Ultrastructure of the bimodal pacemaker neuron in the central nervous system of Helix pomatia L

Fine structure of the identified (RPal) giant neuron of Helix pomatia has been investigated after isolation. The following findings have been established: 1. The cytoplasm of the RPal neuron is electron-microscopically characterized by a strongly invaginated nuclear envelope, a highly-developed rER system, a large number of free ribosomes and by 110-250 nm-sized neurosecretory-like granules with finely-granulated inner content. 2. The fine-structural arrangement of the cytoplasm exhibited a seasonal variation, viz. in winter it was found to be layered, while in spring it showed a homogeneous structure. 3. On the cell surface free axon profiles containing vesicles were sometimes found, which might be afferent connections to the cell body.     

Modulation of endogenous bursting pacemaker activity in a Helix pomatia neuron

Considerable membrane depolarization was shown to arise periodically, at intervals of up to a few minutes, in the PPa1 bursting neuron ofHelix pomatia. Pulses of slow depolarizing current were found by the voltage clamping method. The frequency of the pulses was independent of the holding potential. The equilibrium potential for the slow depolarizing current was about 45 mV. During development of the depolarizing current a region of negative conductivity was observed on the steady-state voltage-current characteristic curve of the membrane. It is suggested that the pulses of slow depolarizing current are associated with the presence of secretory connections between the molluscan neurons.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 9, No. 6, pp. 606–612, November–December, 1977.     

Multiplicity of pacemaker zones in Helix pomatia neurons

Intracellular microelectrode recordings from neurons ofHelix pomatia revealed several local zones of action potential generation both on the soma and on some of the branches of the neurons. Under certain conditions the activity of individual loci of the neuron membrane was synchronized to produce a normal action potential. It is suggested that the somatic membrane of neurons is heterogeneous in structure and consists of separate loci of an electrically excitable membrane, incorporating active and latent pacemaker zones. Neurons ofH. pomatia are characterized by two types of action potential with different triggering mechanisms: one (synaptic) type is generated under the influence of the EPSP, the other (pacemaker) arises through activation of endogenous factors for the neuron (pacemaker potentials). The interaction between synaptic and pacemaker potentials during integrative activity of the neuron is discussed.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 5, No. 1, pp. 88–94, January–February, 1973.     

Effect of x-rays on the pacemaker neurons of Helix pomatia

The effect of superlethal doses of ionizing radiation on exciting and electrical properties of giant neurons of the central nervous system of Helix pomatia has been investigated. At early times following irradiation the excitability does not significantly change whereas the membrane potential, resistance and pump-induced hyperpolarization increase. At later times, a stabilization of these parameters is followed by a diminution of resistance, a decrease of membrane potential and pump-induced hyperpolarization, and even the neuron death.     

Peculiarities of orthodromic inhibition in pacemaker neurons in Helix pomatia

We used the intracellular recording method to study the effect of a group of nerves in the visceral complex on the activity of a pacemaking giantneuron located in the peripheral part of the visceral ganglion in a mollusk. Single excitations of the left and right pallial, the intestinal, and the anal nerves with electrical stimuli evoked similar responses, consisting of phases of rapid depolarization (duration 100 msec, amplitude 3–5 mV) and slower hyperpolarization (duration 400 msec, amplitude 5–8 mV). The excitation also had an aftereffect, which was expressed in inhibition of the background activity of the pacemaker for several seconds. The most interesting of the functional characteristics of that response was the effects of summation. With rhythmic excitation by stimuli of low frequency (0.5–1 c/sec) the result of summation was general hyperpolarization of the neuron and the appearance of giant inhibitory postsynaptic potentials (IPSP's) with an amplitude of 12–16 mV. With higher frequency of excitation (2–3 c/sec and upward) we observed depolarization replacing the hyperpolarization of the neuron, but IPSP's of large amplitude were absent. At the end of rhythmic excitation prolonged inhibition of the pacemaker's activity, lasting some minutes, occurred in all cases. This article discusses the possible mechanisms of that type of prolonged inhibition of the pacemaker's activity, the origin of the phases in biphasic responses, and the reasons for differences in the course of summation of biphasic postsynaptic potentials.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 3, No. 4, pp. 426–433, July–August, 1971.     

Pre- and postsynaptic effects of eugenol and related compounds on Helix pomatia L. neurons

We showed how eugenol blocks the synaptic transmission and gave a possible interpretation how it inhibits the excitation-contraction coupling that several authors described previously. Eugenol acts both in the pre- and postsynaptic side of the neurons. It blocks the Ca2+-currents, decreases the membrane potential of the neurons, increases the inward resistance and decreases the GABA, ACh and glutamate evoked excitatory responses in submillimolar concentration.     

The effect of pentylenetetrazol on the metacerebral neuron of Helix pomatia

The effects of Pentylenetetrazol (PTZ) on the metacerebral giant cell (MCC) of the snail, Helix pomatia were studied. Actions on membrane resistance, time constant, resting and action potentials, outward and inward ionic currents were examined. Superfusion with PTZ in concentrations of 25 to 50 mmol/l, induced a gradually evolving convulsive state, which could be studied by intracellular recording from the MCCs. In the pre-convulsive state an acceleration of the spontaneous activity developed and was followed by paroxysmal depolarization shifts (PDSs), in the convulsive phase. PTZ prolonged the membrane time constant by about 10 percent, but this could not be traced back to alterations in membrane resistance or capacity. The resting membrane potential was not significantly altered; the action potentials were prolonged by slowing down of both the rising and decaying phases. The outward potassium currents were repressed by PTZ in a voltage dependent manner. The decrease of the IA current became more pronounced at increasingly positive command pulses, while IK was relieved from depression especially at longer pulse durations. Inward currents were isolated with the aid of suppression of outward currents by 50 mmol/l TEA. Under these conditions sodium currents, measured in calcium deficient Ringer solution were moderately depressed, while the calcium currents, examined during sodium-free superfusion, were mildly enhanced by PTZ. It is concluded that PTZ effects on ionic conductances, on membrane parameters, on the resting potential and ionic currents explain only modifications of spike potentials occurring in the convulsive state and do not account for the PDS, the central phenomenon of the convulsive electrographic activity, at least in this thoroughly examined type of neuron.     

Diphasic synaptic potentials and prolonged poststimulus hyperpolarization in Helix pomatia neurons

Synaptic activity of neurons giving diphasic excitatory-inhibitory potentials in response to orthodromic stimulation was recorded intracellularly. In response to stimulation of nerves by a single short pulse these neurons developed only the excitatory component of the diphasic potential, but with a longer stimulus a prolonged inhibitory phase, partly suppressing the initial excitatory component, was added. The excitatory phase appeared only when the resting potential reached a certain level. In their response to repetitive stimulation, neurons with a diphasic potential are divided into habituating and nonhabituating. The diphasic potential can also arise in response to application of acetylcholine to the soma of these neurons. It is postulated that this potential reflects the response of different receptors of the postsynaptic membrane to the same mediator. Prolonged poststimulus hyperpolarization can be obtained after repetitive orthodromic or direct stimulation of some neurons. However, as analysis of the results showed, poststimulus hyperpolarization is endogenous in origin and differs in its mechanisms from the diphasic potential.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 5, No. 2, pp. 193–200, March–April, 1973.     

Pre- and postsynaptic effects of dopamine antagonists on dopaminergic synaptic transmission in Helix pomatia

The rate of transmitter mobilization in identified dopaminergic synapses was decreased by the dopamine antagonists pimozide, chlorpromazine, haloperidol, cis-flupenthixol, curare, clozapine and high concentrations of ergometrine. The depolarizing postsynaptic potential was inhibited by pimozide, chlorpromazine, haloperidol, cis-flupenthixol, curare, clozapine, (+)-butaclamol and high concentrations of ergometrine. The hyperpolarizing synaptic potential was inhibited by naloxone, methysergide, (+)-butaclamol, haloperidol, 6-hydroxydopamine and low concentrations of ergometrine, while pimozide, cis-flupenthixol, trans-flupenthixol, curare, clozapine, promethazine, chlorpromazine and (-)-butaclamol had no clear effect. The presynaptic receptors involved in modulation of the mobilization rate showed similarities with the dopamine receptors mediating depolarizations. The dopamine antagonists changed dynamics of synaptic transmission.     

Ouabain-sensitive and insensitive acetylcholine receptors on the membrane of the same neuron in Helix pomatia

Using internally dialyzed neurons of Helix pomatia as a model, the effect of structural analogs of acetylcholine (ACh) were investigated for their cholinomimetic properties on A- and B-types of ACh-responses. Specifically we analyzed choline esters of N-para- and ortho-alkoxybenzoyl-beta-alanines, (CH(3)O-, C(2)H(5)O-, C(3)H(7)O-, iso-C(3)H(7)O-, C(4)H(9)O-, iso-C(4)H(9)O-, C(5)H(11)O-, iso-C(5)H(11)O-), (in all, 16 combinations). The compounds evoked differing sensitivities of response to factors de-activating the Na-K-pump (ouabain and K-free solution). Most compounds resembled ACh: ionic currents caused by these compounds were inhibited by Na-K pump blockers in the case of A-type responses - B-type responses were insensitive to these factors. Ionic currents induced by choline esters of p-, o-propoxy- and iso-propoxybenzoyl-beta-alanines were insensitive to ouabain and K-free solution in the case of A- and B-type responses. Ionic currents induced by the choline ester of p-butoxybenzoyl-beta-alanine were inhibited by ouabain and K-free solution on both types of neuron. The results allow us to classify choline esters of p-, o-propoxy- and iso-propoxybenzoyl-beta-alanines as N-cholinomimetics, while the choline ester of p-butoxybenzoyl-beta-alanine can be considered as M-cholinomimetic. We conclude that both M- and N-type ACh receptors exist on the membrane of the same neurons.     

Mechanism of long lasting synaptic inhibition of a silent neuron of Helix pomatia

• 1.1. Long lasting synaptic inhibitions (LLI) were recorded in the silent cells LPL1 and RPL1 of Helix pomatia.
• 2.2. During LLI the excitability of the cell was strongly reduced.
• 3.3. The membrane conductance changes during LLI were characterized using the voltage-clamp technique.
• 4.4. LLI in the silent cell LPL1 is mediated by a synaptically induced inhibition of the voltage-dependent inward Ca²⁺ -current.

     

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.     

Pharmacological modification of evoked caudate nucleus potentials

To stimulation of the Substantia nigra the nucleus caudatoputamen of the rat responds with the release of a short-latent sum action potential that may serve to characterize the interaction between the two nuclear areas. The partly conflicting results reported in the literature about drug effects on this potential prompted us to examine the effectiveness of dopaminergic and cholinergic substances or their antagonists following systematic administration. Chloropromazine increases the amplitude of the potential, while apomorphine has a distinct antagonistic effect. Interesting appear to be the analogical effects of Tricuran, arecoline and apomorphine, and the furtherance of the chloropromazine effect by arecoline.