Indiana University-Purdue University Indianapolis (IUPUI) / Although paclitaxel is a commonly used anti-neoplastic agent for the treatment of solid tumors, therapy often results in a number of side effects, the most debilitating of which is peripheral neuropathy. Peripheral neuropathy is defined as a pathology of peripheral nerves, and, depending on the type of nerves damaged, the neuropathy can be classified as sensory, motor, or autonomic neuropathy. In the case of peripheral neuropathy induced by paclitaxel, the symptoms are experienced in the extremities and are sensory in nature. Patients undergoing chemotherapy with paclitaxel often report sensory disturbances such as burning, tingling, numbness, a diminished sensation to pain and temperature, loss of vibration sense, loss of proprioception, and loss of deep tendon reflexes. Electrophysiological abnormalities including decreased sensory nerve action potential amplitude and conduction confirm damage to large myelinated fibers. However, the involvement of damage to small diameter sensory neurons in the etiology of paclitaxel – induced peripheral neuropathy is still controversial. Therefore, experiments were performed to determine if paclitaxel alters the function of small diameter sensory neurons and to examine the mechanisms responsible for the change in function. vi Sensory neuron mediated vasodilatation in paclitaxel – injected animals was examined as an indirect measure of calcitonin gene related peptide (CGRP) release and therefore of sensory neuron function. CGRP release was also directly measured from central terminals in the spinal cord. To examine mechanisms of paclitaxel – induced sensory neuron damage, CGRP release and neurite length was examined in paclitaxel – treated sensory neurons in culture. The results demonstrate that (1) paclitaxel decreases the ability of small diameter sensory neurons to produce an increase in blood flow in the skin; (2) paclitaxel alters the release of CGRP from the small diameter sensory neurons; (3) paclitaxel causes the neuronal processes of isolated sensory neurons to degenerate. This dissertation provides novel information showing that paclitaxel alters the function of small diameter sensory neurons and thus provides a better understanding of the mechanisms mediating the sensory disturbances characteristic of peripheral neuropathy resulting from chemotherapy with paclitaxel.

We have developed a highly effective method for in vivo gene silencing in the spinal cord and dorsal root ganglia (DRG) by a cationic lipid facilitated delivery of synthetic, small interfering RNA (siRNA). A siRNA to the delta opioid receptor (DOR), or a mismatch RNA, was mixed with the transfection reagent, i-FectTM (vehicle), and delivered as repeated daily bolus doses (0.5 mug to 4 mug) via implanted intrathecal catheter to the lumbar spinal cord of rats. Twenty-four hours after the last injection, rats were tested for antinociception by the DOR selective agonist, D-Ala2, Glu4]deltorphin II (DELT), or the mu opioid receptor (MOR) selective agonist, D-Ala2, N-Me-Phe4, Gly-ol5]enkephalin (DAMGO). Pretreatment with the siRNA, but not the mismatch RNA or vehicle alone, blocked DELT antinociception dose-dependently. The latter was concomitant with a reduction in the spinal immunoreactivity and receptor density of DOR, and in DOR transcripts in the lumbar DRG and spinal dorsal horn. Neither siRNA nor mismatch RNA pretreatment altered spinal immunoreactivity of MOR or antinociception by spinal DAMGO, and had no effect on the baseline thermal nociceptive threshold. The inhibition of function and expression of DOR by siRNA was reversed by 72 hr after the last RNA injection. The uptake of fluorescence-tagged siRNA was detected in both DRG and spinal cord. The low effective dose of siRNA/i-FectTM complex reflects an efficient delivery of the siRNA to peripheral and spinal neurons, produced no behavioral signs of toxicity. This delivery method may be optimized for other gene targets.

Organisms possess internal sensory systems to detect changes in physiological state. Despite the importance of these sensory systems for maintaining homeostasis, their development, sensory mechanisms, and circuitry are relatively poorly understood. To help address these gaps in knowledge, I used the tracheal dendrite (td) sensory neurons of Drosophila larvae as a model to gain insights into the cellular and molecular organization, developmental regulators, sensory functions and mechanisms, and downstream neural circuitry of internal sensory systems. In this thesis, I present data to show that td neurons comprise defined classes with distinct gene expression and axon projections to the CNS. The axons of one class project to the subesophageal zone (SEZ) in the brain, whereas the other terminates in the ventral nerve cord (VNC). This work identifies expression and a developmental role of the transcription factor Pdm3 in regulating the axon projections of SEZ-targeting td neurons. I find that ectopic expression of Pdm3 alone is sufficient to switch VNC-targeting td neurons to SEZ targets, and to induce the formation of putative synapses in these ectopic target regions. These results define distinct classes of td neurons and identity a molecular factor that contributes to diversification of central axon targeting. I present data to show that td neurons express chemosensory receptor genes and have chemosensory functions. Specifically, I show that td neurons express gustatory and ionotropic receptors and that overlapping subsets of td neurons are activated by decrease in O2 or increase in CO2 levels. I show that respiratory gas-sensitive td neurons are also activated when animals are submerged for a prolonged duration, demonstrating a natural-like condition in which td neurons are activated. I assessed the roles of chemosensory receptor genes in mediating the response of td neurons to O2 and CO2. As a result, I identify Gr28b as a mediator of td responses to CO2. Deletion of Gr28 genes or RNAi knockdown of Gr28b transcripts reduce the response of td neurons to CO2. Thus, these data identify two stimuli that are detected by td neurons, and establish a putative role for Gr28b in internal chemosensation in Drosophila larvae. Finally, I present data to elucidate the neural circuitry downstream of td sensory neurons. I show that td neurons synapse directly and via relays onto neurohormone populations in the central nervous system, providing neuroanatomical basis for internal sensory neuron regulation of hormonal physiology in Drosophila. These results pave the way for future work to functionally dissect the td circuitry to understand its function in physiology and behavior.

Thesis (Ph. D.)--University of Oregon, 2007. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 107-114). Also available for download via the World Wide Web; free to University of Oregon users.

Rapid receptive field (RF) reorganization of somatosensory neurons in the cat dorsal column nuclei (DCN) was studied using electrophysiological and histological methods. Soon after denervation of the peripheral RF by lidocaine injection, every DCN neuron tested exhibited a reorganized RF.

As we interact with the world, we must decide what to do next based on previously acquired and incoming information. The study of perceptual decision-making uses highly controlled sensory stimuli and exploits known properties of sensory and motor systems to understand the processes that occur between sensation and action. Even these relatively simple decisions invoke operations like inference, integration of evidence, attention, appropriate action selection, and the assignment of levels of belief or confidence. Thus, the neurobiology of perceptual decision-making offers a tractable way of studying mechanisms that play a role in higher cognitive function. The controlled nature of perceptual decision-making tasks allows an experimenter to infer the latent processes that give rise to a decision. For example, many decisions are well-described by a process of bounded evidence accumulation, in which sensory evidence is temporally integrated until a terminating threshold is exceeded. This thesis improves our understanding of how these latent processes are implemented at the level of neurobiology. After an introduction to perceptual decision-making in Chapter 1, Chapter 2 focuses on the behavioral observations that corroborate whether a subject’s decisions are governed by bounded evidence accumulation. Through simulations of multiple decision-making models, I show that several commonly accepted signatures of evidence accumulation are also predicted by models that do not posit evidence accumulation. I then dissect these models to uncover the features that underlie their mimicry of evidence accumulation. Using these insights, I designed a novel motion discrimination task that was able to better identify the decision strategies of human subjects. In Chapter 3, I explore how the accumulation of evidence is instantiated by populations of neurons in the lateral intraparietal area (LIP) of the macaque monkey. Recordings from single LIP neurons averaged over many decisions have provided support that LIP represents the accumulation of noisy evidence over time, giving rise to diffusion dynamics. However, this diffusion-like signal has yet to be observed directly because of the inability to record from many neurons simultaneously. I used a new generation of recording technology—neuropixels probes optimized for use in primates—to record simultaneously from hundreds of LIP neurons, elucidating this signal for the first time. Through a variety of analyses, I show that the population’s representation of this signal depends on a small subset of neurons that have response fields that overlap the choice targets. Finally, in Chapter 4, I discover a neural mechanism in the midbrain superior colliculus (SC) involved in terminating perceptual decisions. I show that trial-averaged activity in LIP and SC is qualitatively similar, but that single-trial dynamics in each area are distinct. Unlike LIP, SC fired large bursts of activity at the end of the decision, which were sometimes preceded by smaller bursts. Through simultaneous recordings, I uncover the aspects of the diffusion signal in LIP that are predictive of bursting in SC. These observations led me to hypothesize that bursts in SC are the product of a threshold computation involved in terminating the decision and generating the relevant motor response. I confirmed this hypothesis through focal inactivation of SC, which affected behavior and LIP activity in a way that is diagnostic of an impaired threshold mechanism. In total, this work improves our ability to identify the hidden, intermediate steps that underlie decisions and sheds light on their neural basis. All four chapters have been published or posted as separate manuscripts (Steinemann et al., 2022; Stine et al., 2020; Stine et al., 2022; Stine et al., 2019).

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