Calcium channel blocking agents and potassium-stimulated release of glutamate from cerebellar slices

Calcium Channels As Therapeutic Targets in Neuropathic Pain

Injury to the nerve can produce changes in dorsal horn function and pain. This facilitated processing may be mediated in part by voltage-sensitive calcium channels. Activation of these channels increases intracellular calcium, thereby mediating transmitter release and activating cascades serving to alter membrane excitability and initiate protein transcription. Molecular techniques reveal the complexity and multiplicity of these channels. At the spinal level, blocking of several of these calcium channels, notably those of the N type, can prominently alter pain behavior. These effects are consistent with the high levels of expression on primary afferents and dorsal horn neurons of these channels. More recently, agents binding to auxiliary subunits such as the alpha2delta of these calcium channels diminish excitability of the membrane without completely blocking channel function. Drugs that bind to this site, highly expressed in the superficial dorsal horn, will diminish neuropathic pain states. Continuing developments in our understanding of these channel functions promises to advance the control of aberrant spinal functions initiated by nerve injury.

PERSPECTIVE

Pharmacologic studies showing the role of spinal voltage-sensitive calcium channels in neuropathic pain models provide evidence suggesting their applicability in human pain states.

The involvement of sodium and calcium ions in the release of amino acid neurotransmitters from mouse cortical slices elicited by hyperforin

Hyperforin is currently considered to be the major active antidepressant constituent of the medicinal herb St. John's wort (Hypericum perforatum L.). The mechanism of action however, is still largely unknown, although the involvement of sodium and calcium has been recently inferred. In the present study hyperforin (5 microM) significantly potentiated the release of endogenous aspartate and glutamate from mouse cortical slices when stimulated by veratridine or potassium. Hyperforin (5 microM) also stimulated the release of aspartate, glutamate, serine, glycine and GABA when perfused on its own. Perfusion of the sodium channel blocker, tetrodotoxin (TTX) inhibited the effect of hyperforin, whereas removal of extracellular calcium potentiated the effect. Our observations suggests that hyperforin increases the overflow of neurotransmitters from mouse cerebral cortex possibly through facilitating the entry of sodium into the neurone which leads to the release of calcium from intracellular stores.

Effects of spinally delivered N- and P-type voltage-dependent calcium channel antagonists on dorsal horn neuronal responses in a rat model of neuropathy

Neuropathic pain, due to peripheral nerve damage, can include allodynia (perception of innocuous stimuli as being painful), hyperalgesia (increased sensitivity to noxious stimuli) and spontaneous pain, often accompanied by sensory deficits. Plasticity in transmission and modulatory systems are implicated in the underlying mechanisms. The Kim and Chung rodent model of neuropathy (Kim and Chung, Pain 50 (1992) 355) employed here involves unilateral tight ligation of two (L5 and L6) of the three (L4, L5, and L6) spinal nerves of the sciatic nerve and reproducibly induced mechanical and cold allodynia in the ipsilateral hindpaw over the 14 day post-operative period. In vivo electrophysiological techniques have then been used to record the response of dorsal horn neurones to innocuous and noxious electrical and natural (mechanical and thermal) stimuli after spinal nerve ligation (SNL). Activation of voltage-dependent calcium channels (VDCCs) is critical for neurotransmitter release and neuronal excitability, and antagonists can be antinociceptive. Here, for the first time, the effect of N- and P-type VDCC antagonists (omega-conotoxin-GVIA and omega-agatoxin-IVA, respectively) on the evoked dorsal horn neuronal responses after neuropathy have been investigated. Spinal omega-conotoxin-GVIA (0.1-3.2 microg) produced prolonged inhibitions of both the electrically- and low- and high-intensity naturally-evoked neuronal responses in SNL and control rats. Spinal omega-agatoxin-IVA (0.1-3.2 microg) also had an inhibitory effect but to a lesser extent. After neuropathy the potency of omega-conotoxin-GVIA was increased at lower doses in comparison to control. This indicates an altered role for N-type but not P-type VDCCs in sensory transmission after neuropathy and selective plasticity in these channels after nerve injury. Both pre- and post-synaptic VDCCs appear to be important.

The anticonvulsant effects of the enantiomers of losigamone

1 Losigamone is a novel anticonvulsant undergoing phase III clinical trials in patients with partial and secondary generalized seizures. This study investigated the effects of the S(+)- and R(-)- enantiomers of losigamone on endogenous amino acid release from BALB/c mouse cortical slices, spontaneous depolarizations in the cortical wedge preparation of the DBA/2 mouse and audiogenic seizures in DBA/2 mice. 2 S(+)-losigamone (100 and 200 microM) significantly reduced both potassium- and veratridine-elicited release of glutamate and aspartate from cortical slices. R(-)-losigamone had no effect on release at concentrations up to 400 microM. 3 Cortical wedges exhibit spontaneous depolarizations when perfused with magnesium-free artificial cerebrospinal fluid. S(+)-losigamone significantly reduced these depolarizations at 50-200 microM whilst R(-)-losigamone had a significant effect at 200-800 microM. 4 DBA/2 mice are susceptible to audiogenic seizures and S(+)-losigamone dose-dependently (5, 10 and 20 mg kg-1, i.p.) significantly inhibited clonic/tonic convulsions with 91% of the mice protected at 20 mg kg-1. There was no protection at 20 mg kg-1 with R(-)-losigamone. 5 These results, from both in vitro and in vivo experiments, confirm that the pharmacological activity profiles of the two losigamone enantiomers are not identical and suggest further that excitatory amino acid-mediated processes are involved in the mode of action of S(+)-losigamone whereas R(-)-losigamone does not possess such properties. For the treatment of neurological conditions involving exaggerated excitatory amino acid function the use of S(+)-losigamone might therefore be more effective clinically than losigamone or its R(-)-enantiomer.

Plasma concentration and CNS effects of Ca antagonists darodipine and nimodipine after single-dose oral administration to healthy volunteers

The dynamics at the brain level (quantitative EEG), plasma kinetics and effects on blood pressure and heart rate of the Ca antagonists, darodipine (slow-release, 50- 200 mg) and nimodipine (30 mg), were compared in a double-blind cross-over study on healthy volunteers during a 9-hour period following single drug/placebo administration. Increased EEG total power was observed after 100 and 200 mg daropidine; a concomitant decrease of 14.5-32.0 Hz relative power was observed at 100 mg. The 50-mg dose proved ineffective. These effects were correlated with the darodipine plasma concentration only at the 100-mg dose, with indications of an active concentration interval at approximately 5-10 ng/ml; a reduction in diastolic blood pressure and increased heart rate proved to be linearly correlated with the drug plasma concentration throughout the entire concentration range. Comparable EEG effects were observed after nimodipine, but they did not correlate with the plasma concentration. Implications of the predictability of the brain effect from the drug plasma concentration and differential thresholds for the brain action and effects on (peripheral) circulation are suggested.

Differential modulation of synaptic transmission by calcium chelators in young and aged hippocampal CA1 neurons: evidence for altered calcium homeostasis in aging

The effects of membrane-permeant Ca2+ chelators on field EPSPs (fEPSPs) were measured in the hippocampal CA1 region of brain slices from young (2-4 months) and old (24-27 months) Fischer 344 rats. BAPTA-AM depressed fEPSPs in young slices by up to 70% but enhanced fEPSPs by 30% in aged slices. EGTA-AM, with slower binding kinetics, did not affect fEPSPs from young slices but enhanced fEPSPs in aged slices. BAPTA derivatives with calcium dissociation constants (Kd) of 0.2-3.5 microM reduced or enhanced fEPSPs in young and aged slices, respectively, but 5',5'-dinitro BAPTA-AM (Kd of approximately 7000 microM) had no effect. Frequency facilitation of the fEPSPs occurred in young, but not in aged, slices, except when BAPTA-AM or EGTA-AM was perfused onto aged slices. The differential effects of BAPTA-AM in young and old slices were eliminated by perfusing with a low Ca2+-high Mg2+ saline or with the calcium blocker Co2+. These data suggest that intracellular Ca2+ regulation is altered and raised in aged neurons. Cell-permeant calcium buffers may be able to "ameliorate" deficits in synaptic transmission in the aged brain.

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.

The effect of losigamone (AO-33) on electrical activity and excitatory amino acid release in mouse cortical slices

1. Losigamone is a novel anticonvulsant the mechanism of action of which is not known. This study investigated the effect of losigamone on spontaneous, NMDA- and AMPA-induced depolarizations in the cortical wedge preparation of the DBA/2 mouse (which are susceptible to sound-induced seizures) and on endogenous amino acid release from BALB/c mouse cortical slices. 2. Cortical wedges exhibit spontaneous depolarizations in magnesium-free medium and losigamone was effective in significantly reducing these spontaneous depolarizations at concentrations of 100 microM and above. 3. NMDA-induced depolarizations were significantly reduced by losigamone at concentrations of 25 microM and above. Losigamone had no effect on AMPA-induced depolarizations. 4. Veratridine (20 microM) and potassium (60 mM) were used to stimulate the release of amino acids from mouse cortex. Veratridine-stimulated release of glutamate was significantly reduced by losigamone at concentrations of 100 microM and above, while potassium-stimulated release was significantly reduced by losigamone at 200 microM. 5. NMDA antagonism and inhibition of excitatory amino acid release may contribute to the anticonvulsant effect of losigamone.

Remacemide hydrochloride: a novel antiepileptic agent

1. Remacemide hydrochloride has been shown to possess anticonvulsant activity in a wide range of animal models of epilepsy with ED50s in the 6-60 mg/kg range, depending on the species and route of administration. The compound also has been shown to be effective clinically as add-on therapy for partial seizures. 2. Degradation of remacemide yields the desglycinated metabolite that is approximately 2-fold more potent as an anticonvulsant agent than the parent drug. 3. Both compounds displace [3H]MK801 binding from the cerebral cortical membranes, and the metabolite is approximately 150-fold more potent in doing so than remacemide. This effect, together with the findings that the desglycinate reduces N-methyl-D-aspartate (NMDA)-induced depolarizations in a variety of preparations, suggests that the mechanism of action is through blockade of the channel site of the NMDA-receptor complex. 4. Remacemide and its metabolite, in common with other antiepileptic agents, block sustained repetitive-firing in cultured neurons. The metabolite also has been shown to decrease glutamate release from cortical slices. 5. Remacemide hydrochloride has neuroprotective properties when tested on models of cerebral ischemia. 6. The drug has low toxicity in contrast to other NMDA-channel-blocking compounds, such as MK801 and phencyclidine, probably because of its low affinity for the channel-binding site.

Effects of intrathecally administered nociceptin, an opioid receptor-like1 (ORL1) receptor agonist, on the thermal hyperalgesia induced by unilateral constriction injury to the sciatic nerve in the rat

Nociceptin is a 17 amino acid peptide which acts as a potent endogenous agonist of the opioid receptor-like1 (ORL1) receptor. In the spinal cord, nociceptin is reported to depress glutamatergic transmission and to block the spinally mediated facilitation which is thought to be mediated by the activation of N-methyl-D-aspartate (NMDA) receptor. It has been found that NMDA receptor mediated spinal facilitation is crucial in the maintenance of thermal hyperalgesia evoked by a nerve constriction injury. In the present study, we investigated the effect of intrathecally administered nociceptin on the level of thermal hyperalgesia after unilateral constriction injury to the sciatic nerve in the rat. Intrathecally administered nociceptin attenuated the level of thermal hyperalgesia in a dose dependent manner. These data indicate that spinal ORL1 receptor activation by nociceptin inhibits the spinal facilitation evoked by the nerve constriction injury.

Effects of felbamate on veratridine- and K(+)-stimulated release of glutamate from mouse cortex

Felbamate is a novel anticonvulsant which may modulate the strychnine-insensitive glycine site of the N-methyl-D-aspartate (NMDA) receptor complex. This study examined the effect of felbamate and 5,7-dichlorokynurenic acid on veratridine (20 microM)- and K+ (60 mM)-stimulated release of amino acids in mouse cortical slices. Felbamate significantly decreased veratridine-induced release of glutamate at 400 microM and 800 microM but had no effect on K(+)-stimulated release. 5,7-Dichlorokynurenic acid had no effect on amino-acid release in concentrations up to 200 microM. The inhibitory effect of felbamate on veratridine-induced release of glutamate may be due to inactivation of voltage-sensitive Na+ channels.

The effect of the desglycinyl metabolite of remacemide hydrochloride (FPL 12495AA) and dizocilpine (MK-801) on endogenous amino acid release from mouse cortex

1. In this study the effect of FPL 12495AA, the desglycinyl metabolite of remacemide hydrochloride and dizocilpine (MK-801), on potassium- and veratridine-stimulated release of neurotransmitter amino acids from mouse cortical slices was investigated. 2. Veratridine (20 microM) and potassium (60 mM) produced a preferential release of glutamate and aspartate. Potassium-stimulated release was calcium-dependent, while veratridine-stimulated release was only partially affected by removal of calcium from the medium. 3. FPL 12495AA significantly inhibited veratridine- and potassium-stimulated release of glutamate and aspartate. Lower concentrations of FPL 12495AA were needed to inhibit veratridine-stimulated release of glutamate (12.5 microM) than potassium-stimulated release (100 microM). 4. Dizocilpine significantly inhibited veratridine- and potassium-stimulated release of glutamate and aspartate at concentrations of 100 microM and above. 5. FPL 12495AA and dizocilpine both have an affinity for the ion channel subsite of the N-methyl-D-aspartate (NMDA) receptor. The reduction of potassium-stimulated release of glutamate and aspartate by FPL 12495AA and dizocilpine is probably due to NMDA receptor blockade. 6. FPL 12495AA inhibited veratridine-stimulated release at a concentration of 12.5 microM while dizocilpine was effective only at a concentration of 100 microM. This difference in efficacy is probably due to the higher affinity of FPL 12495AA compared to dizocilpine at the veratridine-binding site on the sodium channel.

Glutamate efflux from rat brain slices and cultures: a comparison of the depolarizing agents potassium, 4-aminopyridine, and veratrine

The major excitatory amino acid neurotransmitter in the mammalian brain is glutamate (GLU). GLU release from nerve terminals is both calcium-dependent and -independent, yet these mechanisms of release are not fully understood. Potassium, 4-aminopyridine (4-AP) and veratrine are commonly used depolarizing agents that were studied for their ability to stimulate GLU efflux from brain slices. These agents produced significant regional variations in GLU efflux from rat brain slices. Potassium was the most potent of the three secretogogues tested. 4-AP produced a significant GLU efflux only in the cerebellum. Veratrine produced consistent stimulation of GLU efflux from all brain regions tested. Potassium was the only depolarizing agent tested that stimulated GLU release from primary astroglial cultures of rat cerebral cortex. All three agents also demonstrated an ability to inhibit GLU reuptake in brain slice preparations. This data suggest that both GLU release and uptake are modulated in a regionally selective manner, and that commonly used depolarizing agents affect not only calcium-dependent neuronal release, but also uptake and glial responses.

Omega-conotoxin GVIA protects against ischemia-induced neuronal death in the Mongolian gerbil but not against quinolinic acid-induced neurotoxicity in the rat

Excessive release of neurotransmitters is reported to contribute to the delayed neuronal death in animal models of cerebral ischemia. Since evidence is accumulating that N-type voltage-sensitive calcium channels (N-channels) regulate the release of neurotransmitters, we investigated the effects of omega-conotoxin GVIA (omega-CTX), an antagonist of N-channels, on delayed neuronal death following transient ischemia in gerbils. Delayed neuronal death in the CA1 subfield of the hippocampus following 5-min ischemia was attenuated by omega-CTX in a dose-dependent manner when the agent was injected intracisternally 1 hr before ischemia was produced. However, omega-CTX failed to prevent neurotoxicity produced by a direct injection of quinolinic acid into the hippocampus in rats. These results suggest that omega-CTX has a neuroprotective effect against ischemic brain injury, which effect probably results from its inhibition of the excessive release of neurotransmitters, including excitatory amino acids, during ischemia.

The effect of neuropeptides on the release of neurotransmitter amino acids from rat striatum

Neuropeptide Y (NPY), peptide YY (PYY), and galanin are found throughout the central nervous system with appreciable levels occurring in the striatum. In this study we have investigated the effects of these peptides on the potassium-stimulated release of endogenous neurotransmitter amino acids from slices of rat striatum. The release of glutamate was significantly reduced by nanomolar concentrations of each peptide, whilst the release of aspartate, gamma-aminobutyric acid (GABA) and glycine was not affected. The reduction in release due to galanin (200 nM) was inhibited by glibenclamide (5 microM). These results support the view that NPY, PYY and galanin modulate neurotransmitter release possibly by a presynaptic action. The results with glibenclamide suggest that the action of galanin is mediated through an ATP-sensitive potassium channel.

Characterization of K(+)-evoked [3H]D-aspartate outflow in the rat hippocampus in vitro

The characteristics of K(+)-evoked outflow of [3H]D-aspartate, a glutamate release marker, were systematically investigated in the rat hippocampus, using 35 mM K(+)-evoked [3H]noradrenaline outflow as a reference. Elevation of external K+ concentrations increased [3H]D-aspartate outflow in a concentration-dependent manner both in slices and synaptosomes. In the absence of external Ca2+, K(+)-evoked [3H]D-aspartate outflow was decreased by approx 60% in synaptosomes and 80% in slices. However, elimination of external Ca2+ in the presence of 2 mM EGTA significantly reduced only 100 mM K(+)-evoked outflow, both in slices and synaptosomes. In the absence of external Ca2+, 35 mM K(+)-evoked [3H]noradrenaline outflow was abolished even when EGTA was present in the solution. Furthermore, the Ca(2+)-channel blockers omega-conotoxin (10 nM) and nifedipine (0.5 microM) did not significantly reduce K(+)-evoked [3H]D-aspartate outflow; [3H]noradrenaline outflow, however, was reduced by more than one third by omega-conotoxin. Finally [3H]D-aspartate overflow was insensitive to tetrodotoxin (0.5 microM) both in synaptosomes and in slices, while that of [3H]noradrenaline was significantly reduced in slices. It is concluded that (1) [3H]D-aspartate outflow is partly Ca(2+)-dependent; (2) differences between K(+)-evoked [3H]D-aspartate and [3H]noradrenaline outflow include sensitivity to stimulation by EGTA, to Ca(2+)-channel blockers and to tetrodotoxin. Some of these discrepancies may be ascribed to the existence of a cytosolic, Ca(2+)-independent pool of releasable glutamate and [3H]D-aspartate. These observations pose some problems as to the experimental approach for the study of Ca(2+)-dependent [3H]D-aspartate release.

Modulation of calcium-dependent and -independent components of veratridine-evoked release of glutamate from rat cerebellum

The entry of Ca2+ into the presynaptic neuronal terminal is considered to be a prerequisite for exocytosis. However, reports suggest that a Ca(2+)-independent component of release can exist for some neurotransmitters. In this study we have used veratridine-stimulated release of glutamate from rat cerebellar slices to investigate Ca(2+)-dependent and -independent release. A 1-min pulse of veratridine (10 microM) induced release of glutamate in both Ca(2+)-replete and Ca(2+)-free ACSF. Both modes of release, however, could be elicited in a sequential manner following a single application of veratridine in Ca(2+)-free ACSF, with return to Ca(2+)-replete conditions 5 min post-pulse. This separation permitted the modulation of either, or both, phases of release. Apamin and dihydrokainate had little effect on Ca(2+)-independent release but produced enhancement of the Ca(2+)-dependent phase. Tetrodotoxin abolished both phases of release when applied with the veratridine pulse, but had no effect on the Ca(2+)-dependent phase alone. The Ca(2+)-dependent phase was partially sensitive to Co2+, although the Ca2+ channel blockers verapamil, amiloride, omega-conotoxin and ruthenium red were ineffective, suggesting a lack of involvement of L-, N- or T-type channels. The possible mechanisms mediating the Ca(2+)-dependent and -independent components of endogenous glutamate release from cerebellar slices are discussed.