Influence of Membrane Receptor Lateral Diffusion on the Short-Term Depression of Acetylcholine-Induced Current in Helix Neurons.

We have studied how various drugs increasing the rate of nicotinic acetylcholine receptors (nAChRs) lateral diffusion affect the depression of ACh-induced current in land snail Helix lucorum neurons responsible for defensive behavior. The acetylcholine (ACh) iontophoretic application protocol imitated the behavioral habituation protocol for the intact animal. We found that the drugs decreasing cholesterol level in cell membranes as methyl-β-cyclodextrin 1 mM and Ro 48-8071 2 µM, and polyclonal antibodies to actin-binding proteins as spectrin 5 µg/ml and merlin 2.

Habituation-Like Decrease of Acetylcholine-Induced Inward Current in Helix Command Neurons: Role of Microtubule Motor Proteins.

The role of kinesin and dynein microtubule-associated molecular motors in the cellular mechanism of depression of acetylcholine-induced inward chloride current (ACh-current) was examined in command neurons of land snails (Helix lucorum) in response to repeated applications of ACh to neuronal soma. This pharmacological stimulation imitated the protocol of tactile stimulation evoking behavioural habituation of the defensive reaction. In this system, a dynein inhibitor (erythro-9-(2-hydroxy-3-nonyl)adenine, 50 µM) decreased the ACh-current depression rate.

[Influence of lateral diffusion on depression of neural cholinsensitivity].

Using mathematical model a comparative analysis of the influence of receptor lateral diffusion, endocytosis and exocytosis of receptors on the change in the number of membrane receptors at rhythmical local applications of a mediator on neural soma was performed. The results allow us to estimate quantitatively the effect of these processes on the length of the period between mediator applications. The necessity of considering the change in the rate of receptor lateral diffusion, while studying the effect of some protein kinases and protein phosphatases on the change in the number of receptors, became evident.

[Role of serine/threonine and tyrosine protein phosphatases in command Helix lucorum neurons at the cellular correlate of habituation].

Effects of some inhibitors of serine/threonine and tyrosine protein phosphatases on the depression and spontaneous recovery of the acetylcholine-induced inward current (ACh-current) in command Helix neurons of defensive behavior at the cellular correlate of habituation were investigated. The following drugs were used: okadaic acid (reduces activity ofphosphatases PP1 and PP2A), endothall (PP2A), cyclosporine A and cypermethrin (PP2B), CCT007093 (PPM1D), dephostatin (blocks tyrosine phosphatases). All used inhibitors modify the depression flow, and endothall reduces spontaneous recovery of ACh-current also.

[Role of myosins in depression of sensitivity of Helix neurons to acetylcholine in a cellular analog of habituation].

We investigated the involvement of cytoskeleton motor proteins, myosins, in the molecular mechanism of sensitivity depression to acetylcholine in Helix command neurons of defensive behavior in a cellular analog of habituation. There were analyzed the effects of several drugs disturbing myosin function: ML-7 and MLCK-IP-18--blockers of myosin light chain kinase, blebbistatin--an inhibitor of non-muscle myosin II, Y-27632--inhibitor of kinases ROCK-I and ROCK-II (activate mainly non-muscle myosin II) on the depression of acetylcholine-induced inward current. It was found that ML-7 and MLCK-IP- 18 weakened current depression; blebbistatin and Y-27632 did not change the depression.

Mobility of acetylcholine receptors in command Helix lucorum neurons in a cellular analog of habituation.

We investigated the role of the mobility of acetylcholine receptors in the depression of an acetylcholine-induced inward current (ACh-current) of Helix lucorum (a land snail) command neurons of defensive behavior in a cellular analog of habituation. The inhibitors of endocytosis and exocytosis, actin microfilaments and cytoskeleton microtubules, serine/threonine protein kinases (PKA, PKG, calcium calmodulin-dependent PK II, p38 mitogen-activated PK), tyrosine kinases (including Src-family kinases), serine/threonine phosphatases (PP1, PP2A, PP2B, PPM1D), and tyrosine protein phosphatases altered the depression of the ACh-current. A comparison of experimentally calculated curves of the ACh-current of these neurons and those obtained by mathematical modeling revealed the following: (a) ACh-current depression is caused by the reduction in the number of membranous ACh-receptors, which results from the shift in the balance of multidirectional transport processes of receptors toward the predominance of ACh-receptor internalization over their recycling; (b) depression of ACh-current depends on the activity of serine/threonine and tyrosine protein kinases and protein phosphatases, whose one of the main targets is the neuron transport system-actin microfilaments and microtubules of cytoskeleton, as well as motor proteins.

Cholinergic receptor exocytosis under conditions of depression of acetylcholine-induced current in edible snail neurons in cellular analogue of habituation.

Exocytosis inhibitor Exo 1 potentiates depression of acetylcholine-induced inward current in defensive behavior command neurons of edible snail, when acetylcholine is applied rhythmically to the soma in cellular correlate of habituation. A mathematical model presupposing different receptor localization in the cell and regularities of their translocations made it possible to analyze the dependence of acetylcholine-induced current depression on a number of intracellular processes. It was concluded that depression of choline-sensitive extrasynaptic zones on the membrane of defensive behavior command neurons in edible snail in cellular correlate of habituation is partially determined by attenuation of exocytosis of internalized cholinergic receptors.

[Role of actin microfilaments in depression of acetylcholine-induced current in Helix lucorum neurons on cellular analogue of habituation].

Toxins that impair the function of actin microfilaments in cytoskeleton, cytochalasin B (disrupts microfilaments by inhibiting actin polymerization) and phalloidin (binds polymeric F-actin, stabilizing it and interfering with the function of actin-rich structures) reduce the depression of acetylcholine-induced inward current in Helix lucorum command neurons of defensive behavior during rhythmical local acetylcholine applications to soma (cellular analogue of habituation). These results and mathematical simulation allow us to suggest that the depression of cholinosensitivity of extrasynaptic membrane zones in command neurons on the cellular analogue of habituation is associated with the involvement of actin microfilaments in reduction of the number of membrane cholinoreceptors.

[The role of serine/threonine and tyrosine protein kinases in the depression of cholinosensitivity in Helix lucorum neurons in the cellular correlate of habituation].

Inhibitor ofadenylate cyclase (SQ 22,536) and inhibitors ofserin/threonine protein kinases A (PKA -Rp-cAMPS), G (PKG - H-Arg-Lys-Arg-Ala-Arg-Lys-Glu-OH), calcium/calmodulin-dependent kinase II (CaMKII - KN-93), p38mitogen-activated (MAPK - PD 169316), and tyrosine protein kinases (genistein), including their Src-family (PP2), weaken the depression of the acetylcholine-induced inward current (ACh-current) in command Helix neurons of defensive behavior under conditions of rhythmical local acetylcholine applications to the soma in the cellular analogue of habituation. Selective inhibitor of protein kinase C (PKC - chelerythrine) does not change the depression of the ACh-current. Mathematical simulation of the influence of the inhibitors applied on a number of membrane-connected acetylcholine receptors made it possible to obtain the design curves consistent with the experimental curves of the ACh-current depression.

[The possible mechanisms of acetylcholine receptor desensitization].

The postsynaptic mechanisms initiating the development of two acetylcholine receptor states (I and D) which determine the desensitization are discussed. Attention is focused on the processes of receptor endocytosis as the major mechanism of desensitization. General processes and differences in the development of the D state during the desensitization and long-term depression are analyzed.

[Endocytosis of cholinoreceptors in the mechanism of depression of cholinosensitivity in Helix lucorum neurons in a cellular model of habituation].

Inhibitors of dynamin-dependent endocytosis (dynamin inhibitory peptide and dynasore) and inhibitors of tubulin (colchicine and vinblastine) decrease the depression of acetylcholine-induced inward current in command neurons of Helix defensive behavior under conditions of rhythmical local applications of acetylcholine to a neuron soma in a cellular model of habituation. Mathematical model which allows for the possibility of different localizations of receptors in a neuron and characteristics of receptor travel makes it possible to analyze the dependence of the current depression on some intracellular processes. We suggest that the cholinosensitivity depression of extrasynaptic membrane zones in command neurons of Helix defensive behavior in the cellular model of habituation under study is associated with dynamin-dependent endocytosis of cholinoreceptors with involvement of cytoskeleton microtubules.

[Model of the interreceptor cluster interaction as the basis for increased neuron sensitivity to a mediator].

Inter receptor relations may result in activation of the majority of cluster receptors, when the mediatory concentration is small then to activate "unclucter" receptors. The neuron activation by the small mediatory concentration, the postsynaptic membrane changes under synaptic modification, the one receptor activation by unconnected with him receptor can be explain as results of the cluster receptor influence.

[The role of cluster receptor state in synaptic plasticity].

Common postsynaptic mechanisms underlying formation and increase in efficiency of glutamate and GABA synapses are discussed. Much attention is given to clusterization of different receptor types as a mechanism of long-term potentiation. A possibility of synchronization of activities of receptors forming the same cluster is discussed.

[The role of different conformational states of receptors in the formation of synaptic plasticity].

A mathematical model was proposed that explains the formation of successive temporal stages of synaptic plasticity in terms of the transition of post-synaptic ionotropic receptors to different conformational states.

[Study of giant depolarizing activity in the neonatal hippocampal neurones].

Spontaneous oscillatory activity is a general feature of developing neural networks. Early in postnatal development, spontaneous network-driven events, termed giant depolarizing potentials (GDPs), occur synchronously over the entire hippocampus. By performing simulation of hippocampal network with using physiology parameters of the neurons and its network from the present experiments and literature dates, we investigated the participation of the different components of network in the generation of GDPs.

Mathematical simulation of the induction of long-term depression in cerebellar Purkinje cells.

The question of the mechanisms underlying the induction of associative and homosynaptic long-term depression in cerebellar Purkinje cells is addressed. Mathematical simulation was used to investigate the possibility that long-term depression, which is associated with a decrease in the efficiency of AMPA receptors, could be induced both by phosphorylation and dephosphorylation of these receptors.

[Modeling features of induction of long term potentiation in pyramidal neurons in the CA3 hippocampal region].

The conditions for inducing the long-term potentiation in the hippocampus were analyzed using the mathematical model of a CA3 pyramidal cell. Their dependence on the physiological characteristics of the neuron, the basic biochemical processes, and inhibitory efficacy was considered. The role of additional membrane depolarization in the origination of the hysteresis of the number of phosphorylated receptors as the basis for the formation of an "intermediate" stage of long-term potentiation was studied.

[Mathematical simulation of induction of long-term depression in cerebellar Purkinje cells].

Mechanisms of associative and homosynaptic long-term depression (LTD) in cerebellar Purkinje cells are discussed. The possibility of LTD induction related to a decrease in efficacy of AMPA receptors through either their dephosphorylation or phosphorylation is investigated by mathematical simulation.

[Hysteresis of synaptic effectiveness--one of the possible mechanisms for associative learning?].

The conditions on the occurrence of hysteresis of the amount of phosphorylated receptors were determined depending on the rhythmic frequency of presynaptic neuronal activity using the full and simplified models of synaptic plasticity for pyramidal hippocampal neurones. It is assumed that this phenomenon lies at the basis of the conservation of long-term potentiation.

[Mathematical description of modifying synapses of hippocampal pyramidal neurons].

The system of differential equations describing the plasticity of the hippocampal pyramidal neuron CA3, developed before, was analyzed. The system was divided into two groups according to magnitudes of the biochemical reaction constants. The first group with large values of the constants was transformed into quasi stationary algebraic equations.

[Modulation of NMDA-receptor efficiency by intracellular agents].

Glutamate in one of the principle transmitters in the CNS. Ionotropic receptors of glutamate selectively activated by N-methyl-d-aspartate (NMDA) play an important role in the processes of development, learning, memory etc. Hyperactivation of these receptors is responsible for a number of pathological processes.

Contrasting of synaptic signals by simultaneous modification of excitatory and inhibitory inputs.

Conditions facilitating long-term contrasting of interneuronal connections were studied using a mathematical model of posttetanic Ca(2+)-dependent postsynaptic processes in pyramidal neurons of hippocampal field CA3. These studies demonstrated that modified inhibition selectively facilitates. long-term potentiation of the efficiency of one of the interneuronal connections when the presynaptic neuron discharges at a given frequency for a short time, while connections formed from the same postsynaptic cell with other presynaptic neurons undergo long-term depression.

Studies of long-term potentiation and depression of inhibitory transmission by mathematical modeling of post-synaptic processes.

A mathematical model of posttetanic processes launched by rhythmic stimulation of the excitatory and inhibitory inputs to the dendritic spine of a pyramidal neuron in hippocampal field CA3 was used to study conditions for modifying the efficiency of the inhibitory input. The level of dephosphorylation of GABAa and GABAb receptors, which determines the GABA sensitivity of these receptors, was shown to depend on the Ca(2+)-dependent ratio of active protein kinases and protein phosphatases; the level of dephosphorylation decreased monotonically as the intracellular Ca2+ increased. Posttetanic increases and decreases in the Ca2+ concentration, as compared with the level achieved during the previous stimulation, led to increases or decreases respectively in the number of dephosphorylated GABA receptors and to induction of long-term potentiation and depression, respectively, in the efficiency of inhibitory transmission.

[Genesis of rhythmic activity in a neuronal net model].

The behavior of the summary activity is learned on the proposed model of neuronal net, taking into account the data of the neuronal interactions in Ca3 hippocampus. The summary activities existence of the own oscillations frequency, depending on the synaptic connection weights is found. The hypothesis of this phenomena role on the theta-activity genesis in the hippocampus is proposed.

[A model of the posttetanic efficiency of a neuron dendritic spine].

A model of postsynaptic processes leading to a prolonged posttetanic modification (potentiation and depression) in the efficacy of the excitatory and inhibitory inputs to the neuron CA3 of the hippocampus was developed. The model enables one to estimate changes in postsynaptic potentials depending on the extent of receptor phosphorylation, which is determined by Ca(2+)-dependent changes in secondary messengers.