Year of Publication

2007

Document Type

Dissertation

College

Medicine

Department

Anatomy and Neurobiology

First Advisor

Guoying Bing

Abstract

Dynorphins, endogenous peptide neurotransmitters expressed in the central nervous system, have been implicated in diverse pathophysiological processes, including excitotoxicity, chronic inflammation, traumatic injury, cognitive impairment, and motor dysfunction, with significant changes with aging or age-related disease processes. This has led to the hypothesis that the suppression of dynorphin expression would produce beneficial effects on learning and memory and motor function. To assess the phenotypic manifestations of chronic suppression of endogenous dynorphin, knockout (KO) mice lacking the coding exons of the gene encoding the prodynorphin (Pdyn) precursor protein, were tested in a series of behavioral, biochemical, and molecular biological studies. Moderately aged Pdyn KO perform comparatively better than similarly aged wild-type (WT) mice in the water maze task, although no Pdyn effect was seen among young adult mice. In addition, young adult Pdyn KO mice show mildly improved performance on a passive avoidance task. Minimal baseline differences were noted in spontaneous locomotor activity in an open-field assay, but Pdyn deletion produced a relative sparing of motor dysfunction induced by the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). To investigate the relationship between aging and brain dynorphin expression in mice, we examined dynorphin peptide levels at varying ages in hippocampus, striatum, and frontal cortex of WT mice by quantitative radioimmunoassay. While aging produces progressive decline in Dyn B in striatum and frontal cortex, Dyn A shows an upward trend in frontal cortex without significant change in striatum. Systemic MPTP produces significant short-term elevations in dynorphin peptides that regress to below baseline by 7 days. HPLC analysis of striatal dopamine shows an age-dependent increase in basal dopamine levels in Pdyn KO mice, an effect that is abolished after MPTP. Western blotting experiments demonstrate that Pdyn deletion is associated with greater phosphorylation at the serine-40 site of tyrosine hydroxylase (TH) despite relatively less total TH immunoreactivity, suggesting a suppressive effect of dynorphins on dopamine synthesis. Microarray analysis of hippocampal tissue from young and aged WT and Pdyn KO mice reveals a number of functional groups of genes demonstrating altered expression. The results of this dissertation support a role of endogenous dynorphins in age-associated cognitive and motor dysfunction.

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