Morphology and function of the identified neuron B3 in the buccal ganglia of Helix pomatia

Epileptogenic drugs in a model nervous system: Electrophysiological effects and incorporation into a phospholipid layer

Mechanisms of epileptiform activity in a model nervous system (buccal ganglia of Helix pomatia) are presented. The ganglia contain the identified giant neurons B1 through B4. For epileptiform activity, pentylenetetrazol (1 mmol/L to 40 mmol/L) or etomidate (12.5 micromol/L to 500 micromol/L) were applied. Membrane pressure was measured using a Wilhelmy film balance. In electrophysiological experiments, both drugs induced several effects in all studied neurons: membrane resistance increased, down-stroke of action potentials declined, and all types of chemical synaptic potentials decreased (the latter concerns pentylenetetrazol only). The threshold was 1 mmol/L of pentylenetetrazol and 12.5 micromol/L of etomidate. Epileptiform potentials developed in neurons that had expressed the membrane mechanisms underlying pacemaker potentials. The threshold of this development was again 1 mmol/L of pentylenetetrazol and 12.5 micromol/L of etomidate. Epileptiform depolarizations appeared with 40 mmol/L of pentylenetetrazol and 500 micromol/L of etomidate. In biochemical experiments, both drugs incorporated into an artificial phospholipids membrane and increased pressure in the membrane. The threshold of pressure increase was 1 mmol/L of pentylenetetrazol and 12.5 micromol/L of etomidate. Pressure increased dose-dependently and was 69% and 63% above starting pressure of 10 mN/m with epileptogenic concentrations of pentylenetetrazol (40 mmol/L) and of etomidate (500 micromol/L), respectively. It is postulated that amphiphilic substances incorporate into cell membranes and increase intramembranous pressure, and that this disturbs several membrane processes mechanically and leads to epileptic depolarizations in pacemaker neurons.

Block of spontaneous termination of paroxysmal depolarizations by forskolin (buccal ganglia, Helix pomatia)

Effects of cAMP-activated protein kinases (PKA) on epileptic activity are at present studied in a model nervous system. Identified neurons in the buccal ganglia of the snail Helix pomatia were recorded with intracellular microelectrodes in a continuously perfused experimental chamber. Epileptiform activity appeared regularly in neuron B3 when the saline contained pentylenetetrazol (20-40 mM). Epileptiform activity consisted of a series of paroxysmal depolarization shifts (PDS). Epileptiform activity was quantified by calculating the percentage of PDS-duration of PDS-periods. High percentage of PDS-duration was regularly found 15-30 min after the start of treatment with pentylenetetrazol. Subsequently, percentage of PDS decreased spontaneously. Adding forskolin (50 microM) to the pentylenetetrazol-containing solution increased percentage of PDS-duration. The increase during forskolin corresponded to the amount of decrease which had taken place spontaneously before. During application of forskolin for up to 4 h, spontaneous PDS decrease was absent, i.e., epileptiform activity corresponded to status epilepticus. Forskolin was not able to induce epileptiform activity when applied without pentylenetetrazol. 1,6-Dideoxy-forskolin (50 microM) did not accelerate epileptiform activity. When pentylenetetrazol was applied twice (1 h each) separated by 2.5 h of control conditions, PDS decrease obtained during the first application was found to be largely preserved during control conditions. When forskolin was applied for 30 min in between both applications of pentylenetetrazol, the second response to pentylenetetrazol did not show a preserved PDS decrease. Results suggest that forskolin blocks an endogenous antiepileptic process and that activation of PKA can maintain epileptic activity and induce status epilepticus.

Endogenous pacemaker potentials develop into paroxysmal depolarization shifts (PDSs) with application of an epileptogenic drug

Well-known invertebrate ganglia (buccal ganglia of Helix pomatia, abdominal ganglia of Aplysia californica) were used to study the contribution of synaptic potentials, central pattern generators, and endogenously generated neuronal potentials to the development of epileptiform activity. Epileptiform activity which was induced with application of pentylenetetrazol (1 to 100 mM) or etomidate (0.12 to 1.0 mM) consisted of paroxysmal depolarization shifts (PDSs) recorded simultaneously from several identified neurons with sharp microelectrodes. With application of an epileptogenic drug, endogenous pacemaker potentials develop into PDSs. With increasing concentration of the drug, (i) amplitude of pacemaker-depolarizations and (ii) delay of pacemaker-repolarization increased progressively finally resulting in PDSs. Additionally, the activation characterists of currents shifted from between -50 and -40 mV (pacemaker potentials, control conditions) to between -100 and -40 mV (PDS, epileptic conditions). Only neurons which generated pacemaker potentials under control conditions could generate PDSs under epileptic conditions. Chemical synaptic inputs triggered or blocked pacemaker potentials as well as PDSs. Activities induced from central pattern generators were identified with simultaneous recordings from several identified neurons. The central pattern generators could trigger or block pacemaker potentials as well as PDSs. Results demonstrate that, in the used model nervous systems, pacemaker potentials which are generated by the single neurons are the physiologic basis of epileptic activity.

Paroxysmal depolarization shifts (PDS) induce non-synaptic responses in neighboured neurons (buccal ganglia, Helix pomatia)

A non-synaptic spread of excitation between neighboured neurons was studied in a model nervous system using epileptiform activity. The identified giant neuron B3 in the buccal ganglia of Helix pomatia reliably generated paroxysmal depolarization shifts (PDS) when treated with pentylenetetrazol or etomidate. Simultaneous recordings of neuron B3 and other neurons showed that each PDS in neuron B3 was accompanied by a depolarization in the other neurons. These related depolarizations (PDS-RD) appeared about 1 to 5 s after the beginning of PDS, their amplitude was up to 20 mV and their duration ca. 1 min. Reduction of extracellular calcium concentration or application of a "high Mg-low Ca" solution blocked PDS-RD. There were, however, no hints for synaptic contacts of the studied neurons. Occasional failures of spontaneous PDS in one neuron B3 of the B3-network of neurons, resulted in a failure of PDS-RD in the neighboured neurons. Block and induction of PDS in one neuron by injection of hyperpolarizing and depolarizing currents, respectively, blocked and induced PDS-RD in the neighboured neurons. As intracellular staining of neurons B1 and B3 showed that their dendritic arborizations were co-localized in the same region of the ganglion, a dendro-dendritic release of substances may cause PDS-RD. Since PDS-RD could themselves trigger PDS, PDS-RD may provide a new basic mechanism of synchronizing epileptic activity of neighboured neurons within an epileptic focus.

Heptanol exerts epileptiform effects in identified neurons of the buccal ganglia of Helix pomatia

1-Heptanol (0.2-5.0 mM) known to block electrical contacts was tested under epileptic and non-epileptic conditions in the buccal ganglia of Helix pomatia. Synchronicity of epileptiform activity was not affected. In concentrations below 1 mM, heptanol accelerated epileptiform activity induced by pentylenetetrazol. In concentrations above 1 mM, it evoked epileptiform activity without admixture of an epileptogenic drug. Coupling coefficient was increased and decreased in low and high concentration ranges of heptanol, respectively. The measured decrease of coupling is interpreted as a result of the activation of 'epileptiform' membrane conductances accompanied by decreased length constants of neuronal fibers.

Epileptic neurons induce augmenting synaptic depolarizations in non-epileptic neurons (buccal ganglia, Helix pomatia)

Spread of epileptic activity was studied by inducing epileptiform activity (pentylenetetrazol, PTZ) in one part of a nervous system and by analyzing responses of neurons in a non-PTZ-treated part (identified neurons, paired buccal ganglia, Helix pomatia). Paroxysmal depolarization shifts (PDS) induced time-locked depolarizations in non-epileptic neurons (latency ca. 5 s, duration ca. 1 min, amplitude < or =20 mV). Amplitudes were augmenting during several hours of epileptic activity. Depolarizations were accompanied by an increase in membrane resistance and they were blocked in 'high Mg-low Ca' solutions. It is assumed that the potentials represent a typical widespread response of non-epileptic neurons to PDS of other neurons. This response may be induced via non-specific releases of substances of the epileptically active neurons thereby activating neighboring neurons which in turn activate neurons in control ganglion.

A method to study the effects of an epileptic focus on non-epileptic nervous tissue

Locally applied epileptogenic drugs directly affect the local cells which project their epileptic activity into the surrounding tissue. This paper describes an experimental set-up which allows differentiation of direct effects of an epileptogenic drug from indirect ones, which are synaptically projected. The set-up consisted of an experimental chamber which enabled the isolated superfusion of a part of a nervous system with a drug. Isolated superfusion was obtained by means of a divider between two compartments of the experimental chamber and of an outlet for the bath solutions below the divider. Quality of isolation was tested with unilateral application of the epileptogenic drug pentylenetetrazole (PTZ) and of the dye methylene blue. Distribution of extraganglionic and intraganglionic PTZ was measured by PTZ-selective microelectrodes. It was found that the drug could not be detected at the non-drug side. Correspondingly, unilateral dye application resulted in a sharp border between the treated and untreated side. The experimental set-up was used to study projected events evoked by sustained epileptic activity of defined parts of a nervous system. It was found that the non-treated cells became progressively coupled to the epileptic neurons in the course of the experiments.

Epileptic discharges induced by pentylenetetrazol: ultrastructural alterations in identified neurons and glial cells (Helix pomatia)

The effects of sustained epileptic activity induced by pentylenetetrazol on morphology of buccal ganglia of Helix pomatia were investigated. Neuronal somata and processes as well as glial cells were evaluated after 5 hours of epileptic activity and after 5 hours under control conditions. After epileptic activity neurons showed signs of degeneration consisting of condensation of nuclear chromatin, decreased activity of Golgi apparatus, increased numbers of lamellar bodies and multivesicular bodies, clusters of vesicles and vacuoles, loss of microtubuli, and scattered lamellar bodies. Neuronal somata and large neuronal processes appeared less affected than the smaller processes. Glial cells showed signs of phagocytotic activity as increased cell size, numerous degenerating neuronal processes within the cytoplasm as well as lysosome like bodies and vacuoles. The changes developing along with epileptic activity were interpreted to indicate degeneration and subsequent phagocytotic activity of neuronal processes in synaptic regions of the ganglia. Thus, evidence is presented for synaptically induced degenerative processes in an intact nervous tissue that is not affected by seizure-induced alterations of respiration or systemic circulation.

Identified neuronal individuals in the buccal ganglia of Helix pomatia

The buccal ganglia of Helix pomatia are used as model nervous structures in neurophysiological and epileptological studies. Many basic problems concerning membrane physics and the functioning of single neurons and neuronal networks can be easily studied using these ganglia. The model character derives mainly from the relative simplicity of this nervous system and the fact that it contains large, visually identifiable neurons. As in other invertebrate nervous systems, the large neurons have proved to be individuals showing the same functional and structural properties from one animal to another.

Effects of valproate in a model nervous system (buccal ganglia of Helix pomatia): II. Epileptogenic actions

High concentrations of valproate (VPA; greater than 20 mM) depolarized identified neuronal individuals in the buccal ganglia of Helix pomatia and transiently induced paroxysmal depolarization shifts (PDS). Threshold concentration of VPA for the induction of PDS was decreased (a) by increased seizure susceptibility, (b) by increased concentrations of derivatives of VPA, and (c) by increased H+ concentrations. Intrasomatic injection of VPA did not induce PDS. The epileptogenic action of VPA is believed to be exerted from the extracellular side of the cell membrane.

Effects of valproate in a model nervous system (buccal ganglia of Helix pomatia): I. Antiepileptic actions

Cellular actions of valproate (VPA) were studied using intracellular recordings of identified neuronal individuals in the buccal ganglia of Helix pomatia. Under nonepileptic conditions, VPA induced (a) a hyperpolarization, (b) slight changes in action potentials (AP), and (c) an increase in membrane resistance. Under epileptic conditions (i.e., during application of an epileptogenic drug), extracellular application of VPA decreased frequency of occurrence of epileptic depolarizations (early effect) and led to a decay in paroxysmal depolarizations (late effect). Intracellular injection of VPA could block epileptic activity in the treated neuron immediately. A metabolite of VPA (trans-2-en VPA) mainly lacked the late effect (decay in epileptic depolarizations) obtained with VPA. Results suggest that the early antiepileptic effect is exerted from the extracellular side of the neuronal membrane and that the late effect results from intracellular actions of VPA being delayed by slow access to an intracellular site.

Effects of the hypnotic drug etomidate in a model nervous system (Buccal ganglia, Helix pomatia)

1. Effects of the hypnotic drug etomidate were studied with intracellular recordings of the identified neurons B1 to B4 of the buccal ganglia of Helix pomatia. 2. At threshold doses of 10 mumol/l, etomidate mainly affected interneuronal networks. 3. In concentrations above 200 mumol/l, the drug induced typical epileptic activities (paroxysmal depolarization shifts, PDS). Neurons B1 to B4 generated epileptic activities in differential concentration ranges. PDS were synchronized via electrical contacts. PDS could be blocked by the "calcium antagonist" verapamil but not by a block of chemical synaptic transmission. 4. In comparison with the epileptogenic drug pentylenetrazol, effective doses of etomidate to induce PDS were about 100 times lower.