The cyclic AMP-dependent protein kinase catalytic subunit selectively enhances calcium currents in rat nodose neurones

Modulation of low-voltage-activated T-type Ca²⁺ channels

Low-voltage-activated T-type Ca²⁺ channels contribute to a wide variety of physiological functions, most predominantly in the nervous, cardiovascular and endocrine systems. Studies have documented the roles of T-type channels in sleep, neuropathic pain, absence epilepsy, cell proliferation and cardiovascular function. Importantly, novel aspects of the modulation of T-type channels have been identified over the last few years, providing new insights into their physiological and pathophysiological roles. Although there is substantial literature regarding modulation of native T-type channels, the underlying molecular mechanisms have only recently begun to be addressed. This review focuses on recent evidence that the Ca(v)3 subunits of T-type channels, Ca(v)3.1, Ca(v)3.2 and Ca(v)3.3, are differentially modulated by a multitude of endogenous ligands including anandamide, monocyte chemoattractant protein-1, endostatin, and redox and oxidizing agents. The review also provides an overview of recent knowledge gained concerning downstream pathways involving G-protein-coupled receptors. This article is part of a Special Issue entitled: Calcium channels.

Functional roles of a C-terminal signaling complex of CaV1 channels and A-kinase anchoring protein 15 in brain neurons

Regulation of CaV1.2 channels in cardiac myocytes by the β-adrenergic pathway requires a signaling complex in which the proteolytically processed distal C-terminal domain acts as an autoinhibitor of channel activity and mediates up-regulation by the β-adrenergic receptor and PKA bound to A-kinase anchoring protein 15 (AKAP15). We examined the significance of this distal C-terminal signaling complex for CaV1.2 and CaV1.3 channels in neurons. AKAP15 co-immunoprecipitates with CaV1.2 and CaV1.3 channels. AKAP15 has overlapping localization with CaV1.2 and CaV1.3 channels in cell bodies and proximal dendrites and is closely co-localized with CaV1.2 channels in punctate clusters. The neuronal AKAP MAP2B, which also interacts with CaV1.2 and CaV1.3 channels, has complementary localization to AKAP15, suggesting different functional roles in calcium channel regulation. Studies with mice that lack the distal C-terminal domain of CaV1.2 channels (CaV1.2ΔDCT) reveal that AKAP15 interacts with neuronal CaV1.2 channels via their C terminus in vivo and is co-localized in punctate clusters of CaV1.2 channels via that interaction. CaV1.2ΔDCT neurons have reduced L-type calcium current, indicating that the distal C-terminal domain is required for normal functional expression in vivo. Deletion of the distal C-terminal domain impairs calcium-dependent signaling from CaV1.2 channels to the nucleus, as shown by reduction in phosphorylation of the cAMP response element-binding protein. Our results define AKAP signaling complexes of CaV1.2 and CaV1.3 channels in brain and reveal three previously unrecognized functional roles for the distal C terminus of neuronal CaV1.2 channels in vivo: increased functional expression, anchoring of AKAP15 and PKA, and initiation of excitation-transcription coupling.

Pharmacology of signaling induced by dopamine D(1)-like receptor activation

Dopamine D(1)-like receptors consisting of D(1) and D(5) subtypes are intimately implicated in dopaminergic regulation of fundamental neurophysiologic processes such as mood, motivation, cognitive function, and motor activity. Upon stimulation, D(1)-like receptors initiate signal transduction cascades that are mediated through adenylyl cyclase or phosphoinositide metabolism, with subsequent enhancement of multiple downstream kinase cascades. The latter actions propagate and further amplify the receptor signals, thus predisposing D(1)-like receptors to multifaceted interactions with various other mediators and receptor systems. The adenylyl cyclase response to dopamine or selective D(1)-like receptor agonists is reliably associated with the D(1) subtype, while emerging evidence indicates that the phosphoinositide responses in native brain tissues may be preferentially mediated through stimulation of the D(5) receptor. Besides classic coupling of each receptor subtype to specific G proteins, additional biophysical models are advanced in attempts to account for differential subcellular distribution, heteromolecular oligomerization, and activity-dependent selectivity of the receptors. It is expected that significant advances in understanding of dopamine neurobiology will emerge from current and anticipated studies directed at uncovering the molecular mechanisms of D(5) coupling to phosphoinositide signaling, the structural features that might enhance pharmacological selectivity for D(5) versus D(1) subtypes, the mechanism by which dopamine may modulate phosphoinositide synthesis, the contributions of the various responsive signal mediators to D(1) or D(5) interactions with D(2)-like receptors, and the spectrum of dopaminergic functions that may be attributed to each receptor subtype and signaling pathway.

Expression of human amyloid precursor protein in rat cortical neurons inhibits calcium oscillations

Synchronous calcium oscillations are observed in primary cultures of rat cortical neurons when mature networks are formed. This spontaneous neuronal activity needs an accurate control of calcium homeostasis. Alteration of intraneuronal calcium concentration is described in many neurodegenerative disorders, including Alzheimer disease (AD). Although processing of amyloid precursor protein (APP) that generates Abeta peptide has critical implications for AD pathogenesis, the neuronal function of APP remains unclear. Here, we report that expression of human APP (hAPP) in rat cortical neurons increases L-type calcium currents, which stimulate SK channels, calcium-dependent K(+) channels responsible for medium afterhyperpolarization (mAHP). In a neuronal network, increased mAHP in some neurons expressing hAPP leads to inhibition of calcium oscillations in all the cells of the network. This inhibition is independent of production and secretion of Abeta and other APP metabolites. In a neuronal network, reduction of endogenous APP expression using shRNA increases the frequency and reduces the amplitude of calcium oscillations. Altogether, these data support a key role for APP in the control of neuronal excitability.

Protein kinase A activity controls the regulation of T-type CaV3.2 channels by Gbetagamma dimers

Low voltage-activated (LVA), T-type, calcium channels mediate diverse biological functions and are inhibited by Gbetagamma dimers, yet the molecular events required for channel inhibition remain unknown. Here, we identify protein kinase A (PKA) as a molecular switch that allows Gbeta(2)gammax dimers to effect voltage-independent inhibition of Ca(v)3.2 channels. Inhibition requires phosphorylation of Ser(1107), a critical serine residue on the II-III loop of the channel pore protein. S1107A prevents inhibition of unitary currents by recombinant Gbeta(2)gamma(2) dimers but does not disrupt dimer binding nor change its specificity. Gbetagamma dimers released upon receptor activation also require PKA activity for their inhibitory actions. Hence, dopamine inhibition of Ca(v)3.2 whole cell current is precluded by Gbetagamma-scavenger proteins or a peptide that blocks PKA catalytic activity. Fittingly, when used alone at receptor-selective concentrations, D(1) or D(2) agonists do not elicit channel inhibition yet together synergize to inhibit Ca(v)3.2 channel currents. We propose that a dual-receptor regulatory mechanism is used by dopamine to control Ca(v)3.2 channel activity. This mechanism, for example, would be important in aldosterone producing adrenal glomerulosa cells where channel dysregulation would lead to overproduction of aldosterone and consequent cardiac, renal, and brain target organ damage.

Comparison of effects of PKA catalytic subunit on I(h) and calcium channel currents in rat dorsal root ganglion cells

We investigated whether PKA-induced phosphorylation was involved in regulation of hyperpolarization-activated current (I(h)) in rat dorsal root ganglion (DRG) cells. We examined the effect of the catalytic subunit of PKA (PKAc) on I(h) and confirmed an effect of PKAc on Ca(2+) channel currents carried by Ba(2+) (I(Ba)) in identical neurons as a positive control of PKA activity. After the start of recording, amplitudes of I(Ba) gradually decreased (rundown). An intracellular application of ATP reduced the rundown of I(Ba) and induced a depolarizing shift of I(h) activation. The former was partially reversed by PKI but the latter was not affected. An intracellular application of PKAc also prevented the rundown of I(Ba) and this effect was potentiated by okadaic acid (OA). The application of PKAc and OA in combination did not change the electrophysiological properties of I(h) although a potentiating effect on I(Ba) was observed in the same neurons. The application of 2-mM ATP in addition to PKAc and OA did not result in an additional potentiation of I(Ba), but shifted the activation curve of I(h) positively. These results suggested that PKA-induced phosphorylation was not involved in the modulatory mechanisms of I(h) in rat DRG neurons.

Activator protein-1 responsive to the group II metabotropic glutamate receptor subtype in association with intracellular calcium in cultured rat cortical neurons

Activation of ionotropic glutamate (Glu) receptors, such as N-methyl-d-aspartate receptors, is shown to modulate the gene transcription mediated by the transcription factor activator protein-1 (AP1) composed of Fos and Jun family proteins in the brain, while little attention has been paid to the modulation of AP1 expression by metabotropic Glu receptors (mGluRs). In cultured rat cortical neurons, where constitutive expression was seen with all groups I, II and III mGluR subtypes, a significant and selective increase was seen in the DNA binding activity of AP1 120 min after the brief exposure to the group II mGluR agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) for 5 min. In cultured rat cortical astrocytes, by contrast, a significant increase was induced by a group I mGluR agonist, but not by either a group II or III mGluR agonist. The increase by DCG-IV was significantly prevented by a group II mGluR antagonist as well as by either an intracellular Ca(2+) chelator or a voltage-sensitive Ca(2+) channel blocker, but not by an intracellular Ca(2+) store inhibitor. Moreover, DCG-IV significantly prevented the increase of cAMP formation by forskolin in cultured neurons. Western blot analysis revealed differential expression profiles of Fos family members in neurons briefly exposed to DCG-IV and NMDA. Prior or simultaneous exposure to DCG-IV led to significant protection against neuronal cell death by NMDA. These results suggest that activation of the group II mGluR subtype would modulate the gene expression mediated by AP1 through increased intracellular Ca(2+) levels in cultured rat cortical neurons.

Critical role of cAMP-dependent protein kinase anchoring to the L-type calcium channel Cav1.2 via A-kinase anchor protein 150 in neurons

The cAMP-dependent protein kinase (PKA) regulates a wide array of cellular functions. In brain and heart PKA increases the activity of the L-type Ca2+ channel Cav1.2 in response to beta-adrenergic stimulation. Cav1.2 forms a complex with the beta2-adrenergic receptor, the trimeric GS protein, adenylyl cyclase, and PKA wherein highly localized signaling occurs [Davare, M. A., Avdonin, V., Hall, D. D., Peden, E. M., Burette, A., Weinberg, R. J., Horne, M. C., Hoshi, T., and Hell, J. W. (2001) Science 293, 98-101]. PKA primarily phosphorylates Cav1.2 on serine 1928 of the central, pore-forming alpha11.2 subunit. Here we demonstrate that the A-kinase anchor protein 150 (AKAP150) is critical for PKA-mediated regulation of Cav1.2 in the brain. AKAP150 and MAP2B specifically co-immunoprecipitate with Cav1.2 from rat brain. Recombinant AKAP75, the bovine homologue to rat AKAP150, binds directly to three different sites of alpha11.2. MAP2B from rat brain also interacts with these same sites in pull-down assays. Gene disruption of AKAP150 in mice dramatically reduces co-immunoprecipitation of PKA with Cav1.2 and prevents phosphorylation of serine 1928 upon beta-adrenergic stimulation in vivo. These results demonstrate the physiological relevance of PKA anchoring by AKAPs in general and AKAP150 specifically in the regulation of Cav1.2 in vivo.

Adrenomedullin stimulates nitric oxide release from SK-N-SH human neuroblastoma cells by modulating intracellular calcium mobilization

We used SK-N-SH human neuroblastoma cells to test the hypothesis that adrenomedullin (ADM), a multifunctional neuropeptide, stimulates nitric oxide (NO) release by modulating intracellular free calcium concentration ([Ca2+]i) in neuron-like cells. We used a nitrite assay to demonstrate that ADM (10 pM to 100 nM) stimulated NO release from the cells, with a maximal response observed with 1 nM at 30 min. This response was blocked by 1 nM ADM(22-52), an ADM receptor antagonist or 2 microM vinyl-L-NIO, a neuronal NO synthase inhibitor. In addition, 5 microM 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester, an intracellular calcium chelator, eliminated the ADM-induced NO release. Similar results were observed when the cells were incubated in calcium-free medium or when L-type calcium channels were inhibited with 5 microM nifedipine or 10 microM nitrendipine. Depletion of calcium stores in the endoplasmic reticulum (ER) with 1 microM cyclopiazonic acid or 150 nM thapsigargin, or inhibition of ryanodine-sensitive receptors in the ER with 10 microM ryanodine attenuated the ADM-induced NO release. NO responses to ADM were mimicked by 1 mM dibutyryl cAMP, a cAMP analog, and were abrogated by 5 microM H-89, a protein kinase A inhibitor. Furthermore, Fluo-4 fluorescence-activated cell sorter analysis showed that ADM (1 nM) significantly increased [Ca2+]i at 30 min. This response was blocked by nifedipine (5 microM) or H-89 (5 microM) and was reduced by ryanodine (10 microM). These results suggest that ADM stimulates calcium influx through L-type calcium channels and ryanodine-sensitive calcium release from the ER, probably via cAMP-protein kinase A-dependent mechanisms. These elevations in [Ca2+)]i cause activation of neuronal NO synthase and NO release.

Activation of mGluR5 modulates Ca2+ currents in retinal amacrine cells from the chick

In the inner plexiform layer, amacrine cells receive glutamatergic input from bipolar cells. Glutamate can depolarize amacrine cells by activation of ionotropic glutamate receptors or mediate potentially more diverse changes via activation of G protein-coupled metabotropic glutamate receptors (mGluR5). Here, we asked whether selective activation of metabotropic glutamate receptor 5 is linked to modulation of the voltage-gated Ca2+ channels expressed by cultured GABAergic amacrine cells. To address this, we performed whole-cell voltage clamp experiments, primarily in the perforated-patch configuration. We found that agonists selective for mGluR5, including (RS)-2-chloro-5-hydroxyphenylglycine (CHPG), enhanced the amplitude of the voltage-dependent Ca2+ current. The voltage-dependent Ca2+ current and CHPG-dependent current enhancement were blocked by nifedipine, indicating that L-type Ca2+ channels, specifically, were being modulated. We have previously shown that activation of mGluR5 produces Ca2+ elevations in cultured amacrine cells (Sosa et al., 2002). Loading the cells with 5 mM BAPTA inhibited the mGluR5-dependent enhancement, suggesting that the cytosolic Ca2+ elevations are required for modulation of the current. Although activation of mGluR5 is typically linked to activation of protein kinase C, we found that direct activation of this kinase leads to inhibition of the Ca2+ current, indicating that stimulation of this enzyme is not responsible for the mGluR5-dependent enhancement. Interestingly, direct stimulation of protein kinase A produced an enhancement of the Ca2+ current similar to that observed with activation of mGluR5. Thus, activation of mGluR5 may modulate the L-type voltage-gated Ca2+ current in these GABAergic amacrine cells via activation of protein kinase A, possibly via direct activation of a Ca2(+)-dependent adenylate cyclase.

Posttranslational mechanisms of peripheral sensitization

The sensation of pain can be dramatically altered in response to injury or disease. This sensitization can occur at the level of the primary sensory neuron, and can be mediated by multiple biochemical mechanisms, including, but not limited to, changes in gene transcription, changes in translation, stability, or subcellular localization of translated proteins, and posttranslational modifications. This review focuses on posttranslational modifications to ion channels expressed in primary sensory neurons that form the machinery driving peripheral sensitization and pain hypersensitivity. Studies published to date show strong evidence for modulation of ion channels involved in transduction and transmission of nociceptive inputs coincident with biophysical and behavioral sensitization. The roles of phosphorylation and oxidation/reduction reactions of voltage-dependent sodium, potassium, and calcium channels are discussed, as well as phosphorylation-mediated modulation of sensory transduction channels.

Dopamine Receptor Signaling

The D1-like (D1, D5) and D2-like (D2, D3, D4) classes of dopamine receptors each has shared signaling properties that contribute to the definition of the receptor class, although some differences among subtypes within a class have been identified. D1-like receptor signaling is mediated chiefly by the heterotrimeric G proteins Galphas and Galphaolf, which cause sequential activation of adenylate cyclase, cylic AMP-dependent protein kinase, and the protein phosphatase-1 inhibitor DARPP-32. The increased phosphorylation that results from the combined effects of activating cyclic AMP-dependent protein kinase and inhibiting protein phosphatase 1 regulates the activity of many receptors, enzymes, ion channels, and transcription factors. D1 or a novel D1-like receptor also signals via phospholipase C-dependent and cyclic AMP-independent mobilization of intracellular calcium. D2-like receptor signaling is mediated by the heterotrimeric G proteins Galphai and Galphao. These pertussis toxin-sensitive G proteins regulate some effectors, such as adenylate cyclase, via their Galpha subunits, but regulate many more effectors such as ion channels, phospholipases, protein kinases, and receptor tyrosine kinases as a result of the receptor-induced liberation of Gbetagamma subunits. In addition to interactions between dopamine receptors and G proteins, other protein:protein interactions such as receptor oligomerization or receptor interactions with scaffolding and signal-switching proteins are critical for regulation of dopamine receptor signaling.

Protein kinase A and calcium/calmodulin-dependent protein kinase II regulate glycine and GABA release in auditory brain stem nuclei

We reported previously that unilateral cochlear ablation (UCA) in young adult guinea pigs induced protein kinase C (PKC)-dependent plastic changes in the electrically evoked release of exogenous [14C]glycine ([14C]Gly) or [14C]-gamma-aminobutyric acid ([14C]GABA) in several brain stem auditory nuclei. The present study assessed whether such changes depended on protein kinase A (PKA) and calcium/calmodulin-dependent protein kinase II (CaMKII). In the major subdivisions of the cochlear nucleus (CN) and the main nuclei of the superior olivary complex (SOC) dissected from intact animals, dibutyryl-cyclic adenosine monophosphate (DBcAMP) (0.2 mM), a PKA activator, elevated release by 1.6-2.3-fold. The PKA inhibitor, H-89 (2 microM), did not alter the release but blocked the stimulatory effects of DBcAMP. These findings suggested that PKA could positively regulate glycinergic and GABAergic release. After UCA, PKA regulation declined and failed in the ventral CN but persisted in the SOC nuclei. After 145 postablation days, H-89 reversed elevations of [14C]GABA release in the medial nucleus of the trapezoid body (MNTB). A CaMKII inhibitor, KN-93, reversed depressions of [14C]Gly release in the DCN. Thus, the postablation plasticities in these nuclei probably depended on PKA or CaMKII. Both H-89 and KN-93 depressed [14C]Gly release in the lateral superior olive (LSO) and ipsilateral medial superior olive (MSO), suggesting that either kinase was used by endogenous mechanisms in these nuclei to upregulate glycinergic release. In contrast, KN-93 elevated [14C]GABA release in the contralateral MNTB, suggesting a downregulatory action of CaMKII, an action opposite to that of PKA.

Increased phosphorylation of the neuronal L-type Ca(2+) channel Ca(v)1.2 during aging

An increase in Ca2+ influx through L-type Ca2+ channels is thought to contribute to neuronal dysfunctions that underlie senile symptoms and Alzheimer's disease. The molecular basis of the age-dependent up-regulation in neuronal L-type Ca2+ channel activity is largely unknown. We show that phosphorylation of the L-type channel Cav1.2 by cAMP-dependent protein kinase is increased >2-fold in the hippocampus of aged rats. The hippocampus is critical for learning and is one of the first brain regions to be affected in Alzheimer's disease. Phosphorylation of Cav1.2 by cAMP-dependent protein kinase strongly enhances its activity. Therefore, increased Cav1.2 phosphorylation may account for a substantial portion of the age-related rise in neuronal Ca2+ influx and its neuropathological consequences.

Protein kinase A and calcium/calmodulin-dependent protein kinase II regulate D-[3H]aspartate release in auditory brain stem nuclei

We noted previously that after unilateral cochlear ablation (UCA) in young adult guinea pigs, plastic changes in glutamatergic transmitter release in several brain stem auditory nuclei depended on protein kinase C. In this study, we assessed whether such changes depended on protein kinase A (PKA) and calcium/calmodulin-dependent protein kinase II (CaMKII). The electrically-evoked release of D-[3H]aspartate (D-[3H]Asp) was quantified in vitro as an index of glutamatergic transmitter release in the major subdivisions of the cochlear nucleus (CN) and the main nuclei of the superior olivary complex (SOC). In tissues from intact animals, dibutyryl-cyclic adenosine monophosphate (DBcAMP), a PKA activator, elevated D-[3H]Asp release by 1.9-3.7-fold. The PKA inhibitor, H-89 (2 microM), did not alter the evoked release but blocked the stimulatory effects of DBcAMP. These findings suggested that PKA could positively regulate glutamatergic transmitter release. Seven days after the ablation of one cochlea and its cochlear nerve, the stimulatory effect of DBcAMP remained evident. After 145 postablation days, H-89 blocked the plastic elevations of D-[3H]Asp release in the ipsilateral CN and lateral (LSO) and medial (MSO) superior olive. A CaMKII inhibitor, KN-93, produced similar blocks, suggesting that the postablation plasticities in these nuclei depended on PKA or CaMKII. Both H-89 and KN-93 elevated release in the medial nucleus of the trapezoid body (MNTB) and the contralateral MSO, suggesting that either kinase could be used by endogenous mechanisms in these nuclei to downregulate glutamatergic release.

Mutation and activation of Galpha s similarly alters pre- and postsynaptic mechanisms modulating neurotransmission

Constitutive activation of Galphas in the Drosophila brain abolishes associative learning, a behavioral disruption far worse than that observed in any single cAMP metabolic mutant, suggesting that Galphas is essential for synaptic plasticity. The intent of this study was to examine the role of Galphas in regulating synaptic function by targeting constitutively active Galphas to either pre- or postsynaptic cells and by examining loss-of-function Galphas mutants (dgs) at the glutamatergic neuromuscular junction (NMJ) model synapse. Surprisingly, both loss of Galphas and activation of Galphas in either pre- or postsynaptic compartment similarly increased basal neurotransmission, decreased short-term plasticity (facilitation and augmentation), and abolished posttetanic potentiation. Elevated synaptic function was specific to an evoked neurotransmission pathway because both spontaneous synaptic vesicle fusion frequency and amplitude were unaltered in all mutants. In the postsynaptic cell, the glutamate receptor field was regulated by Galphas activity; based on immunocytochemical studies, GluRIIA receptor subunits were dramatically downregulated (>75% decrease) in both loss and constitutive active Galphas genotypes. In the presynaptic cell, the synaptic vesicle cycle was regulated by Galphas activity; based on FM1-43 dye imaging studies, evoked vesicle fusion rate was increased in both loss and constitutively active Galphas genotypes. An important conclusion of this study is that both increased and decreased Galphas activity very similarly alters pre- and postsynaptic mechanisms. A second important conclusion is that Galphas activity induces transynaptic signaling; targeted Galphas activation in the presynapse downregulates postsynaptic GluRIIA receptors, whereas targeted Galphas activation in the postsynapse enhances presynaptic vesicle cycling.

Melanin-concentrating hormone depresses L-, N-, and P/Q-type voltage-dependent calcium channels in rat lateral hypothalamic neurons

Melanin-concentrating hormone (MCH), a cyclic 19-amino-acid peptide, is synthesized exclusively by neurons in the lateral hypothalamic (LH) area. It is involved in a number of brain functions and recently has raised interest because of its role in energy homeostasis. MCH axons and receptors are found throughout the brain. Previous reports set the foundation for understanding the cellular actions of MCH by using non-neuronal cells transfected with the MCH receptor gene; these cells exhibited an increase in cytoplasmic calcium in response to MCH, suggesting an excitatory action for the peptide. In the study presented here, we have used whole-cell recording in 117 neurons from LH cultures and brain slices to examine the actions of MCH. MCH decreased the amplitude of voltage-dependent calcium currents in almost all tested neurons. The inhibition desensitized rapidly (18 s to half maximum at 100 nM concentration) and was dose-dependent (IC(50) = 7.8 nM) when activated with a pulse from -80 mV to 0 mV. A priori activation of G-proteins with GTPgammaS completely eliminated the MCH-induced effect at low MCH concentrations and reduced the MCH-induced effect at high MCH concentrations. Inhibition of G-proteins with pertussis toxin (PTX) blocked the MCH-induced inhibitory effect at high MCH concentrations. Pre-pulse depolarization resulted in an attenuation of the MCH-induced inhibition of calcium currents in most neurons. These data suggest that MCH exerts an inhibitory effect on calcium currents via PTX-sensitive G-protein pathways, probably the G(i)/G(o) pathway, in LH neurons. L-, N- and P/Q-type calcium channels were identified in LH neurons, with L- and N-type channels accounting for most of the voltage-activated current (about 40 % each); MCH attenuated each of the three types (mean 50 % depression), with the greatest inhibition found for N-type currents. In contrast to previous data on non-neuronal cells showing an MHC-evoked increase in calcium, our data suggest that the reverse occurs in LH neurons. The attenuation of calcium currents is consistent with an inhibitory action for the peptide in neurons.

Two independent pathways mediated by cAMP and protein kinase A enhance spontaneous transmitter release at Drosophila neuromuscular junctions

cAMP is thought to be involved in learning process and known to enhance transmitter release in various systems. Previously we reported that cAMP enhances spontaneous transmitter release in the absence of extracellular Ca(2+) and that the synaptic vesicle protein neuronal-synaptobrevin (n-syb), is required in this enhancement (n-syb-dependent; Yoshihara et al., 1999). In the present study, we examined the cAMP-induced enhancement of transmitter release in the presence of external Ca(2+). We raised the intracellular concentration of cAMP by application of either forskolin, an activator of adenylyl cyclase, or by 4-chlorophenylthio-(CPT)-cAMP, a membrane-permeable analog of cAMP, in the presence of external Ca(2+), while recording miniature synaptic currents (mSCs) at the neuromuscular junction in n-syb null mutant embryos. The frequency of mSCs increased in response to elevation of cAMP, and this effect of cAMP was completely blocked by Co(2+) (n-syb-independent pathway). In contrast, in wild-type embryos the cAMP-induced mSC frequency increase was partially blocked by Co(2+). In a mutant, DC0, defective in protein kinase A (PKA), nerve-evoked synaptic currents were indistinguishable from the control, but mSCs were less frequent. In this mutant the enhancement by cAMP of both nerve-evoked and spontaneous transmitter release was completely absent, even in the presence of external Ca(2+). Taken together, these results suggest that cAMP enhances spontaneous transmitter release by increasing Ca(2+) influx (n-syb-independent) as well as by modulating the release mechanism without Ca(2+) influx (n-syb-dependent) in wild-type embryos, and these two effects are mediated by PKA encoded by the DC0 gene.

Tonically active protein kinase A regulates neurotransmitter release at the squid giant synapse

1. Electrophysiological and microinjection methods were used to examine the role of cyclic AMP-dependent protein kinase A (PKA) in regulating transmitter release at the squid giant synapse. 2. Excitatory postsynaptic potentials (EPSPs) evoked by presynaptic action potentials were not affected by presynaptic injection of an exogenous active catalytic subunit of mammalian PKA. 3. In contrast, presynaptic injection of PKI-amide, a peptide that inhibits PKA with high potency and specificity, led to a reversible inhibition of EPSPs. 4. Injection of several other peptides that serve as substrates for PKA also reversibly inhibited neurotransmitter release. The ability of these peptides to inhibit release was correlated with their ability to serve as PKA substrates, suggesting that these peptides act by competing with endogenous substrates for phosphorylation by active endogenous PKA. 5. We suggest that the phosphorylation of PKA substrates is maintained at a relatively high state under basal conditions and that this tonic activity of PKA is to a large degree required for evoked neurotransmitter release at the squid giant presynaptic terminal.

Two independent pathways mediated by cAMP and protein kinase A enhance spontaneous transmitter release at Drosophila neuromuscular junctions

cAMP is thought to be involved in learning process and known to enhance transmitter release in various systems. Previously we reported that cAMP enhances spontaneous transmitter release in the absence of extracellular Ca(2+) and that the synaptic vesicle protein neuronal-synaptobrevin (n-syb), is required in this enhancement (n-syb-dependent; Yoshihara et al., 1999). In the present study, we examined the cAMP-induced enhancement of transmitter release in the presence of external Ca(2+). We raised the intracellular concentration of cAMP by application of either forskolin, an activator of adenylyl cyclase, or by 4-chlorophenylthio-(CPT)-cAMP, a membrane-permeable analog of cAMP, in the presence of external Ca(2+), while recording miniature synaptic currents (mSCs) at the neuromuscular junction in n-syb null mutant embryos. The frequency of mSCs increased in response to elevation of cAMP, and this effect of cAMP was completely blocked by Co(2+) (n-syb-independent pathway). In contrast, in wild-type embryos the cAMP-induced mSC frequency increase was partially blocked by Co(2+). In a mutant, DC0, defective in protein kinase A (PKA), nerve-evoked synaptic currents were indistinguishable from the control, but mSCs were less frequent. In this mutant the enhancement by cAMP of both nerve-evoked and spontaneous transmitter release was completely absent, even in the presence of external Ca(2+). Taken together, these results suggest that cAMP enhances spontaneous transmitter release by increasing Ca(2+) influx (n-syb-independent) as well as by modulating the release mechanism without Ca(2+) influx (n-syb-dependent) in wild-type embryos, and these two effects are mediated by PKA encoded by the DC0 gene.

The A-kinase anchor protein MAP2B and cAMP-dependent protein kinase are associated with class C L-type calcium channels in neurons

Phosphorylation by cAMP-dependent protein kinase (PKA) increases the activity of class C L-type Ca(2+) channels which are clustered at postsynaptic sites and are important regulators of neuronal functions. We investigated a possible mechanism that could ensure rapid and efficient phosphorylation of these channels by PKA upon stimulation of cAMP-mediated signaling pathways. A kinase anchor proteins (AKAPs) bind to the regulatory R subunits of PKA and target the holoenzyme to defined subcellular compartments and substrates. Class C channels isolated from rat brain extracts by immunoprecipitation contain an endogenous kinase that phosphorylates kemptide, a classic PKA substrate peptide, and also the main phosphorylation site for PKA in the pore-forming alpha(1) subunit of the class C channel complex, serine 1928. The kinase activity is inhibited by the PKA inhibitory peptide PKI(5-24) and stimulated by cAMP. Physical association of the catalytic C subunit of PKA with the immunoisolated class C channel complex was confirmed by immunoblotting. A direct protein overlay binding assay performed with (32)P-labeled RIIbeta revealed a prominent AKAP with an M(r) of 280,000 in class C channel complexes. The protein was identified by immunoblotting as the microtubule-associated protein MAP2B, a well established AKAP. Class C channels did not contain tubulin and MAP2B association was not disrupted by dilution or addition of nocodazole, two treatments that cause dissociation of microtubules. In vitro experiments show that MAP2B can directly bind to the alpha(1) subunit of the class C channel. Our findings indicate that PKA is an integral part of neuronal class C L-type Ca(2+) channels and suggest that the AKAP MAP2B may mediate this interaction. Neither PKA nor MAP2B were detected in immunoprecipitates of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-type glutamate receptors or class B N-type Ca(2+) channels. Accordingly, MAP2B docked at class C Ca(2+) channels may be important for recruiting PKA to postsynaptic sites.

Mu-opioid and GABA(B) receptors modulate different types of Ca2+ currents in rat nodose ganglion neurons

Whole-cell patch-clamp recordings were obtained from nodose ganglion neurons acutely dissociated from 10-30-day-old rats to characterize the Ca2+ channel types that are modulated by GABA(B) and mu-opioid receptors. Five components of high-threshold current were distinguished on the basis of their sensitivity to blockade by omega-conotoxin GVIA, nifedipine, omega-agatoxin IVA and omega-conotoxin MVIIC. Administration of the mu-opioid agonist H-Tyr-D-Ala-Gly-Phe(N-Me)-Gly-ol (0.3-1 mM) or the GABA(B) agonist baclofen in saturating concentrations suppressed high-threshold Ca2+ currents by 49.9+/-2.4% (n=69) and 18.7+/-2.1% (n=35), respectively. The inhibition by H-Tyr-D-Ala-Gly-Phe(N-Me)-Gly-ol exceeded that by baclofen in virtually all neurons that responded to both agonists (67%), and occlusion experiments revealed that responses to mu-opioid and GABA(B) receptor activation were not linearly additive. In addition, administration of staurosporine, a non-selective inhibitor of protein kinase A and C, did not affect the inhibitory responses to either agonist or prevent the occlusion of baclofen-induced current inhibition by H-Tyr-D-Ala-Gly-Phe(N-Me)-Gly-ol. Blockade of N-type channels by omega-conotoxin GVIA eliminated current suppression by baclofen in all cells tested (n=11). Mu-opioid-induced inhibition in current was abolished by omega-conotoxin GVIA in 12 of 30 neurons tested, but was only partially reduced in the remaining 18 neurons. In the latter cells administration of omega-agatoxin IVA reduced, but did not eliminate the mu-opioid sensitive current component that persisted after blockade of N-type channels. This residual component of mu-opioid-sensitive current was blocked completely by omega-conotoxin MVIIC in nine neurons, whereas responses to H-Tyr-D-Ala-Gly-Phe(N-Me)-Gly-ol were still recorded in the remaining cells after administration of these Ca2+ channel toxins and nifedipine. Dihydropyridine-sensitive (L-type) current was not affected by activation of mu-opioid or GABA(B) receptors in any of the neurons. These data indicate that in nodose ganglion neurons mu-opioid receptors are negatively coupled to N-, P- and Q-type channels as well as to a fourth, unidentified toxin-resistant Ca2+ channel. In contrast, GABA(B) receptors are coupled only to N-type channels. Furthermore, the results do not support a role for either protein kinase C or A in the modulatory pathway(s) coupling mu-opioid and GABA(B) receptors to Ca2+ channels, but rather lend credence to the notion that the signalling mechanisms utilized by these two receptors might simply compete for inhibitory control of a common pool of N-type channels.

The prostacyclin analogue carbacyclin inhibits Ca(2+)-activated K+ current in aortic baroreceptor neurones of rats

1. Previous studies indicate that prostacyclin (PGI2) increases the activity of baroreceptor afferent fibres. The purpose of this study was to test the hypothesis that PGI2 inhibits Ca(2+)-activated K+ current (IK(Ca))in isolated baroreceptor neurones in culture. 2. Rat aortic baroreceptor neurones in the nodose ganglia were labelled in vivo by applying a fluorescent dye (DiI) to the aortic arch 1-2 weeks before dissociation of the neurones. Outward K+ currents in baroreceptor neurones evoked by depolarizing voltage steps from a holding potential of -40 mV were recorded using the whole-cell patch-clamp technique. 3. Exposure of baroreceptor neurones to the stable PGI2 analogue carbacyclin significantly inhibited the steady-state K+ current in a dose-dependent and reversible manner. The inhibition of K+ current was not caused indirectly by changes in cytosolic Ca2+ concentration. The Ca(2+)-activated K+ channel blocker charybdotoxin (ChTX, 10(-7) M) also inhibited the K+ current. In the presence of ChTX or in the absence of Ca2+, carbacyclin failed to inhibit the residual K+ current. Furthermore, in the presence of high concentrations of carbacyclin, ChTX did not cause further reduction of K+ current. 4. Carbacyclin-induced inhibition of IK(Ca) was mimicked by 8-bromo-cAMP and by activation of G-protein with GTP gamma S. The inhibitory effect of carbacyclin on IK(Ca) was abolished by GDP beta S, which blocks G-protein activation, and by a selective inhibitor of cAMP-dependent protein kinase, PKI5-24. 5. The results demonstrate that carbacyclin inhibits ChTX-sensitive IK(Ca) in isolated aortic baroreceptor neurones by a G-protein-coupled activation of cAMP-dependent protein kinase. This mechanism may contribute to the PGI2-induced increase in baroreceptor activity demonstrated previously.

Adenosine A3 receptors potentiate hippocampal calcium current by a PKA-dependent/PKC-independent pathway

The modulation of high-threshold Ca current (I(Ca)) by adenosine receptors was studied using the voltage clamp method on acutely dissociated guinea pig hippocampal CA3 pyramidal neurons. When these neurons were exposed to adenosine in the presence of A1, A2a and A2b receptor antagonists, I(Ca) potentiation occurred at test potentials of -10 mV, but not at -40 mV. Similar potentiation also occurred using the A3 agonist N6-2-(4-aminophenyl)ethyl-adenosine (APNEA), either alone or in the presence of A1 and A2 antagonists. The putative A4 agonist 2-phenylaminoadenosine (CV-1808; Cornfield et al., 1992) did not potentiate I(Ca) at four concentrations tested between 25 nM and 2500 nM. K0.5 for the APNEA-induced potentiation was 25.4 nM, comparable to that determined in binding studies for the cloned receptor (15.5 nM; Zhou et al., 1992). I(Ca) potentiation by APNEA was blocked by intracellular application of WIPTIDE, a PKA inhibitor (p < 0.001), but was not affected by protein kinase C (PKC) inhibitor peptide (19-36). These results indicate that: (1) A3 receptor activation can significantly potentiate I(Ca), and (2) because the A3 receptor has been linked to down-regulation of adenylyl cyclase (Zhou et al., 1992), PKA appears to be negatively coupled to I(Ca).

Modulation of calcium currents by a D1 dopaminergic protein kinase/phosphatase cascade in rat neostriatal neurons

In rat neostriatal neurons, D1 dopamine receptors regulate the activity of cyclic AMP-dependent protein kinase (PKA) and protein phosphatase 1 (PP1). The influence of these signaling elements on high voltage-activated (HVA) calcium currents was studied using whole-cell voltage-clamp techniques. The application of D1 agonists or cyclic AMP analogs reversibly reduced N- and P-type Ca2+ currents. Inhibition of PKA antagonized this modulation, as did inhibition of PP1, suggesting that the D1 effect was mediated by a PKA enhancement of PP1 activity directed toward Ca2+ channels. In a subset of neurons, D1 receptor-mediated activation of PKA enhanced L-type currents. The differential regulation of HVA currents by the D1 pathway helps to explain the diversity of effects this pathway has on synaptic integration and plasticity in medium spiny neurons.

Beta-adrenergic agonists regulate KCa channels in airway smooth muscle by cAMP-dependent and -independent mechanisms

Stimulation of calcium-activated potassium (KCa) channels in airway smooth muscle cells by phosphorylation-dependent and membrane-delimited, G protein actions has been reported (Kume, H. A. Takai, H. Tokuno, and T. Tomita. 1989. Nature [Lond.]. 341:152-154; Kume, H., M. P. Graziano, and M. I. Kotlikoff. 1992. Proc. Natl. Acad. Sci. USA. 89:11051-11055). We show that beta-adrenergic receptor/channel coupling is not affected by inhibition of endogenous ATP, and that activation of KCa channels is stimulated by both alpha S and cAMP-dependent protein kinase (PKA). PKA stimulated channel activity in a dose-dependent fashion with an EC50 of 0.12 U/ml and maximum stimulation of 7.38 +/- 2.04-fold. Application of alpha S to patches near maximally stimulated by PKA significantly increased channel activity to 15.1 +/- 3.65-fold above baseline, providing further evidence for dual regulatory mechanisms and suggesting that the stimulatory actions are independent. Analysis of channel open-time kinetics indicated that isoproterenol and alpha S stimulation of channel activity primarily increased the proportion of longer duration events, whereas PKA stimulation had little effect on the proportion of short and long duration events, but resulted in a significant increase in the duration of the long open-state. cAMP formation during equivalent relaxation of precontracted muscle strips by isoproterenol and forskolin resulted in significantly less cAMP formation by isoproterenol than by forskolin, suggesting that the degree of activation of PKA is not the only determinant of tissue relaxation. We conclude that beta-adrenergic stimulation of KCa channel activity and relaxation of tone in airway smooth muscle occurs, in part, by means independent of cyclic AMP formation.

Protein kinase A modulates an endogenous calcium channel, but not the calcium-activated chloride channel, in Xenopus oocytes

In Xenopus oocytes, Ca2+ influx through an endogenous voltage-gated Ca2+ channel activates a transient outward Cl- current (ICl(Ca)), which is potentiated by cAMP increase. The site of cAMP effect appears to be the Ca2+ channel instead of the Ca(2+)-activated Cl- channel, because cAMP potentiates the Ba2+ current through the Ca2+ channel in a similar way to the ICl(Ca), and cAMP does not potentiate the Ca(2+)-dependent Cl- current in cells treated with Ca2+ ionophore. Using the catalytic subunit of protein kinase A (PKA) and PKA inhibitors, it was shown that PKA is both necessary and sufficient for the cAMP effect on ICl(Ca). Furthermore, the cAMP/PKA-mediated potentiation of ICl(Ca) was inhibited by both type 1 and type 2A protein phosphatases.

Phosphorylation enhances inactivation of N-type calcium channel current in bullfrog sympathetic neurons

We have investigated the effects of phosphatase and protein kinase inhibitors on calcium channel currents of bullfrog sympathetic neurons using the whole cell configuration of the patch clamp technique. Intracellular dialysis with the phosphatase inhibitors okadaic acid and calyculin A markedly enhanced the decline of inward current during a depolarizing voltage step. Tail current analysis demonstrated that this was genuine inactivation of calcium channel current, not activation of an outward current. The rapidly inactivating current is N-type calcium current (blocked by omega-conotoxin and resistant to nifedipine). Staurosporine, a nonselective protein kinase inhibitor, prevented the action of okadaic acid, suggesting that protein phosphorylation is involved. Under control conditions, the time course of inactivation could be described by the sum of two exponentials (tau = 150 ms and 1200 ms), plus a constant (apparently noninactivating) component, during depolarizations lasting 2 s. Okadaic acid induced a rapid inactivation process (tau = 15 ms) that was absent or negligible under control conditions, without obvious effect on the two slower time constants. As in control cells, inactivation in okadaic-acid-treated cells was strongest near -20 mV, with less inactivation at more positive voltages. However, inactivation did not depend on calcium influx. Modulation of calcium channel activity by phosphorylation may underly the spontaneous shift between inactivating and noninactivating modes recently observed for N-type calcium channels. Differences in basal phosphorylation levels could also explain why N-type calcium channels, originally described as rapidly and completely inactivating, inactivate slowly and incompletely in many neurons.

Spike conduction properties of T-shaped C neurons in the rabbit nodose ganglion

The electrical activity of C-type neurons was recorded intracellularly in the rabbit nodose ganglion maintained in vitro. The initial segment of their axon is spirally wound close to the cell body and a primary branching point divides it into a central process (CP) projecting to the nucleus of solitary tract in the medulla oblongata and a peripheral process (PP) which conveys sensory inputs from the viscera. Stimulation of the CP induced either somatic ("S") spikes or low-amplitude axonal ("A") spikes ("A1" or "A2"). In some cases abrupt changes in the latency of "S" or "A" spikes (jumps) were observed by gradually increasing the stimulus intensity. They are discussed in relation to a secondary branching on the central axon located inside or near the ganglion. Collision experiments showed that antidromic "A" spikes are blocked at the primary bifurcation of the axon (T-shaped neuron). Stimulation of the PP induced either "S" spikes or high amplitude "A" spikes ("A3" or "A4"). Orthodromic spikes could be blocked either before or after the primary bifurcation. When blocking occurs after the bifurcation on the stem axon, the spike can invade the central axon without invading the soma. The study of the refractory periods of the two processes and the application of high frequency stimulation showed that the PP allows higher frequencies than the soma and the CP, and thus that branching and the CP act as low-pass filters. These data support the view that the primary branching point and the CP of these T-shaped cells represent a strategic area to modulate visceral afferent messages.

Induction of immediate early genes by cyclic AMP in primary cultures of neurons from rat cerebral cortex

In this paper, we tested whether physiological activators of the cAMP second messenger pathway in primary cultures of neurons from rat cerebral cortex directly induce c-fos and other immediate early gene (IEG) transcription factors. We have found that brief (30 s to 2 min) stimulation of neurons with vasoactive intestinal peptide (VIP) and SKF-38393, a D1-dopaminergic receptor agonist, potently increased mRNA levels for the IEGs c-fos, jun-B, and NGFI-A, with weaker increases for c-jun. This action was mimicked by forskolin and dibutyryl cAMP. IEG induction by VIP and dibutyryl cAMP was not blocked by excitatory amino acid receptor antagonists or by blockers of dihydropyridine-sensitive calcium channels. Moreover, calcium-free medium did not modify IEG induction by dibutyryl cAMP, suggesting that cAMP can directly regulate IEG expression in differentiated neurons independently of calcium.

The peptide CGRP increases a high-threshold Ca2+ current in rat nodose neurones via a pertussis toxin-sensitive pathway

1. The whole-cell variation of the patch clamp technique was used to study the effect of calcitonin gene-related peptide (CGRP) on voltage-gated calcium currents in acutely dissociated rat nodose ganglion neurones and to determine if its effects were mediated via a guanine nucleotide binding (G) protein. 2. Both low- and high-threshold calcium current components were present in nodose ganglion neurones. CGRP had no effect on the isolated low-threshold current component. However, CGRP (1-1000 nM, ED50 = 50 nM) caused a concentration-dependent increase in high-threshold calcium currents. CGRP (1 microM) increased the peak of these calcium currents 21 +/- 4% over controls. 3. CGRP enhanced a transient high-threshold calcium current evoked from a holding potential of -80 mV but did not affect the slowly inactivating high-threshold current evoked from -40 mV. Multiple high-threshold calcium currents have been reported in sensory neurones. We cannot state unequivocally which high-threshold calcium current component was enhanced by CGRP. However, based on the observation that CGRP increased a transient but not the slowly inactivating high-threshold calcium current, we believe the peptide enhanced primarily the N-type calcium current component. 4. CGRP increased the maximal peak current and caused a modest negative shift of < or = 10 mV in the peak of the current-voltage (I-V) relation in three of six neurones. In the remaining three neurones the peptide increased the maximal peak current without a detectable shift in the peak of the I-V relation. 5. To determine if the CGRP-induced enhancement in calcium current was associated with an increase in calcium conductance, we studied the effect of the peptide on the instantaneous current-voltage (I-V) relation when currents were evoked at a clamp potential (Vc) of +30 mV, positive to the observed maximal current (Vc = 0 to +10 mV). CGRP increased the maximal conductance 23 +/- 4%. 6. The enhancement of calcium current by CGRP was not due to a shift in the voltage dependency of steady-state inactivation of the calcium channels. The stimulatory effect of CGRP on calcium current was evaluated by evoking currents from different holding potentials (Vh) at the same Vc (+10 mV). CGRP-induced increases in calcium currents were similar over the range of (Vh) from -60 to -110 mV, suggesting that the peptide did not alter voltage-dependent steady-state inactivation.(ABSTRACT TRUNCATED AT 400 WORDS)

Tetrahydroaminoacridine (THA) reduces voltage-dependent calcium currents in rat sensory neurons

Tetrahydroaminoacridine (THA) is a centrally active anticholinesterase that also interacts with neuronal K+ and Na+ channels and cardiac Ca2+ channels. The effects of THA on neuronal voltage-dependent Ca2+ channels are not known. We tested the effects of THA (25 nM-250 microM) on the Ca2+ current components of acutely dissociated rat nodose ganglion and dorsal root ganglion (DRG) neurons using the whole cell patch clamp recording technique. THA reduced the low-threshold (T) and high-threshold (N/L) Ca2+ current components in a concentration-dependent manner (IC50 approximately equal to 125 microM for T; approximately equal to 80 microM for N/L). Minimal current reduction was seen below approximately 10 microM. Our results show that THA reduces voltage-dependent Ca2+ currents in rodent sensory neurons suggesting another means by which THA may affect Ca(2+)-dependent physiologic processes.

Modulation of vertebrate neuronal calcium channels by transmitters

A large number of neurotransmitters have now been shown to reduce the amplitude and slow the activation kinetics of whole cell HVA ICa in a great diversity of neurons. These transmitters include L-glutamate (AMPA/kainate, metabotropic and NMDA receptors), GABA (via GABAB receptors, NA (via alpha 2 receptors), 5-HT, NA (via alpha 2 receptors), DA and several peptides. Both whole-cell and single-channel studies have demonstrated that the N-channel is the most common channel type to be blocked by transmitters, although an inhibition of the L-type channel has also occasionally been reported. The suppression of the N-type Ca current was commonly shown to be voltage-dependent, with a relief at large positive voltages. Strong evidence has been put forward showing that the transmitter action is mediated by a G-protein, with GDP-beta-S blocking transmitter action, and GTP-gamma-S directly inhibiting the Ca channel. Moreover, pertussis toxin blocked the transmitter action in most neurons, and following such block, injection of the G-protein Go restored transmitter action. A direct link between the G-protein and the Ca channel has been widely theorized to mediate the action of transmitters on certain neurons. There is also some evidence that certain transmitters in specific neurons mediate calcium channel inhibition through a 2nd messenger, perhaps protein kinase C. Transmitters have also been found, although uncommonly, to inhibit HVA L-type and LVA T-type channels. In addition, an enhancement of both HVA and LVA Ca currents by transmitters has been demonstrated, and substantial evidence exists for mediation of this action by cAMP.

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)

Dynorphin A and cAMP-dependent protein kinase independently regulate neuronal calcium currents

The kappa-selective opioid peptide dynorphin A (DYN) inhibits neuronal adenylate cyclase activity and reduces neuronal voltage-dependent calcium currents. It is not yet known, however, whether the regulation of calcium channel activity is dependent on or independent of the adenylate cyclase/cAMP system. We used the whole-cell variation of the patch clamp technique to show that DYN reversibly reduced, in a naloxone-sensitive manner, calcium currents in acutely dissociated rat nodose ganglion neurons. DYN slowed the rate of current activation and had a greater effect on currents evoked from relatively negative holding potentials. These actions were mimicked by guanosine 5'-[gamma-thio]triphosphate, which activates GTP-binding proteins (G proteins), and were blocked by pretreatment with pertussis toxin, which inactivates Gi- and Go-type G proteins. In contrast, calcium currents recorded in the presence of the catalytic subunit of the cAMP-dependent protein kinase (AK-C), included in the recording pipette, increased in magnitude throughout the recording. DYN was applied to neurons before and after the effect of AK-C became apparent; the reduction of calcium currents by DYN was greater in the presence of AK-C than in its absence. We conclude that the acute reduction of neuronal calcium currents by DYN occurred by means of activation of pertussis toxin-sensitive Gi- or Go-type G proteins. The persistence of the action of DYN in the presence of AK-C indicates, however, that this effect was independent of a reduction of the activity of the adenylate cyclase/cAMP system and suggests in addition that phosphorylated channels may be preferentially inhibited by DYN.

Valproic acid selectively reduces the low-threshold (T) calcium current in rat nodose neurons

Valproic acid (VPA) is an antiepileptic drug used in the treatment of a wide variety of human seizures including generalized absence (GA) (petit mal) seizures. The mechanism of action of VPA in controlling GA seizures is not known. We tested the effects of VPA on the Ca2+ current components of acutely dissociated rat nodose ganglion neurons. VPA reduced the low-threshold (T) Ca2+ current at clinically relevant concentrations but had no effect on the high-threshold (N and L) current components. The effect on T current was concentration-dependent and most apparent at peak current. There was little effect seen on late current. VPA did not affect the rate or voltage-dependency of T current activation. The selective reduction of T current may be a means by which VPA is effective in controlling GA seizures.