Activity-dependent changes in voltage-dependent calcium currents and transmitter release

Activity-dependent regulation of the binomial parameters p and n at the mouse neuromuscular junction in vivo

Block of neurotransmission at the mammalian neuromuscular junction triggers an increase in the number of vesicles released (quantal content). The increase occurs whether nerve and muscle activity are both blocked by placement of a tetrodotoxin (TTX) containing cuff on the nerve or whether muscle activity is selectively blocked by injection of α-bungarotoxin (BTX). We used ANOVA to examine whether the mechanism underlying the increase in quantal content differed between the two types of activity blockade. We found that TTX-induced blockade increased the probability of release (p), whereas BTX-induced blockade increased the number of releasable vesicles (n). The lack of increase in p when postsynaptic activity was blocked with BTX suggests that block of presynaptic activity triggers the increase. To determine whether n is regulated by mismatch of pre- and postsynaptic activity introduced by BTX injection we combined BTX and TTX and still found an increase in n. We conclude that block of acetylcholine binding to acetylcholine receptors during spontaneous release triggers the increase in n.

Sensing and expressing homeostatic synaptic plasticity

Chronic changes in the level of neuronal activity (over a period of days) trigger compensatory changes in synaptic function that seem to contribute to the homeostatic restoration of neuronal activity. Changes in both quantal amplitude and vesicle release contribute to homeostatic synaptic plasticity, but they are often considered as the same phenomenon. In this review, we propose a new approach to studying how neuronal activity is sensed and changes in synaptic function are expressed during synaptic compensation. Changes in quantal amplitude and vesicle release should be considered separately in an attempt to identify the sensors that trigger homeostatic synaptic plasticity. Although data are limited, current evidence suggests that the sensors triggering changes in the quantal amplitude and vesicle release exist at different locations. Furthermore, it is important to recognize that at least two different mechanisms underlie changes in quantal amplitude during homeostatic synaptic plasticity: changes in both the number of postsynaptic receptors and loading of synaptic vesicles with neurotransmitter. Finally, modulation of the probability of neurotransmitter release contributes to the changes in vesicle release associated with homeostatic synaptic plasticity. An improved understanding of where and how neuronal activity is sensed, in addition to the types of changes in synaptic function that are induced, will be needed both to design future experiments and to understand the consequences of synaptic compensation following injury to the nervous system.

Decreased synaptic activity shifts the calcium dependence of release at the mammalian neuromuscular junction in vivo

We examined the mechanism underlying increased quantal content after block of activity at the mouse neuromuscular junction in vivo. We found that, when quantal content was measured in solution containing normal extracellular calcium, block of activity had no effect on either quantal content or the response to repetitive stimulation. However, when quantal content was measured in low extracellular calcium, there was a large increase in quantal content after block of activity. The increase in quantal content was accompanied by increased depression during repetitive stimulation. The explanation for these findings was that there was a shift in the calcium dependence of release after block of activity that manifested as an increase in probability of release in low extracellular calcium. Block of presynaptic P/Q channels eliminated the increase in probability of release. We propose that activity-dependent regulation of presynaptic calcium entry may contribute to homeostatic regulation of quantal content.

Dopamine modulation of calcium currents in pyloric neurons of the lobster stomatogastric ganglion

We examined the dopamine (DA) modulation of calcium currents (ICa) that could contribute to the plasticity of the pyloric network in the lobster stomatogastric ganglion. Pyloric somata were voltage-clamped under conditions designed to block voltage-gated Na+, K+, and H currents. Depolarizing steps from -60 mV generated voltage-dependent, inward currents that appeared to originate in electrotonically distal, imperfectly clamped regions of the cell. These currents were blocked by Cd2+ and enhanced by Ba2+ but unaffected by Ni2+. Dopamine enhanced the peak ICa in the pyloric constrictor (PY), lateral pyloric (LP), and inferior cardiac (IC) neurons and reduced peak ICa in the ventricular dilator (VD), pyloric dilator (PD), and anterior burster (AB) neurons. All of these effects, except for the AB, are consistent with DA's excitation or inhibition of firing in the pyloric neurons. Enhancement of ICa in PY and LP neurons and reduction of ICa in VD and PD neurons are also consistent with DA-induced synaptic strength changes via modulation of presynaptic ICa. However, the reduction of ICa in AB suggests that DA's enhancement of AB transmitter release is not directly mediated through presynaptic ICa. ICa in PY and PD neurons was more sensitive to nifedipine block than in AB neurons. In addition, nifedipine blocked DA's effects on ICa in the PY and PD neurons but not in the AB neuron. Thus the contribution of specific calcium channel subtypes carrying the total ICa may vary between pyloric neuron classes, and DA may act on different calcium channel subtypes in the different pyloric neurons.

Inverse relationship between release probability and readily releasable vesicles in depressing and facilitating synapses

We tested the hypothesis that the probability of vesicular exocytosis at synapses is positively correlated with the pools of readily releasable synaptic vesicles, as shown for mammalian neurons grown in tissue culture. We compared synapses of two identified glutamatergic neurons: phasic (high-output, depressing) and tonic (low-output, facilitating) crustacean motor neurons, which differ 100- to 1000-fold in quantal content. Estimates of vesicles available for exocytosis were made from depletion during forced release and from electron microscopic observation of vesicles docked at synaptic membranes near active zones. Both measurements showed a significantly larger pool of readily releasable vesicles in facilitating synapses, despite their much lower quantal output during stimulation. Thus, the probability for release of docked vesicles is very much lower at facilitating synapses, and the presence of more docked vesicles does not predict higher synaptic release probability in these paired excitatory neurons.

The composite neuron: a realistic one-compartment Purkinje cell model suitable for large-scale neuronal network simulations

We present a simple method for the realistic description of neurons that is well suited to the development of large-scale neuronal network models where the interactions within and between neural circuits are the object of study rather than the details of dendritic signal propagation in individual cells. Referred to as the composite approach, it combines in a one-compartment model elements of both the leaky integrator cell and the conductance-based formalism of Hodgkin and Huxley (1952). Composite models treat the cell membrane as an equivalent circuit that contains ligand-gated synaptic, voltage-gated, and voltage- and concentration-dependent conductances. The time dependences of these various conductances are assumed to correlate with their spatial locations in the real cell. Thus, when viewed from the soma, ligand-gated synaptic and other dendritically located conductances can be modeled as either single alpha or double exponential functions of time, whereas, with the exception of discharge-related conductances, somatic and proximal dendritic conductances can be well approximated by simple current-voltage relationships. As an example of the composite approach to neuronal modeling we describe a composite model of a cerebellar Purkinje neuron.

Distribution of mapacalcine receptors in the central nervous system of rat using the [125I]-labeled mapacalcine derivative

Mapacalcine is a dimeric protein of Mr 19041 extracted from the marine sponge Cliona vastifica. Electrophysiological and pharmacological approaches have demonstrated that mapacalcine was blocking a calcium channel different from N-, L-, P-, T- or Q-type calcium channels on mouse intestinal smooth muscle. Recently a [125I]-labeled derivative of mapacalcine has been synthesized and characterized as a tool usable as a probe to investigate mapacalcine receptors. On rat brain membranes, it binds to its receptor with a K(d)=0.35 nM and a maximal binding capacity of 706 fmol/mg protein. We use here [125I]-mapacalcine to study the mapping of its receptors in the rat brain. Data obtained show a practically homogeneous labeling of the brain. Our experiments suggest that mapacalcine receptors are present on neuronal and glial cells. Interestingly, choroid plexus demonstrates a high density of mapacalcine receptors. These data would suggest that mapacalcine sensitive calcium channels could be involved in the control of calcium homeostasis of the cerebrospinal fluid.

Stimulus history alters behavioral responses of neuronal growth cones

Generally, it is assumed that growth cones respond to a specific guidance cue with a single, specific, and stereotyped behavior. However, there is evidence to suggest that previous exposure to a given cue might alter subsequent responses to that cue (Snow and Letourneau, 1992; Shirasaki et al., 1998). We therefore tested the hypothesis that growth cone responses to stimuli are dependent on the history of previous stimulation. Growth cones of chick dorsal root ganglion neurons were exposed to well characterized stimuli: (1) contact with a laminin-coated bead, which causes growth cone turning, or (2) electrical stimulation, which causes growth cone collapse. Although the expected behavioral responses were observed after the initial stimulation, strikingly different responses to a subsequent stimulation were observed. Growth cones that had recovered from electrical stimulation-induced collapse rapidly developed insensitivity to a second identical electrical stimulation. Growth cones that previously turned in response to contact with a laminin-coated bead responded to a second bead with a "stall" or cessation in outgrowth. This stimulus history dependence of growth cone behavior could be generalized across dissimilar stimuli: after contact with a laminin-coated bead, growth cones failed to collapse in response to electrical stimulation. The calcium/calmodulin-dependent protein kinase II (CaMKII) was implicated in this history dependence by pharmacological experiments. Together, these results demonstrate that growth cones can alter their behavioral response rapidly to a given stimulus in a manner dependent on previous history and that knowledge of past events in growth cone navigation may be required to predict future growth cone behavior.

Activity of voltage-operated calcium channels in rat cerebellar granule neurons and neuronal survival

Neuronal activity and Ca2+ channel activation play important roles in neuronal survival and development. In cerebellar granule neurons, the culture conditions can induce differential expression of various membrane receptor proteins. To test the hypothesis that culture conditions might affect the activity of voltage-operated Ca2+ channels, the present study analysed the differences in Ca2+ signalling between granule neurons grown in the presence of normal (5 mM) or high (25 mM) KCl. The Ca2+ transients evoked by 50 mM KCl developed similarly in both cultures, as a function of age. In contrast, when compared with neurons grown in 25 mM KCl, a proportion of the neurons grown in normal KCl showed, between days in vitro 4 and 6, a higher Ca2+ transient in response to 12.5 mM KCl. These neurons were less sensitive to the effect of 10 microM nifedipine and, conversely, more sensitive to the effects of 10 microM omega-conotoxin MVIIC when stimulated with 50 mM KCl, indicating that they express preferentially, at this stage, the N- and/or Q-type Ca2+ channels. This period of maximal activity of the N/Q-type Ca2+ channels was associated with a significant increase in the rate of neuronal apoptosis. The present study also shows, by comparing the rates of neuronal apoptosis, that the long-term maintenance in 25 mM KCl appears to "synchronize" and sensitize the neuronal population to the apoptotic process. These results illustrate the differential effect the culture conditions can have on the expression and activity of Ca2+ channels, which, in turn, can modulate neuronal survival.

Efficacy of Choto-san on vascular dementia and the protective effect of the hooks and stems of Uncaria sinensis on glutamate-induced neuronal death

Two different multicenter studies on the efficacy of Choto-san on patients with vascular dementia, one a well-controlled but non-double blind (60 patients), the other a double-blind controlled study (139 patients), were performed. In the well-controlled study, Choto-san was superior in global improvement rating, utility rating and improvement of subjective symptoms, psychiatric symptoms and disturbance in daily living activities. In the double-blind study, with more objective criteria than the well-controlled study, Choto-san was also superior in global improvement rating, utility rating and improvement of subjective symptoms, psychiatric symptoms and disturbance in daily living activities. These results suggest that Choto-san is effective in the treatment of vascular dementia. Uncaria sinensis (OLIV.) HAVIL. (US) is the main medicinal plant composing Choto-san. Glutamate-induced cell death of cultured cerebellar granule cells was protected by the application of water extract of US in a dose-dependent manner, and concentrations of 10(-5) to 10(-4) g/ml had a significant effect compared to exposure to glutamate only. Further, the increase of 45Ca2+ influx into cells by glutamate was also blocked by the water extract in a dose-dependent manner. These results suggest that US has a protective effect on glutamate-induced neuronal death in cultured cerebellar granule cells through the inhibition of Ca2+ influx.

Differential expression of three classes of voltage-gated Ca(2+) channels during maturation of the rat cerebellum in vitro

Voltage-gated Ca(2+) channels provide a mode of Ca(2+) influx that is essential for intracellular signaling in many cells. Semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) was used to assess the relative amounts of mRNAs encoding three classes of Ca(2+) channels (alpha1A, alpha1B and alpha1E) during development, in cultures established from prenatal rat cerebellar cortex. Ca(2+) channel transcript levels were standardized to a constitutive marker (cyclophilin). For all three classes of Ca(2+) channels, transcript levels were highest at early stages (4-10 days in vitro) and declined with age. This developmental pattern was differentially regulated by a depolarizing agent, tetraethylammonium chloride (TEA, 1 mM). Chronic depolarization yielded a significant elevation in transcript levels for alpha1B (N-type) and alpha1E (R-type) Ca(2+) channels during neuronal maturation (10-21 days in vitro), but dramatically suppressed transcript levels for the alpha1A (P-type) Ca(2+) channel at all stages of development. The effects of TEA on alpha1A, alpha1B and alpha1E transcript levels were mimicked by increasing external K(+) (from 5 to 10 mM). The regulatory effects of depolarization on transcript levels were dependent on extracellular Ca(2+) for alpha1E but not for alpha1A. For alpha1B, transcript levels depended on extracellular Ca(2+) only for increased K(+) as the depolarizing stimulus, but not for TEA. These results suggest that levels of Ca(2+) channel transcripts in rat cerebellum are developmentally regulated in vitro and can be influenced differentially by transmembrane signaling via chronic depolarization and Ca(2+) entry. Dynamic regulation of Ca(2+) channel expression may be relevant to the different functional roles of Ca(2+) channels and their regional localization within neurons.

Evaluation of the protective effects of alkaloids isolated from the hooks and stems of Uncaria sinensis on glutamate-induced neuronal death in cultured cerebellar granule cells from rats

We have previously shown that an aqueous extract of the hooks and stems of Uncaria sinensis (Oliv.) Havil., Uncariae Uncus Cum Ramulusis, protects against glutamate-induced neuronal death in cultured cerebellar granule cells by inhibition of Ca2+ influx. Because it is not known which components of Uncaria sinensis are active, in this study we have evaluated, by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide (MTT) staining, the neuroprotective effects of the oxyindole alkaloids corynoxeine, rhynchophylline, isorhynchophylline and isocorynoxeine, and the indole alkaloids geissoschizine methyl ether, hirsuteine and hirsutine, isolated from the hooks and stems of Uncaria sinensis, on glutamate-induced cell death. We also investigated the inhibitory effects of the compounds on 45Ca2+ influx in cultured rat cerebellar granule cells. Cell viability evaluated by the MTT assay was significantly increased by application of rhynchophylline (10(-3) M), isorhynchophylline (10(-4)-10(-3) M), isocorynoxeine (10(-4)-10(-3) M), hirsuteine (10(-4)-3 x 10(-4) M) or hirsutine (10(-4)-3 x 10(-4) M) compared with exposure to glutamate only, with the effect of isorhynchophylline being the strongest. The increased 45Ca2+ influx into cells induced by glutamate was significantly inhibited by administration of rhynchophylline (10(-3) M), isorhynchophylline (3 x 10(-4)-10(-3) M), isocorynoxeine (3 x 10(-4)-10(-3) M), geissoschizine methyl ether (10(-3) M), hirsuteine (3 x 10(-4)-10(-3) M) or hirsutine (3 x 10(-4)-10(-3) M). These results suggest that oxyindole alkaloids such as isorhynchophylline, isocorynoxeine and rhynchophylline and indole alkaloids such as hirsuteine and hirsutine are the active components of the hooks and stems of Uncaria sinensis which protect against glutamate-induced neuronal death in cultured cerebellar granule cells by inhibition of Ca2+ influx.

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.

Calcium transients and neurotransmitter release at an identified synapse

It is widely accepted that the modulation of the presynaptic Ca2+ influx is one of the main mechanisms by which neurotransmitter release can be controlled. The well-identified cholinergic synapse in the buccal ganglion of Aplysia has been used to study the modulations that affect presynaptic Ca2+ transients and to relate this to quantal evoked neurotransmitter release. Three types of Ca2+ channel (L, N and P) are present in the presynaptic neurone at this synapse. Influxes of Ca2+ through N- and P-type channels trigger the release of ACh with only N-type Ca2+ channels being regulated by presynaptic neuromodulator receptors. In addition, presynaptic Ca2+ stores, via complex mechanisms of Ca2+ uptake and Ca2+ release, control the Ca2+ concentration that triggers this evoked ACh release.

Regulation of Ca2+ influx by a protein kinase C activator in chromaffin cells: differential role of P/Q- and L-type Ca2+ channels

Phorbol esters reduce depolarization-evoked Ca2+ influx in adrenal chromaffin cells, suggesting that voltage-sensitive Ca2+ channels (VSCCs) are inhibited by protein kinase C-mediated phosphorylation. We now address the possibility that L- and P/Q-type Ca2+ channel subtypes might be differentially involved in phorbol ester action. In bovine chromaffin cells, short-term (10 min) incubations with phorbol 12-myristate 13-acetate (PMA) inhibited early high K+-evoked rises in cytosolic free Ca2+ concentration ([Ca2+]i) and the early component of the depolarization-evoked Mn2+ quenching of fura-2 fluorescence in a dose-dependent manner (IC50: 18 and 7 nM; maximal inhibitions: 45 and 48%, respectively). The protein kinase C inhibitor staurosporine (100 nM) reverted the inhibitory action of PMA. PMA (0.1-1 microM) inhibited the early and late phases of the ionomycin (2 microM)-evoked [Ca2+]i transients by 14-23%. Omega-agatoxin IVA, a blocker of P/Q-type Ca2+ channels, inhibited high K+-evoked [Ca2+]i rises in a dose-dependent fashion (IC50 = 50 nM). In contrast, 0.1 microM omega-conotoxin GVIA, a blocker of N-type channels, was without effect. A sizeable (< 45%) component of early Ca2+ influx persisted in the combined presence of omega-agatoxin IVA (100 nM) and nitrendipine (1 microM). Simultaneous exposure to omega-agatoxin IVA and PMA inhibited both the early [Ca2+]i transients and Mn2+ quenching to a much greater extent than each drug separately. Inhibition of the [Ca2+]i transients by nitrendipine and PMA did not significantly exceed that produced by PMA alone. It is concluded that phorbol ester-mediated activation of protein kinase C inhibits preferentially L-type VSCCs over P/Q type channels in adrenal chromaffin cells. However, the possibility cannot be ruled out that dihydropyridine-resistant, non-P/Q type channels might also be negatively regulated by protein kinase C. This may represent an important pathway for the specific control of VSCCs by protein kinase C-linked receptors, not only in paraneurones but presumably also in neurones and other excitable cells.