Photoactivation of intracellular guanosine triphosphate analogues reduces the amplitude and slows the kinetics of voltage-activated calcium channel currents in sensory neurones

Regulation of Presynaptic Calcium Channels

Voltage-gated calcium channels (VGCCs), especially Cav2.1 and Cav2.2, are the major mediators of Ca2+ influx at the presynaptic membrane in response to neuron excitation, thereby exerting a predominant control on synaptic transmission. To guarantee the timely and precise release of neurotransmitters at synapses, the activity of presynaptic VGCCs is tightly regulated by a variety of factors, including auxiliary subunits, membrane potential, G protein-coupled receptors (GPCRs), calmodulin (CaM), Ca2+-binding proteins (CaBP), protein kinases, various interacting proteins, alternative splicing events, and genetic variations.

How Postdoctoral Research in Paul Greengard's Laboratory Shaped My Scientific Career, Although I Never Did Another Phosphorylation Assay

In this short review, I describe from personal experience how every step in the career of any scientist, no matter how disjointed and pragmatic each might seem at the time, will almost inevitably meld together, to help us all tackle novel projects. My postdoctoral research in Paul Greengard's laboratory, where I investigated neurotransmitter-mediated phosphorylation of Synapsin I, was instrumental in my career progression, and Paul's support was instrumental in my ability to make a leap into independent research.

Functions of Presynaptic Voltage-gated Calcium Channels

Voltage-gated calcium channels are the principal conduits for depolarization-mediated Ca²⁺ entry into excitable cells. In this review, the biophysical properties of the relevant members of this family of channels, those that are present in presynaptic terminals, will be discussed in relation to their function in mediating neurotransmitter release. Voltage-gated calcium channels have properties that ensure they are specialized for particular roles, for example, differences in their activation voltage threshold, their various kinetic properties, and their voltage-dependence of inactivation. All these attributes play into the ability of the various voltage-gated calcium channels to participate in different patterns of presynaptic vesicular release. These include synaptic transmission resulting from single action potentials, and longer-term changes mediated by bursts or trains of action potentials, as well as release resulting from graded changes in membrane potential in specialized sensory synapses.

Optogenetic toolkit for precise control of calcium signaling

Calcium acts as a second messenger to regulate a myriad of cell functions, ranging from short-term muscle contraction and cell motility to long-term changes in gene expression and metabolism. To study the impact of Ca2+-modulated 'ON' and 'OFF' reactions in mammalian cells, pharmacological tools and 'caged' compounds are commonly used under various experimental conditions. The use of these reagents for precise control of Ca2+ signals, nonetheless, is impeded by lack of reversibility and specificity. The recently developed optogenetic tools, particularly those built upon engineered Ca2+ release-activated Ca2+ (CRAC) channels, provide exciting opportunities to remotely and non-invasively modulate Ca2+ signaling due to their superior spatiotemporal resolution and rapid reversibility. In this review, we briefly summarize the latest advances in the development of optogenetic tools (collectively termed as 'genetically encoded Ca2+ actuators', or GECAs) that are tailored for the interrogation of Ca2+ signaling, as well as their applications in remote neuromodulation and optogenetic immunomodulation. Our goal is to provide a general guide to choosing appropriate GECAs for optical control of Ca2+ signaling in cellulo, and in parallel, to stimulate further thoughts on evolving non-opsin-based optogenetics into a fully fledged technology for the study of Ca2+-dependent activities in vivo.

Caged molecules: principles and practical considerations

A caged molecule is an inert but photosensitive molecule that is transformed by photolysis into a biologically active molecule at high speed (typically 1 msec). The process is referred to as photorelease. The spatial resolution of photorelease is limited by the properties of light; submicrometer resolution is potentially achievable. Therefore, focal photorelease of caged molecules enables one to control biological processes with high spatio-temporal precision. The principles underlying caged molecules as well as practical considerations for their use are discussed in this unit.

G protein activation by uncaging of GTP-gamma-S in the leech giant glial cell

Glial cells can be activated by neurotransmitters via metabotropic, G protein-coupled receptors. We have studied the effects of 'global' G protein activation by GTP-gamma-S on the membrane potential, membrane conductance, intracellular Ca(2+) and Na(+) of the giant glial cell in isolated ganglia of the leech Hirudo medicinalis. Uncaging GTP-gamma-S (injected into a giant glial cell as caged compound) by moderate UV illumination hyperpolarized the membrane due to an increase in K+ conductance. Uncaging GTP-gamma-S also evoked rises in cytosolic Ca(2+) and Na+, both of which were suppressed after depleting the intracellular Ca(2+) stores with cyclopiazonic acid (20 micromol l(-1)). Uncaging inositol-trisphosphate evoked a transient rise in cytosolic Ca(2+) and Na+ but no change in membrane potential. Injection of the fast Ca(2+) chelator BAPTA or depletion of intracellular Ca(2+) stores did not suppress the membrane hyperpolarization induced by uncaging GTP-gamma-S. Our results suggest that global activation of G proteins in the leech giant glial cell results in a rise of Ca(2+)-independent membrane K+ conductance, a rise of cytosolic Ca(2+), due to release from intracellular stores, and a rise of cytosolic Na+, presumably due to increased Na+/Ca(2+) exchange.

A study comparing the actions of gabapentin and pregabalin on the electrophysiological properties of cultured DRG neurones from neonatal rats

BACKGROUND

Gabapentin and pregabalin have wide-ranging therapeutic actions, and are structurally related to the inhibitory neurotransmitter GABA. Gabapentin, pregablin and GABA can all modulate voltage-activated Ca2+ channels. In this study we have used whole cell patch clamp recording and fura-2 Ca2+ imaging to characterise the actions of pregabalin on the electrophysiological properties of cultured dorsal root ganglion (DRG) neurones from neonatal rats. The aims of this study were to determine whether pregabalin and gabapentin had additive inhibitory effects on high voltage-activated Ca2+ channels, evaluate whether the actions of pregabalin were dependent on GABA receptors and characterise the actions of pregabalin on voltage-activated potassium currents.

RESULTS

Pregabalin (25 nM - 2.5 microM) inhibited 20-30% of the high voltage-activated Ca2+ current in cultured DRG neurones. The residual Ca2+ current recorded in the presence of pregabalin was sensitive to the L-type Ca2+ channel modulator, Bay K8644. Saturating concentrations of gabapentin failed to have additive effects when applied with pregabalin, indicating that these two compounds act on the same type(s) of voltage-activated Ca2+ channels but the majority of Ca2+ current was resistant to both drugs. The continual application of GABA, the GABAB receptor antagonist CGP52432, or intracellular photorelease of GTP-gamma-S had no effect on pregabalin-induced inhibition of Ca2+ currents. Although clear inhibition of Ca2+ influx was produced by pregabalin in a population of small neurones, a significant population of larger neurones showed enhanced Ca2+ influx in response to pregabalin. The enhanced Ca2+ influx evoked by pregabalin was mimicked by partial block of K+ conductances with tetraethylammonium. Pregabalin produced biphasic effects on voltage-activated K+ currents, the inhibitory effect of pregabalin was prevented with apamin. The delayed enhancement of K+ currents was attenuated by pertussis toxin and by intracellular application of a (Rp)-analogue of cAMP.

CONCLUSIONS

Pregabalin reduces excitatory properties of cultured DRG neurones by modulating voltage-activated Ca2+ and K+ channels. The pharmacological activity of pregabalin is similar but not identical to that of gabapentin. The actions of pregabalin may involve both extracellular and intracellular drug target sites and modulation of a variety of neuronal conductances, by direct interactions, and through intracellular signalling involving protein kinase A.

G Protein Modulation of Voltage-Gated Calcium Channels

Calcium influx into any cell requires fine tuning to guarantee the correct balance between activation of calcium-dependent processes, such as muscle contraction and neurotransmitter release, and calcium-induced cell damage. G protein-coupled receptors play a critical role in negative feedback to modulate the activity of the CaV2 subfamily of the voltage-dependent calcium channels, which are largely situated on neuronal and neuro-endocrine cells. The basis for the specificity of the relationships among membrane receptors, G proteins, and effector calcium channels will be discussed, as well as the mechanism by which G protein-mediated inhibition is thought to occur. The inhibition requires free G beta gamma dimers, and the cytoplasmic linker between domains I and II of the CaV2 alpha 1 subunits binds G beta gamma dimers, whereas the intracellular N terminus of CaV2 alpha 1 subunits provides essential determinants for G protein modulation. Evidence suggests a key role for the beta subunits of calcium channels in the process of G protein modulation, and the role of a class of proteins termed "regulators of G protein signaling" will also be described.

mGluR2 postsynaptically senses granule cell inputs at Golgi cell synapses

In the cerebellar circuit, Golgi cells are thought to contribute to information processing and integration via feedback mechanisms. In these mechanisms, dynamic modulation of Golgi cell excitability is necessary because GABA from Golgi cells causes tonic inhibition on granule cells. We studied the role and synaptic mechanisms of postsynaptic metabotropic glutamate receptor subtype 2 (mGluR2) at granule cell-Golgi cell synapses, using whole-cell recording of green fluorescent protein-positive Golgi cells of wild-type and mGluR2-deficient mice. Postsynaptic mGluR2 was activated by glutamate from granule cells and hyperpolarized Golgi cells via G protein-coupled inwardly rectifying K+ channels (GIRKs). This hyperpolarization conferred long-lasting silencing of Golgi cells, the duration and extents of which were dependent on stimulus strengths. Postsynaptic mGluR2 thus senses inputs from granule cells and is most likely important for spatiotemporal modulation of mossy fiber-granule cell transmission before distributing inputs to Purkinje cells.

Slow inhibition of N-type calcium channels with GTP gamma S reflects the basal G protein-GDP turnover rate

The inhibition of N-type Ca channels via a G protein pathway is a rapid mechanism for modulating Ca influx. It has been noted, however, that when G proteins are activated by guanosine 5'- O-(3-thiotriphosphate) (GTPgammaS), the speed of inhibition is greatly reduced, despite the pathway having fewer molecular steps. We explored this anomaly in chick dorsal root ganglion neurons by comparing Ca current inhibition using GTPgammaS with application of the G protein receptor agonist noradrenaline. Noradrenaline caused rapid Ca channel inhibition (tau~5 s), contrasting greatly with the ~70-fold slower rate observed with GTPgammaS. Additionally, the slow rate with GTPgammaS could be accelerated to near agonist-induced rates by application of noradrenaline, demonstrating that the inhibition with GTPgammaS was not perfusion limited and that the rate-limiting step was upstream from GTPgammaS binding. Our results suggest that in the absence of noradrenaline, G protein activation by GTPgammaS is impeded by the slow resting turnover of GDP/GTP. The rate at which inhibition develops with GTPgammaS (tau~350 s) is thus a direct and sensitive measure of resting GDP turnover.

Adrenaline diminishes K+ contractures and Ba2+-current in chicken slow skeletal muscle fibres

The effects of adrenaline and the beta-adrenergic agonist isoprenaline on K+-evoked tension (K+-contracture) and Ba2+ current were investigated in chicken slow (anterior latissimus dorsi (ald)) muscle using isometric-tension measurements and current recording. Addition of adrenaline (10(-7) - 10(-5) M) or isoprenaline (10(-6) - 10(-5) M) to the bath reduced the amplitude of the K+-contractures. These effects were blocked by the beta-antagonist propranolol (5 x 10(-6) M). External application of a cAMP analogue (8-bromo cyclic AMP; 1 x 10(-4) M) also decreased the amplitude of the K+-contractures. To analyze the possible relationship between the induced tension reduction and effects on sarcolemmal Ca2+ channels, a slow action potential and a slow inward membrane current were studied in intact ald chicken muscle fibres. When the ald muscle was immersed in a Na+- and Cl--free solution containing Ba2+ and depolarizing pulses were delivered from a -80 mV holding potential, the muscle fibres exhibited a small, slow Ba2+-dependent potential (observed at about -26 mV, peak amplitude, around -10 mV). The response was blocked by the addition of Co2+ (5 mM) or Cd2+ (2 mM). Using the three-microelectrode voltage-clamp technique, a slow inward membrane current underlying the Ba2+ potential could be discerned. The current had a mean threshold of -60 mV, reached maximum at about -5 mV and ranged from ca. 9 to 19 pA/cm2 (depending on the external Ba2+ concentration). It had a mean reversal potential of +45 mV. The Ba2+ inward current was diminished when adrenaline or isoprenaline was added to the bath (1 x 10(-5) M); however, this decrease did not occur when propranolol was present (5 x 10-6 M). These results suggest that the decreases in the tension of K+-contractures induced by adrenaline and isoprenaline may occur through beta-adrenergic effects on sarcolemmal Ca2+ channels in ald chicken slow muscle fibres.

G-Protein types involved in calcium channel inhibition at a presynaptic nerve terminal

The inhibition of presynaptic calcium channels via G-protein-dependent second messenger pathways is a key mechanism of transmitter release modulation. We used the calyx-type nerve terminal of the chick ciliary ganglion to examine which G-proteins are involved in the voltage-sensitive inhibition of presynaptic N-type calcium channels. Adenosine caused a prominent inhibition of the calcium current that was totally blocked by pretreatment with pertussis toxin (PTX), consistent with an exclusive involvement of G(o)/G(i) in the G-protein pathway. Immunocytochemistry was used to localize these G-protein types to the nerve terminal and its transmitter release face. We used two approaches to test for modulation by other G-protein types. First, we treated the terminals with ligands for a variety of G-protein-linked neurotransmitter receptor types that have been associated with different G-protein families. Although small inhibitory effects were observed, these could all be eliminated by PTX, indicating that in this terminal the G(i) family is the sole transmitter-induced G-protein inhibitory pathway. Second, we examined the kinetics of calcium channel inhibition by uncaging the nonselective and irreversible G-protein activator GTPgammaS, bypassing the receptors. A large fraction of the rapid GTPgammaS-induced inhibition persisted, consistent with a G(o)/G(i)-independent pathway. Immunocytochemistry identified G(q), G(11), G(12), and G(13) as potential PTX-insensitive second messengers at this terminal. Thus, our results suggest that whereas neurotransmitter-mediated calcium channel inhibition is mainly, and possibly exclusively, via G(o)/G(i), other rapid PTX-insensitive G-protein pathways exist that may involve novel, and perhaps transmitter-independent, activating mechanisms.

A low-cost UV laser for flash photolysis of caged compounds

Photolysis of caged compounds has become a standard tool for the rapid application of bioactive molecules. In principle this technique also allows to apply substances in a spatially very restricted manner. An important practical limitation for such experiments, however, is the high cost of UV lasers. Here we describe the assembly of an inexpensive pulsed nitrogen laser which is suitable for photolysis experiments. The laser which can be constructed in less than 1 week and for less than US$ 500 emits light pulses with a duration of approximately 5 ns, an energy of up to 200 microJ (= 40 kW) and a wavelength of 337 nm. Its beam can be focused to roughly 30 microns, a firing frequency of up to 50 Hz can be achieved, and electrical artifacts are minimal. These specifications make the laser optimally suited for most photolysis experiments. Its low price and ease of use should make the technique of spatially restricted flash photolysis amenable to many laboratories.

Different effects of baclofen and GTP gamma S on voltage-activated Ca2+ currents in rat hippocampal neurons in vitro

Reduction of voltage-activated Ca2+ currents by intracellular application of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) through ultraviolet (UV) photolysis of the caged compound, is followed by a re-augmentation to control levels within 10 min, independently of the divalent cation used. The Ca2+ current inhibition by the gamma-aminobutyric acid type B (GABAB) receptor agonist baclofen, which is also thought to be mediated by a GTP-binding protein (G-protein), is potentiated when GTP gamma S is uncaged during agonist superfusion. The authors suggest that GTP gamma S activates G-protein-dependent pathways that are not activated by the baclofen receptor.

Aspects of calcium-activated chloride currents: a neuronal perspective

Ca(2+)-activated Cl- channels are expressed in a variety of cell types, including central and peripheral neurones. These channels are activated by a rise in intracellular Ca2+ close to the cell membrane. This can be evoked by cellular events such as Ca2+ entry through voltage- and ligandgated channels or release of Ca2+ from intracellular stores. Additionally, these Ca(2+)-activated Cl currents (ICl(Ca)) can be activated by raising intracellular Ca2+ through artificial experimental procedures such as intracellular photorelease of Ca2+ from "caged" photolabile compounds (e.g. DM-nitrophen) or by treating cells with Ca2+ ionophores. The potential changes that result from activation of Ca(2+)-activated Cl- channels are dependent on resting membrane potential and the equilibrium potential for Cl-. Ca2+ entry during a single action potential is sufficient to produce substantial after potentials, suggesting that the activity of these Cl- channels can have profound effects on cell excitability. The whole cell ICl(Ca) can be identified by sensitivity to increased Ca2+ buffering capacity of the cell, anion substitution studies and reversal potential measurements, as well as by the actions of Cl- channel blockers. In cultured sensory neurones, there is evidence that the ICl(Ca) deactivates as Ca2+ is buffered or removed from the intracellular environment. To date, there is no evidence in mammalian neurones to suggest these Ca(2+)-sensitive Cl- channels undergo a process of inactivation. Therefore, ICl(Ca) can be used as a physiological index of intracellular Ca2+ close to the cell membrane. The ICl(Ca) has been shown to be activated or prolonged as a result of metabolic stress, as well as by drugs that disturb intracellular Ca2+ homeostatic mechanisms or release Ca2+ from intracellular stores. In addition to sensitivity to classic Cl- channel blockers such as niflumic acid, derivatives of stilbene (4,4'diisothiocyanostilbene-2,2'-disulphonic acid, 4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonic acid) and benzoic acid (5-nitro 2-(3-phenylpropylamino) benzoic acid), ICl(Ca) are also sensitive to polyamine spider toxins and some of their analogues, particularly those containing the amino acid residue arginine. The physiological role of Ca(2+)-activated Cl- channels in neurones remains to be fully determined. The wide distribution of these channels in the nervous system, and their capacity to underlie a variety of events such as sustained or transient depolarization or hyperpolarizations in response to changes in intracellular Ca2+ and variations in intracellular Cl- concentration, suggest the roles may be subtle, but important.

Muscarine inhibits high-threshold calcium currents with two distinct modes in rat embryonic hippocampal neurons

1. Ca2+ channel modulation by muscarine was investigated in primary cultured embryonic rat hippocampal neurons using the whole-cell variant of the patch-clamp technique. 2. Muscarine produced a reversible and concentration-dependent decrease in the Ba2+ current amplitude. In 65% of neurons sensitive to the agonist, current inhibition was time and voltage dependent, being maximal between -20 and 0 mV and decreasing at depolarizing potentials. In the remaining 35% of neurons, the effects of muscarine were voltage independent, inhibition being constant in a wide potential range between -20 and +80 mV. 3. Different receptors might be involved in the two modes of modulation. Muscarine-induced voltage-dependent inhibition of Ba2+ current was best suppressed by the muscarinic receptor antagonist 4-diphenylacetoxy-N-methyl-piperidine methiodide (81% suppression), while voltage-independent inhibition was best suppressed by AFDX116 (75% suppression). 4. In cells treated with omega-conotoxin (omega-CgTX), the voltage-independent mode of inhibition was strongly prevented, suggesting that the two modulatory mechanisms (voltage dependent and voltage independent) operate on separate classes of high-voltage-activated (HVA) Ca2+ channels. 5. A pertussis toxin-sensitive G-protein is involved in both modes of action of muscarine, since both modes were prevented by pretreatment of the cells with 50 ng ml-1 pertussis toxin. 6. Both modes of modulation were mimicked in different cells by intracellular application of GTP-gamma-S. However, the onset of voltage-independent inhibition was about 5 times slower than that of voltage-dependent inhibition, suggesting involvement of a more complex metabolic pathway for the former mode of channel modulation. 7. Relief of the voltage-dependent inhibition was obtained by depolarizing voltage prepulses and occurred with kinetics that depended on agonist concentration. 8. The voltage-dependent inhibition could be simulated by a kinetic model in which the time course of Ca2+ entry was assumed to be regulated by both the concentration of muscarine and membrane potential.

Activation of Ca(2+)-dependent Cl- currents in cultured rat sensory neurones by flash photolysis of DM-nitrophen

1. Voltage-gated Ca2+ currents (ICa) and Ca(2+)-activated Cl- currents (ICl(Ca)) were recorded from cultured rat dorsal root ganglion (DRG) neurones using the whole-cell configuration of the patch clamp technique. Intracellular photorelease of Ca2+ by flash photolysis of DM-nitrophen elicited transient inward currents only in those cells which possessed Ca(2+)-activated Cl- tail currents following ICa. The reversal potential of the flash responses was hyperpolarized when extracellular Cl- was replaced by SCN-. The flash responses and the Ca(2+)-activated Cl- tail currents were inhibited by the Cl- channel blockers niflumic acid (10-100 microM) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) (10 microM). 2. After activation by ICa, the Ca(2+)-activated Cl- current could be reactivated during its decay by photorelease of caged Ca2+. Experiments carried out on neurones held at 0 mV demonstrated that ICl(Ca) could be chronically activated due to residual Ca2+ influx. These data directly demonstrated that the decay of ICl(Ca) is not due to inactivation but rather to deactivation as a result of removal of the Ca2+ load from the cell cytoplasm. 3. Photorelease of caged inositol 1,4,5-trisphosphate (IP3) failed to activate any Ca(2+)-dependent current responses in cultured DRG neurones, although application of caffeine elicited transient inward currents, and responses to photoreleased IP3 could be obtained from freshly dissociated smooth muscle cells. 4. Photorelease of Ca2+ provides a useful method for investigating the properties of ICl(Ca) independently from other physiological parameters. In addition, we have directly demonstrated that ICl(Ca) in DRG neurones does not inactivate, and so may continue to modulate membrane excitability as long as the intracellular Ca2+ concentration ([Ca2+]i) close to the cell membrane is elevated.(ABSTRACT TRUNCATED AT 250 WORDS)

Heterotrimeric G proteins involved in the modulation of voltage-dependent calcium channels of neuroendocrine cells

Various mechanisms have been identified by which hormones and neurotransmitters, interacting with heptahelical receptors, modulate the intracellular Ca2+ concentration in neuronal, endocrine, and neuroendocrine cells. All of them involve heterotrimeric G proteins. Best documented are hormonal stimulations and inhibitions of voltage-dependent Ca2+ channels. Stimulation is caused by agonists interacting with receptors known to induce phosphatidylinositol 4,5-bisphosphate hydrolysis, that is, a PI response. Although the PI response triggers a transient secretion by fast Ca2+ release, the stimulation of Ca2+ channels is assumed to be responsible for prolonged cell responses and for refilling of IP3-sensitive Ca2+ pools after repeated stimulations. Using antisense oligonucleotide microinjection in rat pituitary GH3 cells, Gi2 has been identified as the pertussis toxin-sensitive G protein stimulating Ca2+ channels, whereas Gq/G11 are involved in the concurrent PI response with subsequent protein kinase C activation, which is required for Ca2+ channel stimulation. Inhibitory modulations of Ca2+ channels are assumed to be the basis of inhibitions of transmitter or hormone secretion. Experiments in GH3 cells have revealed that Go subforms composed of alpha o1 x beta 3 x gamma 4 and alpha o2 x beta 1 x gamma 3 are the active G-protein heterotrimers transferring inhibitory signals from muscarinic M4 and somatostatin receptors to the Ca2+ channel, respectively.

Dorsal root ganglion neurons of embryonic chicks contain nitric oxide synthase and respond to nitric oxide

We investigated the function of nitric oxide (NO) in dorsal root ganglion (DRG) neurons from 10 day embryonic chicks and adult birds. NADPH-diaphorase activity, a histochemical marker for nitric oxide synthase (NOS) in paraformaldehyde-fixed neurons, and NOS-like immunoreactivity were localized in all neurons in thoracic and lumbar ganglia from embryos. However, only a subset of neurons from adults contained NOS-like immunoreactivity and NADPH-diaphorase activity. Thus, embryonic chick DRG neurons have the potential to synthesize NO in response to elevated cytoplasmic Ca2+. We also investigated the ability of dissociated embryonic chick DRG neurons to respond to NO by examining the effects of NO donors and 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP) on Ca2+ current (ICa) using the amphotericin-permeabilized patch-clamp technique: sodium nitroprusside (5 microM) reduced ICa to 0.68 +/- 0.06 (mean +/- S.D., n = 5) of control, S-nitroso-N-acetylpenicillamine (1 microM) reduced ICa to 0.44 +/- 0.06 (n = 4) of control, while 8-Br-cGMP (1 mM) reduced ICa to 0.58 +/- 0.22 (n = 5) of control. ICa was reduced in every neuron tested and this effect was partially reversed after approximately 10 min of washing. Thus, ICa of embryonic chick DRG neurons is inhibited by NO, possibly by a cGMP-dependent mechanism. These results indicate that all DRG neurons in embryonic chicks contain NOS-like immunoreactivity and respond to NO. Further, the percentage of NADPH-diaphorase positive neurons is reduced during development.

Modulation of Ca2+ transients by photorelease of caged nucleotides in frog skeletal muscle fibers

Action potentials and intracellular Ca2+ transients were monitored in current-clamped segments of frog skeletal muscle fibers using the triple vaseline-gap technique. Calcium signals were measured with the fluorescent indicator rhod 2. Action potentials produced a transient increase in intracellular Ca2+ that was estimated, by deconvolution of the fluorescence signals, to range between 3 and 12 microM. The comparative effects of flash photolysis of caged adenosine 3',5'-cyclic monophosphate (cAMP) and caged ATP on action potentials and Ca signals in muscle were investigated. The photorelease of both nucleotides produced a reduction in the amplitude of the afterpotential that follows the spike. Photorelease of cAMP and ATP prolonged the rate of decay of the Ca signals. No changes in either the rate of rise or in the latent period between stimulation and onset of the Ca signal were observed. Release of cAMP reduced the amplitude of Ca signals, whereas release of ATP had the opposite effect. Our results show that cAMP and ATP, released above their endogenous levels, modulate intracellular Ca2+ release. The cAMP modulation is more significant and may be of physiological importance.

Tonic inhibition of neuronal calcium channels by G proteins removed during whole-cell patch-clamp experiments

The barium current through voltage-dependent calcium channels was recorded from cultured rat cortical neurons with the whole-cell configuration of the patch-clamp technique. The maximal current evoked by depolarising pulses from -80 mV to 0 mV was divided into inactivating and non-inactivating fractions. During the first minutes of whole-cell recording, the amplitude of the inactivating fraction increased from less than 0.1 nA to an average value of 1 nA, whereas the amplitude of the non-inactivating component remained essentially the same. This increase in amplitude was prevented when the "perforated-patch technique" was used, suggesting that some intracellular factor that inhibited the barium current was lost or destroyed during conventional whole-cell experiments. When GTP[gamma-S] or GTP was added to the pipette solution, no increase or only a weak rise of the inactivating current was seen, whereas GDP[beta-S] accelerated its increase. The results suggest that some of the calcium channels expressed in cultured cortical neurons are inhibited by a G protein even in the absence of added neurotransmitter. The current increase observed during whole-cell recordings may be due to a loss of intracellular GTP and the subsequent inactivation of an inhibitory G protein.

G-proteins involved in the calcium channel signalling system

Various mechanisms have been identified by which hormones and neurotransmitters interacting with seven transmembrane alpha-helical spanning segments receptors modulate the activity of ion channels. All of the mechanisms involve heterotrimeric G-proteins; the best documented are hormonal modulations of voltage-dependent Ca2+ channels in cardiac, neuronal and endocrine cells. Recent studies using antisense oligonucleotide probes allow the exact identification of the G-proteins involved in these signal transduction pathways.

Isoproterenol and GTP gamma S inhibit L-type calcium channels of differentiating rat skeletal muscle cells

In adult skeletal muscle, G-proteins have been shown to modulate the calcium channels both directly and through a cAMP-dependent phosphorylating mechanism. We have investigated the action of G-proteins on the L-type calcium current in cultured rat muscle cells (myoballs) under voltage clamp in whole cell or perforated patch modes. Intracellular photolytic release of 200 microM GTP gamma S inhibited the L-type calcium current. Inclusion of 500 microM uncaged GTP gamma S in the patch pipette in the whole cell configuration reduced the calcium current by a similar amount. Under perforated patch conditions external application of 10 microM of the beta-adrenergic agonist isoproterenol also reduced the calcium current. Pretreatment of the cells with pertussis toxin reversed the effect of GTP gamma S and removed that of isoproterenol. We conclude that rat myoballs contain beta-adrenergic receptors that inhibit the L-type calcium current, and that this inhibition is mediated by a pertussis toxin-sensitive G-protein.

Kinetics of intracellular calcium release by inositol 1,4,5-trisphosphate and extracellular ATP in porcine cultured aortic endothelial cells

Quantitative, time-resolved measurements have been made of intracellular Ca ion release by inositol 1,4,5-trisphosphate (InsP3) and extracellular ATP in porcine aortic endothelial cells in tissue culture. Intracellular free [Ca] was detected with the calcium dye fluo-3 and InsP3 released intracellularly by photolysis of 'caged' InsP3 in whole-cell voltage-clamped aortic endothelial cells. A rise of [Ca] was recorded at InsP3 concentrations greater than 0.2 microM. The timecourse at low InsP3 concentrations comprised a delay of mean 300 ms (range 266-330 ms), a peak in 2-3 s before declining with a half-time of 5-10 s. The delay and time-to-peak decreased with increasing concentrations of InsP3 over the range 0.2-5 microM. At very high concentrations of InsP3 (> 5 microM), the delay in the Ca response was short, always less than 20 ms. The results are consistent with a direct binding and gating action of InsP3 on the Ca channel of the cellular store. Following InsP3 action there is a refractoriness of the InsP3 Ca release process which recovers with a timecourse of half-time about 30 s. A comparison can be made between the timecourse of InsP3 and extracellular ATP actions. High concentrations of ATP (500 microM) acted with a delay of mean 1.8 s (range 1.2-2.5 s), whereas even moderate concentrations of InsP3 acted much more quickly, suggesting that there are slow coupling steps before or during the production of InsP3 in response to extracellular ATP. Both ATP and InsP3 evoked an increase in membrane conductance to K+, probably via Ca.

A functional role for GTP-binding proteins in synaptic vesicle cycling

The squid giant synapse was used to test the hypothesis that guanosine-5'-triphosphate (GTP)-binding proteins regulate the local distribution of synaptic vesicles within nerve terminals. Presynaptic injection of the nonhydrolyzable GTP analog GTP gamma S irreversibly inhibited neurotransmitter release without changing either the size of the calcium signals produced by presynaptic action potentials or the number of synaptic vesicles docked at presynaptic active zones. Neurotransmitter release was also inhibited by injection of the nonhydrolyzable guanosine diphosphate (GDP) analog GDP beta S but not by injection of AIF4-. These results suggest that a small molecular weight GTP-binding protein directs the docking of synaptic vesicles that occurs before calcium-dependent neurotransmitter release. Depletion of undocked synaptic vesicles by GTP gamma S indicates that additional GTP-binding proteins function in the terminal at other steps responsible for synaptic vesicle replenishment.

Activation of L-type Ca2+ currents in cardiac myocytes by photoreleased GTP

L-type calcium currents (ICa) were recorded from isolated ventricular myocytes by using standard patch-clamp methods. In the absence of agonist, photorelease of GTP by flash photolysis of intracellularly applied caged-GTP rapidly increased the amplitude of ICa over a wide range of membrane potentials. Control experiments clearly demonstrated that this effect was not due to either the release of photolytic by-products or to the light flash itself. The timecourse for activation of ICa by photolysis of caged-GTP was markedly altered by intracellular application of either GDP beta S or GTP gamma S. Upon maximal stimulation of ICa by intracellular dialysis with cAMP, photoreleased GTP induced a small, rapid increase in ICa followed by a gradual inhibition. The presence of Rp-cAMPS intracellularly reduced both the magnitude of the response to photoreleased GTP and its time to peak. Similar effects were observed when protein kinase inhibitor dialysed the cell interior, suggesting that both cAMP-dependent and independent processes were involved in this effect. We conclude that rapid release of GTP within ventricular myocytes, in the absence of agonist, causes rapid activation of L-type Ca2+ current. Mechanisms underlying this effect include stimulation of adenylate cyclase, together with other, as yet uncharacterized, GTP-dependent pathways for increasing ICa in the heart.

Kinetics of G protein-mediated modulation of the potassium M-current in bullfrog sympathetic neurons

The inhibition of the voltage-dependent, K+ M-current (IM) following receptor-independent G protein activation with controlled intracellular perfusion of nonhydrolyzable GTP analogs had an exponential time course, with rates hyperbolically dependent on GTP analog concentration, and a limiting value of 0.53 min-1. The inhibitory agonist muscarine caused a concentration-dependent acceleration of the rate of nucleotide-induced inhibition, with a plateau of about 20 min-1 and an exponential time course. In neurons not treated with nucleotide analogs the IM recovery rate following agonist removal was 3-7 min-1. It is proposed that the overall kinetics of the transduction pathway for IM modulation is governed by the agonist-dependent kinetics of nucleotide interaction with G proteins. A simple model of IM modulation based on G proteins' kinetics has been developed. These data suggest a possible cellular process responsible for the time course of slow synaptic potentials caused by IM inhibition in sympathetic neurons.

Inhibition of L-type calcium channels by internal GTP [gamma S] in mouse pancreatic beta cells

Pretreatment of pancreatic beta cells with pertussis toxin resulted in a 30% increase in peak whole-cell Ca2+ currents recorded in the absence of exogenous intracellular guanine nucleotides. Intracellular application of 90 microM GTP[gamma S], by liberation from a caged precursor, resulted in 40% reduction of the peak Ca2+ current irrespective of whether the current was carried by Ca2+ or Ba2+. Effects on the delayed outward K+ current were small and restricted to a transient Ca(2+)-dependent K+ current component. Inhibition by GTP[gamma S] of the Ca2+ current was not mimicked by standard GTP and could not be prevented either by pretreatment with pertussis toxin or by inclusion of GDP[beta S] or cyclic AMP in the intracellular medium. The inhibitory effect of GTP[gamma S] could be counteracted by a prepulse to a large depolarizing voltage. A similar effect of a depolarizing prepulse was observed in control cells with no exogenous guanine nucleotides. These observations indicate that inhibition of beta cell Ca2+ current by G protein activation results from direct interaction with the channel and does not involve second-messenger systems. Our findings also suggest that the beta cell Ca2+ current is subject to resting inhibition by G proteins.

GTP gamma S causes contraction of skinned frog skeletal muscle via the DHP-sensitive Ca2+ channels of sealed T-tubules

We have investigated the involvement of G-proteins in excitation-contraction coupling of fast-twitch skeletal muscle, using a fibre preparation designed to retain intact T-tubules and sarcoplasmic reticulum. The nonhydrolysable analogue of guanosine triphosphate, GTP gamma S (50-500 microM) caused a strong, transient isometric contraction in this preparation. Reduction of ethylene-bis(oxonitrilo)tetraacete (EGTA) in the sealed T-tubules from 5 mM to 0.1 mM lowered the threshold to GTP gamma S and removal of sodium reversibly raised it. The dihydropyridine (DHP) calcium channel antagonists nicardipine and nifedipine allowed a first contraction and then blocked subsequent GTP gamma S action. The phenylalkylamine methoxyverapamil (D-600) did likewise, reversibly, at 10 degrees C. The guanosine diphosphate analogue, GDP beta S, and procaine reversibly blocked the action of GTP gamma S; pertussis toxin also blocked it. Photolytic release of 40-100 microM GTP gamma S within 0.1 s from S-caged GTP gamma S caused contraction after a latent period of 0.3-20 s. We conclude that GTP gamma S can activate contraction in frog skeletal muscle via a route requiring both the integrity of the T-tubular DHP-sensitive calcium channel (DHPr) and the presence of sodium in the sealed T-tubules. We propose that in this preparation GTP gamma S activates a G-protein, which in turn activates the DHPr as a calcium channel and releases stored calcium from within the sealed T-tubule. Implications of these results for the excitation-contraction coupling mechanism in skeletal muscle are discussed.

Ca2+ channel currents in rat sensory neurones: interaction between guanine nucleotides, cyclic AMP and Ca2+ channel ligands

1. The characteristics have been examined of the high threshold calcium channel current in cultured rat dorsal root ganglion (DRG) neurones recorded in the presence of guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S; 200 microM in the patch pipette). This current, termed IBa, GTP gamma S, was slowly activating and showed little inactivation over 100 ms. 2. External application of forskolin (10 microM) to elevate internal cyclic AMP levels increased the amplitude of IBa, GTP gamma S whereas it had no effect on the control IBa. This cyclic AMP-dependent protein kinase (PKI; 25 microM). 3. The cyclic AMP-dependent phosphorylation induced enhancement of IBa, GTP gamma S was voltage dependent and either did not occur or was observed only transiently at a holding potential (VH) of -30 mV. The forskolin-stimulated enhancement seen at VH -80 mV was lost with a t1/2 of about 1 min when VH was depolarized to -30 mV. Cholera toxin pre-treatment also increased the amplitude of IBa, GTP gamma S at VH -80 mV but not at VH -30 mV. 4. The calcium channel antagonist (-)-202-791 (5 microM) increased the amplitude of IBa, GTP gamma S when applied at VH -80 mV, but either not, or only transiently, at VH -30 mV, as previously observed. This 'agonist' effect of (-)-202-791 was prevented by PKI and was occluded by prior enhancement of IBa, GTP gamma S with forskolin. (-)-202-791 did not increase cyclic AMP levels in DRG neurones. 5. The 'agonist' response of IBa, GTP gamma S to D600 (10 microM) was also occluded by application of forskolin (10 microM) in the patch pipette. Forskolin alone, applied in this manner, increased IBa, GTP gamma S to a similar extent to D600 applied alone. 6. The agonist effect of (+)-202-791 (5 microM) on IBa, GTP gamma S was not prevented by prior enhancement with forskolin, nor was it prevented by PKI. 7. In conclusion, internal GTP gamma S activates G proteins which may interact directly with calcium channels to influence the kinetics of activation and to reduce steady-state inactivation of the channels. There is also an indirect effect on the generation of second messengers such as cyclic AMP. It is likely that forskolin enhances IBa, GTP gamma S by increasing activated Gs coupling to adenylyl cyclase and increasing cyclic AMP generation. The mechanism of action of (-)-202-791 to enhance IBa, GTP gamma S also involves cyclic AMP-dependent phosphorylation.(ABSTRACT TRUNCATED AT 400 WORDS)

The reduction of neuronal calcium currents by ATP-gamma-S is mediated by a G protein and occurs independently of cyclic AMP-dependent protein kinase

We studied the effects of ATP-gamma-S on the T, N and L calcium current components of nodose ganglion neurons using the whole cell variation of the patch clamp technique. ATP-gamma-S can serve as a phosphate donor in kinase-mediated reactions, the donated phosphate group being resistant to the action of phosphatases. We therefore compared the effect of ATP-gamma-S to that of the catalytic subunit of the cyclic AMP-dependent protein kinase (AK-C), included in the recording pipette with 5 mM ATP. AK-C (50 micrograms/ml) had no effect on the T current, and caused a approximately 30% increase in currents containing the N and L components during a 20-min recording, as compared to a approximately 45% decrease in control currents. In contrast, in the presence of 2.5 mM ATP-gamma-S, T currents declined approximately 30%, and currents containing the N and L components declined to a greater extent than control currents, about 65%. In addition, the time to peak current was increased from approximately 14 ms to approximately 40 ms. This effect of ATP-gamma-S on calcium currents was similar to that of certain neurotransmitters or GTP-gamma-S, an activator of G proteins, except that the effects of ATP-gamma-S were delayed 5-7 min relative to GTP-gamma-S. The effects of both ATP-gamma-S and GTP-gamma-S were reduced or abolished in neurons treated with pertussis toxin. We conclude that AK-C regulates neuronal calcium currents, presumably by phosphorylation of channels or associated proteins, and that the ATP-gamma-S-induced reduction of calcium currents cannot be due to its serving as a phosphate donor for endogenous AK.(ABSTRACT TRUNCATED AT 250 WORDS)

Muscarinic modulation of calcium current in neurones from the interatrial septum of bull-frog heart

1. The effects of activation of muscarinic receptors on the voltage-dependent calcium current, ICa, in parasympathetic neurones were examined. 2. Neurones were enzymatically isolated from the interatrial septum of bull-frog (Rana catesbeiana) heart, and were maintained in short-term (1-6 day) tissue culture. ICa was recorded from the cells using whole-cell patch-clamp methods (Clark, Tse & Giles, 1990). 3. External application of 2 nM to 10 microM acetylcholine (ACh) reduced the amplitude and slowed the time course of activation of ICa. These effects were dependent on membrane potential; they were most pronounced at potentials near the peak of the current-voltage relation for ICa (i.e. +10 to +15 mV), whereas at more-negative potentials (i.e. -15 to -25 mV) the effects on both amplitude and time course were relatively small. 4. Atropine (1 microM) completely blocked the action of 1 microM-ACh, indicating that the effects of ACh on ICa were mediated by activation of muscarinic receptors. 5. Other muscarinic agonists, such as carbamylcholine (0.1-10 microM), DL-muscarine (0.1-2.5 microM) and oxotremorine (5 microM), had similar effects on ICa to ACh. 6. A guanine nucleotide-binding protein (G-protein) is involved in this muscarinic inhibition of ICa. Inclusion of the non-hydrolysable guanosine triphosphate analogue guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S; 200 microM) in the intracellular solutions mimicked the effects of ACh, and application of external ACh in the presence of internal GTP-gamma-S produced smaller changes in ICa than in control conditions. Inclusion of another non-hydrolysable analogue, guanosine 5'-O-(2-thiodiphosphate) (GDP-beta-S; 0.5-5 mM), blocked the inhibitory effect of ACh on ICa. 7. The G-protein involved in the inhibition of ICa was sensitive to pertussis toxin (islet-activating protein; IAP). The inhibition of ICa by carbamylcholine (5 microM) was reduced by about 90% after incubating cells for 12-15 h in culture medium containing 200 ng/ml IAP. 8. The possible roles of cyclic AMP or cyclic GMP-dependent protein kinases, or protein kinase C, in the muscarinic inhibition of ICa were tested, but these enzymes appear not to be directly involved.

Kinetics of the conductance evoked by noradrenaline, inositol trisphosphate or Ca2+ in guinea-pig isolated hepatocytes

1. Guinea-pig hepatocytes respond to noradrenaline (NA, 5-10 microM) with a large membrane conductance increase to K+ and Cl-. The response has a long initial delay (range 2-30 s). Following the delay, the K+ conductance (studied in Cl(-)-free solutions) rises quickly to a peak in 1-2 s and is maintained in the continued presence of NA, though often with superimposed oscillations of conductance. The roles of intracellular Ca2+ and D-myo-inositol 1,4,5-trisphosphate (InsP3) in this complex response have been investigated by rapid photolytic release of intracellular Ca2+ (from Nitr5-Ca2+ buffers) or InsP3 from 'caged' InsP3. 2. A rapid increase of intracellular [Ca2+] produced an immediate membrane conductance increase which rose approximately exponentially to a new steady level, consistent with a direct activation of Ca2(+)-dependent ion channels. 3. Following a pulse of InsP3, conductance rose after a brief delay (range 70-1500 ms) which was shortest at high [InsP3] or if the initial cytosolic [Ca2+] had been raised above normal levels. The maximum conductance produced by InsP3 was similar in each cell to the peak recorded with NA and could be evoked by InsP3 concentrations of 0.5-1 microM. 4. The rates of rise of conductance increased with InsP3 concentration in the range 0.25-12.5 microM (range 10-90%, rise times 90-1000 ms), indicating that InsP3-evoked Ca2(+)-efflux from stores increases with InsP3 concentration in this range. 5. Photochemically released InsP3 and Ca2+ activate at physiological concentrations the same membrane conductances as NA. The results indicate that the long initial delay in NA action occurs prior to or during generation of InsP3. The mechanism of the delay and the subsequent apparently all-or-none conductance increase during NA action are discussed in terms of the high co-operativity in InsP3 and Ca2+ actions and an additional positive feedback step. 6. Evidence was found of a negative interaction between [Ca2+] and InsP3-evoked Ca2+ release. The time course of the recovery of InsP3-evoked Ca2+ release following a rise of cytosolic [Ca2+] suggests that this interaction may be important in regulating oscillatory responses of [Ca2+] during hormonal stimulation of guinea-pig hepatocytes.

Modulation of neuronal T-type calcium channel currents by photoactivation of intracellular guanosine 5'-O(3-thio) triphosphate

Low voltage-activated T-type Ca2+ channel currents were recorded from cultured rat dorsal root ganglion neurons using the whole-cell clamp technique with Ba2+ as the charge carrier. The T-type Ca2+ channel current was identified by its low threshold of activation (Vc -50 to -20 mV from VH - 90 mV), its kinetics of inactivation and its sensitivity to NiCl2 (100 microM). It was also sensitive to 1-octanol (1 microM). omega-Conotoxin (1 microM) markedly reduced the high threshold voltage-activated Ca2+ channel currents but did not inhibit the T-type Ca2+ channel current. Photorelease of intracellular guanosine 5'-O(3-thio) triphosphate from a photolabile "caged" precursor had dose-dependent effects on the T-type Ca2+ channel current. At a concentration of 6 microM, guanosine 5'-O(3-thio) triphosphate enhanced the current, but further photorelease of guanosine 5'-O(3-thio) triphosphate (up to 20 microM) inhibited the current. Only the inhibitory response was sensitive to pertussis toxin. These data suggest that more than one G-protein is involved in T-type Ca2+ channel current modulation. Inclusion of guanosine 5'-O(2-thio) diphosphate (1 mM) in the patch solution prevented guanosine 5'-O(3-thio) triphosphate from potentiating the current, and greatly attenuated the inhibitory effects observed when larger amounts of guanosine 5'-O(3-thio) triphosphate were photoreleased. Photorelease of guanosine 5'-O(2-thio) diphosphate had no effect on T-type current but did significantly increase the high voltage-activated current. A low concentration of (-)-baclofen (2 microM), potentiated T-type current, while 100 microM(-)-baclofen inhibited T-type current.

Roles of G proteins in coupling of receptors to ionic channels and other effector systems

Guanine nucleotide binding (G) proteins are heterotrimers that couple a wide range of receptors to ionic channels. The coupling may be indirect, via cytoplasmic agents, or direct, as has been shown for two K+ channels and two Ca2+ channels. One example of direct G protein gating is the atrial muscarinic K+ channel K+[ACh], an inwardly rectifying K+ channel with a slope conductance of 40 pS in symmetrical isotonic K+ solutions and a mean open lifetime of 1.4 ms at potentials between -40 and -100 mV. Another is the clonal GH3 muscarinic or somatostatin K+ channel, also inwardly rectifying but with a slope conductance of 55 pS. A G protein, Gk, purified from human red blood cells (hRBC) activates K+ [ACh] channels at subpicomolar concentrations; its alpha subunit is equipotent. Except for being irreversible, their effects on gating precisely mimic physiological gating produced by muscarinic agonists. The alpha k effects are general and are similar in atria from adult guinea pig, neonatal rat, and chick embryo. The hydrophilic beta gamma from transducin has no effect while hydrophobic beta gamma from brain, hRBCs, or retina has effects at nanomolar concentrations which in our hands cannot be dissociated from detergent effects. An anti-alpha k monoclonal antibody blocks muscarinic activation, supporting the concept that the physiological mediator is the alpha subunit not the beta gamma dimer. The techniques of molecular biology are now being used to specify G protein gating. A "bacterial" alpha i-3 expressed in Escherichia coli using a pT7 expression system mimics the gating produced by hRBC alpha k.

Guanosine-5'-O-(3-thiotriphosphate) modifies kinetics of voltage-dependent calcium current in chick sensory neurons

Internal perfusion with the G-protein activator guanosine-5'-O-(3-thiotriphosphate) (GTP-gamma S) mimics the effect of noradrenaline and dopamine on the voltage-dependent calcium current in chick dorsal root ganglion (DRG) cells. With 100 microM GTP-gamma S in the pipette, the current at +10 mV was depressed by approximately 50%, with a 10-fold increase of its time to peak. The activation time course of the control calcium current could be approximated with a single exponential curve, whereas with GTP-gamma S the activation time course was double exponential, with time constants tau 1 and tau 2. 2 mM Mg-ATP in the pipette prevented the GTP-gamma S-induced current decrease in 70% of the cells, but the time course of the current was always double exponential. From -50 mV, the current at +10 mV was best fitted with tau 1 = 1.7 +/- 0.5 and tau 2 = 25.6 +/- 5.5 in seven cells. Both time constants decreased with increasing depolarizations. In the first 2 min of recording, the current changed with time. However, both tau 1 and tau 2 were constant, whereas the relative contribution of the slow component increased from 10 to 70%. In addition, the effect was independent of the holding potential in the range from -100 to -30 mV. These results suggest that the activation of a G-protein causes a fraction of the high-threshold calcium channels to switch to a new closed state, with slower opening kinetics.

Direct modulation of voltage-dependent calcium channels by muscarinic activation of a pertussis toxin-sensitive G-protein in hippocampal neurons

Acetylcholine (Ach) reversibly reduces the high voltage-activated (HVA) calcium (Ca) current in hippocampal neurons. Pretreatment of the cells with pertussis toxin (PTX) abolishes the Ach effect, suggesting that PTX-sensitive GTP-binding regulatory proteins (G-proteins) are involved in the signal transduction mechanism that links Ach receptor activation to inhibition of Ca channel activity. This effect is mimicked by intracellular application of the nonhydrolyzable GTP analog GTP gamma S. Intracellular application of purified G-proteins restored the response to Ach in PTX-treated cells. Furthermore, Ach inhibits the Ca current independently of the presence of cyclic AMP and of the protein kinase C inhibitor H-7 and neither does the Ach effect on the Ca current seem to be correlated to a transient increase in intracellular Ca. Our results suggest that activation of the alpha-subunit of the PTX-sensitive G-protein could directly modulate the HVA Ca channel without involving second messenger systems.

Interaction between calcium channel ligands and guanine nucleotides in cultured rat sensory and sympathetic neurones

1. Voltage-activated Ca2+ channel currents were recorded from cultured rat dorsal root ganglion (DRG) neurones using the whole-cell clamp technique with Ba2+ as the charge carrier. 2. Inclusion of the GTP analogue guanosine 5'-O-3-thiotriphosphate (GTP-gamma-S, 500 microM) or guanylylimidodiphosphate (GMP-PNP, 500 microM) or GTP itself (1 mM) in the patch pipette solution resulted in a smaller, slowly activating Ca2+ channel current which did not inactivate during a 100 ms voltage step. This current was inhibited by CdCl2 (10-100 microM) and omega-conotoxin (1 microM). 3. Nifedipine (5 microM), (-)-(R)-201-791 (5 microM), D600 (10 microM), and diltiazem (30 microM) inhibited Ca2+ channel currents recorded from control neurones, although in some cells a biphasic response was observed, with an initial increase preceding the inhibition of the currents. In the presence of internal GTP-gamma-S, at a holding potential (VH) of -80 mV, only potentiation of the Ca2+ channel current was observed in the presence of all three Ca2+ channel ligands. Internal GMP-PNP, while less effective than GTP-gamma-S, also resulted in D600 showing an agonist response. Similarly, in the presence of internal GTP (1 mM), (-)-(R)-202-791 gave a prolonged agonist response. 4. Nifedipine, whether acting as an antagonist in control cells or as an agonist in GTP-gamma-S-containing cells, induced a shift to more hyperpolarized potentials of the steady-state inactivation curves. 5. Potentiation of Ca2+ channel currents induced by D600 in GTP-gamma-S-containing cells, was not observed when the neurones were pre-treated with pertussis toxin. The presence of internal GDP-beta-S (500 microM) did not significantly alter the maximum inhibitory action of D600 compared with controls. However, 1 mM-GDP-beta-S increased the rate of onset of inhibition by (-)-(R)-202-791. 6. Depolarizing VH to -30 mV accelerated the onset of inhibition induced by the Ca2+ channel ligands in control cells. In the presence of internal GTP-gamma-S at VH -30 mV, biphasic responses were produced by all the Ca2+ channel antagonist ligands with initial stimulation for 1-2 min being followed by inhibition of the Ca2+ channel currents. 7. The agonist actions of (+)-(S)-202-791 were potentiated by the presence of internal GTP-gamma-S. 8. The expression of an agonist response to (-)-(R)-202-791 induced by internal GTP-gamma-S was also present in sympathetic neurones cultured from adult rat superior cervical ganglion (SCG).(ABSTRACT TRUNCATED AT 400 WORDS)

An investigation into the mechanisms of inhibition of calcium channel currents in cultured sensory neurones of the rat by guanine nucleotide analogues and (-)-baclofen

1. The mechanism of inhibition of calcium channel currents by the guanine nucleotide analogue guanosine 5'-O-3 thiotriphosphate (GTP-gamma-S) and by the GABAB agonist (-)-baclofen has been studied in cultured dorsal root ganglion neurones of the rat. The inhibition by GTP-gamma-S is particularly characterized by an abolition of the transient component of calcium channel currents carried either by Ba2+ (IBa) or by Ca2+ (ICa). 2. The effect of agents increasing intracellular cyclic AMP levels has been examined. Neither internal cyclic AMP nor forskolin prevented the inhibition of IBa by baclofen. Neither forskolin nor pretreatment of cells with cholera toxin prevented the inhibition of the transient component of IBa by GTP-gamma-S. However, both these treatments increased the amplitude of the sustained IBa in the presence of GTP-gamma-S. The ATP analogue adenosine imido-diphosphate which inhibits many ATP requiring enzymes did not prevent the effect of GTP-gamma-S although it reduced the amplitude of IBa. 3. Baclofen (100 microM) produced a 22 +/- 2% increase in inositol phosphate production in 30 s, whereas the increase produced by bradykinin (1 microM) was 70 +/- 14%. However, unlike baclofen, bradykinin did not inhibit IBa or ICa in these cells. 4. The effect of protein kinase C inhibitors was examined. Polymixin B (20 microM in patch pipette) had no effect on the inhibition of IBa by baclofen or GTP-gamma-S. A higher concentration (100 microM) alone inhibited IBa and no further inhibition by baclofen was observed. Neither H7 (50 microM) nor staurosporine (100 nM), applied extracellularly, prevented the response to GTP-gamma-S. 5. The protein kinase C activator di-octanoyl glycerol (20 microM) did not inhibit IBa. Arachidonic acid (100 microM) also produced no inhibition of IBa. 6. In conclusion we have obtained no evidence that a second messenger system mediates the inhibition of calcium channel currents by GTP-gamma-S or baclofen in dorsal root ganglion neurones. These results support the hypothesis that GABAB receptors are directly coupled to calcium channels by G proteins.

Flash photolysis of caged compounds: new tools for cellular physiology

Caged compounds are molecules or ions of physiological interest, e.g. ATP, IP3, cAMP, cGMP, GTP and Ca2+ rendered inactive by chemical modification. The modification introduces a photochemically labile bond, which on exposure to ultraviolet light cleaves rapidly, releasing the active compound. This article reviews some of the major advances and applications of the photorelease approach, and illustrates its potential in several areas of interest to cellular neuroscientists.