This project continues studies on the effects of an environmental stressor, randomly presented loud sound, on biochemical indices of the function of the rostrally projecting midbrain 5-hydroxytryptamine (5-HT, serotonin) containing neurons of rat brain. Their activation is monitored by the in vitro activity of tryptophan hydroxylase, the enzyme at is rate-limiting in the synthesis of the transmitter (modulator) 5-HT and thus determines 5-HT availability. This enzyme exhibits two distinct changes in activity to acute or repeated sound stress, 1) a rapidly reversible increase in activity that is abolished in vitro with alkaline phosphatase and 2) a more persistent increase in enzyme activity seen only after repeated sound stress that is insensitive to the action of alkaline phosphatase. Both increases in activity require the presence of glucocorticoid and are mediated over the amygdaloid central nucleus (ACE). In addition, corticotropin releasing factor (CRF) is known to be involved in the response to acute sound stress. The goal is to explore further the neural pathways, neurotransmitters (modulators) that mediate the appearance of the two types of increase in enzyme activity as well as the involvement of steroid hormones. Studies will focus on the ACE and associated lateral bed nucleus of the stria terminalis, their caudal projections as well as known synaptic inputs to the midbrain raphe and the type I and II corticosterone receptors of brain. Localized stereotaxic microinfusions of CRF, other peptides or classical transmitters, their antagonists, and of steroids and steroid antagonists, or discrete lesions (made by radiofrequency, ibotenic acid or the neurochemically selective neurotoxins, 6-hydroxydopamine or 5,7-dihydroxytryptamine) will be used to identify the neuroanatomical and chemical substrates of the 5-HT response to sound stress. HPLC-EC is used to quantitate 5-hydroxytryptophan (5-HTP) formed in the enzyme assay, tissue levels of 5-HT and its metabolite, 5-hydroxyindoleacetic acid, and tissue 5-HTP accumulation after inhibition of decarboxylation in vivo. The molecular mechanisms that may mediate the two changes in enzyme activity namely phosphorylation or an increase in enzyme level will also be explored with an antibody to the enzyme. These studies may have relevance for severe depression, a condition that can be triggered by stress and in which altered levels of 5-HT, CRF and plasma glucocorticoid have been implicated.
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