- Title
- Neurobiological consequences of stress: tyrosine hydroxylase phosphorylation in response to stress
- Creator
- Ong, Lin Kooi
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2012
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Stress is part of our daily life. One of the major cell types involved in the stress response are the catecholaminergic cells in the brain, the peripheral nervous system and the adrenal medulla. These cells, which produce adrenaline, noradrenaline and dopamine, are subject to a range of controls each of which is involved in the stress response. The major subject of this thesis is the effect of stress on one of these controls namely biosynthesis of the catecholamines. Tyrosine hydroxylase (TH) is the rate-limiting enzyme in catecholamine biosynthesis. TH is itself subject to a range of regulatory mechanisms, including feedback inhibition by the catecholamines, phosphorylation of serine residues (Ser19, Ser31 and Ser40) which can contribute directly or indirectly to enzyme activation, as well as mRNA expression and protein synthesis which determine the availability of TH. In response to stress catecholaminergic cells are depolarized and extracellular calcium enters leading to the release of catecholamines from these cells and also to the activation of signal transduction pathways that lead to an increase in TH phosphorylation and TH activity. When catecholamines are released from cells during the stress response it has been shown that the concomitant increase in TH activity and catecholamine synthesis maintains catecholamine levels in the cells at a constant level. The phosphorylation of each serine residue does not affect TH activity equally. Ser19 phosphorylation does not increase TH activity directly, Ser31 phosphorylation increases TH activity modestly and Ser40 phosphorylation, which relives the feedback inhibition by catecholamines, increases TH activity substantially. Three phases of TH activation (acute, sustained and chronic) have been identified and the regulatory mechanisms for each phase have been extensively characterized in vitro and in situ. The acute phase involves TH phosphorylation which occurs and is mostly reversed over the first hour after exposure to stress. The sustained phase also involves TH phosphorylation via different mechanisms but it occurs from 1 to 24 h after exposure to stress. The chronic phase involves mRNA synthesis and TH protein synthesis and this occurs from 4 to 72 h after exposure to stress. To date, there have been only limited studies that have investigated the acute phase of TH activation in response to stress and no studies that have investigated the sustained phase in vivo. Only the chronic phase of TH activation in response to stress has been extensively investigated in vivo. The work presented in this thesis aimed to systematically investigate the different phases of TH activation, especially the acute and sustained phases, by measuring TH phosphorylation and TH protein at different time points in response to a range of stressors in vivo. The adrenal medulla and the locus coeruleus (LC) where chosen as representative catecholaminergic cells for these studies. We have compared the profile of TH phosphorylation and TH protein elicited by two stressors tentatively classified as physical (footshock or glucoprivation stress) and two stressors tentatively classified as psychological (immobilization or social defeat stress) in the adrenal medulla and the LC over a 1 h period. We found that the different stressors all induce the acute phase of TH activation, but provide different temporal profiles of TH phosphorylation at Ser19, Ser31 and Ser40, without TH protein synthesis in the adrenal medulla and the LC over the first hour in vivo. The physical stressors both activated the catecholaminergic cells to a greater extent when compared to the psychological stressors. We have also compared the profile of TH phosphorylation and TH protein elicited by three different stressors social defeat, glucoprivation or LPS stress in the adrenal medulla at 24 h. It should be noted that the LPS studies were undertaken with neonatal rats. We found that social defeat or glucoprivation stress do not induce sustained phosphorylation. However, LPS stress induces the sustained phase of TH activation by inducing sustained TH phosphorylation at Ser31 and Ser40 without TH protein synthesis being increased in neonatal rats’ adrenal medulla at 24 h. The reason for the difference is unknown, but it is possible that sustained phosphorylation only occurs in neonatal animals or perhaps LPS stress activates the adrenal via a different set of intracellular messengers to the other stressors. Whatever the mechanism this is the first study to demonstrate that the sustained phase of TH activation occurs in vivo. Overall we provided evidence that different catecholaminergic cells respond differently in term of the temporal profiles of TH phosphorylation at Ser19, Ser31 and Ser40, presumably due to differences in the frequency of cell firing and/or the nature of the neurotransmitters released onto these cells, which in turn led to differential activation of signal transduction pathways. In addition, we demonstrated that the activation of TH is associated with the enzymes phosphorylation at Ser31 and Ser40 in vivo, an effect that had previously been demonstrated mainly in cultured cells. This thesis has substantially improved our understanding of the mechanism of action of the catecholaminergic cells in mediating stress responses in vivo.
- Subject
- tyrosine hydroxylase; phosphorylation; stress; adrenal medulla; locus coeruleus; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/932235
- Identifier
- uon:11293
- Rights
- Copyright 2012 Lin Kooi Ong
- Language
- eng
- Full Text
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