Effect of omega-conotoxin GVIA on release of 3H-gamma-aminobutyric acid from slices of rat neostriatum

The Effect of Calcium Channel Antagonists on Spontaneous and Evoked Epileptiform Activity in the Rat Neocortex In Vitro

Calcium influx through voltage-activated calcium channels may play a crucial role in the propagation and maintenance of seizure activity. We have examined the contribution of various types of calcium currents to epileptogenesis by studying the effects of various calcium channel blockers on epileptiform activity. N-methyl-d-aspartate receptor-mediated epileptiform activity was induced by removal of magnesium ions superfusing the cortex, or by low-frequency stimulation of the underlying white matter. CoCl2, CdCl2 and omega-conotoxin, acting at the N- and L-type calcium channels, significantly reduced epileptiform activity. L-channel antagonists nifedipine and verapamil, and the agonist BAY K 8644, increased spontaneous bursting in cortical wedges, but had no effect upon evoked activity. The T-channel blocker NiCl2 had variable effects on epileptiform activity, whereas phenytoin consistently reduced such activity. These results suggest that calcium influx underlying epileptiform activity in the rat neocortex may occur at least partially via the activation of the N-type calcium channel. However, contributions from other calcium channel types cannot be excluded.

Transient brain ischemia in rabbits: the effect of omega-conopeptide MVIIC on hippocampal excitatory amino acids

Neurologic injury that occurs after ischemia results from a cascade of events involving the release of various endogenous neurotoxins. A portion of the release of excitatory neurotransmitters is calcium dependent and may be attenuated by administration of calcium channel blockers. Using an in vivo model of ischemia, we studied the effects of omega-conopeptide MVIIC, a voltage-sensitive calcium channel blocker, and hypothermia (32 degrees C) on hippocampal glutamate and aspartate release in the peri-ischemic period. Thirty-four New Zealand white rabbits of either sex were anesthetized with halothane, intubated, and mechanically ventilated. Monitored variables included blood gases, mean arterial blood pressure, and the electroencephalogram. Microdialysis catheters were transversely inserted through the anterior portion of the dorsal hippocampus and perfused with artificial cerebrospinal fluid at a rate of 2 microliters/min. After stabilization period, animals were randomly assigned to one of the following groups: Control group (n = 8), 10 microM omega-conopeptide MVIIC group (n = 7), 100 microM omega-conopeptide MVIIC group (n = 7), Hypothermia group (n = 6; cranial temperature = 32 degrees C), and omega-conopeptide MVIIC + hypothermia group (n = 6; 100 microM omega-conopeptide MVIIC and cranial temperature 32 degrees C). All the rabbits were subjected to 10 minutes of global cerebral ischemia produced by neck tourniquet inflation combined with hypotension during halothane anesthesia. Conopeptide MVIIC was administered in the artificial cerebrospinal fluid used to perfuse the microdialysis catheter. In control animals, ischemia caused a significant increase in glutamate (9.7 fold) and aspartate (11.3 fold) concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of diverse omega-conopeptides on the in vivo release of glutamic and gamma-aminobutyric acids

omega-Conopeptides are antagonists of subtypes of neuronal calcium channels. Two omega-conopeptides, SVIB and MVIIC, have recently been identified which have a novel specificity for these ionophores. We have tested the actions these peptides, as well as the more selective MVIIA, on the release of glutamic acid and gamma-aminobutyric acid (GABA) in the hippocampus in vivo. For the assay of peptide effects on release, we used microdialysis to deliver multiple pulses of elevated potassium to the brain extracellular fluid. Peptide effects were quantitated from the decrement of the release with peptide perfused through the probes, in comparison to that in control experiments. Synthetic MVIIC caused a 40-50% decrement in the release of both glutamate and GABA at a probe concentration of about 200 nM. Synthetic SVI-B caused a 50% block at about 20-40 microM, while about 200 microM of MVIIA was required for 50% block. Chromatographic experiments showed that differences in potency between MVIIC and MVIIA were not explained by differential degradation. Blockade of release was also observed in the thalamus. MVIIC provides a tool for investigating the role of calcium mediated release of glutamate and GABA in physiological and pathological processes in the mammalian brain in vivo.

Effects of stimulus intensity on the inhibition by omega-conotoxin GVIA and neomycin of K(+_-evoked [3H]norepinephrine release from hippocampal brain slices and synaptosomal calcium influx

The effects of various K+ concentrations on the inhibition of [3H]norepinephrine release from rat hippocampal brain slices and evoked synaptosomal 45Ca2+ influx by omega-conotoxin GVIA (omega-CgTx) and neomycin were examined. K+ (15-75 mM) caused a concentration-dependent release of [3H]norepinephrine that was greater than 90% dependent on extracellular calcium. The ability of omega-CgTx to inhibit [3H]norepinephrine release was optimal at 25 mM K+ and was reduced substantially at higher concentrations of K+. omega-CgTx maximally inhibited [3H]norepinephrine release by 49% (15 mM K+), 58% (25 mM K+), 22% (50 mM K+), and 12% (75 mM K+). In contrast, neomycin caused a concentration-dependent and virtually complete inhibition of [3H]norepinephrine release at all concentrations of K+, with IC50 values of 210 microM (15 mM K+), 150 microM (25 mM K+), 450 microM (50 mM K+), and 1500 microM (75 mM K+). omega-CgTx (1 microM) had little effect (10% or less inhibition) on hippocampal synaptosomal 45Ca2+ influx at any concentration of K+, whereas 3 mM neomycin caused at least 75% inhibition of 45Ca2+ influx, with the largest inhibition (96%) occurring at 25 mM K+. The results suggest that increasing stimulus intensity decreases the contribution of N-type voltage-sensitive calcium channels (VSCC) in mediating K(+)-evoked release of [3H]norepinephrine. The comparative absence of omega-CgTx-sensitive synaptosomal 45Ca(2+)-influx sites suggests that N-type calcium channels are a small subset of channels in rat hippocampal synaptosomes. The demonstration that neomycin can inhibit omega-CgTx-sensitive and -insensitive neurotransmitter release and calcium influx suggests that neomycin may block N-type VSCC as well as non-N-type VSCC.

A toxin (Aga-GI) from the venom of the spider Agelenopsis aperta inhibits the mammalian presynaptic Ca2+ channel coupled to glutamate exocytosis

Venom of the funnel web spider Agelenopsis aperta was fractionated and screened for activity against the mammalian presynaptic, voltage-dependent Ca2+ channel coupled to glutamate exocytosis. A purified toxin (Aga-GI) from this venom inhibits glutamate exocytosis evoked by elevated potassium or by 4-aminopyridine but is without effect on ionomycin-evoked release. At the same time a partial inhibition of the depolarisation-evoked elevation of cytoplasmic free Ca2+ is seen. The toxin does not inhibit 4-aminopyridine- or potassium-evoked depolarisation, or block Ca(2+)-dependent, potassium-evoked [3H]noradrenaline release. The results indicate that the venom contains a toxin capable of inhibiting the presynaptic voltage-dependent Ca2+ channel coupled to glutamate exocytosis in the mammalian central nervous system. This channel is resistant to block by either omega-conotoxin GVIA or nifedipine. Thus Aga-GI is a novel tool with which to probe this elusive neuronal calcium channel.

Inhibition of K(+)-evoked [3H]D-aspartate release and neuronal calcium influx by verapamil, diltiazem and dextromethorphan: evidence for non-L/non-N voltage-sensitive calcium channels

The effects of inhibitors of voltage-sensitive calcium channels (VSCC) on K(+)-evoked [3H]D-aspartate release from rat hippocampal slices and the K(+)-evoked increase in intracellular calcium in neocortical neurons in primary culture were examined. K+ caused a concentration-dependent release of [3H]D-aspartate that was approximately 85% dependent on the presence of extracellular calcium. Neither the marine snail toxin, omega-conotoxin GVIA, nor the dihydropyridine VSCC antagonist, nitrendipine, had any effect on K(+)-evoked release of [3H]D-aspartate. omega-Conotoxin GVIA and nitrendipine caused a relatively small (20-30%) inhibition of K(+)-evoked increase in intracellular calcium in neocortical neurons in primary culture. This suggests that K(+)-evoked [3H]D-aspartate release is not dependent on L- or N-type VSCC, whereas K(+)-evoked neuronal calcium influx was only partially dependent on L- and N-type VSCC. Verapamil, dextromethorphan and diltiazem caused a concentration-dependent inhibition of K(+)-evoked release of [3H]D-aspartate with IC50 values of 30, 100 and 120 microM, respectively. The K(+)-evoked increase in intracellular calcium was inhibited with essentially the same rank order of potency, but with slightly greater potencies (IC50 values for verapamil, diltiazem and dextromethorphan were 20, 50 and 50 microM, respectively). At 300 microM, neither verapamil, diltiazem nor dextromethorphan inhibited [3H]D-aspartate release evoked by the calcium ionophore ionomycin, suggesting that these compounds are not acting intracellularly to inhibit the ability of free cytosolic calcium to evoke release.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective hippocampal lesion following omega-conotoxin administration in rats

Histopathological evaluation of rat brains 3 days following unilateral i.c.v. injections of omega-conotoxin GVIA (omega-ctx), 0.032 and 0.1 nmol/kg, was performed. An isolated unilateral lesion confined to the injected hemisphere was found in the hippocampal CA3 neurons. Morphometric analysis of these cells revealed a significant reduction in cell area in both dose groups compared to i.c.v. injected vehicle, and to the contralateral hemisphere. These data indicate a specific degenerative process and suggest that CA3 cells possess omega-ctx-sensitive Ca2+ channels which are essential to their viability.

Subunit structure and localization of dihydropyridine-sensitive calcium channels in mammalian brain, spinal cord, and retina

Monoclonal antibodies that recognize the alpha 2 delta subunits of calcium channels from skeletal muscle immunoprecipitate a complex of alpha 1, alpha 2 delta, beta, and gamma subunits. They also immunoprecipitate 64% of rabbit brain dihydropyridine-sensitive calcium channels. Iodination of partially purified brain calcium channels followed by immunoprecipitation reveals alpha 1-, alpha 2 delta-, and beta-like subunits that have apparent molecular masses of 175, 142, and 57 kd, respectively. A polypeptide of 100 kd is also specifically immunoprecipitated. Immunocytochemical studies identify dihydropyridine-sensitive calcium channels in neuronal somata and proximal dendrites in rat brain, spinal cord, and retina. Staining of many neuronal somata is uneven, revealing relatively high densities of dihydropyridine-sensitive calcium channels at the base of major dendrites. L-type calcium channels in this location may serve to mediate long-lasting increases in intracellular calcium in the cell body in response to excitatory inputs to the dendrites.