Identifying nicotinic and muscarinic cholinoreceptors at the soma of neuron RPa4 inHelix lucorum

Cholinoreceptors were identified at the somatic membrane of theHelix lucorum RPa4 neuron using intracellular recording techniques. Application of specific agonists of nicotinic (nicotine, cytisine) and muscarinic (muscarine, arecoline) cholinoreceptors to the soma produced neuronal depolarization. The depolarization produced by applying acetylcholine to the cell was of short duration and was often replaced by hyperpolarization. Both selective desensitization of receptors by nicotine and muscarine as well as receptor occupancy by cytisine and arecoline reduced acetylchloline-induced response. The nicotinic cholinoblocker d-tubocurarine substantially inhibited responses to nicotinic cholinomimetics, while atropine, a muscarinic cholinoblocker, depressed response to muscarinic cholinomimetics. Acetylcholine-induced response was inhibited by both cholinoblockers more or less equally. The presence at the RPa4 neuronal somatic membrane is postulated of standard nicotinic and muscarinic cholinoreceptors similar to those found in vertebrates.M. V. Lomonsov State University, Moscow. Translated from Neirofiziologiya, Vol. 20, No. 2, pp. 203–212, March–April, 1988.     

Ionic mechanisms of depolarization responses in Helix pomatia neurons to glutamate application

Reversal potentials of transmembrane ionic currents induced by glutamate were determined in various D neurons ofHelix pomatia. Two types of neurons were found with mean reversal potentials of –10.6±1.2 and –40.0±0.6 mV. Neurons of the first group responded under ordinary conditions to glutamate application by a volley of action potentials. Neurons of the second group did not generate action potentials under the same conditions during glutamate application. With an increase in the dose of mediator the amplitude of D responses in these neurons increased only up to a certain limit, without reaching the critical depolarization level of the cell; a fall in the external chloride ion concentration led to a decrease in their reversal potential. The possible ionic mechanisms of glutamate-dependent depolarization responses of these groups of neurons are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 14, No. 6, pp. 572–577, November–December, 1982.     

Structural organization of an identified giant neuron of Helix lucorum

Method of intracellular staining with cobalt was used for detailed study of processes branching of the giant cell in the left parietal ganglion of the snail Helix lucorum L. Dendritic and axonal branches are described and quantitatively characterized. Terminals of axonal collaterals of this neurone innervating presumed neurosecretory bodies are described in the tissue surrounding the ganglion.     

Ionic mechanisms underlying modulating effects of serotonin on acetylcholine-induced responses in Helix pomatia neurons

The ionic mechanisms underlying modulatory effects of serotonin on acetylcholine-response in identified and nonidentifiedHelix pomatia neurons were investigated using voltage-clamping techniques at the neuronal membrane. External application of 10^–5–10^–4 M serotonin to the membrane of neurons responding to application of acetylcholine depending on Na⁺ depolarization (D_Na response) reduced membrane conductivity during response to acetylcholine without changing reversal potential of acetylcholine-induced current. Acetylcholine (10^–6–10^–4 M) administration took place 1–3 min later. Neurons with response to acetylcholine application dependent on Cl⁺ depolarization (D_Cl response) or hyperpolarization (H_Cl response) behaved similarly. Analogous effects could be produced by external application of theophylline which, together with the latency and residual effect characteristic of serotonin action points to the participation of intracellular processes associated with the cellular cyclase system in the changes produced by serotonin in acetylcholineinduced response. Serotonin brought about a shift in reversal potential and an increase in the acetylcholine-induced current in those neurons where this response was associated with changed permeability at the membrane to certain types of ions. During two-stage acetylcholine-induced response of the D_Na-H_K type, serotonin inhibited the inward current stage. Mechanisms underlying modulatory serotonin action on acetylcholine-induced response in test neurons are discussed in the light of our findings.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 20, No. 1, pp. 57–64, January–February, 1988.     

Regulation of neuron cholinoreceptor plasticity of Helix lucorum by second messengers and opioids

The rhythmical local ionophoretic applications of acetylcholine (ACh) to the somatic membrane of Helix lucorum identified neurons evokes the reversible depression of the ACh-induced response which shows cholinoreceptor (ChR) desensitization. ChR desensitization is regulated not by one but by several known second messengers and G-proteins. The endogenous opioids perform the excitation or inhibitory tonic control of the membrane potential in some neurons constantly activating the ionotropic opiate receptors. The direction of neuron ChR desensitization modulation by opioids depends on the type of the activated modulatory opiate receptors (mu or kappa) on the neuron membrane. Second messengers are involved in intracellular mechanism of modulation of the ChR desensitization by opiate kappa-agonist bremazocine.     

The effect of etorphine on nicotine- and muscarine-induced catecholamine secretion from perfused rat adrenal glands

The effect of etorphine on nicotine and muscarine-mediated catecholamine (CA) release from isolated perfused rat adrenal glands was investigated. Nicotine increased CA secretion at the low concentration of 0.5 micrograms while higher concentrations of muscarine (5 micrograms) were required. Moreover, muscarine released primarily epinephrine (EP) from rat adrenal glands while nicotine released norepinephrine (NE) and Ep. Etorphine inhibited NE and EP release evoked by nicotine to the same extent, whereas, muscarine-mediated release of NE and EP was not affected. Mecamylamine and verapamil inhibited nicotine but not muscarine-induced CA secretion. Our results suggest that etorphine preferentially interacts with nicotinic receptors on rat adrenal chromaffin cell membranes.     

Simulation of mechanisms responsible for initiation and modulation of bursting activity in RPa1 neuron of theHelix snail

A mathematical model of bursting activity in the RPa1 neuron of theHelix snail has been developed. The model allowed us to describe the processes of initiation and augmentation of the bursting activity related to transient secretion of a modulatory factor. Based on the analysis of computer simulations of various mechanisms underlying the effect of a modulating factor on the ionic membrane conductances in the bursting neuron, we suggested that modulating factor evokes a transition of non-voltage-dependent sodium channels and hyperpolarization-activated outward current channels to an active state and influences the gating of voltage-dependent sodium channels.Neirofiziologiya/Neurophysiology, Vol. 27, No. 1, pp. 11–17, January–February, 1995.     

Ionic and molecular mechanisms of beta-amyloid-induced depolarization of the mouse skeletal muscle fibres

Excess production and accumulation of beta-amyloid peptide (betaAP) are central for pathogenesis of Alzheimer's disease. Numerous studies showed that betaAP possessed wide range of toxic effects on neurons, however the mechanism of betaAP influence on another types of excitable cells, for example, skeletal muscle fibres, is unknown. In electrophysiological experiments on the mouse diaphragm, we found for the first time that betaAP (25-35 fragment, 10-6 M) disturbs the processes of the resting membrane potential generation in muscle fibres, leading to depolarization by two mechanisms: 1) inhibition of Na+,K(+)-ATPase, which leads to loss of impact of this pump to the resting membrane potential; 2) increase of membrane cationic permeability due to formation of "amyloid" channels blocked with Zn2+ ions. Our results significantly broaden current understanding of mechanisms of motor disturbances and skeletal muscle pathology in Alzheimer's disease, inclusion body myositis and other betaAP-related disorders.     

Selective involvement of opioids in mechanisms of synapse-specific plasticity in Helix lucorum snail during sensitization acquisition

Effects of met-enkephalin (opioid peptide) and naloxone (opioid antagonist) on nociceptive sensitization were studied in L-RP11 Helix neurons. In control snails sensitizing stimulation produced reversible membrane depolarization and depression of neural responses evoked by sensory stimuli during the short-term stage of sensitization and facilitation of these responses at the long-term stage. Met-enkephalin (10 but not 0.1 microM) suppressed the neural responses evoked by nociceptive stimuli. Sensitizing stimulation during metenkephalin application prevented the facilitation of neural responses evoked by tactile stimulation of snail head, whereas facilitation of neural responses evoked by chemical stimulation of head or tactile stimulation of foot were similar to that in control sensitized snails. Sensitizing stimulation during met-enkephalin and/or naloxone application prevented the facilitation of neural responses evoked by chemical stimulation of snail head, whereas responses evoked by tactile stimulation of snail head or foot were facilitated (as in neurons of control sensitized snails). Opioids are suggested to be involved in regulation of nociceptive mechanisms and selective induction of long-term plasticity in L-RP11 neural inputs activated by tactile of chemical stimulation of snail head.     

Ionic mechanisms underlying tonic and phasic firing behaviors in retinal ganglion cells

In the retina, the firing behaviors that ganglion cells exhibit when exposed to light stimuli are very important due to the significant roles they play in encoding the visual information. However, the detailed mechanisms, especially the intrinsic properties that generate and modulate these firing behaviors is not completely clear yet. In this study, 2 typical firing behaviors—i.e., tonic and phasic activities, which are widely observed in retinal ganglion cells (RGCs)—are investigated. A modified computational model was developed to explore the possible ionic mechanisms that underlie the generation of these 2 firing patterns. Computational results indicate that the generation of tonic and phasic activities may be attributed to the collective actions of 2 kinds of adaptation currents, i.e., an inactivating sodium current and a delayed-rectifier potassium current. The concentration of magnesium ions has crucial but differential effects in the modulation of tonic and phasic firings, when the model neuron is driven by N-methyl-D-aspartate (NMDA) -type synaptic input instead of constant current injections. The proposed model has robust features that account for the ionic mechanisms underlying the tonic and phasic firing behaviors, and it may also be used as a good candidate for modeling some other firing patterns in RGCs.     

Growth, reproduction and activity rhythms in two species of edible snails, Helix aspersa and Helix lucorum, in non 24-hour light cycles

Both juvenile and adult Helix aspera and H. lucorum showed significant periodicities in locomotor activity close to an imposed 24-h die1 of 13L: 11D, and to 20 diels of 13L:7D and 11L: 9D. Helix aspersa is active earlier in the night in 24-h diels than H. lucorum . In both 20-h diels H. aspersa showed a lag in attaining peak activity. We discuss the fact that H. aspersa copes better with 13L:7D and H. lucorum copes better with 11L:9D. Although both of them seem to grow faster in the 24-h diel, H. lucorum is indifferent to photoperiod in relation to its growth. Helix aspersa became mature in four months in all the three regimes, but its size and maturity were influenced by the three photoperiod regimes. Egg-laying was also influenced by the photoperiod regimes, but hatching was not.     

Functional role of neuropeptide CNP4 encoded by gene HCS2 in Helix lucorum nervous system

Helix Command Specific 2 (HCS2) gene is constantly expressed in parietal premotor (command) interneurons involved in control of the terrestrial snail Helix lucorum withdrawal behavior as a trigger element. It is also expressed under noxious conditions in other neurons presumably involved in withdrawal behavior. In this study we addressed the role of neuropeptide CNP4, encoded by gene HCS2, in the regulation of activity of the respiratory system, and in the influence on growth of isolated neurons in culture. It was shown that activity of the premotor interneuron elicits a direct effect (pneumostome closure), and a delayed intensification of respiratory movements. Application of CNP4 mimicked the delayed effects. Presence of the peptide CNP4 in solution for cultured neurons led to increase of neuronal growth. Immunochemical localization of the protein precursor encoded by gene HCS2 and peptide CNP4 in the cultured premotor interneurons revealed their preferential presence in the growth cones. The obtained results suggest that CNP4 may be secreted and involved in synergic regulation of behavior of a snail.     

Postembryonic neuronogenesis in the procerebrum of the terrestrial snail,Helix lucorum L.

Neuronogenesis during posthatching development of the procerebrum of the terrestrial snail Helix lucorum was analyzed using bromodeoxyuridine immunohistochemistry to label proliferating cells. Comparison of the distribution of labeled cells in a series of animals which differed in age at the time of incubation with bromodeoxyuridine, in survival time after incubation, and in age at sacrifice reveals a clear pattern and developmental sequence in neuron origin. First, the proliferating cells are located only at the apical portion of the procerebrum. Second, cells which are produced at any particular age remain, for the most part, confined to a single layer in the procerebrum. Third, as development proceeds, each layer of previously produced neurons is displaced toward the basal part of the procerebrum by the production of additional neurons. Our results suggest that the vast majority of the neurons (probably about 70–80%) of the snail procerebrum are produced during the first 1–2 months of posthatching development. © 1998 John Wiley & Sons, Inc. J Neurobiol 35: 271–276, 1998     

Molecular mechanisms underlying sensorimotor cortex pyramidal neuron involvement in the feeding behavior of rabbits

Response in pyramidal neurons belonging to the sensorimotor cortex (37% of total nerve cells investigated in this zone) which were identified by stimulating the bulbar pyramids, were investigated during experiments on unrestrained rabbits. Pyramidal neurons having connections with the lateral hypothalamus were activated during operation of feeding behavior, while activity was inhibited in those unconnected with the lateral hypothalamus. Microiontophoretic application of a protein synthesis inhibitor to pyramidal neurons caused their ability to respond to ascending activating influences from the lateral hypothalamus to disappear. When pentagastrin was applied to these neurons following the protein synthesis blocker, they recovered their ability to participate in hypothalamic feeding reaction. It is suggested that synthesis and release of a gastrin-like peptide into the perineuronal space is required for sensorimotor cortex pyramidal neurons to participate in the organization of feeding behavior.P. K. Anokhin Institute of Normal Physiology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 19, No. 5, 601–606, September–October, 1987.     

On the mechanisms underlying the depolarization block in the spiking dynamics of CA1 pyramidal neurons

Under sustained input current of increasing strength neurons eventually stop firing, entering a depolarization block. This is a robust effect that is not usually explored in experiments or explicitly implemented or tested in models. However, the range of current strength needed for a depolarization block could be easily reached with a random background activity of only a few hundred excitatory synapses. Depolarization block may thus be an important property of neurons that should be better characterized in experiments and explicitly taken into account in models at all implementation scales. Here we analyze the spiking dynamics of CA1 pyramidal neuron models using the same set of ionic currents on both an accurate morphological reconstruction and on its reduction to a single-compartment. The results show the specific ion channel properties and kinetics that are needed to reproduce the experimental findings, and how their interplay can drastically modulate the neuronal dynamics and the input current range leading to a depolarization block. We suggest that this can be one of the rate-limiting mechanisms protecting a CA1 neuron from excessive spiking activity.     

Seasonal periodicity of enteric nitric oxide synthesis and its regulation in the snail, Helix lucorum

Abstract. The snail Helix lucorum has been used as a model to study the adaptation of a nitric oxide (NO)‐forming enteric neural network to the long‐term resting period of summer estivation or winter hibernation. Quantification of the NO‐derived nitrite established that NO formation is confined to the nitric oxide synthase (NOS)‐containing myenteric network of the mid‐intestine. In active snails but not in resting snails, NO production could be enhanced by the NOS substrate l ‐arginine (l ‐ARG, 1 mM). We followed the enteric NO synthesis in a snail population kept at natural conditions for 1 year. Our findings indicate that NO synthesis was depressed in July during entry to the estivation, had a peak in autumn before hibernation, and finally was reduced during hibernation. Monoamines (histamine, serotonin, and adrenalin) could inhibit the NO liberation in active snails. Cofactors of NOS (β‐NADPH, β‐NAD, FAD, FMN, Ca²⁺, TH4) did not alter the low nitrite production in hibernating snails. We conclude that enteric NO synthesis in H. lucorum has a regular seasonal periodicity following the annual physiological cycles of terrestrial snails. During estivation or hibernation, NOS activity is blocked. Monoamines, the levels of which are elevated during hibernation, can trigger decreased NOS activity. The reduced activity of NOS cannot be restored by the administration of NOS cofactors; therefore, their absence cannot be the cause of the temporarily blocked L‐ARG/NO conversion ability of NOS.     

Differences in the habituation of the nicotinic and muscarinic cholinoreceptors of Helix neuron PPa4

Pharmacological division of the snail cholinoreceptors population of the identified neurone RPa4 by cholinoblockators of muscarine (atropine, platyphylline) and nicotine (d-tubocurarine) receptors allowed to reveal differences in the dynamics of reversible reduction of sensitivity of these receptors during their habituation to repeated iontophoretic acetylcholine applications. Maximum desensitization of nicotine receptors is weaker, develops slower and is eliminated faster after the end of rhythmic acetylcholine applications. An assumption is made that a more rapid and deeper lowering of muscarine cholinoreceptors sensitivity is due to an increase of intracellular concentration of free calcium at their activation by the agonist.     

Elementary and compound postsynaptic potentials in the defensive command neurons of Helix lucorum

The present communication concerns with the analysis of elementary and the compound excitatory postsynaptic potentials (eEPSPs and cEPSPs) recorded by intracellular microelectrode from an identified defensive command neuron of the snail Helix lucorum. The eEPSPs were evoked by single presynaptic action potentials (APs) elicited by cationic current injection into one of the identified sensory neurons synapsing on the respective command neuron. The cEPSPs were elicited by local brief tactile stimuli on the skin or internal organs. It was shown that the cEPSPs amplitudes depend mainly on the number of activated sensory neurons. Compound EPSPs depend also on frequency and the number of APs in the bursts occurring in a single neuron. Presynaptic APs having frequency 2-10 Hz evoke high frequency depression of that eEPSPs after an interval is followed by post-tetanic potentiation of single eEPSPs. Preceding stimulation of a pneumostom area facilitates the cEPSPs elicited by repeated stimulation of viscera. The eEPSPs from the same visceral area demonstrate no heterosynaptic facilitation in experiments with double parallel intracellular recording from responsive sensory and command neurons. The different types of the eEPSPs plasticity are discussed according to their contribution cEPSPs plastic changes.     

The effect of plastic restructuring of the local chemoexcitable zone in the somatic membrane of the PPa4 neuron of Helix lucorum on the excitability of neighboring chemo- and electrically excited zones

Rhythmic application of acetylcholine or serotonin to the local zone of somatic membrane was used to study the effect of extinction of RPa4 neuron depolarization in Helix lucorum on the excitability of adjacent chemo- and electroexcitable zone. It has been found that the extinction of response to iontophoretic application of acetylcholine to one somatic zone decreases the sensitivity of serotonin and cholinoreceptors in adjacent zones, as well as the excitability of electroexcitable membrane. The effect on the excitability of adjacent zones does not depend on the type of receptors activated rhythmically, as the extinction of RPa4 response to the repeated application of serotonin also reduces the sensitivity of adjacent cholinoreceptor zones. A cause of this effect may lie in modification of chemoreceptors and ionic channels, by intracellular regulatory systems that become activated by repeated stimulation.