Amperometric analysis of exocytosis at chromaffin cells from genetically distinct mice

Amperometry in Single Cells and Tissue

The release from cells of signaling molecules through the controlled process of exocytosis involves multiple coordinated steps and is essential for the proper control of a multitude of biological pathways across the endocrine and nervous systems. However, these events are minute both temporally and in terms of the minute amounts of neurotransmitters, hormones, growth factors, and peptides released from single vesicles during exocytosis. It is therefore difficult to measure the kinetics of single exocytosis events in real time. One noninvasive method of measuring the release of molecules from cells is carbon-fiber amperometry. In this chapter, we will describe how we undertake such measurements from both single cells and in live tissue, how the subsequent data are analyzed, and how we interpret these results in terms of their relevant physiology.

Adrenergic chromaffin cells are adrenergic even in the absence of epinephrine

Adrenal chromaffin cells release epinephrine (EPI) and norepinephrine (NE) into the bloodstream as part of the homeostatic response to situations like stress. Here we utilized EPI-deficient mice generated by knocking out (KO) the phenylethanolamine N-methyltransferase (Pnmt) gene. These Pnmt-KO mice were bred to homozygosis but displayed no major phenotype. The lack of EPI was partially compensated by an increase in NE, suggesting that EPI storage was optimized in adrenergic cells. Electron microscopy showed that despite the lack of EPI, chromaffin granules retain their shape and general appearance. This indicate that granules from adrenergic or noradrenergic cells preserve their characteristics even though they contain only NE. Acute insulin injection largely reduced the EPI content in wild-type animals, with a minimal reduction in NE, whereas there was only a partial reduction in NE content in Pnmt-KO mice. The analysis of exocytosis by amperometry revealed a reduction in the quantum size (-30%) and I_max (-21%) of granules in KO cells relative to the wild-type granules, indicating a lower affinity of NE for the granule matrix of adrenergic cells. As amperometry cannot distinguish between adrenergic or noradrenergic cells, it would suggest even a larger reduction in the affinity for the matrix. Therefore, our results demonstrate that adrenergic cells retain their structural characteristics despite the almost complete absence of EPI. Furthermore, the chromaffin granule matrix from adrenergic cells is optimized to accumulate EPI, with NE being a poor substitute. Open Science: This manuscript was awarded with the Open Materials Badge For more information see: https://cos.io/our-services/open-science-badges/.

Altered exocytosis in chromaffin cells from mouse models of neurodegenerative diseases

Chromaffin cells from the adrenal gland (CCs) have extensively been used to explore the molecular structure and function of the exocytotic machinery, neurotransmitter release and synaptic transmission. The CC is integrated in the sympathoadrenal axis that helps the body maintain homoeostasis during both routine life and in acute stress conditions. This function is exquisitely controlled by the cerebral cortex and the hypothalamus. We propose the hypothesis that damage undergone by the brain during neurodegenerative diseases is also affecting the neurosecretory function of adrenal medullary CCs. In this context, we review here the following themes: (i) How the discharge of catecholamines is centrally and peripherally regulated at the sympathoadrenal axis; (ii) which are the intricacies of the amperometric techniques used to study the quantal release of single-vesicle exocytotic events; (iii) which are the alterations of the exocytotic fusion pore so far reported, in CCs of mouse models of neurodegenerative diseases; (iv) how some proteins linked to neurodegenerative pathologies affect the kinetics of exocytotic events; (v) finally, we try to integrate available data into a hypothesis to explain how the centrally originated neurodegenerative diseases may alter the kinetics of single-vesicle exocytotic events in peripheral adrenal medullary CCs.

Distinct patterns of exocytosis elicited by Ca2+, Sr2+ and Ba2+ in bovine chromaffin cells

Three divalent cations can elicit secretory responses in most neuroendocrine cells, including chromaffin cells. The extent to which secretion is elicited by the cations in intact depolarized cells was Ba²⁺ > Sr²⁺ ≥ Ca²⁺, contrasting with that elicited by these cations in permeabilized cells (Ca²⁺ > Sr²⁺ > Ba²⁺). Current-clamp recordings show that extracellular Sr²⁺ and Ba²⁺ cause membrane depolarization and action potentials, which are not blocked by Cd²⁺ but that can be mimicked by tetra-ethyl-ammonium. When applied intracellularly, only Ba²⁺ provokes action potentials. Voltage-clamp monitoring of Ca²⁺-activated K⁺ channels (K_Ca) shows that Ba²⁺ reduces outward currents, which were enhanced by Sr²⁺. Extracellular Ba²⁺ increases cytosolic Ca²⁺ concentrations in Fura-2-loaded intact cells, and it induces long-lasting catecholamine release. Conversely, amperometric recordings of permeabilized cells show that Ca²⁺ promotes the longest lasting secretion, as Ba²⁺ only provokes secretion while it is present and Sr²⁺ induces intermediate-lasting secretion. Intracellular Ba²⁺ dialysis provokes exocytosis at concentrations 100-fold higher than those of Ca²⁺, whereas Sr²⁺ exhibits an intermediate sensitivity. These results are compatible with the following sequence of events: Ba²⁺ blocks K_Ca channels from both the outside and inside of the cell, causing membrane depolarization that, in turn, opens voltage-sensitive Ca²⁺ channels and favors the entry of Ca²⁺ and Ba²⁺. Although Ca²⁺ is less permeable through its own channels, it is more efficient in triggering exocytosis. Strontium possesses both an intermediate permeability and an intermediate ability to induce secretion.

An interplay between the serotonin transporter (SERT) and 5-HT receptors controls stimulus-secretion coupling in sympathoadrenal chromaffin cells

Adrenal chromaffin cells (ACCs), the neuroendocrine arm of the sympathetic nervous system, secrete catecholamines to mediate the physiological response to stress. Although ACCs do not synthesize 5-HT, they express the serotonin transporter (SERT). Genetic variations in SERT are linked to several CNS disorders but the role(s) of SERT/5-HT in ACCs has remained unclear. Adrenal glands from wild-type mice contained 5-HT at ≈ 750 fold lower abundance than adrenaline, and in SERT(-/-) mice this was reduced by ≈80% with no change in catecholamines. Carbon fibre amperometry showed that SERT modulated the ability of 5-HT1A receptors to inhibit exocytosis. 5-HT reduced the number of amperometric spikes (vesicular fusion events) evoked by KCl in SERT(-/-) cells and wild-type cells treated with escitalopram, a SERT antagonist. The 5-HT1A receptor antagonist WAY100635 blocked the inhibition by 5-HT which was mimicked by the 5-HT1A agonist 8-OH-DPAT but not the 5-HT1B agonist CP93129. There was no effect on voltage-gated Ca(2+) channels, K(+) channels, or intracellular [Ca(2+)] handling, showing the 5-HT receptors recruit an atypical inhibitory mechanism. Spike charge and kinetics were not altered by 5-HT receptors but were reduced in SERT(-/-) cells compared to wild-type cells. Our data reveal a novel role for SERT and suggest that adrenal chromaffin cells might be a previously unrecognized hub for serotonergic control of the sympathetic stress response.

The interaction between chromogranin A and catecholamines governs exocytosis

Chromogranins (Cgs) are acidic proteins that have been described in the large, dense core vesicles (LDCVs) of adrenal chromaffin cells and that have been shown to promote LDCV formation, even in nonsecretory cells. Catecholamines (CAs) are adsorbed by Cgs in vitro, and the absence of Cgs modifies the storage and exocytosis of CAs in chromaffin cells. In this study, we set out to assess the role of CgA in the accumulation and exocytosis of CAs in cells when the levels of CgA and CA are manipulated. We overexpressed CgA in nonsecretory HEK293 cells and in secretory PC12 cells, to study the formation, movement, and exocytosis of newly formed granules by evanescent wave microscopy. We analyzed the association of Cgs/CA by HPLC and amperometry and their role in the accumulation and exocytosis of amines, both under resting conditions and after l-DOPA overloading. To our knowledge, this is the first demonstration that CgA expression in a nonsecretory cell line facilitates the storage and exocytosis of CA. In addition, CgA overexpression causes a doubling of the accumulation of CA, although it slows down exocytosis in PC12 cells. We propose a model to explain how the CgA/CA complex governs the accumulation and exocytosis of secreted amines.

Huntingtin-associated protein 1 regulates exocytosis, vesicle docking, readily releasable pool size and fusion pore stability in mouse chromaffin cells

Huntingtin-associated protein 1 (HAP1) was initially established as a neuronal binding partner of huntingtin, mutations in which underlie Huntington's disease. Subcellular localization and protein interaction data indicate that HAP1 may be important in vesicle trafficking and cell signalling. In this study, we establish that HAP1 is important in several steps of exocytosis in adrenal chromaffin cells. Using carbon-fibre amperometry, we measured single vesicle exocytosis in chromaffin cells obtained from HAP1(-/-) and HAP1(+/+) littermate mice. Numbers of Ca(2+)-dependent and Ca(2+)-independent full fusion events in HAP1(-/-) cells are significantly decreased compared with those in HAP1(+/+) cells. We observed no change in the frequency of 'kiss-and-run' fusion events or in Ca(2+) entry. Whereas release per full fusion event is unchanged in HAP1(-/-) cells, early fusion pore duration is prolonged, as indicated by the increased duration of pre-spike foot signals. Kiss-and-run events have a shorter duration, indicating opposing roles for HAP1 in the stabilization of the fusion pore during full fusion and transient fusion, respectively. We use electron microscopy to demonstrate a reduction in the number of vesicles docked at the plasma membrane of HAP1(-/-) cells, where membrane capacitance measurements reveal the readily releasable pool of vesicles to be reduced in size. Our study therefore illustrates that HAP1 regulates exocytosis by influencing the morphological docking of vesicles at the plasma membrane, the ability of vesicles to be released rapidly upon stimulation, and the early stages of fusion pore formation.

A microfluidic platform for chemical stimulation and real time analysis of catecholamine secretion from neuroendocrine cells

Release of neurotransmitters and hormones by calcium-regulated exocytosis is a fundamental cellular process that is disrupted in a variety of psychiatric, neurological, and endocrine disorders. As such, there is significant interest in targeting neurosecretion for drug and therapeutic development, efforts that will be aided by novel analytical tools and devices that provide mechanistic insight coupled with increased experimental throughput. Here, we report a simple, inexpensive, reusable, microfluidic device designed to analyze catecholamine secretion from small populations of adrenal chromaffin cells in real time, an important neuroendocrine component of the sympathetic nervous system and versatile neurosecretory model. The device is fabricated by replica molding of polydimethylsiloxane (PDMS) using patterned photoresist on silicon wafer as the master. Microfluidic inlet channels lead to an array of U-shaped "cell traps", each capable of immobilizing single or small groups of chromaffin cells. The bottom of the device is a glass slide with patterned thin film platinum electrodes used for electrochemical detection of catecholamines in real time. We demonstrate reliable loading of the device with small populations of chromaffin cells, and perfusion/repetitive stimulation with physiologically relevant secretagogues (carbachol, PACAP, KCl) using the microfluidic network. Evoked catecholamine secretion was reproducible over multiple rounds of stimulation, and graded as expected to different concentrations of secretagogue or removal of extracellular calcium. Overall, we show this microfluidic device can be used to implement complex stimulation paradigms and analyze the amount and kinetics of catecholamine secretion from small populations of neuroendocrine cells in real time.

Identification of unique release kinetics of serotonin from guinea-pig and human enterochromaffin cells

The major source of serotonin (5-HT) in the body is the enterochromaffin (EC) cells lining the intestinal mucosa of the gastrointestinal tract. Despite the fact that EC cells synthesise ∼95% of total body 5-HT, and that this 5-HT has important paracrine and endocrine roles, no studies have investigated the mechanisms of 5-HT release from single primary EC cells. We have developed a rapid primary culture of guinea-pig and human EC cells, allowing analysis of single EC cell function using electrophysiology, electrochemistry, Ca(2+) imaging, immunocytochemistry and 3D modelling. Ca(2+) enters EC cells upon stimulation and triggers quantal 5-HT release via L-type Ca(2+) channels. Real time amperometric techniques reveal that EC cells release 5-HT at rest and this release increases upon stimulation. Surprisingly for an endocrine cell storing 5-HT in large dense core vesicles (LDCVs), EC cells release 70 times less 5-HT per fusion event than catecholamine released from similarly sized LDCVs in endocrine chromaffin cells, and the vesicle release kinetics instead resembles that observed in mammalian synapses. Furthermore, we measured EC cell density along the gastrointestinal tract to create three-dimensional (3D) simulations of 5-HT diffusion using the minimal number of variables required to understand the physiological relevance of single cell 5-HT release in the whole-tissue milieu. These models indicate that local 5-HT levels are likely to be maintained around the activation threshold for mucosal 5-HT receptors and that this is dependent upon stimulation and location within the gastrointestinal tract. This is the first study demonstrating single cell 5-HT release in primary EC cells. The mode of 5-HT release may represent a unique mode of exocytosis amongst endocrine cells and is functionally relevant to gastrointestinal sensory and motor function.

Exocytosis is impaired in mucopolysaccharidosis IIIA mouse chromaffin cells

Mucopolysaccharidosis IIIA (MPS IIIA) is a lysosomal storage disorder caused by a deficiency in the activity of the lysosomal hydrolase, sulphamidase, an enzyme involved in the degradation of heparan sulphate. MPS IIIA patients exhibit progressive mental retardation and behavioural disturbance. While neuropathology is the major clinical problem in MPS IIIA patients, there is little understanding of how lysosomal storage generates this phenotype. As reduced neuronal communication can underlie cognitive deficiencies, we investigated whether the secretion of neurotransmitters is altered in MPS IIIA mice; utilising adrenal chromaffin cells, a classical model for studying secretion via exocytosis. MPS IIIA chromaffin cells displayed heparan sulphate storage and electron microscopy revealed large electron-lucent storage compartments. There were also increased numbers of large/elongated chromaffin granules, with a morphology that was similar to immature secretory granules. Carbon fibre amperometry illustrated a significant decrease in the number of exocytotic events for MPS IIIA, when compared to control chromaffin cells. However, there were no changes in the kinetics of release, the amount of catecholamine released per exocytotic event, or the amount of Ca(2+) entry upon stimulation. The increased number of large/elongated granules and reduced number of exocytotic events suggests that either the biogenesis and/or the cell surface docking and fusion potential of these vesicles is impaired in MPS IIIA. If this also occurs in central nervous system neurons, the reduction in neurotransmitter release could help to explain the development of neuropathology in MPS IIIA.

Aging differentially affects multiple aspects of vesicle fusion kinetics

How fusion pore formation during exocytosis affects the subsequent release of vesicle contents remains incompletely understood. It is unclear if the amount released per vesicle is dependent upon the nature of the developing fusion pore and whether full fusion and transient kiss and run exocytosis are regulated by similar mechanisms. We hypothesise that if consistent relationships exist between these aspects of exocytosis then they will remain constant across any age. Using amperometry in mouse chromaffin cells we measured catecholamine efflux during single exocytotic events at P0, 1 month and 6 months. At all ages we observed full fusion (amperometric spike only), full fusion preceded by fusion pore flickering (pre-spike foot (PSF) signal followed by a spike) and pure “kiss and run” exocytosis (represented by stand alone foot (SAF) signals). We observe age-associated increases in the size of all 3 modes of fusion but these increases occur at different ages. The release probability of PSF signals or full spikes alone doesn't alter across any age in comparison with an age-dependent increase in the incidence of “kiss and run” type events. However, the most striking changes we observe are age-associated changes in the relationship between vesicle size and the membrane bending energy required for exocytosis. Our data illustrates that vesicle size does not regulate release probability, as has been suggested, that membrane elasticity or flexural rigidity change with age and that the mechanisms controlling full fusion may differ from those controlling “kiss and run” fusion.

Chromogranins A and B are key proteins in amine accumulation, but the catecholamine secretory pathway is conserved without them

Chromogranins are the main soluble proteins in the large dense core secretory vesicles (LDCVs) found in aminergic neurons and chromaffin cells. We recently demonstrated that chromogranins A and B each regulate the concentration of adrenaline in chromaffin granules and its exocytosis. Here we have further studied the role played by these proteins by generating mice lacking both chromogranins. Surprisingly, these animals are both viable and fertile. Although chromogranins are thought to be essential for their biogenesis, LDCVs were evident in these mice. These vesicles do have a somewhat atypical appearance and larger size. Despite their increased size, single-cell amperometry recordings from chromaffin cells showed that the amine content in these vesicles is reduced by half. These data demonstrate that although chromogranins regulate the amine concentration in LDCVs, they are not completely essential, and other proteins unrelated to neurosecretion, such as fibrinogen, might compensate for their loss to ensure that vesicles are generated and the secretory pathway conserved.

Microwell device for targeting single cells to electrochemical microelectrodes for high-throughput amperometric detection of quantal exocytosis

Electrochemical microelectrodes are commonly used to detect spikes of amperometric current that correspond to exocytosis of oxidizable transmitter from individual vesicles, i.e., quantal exocytosis. We are developing transparent multielectrochemical electrode arrays on microchips in order to automate measurement of quantal exocytosis. Here, we report development of an improved device to target individual cells to each microelectrode in an array. Efficient targeting (~75%) is achieved using cell-sized microwell traps fabricated in SU-8 photoresist together with patterning of poly(l-lysine) in register with electrodes to promote cell adhesion. The surface between electrodes is made resistant to cell adhesion using poly(ethylene glycol) in order to facilitate movement of cells to electrode "docking sites". We demonstrate the activity of the electrodes using the test analyte ferricyanide and perform recordings of quantal exocytosis from bovine adrenal chromaffin cells on the device. Multiple cell recordings on a single device demonstrate the consistency of spike measurements, and multiple recordings from the same electrodes demonstrate that the device can be cleaned and reused without degradation of performance. The new device will enable high-throughput studies of quantal exocytosis and may also find application in rapidly screening drugs or toxins for effects on exocytosis.

Automated targeting of cells to electrochemical electrodes using a surface chemistry approach for the measurement of quantal exocytosis

Here we describe a method to fabricate a multi-channel high-throughput microchip device for measurement of quantal transmitter release from individual cells. Instead of bringing carbon-fiber electrodes to cells, the device uses a surface chemistry approach to bring cells to an array of electrochemical microelectrodes. The microelectrodes are small and "cytophilic" in order to promote adhesion of a single cell whereas all other areas of the chip are covered with a thin "cytophobic" film to block cell attachement and facilitate movement of cells to electrodes. This cytophobic film also insulates unused areas of the conductive film, thus the alignment of cell docking sites to working electrodes is automatic. Amperometric spikes resulting from single-granule fusion events were recorded on the device and had amplitudes and kinetics similar to those measured using carbon-fiber microelectrodes. Use of this device will increase the pace of basic neuroscience research and may also find applications in drug discovery or validation.

Chromogranin B gene ablation reduces the catecholamine cargo and decelerates exocytosis in chromaffin secretory vesicles

Chromogranins/secretogranins (Cgs) are the major soluble proteins of large dense-core secretory vesicles (LDCVs). We have recently reported that the absence of chromogranin A (CgA) caused important changes in the accumulation and in the exocytosis of catecholamines (CAs) using a CgA-knock-out (CgA-KO) mouse. Here, we have analyzed a CgB-KO mouse strain that can be maintained in homozygosis. These mice have 36% less adrenomedullary epinephrine when compared to Chgb(+/+) [wild type (WT)], whereas the norepinephrine content was similar. The total evoked release of CA was 33% lower than WT mice. This decrease was not due to a lower frequency of exocytotic events but to less secretion per quantum (approximately 30%) measured by amperometry; amperometric spikes exhibited a slower ascending but a normal decaying phase. Cell incubation with L-DOPA increased the vesicle CA content of WT but not of the CgB-KO cells. Intracellular electrochemistry, using patch amperometry, showed that L-DOPA overload produced a significantly larger increase in cytosolic CAs in cells from the KO animals than chromaffin cells from the WT. These data indicate that the mechanisms for vesicular accumulation of CAs in the CgB-KO cells were saturated, while there was ample capacity for further accumulation in WT cells. Protein analysis of LDCVs showed the overexpression of CgA as well as other proteins apparently unrelated to the secretory process. We conclude that CgB, like CgA, is a highly efficient system directly involved in monoamine accumulation and in the kinetics of exocytosis from LDCVs.

Electrophysiological and morphological features underlying neurotransmission efficacy at the splanchnic nerve-chromaffin cell synapse of bovine adrenal medulla

The ability of adrenal chromaffin cells to fast-release catecholamines relies on their capacity to fire action potentials (APs). However, little attention has been paid to the requirements needed to evoke the controlled firing of APs. Few data are available in rodents and none on the bovine chromaffin cell, a model extensively used by researchers. The aim of this work was to clarify this issue. Short puffs of acetylcholine (ACh) were fast perifused to current-clamped chromaffin cells and produced the firing of single APs. Based on the currents generated by such ACh applications and previous literature, current waveforms that efficiently elicited APs at frequencies up to 20 Hz were generated. Complex waveforms were also generated by adding simple waveforms with different delays; these waveforms aimed at modeling the stimulation patterns that a chromaffin cell would conceivably undergo upon strong synaptic stimulation. Cholinergic innervation was assessed using the acetylcholinesterase staining technique on the supposition that the innervation pattern is a determinant of the kind of stimuli chromaffin cells can receive. It is concluded that 1) a reliable method to produce frequency-controlled APs by applying defined current injection waveforms is achieved; 2) the APs thus generated have essentially the same features as those spontaneously emitted by the cell and those elicited by fast-ACh perifusion; 3) the higher frequencies attainable peak at around 30 Hz; and 4) the bovine adrenal medulla shows abundant cholinergic innervation, and chromaffin cells show strong acetylcholinesterase staining, consistent with a tight cholinergic presynaptic control of firing frequency.

Differences in the quantal release of catecholamines in chromaffin cells of rat embryos and their mothers

Studies on the bulk catecholamine release from fetal and neonatal rat adrenals, adrenal slices, or isolated chromaffin cells stimulated with high K+, hypoxia, hypercapnia, or acidosis are available. However, a study analyzing the kinetics of quantal secretion is lacking. We report here such a study in which we compare the quantal release of catecholamines from immature rat embryo chromaffin cells (ECCs) and their mothers' (MCCs). Cell challenging with a strong depolarizing stimulus (75 mM K+) caused spike bursts having the following characteristics. ECCs released more multispike events and wave envelopes than MCCs. This, together with narrower single-spike events, a faster decay, and a threefold smaller quantal size suggest a faster secretory machinery in ECCs. Furthermore, with a milder stimulus (25 mM K+) enhanced Ca2+ entry by L-type Ca2+ channel activator BAY K 8644 did not change the kinetic parameters of single spikes in ECCs; in contrast, augmentation of Ca2+ entry increased spike amplitude and width, quantal size, and decay time in MCCs. This suggests that in mature MCCs, the last exocytotic steps are more tightly regulated than in immature ECCs. Finally, we found that quantal secretion was fully controlled by L-type voltage-dependent Ca2+ channels (VDCCs) in ECCs, whereas both L- and non-L VDCCs (N and PQ) contributed equally to secretion control in MCCs. Our results have the following physiological, pharmacological, and clinical relevance: 1) they may help to better understand the regulation of adrenal catecholamine release in response to stress during fetal life and delivery; 2) if clinically used, L-type Ca2+ channel blockers may augment the incidence of sudden infant death syndrome (SIDS); and 3) so-called Ca2+ promotors or activators of Ca2+ entry through L-type VDCCs may be useful to secure a healthy catecholamine surge upon violent stress during fetal life, at birth, or to prevent the SIDS in neonates at risk.

Monitoring of vesicular exocytosis from single cells using micrometer and nanometer-sized electrochemical sensors

Communication between cells by release of specific chemical messengers via exocytosis plays crucial roles in biological process. Electrochemical detection based on ultramicroelectrodes (UMEs) has become one of the most powerful techniques in real-time monitoring of an extremely small number of released molecules during very short time scales, owing to its intrinsic advantages such as fast response, excellent sensitivity, and high spatiotemporal resolution. Great successes have been achieved in the use of UME methods to obtain quantitative and kinetic information about released chemical messengers and to reveal the molecular mechanism in vesicular exocytosis. In this paper, we review recent developments in monitoring exocytosis by use of UMEs-electrochemical-based techniques including electrochemical detection using micrometer and nanometer-sized sensors, scanning electrochemical microscopy (SECM), and UMEs implemented in lab-on-a-chip (LOC) microsystems. These advances are of great significance in obtaining a better understanding of vesicular exocytosis and chemical communications between cells, and will facilitate developments in many fields, including analytical chemistry, biological science, and medicine. Furthermore, future developments in electrochemical probing of exocytosis are also proposed.

Amperometric measurements of catecholamine release from single vesicles in MN9D cells

MN9D cells have been used as a successful model to investigate dopamine pharmacology and to test the specific effects of drugs for the treatment of Parkinson's disease. However, quantitative measurements of quantal release from these cells have not been carried out. In this work, we used amperometry to investigate catecholamine release from MN9D cells. Amperometric events were observed in both undifferentiated and differentiated (butyric acid-treated) cells. An increase in quantal size and half-width was observed for differentiated cells versus undifferentiated cells; however, the number of events per cell and the amplitude remained constant. In transmission electron microscopy images, no obvious cluster of small synaptic vesicles was observed, and large dense-core vesicles were present in the cell body of undifferentiated cells; however, after differentiation, vesicles were concentrated in the cell processes. In differentiated cells, l-DOPA caused an increase in quantal size and half-width, which could be blocked by the vesicular monoamine transporter inhibitor, reserpine.

G protein betagamma subunits modulate the number and nature of exocytotic fusion events in adrenal chromaffin cells independent of calcium entry

G-protein-coupled receptors (GPCR) play important roles in controlling neurotransmitter and hormone release. Inhibition of voltage-gated Ca(2+) channels (Ca(2+) channels) by G protein betagamma subunits (Gbetagamma) is one prominent mechanism, but there is evidence for additional effects distinct from those on calcium entry. However, relatively few studies have investigated the Ca(2+)-channel-independent effects of Gbetagamma on transmitter release, so the impact of this mechanism remains unclear. We used carbon fiber amperometry to analyze catecholamine release from individual vesicles in bovine adrenal chromaffin cells, a widely used neurosecretory model. To bypass the effects of Gbetagamma on Ca(2+) entry, we stimulated secretion using ionomycin (a Ca(2+) ionophore) or direct intracellular application of Ca(2+) through a patch pipette. Activation of endogenous GPCR or transient transfection with exogenous Gbetagamma significantly reduced the number of amperometric spikes (the number of vesicular fusion events). The charge ("quantal size") and amplitude of the amperometric spikes were also significantly reduced by GPCR/Gbetagamma. We conclude that independent from effects on calcium entry, Gbetagamma can regulate both the number of vesicles that undergo exocytosis and the amount of catecholamine released per fusion event. We discuss possible mechanisms by which Gbetagamma might exert these novel effects including interaction with the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex.

The crucial role of chromogranins in storage and exocytosis revealed using chromaffin cells from chromogranin A null mouse

Chromogranins (Cgs) are the major soluble proteins of dense-core secretory vesicles. Chromaffin cells from Chga null mice [chromogranin A knock-out (CgA-KO)] exhibited approximately 30% reduction in the content and in the release of catecholamines compared with wild type. This was because of a lower secretion per single exocytotic event, rather than to a lower frequency of exocytotic events. Cell incubation with L-DOPA produced an increase in the vesicular amine content of wild-type, but not CgA-KO vesicles. In contrast, intracellular electrochemistry showed that L-DOPA produced a significantly larger increase in cytosolic amines in CgA-KO cells than in the wild type. These data indicate that the mechanisms for vesicular accumulation in CgA-KO cells were fully saturated. Patch-amperometry recordings showed a delayed initiation of the amperometric signal after vesicle fusion, whereas no changes were observed in vesicle size or fusion pore kinetics despite the smaller amine content. We conclude that intravesicular proteins are highly efficient systems directly implicated in transmitter accumulation and in the control of neurosecretion.

Alpha-synuclein overexpression in PC12 and chromaffin cells impairs catecholamine release by interfering with a late step in exocytosis

Alpha-synuclein (alpha-syn), a protein implicated in Parkinson's disease pathogenesis, is a presynaptic protein suggested to regulate transmitter release. We explored how alpha-syn overexpression in PC12 and chromaffin cells, which exhibit low endogenous alpha-syn levels relative to neurons, affects catecholamine release. Overexpression of wild-type or A30P mutant alpha-syn in PC12 cell lines inhibited evoked catecholamine release without altering calcium threshold or cooperativity of release. Electron micrographs revealed that vesicular pools were not reduced but that, on the contrary, a marked accumulation of morphologically "docked" vesicles was apparent in the alpha-syn-overexpressing lines. We used amperometric recordings from chromaffin cells derived from mice that overexpress A30P or wild-type (WT) alpha-syn, as well as chromaffin cells from control and alpha-syn null mice, to determine whether the filling of vesicles with the transmitter was altered. The quantal size and shape characteristics of amperometric events were identical for all mouse lines, suggesting that overexpression of WT or mutant alpha-syn did not affect vesicular transmitter accumulation or the kinetics of vesicle fusion. The frequency and number of exocytotic events per stimulus, however, was lower for both WT and A30P alpha-syn-overexpressing cells. The alpha-syn-overexpressing cells exhibited reduced depression of evoked release in response to repeated stimuli, consistent with a smaller population of readily releasable vesicles. We conclude that alpha-syn overexpression inhibits a vesicle "priming" step, after secretory vesicle trafficking to "docking" sites but before calcium-dependent vesicle membrane fusion.

Intragranular pH rapidly modulates exocytosis in adrenal chromaffin cells

Several drugs produce rapid changes in the kinetics of exocytosis of catecholamines, as measured at the single event level with amperometry. This study is intended to unveil whether the mechanism(s) responsible for these effects involve changes in the intravesicular pH. Cell incubation with bafilomycin A1, a blocker of the vesicular proton pump, caused both a deceleration in the kinetics of exocytosis and a reduction in the catecholamine content of vesicle. These effects were also observed upon reduction of proton gradient by nigericin or NH4Cl. pH measurements using fluorescent probes (acridine orange, quinacrine or enhanced green fluorescent protein-synaptobrevin) showed a strong correlation between vesicular pH and the kinetics of exocytosis. Hence, all maneuvers tested that decelerated exocytosis also alkalinized secretory vesicles and vice versa. On the other hand, calcium entry caused a transient acidification of granules. We therefore propose that the regulation of vesicular pH is, at least partially, a necessary step in the modulation of the kinetics of exocytosis and quantal size operated by some cell signals.

Rapid change of quantal size in PC-12 cells detected by neural networks

The basic building block of synaptic transmission-the number of molecules released per vesicle (quantal size (QS)) often changes with stimulation, but there is no agreement about what factors regulate it. To throw more light on this problem spontaneous quantal release was recorded amperometrically in PC-12 cells. Amperometric current spikes, representing single vesicle release, were detected by thresholding and were separated from spurious events on the basis of their amplitude and time course using a pattern recognition system based on the principal component neural network methods. The frequency of current spikes, their amplitude, quantal size, rise time and decay time were typically non-stationary, even in the absence of stimulation. Their running values changed much more than those of memoryless stationary random data with the same probability density distribution. Irrespective of how much the quantal size, rise and decay times varied, their amplitude dependence remained constant, or changed with a very different time course. In conclusion, the quantal size is highly labile in PC-12 cells. The lability does not appear to result from the changes of fusion pore dynamics or the mechanism of release of vesicular content, but because of the preferential release of large vesicles.

Inhibition of voltage-gated calcium channels by sequestration of β subunits

The auxiliary Ca(v)beta subunit is essential for functional expression of high-voltage activated Ca(2+) channels. Here, we describe a lure sequence designed to sequester the Ca(v)beta subunits in transfected bovine chromaffin cells. This sequence is composed of the extracellular and transmembrane domains of the alpha chain of the human CD8, the I-II loop of Ca(v)2.1 subunit, and EGFP. We showed that expressing the CD8-I-II-EGFP sequence in chromaffin cells led to a >50% decrease in overall Ca(2+) current density. Although this decrease involved all the Ca(2+) channel types (L, N, P/Q, R), the proportion of each type supporting the remaining current was altered. A similar effect was observed after transfection when measuring the functional role of Ca(2+) channels in catecholamine release by chromaffin cells: global decrease of release and change of balance between the different channel types supporting it. Possible explanations for this apparent discrepancy are further discussed.

Differential control of adrenal and sympathetic catecholamine release by alpha 2-adrenoceptor subtypes

In the adrenergic system, release of the neurotransmitter norepinephrine from sympathetic nerves is regulated by presynaptic inhibitory alpha2-adrenoceptors, but it is unknown whether release of epinephrine from the adrenal gland is controlled by a similar short feedback loop. Using gene-targeted mice we demonstrate that two distinct subtypes of alpha2-adrenoceptors control release of catecholamines from sympathetic nerves (alpha 2A) and from the adrenal medulla (alpha 2C). In isolated mouse chromaffin cells, alpha2-receptor activation inhibited the electrically stimulated increase in cell capacitance (a correlate of exocytosis), voltage-activated Ca2+ current, as well as secretion of epinephrine and norepinephrine. The inhibitory effects of alpha2-agonists on cell capacitance, voltage-activated Ca2+ currents, and on catecholamine secretion were completely abolished in chromaffin cells isolated from alpha 2C-receptor-deficient mice. In vivo, deletion of sympathetic or adrenal feedback control led to increased plasma and urine norepinephrine (alpha 2A-knockout) and epinephrine levels (alpha 2C-knockout), respectively. Loss of feedback inhibition was compensated by increased tyrosine hydroxylase activity, as detected by elevated tissue dihydroxyphenylalanine levels. Thus, receptor subtype diversity in the adrenergic system has emerged to selectively control sympathetic and adrenal catecholamine secretion via distinct alpha2-adrenoceptor subtypes. Short-loop feedback inhibition of epinephrine release from the adrenal gland may represent a novel therapeutic target for diseases that arise from enhanced adrenergic stimulation.

Flow-based amperometric detection of dopamine in an immobilized cell reactor

A protocol is described for immobilizing PC 12 cells on to the lumen of fused silica microbore tubing having an inside diameter of 250 microm and coated with a thin layer of poly-L-lysine. Optimization studies of the immobilization procedure indicated that a 50 microg ml(-1) solution of poly-L-lysine was the best material for cell adhesion to the fused silica tubing. In addition, it was found that the cells become attached to the poly-L-lysine in approximately 2 h, after which they could be maintained inside of the tubing for a period up to 5 days. Importantly, the immobilized cells ability to release neurotransmitters was evident by measuring the Ca(2+)-induced release of dopamine with an in column amperometric detection scheme involving a Nafion modified platinum ultramicroelectrode.