Different effects of omega-conotoxin GVIA at excitatory and inhibitory synapses in rat CA1 hippocampal neurons

Hydrogen peroxide modulates synaptic transmission in ventral horn neurons of the rat spinal cord

KEY POINTS

Excessive production of reactive oxygen species (ROS) is implicated in many central nervous system disorders; however, the physiological role of ROS in spinal ventral horn (VH) neurons remains poorly understood. We investigated how pathological levels of H2O2, an abundant ROS, regulate synaptic transmission in VH neurons of rats using a whole-cell patch clamp approach. H2O2 increased the release of glutamate and GABA from presynaptic terminals. The increase in glutamate release involved N-type voltage-gated calcium channels (VGCCs), ryanodine receptors (RyRs), and inositol trisphosphate receptors (IP3 Rs); the increase in GABA release, which inhibited glutamatergic transmission, involved IP3 R. Inhibiting N-type VGCCs and RyRs attenuates excitotoxicity resulting from increased glutamatergic activity while preserving the neuroprotective effects of GABA, and may represent a novel strategy for treating H2O2-induced motor neuron disorders resulting from trauma or ischaemia-reperfusion injury. Excessive production of reactive oxygen species (ROS) is a critical component of the cellular and molecular pathophysiology of many central nervous system (CNS) disorders, including trauma, ischaemia-reperfusion injury, and neurodegenerative diseases. Hydrogen peroxide (H2O2), an abundant ROS, modulates synaptic transmission and contributes to neuronal damage in the CNS; however, the pathophysiological role of H2O2 in spinal cord ventral horn (VH) neurons remains poorly understood, despite reports that these neurons are highly vulnerable to oxidative stress and ischaemia. This was investigated in the present study using a whole-cell patch clamp approach in rats. We found that exogenous application of H2O2 increased the release of glutamate from excitatory presynaptic terminals and γ-aminobutyric acid (GABA) from inhibitory presynaptic terminals. The increase of glutamate release was induced in part by an increase in Ca(2+) influx through N-type voltage-gated calcium channels (VGCCs) as well as by ryanodine receptor (RyR)- and inositol trisphosphate receptor-mediated Ca(2+) release from the endoplasmic reticulum (ER). In inhibitory presynaptic neurons, increased IP3 R-mediated Ca(2+) release from the ER increased GABAergic transmission, which served to rescue VH neurons from excessive release of glutamate from presynaptic terminals. These findings indicate that inhibiting N-type VGCCs or RyRs may attenuate excitotoxicity resulting from increased glutamatergic activity while preserving the neuroprotective effects of GABA, and may therefore represent a novel and targeted strategy for preventing and treating H2O2-induced motor neuron disorders.

Anticonvulsant characteristics of pyridoxyl-gamma-aminobutyrate, PL-GABA

GABA is the major inhibitory neurotransmitter in the central nervous system, and its concentration in the brain in associated with a variety of neurological disorders, including seizures, convulsions, and epilepsy. The concentration of GABA is modulated by the pyridoxal-5'-phosphate (PLP)-dependent enzymes, GAD and GABA-T. In this study, we generated pyridoxyl-gamma-aminobutyrate (PL-GABA), a novel GABA analogue composed of pyridoxyl and GABA, and have also characterized its anticonvulsant and pharmacological functions in vitro. The results of biodistribution studies revealed that PL-GABA is capable of crossing the blood-brain barrier. PL-GABA evidenced anticonvulsant activity in a wide range of epilepsy models, some of which were electrically-based (MES seizures) and some chemically-based (bicuculline, pentylenetetrazol (PTZ), picrotoxine, 3-mercaptopropionic acid). Following a timed subcutaneous administration of PTZ to mice, PL-GABA consistently increased the latencies to first twitch and clonus. In addition, PL-GABA displayed no signs of tolerance after subchronic (10 day) treatment. PL-GABA appears to exert its anticonvulsant effects by influencing seizure spread and by raising the seizure threshold. Therefore, our results indicate that PL-GABA exerts a broad-spectrum anticonvulsant effect, and identify the potential for reduced PL-GABA tolerance as an additional positive profile for novel antiepileptic drugs.

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.

Direct G protein modulation of Cav2 calcium channels

The regulation of presynaptic, voltage-gated calcium channels by activation of heptahelical G protein-coupled receptors exerts a crucial influence on presynaptic calcium entry and hence on neurotransmitter release. Receptor activation subjects presynaptic N- and P/Q-type calcium channels to a rapid, membrane-delimited inhibition-mediated by direct, voltage-dependent interactions between G protein betagamma subunits and the channels-and to a slower, voltage-independent modulation involving soluble second messenger molecules. In turn, the direct inhibition of the channels is regulated as a function of many factors, including channel subtype, ancillary calcium channel subunits, and the types of G proteins and G protein regulatory factors involved. Twenty-five years after this mode of physiological regulation was first described, we review the investigations that have led to our current understanding of its molecular mechanisms.

Presynaptic Ca2+ channels--integration centers for neuronal signaling pathways

Calcium influx into presynaptic nerve terminals via voltage-gated Ca2+ channels is an essential step in neurotransmitter release. The predominant Ca2+ channel species in synaptic nerve terminals are P/Q-type and N-type channels, with their relative levels of expression varying across the nervous system. The different distributions of these two channel subtypes are reflected in their distinct physiological and pathological roles, yet their activity is regulated by common mechanisms and both function as part of larger signaling complexes that enable their precise regulation and subcellular targeting. Here, we provide a broad overview of molecular and cellular mechanisms that regulate Ca2+ channels, and how these cellular signaling pathways are integrated at the level of the channel protein.

Voltage-Gated Calcium Channels and Idiopathic Generalized Epilepsies

The idiopathic generalized epilepsies encompass a class of epileptic seizure types that exhibit a polygenic and heritable etiology. Advances in molecular biology and genetics have implicated defects in certain types of voltage-gated calcium channels and their ancillary subunits as important players in this form of epilepsy. Both T-type and P/Q-type channels appear to mediate important contributions to seizure genesis, modulation of network activity, and genetic seizure susceptibility. Here, we provide a comprehensive overview of the roles of these channels and associated subunits in normal and pathological brain activity within the context of idiopathic generalized epilepsy.

Scrapie-infected GT1-1 cells show impaired function of voltage-gated N-type calcium channels (Cav 2.2) which is ameliorated by quinacrine treatment

Prions are transmissible pathogens that cause neurodegenerative diseases, although the mechanisms behind the nervous system dysfunctions are unclear. To study the effects of a prion infection on voltage-gated calcium channels, scrapie-infected gonadotropin-releasing hormone neuronal cells (ScGT1-1) in culture were depolarized by KCl and calcium responses recorded. Lower calcium responses were observed in infected compared to uninfected cells. This effect was still observed when L-type calcium channels were blocked by nimodipine. After inhibition of N-type calcium channels with omega-conotoxin GVIA, there was no difference in calcium responses. The calcium responses after nimodipine treatment became progressively lower during infection, but there was no major loss of the cellular prion protein (PrP(C)) or marked increase in accumulation of the abnormal prion protein (PrP(Sc)) in the cultures. These results indicate that scrapie infection causes a dysfunction of voltage-gated N-type calcium channels, which is exacerbated slowly over time. Quinacrine treatment cleared PrP(Sc) and restored calcium responses in the ScGT1-1 cultures.

Excitatory but not inhibitory synaptic transmission is reduced in lethargic (Cacnb4(lh)) and tottering (Cacna1atg) mouse thalami

Excitatory but not inhibitory synaptic transmission is reduced in lethargic (Cacnb4(lh)) and tottering (Cacna1atg) mouse thalami. Recent studies of the homozygous tottering (Cacna1atg) and lethargic mouse (Cacnb4(lh)) models of absence seizures have identified mutations in the genes encoding the alpha1A and beta4 subunits, respectively, of voltage-gated Ca2+ channels (VGCCs). beta subunits normally regulate Ca2+ currents via a direct interaction with alpha1 (pore-forming) subunits of VGCCs, and VGCCs are known to play a significant role in controlling the release of transmitter from presynaptic nerve terminals in the CNS. Because the gene mutation in Cacnb4(lh) homozygotes results in loss of the beta4 subunit's binding site for alpha1 subunits, we hypothesized that synaptic transmission would be altered in the CNS of Cacnb4(lh) homozygotes. We tested this hypothesis by using whole cell recordings of single cells in an in vitro slice preparation to investigate synaptic transmission in one of the critical neuronal populations that generate seizure activity in this strain, the somatosensory thalamus. The primary finding reported here is the observation of a significant decrease in glutamatergic synaptic transmission mediated by both N-methyl-D-aspartate (NMDA) and non-NMDA receptors in somatosensory thalamic neurons of Cacnb4(lh) homozygotes compared with matched, nonepileptic mice. In contrast, there was no significant decrease in GABAergic transmission in Cacnb4(lh) homozygotes nor was there any difference in effects mediated by presynaptic GABAB receptors. We found a similar decrease in glutamatergic but not GABAergic responses in Cacna1atg homozygotes, suggesting that the independent mutations in the two strains each affected P/Q channel function by causing defective neurotransmitter release specific to glutamatergic synapses in the somatosensory thalamus. This may be an important factor underlying the generation of seizures in these models.

The anticonvulsant actions of sigma receptor ligands in the Mg2+-free model of epileptiform activity in rat hippocampal slices

1. The anticonvulsant potency of a series of structurally-dissimilar compounds which possess nanomolar affinities for high-affinity sigma binding sites was examined in the Mg2+-free model of epileptiform activity in rat hippocampal slices. Extracellular field potential recordings in the CA1 region were employed to examine the effects of test compounds on spontaneous epileptiform activity and multiple population spikes evoked by stimulation of the Schaffer collateral-commissural pathway. 2. Applied at sigma site-selective (i.e. nanomolar) concentrations, dextromethorphan, ditolylguanidine, caramiphen and opipramol failed to modify Mg2+-free epileptiform activity; neither pro- nor anticonvulsant effects were observed. However, applied at micromolar concentrations, these and additional test compounds reversibly inhibited orthodromically-evoked epileptiform field potentials with a rank order potency (IC50 values in microM): dextrorphan (1.5) > ifenprodil (6.3) > dextromethorphan (10) > ditolylguanidine (15) > loperamide (28) > carbetapentane (38) > caramiphen (46) > opipramol (52). Micromolar concentrations of the same compounds also inhibited spontaneous epileptiform bursts recorded during perfusion with Mg2+-free medium. 3. Co-application of ropizine (10 microM), an allosteric modulator of dextromethorphan binding to high-affinity sigma receptors, failed to endow dextromethorphan 10 nM with anticonvulsant properties and did not modify the anticonvulsant potency of 10 microM dextromethorphan. 4. The effects of dextrorphan (10 microM), ifenprodil (20 microM), loperamide (50 microM) and caramiphen (100 microM) were examined in the presence of external Mg2+ on field potential input/output (I/O) relationships and paired-pulse facilitation (PPF) of field excitatory postsynaptic potentials. Only caramiphen elicited effects on these parameters, affecting synaptic transmission at the point of synaptic transfer and depressing PPF ratios to below baseline values. The effects of caramiphen on I/O relationships mimicked those of the established anticonvulsant adenosine: in contrast, adenosine evoked an increase in PPF ratios. 5. Because anticonvulsant activity was observed only at micromolar concentrations of the sigma ligands tested, the results indicate that their anticonvulsant actions should not be ascribed to their occupancy, observed at nanomolar concentrations, of high-affinity sigma binding sites. Rather, anticonvulsant activity more likely reflects functional NMDA receptor antagonism and/or blockade of high voltage-activated Ca2+ channels, effects which are associated with micromolar concentrations of the test compounds. Modulation of GABAergic inhibitory mechanisms may also contribute to the anticonvulsant properties of caramiphen.

Either N- or P-type calcium channels mediate GABA release at distinct hippocampal inhibitory synapses

Transmitter release at most central synapses depends on multiple types of calcium channels. Identification of the channels mediating GABA release in hippocampus is complicated by the heterogeneity of interneurons. Unitary IPSPs were recorded from pairs of inhibitory and pyramidal cells in hippocampal slice cultures. The N-type channel antagonist omega-conotoxin MVIIA abolished IPSPs generated by interneurons in st. radiatum, whereas the P/Q-type antagonist omega-agatoxin IVA had no effect. In contrast, omega-agatoxin IVA abolished IPSPs generated by st. lucidum and st. oriens interneurons, but omega-conotoxin MVIIA had no effect. After unitary IPSPs were blocked by toxin, transmission could not be restored by increasing presynaptic calcium entry. The axons of the two types of interneurons terminated within distinct strata of area CA3. Thus, GABA release onto pyramidal cells, unlike glutamate release, is mediated entirely by either N- or P-type calcium channels, depending on the presynaptic cell and the postsynaptic location of the synapse.

The use of invertebrate peptide toxins to establish Ca2+ channel identity of CA3-CA1 neurotransmission in rat hippocampal slices

The relative contribution(s) of different Ca2+ channel subtypes to synaptic transmission between Schaffer collaterals of hippocampal CA3 pyramidal cells and CA1 pyramidal cell dendrites has been assessed using the synthetic invertebrate peptide toxins omega-conotoxin GVIA to block N-type Ca2+ channels, omega-agatoxin-IVA to block P-type Ca2+ channels and omega-conotoxin MVIIC to block N-, P- and Q-type Ca2+ channels. Omega-Agatoxin-IVA, omega-conotoxin GVIA and omega-conotoxin MVIIC all produced dose-dependent inhibitions of the excitatory post-synaptic field potential (fEPSP) recorded from the CA1 region of transverse hippocampal slices. Application of 300 nM omega-conotoxin GVIA generally produced no further inhibition to that observed with 100 nM, resulting in a maximal 50% inhibition of the fEPSP. By contrast, 30 nM omega-agatoxin-IVA reduced the fEPSP slope by only 4.6 +/- 11.1% (mean +/- S.D., n = 3), suggesting the lack of involvement of classical P-type Ca2+ channels, whereas 300 nM omega-agatoxin-IVA reduced the fEPSP slope by 85.7 +/- 15.3% (n = 3) at the end of 44 min application. Similar applications of 100 and 300 nM sigma-conotoxin MVIIC reduced the fEPSP slope by 30.9 +/- 6.6% and 79.7 +/- 5.7% respectively. Application of 30 nM omega-agatoxin-IVA together with omega-conotoxin GVIA (300 nM) produced no greater inhibition of the fEPSP than that observed with omega-conotoxin GVIA alone, suggesting that the omega-agatoxin-IVA-sensitive and omega-conotoxin MVIIC-sensitive component presents a pharmacology similar to the reported Q-type Ca2+ channel. The inhibition produced by omega-conotoxin GVIA and omega-conotoxin MVIIC showed no recovery with prolonged washing (1-2 h) whereas that produced by omega-agatoxin-IVA was slowly reversible. The observation that omega-agatoxin-IVA, which does not effect N-type Ca2+ channels (Mintz et al. (1992a) Neuron 9, 85), is capable of completely suppressing the fEPSP suggests that, whilst N-type Ca2+ channels may contribute to normal synaptic transmission at Schaffer collateral-CA1 synapses, they are not capable of supporting transmission when Q-type channels are blocked.

Diversity and function of presynaptic calcium channels in the brain

Calcium channels in presynaptic nerve terminals are essential for neurotransmitter release, and current research has provided evidence for the involvement of a multitude of Ca2+ channel types. The diversity of Ca2+ channel structure and distribution in the brain suggests specific functional roles. Modulation by interaction with other proteins and/or by phosphorylation/dephosphorylation reactions enhances the regulatory impact of these channels on brain function.

Distribution of alpha 1A, alpha 1B and alpha 1E voltage-dependent calcium channel subunits in the human hippocampus and parahippocampal gyrus

The distribution of voltage-dependent calcium channel subunits in the central nervous system may provide information about the function of these channels. The present study examined the distribution of three alpha-1 subunits, alpha 1A, alpha 1B and alpha 1E, in the normal human hippocampal formation and parahippocampal gyrus using the techniques of in situ hybridization and immunocytochemistry. All three subunit mRNAs appeared to be similarly localized, with high levels of expression in the dentate granule and CA pyramidal layer. At the protein level, alpha 1A, alpha 1B and alpha 1E subunits were differentially localized. In general, alpha 1A-immunoreactivity was most intense in cell bodies and dendritic processes, including dentate granule cells, CA3 pyramidal cells and entorhinal cortex pre-alpha and pri-alpha cells. The alpha 1B antibody exhibited relatively weak staining of cell bodies but stronger staining of neuropil, especially in certain regions of high synaptic density such as the polymorphic layer of the dentate gyrus and the stratum lucidum and radiatum of the CA regions. The alpha 1E staining pattern shared features in common with both alpha 1A and alpha 1B, with strong immunoreactivity in dentate granule, CA3 pyramidal and entorhinal cortex pri-alpha cells, as well as staining of the CA3 stratum lucidum. These findings suggest regions in which particular subunits may be involved in synaptic communication. For example, comparison of alpha 1B and alpha 1E staining in the CA3 stratum lucidum with calbindin-immuno-reactivity suggested that these two calcium channels subunits may be localized presynaptically in mossy fibre terminals and therefore may be involved in neurotransmitter release from these terminals.

Presynaptic metabotropic glutamate receptors modulate omega-conotoxin-GVIA-insensitive calcium channels in the rat medulla

We have previously demonstrated that the metabotropic glutamate receptor (mGluR) agonist (1S,3R)-1 aminocyclopentane-1,3-dicarboxylate (ACPD) presynaptically inhibits evoked glutamatergic EPSCs and GABAergic IPSCs in patch clamped rat nucleus tractus solitarius (NTS) neurons recorded in this slices. The present study investigated the pharmacology of the presynaptic mGluRs, the the voltage dependent Ca2+ channel (VDCC) subtypes supporting neurotransmitter release, and possible interactions between the two. Monosynaptic EPSCs or IPSCs were evoked by electrical stimulation in the region of the tractus solitarius (TS). The effects of the mGluR agonists ACPD, (2S,3S,4S)-alpha-(carboxycyclopropyl)glycine (L-CCG-I) and L-2-amino-4-phosphonobutyrate (AP4) were examined upon EPSCs. The effects of the above compounds and quisqualate (QUIS) were examined upon IPSCs. L-CCG-I proved the most potent inhibitor of EPSCs and IPSCs. The VDCC blockers omega-AGA-IVA (AGA), omega-conotoxin GVIA (GVIA), omega-conotoxin MVIIC (MVIIC) and nimodipine (NIM) were assessed for their ability to inhibit monosynaptic EPSCs and IPSCs. EPSCs were inhibited by GVIA >> AGA > or = MVIIC. IPSCs were inhibited by AGA > or = MVIIC >> GVIA. NIM was without effect on the EPSC or IPSC. The potency of mGluR inhibition of evoked synaptic transmission was assessed in the absence and following treatment with VDCC blockers. mGluR agonists blocked a greater percentage of the EPSC or IPSC following treatment with GVIA, but not the other VDCC antagonists, than under control conditions. We have previously demonstrated that the postsynaptic inhibitory effects of mGluR activation upon GABAA mediated currents can be mimicked by cyclic guanosine monophosphate (cGMP) analogs. The cGMP-dependent protein kinase (PKG) inhibitors H8 and Rp-8-4-chlorophenylthio-guanosine-3',5'-cyclic monophosphorothioate (Rp-cG) blocked mGluR inhibition of GABAA mediated currents without blocking the ability of mGluR agonists to inhibit the IPSC. The effect of L-CCGI was enhanced following treatment with GVIA in the presence of Rp-cG, confirming a presynaptic locus of mGluR mediated inhibition of the IPSC. In contrast, cGMP analogues potentiate postsynaptic responses to glutamate agonists but depress the EPSC. As with the mGluR agonists, the inhibition of the EPSC by cGMP was potentiated following treatment with GVIA. These results suggest that presynaptic mGluR reduce both glutamate release from afferent fibers and GABA release from inhibitory interneurons following electrical stimulation in the region of the TS. Although different VDCCs support the majority of glutamate and GABA release and mGluR effects on release appear to utilize differing intracellular pathways, presynaptic GVIA-insensitive VDCCs are favorably targeted for inhibition by mGluR agonists.

Suppression of hippocampal synaptic transmission by the spider toxin omega-agatoxin-IV-A

Effects of omega-agatoxin-IV-A (AGTX) on synaptic transmissions were examined in thin transverse sections of the guinea pig hippocampus. AGTX suppressed, in a dose-dependent manner, field potentials elicited by mossy fiber stimulation in CA3 pyramidal cell layer. AGTX also suppressed field potentials elicited by perforant fiber stimulation in granular layer without marked changes in antidromic responses. These results suggest that Ca2+ entry into presynaptic terminals via P-like calcium channels is at least partly responsible for release of glutamate.