Pharmacological identification of a novel Ca2+ channel in chicken brain synaptosomes

Zebrafish calls for reinterpretation for the roles of P/Q calcium channels in neuromuscular transmission

A long-held tenet of neuromuscular transmission is that calcium-dependent neurotransmitter release is mediated by N-type calcium channels in frog but P/Q-type channels in mammals. The N-type assignment in frog is based principally on pharmacological sensitivity to ω-conotoxin GVIA. Our studies show that zebrafish neuromuscular transmission is also sensitive to ω-conotoxin GVIA. However, positional cloning of a mutant line with compromised neuromuscular function identified a mutation in a P/Q- rather than N-type channel. Cloning and heterologous expression of this P/Q-type channel confirmed a block by ω-conotoxin GVIA raising the likelihood that all vertebrates, including frog, use the P/Q-type calcium channel for neuromuscular transmission. In addition, our P/Q defective mutant line offered a means of testing the ability of roscovitine, known to potentiate frog neuromuscular transmission, to mediate behavioral and functional rescue. Acute treatment led to rapid improvement of both, pointing to potential therapeutic benefit for myasthenic disorders involving calcium channel dysfunction.

Emerging knowledge of regulatory roles of D-amino acids in bacteria

The d-enantiomers of amino acids have been thought to have relatively minor functions in biological processes. While l-amino acids clearly predominate in nature, d-amino acids are sometimes found in proteins that are not synthesized by ribosomes, and d-Ala and d-Glu are routinely found in the peptidoglycan cell wall of bacteria. Here, we review recent findings showing that d-amino acids have previously unappreciated regulatory roles in the bacterial kingdom. Many diverse bacterial phyla synthesize and release d-amino acids, including d-Met and d-Leu, which were not previously known to be made. These noncanonical d-amino acids regulate cell wall remodeling in stationary phase and cause biofilm dispersal in aging bacterial communities. Elucidating the mechanisms by which d-amino acids govern cell wall remodeling and biofilm disassembly will undoubtedly reveal new paradigms for understanding how extracytoplasmic processes are regulated as well as lead to development of novel therapeutics.

Stimulation of Ca(2+) influx through ATP receptors on rat brain synaptosomes: identification of functional P2X(7) receptor subtypes

1. ATP receptors of the P2X class have previously been identified on autonomic nerve endings and on a limited population of CNS neurons. 2. In the present study P2X receptors on mammalian cortical synaptosomes have been identified by a variety of functional and biochemical studies. In choline buffer ATP analogues caused concentration/time dependent Ca(2+) influx. Relative to the effects caused by ATP, benzoylbenzoyl ATP (BzATP) was about seven times more active than ATP while 2-me-S-ATP and ATPgammaS were much less active. alpha,beta-me- ATP and beta,gamma-me-ATP were virtually inactive. In sucrose buffer, relative to choline buffer, the activity of BzATP was more than doubled while activity in sodium buffer was reduced. Moreover, the P2X antagonists PPADS or Brilliant Blue G both significantly attenuated influx. These observations suggest the presence of P2X receptors on synaptosomes which subserve Ca(2+) influx. This activity profile of the ATP analogues and the response to blocking agents are characteristic of responses of P2X(7) receptors. 3. Influx was unaffected by the VSCC inhibitors omega-CTx-MVIIC and (-) 202 - 791, indicating that ATP induced Ca(2+) influx occurred primarily through P2X receptors. 4. P2X(7) receptor protein was identified by Western blotting and immunohistochemical staining. Purified preparations were devoid of significant concentrations of GFAP or the microglial marker OX-42 but contained greatly enriched amounts of syntaxin and SNAP 25. 5. The various pharmacological and biochemical studies were all consistent with the presence of functional P2X(7) receptors.

Role of RyRs and IP3 receptors after traumatic injury to spinal cord white matter

Calcium influx and elevation of intracellular free calcium (Ca2+i), with subsequent activation of degenerative enzymes is hypothesized to cause cell injury and death after trauma. We examined the effects of traumatic compressive injury on (Ca2+)i dynamics in spinal cord white matter. We conducted electrophysiological studies with ryanodine and inositol (1,4,5)-triphosphate (IP3) receptor agonists and antagonists in an in vitro model of spinal cord injury (SCI). A 25-30-mm length of dorsal column was isolated from the spinal cord of adult rats, pinned in an in vitro recording chamber (37 degrees C) and injured with a modified clip (2-g closing force) for 15 sec. The functional integrity of the dorsal column was monitored electrophysiologically by quantitatively measuring the compound action potential (CAP) with glass microelectrodes. The CAP decreased to 55.2+/-6.8% of control (p < 0.05) after spinal cord injury (SCI). Chelation of Ca2+i with BAPTA-AM (a high-affinity calcium chelator) promoted significantly greater recovery of CAP amplitude (83.2+/-4.2% of control; p < 0.05) after injury. Infusion of caffeine (1 and 10 mM) exacerbated CAP amplitude decline (45.1+/-5.9% of control; p < 0.05; 44.6+/-3.1% of control; p < 0.05) postinjury. Blockade of Ca2+i release through ryanodine-sensitive receptors (RyRs) with dantrolene (10 microM) and ryanodine (50 microM), conferred significant (p < 0.05) improvement in CAP amplitude after injury. On the other hand, blockade of Ca2+i with inositol (1,4,5)-triphosphate receptor (IP3Rs) blocker 2APB (10 microM) also conferred significant improvement in CAP amplitude after injury (82.9+/-7.9%; p < 0.05). In conclusion, the injurious effects of Ca2+i in traumatic central nervous system (CNS) white matter injury appear to be mediated both by RyRs and through IP3Rs calcium-induced calcium release receptors (CICRs).

Role of L- and N-type calcium channels in the pathophysiology of traumatic spinal cord white matter injury

Recent work has suggested a potential role for voltage-gated Ca(2+) channels in the pathophysiology of anoxic central nervous system white matter injury. To examine the relevance of these findings to neurotrauma, we conducted electrophysiological studies with inorganic Ca(2+) channels blockers and L- and N-subtype-specific calcium channel antagonists in an in vitro model of spinal cord injury. Confocal immunohistochemistry was used to examine for localization of L- and N-type calcium channels in spinal cord white matter tracts. A 30-mm length of dorsal column was isolated from the spinal cord of adult rats, pinned in an in vitro recording chamber and injured with a modified clip (2g closing force) for 15s. The functional integrity of the dorsal column was monitored electrophysiologically by quantitatively measuring the compound action potential at two points with glass microelectrodes. The compound action potential decreased to 71.4+/-2.0% of control (P<0. 05) after spinal cord injury. Removal of extracellular Ca(2+) promoted significantly greater recovery of compound action potential amplitude (86.3+/-7.6% of control; P< 0.05) after injury. Partial blockade of voltage-gated Ca(2+) channels with cobalt (20 microM) or cadmium (200 microM) conferred improvement in compound action potential amplitude. Application of the L-type Ca(2+) channel blockers diltiazem (50 microM) or verapamil (90 microM), and the N-type antagonist omega-conotoxin GVIA (1 microM), significantly enhanced the recovery of compound action potential amplitude postinjury. Co-application of the L-type antagonist diltiazem with the N-type blocker omega-conotoxin GVIA showed significantly greater (P<0.05) improvement in compound action potential amplitude than application of either drug alone. Confocal immunohistochemistry with double labelling for glial fibrillary acidic protein, GalC and NF200 demonstrated L- and N-type Ca(2+) channels on astrocytes and oligodendrocytes, but not axons, in spinal cord white matter. In conclusion, the injurious effects of Ca(2+) in traumatic central nervous system white matter injury appear to be partially mediated by voltage-gated Ca(2+) channels. The presence of L- and N-type Ca(2+) channels on periaxonal astrocytes and oligodendrocytes suggests a role for these cells in post-traumatic axonal conduction failure.

Control of glutamate release by calcium channels and kappa-opioid receptors in rodent and primate striatum

The modulation of depolarization (4-aminopyridine, 2 mM)-evoked endogenous glutamate release by kappa-opioid receptor activation and blockade of voltage-dependent Ca2+ -channels has been investigated in synaptosomes prepared from rat and marmoset striatum. 4-Aminopyridine (4-AP)-stimulated, Ca2+ -dependent glutamate release was inhibited by enadoline, a selective kappa-opioid receptor agonist, in a concentration-dependent and norbinaltorphimine (nor-BNI, selective kappa-opioid receptor antagonist)-sensitive manner in rat (IC50 = 4.4+/-0.4 microM) and marmoset (IC50 = 2.9+/-0.7 microM) striatal synaptosomes. However, in the marmoset, there was a significant (approximately 23%) nor-BNI-insensitive component. In rat striatal synaptosomes, the Ca2+ -channel antagonists omega-agatoxin-IVA (P/Q-type blocker), omega-conotoxin-MVIIC (N/P/Q-type blocker) and omega-conotoxin-GVIA (N-type blocker) reduced 4-AP-stimulated, Ca2+ -dependent glutamate release in a concentration-dependent manner with IC50 values of 6.5+/-0.9 nM, 75.5+5.9 nM and 106.5+/-8.7 nM, respectively. In marmoset striatal synaptosomes, 4-AP-stimulated, Ca2+ -dependent glutamate release was significantly inhibited by omega-agatoxin-IVA (30 nM, 57.6+/-2.3%, inhibition), omega-conotoxin-MVIIC (300 nM, 57.8+/-3.1%) and omega-conotoxin-GVIA (1 microM, 56.7+/-2%). Studies utilizing combinations of Ca2+ -channel antagonists suggests that in the rat striatum, two relatively distinct pools of glutamate, released by activation of either P or Q-type Ca2+ -channels, exist. In contrast, in the primate there is much overlap between the glutamate released by P and Q-type Ca2+ -channel activation. Studies using combinations of enadoline and the Ca2+ -channel antagonists suggest that enadoline-induced inhibition of glutamate release occurs primarily via reduction of Ca2+ -influx through P-type Ca2+ -channels in the rat but via N-type Ca2+ -channels in the marmoset. In conclusion, the results presented suggest that there are species differences in the control of glutamate release by kappa-opioid receptors and Ca2+ -channels.

Electrophysiological characterisation of the human N-type Ca2+ channel III: pH-dependent inhibition by a synthetic macrocyclic polyamine

The effects of a novel synthetic macrocyclic polyamine (LY310315) were investigated on recombinant human N-type Ca2+ channels stabley expressed in HEK293 cells. LY310315 proved to be a potent and reversible N-type Ca2+ channel antagonist. Inhibition by this compound was dose-dependent with an IC50 of approximately 0.4 microM at pH 7.35. LY310315 blocked very rapidly at all concentrations tested. Upon washout, recovery of the Ca2+ current developed with a time constant of approximately 30 s. Use-dependence in the development of block indicated that voltage-dependent transitions in the channel protein were required to permit significant inhibition. Application of > 100 times the IC50 dose of LY310315 to the interior of the cell produced no detectable Ca2+ current inhibition. LY310315 had no effects on the kinetics of channel activation or deactivation but did slightly slow the rate of macroscopic inactivation observed during a 300 ms test depolarisation. In the presence of LY310315 the activation curve was significantly shallower. This resulted in a shift in the activation midpoint voltage to a more depolarised levels. LY310315-induced inhibition of human N-type channels was strongly dependent on the extracellular pH, with increased potency seen upon extracellular acidification. Although most effective against N-type Ca2+ channels, LY310315 was also found to inhibit both P-type and L-type Ca2+ channels. LY310315 proved to be a weak blocker of Na+ currents, but produced approximately 50% of the K+ currents of AtT20 cells at a concentration of 0.5 microM.

Chick cerebellar Purkinje cells express omega-conotoxin GVIA-sensitive rather than funnel-web spider toxin-sensitive calcium channels

Voltage-gated calcium channels form a complex family of distinct molecular entities which participate in multiple neuronal functions. In cerebellar Purkinje cells these channels contribute to the characteristic electrophysiological pattern of complex spikes, first described in birds and later in mammals. A specific calcium channel, the P-type channel, has been shown to mediate the majority of the voltage-gated calcium flux in mammalian Purkinje cells. P-type channels play an essential role in synaptic transmission of mammalian cerebellum. It is unclear whether the P-type calcium channel is present in birds. Studies in chick synaptosomal preparations show that the pharmacological profile of calcium channels is complex and suggest a minimal expression of the P-type channel in avian central nervous system. In the present work, we studied voltage-gated calcium channels in dissociated chick cerebellar Purkinje cells to examine the presence of different calcium channel types. Purkinje cells were used because, in mammals, they express predominantly P-type channels and because the morphology of these cells is thought to be phylogenetically conserved. We found that omega-conotoxin GVIA (omega-CgTx GVIA), a specific antagonist of N-type calcium channel, rather than the synthetic funnel-web spider toxin (sFTX), a P-type channel antagonist, blocks the majority of the barium current flowing through calcium channels in chick Purkinje neurons.

Distinct omega-agatoxin-sensitive calcium currents in somata and axon terminals of rat supraoptic neurones

1. Voltage-dependent calcium currents were measured at room temperature using whole-cell patch clamp in acutely isolated somata and axon terminals of the magnocellular neurosecretory cells (MNCs) from the rat supraoptic nucleus. 2. Administration of omega-agatoxin IVA (omega-Aga IVA) blocked a high-threshold non-inactivating current. This current has an IC50 for omega-Aga IVA of 3 nM; no other types of currents were blocked at doses of up to 500 nM. 3. In the axon terminals omega-Aga IVA blocked a high-threshold current that inactivates markedly (tau approximately 448 ms), and has a much lower sensitivity to the toxin, with an IC50 of 270 nM. Unlike the somatic current, the effect of omega-Aga IVA in the terminals is largely prevented by omega-conotoxin GVIA (omega-CgTX). 4. These data suggest that MNC somata express a single type of omega-Aga IVA-sensitive calcium current similar to the P-type calcium current described in other cells. However, the omega-Aga IVA-sensitive current in axon terminals differs from both the P-type and the recently identified Q-type current in that it is also sensitive to omega-CgTX. The distinct biophysical properties of the currents in somata and axon terminals may have important physiological implications.

Comparative actions of synthetic omega-grammotoxin SIA and synthetic omega-Aga-IVA on neuronal calcium entry and evoked release of neurotransmitters in vitro and in vivo

The effects of synthetic omega-grammotoxin SIA (omega-GsTxSIA) and synthetic omega-Aga-IVA were tested in in vitro and in vivo neurochemical assays that are reflective of voltage-sensitive calcium channel function. Synthetic omega-GsTx SIA inhibited K(+)-evoked rat and chick synaptosomal 45Ca2+ flux, K(+)-evoked release of [3H]D-aspartate and [3H]norepinephrine from rat hippocampal brain slices and K(+)-evoked release of [3H]norepinephrine from chick cortical brain slices with potency values that were comparable to those found previously with omega-GsTx SIA purified from the venom of the tarantula spider Grammostola spatulata. These results indicate that trace contaminants do not account for the pharmacology of purified omega-GsTx SIA. omega-GsTx SIA caused a complete inhibition of rat synaptosomal 45Ca2+ flux and hippocampal slice [3H]D-aspartate release, whereas omega-Aga-IVA caused a maximal inhibition of approx 75%. omega-GsTx SIA and omega-Aga-IVA caused an identical partial inhibition of K(+)-evoked increases of intracellular calcium in cortical neurons in primary culture. The addition of nitrendipine to either omega-GsTx SIA or omega-Aga-IVA resulted in an additive and virtually complete inhibition of the cortical neuron intracellular calcium response. In in vivo microdialysis studies, the K(+)-evoked release of glutamate from hippocampus of awake freely moving rats was inhibited with the following rank order of potency: omega-conotoxin GVIA > omega-GsTx SIA > omega-Aga-IVA. Complete inhibition of K(+)-evoked hippocampal glutamate release was observed with 300 nM omega-conotoxin GVIA and 3 microM omega-GsTx SIA. In urethane anesthetized rats, omega-CgTx GVIA caused a partial inhibition, whereas omega-GsTx SIA caused a concentration-dependent and complete inhibition, of basal serotonin release in the hippocampus. Therefore, omega-GsTx SIA was shown to inhibit responses that are sensitive to omega-conotoxin GVIA, omega-Aga-IVA and omega-conotoxin MVIIC, consistent with the notion that omega-GsTx SIA inhibits N-, P- and Q-type high threshold voltage-sensitive calcium channels.

Effects of Ca2+ channel blockers on transmitter release and presynaptic currents at the frog neuromuscular junction

1. The effects of the calcium channel blockers, funnel-web spider toxin (FTX), omega-agatoxin IVA (omega-Aga IVA) and omega-conotoxin GVIA (omega-CgTX), were tested on transmitter release and presynaptic currents in frog motor nerve endings. 2. Evoked transmitter release was blocked by FTX (IC50 = 0.02 microliter ml-1) and omega-CgTX (1 microM) but was not affected by omega-Aga IVA (0.5 microM). When FTX (0.1 microliter ml-1) was assayed on spontaneous release either in normal Ringer solution or in low Ca(2+)-high Mg2+ solution, it was found not to affect miniature endplate potential (MEPP) amplitude but to increase MEPP frequency by approximately 2-fold in both conditions. 3. Presynaptic calcium currents (ICa), measured by the perineurial technique in the presence of 10 mM tetraethylammonium chloride (TEA) and 200 microM BaCl2 to block K+ currents, were blocked by omega-CgTX (5 microM), partially blocked by FTX (1 microliter ml-1) and not affected by omega-Aga IVA (0.5 microM). 4. The presynaptic calcium-activated potassium current (IK(Ca)) measured by the perineurial technique in the presence of 0.5 microM 3,4-aminopyridine (DAP) to block voltage-dependent K+ currents, was strongly affected by charybdotoxin (ChTX) (300 nM) and completely abolished by BaCl2 (200 microM). This current was also blocked by omega-CgTX (5 microM) and by CdCl2 (200 microM) but was not affected by FTX (1 microliter ml-1). The blockade by omega-CgTX could not be reversed by elevating [Ca]o to 10 mM. 5. The results suggest that in frog synaptic terminals two omega-CgTX-sensitive populations might coexist. The transmitter release process seems to be mediated by calcium influx through a omega-CgTX- and FTX-sensitive population.

Extracellular and intracellular effects of polyamines on smooth muscle contractions

The natural polyamines spermine, spermidine and putrescine, and their metabolic products N1-acetylspermine (N-AS) and 5'-deoxi-5'-methyl-thioadenosine (DMT), but not N1-acetylspermidine nor N-acetylputrescine, relax the KCl-induced contraction in rat uterus in a dose-dependent way. This relaxing effect is counteracted by CaCl2 (0.1-6 mM) but not by Bay K 8644. Spermine, N-AS and DMT also inhibit the contraction induced by methacholine and PGF2 alpha. Spermine, DMT and N-AS relax the vanadate-induced contraction in uterus incubated in calcium-free solution plus EDTA. However, in the vanadate contraction, the EC50s are higher than those obtained with other contracturants. These results suggest that polyamines inhibit smooth muscle contraction by action at plasma membrane level decreasing the influx of calcium. However, intracellular actions of polyamines could also be involved in their effects.