Regulation of depolarization-dependent release of neurotransmitters by adenosine: cyclic AMP-dependent enhancement of release from PC12 cells

Purinergic signalling and cancer

Receptors for extracellular nucleotides are widely expressed by mammalian cells. They mediate a large array of responses ranging from growth stimulation to apoptosis, from chemotaxis to cell differentiation and from nociception to cytokine release, as well as neurotransmission. Pharma industry is involved in the development and clinical testing of drugs selectively targeting the different P1 nucleoside and P2 nucleotide receptor subtypes. As described in detail in the present review, P2 receptors are expressed by all tumours, in some cases to a very high level. Activation or inhibition of selected P2 receptor subtypes brings about cancer cell death or growth inhibition. The field has been largely neglected by current research in oncology, yet the evidence presented in this review, most of which is based on in vitro studies, although with a limited amount from in vivo experiments and human studies, warrants further efforts to explore the therapeutic potential of purinoceptor targeting in cancer.

Effects of YT-146 [2-(1-octynyl) adenosine], an adenosine A2A receptor agonist, on cAMP production and noradrenaline release in PC12 cells

Effects of YT-146 [2-(1-octynyl) adenosine], an adenosine A2 receptor agonist, on cAMP production and noradrenaline (NA) release were investigated in PC12 cells. YT-146 caused a concentration-dependent cAMP accumulation (EC50: 1.2+/-0.9 nM). In [3H]NA-prelabeled cells, YT-146 increased the basal NA release and enhanced ATP-evoked NA release in a concentration-dependent manner (EC50: 0.23+/-0.15 nM). YT-146 augmented the maximal response to ATP without affecting the EC50 value of ATP. These effects of YT-146 were inhibited by several adenosine receptor antagonists with a characteristic of adenosine A2A receptor subtype. The effects of YT-146 were mimicked by forskolin, dibutylyl cAMP and Sp-cAMPS, and inhibited by H-89, a cAMP-dependent protein kinase inhibitor. YT-146 had little effect on ATP-induced increase in intracellular Ca2+ concentration. YT-146 enhanced the NA release induced by several different stimuli including Ca2+ ionophore A23187. The present results suggest that YT-146 is a potent agonist on adenosine A2A receptors in PC12 cells and causes a cAMP-dependent enhancement of NA release by affecting the exocytosis process at a point downstream of the intracellular Ca2+ increase.

The regulation of neurotransmitter secretion by protein kinase C

The effect of protein kinase C (PKC) on the release of neurotransmitters from a number preparations, including sympathetic nerve endings, brain slices, synaptosomes, and neuronally derived cell lines, is considered. A comparison is drawn between effects of activation of PKC on neurotransmitter release from small synaptic vesicles and large dense-cored vesicles. The enhancement of neurotransmitter release is discussed in relation to the effect of PKC on: 1. Rearrangement of the F-actin-based cytoskeleton, including the possible role of MARCKS in this process, to allow access of large dense-cored vesicles to release sites on the plasma membrane. 2. Phosphorylation of key components in the SNAP/SNARE complex associated with the docking and fusion of vesicles at site of secretion. 3. Ion channel activity, particularly Ca2+ channels.

Stimulation of the A2A adenosine receptor increases expression of the tyrosine hydroxylase gene

PC12 cells are known to express A2A adenosine receptors that are linked to adenylyl cyclase. We investigated the role played by A2A adenosine receptors in the expression of the rat tyrosine hydroxylase (TH) gene in PC12 cells. The A2A selective adenosine receptor agonist 2-(p-2-carboxyethyl)phenylethylamino)-5'-N-ethylcarboxyamidoade nosine (CGS21680) caused TH mRNA levels to increase to more than twice the level of the untreated control. Transient transfection analysis demonstrated that the transcription of the TH gene was markedly enhanced upon treatment with CGS21680. The adenosine receptor-mediated TH gene expression was confirmed by the inhibitory effects that adenosine receptor antagonists had on the CGS21680 response. Mutational analysis of the 5' upstream region of the TH gene revealed that the cAMP response element (CRE) at -45 to -38 bp was responsible for the CGS21680 effect. Gel mobility shift assays revealed that six CRE-specific DNA-protein complexes were formed, and the amounts of three of them were significantly increased by treatment with CGS21680. Co-transfection with an expression vector containing protein kinase A (PKA) inhibitor markedly decreased the CGS21680 effect. The results suggest that stimulation of the A2A adenosine receptor leads to an elevated expression of the TH gene by changing the binding pattern of DNA binding proteins that interact with CRE through activation of protein kinase A.

The use of the human neuroblastoma SH-SY5Y to study the effect of second messengers on noradrenaline release

1. Recent data suggesting that the human neuroblastoma SH-SY5Y is a suitable cell line in which to study the effect of second messengers on NA release are discussed in the context of current views on exocytosis. 2. Release of NA is evoked by depolarization, as well as activation of muscarinic (M3) and bradykinin (B2) receptors in SH-SY5Y cells which have not been differentiated by the addition of growth factors. 3. Evoked release is enhanced by activation of protein kinase C. 4. Activation of protein kinase C decreases the changes in intracellular calcium evoked by carbachol, bradykinin and 100 mM K+. 5. SH-SY5Y express N-type and L-type voltage sensitive Ca2+ channels. L-Type Ca(2+)-channels are coupled to NA release under conditions of weak depolarization. However with strong depolarization (100 mM K+) both L-type and N-type channels are involved. 6. Muscarinic- and neuropeptide Y receptors are coupled to the inhibition of Ca2+ channel activity.

Inhibition by ethaverine of catecholamine secretion through blocking L-type Ca2+ channels in PC12 cells

Ethaverine, a derivative of papaverine, is used as a vasodilator and antispasmodic drug. We have investigated the effects of ethaverine on the secretion of [3H]norepinephrine from PC12 cells, of neuroendocrine origin. Treatment with ethaverine reduced catecholamine secretion in a concentration-dependent manner. The maximal inhibitory effect (90%) was achieved with 10 microM ethaverine, and the IC50 for secretion was approximately 2 microM. Ethaverine pretreatment for 1 min prior to stimulation by 70 mM K+ also decreased the level of intracellular Ca2+ in a concentration-dependent manner, as measured by fura-2 fluorescence. The IC50 for the inhibition of the increase in intracellular Ca2+ was approximately 2 microM. Nifedipine, a dihydropyridine L-type Ca2+ channel blocker, did not enhance the inhibitory effect of ethaverine on the 70 mM K(+)-induced increase in [Ca2+]i or catecholamine secretion. In contrast, the addition of the N-type voltage-sensitive Ca2+ channel antagonist omega-conotoxin with ethaverine resulted in further reductions in the increase in [Ca2+]i and catecholamine release. Maximally effective concentrations of ethaverine and nifedipine showed the same inhibitory effect on the 70 mM K(+)-evoked responses. However, ethaverine pretreatment did not inhibit the bradykinin-induced secretion and [Ca2+]i rise, which are known to be produced through the receptor-operated Ca2+ channels. We conclude that ethaverine reduces catecholamine secretion by blocking the L-type voltage-sensitive Ca2+ channel.

Role of extracellular adenosine in ethanol-induced desensitization of cyclic AMP production

The decrease in receptor-stimulated cyclic AMP production after chronic ethanol exposure was suggested previously to be secondary to an ethanol-induced increase in extracellular adenosine. The present study was undertaken to ascertain whether a similar mechanism was responsible for the ethanol-induced desensitization of cyclic AMP production in PC12 pheochromocytoma cells. The acute addition of ethanol in vitro significantly increased both basal cyclic AMP content and extracellular levels of adenosine. A 4-day exposure to ethanol decreased basal as well as 2-chloroadenosine- and forskolin-stimulated cyclic AMP contents. No change in cyclic AMP content was observed after a 2-day exposure of PC12 cells to ethanol. Inclusion of adenosine deaminase during the chronic ethanol treatment significantly decreased extracellular levels of adenosine, yet the percentage decrease in 2-chloroadenosine- and forskolin-stimulated cyclic AMP levels after chronic ethanol exposure was not changed by the inclusion of the adenosine deaminase. Similar results were obtained when the chronic treatment was carried out with serum-free defined media. The ethanol-induced desensitization could not be mimicked by chronic exposure of PC12 cells to adenosine analogues. A 24-h exposure of PC12 cells to 2-chloroadenosine resulted in a decrease in the subsequent ability of this adenosine analogue to stimulate cyclic AMP content, but basal and forskolin-stimulated cyclic AMP levels were increased. Similar results were obtained after a 4-day exposure of PC12 cells to 2-chloroadenosine or 5'-N-ethylcarboxamido-adenosine. The present results indicate that the ethanol-induced decrease in receptor-stimulated cyclic AMP content in PC12 cells is not due to an increase in extracellular adenosine.

Vasoactive intestinal polypeptide-related peptides modulate tyrosine hydroxylase gene expression in PC12 cells through multiple adenylate cyclase-coupled receptors

We investigated the receptor mechanisms by which vasoactive intestinal polypeptide (VIP) and related peptides exert their effects on tyrosine hydroxylase (TH) gene expression. VIP, secretin, and peptide histidine isoleucine (PHI) each produced increases in TH gene expression, as measured by increases in TH mRNA levels and TH activity. The concentrations at which the effects of these peptides were maximal differed for TH activity and TH mRNA. Moreover, maximal increases in TH activity were 130-140% of control, whereas maximal increases in TH mRNA were 250% of control. The concentration dependence of the increases in TH mRNA in response to the three peptides was analyzed by fitting the data to nonlinear regression models that assume either one or two components to the response. The data for secretin fit best to a model that assumes a single component to the increase in TH mRNA levels. The data derived for PHI and VIP fit best to models that assumed two components to the TH mRNA response. These data suggested that there may be more than one receptor or signal transduction mechanism involved in the response to the various peptides. We examined whether the peptides exerted their effects through common or multiple second messenger systems. The ability of maximally active concentrations of these peptides to stimulate increases in TH mRNA was not additive, indicating that the peptides work through a common receptor or signal transduction pathway. Each peptide stimulated increases in protein kinase A (PKA) activity. Secretin and VIP were ineffective in increasing TH mRNA levels in a PKA-deficient mutant PC12 cell line (A126-1B2). Moreover, the adenylate cyclase antagonist 2',5'-dideoxyadenosine prevented the increase in TH mRNA produced by each peptide. Thus, each peptide requires an intact cyclic AMP second messenger pathway to produce changes in TH gene expression, suggesting that the complex pattern of response to VIP and PHI revealed by concentration-response analysis was due to the actions of these peptides at multiple receptors. To evaluate this possibility, we examined the effect of several peptide receptor antagonists on the increase in TH gene expression elicited by VIP, PHI, and secretin. The secretin antagonist secretin (5-27) (20 microM) had no significant effect on VIP or PHI stimulation of TH gene expression, but reduced the effect of secretin. The VIP antagonist VIP (10-28) (20 microM) reduced the effect of VIP on increasing TH mRNA, but had no significant effect on the response of TH mRNA to secretin or PHI.(ABSTRACT TRUNCATED AT 400 WORDS)

Nicotinic receptor-elicited sodium flux in rat pheochromocytoma PC12 cells: effects of agonists, antagonists, and noncompetitive blockers

Nicotinic agonists stimulate 22Na flux in rat pheochromocytoma PC12 cells. The stimulatory effect of carbamylcholine is maximal at 1 mM, while the stimulatory effect of nicotine and anatoxin maximize at the same level at 100 microM and 10 microM, respectively. The tertiary amines arecolone and isoarecolone have no effect on flux at 100 microM, while the methiodides at 100 microM stimulate flux to an extent similar to 1 mM carbamylcholine. Dihydro and alcohol analogues of isoarecolone methiodide have markedly smaller effects on flux. A preincubation for 2 to 20 min with carbamylcholine (2 mM), nicotine (300 microM), anatoxin (30 microM) or isoarecolone methiodide (100 microM) causes marked desensitization to a subsequent carbamylcholine-elicited stimulation of flux. d-Tubocurarine, mecamylamine, hexamethonium, and chlorisondamine inhibit carbamylcholine-elicited flux with IC50 values of 1.0, 0.8, 43, and 0.020 microM, respectively. Atropine has no effect at 1 microM, but reduces the response to carbamylcholine by 50% at 8.6 microM, presumably as a noncompetitive blocker. Other noncompetitive blockers of nicotinic acetylcholine-receptors, such as histrionicotoxins, gephyrotoxin, pumiliotoxin C, phencyclidine, bupivacaine and piperocaine, inhibit carbamylcholine-elicited stimulation of 22Na flux with IC50 values from 0.3 to 1.8 microM. In contrast to d-tubocurarine, which inhibits carbamylcholine-elicited desensitization, and mecamylamine, which has no apparent effect on desensitization, chlorisondamine and certain noncompetitive blockers appear to enhance desensitization. The effects of agonists, antagonists and noncompetitive blockers at the neuronal nicotinic acetylcholine receptor-channel of PC12 cells are compared to their effects on binding of [125I]alpha-bungarotoxin to agonist-recognition sites and of [3H]perhydrohistrionicotoxin to noncompetitive blocker sites of the nicotinic acetylcholine receptor-channel of electric ray (Torpedo) electroplax membranes. There are marked differences in relative potencies for the two types of nicotinic acetylcholine receptor-channel.

Effects of different types of stimulation on cyclic AMP content in the rabbit carotid body: functional significance

Cyclic AMP levels in rabbit carotid bodies incubated under control conditions, 100% O2- or 95% O2/5% CO2- equilibrated medium, are close to 1 pmol/mg wet tissue (range 0.4-2.43 pmol/mg). Isobutylmethylxanthine (0.5 mM) increases cyclic AMP levels by a factor of 14 and 8 in HEPES- and CO2/CH3O(-)-buffered medium, respectively. Forskolin (0.5-10 microM) applied during 30 min increases cyclic AMP levels in a dose-dependent manner. Incubation of carotid bodies at low O2 tensions resulted in an elevation of cyclic AMP levels both in the absence and in the presence of isobutymethylxanthine. In the latter conditions cyclic AMP increase was maximum at an O2 tension of 46 mm Hg and tended to decrease at extremely low PO2. In isobutylmethylxanthine-containing Ca2(+)-free medium, cyclic AMP increased linearly with decreasing PO2 from 66 to 13 mm Hg; the absolute cyclic AMP levels attained in Ca2(+)-free medium were smaller than those observed in Ca2(+)-containing medium at any PO2. The differences between Ca2(+)-free and Ca2(+)-containing media appear to be due to the action of released neurotransmitters in the latter conditions, because dopamine and norepinephrine, which are known to be released by hypoxia in a Ca2(+)-dependent manner, increase cyclic AMP in the carotid body. Low pH/high PCO2 and high [K+]e increase cyclic AMP levels only in Ca2(+)-containing medium. Forskolin potentiates the release of catecholamines induced by low PO2. These results suggest that cyclic AMP plays an important role in the modulation of the chemoreception process.

Effects of forskolin and analogues on nicotinic receptor-mediated sodium flux, voltage-dependent calcium flux, and voltage-dependent rubidium efflux in pheochromocytoma PC12 cells

1. Forskolin, a naturally occurring diterpene that activates adenylate cyclase, HL706, a water-soluble derivative of forskolin (6 beta-[(piperidino)acetoxy]-7-desacetylforskolin) that is less potent than forskolin in activating adenylate cyclase, and 1,9-dideoxyforskolin, an analogue that does not activate adenylate cyclase, were examined for effects on the nicotinic receptor-mediated 22Na+ flux, a high potassium-induced 45Ca2+ flux through L-type calcium channels, and a high potassium-induced 86Rb+ efflux through a calcium-dependent potassium channels in PC12 cells. 2. Forskolin and analogues at 30 microM completely blocked carbamylcholine-elicited flux of 22Na+ through the nicotinic receptor-gated channel. 1,9-Dideoxyforskolin had an IC50 value of 1.6 microM with forskolin and HL706 being two- to three fold less potent. 3. Forskolin and its analogues appear to be noncompetitive blockers of the neuronal nicotinic receptor-channel complex in PC12 cells, but unlike many noncompetitive blockers, did not markedly enhance desensitization. Instead, forskolin, but not HL706 or 1,9-dideoxyforskolin, slightly antagonized the desensitization evoked by high concentrations of carbamylcholine. N-Ethylcarboxamidoadenosine, an adenosine analogue that elevates cyclic AMP and 8-bromo-cyclic AMP had no effect on desensitization. 4. Forskolin, HL706, and 1,9-dideoxyforskolin in the presence of carbamylcholine inhibited the binding of a noncompetitive blocker, [3H]perhydrohistrionicotoxin, to the muscle-type nicotinic receptor-channel complex in Torpedo electroplax membranes with IC50 values of 20 microM. Forskolin had no effect on [3H]perhydrohistrionicotoxin binding in the absence of carbamylcholine, while HL706 and 1,9-dideoxyforskolin still inhibited binding in the absence of carbamylcholine. 5. Forskolin, but not HL706 or 1,9-dideoxyforskolin had a slight inhibitory effect on the binding of [125I]alpha-bungarotoxin to acetylcholine recognition sites in Torpedo membranes. 1,9-Dideoxyforskolin at 30 microM, but not forskolin or HL706, markedly inhibited depolarization-evoked 45Ca+ flux and 86Rb+ efflux in PC12 cells, suggesting that 1,9-dideoxyforskolin has nonspecific inhibitory effects on a variety of ion channels.

Effect of ethanol on cyclic AMP levels in intact PC12 cells

Two subclones of the rat pheochromocytoma cell line, PC12, were used to compare the effects of ethanol on adenylate cyclase activity in isolated membranes with its effects on cyclic AMP accumulation in intact cells. Consistent with previous reports, ethanol increased basal and 2-chloroadenosine-stimulated adenylate cyclase activity in isolated membrane preparations from both subclones. However, ethanol had opposite effects on agonist-stimulated cyclic AMP accumulation in intact cells of the two subclones, enhancing accumulation in one subclone, and inhibiting it in the other. The inhibition of cyclic AMP accumulation did not result from stimulation of phosphodiesterase activity, activation of the inhibitory guanyl nucleotide regulatory protein, Gi, or stimulation of protein kinase C. The results indicate that extrapolation of the effects of ethanol from one cell type to another, or from in vitro to in vivo systems, may be complicated by the interaction of ethanol with regulatory processes that influence second messenger systems, and can differ in various types of intact cells.

Adenosine receptor activation and the regulation of tyrosine hydroxylase activity in PC12 and PC18 cells

We compared the response of rat PC12 cells and a derivative PC18 cell line to the effects of adenosine receptor agonists, antagonists, and adenine nucleotide metabolizing enzymes. We found that theophylline (an adenosine receptor antagonist), adenosine deaminase, and AMP deaminase all decreased basal cyclic AMP content and tyrosine hydroxylase activity in the PC12 cells, but not in PC18 cells. Both cell lines responded to the addition of 2-chloroadenosine and 5'-N-ethylcarboxamidoadenosine, adenosine receptor agonists, by exhibiting an increase in tyrosine hydroxylase activity and cyclic AMP content. The latter finding indicates that both cell lines contained an adenosine receptor linked to adenylate cyclase. We found that the addition of dipyridamole, an inhibitor of adenosine uptake, produced an elevation of cyclic AMP and tyrosine hydroxylase activity in both cell lines. Deoxycoformycin, an inhibitor of adenosine deaminase, failed to alter the levels of cyclic AMP or tyrosine hydroxylase activity. This suggests that uptake was the primary inactivating mechanism of adenosine action in these cells. We conclude that both cell types generated adenine nucleotides which activate the adenosine receptor in an autocrine or paracrine fashion. We found that PC12 cells released ATP in a calcium-dependent process in response to activation of the nicotinic receptor. We also measured the rates of degradation of exogenous ATP, ADP, and AMP by PC12 cells. We found that the rates of metabolism of the former two were at least an order of magnitude greater than that of AMP. Any released ATP would be rapidly metabolized to AMP and then more slowly degraded to adenosine.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotransmitter release from bradykinin-stimulated PC12 cells. Stimulation of cytosolic calcium and neurotransmitter release

The effect of bradykinin on intracellular free Ca2+ and neurotransmitter secretion was investigated in the rat pheochromocytoma cell line PC12. Bradykinin was shown to induce a rapid, but transient, increase in intracellular free Ca2+ which could be separated into an intracellular Ca2+ release component and an extracellular Ca2+ influx component. The bradykinin-induced stimulation of intracellular free Ca2+ displayed a similar time course, concentration dependencies and extracellular Ca2+ dependence as that found for neurotransmitter release, indicating an association between intracellular free Ca2+ levels and neurotransmitter secretion. The selective BK1-receptor antagonist des-Arg9,[Leu8]BK (where BK is bradykinin) did not significantly affect the stimulation of intracellular free Ca2+ or neurotransmitter release. In contrast, these effects of bradykinin were effectively blocked by the selective BK2-receptor antagonist [Thi5,8,D-Phe7]BK, and mimicked by the BK2 partial agonist [D-Phe7]BK in a concentration-dependent manner. The stimulation of intracellular free Ca2+ and neurotransmitter release induced by bradykinin was shown not to involve voltage-sensitive Ca2+ channels, since calcium antagonists had no effect on either response at concentrations which effectively inhibit depolarization-induced responses. These results indicate that bradykinin, acting through the interaction with the BK2 receptor, stimulates an increase in intracellular free Ca2+ leading to neurotransmitter secretion. Furthermore, bradykinin-induced responses involve the release of intracellular Ca2+ and the influx of extracellular Ca2+ that is not associated with the activation of voltage-sensitive Ca2+ channels.

Muscarinic receptors modulating acetylcholine release from insect synaptosomes

1. Cholinergic synapses in the central nervous system of insects contain inhibitory muscarinic receptors whose stimulation by agonists leads to a diminished output of acetylcholine; antagonists, like atropine, facilitate acetylcholine release. 2. The receptors involved appear to be of the M2-subtype. Upon activation of presynaptic receptors a significant reduction of the intrasynaptosomal cyclic AMP level as well as a significantly increased membrane potential was observed. 3. The observed membrane hyperpolarization is apparently not a consequence of a lower cyclic AMP level, thus both effects may offer alternative or synergistical mechanisms for modulating transmitter release.

Second-messenger generation in PC12 cells. Interactions between cyclic AMP and Ca2+ signals

Changes in cyclic AMP concentrations were studied in intact PC12 pheochromocytoma cells exposed to a variety of treatments. A marked increase was triggered by N-(L-2-phenylisopropyl)adenosine, the activator of an adenosine receptor, whereas a decrease (observed even after phosphodiesterase blockade) was induced by carbachol, working through a muscarinic receptor inhibited by the selective muscarinic blocker pirenzepine, only at high concentration (Ki 450 nM). A decrease in cyclic AMP was also induced by clonidine, an alpha 2-adrenergic-receptor agonist. Both the alpha 2-adrenergic and the muscarinic inhibitions were prevented by pretreatment of the cells with pertussis toxin, and were unaffected by the phorbol ester 12-O-tetradecanoylphorbol 13-acetate. The latter drug caused a decrease in the resting cyclic AMP concentrations, and a potentiation of the increase induced by adenosine-receptor activation. Except for clonidine, all these treatments were found to be effective in both growing PC12 cells and, although to a smaller degree, in cells that had stopped growing and had acquired a neuron-like phenotype after prolonged treatment with nerve growth factor (NGF). Neither forskolin (a direct activator of adenylate cyclase) nor the activation of adenosine and alpha-adrenergic receptors was able to modify the resting cytosolic Ca2+ concentration [Ca2+]i in PC12 cells. Likewise, the K+-induced [Ca2+]i transients were unchanged after these treatments, whereas the transients induced by carbachol through the activation of a muscarinic receptor highly sensitive to pirenzepine were moderately potentiated by forskolin (and, to a lesser degree, by the adenosine analogue) and attenuated by clonidine. These results characterize in further detail the spectrum and the mutual interrelationships of the intracellular signals induced by receptor activation in PC12 cells, also as a function of the NGF-induced differentiation.

Second-messenger control of catecholamine release from PC12 cells. Role of muscarinic receptors and nerve-growth-factor-induced cell differentiation

The role of various intracellular signals and of their possible interactions in the control of neurotransmitter release was investigated in PC12 cells. To this purpose, agents that affect primarily the cytosolic concentration of Ca2+, [Ca2+]i (ionomycin, high K+), agents that affect cyclic AMP concentrations (forskolin; the adenosine analogue phenylisopropyladenosine; clonidine) and activators of protein kinase C (phorbol esters) were applied alone or in combination to either growing chromaffin-like PC12-cells, or to neuron-like PC12+ cells differentiated by treatment with NGF (nerve growth factor). In addition, the release effects of muscarinic-receptor stimulation (which causes increase in [Ca2+]i, activation of protein kinase C and decrease in cyclic AMP) were investigated. Two techniques were employed to measure catecholamine release: static incubation of [3H]dopamine-loaded cells, and perfusion incubation of unlabelled cells coupled to highly sensitive electrochemical detection of released catecholamines. The results obtained demonstrate that: (1) release from PC12 cells can be elicited by both raising [Ca2+]i and activating protein kinases (protein kinase C and, although to a much smaller extent, cyclic AMP-dependent protein kinase); and (2) these various control pathways interact extensively. Activation of muscarinic receptors by carbachol induced appreciable release responses, which appeared to be due to a synergistic interplay between [Ca2+]i and protein kinase C activation. The muscarinic-induced release responses tended to become inactivated rapidly, possibly by feedback desensitization of the receptor mediated by protein kinase C. Muscarinic inactivation was prevented (or reversed) by agents that increase, and accelerated by agents that decrease, cyclic AMP. Agents that stimulate release primarily through the Ca2+ pathway (ionomycin and high K+) were found to be equipotent in both PC12- and PC12+ cells, whereas the protein kinase C activator 12-O-tetradecanoyl-phorbol 13-acetate was approx. 10-fold less potent in PC12+ cells, when administered either alone or in combination with ionomycin. In contrast, the cell binding of phorbol esters was not greatly modified by NGF treatment. Thus control of neurotransmitter release from PC12 cells is changed by differentiation, with a diminished role of the mechanism mediated by protein kinase C.

Adenosine receptors activate adenylate cyclase and enhance secretion from bovine adrenal chromaffin cells in the presence of forskolin

Cells of the adrenal medulla release not only catecholamines but also high concentrations of neuropeptides and nucleotides. Chromaffin cells, like many neuronal cells, have a diversity of receptors: adrenergic receptors, peptide receptors, histamine receptors, and dopamine receptors. We recently reported that these cells have nucleotide receptors that can mediate inhibition of the secretory response. The present studies show that adenosine, in the presence of enabling concentrations of forskolin, can potently enhance response to nicotinic stimulation. Neither adenosine nor forskolin alone produces a significant effect. A marked rise in intracellular cyclic AMP (cAMP) concentration is associated with the enhancement of secretion caused by forskolin plus adenosine. A phosphodiesterase inhibitor, Ro 20-1724, used together with forskolin produces significant increases in both cellular cAMP content and catecholamine secretion. However, the adenosine agonist 5'-N-ethylcarboxyadenosine elevates cellular cAMP content in the presence of forskolin without having any positive effect on secretion. This finding suggests that the rise in cAMP level may not be the sole cause of the increase in secretion by adenosine.

Calmodulin- and protein phosphorylation-independent release of catecholamines from PC-12 cells

Catecholamine secretion from PC-12 cells can be triggered by agents that increase intracellular Ca2+ and is enhanced by phorbol esters and agents that elevate intracellular cAMP concentrations. In mutant PC-12 cells lacking cAMP-dependent protein kinase (PK-A) in which protein kinase C (PK-C) was down-regulated, Ca2+-dependent secretion occurred normally but was no longer enhanced by cAMP or phorbol esters. In digitonin-permeabilized PC-12 cells that lacked PK-C and PK-A, a range of calmodulin (CaM) inhibitors failed to block Ca2+-triggered catecholamine release. Moreover, Mn2+, a CaM activator, failed to trigger catecholamine release whereas Ba2+, which does not activate CaM, supported secretion. These results indicate that the basic mechanism of stimulus/secretion coupling in PC-12 cells does not absolutely require a regulated protein phosphorylation- or calmodulin-dependent step.

Characterization of adenosine receptors in the PC12 pheochromocytoma cell line using radioligand binding: evidence for A-2 selectivity

Examination of the binding characteristics of the adenosine agonist radioligands [3H]N6-cyclohexyladenosine [( 3H]CHA), [3H]cyclopentyladenosine [( 3H]CPA), and [3H]5'-N-ethylcarboxamido adenosine [( 3H]NECA) to membranes prepared from PC12 cells showed that the A-1-selective ligands (CHA and CPA) had minimal binding, which was not amenable to analysis using curve-fitting programs. However, [3H]NECA, a nonselective A-1/A-2 agonist, gave reproducible binding, which was enhanced by removal of endogenous adenosine, using the catabolic enzyme adenosine deaminase. This binding was of high affinity (KD = 4.7 nM) with limited capacity (263 fmol/mg of protein). Specific binding of [3H]NECA was unaffected by the presence of either CPA (50 nM) or MgCl2 (10 mM) but was sensitive to guanylylimidodiphosphate (100 microM), a finding suggesting involvement of an N-protein mechanism in the coupling of the adenosine receptor labeled by [3H]NECA to other components of the receptor complex. Binding of [3H]NECA to PC12 cell membranes was stereo-selective, with the R isomer of N6-phenylisopropyladenosine (PIA) being approximately 12 times more active than S-PIA. The A-1-selective agonist CPA was a weak inhibitor of [3H]NECA binding (Ki = 251 nM). The rank order of activity of adenosine agonists in displacing specific [3H]NECA binding was NECA greater than or equal to 2-chloroadenosine greater than CHA greater than or equal to 5'-N-methylcarboxamido adenosine greater than or equal to R-PIA greater than CPA greater than S-PIA. Binding was also displaced by the marine adenosine agonist 1-methylisoguanosine and by a series of xanthine antagonists with the activity order being 1,3-dipropyl-8-(2-amino-4-chloro)phenylxanthine greater than 8-phenyltheophylline greater than 8-p-sulfophenyltheophylline.(ABSTRACT TRUNCATED AT 250 WORDS)

Fate of cyclic nucleotides in PC12 cell cultures: uptake, metabolism, and effects of metabolites on nerve growth factor-induced neurite outgrowth

The fate of cyclic AMP (cAMP), dibutyryl-cAMP (Bt2-cAMP), and the (Sp)-isomer of adenosine 3',5'-monophosphorothioate [(Sp)-cAMPS] was studied in the PC12 culture medium by means of HPLC. In the absence of PC12 cells, cAMP and Bt2-cAMP were rapidly degraded by nonspecific esterases and cyclic nucleotide phosphodiesterase both originating from the serum commonly used as a culture medium ingredient, whereas (Sp)-cAMPS was completely stable. Since 5'-AMP, adenosine, inosine, and hypoxanthine appeared in the culture medium after incubation with cAMP or Bt2-cAMP, we have determined their effect on nerve growth factor (NGF)-induced neurite outgrowth. 5'-AMP, adenosine, and inosine were indeed potent agents in producing a potentiating effect on NGF-induced early neurite outgrowth at a concentration of 1 mM. Thus, cAMP metabolites had the capacity to induce an effect that has been described as cAMP-specific. In serum-free culture medium and in the presence of cells, all cyclic nucleotides were taken up by PC12 cells. Uptake was highly correlated with the hydrophobic nature of the compounds, and was accompanied by a simultaneous excretion of metabolites. On incubation with cAMP, NGF had a pronounced effect on the metabolic pattern found in the culture medium. In particular, dephosphorylation of 5'-AMP was specifically enhanced. This effect of NGF on the degradation of cAMP was also apparent when cAMP metabolites were incubated with PC12 cells. Whereas 5'-AMP degradation was greatly increased, NGF had no effect on the metabolism of the other purine compounds.

The role of cAMP in nerve growth factor-promoted neurite outgrowth in PC12 cells

Nerve growth factor (NGF)-mediated neurite outgrowth in rat pheochromocytoma PC12 cells has been described to be synergistically potentiated by the simultaneous addition of dibutyryl cAMP. To elucidate further the role of cAMP in NGF-induced neurite outgrowth we have used the adenylate cyclase activator forskolin, cAMP, and a set of chemically modified cAMP analogues, including the adenosine cyclic 3',5'-phosphorothioates (cAMPS) (Rp)-cAMPS and (Sp)-cAMPS. These diastereomers have differential effects on the activation of cAMP-dependent protein kinases, i.e., (Sp)-cAMPS behaves as a cAMP agonist and (Rp)-cAMPS behaves as a cAMP antagonist. Our data show that the establishment of a neuritic network, as observed from PC12 cells treated with NGF alone, could not be induced by either forskolin, cAMP, or cAMP analogues alone. The presence of NGF in combination with forskolin or cAMP or its agonistic analogues potentiated the initiation of neurite outgrowth from PC12 cells. The (Sp)-cAMPS-induced stimulation of NGF-mediated process formation was successfully blocked by the (Rp)-cAMPS diastereomer. On the other hand, NGF-stimulated neurite outgrowth was not inhibited by the presence of the cAMP antagonist (Rp)-cAMPS. We conclude that the morphological differentiation of PC12 cells stimulated by NGF does not require cAMP as a second messenger. The constant increase of intracellular cAMP, caused by either forskolin or cAMP and the analogues, in combination with NGF, not only rapidly stimulated early neurite outgrowth but also exerted a maintaining effect on the neuronal network established by NGF.

Evaluation of the binding of the A-1 selective adenosine radioligand, cyclopentyladenosine (CPA), to rat brain tissue

The binding of [3H]-Cyclopentyladenosine (CPA), an N6-substituted analog of adenosine, was examined in vitro. CPA bound with high affinity (Kd = 0.48 nmol/l) to rat brain membranes. Specific binding, which represented 90-97% of the total counts bound at a ligand concentration of 1 nmol/l, was saturable, reversible and sensitive to protein denaturation. The pharmacology of binding was consistent with the labeling of an A-1 receptor, the R- and S-diasteromers of N6-phenylisopropyladenosine (PIA) showing a sixteenfold difference in their ability to displace CPA. The prototypic A-1 selective adenosine agonist, N6-cyclohexyladenosine (CHA) was twofold less active than CPA in displacing radiolabeled CPA. Comparison of the ability of cold CHA and CPA to displace [3H]-CPA gave rate dissociation constants of 1.88 and 1.80 X 10(4) s-1, respectively suggesting that both CHA and CPA bound to the same recognition site. In contrast however, comparison of the binding of [3H]-CPA with that of [3H]-CHA showed distinct differences. The Kd for CHA was approximately twice that of CPA while the apparent Bmax was 60% greater. In comparing the pharmacology of CPA binding with that of CHA, it was found that CHA, S-PIA and the antagonist, PACPX were more active in displacing CHA than CPA. In general however, CPA has a binding profile very similar to that observed with CHA.

Inhibition of protein carboxylmethylation and dopamine autoreceptor functioning

S-Adenosyl-L-homocysteine (SAH, 2-100 microM) greatly inhibited protein carboxymethylation (PCM) in rat striatal synaptosomes, but did not alter the ability of apomorphine and other DA agonists to inhibit DA synthesis. SAH (10 microM) also did not significantly alter the ability of either 0.5 or 1.0 microM apomorphine to inhibit DA release from superfused rat striatal tissue slices, but it did antagonize the response to 5.0 microM apomorphine. The former two concentrations of apomorphine predominantly affected only DA release, whereas the latter concentration suppressed both DA and acetylcholine release. These findings are discussed with regard to the possible relationship between DA autoreceptor functioning and PCM activity.

Dopamine autoreceptor stimulation increases protein carboxyl methylation in striatal slices

We have investigated the possibility that protein carboxyl methylation is involved in coupling dopamine autoreceptor stimulation to intracellular events such as inhibition of dopamine synthesis or release. The dopamine agonists apomorphine and TL-99 were found to stimulate methyl ester formation in striatal slices preloaded with [3H]methionine. The stimulatory effects of apomorphine were dose-dependent, were not due to changes in [3H]methionine uptake or S-[3H]-adenosylmethionine formation, and were blocked by the stereospecific dopamine antagonist (+)-butaclamol. Stimulation of methyl ester formation by dopamine agonists is readily observed only when slices are prepared from rats pretreated with reserpine to deplete endogenous brain catecholamines. This suggests that in slices prepared from normal rats endogenous dopamine (DA) released during slice preparation and incubation masks the effects produced by exogenously administered dopamine agonists on protein carboxyl methylase (PCM) activity. Additional experiments suggested that the effects of apomorphine were mediated via an interaction with DA autoreceptors rather than with postsynaptic DA receptors. Destruction of monoamine neurons and their associated autoreceptors by injecting 6-hydroxydopamine into the area of the medial forebrain bundle abolished the stimulatory effects of apomorphine on methyl ester formation in striatal slices. Furthermore the putative selective DA autoreceptor agonist EMD 23 448 was also found to stimulate methyl ester formation in striatal slices. These findings, discussed in terms of calcium-dependent functions, support the hypothesis that PCM may be a key component in the biochemical transduction of DA autoreceptor stimulation.

Heterogeneous distribution of adenosine transport sites labelled by [3H]nitrobenzylthioinosine in rat brain: an autoradiographic and membrane binding study

A highly heterogeneous distribution of [3H]nitrobenzylthioinosine [( 3H]NBI) binding sites was observed using both autoradiographic and membrane binding methodology. Of the 24 brain regions examined in the radio-ligand binding studies, the highest levels of [3H]NBI sites were found in the thalamus, followed by midbrain, superior colliculus, olfactory cortex and hypothalamus. The thalamus contained over 5 times more sites than cerebellum which exhibited the lowest [3H]NBI binding levels. The results obtained from autoradiographic analysis agreed well with quantitative measurements and revealed that subnuclei of thalamus and hypothalamus as well as specific layers of the superior colliculus contained particularly high concentrations of [3H]NBI sites. When the [3H]NBI autoradiograms were compared with the distribution of adenosine deaminase in brain it was found that brain regions richest in neural elements immunoreactive for adenosine deaminase contained the greatest numbers of [3H]NBI sites. In contrast, a poor correlation was found between the distribution of [3H]NBI binding and adenosine receptors labelled with [3H]cyclohexyladenosine. The co-localization of [3H]NBI binding and adenosine deaminase in brain indicates the existence of neural systems having a high capacity to take up and metabolize adenosine.