During prolonged stress or chronic treatment with neurotoxins, robust compensatory mechanisms occur that maintain sufficient levels of catecholamine neurotransmitters in terminal regions. One of these mechanisms is the up-regulation of tyrosine hydroxylase (TH), the enzyme that controls catecholamine biosynthesis. In neurons of the periphery and locus coeruleus, this up-regulation is associated with an initial induction of TH mRNA. In contrast, this induction either does not occur or it is nominal in mesencephalic dopamine neurons. The reasons for this lack of compensatory TH mRNA induction remain obscure, because so little is known about the regulation of TH expression in these neurons. In this study, we test whether activation of the cAMP signaling pathway regulates TH gene expression in two rodent models of midbrain dopamine neurons, ventral midbrain organotypic slice cultures and MN9D cells. Our results demonstrate that elevation of cAMP leads to induction of TH protein and TH activity in both model systems; however, TH mRNA levels are not up-regulated by cAMP. The induction of TH protein is the result of a novel post-transcriptional mechanism that activates TH mRNA translation. This translational activation is mediated by sequences within the 3Ј untranslated region (UTR) of TH mRNA. Our results support a model in which cAMP induces or activates trans-factors that interact with the TH mRNA 3ЈUTR to increase TH protein synthesis. An understanding of this novel regulatory mechanism may help to explain the control of TH gene expression and consequently dopamine biosynthesis in midbrain neurons under different physiological and pathological conditions.

It is well-established that long-term stress leads to induction of tyrosine hydroxylase (TH) mRNA and TH protein in adrenal medulla and brain. This induction is usually associated with stimulation of TH gene transcription rate. However, a number of studies have reported major discrepancies between the stress-induced changes in TH gene transcription, TH mRNA and TH protein. These discrepancies suggest that post-transcriptional mechanisms also play an important role in regulating TH expression in response to stress and other stimuli. In this report we summarize some of our findings and literature reports that demonstrate these discrepancies in adrenal medulla, locus coeruleus and midbrain dopamine neurons. We then describe our recent work investigating the molecular mechanisms that mediate this post-transcriptional regulation in adrenal medulla and midbrain. Our results suggest that trans-acting factors binding to the polypyrimidine-rich region of the 3′UTR of TH mRNA play a role in these post-transcriptional mechanisms. A hypothetical cellular model describing this post-transcriptional regulation is proposed. Keywords tyrosine hydroxylase; stress; adrenal medulla; locus coeruleus; midbrain dopamine neurons; posttranscriptional regulation TextMost forms of repeated or chronic stress induce tyrosine hydroxylase (TH) in adrenal medulla, sympathetic ganglia and locus coeruleus, and some types of stress induce TH in midbrain dopamine neuron 1-4 . Current hypotheses describing the molecular and signaling mechanisms responsible for this response focus primarily on transcriptional regulation and are based primarily on results of studies performed in vivo using adrenal medulla or in cultured PC12 and neuroblastoma cells. According to these models, stress leads to increased firing of afferent nerve fibers, like the splanchnic nerve for the adrenal medulla, resulting in enhanced activation of multiple plasma membrane receptors on postsynaptic catecholaminergic cell bodies by neurotransmitters released from these afferents. This receptor activation causes stimulation of multiple signaling pathways, leading to activation and/or induction of transcription factors that stimulate the TH gene promoter. The resulting increase in TH gene transcription rate leads to induction of TH mRNA. The consequent increase in TH protein is presumably responsible for

l‐Deprenyl is a relatively selective inhibitor of monoamine oxidase (MAO)‐B that delays the emergence of disability and the progression of signs and symptoms of Parkinson's disease. Experimentally, deprenyl has also been shown to prevent neuronal cell death in various models through a mechanism that is independent of MAO‐B inhibition. We examined the effect of deprenyl on cultured mesencephalic dopamine neurons subjected to daily changes of feeding medium, an experimental paradigm that causes neuronal death associated with activation of the NMDA subtype of glutamate receptors. Both deprenyl (0.5–50 µM) and the NMDA receptor blocker MK‐801 (10 µM) protected dopamine neurons from damage caused by medium changes. The nonselective MAO inhibitor pargyline (0.5–50 µM) was not protective, indicating that protection by deprenyl was not due to MAO inhibition. Deprenyl (50 µM) also protected dopamine neurons from delayed neurotoxicity caused by exposure to NMDA. Because deprenyl had no inhibitory effect on NMDA receptor binding, it is likely that deprenyl protects from events occurring downstream from activation of glutamate receptors. As excitotoxic injury has been implicated in neurodegeneration, it is possible that deprenyl exerts its beneficial effects in Parkinson's disease by suppressing excitotoxic damage.

Repeated nicotine administration induces tyrosine hydroxylase (TH) mRNA in rat midbrain. In this study we investigate the mechanisms responsible for this response using two models of midbrain dopamine neurons, rat ventral midbrain slice explant cultures and mouse MN9D cells. In both models nicotine stimulates TH gene transcription rate in a dose‐dependent manner. However, this stimulation is short‐lived, lasting for 1 h, but less than 3 h, and is not sufficient to induce TH mRNA or TH protein. Nicotine elevates circulating glucocorticoids, which induce TH expression in some model systems. We tested the hypothesis that the effect of nicotine on midbrain TH mRNA is mediated by the glucocorticoid receptor. When rats are administered the glucocorticoid receptor antagonist mifepristone, the induction of TH mRNA by nicotine in both substantia nigra and ventral tegmentum is inhibited. Furthermore, the glucocorticoid receptor agonist dexamethasone stimulates TH gene transcription for sustained periods of time in both midbrain slices and MN9D cells, leading to induction of TH mRNA and TH protein. Our results are consistent with the hypothesis that nicotine induces TH mRNA in midbrain by elevating glucocorticoids, which then act on glucocorticoid receptors in dopamine neurons leading to transcriptional activation of the TH gene.

Selegiline [L-(-)-deprenyl], a monoamine oxidase B inhibitor, has been used in the treatment of Parkinson's disease as a putative neuroprotective agent. Selegiline is metabolized rapidly in the gastrointestinal tract and liver to desmethylselegiline (DMS) and methamphetamine. We have previously shown that selegiline protects dopamine neurons in mesencephalic cultures from toxicity resulting from activation of glutamate receptors. In the present study we examined whether DMS has similar neuroprotective effects. Our data show that DMS protects dopamine neurons from N-methyl-o-aspartate receptor-mediated excitotoxic damage. The efficacy of DMS is greater than that of selegiline, as it can cause protection at lower concentrations and provide significantly greater levels of protection at the same concentrations. Our results suggest that DMS might be the active compound responsible for the neuroprotective properties of selegiline. Key Words: Parkinson's disease-Selegiline-LDeprenyl -Desmethylselegiline -Excitotoxicity -Neuroprotection. J. Neurochem. 68, 434-436 (1997)., in doses of 10 mg/day, is a relatively selective inhibitor of monoamine oxidase (MAO) B. As an adjunct to levodopa therapy, selegiline has been shown to provide improved function and reduced motor fluctuations in patients with advanced Parkinson's disease (PD) (Birkmayer and Riederer, 1984;Golbe et al., 1988). Greater interest has centered on selegiline as a putative neuroprotective therapy based on its capacity to block the development of MPTP-induced parkinsonism (Cohen et al., 1984;Heikkila et al., 1984) and oxidative stress secondary to the MAO-B-dependent metabolism of dopamine (Cohen and Spina, 1989;Olanow, 1992). In patients with early untreated PD, controlled clinical trials demonstrated that selegiline delays the emergence of disability necessitating the introduction of levodopa therapy (Tetrud and Langston, 1989; Parkinson Study Group, 1989, l993a) and slows the progression of signs and symptoms (Olanow et al., 1995). Until recently, it has been considered that the benefits of selegiline in PD are due to its capacity to inhibit MAO-B. However, recent laboratory studies have shown that selegiline prevents neuronal degeneration in various in vivo and in vitro experimental models (Mytilineou and Cohen, 1985; Tatton and Geenwood, 1991;Salo and Tatton, 1992;Ansari et al., 1993;Roy and Bedard, 1993), through a mechanism that does not depend on inhibition ofMAO-B activity (Ansari et al., 1993;Mytilineou et al., 1997).In humans and experimental animals, selegiline is rapidly metabolized in the gastrointestinal tract and liver to L-desmethylselegiline (DMS) and L-methamphetamineby the cytochrome P-450 enzyme system (Yoshida et al., 1986;Heinonen et al., 1994;Barrett et al., 1996). DMS is an inhibitor of MAO-B, but it is less potent than selegiline in in vitro and in vivo assays (Borbe et al., 1990).Our recent studies have shown that selegiline protects cultured mesencephalic dopamine (DA) neurons from cell death caused by N-methyl-D-asparta...