Electric signal synchronization as a behavioural strategy to generate social attention in small groups of mormyrid weakly electric fish and a mobile fish robot.

African weakly electric fish communicate at night by constantly emitting and perceiving brief electrical signals (electric organ discharges, EOD) at variable inter-discharge intervals (IDI). While the waveform of single EODs contains information about the sender's identity, the variable IDI patterns convey information about its current motivational and behavioural state. Pairs of fish can synchronize their EODs to each other via echo responses, and we have previously formulated a 'social attention hypothesis' stating that fish use echo responses to address specific individuals and establish brief dyadic communication frameworks within a group. Here, we employed a mobile fish robot to investigate the behaviour of small groups of up to four Mormyrus rume and characterized the social situations during which synchronizations occurred. An EOD-emitting robot reliably evoked social following behaviour, which was strongest in smaller groups and declined with increasing group size. We did not find significant differences in motor behaviour of M. rume with either an interactive playback (echo response) or a random control playback by the robot. Still, the robot reliably elicited mutual synchronizations with other fish. Synchronizations mostly occurred during relatively close social interactions, usually when the fish that initiated synchronization approached either the robot or another fish from a distance. The results support our social attention hypothesis and suggest that electric signal synchronization might facilitate the exchange of social information during a wide range of social behaviours from aggressive territorial displays to shoaling and even cooperative hunting in some mormyrids.

Central connections of the trigeminal motor command system in the weakly electric Elephantnose fish (Gnathonemus petersii).

The highly mobile chin appendage of Gnathonemus petersii, the Schnauzenorgan, is used to actively probe the environment and is known to be a fovea of the electrosensory system. It receives an important innervation from both the trigeminal sensory and motor systems. However, little is known about the premotor control pathways that coordinate the movements of the Schnauzenorgan, or about central pathways originating from the trigeminal motor nucleus. The present study focuses on the central connections of the trigeminal motor system to elucidate premotor centers controlling Schnauzenorgan movements, with particular interest in the possible connections between the electrosensory and trigeminal systems. Neurotracer injections into the trigeminal motor nucleus revealed bilateral, reciprocal connections between the two trigeminal motor nuclei and between the trigeminal sensory and motor nuclei by bilateral labeling of cells and terminals. Prominent afferent input to the trigeminal motor nucleus originates from the nucleus lateralis valvulae, the nucleus dorsalis mesencephali, the cerebellar corpus C1, the reticular formation, and the Raphe nuclei. Retrogradely labeled cells were also observed in the central pretectal nucleus, the dorsal anterior pretectal nucleus, the tectum, the ventroposterior nucleus of the torus semicircularis, the gustatory sensory and motor nuclei, and in the hypothalamus. Labeled terminals, but not cell bodies, were observed in the nucleus lateralis valvulae and the reticular formation. No direct connections were found between the electrosensory system and the V motor nucleus but the central connections identified would provide several multisynaptic pathways linking these two systems, including possible efference copy and corollary discharge mechanisms.

A bio-inspired electric camera for short-range object inspection in murky waters.

Underwater object inspection by optical sensors is usually unreliable in turbid or dark environments. Here, we designed a biomimetic 'electric camera', inspired by weakly electric fish Gnathonemus petersii, which successfully use active electrolocation for this task. The device probed nearby objects with a weak electric field and captured 'electric images' of the targets by processing the object-evoked field modulations. The camera-based electric images strongly resembled those available to G. petersii. Furthermore, by extracting the fish's analytical cues from these images, close objects could be reliably analysed. Based on the level of 'image blurring' short distances of electrolocation targets, spheres of different sizes and material were estimated. Natural targets, fish or plants, were identified irrespective of their size or distance by their two individual 'electric colours' derived from electric images. Furthermore, we introduce an image cue, called the 'electric outline', which provided information resembling a target's optical contour. Our results indicate that bio-inspired electric imaging principles provide promising cues for sensor-based, short-range object inspections in murky waters. By resembling the electric imaging applied by G. petersii our device can also be used for 'reverse biomimetics', revealing imaging cues that so far have not been considered for weakly electric fish.

Electric-Color Sensing in Weakly Electric Fish Suggests Color Perception as a Sensory Concept beyond Vision.

Many sighted animals use color as a salient and reliable cue [1] to identify conspecifics [2-4], predators, or food [5-7]. Similarly, nocturnal, weakly electric fish Gnathonemus petersii might rely on "electric colors" [8] for unambiguous, critical object recognitions. These fish identify nearby targets by emitting electric signals and by sensing the object-evoked signal modulations in amplitude and waveform with two types of epidermal electroreceptors (active electrolocation) [9-12]. Electrical capacitive objects (animals, plants) modulate both parameters; resistive targets (e.g., rocks) modulate only the signal's amplitude [11, 12]. Ambiguities of electrosensory inputs arise when object size, distance, or position vary. While previous reports suggest electrosensory disambiguations when both modulations are combined as electric colors [8, 13, 14], this concept has never been demonstrated in a natural, behaviorally relevant context. Here, we assessed electric-color perception (1) by recording object-evoked signal modulations and (2) by testing the fishes' behavioral responses to these objects during foraging. We found that modulations caused by aquatic animals or plants provided electric colors when combined as a ratio. Individual electric colors designated crucial targets (electric fish, prey insect larvae, or others) irrespective of their size, distance, or position. In behavioral tests, electrolocating fish reliably identified prey insect larvae of varying sizes from different distances and did not differentiate between artificial prey items generating similar electric colors. Our results indicate a color-like perceptual cue during active electrolocation, the computation [15], reliability, and use of which resemble those of color in vision. This suggests "color" perception as a sensory concept beyond vision and passive sensing.

The Mormyrid Optic Tectum Is a Topographic Interface for Active Electrolocation and Visual Sensing.

The African weakly electric fish Gnathonemus petersii is capable of cross-modal object recognition using its electric sense or vision. Thus, object features stored in the brain are accessible by multiple senses, either through connections between unisensory brain regions or because of multimodal representations in multisensory areas. Primary electrosensory information is processed in the medullary electrosensory lateral line lobe, which projects topographically to the lateral nucleus of the torus semicircularis (NL). Visual information reaches the optic tectum (TeO), which projects to various other brain regions. We investigated the neuroanatomical connections of these two major midbrain visual and electrosensory brain areas, focusing on the topographical relationship of interconnections between the two structures. Thus, the neural tracer DiI was injected systematically into different tectal quadrants, as well as into the NL. Tectal tracer injections revealed topographically organized retrograde and anterograde label in the NL. Rostral and caudal tectal regions were interconnected with rostral and caudal areas of the NL, respectively. However, dorsal and ventral tectal regions were represented in a roughly inverted fashion in NL, as dorsal tectal injections labeled ventral areas in NL and vice versa. In addition, tracer injections into TeO or NL revealed extensive inputs to both structures from ipsilateral (NL also contralateral) efferent basal cells in the valvula cerebelli; the NL furthermore projected back to the valvula. Additional tectal and NL connections were largely confirmatory to earlier studies. For example, the TeO received ipsilateral inputs from the central zone of the dorsal telencephalon, torus longitudinalis, nucleus isthmi, various tegmental, thalamic and pretectal nuclei, as well as other nuclei of the torus semicircularis. Also, the TeO projected to the dorsal preglomerular and dorsal posterior thalamic nuclei as well as to nuclei in the torus semicircularis and nucleus isthmi. Beyond the clear topographical relationship of NL and TeO interconnections established here, the known neurosensory upstream circuitry was used to suggest a model of how a defined spot in the peripheral sensory world comes to be represented in a common associated neural locus both in the NL and the TeO, thereby providing the neural substrate for cross-modal object recognition.

Evidence for mutual allocation of social attention through interactive signaling in a mormyrid weakly electric fish.

Mormyrid weakly electric fish produce electric organ discharges (EODs) for active electrolocation and electrocommunication. These pulses are emitted with variable interdischarge intervals (IDIs) resulting in temporal discharge patterns and interactive signaling episodes with nearby conspecifics. However, unequivocal assignment of interactive signaling to a specific behavioral context has proven to be challenging. Using an ethorobotical approach, we confronted single individuals of weakly electric Mormyrus rume proboscirostris with a mobile fish robot capable of interacting both physically, on arbitrary trajectories, as well as electrically, by generating echo responses through playback of species-specific EODs, thus synchronizing signals with the fish. Interactive signaling by the fish was more pronounced in response to a dynamic echo playback generated by the robot than in response to playback of static random IDI sequences. Such synchronizations were particularly strong at a distance corresponding to the outer limit of active electrolocation, and when fish oriented toward the fish replica. We therefore argue that interactive signaling through echoing of a conspecific's EODs provides a simple mechanism by which weakly electric fish can specifically address nearby individuals during electrocommunication. Echoing may thus enable mormyrids to mutually allocate social attention and constitute a foundation for complex social behavior and relatively advanced cognitive abilities in a basal vertebrate lineage.

Physiological evidence of sensory integration in the electrosensory lateral line lobe of Gnathonemus petersii.

Mormyrid fish rely on reafferent input for active electrolocation. Their electrosensory input consists of phase and amplitude information. These are encoded by differently tuned receptor cells within the Mormyromasts, A- and B-cells, respectively, which are distributed over the animal's body. These convey their information to two topographically ordered medullary zones in the electrosensory lateral line lobe (ELL). The so-called medial zone receives only amplitude information, while the dorsolateral zone receives amplitude and phase information. Using both sources of information, Mormyrid fish can disambiguate electrical impedances. Where and how this disambiguation takes place is presently unclear. We here investigate phase-sensitivity downstream from the electroreceptors. We provide first evidence of phase-sensitivity in the medial zone of ELL. In this zone I-cells consistently decreased their rate to positive phase-shifts (6 of 20 cells) and increased their rate to negative shifts (11/20), while E-cells of the medial zone (3/9) responded oppositely to I-cells. In the dorsolateral zone the responses of E- and I-cells were opposite to those found in the medial zone. Tracer injections revealed interzonal projections that interconnect the dorsolateral and medial zones in a somatotopic manner. In summary, we show that phase information is processed differently in the dorsolateral and the medial zones. This is the first evidence for a mechanism that enhances the contrast between two parallel sensory channels in Mormyrid fish. This could be beneficial for impedance discrimination that ultimately must rely on a subtractive merging of these two sensory streams.

Disembodying the invisible: electrocommunication and social interactions by passive reception of a moving playback signal.

Mormyrid weakly electric fish have a special electrosensory modality that allows them to actively sense their environment and to communicate with conspecifics by emitting sequences of electric signals. Electroreception is mediated by different types of dermal electroreceptor organs for active electrolocation and electrocommunication, respectively. During electrocommunication, mormyrids exhibit stereotyped discharge sequences and locomotor patterns, which can be induced by playback of electric signals. This raises the question: what sensory information is required to initiate and sustain social interactions, and which electrosensory pathway mediates such interactions? By experimentally excluding stimuli from vision and the lateral line system, we show that Mormyrus rume proboscirostris can rely exclusively on its electrosensory system to track a mobile source of electric communication signals. Detection of electric playback signals induced discharge cessations, followed by double-pulse patterns. The animals tried to interact with the moving signal source and synchronized their discharge activity to the playback. These behaviors were absent in control trials without playback. Silencing the electric organ in some fish did not impair their ability to track the signal source. Silenced fish followed on trajectories similar to those obtained from intact animals, indicating that active electrolocation is no precondition for close-range interactions based on electrocommunication. However, some silenced animals changed their strategy when searching for the stationary playback source, which indicates passive sensing. Social interactions among mormyrids can therefore be induced and mediated by passive reception of electric communication signals without the need for perception of the location of the signal source through other senses.

Social interactions between live and artificial weakly electric fish: Electrocommunication and locomotor behavior of Mormyrus rume proboscirostris towards a mobile dummy fish.

Mormyrid weakly electric fish produce short, pulse-type electric organ discharges for actively probing their environment and to communicate with conspecifics. Animals emit sequences of pulse-trains that vary in overall frequency and temporal patterning and can lead to time-locked interactions with the discharge activity of other individuals. Both active electrolocation and electrocommunication are additionally accompanied by stereotypical locomotor patterns. However, the concrete roles of electrical and locomotor patterns during social interactions in mormyrids are not well understood. Here we used a mobile fish dummy that was emitting different types of electrical playback sequences to study following behavior and interaction patterns (electrical and locomotor) between individuals of weakly electric fish. We confronted single individuals of Mormyrus rume proboscirostris with a mobile dummy fish designed to attract fish from a shelter and recruit them into an open area by emitting electrical playbacks of natural discharge sequences. We found that fish were reliably recruited by the mobile dummy if it emitted electrical signals and followed it largely independently of the presented playback patterns. While following the dummy, fish interacted with it spatially by displaying stereotypical motor patterns, as well as electrically, e.g. through discharge regularizations and by synchronizing their own discharge activity to the playback. However, the overall emission frequencies of the dummy were not adopted by the following fish. Instead, social signals based on different temporal patterns were emitted depending on the type of playback. In particular, double pulses were displayed in response to electrical signaling of the dummy and their expression was positively correlated with an animals' rank in the dominance hierarchy. Based on additional analysis of swimming trajectories and stereotypical locomotor behavior patterns, we conclude that the reception and emission of electrical communication signals play a crucial role in mediating social interactions in mormyrid weakly electric fish.

Estimation of distance and electric impedance of capacitive objects in the weakly electric fish Gnathonemus petersii.

During active electrolocation, the weakly electric fish Gnathonemus petersii judges the distance and impedance of nearby objects. Capacitive objects, which modulate local amplitude and waveform of the fish's electric probing signals, cast amplitude and waveform images onto the fish's electroreceptive skin. For an unambiguous estimation of the impedance and distance of an object, the animal has to deal with multiple dependencies of object and image parameters. Based on experimentally recorded amplitude and waveform images, we investigated possible strategies of the fish to unequivocally determine both the distance and the impedance of capacitive objects. We show that the relative slope in amplitude images, but not in waveform images, is independent of object impedance and is a measure of object distance. Distance-invariant impedance estimators were obtained by two different analytical strategies. The peak modulations of both image types were 'calibrated' with the relative slope of the amplitude image. Impedance estimators were obtained whenever these pairs of image features (peak and relative slope) were related dynamically over two consecutive distances. A static impedance estimator termed 'electric colour' is postulated to arise from the relationship of an amplitude and waveform image. Our results confirm that electric colour is indeed unaffected by object distance. For electric colour estimation we suggest a minimalistic approach of just relating the peak modulations of both image types to the basal amplitude and waveform condition. Our results are discussed with regard to the anatomical and physiological organization of the fish's electrosensory neuronal pathways and behavioural strategies of electrolocating fish.

Electrosensory capture during multisensory discrimination of nearby objects in the weakly electric fish Gnathonemus petersii.

Animal multisensory systems are able to cope with discrepancies in information provided by individual senses by integrating information using a weighted average of the sensory inputs. Such sensory weighting often leads to a dominance of a certain sense during particular tasks and conditions, also called sensory capture. Here we investigated the interaction of vision and active electrolocation during object discrimination in the weakly electric fish Gnathonemus petersii. Fish were trained to discriminate between two objects using both senses and were subsequently tested using either only vision or only the active electric sense. We found that at short range the electric sense dominates over vision, leading to a decreased ability to discriminate between objects visually when vision and electrolocation provide conflicting information. In line with visual capture in humans, we call this dominance of the electric sense electrosensory capture. Further, our results suggest that the fish are able to exploit the advantages of multiple senses using vision and electrolocation redundantly, synergistically and complementarily. Together our results show that by providing similar information about the environment on different spatial scales, vision and the electric sense of G. petersii are well attuned to each other producing a robust and flexible percept.

An active electrolocation catheter system for imaging and analysis of coronary plaques.

Coronary artery disease-currently one of the most frequent causes of death-is characterized by atherosclerotic plaques grown in the wall of blood vessels and inhibiting blood flow. Preventive assessment focusses on critical sizes of structural plaque parameters like relative lipid core area and cap thickness to identify high-risk plaques called thin cap fibroatheromas. Although state-of-the-art catheter systems were successfully applied in invasive plaque diagnostics, the high costs induced by these devices inhibit usage in daily clinical practice. To overcome this shortcoming, we follow a biomimetic approach to construct a prospective low-cost catheter system that adapts the active electrolocation principles of weakly electric fish Gnathonemus petersii. Only a few and simple parameters relevant for plaque detection and characterization are estimated from plaque-evoked electric images which are projected on the surface of the catheter. Two prototypical electrolocation catheter systems were tested. The first catheter system featured a ring electrode catheter and was used to obtain dynamic 1D electric images of synthetic plaques in an agarose atherosclerosis model. Our proof of concept showed that synthetic plaques could be reliably detected from 1D electric images. Based on a cluster analysis of selected key image features, synthetic plaques could be categorized into four plaque conditions, predefined from thresholds for critical structural parameters, representing high to low risk plaques. In the second recording approach, plaque-evoked dynamic and static spatial electric images were obtained by a multi-electrode catheter system. Based on these recordings, a synthetic plaque with a critical cap thickness could be detected and localized in a pig coronary artery.

Cannabinoid modulation of zebrafish fear learning and its functional analysis investigated by c-Fos expression.

It has been shown that zebrafish fear learning proceeds in the same way as reported for rodents. However, in zebrafish fear learning it is possible to substitute the use of electric shocks as unconditioned stimulus and utilize the inborn fear responses to the alarm substance Schreckstoff, instead. The skin extract Schreckstoff elicits typical fear reactions such as preferred bottom dwelling, swimming in a tighter shoal, erratic movements and freezing. This natural fear behavior can be transferred from Schreckstoff to any other sensory stimulus by associative conditioning (fear learning). We presented Schreckstoff simultaneously with a red light stimulus and tested the effectiveness of fear learning during memory retrieval. The two brain regions known to be relevant for learning in zebrafish are the medial and the lateral pallium of the dorsal telencephalon, both containing rich expressions of the endocannabinoid receptor CB1. To test the influence of the zebrafish endocannabinoid system on fear acquisition learning, an experimental group of ten fish was pretreated with the CB1 receptor agonist THC (Δ9-tetrahydrocannabinol; 100nM for 1h). We found that CB1 activation significantly inhibited acquisition of fear learning, possibly by impairing stimulus encoding processes in pallial areas. This was supported by analyzes of c-Fos expression in the brains of experimental animals. Schreckstoff exposure during fear acquisition learning and memory retrieval during red light presentation increased the number of labelled cells in pallial structures, but in no other brain region investigated (e.g. striatum, thalamus, and habenula). THC administration before fear conditioning significantly decreased c-Fos expression in these structures to a level similar to the control group without Schreckstoff experience, suggesting that Schreckstoff induced fear learning requires brain circuits restricted mainly to pallial regions of the dorsal telencephalon.

Object discrimination through active electrolocation: Shape recognition and the influence of electrical noise.

The weakly electric fish Gnathonemus petersii can recognise objects using active electrolocation. Here, we tested two aspects of object recognition; first whether shape recognition might be influenced by movement of the fish, and second whether object discrimination is affected by the presence of electrical noise from conspecifics. (i) Unlike other object features, such as size or volume, no parameter within a single electrical image has been found that encodes object shape. We investigated whether shape recognition might be facilitated by movement-induced modulations (MIM) of the set of electrical images that are created as a fish swims past an object. Fish were trained to discriminate between pairs of objects that either created similar or dissimilar levels of MIM of the electrical images. As predicted, the fish were able to discriminate between objects up to a longer distance if there was a large difference in MIM between the objects than if there was a small difference. This supports an involvement of MIMs in shape recognition but the use of other cues cannot be excluded. (ii) Electrical noise might impair object recognition if the noise signals overlap with the EODs of an electrolocating fish. To avoid jamming, we predicted that fish might employ pulsing strategies to prevent overlaps. To investigate the influence of electrical noise on discrimination performance, two fish were tested either in the presence of a conspecific or of playback signals and the electric signals were recorded during the experiments. The fish were surprisingly immune to jamming by conspecifics: While the discrimination performance of one fish dropped to chance level when more than 22% of its EODs overlapped with the noise signals, the performance of the other fish was not impaired even when all its EODs overlapped. Neither of the fish changed their pulsing behaviour, suggesting that they did not use any kind of jamming avoidance strategy.

Investigation of Collective Behaviour and Electrocommunication in the Weakly Electric Fish, Mormyrus rume, through a biomimetic Robotic Dummy Fish.

A robotic fish has been developed to create a mixed bio-hybrid system made up of weakly electric fish and a mobile dummy fish. Weakly electric fish are capable of interacting with each other via sequences of self-generated electric signals during electrocommunication. Here we present the design of an artificial dummy fish, which is subsequently tested in behavioural experiments. The robot consists of two parts: a flexible tail that can move at different frequencies and amplitudes, performing a carangiform oscillation, and a rigid head containing the motor for the tail oscillation. The dummy fish mimics the weakly electric fish Mormyrus rume in morphology, size and electric signal generation. In order to study electrical interactions, the dummy fish is equipped with ten electrodes that record electric signals of nearby real fish and generate electric dipole fields around itself that are similar to those produced by real fish in both waveform and sequence. Behavioural experiments demonstrate that the dummy fish is able to recruit both single individuals and groups of M. rume from a shelter into an exposed area. The development of an artificial dummy fish may help to understand fundamental aspects of collective behaviour in weakly electric fish and the properties necessary to initiate and sustain it in closed-loop feedback experiments based on electrocommunication.

Cross-modal object recognition and dynamic weighting of sensory inputs in a fish.

Most animals use multiple sensory modalities to obtain information about objects in their environment. There is a clear adaptive advantage to being able to recognize objects cross-modally and spontaneously (without prior training with the sense being tested) as this increases the flexibility of a multisensory system, allowing an animal to perceive its world more accurately and react to environmental changes more rapidly. So far, spontaneous cross-modal object recognition has only been shown in a few mammalian species, raising the question as to whether such a high-level function may be associated with complex mammalian brain structures, and therefore absent in animals lacking a cerebral cortex. Here we use an object-discrimination paradigm based on operant conditioning to show, for the first time to our knowledge, that a nonmammalian vertebrate, the weakly electric fish Gnathonemus petersii, is capable of performing spontaneous cross-modal object recognition and that the sensory inputs are weighted dynamically during this task. We found that fish trained to discriminate between two objects with either vision or the active electric sense, were subsequently able to accomplish the task using only the untrained sense. Furthermore we show that cross-modal object recognition is influenced by a dynamic weighting of the sensory inputs. The fish weight object-related sensory inputs according to their reliability, to minimize uncertainty and to enable an optimal integration of the senses. Our results show that spontaneous cross-modal object recognition and dynamic weighting of sensory inputs are present in a nonmammalian vertebrate.

Oxidation and Cognitive Impairment in the Aging Zebrafish.

BACKGROUND: The zebrafish has become an established model organism in aging research giving insight into general aging processes in vertebrates. Oxidative stress in aging may damage proteins and lipids in brain cells. Enhanced levels of oxidized macromolecules exert neurotoxic effects that could lead to disturbances in neuronal functioning and cognitive skills. OBJECTIVE: This study aims to investigate a possible relation between oxidative stress and learning deficits during aging in zebrafish. METHODS: We tested zebrafish of different ages in a color discrimination paradigm for associative learning and in a hole board task for spatial learning abilities. Afterwards, we determined the levels of oxidized lipids and proteins as well as the amount of lipofuscin in the learning-relevant brain regions of animals of the same age. RESULTS: While young zebrafish at the age of 1 year successfully completed both learning tasks, cognitive abilities were significantly impaired in older animals. Learning deficits occurred at the age of 2 years, where we also detected significantly increased levels of lipofuscin and oxidized proteins in the zebrafish medial and lateral pallial areas of the dorsal telencephalon. CONCLUSION: These data suggest that enhanced oxidative stress may contribute to behavioral and cognitive impairments in the aging zebrafish.

The endocannabinoid system and associative learning and memory in zebrafish.

In zebrafish the medial pallium of the dorsal telencephalon represents an amygdala homolog structure, which is crucially involved in emotional associative learning and memory. Similar to the mammalian amygdala, the medial pallium contains a high density of endocannabinoid receptor CB1. To elucidate the role of the zebrafish endocannabinoid system in associative learning, we tested the influence of acute and chronic administration of receptor agonists (THC, WIN55,212-2) and antagonists (Rimonabant, AM-281) on two different learning paradigms. In an appetitively motivated two-alternative choice paradigm, animals learned to associate a certain color with a food reward. In a second set-up, a fish shuttle-box, animals associated the onset of a light stimulus with the occurrence of a subsequent electric shock (avoidance conditioning). Once fish successfully had learned to solve these behavioral tasks, acute receptor activation or inactivation had no effect on memory retrieval, suggesting that established associative memories were stable and not alterable by the endocannabinoid system. In both learning tasks, chronic treatment with receptor antagonists improved acquisition learning, and additionally facilitated reversal learning during color discrimination. In contrast, chronic CB1 activation prevented aversively motivated acquisition learning, while different effects were found on appetitively motivated acquisition learning. While THC significantly improved behavioral performance, WIN55,212-2 significantly impaired color association. Our findings suggest that the zebrafish endocannabinoid system can modulate associative learning and memory. Stimulation of the CB1 receptor might play a more specific role in acquisition and storage of aversive learning and memory, while CB1 blocking induces general enhancement of cognitive functions.

More a finger than a nose: the trigeminal motor and sensory innervation of the Schnauzenorgan in the elephant-nose fish Gnathonemus petersii.

The weakly electric fish Gnathonemus petersii uses its electric sense to actively probe the environment. Its highly mobile chin appendage, the Schnauzenorgan, is rich in electroreceptors. Physical measurements have demonstrated the importance of the position of the Schnauzenorgan in funneling the fish's self-generated electric field. The present study focuses on the trigeminal motor pathway that controls Schnauzenorgan movement and on its trigeminal sensory innervation and central representation. The nerves entering the Schnauzenorgan are very large and contain both motor and sensory trigeminal components as well as an electrosensory pathway. With the use of neurotracer techniques, labeled Schnauzenorgan motoneurons were found throughout the ventral main body of the trigeminal motor nucleus but not among the population of larger motoneurons in its rostrodorsal region. The Schnauzenorgan receives no motor or sensory innervation from the facial nerve. There are many anastomoses between the peripheral electrosensory and trigeminal nerves, but these senses remain separate in the sensory ganglia and in their first central relays. Schnauzenorgan trigeminal primary afferent projections extend throughout the descending trigeminal sensory nuclei, and a few fibers enter the facial lobe. Although no labeled neurons could be identified in the brain as the trigeminal mesencephalic root, some Schnauzenorgan trigeminal afferents terminated in the trigeminal motor nucleus, suggesting a monosynaptic, possibly proprioceptive, pathway. In this first step toward understanding multimodal central representation of the Schnauzenorgan, no direct interconnections were found between the trigeminal sensory and electromotor command system, or the electrosensory and trigeminal motor command. The pathways linking perception to action remain to be studied.