Molecular adaptations in catecholamine biosynthesis induced by cold stress and sympathectomy

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.

Dexamethasone Causes Hypertension in Rats Even Under Chemical Blockade of Peripheral Sympathetic Nerves

Synthetic glucocorticoids (GCs) are widely used to treat inflammatory conditions. However, chronic use of GCs can lead to hypertension. The cause of this undesired side effect remains unclear. Previously, we developed an in vivo rat model to study the mechanisms underlying hypertension induced by the chronic administration of the potent synthetic GC, dexamethasone (DEX) and found that the catecholamine biosynthetic pathway plays an important role. In the current study, we used this model to investigate the role of the adrenal medulla, renal nerves, and other peripheral sympathetic nerves in DEX-induced hypertension. After 5 days of baseline telemetric recording of mean arterial pressure (MAP) and heart rate (HR), rats were subjected to one of the following treatments: renal denervation (RDNX), adrenal medullectomy (ADMX), 6-hydroxydopamine (6-OHDA, 20 mg/kg, i.p.) to induce chemical sympathectomy, or a combination of ADMX and 6-OHDA. On day 11, the animals received vehicle (VEH) or DEX in drinking water for 7 days, with the latter causing an increase in MAP in control animals. ADMX and RDNX by themselves exacerbated the pressor effect of DEX. In the chemical sympathectomy group, DEX still caused a rise in MAP but the response was lower (ΔMAP of 6-OHDA/DEX < VEH/DEX, p = 0.039). However, when ΔMAP was normalized to day 10, 6-OHDA + DEX did not show any difference from VEH + DEX, certainly not an increase as observed in DEX + ADMX or RDNX groups. This indicates that sympathetic nerves do not modulate the pressor effect of DEX. TH mRNA levels increased in the adrenal medulla in both VEH/DEX (p = 0.009) and 6-OHDA/DEX (p = 0.031) groups. In the 6-OHDA group, DEX also increased plasma levels of norepinephrine (NE) (p = 0.016). Our results suggest that the activation of catecholamine synthetic pathway could be involved in the pressor response to DEX in animals even under chemical sympathectomy with 6-OHDA.

A brain-sparing diphtheria toxin for chemical genetic ablation of peripheral cell lineages

Conditional expression of diphtheria toxin receptor (DTR) is widely used for tissue-specific ablation of cells. However, diphtheria toxin (DT) crosses the blood-brain barrier, which limits its utility for ablating peripheral cells using Cre drivers that are also expressed in the central nervous system (CNS). Here we report the development of a brain-sparing DT, termed BRAINSPAReDT, for tissue-specific genetic ablation of cells outside the CNS. We prevent blood-brain barrier passage of DT through PEGylation, which polarizes the molecule and increases its size. We validate BRAINSPAReDT with regional genetic sympathectomy: BRAINSPAReDT ablates peripheral but not central catecholaminergic neurons, thus avoiding the Parkinson-like phenotype associated with full dopaminergic depletion. Regional sympathectomy compromises adipose tissue thermogenesis, and renders mice susceptible to obesity. We provide a proof of principle that BRAINSPAReDT can be used for Cre/DTR tissue-specific ablation outside the brain using CNS drivers, while consolidating the link between adiposity and the sympathetic nervous system.

Neuronal activity regulates expression of tyrosine hydroxylase in adult mouse substantia nigra pars compacta neurons

Striatal delivery of dopamine (DA) by midbrain substantia nigra pars compacta (SNc) neurons is vital for motor control and its depletion causes the motor symptoms of Parkinson's disease. While membrane potential changes or neuronal activity regulates tyrosine hydroxylase (TH, the rate limiting enzyme in catecholamine synthesis) expression in other catecholaminergic cells, it is not known whether the same occurs in adult SNc neurons. We administered drugs known to alter neuronal activity to mouse SNc DAergic neurons in various experimental preparations and measured changes in their TH expression. In cultured midbrain neurons, blockade of action potentials with 1 μM tetrodotoxin decreased TH expression beginning around 20 h later (as measured in real time by green fluorescent protein (GFP) expression driven off TH promoter activity). By contrast, partial blockade of small-conductance, Ca(2+) -activated potassium channels with 300 nM apamin increased TH mRNA and protein between 12 and 24 h later in slices of adult midbrain. Two-week infusions of 300 nM apamin directly to the adult mouse midbrain in vivo also increased TH expression in SNc neurons, measured immunohistochemically. Paradoxically, the number of TH immunoreactive (TH+) SNc neurons decreased in these animals. Similar in vivo infusions of drugs affecting other ion-channels and receptors (L-type voltage-activated Ca(2+) channels, GABA(A) receptors, high K(+) , DA receptors) also increased or decreased cellular TH immunoreactivity but decreased or increased, respectively, the number of TH+ cells in SNc. We conclude that in adult SNc neurons: (i) TH expression is activity-dependent and begins to change ∼20 h following sustained changes in neuronal activity; (ii) ion-channels and receptors mediating cell-autonomous activity or synaptic input are equally potent in altering TH expression; and (iii) activity-dependent changes in TH expression are balanced by opposing changes in the number of TH+ SNc cells.

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.

Chronic cold stress sensitizes brain noradrenergic reactivity and noradrenergic facilitation of the HPA stress response in Wistar Kyoto rats

Many psychiatric disorders, including depression, post-traumatic stress disorder and other anxiety disorders, result from an interaction between genetic factors and exposure to a sufficiently sensitizing environmental stressor. The inbred Wistar Kyoto (WKY) rat strain has been proposed as a model of stress vulnerability, exhibiting an exaggerated hypothalamic-pituitary-adrenal (HPA) response to stress and susceptibility to gastric ulceration. Previously, we showed that stress-activation of the brain noradrenergic system was deficient in WKY rats, and they lacked noradrenergic facilitation of the HPA response in the lateral bed nucleus of the stria terminalis (BSTL), compared to outbred Sprague-Dawley (SD) controls. Deficient modulatory function of the noradrenergic system may contribute to the stress susceptibility of WKY rats. Thus, we investigated the influence of a sensitizing stimulus, chronic intermittent cold exposure, on neuroendocrine and noradrenergic stress reactivity, and on noradrenergic facilitation of the HPA response in these two strains. Chronic cold exposure (7 days, 4 h/day, 4 degrees C) potentiated activation of the HPA axis by acute immobilization stress, assessed by measuring plasma adrenocorticotropic hormone (ACTH), in both strains, although to a greater extent in WKY rats, and enhanced stress-induced norepinephrine (NE) release in BSTL of WKY but not SD rats. We then compared the influence of chronic cold exposure on noradrenergic modulation of the HPA stress response in BSTL, by measuring changes in acute stress-induced elevation of plasma ACTH after microinjecting the alpha(1)-adrenoreceptor antagonist benoxathian into the BSTL. As shown previously, benoxathian attenuated stress-induced ACTH secretion in control SD but not control WKY rats. After chronic cold, the ACTH response to acute stress was attenuated by benoxathian administration into BSTL of both strains, such that the WKY response was not different from that of SD rats. Thus, chronic cold not only sensitized the release of NE in BSTL of WKY rats, but also restored noradrenergic facilitation of their already-elevated HPA response. Such functional sensitization of a previously-deficient facilitatory system may be one mechanism whereby exposure to repeated or severe stress may induce pathologic dysregulation of the stress response in susceptible individuals, resulting in psychiatric illness.

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.

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.

Chronic glucocorticoid administration as well as repeated stress affects the subsequent acute immobilization stress-induced expression of immediate early genes but not that of NGFI-A

We reported that repeated immobilization for six days attenuates the subsequent acute immobilization stress-induced expression of the immediate early genes c-fos, fos B, jun B and nerve growth factor-induced gene-B (NGFI-B), but not of NGFI-A, in the rat paraventricular hypothalamic nucleus. In this study, we confirmed these findings by means of a time-course study, and further investigated whether the elevated plasma basal glucocorticoid level induced by repeated stress underlies the attenuated response of immediate early genes and the preserved reactivity of NGFI-A. Rats implanted with 100, 200 or 400 mg corticosterone or placebo pellets (control), were immobilized for 1 h and decapitated seven days later. In control rats acute immobilization induced c-fos, fos B, jun B, NGFI-A and NGFI-B messenger RNA in the paraventricular hypothalamic nucleus, whereas all of them except NGFI-A, were significantly reduced in rats given 200 and 400 mg corticosterone implants. The similarity of the results from the two procedures suggests that glucocorticoid is involved in regulating immediate early genes in the paraventricular hypothalamic nucleus under repeated stress and that the NGFI-A gene is not regulated by this mechanism. However, the plasma basal corticosterone level in repeatedly stressed rats was lower than that of rats implanted with 100 mg corticosterone, suggesting that a repetitive stress-induced corticosterone surge also contributes to this mechanism.

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.

Sympathoadrenal system in stress. Interaction with the hypothalamic-pituitary-adrenocortical system

Exposure of an organism to any of a variety of stressors markedly activates the sympathoadrenal and hypothalamic-pituitary-adrenocortical systems. Interactions of these major stress systems occur at several levels in the periphery and the brain. In the present study, we used sham-operated or adrenalectomized cortisol-treated conscious rats to examine glucocorticoid effects on indices of CA release, metabolism, and synthesis, and on CA biosynthetic enzyme activities and gene expression at baseline and during immobilization stress (IMO). Adrenalectomy (ADX) stimulated basal and stress-induced increments in norepinephrine release, reuptake, metabolism, turnover, and biosynthesis. Loss of adrenomedullary hormones after ADX did not appear to contribute to these increments. Cortisol treatment reversed the ADX effects on CA indices and suppressed catecholaminergic responses to IMO in intact rats. These results suggest that endogenous glucocorticoids restrain responses of catecholamine turnover, synthesis, release, reuptake, and metabolism during stress. In contrast, in intact rats, continuous administration of cortisol lasting for 7 days exaggerated the IMO-induced increases in plasma CA levels. Inhibition of DOPA conversion to dopamine elevated plasma DOPA levels in chronically cortisol-treated stressed rats compared to saline-treated ones, suggesting a cortisol-induced increase in tyrosine hydroxylation. Stress increases TH and PNMT activities and mRNA levels in the adrenal medulla. Hypophysectomy reduced adrenal PNMT but not TH mRNA levels in control and IMO rats. Pretreatment of hypophysectomized animals with ACTH fully restored the control and IMO-induced adrenal PNMT mRNA levels and augmented PNMT but not TH mRNA responses in intact rats. Long-term cortisol administration to intact rats also elevated adrenal PNMT but not TH mRNA levels. The results indicate a suppressive effect of endogenous glucocorticoids and a stimulatory effect of chronically elevated glucocorticoid levels on sympathoadrenal activity during stress. The results also suggest that a nonneuronal, nonpituitary factor contributes to TH gene expression during some forms of stress, whereas pituitary-adrenocortical factors play the essential role in the regulation of PNMT gene expression.

Tyrosine hydroxylase expression in rat adrenal medulla: influence of age and cold

Chronic and cold exposure is associated with an increase in adrenal medullary tyrosine hydroxylase (TH) activity and expression that may be important for the regulatory response to cold. Senescent rats do not maintain their body temperature as well as young rats. We investigated the ability of the catecholaminergic system of older rats to respond to cold stimulus. TH activity, TH immunoreactivity, and TH mRNA were assessed in adrenal medullae of male F-344 rats of 3 and 24 months of age following 48 h of mild (8 degrees C) cold exposure. In control rats, basal levels of TH activity were increased by 2.9-fold, TH immunoreactivity by 1.3-fold, and TH mRNA by 2.3-fold with age. In the young rats there were increases after a 48-h cold exposure in TH activity, TH immunoreactivity, and TH mRNA per pair of adrenal medullae. In contrast, in senescent rats there were no significant changes in these parameters following cold exposure. These data suggest that the induction of TH activity is impaired in senescent rats following cold exposure and that there is a loss of plasticity with respect to the TH gene expression.

Induction of tyrosine hydroxylase gene expression by a nonneuronal nonpituitary-mediated mechanism in immobilization stress

Stress stimulates the sympathoadrenal system, causing activation of the catecholamine biosynthetic enzymes. Here we examine the changes of gene expression of tyrosine hydroxylase (TH; EC 1.14.16.2), the initial enzyme of catecholamine biosynthesis, with stress. A single immobilization of rats led to a large transient elevation in TH mRNA and a small elevation in TH immunoreactive protein and activity. Repeated daily immobilizations triggered more sustained changes in TH mRNA levels. After two immobilizations, the levels remained elevated even 3 days later. The rise in TH mRNA was followed by increased immunoreactive protein but only a small elevation in activity. With seven repeated immobilizations, the animals did not appear to adapt and still manifested a further rise in TH mRNA. TH activity was markedly elevated and returned to control levels 7 days after the immobilization. The rise in TH mRNA with a single immobilization occurred even in adrenals of hypophysectomized rats that underwent splanchnic nerve section. Immobilization for 30 min was sufficient to increase TH mRNA. The effect was abolished by the transcriptional inhibitor actinomycin D. Mobility gel-shift assays revealed increased binding of c-Fos and c-Jun to the AP-1 transcription factor site after a single immobilization, and the binding was not further elevated with repeated stress. This study shows that a single immobilization can activate TH gene expression by a nonneuronal nonpituitary-mediated pathway associated with increased binding of AP-1 transcription factors.

Bovine tyrosine hydroxylase gene-promoter regions involved in basal and angiotensin II-stimulated expression in nontransformed adrenal medullary cells

The tyrosine hydroxylase gene is expressed specifically in catecholaminergic cells, and its activity is regulated by afferent stimuli. To characterize molecular mechanisms underlying those regulations, we have constructed chimeric genes consisting of bovine tyrosine hydroxylase gene promoters (wild-type or deletion mutants) and a luciferase reporter gene. The basal expression of these genes and their regulation by angiotensin II were examined in cultured bovine adrenal medullary cells. Luciferase activity was normalized to the amount of transfected plasmid DNA. A pTHgoodLUC plasmid containing the -428/+21-bp fragment of the tyrosine hydroxylase gene promoter expressed luciferase activity at severalfold higher levels than the promoterless pOLUC plasmid. Deletion of the -194/-54-bp promoter fragment containing POU/Oct, SP1, and other putative regulatory elements increased luciferase expression fivefold. An additional deletion further upstream (-269/-194 bp), including a 12-O-tetradecanoylphorbol 13-acetate (TPA)-responsive element (TRE)-like site, reduced promoter activity. These results indicate the presence of negatively and positively acting regions in the bovine tyrosine hydroxylase gene promoter controlling basal promoter activity in adrenal medullary cells. Angiotensin II stimulated the expression of endogenous tyrosine hydroxylase gene and pTHgood-LUC approximately threefold without affecting the expression of pOLUC. A comparable threefold stimulation was observed following the deletion of the -194/-54-bp promoter region, despite the increase in basal promoter activity. Additional deletion of the -269/-194-bp promoter fragment reduced stimulation by angiotensin II to 1.5-fold. These results indicate that the angiotensin II receptor-responsive element is located in the -269/-194-bp promoter region containing the TRE-like site. Additional angiotensin II-responsive site(s) may be present outside this region. Gel mobility shift assays demonstrated constitutive and angiotensin II-induced protein binding to the tyrosine hydroxylase gene promoter. Some DNA-protein complexes were displaced with c-Fos antibodies. The results suggest that c-Fos-related antigens support basal promoter activity and mediate activation of tyrosine hydroxylase by angiotensin II receptor.

Early induction of rat brain tryptophan hydroxylase (TPH) mRNA following parachlorophenylalanine (PCPA) treatment

Tryptophan hydroxylase (TPH) is the first and presumably rate-limiting enzyme in serotonin (5-HT) biosynthesis. End-product inhibition of rate-limiting enzymes is common and 5-HT is known to inhibit TPH activity in vivo. However, it is not known whether levels of 5-HT could also be involved in the regulation of the TPH gene. In order to determine whether TPH gene regulation is dependent on the 5-HT concentration, 5-HT levels were reduced by the administration of parachlorophenylalanine (PCPA). PCPA is a potent, specific and irreversible inhibitor of TPH activity which drastically reduces 5-HT concentration in the 5-HT neurons and terminals. When PCPA was administered, TPH activity in both cell bodies and nerve terminal areas, was reduced to 10% of control values and recovered to the control levels by day 7 in raphe nucleus, and within 14 days in the hypothalamus. In serotonergic terminal areas, 5-HT could not be detected immunohistochemically at day 1, but slowly recovered within 2 weeks. At all time points examined, aromatic L-amino acid decarboxylase (AADC) levels were not changed either in the cell body or terminal areas. The steady state levels of TPH mRNA estimated by in situ hybridization increased at day 1 and returned to control levels by day 4. AADC message levels were not altered throughout the periods. These data suggest that a decrease in 5-HT concentration may lead to an up-regulation of TPH gene transcription, by an, as yet, unknown mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of tyrosine hydroxylase gene expression in depolarized non-transformed bovine adrenal medullary cells: second messenger systems and promoter mechanisms

Activation of the tyrosine hydroxylase (TH) gene in the adrenal medulla during stress is mediated by trans-synaptic mechanisms and may involve cholinergic receptors. Stimulation of nicotinic receptors in adrenal medullary cells induces cell depolarization, influx of Ca2+ ions and increases levels of cAMP. We have shown that both cAMP and membrane depolarization produce an increase in the expression of the TH gene in cultured bovine adrenal medullary cells (BAMC). Others have proposed that transcriptional activation of the TH gene by cAMP is mediated through the sequence homologous to a cAMP responsive element (CRE) located in the proximal region of the TH gene promoter. In the present study we have examined the mechanisms by which membrane depolarization increases the TH gene activity. Treatment of serum-free BAMC cultures with the depolarizing agent, veratridine, increased the extracellular concentration of catecholamines, Met5-enkephalin, and the relative abundance of TH mRNA. Veratridine treatment also increased the levels of mRNAs for the catecholamine biosynthetic enzyme phenylethanolamine N-methyltransferase (PNMT), and proenkephalin A (PEK). Treatment for longer than 3 h was required to increase TH mRNA levels. By contrast, our previous studies indicated that cAMP stimulation for 2 h produces a maximal increase in TH mRNA levels in BAMC. The effects of veratridine and forskolin on TH mRNA levels were additive, further indicating that depolarization and cAMP activate TH gene expression via different pathways. Calmidazolium, an antagonist of calmodulin, had no effect on the veratridine-induced increase in TH mRNA levels. Similarly sphingosine treatment or preincubation with PMA, which reduce protein kinase C (PKC) activity and attenuate the induction of TH mRNA by PMA or the hormone, angiotensin II, did not affect the induction by veratridine. To identify promoter mechanisms of TH gene activation in depolarized cells we transfected BAMC with a plasmid pTHgoodLuc and treated with veratridine for 24 h. pTHgoodLUC contains a luciferase reporter gene linked to a -428/+21 bp fragment of the bovine TH gene promoter (relative to the transcription start site). Veratridine increased the expression of luciferase from the TH promoter 2.5-fold. Deletion of the -194/-54 bp promoter region containing SP-1 and POU/Oct sites reduced veratridine stimulation by 40%. Additional deletion of the -269 to -190 bp promoter segment, including an AP-1 element, further reduced veratridine stimulation to a statistically non-significant level. In conclusion, activation of TH gene expression upon depolarization is not mediated by calmodulin and PKC. Promoter sequences involved in this activation are located upstream from the CRE. Depolarization may activate TH gene transcription by acting on more than one regulatory region.

Repeated stress reduces the subsequent stress-induced expression of Fos in rat brain

Repeated stress is known to potentiate the CNS response to subsequent stress. Various stressful stimuli can induce Fos expression in discrete regions of the brain, such as the lateral septum, the hypothalamic paraventricular nucleus and the locus coeruleus. We investigated by immunohistochemistry the effect of the stress of repeated immobilization on Fos expression in those regions of the brain in adult male rats. Six daily immobilizations suppressed the expression of Fos in all regions when immobilization was subsequently applied, suggesting that Fos does not play a major role in potentiating the stress response under repeated stressed conditions.

Unilateral stimulation or removal of rat vibrissae: analysis of nerve growth factor and tyrosine hydroxylase mRNA in the brain

Previous work has shown that unilateral manipulation of vibrissae in the rat can lead to behavioral asymmetries and to neuronal changes in the basal ganglia: in brief, vibrissae stimulation led to increases in neostriatal dopamine release, whereas unilateral removal of vibrissae led to asymmetries in striatal afferents and to bilateral changes in mesencephalic dopamine mechanisms which were related to the occurrence of behavioral asymmetries and the later recovery therefrom. In the present study, the analysis of neuronal mechanisms possibly affected by vibrissae manipulation was extended to the nerve growth factor and the expression of tyrosine hydroxylase mRNA. Unilateral stimulation or removal of the vibrissae did not lead to significant changes in tissue levels of nerve growth factor in the neostriatum, parietal cortex (including the barrel cortex) or the hippocampus. In contrast, tyrosine hydroxylase mRNA in the substantia nigra and ventral tegmental area was affected by vibrissae removal but not by stimulation, as a bilateral increase in labeling was observed on the level of individual neurons. This effect was only observed in animals tested 4 h after vibrissae removal but not after 10 days. The results are discussed with respect to the interaction of vibrissae function with the basal ganglia, the neurotransmitter dopamine and mechanism of functional recovery.

Stress, antidepressant drugs, and the locus coeruleus

This review presents a synthesis of a large body of seemingly inconsistent literature on the role of the locus coeruleus-norepinephrine (LC-NE) system and the corticotropin-releasing hormone (CRH)-median eminence system in mediating the CNS effects of stress and the therapeutic effects of antidepressant drugs. The clinical implications of these findings for the etiology and treatment of stress-related psychiatric disorders such as depression will be discussed.

Correlation of GAP-43 immunoreactivity with subpopulations of chromaffin cells in rat adrenal medulla

The neuronal growth-associated protein (GAP-43) is widely expressed during embryonic growth and axonal regeneration and has been thought to contribute to synaptic plasticity in adult animals. In contrast to the exclusively presynaptic pattern of GAP-43 immunoreactivity in sympathetic ganglia, GAP-43 intensely and selectively labeled the noradrenergic subpopulation of adult rat adrenal medullary chromaffin cell bodies, a pattern which persisted with adrenal denervation. Adjacent adrenergic islands containing neuropeptide Y and phenylethanolamine-N-methyl transferase immunoreactivity failed to express GAP-43. The immunohistochemical appearance of GAP-43 was qualitatively unchanged in the adrenal medulla of aged and diabetic rats, conditions in which the sympathoadrenal axis is thought to be dysfunctional.

Differential regulation of tyrosine hydroxylase protein and activity in rabbit sympathetic neurones after long-term cold exposure: altered responses in ageing

The aim of this study was to investigate the response of sympathetic neurones to prolonged neural stimulation, using cold exposure as a non-invasive experimental paradigm. We examined the effects of prolonged (8 days and 4 wk) cold exposure on tyrosine hydroxylase (TH) protein and activity and neuropeptide Y (NPY) levels in sympathetic neurones of the superior cervical ganglion (SCG), together with NPY levels in the ear artery from young and aged rabbits. The main findings were as follows. In young rabbits, TH levels and TH activity were differentially regulated in response to prolonged cold exposure. TH levels rose whilst TH activity tended to decline. Decentralization of SCG from young animals before cold exposure abolished the rise in TH levels. TH activity in SCG from young rabbits was reduced by decentralization whilst cold exposure resulted in an increase in TH activity. Thus, TH activity was induced in the SCG in the absence of pre-ganglionic input, demonstrating a non-synaptic regulatory mechanism. In old rabbits, cold-induced changes were either delayed or failed to occur, indicating that the responses of sympathetic neurones to cold stress are impaired in old age.

Cold-induced increases in phenylethanolamine N-methyltransferase (PNMT) mRNA are mediated by non-cholinergic mechanisms in the rat adrenal gland

Previously, we reported that cold stress induces a rapid increase in adrenomedullary PNMT mRNA levels, followed by concomitant increases in PNMT immunoreactivity (10). In the present study, the extracellular signals mediating this adaptive response to stress were investigated using northern analysis and RNA slot-blot hybridization. Although adrenal denervation significantly diminished cold-induced increments in adrenomedullary PNMT mRNA levels, it did not completely abolish the cold stress response. In contrast to these results, splanchnectomy completely inhibited cold-induced increments in TH mRNAs in the same tissue samples. These findings indicate that the effects of cold exposure on PNMT mRNA levels are mediated by both neural and non-neural mechanisms, and that adrenal PNMT and TH are differentially regulated in response to cold stress. Surprisingly, the neural component of the PNMT stress response could not be attenuated by peripheral administration of chlorisondamine, a powerful nicotinic ganglionic blocking agent. In contrast, chlorisondamine was effective in inhibiting sympathetic neural activity, as judged by the drug's ability to completely block increases in blood pressure, heart rate, and plasma catecholamines resulting from spinal cord stimulation in pithed rats. The administration of atropine, a muscarinic receptor antagonist, also failed to inhibit cold-induced alterations in adrenal PNMT mRNA. These results suggest that the trans-synaptic induction of adrenal PNMT mRNA involves a non-cholinergic component, and that cold-induced increases in PNMT mRNA are not coupled to acetylcholine-mediated adrenal catecholamine release.

Repeated immobilization stress alters tyrosine hydroxylase, corticotropin-releasing hormone and corticosteroid receptor messenger ribonucleic Acid levels in rat brain

In situ hybridization histochemistry was used to localize and quantify the effects of acute and repeated immobilization stress on mRNA levels of tyrosine hydroxylase (TH) in catecholaminergic neurons in the locus ceruleus and substantia nigra and on mRNA levels of relevant markers of the hypothalamic-pituitary-adrenal axis, namely corticotropin-releasing hormone (CRH) in the hypothalamic paraventricular nucleus (PVN), proopiomelanocortin in the pituitary, and mineralocorticoid receptors (MR, type I) and glucocorticoid receptors (GR, type II) in the hippocampus, PVN and pituitary. Control, acutely stressed (1 × lMO, sacrificed immediately after 2 h of immobilization), and repeatedly stressed (6 × IMO plus delay, sacrificed 24 h after 6 daily 2-h immobilizations and 6 × lMO plus challenge, sacrificed immediately after the seventh daily 2-h immobilization) male Sprague-Dawley rats were examined. TH mRNA expression was increased in the locus ceruleus in the acutely stressed and repeatedly stressed animals. The increase in TH mRNA levels was greatest in the repeatedly stressed (6 × IMO plus challenge) group. TH mRNA levels were not altered in the substantia nigra. CRH mRNA levels in the PVN were significantly increased in the three stressed groups and the increase was greatest in the 6 × IMO plus challenge group. CRH mRNA levels were increased in the central nucleus of the amygdala only after acute stress. Proopiomelanocortin mRNA levels were elevated in the anterior pituitary during acute and repeated stress, but the magnitude of the effect was largest after acute stress. The changes in the hypothalamic-pituitary-adrenal axis were accompanied by an acute stress-induced increase in MR mRNA levels in the hippocampus, MR and GR mRNA levels in the PVN and GR mRNA levels in the pituitary. MR mRNA levels continued to be elevated in the PVN in the 6 × IMO plus challenge animals. Plasma corticosterone levels were elevated in the acute and repeated stress conditions. The results show that repeated immobilization stress produces a rapid and persistent increase in mRNA expression of TH in the locus ceruleus, CRH in the PVN, and proopiomelanocortin in the anterior pituitary. The TH-containing neurons in the locus ceruleus and the CRH-containing neurons in the PVN appear to preserve the capability to respond to repeated stimulation (6 × IMO plus challenge) indicating altered feedback mechanisms under repeated stress conditions. GR and MR mRNA levels are differentially regulated in the hippocampus, PVN and pituitary by acute and repeated stress. It is of interest that the central nervous system systems which are activated during repeated stress, namely the locus ceruleus-norepinephrine system and hypothalamic-pituitary-adrenal axis, are dysregulated in melancholic depression. Further studies of the central nervous system effects of prolonged exposure to stress may help elucidate the mechanisms underlying dysregulation of the locus ceruleus-norepinephrine system and hypothalamic-pituitary-adrenal axis in depression and other stress-related psychiatric diseases.

Molecular aspects of the regulation of tyrosine hydroxylase by testosterone

Previous studies have demonstrated that the sympathetic hypogastric ganglia (HG) are dependent upon the continued presence of testosterone for normal development and maintenance of tyrosine hydroxylase (TH) activity. The regulation of TH by testosterone has been examined further to determine whether the reduction in TH activity following castration is associated with changes in levels of TH protein and mRNA. TH protein was measured by immunotitration of HG homogenates using a TH-specific antibody, and TH-specific mRNA was detected by hybridization of dot blots of total RNA isolated from HG with a cDNA probe coding for TH. The results show that tyrosine hydroxylase activity, protein and mRNA are coordinately reduced in a graded fashion at 1, 2 and 4 weeks following castration. Testosterone replacement therapy immediately following castration prevents the decrease in TH levels. The results indicate that gonadal steroids regulate the biosynthesis of TH in the HG. Testosterone may control TH either directly by interacting with neurons of the HG, or indirectly by altering levels of trophic factors in the target tissues.

Cold-induced alterations in the binding of adrenomedullary nuclear proteins to the promoter region of the tyrosine hydroxylase gene

It is well documented that cold stress induces a rapid trans-synaptically mediated increase in the relative abundance of rat adrenomedullary tyrosine hydroxylase (TH) mRNA. To investigate the transcriptional mechanisms regulating the cold stress response, we have employed a gel mobility shift assay, using DNA fragments prepared from the proximal 5' flanking region of the bovine TH gene as a heterologous molecular probe. In pilot studies, this region of the bovine TH promoter (nucleotides -246 to +21) was fused to the bacterial reporter gene, chloramphenicol acetyltransferase, and the chimeric construct transfected into human neuroblastoma SK-N-BE(2)-C, hepatoma HepG2, and rat pheochromocytoma PC-12 cells. Results of this analysis indicate that the proximal 5' flanking region of the bovine TH gene contains sufficient information to drive transient reporter gene expression in both human and rat catecholaminergic clonal cell lines. The findings derived from the gel mobility shift studies demonstrate that cold exposure causes rapid and selective alterations in the binding of adrenomedullary nuclear proteins to the proximal 5' flanking region of the TH gene. The most striking cold stress-induced alteration in DNA/nucleoprotein binding occurs in a region of the TH promoter (nucleotides -246 to -189) which contains an element bearing marked sequence similarity to an AP1 binding site and is highly conserved among animal species. This alteration occurs within 1 hr of cold exposure and persists for up to 48 hr after the onset of stress. The results of adrenal denervation experiments indicate that the cold-induced change in DNA/nucleoprotein binding is neurally mediated, requiring intact sympathetic innervation of the gland.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute and repeated administration of fluphenazine-N-mustard alters levels of tyrosine hydroxylase mRNA in subsets of mesencephalic dopaminergic neurons

Changes in striatal dopamine turnover and levels of tyrosine hydroxylase messenger RNA were examined in mice injected with D2 selective doses of fluphenazine-N-mustard, an irreversible blocker of dopaminergic receptors. The animals were killed at different times after acute and repeated injections of the drug and dopamine turnover was assessed by measuring dopamine and its metabolite, dihydroxyphenylalanine, in the striatum. Tyrosine hydroxylase mRNA was measured at the single-cell level in neurons of the substantia nigra pars compacta and the ventral tegmental area with quantitative in situ hybridization histochemistry. Acute treatment with fluphenazine-N-mustard induced an increase in both striatal dopamine turnover and the level of tyrosine hydroxylase mRNA in the substantia nigra but not the ventral tegmental area. After two days of repeated drug injections (twice daily), tyrosine hydroxylase mRNA was decreased in the substantia nigra despite the persistence of an elevated dopamine turnover in the striatum. The decrease in mRNA was still observed after four days of repeated treatment while, at that time, turnover values were not different from control. No changes were observed in the ventral tegmental area. The initial increase in tyrosine hydroxylase mRNA in substantia nigra pars compacta suggests that activation of nigrostriatal neurons triggers a very rapid increase in genomic expression of the enzyme. The following decrease in mRNA levels precedes desensitization to the effects of the drug on dopamine turnover, further illustrating a lack of correspondence between increased neurotransmission and levels of tyrosine hydroxylase mRNA in catecholaminergic neurons of the central nervous system.

Modulation of tyrosine hydroxylase gene expression in the rat adrenal gland by exercise: effects of age

Both aging and exercise are associated with alterations in circulating levels of catecholamines. To determine the interactions of age and exercise on tyrosine hydroxylase (TH) activity and TH mRNA, Fischer-344 female rats aged 5 months (young) and 25 months (old) were trained by treadmill running for 10 weeks. The elevation in maximum oxygen consumption in both groups was equivalent following exercise, indicating that training had occurred. In control rats, both TH activity and TH mRNA were greater in the older groups when compared with the younger animals. In young rats, exercise decreased TH activity by 25% and TH mRNA by 27%. In older rats, exercise was not associated with a decrease in TH activity and TH mRNA. Choline acetyltransferase activity (ChAT) was decreased and glutamic acid decarboxylase activity (GAD) was increased by exercise in young rats. The decrease in ChAT activity and increase in GAD activity suggest that trans-synaptic mechanisms play a role in the exercise-induced alteration of TH gene expression. Neither ChAT nor GAD was altered by exercise in older groups. Our data suggest that the previously reported diminution in catecholamines associated with exercise may be due to a decrease in TH mRNA and a resulting decrease in TH activity. There was no effect of exercise in the old rats, supporting previous observations that the plasticity of the sympathoadrenal system diminishes with age.

Differential effects of reserpine on brainstem catecholaminergic neurons revealed by Fos protein immunohistochemistry

The effects of a single systemic injection of reserpine on c-fos proto-oncogene expression in catecholaminergic neurons of the rat brainstem were studied by immunohistochemistry for Fos proteins (Fos). In control rats, a few Fos immunoreactive neuronal nuclei were observed in the tectum and mesencephalic central gray. Within hours after drug injection, a substantial number of brainstem neurons stained intensely for Fos. The staining was maximal at 6 h and returned to control levels within 24 h. Double-immunohistochemical staining with antibodies to tyrosine hydroxylase revealed that in all noradrenergic (NA) neuron subgroups except the A2 group, the majority of NA neurons stained for Fos. Most adrenergic neurons were also labeled. In contrast, aside from some cells in the ventral tegmental area, reserpine did not induce Fos immunoreactivity in dopaminergic neurons. Numerous non-catecholaminergic neurons were intensely stained with Fos in the substantia nigra pars reticulata, ventral tegmental area, mesencephalic central gray, pontine nuclei and tectum. A small number of Fos immunoreactive neurons was also observed in raphe nuclei. Injection of saline (i.p.) resulted in a moderate increase in Fos immunoreactivity in the locus ceruleus, in A1/C1 neurons and in the mesencephalic central gray. The results demonstrate that acute reserpine treatment induces Fos expression in distinct populations of brainstem neurons, comprising both catecholaminergic and non-catecholaminergic neurons. Thus, induction of Fos by reserpine does not coincide with the site of action of this drug. The distribution of Fos immunoreactive NA neurons after reserpine treatment is comparable to that reported after application of stressful stimuli.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitation of tyrosine hydroxylase and neuropeptide Y immunoreactivity in single rat sympathetic neurons: effects of preganglionic nerve activity

Using computerised densitometry to measure immunocytochemical reaction product in a model system, we established conditions that produced a linear relationship between the logarithm of antigen concentration and the measured intensity of staining. We then applied the densitometric technique to assess the changes in tyrosine hydroxylase (TH) and neuropeptide tyrosine (NPY) within sympathetic neurons of rat superior cervical ganglion following chronic decentralization and following reserpine treatment. One week after surgical or pharmacological decentralization, there was appreciable reduction of neuronal levels of both TH and NPY. However, there remained considerable variation in the immunoreactivities of individual cells. Three days of treatment with reserpine elevated TH levels but substantially reduced NPY. Both these effects were prevented by prior decentralization of the ganglia. No differences were seen between normotensive and the Otago strain of genetically hypertensive rats, either in basal TH or NPY immunoreactivities or in responses to the maneuvers performed. Comparison of our findings with previous biochemical data indicate that densitometric immunocytochemistry provides an accurate index of neuronally localised antigen concentrations but also allows analysis of interneuronal differences that are not otherwise apparent.

Effects of stress and adrenalectomy on tyrosine hydroxylase mRNA levels in the locus ceruleus by in situ hybridization

The locus ceruleus-norepinephrine system is one of the principal effectors of the stress response. Acute stress induces norepinephrine synthesis and release, and noradrenergic cells compensate by increasing the activity of tyrosine hydroxylase, the rate-limiting enzyme in catecholamine synthesis. Here we use in situ hybridization histochemistry to show the effects of acute and chronic intermittent stress on the expression of tyrosine hydroxylase mRNA in the rat locus ceruleus. Restraint stress increased tyrosine hydroxylase mRNA in the locus ceruleus but not in dopaminergic nuclei such as the substantia nigra or ventral tegmental area. One hour of footshock or restraint caused a rapid increase in locus ceruleus tyrosine hydroxylase mRNA which returned to basal levels within 24 h. Chronic intermittent stress (1 hour of restraint or footshock per day for 14 days) produced no change in tyrosine hydroxylase mRNA. Neither adrenalectomy nor dexamethasone replacement significantly affected mRNA expression. These findings indicate that acute stress can increase the expression of tyrosine hydroxylase mRNA in the locus ceruleus but that adaptation occurs to repeated stress, and that the expression of tyrosine hydroxylase mRNA in the locus ceruleus is independent of direct glucocorticoid modulation.

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

Long-term cold exposure (5-7 days) is known to induce concomitant increases in the levels of adrenomedullary tyrosine hydroxylase (TH) RNA, protein, and enzyme activity. In this report, we compare the time courses of these changes and investigate the effects of cold exposure on the levels of biopterin, the cofactor required for tyrosine hydroxylation. After only 1 h of cold exposure, TH mRNA abundance increased 71% compared with nonstressed controls. Increases in total cellular TH RNA levels were maximal (threefold over control values) within 3-6 h of cold exposure and remained elevated throughout the duration of the experiment (72 h). TH protein levels increased rapidly after 24 h of cold exposure and reached a maximal value threefold above that of controls at 48-72 h. Despite the relatively rapid and large elevations in TH RNA and protein content, only modest increases in TH activity were detected during the initial 48 h of cold exposure. Adrenomedullary biopterin increased rapidly after the onset of cold exposure, rising to a level approximately twofold that of the nonstressed controls at 24 h, and remained at this level throughout the duration of the stress period. Taken together, the results of this time course study indicate that cold-induced alterations in adrenal TH activity are mediated by multiple cellular control mechanisms, which may include pre- and posttranslational regulation. Our findings also suggest that cold stress-induced increases in the levels of the TH cofactor may represent another key event in the sympathoadrenal system's response to cold stress.

Coordinate and differential regulation of phenylethanolamine N-methyltransferase, tyrosine hydroxylase and proenkephalin mRNAs by neural and hormonal mechanisms in cultured bovine adrenal medullary cells

Primary cultures of bovine adrenal medullary cells (AM) in a chemically defined media were used to examine the role of neural and hormonal factors in the expression of proenkephalin A (pEK), phenylethanolamine N-methyltransferase (PNMT) and tyrosine hydroxylase (TH) genes. Acetylcholine or nicotine reduced cellular content of catecholamines by 30% and increased the relative abundance of pEK, TH, and PNMT mRNAs. The increases produced by acetylcholine were +129%, +147%, and +43% for pEK, TH, and PNMT mRNA, respectively. The kinetics of increases produced by nicotine were different for the 3 mRNAs, with pEK and TH showing enhanced levels over 48 h incubation, while PNMT showed increase during the initial 18 h (+90%) followed by decline to control levels at 48 h. 8-Br cAMP and forskolin elicited a similar pattern of changes as nicotine, suggesting that cyclic AMP may be involved in the mediation of the nicotinic effects. To examine the role of depletion of cellular catecholamines in the regulation of mRNA levels, cells were exposed to tetrabenazine or reserpine. Decreases in cellular catecholamine contents were accompanied by increases in TH and pEK mRNA levels, while the expression of PNMT gene exhibited a transient 4-fold increase and then profound inhibition (60-95%) over a 48-h period. The tetrabenazine effect on TH and pEK mRNA was reduced by alpha-amanitin, suggesting transcriptionally-mediated regulation. Inductions of pEK but not TH or PNMT mRNAs were inhibited by cycloheximide. Hormonal regulation of TH, PNMT, and pEK mRNAs was examined by incubation of cells with dexamethasone. Low concentrations of dexamethasone (0.1, 10 nM) were effective to increase PNMT (+35%, +90%) and pEK (+27%, 45%) mRNA levels. TH mRNA was not affected by similar concentrations of dexamethasone, however, there was a 45% increase at 1 microM. Dexamethasone-elicited increases in PNMT mRNA levels were observed at 48 h and persisted up to 7 days, suggesting that hormonal mechanisms may be distinct from those mediating effects of nicotine, cAMP or tetrabenazine. Taken together, these results indicate that (1) the level of TH, PNMT, and pEK mRNAs are regulated by direct neural (acetylcholine) and hormonal (glucocorticoid) inputs to adrenal medullary cells; (2) effects of acetylcholine could be mediated by cyclic AMP and alterations in catecholamine content; and (3) expression of individual genes is regulated differentially. Such differential regulation of TH, PNMT, and pEK mRNAs may contribute to the long-term selective control of hormonal output from adrenomedullary cells.

Bimodal regulation of adrenal opiate peptides by cholinergic mechanisms

Physiologic stressors increase trans-synaptic impulse activity and result in adrenal catecholamine release and biosynthesis. To determine the effects of stress on the co-localized opiate peptide system, rats were cold stressed at 4 degrees C. While cold stress slightly decreased enkephalin levels, a more severe stress (wetting and cold) increased enkephalin levels by 95%. Examining trans-synaptic-cholinergic mechanisms, treatment with either nicotinic or muscarinic agonists alone resulted in no change in adrenal enkephalin content. However, treatment with both nicotinic and muscarinic agonists together resulted in a three-fold rise in enkephalin levels. To further examine cellular mechanisms, medullae were explanted in the presence of agents that increase second messenger cyclic nucleotide levels. Treatments that increase the levels of cAMP, the cyclic nucleotide associated with nicotinic receptor activation, prevented the rise in medullary enkephalin relative to control explants. In contrast, treatments that increased cGMP levels, the cyclic nucleotide associated with muscarinic receptor activation, had no effect on enkephalin content compared to control explants. However, in the presence of both forskolin (10 microM) plus db-cGMP (5 mM), enkephalin content rose three-fold over control explants. These data suggest that, distinct from catecholamine pathways, enkephalin levels can be positively or negatively regulated by the severity of a stressful stimulus, by cholinergic receptor mechanisms and by an interaction of cyclic nucleotide second-messenger pathways.

Isolation of a rat adrenal cDNA clone encoding phenylethanolamine N-methyltransferase and cold-induced alterations in adrenal PNMT mRNA and protein

Cold stress is known to increase the synthesis and release of catecholamines in the sympathoadrenal system. Previously, we have demonstrated that cold exposure results in a 3- to 4-fold increase in adrenomedullary tyrosine hydroxylase (TH) activity, which is mediated by concomitant alterations in TH mRNA and protein levels. To further investigate the effects of stress on the expression of the catecholamine biosynthetic enzymes, we have isolated a rat cDNA clone encoding the epinephrine-synthesizing enzyme phenylethanolamine N-methyltransferase (PNMT). The cDNA clone is 905 nucleotides in length and contains a single open reading frame corresponding to 270 amino acids. The amino acid sequence predicted from this nearly full-length cDNA is 89% and 86% identical to that of bovine and human PNMT, respectively. Using the rat PNMT cDNA as a hybridization probe, we have measured the effects of cold stress on the relative abundance of adrenomedullary PNMT mRNA. Levels of PNMT protein were also estimated using an immunoblot analysis. As in the case of TH, cold exposure resulted in a rapid and prolonged increase in PNMT mRNA abundance, followed by concomitant increases in PNMT immunoreactivity. However, there appear to be quantitative and qualitative differences in the adaptive response of TH and PNMT to cold stress.

Preganglionic nerve stimulation increases mRNA levels for tyrosine hydroxylase in the rat superior cervical ganglion

Increased synaptic stimulation of sympathetic neurons in vivo causes a delayed increase in the activity and the amount of tyrosine hydroxylase (TH). To determine whether these changes result from an increase in the messenger RNA for TH, the rat preganglionic cervical sympathetic trunk was electrically stimulated unilaterally for 90 min, and 48 h later RNA was extracted from stimulated and contralateral control superior cervical ganglia. Northern blots probed with a cDNA for TH demonstrated that nerve stimulation produced about a 2.5-fold increase in the amount of TH mRNA in the ganglion. These results indicate that synaptic stimulation leads to an increase in TH mRNA, either by increasing the rate of transcription of the TH gene or by increasing the stability of its mRNA.

Transcriptional control of adrenal catecholamine and opiate peptide transmitter genes

In the rat, decreasing transsynaptic activity through adrenal denervation, nicotinic receptor blockade, or explanation is associated with an increase in preproenkephalin mRNA, enkephalin prohormone and peptide. In contrast, catecholamine pathways remain unchanged under similar conditions. Since it is not known whether changes in messenger RNA result from stabilization or increased synthesis, we exploited transcription 'run-on' assays to measure the rate of transmitter gene read out. Tyrosine hydroxylase message (TH-mRNA) was found to be the most abundantly produced transcript in the unmanipulated control rat adrenal medulla. TH-mRNA was produced in excess of twice the rate of transcription of the structural gene beta-actin. In contrast, preproenkephalin transcription occurred at a much lower rate (60% of the actin gene and only 25% of tyrosine hydroxylase gene transcription). All transcripts were inhibited by the polymerase II inhibitor, alpha-amanitin. After two days in explant culture, the rate of enkephalin transcription increased approximately 2-fold (to the same level as actin transcription); while tyrosine hydroxylase transcriptional activity fell to 30% of actin level. To analyze cellular mechanisms, explants were depolarized with potassium chloride. Enkephalin gene transcription was observed to be 2.5-fold less when grown under depolarizing conditions (50 mM KCl) than in control explants. On the other hand, tyrosine hydroxylase gene read-out was unchanged, similar to results obtained when TH catalytic activity was measured. These data indicate that membrane depolarization can selectively regulate expression of a transmitter gene product and are consistent with a proposed transsynaptic regulatory mechanism controlling biosynthesis of adrenal opiate peptides.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of tyrosine hydroxylase and phenylethanolamine N-methyltransferase mRNA levels in the sympathoadrenal system by the pituitary-adrenocortical axis

The pituitary-adrenocortical axis plays a complex role in the regulation of the levels of enzymes of the catecholamine biosynthetic pathway. In this report we have explored molecular mechanisms of these regulations, by examining the effects of hypophysectomy (HPX) and dexamethasone (DEX) on tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT) mRNA levels in the adrenal medulla (AM) and superior cervical ganglia (SCG). Three weeks after hypophysectomy weights (-48%), total RNA (-49%), and DNA (-22%) contents in AM were significantly reduced, when compared to sham-operated animals (SO). In SCG decreases in weight (-23%) and in the ratio of RNA/DNA (-25%) were also found. TH mRNA contents paralleled decreases in total RNA levels and no significant change in the relative abundance of TH mRNA was found. When HPX rats were injected for 5 days with DEX (1 mg/kg, i.p.), TH mRNA levels in the SCG (+51%) and in the AM (+74%) were significantly increased when compared to saline-treated HPX animals. DEX given to SO rats increased TH mRNA in SCG (+49%); a 27% increase in TH mRNA in the AM was also observed. The relative abundance of PNMT mRNA in the AM was reduced after hypophysectomy (-64%). This decrease was completely reversed by DEX. In contrast, DEX did not affect PNMT mRNA levels in the AM of SO rats. PNMT mRNA was not detected in SCG of saline- or DEX-treated rats. In conclusion, our findings suggest that the pituitary-adrenocortical axis is involved in the regulation of the steady-state levels of TH and PNMT mRNAs. This regulation involves: (1) induction of TH mRNA contents in AM and SCG by increased plasma glucocorticoid levels; and (2) maintenance of the steady-state levels of PNMT mRNA in AM by glucocorticoid-dependent mechanisms.

Roles of the pituitary-adrenocortical axis in control of the native and cryptic enkephalin levels and proenkephalin mRNA in the sympathoadrenal system of the rat

The effects of hypophysectomy (HPX) and dexamethasone (DEX) on the levels of Met5-enkephalin (ME), ME precursors, and the abundance of proenkephalin (pEK) mRNA, were examined in the adrenal medulla (AM) and superior cervical ganglia (SCG). To assess possible changes in enkephalin processing, both cryptic (after trypsin and carboxypeptidase B digestions) and native (without enzyme digestions) ME-like immunoreactivity (ME-LI) was measured. Three weeks after HPX the proportion of pEK mRNA to the total RNA content in the AM was not significantly changed when compared to sham-operated (SO) animals. Total (native + cryptic) ME-LI was decreased by 45% in the AM of HPX rats. This decrease was paralleled by a 58% depletion of AM proteins. Cryptic ME-LI was also reduced by 43%. In contrast, native ME-LI was not altered after HPX, indicating enhanced processing of ME precursors. Treatment with DEX (5 daily injections--1 mg/kg, i.p.) increased the relative abundance of pEK mRNA (+27%) and total ME-LI in the AM of HPX group, but not in SO group. Native ME-LI, cryptic ME-LI, and their ratio were not significantly affected by DEX in the AM of HPX or SO rats. In SCG, the relative abundance of pEK mRNA decreased by 25% after hypophysectomy. Total and cryptic ME-LI in the SCG of HPX rats were not changed when compared to SO rats. In contrast, HPX reduced native ME-LI suggesting decreased processing of ME precursors. Similarly, as in AM, DEX produced increase in the SCG pEK mRNA only in HPX (+68%) and not in the SO rats. In SCG, DEX produced decreases in total ME-LI which could be attributed to an increased enkephalin release. An overall reduction of cryptic ME-LI was also observed after DEX, whereas native ME-LI remained unchanged suggesting increased processing of enkephalins. Our findings indicate that the pituitary adrenocortical axis controls the relative proportions of ME to its precursors, and that this control involves both glucocorticoid-dependent (SCG) and glucocorticoid-independent (AM) mechanisms. In contrast, our studies do not suggest specific control of pEK synthesis by the pituitary adrenocortical axis. The pituitary adrenocortical axis may also influence the relative contents of ME and catecholamines in the AM and SCG. The ratio of ME/catecholamines increased after HPX (AM and SCG) and after DEX (SCG). Such regulation may contribute to the control of co-transmitter output in the sympathoadrenal system.

Characterization of enkephalins in rat adrenal medullary explants

In the rat, removal of depolarizing stimuli to the adrenal medulla by surgical denervation in vivo or by explanting adrenal medullae has been shown to dramatically increase preproenkephalin mRNA, and enkephalin-containing (EC) peptides. To further elucidate the cellular basis of these effects and the role of transsynaptic influences on post-translational processing, we have defined the time course, and characterized EC peptides in rat adrenal medullary explants in control and depolarized states. The rise in EC peptides begins after 1 day in culture and reaches a peak at 4-7 days. Although the onset of the increase in EC peptides in culture is delayed by 12-24 h compared to the changes seen in vivo, following surgical denervation, the time course of peak and duration is remarkably similar. Size exclusion chromatography (SEC) revealed that the major species of newly appearing EC peptides in explanted glands is a high molecular weight peptide of approximately 18,000 with a Met-/Leu-enkephalin ratio of approximately 6. These results suggest that proenkephalin, the initial precursor of the EC peptide family, is the major EC peptide that accumulates in rat adrenal medullary explants. A low-molecular weight EC peptide, found by high-performance liquid chromatography to be free Met-enkephalin, is a minor component of the culture induced increase in EC peptides. Culturing of medullae in the presence of depolarizing concentrations of K+ prevents the accumulation of the proenkephalin-like EC peptides and free enkephalins.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of tyrosine hydroxylase gene expression in rat brain and adrenals by exposure to cold

The long-term changes in tyrosine hydroxylase (TH) activity induced by chronic exposure to cold in brain noradrenergic neurons of the locus coeruleus (LC) were analyzed and compared to those measured in a peripheral tissue such as adrenals. This analysis was made possible at the level of one single tissue corresponding to one animal by the use of sensitive methods that allow assay of TH activity, protein, and mRNA levels in parallel from the same homogenate. The three parameters were measured in brain structures and adrenals of rats maintained at 4 degrees C during 4 days and were compared to those of control animals kept at normal housing temperature (22 degrees C). LC of rats exposed to cold contained 200% more TH mRNA than controls. The amount of TH protein in this area rose to as much as 164% that of controls. Similarly, the activity of the enzyme increased to 140% of the normal value. Thus, these observations show that 1) the increase in TH mRNA was much higher than the increase in protein levels, and that 2) the newly synthesized molecules have about the same activity as that present under normal conditions. In contrast to the LC, no variation of these parameters was observed in the substantia nigra. In the adrenals, the variations in the different parameters were qualitatively similar to that observed in the LC, although they were quantitatively higher: TH mRNA, TH protein, and TH activity levels were respectively 330%, 182%, and 167% that of control adrenals. Altogether, these results demonstrate that exposure to cold induces an alteration in TH synthesis in brain noradrenergic neurons as well as in adrenals.

A cholinergic antagonist blocks cold stress-induced alterations in rat adrenal tyrosine hydroxylase mRNA

The role of nicotinic cholinergic transmission in cold stress-induced alterations in rat adrenomedullary tyrosine hydroxylase (TH) mRNA was investigated by RNA dot-blot hybridization, using a cloned TH cDNA probe. Chlorisondamine, a ganglionic blocking agent, greatly attenuated the induction of TH mRNA levels caused by cold exposure, whereas carbachol and nicotine, cholinergic agonists, increased TH mRNA in control animals. These results suggest that cholinergic nicotinic receptors play a key role in the transsynaptic induction of adrenal TH gene expression.