Effects of cold exposure on rat adrenal tyrosine hydroxylase: an analysis of RNA, protein, enzyme activity, and cofactor levels

Adaptive remodeling of the stimulus-secretion coupling: Lessons from the 'stressed' adrenal medulla

Stress is part of our daily lives and good health in the modern world is offset by unhealthy lifestyle factors, including the deleterious consequences of stress and associated pathologies. Repeated and/or prolonged stress may disrupt the body homeostasis and thus threatens our lives. Adaptive processes that allow the organism to adapt to new environmental conditions and maintain its homeostasis are therefore crucial. The adrenal glands are major endocrine/neuroendocrine organs involved in the adaptive response of the body facing stressful situations. Upon stress episodes and in response to activation of the sympathetic nervous system, the first adrenal cells to be activated are the neuroendocrine chromaffin cells located in the medullary tissue of the adrenal gland. By releasing catecholamines (mainly epinephrine and to a lesser extent norepinephrine), adrenal chromaffin cells actively contribute to the development of adaptive mechanisms, in particular targeting the cardiovascular system and leading to appropriate adjustments of blood pressure and heart rate, as well as energy metabolism. Specifically, this chapter covers the current knowledge as to how the adrenal medullary tissue remodels in response to stress episodes, with special attention paid to chromaffin cell stimulus-secretion coupling. Adrenal stimulus-secretion coupling encompasses various elements taking place at both the molecular/cellular and tissular levels. Here, I focus on stress-driven changes in catecholamine biosynthesis, chromaffin cell excitability, synaptic neurotransmission and gap junctional communication. These signaling pathways undergo a collective and finely-tuned remodeling, contributing to appropriate catecholamine secretion and maintenance of body homeostasis in response to stress.

Neonatal overfeeding increases capacity for catecholamine biosynthesis from the adrenal gland acutely and long-term in the male rat

A poor nutritional environment during early development has long been known to increase disease susceptibility later in life. We have previously shown that rats that are overfed as neonates (i.e. suckled in small litters (4 pups) relative to control conditions (12 pups)) show dysregulated hypothalamic-pituitary-adrenal axis responses to immune stress in adulthood, particularly due to an altered capacity of the adrenal to respond to an immune challenge. Here we hypothesised that neonatal overfeeding similarly affects the sympathomedullary system, testing this by investigating the biochemical function of tyrosine hydroxylase (TH), the first rate-limiting enzyme in the catecholamine synthesis. We also examined changes in adrenal expression of the leptin receptor and in mitogen-activated protein kinase (MAPK) signalling. During the neonatal period, we saw age-dependent changes in TH activity and phosphorylation, with neonatal overfeeding stimulating increased adrenal TH specific activity at postnatal days 7 and 14, along with a compensatory reduction in total TH protein levels. This increased TH activity was maintained into adulthood where neonatally overfed rats exhibited increased adrenal responsiveness 30 min after an immune challenge with lipopolysaccharide, evident in a concomitant increase in TH protein levels and specific activity. Neonatal overfeeding significantly reduced the expression of the leptin receptor in neonatal adrenals at postnatal day 7 and in adult adrenals, but did not affect MAPK signalling. These data suggest neonatal overfeeding alters the capacity of the adrenal to synthesise catecholamines, both acutely and long term, and these effects may be independent of leptin signalling.

Investigation of the therapeutic potential of N-acetyl cysteine and the tools used to define nigrostriatal degeneration in vivo

The glutathione precursor N-acetyl-L-cysteine (NAC) is currently being tested on Parkinson's patients for its neuroprotective properties. Our studies have shown that NAC can elicit protection in glutathione-independent manners in vitro. Thus, the goal of the present study was to establish an animal model of NAC-mediated protection in which to dissect the underlying mechanism. Mice were infused intrastriatally with the oxidative neurotoxicant 6-hydroxydopamine (6-OHDA; 4 μg) and administered NAC intraperitoneally (100mg/kg). NAC-treated animals exhibited higher levels of the dopaminergic terminal marker tyrosine hydroxylase (TH) in the striatum 10d after 6-OHDA. As TH expression is subject to stress-induced modulation, we infused the tracer FluoroGold into the striatum to retrogradely label nigrostriatal projection neurons. As expected, nigral FluoroGold staining and cell counts of FluoroGold(+) profiles were both more sensitive measures of nigrostriatal degeneration than measurements relying on TH alone. However, NAC failed to protect dopaminergic neurons 3 weeks following 6-OHDA, an effect verified by four measures: striatal TH levels, nigral TH levels, nigral TH(+) cell counts, and nigral FluoroGold levels. Some degree of mild toxicity of FluoroGold and NAC was evident, suggesting that caution must be exercised when relying on FluoroGold as a neuron-counting tool and when designing experiments with long-term delivery of NAC--such as clinical trials on patients with chronic disorders. Finally, the strengths and limitations of the tools used to define nigrostriatal degeneration are discussed.

Impaired adrenal medullary function in a mouse model of depression induced by unpredictable chronic stress

Stress has been considered determinant in the etiology of depression. The adrenal medulla plays a key role in response to stress by releasing catecholamines, which are important to maintain homeostasis. We aimed to study the adrenal medulla in a mouse model of depression induced by 21 days of unpredictable chronic stress (UCS). We observed that UCS induced a differential and time-dependent change in adrenal medulla. After 7 days of UCS, mice did not show depressive-like behavior, but the adrenal medullae show increased protein and/or mRNA levels of catecholamine biosynthetic enzymes (TH, DβH and PNMT), Neuropeptide Y, the SNARE protein SNAP-25, the catecholamine transporter VMAT2 and the chromaffin progenitor cell markers, Mash1 and Phox2b. Moreover, 7 days of UCS induced a decrease in the chromaffin progenitor cell markers, Sox9 and Notch1. This suggests an increased capacity of chromaffin cells to synthesize, store and release catecholamines. In agreement, after 7 days, UCS mice had higher NE and EP levels in adrenal medulla. Opposite, when mice were submitted to 21 days of UCS, and showed a depressive like behavior, adrenal medullae had lower protein and/or mRNA levels of catecholamine biosynthetic enzymes (TH, DβH, PNMT), catecholamine transporters (NET, VMAT1), SNARE proteins (synthaxin1A, SNAP25, VAMP2), catecholamine content (EP, NE), and lower EP serum levels, indicating a reduction in catecholamine synthesis, re-uptake, storage and release. In conclusion, this study suggests that mice exposed to UCS for a period of 21 days develop a depressive-like behavior accompanied by an impairment of adrenal medullary function.

Treadmill exercise does not change gene expression of adrenal catecholamine biosynthetic enzymes in chronically stressed rats

ABSTRACT Chronic isolation of adult animals represents a form of psychological stress that produces sympatho-adrenomedullar activation. Exercise training acts as an important modulator of sympatho-adrenomedullary system. This study aimed to investigate physical exercise-related changes in gene expression of catecholamine biosynthetic enzymes (tyrosine hydroxylase, dopamine-ß-hydroxylase and phenylethanolamine N-methyltransferase) and cyclic adenosine monophosphate response element-binding (CREB) in the adrenal medulla, concentrations of catecholamines and corticosterone (CORT) in the plasma and the weight of adrenal glands of chronically psychosocially stressed adult rats exposed daily to 20 min treadmill running for 12 weeks. Also, we examined how additional acute immobilization stress changes the mentioned parameters. Treadmill running did not result in modulation of gene expression of catecholamine synthesizing enzymes and it decreased the level of CREB mRNA in the adrenal medulla of chronically psychosocially stressed adult rats. The potentially negative physiological adaptations after treadmill running were recorded as increased concentrations of catecholamines and decreased morning CORT concentration in the plasma, as well as the adrenal gland hypertrophy of chronically psychosocially stressed rats. The additional acute immobilization stress increases gene expression of catecholamine biosynthetic enzymes in the adrenal medulla, as well as catecholamines and CORT levels in the plasma. Treadmill exercise does not change the activity of sympatho-adrenomedullary system of chronically psychosocially stressed rats.

Effects of chronic social defeat on expression of dopamine β-hydroxylase in rat brains

It is documented that stress activates the locus coeruleus-norepinephrine system. However, there are far few reports regarding effects of stress on the expression of dopamine β-hydroxylase, a hallmark enzyme of the noradrenergic neuron. In the present study, adult Fischer 344 rats were subjected to chronic social defeat for 4 weeks. Dopamine β-hydroxylase expressional levels in the locus coeruleus and its terminal regions were measured by in situ hybridization and western blotting. The results showed that immediately following chronic social defeat there are significantly increased mRNA and protein levels of dopamine β-hydroxylase in the locus coeruleus, and dopamine β-hydroxylase protein levels in the hippocampus, frontal cortex and amygdala, compared with those in the control. This chronic social defeat-induced upregulation of dopamine β-hydroxylase was completely abolished by adrenalectomy, and/or by treatment with corticosteroid receptor antagonists, mifepristone and spironolactone, either alone or in combination. Furthermore, treatment with desipramine, an antidepressant with specific inhibitory effects on norepinephrine transport, prevented an increased dopamine β-hydroxylase expression by chronic social defeat in the locus coeruleus and its main terminal regions such as the hippocampus, frontal cortex and amygdala. However, treatment with fluoxetine, an antidepressant with specific inhibition for serotonin transport, only selectively blocked increased dopamine β-hydroxylase protein levels in the hippocampus caused by CSD. The present findings indicate that chronic social defeat activates the locus coeruleus-norepinephrine system by upregulating the expression of dopamine β-hydroxylase, which may increase norepinephrine synthesis. This chronic social defeat induced upregulation of DBH expression was mediated through corticosterone and corticosteroid receptors, with possible interference from antidepressants.

Does exposure to chronic stress influence blood pressure in rats?

The principal aim of this study was to determine whether prolonged chronic footshock stress can evoke sustained changes in blood pressure in rats and to elucidate possible underlying neurochemical mechanisms as mediated by the sympathoadrenal system. Adult male Wistar rats instrumented for telemetric recording of arterial pressure, heart rate and locomotor activity were subjected to six weeks of inescapable unpredictable electrical footshocks (FS+) or were exposed to shock chambers but were not shocked (FS-). Compared to FS- animals, FS+ animals had significantly reduced body weight gain (by 30%), locomotor activity (by 25%) and social interaction time (by 30%)--symptoms commonly induced by chronic stress and depression in humans. These changes were associated with small, but significant increases in systolic blood pressure (by 7%) and pulse pressure (by 11%) in FS+ rats relative to FS- rats. We have also found neurochemical alterations in sympathoadrenal pathways (that lasted for at least one week post-stress) including about 2-3 fold increases in the levels of tyrosine hydroxylase phosphorylation in the sympathetic ganglia and adrenal gland and a 1.8-fold increase in the expression of the Angiotensin II receptor type 1 protein in the adrenal gland of FS+ rats relative to FS- rats. We conclude that uncontrollable and unpredictable footshock stress can lead to elevation in systolic blood pressure when applied for an extended period of time (six weeks) in Wistar rats, and that these changes could be mediated by stress-induced modifications in sympathoadrenal pathways.

The neurobiology of tetrahydrobiopterin biosynthesis: a model for regulation of GTP cyclohydrolase I gene transcription within nigrostriatal dopamine neurons

Within the brain, the reduced pteridine cofactor 6R-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4) is absolutely required for the synthesis of the monoamine (MA) neurotransmitters dopamine (DA), norepinephrine, epinephrine (E), and serotonin (5-HT), the novel gaseous neurotransmitter nitric oxide and the production of yet to be identified 1-O-alkylglycerol-derived lipids. GTP cyclohydrolase I (GTPCH) catalyzes the first and limiting step in the BH4 biosynthetic pathway, which is now thought to involve up to eight different proteins supporting six alternate de novo and two alternate salvage pathways. Gene expression analysis across different regions of the human brain shows the abundance of transcripts coding for all eight of these proteins to be highly correlated with each other and to be enriched within human MA neurons. The potential for multiple routes for BH4 synthesis therefore exists within the human brain. GTPCH expression is particularly heterogeneous across different populations of human and rodent MA-containing neurons, with low expression levels and therefore BH4 being a characteristic of nigrostriatal DA (NSDA) neurons. Basic knowledge of how GCH1 gene transcription is controlled within NSDA neurons may explain the distinctive susceptibility of these neurons to human genetic mutations that result in BH4 deficiency. A model for cyclic adenosine monophosphate-dependent GCH1 transcription is described that involves a unique combination of DNA regulatory sequences and transcription factors. This model proposes that low levels of GCH1 transcription within NSDA neurons are driven by their distinctive physiology, suggesting that pharmacological manipulation of GCH1 gene transcription can be used to modify BH4 levels and therefore DA synthesis in the basal ganglia.

Functional programming of the autonomic nervous system by early life immune exposure: implications for anxiety

Neonatal exposure of rodents to an immune challenge alters a variety of behavioural and physiological parameters in adulthood. In particular, neonatal lipopolysaccharide (LPS; 0.05 mg/kg, i.p.) exposure produces robust increases in anxiety-like behaviour, accompanied by persistent changes in hypothalamic-pituitary-adrenal (HPA) axis functioning. Altered autonomic nervous system (ANS) activity is an important physiological contributor to the generation of anxiety. Here we examined the long term effects of neonatal LPS exposure on ANS function and the associated changes in neuroendocrine and behavioural indices. ANS function in Wistar rats, neonatally treated with LPS, was assessed via analysis of tyrosine hydroxylase (TH) in the adrenal glands on postnatal days (PNDs) 50 and 85, and via plethysmographic assessment of adult respiratory rate in response to mild stress (acoustic and light stimuli). Expression of genes implicated in regulation of autonomic and endocrine activity in the relevant brain areas was also examined. Neonatal LPS exposure produced an increase in TH phosphorylation and activity at both PNDs 50 and 85. In adulthood, LPS-treated rats responded with increased respiratory rates to the lower intensities of stimuli, indicative of increased autonomic arousal. These changes were associated with increases in anxiety-like behaviours and HPA axis activity, alongside altered expression of the GABA-A receptor α2 subunit, CRH receptor type 1, CRH binding protein, and glucocorticoid receptor mRNA levels in the prefrontal cortex, hippocampus and hypothalamus. The current findings suggest that in addition to the commonly reported alterations in HPA axis functioning, neonatal LPS challenge is associated with a persistent change in ANS activity, associated with, and potentially contributing to, the anxiety-like phenotype. The findings of this study reflect the importance of changes in the perinatal microbial environment on the ontogeny of physiological processes.

Functional characterization of alpha9-containing cholinergic nicotinic receptors in the rat adrenal medulla: implication in stress-induced functional plasticity

An increase in circulating adrenal catecholamine levels constitutes one of the mechanisms whereby organisms cope with stress. Accordingly, stimulus-secretion coupling within the stressed adrenal medullary tissue undergoes persistent remodeling. In particular, cholinergic synaptic neurotransmission between splanchnic nerve terminals and chromaffin cells is upregulated in stressed rats. Since synaptic transmission is mainly supported by activation of postsynaptic neuronal acetylcholine nicotinic receptors (nAChRs), we focused our study on the role of alpha9-containing nAChRs, which have been recently described in chromaffin cells. Taking advantage of their specific blockade by the alpha-conotoxin RgIA (alpha-RgIA), we unveil novel functional roles for these receptors in the stimulus-secretion coupling of the medulla. First, we show that in rat acute adrenal slices, alpha9-containing nAChRs codistribute with synaptophysin and significantly contribute to EPSCs. Second, we show that these receptors are involved in the tonic inhibitory control exerted by cholinergic activity on gap junctional coupling between chromaffin cells, as evidenced by an increased Lucifer yellow diffusion within the medulla in alpha-RgIA-treated slices. Third, we unexpectedly found that alpha9-containing nAChRs dominantly (>70%) contribute to acetylcholine-induced current in cold-stressed rats, whereas alpha3 nAChRs are the main contributing channels in unstressed animals. Consistently, expression levels of alpha9 nAChR transcript and protein are overexpressed in cold-stressed rats. As a functional relevance, we propose that upregulation of alpha9-containing nAChR channels and ensuing dominant contribution in cholinergic signaling may be one of the mechanisms whereby adrenal medullary tissue appropriately adapts to increased splanchnic nerve electrical discharges occurring in stressful situations.

Catecholaminergic systems in stress: structural and molecular genetic approaches

Stressful stimuli evoke complex endocrine, autonomic, and behavioral responses that are extremely variable and specific depending on the type and nature of the stressors. We first provide a short overview of physiology, biochemistry, and molecular genetics of sympatho-adrenomedullary, sympatho-neural, and brain catecholaminergic systems. Important processes of catecholamine biosynthesis, storage, release, secretion, uptake, reuptake, degradation, and transporters in acutely or chronically stressed organisms are described. We emphasize the structural variability of catecholamine systems and the molecular genetics of enzymes involved in biosynthesis and degradation of catecholamines and transporters. Characterization of enzyme gene promoters, transcriptional and posttranscriptional mechanisms, transcription factors, gene expression and protein translation, as well as different phases of stress-activated transcription and quantitative determination of mRNA levels in stressed organisms are discussed. Data from catecholamine enzyme gene knockout mice are shown. Interaction of catecholaminergic systems with other neurotransmitter and hormonal systems are discussed. We describe the effects of homotypic and heterotypic stressors, adaptation and maladaptation of the organism, and the specificity of stressors (physical, emotional, metabolic, etc.) on activation of catecholaminergic systems at all levels from plasma catecholamines to gene expression of catecholamine enzymes. We also discuss cross-adaptation and the effect of novel heterotypic stressors on organisms adapted to long-term monotypic stressors. The extra-adrenal nonneuronal adrenergic system is described. Stress-related central neuronal regulatory circuits and central organization of responses to various stressors are presented with selected examples of regulatory molecular mechanisms. Data summarized here indicate that catecholaminergic systems are activated in different ways following exposure to distinct stressful stimuli.

Post-transcriptional regulation of tyrosine hydroxylase expression in adrenal medulla and brain

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 the 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 ceruleus, 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' untranslated region of TH mRNA play a role in these post-transcriptional mechanisms. A hypothetical cellular model describing this post-transcriptional regulation is proposed.

Functional remodeling of gap junction-mediated electrical communication between adrenal chromaffin cells in stressed rats

An increase in circulating catecholamine levels represents one of the mechanisms whereby organisms cope with stress. In the periphery, catecholamines mainly originate from the sympathoadrenal system. As we reported, in addition to the central control through cholinergic innervation, a local gap junction-delineated route between adrenal chromaffin cells contributes to catecholamine exocytosis. Here, we investigated whether this intercellular communication is modified when the hormonal demand is increased as observed during cold stress. Our results show that in cold exposed rats, gap-junctional communication undergoes a functional plasticity, as evidenced by an increased number of dye-coupled cells. Of a physiological interest is that this upregulation of gap-junctional coupling results in the appearance of a robust electrical coupling between chromaffin cells that allows the transmission of action potentials between coupled cells. This enhancement of gap-junctional communication parallels an increase in expression levels of connexin36 (Cx36) and connexin43 (Cx43) proteins. Both transcriptional and posttranslational mechanisms are involved because Cx36 transcripts are increased in stressed rats and the expression of the scaffolding protein zonula occludens-1, known to interact with both Cx36 and Cx43, is also upregulated. Consistent with an upregulated coupling extent in stressed rats, the cytosolic Ca(2+) concentration rises triggered in a single cell by an iontophoretic application of nicotine occur simultaneously in several neighboring cells. These results describe for the first time a functional plasticity of junctional coupling between adult chromaffin cells that should be crucial for adaptation to stress or sensitization to subsequent stressors.

Stress-induced catecholaminergic function: transcriptional and post-transcriptional control

This review summarizes knowledge on the effects of stress on two catecholamine biosynthetic enzymes, tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT). Information is presented on differential responses of the enzymes to a variety of stressors as well as differential responses of the enzymes localized to the central nervous system vs. peripheral nervous system and tissues. Changes in mRNA and protein or activity are described, including species- and stressor-specific effects. While temporal changes in these parameters may differ for the particular stressor or enzyme, in general, maximal changes in mRNA and protein content occur at 6-8 and 24 h after stressor exposure, respectively. Elevation of TH and PNMT transcriptional activators prior to mRNA induction and nuclear run-on assays show that stress activates the genes encoding these enzymes. Yet, extents of induction of mRNA, protein and enzyme activity are often discordant depending on the stress, its duration and repetition of exposure. The extremes are concordant changes in mRNA and protein/activity vs. highly elevated mRNA with no change in protein/activity. Post-transcriptional and/or post-translational regulatory influences that may contribute to the complex effects of stress on TH, PNMT and the stress hormone epinephrine are explored.

Influence of prior experience with homotypic or heterotypic stressor on stress reactivity in catecholaminergic systems

Here we review how prior experience with stress alters the response to a subsequent homotypic or heterotypic stressor, focusing on the catecholaminergic systems in the adrenal medulla and the locus coeruleus (LC). The changes in response to homotypic stress differ depending on the stressor applied. With immobilization stress (IMO), transcriptional responses in the adrenal medulla to a single exposure are pronounced and several of the transcription factors and signaling kinases induced or activated are reviewed and compared to the longer term alterations with repeated stress, consistent with persistent activation of gene expression of catecholamine (CA) biosynthetic enzymes. In the LC, transcriptional and post-transcriptional activation of gene expression are shown to be important. Repeated IMO stress triggers further activation of a number of signalling pathways. Neither adrenal medulla nor LC display habituation to long term repeated stress. In contrast, gene expression for CA biosynthetic enzymes habituates to prolonged cold stress in the adrenal medulla and LC, but displays an exaggerated response with exposure to a novel or heterotypic stressor such as IMO. Some of the transcriptional pathways displaying sensitization are described.

Evidence for regulation of tyrosine hydroxylase mRNA translation by stress in rat adrenal medulla

Long-term stress leads to the induction of tyrosine hydroxylase (TH) protein and enzymatic activity in the adrenal medulla. This adaptive response is necessary to maintain the catecholamine biosynthetic capacity of adrenal chromaffin cells during periods of sustained catecholamine secretion. In this report we demonstrate that when rats are subjected to short-term stress, TH mRNA is induced for at least 24 h, but TH protein and TH activity (assayed under Vmax conditions) are not increased. In contrast, adrenal TH mRNA, TH protein and TH activity are induced in rats subjected to long-term stress. Using sucrose gradient fractionation, we show that the lack of induction of TH protein after one type of short-term stressor, a single 2-h immobilization stress is associated with a decrease in the percentage of TH mRNA molecules associated with polysomes. In contrast, after repeated immobilizations the polysome profile of TH mRNA is identical to that observed in control animals, even though TH mRNA is induced 2- to 3-fold. These results are consistent with the hypothesis that even though TH mRNA is induced by short-term stressors, mechanisms that control TH mRNA translation must also be appropriately regulated for TH protein to be induced.

Nicotinic Cholinergic Regulation of Tyrosine Hydroxylase Gene Expression and Catecholamine Synthesis in Isolated Bovine Adrenal Chromaffin Cells

Isolated bovine adrenal chromaffin cells were used to study the nicotinic regulation of tyrosine hydroxylase (TH) gene expression. Continuous exposure of the cells to carbachol or the nicotinic receptor agonist 1,1-dimethyl-4-phenylpiperazinium (DMPP) produces a time- and concentration-dependent increase in TH enzyme activity, whereas muscarine has no effect. DMPP at 1 microM (EC50 = 0.3 microM) elicits a two- to threefold elevation of both TH activity and TH immunoreactive protein level after 3-5 days in the presence of 2.5 mM calcium; the increase in enzyme levels is significantly less at lower extracellular calcium levels. The rate of hydroxylation of tyrosine to dopamine (DA) in intact cells, an index of endogenous TH activity, increases in parallel with the rise in TH levels. The TH mRNA level is elevated before the increase in protein levels. As determined by nuclear run-on assays, TH gene transcription is stimulated two- to threefold within 30 min of addition of 1 microM DMPP to the cells; transcription returns to basal levels by 2 h. Nitrendipine (20 microM) blocks the stimulation of transcription by DMPP. Pretreatment of the cells with cycloheximide (5 microM) does not prevent the DMPP stimulation of transcription. Forskolin (10 microM) also increases TH transcription (fourfold in 15 min) by a mechanism that is not blocked by cycloheximide. These results show that nicotinic receptor stimulation increases TH mRNA synthesis, TH protein levels, and TH activity in a calcium-dependent manner. Furthermore, the nicotinic influence on TH gene expression does not appear to require the synthesis of a protein factor for its effects. That in situ DA synthesis rates are elevated consequent to the rise in TH levels demonstrates that TH induction serves as a mechanism for enhancing the catecholamine-synthesizing capacity of the chromaffin cell on a long-term basis.

Regulation of the transcriptional activity of the tyrosine hydroxylase gene by androgen receptor

Dopamine and the sex hormone testosterone are important factors regulating male sexual behavior. To investigate the possibility that these two factors are functionally interrelated, we investigated the potential role of the androgen receptor (AR) on transcriptional activity of the tyrosine hydroxylase (TH) gene that encodes the rate-limiting enzyme of the dopamine biosynthesis pathway. In this study, using transient co-transfection assays in TH-positive SK-N-BE(2)C and MN9D cells, we show that AR prominently transactivates TH promoter function in a ligand-dependent manner. Deletional and site-directed mutational analyses have mapped a putative androgen response element (ARE) in a region from -1562 to -1328 base pairs in the upstream TH promoter. We also found that DJ-1, one of recently identified genes whose mutations cause Parkinson's disease, down-regulated AR-dependent TH activation by approximately 50% in SK-N-BE(2)C cells. Based on these data, we propose that AR activates TH gene expression and that DJ-1 may modulate AR activity as a transcriptional co-repressor.

The effects of enalapril maleate and cold stress exposure on tyrosine hydroxylase activity in some rat tissues

Enalapril is a highly specific and competitive inhibitor of angiotensin-I converting enzyme (ACE) and thus belongs to the category of ACE inhibitors. The beneficial effects of ACE inhibitors appear to result primarily from the suppression of the plasma renin-angiotensin-aldesterone system. This study was designed to detect the effects of enalapril maleate and cold stress on tyrosine hydroxylase (TH) activity in adrenal medulla, heart and hypothalamus in rat. In cold stress treatment (exposed to 8 degrees C cold for 48 h) TH activity was found to be raised significantly (p < 0.05) in adrenal medulla, hypothalamus and heart tissues. In the adrenal medulla, hypothalamus and heart tissues, TH activity of enalapril maleate treated rats (10 mg kg(-1) body weight) group was not raised significantly (p > 0.05). Following intraperitoneal injection of enalapril maleate (10 mg kg(-1) body weight) the rats were exposed to 8 degrees C cold for 48 h. After cold stress and enalapril maleate treatment no statistically significant change in tyrosine hydroxylase activity was detected in adrenal medulla, hypothalamus or heart (p > 0.05). The results of our studies show that enalapril maleate blocks the effect of cold stress on the regulation of TH activity.

Effect of prolonged nicotine infusion on response of rat catecholamine biosynthetic enzymes to restraint and cold stress

There is a paradoxical relationship between nicotine and stress. To help elucidate their relationship on catecholamine biosynthesis, rats were infused with nicotine for 7-14 days before exposure to cold or restraint stress. Nicotine (5 mg/kg/day, 14 days) did not alter basal plasma corticosterone or its elevation with 24 h cold stress, but prevented corticosterone elevation following 2 h restraint stress. In adrenal medulla (AM), response of dopamine beta-hydroxylase (DBH), but not tyrosine hydroxylase (TH) mRNA, to both stressors was attenuated in nicotine-infused rats. In locus coeruleus (LC), restraint stress elevated TH and DBH mRNA in saline-, but not in nicotine-infused rats. Cold stress triggered a similar response of TH and DBH mRNAs in LC with and without nicotine infusion. With shorter nicotine infusion (8 mg/kg/day, 7 days), TH mRNA in AM was not induced by restraint stress on one (1x) or two (2x) consecutive days nor was DBH mRNA in AM or LC by 2x. The findings demonstrate that constant release of nicotine can modulate, or even prevent, some stress responses at the level of the HPA axis and gene expression of catecholamine biosynthetic enzymes in LC and AM.

Induction of tyrosine hydroxylase in the locus coeruleus of transgenic mice in response to stress or nicotine treatment: lack of activation of tyrosine hydroxylase promoter activity

Prolonged stress or chronic nicotine administration leads to induction of tyrosine hydroxylase (TH) in adrenal medulla and locus coeruleus (LC) of the rat. In this study we use mice that express a transgene encoding 4.5 kb of TH gene 5'-flanking region fused upstream of the reporter gene, human alkaline phosphatase (hAP) to test whether TH gene promoter activity is stimulated by immobilization stress, cold exposure or nicotine administration in adrenal medulla and LC. TH-hAP transgene expression is increased in response to all three stimuli in the adrenal medulla. In contrast, TH-hAP expression does not increase in response to either immobilization stress or nicotine administration in the LC and only a small induction of LC TH-hAP mRNA is observed in response to cold exposure. TH mRNA is induced 2-3 fold and TH activity is increased significantly by all three stimuli in both the adrenal and LC. These results support the hypothesis that TH expression is induced by stress or nicotine treatment in both the adrenal medulla and LC of the mouse. The induction in the adrenal is dependent primarily on transcriptional mechanisms, whereas that in the LC is apparently dependent primarily on post-transcriptional mechanisms.

Effects of diabetes and recurrent hypoglycemia on the regulation of the sympathoadrenal system and hypothalamo-pituitary-adrenal axis

Epinephrine, norepinephrine, and corticosterone responses to hypoglycemia are impaired in diabetic rats. Recurrent hypoglycemia further diminishes epinephrine responses. This study examined the sympathoadrenal system and hypothalamo-pituitary-adrenal axis for molecular adaptations underlying these defects. Groups were normal (N) and diabetic (D) rats and diabetic rats exposed to 4 days of 2 episodes/day of hyperinsulinemic hypoglycemia (D-hypo) or hyperinsulinemic hyperglycemia (D-hyper). D-hypo and D-hyper rats differentiated effects of hypoglycemia and hyperinsulinemia. Adrenal tyrosine hydroxylase (TH) mRNA was reduced (P < 0.05 vs. N) 25% in all diabetic groups. Remarkably, mRNA for phenylethanolamine N-methyltransferase (PNMT), which converts norepinephrine to epinephrine, was reduced (P < 0.05 vs. all) 40% only in D-hypo rats. Paradoxically, dopamine beta-hydroxylase mRNA was elevated (P < 0.05 vs. D, D-hyper) in D-hypo rats. Hippocampal mineralocorticoid receptor (MR) mRNA was increased (P < 0.05 vs. N) in all diabetic groups. Hippocampal glucocorticoid receptor (GR), hypothalamic paraventricular nucleus (PVN) GR and corticotropin-releasing hormone (CRH), and pituitary GR and proopiomelanocortin (POMC) mRNA levels did not differ. We conclude that blunted corticosterone responses to hypoglycemia in diabetic rats are not due to altered basal expression of GR, CRH, and POMC in the hippocampus, PVN, and pituitary. The corticosterone defect also does not appear to be due to increased hippocampal MR, since we have reported normalized corticosterone responses in D-hypo and D-hyper rats. Furthermore, impaired epinephrine counterregulation in diabetes is associated with reduced adrenal TH mRNA, whereas the additional epinephrine defect after recurrent hypoglycemia is associated with decreases in both TH and PNMT mRNA.

Functional allelic heterogeneity and pleiotropy of a repeat polymorphism in tyrosine hydroxylase: prediction of catecholamines and response to stress in twins

Tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, has a common tetranucleotide repeat polymorphism, (TCAT)(n). We asked whether variation at (TCAT)(n) may influence the autonomic nervous system and its response to environmental stress. To understand the role of heredity in such traits, we turned to a human twin study design. Both biochemical and physiological autonomic traits displayed substantial heritability (h(2)), up to h(2) = 56.8 +/- 7.5% (P < 0.0001) for norepinephrine secretion, and h(2) = 61 +/- 6% (P < 0.001) for heart rate. Common (TCAT)(n) alleles, particularly (TCAT)(6) and (TCAT)(10i), predicted such traits (including catecholamine secretion, as well as basal and poststress heart rate) in allele copy number dose-dependent fashion, although in directionally opposite ways, indicating functional allelic heterogeneity. (TCAT)(n) diploid genotypes (e.g., [TCAT](6)/[TCAT](10i)) predicted the same physiological traits but with increased explanatory power for trait variation (in contrast to allele copy number). Multivariate ANOVA documented genetic pleiotropy: joint effects of the (TCAT)(10i) allele on both biochemical (norepinephrine) and physiological (heart rate) traits. (TCAT)(6) allele frequencies were lower in normotensive twins at genetic risk of hypertension, consistent with an effect to protect against later development of hypertension, and suggesting that the traits predicted by these variants in still-normotensive subjects are early, heritable, "intermediate phenotypes" in the pathogenetic scheme for later development of sustained hypertension. We conclude that common allelic variation within the tyrosine hydroxylase locus exerts a powerful, heritable effect on autonomic control of the circulation and that such variation may have implications in later development of cardiovascular disease traits such as hypertension.

The effect of adrenomedullin (ADM) on tyrosine hydroxylase (TH) enzyme activity and blood pressure in cold exposed rats

It is known that, under stress conditions the hypothalamo-pituitary-adrenal (HPA) axis is stimulated and catecholamine production is increased. Adrenomedullin (ADM) is a novel peptide that elicits a long-vasorelaxation, and participates in blood pressure regulation via different mechanisms. In the present study, we investigated the administration of ADM on tyrosine hydroxylase (TH) enzyme activity in cold exposed rats. Four groups of Sprague-Dawley rats were studied for their TH enzyme activity in the adrenal medulla and hypothalamus. In addition to measuring blood pressure in these rats, TH enzyme activity in both the adrenal medulla and hypothalamus were examined in four groups of Sprague-Dawley rats: animals exposed to room temperature and cold stress (8 masculine C, 48 h), and rats injected with ADM (1.0 nmol/kg, i.v.) alone and/or together with cold stress. TH activity was shown to be increased in cold treated groups and decreased in ADM and ADM + cold stress group. Our findings appear to suggest that external ADM application caused an opposite effect on the same system in rats, decreasing the activity of tyrosine hydroxylase (TH) enzyme activity. Furthermore, externally applied ADM was shown to produce its expected hypotensive effect in cold-stressed rats. Our results suggest that a possible explanation for the effects of ADM is that, the uptake of ADM under cold stress may effect TH activity in studied tissues.

Interactions between Egr1 and AP1 factors in regulation of tyrosine hydroxylase transcription

Several treatments which regulate tyrosine hydroxylase (TH) transcription, such as stress in vivo, or 12-O-tetradecanoylphorbol-13-acetate (TPA) in cell culture, induce both Egr1 and AP1 factors. Previously, we identified a functional Egr1 motif overlapping with Sp1 site in the rat TH promoter. Its response to Egr1 also required the presence of an AP1/Ebox motif. Here, we further examined the cross-talk between these sites. Insertion of 10- or 20-bp between the Sp1/Egr1 and AP1/Ebox elements, reduced the ability of Egr1 to upregulate luciferase reporter activity controlled by the proximal 272 nucleotides of the rat TH promoter in PC12 cells. Electrophoretic mobility shift assays with nuclear extracts from TPA treated cells were used to identify the composition of the factors which bound the AP1/Ebox motif and whether there is competition with factors which bind the Sp1/Egr1 motif. The complexes formed with labeled AP1/E box oligonucleotide were reduced or supershifted with antisera to Fos family, c-Fos, Fra-2, and Jun D. Excess Sp1/Egr1 oligonucleotide or anti Egr1 antisera did not compete. Fra-2 was a major component of the complex after 2-4 h TPA. Transfection of PC12 cells with Fra-2 induced reporter activity requiring the AP1, but not the Egr1 motif. However, when cotransfected with Fra-2, Egr1 expression plasmids elicited lower induction of luciferase activity than observed with Egr1 alone. Our results suggest that although it does not compete for binding to the promoter, Egr1 can modulate the regulation of TH transcription by AP1 factors.

Distinct protein kinases regulate SNAP-25 expression in chromaffin cells

The contribution of distinct Ca(2+)-sensitive protein kinases to the regulation of the expression of the synaptosomal-associated protein SNAP-25 was examined in bovine chromaffin cells. Prolonged incubation with high K(+) (38 mM) or 1,1-dimethyl-4-phenyl-piperazinium (DMPP), a nicotinic receptor agonist, significantly increased SNAP-25 protein and mRNA expression, as assessed by immunoblotting and semi-quantitative RT-PCR analysis. Both stimuli preferentially enhanced mRNA coding for the SNAP-25a isoform. Increase of SNAP-25 expression induced by K(+) or DMPP was inhibited over 70% by KN-62 and KN-93, two Ca(2+)/calmodulin-dependent protein kinase (CaMK) inhibitors, whereas the inactive analogue KN-92 only reduced the expression by 34%. The three compounds also inhibited the high K(+)-elicited [Ca(2+)](i) signal by 40%, suggesting that the effect of KN-62 and KN-93 was a combination of CaMK/ Ca(2+) influx inhibitory actions. Incubation of the cells with mitogen-activated protein kinase (MAPK) inhibitors PD98059 and U0126 reduced protein expression elicited by high K(+) by 50%, but did not modify the response to DMPP. Interestingly, although protein kinase A (PKA) inhibition by H-89 did not affect the high K(+) or DMPP-induced SNAP-25 expression, basal protein levels were significantly modified upon activation or inhibition of this pathway. Basal expression of SNAP-25 was also modified by the protein kinase C (PKC) activator, phorbol 12-myristate 13-acetate, but not by Gö6976, a PKC-alpha inhibitor, suggesting that the Ca(2+)-insensitive PKC-epsilon isoform control basal expression of SNAP-25 in these cells. Taken together, these results provide the first evidence that diverse protein kinases might converge in the induction of SNAP-25 expression in chromaffin cells. The preferential contribution of one or another kinase would depend on the physiological or experimental conditions.

Regulation of expression and enzymatic activities of tyrosine and tryptophan hydroxylases in rat brain after acute electroconvulsive shock

Acute electroconvulsive shock (ECS) causes a significant increase of protein synthesis in depressive patients and such an increase raises the possibility that the regulation of specific proteins and enzymatic activities in the brain might be one of the mechanisms required for the induction of long-term adaptive neurochemical changes after electroconvulsive therapy. In current studies, we investigated and compared simultaneously the short- and long-term effects of an acute ECS on the expression and enzymatic activities of both tyrosine and tryptophan hydroxylases (TH and TpOH, respectively) in different rat brain areas. Our results demonstrated that an acute ECS produced: (1) a long-lasting decrease in TH and TpOH protein levels in locus ceruleus (LC), ventral tegmental area (VTA) and in TpOH protein level in the raphe centralis (RC), maximal at 72 h, with concomitant changes in mRNA levels and enzymatic activities in the LC only; (2) large increase of TpOH protein levels in the frontal cortex (Cxf) (+145%) and increase of TH protein levels in the hippocampus (Hip) (+207%), maximal at 72 h and 7 days which was not accompanied by corresponding increase of in vivo enzymatic activities. Furthermore, a second ECS increased in vivo TpOH activity in the Cxf (+19%) while decreasing K(m) value (-50%) for tetrahydrobiopterin cofactor. A stability of the observed findings on TpOH activity in the Cxf after repeated ECS might be one of the mechanisms for the antidepressant effects of electroconvulsive therapy.

Stress-triggered activation of gene expression in catecholaminergic systems: dynamics of transcriptional events

Stress triggers important adaptive responses that enable an organism to cope with a changing environment. However, when prolonged or repeated, stress can be extremely harmful. The release of catecholamines is a key initial event in responses to stressors and is followed by an increase in the expression of genes that encode catecholamine-synthesizing enzymes. This process is mediated by transcriptional mechanisms in the adrenal medulla and the locus coeruleus. The persistence of transcriptional activation depends on the duration and repetition of the stress. Recent work has begun to identify the various transcription factors that are associated with brief or intermediate duration of a single or repeated stress. These studies suggest that dynamic interplay is involved in converting the transient increases in the rate of transcription into prolonged (potentially adaptive or maladaptive) changes in gene expression.

Identification of a potential nurr1 response element that activates the tyrosine hydroxylase gene promoter in cultured cells

Expression of the gene encoding tyrosine hydroxylase (TH), the initial and rate-limiting enzyme of catecholamine biosynthesis, is regulated at the transcriptional level during neuronal development and in response to a variety of environmental stimuli. Nur-related factor 1 (Nurr1), a member of the orphan nuclear receptor superfamily, is required for development of dopamine-producing neurons in the ventral midbrain and for expression of TH in these neurons. In the present study, we found a direct activation of the rat TH gene promoter by Nurr1 in cultured cell lines. This activation appeared to be dependent on multiple regulatory elements conferring Nurr1 responsiveness to the promoter. We identified a Nurr1 response element (TH-NBRE1) in the proximal region of the TH promoter that mediates a moderate activation of the promoter. The sequence of TH-NBRE1 was highly homologous to that of the typical NGFI-B response element. Our findings suggest that Nurr1 may be implicated in the transcriptional control of TH gene expression during development or in response to altered physiological states.

Catecholamine biosynthesis and physiological regulation in neuroendocrine cells

The catecholamines are widely distributed in mammals and their levels and physiological functions are regulated at many sites. These include their release from neuroendocrine cells, the type and sensitivity of the multiple receptors in target cells, the efficacy of the reuptake system in the secretory cells, and the rates of catecholamine biosynthesis and degradation. In the present review the main focus will be on the more recent studies on the biosynthesis in neuroendocrine cells which involves a specific set of enzymes, with special reference to physiologically important regulatory mechanisms. Eight enzymes of the biosynthetic pathway have now been identified, cloned, expressed as recombinant proteins, characterized with respect to catalytic and regulatory properties, and some of them also crystallized. The identification of the tyrosine hydroxylase catalysed reaction as the rate-limiting step in the normal catecholamine biosynthesis has attracted most attention, both in terms of transcriptional and post-translational regulation. In certain human genetic disorders of catecholamine biosynthesis other enzymes in the pathway may become rate-limiting, notably those involved in the biosynthesis/regeneration of the natural co-factor tetrahydrobiopterin in the tyrosine hydroxylase reaction. The enzymes involved seem to be regulated by a variety of physiological factors, both on a long-term scale and a short-term basis, and include the relative rates of synthesis, degradation and state of activation of the biosynthetic enzymes, notably of tyrosine hydroxylase. Multiple surface receptors and signalling pathways are activated in response to extracellular stimuli and play an essential role in the regulation of catecholamine biosynthesis.

Multiple signalling pathways exist in the stress-triggered regulation of gene expression for catecholamine biosynthetic enzymes and several neuropeptides in the rat adrenal medulla

A critical component of the response to stress is the coincident activation of the hypothalamic-pituitary-adrenal axis and the sympathoadrenal system - comprised of sympathetic ganglia and the adrenal medullae. The sympathoadrenal system produces the catecholamines - noradrenaline and adrenaline, and several neuropeptides, involved in the homeostatic mechanisms that govern the adaptation to stress. This brief survey aims to provide a general overview of the present knowledge about the impact of stress on neurotransmitter gene expression in the adrenal medulla, with particular attention paid to the apparent heterogeneity in stress-evoked signals and regulatory pathways.

Protein synthesis blockade differentially affects the degradation of constitutive and nicotinic receptor-induced tyrosine hydroxylase protein level in isolated bovine chromaffin cells

Continuous incubation of bovine adrenal chromaffin cells with the nicotinic receptor agonist 1,1-dimethyl-4-phenylpiperazinium (DMPP) causes a twofold increase in the steady-state level of catalytically active tyrosine hydroxylase (TH) protein by 3-4 days. The present study examined the processes that control the time course of enzyme induction. In cells exposed to DMPP for 36 or 54 h, incorporation of [3H]leucine into TH was increased 1.9- and 2.2-fold, respectively, compared with control (non-DMPP-treated) cells. The increase correlated with a twofold rise in TH mRNA level, indicating the absence of translational control of TH synthesis by DMPP. Also absent was an effect by DMPP on the rate of degradation of TH protein because pulse-chase analysis estimated a half-life for TH of 26 +/- 5 h in DMPP-treated cells, a value that was (a) essentially the same as that estimated in control cells (29 +/- 3 h), (b) within the same range as that estimated by approach to steady state (t(1/2) = 19 +/- 4 h), which measured the decline of TH protein content from the DMPP-induced steady-state level back to the basal value during deinduction with the nicotinic antagonist hexamethonium, and (c) consistent with the time course of accumulation of TH protein to a new steady-state level in response to DMPP. However, different rates of degradation for TH protein were observed in control and DMPP-treated cells under conditions in which protein synthesis was blocked. In control cells incubated with 100 microM puromycin or 20 microM cycloheximide for 3 days, the level of catalytically active TH protein failed to decline and exhibited a half-life of > or = 250 h. This finding indicated that TH protein was stabilized. TH protein level also failed to decline when cells were incubated for 3 days with a concentration of the transcription inhibitor alpha-amanitin that caused a >90% loss of TH mRNA. Thus, degradation of constitutively expressed TH protein appears to be controlled by processes dependent on ongoing transcription and translation. In contrast, the increased amount of TH induced by DMPP was not stabilized but instead underwent a decline to the basal level following addition of puromycin or cycloheximide. It is important to note, however, that the decline occurred at a slower rate (t(1/2) > or = 45 h) than that measured during deinduction. Taken together, these data suggest that alterations in the rate of degradation of TH protein may play a role in controlling TH level when TH synthesis is blocked but not when TH synthesis is increased, such as during nicotinic receptor stimulation.

Electron-immunocytochemical studies of perivascular nerves of mesenteric and renal arteries of golden hamsters during and after arousal from hibernation

Electron immunocytochemistry was used to examine perivascular nerves of hamster mesenteric and renal arteries during hibernation and 2 h after arousal from hibernation. Vessels from cold-exposed but nonhibernating, and normothermic control hamsters were also examined. During hibernation the percentage of axon profiles in mesenteric and renal arteries that were immunopositive for markers of sympathetic nerves, tyrosine hydroxylase (TH) and neuropeptide Y (NPY), were increased 2-3 fold compared with normothermic and cold control animals. This increase was reduced markedly only 2 h after arousal from hibernation. The small percentage of nitric oxide synthase-1-positive axon profiles found in mesenteric (but not renal) arteries was also increased during hibernation and returned towards control values after arousal. In contrast, the percentage of perivascular axons immunostaining for vasoactive intestinal polypeptide (VIP), a marker for parasympathetic nerves, was reduced in mesenteric arteries during hibernation. There was no labelling of perivascular nerves for substance P in either mesenteric or renal arteries. It is suggested that the increase in percentage of TH- and NPY-immunostained perivascular nerves may account for the increased vasoconstriction associated with high vascular resistance that is known to occur during hibernation. The reduction in the percentage of axons positive for VIP in hibernating animals would contribute to this mechanism since this neuropeptide is a vasodilator.

Regulation of tyrosine hydroxylase gene expression by muscarinic agonists in rat adrenal medulla

Tyrosine hydroxylase (TH) gene expression in the adrenal medulla is regulated by numerous stimuli via transsynaptic mechanisms. The adrenal chromaffin cell receptors that mediate this transsynaptic response remain unidentified. In this report we demonstrate that the muscarinic acetylcholine receptor agonist bethanechol stimulates the TH gene transcription rate in both innervated and denervated adrenal glands. Hence, this muscarinic response is not dependent on transsynaptic influences, suggesting that agonist occupation of adrenal chromaffin cell muscarinic receptors is sufficient to activate intracellular signaling pathways that stimulate the TH gene. When bethanechol is administered repeatedly over a 3-h interval (four injections spaced 1 h apart), TH mRNA levels are increased two- to threefold at 6 and 12 h after the initial injection of drug. It is surprising that this induction of TH mRNA does not lead to increases in TH activity or TH protein level. These results are consistent with the hypothesis that both transcriptional and posttranscriptional mechanisms must be regulated to induce TH protein and that muscarinic agonists activate only a subset of these mechanisms.

Region-specific alterations in tyrosine hydroxylase activity in rats exposed to lead

Previous studies from our laboratory showed that subchronic exposure to low levels of Pb resulted in significant decrease in dopamine (DA) content, attenuation of stimulus-induced release of DA in the dopaminergic projection area of nucleus accumbens (NA), and alterations in tyrosine hydroxylase (TH) activity in rat whole brain homogenates. The present study reported here was conducted to assess the functional integrity of DA synthesis in different brain regions of rats subchronically (90-days) exposed to 50 ppm Pb by measuring the activity of the rate limiting enzyme, tyrosine hydroxylase, in seven brain regions. In Pb-exposed rats, TH activity was reduced in two of the seven brain regions investigated, i.e., nucleus accumbens (42% reduction) and frontal cortex (61% reduction) when compared to controls. In contrast, Pb exposure did not affect the TH activity in cerebellum, brainstem, hippocampus, hypothalamus and striatum. The changes in TH activity in nucleus accumbens (NA) and frontal cortex (FC) in Pb-exposed rats were further confirmed by Western blot analysis using TH polyclonal antibody. Collectively, these results indicate that low level subchronic Pb exposure may affect TH protein in these brain regions.

Vasoactive intestinal peptide induces both tyrosine hydroxylase activity and tetrahydrobiopterin biosynthesis in PC12 cells

Vasoactive intestinal peptide plays an important role in the trans-synaptic activation of tyrosine hydroxylase in sympathoadrenal tissues in response to physiological stress. Since tyrosine hydroxylase is thought to be subsaturated with its cofactor, tetrahydrobiopterin, we tested the hypothesis that up-regulation of tyrosine hydroxylase gene expression following vasoactive intestinal peptide treatment is accompanied by a concomitant elevation of intracellular tetrahydrobiopterin biosynthesis. We also investigated the second messenger systems involved in vasoactive intestinal peptide's effects on tetrahydrobiopterin metabolism. Our results demonstrate that treatment of PC12 cells for 24 h with vasoactive intestinal peptide induced intracellular tetrahydrobiopterin levels 3.5-fold. This increase was due to increased expression of the gene encoding GTP cyclohydrolase, the initial and rate-limiting enzyme in tetrahydrobiopterin biosynthesis, which was blocked by the transcriptional inhibitor, actinomycin D. Activation of tyrosine hydroxylase and GTP cyclohydrolase by vasoactive intestinal peptide was mediated by cyclic-AMP. Furthermore, stimulation of cyclic-AMP-mediated responses or protein kinase C activity induced the maximal in vitro activities of both tyrosine hydroxylase and GTP cyclohydrolase; the responses were additive when both treatments were combined. Induction of sphingolipid metabolism had no effect on the activation of tyrosine hydroxylase, while it induced GTP cyclohydrolase in a protein kinase C-independent manner. Our results support the hypothesis that intracellular tetrahydrobiopterin levels are tightly linked to tyrosine hydroxylation and that tetrahydrobiopterin bioavailability modulates catecholamine synthesis.

AP-1 mediates trans-synaptic induction of tyrosine hydroxylase gene expression in adrenal medulla but not in superior cervical ganglia

Reserpine treatment leads to a rapid trans-synaptic increase of the tyrosine hydroxylase (TH) gene transcription rate and mRNA levels in catecholaminergic tissues including the adrenal medulla (AM) and the superior cervical ganglia (SCG). In the AM, the formation of a specific protein complex with the TPA-responsive element located in the proximal region of the TH gene was enhanced between 30 min and 8 hr following the injection. This complex appears to contain a member of the Fos family and an antigenically related Jun protein. Moreover, the prolonged and enhanced expression of the c-Fos protein in the AM and its phosphorylation are likely to contribute to the increased TH transcription following reserpine treatment. Most strikingly, in the SCG, the trans-synaptic induction of TH transcription is transduced by totally different mechanisms, since no AP-1 complex and only minute amounts of c-Fos immunoreactivity were detected. Our study provides the first demonstration that, following the same stimulus, the induced expression of a single gene is mediated by different cis- and trans-acting factors in two distinct tissues sharing the same embryonic origin.

Immobilization Stress Elevates GTP Cyclohydrolase I mRNA Levels in Rat Adrenals Predominantly by Hormonally Mediated Mechanisms

GTP cyclohydrolase I (GTPCH) is the rate-limiting enzyme in the biosynthesis of tetrahydrobiopterin, the cofactor for catecholamine, indolamine and nitric oxide biosynthesis. In this study we examined the effect of immobilization stress on GTPCH mRNA levels and the mechanism(s) of stress-induced changes in adrenomedullary GTPCH mRNA levels. We used reverse-polymerase chain reaction to isolate and clone a cDNA corresponding to nucleotides 269 to 570 of rat GTPCH. Northern blot analysis with a cRNA probe revealed two species of GTPCH mRNA (about 3.6 and 1.2 kb) in rat adrenal medulla and cortex, and in PC12 cells. The levels of both forms of GTPCH mRNA were significantly increased 3-5 fold in adrenal medulla by a single 2 hour immobilization and by repeated immobilizations (2 hours a day for 2 days). Hypophysectomy had little effect on their basal levels but prevented the stress elicited rise in both GTPCH mRNAs. In contrast, unilateral transection of the splanchnic nerve did not affect induction of the 3.6 kb GTPCH mRNA by stress. Combined denervation with hypophysectomy completely blocked the induction of both GTPCH mRNA species by immobilization stress. Thus, stress elicits elevation of both forms of GTPCH mRNA by a mechanism requiring an intact pituitary-adrenocortical axis.

Induction of tyrosine hydroxylase by forskolin: modulation with age

With aging, circulating catecholamines are elevated in both humans and animals. This may be related to the increased basal levels of tyrosine hydroxylase messenger RNA (mRNA) levels and tyrosine hydroxylase enzyme activity in the adrenal medulla of senescent compared with younger animals. In addition, tyrosine hydroxylase gene expression in the senescent rat is resistant to further stimulation by cold exposure as compared with younger animals. Collectively, these observations suggest either that tyrosine hydroxylase expression is already maximally stimulated in senescent rats or that tyrosine hydroxylase gene induction pathways are impaired with senescence. To help distinguish between these possibilities, we examined the induction of tyrosine hydroxylase mRNA, tyrosine hydroxylase immunoreactivity and tyrosine hydroxylase enzyme activity in the adrenal medulla following forskolin administration to young and old F-344 rats. Forskolin at doses of 1.8 and 3.5 mg/kg increased tyrosine hydroxylase mRNA levels 2.5-fold in adrenal medulla from young rats but did not increase either tyrosine hydroxylase immunoreactivity or tyrosine hydroxylase enzyme activity 5 h after administration. Prolonged treatment with forskolin (3 doses, 12 h apart) increased tyrosine hydroxylase mRNA levels and tyrosine hydroxylase immunoreactivity and tyrosine hydroxylase enzyme activity. In senescent rats, the baseline level of tyrosine hydroxylase mRNA was more than 2-fold higher compared with young rats. A single injection of the lower dose of forskolin increased tyrosine hydroxylase mRNA levels by the same increment in senescent as compared with young rats. These data indicate that the tyrosine hydroxylase gene in the adrenal medulla from senescent rats is still capable of further stimulation.

Glucoprivation by insulin leads to trans-synaptic increase in rat adrenal tyrosine hydroxylase mRNA levels

Effects of single or repeated insulin or 2-deoxy-D-glucose administration on adrenal tyrosine hydroxylase mRNA and protein levels were examined in rats. Insulin produced hypoglycemia and an elevation in plasma epinephrine and norepinephrine levels. A significant increase (3-5-fold) in tyrosine hydroxylase mRNA levels was found at 5 h, decreasing to near basal levels at 24 h following the first and also the sixth consecutive injection of insulin or 2-deoxy-D-glucose. Whereas insulin treatment raised tyrosine hydroxylase mRNA levels in intact adrenals, no increase in tyrosine hydroxylase mRNA levels occurred following adrenal denervation by splanchnic nerve transection. Western blot analysis showed that although a single insulin treatment did not affect tyrosine hydroxylase protein levels, a significant increase was observed following the seventh administration. This study shows that insulin-induced hypoglycemia increases tyrosine hydroxylase gene expression and that this process is regulated by a central mechanism via the splanchnic nerve.

Regulation of tyrosine hydroxylase and tetrahydrobiopterin biosynthetic enzymes in PC12 cells by NGF, EGF and IFN-gamma

The regulation of catecholamine and tetrahydrobiopterin synthesis was investigated in cultured rat pheochromocytoma PC12 cells following treatments with nerve growth factor (NGF), epidermal growth factor (EGF) and interferon-gamma (IFN-gamma). NGF and EGF, but not IFN-gamma, caused an increase after 24 h in the levels of BH4 and catecholamines, and the activities of tyrosine hydroxylase and GTP cyclohydrolase, the rate-limiting enzymes in catecholamine and BH4 synthesis, respectively. Actinomycin D, a transcriptional inhibitor, blocked treatment-induced elevations in tyrosine hydroxylase and GTP cyclohydrolase activities. NGF, EGF or IFN-gamma did not affect the activity of sepiapterin reductase, the final enzyme in BH4 biosynthesis. Rp-cAMP, an inhibitor of cAMP-mediated responses, blocked the induction of tyrosine hydroxylase by NGF or EGF; inhibition of protein kinase C partially blocked the EGF effect, but not the NGF effect, NGF also induced GTP cyclohydrolase in a cAMP-dependent manner, while the EGF effect was not blocked by Rp-cAMP or protein kinase C inhibitors. Sphingosine induced GTP cyclohydrolase in a protein kinase C-independent manner without affecting tyrosine hydroxylase activity. Our results suggest that both tyrosine hydroxylase and GTP cyclohydrolase are induced in a coordinate and transcription-dependent manner by NGF and EGF, while conditions exist where the induction of tyrosine hydroxylase and GTP cyclohydrolase is not coordinately regulated.