Opposing effects of the purinergic P2X7 receptor on seizures in neurons and microglia in male mice

BACKGROUND

The purinergic ATP-gated P2X7 receptor (P2X7R) is increasingly recognized to contribute to pathological neuroinflammation and brain hyperexcitability. P2X7R expression has been shown to be increased in the brain, including both microglia and neurons, in experimental models of epilepsy and patients. To date, the cell type-specific downstream effects of P2X7Rs during seizures remain, however, incompletely understood.

METHODS

Effects of P2X7R signaling on seizures and epilepsy were analyzed in induced seizure models using male mice including the kainic acid model of status epilepticus and pentylenetetrazole model and in male and female mice in a genetic model of Dravet syndrome. RNA sequencing was used to analyze P2X7R downstream signaling during seizures. To investigate the cell type-specific role of the P2X7R during seizures and epilepsy, we generated mice lacking exon 2 of the P2rx7 gene in either microglia (P2rx7:Cx3cr1-Cre) or neurons (P2rx7:Thy-1-Cre). To investigate the protective potential of overexpressing P2X7R in GABAergic interneurons, P2X7Rs were overexpressed using adeno-associated virus transduction under the mDlx promoter.

RESULTS

RNA sequencing of hippocampal tissue from wild-type and P2X7R knock-out mice identified both glial and neuronal genes, in particular genes involved in GABAergic signaling, under the control of the P2X7R following seizures. Mice with deleted P2rx7 in microglia displayed less severe acute seizures and developed a milder form of epilepsy, and microglia displayed an anti-inflammatory molecular profile. In contrast, mice lacking P2rx7 in neurons showed a more severe seizure phenotype when compared to epileptic wild-type mice. Analysis of single-cell expression data revealed that human P2RX7 expression is elevated in the hippocampus of patients with temporal lobe epilepsy in excitatory and inhibitory neurons. Functional studies determined that GABAergic interneurons display increased responses to P2X7R activation in experimental epilepsy. Finally, we show that viral transduction of P2X7R in GABAergic interneurons protects against evoked and spontaneous seizures in experimental temporal lobe epilepsy and in mice lacking Scn1a, a model of Dravet syndrome.

CONCLUSIONS

Our results suggest a dual and opposing action of P2X7R in epilepsy and suggest P2X7R overexpression in GABAergic interneurons as a novel therapeutic strategy for acquired and, possibly, genetic forms of epilepsy.

María Teresa Miras Portugal: a pioneer in the study of purinoceptors in chromaffin cells

María Teresa Miras Portugal devoted most of her scientific life to the study of purinergic signalling. In an important part of her work, she used a model system: the chromaffin cells of the adrenal medulla. It was in these cells that she identified diadenosine polyphosphates, from which she proceeded to the study of adrenomedullary purinome: nucleotide synthesis and degradation, adenosine transport, nucleotide uptake into chromaffin granules, exocytotic release of nucleotides and autocrine regulation of chromaffin cell function via purinoceptors. This short review will focus on the current state of knowledge of the purinoceptors of adrenal chromaffin cells, a subject to which María Teresa made seminal contributions and which she continued to study until the end of her scientific life.

β-Lactam TRPM8 Antagonist RGM8-51 Displays Antinociceptive Activity in Different Animal Models

Transient receptor potential melastatin subtype 8 (TRPM8) is a cation channel extensively expressed in sensory neurons and implicated in different painful states. However, the effectiveness of TRPM8 modulators for pain relief is still a matter of discussion, since structurally diverse modulators lead to different results, depending on the animal pain model. In this work, we described the antinociceptive activity of a β–lactam derivative, RGM8-51, showing good TRPM8 antagonist activity, and selectivity against related thermoTRP channels and other pain-mediating receptors. In primary cultures of rat dorsal root ganglion (DRG) neurons, RGM8-51 potently reduced menthol-evoked neuronal firing without affecting the major ion conductances responsible for action potential generation. This compound has in vivo antinociceptive activity in response to cold, in a mouse model of oxaliplatin-induced peripheral neuropathy. In addition, it reduces cold, mechanical and heat hypersensitivity in a rat model of neuropathic pain arising after chronic constriction of the sciatic nerve. Furthermore, RGM8-51 exhibits mechanical hypersensitivity-relieving activity, in a mouse model of NTG-induced hyperesthesia. Taken together, these preclinical results substantiate that this TRPM8 antagonist is a promising pharmacological tool to study TRPM8-related diseases.

Regulation of the voltage-dependent sodium channel NaV1.1 by AKT1

The voltage-sensitive sodium channel Na_V1.1 plays a critical role in regulating excitability of GABAergic neurons and mutations in the corresponding gene are associated to Dravet syndrome and other forms of epilepsy. The activity of this channel is regulated by several protein kinases. To identify novel regulatory kinases we screened a library of activated kinases and we found that AKT1 was able to directly phosphorylate Na_V1.1. In vitro kinase assays revealed that the phosphorylation site was located in the C-terminal part of the large intracellular loop connecting domains I and II of Na_V1.1, a region that is known to be targeted by other kinases like PKA and PKC. Electrophysiological recordings revealed that activated AKT1 strongly reduced peak Na⁺ currents and displaced the inactivation curve to more negative potentials in HEK-293 cell stably expressing Na_V1.1. These alterations in current amplitude and steady-state inactivation were mimicked by SC79, a specific activator of AKT1, and largely reverted by triciribine, a selective inhibitor. Neurons expressing endogenous Na_V1.1 in primary cultures were identified by expressing a fluorescent protein under the Na_V1.1 promoter. There, we also observed a strong decrease in the current amplitude after addition of SC79, but small effects on the inactivation parameters. Altogether, we propose a novel mechanism that might regulate the excitability of neural networks in response to AKT1, a kinase that plays a pivotal role under physiological and pathological conditions, including epileptogenesis.

The Adrenal Medulla Modulates Mechanical Allodynia in a Rat Model of Neuropathic Pain

We have investigated whether the stress response mediated by the adrenal medulla in rats subjected to chronic constriction injury of the sciatic nerve (CCI) modulates their nocifensive behavior. Treatment with SK29661 (300 mg/kg; intraperitoneal (I.P.)), a selective inhibitor of phenylethanolamine N-methyltransferase (PNMT) that converts noradrenaline (NA) into adrenaline (A), fully reverted mechanical allodynia in the injured hind paw without affecting mechanical sensitivity in the contralateral paw. The effect was fast and reversible and was associated with a decrease in the A to NA ratio (A/NA) in the adrenal gland and circulating blood, an A/NA that was elevated by CCI. 1,2,3,4-tetrahydroisoquinoline-7-sulfonamide (SKF29661) did not affect exocytosis evoked by Ca²⁺ entry as well as major ionic conductances (voltage-gated Na⁺, Ca²⁺, and K⁺ channels, nicotinic acetylcholine receptors) involved in stimulus-secretion coupling in chromaffin cells, suggesting that it acted by changing the relative content of the two adrenal catecholamines. Denervation of the adrenal medulla by surgical splanchnectomy attenuated mechanical allodynia in neuropathic animals, hence confirming the involvement of the adrenal medulla in the pathophysiology of the CCI model. Inhibition of PNMT appears to be an effective and probably safe way to modulate adrenal medulla activity and, in turn, to alleviate pain secondary to the injury of a peripheral nerve.

Live Imaging Reveals Cerebellar Neural Stem Cell Dynamics and the Role of VNUT in Lineage Progression

Little is known about the intrinsic specification of postnatal cerebellar neural stem cells (NSCs) and to what extent they depend on information from their local niche. Here, we have used an adapted cell preparation of isolated postnatal NSCs and live imaging to demonstrate that cerebellar progenitors maintain their neurogenic nature by displaying hallmarks of NSCs. Furthermore, by using this preparation, all the cell types produced postnatally in the cerebellum, in similar relative proportions to those observed in vivo, can be monitored. The fact that neurogenesis occurs in such organized manner in the absence of signals from the local environment, suggests that cerebellar lineage progression is to an important extent governed by cell-intrinsic or pre-programmed events. Finally, we took advantage of the absence of the niche to assay the influence of the vesicular nucleotide transporter inhibition, which dramatically reduced the number of NSCs in vitro by promoting their progression toward neurogenesis.

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We present a preparation that allows monitoring the behavior of cerebellar NSCs

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Isolated NSCs maintain their neurogenic nature in absence of niche factors

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The model enables monitoring the three postnatal cerebellar niches simultaneously

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VNUT influences the balance between quiescence and activation of cerebellar NSCs

Identification of IQM-266, a Novel DREAM Ligand That Modulates KV4 Currents

Downstream Regulatory Element Antagonist Modulator (DREAM)/KChIP3/calsenilin is a neuronal calcium sensor (NCS) with multiple functions, including the regulation of A-type outward potassium currents (I _A). This effect is mediated by the interaction between DREAM and K_V4 potassium channels and it has been shown that small molecules that bind to DREAM modify channel function. A-type outward potassium current (I _A) is responsible of the fast repolarization of neuron action potentials and frequency of firing. Using surface plasmon resonance (SPR) assays and electrophysiological recordings of K_V4.3/DREAM channels, we have identified IQM-266 as a DREAM ligand. IQM-266 inhibited the K_V4.3/DREAM current in a concentration-, voltage-, and time-dependent-manner. By decreasing the peak current and slowing the inactivation kinetics, IQM-266 led to an increase in the transmembrane charge ( Q K V 4.3 / DREAM ) at a certain range of concentrations. The slowing of the recovery process and the increase of the inactivation from the closed-state inactivation degree are consistent with a preferential binding of IQM-266 to a pre-activated closed state of K_V4.3/DREAM channels. Finally, in rat dorsal root ganglion neurons, IQM-266 inhibited the peak amplitude and slowed the inactivation of I _A. Overall, the results presented here identify IQM-266 as a new chemical tool that might allow a better understanding of DREAM physiological role as well as modulation of neuronal I _A in pathological processes.

Overexpression of P2X3 and P2X7 Receptors and TRPV1 Channels in Adrenomedullary Chromaffin Cells in a Rat Model of Neuropathic Pain

We have tested the hypothesis that neuropathic pain acting as a stressor drives functional plasticity in the sympathoadrenal system. The relation between neuropathic pain and adrenal medulla function was studied with behavioral, immunohistochemical and electrophysiological techniques in rats subjected to chronic constriction injury of the sciatic nerve. In slices of the adrenal gland from neuropathic animals, we have evidenced increased cholinergic innervation and spontaneous synaptic activity at the splanchnic nerve–chromaffin cell junction. Likewise, adrenomedullary chromaffin cells displayed enlarged acetylcholine-evoked currents with greater sensitivity to α-conotoxin RgIA, a selective blocker of α9 subunit-containing nicotinic acetylcholine receptors, as well as increased exocytosis triggered by voltage-activated Ca²⁺ entry. Altogether, these adaptations are expected to facilitate catecholamine output into the bloodstream. Last, but most intriguing, functional and immunohistochemical data indicate that P2X3 and P2X7 purinergic receptors and transient receptor potential vanilloid-1 (TRPV1) channels are overexpressed in chromaffin cells from neuropathic animals. These latter observations are reminiscent of molecular changes characteristic of peripheral sensitization of nociceptors following the lesion of a peripheral nerve, and suggest that similar phenomena can occur in other tissues, potentially contributing to behavioral manifestations of neuropathic pain.

MicroRNA-22 Controls Aberrant Neurogenesis and Changes in Neuronal Morphology After Status Epilepticus

Prolonged seizures (status epilepticus, SE) may drive hippocampal dysfunction and epileptogenesis, at least partly, through an elevation in neurogenesis, dysregulation of migration and aberrant dendritic arborization of newly-formed neurons. MicroRNA-22 was recently found to protect against the development of epileptic foci, but the mechanisms remain incompletely understood. Here, we investigated the contribution of microRNA-22 to SE-induced aberrant adult neurogenesis. SE was induced by intraamygdala microinjection of kainic acid (KA) to model unilateral hippocampal neuropathology in mice. MicroRNA-22 expression was suppressed using specific oligonucleotide inhibitors (antagomir-22) and newly-formed neurons were visualized using the thymidine analog iodo-deoxyuridine (IdU) and a green fluorescent protein (GFP)-expressing retrovirus to visualize the dendritic tree and synaptic spines. Using this approach, we quantified differences in the rate of neurogenesis and migration, the structure of the apical dendritic tree and density and morphology of dendritic spines in newly-formed neurons.SE resulted in an increased rate of hippocampal neurogenesis, including within the undamaged contralateral dentate gyrus (DG). Newly-formed neurons underwent aberrant migration, both within the granule cell layer and into ectopic sites. Inhibition of microRNA-22 exacerbated these changes. The dendritic diameter and the density and average volume of dendritic spines were unaffected by SE, but these parameters were all elevated in mice in which microRNA-22 was suppressed. MicroRNA-22 inhibition also reduced the length and complexity of the dendritic tree, independently of SE. These data indicate that microRNA-22 is an important regulator of morphogenesis of newly-formed neurons in adults and plays a role in supressing aberrant neurogenesis associated with SE.

Alpha2-adrenoceptors in adrenomedullary chromaffin cells: functional role and pathophysiological implications

Chromaffin cells from the adrenal medulla participate in stress responses by releasing catecholamines into the bloodstream. Main control of adrenal catecholamine secretion is exerted both neurally (by the splanchnic nerve fibers) and humorally (by corticosteroids, circulating noradrenaline, etc.). It should be noted, however, that secretory products themselves (catecholamines, ATP, opioids, ascorbic acid, chromogranins) could also influence the secretory response in an autocrine/paracrine manner. This form of control is activity-dependent and can be either inhibitory or excitatory. Among the inhibitory influences, it stands out the one mediated by α₂-adrenergic autoreceptors activated by released catecholamines. α₂-adrenoceptors are G protein-coupled receptors capable to inhibit exocytotic secretion through a direct interaction of Gβγ subunits with voltage-gated Ca²⁺ channels. Interestingly, upon intense and/or prolonged stimulation, α₂-adrenergic receptors become desensitized by the intervention of G protein-coupled receptor kinase 2 (GRK2). In several experimental models of heart failure, there has been reported the up-regulation of GRK2 and the loss of functioning of inhibitory α₂-adrenoceptors resulting in enhanced release of adrenomedullary catecholamines. Given the importance of circulating catecholamines in the pathophysiology of heart failure, the recovery of α₂-adrenergic modulation of the secretory response from chromaffin cells appears as a novel strategy for a better control of the patients with this cardiac disease.

Neurodevelopmental alterations and seizures developed by mouse model of infantile hypophosphatasia are associated with purinergic signalling deregulation

Hypomorphic mutations in the gene encoding the tissue-nonspecific alkaline phosphatase (TNAP) enzyme, ALPL in human or Akp2 in mice, cause hypophosphatasia (HPP), an inherited metabolic bone disease also characterized by spontaneous seizures. Initially, these seizures were attributed to the impairment of GABAergic neurotransmission caused by altered vitamin B6 (vit-B6) metabolism. However, clinical cases in human newborns and adults whose convulsions are refractory to pro-GABAergic drugs but controlled by the vit-B6 administration, suggest that other factors are involved. Here, to evaluate whether neurodevelopmental alterations are underlying the seizures associated to HPP, we performed morphological and functional characterization of postnatal homozygous TNAP null mice, a model of HPP. These analyses revealed that TNAP deficient mice present an increased proliferation of neural precursors, an altered neuronal morphology, and an augmented neuronal activity. We found that these alterations were associated with a partial downregulation of the purinergic P2X7 receptor (P2X7R). Even though deficient P2X7R mice present similar neurodevelopmental alterations, they do not develop neonatal seizures. Accordingly, we found that the additional blockage of P2X7R prevent convulsions and extend the lifespan of mice lacking TNAP. In agreement with these findings, we also found that exogenous administration of ATP or TNAP antagonists induced seizures in adult wild-type mice by activating P2X7R. Finally, our results also indicate that the anticonvulsive effects attributed to vit-B6 may be due to its capacity to block P2X7R. Altogether, these findings suggest that the purinergic signalling regulates the neurodevelopmental alteration and the neonatal seizures associated to HPP.

microRNA targeting of the P2X7 purinoceptor opposes a contralateral epileptogenic focus in the hippocampus

The ATP-gated ionotropic P2X7 receptor (P2X7R) modulates glial activation, cytokine production and neurotransmitter release following brain injury. Levels of the P2X7R are increased in experimental and human epilepsy but the mechanisms controlling P2X7R expression remain poorly understood. Here we investigated P2X7R responses after focal-onset status epilepticus in mice, comparing changes in the damaged, ipsilateral hippocampus to the spared, contralateral hippocampus. P2X7R-gated inward currents were suppressed in the contralateral hippocampus and P2rx7 mRNA was selectively uploaded into the RNA-induced silencing complex (RISC), suggesting microRNA targeting. Analysis of RISC-loaded microRNAs using a high-throughput platform, as well as functional assays, suggested the P2X7R is a target of microRNA-22. Inhibition of microRNA-22 increased P2X7R expression and cytokine levels in the contralateral hippocampus after status epilepticus and resulted in more frequent spontaneous seizures in mice. The major pro-inflammatory and hyperexcitability effects of microRNA-22 silencing were prevented in P2rx7^−/− mice or by treatment with a specific P2X7R antagonist. Finally, in vivo injection of microRNA-22 mimics transiently suppressed spontaneous seizures in mice. The present study supports a role for post-transcriptional regulation of the P2X7R and suggests therapeutic targeting of microRNA-22 may prevent inflammation and development of a secondary epileptogenic focus in the brain.

Nucleotides in neuroregeneration and neuroprotection

Brain injury generates the release of a multitude of factors including extracellular nucleotides, which exhibit bi-functional properties and contribute to both detrimental actions in the acute phase and also protective and reparative actions in the later recovery phase to allow neuroregeneration. A promising strategy toward restoration of neuronal function is based on activation of endogenous adult neural stem/progenitor cells. The implication of purinergic signaling in stem cell biology, including regulation of proliferation, differentiation, and cell death has become evident in the last decade. In this regard, current strategies of acute transplantation of ependymal stem/progenitor cells after spinal cord injury restore altered expression of P2X4 and P2X7 receptors and improve functional locomotor recovery. The expression of both receptors is transcriptionally regulated by Sp1 factor, which plays a key role in the startup of the transcription machinery to induce regeneration-associated genes expression. Finally, general signaling pathways triggered by nucleotide receptors in neuronal populations converge on several intracellular kinases, such as PI3K/Akt, GSK3 and ERK1,2, as well as the Nrf-2/heme oxigenase-1 axis, which specifically link them to neuroprotection. In this regard, regulation of dual specificity protein phosphatases can become novel mechanism of actions for nucleotide receptors that associate them to cell homeostasis regulation. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Role of P2X7 and P2Y2 receptors on α-secretase-dependent APP processing: Control of amyloid plaques formation "in vivo" by P2X7 receptor

Amyloid precursor protein (APP) is expressed in a large variety of neural and non-neural cells. The balance between non-pathogenic and pathologic forms of APP processing, mediated by α-secretase and β-secretase respectively, remains a crucial step to understand β-amyloid, Aβ42 peptide, formation and aggregation that are at the origin of the senile plaques in the brain, a characteristic hallmark of Alzheimer's disease (AD). In Neuro-2a, a neuroblastoma cell line that constitutively expresses APP, activation of the P2X7 receptor leads to reduction of α-secretase activity, the opposite effect being obtained by P2Y₂ receptor activation.

The in vivo approach was made possible by the use of J20 mice, a transgenic mouse model of familial Alzheimer's disease (FAD) expressing human APP mutant protein. This animal exhibits prominent amyloid plaques by six months of age. In vivo inhibition of the P2X7 receptor induced a significant decrease in the number and size of hippocampal amyloid plaques. This reduction is mediated by an increase in the proteolytic processing of APP through α-secretase activity, which correlates with an increase in the phosphorylated form of GSK-3, a less active form of this enzyme. The in vivo findings corroborate the therapeutic potential of P2X7 antagonists in the treatment of FAD.

Purinergic P2X7 receptors mediate cell death in mouse cerebellar astrocytes in culture

The brain distribution and functional role of glial P2X7 receptors are broader and more complex than initially anticipated. We characterized P2X7 receptors from cerebellar astrocytes at the molecular, immunocytochemical, biophysical, and cell physiologic levels. Mouse cerebellar astrocytes in culture express mRNA coding for P2X7 receptors, which is translated into P2X7 receptor protein as proven by Western blot analysis and immunocytochemistry. Fura-2 imaging showed cytosolic calcium responses to ATP and the synthetic analog 3'-O-(4-benzoyl)benzoyl-ATP (BzATP) exhibited two components, namely an initial transient and metabotropic component followed by a sustained one that depended on extracellular calcium. This latter component, which was absent in astrocytes from P2X7 receptor knockout mice (P2X7 KO), was modulated by extracellular Mg(2+), and was sensitive to Brilliant Blue G (BBG) and 3-(5-(2,3-dichlorophenyl)-1H-tetrazol-1-yl)methyl pyridine (A438079) antagonism. BzATP also elicited inwardly directed nondesensitizing whole-cell ionic currents that were reduced by extracellular Mg(2+) and P2X7 antagonists (BBG and calmidazolium). In contrast to that previously reported in rat cerebellar astrocytes, sustained BzATP application induced a gradual increase in membrane permeability to large cations, such as N-methyl-d-glucamine and 4-[3-methyl-2(3H)-benzoxazolylidene)-methyl]-1-[3-(triethylammonio)propyl]diiodide, which ultimately led to the death of mouse astrocytes. Cerebellar astrocyte cell death was prevented by BBG but not by calmidazolium, removal of extracellular calcium, or treatment with the caspase-3 inhibitor, benzyloxycarbonyl-Asp(OMe)-Glu(OMe)-Val-Asp(OMe)-fluoromethylketone, thus suggesting a necrotic-type mechanism of cell death. Since this cellular response was not observed in astrocytes from P2X7 KO mice, this study suggests that stimulation of P2X7 receptor may convey a cell death signal to cerebellar astrocytes in a species-specific manner.

P2X7 receptors trigger ATP exocytosis and modify secretory vesicle dynamics in neuroblastoma cells

Previously, we reported that purinergic ionotropic P2X7 receptors negatively regulate neurite formation in Neuro-2a (N2a) mouse neuroblastoma cells through a Ca²⁺/calmodulin-dependent kinase II-related mechanism. In the present study we used this cell line to investigate a parallel though faster P2X7 receptor-mediated signaling pathway, namely Ca²⁺-regulated exocytosis. Selective activation of P2X7 receptors evoked exocytosis as assayed by high resolution membrane capacitance measurements. Using dual-wavelength total internal reflection microscopy, we have observed both the increase in near-membrane Ca²⁺ concentration and the exocytosis of fluorescently labeled vesicles in response to P2X7 receptor stimulation. Moreover, activation of P2X7 receptors also affects vesicle motion in the vertical and horizontal directions, thus, involving this receptor type in the control of early steps (docking and priming) of the secretory pathway. Immunocytochemical and RT-PCR experiments evidenced that N2a cells express the three neuronal SNAREs as well as vesicular nucleotide and monoamine (VMAT-1 and VMAT-2) transporters. Biochemical measurements indicated that ionomycin induced a significant release of ATP from N2a cells. Finally, P2X7 receptor stimulation and ionomycin increased the incidence of small transient inward currents, reminiscent of postsynaptic quantal events observed at synapses. Small transient inward currents were dependent on extracellular Ca²⁺ and were abolished by Brilliant Blue G, suggesting they were mediated by P2X7 receptors. Altogether, these results suggest the existence of a positive feedback mechanism mediated by P2X7 receptor-stimulated exocytotic release of ATP that would act on P2X7 receptors on the same or neighbor cells to further stimulate its own release and negatively control N2a cell differentiation.

Ca2+/calmodulin-dependent kinase II signalling cascade mediates P2X7 receptor-dependent inhibition of neuritogenesis in neuroblastoma cells

ATP, via purinergic P2X receptors, acts as a neurotransmitter and modulator in both the central and peripheral nervous systems, and is also involved in many biological processes, including cell proliferation, differentiation and apoptosis. Previously, we have reported that P2X7 receptor inhibition promotes axonal growth and branching in cultured hippocampal neurons. In this article, we demonstrate that the P2X7 receptor negatively regulates neurite formation in mouse Neuro-2a neuroblastoma cells through a Ca2+/calmodulin-dependent kinase II-related mechanism. Using both molecular and immunocytochemical techniques, we characterized the presence of endogenous P2X1, P2X3, P2X4 and P2X7 subunits in these cells. Of these, the P2X7 receptor was the only functional receptor, as its activation induced intracellular calcium increments similar to those observed in primary neuronal cultures, exhibiting pharmacological properties characteristic of homomeric P2X7 receptors. Patch-clamp experiments were also conducted to fully demonstrate that ionotropic P2X7 receptors mediate nonselective cation currents in this cell line. Pharmacological inhibition of the P2X7 receptor and its knockdown by small hairpin RNA interference resulted in increased neuritogenesis in cells cultured in low serum-containing medium, whereas P2X7 overexpression significantly reduced the formation of neurites. Interestingly, P2X7 receptor inhibition also modified the phosphorylation state of focal adhesion kinase, Akt and glycogen synthase kinase 3, protein kinases that participate in the Ca2+/calmodulin-dependent kinase II signalling cascade and that have been related to neuronal differentiation and axonal growth. Taken together, our results provide the first mechanistic insight into P2X7 receptor-triggered signalling pathways that regulate neurite formation in neuroblastoma cells.

P2X7 and P2Y13 purinergic receptors mediate intracellular calcium responses to BzATP in rat cerebellar astrocytes

Previous work has established the presence of functional P2X(7) subunits in rat cerebellar astrocytes, which after stimulation with 3'-O-(4-benzoyl)benzoyl ATP (BzATP) evoked morphological changes that were not reproduced by any other nucleotide. To further characterize the receptor(s) and signaling mechanisms involved in the action of BzATP, we have employed fura-2 microfluorometry and the patch-clamp technique. BzATP elicited intracellular calcium responses that typically exhibited two components: the first one was transient and metabotropic in nature--sensitive to phospholipase C inhibition and pertussis toxin treatment, whereas the second one was sustained and depended on the presence of extracellular calcium. The ionotropic nature of this latter component was corroborated by measurements of Mn(2+) entry and macroscopic non-selective cation currents evoked by either BzATP (100 muM) or ATP (1 mM). The two components of the calcium response to BzATP differed in their pharmacological sensitivity. The metabotropic component was partially sensitive to pyridoxalphosphate-5'-phosphate-6-azo-(-2-chloro-5-nitrophenyl)-2,4-disulfonate, a selective antagonist of P2Y(13) receptors, while the ionotropic component was modulated by external magnesium and markedly reduced by brilliant blue G and 3-(5-(2,3-dichlorophenyl)-1H-tetrazol-1-yl)methyl pyridine (A438079), thus implying the involvement of P2X(7) purinergic receptors. It is concluded that P2Y(13) and P2X(7) purinergic receptors are functionally expressed in rat cerebellar astrocytes and mediate the increase in intracellular calcium elicited by BzATP in these cells.

Muscarinic excitation-secretion coupling in chromaffin cells

Excitation-secretion coupling in adrenomedullary chromaffin cells physiologically commences when acetylcholine molecules released from splanchnic nerve terminals bind to cholinergic receptors located at the cell's plasma membrane. While nicotinic acetylcholine receptors ensure a rapid and efficacious transmission of preganglionic impulses, muscarinic acetylcholine receptors are considered to play a subsidiary role mostly by facilitating the nicotinic responses. Nevertheless, the variety of effects brought about by muscarinic stimulation in chromaffin cells (release of intracellular Ca2+, activation of Ca2+ entry through non-selective cation channels and voltage-dependent Ca2+ channels, impairment and/or enhancement of action potential firing, etc.) and the long-lasting nature of many of them suggests that muscarinic receptors might contribute to the fine tuning of the catecholamine secretory response upon graded preganglionic stimulation and prolonged periods of time. Such a variety of effects probably reflects not only the diversity of muscarinic receptors expressed in chromaffin cells but also the existence of differences among the animal species employed in the reported investigations. Accordingly, we first review on an animal species-based approach the most relevant features of the muscarinic response in chromaffin cells from a set of mammals, and finally present a unified picture of the mechanisms of muscarinic excitation-secretion coupling in chromaffin cells.

ATP from subplasmalemmal mitochondria controls Ca2+-dependent inactivation of CRAC channels

A sustained Ca2+ entry is the primary signal for T lymphocyte activation after antigen recognition. This Ca2+ entry mainly occurs through store-operated Ca2+ channels responsible for a highly selective Ca2+ current known as I(CRAC). Ca2+ ions act as negative feedback regulators of I(CRAC), promoting its inactivation. Mitochondria, which act as intracellular Ca2+ buffers, have been proposed to control all stages of CRAC current and, hence, intracellular Ca2+ signaling in several types of non-excitable cells. Using the whole-cell configuration of the patch clamp technique, which allows control of the intracellular environment, we report here that respiring mitochondria located close to CRAC channels can regulate slow Ca2+-dependent inactivation of I(CRAC) by increasing the Ca2+-buffering capacity beneath the plasma membrane, mainly through the release of ATP.

Intracellular Ca2+ microdomain-triggered exocytosis in neuroendocrine cells

Colocalization of voltage-gated Ca2+ channels and exocytotic sites at the active zones of nerve terminals underlies 'synchronous' action potential discharge and synaptic vesicle exocytosis, thus allowing fast interneuronal signalling. Such a demand for a rapid release is not expected in neuroendocrine cells whose secretory products act throughout the entire organism. Nevertheless, by using evanescent field imaging of near-membrane Ca2+ concentrations and fluorescently labelled vesicles, Becherer et al. have recently reported exocytosis of individual large dense-core vesicles triggered by Ca2+ microdomains formed around clusters of open L-type Ca2+ channels in chromaffin cells from the adrenal medulla. This finding, besides illustrating the power of new microscopy imaging techniques, directly demonstrates in neuroendocrine cells a functional interaction between Ca2+ channels and secretory vesicles very much reminiscent of that in neurons.

Distinct potentiation of L-type currents and secretion by cAMP in rat chromaffin cells

We have investigated the potentiating action of cAMP on L-currents of rat chromaffin cells and the corresponding increase of Ca(2+)-evoked secretory responses with the aim of separating the action of cAMP on Ca(2+) entry through L-channels and the downstream effects of cAMP/protein kinase A (PKA) on exocytosis. In omega-toxin-treated rat chromaffin cells, exposure to the permeable cAMP analog 8-(4-chlorophenylthio)-adenosine 3',5'-monophosphate (pCPT-cAMP; 1 mM, 30 min) caused a moderate increase of Ca(2+) charge carried through L-channels (19% in 10 mM Ca(2+) at +10 mV) and a drastic potentiation of secretion ( approximately 100%), measured as membrane capacitance increments (deltaC). The apparent Ca(2+) dependency of exocytosis increased with pCPT-cAMP and was accompanied by 83% enhancement of the readily releasable pool of vesicles with no significant change of the probability of release, as evaluated with paired-pulse stimulation protocols. pCPT-cAMP effects could be mimicked by stimulation of beta(1)-adrenoreceptors and reversed by the PKA inhibitor H89, suggesting strict PKA dependence. For short pulses to +10 mV (100 ms), potentiation of exocytosis by pCPT-cAMP was proportional to the quantity of charge entering the cell and occurred independently of whether L, N, or P/Q channels were blocked, suggesting that cAMP acts as a constant amplification factor for secretion regardless of the channel type carrying Ca(2+). Analysis of statistical variations among depolarization-induced capacitance increments indicates that pCPT-cAMP acts downstream of Ca(2+) entry by almost doubling the mean size of unitary exocytic events, most likely as a consequence of an increased granule-to-granule rather than a granule-to-membrane fusion.

Extracellular ATP regulates exocytosis in inhibiting multiple Ca(2+) channel types in bovine chromaffin cells

Feedback modulation of voltage-dependent Ca2+ channels by ATP is a well documented phenomenon in bovine chromaffin cells. However, its influence in the control of hormone release is at present poorly understood. By using combined patch-clamp and fura-2 fluorescence measurements we provide evidence that the three Ca2+ channel types (L, N and P/Q) expressed in bovine chromaffin cells are inhibited by ATP (30 microM), and that their involvement in the secretory response, as assayed by capacitance measurements, is roughly proportional to their contribution to the whole-cell Ca2+ current (ICa) both in the absence and presence of ATP. ATP did not modify the capacitance increase observed in cells dialyzed with Ca(2+)-EGTA buffers (1.5 microM free Ca2+), thus excluding a direct effect of ATP on the secretory machinery. Voltage predepolarizations or long chemical (2 s, 70 mM KCl) depolarizations attenuate the effect of ATP on exocytosis by partially relieving the inhibition of ICa Likewise, a strong stimulation that depletes the readily releasable pool of vesicles prevents an inhibitory effect of ATP on the secretory response. While these results lend support to the hypothesis of autocrine modulation of exocytosis by endogenously released ATP acting on P2y-purinoceptors to inhibit ICa, feedback regulation of the rate of release will be a complex function of the occupancy of those receptors and of the electrical and secretory activity of the cell.

Inhibition of SNARE complex assembly differentially affects kinetic components of exocytosis

In chromaffin cells, an increase in intracellular Ca2+ leads to an exocytotic burst followed by sustained secretion. The burst can be further resolved into two kinetically distinct components, which suggests the presence of two separate pools of vesicles. To investigate how these components relate to SNARE complex formation, we introduced an antibody that blocks SNARE assembly but not disassembly. In the presence of the antibody, the sustained component was largely blocked, the burst was slightly reduced, and one of its kinetic components was eliminated. We conclude that SNARE complexes form before Ca(2+)-triggered membrane fusion and exist in a dynamic equilibrium between a loose and a tight state, both of which support exocytosis. Interaction of the antibody with preformed SNARE complexes favors the loose state.

Characterization of adrenal medullary chromaffin cells by flow cytometry

BACKGROUND

Adrenomedullary chromaffin cells are neural crest derivatives widely used as a model system to study neurosecretory mechanisms. Morphological, immunohistochemical, and functional data indicate that chromaffin cells are heterogeneous and support the distinction between adrenaline (A)- and noradrenaline (NA)-producing and secreting cells. The aim of this study was to characterize by flow cytometry the two main chromaffin cell subtypes in suspensions of cultured bovine chromaffin cells.

METHODS

An indirect immunofluorescence method was used for the specific labeling of two intracellular enzymes, dopamine beta-hydroxylase (DBH) and phenylethanolamine N-methyltransferase (PNMT), involved in the synthesis of NA and A, respectively. Flow cytometry analysis of fluorescence labeling was performed in two chromaffin cell fractions differentially enriched in A-containing cells by centrifugation through density gradients. PNMT and DBH-related fluorescence was also correlated with the A and NA content of the cells assayed by HPLC measurements.

RESULTS

No significant differences were found in forward-side scatter plots between the two cell fractions (A-enriched cells and mixed cells); however, the degree of labeling of the enzymes and the corresponding PNMT/DBH-related fluorescence ratio was significantly greater in the A-enriched cell fraction. The existence of changes in DBH and PNMT content of chromaffin cells over time (1 week) in culture was also examined. No significant variation in enzyme related fluorescence values was detected in any of the two cell fractions, and this result correlated well with HPLC determinations of the catecholamine content (A and NA) of the cells.

CONCLUSIONS

Flow cytometry appears to be a useful technique to characterize chromaffin cell subtypes and to follow their phenotypic changes in response to growth factors.

Ca2+-dependent capacitance increases in rat basophilic leukemia cells following activation of store-operated Ca2+ entry and dialysis with high-Ca2+-containing intracellular solution

Ca2+-dependent vesicular fusion was studied in single whole-cell patch-clamped rat basophilic leukemia (RBL) cells using the capacitance technique. Dialysis of the cells with 10 microM free Ca2+ and 300 microM guanosine 5'-O-(3-thiotriphosphate) (GTP[gamma-S]) resulted in prominent capacitance increases. However, dialysis with either Ca2+ (225 nM to 10 microM) or GTP[gamma-S] alone failed to induce a capacitance change. Under conditions of weak Ca2+ buffering (0.1 mM EGTA), activation of Ca2+-release-activated Ca2+ (CRAC) channels by dialysis with inositol 1,4,5-trisphosphate (InsP3) failed to induce a capacitance increase even in the presence of GTP[gamma-S]. However, when Ca2+ATPases were inhibited by thapsigargin, InsP3 and GTP[gamma-S] led to a pronounced elevation in membrane capacitance. This increase was dependent on a rise in intracellular Ca2+ because it was abolished when cells were dialysed with a high level of EGTA (10 mM) in the recording pipette. The increase was also dependent on Ca2+ influx because it was effectively suppressed when external Ca2+ was removed. Our results demonstrate that ICRAC represents an important source of Ca2+ for triggering a secretory response.

Ultrastructural organization of bovine chromaffin cell cortex-analysis by cryofixation and morphometry of aspects pertinent to exocytosis

We have analyzed ultrathin sections from isolated bovine chromaffin cells grown on plastic support, after fast freezing, by quantitative electron microscopy. We determined the size and intracellular distribution of dense core vesicles (DVs or chromaffin granules) and of clear vesicles (CVs). The average diameter of DVs is 356 nm, and that of CVs varies between 35-195 nm (average 90 nm). DVs appear randomly packed inside cells. When the distance of the center of DVs to the cell membrane (CM) is analyzed, DV density is found to decrease as the CM is approached. According to Monte Carlo simulations performed on the basis of the measured size distribution of DVs, this decay can be assigned to a "wall effect." Any cortical barrier, regardless of its function, seems to not impose a restriction to a random cortical DV packing pattern. The number of DVs closely approaching the CM (docked DVs) is estimated to be between 364 and 629 (average 496), i.e., 0.45 to 0.78 DVs/micron2 CM. Deprivation of Ca2+, priming by increasing [Ca2+]i, or depolarization by high [K+]e for 10 s (the effect of which was controlled electrophysiologically and predicted to change the number of readily releasable granules [RRGs]) does not significantly change the number of peripheral DVs. The reason may be that (a) structural docking implies only in part functional docking (capability of immediate release), and (b) exocytosis is rapidly followed by endocytosis and replenishment of the pool of docked DVs. Whereas the potential contribution of DVs to CM area increase by immediate release can be estimated at 19-33%, that of CVs is expected to be in the range of 5.6-8.0%.

Veratridine-induced oscillations of cytosolic calcium and membrane potential in bovine chromaffin cells

1. Veratridine (VTD) induced large oscillations of the cytosolic Ca2+ concentration ([Ca2+]i) and the membrane potential (Vm) in otherwise silent bovine chromaffin cells loaded with fura-2. 2. Depletion of the intracellular Ca2+ stores by thapsigargin or ryanodine did not affect these oscillations. Caffeine had a complex effect, decreasing them in cells with high activity but increasing them in cells with low activity. 3. The [Ca2+]i oscillations required extracellular Ca2+ and Na+ and were blocked by Ni2+ or tetrodotoxin. They were antagonized by high external concentrations of Mg2+ and/or Ca2+. 4. The oscillations of Vm had three phases: (i) slow depolarization (20 mV in 10-40 s); (ii) further fast depolarization (30 mV in 1 s); and (iii) rapid (5 s) repolarization. [Ca2+]i decreased during (i), increased quickly during (ii) with a 1 s delay with regard to the peak depolarization, and decreased during (iii). 5. Slight depolarizations increased the frequency of the oscillations whereas large depolarizations decreased it. 6. The Ca(2+)-dependent K+ channel blocker apamin increased the duration and decreased the frequency of the oscillations. 7. We propose the following mechanism for the oscillations: (i) the membrane depolarizes slowly by a decrease of potassium conductance (gK), perhaps due to a gradual decrease of [Ca2+]i; (ii) the threshold for activation of Na+ channels (decreased by VTD) is reached, producing further depolarization and recruiting Ca2+ channels, and inactivation of both Ca2+ and VTD-poisoned Na+ channels is slow; and (iii) gK increases, aided by activation of Ca(2+)-dependent K+ channels by the increased [Ca2+]i, and the membrane repolarizes. The contribution of the Na+ channels seems essential for the generation of the oscillations. 8. Bovine chromaffin cells have the machinery required for [Ca2+]i oscillations even though the more physiological stimulus tested here (high K+, field electrical stimulation, nicotinic or muscarinic agonists) produced mainly non-oscillatory responses.

Permeation by zinc of bovine chromaffin cell calcium channels: relevance to secretion

Zn2+ increased the rate of spontaneous release of catecholamines from bovine adrenal glands. This effect was Ca2+ independent; in fact, in the absence of extracellular Ca2+, the secretory effects of Zn2+ were enhanced. At low concentrations (3-10 microM), Zn2+ enhanced the secretory responses to 10-s pulses of 100 microM 1,1-dimethyl-4-phenylpiperazinium (DMPP, a nicotinic receptor agonist) or 100 mM K+. In the presence of DMPP, secretion was increased 47% above controls and in high-K+ solutions, secretion increased 54% above control. These low concentrations of Zn2+ did not facilitate the whole-cell Ca2+ (ICa) or Ba2+ (IBa) currents in patch-clamped chromaffin cells. Higher Zn2+ concentrations inhibited the currents (IC50 values, 346 microM for ICa and 91 microM for IBa) and blocked DMPP- and K(+)-evoked secretion (IC50 values, 141 and 250 microM, respectively). Zn2+ permeated the Ca2+ channels of bovine chromaffin cells, although at a much slower rate than other divalent cations. Peak currents at 10 mM Ba2+, Ca2+, Sr2+ and Zn2+ were 991, 734, 330 and 7.4 pA, respectively. Zn2+ entry was also evidenced using the fluorescent Ca2+ probe fura-2. This was possible because Zn2+ causes an increase in fura-2 fluorescence at the isosbestic wave-length for Ca2+, i.e. 360 nm. There was a slow resting entry of Zn2+ which was accelerated by stimulation with DMPP or high-K+ solution. The entry of Zn2+ was concentration dependent, slightly antagonized by 1 mM Ca2+ and completely blocked by 5 mM Ni2+. The entry of Ca2+ evoked by depolarization with high-K+ solution was antagonized by Zn2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions between Ca2+, PCA50941 and Bay K 8644 in bovine chromaffin cells

We describe here the effects of PCA50941 (a novel 1,4-dihydropyridine derivative) comparatively with Bay K 8644 on various parameters in bovine adrenal chromaffin cells. The binding of [3H](+)-isradipine to bovine adrenal medulla plasma membranes was inhibited similarly by PCA50941 and Bay K 8644 at various [Ca2+]o suggesting a common binding site for both compounds on the dihydropyridine receptor. In voltage-clamped chromaffin cells PCA50941 (1 microM) and Bay K 8644 (1 microM) shifted the I-V relationship of whole-cell Ca2+ currents by about 5-10 mV towards more hyperpolarizing potentials. At -20 mV, PCA50941 enhanced ICa by 195 +/- 16% and Bay K 8644 by 288 +/- 51%. Stimulation of fura 2-loaded chromaffin cell suspensions with 17.7 K+/0.5 Ca2+ increased 3-fold the basal [Ca2+]i. PCA50941 increased further the K(+)-evoked peak to 655 nM, and Bay K 8644 to 1129 nM. In the presence of 5 mM Ca2+, PCA50941 or Bay K 8644 increased the [Ca2+] peaks to 427 and 350 nM, respectively. PCA50941 potentiated the release of catecholamines from perfused bovine adrenal glands evoked by 30 s pulses of 17.7 mM K+ in a manner dependent on the [Ca2+]o. Thus at 1, 2.5, 5 and 10 mM Ca2+, secretion was 2.3-, 3.8-, 5- and 4-fold greater than in control glands. Bay K 8644 enhanced the K(+)-induced response 3- and 9-fold at [Ca2+]o of 0.25 or 0.5 mM, respectively; at higher [Ca2+]o the potentiation was similar to that of PCA50941.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of Ca2+ channel antagonists on chromaffin cell death and cytosolic Ca2+ oscillations induced by veratridine

Exposure of bovine chromaffin cells to 30 microM veratridine for 24 h led to 70-80% cell death as reflected by phase contrast microscopy, trypan blue exclusion, lactate dehydrogenase (LDH) release and cell catecholamine contents. Na+ deprivation, Ca2+ deletion or tetrodotoxin (5 microM) prevented the veratridine-induced cell damage. Nimodipine and verapamil, but not omega-conotoxin GVIA afforded 20-30% protection. Flunarizine protected the cells by 80% and R56865 by 60%. Stimulation of fura-2-loaded single bovine chromaffin cells with 30 microM of 1,1-dimethyl-4-phenylpiperazinium (DMPP) or 59 mM K+ caused fast increases in cytosolic Ca2+ concentrations, ([Ca2+]i). The [Ca2+]i rose from 0.1 to peaks of 1.9 microM, which quickly declined to near basal levels with a t1/2 of around 30 s. In spite of sustained stimulation with these two depolarizing agents, the [Ca2+]i remained low and did not undergo oscillations. In contrast, veratridine (30 microM) caused large and frequent oscillatory changes in the [Ca2+]i which were long-lasting and did not disappear even 30 min after washing out the toxin. The [Ca2+]i oscillations were reversibly suppressed by Na+ or Ca2+ removal and by 5 microM tetrodotoxin. Selective L-type Ca2+ channel blockers (10 microM nimodipine or verapamil) or N-type Ca2+ channel blockers (1 microM omega-conotoxin GVIA) did not affect the [Ca2+]i oscillations. In contrast, flunarizine or R56865 (10 microM each) suppressed the oscillations of [Ca2+]i. The results demonstrate that bovine chromaffin cells have the necessary machinery to develop prolonged and repetitive [Ca2+]i oscillations in the presence of veratridine; however, 'physiological' depolarizing stimuli did not cause oscillations.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium channel subtypes in cat chromaffin cells

1. Using the patch-clamp technique we have investigated the kinetic and pharmacological properties of high-voltage-activated (HVA) Ca2+ channels in short-term-cultured cat chromaffin cells. 2. In 10 mM Ba2+, HVA currents activated around -40 mV, reached maximal amplitude at 0 mV and reversed at about +60 mV. At 0 mV, HVA current activation was fast (mean tau act, 2.45 ms), and followed by either an incomplete inactivation or by a second slow phase of activation (mean tau slow, 36.8 ms) that was lost when Ba2+ was replaced by Ca2+. HVA Ba2+ currents deactivate quickly on repolarization to -50 mV (mean tau deact, 0.36 ms). 3. In most cells, HVA currents were sensitive to common dihydropyridine (DHP) derivatives. Nisoldipine blocked the currents maximally at low membrane potentials (mean block 76% at -30 mV, 3 microM) and gradually less at higher voltages. Nisoldipine block was clearly time dependent (33 and 56% after 30 and 600 ms, respectively, to 0 mV). 4. Bay K 8644 (3 microM) action was variable and caused (1) a 2- to 4-fold increase of Ba2+ currents at -40 to -20 mV, (2) a -15 mV shift of the current-voltage relationship and (3) a 10- to 20-fold prolongation of HVA channel deactivation at -50 mV. 5. Nisoldipine block and Bay K 8644 potentiation of HVA currents increased markedly in omega-conotoxin GVIA (omega-CgTX)-pretreated cells, suggesting an increased fraction of DHP-sensitive currents in these cells. Nisoldipine block of residual omega-CgTX-resistant currents was almost complete (mean block, 82%) during pulses of 1 s to 0 mV. 6. The degree of inhibition produced by omega-CgTX (2 microM for 1 min) varied from cell to cell (mean block, 46%) and was partly reversible. Residual omega-CgTX-resistant currents exhibited faster activation-deactivation kinetics than control currents. 7. The slow phase of HVA current activation was abolished if a conditioning prepulse of 40 ms to +70 mV preceded a test pulse to 0 mV. Double-pulse protocols caused an average current increase (facilitation) of 37% that was voltage dependent and which correlated with the slow phase of Ca2+ channel activation. Facilitation was lost in most omega-CgTX-treated cells and was little affected by nisoldipine (3 microM) and Bay K 8644 (1 microM). Facilitation was potentiated in cells dialysed with 100 microM guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) and fully prevented by 1 mM guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S).(ABSTRACT TRUNCATED AT 400 WORDS)

Localized L-type calcium channels control exocytosis in cat chromaffin cells

Depolarizing 1-s pulses to 0 mV from a holding potential of -70 mV, induced whole-cell currents through Ca2+ channels (ICa) in patch-clamped cat adrenal medulla chromaffin cells. The dihydropyridine (DHP) furnidipine (3 microM) reduced the peak current by 47% and the late current by 80%. omega-Conotoxin GVIA (CgTx, 1 microM) reduced the peak ICa by 42% and the late ICa by 55%. Pulses (10 s duration) with 70 mM K+/2.5 mM Ca2+ solution (70 K+/2.5 Ca2+), applied to single fura-2-loaded cat chromaffin cells increased the cytosolic Ca2+ concentration ([Ca2+]i) from 0.1 to 2.21 microM; this increase was reduced by 43.7% by furnidipine and by 42.5% by CgTx. In the perfused cat adrenal gland, secretion evoked by 10-s pulses of 70 K+/2.5 Ca2+ was reduced by 25% by CgTx and by 96% by furnidipine. Similar results were obtained when secretion from superfused isolated cat adrenal chromaffin cells was studied and when using a tenfold lower [Ca2+]o. The results are compatible with the existence of DHP-sensitive (L-type) as well as CgTx-sensitive (N-type) voltage-dependent Ca2+ channels in cat chromaffin cells. It seems, however, that though extracellular Ca2+ entry through both channel types leads to similar increments of averaged [Ca2+]i, the control of catecholamine release is dominated only by Ca2+ entering through L-type Ca2+ channels. This supports the idea of a preferential segregation of L-type Ca2+ channels to localized "hot spots" in the plasmalemma of chromaffin cells where exocytosis occurs.

Modulation by L-type Ca2+ channels and apamin-sensitive K+ channels of muscarinic responses in cat chromaffin cells

In the perfused cat adrenal gland stimulated with the muscarinic agonist methacholine chloride (100 microM for 3 min), two components were detected in the catecholamine secretory response: 1) an early phasic component that peaked at 300 ng/5 s catecholamine release and 2) a tonic component whose peak was transient and declined to a plateau of about 140 ng/5 s. Apamin (0.1 microM) increased the phasic component to 1,200 ng/5 s and the tonic component to approximately 350 ng/5 s. In single fura 2-loaded cat adrenal chromaffin cells, the cytosolic Ca2+ concentration ([Ca2+]i) also followed a biphasic pattern after stimulation with methacholine. Depletion of extracellular Ca2+ reduced the phasic [Ca2+]i peak by > 50% and the phasic secretory peak by approximately 90%; both the tonic components of [Ca2+]i and secretion were abolished. Depletion of intracellular Ca2+ pools decreased the phasic and tonic components of [Ca2+]i and secretion with respect to control values; however, the phasic components diminished more than the tonic components of [Ca2+]i and secretion. Although 3 microM furnidipine (a dihydropyridine L-type Ca2+ channel blocker) inhibited the phasic component of [Ca2+]i and secretion, its effects were more pronounced on the tonic component. omega-Conotoxin GVIA (1 microM, an N-type Ca2+ channel blocker) did not affect the [Ca2+]i or the methacholine secretory responses. The secretion peak seems to depend on both extracellular free Ca2+ (Cao2+) entry through L-type Ca2+ channels as well as on the mobilization of Ca2+ from intracellular stores; the plateau depends only on Cao2+ entry through L-type Ca2+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

R56865 inhibits catecholamine release from bovine chromaffin cells by blocking calcium channels

1. The effects of R56865 (a new class of cardioprotective agent which prevents Na+ and Ca2+ overload in cardiac myocytes) on catecholamine release, whole-cell current through Ca2+ channels (IBa) and cytosolic Ca2+ concentrations, [Ca2+]i, have been studied in bovine chromaffin cells. 2. R56865 caused a time- and concentration-dependent blockade of catecholamine release from superfused cells stimulated intermittently with 5 s pulses of 59 mM K+. After 5 min superfusion, a 3 microM concentration inhibited secretion by 20%; the blockade increased gradually with perfusion time, to reach 85% after 40 min. The IC50 to block secretion after 5 min periods of exposure to increasing concentrations of R56865 was around 3.1 microM. The blocking effects of R56865 were reversible after 5-15 min wash out. In high Ca2+ solution (10 mM Ca2+), the degree of blockade of secretion diminished by 20% in comparison with 1 mM Ca2+. 3. In electroporated cells, R56865 (10 microM) did not modify the secretory response induced by the application of 10 microM free Ca2+. 4. R56865 blocked the peak IBa current in a concentration- and time-dependent manner; its IC50 was very similar to that obtained for secretion (3 microM). The compound not only reduced the size of the peak current but also promoted its inactivation; when the effects of R56865 were measured at the most inactivated part of the current, its IC50 was 1 microM. Both the inactivation and the reduction of the peak currents were reversible upon washing out the drug. 5. In fura-2-loaded single chromaffin cells the basal [Ca2+]i of around 100 nM was elevated to a peak of1.5 microM by the application of a 5 s pulse of 59 mM K+. R56865 (10 microM) did not affect the basal [Ca2+]but blocked by 90% the K+-evoked increase. This effect was fully reversible after 5-10 min of wash out.6. The results are compatible with the idea that R56865 blocks Ca2+ entry into K+-depolarized chromaffin cells by promoting the inactivation of voltage-dependent Ca2+ channels; this would lead to the limitation of the rise in [Ca2+]i and of the release of catecholamines. The restriction of catecholamine release may favour indirectly the known direct beneficial cardioprotective actions of R56865.

Separation between cytosolic calcium and secretion in chromaffin cells superfused with calcium ramps

This paper describes experiments in which cytosolic Ca2+ concentrations ([Ca2+]i) and catecholamine release were measured in two populations of chromaffin cells stimulated with a solution enriched in K+ (100 mM). Once depolarized, external Ca2+ or Ba2+ ions were offered to cells either as a single 2.5 mM step or as a ramp that linearly increased the concentration from 0 to 2.5 mM over a 10-min period. A clear separation between the changes of the [Ca2+]i and the time course of secretion was observed. Specifically, secretion and [Ca2+]i rose in parallel when a Ca2+ step was used to reach a peak in a few seconds; however, while secretion declined to the basal level, [Ca2+]i remained elevated at a plateau of 400 nM. With a Ca2+ ramp, only a transient small peak of secretion was observed, yet the [Ca2+]i remained elevated throughout the 10-min stimulation period. The separation between secretion and [Ca2+]i was observed even when voltage-dependent Ca2+ channels were expected to remain open (mild depolarization in the presence of 1 microM Bay K 8644). By using Ba2+ steps or ramps, sustained noninactivating secretory responses were obtained. The results suggest that the rate and extent of secretion are not a simple function of the [Ca2+]i at a given time; they are compatible with the following conclusions: (i) A steep extracellular-to-cytosolic Ca2+ gradient is required to produce a sharp increase in the [Ca2+]i at exocytotic sites capable of evoking a fast but transient secretory response. (ii) As a result of Cai(2+)-dependent inactivation of Ca2+ channels, those high [Ca2+]i are possible only at early times after cell depolarization. (iii) The Cai(2+)-dependent supply of storage granules to the secretory machinery cooperates with the supply of Ca2+ through Ca2+ channels to regulate the rate and extent of secretion.

Small-conductance Ca(2+)-activated K+ channels in bovine chromaffin cells

Simultaneous whole-cell patch-clamp and fura-2 fluorescence [Ca2+]i measurements were used to characterize Ca(2+)-activated K+ currents in cultured bovine chromaffin cells. Extracellular application of histamine (10 microM) induced a rise of [Ca2+]i concomitantly with an outward current at holding potentials positive to -80 mV. The activation of the current reflected an increase in conductance, which did not depend on membrane potential in the range -80 mV to -40 mV. Increasing the extracellular K+ concentration to 20 mM at the holding potential of -78 mV was associated with inwardly directed currents during the [Ca2+]i elevations induced either by histamine (10 microM) or short voltage-clamp depolarizations. The current reversal potential was close to the K+ equilibrium potential, being a function of external K+ concentration. Current fluctuation analysis suggested a unit conductance of 3-5 pS for the channel that underlies this K+ current. The current could be blocked by apamin (1 microM). Whole-cell current-clamp recordings showed that histamine (10 microM) application caused a transient hyperpolarization, which evolved in parallel with the [Ca2+]i changes. It is proposed that a small-conductance Ca(2+)-activated K+ channel is present in the membrane of bovine chromaffin cells and may be involved in regulating catecholamine secretion by the adrenal glands of various species.

Two components in the adrenal nicotinic secretory response revealed by cobalt ramps

Prolonged stimulation with nicotine (50 microM) enhanced the secretion of catecholamines from perfused cat adrenal glands. The profile of secretion consisted of a quick activation phase to a peak of 7.68 micrograms/min followed by a second inactivation phase which exhibited a t1/2 of 3.75 min. Sustained stimulation with a solution enriched in K+ (59 mM) also evoked a transient secretory response, with a peak release of 8.62 micrograms/2 min and a t1/2 for inactivation of 4.8 min. Co2+ (10 mM) blocked the nicotinic response by 58% and the K(+)-evoked secretory response by over 96%. In the presence of Co2+ (5 mM), continuous perfusion with nicotine produced a transient but large initial secretory response; the gradual decrease of the extracellular Co2+ concentration, [Co2+]0, as a continuous ramp allowed the development of a second component of secretion which inactivated later on. When the glands were continuously stimulated with 59 mM K+ in the presence of Co2+, the first component of secretion was missing; the second component appeared as [Co2+]0 decreases as a ramp. In similar experiments performed in low-Na+ solution (10 mM Na+), only the first secretion component evoked by nicotine was observed. This finding suggests that the second component of secretion depends on Na+ entry through the nicotinic receptor, on the ensuing cell depolarization and on Ca2+ entry through voltage-dependent Ca2+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca(2+)-activated K+ channels modulate muscarinic secretion in cat chromaffin cells

1. This study was aimed at testing the hypothesis that Ca(2+)-dependent K+ channels regulate the release of catecholamines mediated by muscarinic stimulation of cat adrenal chromaffin cells. Two parameters were measured: the secretory response to brief pulses of methacholine (100 microM for 10 s) in intact cat adrenal glands perfused at a high rate with oxygenated Krebs solution; and the changes in cytosolic Ca2+ concentrations, [Ca2+]i, produced by puff applications of methacholine pulses (also 100 microM for 10 s) in isolated single cat adrenal chromaffin cells loaded with Fura-2. 2. A pulse of methacholine released 805 +/- 164 ng of catecholamines (mean of thirty-two pulses). d-Tubocurarine (DTC) increased the secretory response in a concentration-dependent manner. The maximum increase (around 1000 ng catecholamines over control values) was reached at 100 microM-DTC and the EC50 was around 10 microM. 3. The secretory responses to methacholine alone, or to the combination of methacholine plus DTC, were strongly dependent on the extracellular Ca2+ concentration, [Ca2+]o. Thus Ca2+o removal from the perfusing solution for 5-10 min abolished catecholamine release. 4. At 0.1 microM, isradipine (an L-type Ca2+ channel blocker) inhibited by 71% the secretory response to DTC plus methacholine. At 1 microM, Bay K 8644 (an L-type Ca2+ channel activator) increased 2-fold the secretory response to DTC plus methacholine (2746 ng of catecholamines). 5. Apamin (1 microM) increased 3.5-fold the secretory response to methacholine pulses (from 500 to 1800 ng of catecholamines). 6. Methacholine pulses enhanced [Ca2+]i from the resting level of 100 nM to a peak of 1000 nM which quickly declined to basal level. DTC (100 microM) enhanced by 20% the [Ca2+]i peak and substantially prolonged its duration. 7. Apamin (1 microM) increased by 60% the [Ca2+]i peak evoked by methacholine, and delayed the initiation of decline of the [Ca2+]i peak. 8. These results are compatible with the idea that muscarinic stimulation depolarizes the cat adrenal chromaffin cell through an unidentified mechanism. Depolarization is probably counteracted by activation of Ca2+i-dependent K+ channels. Therefore, inhibition of these channels enhances depolarization and firing of action potentials which activate voltage-dependent L-type Ca2+ channels to increase further the Ca2+i signal and the secretory response. Thus Ca2+i-dependent K+ channels, probably of the small-conductance type (SK), seem to be involved in the modulation of muscarinic-evoked catecholamine release responses in cat adrenal chromaffin cells.

Alamethicin-evoked catecholamine release from cat adrenal glands

Alamethicin enhances the rate of catecholamine output from perfused cat adrenal glands in a concentration-dependent manner. At 37 degrees C, catecholamine released went from 4.29 +/- 0.25 to 20.51 +/- 0.63 micrograms/stimulus at ionophore concentrations ranging from 20 to 100 micrograms/ml. Secretion was abolished at 22 degrees C or in the absence of extracellular Ca. The time-course of secretion (quick activation followed by a decline) evoked by alamethicin considerably differs from the catecholamine release pattern seen with A23187, X537A or ionomycin, which evoke a slowly developing, non-inactivating secretory response. In fact, its transient secretion pattern resembles that of nicotinic or high-K stimulation of cat adrenal glands, thus suggesting that alamethicin might form Ca permeable artificial channels in chromaffin cell plasma membranes.

Sodium-dependent inhibition by PN200-110 enantiomers of nicotinic adrenal catecholamine release

1. Dimethylphenylpiperazinium (DMPP) or high K concentrations evoke catecholamine release from perfused cat adrenal glands; in both cases the secretory response was significantly enhanced in the absence of Na. Tetrodotoxin did not modify the nicotinic secretory response. 2. The (+)- and (-)-enantiomers of the dihydropyridine Ca channel blocker PN200-110 show a high degree of stereoselectivity in the inhibition of catecholamine secretion evoked by high K or by DMPP in the presence of Na, the (+)-enantiomer being 57 and 80 times more potent, respectively, than the (-)-enantiomer. Both, noradrenaline and adrenaline release were equally depressed by PN200-110. 3. The IC50 values for (+)- and (-)-PN200-110 for blockade of the secretory response induced by K or DMPP in the presence of Na are in the same range. In the absence of Na, (-)-PN200-110 did not affect DMPP-evoked secretion; however, the (+)-enantiomer partially inhibited it. 4. The results suggest that the physiological catecholamine release from chromaffin cells is preceded by Na entry through the nicotinic receptor-associated ionophore; this causes cell depolarization, opening of voltage-dependent, dihydropyridine-sensitive Ca channels and Ca entry into the cell. In the absence of Na, additional Ca influx through an alternative pathway (the nicotinic cholinoceptor ionophore?) might also activate secretion.

Mobilization of intracellular calcium by extracellular ATP and by calcium ionophores in the Ehrlich ascites-tumour cell

We have studied the changes of the intracellular free calcium concentration ([Ca2+]i) effected by external ATP, which induces formation of inositol trisphosphate, and by the divalent cation ionophores ionomycin and A23187. Both, ATP (40 microM) and ionophores (1-80 mumol/l cells ionomycin; 20-400 mumol/l cells A23187), produced a transient rise of [Ca2+]i which reached its maximum within 15-30 s and declined near resting values (about 200 nM) within 1-3 min. When the [Ca2+]i peak surpassed 500 nM a transient cell shrinkage due to simultaneous activation of Ca2+-dependent K+ and Cl- channels was also observed. The cell response was similar in medium containing 1 mM Ca2+ and in Ca2+-free medium, suggesting that the Ca mobilized to the cytosol comes preferently from the intracellular stores. Treatment with low doses of ionophore (1 mumol/l cells for ionomycin; 20 mumol/l cells for A23187) depressed the response to a subsequent treatment, either with ionophore or with ATP. Treatment with ATP did also inhibit the subsequent response to ionophore, but in this case the inhibition was dependent on time, the stronger the shorter the interval between both treatments. This result suggests that the permeabilization of Ca stores by ATP is transient and that Ca can be taken up again by the intracellular stores. Refill was most efficient when Ca2+ was present in the incubation medium. Addition of either ATP or ionomycin (1-25 mumol/l cells) to cells incubated in medium containing 1 mM Ca2+ decreased drastically the total cell Ca content during the following 3 min of incubation. In the case of ATP the total cell levels of Ca returned to the initial values after 7-15 min, whereas in the case of the ionophore they remained decreased during the whole incubation period. These results indicate that Ca released from the intracellular stores by either ATP or ionophores is quickly extruded by active mechanisms located at the plasma membrane. They also suggest that, under the conditions studied here, with 1 mM Ca2+ outside, the Ca-mobilizing effect of ionophores is stronger in endomembranes than in the plasma membrane.

A dopaminergic receptor in adrenal medulla as a possible site of action for the droperidol-evoked hypertensive response

Recently, an inhibitory dopaminergic receptor has been described that modulates catecholamine release from adrenal medulla. It has also been reported that low doses of droperidol increase arterial pressure in some patients with pheochromocytoma. The authors investigated whether an effect of droperidol on such a receptor could be one of the mechanisms involved in the hypertensive response. Isolated cat adrenal glands were perfused with Krebs-bicarbonate solution, and the catecholamine release was measured in the effluent. Then, the glands were stimulated by activation of the nicotinic receptor (nicotine, 5 microM), and the effect of low and high doses of droperidol and/or apomorphine on the catecholamine secretory responses evoked by nicotine was investigated. Low concentrations of droperidol (0.05 microM) (a dopaminergic antagonist) markedly increased the secretory response induced by nicotine whereas higher concentrations (50 microM) decreased it. Apomorphine (1 microM) (a dopaminergic agonist) inhibits the catecholamine release produced by nicotine, and this inhibitory effect was completely reversed by the lowest concentration of droperidol but not by the highest. In fact, the high concentration of droperidol further inhibited the catecholamine release induced by nicotine. The results suggest that the hypertensive responses evoked by low doses of droperidol in some patients with pheochromocytoma could be due to the inactivation of a dopaminergic inhibitory system present in the adrenal medulla that, under physiologic conditions, limits the amount of catecholamines released by the gland. Such as an inhibitory mechanism could operate in an exaggerated manner in patients with pheochromocytoma.

Characterization of a dopaminergic receptor that modulates adrenomedullary catecholamine release

Nicotine evokes the release of catecholamines from bovine adrenal glands perfused with oxygenated Krebs-bicarbonate solution. Two 2-min pulses of 5 microM nicotine, at 40-min intervals (S1 and S2), gave net catecholamine outputs of 45.2 +/- 3.6 and 29.1 +/- 3.5 micrograms/8 min, respectively. Apomorphine (1 or 10 microM) markedly inhibited catecholamine release during S2 to 9.1 +/- 2.2 and 0.5 micrograms/8 min, respectively. Haloperidol (0.5 microM) reversed the inhibitory effects of apomorphine. Haloperidol alone enhanced catecholamine release induced by nicotine to 67.9 +/- 7.9 micrograms/8 min. [3H]Spiperone binds to adrenomedullary membranes with a KD of 0.24 nM and a Bmax of 117 fmol/mg of protein. Whereas spiperone and haloperidol potently displaced such binding, 3,4-dihydroxyphenylethylamine (dopamine) and sulpiride were poorer displacers, and SCH23390, prazosin, phenoxybenzamine, propranolol, BAY-K-8644, and nitrendipine did not displace [3H]spiperone bound. These data strongly suggest that, as in the cat, the bovine adrenal medulla chromaffin cell contains a dopaminergic receptor that modulates the catecholamine secretory process triggered by stimulation of the nicotinic cholinoceptor. Such a receptor seems to be of the D2 type and might be involved in a sympatho-adrenal cooperative mechanism contributing to the maintenance of cardiovascular homeostasis during stressful situations as well as to the pathogenesis of hypertension. If so, selective dopaminergic agonists might prove clinically useful in the treatment of hypertension.

A dopaminergic receptor modulates catecholamine release from the cat adrenal gland

Nicotine evokes the release of catecholamines from perfused cat adrenal glands in a concentration-dependent manner, the median effective concentration for nicotine being 5 microM. Two 2 min pulses of 5 microM-nicotine, 40 min apart (S1 and S2) gave net catecholamine outputs of 7.64 and 3.55 micrograms/8 min, respectively. The ratio S2/S1 in control glands was 0.5. Increasing concentrations of apomorphine (1-10 microM) markedly inhibited catecholamine release during the second nicotine pulse (S2). At 1 microM-apomorphine, the release during S2 was significantly reduced to 16% of S1; with 10 microM-apomorphine, the secretory response was reduced further to only 3% of S1, the ratio S2/S1 being 0.03. The presence of haloperidol, sulpiride or picobenzide (each 0.5 microM) during S2, completely reversed the inhibition of catecholamine release produced by apomorphine. Haloperidol itself increased the nicotinic secretory response during S2; so, while the ratio S2/S1 was 0.5 in control conditions, this ratio increased significantly to 0.95 if haloperidol (0.5 microM) was present during S2, suggesting that the presence of this dopaminergic antagonist removed a negative feed-back mechanism that inhibits nicotine-evoked catecholamine release. If present during S2, dopamine (1 microM) also markedly inhibited catecholamine release evoked by nicotine; this inhibition was again reversed by 0.5 microM-haloperidol. Neither the opiate antagonist naloxone nor the alpha-adrenoceptor blocking agent phentolamine (at concentrations of 0.5-5 microM) affected the inhibition by apomorphine of the secretory response to nicotine. These data strongly suggest that the cat adrenal medulla chromaffin cell membrane contains a dopaminergic receptor which modulates the catecholamine secretory process triggered by stimulation of the nicotinic cholinoceptor. The fact that dopamine is released in measurable amounts, together with adrenaline and noradrenaline, from perfused cat adrenal glands in response to nicotinic stimulation (V. Ceña, unpublished results), favours a role for this dopaminergic receptor in modulating catecholamine release from the chromaffin cell.

Effects of the novel dihydropyridine BAY-K-8644 on adrenomedullary catecholamine release evoked by calcium reintroduction

Reintroduction of Ca ions to cat adrenal glands perfused at room temperature with Krebs solution lacking Ca and Mg, evoked a catecholamine secretory response that was directly proportional to the concentration of Ca reintroduced. This secretory response inactivated quickly, was abolished by nM concentrations of nifedipine and was potentiated dramatically by nM concentrations of BAY-K-8644. Excess Ca antagonized the inhibitory effects of nifedipine and this drug inhibited competitively the potentiating effects of BAY-K-8644. These data suggest 1st) that extracellular divalent cations deprivation activates specific Ca channels; 2nd) that the dihydropyridine BAY-K-8644 increases the release of catecholamines by Ca reintroduction by activating and/or delaying the inactivation of Ca channels; and 3rd) that the access of the dihydropyridine-type Ca agonist and antagonists to their "intra-channel" site of action requires the pre-activation of Ca channels.