Factors promoting the release of picrotoxin from the trap in the GABA(A) receptor pore.

Picrotoxin (PTX), a convulsant of plant origin, has been used in many studies as research tool. PTX is the open channel blocker of the GABAA receptor (GABAAR). Being in the pore, PTX initiates transfer of the channel to the closed state and thus it falls into the "trap". The consequence of this PTX trapping is so-called aftereffect, i.e. continuation of the blockade of the GABA-induced chloride current (IGABA) after removal of PTX from the external solution. The present work shows that the positive allosteric modulators (PAMs) of the GABAA receptor, allopregnanolone (Allo) and zolpidem (Zolp) as well as a high concentration of GABA shortened the PTX aftereffect. Experiments were carried out on isolated Purkinje neurons of the rat cerebellum using the whole-cell patch-clamp method. IGABA was induced by applications of 5 µM GABA (EC30) for 1 s with 30 s intervals. 50 µM PTX completely blocked IGABA, and recovery upon PTX washout occurred with a time constant (τrec) of 20.2 min. 1 µM Allo reduced the blocking effect of PTX by 30% and accelerated the recovery of IGABA by almost 10 times (τrec = 2.4 min). 0.5 µM Zolp did not change the IGABA block in the presence of PTX but accelerated the recovery of IGABA by more than 3 times (τrec = 5.6 min). Increasing the GABA concentration to 20 µM did not change the blocking effect of PTX, but accelerated recovery by 6 times (τrec = 3.3 min). The mechanism of the shortening of the PTX aftereffect is presumably the expansion of the GABAAR pore in the presence of PAMs and a high concentration of the agonist and, as a consequence, the escape of PTX from the "trap". The work describes new pharmacological properties of Allo and Zolp.

Effect of nootropic dipeptide noopept on CA1 pyramidal neurons involves α7AChRs on interneurons in hippocampal slices from rat.

Noopept (NP) is a proline-containing dipeptide with nootropic and neuroprotective properties. We have previously shown that NP significantly increased the frequency of spontaneous IPSCs in hippocampal CA1 pyramidal cells mediated by the activation of inhibitory interneurons in stratum radiatum. The cholinergic system plays an important role in the performance of cognitive functions, furthermore multiple behavioral and clinical facts link NP with the cholinergic system. The present study was undertaken to reveal the possible interaction of NP with neuronal nicotinic acetylcholine receptors (nAChRs). Currents were recorded from rat hippocampal neurons using the whole-cell, patch-clamp technique. NP (5 µM) increased the action potential firing frequency recorded from GABAergic interneurons in the stratum radiatum (SR) of CA1 region. This effect was almost completely abolished by the application of the α7 nAChR-selective antagonists α-bungarotoxin (α-BGT; 6 nM) and methyllycaconitine (MLA; 20 nM). The increase in the frequency of spontaneous IPSCs in CA1 pyramidal cells induced by NP was also eliminated by α7 nAChRs antagonists. These results imply the involvement of α7 nAChRs in the modulation of hippocampal neuronal activity caused by NP and indicate that a7 nAChRs are an important site of action of NP.

Pregnane neurosteroids exert opposite effects on GABA and glycine-induced chloride current in isolated rat neurons.

The ability of endogenous neurosteroids (NSs) with pregnane skeleton modified at positions C-3 and C-5 to modulate the functional activity of inhibitory glycine receptors (GlyR) and ionotropic É£-aminobutyric acid receptors (GABAA R) was estimated. The glycine and GABA-induced chloride current (IGly and IGABA ) were measured in isolated pyramidal neurons of the rat hippocampus and in isolated rat cerebellar Purkinje cells, respectively. Our experiments demonstrated that pregnane NSs affected IGABA and IGly in a different manner. At low concentrations (up to 5 µM), tested pregnane NSs increased or did not change the peak amplitude of the IGABA , but reduced the IGly by decreasing the peak amplitude and/or accelerating desensitization. Namely, allopregnanolone (ALLO), epipregnanolone (EPI), pregnanolone (PA), pregnanolone sulfate (PAS) and 5ß-dihydroprogesterone (5ß-DHP) enhanced the IGABA in Purkinje cells. Dose-response curves plotted in the concentration range from 1 nM to 100 µM were smooth for EPI and 5ß-DHP, but bell-shaped for ALLO, PA and PAS. The peak amplitude of the IGly was reduced by PA, PAS, and 5α- and 5ß-DHP. In contrast, ALLO, ISO and EPI did not modulate it. Dose-response curves for the inhibition of the IGly peak amplitude were smooth for all active compounds. All NSs accelerated desensitization of the IGly . The dose-response relationship for this effect was smooth for ALLO, PA, PAS and 5ß-DHP, but it was U-shaped for EPI, 5α-DHP and ISO. These results, together with our previous results on NSs with androstane skeleton, offer comprehensive overview for understanding the mechanisms of effects of NSs on IGly and IGABA .

Human Three-Finger Protein Lypd6 Is a Negative Modulator of the Cholinergic System in the Brain.

Lypd6 is a GPI-tethered protein from the Ly-6/uPAR family expressed in the brain. Lypd6 enhances the Wnt/ß-catenin signaling, although its action on nicotinic acetylcholine receptors (nAChRs) have been also proposed. To investigate a cholinergic activity of Lypd6, we studied a recombinant water-soluble variant of the human protein (ws-Lypd6) containing isolated "three-finger" LU-domain. Experiments at different nAChR subtypes expressed in Xenopus oocytes revealed the negative allosteric modulatory activity of ws-Lypd6. Ws-Lypd6 inhibited ACh-evoked currents at α3ß4- and α7-nAChRs with IC50 of ∼35 and 10 µM, respectively, and the maximal amplitude of inhibition of 30-50%. EC50 of ACh at α3ß4-nAChRs (∼30 µM) was not changed in the presence of 35 µM ws-Lypd6, while the maximal amplitude of ACh-evoked current was reduced by ∼20%. Ws-Lypd6 did not elicit currents through nAChRs in the absence of ACh. Application of 1 µM ws-Lypd6 significantly inhibited (up to ∼28%) choline-evoked current at α7-nAChRs in rat hippocampal slices. Similar to snake neurotoxin α-bungarotoxin, ws-Lypd6 suppressed the long-term potentiation (LTP) in mouse hippocampal slices. Colocalization of endogenous GPI-tethered Lypd6 with α3ß4- and α7-nAChRs was detected in primary cortical and hippocampal neurons. Ws-Lypd6 interaction with the extracellular domain of α7-nAChR was modeled using the ensemble protein-protein docking protocol. The interaction of all three Lypd6 loops ("fingers") with the entrance to the orthosteric ligand-binding site and the loop C of the primary receptor subunit was predicted. The results obtained allow us to consider Lypd6 as the endogenous negative modulator involved in the regulation of the cholinergic system in the brain.

Electrophysiological Activity and Survival Rate of Rats Nervous Tissue Cells Depends on D/H Isotopic Composition of Medium.

The deuterium content modification in an organism has a neuroprotective effect during the hypoxia model, affecting anxiety, memory and stress resistance. The aim of this work was to elucidate the possible mechanisms of the medium D/H composition modification on nerve cells. We studied the effect of an incubation medium with a 50 ppm deuterium content compared to a medium with 150 ppm on: (1) the activity of Wistar rats' hippocampus CA1 field neurons, (2) the level of cultured cerebellar neuron death during glucose deprivation and temperature stress, (3) mitochondrial membrane potential (MMP) and the generation of reactive oxygen species in cultures of cerebellar neurons. The results of the analysis showed that the incubation of hippocampal sections in a medium with a 50 ppm deuterium reduced the amplitude of the pop-spike. The restoration of neuron activity was observed when sections were returned to the incubation medium with a 150 ppm deuterium content. An environment with a 50 ppm deuterium did not significantly affect the level of reactive oxygen species in neuron cultures, while MMP decreased by 16-20%. In experiments with glucose deprivation and temperature stress, the medium with 50 ppm increased the death of neurons. Thus, a short exposure of nerve cells in the medium with 50 ppm deuterium acts as an additional stressful factor, which is possibly associated with the violation of the cell energy balance. The decrease in the mitochondrial membrane potential, which is known to be associated with ATP synthesis, indicates that this effect may be associated with the cell energy imbalance. The decrease in the activity of the CA1 field hippocampal neurons may reflect reversible adaptive changes in the operation of fast-reacting ion channels.

The mechanisms of potentiation and inhibition of GABAA receptors by non-steroidal anti-inflammatory drugs, mefenamic and niflumic acids.

Fenamates mefanamic and niflumic acids (MFA and NFA) induced dual potentiating and inhibitory effects on GABA currents recorded in isolated cerebellar Purkinje cells using the whole-cell patch-clamp and fast-application techniques. Regardless of the concentration, both drugs induced a pronounced prolongation of the current response. We demonstrated that the same concentration of drugs can produce both potentiating and inhibitory effects, depending on the GABA concentration, which indicates that both processes take place simultaneously and the net effect depends on the concentrations of both the agonist and fenamate. We found that the NFA-induced block is strongly voltage-dependent. The Woodhull analysis of the block suggests that NFA has two binding sites in the pore - shallow and deep. We built a homology model of the open GABAAR based on the cryo-EM structure of the open α1 GlyR and applied Monte-Carlo energy minimization to optimize the ligand-receptor complexes. A systematic search for MFA/NFA binding sites in the GABAAR pore revealed the existence of two sites, the location of which coincides well with predictions of the Woodhull model. In silico docking suggests that two fenamate molecules are necessary to occlude the pore. We showed that MFA, acting as a PAM, competes with an intravenous anesthetic etomidate for a common binding site. We built structural models of MFA and NFA binding at the transmembrane ß(+)/α(-) intersubunit interface. We suggested a hypothesis on the molecular mechanism underlying the prolongation of the receptor lifetime in open state after MFA/NFA binding and ß subunit specificity of the fenamate potentiation.

Effects of copper on viability and functional properties of hippocampal neurons in vitro.

Copper (Cu2+) is an essential metal presented in the mammalian brain and released from synaptic vesicles following neuronal depolarization. However, the disturbance of Cu2+ homeostasis results in neurotoxicity. In our study we performed for the first time a combined functional investigation of cultured hippocampal neurons under Cu2+ exposure, its effect on spontaneous spike activity of hippocampal neuronal network cultured on multielectrode array (MEA), and development of long-term potentiation (LTP) in acute hippocampal slices in the presence of Cu2+. Application of 0.2mM CuCl2 for 24h reduced viability of cultured neurons to 40±6%, whereas 0.01mM CuCl2 did not influence significantly on the neuronal survival. However, exposure to the action of 0.01mM Cu2+ resulted in pronounced reduction of network spike activity and abolished LTP induced by high-frequency stimulation of Schaffer's collaterals in CA1 pyramidal neurons of hippocampal slices. Antioxidant Trolox, the hydrosoluble vitamin E analogue, prevented neurotoxic effect and alterations of network activity under Cu2+ exposure, but didn't change the impairment of LTP in Cu2+-exposured hippocampal slices. We hypothesized that spontaneous network neuronal activity probably is one of the potential targets of Cu2+-induced neurotoxicity, in which free radicals can be involved. At the same time, it may be suggested that Cu2+-induced alterations of long-lasting trace processes (like LTP) are not mediated by oxidative damage.

Mitochondria-targeted antioxidant SkQT1 decreases trauma-induced neurological deficit in rat and prevents amyloid-ß-induced impairment of long-term potentiation in rat hippocampal slices.

This study assesses a protective effect of a mitochondria-targeted antioxidant SkQT1 (a mixture of 10-(6'-toluquinonyl) decyltriphenylphosphonium and 10-(5'-toluquinonyl) decyltriphenylphosphonium in proportion of 1.4:1), using an open focal trauma model of the rat brain sensorimotor cortex and a model of amyloid-beta1-42 (Abeta)-induced impairment of hippocampal long-term potentiation (LTP), a kind of synaptic plasticity associated with learning and memory. It was found that a trauma-induced neurological deficit could be partially improved with daily intraperitoneal injections of SkQT1 (250 nmol/kg) for 5 days after the trauma. Neither an analog of SkQT1 without thymoquinone (C12TPP) nor original thymoquinone without a cation residue was effective to improve such conditions. In the SkQ molecule, the phosphonium cation can be replaced by the rhodamine 19 cation, with the SkQTR1 being still active in the treatment of the neurological deficit. Application of 200 nM Abeta to rat hippocampal slices impaired the induction of LTP in the hippocampal CA1 pyramidal layer. A single intraperitoneal injection of SkQT1 (250 nmol/kg body weight) made 24 h before the slice preparation prevented the harmful effect of Abeta on the LTP. Thus mitochondria-targeted antioxidants, containing thymoquinone, have neuroprotective properties.

Block of GABA(A) receptor ion channel by penicillin: electrophysiological and modeling insights toward the mechanism.

GABA(A) receptors (GABA(A)R) mainly mediate fast inhibitory neurotransmission in the central nervous system. Different classes of modulators target GABA(A)R properties. Penicillin G (PNG) belongs to the class of noncompetitive antagonists blocking the open GABA(A)R and is a prototype of ß-lactam antibiotics. In this study, we combined electrophysiological and modeling approaches to investigate the peculiarities of PNG blockade of GABA-activated currents recorded from isolated rat Purkinje cells and to predict the PNG binding site. Whole-cell patch-сlamp recording and fast application system was used in the electrophysiological experiments. PNG block developed after channel activation and increased with membrane depolarization suggesting that the ligand binds within the open channel pore. PNG blocked stationary component of GABA-activated currents in a concentration-dependent manner with IC50 value of 1.12mM at -70mV. The termination of GABA and PNG co-application was followed by a transient tail current. Protection of the tail current from bicuculline block and dependence of its kinetic parameters on agonist affinity suggest that PNG acts as a sequential open channel blocker that prevents agonist dissociation while the channel remains blocked. We built the GABA(A)R models based on nAChR and GLIC structures and performed an unbiased systematic search of the PNG binding site. Monte-Carlo energy minimization was used to find the lowest energy binding modes. We have shown that PNG binds close to the intracellular vestibule. In both models the maximum contribution to the energy of ligand-receptor interactions revealed residues located on the level of 2', 6' and 9' rings formed by a bundle of M2 transmembrane segments, indicating that these residues most likely participate in PNG binding. The predicted structural models support the described mechanism of PNG block.

Glycine receptor in hippocampal neurons as a target for action of extracellular cyclic nucleotides.

Cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) are well known intracellular second messengers. At present study, we describe the effects of extracellularly applied cAMP and cGMP on glycine-induced chloride currents (I(Gly)) in isolated rat hippocampal pyramidal neurons. 50 or 500 µM glycine was applied for 600 ms with 1 min intervals. cAMP and cGMP were co-applied with glycine. We found that both cAMP and cGMP rapidly, reversibly and in a dose-dependent manner accelerated the I(Gly) desensitization. The effect was more prominent on I(Gly) induced by 500 µM than by 50 µM glycine. Dose-response curves were constructed in the 0.1-100,000 nM range of cAMP and cGMP concentrations. They demonstrate that threshold concentration of both compounds was about 1 nM and maximal effect was manifested at 100 nM. When cAMP and cGMP were added to the recording pipette, their extracellular application caused the effects similar to those obtained with normal intracellular medium. The effects of cyclic nucleotides remained unchanged in the presence of the antagonist of adenosine receptors in extracellular solution, and the agonist of adenosine receptors did not mimic the effect of cyclic nucleotides. The changes in the decay kinetics were equally pronounced at negative and positive membrane potentials. When co-administered 1 nM cAMP and 1 nM cGMP caused a weaker effect than either of the compounds alone which suggests a negative interaction between binding sites for cAMP and cGMP. This work describes a novel mode of action of cyclic nucleotides, namely, the modulation of GlyRs functions from extracellular side.

Mitochondria-targeted plastoquinone antioxidant SkQ1 prevents amyloid-ß-induced impairment of long-term potentiation in rat hippocampal slices.

Bath application of 200 nM amyloid-ß1-42 (Aß) to rat hippocampal slices impairs induction of long-term potentiation (LTP) of the population spike in pyramidal layer of the CA1 field of the hippocampus. Intraperitoneal injection of mitochondria-targeted plastoquinone derivative SkQ1 at very low concentrations (250 nmol/kg body weight) given 24 h before the slice preparation or 1 h treatment of hippocampal slices with 250 nM SkQ1 prevents the deleterious effect of Aß on LTP. To elucidate which part of the molecule is responsible for this type of neuroprotective activity, the effect of the analog of SkQ1 lacking plastoquinone (C12TPP) was studied. It was found that C12TPP was much less efficient in LTP protection than SkQ1 itself. It means that the plastoquinone part of the SkQ1 molecule is responsible for the LTP rescue. To summarize, in vivo and in vitro injection of SkQ1 compensates for Aß-induced oxidative damage of long-term synaptic plasticity in the hippocampus, which is considered to be the main reason of memory loss and impairment of other cognitive functions associated with Alzheimer's disease. Therefore, SkQ1 may be considered as a promising candidate for the treatment of early-stage Alzheimer's disease.

Donepezil in low micromolar concentrations modulates voltage-gated potassium currents in pyramidal neurons of rat hippocampus.

Donepezil is a cholinesterase inhibitor widely used for the treatment of Alzheimer's disease. Voltage-gated K(+)-channels are discussed as possible targets for the drug, but the results obtained by different authors are contradictory. In the present study performed on pyramidal cells isolated from rat's hippocampus, we investigated the effect of donepezil on delayed rectifier K(+)-current (I(K(DR))) and transient outward K(+)-current (I(K(A))) using patch-clamp technique. The inhibitory effect of donepezil on I(K(DR)) was found in all the cells tested, but its strength varied in different cells. Two groups of neurons were differing in their sensitivity to donepezil: more sensitive (IC(50)=8.9 µM) and less sensitive (IC(50)=114.9 µM). The effect of the drug on I(K(DR)) was rapid, reversible and voltage-dependent, increasing with depolarization. Donepezil modulated I(K(A)) in two different ways: in some cells it suppressed the current with the IC(50) value of 23.4 µM, while in other cells it augmented the current with a bell-shaped dose-response curve. Maximal (about twofold) enhancement of I(K(A)) amplitude was caused by 10 µM donepezil. Augmentation of I(K(A)) increased with membrane depolarization. Our results show for the first time that voltage-dependent potassium channels in mammals' neurons are effectively modulated by low micromolar concentrations of donepezil.

Donepezil in a narrow concentration range augments control and impaired by beta-amyloid peptide hippocampal LTP in NMDAR-independent manner.

Acetylcholinesterase (AChE) inhibitor donepezil is widely used for the treatment of Alzheimer's disease (AD). The mechanisms of therapeutic effects of the drug are not well understood. The ability of donepezil to reverse a known pathogenic effect of ß-amyloid peptide (Abeta), namely, the impairment of hippocampal long-term potentiation (LTP), was not studied yet. The goal of the present study was to study the influence of donepezil in 0.1-10 µM concentrations on control and Abeta-impaired hippocampal LTP. Possible involvement of N-methyl-D: -aspartate receptors (NMDARs) into mechanisms of donepezil action was also studied. LTP of population spike (PS) was studied in the CA1 region of rat hippocampal slices. Change of LTP by donepezil treatment had a bell-shaped dose-response curve. The drug in concentrations of 0.1 and 1 µM did not change LTP while in concentration of 0.5 µM significantly increased it, and in concentration of 5 and 10 µM suppressed LTP partially or completely. Abeta (200 nM) markedly suppressed LTP. Addition of 0.1, 0.5 or 1 µM donepezil to Abeta solution caused a restoration of LTP. N-methyl-D: -aspartate (NMDA) currents were studied in acutely isolated pyramidal neurons from CA1 region of rat hippocampus. Neither Abeta, nor 0.5 µM donepezil were found to change NMDA currents, while 10 µM donepezil rapidly and reversibly depressed it. Results suggest that donepezil augments control and impaired by Abeta hippocampal LTP in NMDAR-independent manner. In general, our findings extend the understanding of mechanisms of therapeutic action of donepezil, especially at an early stage of AD, and maybe taken into account while considering the possibility of donepezil overdose.

Novel nootropic dipeptide Noopept increases inhibitory synaptic transmission in CA1 pyramidal cells.

Effects of newly synthesized nootropic and anxiolytic dipeptide Noopept on inhibitory synaptic transmission in hippocampal CA1 pyramidal cells were investigated using patch-clamp technique in whole-cell configuration. Bath application of Noopept (1 microM) significantly increased the frequency of spike-dependant spontaneous IPSCs whereas spike-independent mIPSCs remained unchanged. It was suggested that Noopept mediates its effect due to the activation of inhibitory interneurons terminating on CA1 pyramidal cells. Results of current clamp recording of inhibitory interneurons residing in stratum radiatum confirmed this suggestion.

The binding of donepezil with external mouth of K+-channels of molluscan neurons.

Earlier, we have shown a strong inhibitory effect of donepezil on K+-current of molluscan neurons (Solntseva et al., Comp Biochem Physiol 144, 319-326, 2007). In the present work, a possible interaction of donepezil with the external mouth of the channel was examined using, as a tool, tetraethylammonium (TEA), a classical antagonist of potassium channels. Experiments were conducted in isolated neurons of snail Helix aspersa using the two-microelectrode voltage-clamp technique. A high-threshold slow-inactivating K+-current involving Ca2+-dependent (I (C)) and Ca2+-independent (I (K)) components was recorded. The I (C) was estimated at 30 mV, and I (K) at 100 mV. The IC(50) values for blocking effect of donepezil on I (C) varied from 5.0 to 8.9 microM in different cells. Corresponding values for I (K) varied from 4.9 to 9.9 microM. The IC(50) values for blocking effect of TEA on I (C) lied in the range of 200 to 910 microM, and on I (K) lied in the range of 100 to 990 microM. The comparison of the effects of donepezil and TEA on the same cells revealed significant correlation between IC(50) values of these effects. The value of Spearman coefficient of correlation (r) was 0.77 for I (C) (P < 0.05), and 0.82 for I (K) (P < 0.05). In the presence of TEA, the effect of donepezil, both on I (C) and I (K), appears significantly weaker than in control solution. Dose-response curves of donepezil effect both on I (C) and I (K) were shifted right along horizontal axis when donepezil was applied in combination with TEA. Results suggest that TEA interferes with donepezil and precludes the occupation by donepezil of its own site. We suppose that the site for donepezil is situated near the TEA site with possible overlap.

The effects of ferric iron on voltage-gated potassium currents in molluscan neurons.

In isolated neurons of Helix pomatia, a two-microelectrode voltage clamp technique was used to study the effect of Fe3+ on voltage-gated potassium currents: a low-threshold fast-inactivating current (I(A)) and a high threshold slow-inactivating current with calcium-dependent (I(C)) and calcium-independent (I(DR)) components. Extracellular application of FeCl3 rapidly, reversibly and dose-dependently reduced the amplitude of I(A), I(C) and I(DR) with IC50 values of 49, 45 and 70 microM, respectively. Complete inhibition of K+ currents was reached at 100-500 microM Fe3+. The threshold for the total slow-inactivating potassium current shifted in a positive direction by 10-30 mV in the presence of Fe3+ (50-300 microM). Our work is the first demonstration of the strong blocking effect of Fe3+ on potassium currents of neuronal membrane.

Donepezil is a strong antagonist of voltage-gated calcium and potassium channels in molluscan neurons.

Donepezil is an acetylcholinesterase inhibitor used in Alzheimer's disease therapy. The neuroprotective effect of donepezil has been demonstrated in a number of different models of neurodegeneration including beta-amyloid toxicity. Since the mechanisms of neurodegeneration involve the activation of both Ca(2+)- and K(+)-channels, the study of donepezil action on voltage-gated ionic currents looked advisable. In the present study, the action of donepezil on voltage-gated Ca(2+)- and K(+)-channels was investigated on isolated neurons of the edible snail (Helix pomatia) using the two-microelectrodes voltage-clamp technique. Donepezil rapidly and reversibly inhibited voltage activated Ca(2+)-current (I(Ca)) (IC(50)=7.9 microM) and three types of high threshold K(+)-current: Ca(2+)-dependent K(+)-current (I(C)) (IC(50)=6.4 microM), delayed rectifier K(+)-current (I(DR)) (IC(50)=8.0 microM) and fast transient K(+)-current (I(Adepol)) (IC(50)=9.1 microM). The drug caused a dual effect on low-threshold fast transient K(+)-current (I(A)), potentiating it at low (5 microM) concentration, but inhibiting at higher (7 microM and above) concentration. Donepezil also caused a significant hyperpolarizing shift of the voltage-current relationship of I(Ca) (but not of any type of K(+)-current). Results suggest the possible contribution of the blocking effect of donepezil on the voltage-gated Ca(2+)- and K(+)-channels to the neuroprotective effect of the drug.

Cyclic nucleotides induce long-term augmentation of glutamate-activated chloride current in molluscan neurons.

1. Literature data indicate that serotonin induces the long-term potentiation of glutamate (Glu) response in molluscan neurons. The aim of present work was to elucidate whether cyclic nucleotides can cause the same effect. 2. Experiments were carried out on isolated neurons of the edible snail (Helix pomatia) using a two-microelectrode voltage-clamp method. 3. In the majority of the cells examined, the application of Glu elicited a Cl- -current. The reversal potential (Er) of this current lied between -35 and -55 mV in different cells. 4. Picrotoxin, a blocker of Cl- -channels, suppressed this current equally on both sides of Er. Furosemide, an antagonist of both Cl- -channels and the Na+/K+/Cl- -cotransporter, had a dual effect on Glu-response: decrease in conductance, and shift of Er to negative potentials. 5. A short-term (2 min) cell treatment with 8-Br-cAMP or 8-Br-cGMP caused long-term (up to 30 min) change in Glu-response. At a holding potential of -60 mV, which was close to the resting level, an increase in Glu-activated inward current was observed. This potentiation seems to be related to the right shift of Er of Glu-activated Cl- -current rather than to the increase in conductance of Cl- -channels. The blocking effect of picrotoxin rested after 8-Br-cAMP treatment. 6. The change in the Cl- -homeostasis as a possible mechanism for the observed effect of cyclic nucleotides is discussed.

Selective suppression of the slow-inactivating potassium currents by nootropics in molluscan neurons.

The role of the voltage-gated K+ channels in the effect of some nootropics was investigated. Earlier, the multiple effect of high concentrations of two nootropics, piracetam and its peptide analogue GVS-111 [Seredenin et al. (1995), US Patent No. 5,439,930], on Ca2+ and K+ currents of molluscan neurons was shown [Solntseva et al. (1997), General Pharmacology 29, 85-89]. In the present work, we describe the selective effect of low concentrations of these nootropics as well as vinpocetine on certain types of K+ current. The experiments were performed on isolated neurons of the land snail Helix pomatia using a two-microelectrode voltage-clamp method. The inward voltage-gated Ca2+ current (ICa) and three subtypes of the outward voltage-gated K+ current were recorded: Ca2+-dependent K+ current (IK(Ca)), delayed rectifying current (IKD), and fast-inactivating K+ current (IA). It has been found that I Ca was not changed in the presence of 30 microM vinpocetine, 100 microM piracetam or 10 nM GVS-111, while slow-inactivating, TEA-sensitive IK(Ca) and IKD were inhibited (IK(Ca) more strongly than IKD). In contrast, the fast-inactivating, 4-AP-sensitive K+ current (IA) was not diminished by low concentrations of piracetam and GVS-111, while vinpocetine even augmented it. A possible role of slow-inactivating subtypes of the K+ channels in the development of different forms of dementia is discussed.

Mechanisms of GABA(A) receptor blockade by millimolar concentrations of furosemide in isolated rat Purkinje cells.

The action of diuretic furosemide on the GABA(A) receptor was studied in acutely isolated Purkinje cells using the whole-cell recording and fast application system. Furosemide blocked stationary component of GABA-activated currents in a concentration-dependent manner with IC(50) value > 5 mM at -70 mV. The inhibition was rapid in the onset, fully reversible and did not require drug pre-perfusion. The termination of GABA and furosemide co-application was followed by transient increase in the inward current 'tail' current, which was not observed when furosemide was continuously present in the solution. The degree of furosemide block did not depend on GABA concentration. Furosemide block increased with membrane depolarization. Five millimolar furosemide depressed GABA currents by 32.4+/-1.3% at -70 mV and by 76.7+/-5.0% at +70 mV. Analysis of the voltage dependence of the block suggests that furosemide binds at the site located within GABA(A) channel pore with a dissociation constant of 5.3+/-0.5 mM at 0 mV and electric distance of 0.27. Our results provide evidence that furosemide interacts with Purkinje cell GABA(A) receptors (most probably composed of alpha1beta2/3gamma2 subunits) through a low affinity site located in channel pore and suggest that furosemide acts as a sequential open channel blocker, which prevents the dissociation of agonist while the channel is blocked.