Differing temporal roles of Ca2+ and cAMP in nicotine-elicited elevation of tyrosine hydroxylase mRNA

Nicotinic Acetylcholine Receptors of PC12 Cells

Nicotinic acetylcholine receptors (nAChRs) have gained much attention in the scientific community since they play a significant role in multiple physiological and pathophysiological processes. Multiple approaches to study the receptors exist, with characterization of the receptors' functionality at a single cellular level using cell culturing being one of them. Derived from an adrenal medulla tumor, PC12 cells express nicotinic receptor subunits and form functional nicotinic receptors. Thus, the cells offer a convenient environment to address questions related to the functionality of the receptors. The review summarizes the findings on nicotinic receptors' expression and functions which were conducted using PC12 cells. Specific focus is given to α3-containing receptors as well as α7 receptor. Critical evaluation of findings is provided alongside insights into what can still be learned about nAChRs, using PC12 cells.

Inflammatory Signaling in Hypertension: Regulation of Adrenal Catecholamine Biosynthesis

The immune system is increasingly recognized for its role in the genesis and progression of hypertension. The adrenal gland is a major site that coordinates the stress response via the hypothalamic-pituitary-adrenal axis and the sympathetic-adrenal system. Catecholamines released from the adrenal medulla function in the neuro-hormonal regulation of blood pressure and have a well-established link to hypertension. The immune system has an active role in the progression of hypertension and cytokines are powerful modulators of adrenal cell function. Adrenal medullary cells integrate neural, hormonal, and immune signals. Changes in adrenal cytokines during the progression of hypertension may promote blood pressure elevation by influencing catecholamine biosynthesis. This review highlights the potential interactions of cytokine signaling networks with those of catecholamine biosynthesis within the adrenal, and discusses the role of cytokines in the coordination of blood pressure regulation and the stress response.

Imidacloprid, a neonicotinoid insecticide, facilitates tyrosine hydroxylase transcription and phenylethanolamine N-methyltransferase mRNA expression to enhance catecholamine synthesis and its nicotine-evoked elevation in PC12D cells

Imidacloprid is a neonicotinoid insecticide acting as an agonist of nicotinic acetylcholine receptors (nAChRs) in the target insects. However, questions about the safety to mammals, including human have emerged. Overactivation of mammalian peripheral catecholaminergic systems leads to onset of tachycardia, hypertension, vomiting, etc., which have been observed in acutely imidacloprid-poisoned patients as well. Physiological activation of the nAChRs is known to drive catecholamine biosynthesis and secretion in mammalian adrenal chromaffin cells. Yet, the impacts of imidacloprid on the catecholaminergic function of the chromaffin cells remain to be evaluated. In this study using PC12D cells, a catecholaminergic cell line derived from the medulla chromaffin-cell tumors of rat adrenal gland, we examined whether imidacloprid itself could impact the catecholamine-synthesizing ability. Imidacloprid alone did facilitate tyrosine hydroxylase (TH) transcription via activation of α3β4 nAChR and the α7 subunit-comprising receptor. The insecticide showed the TH transcription-facilitating ability at the concentrations of 3 and 30 μM, at which acetylcholine is known to produce physiological responses, including catecholamine secretion through the nAChRs in adrenal chromaffin cells. The insecticide-facilitated TH transcription was also dependent on PKA- and RhoA-mediated signaling pathways. The insecticide coincidentally raised levels of TH and phenylethanolamine N-methyltransferase (PNMT) mRNA, and as a consequence, increased catecholamine production, although the efficacy of the neonicotinoid was lesser than that of nicotine, indicating its partial agonist-like action. Intriguingly, in cultured rat adrenal chromaffin cells, imidacloprid did increase levels of TH and PNMT protein. When the chromaffin cells were treated with nicotine in the presence of the insecticide, nicotine-elevated adrenaline production was enhanced due to facilitation of nicotine-increased TH and PNMT protein expression, and simultaneous enhancement of nicotine-elevated adrenaline secretion also took place. These findings thus suggest that imidacloprid may facilitate the physiological functions of adrenal glands in mammals.

Nicotinic acetylcholine receptor antagonists alter the function and expression of serine racemase in PC-12 and 1321N1 cells

Western blot analysis demonstrated that PC-12 cells express monomeric and dimeric forms of serine racemase (m-SR, d-SR) and that 1321N1 cells express m-SR. Quantitative RT-PCR and functional studies demonstrated that PC-12 cells express homomeric and heteromeric forms of nicotinic acetylcholine receptors (nAChR) while 1321N1 cells primarily express the α7-nAChR subtype. The effect of nAChR agonists and antagonists on SR activity and expression was examined by following concentration-dependent changes in intracellular d-Ser levels and SR protein expression. Incubation with (S)-nicotine increased d-Ser levels, which were attenuated by the α7-nAChR antagonist methyllycaconitine (MLA). Treatment of PC-12 cells with mecamylamine (MEC) produced a bimodal reduction of d-Ser reflecting MEC inhibition of homomeric and heteromeric nAChRs, while a unimodal curve was observed with 1321N1 cells, reflecting predominant expression of α7-nAChR. The nAChR subtype selectivity was probed using α7-nAChR selective inhibitors MLA and (R,S)-dehydronorketamine and α3β4-nAChR specific inhibitor AT-1001. The compounds reduced d-Ser in PC-12 cells, but only MLA and (R,S)-dehydronorketamine were effective in 1321N1 cells. Incubation of PC-12 and 1321N1 cells with (S)-nicotine, MEC and AT-1001 did not affect m-SR or d-SR expression, while MLA and (R,S)-dehydronorketamine increased m-SR expression but not SR mRNA levels. Treatment with cycloheximide indicated that increased m-SR was due to de novo protein synthesis associated with phospho-active forms of ERK1/2, MARCKS, Akt and rapamycin-sensitive mTOR. This effect was attenuated by treatment with the pharmacological inhibitors U0126, LY294002 and rapamycin, which selectively block the activation of ERK1/2, Akt and mTOR, respectively, and siRNAs directed against ERK1/2, Akt and mTOR. We propose that nAChR-associated changes in Ca(2+) flux affect SR activity, but not expression, and that MLA and (R,S)-dehydronorketamine bind to allosteric sites on the α7-nAChR and promote multiple signaling cascades that converge at mTOR to increase m-SR levels.

Human colon adenocarcinoma HT-29 cell: electrochemistry and nicotine stimulation

Recently, it was demonstrated that colorectal cancer HT-29 cells can secrete epinephrine (adrenaline) in an autocrine manner to auto-stimulate cellular growth by adrenoreceptors activation, and that this secretion is enhanced by nicotine, showing an indirect relation between colorectal cancer and tobacco. The electrochemical behaviour of human colon adenocarcinoma HT-29 cells from a colorectal adenocarcinoma cell line, the hormone and neurotransmitter epinephrine, and nicotine, were investigated by cyclic voltammetry, using indium tin oxide (ITO), glassy carbon (GC) and screen printed carbon (SPC) electrodes. The oxidation of the HT-29 cells, previously grown onto ITO or SPC surfaces, followed an irreversible oxidation process that involved the formation of a main oxidation product that undergoes irreversible reduction, as in the epinephrine oxidation mechanism. The effect of nicotine stimulation of the HT-29 cells was also investigated. Nicotine, at different concentration levels 1, 2 and 15 mM, was introduced in the culture medium and an increase with incubation time, 0 to 3h and 30 min, of the HT-29 cells oxidation and reduction peaks was observed. The interaction of nicotine with the HT-29 cells stimulated the epinephrine secretion causing an increase in epinephrine release concentration, and enabling the conclusion that epinephrine and nicotine play an important role in the colorectal tumour growth.

Evidence for functional atypical nicotinic receptors that activate K+-dependent Cl- secretion in mouse tracheal epithelium

The present study focused on the influence of nicotinic acetylcholine receptors (nAChR) on ion transport processes in mouse tracheal epithelium. RT-PCR experiments revealed expression of the α3, α4, α5, α7, α9, α10, β2, and β4 nAChR subunits in mouse tracheal epithelium. In Ussing chamber recordings of mouse tracheae, apically applied nicotine (100 μM) induced a dose-dependent increase of the transepithelial short-circuit current (EC(50): 14.6 μM). The nicotine-induced effect (I(NIC)) was attenuated by mecamylamine (25 μM, apical) and methyllycaconitine (1 μM, apical). The nAChR agonist 1.1-dimethyl-4-phenylpiperatinium iodide (DMPP) (100 μM) revealed apical and basolateral location of the receptors. I(NIC) was not affected by the sodium channel inhibitor amiloride (10 μM, apical) or the cystic fibrosis transmembrane conductance regulator inhibitor CFTR(inh)-172 (20 μM, apical) but was reduced by the chloride channel inhibitor 5-nitro-2-(3-phenylpropylamino)benzoic acid (100 μM, apical), the Na(+)/K(+)/2Cl(-) cotransporter inhibitor bumetanide (200 μM, basolateral), the potassium channel inhibitor Ba(2+) (5 mM, basolateral), and 4.4'-diisothiocyanatostilbene-2.2'-disulfonate (100 μM, apical), indicating a contribution of Ca(2+)-activated chloride channels and potassium channels. Removal of extracellular Na(+) (apical) or Ca(2+) (apical) did not influence I(NIC) but reduced the DMPP effect. Experiments with the Ca(2+)-ionophore A23187, a mix of 3-isobutyl-1-methylxanthine and forskolin, or the inositol-1,4,5-triphospate (IP(3)) receptor inhibitor 2-aminoethyl-diphenyl-borinate (75 μM, apical) decreased I(NIC), indicating a nicotine-mediated increase of intracellular Ca(2+) and cAMP levels involving the IP(3) signaling pathway. These findings indicate the activity of Ca(2+)-permeable nAChRs and alternative metabotropic pathways by nAChR activation that mediate Cl(-) and K(+) transport in tracheal epithelium.

Isoform-specific regulation of the inositol 1,4,5-trisphosphate receptor by O-linked glycosylation

The inositol 1,4,5-trisphosphate receptor (InsP(3)R), an intracellular calcium channel, has three isoforms with >65% sequence homology, yet the isoforms differ in their function and regulation by post-translational modifications. We showed previously that InsP(3)R-1 is functionally modified by O-linked β-N-acetylglucosamine glycosylation (O-GlcNAcylation) (Rengifo, J., Gibson, C. J., Winkler, E., Collin, T., and Ehrlich, B. E. (2007) J. Neurosci. 27, 13813-13821). We now report the effect of O-GlcNAcylation on InsP(3)R-2 and InsP(3)R-3. Analysis of AR4-2J cells, a rat pancreatoma cell line expressing predominantly InsP(3)R-2, showed no detectable O-GlcNAcylation of InsP(3)R-2 and no significant functional changes despite the presence of the enzymes for addition (O-β-N-acetylglucosaminyltransferase) and removal (O-β-N-acetylglucosaminidase) of the monosaccharide. In contrast, InsP(3)R-3 in Mz-ChA-1 cells, a human cholangiocarcinoma cell line expressing predominantly InsP(3)R-3, was functionally modified by O-GlcNAcylation. Interestingly, the functional impact of O-GlcNAcylation on the InsP(3)R-3 channel was opposite the effect measured with InsP(3)R-1. Addition of O-GlcNAc by O-β-N-acetylglucosaminyltransferase increased InsP(3)R-3 single channel open probability. Incubation of Mz-ChA-1 cells in hyperglycemic medium caused an increase in the InsP(3)-dependent calcium release from the endoplasmic reticulum. The dynamic and inducible nature of O-GlcNAcylation and the InsP(3)R isoform specificity suggest that this form of modification of InsP(3)R and subsequent changes in intracellular calcium transients are important in physiological and pathophysiological processes.

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.

α4β2 nicotinic receptors partially mediate anti-inflammatory effects through Janus kinase 2-signal transducer and activator of transcription 3 but not calcium or cAMP signaling

Despite evidence that smoking confers protection against neurological disorders, how and whether specific nicotinic receptor subtypes are involved is unknown. We reported previously that nicotine suppresses constitutive nuclear factor κB (NF-κB) activity and thereby proinflammatory cytokine (PIC) production in SHEP1 cells stably transfected with α4β2 nicotinic receptors. Here, we report the anti-inflammatory effects of nicotine pretreatment in lipopolysaccharide (LPS)-stimulated SHEP1 cells. Nicotine (100-300 nM, concentrations found in smoker's blood) blocked LPS-induced NF-κB translocation and production of PICs interleukin (IL)-1β and IL-6 but only partially blocked inhibitor of nuclear factor-κBα (IκBα) phosphorylation. These effects were exclusively in cells transfected with α4β2 receptors but not in wild types. The cell-permeable calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester, the adenylate cyclase stimulant forskolin, and a specific protein kinase A (PKA) inhibitor PKI 14-22-amide failed to block the effect of nicotine on LPS-induced NF-κB translocation and IκBα phosphorylation. However, the effects of nicotine on NF-κB activity were significantly blocked by the highly specific janus kinase 2 (JAK2) inhibitor α-cyano-(3,4-dihydroxy)-N-benzylcinnamide (AG-490) and the signal transducer and activator of transcription 3 (STAT3) inhibitor 2-hydroxy-4-[[[[(4-methylphenyl)sulfonyl]oxy]acetyl]amino]-benzoic acid (NSC74859). These findings reveal a calcium- and cAMP-PKA-independent signaling cascade and suggest a role for JAK2-STAT3 transduction in α4β2-mediated attenuation of LPS-induced inflammation. Anti-inflammatory effects of nicotine may therefore be mediated through α4β2 receptors, the predominant high-affinity binding sites for nicotine in the central nervous system, in addition to the better-established α7 receptors.

Nicotinic acetylcholine receptor-mediated calcium signaling in the nervous system

Based on the composition of the five subunits forming functional neuronal nicotinic acetylcholine receptors (nAChRs), they are grouped into either heteromeric (comprising both alpha and beta subunits) or homomeric (comprising only alpha subunits) receptors. The nAChRs are known to be differentially permeable to calcium ions, with the alpha7 nAChR subtype having one of the highest permeabilities to calcium. Calcium influx through nAChRs, particularly through the alpha-bungarotoxin-sensitive alpha7-containing nAChRs, is a very efficient way to raise cytoplasmic calcium levels. The activation of nAChRs can mediate three types of cytoplasmic calcium signals: (1) direct calcium influx through the nAChRs, (2) indirect calcium influx through voltage-dependent calcium channels (VDCCs) which are activated by the nAChR-mediated depolarization, and (3) calcium-induced calcium release (CICR) (triggered by the first two sources) from the endoplasmic reticulum (ER) through the ryanodine receptors and inositol (1,4,5)-triphosphate receptors (IP(3)Rs). Downstream signaling events mediated by nAChR-mediated calcium responses can be grouped into instantaneous effects (such as neurotransmitter release, which can occur in milliseconds after nAChR activation), short-term effects (such as the recovery of nAChR desensitization through cellular signaling cascades), and long-term effects (such as neuroprotection via gene expression). In addition, nAChR activity can be regulated by cytoplasmic calcium levels, suggesting a complex reciprocal relationship. Further advances in imaging techniques, animal models, and more potent and subtype-selective ligands for neuronal nAChRs would help in understanding the neuronal nAChR-mediated calcium signaling, and lead to the development of improved therapeutic treatments.

Preconditioning stimuli that augment chromaffin cell secretion

We have investigated here whether a preconditioned stimulation of nicotinic and muscarinic receptors augmented the catecholamine release responses elicited by supramaximal 3-s pulses of 100 muM acetylcholine (100ACh) or 100 mM K(+) (100K(+)) applied to fast-perifused bovine adrenal chromaffin cells. Threshold concentrations of nicotine (1-3 muM) that caused only a tiny secretion did, however, augment the responses elicited by 100ACh or 100K(+) by 2- to 3.5-fold. This effect was suppressed by mecamylamine and by Ca(2+) deprivation, was developed with a half-time (t(1/2)) of 1 min, and was reversible. The nicotine effect was mimicked by threshold concentrations of ACh, choline, epibatidine, and oxotremorine-M but not by methacholine. Threshold concentrations of K(+) caused lesser potentiation of secretion compared with that of threshold nicotine. The data are compatible with an hypothesis implying 1) that continuous low-frequency sympathetic discharge places chromaffin cells at the adrenal gland in a permanent "hypersensitive" state; and 2) this allows an explosive secretion of catecholamines by high-frequency sympathetic discharge during stress.

Abnormalities of sarcoplasmic reticulum Ca2+ mobilization in aortic smooth muscle cells from streptozotocin-induced diabetic rats

1. Previously, we found that contractions in response to receptor-dependent (i.e. a(1)-adrenoceptor agonist phenylephrine) and -independent (i.e. cyclopiazonic acid) stimuli are decreased in rat aorta during late diabetes. The aim of the present study was to further investigate the changes of intracellular Ca(2+) homeostasis in diabetic aortic smooth muscle cells. Functional changes of inositol 1,4,5-trisphosphate (IP(3))- and ryanodine-sensitive Ca(2+) stores of the sarcoplasmic reticulum (SR) were evaluated using Fluo-3 acetoxymethyl ester fluorescence, western blot and organ bath techniques. 2. In aortic smooth muscle cells from diabetic rats, the Ca(2+) release and Ca(2+) influx caused by both 10 mmol/L phenylephrine (depletion of IP(3)-sensitive Ca(2+) stores) and 1 mmol/L ryanodine (depletion of ryanodine-sensitive Ca(2+) stores) were both significantly decreased compared with control. Moreover, protein expression levels of IP(3) (260 kDa) and ryanodine receptors (500 kDa) were reduced by 31.8 +/- 7.7 and 69.2 +/- 8.4%, respectively, in aortas from diabetic rats compared with those from control rats. 3. In diabetic rat aorta, phenylephrine-induced contractility was decreased to approximately two-thirds of that in controls, whereas ryanodine alone did not cause obvious contraction in aortas from either control or diabetic rats. 4. The present results suggest that the hyporeactivity of aortic smooth muscle to vasoconstrictors in diabetes results mainly from changes to the IP(3)-sensitive Ca(2+) release pathway. The SR Ca(2+) signalling pathway plays a crucial role in the development of diabetic vascular complications.

Muscle and neuronal nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are integral membrane proteins and prototypic members of the ligand-gated ion-channel superfamily, which has precursors in the prokaryotic world. They are formed by the assembly of five transmembrane subunits, selected from a pool of 17 homologous polypeptides (alpha1-10, beta1-4, gamma, delta, and epsilon). There are many nAChR subtypes, each consisting of a specific combination of subunits, which mediate diverse physiological functions. They are widely expressed in the central nervous system, while, in the periphery, they mediate synaptic transmission at the neuromuscular junction and ganglia. nAChRs are also found in non-neuronal/nonmuscle cells (keratinocytes, epithelia, macrophages, etc.). Extensive research has determined the specific function of several nAChR subtypes. nAChRs are now important therapeutic targets for various diseases, including myasthenia gravis, Alzheimer's and Parkinson's diseases, and schizophrenia, as well as for the cessation of smoking. However, knowledge is still incomplete, largely because of a lack of high-resolution X-ray structures for these molecules. Nevertheless, electron microscopy studies on 2D crystals of nAChR from fish electric organs and the determination of the high-resolution X-ray structure of the acetylcholine binding protein (AChBP) from snails, a homolog of the extracellular domain of the nAChR, have been major steps forward and the data obtained have important implications for the design of subtype-specific drugs. Here, we review some of the latest advances in our understanding of nAChRs and their involvement in physiology and pathology.

Nicotine promotes cell proliferation via α7-nicotinic acetylcholine receptor and catecholamine-synthesizing enzymes-mediated pathway in human colon adenocarcinoma HT-29 cells

Cigarette smoking has been implicated in colon cancer. Nicotine is a major alkaloid in cigarette smoke. In the present study, we showed that nicotine stimulated HT-29 cell proliferation and adrenaline production in a dose-dependent manner. The stimulatory action of nicotine was reversed by atenolol and ICI 118,551, a beta(1)- and beta(2)-selective antagonist, respectively, suggesting the role of beta-adrenoceptors in mediating the action. Nicotine also significantly upregulated the expression of the catecholamine-synthesizing enzymes [tyrosine hydroxylase (TH), dopamine-beta-hydroxylase (DbetaH) and phenylethanolamine N-methyltransferase]. Inhibitor of TH, a rate-limiting enzyme in the catecholamine-biosynthesis pathway, reduced the actions of nicotine on cell proliferation and adrenaline production. Expression of alpha7-nicotinic acetylcholine receptor (alpha7-nAChR) was demonstrated in HT-29 cells. Methyllycaconitine, an alpha7-nAChR antagonist, reversed the stimulatory actions of nicotine on cell proliferation, TH and DbetaH expression as well as adrenaline production. Taken together, through the action on alpha7-nAChR nicotine stimulates HT-29 cell proliferation via the upregulation of the catecholamine-synthesis pathway and ultimately adrenaline production and beta-adrenergic activation. These data reveal the contributory role alpha7-nAChR and beta-adrenoceptors in the tumorigenesis of colon cancer cells and partly elucidate the carcinogenic action of cigarette smoke on colon cancer.

Differential coupling of α7 and non-α7 nicotinic acetylcholine receptors to calcium-induced calcium release and voltage-operated calcium channels in PC12 cells

Neuronal nicotinic acetylcholine receptors (nAChRs) are ligand-gated cation channels that can modulate various neuronal processes by altering intracellular Ca(2+) levels. Following nAChR stimulation Ca(2+) can enter cells either directly, through the intrinsic ion channel, or indirectly following voltage-operated Ca(2+) channel (VOCC) activation; Ca(2+) levels can subsequently be amplified via Ca(2+)-induced Ca(2+) release from intracellular stores. We have used subtype-selective nAChR agonists to investigate the Ca(2+) sources contributing to alpha7 and non-alpha7 nAChR-mediated increases in intracellular Ca(2+) in PC12 cells. Application of the alpha7 nAChR positive allosteric modulator PNU 120596 (10 mum), in conjunction with the alpha7 nAChR agonist, compound A [(R)-N-(1-azabicyclo[2.2.2]oct-3-yl)(5-(2-pyridyl)thiophene-2-carboxamide), 10 nm], produces a rapid increase in fluo-3 fluorescence that is prevented by the selective alpha7 nAChR antagonist alpha-bungarotoxin. The non-alpha7 nAChR agonist 5-Iodo-A-85380 produces alpha-bungarotoxin-insensitive increases in intracellular Ca(2+) (EC(50) = 11.2 mum). Using these selective agonists or KCl in conjunction with general and selective VOCC inhibitors, we demonstrate that the primary route of Ca(2+) entry following either non-alpha7 nAChR activation or KCl stimulation is via L-type VOCCs. In contrast, the alpha7 nAChR-mediated response is unaffected by VOCC blockers but is inhibited by modulators of intracellular Ca(2+) stores. These results indicate that alpha7 and non-alpha7 nAChRs are differentially coupled to Ca(2+)-induced Ca(2+) release and VOCCs, respectively.

Sustained phosphorylation of tyrosine hydroxylase at serine 40: a novel mechanism for maintenance of catecholamine synthesis

Tyrosine hydroxylase (TH) is the rate-limiting enzyme in catecholamine synthesis. Its activity is known to be controlled acutely (minutes) by phosphorylation and chronically (days) by protein synthesis. Using bovine adrenal chromaffin cells we found that nicotine, acting via nicotinic receptors, sustained the phosphorylation of TH at Ser40 for up to 48 h. Nicotine also induced sustained activation of TH, which for the first 24 h was completely independent of TH protein synthesis, and the phosphorylation of TH at Ser31. Imipramine did not inhibit the acute phosphorylation of TH at Ser40 or TH activation induced by nicotine, but did inhibit the sustained responses to nicotine seen at 24 h. The protein kinase(s) responsible for TH phosphorylation at Ser40 switched from being protein kinase C (PKC) independent in the acute phase to PKC dependent in the sustained phase. Sustained phosphorylation and activation of TH were also observed with histamine and angiotensin II. Sustained phosphorylation of TH at Ser40 provides a novel mechanism for increasing TH activity and this leads to increased catecholamine synthesis. Sustained phosphorylation of TH may be a selective target for drugs or pathology in neurons that contain TH and synthesize dopamine, noradrenaline or adrenaline.

L-type calcium channel ligands block nicotine-induced signaling to CREB by inhibiting nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) are inhibited by several drugs that are commonly thought to be specific for L-type calcium channels (LTCCs). In neurons, LTCCs are activated by nicotine-induced depolarization to engage downstream signaling events; however, the role of LTCC drug interactions with nAChRs in signaling has not been examined in detail. We investigated the effects of LTCC ligands on nAChR currents and downstream signaling in rat superior cervical ganglion (SCG) neurons. We found that 10microM nicotine and 40mM K(+) both reversibly depolarize SCG neurons to -20mV, sufficient to activate LTCCs and downstream signaling, including induction of nuclear phospho-CREB (pCREB); this induction was blocked by LTCC antagonists. Interestingly, the effects of LTCC antagonists on nicotine-induced signaling to CREB are not mediated by their actions on LTCCs, but rather via inhibition of nAChRs, which prevents nicotine-induced depolarization. We show that this effect is sufficient to block pCREB induction in neurons expressing an antagonist-insensitive LTCC. Taken together, our data show that, at concentrations typically used to block LTCCs, these antagonists inhibit nAChR currents and downstream signaling. These findings serve as a caution in attributing a role for LTCCs when using these drugs experimentally or therapeutically.

Nicotine-induced upregulation of human neuronal nicotinic alpha7-receptors is potentiated by modulation of cAMP and PKC in SH-EP1-halpha7 cells

Chronic nicotine exposure induces upregulation of nicotinic receptors, but the mechanisms underlying this phenomenon are not well understood. The aim of this study was to examine the role of different second messenger systems in the nicotine-induced upregulation of alpha7-nicotinic receptors in SH-EP1-halpha7 human epithelial cells. We show here that chronic exposure to nicotine results in accumulation of cAMP. Furthermore, an enhanced cAMP signalling potentiates nicotine-induced upregulation of alpha7-nicotinic receptors measured by [3H]methyllycaconitine ([3H]MLA) binding suggesting that cAMP is involved in the alpha7-nicotinic receptor upregulation. Down-regulation of protein kinase C (PKC) with a phorbol ester abolishes the nicotine-induced upregulation of alpha7-nicotinic receptors. Furthermore, overexpression of PKCalpha in SH-EP1-halpha7 cells results in potentiation of nicotine-evoked upregulation indicating that PKC has a role in regulation of alpha7-nicotinic receptor number. The Ca2+-calmodulin kinase II (CaMKII) and extracellular signal regulated kinase 1/2 (ERK1/2) appear not to participate in alpha7-nicotinic receptor upregulation since the specific inhibitors of these kinases did not have an effect on the nicotine-induced upregulation. Taken together this study provides evidence that nicotine induces accumulation of cAMP and that the upregulation mechanisms of alpha7-nicotinic receptors are potentiated both by cAMP and PKC. As nicotine-evoked upregulation of heteromeric nicotinic receptors in SH-SY5Y cells was unaffected by the treatment with drugs affecting cAMP signalling or PKC activity, our results suggest that the upregulation mechanisms of homomeric alpha7-nicotinic receptors and heteromeric nicotinic receptors differ from each other.

Prolonged Activation of cAMP-response Element-binding Protein and ATF-2 Needed for Nicotine-triggered Elevation of Tyrosine Hydroxylase Gene Transcription in PC12 Cells

Phosphorylation (P-) of cAMP-response element-binding protein (CREB) by protein kinase A or mitogen-activated protein kinases was implicated in mediating the increased tyrosine hydroxylase (TH) gene expression after prolonged exposure to nicotine in vivo and in cell culture. We examined the time course and signaling pathways for phosphorylation of CREB and possible involvement of ATF-2. Treatment of PC12 cells with 200 microm nicotine triggered rapid but transient elevation of P-CREB followed by a second sustained rise after 2-5 h of continuous nicotine. In contrast, ERK1/2 was only phosphorylated with short term nicotine exposure. MEK inhibitor U0126 abolished nicotine-induced rise in P-ERK1/2, but not P-CREB, nor did it inhibit nicotine-evoked elevation in TH promoter activity, indicating that ERK1/2 was not needed for induction of TH gene expression by nicotine. In contrast, protein kinase A inhibitor H-89 or Ca(2+)/calmodulin-activated protein kinase inhibitor KN-93 reduced the nicotine-triggered rise in P-CREB and TH promoter activity. There was a delayed elevation of P-ATF-2 after 1 h of nicotine treatment, accompanied by increased ATF-2 protein. Upstream kinase JNK, but not p38, was phosphorylated especially after 5 min to 2 h of nicotine exposure. To examine the requirement for CREB and ATF-2, cells were transfected with dominant negative forms of ATF-2 or CREB. Both reduced the basal TH promoter activity and the response to nicotine. Knockdown of ATF-2 or CREB with siRNA did not alter basal TH promoter activity or mRNA but greatly attenuated the response to nicotine. The results suggest that both ATF-2 and CREB mediate activation of TH gene transcription by nicotine.

Prenatal morphine exposure affects sympathoadrenal axis activity and serotonin metabolism in adult male rats both under basal conditions and after an ether inhalation stress

We have previously shown that prenatal morphine exposure inhibited the hypothalamo-pituitary-adrenal (HPA) axis and altered the hypothalamic metabolism of serotonin during the early postnatal period in the rat and induced a chronic sympathoadrenal hyperactivity under resting conditions in adult male rats. In this study, we examined the effects of prenatal morphine exposure on the responsiveness to an acute ether inhalation stress of the sympathoadrenal and HPA axis and the hippocampal and hypothalamic concentrations of serotonin (5HT) and 5-hydroxylindoleacetic acid (5HIAA) in 3-month-old male rats. The plasma levels of adrenocorticopic hormone (ACTH) and corticosterone (B) did not differ between the two groups both under resting conditions and after ether exposure. Ether inhalation increased adrenal tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT) mRNA expression as well as adrenal epinephrine (E) concentration in control rats but not in prenatally morphine-exposed (PM) animals. Under basal conditions, hypothalamic concentrations of 5HT and 5HIAA increased in PM animals. In contrast to control animals, PM rats showed, in response to stress, an increased level of 5HT and 5HIAA in both the hypothalamus and in the hippocampus. In conclusion, prenatal morphine exposure produces long-lasting alterations in brain serotonin transmission and in the sympathoadrenal responsiveness to an acute systemic stress.

Multiple calcium channels and kinases mediate alpha7 nicotinic receptor neuroprotection in PC12 cells

alpha7 Nicotinic receptors are calcium permeant and provide neuroprotection against many insults. We investigated the roles of intracellular calcium ions and downstream calcium channels in this protection. The alpha7 agonist GTS-21 prevented pheochromocytoma cell death induced by nerve growth factor + serum deprivation over a 3-day interval. This effect was blocked by the intracellular calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid in a manner that did not appear to involve changes in receptor density. 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid blocked GTS-21-induced protein kinase C activation, a necessary process for protection. The insositol triphosphate calcium-channel blocker xestospongin C and the phospholipases C inhibitor U-73122 blocked protection, ryanodine partially attenuated protection, but the L-type channel antagonist nifedipine had no effect. ERK1/2 but not JNK and p38 were activated by GTS-21, and the ERK phosphorylation inhibitors PD98059 and U0126 blocked protection.

Nicotine and epibatidine triggered prolonged rise in calcium and TH gene transcription in PC12 cells

The effect of epibatidine on regulation of [Ca2+]i and tyrosine hydroxylase (TH) transcription was examined. Epibatidine triggers a biphasic rise in [Ca2+]i in PC12 cells similar to that observed with nicotine. There was an immediate transient increase in [Ca2+]i and a subsequent sustained second elevation. In contrast to nicotine, the epibatidine-triggered increase in [Ca2+]i was independent of activation of alpha7 nicotinic acetylcholine receptors, as it was not altered by either methyllycaconitine or alpha-bungarotoxin. The second [Ca2+]i elevation involves calcium release from intracellular stores and is inhibited by dantrolene or xestospongin C. Epibatidine, like nicotine, elevated TH promoter driven reporter transcription, mostly mediated by the cyclic-AMP responsive motifs. Elevation in TH promoter activity requires Ca2+ and cAMP since it is inhibited by 1,2-bis(o-Aminophenoxy)ethane-N,N,N',N'-tetraacetic Acid Tetra (acetoxymethyl ester) (BAPTA-AM) or 2',5'-dideoxyadenosine (DDA). The results reveal that epibatidine can elevate [Ca2+]i in an alpha7 independent manner and nevertheless induce TH transcription.

Changes in IP3 receptor are associated with altered calcium response to cholecystokinin in diabetic rat pancreatic acini

OBJECTIVES

Pancreatic acini of diabetic rats release amylase less than normal acini on cholecystokinin (CCK) stimulation. Pancreatic enzyme secretion by CCK is closely related to the second messenger inositol 1,4,5-trisphosphate (IP3), which mobilizes intracellular calcium stores via the endoplasmic reticulum-located receptor IP3 (IP3R). Recently, we observed altered intracellular calcium response on CCK-8 stimulation in streptozotocin (STZ)- treated diabetic rat acini.

METHODS

To determine whether IP3R is involved in altered calcium response, we measured inositol phosphate (IP) formation and the expression and phosphorylation of type III IP3R protein in diabetic acini. Also, CCK receptor mRNA expression was examined to determine whether the changes in IP formation and IP3R protein phosphorylation in diabetic acini might result from the defect at the postreceptor level.

RESULTS

CCK-8-induced IP formation at all concentrations used was significantly reduced in diabetic acini, though IP formation was increased in a concentration-dependent manner. The expression of type III IP3R protein was significantly reduced in diabetic acini. Additionally, CCK-8-stimulated phosphorylation of type III IP3R protein was not observed in diabetic acini. However, the reduction of CCK receptor mRNA expression was not detected in diabetic acini.

CONCLUSION

Our results indicate that altered calcium response to CCK-8 in diabetic acini might be associated with a post-CCK receptor defect including the changes in IP formation, type III IP3R protein expression, and phosphorylation of type III IP3R protein.

Role of Ca2+ in induction of neurotransmitter-related gene expression by butyrate

We examined the effect of butyrate on neurotransmitter-related gene expression and calcium homeostasis in PC12 cells. Pretreatment with Ca2+ chelators (EGTA or BAPTA-AM) attenuated the butyrate-triggered accumulation of TH and ppEnk mRNA indicating that Ca2+ plays a role in butyrate-induced regulation of neuronal genes. Butyrate alone did not alter intracellular Ca2+ levels as determined by Fura-PE3 fluorescence; however, pretreatment with butyrate (18-24 h) reduced the first Ca2+ peak and prevented the second sustained rise in [Ca2+]i as induced by nicotine or ryanodine. In contrast, butyrate had no effect on Ca2+ transients when added shortly before or during nicotine or ryanodine stimulation. These results suggest that chronic butyrate exposure can modulate cell responses by affecting intracellular Ca2+ signaling.

Nicotinic acetylcholine receptors and the regulation of neuronal signalling

Neuronal nicotinic acetylcholine (nACh) receptors in the brain are more commonly associated with modulatory events than mediation of synaptic transmission. nACh receptors have a high permeability for Ca(2+), and Ca(2+) signals are pivotal in shaping nACh receptor-mediated neuromodulatory effects. In this review, we consider the mechanisms through which nACh receptors convert rapid ionic signals into sustained, wide-ranging phenomena. The complex Ca(2+) responses that are generated after activation of nACh receptors can transmit information beyond the initial domain and facilitate the interface with many intracellular processes. These mechanisms underlie the diverse repertoire of neuronal activities of nicotine in the brain, from the enhancement of learning and memory, to addiction and neuroprotection.

Weak synaptic activity induces ongoing signaling to the nucleus that is enhanced by BDNF and suppressed by low-levels of nicotine

The developing nervous system adapts to a wide array of stimuli, in part, by evoking activity-dependent mechanisms that signal to the nucleus and induce long-term modifications in neuronal function. It is well established that one such stimulus is strong synaptic activity. Our interest, however, is whether weak activity generated at developing synapses also signals to the nucleus and if so, can these signals be modulated by extrinsic factors. Using cultured hippocampal neurons and a highly sensitive readout of CRE-mediated gene expression, we demonstrate that weak synaptic transmission, including non-evoked, spontaneous transmitter release, induces ongoing gene expression. These weak synaptic stimuli, acting through NMDA receptors, signal to the nucleus through a MAPK pathway, without a significant contribution of L-type Ca2+ channels. In addition, we show that BDNF, a molecule that has clear effects on synaptic plasticity, enhances this CRE-dependent gene expression by acting upstream of NMDA receptors. On the other hand, low levels of nicotine, which also effects synaptic plasticity, suppress ongoing CRE-mediated gene expression indirectly by acting on GABAergic neurons; this indirect action on gene expression suggests an alternative mechanism for how nicotine produces long-lasting changes.

Application of a customized pathway-focused microarray for gene expression profiling of cellular homeostasis upon exposure to nicotine in PC12 cells

Maintenance of cellular homeostasis is integral to appropriate regulation of cellular signaling and cell growth and division. In this study, we report the development and quality assessment of a pathway-focused microarray comprising genes involved in cellular homeostasis. Since nicotine is known to have highly modulatory effects on the intracellular calcium homeostasis, we therefore tested the applicability of the homeostatic pathway-focused microarray on the gene expression in PC-12 cells treated with 1 mM nicotine for 48 h relative to the untreated control cells. We first provided a detailed description of the focused array with respect to its gene and pathway content and then assessed the array quality using a robust regression procedure that allows for the exclusion of unreliable measurements while decreasing the number of false positives. As a result, the mean correlation coefficient between duplicate measurements of the arrays used in this study (control vs. nicotine treatment, three samples each) has increased from 0.974+/-0.017 to 0.995+/-0.002. Furthermore, we found that nicotine affected various structural and signaling components of the AKT/PKB signaling pathway and protein synthesis and degradation processes in PC-12 cells. Since modulation of intracellular calcium concentrations ([Ca(2+)](i)) and phosphatidylinositol signaling are important in various biological processes such as neurotransmitter release and tissue pathogenesis including tumor formation, we expect that the homeostatic pathway-focused microarray potentially can be used for the identification of unique gene expression profiles in comparative studies of drugs of abuse and diverse environmental stimuli, such as starvation and oxidative stress.

Involvement of calcineurin in transforming growth factor-beta-mediated regulation of extracellular matrix accumulation

Calcineurin is a calcium-dependent, serine/threonine phosphatase that functions as a signaling intermediate. In this study, we investigated the role of calcineurin in transforming growth factor-beta (TGF-beta)-mediated cellular effects and examined the signaling pathway involved in activation of calcineurin. Calcineurin is activated by TGF-beta in a time- and dose-dependent manner. Consistent with increased phosphatase activity, the calcineurin substrate, NFATc1, is dephosphorylated and transported to the nucleus. Inhibition of calcineurin prior to the addition of TGF-beta revealed that calcineurin is required for TGF-beta-mediated accumulation of extracellular matrix (ECM) proteins but not cell hypertrophy. Conversely, overexpression of constitutively active calcineurin was sufficient to induce ECM protein expression. The mechanism of calcineurin activation by TGF-beta was found to be induction of a low, sustained increase of intracellular calcium. Chelation of extracellular calcium blocked both TGF-beta-mediated calcium influx and calcineurin activity. Finally, calcium entry was found to be dependent upon generation of reactive oxygen species (ROS) including superoxide anion and hydrogen peroxide. Accordingly, inhibition of ROS generation also blocked TGF-beta-mediated calcineurin phosphatase activity and decreased ECM accumulation. In conclusion, this study describes a new pathway for TGF-beta-mediated regulation of ECM via generation of ROS, calcium influx, and activation of calcineurin.

Effect of beta-amyloid peptide 1-42 on the cytoprotective action mediated by alpha7 nicotinic acetylcholine receptors in growth factor-deprived differentiated PC-12 cells

Brain deposition of beta-amyloid peptide (Abeta1-42)-containing senile plaques is a consistent finding in Alzheimer's disease (AD). However, the link between Abeta1-42 and neuronal degeneration remains unclear. It has been reported that Abeta peptides bind with selectivity to alpha7 nicotinic acetylcholine receptors (alpha7nAChRs) and that the two proteins are associated in human AD brain tissue. A potential functional interaction between alpha7nAChRs and Abeta1-42 also has been suggested through the ability of nicotine to inhibit Abeta1-42-induced cytotoxicity. Differentiated PC-12 cells share several features in common with cholinergic basal forebrain neurons. The cells express alpha7nAChRs, they require growth factor stimulation for their maintenance and survival, and nicotine protects against cytotoxicity subsequent to growth factor withdrawal. Using these cells as a model system, we designed experiments to more directly determine whether Abeta peptides (Abeta1-42 and Abeta1-40) interfere with a potential nicotinic cytoprotective action and with the ability of nicotine to increase intracellular Ca2+. Differentiated PC-12 cells were preloaded with fura 2/acetoxymethyl ester and intracellular free Ca2+ levels were determined by fluorescent imaging. Nicotine-induced Ca2+ signals were inhibited by pretreatment with the alpha7nAChR-selective antagonists alpha-bungarotoxin and methyllycaconitine, and they were completely absent in cells maintained in Ca2+-free medium. The nicotine response also was blocked by pretreatment with 100 nM Abeta1-42. Nicotine (1-1000 muM) produced a concentration-dependent increase in cell viability in differentiated PC-12 cells that underwent nerve growth factor withdrawal for 24 h. Cell viability was maintained near 100% by 100 muM nicotine. The cytoprotective action of nicotine was efficiently antagonized by cotreatment with alpha7nAChR antagonists. A concentration-dependent inhibition of the cytoprotective action of nicotine also was produced by cotreatment with Abeta1-42 (1-100 nM), but not with Abeta40 -1. It is possible, therefore, that in AD, as growth factor support to basal forebrain cholinergic neurons declines, the interaction of Abeta peptides with alpha7nAChRs may enhance toxicity by interfering with an important nicotinic signal for neuronal viability.

Nicotinic acetylcholine receptor alpha 7 regulates cAMP signal within lipid rafts

Neuronal nicotinic acetylcholine receptors (nAChRs) are made of multiple subunits with diversified functions. The nAChR alpha 7-subunit has a property of high Ca2+ permeability and may have specific functions and localization within the plasma membrane as a signal transduction molecule. In PC-12 cells, fractionation by sucrose gradient centrifugation revealed that nAChR alpha 7 existed in low-density, cholesterol-enriched plasma membrane microdomains known as lipid rafts where flotillin also exists. In contrast, nAChR alpha 5- and beta2-subunits were located in high-density fractions, out of the lipid rafts. Type 6 adenylyl cyclase (AC6), a calcium-inhibitable isoform, was also found in lipid rafts and was coimmunoprecipitated with nAChR alpha 7. Cholesterol depletion from plasma membranes with methyl-beta-cyclodextrin redistributed nAChR alpha 7 and AC6 diffusely within plasma membranes. Nicotine stimulation reduced forskolin-stimulated AC activity by 35%, and this inhibition was negated by either treatment with alpha-bungarotoxin, a specific antagonist of nAChR alpha 7, or cholesterol depletion from plasma membranes. The effect of cholesterol depletion was negated by the addition of cholesterol. These data suggest that nAChR alpha 7 has a specific membrane localization relative to other nAChR subunits and that lipid rafts are necessary to localize nAChR alpha 7 with AC within plasma membranes. In addition, nAChR alpha 7 may regulate the AC activity via Ca2+ within lipid rafts.

Development of catecholaminergic systems in the spinal cord of the dogfish Scyliorhinus canicula (Elasmobranchs)

The development of catecholamine-synthesizing cells and fibers in the spinal cord of dogfish (Scyliorhinus canicula L.) was studied by means of immunohistochemistry using antibodies against tyrosine hydroxylase (TH). The only TH-immunoreactive (TH-ir) cells already present in the spinal cord of stage 26 embryos were of cerebrospinal fluid-contacting (CSF-c) type. These cells were the first catecholaminergic neurons of the dogfish CNS. The number of these TH-ir cells increased very considerably in later embryos and adult dogfish. In later embryos (stage 33; prehatching), faintly TH-ir non-CSF-contacting neurons were observed in the ventral horn throughout most of the spinal cord. In adult dogfish, some non-CSF-contacting TH-ir cells were observed ventral or lateral to the central canal. In the rostral spinal cord, the catecholaminergic neurons observed in dorsal regions were continuous with caudal rhombencephalic populations. Numerous TH-ir fibers were observed in the spinal cord of later embryos and in adults, both intrinsic and descending from the brain, innervating many regions of the cord including the dorsal and ventral horns. In addition, some TH-ir fibers innervated the marginal nucleus of the spinal cord. The early appearance of catecholaminergic cells and fibers in the embryonic spinal cord of the dogfish, and the large number of these elements observed in adults, suggests an important role for catecholamines through development and adulthood in sensory and motor functions.

Nicotinic induction of preproenkephalin and tyrosine hydroxylase gene expression in butyrate-differentiated rat PC12 cells: a model for adaptation to gut-derived environmental signals

Accelerated maturation of peripheral sympathoadrenal transmitter levels and function occurs at 7-10 postnatal days in the rat. This event is temporally disconnected from the timing of major changes in physiologic stimuli evident after the birthing process (i.e. temperature, oxygen, sound, light, etc.). Colonization of the gut, fermentation of carbohydrates, and production of short-chain fatty acids (e.g. butyrate) mirrors this postnatal time course. In this report, we examined the interaction between butyrate differentiation of rat pheochromocytoma cells and cholinergic-nicotinic induction of the neuropeptide (enkephalin) and catecholamine-related biosynthetic enzymes (tyrosine hydroxylase, dopamine beta-hydroxylase, phenylethanolamine N-methyltransferase). Our results show that butyrate induces both preproenkephalin and tyrosine hydroxylase mRNA through a proximal promoter region and that this regulatory step is time and dose dependent. Moreover, there is an additional interaction with cholinergic-nicotinic inducible mechanisms consistent with classically described transsynaptic cholinergic regulation of these genes. Dopamine beta-hydroxylase and phenylethanolamine N-methyltransferase promoters were not affected by butyrate treatment. We speculate that colonization of the human gut (along with the attendant fermentation of enteral carbohydrates to short-chain fatty acids) may represent a mechanism through which environmental signals affect postnatal maturation of sympathoadrenal transmitter systems.

Effects of short- and long-term nicotine treatment on intracellular calcium and tyrosine hydroxylase gene expression

It is important to determine how the signaling pathways for the short-term effects of nicotine (catecholamine secretion, phosphorylation of tyrosine hydroxylase) differ from those required for changes in gene expression. Our aim was to distinguish the pathways involved in short- and long-term nicotinic stimulation. PC12 cells were treated with several concentrations of nicotine from 10 micro M to 1 mM. All elicited a rapid and transient rise in [Ca(2+)](i), which was concentration dependent. After several minutes of continued exposure, a second smaller sustained rise in [Ca(2+)](i) was only observed with intermediate concentrations of nicotine (50-200 micro M). This sustained rise was not observed in cells pretreated with alpha-bungarotoxin (alpha-BTX). alpha-BTX also prevented the elevation of tyrosine hydroxylase mRNA by nicotine. The effects of brief and prolonged treatment with nicotine on the signaling pathways involved in changes in [Ca(2+)](i) and induction of tyrosine hydroxylase gene expression are summarized. The results indicate that nicotine may elicit different signaling pathways depending on the concentration. The sustained elevation of [Ca(2+)](i) via activation of alpha7 nAChRs is proposed as the mechanism leading to increased tyrosine hydroxylase gene expression.

Nicotine increases hepatic oxygen uptake in the isolated perfused rat liver by inhibiting glycolysis

Nicotine influences energy metabolism, yet mechanisms remain unclear. Since the liver is one of the largest organs and performs many metabolic functions, the goal of this study was to determine whether nicotine would affect respiration and other metabolic functions in the isolated perfused liver. Infusion of 85 microM nicotine caused a rapid 10% increase in oxygen uptake over basal values of 105 +/- 5 micromol/g/h in perfused livers from fed rats, and an increase of 27% was observed with 850 microM nicotine. Concomitantly, rates of glycolysis of 105 +/- 8 micromol/g/h were decreased to 52 +/- 9 micromol/g/h with nicotine, whereas ketone body production was unaffected. Nicotine had no effect on oxygen uptake in glycogen-depleted livers from 24-h fasted rats. Furthermore, addition of glucose to perfused livers from fasted rats partially restored the stimulatory effect of nicotine. Infusion of atractyloside, potassium cyanide, or glucagon blocked the nicotine-induced increase in respiration. Intracellular calcium was increased in isolated hepatocytes by nicotine, a phenomenon prevented by incubation of cells with d-tubocurarine, a nicotinic acetylcholine receptor antagonist. Respiration was also increased approximately 30% in hepatocytes isolated from fed rats by nicotine, whereas hepatocytes isolated from fasted rats showed little response. In the presence of N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H-89), an inhibitor of cyclic AMP-dependent protein kinase A, nicotine failed to stimulate respiration. These data support the hypothesis that inhibition of glycolysis by nicotine increases oxygen uptake due to an ADP-dependent increase in mitochondrial respiration.

Intracellular Ca2+ signals evoked by stimulation of nicotinic acetylcholine receptors in SH-SY5Y cells: contribution of voltage-operated Ca2+ channels and Ca2+ stores

Neuronal nicotinic acetylcholine receptors (nAChR) can regulate several neuronal processes through Ca2+-dependent mechanisms. The versatility of nAChR-mediated responses presumably reflects the spatial and temporal characteristics of local changes in intracellular Ca2+ arising from a variety of sources. The aim of this study was to analyse the components of nicotine-evoked Ca2+ signals in SH-SY5Y cells, by monitoring fluorescence changes in cells loaded with fluo-3 AM. Nicotine (30 microm) generated a rapid elevation in cytoplasmic Ca2+ that was partially and additively inhibited (40%) by alpha7 and alpha3beta2* nAChR subtype selective antagonists; alpha3beta4* nAChR probably account for the remaining response (60%). A substantial blockade (80%) by CdCl2 (100 microm) indicates that voltage-operated Ca2+ channels (VOCC) mediate most of the nicotine-evoked response, although the alpha7 selective antagonist alpha-bungarotoxin (40 nm) further decreased the CdCl2- resistant component. The elevation of intracellular Ca2+ levels provoked by nicotine was sustained for at least 10 min and required the persistent activation of nAChR throughout the response. Intracellular Ca2+ stores were implicated in both the initial and sustained nicotine-evoked Ca2+ responses, by the blockade observed after ryanodine (30 microm) and the inositoltriphosphate (IP3)-receptor antagonist, xestospongin-c (10 microm). Thus, nAChR subtypes are differentially coupled to specific sources of Ca2+: activation of nAChR induces a sustained elevation of intracellular Ca2+ levels which is highly dependent on the activation of VOCC, and also involves Ca2+ release from ryanodine and IP3-dependent intracellular stores. Moreover, the alpha7, but not alpha3beta2* nAChR, are responsible for a fraction of the VOCC-independent nicotine-evoked Ca2+ increase that appears to be functionally coupled to ryanodine sensitive Ca2+ stores.

Integrative nuclear FGFR1 signaling (INFS) pathway mediates activation of the tyrosine hydroxylase gene by angiotensin II, depolarization and protein kinase C

The integrative nuclear FGFR1 signaling (INFS) pathway functions in association with cellular growth, differentiation, and regulation of gene expression, and is activated by diverse extracellular signals. Here we show that stimulation of angiotensin II (AII) receptors, depolarization, or activation protein kinase C (PKC) or adenylate cyclase all lead to nuclear accumulation of fibroblast growth factor 2 (FGF-2) and FGFR1, association of FGFR1 with splicing factor-rich domains, and activation of the tyrosine hydroxylase (TH) gene promoter in bovine adrenal medullary cells (BAMC). The up-regulation of endogenous TH protein or a transfected TH promoter-luciferase construct by AII, veratridine, or PMA (but not by forskolin) is abolished by transfection with a dominant negative FGFR1TK-mutant which localizes to the nucleus and plasma membrane, but not by extracellularly acting FGFR1 antagonists suramin and inositolhexakisphosphate (IP6). Mechanism of TH gene activation by FGF-2 and FGFR1 was further investigated in BAMC and human TE671 cultures. TH promoter was activated by co-transfected HMW FGF-2 (which is exclusively nuclear) but not by cytoplasmic FGF-1 or extracellular FGFs. Promoter transactivation by HMWFGF-2 was accompanied by an up-regulation of FGFR1 specifically in the cell nucleus and was prevented FGFR1(TK-) but not by IP6 or suramin. The TH promoter was also transactivated by co-transfected wild-type FGFR1, which localizes to both to the nucleus and the plasma membrane, and by an exclusively nuclear, soluble FGFR1(SP-/NLS) mutant with an inserted nuclear localization signal. Activation of the TH promoter by nuclear FGFR1 and FGF-2 was mediated through the cAMP-responsive element (CRE) and was associated with induction of CREB- and CBP/P-300-containing CRE complexes. We propose a new model for gene regulation in which nuclear FGFR1 acts as a mediator of CRE transactivation by AII, cell depolarization, and PKC.

Stimulation of non-alpha7 nicotinic receptors partially protects dopaminergic neurons from 1-methyl-4-phenylpyridinium-induced toxicity in culture

Previous work has shown that nicotine treatment protects against nigrostriatal degeneration in rodents, findings that may be of relevance to the decreased incidence of Parkinson's disease in cigarette smokers. In the present studies, we investigated the effect of nicotine against 1-methyl-4-phenylpyridinium-induced toxicity in dopaminergic ventral mesencephalic cultures to identify the nicotinic receptor population that may be involved. [3H]Epibatidine, a ligand that binds to receptors containing alpha2-alpha6 subunits, bound to at least two populations of sites that were up-regulated by nicotine in a time and dose dependent manner. We next examined the effect of nicotine on cultures treated with 1-methyl-4-phenylpyridinium, a neurotoxin that selectively damages nigrostriatal dopaminergic neurons. Pre-treatment with nicotine, at 10(-7)-10(-4) M, partially prevented the toxin-induced decline in dopaminergic cells. Pre-exposure to nicotine for 24 h resulted in optimal protection, suggesting that receptor up-regulation may contribute to the observed neuroprotective effect. Nicotine-mediated protection was blocked by pre-incubation with the nicotinic receptor antagonist d-tubocurarine (10(-4) M), but not the alpha7 receptor-selective antagonist alpha-bungarotoxin (10(-7) M). Our results show that nicotinic receptor activation partially protects nigral dopaminergic neurons from 1-methyl-4-phenylpyridinium-induced toxicity in culture and that this appears to occur through an interaction at non-alpha7 containing receptors.

Relationship between the increased cell surface alpha7 nicotinic receptor expression and neuroprotection induced by several nicotinic receptor agonists

Nicotine and other nicotinic acetylcholine receptor agonists have been shown to exert neuroprotective actions in vivo and in vitro by an as yet unknown mechanism. Even the identification of the subtype of nicotinic receptor(s) mediating this action has not been determined. In neural cell lines, the induction of cytoprotection often requires exposure to nicotine for up to 24 hr to produce a full protective effect. One phenomenon associated with chronic exposure of neural cells to nAChR agonists is the increased expression of nAChRs (upregulation), possibly as a response to desensitization. Because nicotinic receptors desensitize rapidly in the continuous presence of agonist, we investigated whether the neuroprotective actions produced by different nicotinic receptor agonists was related to their ability to induce nicotinic receptor upregulation. Differentiated PC12 cells were preincubated for 24 hr with various nAChR ligands, and the cells were subsequently deprived of both NGF and serum to induce cytotoxicity. Under control conditions cell viability was reduced to 66.5 +/- 5.4% of control by trophic factor withdrawal. For those cells pretreated with nicotine (1 nM-100 microM) cell viability increased from 74.2 +/- 1.5 to 97.3 +/- 4%. The neuroprotective action of nicotine was blocked by co-treatment with either 5 microM mecamylamine or 10 nM methyllycaconitine (MLA). The high potency blockade by MLA suggested that neuroprotection was mediated through the alpha7 nicotinic receptor subtype. For the seven agonists examined for neuroprotective activity, only nicotine was capable of evoking a near maximal (near 100% cell viability) neuroprotective action. The next most effective group included epibatidine, 4OHGTS-21, methycarbamylcholine, and 1,1-dimethyl-4-phenyl-piperazinium iodide. These least effective group included cytisine and tetraethylammonium. Incubation of differentiated PC12 cells with 10 microM nicotine increased the number of [(125)I]alpha bungarotoxin ([(125)I]alphaBGTbinding sites by 41% from 82.6 +/- 3.67 to 117 +/- 10.3 fmol/mg protein). Under similar conditions of incubation, the nicotinic receptor agonist cytisine (that was least effective in terms of neuroprotection) failed to increase the number of [(125)I]alphaBGT binding sites. Cells expressing increased levels of cell surface [(125)I]alphaBGT binding sites received added neuroprotective benefit from nicotine. Thus the induced upregulation of the alpha7 subtype of nicotinic receptors during chronic exposure to nicotine may be responsible for the drug's neuroprotective action.

Prenatal nicotine affects catecholamine gene expression in newborn rat carotid body and petrosal ganglion

Nicotine exposure modifies the expression of catecholamine and opioid neurotransmitter systems involved in attenuation of hypoxic chemosensitivity. We used in situ hybridization histochemistry to determine the effect of prenatal and early postnatal nicotine exposure on tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DbetaH), preproenkephalin (PPE), and D2-dopamine receptor mRNA levels in the rat carotid body and petrosal ganglion during postnatal development. In the carotid body, nicotine increased TH mRNA expression in animals at 0 and 3 postnatal days (both, P < 0.05 vs. control) without affecting TH mRNA levels at 6 and 15 days. At 15 postnatal days, DbetaH mRNA levels were increased in the carotid body of nicotine-exposed animals. Dopamine D2-receptor mRNA levels in the carotid body increased with postnatal age but were unaffected by nicotine exposure. PPE was not expressed in the carotid body at any of the ages studied in control or treated animals. In the petrosal ganglion, nicotine increased the number of ganglion cells expressing TH mRNA in animals at 3 days (P < 0.01 vs. control). DbetaH mRNA expression was not induced nor was PPE mRNA expression increased in the petrosal ganglion in treated animals. Prenatal nicotine exposure upregulates mRNAs involved in the synthesis of two inhibitory neuromodulators, dopamine and norepinephrine, in peripheral arterial chemoreceptors, which may contribute to abnormalities in cardiorespiratory control observed in nicotine exposed animals.

Nicotine-induced phosphorylation of extracellular signal-regulated protein kinase and CREB in PC12h cells

We have investigated mechanisms of nicotine-induced phosphorylation of extracellular signal-regulated protein kinase (p42/44 MAP kinase, ERK) and cAMP response element binding protein (CREB) in PC12h cells. Nicotine transiently induced ERK phosphorylation at more than 1 microM. The maximal level of nicotine-induced ERK phosphorylation was lower than that of the membrane depolarization induced and, to a great extent, the nerve growth factor (NGF)-induced ERK phosphorylation. Nicotinic acetylcholine receptor (nAChR) alpha7 subunit-selective inhibitors had no significant effect on nicotine-induced ERK phosphorylation. L-Type voltage-sensitive calcium channel antagonists inhibited nicotine-induced ERK phosphorylation. Calcium imaging experiments showed that alpha7-containing nAChR subtypes were functional at 1 microM of nicotine in the nicotine-induced calcium influx, and non-alpha7 nAChRs were prominent in the Ca(2+) influx at 50 microM of nicotine. An expression of dominant inhibitory Ras inhibited nicotine-induced ERK phosphorylation. A calmodulin antagonist, a CaM kinase inhibitor, a MAP kinase kinase inhibitor inhibited nicotine-induced ERK and CREB phosphorylation. The time course of the phosphorylation of CREB induced by nicotine was similar to that of ERK induced by nicotine. These results suggest that non-alpha7 nAChRs are involved in nicotine-induced ERK phosphorylation through CaM kinase and the Ras-MAP kinase cascade and most of the nicotine-induced CREB phosphorylation is mediated by the ERK phosphorylation in PC12h cells.

Region-specific transcriptional response to chronic nicotine in rat brain

Even though nicotine has been shown to modulate mRNA expression of a variety of genes, a comprehensive high-throughput study of the effects of nicotine on the tissue-specific gene expression profiles has been lacking in the literature. In this study, cDNA microarrays containing 1117 genes and ESTs were used to assess the transcriptional response to chronic nicotine treatment in rat, based on four brain regions, i.e. prefrontal cortex (PFC), nucleus accumbens (NAs), ventral tegmental area (VTA), and amygdala (AMYG). On the basis of a non-parametric resampling method, an index (called jackknifed reliability index, JRI) was proposed, and employed to determine the inherent measurement error across multiple arrays used in this study. Upon removal of the outliers, the mean correlation coefficient between duplicate measurements increased to 0.978+/-0.0035 from 0.941+/-0.045. Results from principal component analysis and pairwise correlations suggested that brain regions studied were highly similar in terms of their absolute expression levels, but exhibited divergent transcriptional responses to chronic nicotine administration. For example, PFC and NAs were significantly more similar to each other (r=0.7; P<10(-14)) than to either VTA or AMYG. Furthermore, we confirmed our microarray results for two representative genes, i.e. the weak inward rectifier K(+) channel (TWIK-1), and phosphate and tensin homolog (PTEN) by using real-time quantitative RT-PCR technique. Finally, a number of genes, involved in MAPK, phosphatidylinositol, and EGFR signaling pathways, were identified and proposed as possible targets in response to nicotine administration.

Activation of angiotensin II subtype 2 receptor induces catecholamine release in an extracellular Ca(2+)-dependent manner through a decrease of cyclic guanosine 3',5'-monophosphate production in cultured porcine adrenal medullary chromaffin Cells

We have previously demonstrated that CGP 42112 (AT(2) agonist > or =1 nM) markedly reduces catecholamine biosynthesis through AT(2), which is the major angiotensin II (AngII) receptor subtype in cultured porcine chromaffin cells. Also, we have shown that CGP 42112 (> or =1 nM) induces a reduction in cGMP production in these cells. The present study showed that AngII reduced cGMP production via AT(2) in a manner similar to that found with CGP 42112. AngII (1 nM) significantly increased catecholamine secretion from cultured porcine adrenal medullary chromaffin cells. The stimulation was significantly inhibited by PD 123319 (AT(2) antagonist). The stimulation was moderately, but significantly, attenuated by CV-11974 (AT(1) antagonist, > or =10 nM), suggesting an involvement of AT(1). Moreover, CGP 42112 (> or =10 nM) markedly increased catecholamine release from these cells. The stimulation by CGP 42112 was abolished by PD 123319, whereas CV-11974 had no effect, indicating that this response is also mediated by AT(2). We further examined whether extracellular Ca(2+) is involved in the stimulatory effect of AT(2) on catecholamine secretion. Removal of external Ca(2+) significantly suppressed either AngII plus CV-11974 (100 nM; which simulates specific AT(2) stimulation) or CGP 42112- induced catecholamine secretion. AngII plus CV-11974 or CGP 42112 caused a sustained increase in intracellular Ca(2+) ([Ca(2+)](i)), as determined in fura-2-loaded chromaffin cells in an extracellular Ca(2+)-dependent manner. In the presence of EGTA, the subsequent addition of AngII with CV-11974 and CGP 42112 did not cause any increase in [Ca(2+)](i) levels. Consistent with this finding, CGP 42112 (10 nM to 1 microM) did not alter inositol triphosphate (IP(3)) production, a messenger for mobilization of Ca(2+) from intracellular storage sites. In addition, the intracellular Ca(2+) chelator 1,2-bis(2-amino-phenoxy)ethane-N,N,N',N'- tetraacetic acid acetoxymethylester (BAPTA) did not affect CGP 42112-induced catecholamine release. We tested whether a decrease in cGMP was the cause of the stimulatory effect of AT(2) on catecholamine secretion. Pretreatment with 8-bromo-cGMP (1 mM) prevented the stimulatory effect of AngII plus CV-11974 and CGP 42112 on both catecholamine secretion and [Ca(2+)](i). When 8-bromo-cGMP was added after application of AngII plus CV-11974 or CGP 42112, [Ca(2+)](i) induced by these agents was gradually reduced toward the baseline values. Similarly, guanylin completely abolished the AngII- plus CV-11974-induced increase in both NE secretion and [Ca(2+)](i). The Ca(2+) channel blockers, nicardipine and omega-conotoxin G VIA, at 1 microM in both cases, were also effective in inhibiting AT(2) stimulation-induced secretion. On the other hand, neither T-type voltage-dependent Ca(2+) channel blockers, flunarizine, nor Ni(2+) affected catecholamine release caused by AT(2) stimulation. These findings demonstrate that AT(2) stimulation induces catecholamine secretion by mobilizing Ca(2+) through voltage-dependent Ca(2+) channels without affecting intracellular pools and that these effects could be mediated by a decrease in cGMP production.

Endogenous and exogenous ARC in serum withdrawal mediated PC12 cell apoptosis: a new pro-apoptotic role for ARC

The role of ARC (Apoptosis Repressor with Caspase Recruitment Domain) in PC12 cell serum withdrawal driven apoptosis was studied. A progressive and massive increase in ARC protein occurs during serum withdrawal that correlates with declining survival and processing of caspase-2, previously shown to associate with ARC. This accumulation of ARC occurs in a transcriptional and translational independent manner. Additionally, ARC is localized exclusively in the nucleus of PC12 cells. Furthermore, transfection of PC12 cells with hARC-Flag promotes death and fails to protect the cells from apoptosis by serum withdrawal. Therefore, ARC functions in a pro-apoptotic manner in PC12 cell serum withdrawal induced apoptosis.

Interactions between the inositol 1,4,5-trisphosphate and cyclic AMP signaling pathways regulate larval molting in Drosophila

Larval molting in Drosophila, as in other insects, is initiated by the coordinated release of the steroid hormone ecdysone, in response to neural signals, at precise stages during development. In this study we have analyzed, using genetic and molecular methods, the roles played by two major signaling pathways in the regulation of larval molting in Drosophila. Previous studies have shown that mutants for the inositol 1,4,5-trisphosphate receptor gene (itpr) are larval lethals. In addition they exhibit delays in molting that can be rescued by exogenous feeding of 20-hydroxyecdysone. Here we show that mutants for adenylate cyclase (rut) synergize, during larval molting, with itpr mutant alleles, indicating that both cAMP and InsP(3) signaling pathways function in this process. The two pathways act in parallel to affect molting, as judged by phenotypes obtained through expression of dominant negative and dominant active forms of protein kinase A (PKA) in tissues that normally express the InsP(3) receptor. Furthermore, our studies predict the existence of feedback inhibition through protein kinase A on the InsP(3) receptor by increased levels of 20-hydroxyecdysone.

Involvement of alpha7 nicotinic acetylcholine receptors in activation of tyrosine hydroxylase and dopamine beta-hydroxylase gene expression in PC12 cells

Nicotine treatment increases intracellular free Ca(2+) concentration [Ca(2+)](i), stimulates catecholamine release, and elevates gene expression for the catecholamine biosynthetic enzymes tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH). However, the type of nicotinic acetylcholine receptors (nAChRs) mediating these events is unclear. The nAChR receptor antagonists alpha-bungarotoxin (alphaBTX) and methyllycaconitine greatly reduced the nicotine-triggered initial transient rise in [Ca(2+)](i) and prevented the second prolonged elevation of [Ca(2+)](i), suggesting the involvement of alpha7 nAChRs. Two specific alpha7 nicotinic agonists, 3-(2,4-dimethoxybenzilidene)anabaseine (DMXB) and E, E-3-(cinnamylidene)anabaseine (3-CA), were found to elicit a small, delayed increase in [Ca(2+)](i) with kinetics and magnitude similar to the second elevation observed with nicotine. This increase was inhibited by the inositol trisphosphate receptor antagonist xestospongin C. Exposure to 3-CA or DMXB for 6 or 24 h elevated TH and DBH mRNA levels two- to fourfold over control levels. These agonists were more effective than nicotine alone in increasing TH and DBH gene expression and significantly elevated [Ca(2+)](i) for up to 6 h. The increase in [Ca(2+)](i) or the elevation in TH mRNA by 3-CA was completely inhibited by alphaBTX. This study, for the first time, implicates stimulation of alpha7 nAChRs in the activation of TH and DBH gene expression.

Identification of type 1 IP(3) receptors in the rat kidney and their modulation by immobilization stress

Inositol 1,4,5-trisphosphate receptor (IP(3)-receptor) is a calcium channel, transporting calcium from intracellular stores to the cytoplasm. In kidney, IP(3)-receptors are involved in the signal transduction of various hormones. In our work we studied the effect of immobilization stress on the IP(3)-receptor's protein content in renal cortex and the medulla of normotensive and hypertensive rats. We detected both mRNA and type 1 IP(3)-receptor protein in medulla, but not in renal cortex. We found that this receptor was approximately twice as abundant in normotensive as in genetically hypertensive rat kidney. Immobilization stress decreased the amount of type 1 IP(3)-receptor in the renal medulla of normotensive rats approximately five times, while no effect due to single and/or repeated stress was observed in the renal medulla of spontaneously hypertensive rats. The results indicate that expression of type 1 IP(3)-receptor in renal medulla is modulated by hypertension and immobilization stress.