Spatio-temporal dynamics of the egg-laying-inducing peptides during an egg-laying cycle: a semiquantitative matrix-assisted laser desorption/ionization mass spectrometry approach

The activity-dependent release of peptides from the neuro-endocrine caudodorsal cell (CDC) system of the freshwater snail Lymnaea stagnalis regulates egg laying and related behaviors. In this study, we optimized a mass spectrometry-based approach to study the spatio-temporal dynamics of peptides that are largely derived from the CDC hormone precursor during an egg-laying cycle and a CDC discharge in vitro. Semi-quantitative peptide mass profiling using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) indicated a massive depletion of peptides from the neurohemal area in the cerebral commissure (COM) during egg laying and the existence of a reserve pool of peptides in the CDC somata that were transported to the COM to restore peptide levels. The depletion of CDC peptides from the COM was correlated to their release during an induced electrical discharge in vitro. Moreover, MALDI-MS of the releasate revealed extensive truncation of the carboxyl terminal peptide. Finally, two novel peptides of 1788 and 5895 Da, not encoded by the CDC hormone precursor, also exhibited temporal quantitative changes similar to those of CDC peptides. Sequencing of the peptide of 1788 Da by tandem mass spectrometry yielded the novel sequence HF(FH)FYGPYDVFQRDVamide. Together, this implicates a more complex set of CDC peptides for the regulation of egg laying than previously anticipated.

"Allohormones": a class of bioactive substances favoured by sexual selection

During close bodily contact, many species transfer substances that influence the behaviour or physiology of conspecifics. Such transfer is especially common during courtship and copulation. When this is the case the involved bioactive substances are favoured by sexual selection because their effects include increased egg production, inhibited remating, and changed sperm transport or storage in the partner. The direct mode of action of these substances is fundamentally different from that of pheromones and nuptial gifts. Therefore, the term allohormone is introduced here. An allohormone is defined as a substance that is transferred from one individual to another free-living member of the same species and that induces a direct behavioural or physiological response, bypassing external sensory organs. Although we emphasise the importance of allohormones in reproductive processes, allohormones may also have important functions outside of copulation.

NPY in invertebrates: molecular answers to altered functions during evolution

As in Lymnaea stagnalis NPY plays a key role in regulating energy flows but has no effect on food intake, two important questions arise: 1) How is the amount of food consumed related to energy storage? 2) Can we give a molecular explanation for this alteration in function of NPY during evolution? Recent data have shown that also in Lymnaea a leptin-like factor is produced by glycogen storing cells which inhibits food intake, a Lymnaea storage feedback factor (LySFF). So, food consumption seems in balance with the amount of energy stored in this animal. We suppose that NPY neurons in Lymnaea have receptors for LySFF so that their activity in regulating energy homeostasis reflects the amount of stored energy. By comparing the molecular structure of NPYs in invertebrates it became clear that only molluscan and arthropod NPY are synthesized from a prohormone similar to vertebrate NPYs and should be considered as real invertebrate homologs of NPY. Based on pharmacological data we suppose that the identified Lymnaea NPY receptor is a Y1 subtype. This might explain that LyNPY has no effect on food intake in Lymnaea as this function of NPY in mammals is regulated through the Y5 subtype receptor.

Pattern generation in the buccal system of freely behaving Lymnaea stagnalis

Central pattern generators (CPGs) are neuronal circuits that drive active repeated movements such as walking or swimming. Although CPGs are, by definition, active in isolated central nervous systems, sensory input is thought play an important role in adjusting the output of the CPGs to meet specific behavioral requirements of intact animals. We investigated, in freely behaving snails (Lymnaea stagnalis), how the buccal CPG is used during two different behaviors, feeding and egg laying. Analysis of the relationship between unit activity recorded from buccal nerves and the movements of the buccal mass showed that electrical activity in laterobuccal/ventrobuccal (LB/VB) nerves was as predicted from in vitro data, but electrical activity in the posterior jugalis nerve was not. Autodensity and interval histograms showed that during feeding the CPG produces a much stronger rhythm than during egg laying. The phase relationship between electrical activity and buccal movement changed little between the two behaviors. Fitting the spike trains recorded during the two behaviors with a simple model revealed differences in the patterns of electrical activity produced by the buccal system during the two behaviors investigated. During egg laying the bursts contained less spikes, and the number of spikes per burst was significantly more variable than during feeding. The time between two bursts of in a spike train was longer during egg laying than during feeding. The data show what the qualitative and quantitative differences are between two motor patterns produced by the buccal system of freely behaving Lymnaea stagnalis.

Transmitter identification in neurons involved in male copulation behavior in Lymnaea stagnalis

In this paper, we have mapped the cellular localization of various transmitters onto the central neurons which are involved in male copulation behavior in Lymnaea stagnalis, by combining retrograde tracing with immunocytochemistry and in situ hybridization. Evidence is provided that neurons which were backfilled from the penis nerve, the sole nerve to innervate the male copulatory organ, synthesize a multitude of neuropeptides (APGWamide, Lymnaea neuropeptide tyrosin [LNPY], conopressin, pedal peptide, SEEPLY, DEILSR, myomodulin, and Lymnaea inhibitory peptide [LIP]) as well as the classical neurotransmitter, serotonin. In the anterior lobe, the backfilled neurons mainly contain the tetrapeptide APGWamide and conopressin, and not LNPY or pedal peptide. The results suggest a central role in the regulation of copulation activity for the anterior lobe neurons that produce APGWamide and conopressin. Immunostainings of backfilled nervous systems revealed immunopositive axons originating from these neurons to form varicosities on the cell somata of neurons in the other clusters contributing to the innervation of the male sexual system. Neurons from the right parietal ganglion projecting into the penis nerve were electrophysiologically and morphologically identified by simultaneously recording from the cell body intracellularly and the penis nerve extracellularly and subsequently filling them with an anterograde tracer and subjecting them to immunocytochemistry. This method has provided links between morphology, physiology, and the transmitter contents of these neurons.

Behavior-dependent activities of a central pattern generator in freely behaving Lymnaea stagnalis

Cyclic or repeated movements are thought to be driven by networks of neurons (central pattern generators) that are dynamic in their connectivity. During two unrelated behaviors (feeding and egg laying), we investigated the behavioral output of the buccal pattern generator as well as the electrical activity of a pair of identified interneurons that have been shown to be involved in setting the level of activity of this pattern generator (PG). Analysis of the quantile plots of the parameters that describe the behavior (movements of the buccal mass) reveals that during egg laying, the behavioral output of the PG is different compared with that during feeding. Comparison of the average durations of the different parts of the buccal movements showed that during egg laying, the duration of one specific part of buccal movement is increased. Correlated with these changes in the behavioral output of the PG were changes in the firing rate of the cerebral giant neurons (CGC), a pair of interneurons that have been shown to modulate the activity of the PG by means of multiple synaptic contacts with neurons in the buccal ganglion. Interval- and autocorrelation histograms of the behavioral output and CGC spiking show that both the PG output and the spiking properties of the CGCs are different when comparing egg-laying animals with feeding animals. Analysis of the timing relations between the CGCs and the behavioral output of the PG showed that both during feeding and egg laying, the electrical activity of the CGCs is largely in phase with the PG output, although small changes occur. We discuss how these results lead to specific predictions about the kinds of changes that are likely to occur when the animal switches the PG from feeding to egg laying and how the hormones that cause egg laying are likely to be involved.

Role of neuropeptides encoded on CDCH-1 gene in the organization of egg-laying behavior in the pond snail, Lymnaea stagnalis

Egg laying in the pond snail Lymnaea stagnalis is triggered by a discharge of the neuroendocrine caudodorsal cells (CDCs). The CDCs expresses three different caudorsal cell hormone (CDCH) genes. This gene family expresses, in total, 11 different peptides among which is the ovulation hormone. Besides the CDCs, the CDCH gene family is expressed in other central and peripheral neurons. In this study, we investigated the roles the different CDCH peptides play in the organization of egg-laying behavior. Egg-laying behavior is a sequence of stereotyped movements in which three phases can be distinguished: resting, turning, and oviposition. We have used the excitation of right pedal N (RPeN) motor neurons as a simple analogue of shell-turning behavior, one of the elements of egg-laying behavior. RPeN motor neurons were inhibited during the resting phase of egg laying but were subsequently excited at the onset of and during the turning phase. The excitatory effect could be evoked by application of beta3-CDCP on RPeN motor neurons in the CNS as well as in isolation but not by the ovulation hormone, alpha-CDCP or Calfluxin, the other CDCH-1 peptides tested. The ovulation hormone itself caused inhibition of RPeN motor neurons. Anti-CDCH-1 positive fiber tracts were found close to the cell bodies and axons of the RPeN motor neurons. Electrical stimulation of a nerve that contains these fibers resulted in excitation of the RPeN motor neurons. The effects of injection of CDCH-1 peptides into intact animals correlated well with the effects of these peptides on RPeN motor neurons. Injection of beta3-CDCP or alpha-CDCP into intact animals resulted in immediate turning behavior in the absence of egg laying itself. The ovulation hormone and Calfluxin had no immediate effect on the behavior. Furthermore, our data indicate that the individual CDCH-1 peptides act on different targets.

Spontaneous switching between ortho- and antidromic spiking as the normal mode of firing in the cerebral giant neurons of freely behaving Lymnaea stagnalis

1. Action-potential generation at sites remote from the cell body leads to antidromic firing and occurs in a wide variety of animals and experimental circumstances. Remote sites of spike generation may play a role in the functional subdivision of the axonal branches of a neuron and are also thought to play a role in synaptic integration. 2. Spontaneous ortho- and antidromic firing was investigated by recording the electrical activity of somata and axons of a pair of identified giant neurons [cerebral giant cells (CGCs)] in freely behaving animals. 3. At the soma of each CGC, the shape of the extracellular action potential was not constant but jumped between two well-defined levels. Subsequent recordings of synchronous firing in both cell bodies showed that the shape of the extracellular action potential depended on the firing sequence of the two CGCs. 4. Simultaneous recordings of the cell body and the main axon of a single CGC showed that spontaneous changes in the direction of spike conduction (orthodromic or antidromic) occurred. These changes in the direction of spike conduction coincided with the changes in the shapes of the extracellular action potentials recorded from the somata. 5. These results show that, under physiological conditions, spontaneous switching occurs between ortho- and antidromic spiking in the CGCs, and that action-potential generation at sites remote from the cell body is a physiologically relevant mechanism.

The neuropeptide schistosomin and haemolymph from parasitized snails induce similar changes in excitability in neuroendocrine cells controlling reproduction and growth in a freshwater snail

Infection of the snail Lymnaea stagnalis with the schistosome parasite Trichobilharzia ocellata results in inhibition of reproduction and in giant growth. Parasite-related effects on the neuroendocrine centres that control these processes were studied electrophysiologically. Haemolymph from infected snails reduced the excitability of the caudodorsal cells, which control egg laying. In contrast, the excitability of the growth-controlling Light Green Cells was increased under these conditions. The endogenous anti-gonadotropic neuropeptide schistosomin, the presence of which is strongly enhanced in parasitized snails, induced similar effects. Schistosomin apparently plays an important role in the balance between reproduction and growth in Lymnaea. This balance is severely disturbed during parasitic infection, probably as a result of the release of the peptide.

Inhibitory modulation by FMRFamide of the voltage-gated sodium current in identified neurones in Lymnaea stagnalis

1. The putative neurotransmitter FMRFa (Phe-Met-Arg-Phe-amide) caused an inhibitory modulation of the voltage-gated sodium current (INa) in central neurones, the peptidergic caudo dorsal cells (CDCs) of the mollusc Lymnaea stagnalis. FMRFa reduced INa at all command potentials tested (ranging from -35 to +20 mV), but the amplitude of the effect of FMRFa was voltage dependent, inhibition being stronger at more negative potentials (50 +/- 5% reduction at half-maximal INa activation versus 25 +/- 8% at the peak of the I-V curve). 2. INa current traces were well fitted by a Hodgkin & Huxley based model, using m3 activation kinetics and two time constants for inactivation. 3. The steady-state inactivation curve of INa was characterized by half-maximal inactivation at -42.5 +/- 1.81 mV and a slope factor of 4.6 +/- 0.28 mV. The fastest time constant of inactivation ran from 100 +/- 5 to 0.8 +/- 0.32 ms and the slower time constant from 505 +/- 45 to 4.8 +/- 1.40 ms in the range -40 to -5 mV. 4. FMRFa had no significant effect on either component of inactivation, nor on the voltage dependence of steady-state inactivation, nor on the maximal conductance. 5. FMRFa affected the activation of INa. The activation time constant was increased, ranging from 0.75 +/- 0.050 to 0.22 +/- 0.017 ms under control and from 0.91 +/- 0.043 to 0.31 +/- 0.038 ms with FMRFa in the voltage range -25 to +5 mV. The steady-state activation curve was shifted to less negative potentials: half-maximal activation occurred at -26.5 +/- 1.2 mV under control and at 23.6 +/- 1.4 mV with FMRFa; the slope factor (4.6 +/- 1.4 mV in control experiments) was not affected. The combination of slower activation kinetics and a shift in the voltage dependence of activation in the Hodgkin & Huxley based model, adequately explained the reduction of INa by FMRFa. 6. The physiological consequence is that the spiking threshold is increased, causing an arrest of on-going firing activity and a decrease in excitability.

Quantitative ultrastructural tannic acid study of the relationship between electrical activity and peptide secretion by the bag cell neurons of Aplysia californica

Release of neurosecretory material by the neuroendocrine bag cells (BC) of the gastropod Aplysia californica was studied, using quantitative electron microscopy and the tannic acid method for the demonstration of exocytosis of neuropeptides. Axon terminals of electrically inactive BC located in the periphery of the pleurovisceral connectives are filled with secretory granules and show low exocytosis activity (one exocytosis figure per 8 terminal profiles). In terminals of BC stimulated to an electrical discharge, in contrast, granules are scarce or absent and exocytosis activity has increased 24-fold. During rest and, particularly, during electrical discharge, BC apparently release secretory material into the hemolymph by exocytosis from axon terminals. Release furthermore takes place from axons running in the connective tissue surrounding the connectives.

One receptor type mediates two independent effects of FMRFa on neurosecretory cells of Lymnaea

Structure activity relations (SAR) of FMRFa on the transient hyperpolarizing response and long lasting depression of excitability of neurosecretory caudo dorsal cells (CDCs) of the pond snail Lymnaea stagnalis were examined. Although these effects to FMRFa occur independently, the SARs for the induction of both responses were identical suggesting that CDCs possess a single type of FMRFa receptors. Native GDPFLRFa and SDPFLRFa were equipotent to FMRFa receptors. It is concluded that activation of the receptor requires [Arg3-Phe4]-NH2, whereas N-terminal amino acids are involved in binding.

The role of cAMP in regulation of electrical activity of the neuroendocrine caudodorsal cells of Lymnaea stagnalis

The neuroendocrine caudodorsal cells (CDCs) of the pond snail Lymnaea stagnalis release a number of peptides, including the ovulation hormone, caudodorsal cell hormone (CDCH), during a period of high electrical activity (the CDC-discharge). Earlier studies have shown that during the CDC-discharge adenylate cyclase activity is increased, and that the cyclic adenosine monophosphate (cAMP) analogue 8-chlorophenylthio (8-CPT)-cAMP induces exocytosis and release of peptides from the CDCs. Here, we have investigated the role of cAMP, adenylate cyclase and phosphodiesterase in determining the state of excitability of the CDCs. The cAMP analogue 8-CPT-cAMP induced long-lasting discharges in CDCs. Simultaneous inhibition of the phosphodiesterase by 3-isobutyl-1-methylxanthine (IBMX) and activation of the adenylate cyclase by forskolin gave similar results. These agents also induced discharges of CDCs in dissociated cell culture, indicating that the responses to an increase of cAMP were an endogenous property of the cells. The CDC-afterdischarge can be induced by a period of repetitive electrical stimulation. Inhibition of the phosphodiesterase-activity by IBMX did not change the resting membrane potential, but increased the proportion of preparations that responded to this stimulation with an afterdischarge by more than 200%. It is suggested that cAMP-regulating enzymes are involved in stimulus-response coupling of the afterdischarge in CDCs. The induction of an after discharge probably requires both a low phosphodiesterase-activity and the activation of the adenylate cyclase. The low excitability of the CDCs following an afterdischarge might originate from a refractoriness in the activation of the adenylate cyclase.

Chemically mediated positive feedback generates long-lasting afterdischarge in a molluscan neuroendocrine system

The peptidergic neuroendocrine caudodorsal cells (CDCs) of Lymnaea stagnalis control egg laying. The CDC network consists of 100 electrotonically coupled neurons that form two clusters in the cerebral ganglia. Upon prolonged, repeated, intracellular stimulation of one CDC, excitation spreads over the network and leads to a 30-min period of spiking activity: the afterdischarge. During the afterdischarge a number of peptides, including the ovulation hormone, are released. When two ganglia rings from different animals were pinned down next to each other, an afterdischarge initiated in the CDCs of one CNS activated the CDCs of the other CNS, indicating that excitation spreads in the absence of physical contact between the CDCs. A single isolated intercerebral commissure (COM), the neurohaemal area of the CDCs, displayed the same discharge-inducing capability when brought in the vicinity of a second, intact, CNS. Other parts of the CNS did not possess this property. CDC afterdischarges could also induce repetitive spiking in adjacent isolated CDC somata showing that the effect can be directly on the CDCs themselves. The discharge-inducing factor was well separated from the ovulation hormone on a Bio-Gel P-6 column. The factor was pronase-degradable and inhibitors of proteolytic enzymes increased the factor's longevity. It is concluded that, contingent upon the CDC-discharge, a small (less than or equal to 1500 Da) excitatory peptide is released that acts directly on the CDCs. Its function is argued to be: (1) the spread of excitation from a subset of CDCs, receiving external input, over the entire CDC network; and (2) to provide a positive feedback to generate a maximum (all-or-none) response.

Spontaneous and elicited bag cell discharges in gonadectomized Aplysia

The neuroendocrine bag cells of the hermaphroditic marine gastropod, Aplysia, secrete peptide hormones that induce release of ripe eggs from the ovotestis. The egg string is subsequently deposited on the substrate by means of a complex sequence of rhythmic head and neck movements. Gonadectomy (removal of the ovotestis) was performed in two closely related species of Aplysia to prevent completely the synthesis, build-up and release of eggs. Chronically implanted electrodes were used either to monitor spontaneous bag cell discharges (A. brasiliana) or to selectively elicit bag cell discharges (A. californica) in gonadectomized and mock-operated animals. Gonadectomized animals showed the normal occurrence of spontaneous bag cell discharges in the complete absence of eggs, indicating that feedback from ripe eggs in the ovotestis is not necessary for normal activation of the bag cells. However, gonadectomized animals showed a significant decrease in specific head and neck movements following elicited bag cell discharges. This finding indicates that, once the bag cells fire and the eggs are released, input from the eggs is necessary for normal expression of the behaviour associated with egg deposition.

Membrane mechanism of neuroendocrine caudo-dorsal cell inhibition by the ring neuron in the pond snail Lymnaea stagnalis

Ovulation in the pond snail Lymnaea stagnalis is controlled by the neuroendocrine caudo-dorsal cells (CDCs) in the cerebral ganglia, which release an ovulation hormone during a period of impulse activity. Firing of the single RN in the right cerebral ganglion hyperpolarizes the CDCs. This hyperpolarization is caused by the opening of potassium channels in the axons that connect both the CDC clusters. By this action, that presumably is mediated by axonal branches of the RN in the intercerebral commissure closely associated with these CDC axons, the RN decouples both the CDC clusters. Although the RN has negative feedback on the CDC, it does not control afterdischarge characteristics. The authors suggest that the RN, next to the egg-laying behavior, is involved in another behavior, not compatible with ovulation. Male reproductive activity is presented as a possible candidate for such behavior.

Ring neuron control of columellar motor neurons during egg-laying behavior in the pond snail

Egg-laying in Lymnaea is characterized by the stereotyped egg-laying behavior (ELB), composed of foot contractions and shell movements. Egg-laying can be induced by a clean water stimulus, that triggers a discharge of the neuroendocrine caudo-dorsal cells (CDCs), which release the ovulation hormone into the blood. A part of the behavior is lost when egg-laying is triggered by hormone injection, indicating that during natural stimulus-induced or spontaneous egg-laying this part (the first phase) may be controlled by neuronal events in the CNS triggered by (a) factor(s) not released to the blood. The authors have identified an unpaired neuron, the ring neuron, that is excited during an in vitro afterdischarge of the CDCs, and which, by its numerous axonal branches in the pedal ganglia, modulates motorneurons of the columellar muscle, which controls shell movements. These motor-neurons, identified as such in reduced preparations by 1 for 1 muscle potentials and elements in the connecting nerve, all receive either excitatory or inhibitory input from the ring neuron, as well as from an unknown neuron which has common input of the ring neuron and the motorneurons. The action of the CDCs on the ring neuron cannot be mimicked by the ovulation hormone, and we therefore conclude that the first part of the ELB is probably caused by a nonhormonal local action of the CDCs on the ring neuron and possibly the common input neuron.

Selective recording and stimulation of individual identified neurons in freely behaving Aplysia

A neuroethological technique is described for selective recording and stimulation of an individual neuron in freely behaving Aplysia by means of a fine wire glued into the connective tissue sheath above the identified cell body. A whole-nerve cuff electrode simultaneously monitored functionally related multiunit axon activity. For biophysical analysis the soma was impaled with a microelectrode when the ganglion was subsequently exposed. The technique is illustrated for several identified neurons involved in different behaviors.

Integration of biphasic synaptic input by electrotonically coupled neuroendocrine caudodorsal cells in the pond snail

The ovulation hormone-producing caudodorsal cells (CDCs) of the pond snail Lymnaea stagnalis form two clusters of electrotonically coupled cells, each containing a few specialized (ventral) cells that connect the clusters. The hormone is secreted during a pacemaker-driven discharge. The CDCs receive a biphasic cholinergic postsynaptic potential (PSP), consisting of a rapid excitatory postsynaptic potential (EPSP) and a slow inhibitory postsynaptic potential (IPSP) that is elicited by stimulation of nerves. The effect of the synaptic input on the discharge of the CDCs is described and the location of the synapse investigated by a combination of electrophysiological recordings and morphological techniques. The PSP interrupts the discharge and hastens its termination. In addition, it causes a reversal of the temporal order of the spikes of ventral cells (that normally lead) and dorsal cells (that lead only after the PSP). Ion-substitution experiments indicate that the ionic mechanism underlying the biphasic PSP is conventional, involving a conductance increase for Na+ (EPSP) and K+ (IPSP). Receptors mediating the inhibitory component occur only on the proximal axons of the ventral cells, both components are larger and reverse more readily in ventral cells. These findings suggest that the PSP is generated in the ventral cells. The biphasic PSP has no effect on electrical coupling, suggesting that it is not generated along the electrical pathways among the cells. Horseradish peroxidase (HRP) staining reveals that the lateral branches emerge from the proximal axons of the ventral cells only. In HRP-filled preparations processed for electron microscopy (EM) acetylcholinesterase is demonstrated at these branches where it occurs associated with synapses. The location on fine branches of the ventral cells explains the absence of an effect on electrotonic transmission, whereas the reluctance of components of the PSP to reverse at the expected potentials is due to the distribution of the synapses over more than one cell. It is concluded that the biphasic PSP is received only by the ventral cells and that it is conveyed electrotonically to the other cells.

Morphology and electrophysiology of the ovulation hormone producing neuro-endocrine cells of the freshwater snail Lymnaea stagnalis (L.)

The ovulation hormone producing neuro-endocrine cells of Lymnaea stagnalis, the caudo-dorsal cells (CDC), are comparable to the bag cells of Aplysia. Both cell types are capable of the production of a long-lasting activity (afterdischarge) during which an ovulation hormone is released. The CDC (30 cells in the left cerebral ganglion and 70 cells in the right) are usually electrically silent but an afterdischarge can be brought about in all cells of both groups by direct, repetitive electrical stimulation of single CDC. This is not possible in every preparation, indicating that the CDC can be in different states of excitability. All cells participate in the afterdischarge and fire approximately synchronously. All CDC are electrotonically connected. Results of experiments in which neurones were injected with horseradish peroxidase suggest that the demonstrated electrotonic connexions between the two opposite groups of CDC are brought about by 10-12 special axons.

ACTH-like immunoreactivity in two electronically coupled giant neurons in the pond snail Lymnaea stagnalis

Two giant neurons (diameter 130 micron) were identified immunocytochemically by means of the unlabeled antibody enzyme technique with anti-ACTH 1-39 and 1-24 in the pond snail Lymnaea stagnalis. The cells are located in the visceral and the right parietal ganglion, respectively. They contain moderately electron dense elementary granules (diameter 150-160 nm). By means of the intracellular horseradish peroxidase injection technique it was shown that the cells send fibres into the neuropiles of various ganglia and into nerves. Synapses occur on the fine fibre branches in the neuropile. Synapse-like structures were found on the cell bodies and on the major fibres. The giant neurons are electrotonically coupled. With toluidine blue staining for small peptides it was demonstrated that in the central nervous system of the pond snail numerous peptidergic neurons occur in addition to those identified with the classical staining methods for neurosecretion.