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.
