Regulation of intracellular calcium and calcium buffering properties of rat isolated neurohypophysial nerve endings

Synaptic vesicle recruitment for release explored by Monte Carlo simulation at the crayfish neuromuscular junction

Neurotransmission at chemically transmitting synapses requires calcium-mediated fusion of synaptic vesicles with the presynaptic membrane. Utilizing ultrastructural information available for the crustacean excitatory neuromuscular junction, we developed a model that employs the Monte Carlo simulation technique to follow the entry and movement of Ca2+ ions at a presynaptic active zone, where synaptic vesicles are preferentially docked for release. The model includes interaction of Ca2+ with an intracellular buffer, and variable separation between calcium channels and vesicle-associated Ca2+-binding targets that react with Ca2+ to trigger vesicle fusion. The end point for vesicle recruitment for release was binding of four Ca2+ ions to the target controlling release. The results of the modeling experiments showed that intracellular structures that interfere with Ca2+ diffusion (in particular synaptic vesicles) influence recruitment or priming of vesicles for release. Vesicular recruitment is strongly influenced by the separation distance between an opened calcium channel and the target controlling release, and by the concentration and binding properties of the intracellular buffers, as in previous models. When a single opened calcium channel is very close to the target, a single synaptic vesicle can be recruited. However, many of the single-channel openings actuated by a nerve impulse are likely to be ineffective for release, although they contribute to the buildup of total intracellular Ca2+. Thus, the overall effectiveness of single calcium channels in causing vesicles to undergo exocytosis is likely quite low.Key words: synapse, Monte Carlo simulation, synaptic vesicle, active zone, vesicle recruitment, crayfish, calcium, calcium buffer.

Pulsed laser imaging of Ca(2+) influx in a neuroendocrine terminal

The surge of Ca(2+) that triggers vesicle fusion is shaped by the distribution of Ca(2+) channels and the physical relationship between those channels and the exocytotic apparatus. Although channels and the release apparatus are thought to be tightly associated at fast synapses, the arrangement at neuroendocrine cells is less clear. The distribution of Ca(2+) influx near release sites is difficult to determine because of spatial and temporal limitations on Ca(2+) imaging techniques. We now present spatially resolved images of Ca(2+) influx into rat neuroendocrine terminals on a millisecond time scale. Images of voltage-dependent Ca(2+) influx into neurohypophysial terminals were captured after excitation of Ca(2+)-sensitive dyes with pulses of laser light lasting a fraction of a microsecond. Submembranous Ca(2+) increases were detected during the first millisecond of an evoked Ca(2+) tail current. Steep gradients of Ca(2+) were evident, with concentrations near the membrane reaching above 1 microM during a 30 msec depolarization. Ca(2+) influx appeared evenly distributed, even when diffusion was restricted with an exogenous Ca(2+) chelator. During longer depolarizations, mean and peak Ca(2+) concentrations reached an asymptote in parallel, suggesting that Ca(2+) binding proteins near the membrane rapidly buffer Ca(2+) and do not become saturated during prolonged influx. These data support the hypothesis that exocytosis is activated in these terminals by the summation of influx through multiple, randomly spaced Ca(2+) channels.

Mitochondrial clearance of cytosolic Ca(2+) in stimulated lizard motor nerve terminals proceeds without progressive elevation of mitochondrial matrix [Ca(2+)]

This study used fluorescent indicator dyes to measure changes in cytosolic and mitochondrial [Ca(2+)] produced by physiological stimulation of lizard motor nerve terminals. During repetitive action potential discharge at 10-50 Hz, the increase in average cytosolic [Ca(2+)] reached plateau at levels that increased with increasing stimulus frequency. This stabilization of cytosolic [Ca(2+)] was caused mainly by mitochondrial Ca(2+) uptake, because drugs that depolarize mitochondria greatly increased the stimulation-induced elevation of cytosolic [Ca(2+)], whereas blockers of other Ca(2+) clearance routes had little effect. Surprisingly, during this sustained Ca(2+) uptake the free [Ca(2+)] in the mitochondrial matrix never exceeded a plateau level of approximately 1 microM, regardless of stimulation frequency or pattern. When stimulation ceased, matrix [Ca(2+)] decreased over a slow ( approximately 10 min) time course consisting of an initial plateau followed by a return to baseline. These measurements demonstrate that sustained mitochondrial Ca(2+) uptake is not invariably accompanied by progressive elevation of matrix free [Ca(2+)]. Both the plateau of matrix free [Ca(2+)] during stimulation and its complex decay after stimulation could be accounted for by a model incorporating reversible formation of an insoluble Ca salt. This mechanism allows mitochondria to sequester large amounts of Ca(2+) while maintaining matrix free [Ca(2+)] at levels sufficient to activate Ca(2+)-dependent mitochondrial dehydrogenases, but below levels that activate the permeability transition pore.

Mitochondrial clearance of cytosolic Ca(2+) in stimulated lizard motor nerve terminals proceeds without progressive elevation of mitochondrial matrix [Ca(2+)]

This study used fluorescent indicator dyes to measure changes in cytosolic and mitochondrial [Ca(2+)] produced by physiological stimulation of lizard motor nerve terminals. During repetitive action potential discharge at 10-50 Hz, the increase in average cytosolic [Ca(2+)] reached plateau at levels that increased with increasing stimulus frequency. This stabilization of cytosolic [Ca(2+)] was caused mainly by mitochondrial Ca(2+) uptake, because drugs that depolarize mitochondria greatly increased the stimulation-induced elevation of cytosolic [Ca(2+)], whereas blockers of other Ca(2+) clearance routes had little effect. Surprisingly, during this sustained Ca(2+) uptake the free [Ca(2+)] in the mitochondrial matrix never exceeded a plateau level of approximately 1 microM, regardless of stimulation frequency or pattern. When stimulation ceased, matrix [Ca(2+)] decreased over a slow ( approximately 10 min) time course consisting of an initial plateau followed by a return to baseline. These measurements demonstrate that sustained mitochondrial Ca(2+) uptake is not invariably accompanied by progressive elevation of matrix free [Ca(2+)]. Both the plateau of matrix free [Ca(2+)] during stimulation and its complex decay after stimulation could be accounted for by a model incorporating reversible formation of an insoluble Ca salt. This mechanism allows mitochondria to sequester large amounts of Ca(2+) while maintaining matrix free [Ca(2+)] at levels sufficient to activate Ca(2+)-dependent mitochondrial dehydrogenases, but below levels that activate the permeability transition pore.

Pulsed laser imaging of Ca(2+) influx in a neuroendocrine terminal

The surge of Ca(2+) that triggers vesicle fusion is shaped by the distribution of Ca(2+) channels and the physical relationship between those channels and the exocytotic apparatus. Although channels and the release apparatus are thought to be tightly associated at fast synapses, the arrangement at neuroendocrine cells is less clear. The distribution of Ca(2+) influx near release sites is difficult to determine because of spatial and temporal limitations on Ca(2+) imaging techniques. We now present spatially resolved images of Ca(2+) influx into rat neuroendocrine terminals on a millisecond time scale. Images of voltage-dependent Ca(2+) influx into neurohypophysial terminals were captured after excitation of Ca(2+)-sensitive dyes with pulses of laser light lasting a fraction of a microsecond. Submembranous Ca(2+) increases were detected during the first millisecond of an evoked Ca(2+) tail current. Steep gradients of Ca(2+) were evident, with concentrations near the membrane reaching above 1 microM during a 30 msec depolarization. Ca(2+) influx appeared evenly distributed, even when diffusion was restricted with an exogenous Ca(2+) chelator. During longer depolarizations, mean and peak Ca(2+) concentrations reached an asymptote in parallel, suggesting that Ca(2+) binding proteins near the membrane rapidly buffer Ca(2+) and do not become saturated during prolonged influx. These data support the hypothesis that exocytosis is activated in these terminals by the summation of influx through multiple, randomly spaced Ca(2+) channels.

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.

Upregulation of calcium homeostatic mechanisms in chronically depolarized rat myenteric neurons

Perturbations of intracellular Ca2+ ion concentration ([Ca2+]i) have important effects on numerous neuronal processes and influence development and survival. Neuronal [Ca2+]i is, in large part, dependent on activity, and changes in activity levels can alter how neurons handle calcium (Ca). To investigate the ability of neuronal Ca homeostatic mechanisms to adapt to the persistent elevation of [Ca2+]i, we used optical and electrophysiological recording techniques to measure [Ca2+]i transients in neurons from the rat myenteric plexus that had been chronically depolarized by growth in culture medium containing elevated (25 mM) KCl. When studied in normal saline, neurons that had previously been chronically depolarized for 3-5 days had briefer action potentials than control neurons, their action potentials produced smaller, more rapidly decaying increases in [Ca2+]i, and voltage-clamp pulses with action potential waveforms evoked smaller Ca currents than in control neurons. Simultaneous voltage-clamp measurements and calcium imaging revealed that increases in the Ca handling capacities of the chronically depolarized neurons permitted them to limit the amplitudes of action potential-evoked [Ca2+]i transients and to restore [Ca2+]i to basal levels more rapidly than control neurons. Release of Ca from endoplasmic reticulum-based Ca stores made smaller contributions to action potential-evoked [Ca2+]i transients in chronically depolarized neurons even though those neurons had larger caffeine-releasable Ca stores. Endoplasmic reticulum-based Ca sequestration mechanisms appeared to contribute to the faster decay of [Ca2+]i transients in chronically depolarized neurons. These results demonstrate that when neurons experience prolonged perturbations of [Ca2+]i, they can adjust multiple components of their Ca homeostatic machinery. Appropriate utilization of this adaptive capability should help neurons resist potentially lethal metabolic and environmental insults.

Astroglial modulation of neurotransmitter/peptide release from the neurohypophysis: present status

Reviewed in this article are those studies that have contributed heavily to our current conceptualizations of glial participation in the functioning of the magnocellular hypothalamo-neurohypophysial system. This system undergoes remarkable morphological and functional reorganization induced by increased demand for peptide synthesis and release, and this reorganization involves the astrocytic elements in primary roles. Under basal conditions, these glia appear to be vested with the responsibility of controlling the neuronal microenvironment in ways that reduce neuronal excitability, restrict access to neuronal membranes by neuroactive substances and deter neuron neuron interactions within the system. With physiological activation, the glial elements, via receptor-mediated mechanisms, take up new positions. This permissively facilitates neuron neuron interactions such as the exposure of neuronal membranes to released peptides and the formation of gap junctions and new synapses, enhances and prolongs the actions of those excitatory neurotransmitters for which there are glial uptake mechanisms, and facilitates the entry of peptides into the blood. In addition, subpopulations of these glia either newly synthesize or increase synthesis of neuroactive peptides for which their neuronal neighbors have receptors. Release of these peptides by the glia or their functional roles in the system have not yet been demonstrated.

Roles of Na(+)-Ca2+ exchange and of mitochondria in the regulation of presynaptic Ca2+ and spontaneous glutamate release

The release of neurotransmitter from presynaptic terminals depends on an increase in the intracellular Ca2+ concentration ([Ca2+]i). In addition to the opening of presynaptic Ca2+ channels during excitation, other Ca2+ transport systems may be involved in changes in [Ca2+]i. We have studied the regulation of [Ca2+]i in nerve terminals of hippocampal cells in culture by the Na(+)-Ca2+ exchanger and by mitochondria. In addition, we have measured changes in the frequency of spontaneous excitatory postsynaptic currents (sEPSC) before and after the inhibition of the exchanger and of mitochondrial metabolism. We found rather heterogeneous [Ca2+]i responses of individual presynaptic terminals after inhibition of Na(+)-Ca2+ exchange. The increase in [Ca2+]i became more uniform and much larger after additional treatment of the cells with mitochondrial inhibitors. Correspondingly, sEPSC frequencies changed very little when only Na(+)-Ca2+ exchange was inhibited, but increased dramatically after additional inhibition of mitochondria. Our results provide evidence for prominent roles of Na(+)-Ca2+ exchange and mitochondria in presynaptic Ca2+ regulation and spontaneous glutamate release.

Calcium buffering capacity of neuronal cell cytosol measured by flash photolysis of calcium buffer NP-EGTA

N1E-115 mouse neuroblastoma cells were injected with a calcium buffer/indicator solution to allow both ratiometric measurement of free calcium concentration and the release of calcium ions upon UV flash. The solution contained sulforhodamine, a marker dye used to estimate the volume injected; fluo-3, a calcium indicator, and NP-EGTA, a high affinity calcium-selective buffer that is converted by UV flash to products with negligible calcium affinity. The calcium increase recorded upon UV irradiation (Delta[Ca2+]i) was small for small injection volumes, increased with larger injection volumes, but approached a plateau at the largest injection volumes. From this relation we estimate the buffering capacity of the cytosol as 1700 ions bound per ion free.

ATP-evoked increases in [Ca2+]i and peptide release from rat isolated neurohypophysial terminals via a P2X2 purinoceptor

1. The effect of externally applied ATP on cytosolic free Ca2+ concentration ([Ca2+]i) was tested in single isolated rat neurohypophysial nerve terminals by fura-2 imaging. The release of vasopressin (AVP) and oxytocin (OT) upon ATP stimulation was also studied from a population of terminals using specific radioimmunoassays. 2. ATP evoked a sustained [Ca2+]i increase, which was dose dependent in the 1-100 microM range (EC50 = 4.8 microM). This effect was observed in only approximately 40 % of the terminals. 3. Interestingly, ATP, in the same range (EC50 = 8.6 microM), evoked AVP, but no significant OT, release from these terminals. 4. Both the [Ca2+]i increase and AVP release induced by ATP were highly and reversibly inhibited by suramin, suggesting the involvement of a P2 purinergic receptor in the ATP-induced responses. Pyridoxal-5-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), another P2 purinergic receptor antagonist, strongly reduced the ATP-induced [Ca2+]i response. 5. To further characterize the receptor, different agonists were tested, with the following efficacy: ATP = 2-methylthio-ATP > ATP-gamma-S > alpha, beta-methylene-ATP > ADP. The compounds adenosine, AMP, beta, gamma-methylene-ATP and UTP were ineffective. 6. The ATP-dependent [Ca2+]i increase was dependent on extracellular Ca2+ concentration ([Ca2+]o). Fluorescence-quenching experiments with Mn2+ showed that externally applied ATP triggered a Mn2+ influx. The ATP-induced [Ca2+]i increase and AVP release were independent of and additive to a K+-induced response, in addition to being insensitive to Cd2+. The ATP-induced [Ca2+]i increase was strongly reduced in the presence of Gd3+. These results suggest that the observed [Ca2+]i increases were elicited by Ca2+ entry through a P2X channel receptor rather than via a voltage-dependent Ca2+ channel. 7. We propose that ATP, co-released with neuropeptides, could act as a paracrine-autocrine messenger, stimulating, via Ca2+ entry through a P2X2 receptor, the secretion of AVP, in particular, from neurohypophysial nerve terminals.

Endogenous calcium buffering in motoneurones of the nucleus hypoglossus from mouse

1. Simultaneous patch clamp and rapid microfluorometric calcium measurements were performed on sixty-five motoneurones in slices of the nucleus hypoglossus in the brainstem of 2- to 6-day-old mice. 2. Hypoglossal motoneurones were particularly vulnerable to mechanical or metabolic stress during isolation of in vitro slice preparations. Therefore, experimental conditions were optimized for functional integrity, as judged by spontaneous rhythmic activity of hypoglossal nerves (XII). 3. Calcium concentrations in the cell soma were monitored with a temporal resolution in the millisecond time domain during depolarizing voltage steps. Ratiometric fluorescence measurements were made using a rapid monochromator (switching tau < 10 ms), a photomultiplier tube and the calcium sensitive dyes fura-2 and mag-fura-5. 4. Dynamics of somatic calcium transients were investigated as a function of the concentration of calcium indicator dye in the cell. Decays of calcium transients were approximated to a single exponential component and decay time constants showed a linear dependence on dye concentration. The extrapolated decay time in the absence of indicator dye was 0.7 +/- 0.2 s, suggesting rapid somatic calcium dynamics under physiological conditions. 5. By a process of back-extrapolation, the 'added buffer' method, a calcium binding ratio of 41 +/- 12 (9 cells) was obtained indicating that 98% of the calcium ions entering a hypoglossal motoneurone were bound by endogenous buffers. 6. Endogenous calcium binding ratios in hypoglossal motoneurones were small compared with those of other neurones with comparable size or geometry. Accordingly, our measurements suggest that the selective vulnerability of hypoglossal motoneurones to calcium-related excitotoxicity might partially result from low concentrations of calcium buffers in these cells.

Intracellular calcium clearance in Purkinje cell somata from rat cerebellar slices

1. The mechanisms governing the return of intracellular calcium (Cai2+) to baseline levels following depolarization-evoked [Ca2+]i rises were investigated in Purkinje cell somata using tight-seal whole-cell recordings and fura-2 microfluorometry, for peak [Ca2+]i ranging from 50 nm to 2 microM. 2. Cai2+ decay was well fitted by a double exponential with time constants of O.6 and 3 s. Both time constants were independent of peak [Ca2+]i but the contribution of the faster component increased with [Ca2+]i. 3. Thapsigargin (10 microM) and cyclopiazonic acid (50 microM) prolonged Cai2+ decay indicating that sarco-endoplasmic reticulum Ca2+ (SERCA) pumps contribute to Purkinje cell Cai2+ clearance. 4. A modest participation in clearance was found for the plasma membrane Ca2+ (PMCA) pumps using 5,6-succinimidyl carboxyeosin (40 microM). 5. The Na(+)-Ca2+ exchanger also contributed to the clearance process, since replacement of extracellular Na+ by Li+ slowed Cai2+ decay. 6. Carbonyl cyanide m-chlorophenylhydrazone (CCCP, 2 microM) and rotenone (10 microM) increased [Ca2+]i and elicited large inward currents at -60 mV. Both effects were also obtained with CCCP in the absence of external Ca2+, suggesting that mitochondrial Ca2+ uptake uncouplers release Ca2+ from intracellular stores and may alter the membrane permeability to Ca2+. These effects were irreversible and impeded tests on the role of mitochondria in Cai2+ clearance. 7. The relative contribution of the clearance systems characterized in this study varied as a function of [Ca2+]i. At 0.5 microM Cai2+, SERCA pumps and the Na(+)-Ca2+ exchanger contribute equally to removal and account for 78% of the process. Only 45% of the removal at 2 microM Cai2+ can be explained by these systems. In this high [Ca2+]i range the major contribution is that of SERCA pumps (21%) and of the Na(+)-Ca2+ exchanger (18%), whereas the contribution of PMCA pumps is only 6%.

Increased calcium buffering in basal forebrain neurons during aging

Increased calcium buffering in basal forebrain neurons during aging. J. Neurophysiol. 80: 350-364, 1998. Alterations of neuronal calcium (Ca2+) homeostasis are thought to underlie many age-related changes in the nervous system. Basal forebrain neurons are susceptible to changes associated with aging and to related dysfunctions such as Alzheimer's disease. It recently was shown that neurons from the medial septum and nucleus of the diagonal band (MS/nDB) of aged (24-27 mo) F344 rats have an increased current influx through voltage-gated Ca2+ channels (VGCCs) relative to those of young (1-4. 5 mo) rats. Possible age-related changes in Ca2+ buffering in these neurons have been investigated using conventional whole cell and perforated-patch voltage clamp combined with fura-2 microfluorimetric techniques. Basal intracellular Ca2+ concentrations ([Ca2+]i), Ca2+ influx, Ca2+ transients (Delta[Ca2+]i), and time course of Delta[Ca2+]i were quantitated, and rapid Ca2+ buffering values were calculated in MS/nDB neurons from young and aged rats. The involvement of the smooth endoplasmic reticulum (SER) was examined with the SER Ca2+ uptake blocker, thapsigargin. An age-related increase in rapid Ca2+ buffering and Delta[Ca2+]i time course was observed, although basal [Ca2+]i was unchanged with age. The SER and endogenous diffusible buffering mechanisms were found to have roles in Ca2+ buffering, but they did not mediate the age-related changes. These findings suggest a model in which some aging central neurons could compensate for increased Ca2+ influx with greater Ca2+ buffering.

Effects of mitochondrion on calcium transients at intact presynaptic terminals depend on frequency of nerve firing

The rate and the total amount of Ca2+ elevation in the presynaptic terminals of bullfrog sympathetic ganglia depend on the firing frequency of the terminals. Carbonyl cyanide m-chlorophenylhydrazone (CCCP), a mitochondrial uncoupler, was used for testing whether mitochondrial Ca2+ uptake is one of the mechanisms that underlie this frequency dependence. Fura-2 fluorimetry was used for measurement of intraterminal Ca2+. When stimulations of different durations (30 and 1.5 s) and frequencies (4 and 20 Hz) evoked Ca2+ transients with similar peak amplitudes (264 +/- 22 nM vs. 251 +/- 18 nM, means +/- SE), CCCP augmented the responses to the 4-Hz stimulation 8.9 times more strongly than it did the responses to the 20-Hz stimulation (249.7 +/- 81.5% vs. 25.3 +/- 10.2%). When stimulations delivered at the two frequencies had the same durations (1.5, 3, 6, 10, 20, and 30 s), CCCP enlarged the responses to the 4-Hz stimulations up to 4.2 times more than it did the responses to the 20-Hz stimulations. When the same number of stimuli (120) was delivered at the two frequencies, the effects of CCCP on the responses evoked by the 4-Hz train were again 6.8 times stronger than its effects on the responses to the 20-Hz stimulation. Therefore neither the peak amplitudes of the responses nor the durations of the stimulations dictated the extent to which the mitochondria modulated the peak [Ca2+]i. Instead, the extent of the modulation was governed by the frequency of stimulation. Specifically, the less frequent the Ca2+ influx, the stronger the mitochondrial modulation. Also, during nerve firing Ca2+ release from the ryanodine-sensitive store had a higher potential to influence the [Ca2+]i transients than did Ca2+ removal by the mitochondria for the first 6 s of the responses. On cessation of stimulation, CCCP reduced the initial rapid rate of Ca2+ decay. Thus uptake by the mitochondria was an important mechanism for Ca2+ removal after repetitive firing at the presynaptic terminals.

Evidence that mitochondria buffer physiological Ca2+ loads in lizard motor nerve terminals

1. Changes in cytosolic and mitochondrial [Ca2+] produced by brief trains of action potentials were measured in motor nerve terminals using a rapidly scanning confocal microscope. Cytosolic [Ca2+] was measured using ionophoretically injected Oregon Green BAPTA 5N (OG-5N). Mitochondrial [Ca2+] was measured using rhod-2, bath loaded as dihydrorhod-2. 2. In response to 100-250 stimuli at 25-100 Hz the average cytosolic [Ca2+] showed an initial rapid increase followed by a much slower rate of increase. Mitochondrial [Ca2+] showed no detectable increase during the first fifteen to twenty stimuli, but after this initial delay also showed an initially rapid rise followed by a slower rate of increase. The onset of the increase in mitochondrial [Ca2+] coincided with the slowing of the rate of rise of cytosolic [Ca2+]. The peak levels of cytosolic and mitochondrial [Ca2+] both increased with increasing frequencies of stimulation. 3. When stimulation terminated, the initial rate of decay of cytosolic [Ca2+] was much more rapid than that of mitochondrial [Ca2+]. 4. After addition of carbonyl cyanide m-chlorophenyl hydrazone (CCCP, 1-2 microM) to dissipate the proton electrochemical gradient across the mitochondrial membrane, cytosolic [Ca2+] rose rapidly throughout the stimulus train, reaching levels much higher than normal. CCCP inhibited the increase in mitochondrial [Ca2+]. 5. These results suggest that mitochondrial uptake of Ca2+ contributes importantly to buffering presynaptic cytosolic [Ca2+] during normal neuromuscular transmission.

Membrane excitability and secretion from peptidergic nerve terminals

1. Thin slices of the posterior pituitary can be used as a preparation for the study of biophysical mechanisms underlying neuropeptide secretion. Patch-clamp techniques in this preparation have revealed the properties of ion channels that control the excitability of the nerve terminal membrane and have clarified the relation between Ca2+ and exocytosis. 2. Repetitive electrical activity at high frequencies broadens action potentials to allow more Ca2+ entry and thus enhance exocytosis. Action potential broadening results from the inactivation of a voltage-dependent K+ channel. 3. When repetitive electrical activity is sustained, secretion is depressed. This depression can be attributed in part to action potential failure caused by the opening of a Ca(2+)-activated K+ channel. This channel can be modulated by protein kinases, phosphatases, and G-proteins. 4. The inhibitory neurotransmitter GABA activates a GABAA receptor in the nerve terminal membrane. The gating of the associated Cl- channel depolarizes the membrane slightly to inactivate voltage-gated Na+ channels and block action potential propagation. 5. The response of the nerve terminal GABAA receptor is enhanced by neuroactive steroids and this can potentiate the inhibition of neurosecretion by GABA. The action of neurosteroids at this site could play a role in changes in neuropeptide secretion associated with reproductive transitions. 6. Ca2+ channels in the nerve terminal membrane are inactivated by sustained depolarization and by trains of brief pulses. Ca2+ entry promotes Ca2+ channel inactivation during trains by inhibiting the recovery of Ca2+ channels from inactivation. The inactivation of Ca2+ channels can play a role in defining the optimal frequency and train duration for evoking neuropeptide secretion. 7. Measurements of membrane capacitance in peptidergic nerve terminals have revealed rapid exocytosis and endocytosis evoked by Ca2+ entry through voltage-gated Ca2+ channels. Exocytosis is too rapid to account for the delays in neuropeptide secretion evoked by trains of action potentials. Endocytosis sets in rapidly after exocytosis with a time course comparable to that of the rapid endocytosis observed in nerve terminals at rapid synapses. Our results support the finding in rapid synaptic nerve terminals that endocytosis is inhibited by intracellular Ca2+. Multiple pools of vesicles were revealed, and these pools may reflect different stages in the mobilization and release of neuropeptide.

Short-term changes in the Ca2+-exocytosis relationship during repetitive pulse protocols in bovine adrenal chromaffin cells

Stimulus-secretion coupling was monitored with capacitance detection in bovine chromaffin cells recorded in perforated patch mode and stimulated with trains of depolarizing pulses. A subset of stimulus trains evoked a response with a Ca2+-exocytosis relationship identical to that obtained for single depolarizing pulses (Engisch and Nowycky, 1996). Other trains evoked responses with enhanced or diminished Ca2+ efficacy relative to this input-output function. The probability of obtaining a particular Ca2+-exocytosis relationship was correlated with the amount of Ca2+ entry per pulse, such that shorter pulses or smaller currents were associated with the greatest efficacy, and longer pulses and larger currents with the lowest efficacy. Apparent enhancements in Ca2+ efficacy were not caused by residual Ca2+ summing between pulses, because decreasing the interval between pulses usually reduced efficacy in the same cell; conversely, increasing the interval between pulses did not prevent an enhanced Ca2+-exocytosis relationship. Apparent decreases in Ca2+ efficacy were not caused by depletion of an available pool of release-ready vesicles, because an equivalent amount of total Ca2+ entry during a single long depolarizing pulse usually evoked a much larger secretory response in the same cell. Finally, there were no striking differences in global Ca2+ levels monitored with the fluorescent indicator Fura Red that could account for apparent changes in Ca2+ efficacy during repetitive stimulus protocols. It appears that in chromaffin cells, the Ca2+-exocytosis relationship is subject to activity-dependent changes during a stimulus train and can be modulated up or down from a basal state accessed by single pulse stimulations.

Short-term changes in the Ca2+-exocytosis relationship during repetitive pulse protocols in bovine adrenal chromaffin cells

Stimulus-secretion coupling was monitored with capacitance detection in bovine chromaffin cells recorded in perforated patch mode and stimulated with trains of depolarizing pulses. A subset of stimulus trains evoked a response with a Ca2+-exocytosis relationship identical to that obtained for single depolarizing pulses (Engisch and Nowycky, 1996). Other trains evoked responses with enhanced or diminished Ca2+ efficacy relative to this input-output function. The probability of obtaining a particular Ca2+-exocytosis relationship was correlated with the amount of Ca2+ entry per pulse, such that shorter pulses or smaller currents were associated with the greatest efficacy, and longer pulses and larger currents with the lowest efficacy. Apparent enhancements in Ca2+ efficacy were not caused by residual Ca2+ summing between pulses, because decreasing the interval between pulses usually reduced efficacy in the same cell; conversely, increasing the interval between pulses did not prevent an enhanced Ca2+-exocytosis relationship. Apparent decreases in Ca2+ efficacy were not caused by depletion of an available pool of release-ready vesicles, because an equivalent amount of total Ca2+ entry during a single long depolarizing pulse usually evoked a much larger secretory response in the same cell. Finally, there were no striking differences in global Ca2+ levels monitored with the fluorescent indicator Fura Red that could account for apparent changes in Ca2+ efficacy during repetitive stimulus protocols. It appears that in chromaffin cells, the Ca2+-exocytosis relationship is subject to activity-dependent changes during a stimulus train and can be modulated up or down from a basal state accessed by single pulse stimulations.

Calcium homeostatic mechanisms operating in cultured postnatal rat hippocampal neurones following flash photolysis of nitrophenyl-EGTA

1. We examined Ca2+ homeostatic mechanisms in cultured postnatal rat hippocampal neurones by monitoring the recovery of background-subtracted fluo-3 fluorescence levels at 20-22 degrees C immediately following a rapid increase in Ca2+ levels induced by flash photolysis of the caged Ca2+ compound nitrophenyl-EGTA (NP-EGTA). 2. A variety of methods or drugs were used in attempt to block specifically efflux of Ca2+ by the plasmalemmal Na(+)-Ca2+ exchanger or uptake of Ca2+ into mitochondria. 3. Many of the experimental manipulations produced a decrease in intracellular pH (pHi) measured in sister cultures using the pH-sensitive dye 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF). Accordingly, in each case, we determined the appropriate amount of the weak base trimethylamine (TMA) required to restore baseline pHi prior to flash photolysis. 4. Blockade of the plasmalemmal Na(+)-Ca2+ exchanger by replacement of external Na+ with either Li+ or N-methyl-D-glucamine (NMDG) markedly reduced pHi but did not affect the rate of recovery of fluo-3 fluorescence intensities once pHi was restored. 5. Inhibition of mitochondrial Ca2+ uptake, using the protonophore carbonyl cyanide m-chloro-phenylhydrazone (CCCP), resulted in a reduction in pHi, which could be restored by the addition of 2 mM TMA. Under these conditions the rate of recovery of Ca2+ levels was significantly slower than in the controls. Similar results were found using the respiratory chain inhibitor rotenone. 6. We conclude that, when the potential effects of changes in pHi are taken into account, mitochondria appear to sequester significant amounts of Ca2+ in the neuronal preparations used.

Ca2+- and voltage-dependent inactivation of Ca2+ channels in nerve terminals of the neurohypophysis

Ca2+ channel inactivation was investigated in neurohypophysial nerve terminals by using patch-clamp techniques. The contribution of intracellular Ca2+ to inactivation was evaluated by replacing Ca2+ with Ba2+ or by including BAPTA in the internal recording solution. Ca2+ channel inactivation during depolarizing pulses was primarily voltage-dependent. A contribution of intracellular Ca2+ was revealed by comparing steady-state inactivation of Ca2+ channels with Ca2+ current and with intracellular [Ca2+]. However, this contribution was small compared to that of voltage. In contrast to voltage-gated Ca2+ channels in other preparations, in the neurohypophysis Ba2+ substitution or intracellular BAPTA increased the speed of inactivation while reducing the steady-state level of inactivation. Ca2+ channel recovery from inactivation was studied by using a paired-pulse protocol. The rate of Ca2+ channel recovery from inactivation at negative potentials was increased dramatically by Ba2+ substitution or intracellular BAPTA, indicating that intracellular Ca2+ inhibits recovery. Stimulation with trains of brief pulses designed to mimic physiological bursts of electrical activity showed that Ca2+ channel inactivation was much greater with 20 Hz trains than with 14 Hz trains. Inactivation induced by 20 Hz trains was reduced by intracellular BAPTA, suggesting an important role for Ca2+-dependent inactivation during physiologically relevant forms of electrical activity. Inhibitors of calmodulin and calcineurin had no effect on Ca2+ channel inactivation, arguing against a mechanism of inactivation involving these Ca2+-dependent proteins. The inactivation behavior described here, in which voltage effects on Ca2+ channel inactivation predominate at positive potentials and Ca2+ effects predominate at negative potentials, may be relevant to the regulation of neuropeptide release.

Kappa-opioid receptor activation modulates Ca2+ currents and secretion in isolated neuroendocrine nerve terminals

Whole-cell patch-clamp recordings were performed together with time-resolved measurements of membrane capacitance (Cm) in nerve terminals acutely dissociated from neurohypophysis of adult rats to investigate modulation of Ca2+ currents and secretion by activation of opioid receptors. Bath superfusion of the kappa-opioid agonists U69,593 (0.3-1 microM), dynorphin A (1 microM), or U50,488H (1-3 microM) reversibly suppressed the peak amplitude of Ca2+ currents 32. 7 +/- 2.7% (in 41 of 56 terminals), 37.4 +/- 5.3% (in 5 of 8 terminals), and 33.5 +/- 8.1% (in 5 of 10 terminals), respectively. In contrast, tests in 11 terminals revealed no effect of the mu-opioid agonist [D-Pen2,5]-enkephalin (1-3 microM; n = 7) or of the delta-agonist Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol (1 microM; n = 4) on Ca2+ currents. Three components of high-threshold current were distinguished on the basis of their sensitivity to blockade by omega-conotoxin GVIA, nicardipine, and omega-conotoxin MVIIC: N-, L-, and P/Q-type current, respectively. Administration of U69,593 inhibited N-type current in these nerve terminals on average 32%, whereas L-type current was reduced 64%, and P/Q-type current was inhibited 28%. Monitoring of changes in Cm in response to brief depolarizing steps revealed that the kappa-opioid-induced reductions in N-, L-, or P/Q-type currents were accompanied by attenuations in two kinetically distinct components of Ca2+-dependent exocytotic release. These data provide strong evidence of a functional linkage between blockade of Ca2+ influx through voltage-dependent Ca2+ channels and inhibitory modulation of release by presynaptic opioid receptors in mammalian central nerve endings.

Kappa-opioid receptor activation modulates Ca2+ currents and secretion in isolated neuroendocrine nerve terminals

Whole-cell patch-clamp recordings were performed together with time-resolved measurements of membrane capacitance (Cm) in nerve terminals acutely dissociated from neurohypophysis of adult rats to investigate modulation of Ca2+ currents and secretion by activation of opioid receptors. Bath superfusion of the kappa-opioid agonists U69,593 (0.3-1 microM), dynorphin A (1 microM), or U50,488H (1-3 microM) reversibly suppressed the peak amplitude of Ca2+ currents 32. 7 +/- 2.7% (in 41 of 56 terminals), 37.4 +/- 5.3% (in 5 of 8 terminals), and 33.5 +/- 8.1% (in 5 of 10 terminals), respectively. In contrast, tests in 11 terminals revealed no effect of the mu-opioid agonist [D-Pen2,5]-enkephalin (1-3 microM; n = 7) or of the delta-agonist Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol (1 microM; n = 4) on Ca2+ currents. Three components of high-threshold current were distinguished on the basis of their sensitivity to blockade by omega-conotoxin GVIA, nicardipine, and omega-conotoxin MVIIC: N-, L-, and P/Q-type current, respectively. Administration of U69,593 inhibited N-type current in these nerve terminals on average 32%, whereas L-type current was reduced 64%, and P/Q-type current was inhibited 28%. Monitoring of changes in Cm in response to brief depolarizing steps revealed that the kappa-opioid-induced reductions in N-, L-, or P/Q-type currents were accompanied by attenuations in two kinetically distinct components of Ca2+-dependent exocytotic release. These data provide strong evidence of a functional linkage between blockade of Ca2+ influx through voltage-dependent Ca2+ channels and inhibitory modulation of release by presynaptic opioid receptors in mammalian central nerve endings.

Ca2+- and voltage-dependent inactivation of Ca2+ channels in nerve terminals of the neurohypophysis

Ca2+ channel inactivation was investigated in neurohypophysial nerve terminals by using patch-clamp techniques. The contribution of intracellular Ca2+ to inactivation was evaluated by replacing Ca2+ with Ba2+ or by including BAPTA in the internal recording solution. Ca2+ channel inactivation during depolarizing pulses was primarily voltage-dependent. A contribution of intracellular Ca2+ was revealed by comparing steady-state inactivation of Ca2+ channels with Ca2+ current and with intracellular [Ca2+]. However, this contribution was small compared to that of voltage. In contrast to voltage-gated Ca2+ channels in other preparations, in the neurohypophysis Ba2+ substitution or intracellular BAPTA increased the speed of inactivation while reducing the steady-state level of inactivation. Ca2+ channel recovery from inactivation was studied by using a paired-pulse protocol. The rate of Ca2+ channel recovery from inactivation at negative potentials was increased dramatically by Ba2+ substitution or intracellular BAPTA, indicating that intracellular Ca2+ inhibits recovery. Stimulation with trains of brief pulses designed to mimic physiological bursts of electrical activity showed that Ca2+ channel inactivation was much greater with 20 Hz trains than with 14 Hz trains. Inactivation induced by 20 Hz trains was reduced by intracellular BAPTA, suggesting an important role for Ca2+-dependent inactivation during physiologically relevant forms of electrical activity. Inhibitors of calmodulin and calcineurin had no effect on Ca2+ channel inactivation, arguing against a mechanism of inactivation involving these Ca2+-dependent proteins. The inactivation behavior described here, in which voltage effects on Ca2+ channel inactivation predominate at positive potentials and Ca2+ effects predominate at negative potentials, may be relevant to the regulation of neuropeptide release.

Mitochondrial involvement in post-tetanic potentiation of synaptic transmission

Posttetanic potentiation (PTP) is an essential aspect of synaptic transmission that arises from a persistent presynaptic [Ca2+]i following tetanic stimulation. At crayfish neuromuscular junctions, several inhibitors of mitochondrial Ca2+ uptake and release (tetraphenylphosphonium or TPP+, carbonyl cyanide m-chlorophenylhydrazone or CCCP, and ruthenium red) blocked PTP and the persistence of presynaptic residual [Ca2+]i, while endoplasmic reticulum (ER) Ca2+ pump inhibitors and release channel activators (thapsigargin, 2,5-di-(tert-butyl)-1,4-benzohydroquinone or BHQ, and caffeine) had no effects. PTP apparently results from the slow efflux of tetanically accumulated mitochondrial Ca2+.

Calcium dynamics associated with a single action potential in a CNS presynaptic terminal

Calcium dynamics associated with a single action potential were studied quantitatively in the calyx of Held, a large presynaptic terminal in the rat brainstem. Terminals were loaded with different concentrations of high- or low-affinity Ca2+ indicators via patch pipettes. Spatially averaged Ca2+ signals were measured fluorometrically and analyzed on the basis of a single compartment model. A single action potential led to a total Ca2+ influx of 0.8-1 pC. The accessible volume of the terminal was about 0.4 pl; thus the total calcium concentration increased by 10-13 microM. The Ca(2+)-binding ratio of the endogenous buffer was about 40, as estimated from the competition with Fura-2, indicating that 2.5% of the total calcium remained free. This is consistent with the peak increase in free calcium concentration of about 400 nM, which was measured directly with MagFura-2. The decay of the [Ca2+]i transients was fast, with time constants of 100 ms at 23 degrees C and 45 ms at 35 degrees C, indicating Ca2+ extrusion rates of 400 and 900 s-1, respectively. The combination of the relatively low endogenous Ca(2+)-binding ratio and the high rate of Ca2+ extrusion provides an efficient mechanism for rapidly removing the large Ca2+ load of the terminal evoked by an action potential.

Regulation of secretory granule recruitment and exocytosis at rat neurohypophysial nerve endings

1. Time-resolved cell membrane capacitance (Cm) measurements were used in combination with fura-2 microfluorometry under whole-cell patch clamp recording to investigate the kinetics and Ca2+ sensitivity of exocytotic granule fusion evoked by depolarizing stimuli at single, isolated nerve endings of the rat neurohypophysis. 2. Single step depolarizations or trains of depolarizing pulses evoked voltage-dependent, inward Ca2+ currents (ICa) and induced both Ca(2+)-dependent and Ca(2+)-independent changes in Cm. Three distinct Cm responses were observed and were differentiated by their kinetics and Ca2+ sensitivity: a non-exocytotic transient (delta Cm,t) and an exocytotic Cm 'jump' (delta Cm,J) and a slower, often latent, exocytotic Cm rise (delta Cm,s) that outlasted the depolarizing pulse stimulus. 3. The delta Cm,t was characterized by a rapid, transient component observed in 70% of nerve endings and a voltage-activation relationship that preceded that of the ICa. The amplitude and kinetics of the delta Cm,t were unaffected by ICa block by Cd2+, Ca2+ load reduction, or alterations in intracellular Ca2+ buffering. 4. In contrast to the delta Cm,t, both the delta Cm,J and delta Cm,s were Ca2+ dependent as evidenced by their sensitivity to Cd2+ block of ICa, intraterminal application of 10 mM BAPTA and reduced [Ca2+]o or replacement of Ca2+ as the charge carrier with Ba2+. 5. The delta Cm,J was proportional to depolarization-evoked Ca2+ influx with initial exocytotic rate of approximately 350 granule fusions s-1. The amplitude of the delta Cm,J rose exponentially (tau = 40 ms) and approached an asymptote (15.5 fF) with longer duration depolarizations indicating the fusion from and depletion of an immediately releasable pool (IRP) estimated at nineteen docked and primed secretory granules. 6. The delta Cm,s was induced by the application of repetitive long duration pulses and defined as the exocytosis of secretory granules from a readily releasable granule pool (RRP). The delta Cm,s response occurred only after exceeding a [Ca2+]i threshold value and rose thereafter in proportion to Ca2+ influx with a mean initial secretory rate of 36 granule fusions s-1. The mean latency for delta Cm,s activation was 850 ms following the initiation of the step depolarizations. The delta Cm,s response magnitude, reflecting the size of the RRP, was dependent on the resting [Ca2+]i and the nerve ending size, and was depletable using repetitive depolarizations of long duration. 7. Recruitment into and release from the RRP and IRP were differentially sensitive to changes in intraterminal Ca2+ buffering conditions. For example, introduction of 5 mM EGTA was shown to have no effect on the evoked IRP but significantly reduced the RRP. In comparison, diminishment of the endogenous Ca2+ buffering capacity of nerve endings by treatment with the mitochondrial Ca2+ uniporter blocker Ruthenium Red (10 microM) potentiated the RRP size but had no significant effect on the IRP size. 8. The present study indicates that the Ca(2+)-dependent recruitment of and release from functionally distinct pools of peptide-containing secretory granules in combination with the [Ca2+]i regulatory properties of neurohypophysial nerve endings may explain both the depletion of peptide release under prolonged stimulus and the potentiation of peptide release observed to occur during recurrent phasic action potential activity in this system.

Localization of voltage-sensitive Ca2+ fluxes and neuropeptide Y immunoreactivity to varicosities in SH-SY5Y human neuroblastoma cells differentiated by treatment with the protein kinase inhibitor staurosporine

The distribution of voltage-sensitive elevations of the level of Ca2+ in untreated SH-SY5Y cells and cells that had been induced to differentiate with staurosporine was investigated by monitoring fura-2 fluorescence in cell suspensions, and by using microfluorometry and quantitative fluorescence imaging on cell bodies and on cellular processes. Cell bodies of both types of cells displayed small Ca2+ elevations, which were composed of transient and sustained components. Elevations were partially sensitive to the L- and N-channel blockers nifedipine (1 microM) and omega-conotoxin GVIA (100 nM) respectively. Up to ten times Ca2+ elevations were observed in varicosities of treated cells than in cell bodies of treated and cells. These elevations were insensitive to compounds known to release Ca2+ from intracellular stores. Elevations of Ca2+ were sustained, and they were insensitive to 5 microM nifedipine, 100 nM omega-agatoxin IVA and 100 nM omega-conotoxin GVIA, and partially sensitive to 2 microM omega-conotoxin GVIA, indicating predominance of non-L-type, non-N-type, non-P-type channel activity. The intracellular localization of neuropeptide Y, a marker of differentiation in these cells, was also investigated by fluorescence immunocytochemistry. Varicosities of treated cells displayed marked fluorescence when viewed in a confocal microscope. These findings show that the varicosities of staurosporine-treated cells exhibit some of the functional properties of nerve terminals. The varicosities resemble boutons en passant nerve endings and they seem to express Ca2+ channels different from those in the cell body.

Mitochondria accumulate Ca2+ following intense glutamate stimulation of cultured rat forebrain neurones

1. In cultures of rat forebrain neurones, mitochondria buffer glutamate-induced, NMDA receptor-mediated Ca2+ influx. Here, we have used the fluorescent calcium indicator, indo-1 AM to record [Ca2+]i from single cells. We varied either the glutamate concentration or the duration of exposure to investigate the cellular mechanisms recruited to buffer [Ca2+]i within different stimulation protocols. 2. For a 15 s stimulus, the recovery time doubled as the glutamate concentration was raised from 3 to 300 microM. Changing the duration of exposure from 15 s to 5 min increased the recovery time tenfold even when the glutamate concentration was held at 3 microM. 3. We used a selective inhibitor of the mitochondrial Na(+)-Ca2+ exchange, CGP-37157. When applied immediately after a 15 s, 100 microM glutamate challenge, CGP-37157 consistently caused a rapid fall in [Ca2+]i followed by a slow rise after the drug was washed out. A similar pattern was seen with the 5 min, 3 microM glutamate stimulus. The effects of CGP-37157 are consistent with the release of substantial mitochondrial Ca2+ stores during recovery from an intense glutamate stimulus. 4. These studies suggest that mitochondria become progressively more important for buffering glutamate-induced Ca2+ loads as the stimulus intensity increases. The recovery of [Ca2+]i to baseline following glutamate removal is critically regulated by the release of Ca2+ from mitochondrial stores via mitochondrial Na(+)-Ca2+ exchange. The data highlight a previously under-appreciated role for [Na+]i in the regulation of [Ca2+]i in central neurones.

Calbindin-D28k mRNA expression in magnocellular hypothalamic neurons of female rats during parturition, lactation and following dehydration

Recent studies indicate that calcium binding proteins may play a role in determining the electrical firing patterns of the hypothalamic magnocellular oxytocin (OT) and vasopressin (VP) neurons. In this study we have examined the calbindin-D28k mRNA content of magnocellular neurons in the supraoptic (SON) and paraventricular (PVN) nuclei and determined whether changes in expression correlate with the specific patterns of electrical activity displayed by these cells under different physiological circumstances. In situ hybridization with [35S]-labelled oligonucleotides revealed a heterogeneous pattern of calbindin-D28k mRNA expression in the SON and magnocellular PVN. Quantitative analysis demonstrated that the number of silver grains/cell in the dorsal half of the SON was approximately 30% higher (P < 0.05) than that of the ventral half of the nucleus. Within the PVN, calbindin-D28k mRNA-expressing neurons were detected in the medial magnocellular division of the PVN but not in magnocellular cells forming the core of the lateral magnocellular division. Dehydration for 24 h did not alter calbindin-D28k mRNA expression in the SON, PVN or cingulate cortex. In parturient and lactating rats, calbindin-D28k mRNA levels were significantly (P < 0.05) reduced in the medial magnocellular division of the PVN compared with virgin animals. No significant differences in calbindin-D28k mRNA expression were observed in either ventral or dorsal halves of the SON, or in the cingulate cortex of these animals. These results provide evidence for the differential expression of calbindin-D28k mRNA by hypothalamic magnocellular neurons and suggest that OT cells may express more calbindin-D28k mRNA than VP neurons. The reduction in calbindin-D28k mRNA expression by putative OT neurons of the PVN at the time of parturition and lactation supports the hypothesis of Li and colleagues (J. Physiol., 488 (1995) 601-608) that calbindin may play a part in determining the electrical firing patterns of magnocellular neurons. However, the absence of any similar decrease in the SON suggests that changes in calbindin-D28k mRNA expression are not essential for OT neurons to exhibit episodic bursting behavior.

High endogenous calcium buffering in Purkinje cells from rat cerebellar slices

1. The ability of Purkinje cells to rapidly buffer depolarization-evoked intracellular calcium changes (delta [Ca2+]i) was estimated by titrating the endogenous buffer against incremental concentrations of the Ca(2+)-sensitive dye fura-2. 2. In cells from 15-day-old rats, pulse-evoked delta [Ca2+]i were stable during the loading with 0.5 mM fura-2 through the patch pipette. In cells from 6-day-old rats, delta [Ca2+]i decreased by approximately 50% during equivalent experiments. This decrease was not related to changes in Ca2+ influx, since the integral of the Ca2+ currents remained constant throughout the recording. 3. Experiments with high fura-2 concentrations (1.75-3.5 mM) were performed in order to obtain for each cell the curve relating delta [Ca2+]i to fura-2 concentration. From this relationship, values for the Ca2+ binding ratio (the ratio of buffer-bound Ca2+ changes over free Ca2+ changes) were calculated. 4. In Purkinje cells from 15-day-old rats, the Ca2+ binding ratio was approximately 2000, an order of magnitude larger than that of other neurones and neuroendocrine cells studied to date. This Ca2+ binding ratio was significantly smaller (approximately 900) in Purkinje cells from 6-day-old rats. 5. We propose that the large Ca2+ binding ratio of Purkinje cells is related to the presence of large concentrations of Ca2+ binding proteins and that these cells regulate their ability to handle Ca2+ loads during development through changes in the concentration of Ca2+ binding proteins.

Rapid exocytosis and endocytosis in nerve terminals of the rat posterior pituitary

1. Ca(2+)-induced exocytosis and endocytosis were studied by measuring the membrane capacitance of voltage-clamped peptidergic nerve terminals in slices prepared from the rat posterior pituitary. 2. Depolarizing pulses produced rapid increases in capacitance. These increases varied in parallel with Ca2+ current as voltage was varied. Elimination of Ca2+ current blocked depolarization-induced capacitance changes. 3. Depolarization-induced capacitance changes increased with pulse duration. Capacitance changes also increased with integrated Ca2+ influx, but saturated at high levels of Ca2+ entry. This saturation allowed us to estimate a pool size of 190 vesicles, assuming each vesicle has a capacitance of 1 fF. Vesicles from this pool fused with a time constant of 0.43 s. The capacitance change increased with the first power of integrated Ca2+ influx. 4. Experiments with briefer pulses revealed a rapid component of exocytosis comprising a pool of forty vesicles that fuse with a time constant of 14 ms. This rapid process may reflect a final Ca(2+)-regulated triggering step, which is distinct from the slower kinetic step revealed by longer duration pulses. The slower step may reflect a priming of vesicles prior to exocytosis. 5. Depolarization-induced capacitance increases in most cases were followed by a rapid decay in capacitance, reflecting membrane reuptake tightly coupled to exocytosis. A variable amount of rapid endocytosis followed depolarization-induced capacitance increases. The time constant for rapid endocytosis to baseline was 0.44 s. Excess endocytosis was occasionally observed, with capacitance decaying below the pre-stimulus baseline with a time constant of 2.1 s. 6. Rapid endocytosis was slower after pulses that produced greater increases in intracellular Ca2+, consistent with the hypothesis that intracellular Ca2+ inhibits rapid endocytosis. 7. Exocytosis follows depolarization with no detectable delay, indicating that Ca2+ triggers neuropeptide secretion from nerve terminals with kinetics comparable to that observed in other rapidly secreting systems.

Effects of injecting calcium-buffer solution on [Ca2+]i in voltage-clamped snail neurons

We have investigated why fura-2 and Ca(2+)-sensitive microelectrodes report different values for the intracellular free calcium ion concentration ([Ca(2+)]i or its negative log, pCa(i)) of snail neurons voltage-clamped to -50 or -60 mV. Both techniques were initially calibrated in vitro, using calcium calibration solutions that had ionic concentrations similar to those of snail neuron cytoplasm. Pressure injections of the same solutions at resting and elevated [Ca(2+)]i were used to calibrate both methods in vivo. In fura-2-loaded cells these pressure injections generated changes in [Ca(2+)]i that agreed well with those expected from the in vitro calibration. Thus, using fura-2 calibrated in vitro, the average resting [Ca(2+)]i was found to be 38 nM (pCa(i) 7.42 +/- 0.05). With Ca(2+)-sensitive microelectrodes, the first injection of calibration solutions always caused a negative shift in the recorded microelectrode potential, as if the injection lowered [Ca2+]i. No such effects were seen on the fura-2 ratio. When calibrated in vivo the Ca(2+)-sensitive microelectrode gave an average resting [Ca2+]i of approximately 25 nM (pCa(i) 7.6 +/- 0.1), much lower than when calibrated in vitro. We conclude that [Ca(2+)]i in snail neurons is approximately 40 nM and that Ca(2+)-sensitive microelectrodes usually cause a leak at the point of insertion. The effects of the leak were minimized by injection of a mobile calcium buffer.

Relationship between intracellular calcium and its muffling measured by calcium iontophoresis in snail neurones

1. We have measured intracellular free calcium ion concentration ([Ca2+]i) with fura-2, and intracellular chloride with chloride-sensitive microelectrodes, in voltage-clamped snail neurones. By making iontophoretic injections of CaCl2 we have investigated calcium muffling, the sum of the processes which minimize the calcium transient, at different values of [Ca2+]i. 2. By injection of calcium into cell-sized droplets of buffer we measured the calcium transport index. It was stable over the range pCa 6-7.4 (0.48 +/- 0.06 measured at pCa 6.70 +/- 0.12, n = 5). 3. Measurement of intracellular chloride activity during a series of fura-2-KCl pressure injections revealed a nearly linear relationship between fura-2 Ca(2+)-insensitive fluorescence and the sum of the increments in intracellular chloride. This allowed us to calculate the intracellular fura-2 concentration ([fura-2]i). 4. The rate of recovery of [Ca2+]i following a depolarization-induced load was increased by low [fura-2]i (10-20 microM) but decreased by higher [fura-2]i (40-80 microM). These effects are consistent with the addition of a mobile buffer to the cytoplasm. 5. Iontophoresis of Ca2+ at various membrane potentials allowed us to calculate the intracellular calcium muffling power (the amount of calcium required to cause a transient tenfold increase in [Ca2+]i per unit volume) and calcium muffling ratio (number of Ca2+ ions injected divided by the maximum increase in [Ca2+]i per unit volume) at different values of [Ca2+]i. 6. Calcium muffling power at resting [Ca2+]i was approximately 40 microM Ca2+ (pCa unit)-1, (about 250 times less than for hydrogen ions). It increased linearly about fivefold with [Ca2+]i over the range 20-120 nM (10 cells, 153 measurements) and therefore exponentially with decreasing pCa. 7. The calcium muffling ratio appeared to be constant (361 +/- 14, n = 10 cells, 130 measurements) over the range 20-120 nM Ca2+. 8. In three experiments we modelled the additional calcium buffering power produced by multiple pressure injections of fura-2 into voltage-clamped snail neurones. Back-extrapolation of the increases in calcium buffering power allowed us to calculate the calcium muffling power of the neurones. 9. Small increases in [fura-2]i (approximately 10 microM) significantly increased intracellular calcium muffling power in individual experiments. However, the variability among neurones in intracellular calcium muffling power was large enough to obscure the additional buffering produced by fura-2 in pooled experiments.

An NMDA receptor on isolated secretory nerve endings

While the presence of post-synaptic NMDA receptors in the CNS is well-established, the present study addressed the question of whether NMDA receptors may also be present on secretory nerve endings. Using microspectrofluorometry of fura-2 loaded isolated neurohypophysial nerve endings of the rat, we found that both glutamate (EC50 = 50 microM) and NMDA (EC50 = 30 microM) induced a rapid rise in (Ca2+]i. These responses were glycine-dependent and abolished by 1 mM Mg2+, 1 microM dizocilpine, and removal of extracellular Ca2+. Responses were not significantly affected by treatment with Ca2+ channel blockers or 10 microM CNQX.

Influence of conductance changes on patch clamp capacitance measurements using a lock-in amplifier and limitations of the phase tracking technique

We characterized the influence of conductance changes on whole-cell patch clamp capacitance measurements with a lock-in amplifier and the limitations of the phase-tracking method by numerical computer simulations, error formulas, and experimental tests. At correct phase setting, the artifacts in the capacitance measurement due to activation of linear conductances are small. The cross talk into the capacitance trace is well approximately by the second-order term in the Taylor expansion of the admittance. In the case of nonlinear current-voltage relationships, the measured conductance corresponds to the slope conductance in the range of the sine wave amplitude, and the cross talk into the capacitance trace corresponds to the second-order effect of the slope conductance. The finite gating kinetics of voltage-dependent channels generate phase-shifted currents. These lead to major artifacts in the capacitance measurements when the angular frequency of the sine wave is close to the kinetic rate constant of the channel. However, when the channel kinetics are sufficiently slow, or sufficiently fast, the cross talk is still close to the second-order effect of the measured conductance. The effects of activation of voltage-dependent currents on the capacitance measurements may be estimated, provided a detailed characterization of the kinetics and voltage dependence is available. A phase error of the lock-in amplifier of a few degrees leads to significant projections. The phase-tracking method can be used to keep the phase aligned only during periods of low membrane conductance. However, nonideal properties of the equivalent circuit, in particular the fast capacitance between the pipette and the bath solutions, may lead to large phase errors when the phase-tracking method is used, depending on the electrical properties of the cell. In this article we provide practical values, setting the range where possible artifacts are below defined limits. For proper evaluation of capacitance measurements, the capacitance and conductance traces should always be displayed together.

The use of fura-2 for estimating Ca buffers and Ca fluxes

The compound fura-2 (Grynkiewicz et al., J. Biol. Chem. 260, 3440-3450, 1985) is generally known as an indicator dye for measuring the concentration of free calcium ([Ca2+]) inside living cells. It should be appreciated, however, that this is not what it actually is. More accurately, it is a divalent metal ion chelator which changes its fluorescence properties upon complexation. Thus, [Ca2+] has to be inferred indirectly by means of the law of mass action. As a chelator, fura-2 may influence the quantity of interest, the Ca signal. On the other hand, the chelator action may be used for a number of other purposes, some of them more directly related to its molecular properties: as a chelator, competing with endogenous Ca buffers, it can be used to estimate endogenous buffers and their properties. When present at sufficiently high concentration, such that it outcompetes endogenous buffers, fura-2 reports total Ca changes and is a probe for Ca fluxes across the membrane. Here, theory and methodological considerations of such applications of fura-2 will be summarized and results on Ca buffer and Ca flux measurements derived from various methods will be compared.

Alcohol inhibits the activation of NAD-linked dehydrogenases by calcium in brain and heart mitochondria

The effect of ethanol on the Ca(2+)-dependent activation of mitochondrial dehydrogenases in rat brain and heart mitochondria was investigated. ADP-stimulated respiration of isolated brain and heart mitochondria (state 3) was stimulated further by submicromolar concentrations of free calcium when respiring on non-saturating concentrations of NAD-linked substrates. The stimulation of oxidative phosphorylation by Ca2+ was correlated with an increase of the mitochondrial matrix free calcium concentration ([Ca2+]m), as measured by fura-2, and with an increased reduction of the mitochondrial NAD(P) pool, indicating an activation of Ca(2+)-dependent dehydrogenases. Sodium inhibited Ca(2+)-dependent stimulation of state 3 respiration and NAD(P) reduction as a result of stimulation of Ca2+ efflux through the Na+/Ca2+ antiporter which reduced the steady-state value of [Ca2+]m. Ethanol stimulated the Na+/Ca2+ antiporter both in brain and heart mitochondria. As a result of this stimulation, ethanol, at pharmacological concentrations (50-300 mM), enhanced the sodium-dependent reduction of [Ca2+]m, and thus attenuated the activation of NAD-linked dehydrogenases and the stimulation of oxidative phosphorylation, by submicromolar concentrations of Ca2+, both in brain and heart mitochondria. This pharmacological effect of ethanol, on brain and heart mitochondria, may be responsible, in part, for the acute and chronic effects of ethanol on brain and heart function and metabolism.