Photoinactivation of neurones axonally filled with the fluorescent dye 5(6)-carboxyfluorescein in the pond snail, Lymnaea stagnalis

Crustacean cardioactive peptide (CCAP)-related molluscan peptides (M-CCAPs) are potential extrinsic modulators of the buccal feeding network in the pond snail Lymnaea stagnalis

We combine electrophysiological and immunocytochemical analyses in the snail Lymnaea stagnalis of M-CCAP1 and M-CCAP2, two molluscan peptides with structure similar to crustacean cardioactive peptide CCAP, originally isolated from the snail Helix pomatia. Both M-CCAP peptides (M-CCAP1 and M-CCAP2, 1 microM) had an excitatory effect, depolarizing all the identified neurons of the buccal feeding network (including motoneurons: B1, B2, B4 and modulatory interneurons SO, OC: 62 neurons in 33 preparations). Additionally, in 67% of preparations, rhythmic activity (fictive feeding) was recorded with a mean rate of 7 cycles/min. No significant difference in the proportion of preparations showing fictive feeding or mean feeding rate was found between M-CCAP1 and M-CCAP2. The extrinsic feeding modulator, the serotonergic CGC neuron, responds by increase of the spontaneous activity after M-CCAP application (9 of 18 preparations). Crustacean CCAP (1 microM) evokes a slight membrane depolarization in 3 out of 8 preparations but never evokes fictive feeding. Immunostaining revealed no cell bodies in the buccal ganglia, but a dense network of CCAP immunopositive fibers arborizing in the buccal neuropil. Many of these fibers originate from a symmetrical pair of CCAP-immunoreactive cerebro-buccal interneurons, which are the most likely candidates for extrinsic modulatory interneurons in the buccal feeding network. Our data are the first results suggesting that M-CCAP-peptides exist as effective modulators in mollusc.

A cellular mechanism for prepulse inhibition

In prepulse inhibition (PPI), startle responses to sudden, unexpected stimuli are markedly attenuated if immediately preceded by a weak stimulus of almost any modality. This experimental paradigm exposes a potent inhibitory process, present in nervous systems from invertebrates to humans, that is widely considered to play an important role in reducing distraction during the processing of sensory input. The neural mechanisms mediating PPI are of considerable interest given evidence linking PPI deficits with some of the cognitive disorders of schizophrenia. Here, in the marine mollusk Tritonia diomedea, we describe a detailed cellular mechanism for PPI--a combination of presynaptic inhibition of startle afferent neurons together with distributed postsynaptic inhibition of several downstream interneuronal sites in the startle circuit.

Distribution of sensorin immunoreactivity in the central nervous system ofHelix pomatia: Functional aspects

Land snails belonging to the genus Helix are commonly used to study several behaviors and their plasticity at the cellular level. Because the knowledge of sensory neurons in these species is far from being complete, we have investigated the presence and distribution in Helix pomatia central nervous system of the immunoreactivity for sensorin, a peptide specific for mechanosensory neurons in Aplysia. We found that the majority of immunopositive cells were grouped in clusters located in all the central ganglia, except for the pedal ganglion, where only a single large neuron was stained. A symmetrical cluster of stained cells in the cerebral ganglia showed homology with the cerebral J clusters in Aplysia. Most of the somata of these Helix cerebral clusters send their axons in the ipsilateral cerebropedal connective and lip nerves and make monosynaptic connections with cells located in a medial adjacent cluster. This monosynaptic circuit can be reestablished in culture, where it shows homosynaptic depression as it does in the ganglionic preparation.

Distinct responses of osphradial neurons to chemical stimuli and neurotransmitters in Lymnaea stagnalis L

1. In Lymnaea stagnalis L. (Pulmonata, Basommatophora) the neurons in the osphradium were visualized by staining through the inner right parietal nerve by 5,6-carboxyfluorescein (5,6-CF). Three types of neurons were identified: three large ganglionic cells (GC1-3; 80-100 microns), the small putative sensory neurons (SC; 20 microns) and very small sensory cells (3-5 microns). 2. The ganglionic and putative sensory neurons were investigated by whole cell patch-clamp method in current-clamp condition. The three giant ganglionic neurons (GC1-3) located closely to the root of osphradial nerve, had a membrane potential (MP) between -30 and -70 mV and showed tonic or bursting activities. The small putative sensory cells (SCs) scattered throughout the osphradial ganglion, possessed a MP between -25 and -55 mV and showed an irregular firing pattern with membrane oscillations. At resting MP the GC1-3 cells were depolarized and increased the frequency of their firing, while the SCs were hyperpolarized and inhibited by NaCl (10(-2) M) and L-aspartate (10(-5) M) applied to the osphradium. 3. 5-Hydroxytryptamine (5HT, 10(-6) M), gamma-aminobutyric acid (GABA; 10(-6) M) and the GABAB agonist baclofen (10(-6) M) depolarized the neurons GC1-3 and increased their firing frequency. In contrast, on the GC1-3 neurons, acetylcholine (Ach; 10(-6) M) and FMRFamide (10(-6) M) caused hyperpolarization and cessation of the firing activity. The 5HT effect was blocked by mianserin (10(-6) M) but picrotoxin (10(-5) M) failed to block the GABA-induced effect on the GC1-3 cells. 4. The small putative sensory neurons (SCs) were excited by Ach (10(-6) M) and 5HT (10(-6) M) but were inhibited by GABA (10(-6) M). FMRFamide (10(-6) M) had a biphasic response. The Ach effect was blocked by hexamethonium (10(-6) M) and tetraethylammonium (10(-6) M), indicating the involvement of nicotinic cholinergic receptors. 5. The distinct responses of the two populations of osphradial neurons to chemical stimuli and neurotransmitters suggest that they can differently perceive signals from environment and hemolymph.

Histological characterization of lip and tentacle nerves in Lymnaea stagnalis

The lip and tentacle nerves of the pond snail, Lymnaea stagnalis, were characterized using histological techniques. Anatomical drawings showed the detailed distributions of the superior lip, median lip, and tentacle nerves in the lip and tentacle; in particular it was found that the mouth is mainly innervated by the superior lip nerve. The tentacle nerve was clarified to form a zigzag structure along the extension direction in a shrinking tentacle. By backfilling of the superior lip nerve and/or the median lip nerve with fluorescent dyes, the neurons in the CNS made some clusters, whereas those stained from the tentacle nerve made other clusters. These stained neurons were not part of the central pattern generator or its regulatory neurons for feeding. The present results, therefore, suggest that the superior lip nerve may be employed as a principal factor in the chemosensory transduction from the mouth, and that no direct inputs occur through the lip and tentacle nerves to the central pattern generator or its regulatory neurons for feeding.

Histochemical study on the relation between NO-generative neurons and central circuitry for feeding in the pond snail, Lymnaea stagnalis

To examine whether nitric oxide (NO)-generative neurons are included in the central circuitry for generation of feeding pattern in the pond snail, Lymnaea stagnalis, two staining techniques for NADPH diaphorase and serotonin (5-HT) were applied for its central nervous system (CNS). The former technique is known to show localization of NO synthase; the latter is well employed as a marker for the feeding circuitry because 5-HT is a main transmitter in it. In the buccal ganglion, B2 motoneuron was found to be a putative NO-generative neuron. This motoneuron is not involved directly in the coordination of feeding pattern but is activated simultaneously with the feeding to control the oesophageal and gut tissues for the digestion. Taking account of the diffusion effects of NO, the NO released from B2 motoneuron, when the feeding is started, is thought to sufficiently modulate the feeding circuitry. In the cerebral ganglion, the superior lip nerve, the median lip nerve and the tentacle nerve included both putative NO-generative fibers and serotonergic fibers. These fibers are not identical, but the NO released in the nerves may activate the serotonergic fibers, resulting in the influence upon the initiation of the feeding. Therefore, our present findings clearly showed that NO is not involved in transmission within the central circuitry for the feeding, but suggested that NO can crucially affect the feeding behavior, such as initiation and modulation of the feeding pattern.

Pattern-generating role for motoneurons in a rhythmically active neuronal network

The role of motoneurons in central motor pattern generation was investigated in the feeding system of the pond snail Lymnaea stagnalis, an important invertebrate model of behavioral rhythm generation. The neuronal network responsible for the three-phase feeding motor program (fictive feeding) has been characterized extensively and divided into populations of central pattern generator (CPG) interneurons, modulatory interneurons, and motoneurons. A previous model of the feeding system considered that the motoneurons were passive followers of CPG interneuronal activity. Here we present new, detailed physiological evidence that motoneurons that innervate the musculature of the feeding apparatus have significant electrotonic motoneuron-->interneuron connections, mainly confined to cells active in the same phase of the feeding cycle (protraction, rasp, or swallow). This suggested that the motoneurons participate in rhythm generation. This was assessed by manipulating firing activity in the motoneurons during maintained fictive feeding rhythms. Experiments showed that motoneurons contribute to the maintenance and phase setting of the feeding rhythm and provide an efficient system for phase-locking muscle activity with central neural activity. These data indicate that the distinction between motoneurons and interneurons in a complex CNS network like that involved in snail feeding is no longer justified and that both cell types are important in motor pattern generation. This is a distributed type of organization likely to be a general characteristic of CNS circuitries that produce rhythmic motor behavior.

In vivo neuropharmacological and in vitro laser ablation techniques as tools in the analysis of neuronal circuits underlying behavior in a molluscan model system

1. This paper reviews the selective lesioning techniques employed to elucidate the role of the neurotransmitters dopamine and serotonin and single, identified interneurons in the feeding system of the pond snail Lymnaea stagnalis. 2. The pathway lesioning work reviewed in this paper showed that dopamine is necessary for the feeding response to occur and serotonin has a mainly modulatory role in the feeding system of Lymnaea. 3. The photoinactivation results reviewed here assist in the elucidation of the different roles that different types of interneurons play in the initiation and modulation of patterned neuronal activity underlying feeding.

Cholinergic interneurons in the feeding system of the pond snail Lymnaea stagnalis. II. N1 interneurons make cholinergic synapses with feeding m otoneurons

The N1 neurons are a population of interneurons active during the protraction phase of the feeding rhythm . All the N1 neurons are coupled by electrical synapses which persist in a high Mg/low Ca saline which blocks chemical synapses. Individual N1 spikes produce discrete electrotonic postsynaptic potentials (PSPS) in other N1 cells, but the coupling is not strong enough to ensure 1:1 firing. Bursts of N1 spikes generate com pound PSPS in the feeding motoneurons. The sign (excitation or inhibition) of the N1 input corresponds with the synaptic barrage recorded during the protraction phase. Discrete PSPS are only resolved in a Hi-Di saline. Their variation in latency and number can be explained by variation in electrotonic propagation within the electrically coupled network of N1 cells. The excitatory postsynaptic potentials (EPSPS) in the 1 cell are reduced by 0.5 mM antagonists hexamethonium (HMT), atropine (ATR), curare (d-TC) and by methylxylocholine (MeXCh), all of which block the excitatory cholinergic receptor (Elliott et al. ( Phil. Trans. R. Soc. Lond. 336, 157-166 (Preceding paper.) (1992)). The 1 cell EPSPS were transiently blocked by phenyltrimethylammonium (PTMA), which is both an agonist and antagonist at the 1 cell excitatory acetylcholine (ACh) receptor (Elliott et al. 1992). The inhibitory postsynaptic potential (IPSP) in the 3 cell is blocked by bath applications of MeXCh and PTM A , which both abolish the response of the 3 cell to ACh (Elliott et al. 1992). It is concluded that the population of N1 cells are multiaction, premotor cholinergic interneurons.

Simultaneous measurements of intracellular pH in the leech giant glial cell using 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein and ion-sensitive microelectrodes

We have employed two independent techniques to measure the intracellular pH (pHi) in giant glial cells of the leech Hirudo medicinalis, using the fluorescent dye 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF) and double-barreled neutral-carrier, pH-sensitive microelectrodes, which also record the membrane potential. We have compared two procedures for calibrating the ratio of the BCECF signal, excited at 440 nm and 495 nm: 1) the cell membrane was H(+)-permeabilized with nigericin in high-K+ saline at different external pH (pHo) values, and 2) the pHi of intact cells was perturbed in CO2/HCO3(-) -buffered saline of different pH, and the BCECF ratio was calibrated according to a simultaneous microelectrode pH reading. As indicated by the microelectrode measurements, the pHi did not fully equilibrate to the pHo values in nigericin-containing, high-K+ saline, but deviated by -0.12 +/- 0.02 (mean +/- SEM, n = 37) pH units. In intact cells, the microelectrode readings yielded up to 0.15 pH unit lower values than the calibrated BCECF signal. In addition, larger dye injections into the cells (> 100 microM) caused an irreversible membrane potential loss indicative of some damage to the cells. The amplitude and kinetics of slow pHi changes were equally followed by both sensors, and the dye ratio recorded slightly higher amplitudes during faster pHi shifts as induced by the addition and removal of NH4+.

Structure and pharmacological properties of a molluscan glutamate-gated cation channel and its likely role in feeding behavior

We describe the isolation of a molluscan (Lymnaea stagnalis) full-length complementary DNA that encodes a mature polypeptide (which we have named Lym-eGluR2) with a predicted molecular weight of 105 kDa that exhibits 44-48% identity to the mammalian kainate-selective glutamate receptor GluR5, GluR6, and GluR7 subunits. Injection of in vitro-transcribed RNA from this clone into Xenopus laevis oocytes results in the robust expression of homo-oligomeric cation channels that can be gated by L-glutamate (EC50 = 1.2 +/- 0.3 micron) and several other glutamate receptor agonists; rank order of potency: glutamate >> kainate > ibotenate > AMPA. These currents can be blocked by the mammalian non-NMDA receptor antagonists 6,7-dinitroquinoxaline-2,3-dione, 6-cyano-7-nitroquinoxaline-2,3-dione, and 1-(4-chlorobenzoyl)piperazine-2,3-dicarboxylic acid. Ionic-replacement experiments have shown that the agonist-induced current is carried entirely by sodium and potassium ions. In situ hybridization has revealed that the Lym-eGluR2 transcript is present in all 11 ganglia of the Lymnaea CNS, including the 4-cluster motorneurons within the paired buccal ganglia. The pharmacological properties and deduced location of Lym-eGluR2 are entirely consistent with it being (a component of) the receptor, which has been identified previously on buccal motorneurons, that mediates the excitatory effects of glutamate released from neurons within the feeding central pattern generator.