Ziconotide, an intrathecally administered N-type calcium channel antagonist for the treatment of chronic pain

Ziconotide is a novel peptide that blocks the entry of calcium into neuronal N-type voltage-sensitive calcium channels, preventing the conduction of nerve signals. N-type calcium channels are present in the superficial laminae of the dorsal horn of the spinal cord. In various animal models of pain, intrathecal administration of ziconotide blocked nerve transmission and nociception. The United States Food and Drug Administration recently approved ziconotide intrathecal infusion for the management of severe chronic pain in patients who require intrathecal therapy and who are intolerant of or refractory to other treatment, such as systemic analgesics, adjunctive therapies, or intrathecal morphine. The drug has a narrow therapeutic window and a lag time for the onset and offset of analgesia and adverse events. In early clinical trials, frequent and severe psychiatric and central nervous system adverse effects were associated with rapid intrathecal infusion (0.4 microg/hr) and frequent up-titration (every 12 hrs). Therefore, patients with psychiatric symptoms are not candidates for this drug. Drug trials of external intrathecal catheters and microinfusion devices demonstrated a 3% risk of meningitis. A low initial infusion rate of 0.1 microg/hour and limiting infusion rate increases to 2-3 times/week are now recommended. Patients responsive to intrathecal ziconotide require an implanted infusion system to receive long-term therapy.

Activity-dependent decrease of excitability in rat hippocampal neurons through increases in Ih

Hippocampal long-term potentiation (LTP) induced by theta-burst pairing of Schaffer collateral inputs and postsynaptic firing is associated with localized increases in synaptic strength and dendritic excitability. Using the same protocol, we now demonstrate a decrease in cellular excitability that was blocked by the h-channel blocker ZD7288. This decrease was also induced by postsynaptic theta-burst firing alone, yet it was blocked by NMDA receptor antagonists, postsynaptic Ca2+ chelation, low concentrations of tetrodotoxin, omega-conotoxin MVIIC, calcium/calmodulin-dependent protein kinase II (CaMKII) inhibitors and a protein synthesis inhibitor. Increasing network activity with high extracellular K+ caused a similar reduction of cellular excitability and an increase in h-channel HCN1 protein. We propose that backpropagating action potentials open glutamate-bound NMDA receptors, resulting in an increase in I(h) and a decrease in overall excitability. The occurrence of such a reduction in cellular excitability in parallel with synaptic potentiation would be a negative feedback mechanism to normalize neuronal output firing and thus promote network stability.

Asynchronous GABA release generates long-lasting inhibition at a hippocampal interneuron-principal neuron synapse

Hippocampal GABAergic interneurons show diverse molecular and morphological properties. The functional significance of this diversity for information processing is poorly understood. Here we show that cholecystokinin (CCK)-expressing interneurons in rat dentate gyrus release GABA in a highly asynchronous manner, in contrast to parvalbumin (PV) interneurons. With a gamma-frequency burst of ten action potentials, the ratio of asynchronous to synchronous release is 3:1 in CCK interneurons but is 1:5 in parvalbumin interneurons. N-type channels trigger synchronous and asynchronous release in CCK interneuron synapses, whereas P/Q-type Ca(2+) channels mediate release at PV interneuron synapses. Effects of Ca(2+) chelators suggest that both a long-lasting presynaptic Ca(2+) transient and a large distance between Ca(2+) source and sensor of exocytosis contribute to the higher ratio of asynchronous to synchronous release in CCK interneuron synapses. Asynchronous release occurs at physiological temperature and with behaviorally relevant stimulation patterns, thus generating long-lasting inhibition in the brain.

Medullary N-type and P/Q-type calcium channels contribute to neuropathy-induced allodynia

The present study was designed to determine the contribution of N-type, P/Q-type and L-type calcium channels in the rostral ventromedial medulla to tactile allodynia following peripheral nerve injury. L5/L6 spinal nerve ligation in rats produced tactile allodynia, which was dose-dependently inhibited by intrarostral ventromedial medulla microinjection of the N-type calcium channel antagonist omega-conotoxin MVIIA. Similarly, intrarostral ventromedial medulla microinjection of the P/Q-type calcium channel antagonist omega-agatoxin IVA inhibited spinal nerve ligation-induced tactile allodynia, whereas intrarostral ventromedial medulla microinjection of the L-type calcium channel antagonist nimodipine had no effect. These results demonstrate that N-type and P/Q-type calcium channels in the rostral ventromedial medulla contribute to tactile allodynia following peripheral neuropathy, likely via neurotransmitter-mediated activation of descending facilitatory systems from the rostral ventromedial medulla.

Contribution of L‐type channels to Ca 2+ regulation of neuronal properties in early developing Purkinje neurons

Activity driven Ca2+ signaling is an important regulator of neuronal development. Early developing Purkinje neurons (postnatal day 5-7) prior to the stage of dendritic development express a somatic Ca2+ signaling pathway that is electrically driven and communicates information from the cell membrane to the cytosol and nucleus. In the current studies, we examined the properties and potential functional role of this pathway using acutely isolated Purkinje neurons from postnatal day 5-7 rat pups and brief K+ stimulation to activate the pathway. Results show that the amplitude of the nuclear Ca2+ signal increases as a function of the cytosolic Ca2+ signal but is larger than the cytosolic Ca2+ signal at strong K+ stimulations. Both L-type and P-type Ca2+ channels contribute to the Ca2+ signal. We also show using semiquantitative immunohistochemical methods that activation of this Ca2+ signaling pathway results in activation the transcription factor CREB and that L-type Ca2+ channels play a prominent role in this effect. The level of cfos, a transcription factor whose expression is regulated by CREB, was also increased by K+ stimulation. K+ stimulation also altered the level of the Ca2+ binding protein calbindin, an effect that involved L-type Ca2+ channels. The relationship between increases in Ca2+ and calbindin expression was bell-shaped, with high levels of Ca2+ decreasing calbindin expression. The level of the transmitter GABA was also increased by K+ stimulation but this effect was not dependent on L-type Ca2+ channels. Taken together, these results support a role for L-type channels in the phenotypic expression of Purkinje neuron properties during early development and suggest that the different activity patterns of early developing Purkinje neurons could be one mechanism for signaling the induction of specific genes through differences in cytosolic or nuclear Ca2+.

Ginkgolide B, a constituent of Ginkgo biloba, facilitates glutamate exocytosis from rat hippocampal nerve terminals

Although previous studies have demonstrated that Ginkgo biloba extract has modest effects in the improvement of memory and cognitive function of the Alzheimer's disease patients, the mechanism(s) underlying its beneficial effects remain(s) unclear. In this study, the effect of ginkgolide B, one of the major constituents of Ginkgo biloba extract, on the release of endogenous glutamate from rat hippocampal nerve terminals (synaptosomes) was studied. Ginkgolide B facilitated the Ca2+-dependent release of glutamate evoked by 4-aminopyridine in a concentration-dependent manner. The facilitatory action of ginkgolide B was not due to it increasing synaptosomal excitability because ginkgolide B did not alter the 4-aminopyridine-evoked depolarization of the synaptosomal plasma membrane potential. Rather, examination of the effect of ginkgolide B on cytosolic free Ca2+ concentration revealed that the facilitation of glutamate release could be attributed to an enhancement of presynaptic voltage-dependent Ca2+ influx. Consistent with this, the ginkgolide B-mediated facilitation of glutamate release was significantly prevented in synaptosomes pretreated with a wide spectrum blocker of N-, P-, and Q-type Ca2+ channels, omega-conotoxin MVIIC. Moreover, the facilitation produced by ginkgolide B was completely abolished by the protein kinase A inhibitor, but not by the protein kinase C inhibitor. These results suggest that ginkgolide B effects a increase in protein kinase A activation, which subsequently enhances the Ca2+ entry through voltage-dependent N- and P/Q-type Ca2+ channels to cause a increase in evoked glutamate release from rat hippocampal nerve terminals. In addition, glutamate release elicited by Ca2+ ionophore (ionomycin) was also facilitated by ginkgolide B, which suggests that ginkgolide B may have a direct effect on the secretory apparatus downstream of Ca2+ entry. These actions of ginkgolide B may provide some information regarding the beneficial effects of Ginkgo biloba in the central nervous system.

Characteristics of Omega-conotoxin GVI A and MVIIC Binding to Cav 2.1 and Cav 2.2 Channels Captured by Anti-Ca2+ Channel Peptide Antibodies

A New Binding Method (NBM) was used to investigate the characteristics of the specific binding of 125I-omega-conotoxin (omega-CTX) GVIA and 125I-omega-CTX MVIIC to Cav2.1 and Cav2.2 channels captured from chick brain membranes by antibodies against B1Nt (a peptide sequence in Car2.1 and Cav2.2 channels). The results for the NBM were as follows. (1) The ED50 values for specific binding of 125I-omega-CTX GVIA and 125I-omega-CTX MVIIC to Cav2.1 and Cav2.2 channels were about 68 and 60 pM, respectively, and very similar to those (87 and 35 pM, respectively) to crude membranes from chick brain. (2) The specific 125I-omega-CTX GVIA (100 pM) binding was inhibited by omega-CTX GVIA (0.5 nM), dynorphine A (Dyn), gentamicin (Gen), neomycin (Neo) and tobramicin (Tob) (100 microM each), but not by omega-agaconotoxin (Aga) IVA, calciseptine, omega-CTX SVIB, omega-CTX MVIIC (0.5 nM each), PN200-110 (PN), diltiazem (Dil) or verapamil (Ver) (100 microM each). Calmodulin (CaM) inhibited the specific binding in a dose-dependent manner (IC50 value of about 100 microg protein/ml). (3) The specific 125I-omega-CTX MVIIC (60 pM) binding was inhibited by omega-CTX MVIIC, omega-CTX GVIA, omega-CTX SVIB (0.5 nM each), Dyn, Neo and Tob (100 microM, each), but not by omega-Aga IVA, calciseptine (0.5 nM each), PN, Dil, Ver (100 microM each) or 100 microg protein/ml CaM. These results suggested that the characteristics of the specific binding of 125I-omega-CTX GVIA and 125I-omega-CTX MVIIC to Cav2.1 and Cav2.2 channels in the NBM were very similar to those to crude membranes from chick brain, although the IC50 values for CaM and free Ca2+ of CaM were about 33- and 5000-fold higher, respectively, than those for the specific binding of 125I-omega-CTX GVIA and 125I-omega-CTX MVIIC to crude membranes.

Antinociceptive effects of intrathecally administered huwentoxin-I, a selective N-type calcium channel blocker, in the formalin test in conscious rats

The present study was undertaken to elucidate the antinociceptive effect of intrathecal administration of huwentoxin-I (HWTX-I), a N-type calcium channel blocker from the venom of the Chinese bird spider Ornithoctonus huwena (Wang) [=Selenocosmia huwena wang], by comparison with omega-Conotoxin-MVIIA (omega-CTX-MVIIA) and morphine hydrochloride in the formalin test in conscious rats. Similar to omega-CTX-MVIIA and morphine, intrathecal pre-treatment with HWTX-I resulted in suppression of nociceptive behavior in a dose-dependent manner. The ED50 values of HWTX-I and omega-CTX-MVIIA were 0.28 and 0.19 microg/kg, respectively. It was also found that, at lower doses (0.1 and 0.5 microg/kg), the antinociceptive effect of HWTX-I was identical to that of omega-CTX-MVIIA, while omega-CTX-MVIIA acted more remarkably than HWTX-I at higher dose (1.0 microg/kg). However, the antinociception induced by omega-CTX-MVIIA were companied with motor dysfunction, and these side-effects became more evident with the doses of omega-CTX-MVIIA increasing. In contrast, HWTX-I did not show these side-effects at the doses of 0.5-1.0 microg/kg. Compared with HWTX-I and omega-CTX-MVIIA, the analgesic effect of intrathecal morphine hydrochloride was initiated faster, but lasted for a shorter time (about 2-3 h at 1.0 microg/kg) than that of HWTX-I and omega-CTX-MVIIA (about 4- 5 h at 1.0 microg/kg). Therefore, the present results show that, like omega-CTX-MVIIA, the intrathecal administration of HWTX-I is effective in antinociception in the rat model of the formalin test.

Different effects of L-type and L+N-type calcium channel blockers on hamster cheek pouch venules

Dihydropyridine calcium channel blockers are not uniform in terms of their action on calcium channel. L-type calcium channel blockers dilate the resistance arterioles. Cilnidipine is a dihydropyridine calcium channel blocker that also acts on N-type calcium channels, and may dilate venules through its effect on the sympathetic receptor. The influence of an L-type calcium channel blocker (nifedipine) or this L+N type blocker at 10(-7) mol to 10(-4) mol on venular diameter was examined by superfusion of male Syrian hamster cheek pouches. Nifedipine dose dependently dilated the arterioles alone, whereas cilnidipine dilated both arterioles and venules. Application of 10(-7) mol omega conotoxin, an inhibitor of N-type channels, after nifedipine led to significant dilation of venules, while it had no influence when administered after cilnidipine. These findings indicate that the effects of calcium channel blockers on the venules differ according to the action on N-type calcium channels, and that cilnidipine (an L+N type calcium channel blocker) dilates venules through its additional action on N-type channels.

Two pathways for the activation of small-conductance potassium channels in neurons of substantia nigra pars reticulata

Neurons in substantia nigra pars reticulata express the messenger RNA for SK2 but not for SK3 subunits that form small-conductance, Ca2+-dependent K+ channels in dopamine neurons. To determine pathways for the activation of small-conductance, Ca2+-dependent K+ channels in substantia nigra pars reticulata neurons of rats and mice, we studied effects of the selective blocker of small-conductance, Ca2+-dependent K+ channels, apamin (0.01 or 0.3 microM). Apamin diminished the afterhyperpolarization following each action potential and induced burst discharges in substantia nigra pars reticulata neurons. Apamin had a robust effect already at a low (10 nM) concentration consistent with the expression of the SK2 subunit. Afterhyperpolarizations were also reduced by the Ca2+ channel blockers Ni2+ (100 microM) and omega-conotoxin GVIA (1 microM). Depletion of intracellular Ca2+ stores did not change the afterhyperpolarization. However, we observed outward current pulses that occurred independently from action potentials and were abrogated by apamin. Apart from a faster time course, they shared all properties with spontaneous hyperpolarizations or outward currents that ryanodine receptor-mediated Ca2+ release from intracellular stores induces in juvenile dopamine neurons. Sensitization of ryanodine receptors by caffeine silenced substantia nigra pars reticulata neurons. This effect was abolished by the depletion of intracellular Ca2+ stores. We conclude that SK2 channels in substantia nigra pars reticulata neurons are activated by Ca2+ influx through at least two types of Ca2+ channels in the membrane and by ryanodine receptor-mediated Ca2+ release from intracellular stores. Ryanodine receptors do not amplify small-conductance, Ca2+-dependent K+ channel activation by the Ca2+ influx during a single spike. Yet, ryanodine receptor-mediated Ca2+ release and, thereby, an activation of small-conductance, Ca2+-dependent K+ channels by intracellular Ca2+ are available for excitability modulation in these output neurons of the basal ganglia system.

Gabapentin fails to alter P/Q-type Ca2+ channel-mediated synaptic transmission in the hippocampus in vitro

Gabapentin (Neurontin) has been successfully used in the treatment of both epilepsy and neuropathic pain. Despite its widespread clinical use, its mechanism of action is very poorly understood. Indeed, the only protein it is known to interact with is the alpha2delta subunit of the voltage-gated Ca(2+) channel complex. In a recent article, gabapentin was reported to inhibit synaptic transmission in the spinal cord through an inhibitory effect on presynaptic P/Q-type Ca(2+) channels in both glutamatergic primary afferents and glycinergic interneuones. To examine if such inhibition of P/Q-channel-mediated synaptic transmission by gabapentin generalised to other synaptic pathways, we tested the actions of gabapentin of P/Q-type Ca(2+) channel-mediated synaptic responses in the CA1 subfield of the hippocampus. We found that gabapentin was completely inactive on such synaptic responses even at 10 times the maximally effective concentration used in the spinal cord. A small ( approximately 10%) but consistent depression of control synaptic responses was elicited by 10 microM gabapentin. No greater response was observed at a 10 times higher concentration. From these data we conclude that gabapentin is not a generic inhibitor of presynaptic P/Q-type channels and its actions at the spinal level must represent a feature of the P/Q-type channel not present in the hippocampus. Given the known interactions of this compound, the best candidate for this is the presence, subtype, or state of the alpha2delta subunit.

Activation of neuropeptide Y Y1 receptors inhibits glutamate release through reduction of voltage-dependent Ca2+ entry in the rat cerebral cortex nerve terminals: Suppression of this inhibitory effect by the protein kinase C-dependent facilitatory pathway

Neuropeptide Y (NPY) is known to regulate the presynaptic glutamate release and neuronal responses to excitatory neurotransmission. The aim of this study was to investigate the effect of NPY on the release of endogenous glutamate from rat cerebrocortical nerve terminals (synaptosomes). NPY inhibited the Ca2+-dependent glutamate release evoked by 4-aminopyridine, and this inhibitory effect was mediated via NPY Y1 receptors, because it was mimicked by the specific NPY Y1 receptor agonist [Leu31 Pro34] NPY and blocked by the NPY Y1 receptor antagonist GR 231118. The inhibitory action of NPY was not due to it decreasing synaptosomal excitability or directly interfering with the release process at some point subsequent to Ca2+ influx, because NPY did not alter the 4-aminopyridine-evoked depolarization of the synaptosomal plasma membrane potential or ionomycin and hypertonic solution-induced glutamate release. Examination of the effect of NPY on the cytosolic [Ca2+] revealed that the inhibition of glutamate release could be attributed to a reduction in voltage-dependent Ca2+ influx. Consistent with this, the NPY-mediated inhibition of glutamate release was completely abolished in synaptosomes pretreated with N- and P/Q-type Ca2+ channel blocker, omega-conotoxin MVIIC. Moreover, NPY-mediated inhibition of 4-aminopyridine-evoked glutamate release was insensitive to KT 5720 and Ro32-0432 but was suppressed when protein kinase C was stimulated with phorbol ester. Together, these results suggest that NPY acting predominantly on NPY Y1 receptors inhibits glutamate release from rat cerebrocortical synaptosomes, likely by a mechanism involving direct coupling of receptors to N- and P/Q-type Ca2+ channels, and this coupling is subject to regulation by protein kinase C-dependent pathway. This implies that selective ligand for NPY receptors may be of value for treatment of conditions characterized by excessive glutamate release in the cerebral cortex.

Mediators of non-adrenergic non-cholinergic inhibitory neurotransmission in porcine jejunum

The purpose of this study was to determine the non-adrenergic non-cholinergic inhibitory neurotransmitter in pig jejunum. Intracellular electrical activity was recorded from circular smooth muscle cells. Inhibitory junction potentials (IJPs) evoked by electrical field stimulation were inhibited by tetrodotoxin (1 micromol L(-1)), omega-conotoxin GVIA (0.1 micromol L(-1)) tetrodotoxin, apamin (1 micromol L(-1)), 1-[6-((17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl]-1H-pyrrole-2,5-dione (U-73122; 10 micromol L(-1)) but not by N omega-nitro-l-arginine (l-NNA; 100 micromol L(-1)), haemoglobin (10 micromol L(-1)), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 10 micromol L(-1)) or 9-(tetrahydro-2-furyl)adenine (SQ-22536; 10 micromol L(-1)). S-nitroso-N-acetylpenicillamine (SNAP) hyperpolarized the membrane potential. This was inhibited by ODQ (3 micromol L(-1)) and charybdotoxin (0.1 micromol L(-1)). Adenosine-5-triphosphate (ATP; 100 micromol L(-1)) and 2-methylthio ATP (2-MeS-ATP; 100 micromol L(-1)) did not hyperpolarize the membrane potential and 6-N-N-diethyl-beta- gamma -dibromomethylene-d-adenosine-5'-triphosphate (ARL67156; 100 micromol L(-1)) did not modify IJPs. Carbon monoxide (CO; 10%) and tricarbonyl dichlororuthenium dimer ([Ru(CO3Cl2)]2; 100 micromol L(-1)) hyperpolarized the membrane potential however zinc, copper and tin protoporphyrin IX (100 micromol L(-1)) did not alter IJPs. Vasoactive intestinal peptide (VIP) hyperpolarized the membrane potential but 4-Cl-d-Phe6-Leu17-VIP (1 micromol L(-1)) did not modify IJPs. Pituitary adenylate cyclase activating peptide (PACAP)38 (0.5 micromol L(-1)) hyperpolarized the membrane potential. This was inhibited by apamin (1 micromol L(-1)) but not by tetrodotoxin (1 micromol L(-1)). Pituitary adenylate cyclase activating peptide6-38 (1 micromol L(-1)) inhibited IJPs. These data suggest that inhibitory neurotransmission in pig jejunum is mediated partly by PACAP.

Testosterone enhances calcium reabsorption by the kidney

The kidney is a target tissue for androgens, but the role of these hormones in the regulation of calcium (Ca2+) reabsorption remains unclear. The present study examines the effects of testosterone on Ca2+ transport by the luminal membranes of proximal and distal nephrons of the rabbit kidney. Tubule suspensions were pre-incubated in the presence or absence of the hormone, and 45C2+ uptake by the luminal membranes was measured using the rapid filtration technique. In the proximal tubules, testosterone did not influence Ca2+ uptake. In the distal tubules, a 5 min incubation with the hormone increased this uptake with a maximal response at 10(-10)M. Ca2+ transport by the distal membranes shows a dual kinetics. Testosterone enhanced the Vmax value of the low affinity component. In an attempt to identify the underlying mechanisms involved in this action, several messenger inhibitors were introduced in the tubule suspension. PD 98059 and U0 126 as well as AG 99 and genistein interfered with the hormone action suggesting the implication of a MEK kinase and a tyrosine kinase. To determine the type of the channels involved in this effect, Ca2+ uptake was measured in the presence of diltiazem, omega-conotoxin MVIIC and mibefradil, i.e. selective inhibitors of the L-type, P/Q type and T-type channels. An inhibition of Ca2+ transport was observed exclusively with mibefradil. These results indicate that testosterone enhances Ca2+ transport by opening a T-type Ca2+ channel in the distal luminal membrane, via MEK kinase and tyrosine kinase dependent mechanisms.

Depolarization-induced calcium influx in rat mesenteric small arterioles is mediated exclusively via mibefradil-sensitive calcium channels

1. In this study, intracellular Ca(2+) was measured as the Fura-2 ratio (R) of fluorescence excited at 340 and 380 nm (F(340)/F(380)) in nonpressurized rat mesenteric small arterioles ( (lumen diameter) 10-25 microm). 2. The response to depolarization using 75 mm KCl was an increase in R from a baseline of 0.96+/-0.01 ([Ca(2+)](i) approximately 74 nm) to 1.04+/-0.01 ( approximately 128 nm) (n=80). The response to 75 mm K(+) was reversibly abolished in Ca(2+)-free physiological saline solution, whereas phentolamine (10 microm) or tetrodotoxin (1 microm) had no effects. LaCl(3) (200 microm) inhibited 61+/-9% of the response. 3. A [K(+)]-response curve indicated that the Ca(2+) response was activated between 15 and 25 mm K(+). The data suggest that the Ca(2+) response was caused by the activation of voltage-dependent Ca(2+) channels. 4. Mibefradil use dependently inhibited the Ca(2+) response to 75 mm K(+) by 29+/-2% (100 nm), 73+/-7% (1 microm) or 89+/-7% (10 microm). Pimozide (500 nm) use dependently inhibited the Ca(2+) response by 85+/-1%. 5. Nifedipine (1 microm) inhibited the Ca(2+) response to 75 mm K(+) by 41+/-12%. The response was not inhibited by calciseptine (500 nm), omega-agatoxin IVA (100 nm), omega-conotoxin MVIIA (500 nm), or SNX-482 (100 nm). 6. Using reverse transcriptase-polymerase chain reaction, it was shown that neither Ca(V)2.1a (P-type) nor Ca(V)2.1b (Q-type) voltage-dependent Ca(2+) channels were expressed in mesenteric arterioles, whereas the Ca(V)3.1 (T-type) channel was expressed. Furthermore, no amplification products were detected when using specific primers for the beta(1b), beta(2), or beta(3) auxiliary subunits of high-voltage-activated Ca(2+) channels. 7. The results suggest that the voltage-dependent Ca(2+) channel activated by sustained depolarization in mesenteric arterioles does not classify as any of the high-voltage-activated channels (L-, P/Q-, N-, or R-type), but is likely to be a T-type channel. The possibility that the sustained Ca(2+) influx observed was the result of a T-type window current is discussed.

Regulation of calcium signalling by 1-O-alkylglycerols in human Jurkat T lymphocytes

We studied the role of natural occurring 1-O-alkylglycerols on the calcium signalling in Jurkat T-cells. Alkylglycerols evoked an increase in free intracellular calcium concentration [Ca2+]i, in a dose-dependent manner. When the experiments were performed in calcium-free buffer, the alkylglycerol response on the rise of [Ca2+]i was wholly abolished compared with the one in calcium-containing buffer, suggesting that these etherlipids induce a calcium influx by the opening of Ca2+ channels. We further employed inhibitors of voltage-gated calcium channels. We observed that omega-conotoxin, a blocker of N-type voltage-activated Ca2+ channels, but not verapamil, a blocker of L-type voltage-activated Ca2+ channels, curtailed significantly the calcium rise evoked by the lipid agents. Alkylglycerols also induced plasma membrane depolarisation, known to be involved in the opening of the voltage-gated calcium channels. Our study shows that alkylglycerols increase [Ca2+]i influx in human Jurkat T-cells possibly by modulating the permeability of calcium channels.

Effects of Ca2+ channel antagonists on acetylcholine and catecholamine releases in the in vivo rat adrenal medulla

To elucidate the types of voltage-dependent Ca(2+) channels controlling ACh and catecholamine releases in the in vivo adrenal medulla, we implanted microdialysis probes in the left adrenal medulla of anesthetized rats and investigated the effects of Ca(2+) channel antagonists on ACh, norepinephrine, and epinephrine releases induced by nerve stimulation. The dialysis probes were perfused with Ringer solution containing a cholinesterase inhibitor, neostigmine. The left splanchnic nerves were electrically stimulated at 2 and 4 Hz before and after intravenous administration of Ca(2+) channel antagonists. omega-Conotoxin GVIA (an N-type Ca(2+) channel antagonist, 10 microg/kg) inhibited ACh release at 2 and 4 Hz by approximately 40%, norepinephrine release at 4 Hz by approximately 50%, and epinephrine release at 2 and 4 Hz by approximately 45%. A fivefold higher dose of omega-conotoxin GVIA (50 microg/kg) did not further inhibit these releases. omega-Conotoxin MVIIC (a P/Q-type Ca(2+) channel antagonist, 50 microg/kg) inhibited ACh and epinephrine releases at 4 Hz by approximately 30%. Combined omega-conotoxin GVIA (50 microg/kg) and MVIIC (250 microg/kg) inhibited ACh release at 2 and 4 Hz by approximately 70% and norepinephrine and epinephrine releases at 2 and 4 Hz by approximately 80%. Nifedipine (an L-type Ca(2+) channel antagonist, 300 and 900 microg/kg) did not change ACh release at 2 and 4 Hz; however, nifedipine (300 microg/kg) inhibited epinephrine release at 4 Hz by 20%, and nifedipine (900 microg/kg) inhibited norepinephrine and epinephrine releases at 4 Hz by 30%. In conclusion, both N- and P/Q-type Ca(2+) channels control ACh release on preganglionic splanchnic nerve endings while L-type Ca(2+) channels do not. L-type Ca(2+) channels are involved in norepinephrine and epinephrine releases on chromaffin cells.

Differential involvement of N-type calcium channels in transmitter release from vasoconstrictor and vasodilator neurons

1. The effects of calcium channel blockers on co-transmission from different populations of autonomic vasomotor neurons were studied on isolated segments of uterine artery and vena cava from guinea-pigs. 2. Sympathetic, noradrenergic contractions of the uterine artery (produced by 200 pulses at 1 or 10 Hz; 600 pulses at 20 Hz) were abolished by the N-type calcium channel blocker omega-conotoxin (CTX) GVIA at 1-10 nm. 3. Biphasic sympathetic contractions of the vena cava (600 pulses at 20 Hz) mediated by noradrenaline and neuropeptide Y were abolished by 10 nm CTX GVIA. 4. Neurogenic relaxations of the uterine artery (200 pulses at 10 Hz) mediated by neuronal nitric oxide and neuropeptides were reduced <50% by CTX GVIA 10-100 nm. 5. Capsaicin (3 microm) did not affect the CTX GVIA-sensitive or CTX GVIA-resistant neurogenic relaxations of the uterine artery. 6. The novel N-type blocker CTX CVID (100-300 nm), P/Q-type blockers agatoxin IVA (10-100 nm) or CTX CVIB (100 nm), the L-type blocker nifedipine (10 microm) or the 'R-type' blocker SNX-482 (100 nm), all failed to reduce CTX GVIA-resistant relaxations. The T-type channel blocker NiCl(2) (100-300 microm) reduced but did not abolish the remaining neurogenic dilations. 7. Release of different neurotransmitters from the same autonomic vasomotor axon depends on similar subtypes of calcium channels. N-type channels are responsible for transmitter release from vasoconstrictor neurons innervating a muscular artery and capacitance vein, but only partly mediate release of nitric oxide and neuropeptides from pelvic vasodilator neurons.

Cystine/glutamate exchange serves as the source for extracellular glutamate: modifications by repeated cocaine administration

Repeated administration of cocaine lowers the basal extracellular levels of glutamate in the nucleus accumbens as measured by microdialysis. The studies presented reveal that this long-term neuroadaptation elicited by repeated cocaine results from a decrease in the activity of cystine/glutamate exchange.

Role of Lipid Microdomains in P/Q-type Calcium Channel (Cav2.1) Clustering and Function in Presynaptic Membranes

Lipid microdomains can selectively include or exclude proteins and may be important in a variety of functions such as protein sorting, cell signaling, and synaptic transmission. The present study demonstrates that two different voltage-gated calcium channels, which both interact with soluble N-ethyl-maleimide-sensitive fusion protein attachment protein receptor (SNARE) proteins but have distinct subcellular distributions and roles in synaptic transmission, are differently distributed in lipid microdomains; presynaptic P/Q (Cav2.1) but not Lc (Cav1.2) calcium channel subtypes are mainly accumulated in detergent-insoluble complexes. The immunoisolation of multiprotein complexes from detergent-insoluble or detergent-soluble fractions shows that the alpha1A subunits of Cav2.1 colocalize and interact with SNARE complexes in lipid microdomains. The altered organization of these microdomains caused by saponin and methyl-beta-cyclodextrin treatment largely impairs the buoyancy and distribution of Cav2.1 channels and SNAREs in flotation gradients. On the other hand, cholesterol reloading partially reverses the drug effects. Methyl-beta-cyclodextrin treatment alters the colocalization of Cav2.1 with the proteins of the exocytic machinery and also impairs calcium influx in nerve terminals. These results show that lipid microdomains in presynaptic terminals are important in organizing membrane sites specialized for synaptic vesicle exocytosis. The cholesterol-enriched microdomains contribute to optimizing the compartmentalization of exocytic machinery and the calcium influx that triggers synaptic vesicle exocytosis.

Appearance of voltage-gated calcium channels following overexpression of ATPase II cDNA in neuronal HN2 cells

ATPase II (a Mg2+-ATPase) is also believed to harbor aminophospholipid translocase (APTL) activity, which is responsible for the translocation of phosphatidylserine (PS) from the outer leaflet of the plasma membrane to the inner. To test this hypothesis we overexpressed the mouse ATPase II cDNA in the neuronal HN2 cells. In addition to a dramatic increase in APTL activity, we also made the unexpected observation that expression of the mouse ATPase II cDNA from the vector pCMV6 resulted in the appearance of calcium current. Although the hybrid cell line HN2 or a line (HN2V32) obtained by expressing a heterologous gene from the same expression vector showed no calcium current, both ATPase II-overexpressing clones (HN2A12 and HN2A22) showed significant barium conductance. This current was due to calcium channels because it was blocked almost completely by 100 microM CdCl2 and it had a significant N-type component since it was blocked by 38.5% in the presence of 5 microM omega-conotoxin (omega-CTX). Western blot analysis using an antibody against the N-type calcium-channel alpha1B subunit revealed a dramatic increase in expression of this protein in the HN2A12 and HN2A22 cell lines. Our results suggest that ATPase II also harbors APTL activity. In view of the prior knowledge that APTL activity is inhibited by an increase in calcium, our results also suggest that APTL expression exerts a negative feedback regulation on itself by inducing expression of channels that cause an influx of calcium ions. The mechanism of this regulation could reveal important information on a possible cross-regulation between these two families of proteins in neuronal cells.

Calcium current subtypes in GnRH neurons

Calcium plays roles in excitability, rhythm generation, and neurosecretion. Identifying channel subtypes that regulate calcium influx is thus important to understanding rhythmic GnRH secretion, which is a prerequisite for reproduction. Whole-cell voltage-clamp recordings were made from short-term dissociated GnRH adult ovariectomized (OVX) mice (n = 21) to identify channel subtypes that carry calcium current using selective channel blockers and voltage characteristics. Low-voltage activated (LVA) currents were not observed in 42 GnRH neurons tested, although most non-GnRH neurons (4/6) displayed LVA current. The L-type component of the high-voltage activated (HVA) calcium current was 25% +/- 2%. The remaining HVA calcium current passed through N-type (27% +/- 3%), P-type (15% +/- 1%), Q-type (18% +/- 3%), and R-type (15% +/- 1%) channels. Because these data differ substantially from reports on cultured GnRH neurons, which may represent reproductively immature models, we also examined GnRH neurons from gonadal-intact young (Postnatal Days 4-10, n = 8 mice) mice. LVA currents were still rare (2/28) in young mice. Although the same HVA components were observed, the proportions were shifted toward significantly more L-type and less N-type current, suggesting a possible developmental shift in calcium currents in GnRH neurons. These data suggest that calcium channel subtypes in GnRH neurons prepared in the short term from brain slices differ substantially from those in long-term cultured GnRH models. These findings provide a vital foundation to examine the role of calcium channels in the secretory and rhythmic machinery of GnRH neurons.

Glucose-Deprivation-Induced [3H]D-Aspartate Release from Isolated Bovine and Human Retinae

The glucose deprivation-induced release of [3H]D-aspartate was studied in bovine and human retinas in a superfusion apparatus. [3H]D-aspartate release was significantly increased upon omitting glucose in the superfusion buffer. This effect was dependent on external Ca2+ because L- and N-type Ca2+-channel blockers, such as diltiazem (1 microM), nitrendipine (1 microM), and omega-conotoxin (100 nM), significantly reduced the effect of glucose-deprivation induced release of [3H]D-aspartate. Furthermore, while glutamate receptor agonists (L-glutamate, N-methyl-D-aspartate, but not kainate) potentiated the effects of glucose deprivation, antagonists (MK-801, MCPG, ifenprodil, and L-AP3) at these receptors blocked the glucose deprivation-induced release process. Taken together, these studies have demonstrated that under conditions of glucose deprivation, as may happen during ischemic events in vivo, the retinal glutamatergic nerve endings and/or glial cells promote the efflux of [3H]D-aspartate into the extracellular environment. This process appears to be receptor-mediated and dependent on extracellular Ca2+ and is similar to previous reports pertaining to brain tissues.