Possible role during exocytosis of a Ca(2+)-activated channel in neurohypophysial granules

Functional ryanodine receptors in the membranes of neurohypophysial secretory granules

Highly localized Ca(2+) release events have been characterized in several neuronal preparations. In mouse neurohypophysial terminals (NHTs), such events, called Ca(2+) syntillas, appear to emanate from a ryanodine-sensitive intracellular Ca(2+) pool. Traditional sources of intracellular Ca(2+) appear to be lacking in NHTs. Thus, we have tested the hypothesis that large dense core vesicles (LDCVs), which contain a substantial amount of calcium, represent the source of these syntillas. Here, using fluorescence immunolabeling and immunogold-labeled electron micrographs of NHTs, we show that type 2 ryanodine receptors (RyRs) are localized specifically to LDCVs. Furthermore, a large conductance nonspecific cation channel, which was identified previously in the vesicle membrane and has biophysical properties similar to that of an RyR, is pharmacologically affected in a manner characteristic of an RyR: it is activated in the presence of the RyR agonist ryanodine (at low concentrations) and blocked by the RyR antagonist ruthenium red. Additionally, neuropeptide release experiments show that these same RyR agonists and antagonists modulate Ca(2+)-elicited neuropeptide release from permeabilized NHTs. Furthermore, amperometric recording of spontaneous release events from artificial transmitter-loaded terminals corroborated these ryanodine effects. Collectively, our findings suggest that RyR-dependent syntillas could represent mobilization of Ca(2+) from vesicular stores. Such localized vesicular Ca(2+) release events at the precise location of exocytosis could provide a Ca(2+) amplification mechanism capable of modulating neuropeptide release physiologically.

Voltage-dependent kappa-opioid modulation of action potential waveform-elicited calcium currents in neurohypophysial terminals

Release of neurotransmitter is activated by the influx of calcium. Inhibition of Ca(2+) channels results in less calcium influx into the terminals and presumably a reduction in transmitter release. In the neurohypophysis (NH), Ca(2+) channel kinetics, and the associated Ca(2+) influx, is primarily controlled by membrane voltage and can be modulated, in a voltage-dependent manner, by G-protein subunits interacting with voltage-gated calcium channels (VGCCs). In this series of experiments we test whether the kappa- and micro-opioid inhibition of Ca(2+) currents in NH terminals is voltage-dependent. Voltage-dependent relief of G-protein inhibition of VGCC can be achieved with either a depolarizing square pre-pulse or by action potential waveforms. Both protocols were tested in the presence and absence of opioid agonists targeting the kappa- and micro-receptors in neurohypophysial terminals. The kappa-opioid VGCC inhibition is relieved by such pre-pulses, suggesting that this receptor is involved in a voltage-dependent membrane delimited pathway. In contrast, micro-opioid inhibition of VGCC is not relieved by such pre-pulses, indicating a voltage-independent diffusible second-messenger signaling pathway. Furthermore, relief of kappa-opioid inhibition during a physiologic action potential (AP) burst stimulation indicates the possibility of activity-dependent modulation in vivo. Differences in the facilitation of Ca(2+) channels due to specific G-protein modulation during a burst of APs may contribute to the fine-tuning of Ca(2+)-dependent neuropeptide release in other CNS terminals, as well.

Individual calcium syntillas do not trigger spontaneous exocytosis from nerve terminals of the neurohypophysis

Recently, highly localized Ca(2+) release events, similar to Ca(2+) sparks in muscle, have been observed in neuronal preparations. Specifically, in murine neurohypophysial terminals (NHT), these events, termed Ca(2+) syntillas, emanate from a ryanodine-sensitive intracellular Ca(2+) pool and increase in frequency with depolarization in the absence of Ca(2+) influx. Despite such knowledge of the nature of these Ca(2+) release events, their physiological role in this system has yet to be defined. Such localized Ca(2+) release events, if they occur in the precise location of the final exocytotic event(s), may directly trigger exocytosis. However, directly addressing this hypothesis has not been possible, since no method capable of visualizing individual release events in these CNS terminals has been available. Here, we have adapted an amperometric method for studying vesicle fusion to this system which relies on loading the secretory granules with the false transmitter dopamine, thus allowing, for the first time, the recording of individual exocytotic events from peptidergic NHT. Simultaneous use of this technique along with high-speed Ca(2+) imaging has enabled us to establish that spontaneous neuropeptide release and Ca(2+) syntillas do not display any observable temporal or spatial correlation, confirming similar findings in chromaffin cells. Although these results indicate that syntillas do not play a direct role in eliciting spontaneous release, they do not rule out indirect modulatory effects of syntillas on secretion.

Spectral imaging microscopy demonstrates cytoplasmic pH oscillations in glial cells

Glial cells exhibit distinct cellular domains, somata, and filopodia. Thus the cytoplasmic pH (pH(cyt)) and/or the behavior of the fluorescent ion indicator might be different in these cellular domains because of distinct microenvironments. To address these issues, we loaded C6 glial cells with carboxyseminaphthorhodafluor (SNARF)-1 and evaluated pH(cyt) using spectral imaging microscopy. This approach allowed us to study pH(cyt) in discrete cellular domains with high temporal, spatial, and spectral resolution. Because there are differences in the cell microenvironment that may affect the behavior of SNARF-1, we performed in situ titrations in discrete cellular regions of single cells encompassing the somata and filopodia. The in situ titration parameters apparent acid-base dissociation constant (pK'(a)), maximum ratio (R(max)), and minimum ratio (R(min)) had a mean coefficient of variation approximately six times greater than those measured in vitro. Therefore, the individual in situ titration parameters obtained from specific cellular domains were used to estimate the pH(cyt) of each region. These studies indicated that glial cells exhibit pH(cyt) heterogeneities and pH(cyt) oscillations in both the absence and presence of physiological HCO(3)(-). The amplitude and frequency of the pH(cyt) oscillations were affected by alkalosis, by acidosis, and by inhibitors of the ubiquitous Na(+)/H(+) exchanger- and HCO(3)(-)-based H(+)-transporting mechanisms. Optical imaging approaches used in conjunction with BCECF as a pH probe corroborated the existence of pH(cyt) oscillations in glial cells.

Hydrogen ions control synaptic vesicle ion channel activity in Torpedo electromotor neurones

During exocytosis the synaptic vesicle fuses with the surface membrane and undergoes a pH jump. When the synaptic vesicle is inside the presynaptic nerve terminal its internal pH is about 5.5 and after fusion, the inside of the vesicle comes in contact with the extracellular medium with a pH of about 7.25. We examined the effect of such pH jump on the opening of the non-specific ion channel in the synaptic vesicle membrane, in the context of the post-fusion hypothesis of transmitter release control. The vesicles were isolated from Torpedo ocellata electromotor neurones. The pH dependence of the opening of the non-specific ion channel was examined using the fused vesicle-attached configuration of the patch clamp technique. The rate of opening depends on both pH and voltage. Increasing the pH from 5.5 to 7.25 activated dramatically the non-specific ion channel of the vesicle membrane. The single channel conductance did not change significantly with the alteration in the pH, and neither did the mean channel open time. These results support the hypothesis that during partial fusion of the vesicle with the surface membrane, ion channels in the vesicle membrane open, admit ions and thus help in the ion exchange process mechanism, leading to the release of the transmitter from the intravesicular ion exchange matrix. This process may have also a pathophysiological significance in conditions of altered pH.

Ca2+ syntillas, miniature Ca2+ release events in terminals of hypothalamic neurons, are increased in frequency by depolarization in the absence of Ca2+ influx

Localized, brief Ca2+ transients (Ca2+ syntillas) caused by release from intracellular stores were found in isolated nerve terminals from magnocellular hypothalamic neurons and examined quantitatively using a signal mass approach to Ca2+ imaging. Ca2+ syntillas (scintilla, L., spark, from a synaptic structure, a nerve terminal) are caused by release of approximately 250,000 Ca ions on average by a Ca2+ flux lasting on the order of tens of milliseconds and occur spontaneously at a membrane potential of -80 mV. Syntillas are unaffected by removal of extracellular Ca2+, are mediated by ryanodine receptors (RyRs) and are increased in frequency, in the absence of extracellular Ca2+, by physiological levels of depolarization. This represents the first direct demonstration of mobilization of Ca2+ from intracellular stores in neurons by depolarization without Ca2+ influx. The regulation of syntillas by depolarization provides a new link between neuronal activity and cytosolic [Ca2+] in nerve terminals.

Isoflurane and propofol inhibit voltage-gated sodium channels in isolated rat neurohypophysial nerve terminals

Mounting electrophysiological evidence indicates that certain general anesthetics, volatile anesthetics in particular, depress excitatory synaptic transmission by presynaptic mechanisms. We studied the effects of representative general anesthetics on voltage-gated Na+ currents (INa) in nerve terminals isolated from rat neurohypophysis using patch-clamp electrophysiological analysis. Both isoflurane and propofol inhibited INa in a dose-dependent and reversible manner. At holding potentials of -70 or -90 mV, isoflurane inhibited peak INa with IC50 values of 0.45 and 0.56 mM, and propofol inhibited peak INa with IC50 values of 4.1 and 6.0 microM, respectively. Isoflurane (0.8 mM) did not significantly alter the V1/2 of activation; propofol caused a small positive shift. Isoflurane (0.8 mM) or propofol (5 microM) produced a negative shift in the voltage dependence of inactivation. Recovery of INa from inactivation was slower from a holding potential of -70 mV than from -90 mV; isoflurane and propofol further delayed recovery from inactivation. In conclusion, the volatile anesthetic isoflurane and the intravenous anesthetic propofol inhibit voltage-gated Na+ currents in isolated neurohypophysial nerve terminals in a concentration- and voltage-dependent manner. Marked effects on the voltage dependence and kinetics of inactivation and minimal effects on activation support preferential anesthetic interactions with the fast inactivated state of the Na+ channel. These results are consistent with direct inhibition of oxytocin and vasopressin release from the neurohypophysis by isoflurane and propofol. Inhibition of voltage-gated Na+ channels may contribute to the presynaptic effects of general anesthetics on nerve terminal excitability and neurotransmitter release.

Ca(2+)-regulated, neurosecretory granule channel involved in release from neurohypophysial terminals

Ion channels from bovine neurohypophysial secretory granules (NSG) were incorporated into artificial lipid bilayers. Specific antibodies against identified synaptic vesicle proteins were tested on such incorporated channel activity and on peptide release from rat permeabilized neurohypophysial terminals. Both the NSG cation channel and Ca(2+)-dependent release were inhibited by only SY-38, a monoclonal antibody directed against the C-terminus of synaptophysin. SY-38 and Ca(2+) altered both the gating and conductance of the NSG cation channel, but in opposite ways. The close correlation between SY-38 effects on Ca(2+)-dependent channel activity and release leads us to conclude that this synaptophysin-like NSG channel is directly involved in peptide secretion from these central nervous system terminals.

Single granule pH cycling in antigen-induced mast cell secretion

The pH cycling of individual granules in secreting (serotonin-loaded) mast cells is quantitatively examined using multicolor multiphoton fluorescence microscopy. A typical exocytosis event consists of maximal calcium rise at time zero, granule alkalization a few seconds later and, finally, complete contents release at a fraction of a second after alkalization. Membrane fusion is either transient, as indicated by subsequent granule reacidification, or ‘full’, as indicated by a granule disappearance with a collapse of its membrane into the plasma membrane. The relative frequency of these two coexisting behaviors (the ‘kiss-to-collapse’ ratio) is approximately 2:1. A typical transiently fusing granule experiences multiple alkalization/acidification cycles after addition of exogenous antigen. Between recycling granules, coalescence events are frequent, with 80% resulting in a collapse of the formed granule complex to the plasma membrane. The full dynamics of secretion encompass a complex combination of these granule activities.

Building a bilayer model of the neuromuscular synapse

Progress over the past 10 years has made it possible to construct a simple model of neurotransmitter release. Currently, some models use artificially formed vesicles to represent synaptic vesicles and a planar lipid bilayer as a presynaptic membrane. Fusion of vesicles with the bilayer is via channel proteins in the vesicle membrane and an osmotic gradient. In this paper; a framework is presented for the successful construction of a more complete model of synaptic transmission. This model includes real synaptic vesicles that fuse with a planar bilayer. The bilayer contains acetylcholine receptor (AChR) channels which function as autoreceptors in the membrane. Vesicle fusion is initiated following a Ca2+ flux through voltage-gated Ca2+ channels. Key steps in the plan are validated by mathematical modeling. Specifically, the probability that a reconstituted AChR channel opens following the release of ACh from a fusing vesicle, is calculated as a function of time, quantal content, and number of reconstituted AChRs. Experimentally obtainable parameters for construction of a working synapse are given. The inevitable construction of a full working model will mean that the minimal structures necessary for synaptic transmission are identified. This will open the door in determining regulatory and modulatory factors of transmitter release.

Ion channels in presynaptic nerve terminals and control of transmitter release

The primary function of the presynaptic nerve terminal is to release transmitter quanta and thus activate the postsynaptic target cell. In almost every step leading to the release of transmitter quanta, there is a substantial involvement of ion channels. In this review, the multitude of ion channels in the presynaptic terminal are surveyed. There are at least 12 different major categories of ion channels representing several tens of different ion channel types; the number of different ion channel molecules at presynaptic nerve terminals is many hundreds. We describe the different ion channel molecules at the surface membrane and inside the nerve terminal in the context of their possible role in the process of transmitter release. Frequently, a number of different ion channel molecules, with the same basic function, are present at the same nerve terminal. This is especially evident in the cases of calcium channels and potassium channels. This abundance of ion channels allows for a physiological and pharmacological fine tuning of the process of transmitter release and thus of synaptic transmission.

Stimulus-secretion coupling in neurohypophysial nerve endings: a role for intravesicular sodium?

It is generally accepted that Ca is essentially involved in regulated secretion, but the role of this cation, as well as others such as Na, is not well understood. An illustrative example occurs in neurohypophysial secretion, where an experimentally induced increase in the cytosolic concentration of Na+ can induce continuous neuropeptide release. In contrast, an increase in cytosolic Ca2+ will have only a transient stimulatory effect. The secretion-promoting targets for Ca2+ are not known; they may be cytosolic, as is usually assumed, but they may also be intravesicular, especially in view of evidence that Ca-rich secretory vesicles are preferentially secreted. In the present work, we have investigated the movements of these cations into and out of secretory vesicles during stimulus-secretion coupling. Isolated rat neurohypophysial nerve endings were stimulated by potassium (55 mM) depolarization, and at 6 min (peak secretion) and 20 min after the onset of stimulation, the elemental content of individual secretory vesicles was measured by quantitative x-ray microanalysis. A depolarization-induced transient increase in intravesicular Na+ concentration was found to coincide with the onset of secretion. Moreover, only a predicted small fraction of peripheral vesicles-presumably the docked ones-were Na+-loaded. The low sulfur concentration of Na+-rich vesicles most likely resulted from vesicle swelling. The results suggest that high intravesicular Na+ concentrations in docked vesicles, occurring by Na+/Ca2+ exchange or by transient fusion pore opening, is a proximal event in exocytosis.

Multitude of ion channels in the regulation of transmitter release

The presynaptic nerve terminal is of key importance in communication in the nervous system. Its primary role is to release transmitter quanta on the arrival of an appropriate stimulus. The structural basis of these transmitter quanta are the synaptic vesicles that fuse with the surface membrane of the nerve terminal, to release their content of neurotransmitter molecules and other vesicular components. We subdivide the control of quantal release into two major classes: the processes that take place before the fusion of the synaptic vesicle with the surface membrane (the pre-fusion control) and the processes that occur after the fusion of the vesicle (the post-fusion control). The pre-fusion control is the main determinant of transmitter release. It is achieved by a wide variety of cellular components, among them the ion channels. There are reports of several hundred different ion channel molecules at the surface membrane of the nerve terminal, that for convenience can be grouped into eight major categories. They are the voltage-dependent calcium channels, the potassium channels, the calcium-gated potassium channels, the sodium channels, the chloride channels, the non-selective channels, the ligand gated channels and the stretch-activated channels. There are several categories of intracellular channels in the mitochondria, endoplasmic reticulum and the synaptic vesicles. We speculate that the vesicle channels may be of an importance in the post-fusion control of transmitter release.

Ca2+-induced deprotonation of peptide hormones inside secretory vesicles in preparation for release

The acidic environment inside secretory vesicles ensures that neuropeptides and peptide hormones are packaged in a concentrated condensed form. Although this is optimal for storage, decondensation limits release. Thus, it would be advantageous to alter the physical state of peptides in preparation for exocytosis. Here, we report that depolarization of the plasma membrane rapidly increases enhanced green fluorescent protein (EGFP)-tagged hormone fluorescence inside secretory vesicles. This effect requires Ca2+ influx and persists when exocytosis is inhibited by N-ethylmaleimide. Peptide deprotonation appears to produce this response, because it is not seen when the vesicle pH gradient is collapsed or when a pH-insensitive GFP variant is used. These data demonstrate that Ca2+ evokes alkalinization of the inside of secretory vesicles before exocytosis. Thus, Ca2+ influx into the cytoplasm alters the physical state of intravesicular contents in preparation for release.

Ca2+ influx and activation of a cation current are coupled to intracellular Ca2+ release in peptidergic neurons of Aplysia californica

1. Stimulation of inputs to bag cell neurons in the abdominal ganglion of Aplysia californica causes an increase in their intracellular Ca2+ concentration ([Ca2+]i). We have used thapsigargin, a specific inhibitor of the endoplasmic reticulum Ca2+ pump, to analyse the effects of Ca2+ released from intracellular stores on the electrophysiological responses of bag cell neurons. 2. Using digital imaging of fura-2-loaded isolated bag cell neurons we found that thapsigargin rapidly evoked an increase in [Ca2+]i in somata, with smaller increases in neurites. Thapsigargin-induced elevation of [Ca2+]i peaked at about 1 microM within 5-10 min and then decayed to basal levels by 30 min. 3. Placement of an extracellular vibrating Ca(2+)-selective microelectrode to within 1 micron of somata revealed a relatively large steady-state Ca2+ efflux. Thapsigargin produced a rapid increase in Ca2+ influx. Changes in Ca2+ flux were not detected at neurites. 4. Thapsigargin produced a small depolarization in isolated bag cell neurons in artificial sea water (ASW). Sometimes enhanced depolarizations were observed when extracellular Na+ was replaced by TEA or Tris, but not N-methyl-D-glucamine (NMDG). The depolarization was not blocked by 100 microM tetrodotoxin (TTX), removal of extracellular Ca2+ (0.5 mM EGTA) or addition of 10 mM Co2+ to the bath solution. 5. In voltage-clamp experiments, thapsigargin induced an inward current (ITg) that was recorded in Ca(2+)-free media containing TEA or Tris substituted for Na+. The apparent reversal potential of ITg was -16.8 +/- 1.2 mV in TEA-ASW. Induction of ITg was inhibited in neurons that were microinjected with the Ca2+ chelator BAPTA-Dextran70 or treated with the membrane-permeant analogue BAPTA AM. Activation of ITg was not observed when Na+ was replaced with NMDG. Manipulation of [Na+]o and [K+]o produced shifts in the reversal potential of ITg consistent with the underlying channels being permeable to both Na+ and K+. 6. Thapsigargin did not alter the amplitude or kinetics of voltage-activated Ba2+ currents, but in some experiments it did increase the amplitude of a component of outward K+ current. 7. Thapsigargin neither induced bag cell neurons within the intact ganglion to depolarize and fire spontaneously, nor did it alter the frequency or duration of firing of an electrically stimulated bag cell after-discharge. 8. We conclude that thapsigargin-sensitive Ca2+ pools are present predominantly in the somata of bag cell neurons. Ca2+ that is released from thapsigargin-sensitive Ca2+ stores activates a non-selective cation current that may help sustain depolarization of the somata, but does not by itself trigger an after-discharge.

The non-specific ion channel in Torpedo ocellata fused synaptic vesicles

1. Synaptic vesicles were isolated and fused into large structures with a diameter of more than 20 microns to characterize their ionic channels. The 'cell'-attached and inside-out configurations of the patch clamp technique were used. 2. Two types of ion channels were most frequently observed: a low conductance chloride channel and a high conductance non-specific channel. 3. The non-specific channel has a main conducting state and a substate. The main conducting state has a slope conductance of 246 +/- 15 pS (+/- S.E.M., n = 15), in the presence of different combinations of KCl and potassium glutamate. 4. From the reversal potentials of the current-voltage (I-V) relation, it was concluded that this channel conducts both Cl- and K+. 5. The non-specific channel is highly voltage dependent: under steady-state voltages it has a high open probability near 0 mV and does not inactivate; when the membrane is hyperpolarized (pipette side more positive), the open probability decreases dramatically. 6. Voltage pulses showed that upon hyperpolarization (from holding potentials between -20 and + 20 mV), the channels deactivated; when the membrane was stepped back to the holding potential, the channels reactivated rapidly. 7. In a number of experiments, when the pipette side was made more negative than the bath, the open probability also decreased. 8. Frequently, a substate with a conductance of about 44 +/- 4% (+/- S.E.M., n = 3) of the main state was detected. 9. We speculate that this non-specific ion channel may have different roles at the various stages of the life cycle of the synaptic vesicle. When the synaptic vesicle is an intracellular structure, it might help its transmitter-concentrating capacity by dissipating the polarization. After fusion with the surface membrane, it might constitute an additional conductance pathway, taking part in frequency modulation of synaptic transmission.

Evaluation of the evidence for ion channels in synaptic vesicles

Synaptic vesicles (SVs) have been the focus of much research for many years, however only recently have ion channels from SV membranes been reported. There is now convincing evidence that SVs contain ion channels. This conclusion is based on direct experimental results from several different laboratories using the patch clamp or planar lipid bilayer technique on SVs and neurosecretory granules (NSG). Some limitations of patch clamping and of fusing synamptic vesicles to a bilayer are described and the advantages of the nystatin/ergosterol fusion method are presented. Six different channels appear to exist in SV (or NSG) membranes. Two large channels (250 and 154 pS) have been observed in SVs isolated from mammalian brain, two channels (180 and 13 pS) from Torpedo electric organ, and two channels (130 and 30-40 pS) from NSG. The three larger channels from each set (250, 180 and 130 pS7) are novel in that they have a subconductance state. The 154 pS channel has been identified as synaptophysin but the identity and function of the other channels is unknown. Although some of the channels are gated by voltage, only the 130 pS channel is modulated by Ca2+. Further knowledge of what regulates these channels is mandatory if we are to determine the physiological significance of these channels.

Patch clamp studies of single intact secretory granules

The membrane of secretory granules is involved in the molecular events that cause exocytotic fusion. Several of the proteins that have been purified from the membrane of secretory granules form ion channels when they are reconstituted in lipid bilayers and, therefore, have been thought to form part of the molecular structure of the exocytotic fusion pore. We have used the patch clamp technique to study ion conductances in single isolated secretory granules from beige mouse mast cells. We found that the membrane of the intact granule had a conductance of < 50 pS. No abrupt changes in current corresponding to the opening and closing of ion channels were observed, even under conditions where exocytotic fusion occurred. However, mechanical tension or a large voltage pulse caused the breakdown of the granule membrane resulting in the abrupt opening of a pore with an ion conductance of about 1 nS that fluctuated rapidly and could expand to an immeasurably large conductance or close completely. Surprisingly, the behavior of these pores resembled the pattern of conductance changes of exocytotic fusion pores observed in degranulating beige mast cells. This similarity supports the view that the earliest fusion pore is formed upon the breakdown of a bilayer such as that formed during hemifusion.

Connexin43 in rat pituitary: localization at pituicyte and stellate cell gap junctions and within gonadotrophs

Immunohistochemical methods were employed to investigate the cellular and ultrastructural localization of the gap junction protein connexin43 (Cx43) in rat pituitary. Western blots of pituitary homogenates probed with anti-Cx43 antibodies showed the presence of Cx43 in both anterior and posterior pituitary lobes. By light microscopy (LM), Cx43-immunoreactive (Cx43-IR) puncta were found in all areas of the posterior lobe, but at greater concentrations in peripheral regions of this structure. By electron microscopy (EM), immunogold labelling for Cx43 was seen at gap junctions between thin cytoplasmic processes of pituicytes. No immunoreactivity was detected in the intermediate lobe. The anterior lobe contained puncta similar to but more sparsely scattered than those in the posterior lobe, and by EM analysis these were demonstrated to correspond to labelled gap junctions between stellate cells. In addition, anti-Cx43 antibodies produced intracellular labelling in a small percentage of endocrine cells, which were distributed throughout the anterior lobe and determined by double immunostaining methods to be cells containing luteinizing hormone. By EM, labelling within these cells was associated with predominantly large secretory granules and other loosely organized organelles. The results indicate that gap junctions in the pituitary are composed of Cx43 and that this or a related protein may have a novel intracellular function within gonadotrophs.

Reconstitution of transmitter secretion

Molecular mechanisms involved in the various stages of transmitter secretion have been studied by perturbing the composition of secretory cells using pharmacological and biochemical agents. An emerging approach is to reconstitute individual steps or the entire sequence of secretion mechanisms in non-secretory cells by loading the cell with presynaptic components or their mRNAs.

Release of LHRH is linearly related to the time integral of presynaptic Ca2+ elevation above a threshold level in bullfrog sympathetic ganglia

To study the Ca2+ dependency of luteinizing hormone-releasing hormone (LHRH) release in the bullfrog sympathetic ganglia, a method was developed to fill the preganglionic nerve terminal boutons with membrane-impermeant fura-2. We found that as stimulation frequency increased from 0.5 to 40 Hz, the peak [Ca2+]i ([Ca2+]p) and the rate of rise in [Ca2+]i increased, the decay of [Ca2+]i transients followed up to three exponentials, and release of LHRH was linearly related to integral of ([Ca2+]i--[Ca2+]t)dt. The threshold level of [Ca2+]i for LHRH release for a given set of boutons on a C cell, [Ca2+]t, was estimated by the [Ca2+]p evoked by 0.5 Hz stimulation that does not induce LHRH release.