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 in epileptological studies. Many basic problems concerning membrane physics, functioning of the single neurons and of neuronal networks can be studied easily using these ganglia. The model character mainly comes from the relative simplicity of this nervous system and 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 the next.     

Synapses identifiable in the parietal ganglia of the snail Helix lucorum

The synaptic plasticity is a background for learning and memory. Identifiable synapses that are the synapses between individually identifiable neurons are a very convenient model for studying plasticity. Synapses between the interoceptive mechanosensory neurons and the command neurons of the withdrawal behavior were identified in the Helix lucorum brain. It was shown that synaptic plasticity estimated by the dynamics of the elementary postsynaptic potentials elicited by single presynaptic spikes differed from the synaptic plasticity estimated by the dynamics of compound synaptic responses of the same neurons to sensory stimulation. Habituation and heterosynaptic facilitation phenomena are discussed in terms of the dynamics of the elementary postsynaptic potentials.     

Morphological and electrophysiological mapping of giant neurons in the suboesophageal ganglia of Helix pomatia

1. To reveal the morphology of the suboesophageal ganglia of Helix pomatia, the connective tissue was completely removed and the preparations stained whole-mount with methyl green-pyronin G to display the relative locations of the neurons. 2. Fifteen large cells which could be recognized as individuals in at least 75% of the preparations investigated, were morphologically identified by size, position and color. 3. The cells were electrophysiologically characterized with respect to spontaneous activity, synaptic input from peripheral nerves, and response to application of drugs (e.g. ACh, DA and 5-HT). 4. The peripheral axonal projections of eight of the major identified cells were investigated by intracellular CoCl2 injection and by cobalt backfilling of the peripheral nerves.     

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.     

Pacemaker potentials are the physiologic basis of epileptiform activity in the buccal ganglia of Helix pomatia

Mechanisms of epileptic activity in nervous systems were studied using the identified neurons B1 through B4 in the buccal ganglia of the snail Helix pomatia as a model system. Activities were recorded with intracellular microelectrodes. Epileptiform activity was induced by bath application of an epileptogenic drug (pentylenetetrazol: 1 mM to 40 mM, or etomidate: 0.1 mM to 1.0 mM). Epileptiform potentials recorded from the somata of neurons consisted of paroxysmal depolarization shifts (PDSs). With increasing concentration of an epileptogenic drug, pacemaker potentials in neuron B3 developed into PDS. Simultaneously several types of chemical post-synaptic potentials were suppressed in amplitude. Since on the one hand epileptic seizures only appear when PDS are synchronized in many neurons and since on the other hand synaptic potentials were found to be suppressed during epileptic conditions, mechanisms underlying neuronal synchronization were studied. Evidence was found that, under epileptogenic conditions only, neurons were synchronized by an non-synaptic release of substances. Strong depolarizations accompanied by an increase in intracellular calcium concentration are known to induce an unspecific exocytosis. Thus, an unspecific exocytosis from the dendrites of PDS-generating neurons probably appears under epileptic conditions and synchronizes neighbouring neurons.     

Electron microscope investigation of synapses,synapse-like structures,and possible sites of release of neurosecretory material in the buccal ganglia of Helix pomatia (Mollusca)

Summary In the buccal ganglia of Helix pomatia synapses and sites of possible release of neurosecretory material were investigated electron microscopically. There is one chemical synapse and one electrotonic synapse in the neuropile of the ganglion. No synapses could be detected in the buccal nerves, cerebro-buccal connectives, or in the buccal commissure. The synaptic cleft of the chemical synapse is about 25 nm wide and contains electron-dense material whereas the cleft of the electrotonic synapse is only 5 nm wide. The presynaptic fibre of the chemical synapse contains clear vesicles and dense core vesicles. The release sites of neurosecretory material are found at the initial segment of the axons, at perikarya of neurones, and at the perineurium of the ganglion. If the terminals are located at the plasmalemma of a nerve cell, these release sites are called synapse-like structures according to Roubos and Moorer-van Delft (1979). The synapse-like structures show all structural elements of synapses, except the 25 nm cleft containing dense material; the cleft is only 15–20 nm wide here like the normal cleft between neurones and glial cells or between two fibres. If the secretory material is released at the periphery through the perineurium the terminal is called synaptoid according to Scharrer (1970). In all cases, i.e. synapses, synapse-like structures, and synaptoids, clear vesicles were found in the axon terminal. This finding provides further evidence that clear vesicles always accompany the release of substances from axon endings.     

Cytological aspects of different nerve cell somata in the buccal ganglia of Helix pomatia L.

In early September most of the neurons of the buccal ganglia of Helix pomatia contain neurosecretory material as membrane bound granules. There is only one, in exceptional cases 2 types of granules per cell. This suggests that different types of granules do not change into one another, and that each granule type contains a different secretory product. One granule type contains PAF-positive neurosecretory material, another one catecholamines, but most of the granules cannot be associated with special substances. The identified giant neurons B1-B4 contain granules in less density than the smaller neurons. B1 and B2 resemble each other in their granule type, whereas both B3 and B4 differ from B1 and B2.     

Processing of mechano- and chemosensory information in the lip nerve and cerebral ganglia of the snail Helix pomatia L.

Neurophysiologists have long been seeking out simple model systems in which to analyse the neuronal mechanisms underlying the organisation of behaviour. The feeding behaviour of molluscs has proved to be one of the most useful simple systems for the analysis of cyclical motor patterns, the interactions of central pattern generating interneurones and the role of sensory inputs in the initiation and maintenance of the behaviour. Considerable progress has been made in one or both of the first two aspects of this research in Lymnaea, Helisoma, Limax, Planorbarius, Pleurobranchaea and Tritonia (for reviews see [3, 7, 8, 15]) and more recently, in Aplysia [39] and Planorbis [1]. The role of mechano- and chemosensory inputs in the organisation of the feeding behaviour was studied in at least twenty molluscan species (for a review see [3]). However, in only less than half of them was the analysis extended to the effect of tactile and chemical inputs on identified neurones in the buccal and cerebral ganglia which contain the feeding circuitry (Aplysia: [12, 22, 36, 41]; Pleurobranchaea: [9, 16, 17]; Tritonia: [2]; Helisona: [21]; Limax: [11, 14, 35]; Helix: [6, 19, 24-26, 32, 38]). In present chapter I would like to review our earlier findings on the processing of mechano- and chemosensory information in the lip nerves and cerebral ganglia of Helix pomatia L. These findings were published in a series of papers between 1982 and 1987 [19, 20, 24-26]. The results reviewed here prepared the way for the development of new lines of research in our laboratory on the plasticity and serotonergic modulation of feeding in this widely used experimental animal [27, 40].     

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.     

Parvalbumin-immunoreactive neurons in the brain of Helix pomatia

Parvalbumin-immunoreactive material was detected in the central nervous system of the snail, Helix pomatia. Each ganglion investigated contained parvalbumin-immunoreactive neurons. The molecular weight of Helix parvalbumin-immunoreactive material as determined by Western blots is about 40 kilodaltons. ⁴⁵Ca²⁺ overlays showed that this protein binds Ca²⁺. In contrast to vertebrates, in Helix neurons parvalbuminlike material was not colocalized with the neurotransmitter gamma-aminobutyric acid (GABA).