Differential regulation of N- and Q-type Ca2+ channels by cyclic nucleotides and G-proteins

The Signaling Pathways Involved in the Anticonvulsive Effects of the Adenosine A1 Receptor

Adenosine acts as an endogenous anticonvulsant and seizure terminator in the brain. Many of its anticonvulsive effects are mediated through the activation of the adenosine A₁ receptor, a G protein-coupled receptor with a wide array of targets. Activating A₁ receptors is an effective approach to suppress seizures. This review gives an overview of the neuronal targets of the adenosine A₁ receptor focusing in particular on signaling pathways resulting in neuronal inhibition. These include direct interactions of G protein subunits, the adenyl cyclase pathway and the phospholipase C pathway, which all mediate neuronal hyperpolarization and suppression of synaptic transmission. Additionally, the contribution of the guanyl cyclase and mitogen-activated protein kinase cascades to the seizure-suppressing effects of A₁ receptor activation are discussed. This review ends with the cautionary note that chronic activation of the A₁ receptor might have detrimental effects, which will need to be avoided when pursuing A₁ receptor-based epilepsy therapies.

μ-Opioid inhibition of Ca2+ currents and secretion in isolated terminals of the neurohypophysis occurs via ryanodine-sensitive Ca2+ stores

μ-Opioid agonists have no effect on calcium currents (I(Ca)) in neurohypophysial terminals when recorded using the classic whole-cell patch-clamp configuration. However, μ-opioid receptor (MOR)-mediated inhibition of I(Ca) is reliably demonstrated using the perforated-patch configuration. This suggests that the MOR-signaling pathway is sensitive to intraterminal dialysis and is therefore mediated by a readily diffusible second messenger. Using the perforated patch-clamp technique and ratio-calcium-imaging methods, we describe a diffusible second messenger pathway stimulated by the MOR that inhibits voltage-gated calcium channels in isolated terminals from the rat neurohypophysis (NH). Our results show a rise in basal intracellular calcium ([Ca(2+)]i) in response to application of [D-Ala(2)-N-Me-Phe(4),Gly5-ol]-Enkephalin (DAMGO), a MOR agonist, that is blocked by D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), a MOR antagonist. Buffering DAMGO-induced changes in [Ca(2+)]i with BAPTA-AM completely blocked the inhibition of both I(Ca) and high-K(+)-induced rises in [Ca(2+)]i due to MOR activation, but had no effect on κ-opioid receptor (KOR)-mediated inhibition. Given the presence of ryanodine-sensitive stores in isolated terminals, we tested 8-bromo-cyclic adenosine diphosphate ribose (8Br-cADPr), a competitive inhibitor of cyclic ADP-ribose (cADPr) signaling that partially relieves DAMGO inhibition of I(Ca) and completely relieves MOR-mediated inhibition of high-K(+)-induced and DAMGO-induced rises in [Ca(2+)]i. Furthermore, antagonist concentrations of ryanodine completely blocked MOR-induced increases in [Ca(2+)]i and inhibition of I(Ca) and high-K(+)-induced rises in [Ca(2+)]i while not affecting KOR-mediated inhibition. Antagonist concentrations of ryanodine also blocked MOR-mediated inhibition of electrically-evoked increases in capacitance. These results strongly suggest that a key diffusible second messenger mediating the MOR-signaling pathway in NH terminals is [Ca(2+)]i released by cADPr from ryanodine-sensitive stores.

AKAP-dependent sensitization of Ca(v) 3.2 channels via the EP(4) receptor/cAMP pathway mediates PGE(2) -induced mechanical hyperalgesia

BACKGROUND AND PURPOSE

The Ca(v) 3.2 isoform of T-type Ca(2+) channels (T channels) is sensitized by hydrogen sulfide, a pro-nociceptive gasotransmitter, and also by PKA that mediates PGE(2) -induced hyperalgesia. Here we examined and analysed Ca(v) 3.2 sensitization via the PGE(2) /cAMP pathway in NG108-15 cells that express Ca(v) 3.2 and produce cAMP in response to PGE(2) , and its impact on mechanical nociceptive processing in rats.

EXPERIMENTAL APPROACH

In NG108-15 cells and rat dorsal root ganglion (DRG) neurons, T-channel-dependent currents (T currents) were measured with the whole-cell patch-clamp technique. The molecular interaction of Ca(v) 3.2 with A-kinase anchoring protein 150 (AKAP150) and its phosphorylation were analysed by immunoprecipitation/immunoblotting in NG108-15 cells. Mechanical nociceptive threshold was determined by the paw pressure test in rats.

KEY RESULTS

In NG108-15 cells and/or rat DRG neurons, dibutyryl cAMP (db-cAMP) or PGE(2) increased T currents, an effect blocked by AKAP St-Ht31 inhibitor peptide (AKAPI) or KT5720, a PKA inhibitor. The effect of PGE(2) was abolished by RQ-00015986-00, an EP(4) receptor antagonist. AKAP150 was co-immunoprecipitated with Ca(v) 3.2, regardless of stimulation with db-cAMP, and Ca(v) 3.2 was phosphorylated by db-cAMP or PGE(2) . In rats, intraplantar (i.pl.) administration of db-cAMP or PGE(2) caused mechanical hyperalgesia, an effect suppressed by AKAPI, two distinct T-channel blockers, NNC 55-0396 and ethosuximide, or ZnCl(2) , known to inhibit Ca(v) 3.2 among T channels. Oral administration of RQ-00015986-00 suppressed the PGE(2) -induced mechanical hyperalgesia.

CONCLUSION AND IMPLICATIONS

Our findings suggest that PGE(2) causes AKAP-dependent phosphorylation and sensitization of Ca(v) 3.2 through the EP(4) receptor/cAMP/PKA pathway, leading to mechanical hyperalgesia in rats.

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

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

Site-specific regulation of CA(V)2.2 channels by protein kinase C isozymes betaII and epsilon

Ca(v)2.2 high voltage-gated calcium channels are regulated by phorbol-12-myristae, 13-acetate (PMA) via Ser/Thr protein kinase C (PKC) phosphorylation sites in the I-II linker and C-terminus of the alpha(1) 2.2 subunit. Here we show that PMA enhancement of Ca(v)2.2 currents expressed in Xenopus oocytes can be blocked by inhibitors of PKC betaII or PKC epsilon isozymes, as shown previously for Ca(v)2.3 currents, and that microinjection of PKC betaII or PKC epsilon isozymes in the oocytes expressing the WT Ca(v)2.2 channels increases the basal barium current (I(Ba)). The I-V plot shows a large increase in current amplitude with PKC betaII and PKC epsilon isozymes with only a small shift in the peak I(Ba) in the hyperpolarizing direction. The potentiation of Ca(v)2.2 currents by microinjection of PKC betaII and PKC epsilon isozymes was not altered by the inhibition of G proteins with GDPbetaS. The combination of isozyme specific inhibitors with previously generated Ser/Thr to Ala mutants of alpha(1) 2.2 subunit revealed that PKC betaII or PKC epsilon isozymes (but not PKC alpha or delta) can provide full enhancement through the stimulatory site (Thr-422) in the I-II linker but that PKC epsilon is better at decreasing channel activity through the inhibitory site Ser-425. The enhancing effect of PKC betaII or epsilon at Thr-422 is dominant over the inhibitory effect at Ser-425. Injected PKC betaII also enhances Ca(v)2.2 current when any of the potential stimulatory sites (Ser-1757, Ser-2108 and Ser-2132) are available in the C-terminus. PKC epsilon provides lesser enhancement with C-terminal sites and only with Ser-2108 and Ser-2132. Sites Ser-1757 and Ser-2132, but not Ser-2108, are dominant over the inhibitory site Ser-425. Collectively, these results reveal a hierarchy of regulatory sites in Ca(v)2.2 channels. Site-specific regulation by different PKC isozymes may allow graded levels of channel activation and susceptibility or resistance to subsequent stimulatory events.

Signaling complexes of voltage-gated calcium channels and G protein-coupled receptors

Activation of opioid or opioid-receptor-like (ORL1 a.k.a. NOP or orphanin FQ) receptors mediates analgesia through inhibition of N-type calcium channels in dorsal root ganglion (DRG) neurons (1, 2). Unlike the three types of classical mu, delta, and kappa opioid receptors, ORL1 mediates an agonist-independent inhibition of N-type calcium channels. This is mediated via the formation of a physical protein complex between the receptor and the channel, which in turn allows the channel to effectively sense a low level of constitutive receptor activity (3). Further inhibition of N-type channel activity by activation of other G protein-coupled receptors is thus precluded. ORL1 receptors, however, also undergo agonist-induced internalization into lysosomes, and channels thereby become cointernalized in a complex with ORL1. This then results in removal of N-type channels from the plasma membrane and reduced calcium entry (4). Similar signaling complexes between N-type channels and GABA(B) receptors have been reported (5). Moreover, both L-type and P/Q-type channels appear to be able to associate with certain types of G protein-coupled receptors (6, 7). Hence, interactions between receptors and voltage-gated calcium channels may be a widely applicable means to optimize receptor channel coupling.

Presynaptic calcium channels: structure, regulators, and blockers

The central and peripheral nervous systems express multiple types of ligand and voltage-gated calcium channels (VGCCs), each with specific physiological roles and pharmacological and electrophysiological properties. The members of the Ca(v)2 calcium channel family are located predominantly at presynaptic nerve terminals, where they are responsible for controlling evoked neurotransmitter release. The activity of these channels is subject to modulation by a number of different means, including alternate splicing, ancillary subunit associations, peptide and small organic blockers, G-protein-coupled receptors (GPCRs), protein kinases, synaptic proteins, and calcium-binding proteins. These multiple and complex modes of calcium channel regulation allow neurons to maintain the specific, physiological window of cytoplasmic calcium concentrations which is required for optimal neurotransmission and proper synaptic function. Moreover, these varying means of channel regulation provide insight into potential therapeutic targets for the treatment of pathological conditions that arise from disturbances in calcium channel signaling. Indeed, considerable efforts are presently underway to identify and develop specific presynaptic calcium channel blockers that can be used as analgesics.

Role of the cyclic-AMP/PKA cascade and of P/Q-type Ca++ channels in endocannabinoid-mediated long-term depression in the nucleus accumbens

Glutamate transmission between prefrontal cortex (PFC) and accumbens (NAc) plays a crucial role in the establishment and expression of addictive behaviors. At these synapses exogenous cannabinoid receptor 1 (CB1R) agonists reversibly inhibit excitatory transmission, and the sustained release of endogenous cannabinoids (eCB) following prolonged cortical stimulation leads to long-term depression (LTD). Activation of presynaptic K(+) channels mediates the effects of exocannabinoids, but the transduction pathway underlying the protracted phase of eCB-LTD is unknown. Here we report that the maintenance of eCB-LTD does not involve presynaptic K(+) channels: eCB-LTD was not affected by blockade of K(+) channels with 4-AP (100 microM) and BaCl(2) (300 microM) (fEPSP=78.9+/-5.4% of baseline 58-60 min after tetanus, compared to 78.9+/-5.9% in control slices). In contrast, eCB-LTD was blocked by treatment of the slices with the adenylyl cyclase (AC) activator forskolin (10 microM), and with the protein kinase A (PKA) inhibitor KT5720 (1 microM) (fEPSP=108.9+/-5.7% in forskolin and 110.5+/-7.7% in KT5720, compared to 80.6+/-3.9% in control conditions). Additionally, selective blockade of P/Q-type Ca(2+) channels with omega-agatoxin-IVA (200 nM) occluded the expression of eCB-LTD (fEPSP=113.4+/-15.9% compared to 78.6+/-4.4% in control slices), while blockade of N- with omega-conotoxin-GVIA (1 microM) or L-type Ca(2+) channels with nimodipine (1 microM), was without effect (fEPSP was 83.7+/-5.3% and 87+/-8.9% respectively). These data show that protracted inhibition of AC/PKA activity and P/Q-type Ca(2+) channels are necessary for expression of eCB-LTD at NAc synapses.

Transcriptome analysis of frontal cortex in alcohol-preferring and nonpreferring rats

Although it is widely accepted that alcohol abuse and alcoholism have a significant genetic component of risk, the identities of the genes themselves remain obscure. To illuminate such potential genetic contributions, DNA macroarrays were used to probe for differences in normative cortical gene expression between rat strains genetically selected for alcohol self-administration preference, AA (Alko, alcohol) and P (Indiana, preferring), or avoidance, ANA (Alko, nonalcohol) and NP (Indiana, nonpreferring). Among 1,176 genes studied, six demonstrated confirmable, differential expression following comparison of ethanol-naive AA and ANA rats. Specifically, the mRNA level for metabotropic glutamate receptor 3 (mGluR3) was down-regulated in the AA vs. ANA lines. In contrast, calcium channel subunit alpha2delta1 (cacna2d1), vesicle-associated membrane protein 2 (VAMP2), syntaxin 1 (both syntaxin 1a and 1b; STX1a and STX1b), and syntaxin binding protein (MUNC-18) mRNAs were found to be increased in frontal cortex following comparison of AA with ANA animals. Bioinformatic analysis of these molecular targets showed that mGluR3 and cacna2d1 fall within chromosomal locations reported to be alcohol-related by the Collaborative Study on the Genetics of Alcoholism (COGA) as well as quantitative trait loci (QTL) studies. To determine further whether these differences were strain specific, the above-mentioned genes were compared in ethanol-preferring (P) and -nonpreferring (NP) selected lines. VAMP2 was the only gene that displayed statistically different mRNA levels in a comparison of P and NP rats. In conclusion, the altered cortical gene expression illuminated here would have the effect of altering neurotransmitter release in AA rats (compared with ANA rats). Such alterations, however, might not be a universal characteristic of all animal models of alcohol abuse and will also require further investigation in post-mortem human samples.

Role of p/q-Ca2+ channels in metabotropic glutamate receptor 2/3-dependent presynaptic long-term depression at nucleus accumbens synapses

The nucleus accumbens (NAc) is an important cerebral area involved in reward and spatial memory (Pennartz et al., 1994), but little is known about synaptic plasticity in this region. Here, electron microscopy revealed that, in the NAc, metabotropic glutamate receptors 2/3 (mGlu2/3) immunostaining was essentially associated with axonal terminals and glial processes, whereas postsynaptic dendrites and neuronal cell bodies were unstained. Electrophysiological techniques in the NAc slice preparation demonstrated that activation of mGlu2/3 with synaptically released glutamate or specific exogenous agonist, such as LY354740 (200 nm, 10 min), induced long-term depression of excitatory synaptic transmission (mGlu2/3-LTD). Tetanic-LTD and pharmacological mGlu2/3-LTD occluded each other, suggesting common mechanisms. The mGlu2/3-LTD did not require synaptic activity but depended on the cAMP-protein kinase A cascade. Selective inhibition of P/Q-type Ca(2+) channels with omega-agatoxin-IVA occluded the expression of mGlu2/3-LTD, and, conversely, the inhibitory effects of omega-agatoxin-IVA were abolished during mGlu2/3-LTD. Thus, mGlu2/3 play an important role in the control of use-dependent synaptic plasticity at prelimbic cortex-NAc synapses: their activation causes a form of LTD mediated by the long-lasting reduction of P/Q-type Ca(2+)channels contribution to transmitter release.

Peptidergic modulation of insect voltage-gated Ca(2+) currents: role of resting Ca(2+) current and protein kinases A and C

The modulation of voltage-gated Ca(2+) currents in isolated dorsal unpaired median (DUM) neurons of cockroach was investigated using whole cell patch clamp. The neuropeptide neurohormone D (NHD), a member of the adipokinetic hormone family, affected Ca(2+) currents at pico- to nanomolar concentrations. It strongly enhanced currents activating at lower depolarizations, whereas those activating at strong depolarizations were slightly attenuated. The first effect results from upregulation of a previously characterized omega-conotoxin MVIIC- and omega-agatoxin IVA-sensitive "mid/low voltage-activated" (M-LVA) Ca(2+) current. The cAMP-analogue 8-bromo-cAMP, forskolin, and the catalytic subunit of protein kinase A (PKA) mimicked the stimulating action of NHD. In addition, preincubation of neurons with the PKA inhibitor KT 5720 abolished the action of NHD. Thus NHD seems to upregulate the M-LVA current via channel phosphorylation by PKA. Activation of protein kinase C by oleoylacetylglycerol (OAG) mimicked the effect of NHD, and subsequent NHD application only enhanced the current to a moderate extent. On the other hand, inhibition of protein kinase C (PKC) by Gö 6976 abolished the NHD effect. These results indicate that also PKC, too, may play a role in the peptidergic modulation of the M-LVA Ca(2+) current. The reduction of Ca(2+) currents in the high-voltage-range is caused by the NHD-induced upregulation of a voltage-independent Ca(2+) resting current, I(Ca,R), which most probably leads to enhanced Ca(2+)-dependent inactivation of voltage-gated Ca(2+) currents. To assess the major consequences of the Ca(2+) current changes, current-clamp investigations were performed. Experiments with iberiotoxin, a specific blocker of BK-type Ca(2+)-dependent K(+) currents, and the M-LVA current-blocking omega-toxins suggested that NHD causes-via increasing Ca(2+)-dependent K(+) currents-a larger hyperpolarization of action potentials. The lowering in the action potential threshold produced by NHD, however, seems to be a direct consequence of the hyperpolarizing shift of the activation curve of total Ca(2+) current resulting from NHD-induced upregulation of the M-LVA current component.

Adenosine receptor subtypes modulate two major functional pathways for hippocampal serotonin release

To clarify the mechanisms of interaction between adenosine A(1) receptor (A1-R) and adenosine A(2) receptor (A2-R) on neurotransmitter release, this study determined the functional interactions among adenosine receptors (AD-Rs), voltage-sensitive Ca(2+) channels (VSCCs), protein kinases (PKs), and synaptic proteins [N-ethylmaleimide-sensitive factor (NSF) attachment protein (SNAP) receptors] on hippocampal serotonin release using in vivo microdialysis in freely moving rat. Basal serotonin release was regulated by two functional complexes: N-type VSCC (N-VSCC)/calcium-phospholipid-dependent protein kinase (PKC)/syntaxin (major pathway) and P-type VSCC (P-VSCC)/cyclic AMP-dependent protein kinase (PKA)/synaptobrevin (minor pathway). However, K(+)-evoked serotonin release was regulated by N-VSCC/PKC/syntaxin (minor pathway) and P-VSCC/PKA/synaptobrevin (major pathway). A1-R antagonists increased basal serotonin release, which was reduced by inhibitors of N-VSCC, PKC, and syntaxin predominantly and by inhibitors of PKA and synaptobrevin weakly, but was not affected by P-VSCC inhibitor. In the presence of A1-R antagonist, A2-R agonists increased basal serotonin release, which was inhibited by inhibitors of P-VSCC, PKA, and synaptobrevin predominantly and reduced by inhibitors of N-VSCC, PKC, and syntaxin weakly. Under the condition of activation of adenylate cyclase in the absence of A1-R antagonists, A2-R agonists increased basal serotonin release. A1-R antagonist and A2-R agonist enhanced K(+)-evoked serotonin release, which was inhibited by inhibitors of P-VSCC, PKA, and synaptobrevin predominantly. These results suggest that an activation of A1-R suppresses serotonin release via inhibition of both N-VSCC/PKC/syntaxin and P-VSCC/PKA/synaptobrevin pathways, and an activation of A2-R stimulates serotonin release via enhancement of the P-VSCC/PKA/synaptobrevin pathway. Therefore, PKA activity plays an important role in the interaction between A1-R and A2-R on hippocampal serotonin release.