Lymnaea as a neurobiological system
The pond snail Lymnaea stagnalis is a model molluscan system used in many laboratories to study a wide range of fundamental neurobiological problems (Kemenes & Benjamin, 2009). The compact central nervous system of Lymnaea, with its beautiful brightly-pigmented orange neurons has been attractive to many neuroscientists interested in relating the molecular and electrical properties of neurons to behavior. Individual neurons can be identified as parts of behavioral circuits and their synaptic connectivities determined by electrophysiological recording. The ability to grow Lymnaea neurons in cell culture has allowed the in vitro reconstruction of synapses and neural circuits. Early work focused on using Lymnaea giant neurons to analyse the biophysical properties of neurons or investigated the organization of neuroendocrine peptidergic neural networks involved in growth and reproduction. Since then the neural networks underlying a variety of reflexive and rhythmic behaviors have been investigated including feeding, respiration, defensive withdrawal, locomotion, gravity orientation, and reproduction (copulation and egg-laying). Many of these behaviours are highly dynamic leading to exciting recent studies on the neural and molecular mechanisms underlying simple forms of memory formation (Benjamin et al., 2000; Benjamin & Kemenes, 2010).
The biology of Lymnaea
Lymnaea stagnalis Linneus (L.) (Pulmonata, Basomatophora) is a widely distributed inhabitant of freshwater ponds, lakes and rivers rich in vegetation and is found throughout Europe, the Northern United States and parts of Asia. The snails are from 2 to 5 cm in shell length and typically are found in large numbers close to the water surface (Fig. 1) feeding on floating pond weed. They also feed on algae, detritus and carrion and so are generalists in their feeding habits. Lymnaea has a gas-filled lung and ventilation is accomplished by opening and closing movements of the apex of a muscular tube (the pneumostome) that forms the entrance to the lung (Fig. 1). Skin respiration is also important in Lymnaea and its relative importance compared to lung ventilation depends on the oxygen content of water. Locomotion is mainly carried out by the coordinated front-to-back beating of cilia on the sole of the foot although associated cyclical shell movements and body-turning involve muscular contractions (Winlow et al., 1992). If food is placed beneath the water surface, say at the bottom of an aquarium tank, then the snails carry out regular cycles of locomotion to and from the water surface to alternate feeding with breathing. The reproductive biology of Lymnaea has been well-studied. It is a simultaneous hermaphrodite but during mating behavior one individual acts as the male and the other the female. Often this is immediately followed by a reversal of sexual roles by the same pair of snails. During oviposition, gelatinous egg masses, each containing 100 eggs or more, are deposited on the substrate and tiny snails in adult form eventually emerge without any free-living veliger larval stage. This makes it simple to breed snails in the laboratory, a major advantage for behavioral and molecular studies where large numbers of animals are required.
Organization of the nervous system
Lymnaea has a ring of nine ganglia that form a Central Nervous System (CNS) or brain (Fig. 2B) around the anterior gut with two further (buccal) ganglia (Fig. 2A) that lie more distant from the rest of the brain on the surface of the feeding apparatus, the buccal mass. There is also a peripheral nervous system that consists of scattered neurons that lie beneath the epithelia of the skin, small numbers of neurons that lie at the branches of peripheral nerves and larger aggregates of sensory nerve cells associated with special sense organs such as the eyes, osphradium and tentacles. Many of the sub-epithelial cells are clearly sensory neurons based on their morphology (they have apical dendrites projecting through to the surface of the epithelial layer) but others may have a motor function mediating local reflexes, although these have not been studied in any detail. In the CNS, it is possible to identify pairs of (left and right) symmetrical ganglia with similar general functions. These are the buccal, cerebral, pleural, pedal and parietal ganglia (Fig. 2B). The visceral ganglion and right parietal ganglion tend to be ‘fused’ compound structures and the symmetries are less clear. The cerebral ganglia are the largest and most lobed ganglia in the CNS and are involved in the sensory and motor control of feeding, head waving and copulation (right side only) and the sensory processing of information from the eyes, tentacle and lips. The pedal ganglia control locomotion and conjoint muscle movements involving the shell. The pleural ganglia have no nerves and their behavioral function is generally unknown although there are peptidergic neurons of several different types present. The visceral/parietal ganglia have been extensively investigated and are involved in respiration, control of heartbeat and other visceral functions (nerves innervate the gut, kidney and reproductive system). Many more complex actions, such as locomotion, egg-laying and whole body-withdrawal require control of the whole body and involve several different ganglia.
Chemical and tactile skin sensory modalities are very important in Lymnaea but other sensory systems involved in gravity orientation also have been investigated in some detail. Until recently the visual capabilities of Lymnaea have been underestimated but now it is realized that these snails have an efficiently organized eye with special optics that allow visual images to be formed (Fig. 3)
Snails live in a chemical world
Using feeding as an assay for chemical sensitivity, Lymnaea shows selective feeding responses to different chemicals. Sugars like sucrose and maltose stimulate feeding responses but feeding is inhibited by quinine (Kemenes et al., 1986). Snails can rapidly be trained to respond to previously feeding-neutral stimuli like amyl acetate (Alexander et al., 1984; Kemenes et al., 2002). Snails will detect point food stimuli from a distance in still water and this is likely to involve tropotaxis, a simultaneous comparison of chemical signals at symmetrical paired receptor sites followed by a turn directly to the more strongly stimulated side (classification of orientation mechanisms based on Fraenkel & Gunn, 1961). Closer to the food, where stronger concentration gradients are more likely to be present, zig-zag head motions have been seen in another basomatophoran pond snail (Townsend, 1973). This is associated with a form of orientation called klinotaxis where the snails are comparing chemical intensity at successive symmetrical points in space. Spatial sampling of the type required for distance chemical orientation is likely to involve the pair of bilaterally symmetrical tentacles located on left and right front sides of the head but whether the chemoreceptors are located on the tentacle itself or at its base is still an open question. Distance orientation to chemical food stimuli is unlikely to involve chemoreceptors in the special sense organ, the osphradium, because it is a single sense organ located to the rear of the animal, on the right side only, making it difficult to carry out spatial comparisons of sensory information using this sense organ, even though it can respond to chemical stimuli (Wedermeyer & Schild, 1995). Localization of food by moving up-stream to chemicals (positive rheotaxis) is another mechanism available for locating distant food available in flowing water and it has been reported to occur in Lymnaea (Kemenes & Benjamin, 1989). Finally, local ‘taste’ chemoreceptors on the lips of Lymnaea respond to applied sugars by initiating feeding ingestive movements.
Mainly about touch
Touch to the skin is mainly an aversive stimulus mediated by primary touch receptors with local receptive fields covering the whole surface of the body. Weak point tactile stimuli cause local withdrawal responses on structures like the tentacles but recruit progressively larger areas of the body as stimulus intensities increase. Strong pokes to the animal cause whole-body withdrawal responses (Ferguson & Benjamin, 1991a). Gentle touch to the lips is treated differently by the snail. It is a component of the normal stimulus required for feeding acting during the rasp phase of the feeding cycle when the radula is scraped across the food substrate, increasing the regularity and frequency of motoneuron firing (Staras et al., 1999). Other types of mechanoreceptors are involved with feeding satiety. Those innervating the oesophagus respond to stretching caused by food ingestion and inhibit neurons in the feeding circuit (Elliott & Benjamin, 1989).
The ups and downs of geotactic behavior
Gravity-mediated orientation and its sensory control in Lymnaea is intriguing and quite complex. Lymnaea is either positively or negatively geotactic depending on the oxygen content of the lung and surrounding pond water (Janse, 1982). In O2-rich conditions snails are positively geotactic and they move away from the surface of the water. In O2-poor condition the snails switch to being negatively geotactic and they move towards the surface of the water to carry out aerial breathing. There is a consensus that the statocysts are involved in both types of geotactic responses. These are paired sense organs that lie on the surface of the pedal ganglia. Statocysts are simple spherical chambers with a central fluid filled cavity formed by beating ciliated epithelial hair cells (sensory cells) and supporting cells. Calcareous ‘stones’, move round in the less dense fluid and mechanically stimulate particular sets of hair cells depending on the tilt of the animal. Whether particular subsets of hair cells are responsible for either positive or negative geotaxis is unknown although different oxygen levels can modulate the electrical properties of some of the hair cells. However, snails without statocysts still move to the water surface (although their crawling tracts are longer and velocity is reduced) so there must be a second mechanism involved in geotaxis. Janse (1982) showed the presence of a buoyancy orientation mechanism involving the shell and its contained air-filled lung, probably mediated by proprioception in the lung or body wall. The buoyancy of the shell and its contained organs depends on the ‘filling condition’ of the lung. After long periods when the animal is prevented from ventilating the bouyancy is low (presumably less gas in the lung) and the buoyancy mechanism makes the snail negatively geotactic. With high buoyancy immediately after ventilation the animal becomes positively geotactic. Under normal conditions buoyancy orientation and statocyst orientation will steer the animal in the same direction as the two systems act cooperatively.
Snails can see quite well
It is well-known that Lymnaea are positively phototactic. Because this orientation towards light depends on the presence of both eyes, located at the base of the tentacles, this is likely to be due to the mechanism of tropotaxis. There are also non-ocular photoreceptors located in the skin and these mediate ‘shadow’ responses when the movement of a shadow across the snail causes a whole-body withdrawal response into the shell. This occurs in the absence of the eyes and electrical responses to general ‘light off’ stimuli can be recorded in the CNS as long as the nerves to the skin are intact (Ferguson & Benjamin, 1991b). It has recently been shown that the Lymnaea eye is capable of forming an image on the retina underwater (Fig. 3C) (Gál et al., 2004) and visual approach and reaching behaviour can be evoked by a black and white check pattern. Visual discrimination experiments by Andrew & Savage (2000) have shown that successful reward conditioning was achieved with the check pattern that could be discriminated from a grey pattern of equal luminance. We can speculate that Lymnaea may use this visual capacity to approach food objects providing ‘patterned’ light and dark stimulation like floating pond-weed. Indeed, one of its two retinal ‘foveal’ regions, where the visual image is best, ‘points’ upwards presumably allowing the surface of the water to be screened visually.
The motor networks underlying whole-body withdrawal, feeding, respiration, heartbeat control and reproduction are well-known with less information available on locomotion.
Whole-body withdrawal reflex involves multiple ganglion integration
This is a fast defensive reflex response induced by photic (shadow or light off) and strong tactile stimuli to the skin. Both stimuli produce contractions in two well developed muscular systems, the columellar muscle (CM) and the dorsal longitudinal muscle (DLM) (Fig. 4A) that cover most of the internal surfaces of the body wall (Ferguson & Benjamin, 1991a). There are two components of the response, shortening of the head-foot and the pulling down of the shell to protect the body. Contraction of the CM pulls the shell down over the body and shortens the ventral part of the head-foot. During whole-body withdrawal the DLM contracts synchronously with the CM and shortens the dorsal part of the head-foot. Both muscles therefore act together to produce the whole behavioral response. One of the interesting features of the motor control network is that the CM and DLM motoneurons (4 and 19 in number, respectively) are widely-distributed in all nine ganglia of the CNS (apart from the buccals) and innervate separate and discrete parts of the muscles via local nerve projections. The question that arises is how the electrical activity in the these scattered neurons is synchronized to produce the coordinated contractions in the CM and DLM muscles? One mechanism is via the extensive network of electrotonic (electrical) synaptic connections that couple the motoneurons in the different ganglia together (Fig. 4A) (Ferguson & Benjamin, 1991a). Due to this type of synaptic mechanism, excitatory signals due to sensory stimulation transmit rapidly throughout the motoneuronal network. A second mechanism, probably more important, is that sensory excitatory synaptic responses to local skin stimulation can be recorded in all the motoneurons independent of their location in the CNS (Ferguson & Benjamin, 1991b). This conjoint response in the motoneuron network is probably mediated by interneurons (Fig. 4A) in the CNS that first diverge excitation from the source of stimulation and then converge on individual motoneurons. A candidate interneuron for this role is the pedal neuron named PeB11. This neuron excites many of the withdrawal response motoneurons and fires during the whole-body withdrawal response (Syed & Winlow, 1991).
Rhythmic ingestion of food involves a central pattern generator with complex modulatory controlFeeding in Lymnaea is a rhythmic motor behavior consisting of a repetitive sequence of movements called rasps. During each rasp, the mouth opens and a toothed radula (or tongue) is scraped forward over the food substrate (protraction phase). Food is then lifted into the mouth (retraction phase), which closes while the food is being swallowed (swallow phase) and the sequence is repeated (Benjamin & Elliott, 1989). Rhythmic movements of the feeding muscles are driven by a network of motoneurons (B1 to B10) that, in turn, are driven by synaptic inputs from a feeding central pattern generator (CPG) network of interneurons (Fig. 4C) (maps of cells in Brierley et al., 1997). Each phase of the feeding rhythm is generated by one of three main types of CPG interneurons, N1 (protraction), N2 (retraction), N3 (swallow), providing sequences of excitatory and inhibitory synaptic inputs to motoneurons active in different phases of the feeding rhythm. The N cells of the CPG produce the basic three-phase motor program. No single factor is involved, but rhythmic activity relies on a combinations of properties that include both synaptic connections and the electrical (intrinsic) properties of the neurons themselves. Work on isolated CPG neurons in cell culture (Straub et al., 2002) showed that only the N1 cells (plateauing) and the N3 cells (postinhibitory rebound) have important intrinsic properties within the CPG but extensive, mainly inhibitory, synaptic connections occur between the N cells (Fig. 4C), indicating that rhythmicity mainly depends on synaptic connectivity, a network property. Activity in the motoneurons and CPG neurons is modulated by identified higher order interneurons, such as the cerebral giant cells (CGCs), the cerebral ventral 1 cells (CV1s) and the slow oscillator (SO) (Fig. 4C). The CGCs (equivalent to the metacerebral giant cells in other gastropods) act as gating neurons in the feeding circuit. Increased CGC spiking activity during feeding facilitates feeding responses to food. The CV1 cells are members of a larger population of neurons called the cerebro-buccal interneurons (CBIs) that are command-like neurons involved in the activation of feeding. The SO is unusual in that it is a single neuron randomly occurring in either the left or right buccal ganglion. Its role is to control the frequency of the CPG-driven rhythm and maintain its regularity (Kemenes et al., 2001). Many of the neurons in the feeding system contribute to more than one function suggesting a distributed type of network organization(Benjamin, 2012). Multi-tasking would make 'economic sense' where only about 100 neurons are available to carry out a variety of quite complex tasks performed by millions of neurons in the vertebrate brain.
Respiration in hypoxic environments
As their name implies, Lymnaea stagnalis populations often live in stagnant water and when the environment becomes hypoxic the snails orientate at the surface and perform rhythmic opening and closing movements of their pulmonary opening, the pneumostome. This aerial respiration behavior is controlled by a respiratory CPG, the three main components of which are the right pedal dorsal 1 (RPeD1), input 3 (IP3), and visceral dorsal 4 (VD4) interneurons (Fig. 4D). These provide synaptic inputs to identified motoneurons (I and J, opener and K, closer) innervating pneumostome opener and closer muscles (Fig. 4D) (Syed & Winlow, 1991). The chemosensory stimulus (hypoxia) that triggers pneumostome opening first activates sensory cells in the pneumostome-osphradial area, which in turn provide excitatory afferent inputs to RPeD1. Through its synaptic connections with the other members of the CPG network, activation of RPeD1 initiates CPG activity which underlies the respiratory rhythm. There are further reciprocal inhibitory synaptic connections between the two other members of the network that help generate the respiratory rhythm.
The well-modulated snail heart
The Lymnaea heart is an excellent organ for studying chemical modulation. Whilst heartbeat is generated by a muscle pacemaker located in the heart (myogenic), it is controlled by several types of motoneurons that release a rich cocktail of chemicals into the heart. The peptide FMRFamide is a well-known cardioexcitatory molecule in molluscs but uniquely in Lymnaea it is known to be present in a pair of excitatory motoneurons, the Ehe cells (Buckett et al., 1990), that use the peptide to increase the rate of heartbeat (Fig. 4B) and its strength via second messenger-mediated effects on calcium channels. No classical transmitter appears to be involved. More recently it has been shown that the mRNA that encodes for FMRFamide also codes for several other peptides (Benjamin & Kemenes, 2013) that are also present in the Ehe heart motoneurons (shown by single cell MALDI-TOF mass spectrometry, Worster et al., 1998) and these are also released into the heart to influence heartbeat (Santama & Benjamin, 2000). One of these co-released peptides, ‘SEEPLY’, modulates the effects of FMRFamide by delaying the mobilising effects of FMRFamide on the inositol phosphate pathway (Willoughby et al., 1999a). The other two, the so-called isoleucine peptides (EFLRIamide and pQFYRIamide), act via a separate cyclic AMP-mediated pathway to produce additional excitatory modulation of the heartbeat (Willoughby et al., 1999b). There are also some further inhibitory effects of the isoleucine peptides not mediated by cyclic AMP making the control of heartbeat by the Ehe cells highly interactive. There are two further types of heart excitatory motoneurons, the She and Hhe cells that utilise serotonin (5-HT) as their transmitter (Fig. 4B). Finally there is a heart motoneuron type, the Khi, that inhibits heartbeat using acetylcholine as its mediating transmitter (Fig. 4B). How the action of these various motoneurons are controlled and linked to the behavioral requirements of the animal is largely unknown. However, there is a modulatory interneuronal network in the CNS (Fig. 4B) that appears to be involved (Buckett et al., 1990b). It provides synaptic inputs to the motoneurons. Of these, the so-called ‘input 3’ is the most significant . This provides bursts of excitation to the serotonergic Hhe cells which excites the heart in a cyclical manner. At the same time input 3 inhibits the cholinergic Khi cells which are the only inhibitory motoneuron cell type innervating the heart. These reciprocal effects of input 3 reduce Khi inhibitory effects in favour of the Hhe excitation thus increasing the frequency and strength of heartbeat. One interesting feature of the interneuronal system that controls Lymnaea heartbeat is that the same interneurons that control heartbeat also form the respiratory CPG (they have different target motoneurons for respiration compared with heartbeat control, compare Fig. 4D and Fig. 4B). This shows that the mechanisms that control visceral function are overlapping and multifunctional.
Male mating behavior and preputium eversion
The snail playing the male role climbs on the shell of the prospective female, moves over the shell in a counterclockwise direction (circling) until he reaches the area of the female gonophore. The preputium (muscular structure that surrounds the penis) is then partially everted through the male pore. This is followed by probing for the female pore by the preputium, insertion of this organ into this pore followed by penis eversion and intromission (de Boer et al., 1996). Each of the four stages prior to intromission is variable in duration but the intromission is more constant and lasts for about 36 minutes. The whole mating behavior may last for several hours. Most neural information on mating concerns the motor control of the preputium. Five groups of neurons on the right-hand side of the CNS are likely to be involved in the control of the preputium and other parts of the male system because they project along the penis nerve that exits from the right cerebral ganglion. The role of one of these five groups of neurons located in the ventral part of the right anterior lobe (rAL) of the cerebral ganglion has been investigated in detail. Recording form these rAL cells in vivo using fine wire electrodes shows that the cells are normally silent but increase their spiking during preputium eversion and throughout intromission (de Boer et al. 1997). Artifical electrical stimulation of the rAL neurons causes at least partial eversion of the preputium in all the animals tested. Eversion of the preputium involves relaxation of preputial retraction muscle bands and these muscles are innervated by nerve fibres from rAL neurons that contain the peptide APGWamide. Significantly, injection of the peptide into intact snails causes reliable eversion of the preputium. So one of the roles of the APGW peptide is to relax the preputial muscle bands to cause preputial eversion but as the rAL neurons that contain APGW also release other co-expressed peptides (e.g. conopressin) these presumably will be involved as well.
Egg-laying and shell turning
Like mating, egg-laying consists of a sequence of behavioral events beginning with a rest period when the animal ceases to locomote, then a turning phase characterized by counterclockwise shell movements and high frequency rasping to clean the substrate, followed by oviposition and a final phase called inspection when the snail moves along the length of the egg-mass brushing it with lips and tentacles. Resting and turning last for about an hour each, oviposition 10 minutes and inspection about 2 minutes. Egg-laying in Lymnaea is an example of a complex behavior that is triggered by the release of multiple neuropeptide transmitters from neuroendocrine centres within the CNS that act on other neural circuits controlling egg-laying behavior. Egg laying is triggered by synchronous electrical activity in several hundred caudodorsal cells (CDC) located in the cerebral ganglion of Lymnaea. The CDCs express three related caudodorsal cell hormone (CDCH) genes giving rise to at least nine different peptides. The best understood of these peptides is the caudodorsal cell hormone-1 (CDCH-1), the so-called ovulation hormone. This peptide is released into the blood and when injected into the animal evokes ovulation and egg-mass formation and triggers the last two phases of egg-laying behavior. How this hormone and the other peptides act on the neural circuits responsible for the various behaviors associated is beginning to be understood. Motoneurons in the pedal ganglia called the N cells fire on the right side to cause the counterclockwise movements of the shell seen during the turning phase of egg-laying behavior (Hermann et al., 1994). These neurons are excited at the onset and during the turning phase of the behavior but are inhibited during the earlier stationary phase when no turning occurs (Hermann et al., 1997). The excitatory phase can be evoked by application of beta3-CDCP peptide but not by other CDC peptides, ovulation hormone, alpha-CDCP or calfluxin. The ovulation hormone inhibits the N cells' electrical activity. This might be expected as no turning behavior occurs during oviposition. The effects of injecting these peptides into the whole animal correlates well with the effects of the N cells obtained in more isolated preparations. Injection of Beta3-CDCP and alpha-CDCP also increase the rate of rasping movements as well as increasing shell turning consistent with the normal increases in rasping seen during the turning phase of natural egg-laying behavior. These data on the peptidergic control of egg-laying behavior, although not complete, indicate that different individual CDC peptides control the complex behavioral sequence by acting on different targets inside and outside of the nervous system. In addition sensory feedback mechanisms from target structures in the female reproductive system also appear to be involved in the timing of the behavioral switches occurring at different phases of the egg-laying behavior (Hermann et al., 1997).
- Taxonomy ID: 6523 in NCBI Taxonomy Browser
- The American Malacological Society
- The Malacological Society of London
- Man and Mollusc: An all-purpose resource site about molluscs
- The Lymnaea stagnalis Sequencing Consortium
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I thank Dr. Ildiko Kemenes for her help in producing the figures.