Effects of lacosamide, a novel sodium channel modulator, on dorsal horn neuronal responses in a rat model of neuropathy

Various mechanisms underlie the complexity of neuropathic pain (pain due to disease of the somatosensory system), with each mechanism bearing a different order of relevance from one person and pain state to the next. Successful treatment is contingent on sound knowledge of underlying mechanisms that may occur at peripheral, spinal and/or supraspinal sites. In particular, ion channels throughout the nervous system are known to play an intimate part in neuropathic pain, and thus stand as good targets for analgesic drugs. Agents that modulate voltage-gated sodium channel function can reduce action potential propagation along sensory neurones to reduce the transmission and perception of nociceptive signals. Lacosamide is a functionalised amino acid that affects voltage-gated sodium channels in a novel way by enhancing the slow inactivating ‘braking’ state of these channels. To validate lacosamide's inhibitory efficacy in vivo, we unilaterally ligated spinal nerves L5 and L6 in rats to induce a state of neuropathy, and on post-operative days 14-17 recorded evoked-responses of deep dorsal horn neurones before and after spinal or systemic lacosamide delivery. Lacosamide's effects on various measures in spinal nerve-ligated rats were compared to rats that underwent sham surgery. Our results show that neuropathy induced novel inhibitory effects of lacosamide on mechanical and electrical responses, and enhanced inhibitory effects on thermal responses after systemic or spinal administration, suggesting state-preference actions of lacosamide.     

Stereoselective effects of the novel anticonvulsant lacosamide against 4-AP induced epileptiform activity in rat visual cortex in vitro

We examined effects of the novel anticonvulsant lacosamide and its inactive isomer (SPM 6953) in an in vitro model of epileptiform activity. Focal field potential recordings (34+/-0.2 degrees C) were obtained from 17 to 22 day old rat brain slices. Physiological synaptic transmission (fEPSP amplitude and duration) in CA1 of rat hippocampus was not significantly altered (P > 0.05, n = 4) by lacosamide (1 microM-1 mM). Recording from visual cortex during application of 4-aminopyridine (4-AP; 100 microM) revealed both spontaneous and evoked 'ictal like' discharges. Spontaneous ictal like discharges in the visual cortex were blocked by 100 microM carbamazepine (CBZ), 100 microM pentobarbital and 200 microM phenobarbital (PHB) but were insensitive to the anti-absence drug ethosuximide (750 microM; n = 4, P > 0.05). Lacosamide reduced tonic duration and maximal firing frequency with EC(50)s of 41 and 71 microM, respectively. In contrast, the S stereoisomer (100-320 microM) produced no significant effect on spontaneous ictal activity (n = 3-4, P > 0.05). Seizures induced by high frequency (100 Hz, 1s) stimulation were selectively reduced in amplitude by PHB (200 microM) and frequency by CBZ (100 microM; n = 6) and lacosamide (100 microM; n = 4). GABAergic negative going potentials were attenuated by CBZ (irreversible with washing) and lacosamide (reversible) but not by PHB. We conclude that lacosamide blocks 4-AP induced epileptiform activity in the visual cortex. This novel anticonvulsant drug appears to inhibit epileptogenesis (seizure spread) by interacting with a stereoselective, but as yet unidentified, target site in rodent neocortex in the mid-micromolar range.     

The role of glutamatergic modulation in the mechanism of action of ketamine,a prototype rapid-acting antidepressant drug

Over the past decade, ketamine has been one of the most commonly studied potential antidepressants. This is because it produces a spectacularly rapid and persistent therapeutic effect in people suffering from severe treatment-resistant depression (TRD), for which classical drugs are ineffective. Similar efficacy was demonstrated for scopolamine, a drug belonging to a completely different pharmacological group. This interesting coincidence piqued the interest of psychopharmacologists and prompted them to search for a possible common mechanism of these rapid acting antidepressant drugs (RAADs). A thorough explanation of this mechanism is also important because each of these substances induces serious side effects. Knowing the mechanism responsible for the therapeutic efficacy of RAADs could lead to minimizing, or even avoiding certain undesirable effects. This review provides an overview of the mechanism of action of a prototype RAAD, ketamine, in animal models, with a particular focus on the roles of NMDA receptors, AMPA receptors, synaptogenesis, and modulation of glutamate transmission by other modulators of this system, such as mGlu receptor ligands. Recently studied roles for ketamine enantiomers and metabolites in its rapid antidepressant effect are also considered. Finally, the results of multiple clinical trials are reported and discussed in relation to basic research. This review concludes that success in introducing novel therapeutic RAADs will depend on better cooperation and integration of neuroscience research and clinical practice.     

Selectivity of antagonists for the Cys‐loop native receptors for ACh, 5‐HT and GABA in guinea‐pig myenteric neurons

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Antihyperalgesic efficacy of lacosamide in a rat model for muscle pain induced by TNF

Chronic muscle pain is a problem with high prevalence in clinical practice and its pharmacological treatment is difficult. There is a lack of animal models which reliably predict analgesic activity of drugs on muscle pain. Here we used intramuscular injection of tumor necrosis factor-alpha (TNF) in rats as a model of muscle pain. In this model we tested the antihyperalgesic action of lacosamide in comparison to the analgesics pregabalin and gabapentin. Mechanical withdrawal thresholds to muscle pressure were measured with an algesimeter exerting pressure on the gastrocnemius muscles previously injected with TNF. Fore limb grip strength was measured with a digital grip force meter after TNF injection into the biceps brachii muscles. A complete reversal of hyperalgesia was seen with lacosamide at 30mg/kg. Significant effects were also seen for pregabalin at 30 and 100mg/kg and gabapentin at 100mg/kg. In biceps muscle hyperalgesia, a significant reversal of hyperalgesia was seen with lacosamide at 10mg/kg. Significant effects were also seen for pregabalin and gabapentin at 100mg/kg. We could thus demonstrate in a rat model for myalgia that lacosamide effectively reduces muscular hyperalgesia and is somewhat more potent than gabapentin and pregabalin.     

Identification of a novel high affinity adenosine binding protein from bovine striatum

Summary In solubilized extracts from bovine striatal membranes three different binding sites for 5-N-ethylcarboxamidoadenosine ([³H]NECA) were observed after separation of the extract by gel filtration on Sepharose CL-6B. The first peak was eluted in the void volume and contained the AZ adenosine receptor. In the second peak, [³H]NECA binding sites were eluted with a pharmacological profile characteristic of adenotin, a low affinity non-receptor adenosine binding protein. The third peak represented approximately 50% of the [³H]NECA binding activity. This site bound [³H]NECA in a reversible and saturable manner with K _D of 17 nmol/l and a binding capacity of 11.3 pmol/mg protein. In competition experiments, adenosine, NECA, NAD, nnosine, 5-AMP and S-adenosyl-L-homocysteine were the most potent ligands. In contrast to adenosine receptors, this site did neither bind adenosine receptor antagonists nor the A₂ selective agonist CGS 21,680 (2-[p-(2-carboxyethyl)phenethylamino]5-N-ethylcarboxamidoadenosine). These results suggest the existence of a novel high affinity binding site for adenosine of unknown function in bovine striatum.Abbreviations AMPPCP ,-methyleneadenosine-5-triphosphonate - CCPA 2-chloro-N⁶-cyclopentyladenosine - CHAPS 3-[3-(cholamidopropyl)dimethylammonio]-1-propanesulfonate - CGS 21,680 2-[p-(2-carboxyethyl)phenethylamino]-5-N-ethylcarbox-amidoadenosine - CPA N⁶-cyclopentyladenosine - DPCPX 1,3-dipropyl-8-cyclopentylxanthine - GppNHp guanosine-5-[,-imido]triphosphate - GTP[S] guanosine-5-O-(3-thiotriphosphate) - NBTI S-4-nitrobenzyl-6-thioinosine - NECA 5-N-ethylcarbox-amidoadenosine - NECG 5-N-ethylcarboxamidoguanosine - PIA N⁶-phenylisopropyladenosine - SAH S-adenosyl-L-homocysteine - XAC 8-{4-[([{(2-aminoethyl)-amino}carbonyl]-methyl)oxy]-phenyl}-1,3-dipropylxanthine Send offprint requests to A. Lorenzen at the above address     

Group III mGlu receptor agonists potentiate the anticonvulsant effect of AMPA and NMDA receptor block

We report the anticonvulsant action in DBA/2 mice of two mGlu Group III receptor agonists: (R,S)-4-phosphonophenylglycine, (R,S)-PPG, a compound with moderate mGlu8 selectivity, and of (1S,3R,4S)-1-aminocyclopentane-1,2,4-tricarboxylic acid, ACPT-1, a selective agonist for mGlu4alpha receptors. Both compounds, given intracerebroventricularly at doses which did not show marked anticonvulsant activity, produced a consistent shift to the left of the dose-response curves (i.e. enhanced the anticonvulsant properties) of 1-(4'-aminophenyl)-3,5-dihydro-7,8-dimethoxy-4H-2,3-benzodiazepin-4-one hydrochloride, CFM-2, a noncompetitive AMPA receptor antagonist, and 3-((+/-)-2-carboxypiperazin-4-yl)-1-phosphonic acid, CPPene, a competitive NMDA receptor antagonist, in DBA/2 mice. In addition, (R,S)-PPG and ACPT-1 administered intracerebroventricularly prolonged the time course of the anticonvulsant properties of CFM-2 (33 micromol/kg, i.p.) and CPPene (3.3 micromol/kg, i.p.) administered intraperitoneally. We conclude that modest reduction of synaptic glutamate release by activation of Group III metabotropic receptors potentiates the anticonvulsant effect of AMPA and NMDA receptor blockade.     

The neuroprotective effects of the decahydroisoquinoline, LY 215490; a novel AMPA antagonist in focal ischaemia

LY 215490 (3 RS,4aRS,6RS,8aRS)-6-[2-(1(2)H- tetrazole- 5-yl)ethyl]decahydroisoquinoline -3-carboxylic acid), a novel, selective, competitive and systemically active AMPA receptor antagonist was tested as a neuroprotective agent against focal ischaemia in a model of permanent MCA occlusion in the rat. LY 215490 was administered at a dose of 10, 30 or 100 mg/kg 30 min prior to and post-MCA occlusion. The animals were allowed to survive for 24 hr, following which time the brains were processed for volumetric analysis of the infarct size. The low dose of LY 215490 was not effective against the infarct volume in the hemisphere, cortex or caudate. The 2 × 30 mg/kg dose of LY 215490 resulted in 25 and 31% protection against the volume of hemispheric and cortical ischaemic damage, respectively. The highest dose of LY 215490 resulted in a reduced neuroprotective effect with 23 and 27% protection against the volume of hemispheric and cortical ischaemic damage, respectively. The slightly reduced neuroprotective effect of the highest dosing regimen may be due to the respiratory problems seen with this dose. Neither of the two neuroprotective doses of LY 215490 produced any reduction in the volume of caudate damage which represents the core of the infarct.     

Receptor binding profile suggests multiple mechanisms of action are responsible for ibogaine's putative anti-addictive activity

The indole alkaloid ibogaine (NIH 10567, Endabuse) is currently being examined for its potential utility in the treatment of cocaine and opioid addiction. However, a clearly defined molecular mechanism of action for ibogaine's putative anti-addictive properties has not been delineated. Radioligand binding assays targeting over 50 distinct neurotransmitter receptors, ion channels, and select second messenger systems were employed to establish a broad in vitro pharmacological profile for ibogaine. These studies revealed that ibogaine interacted with a wide variety of receptors at concentrations of 1–100 µM. These included the mu, delta, kappa, opiate, 5HT₂, 5HT₃, and muscarinic_{1 and 2} receptors, and the dopamine, norepinephrine, and serotonin uptake sites. In addition, ibogaine interacted withN-methyl-d-aspartic acid (NMDA) associated ion and sodium ion channels as determined by the inhibition of [³H]MK-801 and [³H]bactrachotoxin A 20--benzoate binding (BTX-B), respectively. This broad spectrum of activity may in part be responsible for ibogaine's putative anti-addictive activity.     

3H-TVX Q 7821: identification of 5-HT1 binding sites as target for a novel putative anxiolytic

Summary In order to clarify the mechanism of action of the putative nonbenzodiazepine anxiolytic TVX Q 7821 [2-(4-(4-(2-pyrimidinyl)-1-piperazinyl)butyl)-1,2-benzisothiazol-3-(2H)one-1,1-dioxidehydrochloride], binding studies with the radio labelled compound were performed.³H-TVX Q 7821 bound apidly, reversibly and in a saturable manner with high affinity to calf brain structures with preference for the hippocampus (K _D 1.62 nmol/l;B _max 320 fmol/mg protein).³H-TVX Q 7821 binding was displaced only by 5-hydroxytryptamine and its agonists and antagonists including spiperone, but was not displaced by a variety of other neurotransmitters and drugs. The 5-HT₂ receptor antagonist ketanserin was a weak displacer. The hippocampal binding sites for³H-TVX Q 7821 were pharmacologically very similar to the 5-HT₁-binding sites in this region. TVX Q 7821 is likely to be an important tool in research on functional aspects of 5-HT₁ binding sites.     

Pharmacological characterisation of the GlyT-1 glycine transporter using two novel radioligands

Inhibitors of the glycine transporter GlyT-1 are being developed as potential treatments for schizophrenia. Here we report on the use of two novel radioligands, [³H]-SB-733993 and [³H]-GSK931145, for the characterisation of GlyT-1 in both cells and native tissue. Binding was evaluated in membranes either from HEK293 cells expressing recombinant human GlyT-1 (hGlyT-1) or from rat cerebral cortex. Specific binding of both [³H]-SB-733993 and [³H]-GSK931145 to hGlyT-1 HEK293 cell membranes and rat cerebral cortex membranes was saturable and comprised >90% of total binding. K_d and B_max values for the two radioligands were fairly similar, with K_d values of 1-2 nM and B_max values of around 7000 fmol/mg protein in hGlyT-1 membranes and 3000 fmol/mg protein in rat cortex membranes. Association of [³H]-SB-733993 was faster, with binding reaching equilibrium within 30 min compared with 90 min for [³H]-GSK931145. Dissociation was also much slower for [³H]-GSK931145 than for [³H]-SB-733993, with 50% of specific binding being dissociated by approximately 40 min and 5 min, respectively. Autoradiography studies with [³H]-GSK931145 showed widespread distribution of binding in rat brain, with generally higher binding in caudal compared with rostral areas. Initial studies in human frontal cortex membranes showed clear specific binding of [³H]-GSK931145, though with much lower density (B_max 570 fmol/mg protein) and slightly lower affinity (K_d 4.5 nM) compared with rat cortex. A human brain autoradiography study showed higher specific binding in cerebellum compared with frontal cortex. All GlyT-1 inhibitors tested, as well as glycine itself, competed fully for the binding of both [³H]-SB-733993 and [³H]-GSK931145 in both hGlyT-1 and rat cortex membranes. Studies on the effect of varying NaCl concentration showed that [³H]-SB-733993 binding was reduced by >90% in the absence of added Na⁺ ions, whilst [³H]-GSK931145 binding was unaffected. Glycine produced concentration-dependent decreases in binding affinity of both radioligands without major changes in B_max values, suggesting that both [³H]-SB-733993 and [³H]-GSK931145 bind to sites on GlyT-1 that are orthosteric to the site at which glycine itself binds. Overall, these results show that both [³H]-SB-733993 and [³H]-GSK931145 are useful radioligands for studies on GlyT-1 in both cell lines and native tissues, with [³H]-GSK931145 being the radioligand of choice for further studies on GlyT-1 expression and pharmacology.     

Isolation and characterization of Jingzhaotoxin-V, a novel neurotoxin from the venom of the spider Chilobrachys jingzhao.

Jingzhaotoxin-V (JZTX-V), a 29-residue polypeptide, is derived from the venom of the spider Chilobrachys jingzhao. Its cDNA determined by rapid amplification of 3' and 5'-cDNA ends encoded an 83-residue precursor with a pro-region of 16 residues. JZTX-V inhibits tetrodotoxin-resistant and tetrodotoxin-sensitive sodium currents in rat dorsal root ganglion neurons with IC50 values of 27.6 and 30.2 nM, respectively. Moreover, the toxin exhibits high affinity to the resting closed states of the channels. JZTX-V also inhibits Kv4.2 potassium currents expressed in Xenpus Laevis oocytes (IC50=604.2 nM), but has no effects on outward delay-rectified potassium channels expressed in Xenopus laevis oocytes. JZTX-V alters the gating properties of sodium channels by shifting the activation curves to the depolarizing direction and the inactivation curves to the hyperpolarizing direction. Small unilamellar vesicles binding assays show that the partitioning of JZTX-V into lipid bilayer requires negatively charged phospholipids. The phospholipid membrane binding activity of JZTX-V is also verified using intrinsic tryptophan fluorescence analysis as well as acrylamide-quenching assays. Importantly, human multiple sodium channel subtypes are attractive targets for treatment of pain, highlighting the importance of JZTX-V as potential lead for drug development.     

GABAA receptor modulation by the novel intravenous general anaesthetic E-6375

E-6375 (4-butoxy-2-[4-(2-cyanobenzoyl)-1-piperazinyl] pyrimidine hydrochloride) is a new intravenous general anaesthetic with an anaesthetic potency, in mice, comparable to propofol, or etomidate. Here, we examined the effect of E-6375 upon the GABA_A receptor, a putative target of intravenous anaesthetic action. E-6375 reversibly enhanced GABA-evoked currents mediated by recombinant GABA_A (₁β₂γ_2L) receptors expressed in Xenopus laevis oocytes, with little effect on NMDA- and kainate-evoked currents mediated by NR1a/NR2A and GluR1o/GluR2o glutamate receptors, respectively. E-6375 prolonged the decay of GABA-evoked miniature inhibitory postsynaptic currents recorded from rat Purkinje neurones demonstrating the anaesthetic also enhanced the activity of synaptic GABA_A receptors. The GABA enhancing action of E-6375 on recombinant GABA_A receptors was unaffected by the subtype of the isoform (i.e. _xβ₂γ_2L; x=1–3) within the receptor, but was increased by the omission of the γ_2L subunit. Receptors incorporating β₂, or β₃, subunits were more sensitive to modulation by E-6375 than those containing the β₁ subunit. The selectivity of E-6375 was largely governed by the identity (serine or asparagine) of a single amino acid residue within the second transmembrane domain of the β-subunit. The various in vivo actions of general anaesthetics may be mediated by GABA_A receptor isoforms that have a differential distribution within the CNS. The identification of agents, such as E-6375, that discriminate between GABA_A receptor subtypes may augur the development of general anaesthetics with an improved therapeutic profile.     

The tail wags the dog: possible mechanism for reverse allosteric control of ligand-activated channels

In this issue of the British Journal of Pharmacology, a new article by Kozuska et al. discusses the multiple salt bridges in the intracellular domain of the 5HT_3A receptor. These interactions increase the overall rigidity of the receptor, stabilize its low conducting state and affect the ligand cooperativity. The authors suggest that the allosteric effects of these regions on the receptor may be involved in a possible ‘reverse’ allosteric modulation of 5HT₃ receptors.

Linked Article

This article is a Commentary on Kozuska et al., pp. 1617–1628 volume 171 issue 7. To view this paper visit http://dx.doi.org/10.1111/bph.12536     

Cysteine accessibility analysis of the human alpha7 nicotinic acetylcholine receptor ligand-binding domain identifies L119 as a gatekeeper

A large number of structurally diverse ligands have been produced to selectively target α7 nicotinic acetylcholine receptors (nAChRs). We applied the method of scanning cysteine accessibility mutations (SCAM) to the ligand-binding domain of the α7 nAChR to identify subdomains of particular importance to the binding and subsequent activation by select agonists. We evaluated the activity of four structurally distinct α7 agonists on wild-type human α7 and 44 targeted mutants expressed in Xenopus oocytes. Responses were measured prior and subsequent to the application of the sulfhydryl reagent methanethiosulfonate ethylammonium (MTSEA). One mutant (C116S) served as a Cys-null control, and the additional mutants were made in the C116S background. In many cases, the insertion of free cysteines into the agonist-binding site had a negative effect on function, with 12 of 44 mutants showing no detectable responses to ACh, and with only 19 of the 44 mutants showing sufficiently large responses to permit further study. Several of the cysteine mutations, including W55C, showed selectively reduced responses to the largest agonist tested, 2-methoxy,4-hydroxy-benzylidene anabaseine. Interestingly, although homology models suggest that most of the introduced cysteine mutations should have had good solvent accessibility, application of MTSEA had no effect or produced only modest changes in the agonist response profile of most mutants. Consistent with previous studies implicating W55 to play important roles in agonist activation, MTSEA treatment further decreased the functional responses of W55C to all the test agonists. While the cysteine mutation at L119 itself had relatively little effect on receptor function, treatment of L119C receptors with MTSEA or alternative cationic sulfhydryl reagents profoundly decreased activation by all agonists tested, suggesting a general block of gating. The homologous mutation in heteromeric nAChRs produced similar results, provided that the mutation was placed in the beta subunit complementary surface of the ligand-binding domain. Structural models locate the L119 residue directly across the subunit interface from the C-loop of the primary face of the binding domain. Our data suggest that a covalent modification of L119C by MTSEA or other cationic reagents might block the binding of even small agonists such as TMA through electrostatic interactions. Reaction of L119C with small non-polar reagents increases activation by small agonists but can block the access of large ligands such as benzylidene anabaseines to the ligand-binding domain.     

The anticonvulsant action of the carbonic anhydrase inhibitor methazolamide: possible involvement of a noradrenergic mechanism

In contrast to previous concepts, the noradrenergic mechanism required for the anticonvulsant action of inhibitors of carbonic anhydrase in mice is extracerebral on the basis of the following: (1) the l-aromatic amino acid decarboxylase inhibitor Ro 4-4602/1 abolished the restorative action of d,1-dopa in reserpine- and phenoxy-benzamine-treated mice without evidence of significant inhibition of 1-aromatic amino acid decarboxylase in brain, (2) depletion of extracerebral stores of norepinehrine by the i.v. administration of 6-hydroxydopamine abolished the anticonvulsant effect of methazolamide, and (3) the anticonvulsant action of the carbonic anhydrase inhibitor in 6-hydroxydopamine-treated mice was restored by the administration of d,1-dopa. The noradrenergic mechanism appears to be of the α-type. Consistent antagonist action was shown by the α-blockers varied in action. The antagonist action of the α-blockers studied appeared to be specifically related to α-adrenergic blockade because (1) potency and duration of antagonist action correspond with potency and duration of α-adrenergic blockade, (2) phenoxybenzamine failed to antagonize the anticonvulsant action of diphenylhydantoin, (3) the anticonvulsant action of methazolamide was not abolished when phenoxybenzamine was given after the carbonic anhydrase inhibitor, (4) d,1-dopa administration reversed the antagonist action of phenoxybenzamine, and (5) the dopamine β-hydroxylase inhibitor U-14624 annulled the reversal of phenoxybenzamine's antagonist action by d,1-dopa. The anticonvusant action of methazolamide in rats was not affected by depletion of cerebral or extracerebral stores of norepinephrine by 6-hydroxydopamine. Anticonvulsant action was abolished by the administration of reserpine to rats injected i.vent. with 6-hydroxydopamine. The data suggest that norepinephrine and serotonin both are involved in the anticonvulsant action of inhibitors of carbonic anhydrase in rats.     

Nonclinical safety assessment of SPX-101, a novel peptide promoter of epithelial sodium channel internalization for the treatment of cystic fibrosis

Background: ENaC inhibition has long been an attractive therapeutic target for the treatment of cystic fibrosis. However, previous attempts at developing ENaC inhibitors have been unsuccessful due to complications arising from systemic circulation of the compounds. Here, we describe the preclinical toxicology assessment of a new inhaled peptide promoter of ENaC internalization delivered as a nebulized aerosol.

Methods: Preclinical assessment of SPX-101 safety was determined using an in vitro hERG assay, bolus injection of SPX-101 in a canine cardiovascular and respiratory safety pharmacology model and 28-day inhalation toxicology studies of nebulized drug in rats and dogs.

Results: SPX101 had no effects on the respiratory, cardiac or central nervous systems. The 28-day inhalation toxicology studies of nebulized SPX-101 in rats and dogs revealed no drug-related adverse events. Plasma levels of SPX-101 peaked less than 1?h after the end of treatment in rats and were below the limit of detection in canine models.

Conclusions: SPX-101, a novel peptide promoter of ENaC internalization, elicited no adverse effects at doses up to the MFD and in excess of the highest preclinical efficacious and expected clinical doses. In contrast to channel blockers like amiloride and derivative small molecules, SPX-101 does not achieve significant systemic circulation, thus doses are not limited due to toxic side effects like hyperkalemia and weight loss.     

Neutral effects of the novel analgesic tapentadol on cardiac repolarization due to mixed ion channel inhibitory activities

Tapentadol is a novel analgesic with µ‐opioid receptor agonistic and norepinephrine reuptake inhibiting activities at submicromolar concentrations. Given that inhibition of cardiac potassium currents by noncardiovascular pharmaceuticals is a critical issue in drug development, the potential effects of tapentadol on myocardial repolarization were evaluated. Tapentadol concentration‐dependently inhibited hERG‐related currents in CHO cells with 250 and 750 times lower potency than haloperidol and sertindole. In electrically stimulated guinea pig papillary muscles, tapentadol at 10 and 100 µM shortened the action potential duration at 30% of repolarization (APD₃₀) and APD₉₀. Maximum effects (100 µM) were more pronounced on APD₃₀ than on APD₉₀. In contrast, the hERG inhibitor, d,l‐sotalol concentration‐dependently (1–100 nM) prolonged APD₉₀ without affecting APD₃₀. In isolated, perfused, spontaneously beating guinea pig hearts, volume‐conducted ECGs revealed that tapentadol reduced heart rate (HR) and prolonged uncorrected QT time, but did not affect HR‐corrected QTc time. The hERG inhibitor, dofetilide (0.01 and 0.1 µM) reduced HR and prolonged QT and QTc time. PR time was prolonged by tapentadol but not by dofetilide. Intravenous infusion of tapentadol (3, 6, 9mg/kg) in conscious dogs led to suprapharmacological plasma concentrations of up to 12 µM (2,531 ng/ml). QT time decreased in tapentadol‐treated dogs in parallel to an increase in HR, whereas HR‐corrected QTc time was not affected. In conclusion, the in vitro effects of tapentadol suggest mixed ion channel activities on potassium, calcium, and sodium channels at supra‐pharmacological concentrations. These activities may be neutralizing, resulting in lack of a net effect of tapentadol on cardiac repolarization. Drug Dev Res 1–12, 2010 © 2010 Wiley‐Liss, Inc.     

Glycine induces a novel form of long-term potentiation in the superficial layers of the superior colliculus

1. The mammalian superior colliculus (SC) is a midbrain nucleus containing space maps of different sensory modalities which show various forms of age- and activity-dependent plasticity in vivo and in vitro. In the present study, we aimed to characterize the role of glycine (Gly) receptors in the SC, and we observed that application of glycine (Gly; 500 μM and 3 mM) for 7 min to SC slices of adult guinea-pigs caused a novel form of long-term potentiation (termed LTP_gly) of evoked excitatory postsynaptic potentials recorded in the superficial layers.
2. The strength of potentiation was found to be concentration-dependent and partially independent from synaptic stimulation.
3. LTP_gly did not involve NMDA receptor activation as proven by the lack of inhibition by 100 μM D,L-2-amino-5-phosphonovaleric acid (APV) and 10 μM MK-801.
4. LTP_gly could only be masked but not prevented by strychnine (100 μM) and remained undisturbed in the presence of picrotoxin (100 μM).
5. Inhibition of carbonic anhydrase by acetazolamide (20 μM) had no effect on LTP_gly suggesting that the excitatory action of Gly is not due to a differential breakdown of the Cl⁻/HCO₃⁻ gradients.
6. As indicated by the inhibition of LTP_gly of the fEPSP slope by the L-type calcium channel blocker nifedipine (20 μM), voltage-dependent calcium channels are the source for Ca²⁺ elevation as the intracellular trigger.
7. Our data provide the first evidence for a role of Gly in SC synaptic transmission. They illustrate a so far unknown action of Gly which can lead to long-lasting changes of synaptic efficacy and which is not mediated via NMDA-related or strychnine-sensitive binding sites.