Neuropeptide Y Y5 receptors inhibit kindling acquisition in rats

Neuropeptide Y inhibits neuronal excitability and seizures in various experimental models. This peptide delays kindling epileptogenesis but the receptors involved in this action are unknown. We have studied the role of Y5 receptors in kindling using the selective antagonist GW438014A (IC50=210 nM), a small heterocycle molecule that crosses the blood-brain barrier, and the selective peptide agonist Ala31Aib34 NPY (IC50=6.0 nM). Intraperitoneal injection of GW438014A (10 mg/kg), 30 min before the beginning of a rapid-kindling protocol, significantly accelerated the rate of kindling acquisition as compared to vehicle-injected rats. Thus, the number of electrical stimuli required to reach stages 3 and 4-5 of kindling were reduced by 50% and 25%, respectively. The average afterdischarge duration in the stimulated hippocampus was prolonged by 2-fold. Conversely, kindling rate was delayed by intracerebroventricular administration of 24 nmol Ala31Aib32 NPY. Thus, the number of stimuli necessary to reach stages 2 and 3 of kindling was increased by 3- and 4-fold, respectively. During the stimulation protocol (40 stimuli) none of the rats treated with the Y5 agonist showed stages 4-5 seizures. Twenty-four hours after the last kindling stimulation, thus during the re-test session, Y5 agonist- or antagonist-treated rats had stages 4-5 seizures as their controls. In rats treated with both the antagonist and the agonist, kindling rate was similar to vehicle-injected rats. These data indicate that Y5 receptors mediate inhibitory effects of NPY in kindling and display anticonvulsant rather then antiepileptogenic effects upon agonist stimulation.

Food intake inhibition and reduction in body weight gain in lean and obese rodents treated with GW438014A, a potent and selective NPY-Y5 receptor antagonist

Numerous reports have implicated theY5 receptor as the 'feeding' receptor mediating the orexigenic action of neuropeptide Y (NPY). This notion is supported by the correlation between the in vitro functional and binding activities of different peptide agonists and their potent stimulation of food intake in rodents. We have discovered a series of small molecule heterocycles with high affinity, selectivity, and functional antagonism for Y5 receptors. Intraperitoneal (i.p.) administration of GW438014A into rodents, resulted in a potent reduction of NPY-induced and normal overnight food intake. Brain levels of GW438014A were detected well in excess of its binding IC(50) for up to 3 h post-dosing. Daily (i.p., BID, 10 mg/kg) administration of this compound to Zucker Fatty rats for a period of 4 days resulted in a marked decrease in the rate of weight gain and a reduction in fat mass. No effect on food intake was observed following oral administration of GW438014A (25-100 mg/kg), consistent with the poor oral bioavailability (<3%) and low brain levels observed.

Food intake inhibition and reduction in body weight gain in rats treated with GI264879A, a non-selective NPY-Y1 receptor antagonist

Neuropeptide Y has been proposed to play a major role in the hypothalamic regulation of feeding behavior through the activation of specific, central NPY receptor(s). In an effort to design small molecule antagonists of NPY receptors, we have synthesized a series of substituted dipeptides based on defined pharmacophores, previously identified by us and others as essential for the interaction with the peptide receptors. GI264879A behaves as a functional antagonist of Y1 receptors while displaying no binding selectivity for the different NPY receptor subtypes. We demonstrate here that administration of GI264879A to rats causes a significant decrease in food intake and body weight partly through a mechanism dependent on the integrity of the vagus nerve.

Pharmacological characterization and selectivity of the NPY antagonist GR231118 (1229U91) for different NPY receptors

Neuropeptide Y (NPY) is widely distributed throughout the central and peripheral nervous system and exerts a wide range of physiological responses by activating specific receptors. In this study we have characterized the potency of the high affinity peptide dimer antagonist, GR231118, to displace radiolabeled NPY/PYY from different tissues and cell lines expressing Y1 or Y2 receptors and from CHO cells stably transfected with human cDNA encoding for Y1, Y2 and Y4 receptors. GR231118 displays high affinity for Y1 and Y4 receptors, equal or better than that of NPY itself, while its activity is several fold weaker for Y2 receptors. Displacement of radiolabeled PYY from rat hypothalamic membranes by GR231118, reveals the existence of high and low affinity binding sites which may be equated to Y1 and Y2 receptors respectively suggesting that the compound maybe used as a tool to dissect central NPY receptors.

Novel neuropeptide Y receptor antagonists block vasoconstriction in the hamster cheek pouch microcirculation

We investigated the efficacy of novel neuropeptide Y (NPY) antagonists to inhibit the microcirculatory dynamics of NPY in the hamster cheek pouch microcirculation using intravital microscopy and computer-assisted image analysis. Changes in arteriolar diameter served as an index of vasomotor alterations. Fluorescein isothiocyanate-labeled Dextran 150 served as a tracer for measurements of macromolecular transport. GW 383 and GW 1229, two novel NPY receptor antagonists, were applied topically in separate experiments. Pretreatment with 10(-5), 10(-6) and 10(-7) M GW 383 and with 10(-6) and 10(-8) M GW 1229 attenuated the vasoconstriction induced by 10(-7) M NPY in a dose-dependent manner. Furthermore, pretreatment with 10(-7) and 10(-8) M GW 1229 significantly inhibited the 10(-9) M NPY-induced vasoconstriction. At these doses, the NPY antagonists did not alter microvascular permeability. Our results demonstrate that the novel NPY antagonists inhibit the vasoconstriction induced by NPY in the hamster check pouch microcirculation. We suggest that the inhibition is due to binding of antagonists to Y1-type NPY receptors.

GW1229, a novel neuropeptide Y Y1 receptor antagonist, inhibits the vasoconstrictor effect on neuropeptide Y in the hamster microcirculation

We studied the effect of GW1229, a novel neuropeptide Y Y1 receptor antagonists, on the vasoconstriction induced by neuropeptide Y and structurally related analogs in the hamster cheek pouch microcirculation. Changes in arteriolar diameter and microvascular conductance were assessed by intravital microscopy and measurement of sodium22 clearance. GW1229 did not affect basal vascular conductance but inhibited, concentration dependently, the reduction in arteriolar diameter and vascular conductance induced by 100 nM neuropeptide Y. GW1229 also counteracted the vasoconstrictor effect of 100 nM [Leu31,Pro34]neuropeptide Y, and that of 300 nM neuropeptide Y-[(13-36). In contrast, GW1229 had no effect on the vasoconstriction induced by noradrenaline. We conclude that the vasoconstrictor effect on neuropeptide Y in the hamster cheek pouch is mediated by neuropeptide Y Y1 receptors. The maintenance of physiological tone in this vascular bed does not involve the participation of endogenous neuropeptide Y.

Hemodynamic profile of neuropeptide Y in dogs: effect of ganglionic blockade

The hemodynamic effects of neuropeptide Y (NPY) were examined over a dose range of 0.03-30 nmol/kg, i.v., in anesthetized open-chest, ventilated dogs with and without ganglionic blockade. In normal (non-ganglion-blocked) animals, NPY produced significant, dose-dependent, and sustained (lasting 15-45 min) increases in mean arterial blood pressure and systemic vascular resistance (SVR) with a threshold dose of 0.3 nmol/kg and a maximum effective dose of 10 nmol/kg. Cardiac index (CI) decreased at doses > 1 nmol/kg, but stroke volume was not altered; heart rate (HR) decreased significantly at and above the 3 nmol/kg dose. No significant changes were observed in the left ventricular dP/dt (LVdP/dt) or the contractility index (LVdP/dt divided by systolic pressure). In ganglion-blocked animals, pressor and SVR responses to NPY were similar to those seen in normal animals but HR was not affected and a small but significant decrease in CI was seen only at the 30 nmol/kg. Furthermore, whereas LVdP/dt of ganglion-blocked dogs increased significantly at and above the 1 nmol/kg dose, the contractility index increased slightly only with the 10 and 30 nmol/kg doses. These data indicate that NPY produces sustained hypertension in dogs secondary to peripheral vasoconstriction, has a weak, direct positive inotropic action on the heart, and lacks chronotropic effects.

Hemodynamic characterization of a novel neuropeptide Y receptor antagonist

Defining the roles of the vasoconstrictor peptide neuropeptide Y (NPY) in the cardiovascular system is difficult due to lack of availability of specific NPY receptor antagonists. We report the in vivo NPY receptor blocking actions of a novel nonapeptide dimer, 1229U91 {(IleGluProDprTyrArgLeuArgTyrNH(2)(2)}, and describe its hemodynamic effects. In anesthetized normotensive rats, 1229U91 produced significant and dose-dependent reductions in NPY-reduced hemodynamic responses. 1229U91 (3-30 nmol/kg intravenously, i.v.) attenuated the pressor response (34 +/- 6-84 +/- 1%) and the increases in renal vascular resistance (RVR, 56 +/- 9-94 +/- 2%) produced by NPY (1 nmol/kg i.v.). Intravenous norepinephrine (NE)-induced hemodynamic responses were not altered by 1229U91. 1229U91 also produced dose-dependent inhibition of NPYinduced vasoconstrictor responses in anesthetized dogs and spontaneously hypertensive rats (SHR). These data demonstrate that 1229U91 is a selective NPY receptor antagonist. 1229U91 had no effect on resting hemodynamic variables in these preparations. In conscious SHR, 1229U91 did not produce significant changes in blood pressure (BP) or heart rate (HR) over a wide dose-range (15-1,500 nmol/kg i.v.). Lack of effect of the NPY receptor antagonist in SHR suggests that NPY does not contribute to the maintenance of BP in this hypertension model.

Synthesis, structure and stability of novel dimeric peptide-disulfides

Oxidation of nonapeptide dithiol (2) with K3Fe(CN)6 leads to either monomeric disulfide (4) or antiparallel and parallel dimeric disulfides (3a and 3b) depending upon reaction conditions. When exposed to small amounts of thiols or cyanide in aqueous solution, these three species interconvert to an equilibrium mixture of 2:1:8 (3a:3b:4).

High-affinity neuropeptide Y receptor antagonists

Neuropeptide Y (NPY) is one of the most abundant peptide transmitters in the mammalian brain. In the periphery it is costored and coreleased with norepinephrine from sympathetic nerve terminals. However, the physiological functions of this peptide remain unclear because of the absence of specific high-affinity receptor antagonists. Three potent NPY receptor antagonists were synthesized and tested for their biological activity in in vitro, ex vivo, and in vivo functional assays. We describe here the effects of these antagonists inhibiting specific radiolabeled NPY binding at Y1 and Y2 receptors and antagonizing the effects of NPY in human erythroleukemia cell intracellular calcium mobilization perfusion pressure in the isolated rat kidney, and mean arterial blood pressure in anesthetized rats.

Structure-activity relationship of novel pentapeptide neuropeptide Y receptor antagonists is consistent with a noncontinuous epitope for ligand-receptor binding

We report the first systematic study on short peptide structure affinity and activity for the neuropeptide Y (NPY) receptor. A series of linear pentapeptides has been synthesized that display affinities in the low micromolar range toward rat brain NPY receptors. Furthermore, some of these compounds competitively antagonize the Y1-type NPY receptor-mediated increase in cytosolic Ca2+ in human erythroleukemic (HEL) cells. The inactive NPY carboxyl-terminal pentapeptide (Thr-Arg-Gln-Arg-Tyr-NH2; IC50 > 100 microM) was modified by replacing threonine with an aromatic amino acid and glutamine with leucine. This resulted in a series of pentapeptides with dramatically improved affinity (IC50 = 0.5-4 microM) for the rat brain receptor. The structure-affinity data suggest that these peptides may represent a noncontinuous epitope containing the amino-terminal tyrosine and the carboxyl-terminal residues Arg-35 and Tyr-36 of NPY.

Novel modified carboxy terminal fragments of neuropeptide Y with high affinity for Y2-type receptors and potent functional antagonism at a Y1-type receptor

Peptide analogs of neuropeptide Y (NPY) with a Tyr-32 and Leu-34 replacement resulted in the decapeptide TyrIleAsnLeuIleTyrArgLeuArgTyr-NH2 (9; Table 1) and a 3700-fold improvement in affinity at Y2 (rat brain; IC50 = 8.2 +/- 3 nM) receptors when compared to the native NPY(27-36) C-terminal fragment. In addition, compound 9 was an agonist at Y1 (human erythroleukemia (HEL) cell; ED50 = 8.8 +/- 0.5 nM) receptors with potency comparable to that of NPY(1-36) (ED50 = 5 nM). Molecular dynamics and 1H-NMR were used to propose a solution structure of decapeptide 9 and for subsequent analog design. The replacement of Leu with Pro at position 4 of decapeptide 9 afforded an antagonist of NPY in HEL cells (18, TyrIleAsnProIleTyrArgLeuArgTyr-NH2; IC50 = 100 +/- 5 nM). Deletion of the N-terminal tyrosine of 18 resulted in a 10-fold improvement in antagonistic activity with a parallel 4-fold decrease in Y2 affinity. This potent antagonist may provide further insight into the physiological role(s) for NPY in the mammalian and peripheral nervous system.

The role of extracellular calcium in the neuropeptide-Y-induced increase in cytosolic calcium in human erythroleukemic (HEL) cells

Cytosolic calcium changes were followed in human erythroleukemic (HEL) cells loaded with the fluorescent probe fura-2. Peak increases in cytosolic calcium were reduced by two-thirds in cells suspended in Ca(2+)-free medium, suggesting that calcium entry significantly contributes to the increases in cytosolic calcium after NPY receptor stimulation. To establish if Ca2+ entry was a direct consequence of receptor stimulation or indirectly via depletion of Ca2+ stores, the latter were totally or partially depleted by treatment with cyclopiazonic acid or alpha-thrombin, respectively, in Ca(2+)-free medium. Partial depletion markedly diminished and full depletion suppressed the NPY-induced response in Ca(2+)-free medium. After full depletion, the recovery of the NPY-induced increase in cytosolic calcium was dependent on the length of [Ca2+]e reexposure, suggesting a direct entry of Ca2+ to the storage sites followed by release to the cytosol. After partial depletion, transient reexposure to [Ca2+]e did not by itself increase cytosolic calcium levels or refill the stores as NPY stimulation did not increase cytosolic calcium if [Ca2+]e was chelated prior to stimulation. However, if partially depleted cells were exposed to NPY in the presence of readded [Ca2+]3, the peak calcium response was similar to that of control cells, indicating that partially depleted calcium stores can be refilled from extracellular sources only if NPY receptors are stimulated. Analysis of the data suggests that in HEL cells the entry of calcium and mobilization from intracellular stores are in series processes and that entry is triggered by intracellular levels only under extreme depletion, while under physiological conditions calcium entry is coupled to receptor stimulation.

Microcirculatory dynamics of neuropeptide Y

We used the hamster cheek pouch microcirculation to investigate by intravital microscopy the effects of neuropeptide Y (NPY) on arteriolar diameter, leukocyte adhesion to microvascular endothelium, and postcapillary venular permeability. We applied NPY topically for 3 min at concentrations of 10(-7), 10(-9), and 10(-11) M. We quantified arteriolar diameter and permeability changes by digital image analysis. We used the mass of fluorescein isothiocyanate-Dextran 150 accumulated around postcapillary venules (10-30 microns) to calculate extravasation rates of macromolecules. We also measured the number of adhering white cells per 100-microns length of postcapillary venules using acridine orange to label white blood cells. At the applied doses, NPY did not alter either microvascular permeability to macromolecules or leukocyte adhesion to microvascular walls. NPY, in a dose-dependent manner, constricted arterioles ranging in control diameter from 10 to 60 microns. Vasoconstriction was strongest in arterioles ranging in diameter from 30 to 39 microns at a concentration of NPY of 10(-7) M. The Y1-type NPY receptor agonist, Leu31, Pro34-NPY, was as potent as NPY, whereas the carboxy-terminal fragment NPY 13-36 had no activity, indicating that the hamster cheek pouch microvasculature expresses the Y1 type of NPY receptor. We also blocked alpha-adrenergic receptors to test whether norepinephrine is required for NPY-induced vasoconstriction. This blockade did not inhibit the vasoconstriction caused by exogenous NPY. Our results demonstrate that (1) NPY modulates microvascular hemodynamics by changes in arteriolar diameter, (2) the NPY receptor on the hamster cheek pouch microvasculature is of the Y1 type, and (3) exogenous NPY-induced vasoconstriction is independent of the activity of endogenous norepinephrine.

Differences in the reserpine-sensitive storage in vivo of 1-methyl-4-phenylpyridinium in rats and mice may explain differences in catecholamine toxicity to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

Administration of reserpine, an inhibitor of vesicular catecholamine storage, differentially reduced the accumulation of MPP+ formed from MPTP in rats and mice. The effects were most pronounced in the adrenal gland for either species. In rats, reserpine decreased striatal and hippocampal MPP+ levels while in mice reserpine did not affect the disposition of MPP+ in the striatum but decreased hippocampal MPP+. The data suggest that mice may be more sensitive to the toxicant because less striatal MPP+ appears to be stored in the reserpine-sensitive storage vesicle.

Characterization of the neuropeptide Y-induced intracellular calcium release in human erythroleukemic cells

Human erythroleukemic (HEL) cells, loaded with fura-2, respond to neuropeptide Y (NPY) with a fast and transient increase in intracellular calcium. The Y1 receptor-specific agonist (Leu-31,Pro-34)-NPY is 4-fold more potent and the carboxyl-terminal fragment NPY13-36 is 150-fold less potent than NPY. Thus, it is concluded that the response is mediated through the activation of a Y1 type of NPY receptor. HEL cells do not respond to a second addition of NPY but do respond to a further addition of alpha-thrombin (alpha-T). However, in a calcium-free medium, prior stimulation with NPY largely inhibits a subsequent response to alpha-T. Moreover, prior stimulation with alpha-T in the absence of external calcium completely prevents the response to the addition of NPY, indicating a common effector pathway. The latter is further reinforced by using thapsigargin (TG), which has been shown to deplete the Inositol 1,4,5-trisphosphate-dependent calcium pool in other systems. HEL cells preincubated with TG in calcium-free medium fail to respond to either NPY or alpha-T. Likewise, prior stimulation with NPY or alpha-T in calcium-free medium significantly inhibits the response to TG. Preincubation of cells with phorbol esters strongly inhibits the NPY-induced release of intracellular Ca2+ in HEL cells, an effect that is partially prevented by preincubation of the cells with H7, a protein kinase C inhibitor. However, neither the homologous nor the apparent heterologous desensitization of the NPY receptor can be prevented by H7. It is concluded that NPY releases intracellular Ca2+ from an inositol 1,4,5-trisphosphate-sensitive calcium pool, which is restored by external calcium, and that NPY receptor desensitization is protein kinase C independent.

Multicompartmental secretion of ascorbate and its dual role in dopamine beta-hydroxylation

The neurobiological functions of ascorbate have both intra- and extracellular sites of action. Intracellularly, it participates predominantly in enzymic and transport reactions for neurotransmitter and hormone biosynthesis. Ascorbate is the cofactor for the dopamine beta-hydroxylase and peptidylglycine alpha-amidating monooxygenase systems, which catalyze the synthesis of norepinephrine and a variety of alpha-amidated peptides, respectively. The localization of these enzymes within the neurotransmitter- or hormone-containing storage vesicle requires a system for the constant regeneration of ascorbate to the reduced form. In fact, ascorbate participates in its own regeneration as a component of the vesicular electron-transport system. In addition to the roles of ascorbate in messenger synthesis, it is secreted from cells from different subcellular compartments. The extracellular role(s) of ascorbate are still unknown, although its interaction with and modification of plasma membrane proteins suggests some modulatory function.

Effect of intranigral Mn2+ on striatal and nigral synthesis and levels of dopamine and cofactor

A single injection of manganese chloride into the rat substantia nigra caused a significant and reversible drop in nigral and striatal dopamine and cofactor content ipsilateral to the lesion. Maximal decrease, in both tissues, was observed 60 days after the lesion, and showed complete recovery at 90 days. In vivo striatal tyrosine hydroxylation and GTP cyclohydrolase activities were also decreased maximally at 60 days and were recovered by 90 days after the lesion. No effects were observed on the side contralateral to the injection.

Diethyldithiocarbamate potentiates the neurotoxicity of in vivo 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and of in vitro 1-methyl-4-phenylpyridinium

Diethyldithiocarbamic acid (DDC) potentiates in vivo neurotoxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and in vitro neurotoxicity of 1-methyl-4-phenylpyridinium (MPP+). Male C57B1/6 mice were given two or five injections of MPTP (30 mg/kg i.p.) preceded 0.5 h by DDC (400 mg/kg i.p.). The mice were tested for catalepsy, akinesia, or motor activity during and after the period of dosing. Striatal and hippocampal tissues were obtained at 2 and 7 days following the last injection and evaluated for dopamine and norepinephrine levels, respectively. These same tissues were also analyzed for the levels of glial fibrillary acidic protein (GFAP), an astrocyte-localized protein known to increase in response to neural injury. Pretreatment with DDC potentiated the effect of MPTP in striatum and resulted in substantially greater dopamine depletion, as well as a more pronounced elevation in GFAP. In hippocampus, the levels of norepinephrine and GFAP were not different from controls in mice receiving only MPTP, but pretreatment with DDC resulted in a sustained depletion of norepinephrine and an elevation of GFAP, suggesting that damage was extended to this brain area by the combined treatment. Mice receiving MPTP preceded by DDC also demonstrated a more profound, but reversible, catalepsy and akinesia compared to those receiving MPTP alone. Systemically administered MPP+ decreased heart norepinephrine, but did not alter the striatal levels of dopamine or GFAP, and pretreatment with DDC did not alter these effects, but did increase lethality. DDC is known to increase brain levels of MPP+ after MPTP, but our data indicate that this is not due to a movement of peripherally generated MPP+ into CNS.(ABSTRACT TRUNCATED AT 250 WORDS)

Mechanisms of toxicity and cellular resistance to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and 1-methyl-4-phenylpyridinium in adrenomedullary chromaffin cell cultures

Bovine adrenomedullary chromaffin (BAMC) cells, cultured in a defined medium, were used to study the mechanisms of toxicity and cellular resistance to the catecholamine neuron toxicants 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridinium (MPP+). The viability of the cells was assessed biochemically [cellular catecholamine content and the catalytic activities of tyrosine hydroxylase (TH) and lactate dehydrogenase (LDH)] and anatomically (by electron microscopy). When cultures of BAMC cells were exposed to MPTP or MPP+ for 3 days, a marked loss of cellular catecholamines and TH activity was observed. The addition of an inhibitor of monoamine oxidase (MAO) B (Ro 19-6327), but not MAO A (clorgyline), prevented the toxicity of MPTP but not that of MPP+. In addition, the cellular toxicity of MPP+, but not MPTP, was antagonized by desmethylimipramine, an inhibitor of cellular catecholamine uptake. The toxicity of MPP+ was time dependent, with losses of TH and the release of cellular LDH occurring after 48 h in culture. Catecholamine depletion occurred somewhat sooner, being evident after 24 h of exposure to MPP+. The cellular toxicity of MPP+ was concentration dependent and significantly enhanced by inhibitors of catecholamine vesicular uptake (reserpine, tetrabenazine, or Ro 4-1284). Electron microscopic examination of cells treated with either MPP+, tetrabenazine, or their combination revealed that MPP+ damaged BAMC cells and that this damage was markedly potentiated by the inhibition of vesicular uptake by tetrabenazine. The concentration of glucose in the culture media of untreated cells slowly decreased as a function of time. The rate of glucose consumption was markedly accelerated by MPP+ treatment and the losses in cell TH and the release of LDH into the media were preceded by a 99% depletion of glucose from the media. In cultures not treated with MPP+, lactate accumulated in the media as a function of time. Addition of MPP+ to the media increased the formation of lactate, in a concentration-dependent manner. Reserpine pretreatment further enhanced the production of lactate in response to MPP+. Culturing cells in glucose-free medium greatly potentiated the effects of MPP+ on cellular TH and catecholamines. The toxicity observed after 3 days' exposure of BAMC cells to MPP+ could be prevented when the medium was replaced with fresh medium every 24 h. The effects of glucose deprivation and reserpine were observed to be additive. The ability of MPP+ to affect mitochondrial function is determined by the capacity of the storage vesicle to sequester the pyridinium, acting as a cytosolic "buffer."(ABSTRACT TRUNCATED AT 400 WORDS)

Carrier-independent entry of 1-methyl-4-phenylpyridinium (MPP+) into adrenal chromaffin cells as a consequence of charge delocalization

The administration of 1-methyl-4-phenylpyridinium (MPP+) to cultures of adrenal medullary chromaffin cells resulted in time and concentration-dependent increases in the cellular content of MPP+. Co-incubation of cells with MPP+, in the presence of desmethylimipramine (DMI), reduced but did not prevent the accumulation of the pyridinium in these cells. Similarly, DMI and MPP+ co-administration reduced but did not prevent the neurotoxicant-induced release of a cytosolic marker, lactate dehydrogenase, into the media. Molecular orbital calculations reveal that the positive charge of MPP+ is highly delocalized throughout the pyridinium ring and consequently MPP+ may be able to diffuse down concentration or charge gradients. Thus, these data provide a basis for the entry of MPP+ into cells and subcellular organelles that lack a catecholamine transporter, e.g. mitochondria.

Neuropeptide Y mobilizes intracellular Ca2+ and increases inositol phosphate production in human erythroleukemia cells

The intracellular concentration of free Ca2+ was monitored by measuring the fluorescence of fura-2 loaded Human Erythroleukemia Cells. Neuropeptide Y (NPY) increased intracellular Ca2+ in a dose-dependent manner and the 50% effective concentration was 2 nM. Chelation of extracellular Ca2+ by EGTA did not reduce the NPY-mediated increase in cytoplasmic Ca2+, indicating that the increase in fluorescence was due to the release of intracellular Ca2+. A second dose of NPY, after intracellular Ca2+ had returned to basal levels, failed to elicit a response, indicating that the NPY receptor had undergone desensitization. In similar experiments, NPY increased the formation of inositol phosphates, suggesting that the mobilization of Ca2+ from intracellular stores in HEL cells was secondary to the generation of inositol phosphates and stimulation of phospholipase C.

Characterization of cellular transport, subcellular distribution, and secretion of the neurotoxicant 1-methyl-4-phenylpyridinium in bovine adrenomedullary cell cultures

Cultures of bovine adrenomedullary chromaffin cells accumulated 1-[methyl-3H]methyl-4-phenylpyridinium ([3H]MPP+) in a time- and concentration-dependent manner with an apparent Km of 0.7 microM and a Vmax of 3 pmol/min/10(6) cells. The uptake was sodium dependent and sensitive to inhibitors of the cell-surface catecholamine transporter. At low concentrations of MPP+, the subcellular distribution was identical to that of endogenous catecholamines in the catecholamine-containing chromaffin vesicles. However, at a higher concentration of MPP+, a larger proportion of the toxicant was recovered in the cytosolic fraction, with less in the chromaffin vesicle fractions. When cells were prelabeled with [3H]MPP+, at 1 and 300 microM, and then permeabilized with digitonin in the absence of Ca2+, there was a proportionally greater release of MPP+ from the cells labeled at the higher concentration of the toxicant. In the presence of Ca2+, cell permeabilization induced a time-dependent secretion of catecholamines and a parallel secretion of MPP+. Under these conditions, the secretion of endogenous catecholamines was unaffected by the presence of MPP+. When the permeabilization studies were carried out in the presence of tetrabenazine, a massive release of MPP+ was observed in the absence of Ca2+ and was not further increased by Ca2+. In intact cells prelabeled with 300 microM [3H]MPP+, the secretagogues nicotine and veratridine elicited a Ca2+ -dependent secretion of catecholamines and MPP+ from the cells in similar proportions to their cellular contents. Barium-induced release of both species was independent of external Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Accumulation of 1-methyl-4-phenylpyridinium (MPP+) into bovine chromaffin granules results in a large restriction of its molecular motion: a 13C and 31P NMR study

13C labelled 1-methyl-4-phenylpyridinium ([13C-methyl]MPP+) is taken up by chromaffin granules and a signal from the methyl group within the granular matrix appears at 49.8 ppm. No shift of the [13C-Me] signal is observed upon incorporation into the granules. However the linewidth of the signal is significantly increased suggesting a restriction in the molecular motion of MPP+. Confirmation of the above derives from the determination of the T1 relaxation time of the [13C-Me] carbon of MPP+ which shows a four fold decrease upon incorporation of MPP+ into the granule. 31P NMR spectra of ATP in the presence of MPP+ reveals that the toxicant has low affinity for the nucleotide, suggesting that it is the viscosity of the granular matrix that restricts MPP+ mobility rather than a specific interaction with intragranular ATP.

Potentiation by reserpine and tetrabenazine of brain catecholamine depletions by MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) in the mouse; evidence for subcellular sequestration as basis for cellular resistance to the toxicant

Administration to mice of the neurotoxicant MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) decreased striatal dopamine and, to a lesser extent, hippocampal noradrenaline levels when measured 2 weeks after the last dose of MPTP. Reserpine and tetrabenazine, inhibitors of catecholamine vesicular transporter, potentiated the catecholamine depletions produced by MPTP in the hippocampus and striatum, respectively. These results are compatible with our hypothesis that sequestration of the toxic MPTP metabolite MPP+ (1-methyl-4-phenylpyridinium) in the catecholamine storage vesicle retards the catecholaminergic toxicity of MPTP.

Energy-driven uptake of the neurotoxin 1-methyl-4-phenylpyridinium into chromaffin granules via the catecholamine transporter

Chromaffin granules take up and concentrate 1-methyl-4-phenylpyridinium (MPP+) through a temperature-sensitive and saturable mechanism. The uptake displays an apparent Km of 51.2 microM and a Vmax of 7.1 nmol/min/mg of protein. MPP+ uptake is markedly depressed in the absence of ATP or by inhibition of the membrane Mg2+-dependent ATPase, and it is completely blocked by reserpine. Reversal of the membrane potential by carbonyl cyanide m-chlorophenylhydrazone or dissipation of the pH gradient in the presence of nigericin plus potassium ions produces a marked inhibition of MPP+ uptake indicating that the process is dependent upon the integrity of the transmembrane proton electrochemical gradient generated and maintained by the membrane Mg2+-dependent ATPase. Furthermore, the data shows that a permanently charged compound is capable of entering the granule through the catecholamine carrier.

Subcellular compartmentalization of 1-methyl-4-phenylpyridinium with catecholamines in adrenal medullary chromaffin vesicles may explain the lack of toxicity to adrenal chromaffin cells

Cultures of bovine adrenomedullary chromaffin cells accumulated 1-methyl-4-phenylpyridinium (MPP+) in a time- and concentration-dependent manner by a process that was prevented by desmethylimipramine. The subcellular localization of the incorporated [methyl-3H]MPP+ was examined by differential centrifugation and sucrose density gradient fractionation and was found to be predominantly colocalized with catecholamines in chromaffin vesicles, and negligible amounts were detected within the mitochondrial fraction. When chromaffin cell membranes were made permeable with the detergent digitonin in the absence of calcium, there was no increase in the release of [3H]MPP+, indicating that there is negligible accumulation of the neurotoxin in the cytosol. Simultaneous exposure to digitonin and calcium induced cosecretion of MPP+ and catecholamines. Stimulation of the cells with nicotine released both catecholamines and MPP+ at identical rates and percentages of cellular content in a calcium-dependent manner. Last, when cells were incubated with MPP+ in the presence of tetrabenazine (an inhibitor of vesicular uptake), the chromaffin cell toxicity of MPP+ was potentiated. We submit that the ability of the chromaffin cells to take up and store MPP+ in the chromaffin vesicle prevents the toxin's interaction with other structures and, thus, prevents cell damage. As an extension of this hypothesis, the relative resistance of some brain monoaminergic neurons to the toxic actions of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine may result from the subcellular sequestration of MPP+ in the storage vesicle.

Rat striatal dopamine and tetrahydrobiopterin content following an intrastriatal injection of manganese chloride

Injection of manganese into the rat corpus striatum causes a rapid fall in the biopterin and dopamine (DA) content ipsilateral to the lesion. Two weeks after the lesion both biopterin and DA are partially recovered. Controls, injected with saline or magnesium, do not show alterations in their DA or cofactor levels. It is proposed that the fall in DA levels results from a rapid displacement of the amine from its storage sites by manganese followed by a decrease in the rate of DA synthesis causes by the drop in cofactor levels.

Uptake of magnesium by chromaffin granules in vitro: role of the proton electrochemical gradient

The chromaffin granule membrane in vitro is impermeable to protons as well as to Mg2+; however, when granules are incubated in the presence of the proton ionophore carbonyl cyanide p-trifluoromethoxy-phenylhydrazone or an inhibitor of the granule membrane Mg2+-dependent ATPase, the metal ion is accumulated inside the granules. This accumulation is dependent upon the granule transmembrane potential. The simultaneous presence of the ATPase inhibitor and the proton ionophore markedly increases metal ion incorporation. Mg2+ incorporation is also promoted by nigericin in the presence of potassium or sodium ions, indicating that Mg2+ accumulation is also dependent upon the transmembrane pH gradient. Concomitant with the Mg2+ accumulation, there is a significant loss of endogenous catecholamines. It is concluded that Mg2+ accumulation is determined by the electrochemical gradient maintained across the membrane. Once the metal ion has accumulated into the granules it displaces catecholamines from their storage sites.

Characterization of ascorbic acid transport by adrenomedullary chromaffin cells. Evidence for Na+-dependent co-transport

Ascorbic acid transport by bovine adrenomedullary chromaffin cells in primary cultures has been characterized. Ascorbic acid uptake can be measured by either high performance liquid chromatography with electrochemical detection or radiometric techniques with L-[1-14C]ascorbic acid. The transport system is temperature- and energy-dependent and exhibits Michaelis-Menten kinetics with an apparent Km of 29 microM when the external Na+ concentration is 150 mM. Uptake of ascorbate by chromaffin cells is ouabain-sensitive and dependent on the presence of external Na+. Ascorbate transport by chromaffin cells is, thus, an active process driven by the Na+ electrochemical gradient. The kinetics of this co-transport system fits an "affinity type" model where binding of Na+ to the carrier increases the affinity to ascorbate and vice versa. Thus, the data suggest that binding of either Na+ or ascorbate induces a conformational change in the transporter, which results in a change in the association constant for the second ligand while the mobility of the carrier remains unchanged. Cellular uptake of ascorbate into adrenomedullary chromaffin cells appears to be followed by its distribution into several subcellular compartments. One subcellular compartment for concentration of ascorbate is the chromaffin vesicle where it accumulates at a relatively slow rate. The interrelationships between ascorbate transport and other aspects of ascorbate metabolism and chromaffin vesicle function and dopamine beta-hydroxylation are also considered.

Biochemical and functional evidence for the cosecretion of multiple messengers from single and multiple compartments

Examination of the secretory profile and subcellular localization of some of the multiple export products of the adrenal medullary chromaffin cells indicates that several compartments (chromaffin vesicle, lysosomes, endoplasmic reticulum) are coupled to specific receptors and to cell depolarization through Ca2+-dependent mechanism(s). The activation of the release process results in the concerted cosecretion of endogenous catecholamines, newly incorporated catecholamines, adenine nucleotides, chromogranins, dopamine beta-hydroxylase (EC 1.14.17.1), enkephalins and related opioid peptides, stored ascorbate and newly incorporated ascorbate, lysosomal hydrolases, and soluble acetylcholinesterase. This complex organization for the coexistence of these multiple putative messengers and their cosecretion may be relevant to other endocrine cells and neurons where coexistence of transmitters has been found. This coexistence in multiple secretory compartments may provide the subcellular basis for independent regulation of the synthesis, packaging, and secretion of individual transmitters within the multiplicity of putative messengers secreted by a particular endocrine cell or nerve terminal.

Secretion of newly taken up ascorbic acid by adrenomedullary chromaffin cells originates from a compartment different from the catecholamine storage vesicle

Chromaffin cells in primary culture take up [14C]ascorbic acid from the incubation medium. Cells, stimulated immediately after a short labeling period with [14C]ascorbate, secrete ascorbic acid concomitantly with catecholamines (CA) through a nicotinic receptor-mediated Ca2+-dependent process. A proportional release of CA and [14C]ascorbic acid was observed through a large range of secretion rates obtained by varying the concentration of nicotine or by changing the concentrations of Ca2+ and Na+ in the external medium. However, under the same conditions of stimulation, different cell preparations secrete 2-10 times more CA than [14C]ascorbate (as percentage of cell content). Furthermore, a different time course of secretion was observed for CA and [14C]ascorbate for each of several secretagogues. In addition, Ba2+ is a much more potent stimulus for CA secretion than for secretion of [14C]ascorbate, and Ca2+ channel blockers are more potent in inhibiting CA secretion than [14C]ascorbate secretion. These data suggested the possibility that newly taken up ascorbate was being secreted from a compartment altogether distinct from the chromaffin vesicle. This hypothesis was confirmed by subcellular distribution studies, where only a minor fraction of newly taken up [14C]ascorbate was found in the vesicular fraction (P2) from homogenates of chromaffin cells prepared after a short incubation with [14C]ascorbate. However, the subcellular distribution of [14C]ascorbate follows that of endogenous ascorbate when a short pulse with the label is chased by a prolonged equilibration period in the absence of ascorbate, indicating that a transfer has occurred from the extravesicular compartment(s) to the CA storage organelle. Endogenous ascorbate, which is found both inside and outside the chromaffin vesicle, was also found to be secreted from chromaffin cells, indicating that ascorbic acid could be released simultaneously from two different subcellular compartments.

Secretion of newly taken-up ascorbic acid by adrenomedullary chromaffin cells

Primary cultures of bovine adrenomedullary cells accumulate carbon-14-labeled ascorbic acid through a saturable and energy-dependent process. The newly taken-up ascorbate is released concomitantly with catecholamines upon stimulation of chromaffin cell secretion. The release of ascorbate is Ca2+-dependent and mediated through activation of nicotinic receptors. These results indicate that exogenous ascorbate taken up into chromaffin cells is incorporated in situ into a secretable compartment, probably the catecholamine-containing chromaffin vesicles.

Intraneuronal site of action for imipramine in rat striatal slices

The uptake of 3H-labeled imipramine ([3H]IMI) in rat carpus striatum slices was found to be strongly temperature-dependent. The accumulation was shown to be saturable and two apparent Km's were found: 2.2 x 10(-8) and 3.5 x 10(-7) M. Once incorporated, the labeled drug was released from superfused slices by K+ (55 mM) depolarization in the presence of calcium ions. Imipramine was also studied for its ability to induce the release of [3H]dopamine ([2H]DA) which had been previously accumulated by striatal slices. It was found that striatal slices superfused during 1 or 6 min with imipramine (10(-6) - 10(-4) M) release substantial amounts of radioactive dopamine, independently of the presence of Ca2+ in the medium. This release is completely abolished after reserpine pretreatment. It is proposed that imipramine enters the dopaminergic storage vesicles and displaces dopamine. An intraneuronal mechanism of action for imipramine is discussed.