Inhibition of voltage-gated Ca2+ channels by antazoline

Combined Treatment With 2-(2-Benzofu-Ranyl)-2-Imidazoline and Recombinant Tissue Plasminogen Activator Protects Blood-Brain Barrier Integrity in a Rat Model of Embolic Middle Cerebral Artery Occlusion

Recombinant tissue plasminogen activator (rt-PA) is used to treat acute ischemic stroke but is only effective if administered within 4.5 h after stroke onset. Delayed rt-PA treatment causes blood-brain barrier (BBB) disruption and hemorrhagic transformation. The compound 2-(-2-benzofuranyl)-2-imidazoline (2-BFI), a newly discovered antagonist of high-affinity postsynaptic N-methyl-D-aspartate (NMDA) receptors, has been shown to have neuroprotective effects in ischemia. Here, we investigated whether combining 2-BFI and rt-PA can ameliorate BBB disruption and prolong the therapeutic window in a rat model of embolic middle cerebral artery occlusion (eMCAO). Ischemia was induced in male Sprague Dawley rats by eMCAO, after which they were treated with 2-BFI (3 mg/kg) at 0.5 h in combination with rt-PA (10 mg/kg) at 6 or 8 h. Control rats were treated with saline or 2-BFI or rt-PA. Combined therapy with 2-BFI and rt-PA (6 h) reduced the infarct volume, denatured cell index, BBB permeability, and brain edema. This was associated with increased expression of aquaporin 4 (AQP4) and tight junction proteins (occludin and ZO-1) and downregulation of intercellular adhesion molecule 1 (ICAM-1) and matrix metalloproteinases 2 and 9 (MMP2 and MMP9). We conclude that 2-BFI protects the BBB from damage caused by delayed rt-PA treatment in ischemia. 2-BFI may therefore extend the therapeutic window up to 6 h after stroke onset in rats and may be a promising therapeutic strategy for humans. However, mechanisms to explain the effects oberved in the present study are not yet elucidated.

Histamine H3 receptor activation counteracts adenosine A2A receptor-mediated enhancement of depolarization-evoked [3H]-GABA release from rat globus pallidus synaptosomes

High levels of histamine H3 receptors (H3Rs) are found in the globus pallidus (GP), a neuronal nucleus in the basal ganglia involved in the control of motor behavior. By using rat GP isolated nerve terminals (synaptosomes), we studied whether H3R activation modified the previously reported enhancing action of adenosine A2A receptor (A2AR) stimulation on depolarization-evoked [(3)H]-GABA release. At 3 and 10 nM, the A2AR agonist CGS-21680 enhanced [(3)H]-GABA release induced by high K(+) (20 mM) and the effect of 3 nM CGS-21680 was prevented by the A2AR antagonist ZM-241385 (100 nM). The presence of presynaptic H3Rs was confirmed by the specific binding of N-α-[methyl-(3)H]-histamine to membranes from GP synaptosomes (maximum binding, Bmax, 1327 ± 79 fmol/mg protein; dissociation constant, Kd, 0.74 nM), which was inhibited by the H3R ligands immepip, clobenpropit, and A-331440 (inhibition constants, Ki, 0.28, 8.53, and 316 nM, respectively). Perfusion of synaptosomes with the H3R agonist immepip (100 nM) had no effect on K(+)-evoked [(3)H]-GABA release, but inhibited the stimulatory action of A2AR activation. In turn, the effect of immepip was blocked by the H3R antagonist clobenpropit, which had no significant effect of its own on K(+)-induced [(3)H]-GABA release. These data indicate that H3R activation selectively counteracts the facilitatory action of A2AR stimulation on GABA release from striato-pallidal projections.

Fast, non-competitive and reversible inhibition of NMDA-activated currents by 2-BFI confers neuroprotection

Excessive activation of the N-methyl-D-aspartic acid (NMDA) type glutamate receptors (NMDARs) causes excitotoxicity, a process important in stroke-induced neuronal death. Drugs that inhibit NMDA receptor-mediated [Ca(2+)]i influx are potential leads for development to treat excitotoxicity-induced brain damage. Our previous studies showed that 2-(2-benzofu-ranyl)-2-imidazoline (2-BFI), an immidazoline receptor ligand, dose-dependently protects rodent brains from cerebral ischemia injury. However, the molecular mechanisms remain unclear. In this study, we found that 2-BFI transiently and reversibly inhibits NMDA, but not AMPA currents, in a dose-dependent manner in cultured rat cortical neurons. The mechanism of 2-BFI inhibition of NMDAR is through a noncompetitive fashion with a faster on (Kon = 2.19±0.33×10(-9) M(-1) sec(-1)) and off rate (Koff = 0.67±0.02 sec(-1)) than those of memantine, a gold standard for therapeutic inhibition NMDAR-induced excitotoxicity. 2-BFI also transiently and reversibly blocked NMDA receptor-mediated calcium entry to cultured neurons and provided long-term neuroprotection against NMDA toxicity in vitro. Collectively, these studies demonstrated a potential mechanism of 2-BFI-mediated neuroprotection and indicated that 2-BFI is an excellent candidate for repositioning as a drug for stroke treatment.

P/Q-type calcium channel modulators

P/Q-type calcium channels are high-voltage-gated calcium channels contributing to vesicle release at synaptic terminals. A number of neurological diseases have been attributed to malfunctioning of P/Q channels, including ataxia, migraine and Alzheimer's disease. To date, only two specific P/Q-type blockers are known: both are peptides deriving from the spider venom of Agelenopsis aperta, ω-agatoxins. Other peptidic calcium channel blockers with activity at P/Q channels are available, albeit with less selectivity. A number of low molecular weight compounds modulate P/Q-type currents with different characteristics, and some exhibit a peculiar bidirectional pattern of modulation. Interestingly, there are a number of therapeutics in clinical use, which also show P/Q channel activity. Because selectivity as well as the exact mode of action is different between all P/Q-type channel modulators, the interpretation of clinical and experimental data is complicated and needs a comprehensive understanding of their target profile. The situation is further complicated by the fact that information on potency varies vastly in the literature, which may be the result of different experimental systems, conditions or the splice variants of the P/Q channel. This review attempts to provide a comprehensive overview of the compounds available that affect the P/Q-type channel and should help with the interpretation of results of in vitro experiments and animal models. It also aims to explain some clinical observations by implementing current knowledge about P/Q channel modulation of therapeutically used non-selective drugs. Chances and challenges of the development of P/Q channel-selective molecules are discussed.

Synthetic Aβ oligomers (Aβ(1-42) globulomer) modulate presynaptic calcium currents: prevention of Aβ-induced synaptic deficits by calcium channel blockers

Alzheimer's disease is accompanied by increased brain levels of soluble amyloid-β (Aβ) oligomers. It has been suggested that oligomers directly impair synaptic function, thereby causing cognitive deficits in Alzheimer's disease patients. Recently, it has been shown that synthetic Aβ oligomers directly modulate P/Q-type calcium channels, possibly leading to excitotoxic cascades and subsequent synaptic decline. Using whole-cell recordings we studied the modulation of recombinant presynaptic calcium channels in HEK293 cells after application of a stable Aβ oligomer preparation (Aβ1-42 globulomer). Aβ globulomer shifted the half-activation voltage of P/Q-type and N-type calcium channels to more hyperpolarized values (by 11.5 and 7.5 mV). Application of non-aggregated Aβ peptides had no effect. We then analyzed the potential of calcium channel blockers to prevent Aβ globulomer-induced synaptic decline in hippocampal slice cultures. Specific block of P/Q-type or N-type calcium channels with peptide toxins completely reversed Aβ globulomer-induced deficits in glutamatergic neurotransmission. Two state-dependent low molecular weight P/Q-type and N-type calcium channel blockers also protected neurons from Aβ-induced alterations. On the contrary, inhibition of L-type calcium channels failed to reverse the deficit. Our data show that Aβ globulomer directly modulates recombinant P/Q-type and N-type calcium channels in HEK293 cells. Block of presynaptic calcium channels with both state-dependent and state-independent modulators can reverse Aβ-induced functional deficits in synaptic transmission. These findings indicate that presynaptic calcium channel blockers may be a therapeutic strategy for the treatment of Alzheimer's disease.

Neurovascular protection conferred by 2-BFI treatment during rat cerebral ischemia

Stroke is caused by vascular dysfunction and currently there are no effective therapeutics to stroke induced brain damage. In contrast to an intense emphasis on neuroprotection, relatively few studies have addressed means of vascular protection in cerebral ischemia. Here we discovered that the ligand to immidazolin receptor, 2-BFI, not only provided potent neuroprotection during middle cerebral artery occlusion in rat, which confirmed our previous reports, but also protected the integrity of the cerebral vasculature. Treatment with 2-BFI twice daily after the occlusion of the middle cerebral artery for 14 d significantly improved the neurological deficits, reduced brain infarction, and importantly, protected the cerebral vasculature as evidenced by the increased expression of an endothelial marker, von Willebrand factor, and better preservation of the cerebral vasculature, as viewed under a confocal microscope on rat brain perfused with FITC-labeled dextran. These results indicated that 2-BFI contributes to protection of neurovasculature. Understanding the molecular mechanisms could eventually lead to development of more effective therapies for stroke.

Pentobarbital inhibition of human recombinant alpha1A P/Q-type voltage-gated calcium channels involves slow, open channel block

BACKGROUND AND PURPOSE

Pre-synaptic neurotransmitter release is largely dependent on Ca(2+) entry through P/Q-type (Ca(V)2.1) voltage-gated Ca(2+) channels (PQCCs) at most mammalian, central, fast synapses. Barbiturates are clinical depressants and inhibit pre-synaptic Ca(2+) entry. PQCC barbiturate pharmacology is generally unclear, specifically in man. The pharmacology of the barbiturate pentobarbital (PB) in human recombinant alpha(1A) PQCCs has been characterized.

EXPERIMENTAL APPROACH

PB effects on macroscopic Ca(2+)(I(Ca)) and Ba(2+)(I(Ba)) currents were studied using whole-cell patch clamp recording in HEK-293 cells heterologously expressing (alpha(1A))(human)(beta(2a)alpha(2)delta-1)(rabbit) PQCCs.

KEY RESULTS

PB reversibly depressed peak current (I(peak)) and enhanced apparent inactivation (fractional current at 800 ms, r(800)) in a concentration-dependent fashion irrespective of charge carrier (50% inhibitory concentration: I(peak), 656 microM; r(800), 104 microM). Rate of mono-exponential I(Ba) decay was linearly dependent on PB concentration. PB reduced channel availability by deepening non-steady-state inactivation curves without altering voltage dependence, slowed recovery from activity-induced unavailable states and produced use-dependent block. PB (100 microM) induced use-dependent block during physiological, high frequency pulse trains and overall depressed PQCC activity by two-fold.

CONCLUSION AND IMPLICATIONS

The results support a PB pharmacological mechanism involving a modulated receptor with preferential slow, bimolecular, open channel block (K(d)= 15 microM). Clinical PB concentrations (<200 microM) inhibit PQCC during high frequency activation that reduces computed neurotransmitter release by 16-fold and is comparable to the magnitude of Ca(2+)-dependent facilitation, G-protein modulation and intrinsic inactivation that play critical roles in PQCC modulation underlying synaptic plasticity. The results are consistent with the hypothesis that PB inhibition of PQCCs contributes to central nervous system depression underlying anticonvulsant therapy and general anaesthesia.

Pre-synaptic histamine H₃ receptors modulate glutamatergic transmission in rat globus pallidus

The globus pallidus, a neuronal nucleus involved in the control of motor behavior, expresses high levels of histamine H(3) receptors (H(3)Rs) most likely located on the synaptic afferents to the nucleus. In this work we studied the effect of the activation of rat pallidal H(3)Rs on depolarization-evoked neurotransmitter release from slices, neuronal firing rate in vivo and turning behavior. Perfusion of globus pallidus slices with the selective H(3)R agonist immepip had no effect on the release of [(3)H]-GABA ([(3)H]-γ-aminobutyric acid) or [(3)H]-dopamine evoked by depolarization with high (20 mM) K(+), but significantly reduced [(3)H]-d-aspartate release (-44.8 ± 2.6% and -63.7 ± 6.2% at 30 and 100 nM, respectively). The effect of 30 nM immepip was blocked by 10 μM of the selective H(3)R antagonist A-331440 (4'-[3-[(3(R)-dimethylamino-1-pyrrolidinyl]propoxy]-[1,1-biphenyl]-4'-carbonitrile). Intra-pallidal injection of immepip (0.1 μl, 100 μM) decreased spontaneous neuronal firing rate in anaesthetized rats (peak inhibition 68.8±10.3%), and this effect was reversed in a partial and transitory manner by A-331440 (0.1 μl, 1 mM). In free-moving rats the infusion of immepip (0.5 μl; 10, 50 and 100 μM) into the globus pallidus induced dose-related ipsilateral turning following systemic apomorphine (0.5 mg/kg, s.c.). Turning behavior induced by immepip (0.5 μl, 50 μM) and apomorphine was partially prevented by the local injection of A-331440 (0.5 μl, 1 mM) and was not additive to the turning evoked by the intra-pallidal injection of antagonists at ionotropic glutamate receptors (0.5 μl, 1 mM each of AP-5, dl-2-amino-5-phosphonovaleric acid, and CNQX, 6-nitro-7-sulphamoylbenzo[f]quinoxaline-2,3-dione). These results indicate that pre-synaptic H(3)Rs modulate glutamatergic transmission in rat globus pallidus and thus participate in the control of movement by basal ganglia.

Noradrenergic agonists and antagonists influence migration of cortical spreading depression in rat-a possible mechanism of migraine prophylaxis and prevention of postischemic neuronal damage

Cortical spreading depression (CSD) is thought to be a neuronal mechanism that expands the penumbra zone after focal brain ischemia and that causes migraine aura. Both adrenergic agonists and antagonists significantly influence the size of the penumbra zone and decline the frequency of migraine. To study whether these compounds act by influencing CSD, we applied different drugs topically to an area of the exposed cortex of anesthetized adult rats and observed the migration of CSD-related DC potential deflections across the treated area. The adrenergic agonist norepinephrine (1 mmol/L) and the alpha(2)-agonist clonidine (0.56 mmol/L) blocked reversibly the migration of CSD. The beta-blocker propranolol (250 micromol/L to 1 mmol/L) dose-dependently diminished migration velocity or even blocked migration of CSD. The CSD blockade by the alpha(2)-antagonist yohimbine (1.75 mmol/L) was because of its action on inhibitory 5-HT(1A) receptors. None of the substances in the concentrations used had influence on regional cerebral blood flow or on systemic arterial blood pressure. The data suggest that the interference of these compounds with CSD may contribute to their beneficial therapeutic effect. The effect of beta-receptor antagonists in human migraine needs further exploration, since these drugs also work in migraine without aura.