies. Predicting
how modulatory influences will alter neuronal activity is challenging
because the underlying cellular and circuit properties are delicately
balanced and often respond in nonlinear and multifaceted ways to
modulatory influences. In my thesis I address how several types of
modulatory influences affect neuronal activity in the crustacean
stomatogastric nervous system, and seek to characterize the circuit and
cellular mechanisms that underlie them. In Chapter II I show that the
activation of chemosensory pathway alters the frequency of
backpropagating action potentials in a proprioceptive sensory neuron
that measures muscle tension when the animal chews. These
backpropagating action potentials invade the most distant regions of the
proprioceptive neuron where muscle tension is encoded, including the
spike initiation site and sensory dendrites. They alter the latency, the
number, and the frequency of action potentials in response to muscle
stimuli. When the chemosensory neurons become active, backpropagating
action potential frequency decreases, thereby granting greater
sensitivity to the muscle tension receptor. Since the chemosensory
pathway is activated by food before the chewing starts, the modulation
of backpropagating action potentials prepares the muscle receptor for
future changes in muscle tension. Thus, my results demonstrate that one
sensory pathway can prime another for upcoming tasks via the modulation
of backpropagating action potentials. In Chapter III I show two ways
that neuronal activity can be sustained during temperature modulation.
First, I show that axons of different pyloric neurons maintain action
potential timing between them over a large temperature range, despite
their distinct morphological and intrinsic properties. I used
computational model axons to determine if, and if so, how, axons with
different diameters that are exposed to varying temperatures can
maintain action potential timing with one another. I found that the
temperature sensitivity of most ion channel properties mattered little
to action potential timing. Conversely, the ratio of two Sodium channel
parameters were critical: how much the maximum conductance and
activation gate time constant in one axon changed with temperature
relative to the other axon strongly influenced action potential timing
between two. Since the ratio was critical, but not the actual values,
this demonstrated that even highly temperature-sensitive ion channels
can support temperature-robust action potential timing between neurons.
Second, I show that acutely warming the stomatogastric ganglion by 3°C
disrupts a gastric mill rhythm by diminishing the spread of electrical
signals in the dendrites of the Lateral Gastric neuron (LG). I also show
that a substance P-related peptide restores dendritic electrical spread
and consequently the gastric mill rhythm at the warmer temperature.
Specifically, the peptide rescues electrical spread through the
activation of a modulatory cation current (the 'modulatory induced
current' (IMI)). These data demonstrate the cellular mechanisms by which
this peptide neuromodulator induces temperature-robust neuronal
activity. A realization during my work on the previous chapters was
that few peer-reviewed protocols exist that provide detailed and
reproducible workflows of electrophysiological and molecular approaches
for the study of modulatory influences. Many laboratories use
'homegrown' protocols or protocols that were inherited by word of mouth
and are not widely available. This leads to a lack or reproducibility of
research approaches and results and impedes the widespread use of these
techniques. Chapters IV and V address these issues. In Chapter IV, I
first, provide detailed protocols on how to generate action potentials
in an axon using extracellular stimulation. Second, I provide a detailed
protocol on how to measure action potential conduction velocity using
extracellular recordings. In Chapter V, I expand on the concept of
providing easily understandable and reproducible protocols to the
processes of integrating genetic and molecular techniques with
electrophysiological one in both lab and classroom settings. I establish
a workflow that guides undergraduates or physiologists in the manual
identification, confirmation, and curation of putative genes involved in
neuronal function. I implement this workflow in a Course in
Undergraduate Research Education (CURE) – like setting, that brings
undergraduate students of all levels to actively participate in research
labs by allowing students to work under supervision of graduate
students and faculty mentors. The workflow outlines a efficient protocol
for gene identification in marbled crayfish, clear leaning objectives,
and several quality control and assessment processes that enable
students to conceptualize the interconnectedness of genetics, molecular,
and physiological neuroscience. By following this workflow, I
identified the transcript and gene sequences for two Gamma Aminobutyric
Acid (GABA) receptors subunits in the marbled crayfish (Procambarus
virginalis). In addition to its educational purpose, the provided
protocol serves as a first step toward integrating genetic and molecular
techniques with electrophysiological ones to study the impact of
receptor diversity for the cellular mechanisms of modulation in the
marbled crayfish.
Keywords: None provided.
Note: This document is embargoed until June 28, 2022.
