The neural basis of dominance hierarchy formation in crayfish

The processing of proprioceptive signals in distributed networks: insights from insect motor control

The integration of sensory information is required to maintain body posture and to generate robust yet flexible locomotion through unpredictable environments. To anticipate required adaptations in limb posture and enable compensation of sudden perturbations, an animal's nervous system assembles external (exteroception) and internal (proprioception) cues. Coherent neuronal representations of the proprioceptive context of the body and the appendages arise from the concerted action of multiple sense organs monitoring body kinetics and kinematics. This multimodal proprioceptive information, together with exteroceptive signals and brain-derived descending motor commands, converges onto premotor networks - i.e. the local neuronal circuitry controlling motor output and movements - within the ventral nerve cord (VNC), the insect equivalent of the vertebrate spinal cord. This Review summarizes existing knowledge and recent advances in understanding how local premotor networks in the VNC use convergent information to generate contextually appropriate activity, focusing on the example of posture control. We compare the role and advantages of distributed sensory processing over dedicated neuronal pathways, and the challenges of multimodal integration in distributed networks. We discuss how the gain of distributed networks may be tuned to enable the behavioral repertoire of these systems, and argue that insect premotor networks might compensate for their limited neuronal population size by, in comparison to vertebrate networks, relying more heavily on the specificity of their connections. At a time in which connectomics and physiological recording techniques enable anatomical and functional circuit dissection at an unprecedented resolution, insect motor systems offer unique opportunities to identify the mechanisms underlying multimodal integration for flexible motor control.

Combining Old and New Tricks: The Study of Genes, Neurons, and Behavior in Crayfish

For over a century the nervous system of decapod crustaceans has been a workhorse for the neurobiology community. Many fundamental discoveries including the identification of electrical and inhibitory synapses, lateral and pre-synaptic inhibition, and the Na⁺/K⁺-pump were made using lobsters, crabs, or crayfish. Key among many advantages of crustaceans for neurobiological research is the unique access to large, accessible, and identifiable neurons, and the many distinct and complex behaviors that can be observed in lab settings. Despite these advantages, recent decades have seen work on crustaceans hindered by the lack of molecular and genetic tools required for unveiling the cellular processes contributing to neurophysiology and behavior. In this perspective paper, we argue that the recently sequenced marbled crayfish, Procambarus virginalis, is suited to become a genetic model system for crustacean neuroscience. P. virginalis are parthenogenetic and produce genetically identical offspring, suggesting that germline transformation creates transgenic animal strains that are easy to maintain across generations. Like other decapod crustaceans, marbled crayfish possess large neurons in well-studied circuits such as the giant tail flip neurons and central pattern generating neurons in the stomatogastric ganglion. We provide initial data demonstrating that marbled crayfish neurons are accessible through standard physiological and molecular techniques, including single-cell electrophysiology, gene expression measurements, and RNA-interference. We discuss progress in CRISPR-mediated manipulations of the germline to knock-out target genes using the ‘Receptor-mediated ovary transduction of cargo’ (ReMOT) method. Finally, we consider the impact these approaches will have for neurophysiology research in decapod crustaceans and more broadly across invertebrates.

From Biological Synapses to “Intelligent” Robots

This selective review explores biologically inspired learning as a model for intelligent robot control and sensing technology on the basis of specific examples. Hebbian synaptic learning is discussed as a functionally relevant model for machine learning and intelligence, as explained on the basis of examples from the highly plastic biological neural networks of invertebrates and vertebrates. Its potential for adaptive learning and control without supervision, the generation of functional complexity, and control architectures based on self-organization is brought forward. Learning without prior knowledge based on excitatory and inhibitory neural mechanisms accounts for the process through which survival-relevant or task-relevant representations are either reinforced or suppressed. The basic mechanisms of unsupervised biological learning drive synaptic plasticity and adaptation for behavioral success in living brains with different levels of complexity. The insights collected here point toward the Hebbian model as a choice solution for “intelligent” robotics and sensor systems.

Stress, Subordination, and Anomalies of Feeding Across the Tree of Life: Implications for Interpreting Human Eating Disorders

Eating behaviors of animals living in naturalistic environments offer unique insights into several dysregulated eating patterns observed in humans. Social subordination is a known precipitant of hyperphagia and hypophagia in human beings, and examples of similar responses have been identified in a phylogenetically widespread range of vertebral species. This points to potentially conserved, patterned responses to animals navigating lives within social hierarchies. Self-imposed food restriction in subordinate fish and hyperphagic responses in socially subordinated bird and primate individuals may represent evolved adaptations to the stress of social subordination. As such, hyperphagic and hypophagic responses to social subordination in these species may model the natural history, neurobiology, and behavioral ecology of human dieting and bingeing more accurately than some current animal models. Phylogenetically widespread similarities in eating patterns under the stress of social subordination point to potentially shared biological benefits of these behaviors across species and the role of evolutionary trade-offs, adaptations, and other processes in shaping them. The application of a broadly comparative lens to disordered eating behaviors in other species exposes important similarities and differences between neurophysiology of eating across species. In doing so, it highlights the value of phylogenetic analyses and macroevolution as tools for identifying novel, naturally occurring models for understanding disordered human eating. Moreover, this approach introduces the intriguing possibility that human cultural influences on disordered eating may have far more ancient origins than previously considered.

No detectable changes in crayfish behavior due to sublethal dietary mercury exposure

Mercury, and particularly its organic form, methylmercury (MeHg), is a ubiquitous environmental contaminant with documented dosage-dependent adverse effects on endpoints spanning many levels of biological organization. However, relatively little is known about the sublethal impacts of environmentally-relevant exposures on behavioral characteristics that may impact predator-prey relationships, and thus the potential for Hg bioaccumulation within food webs due to behavioral impairments. This study investigated the potential for dietary mercury exposure to impair two behavioral outcomes in the highly invasive rusty crayfish, Faxonius rusticus, which are expected to influence interactions with their fish predators: the tail-flip escape response and chelae pinch strength. Field-caught animals were randomly assigned to four dietary treatments with mean (±1 SE) dry-weight total Hg (THg) concentrations of 3.52 ± 0.57, 114.01 ± 4.05, 274.10 ± 0.38, and 565.79 ± 1.33 ng/g in the control, low, moderate and high exposure treatments, respectively, for 16 weeks. After initial observations, exposures began and mass and behavior were measured every two weeks. At the end of the experiment, THg concentrations in tail muscle tissue were significantly higher in the high exposure treatment than in the control and low exposure treatments (Tukey's HSD, family-wise α = 0.05). Exposure-dependent declines in survival, mass, pinch strength, or tail-flip escape response velocity were not detected within the 12- to 16-week experimental exposure period, which represents one season of the crayfish's 3-4 year lifespan. This suggests that crayfish may be relatively tolerant of dietary exposure to sublethal concentrations of mercury within a single season. Further investigation of the physiological underpinnings of this tolerance is warranted.

Characterization of locomotor response to psychostimulants in the parthenogenetic marbled crayfish (Procambarus fallax forma virginalis): A promising model for studying the neural and molecular mechanisms of drug addiction

Although scientific research using mammalian models has made great strides in uncovering the enigmatic neural and molecular mechanisms orchestrating the state of drug addiction, a complete understanding has thus far eluded researchers. The complexity of the task has led to the use of invertebrate model systems to complement the research of drug-induced reward in mammalian systems. Invertebrates, such as crayfish, offer excellent model systems to help reveal the underlying mechanisms of drug addiction as they retain the ancestral neural reward circuit that is evolutionarily conserved across taxa, and they possess relatively few, large neurons, laid out in an accessible, modularly organized nervous system. Crayfish offer the benefits of delineated developmental life stages, a large body size suitable for a variety of experimental methods, and stereotyped behaviors. Unique among crayfish is the parthenogenetic marbled crayfish (Procambarus fallax forma virginalis), a species of asexually reproducing, genetically identical clones. With the benefits of reduced individual variation, high fecundity, and easy lab husbandry, the marbled crayfish would make a particularly powerful addition to the animal model repertoire. Here we characterize the locomotor response of juvenile P. f. f. virginalis exposed to the psychostimulant, d-amphetamine sulfate. Custom video-tracking software was used to record the movement patterns of juveniles exposed to water infused with varying concentrations of d-amphetamine sulfate. ANOVA demonstrated that crayfish locomotion was significantly impacted by drug concentration. These psychostimulant effects provide the foundation of P. f. f. virginalis as a model for parsing the neural and molecular mechanisms of drug addiction.

The Sensitivity of the Crayfish Reward System to Mammalian Drugs of Abuse

The idea that addiction occurs when the brain is not able to differentiate whether specific reward circuits were triggered by adaptive natural rewards or falsely activated by addictive drugs exist in several models of drug addiction. The suitability of crayfish (Orconectes rusticus) for drug addiction research arises from developmental variation of growth, life span, reproduction, behavior and some quantitative traits, especially among isogenic mates reared in the same environment. This broad spectrum of traits makes it easier to analyze the effect of mammalian drugs of abuse in shaping behavioral phenotype. Moreover, the broad behavioral repertoire allows the investigation of self-reinforcing circuitries involving appetitive and exploratory motor behavior, while the step-wise alteration of the phenotype by metamorphosis allows accurate longitudinal analysis of different behavioral states. This paper reviews a series of recent experimental findings that evidence the suitability of crayfish as an invertebrate model system for the study of drug addiction. Results from these studies reveal that unconditioned exposure to mammalian drugs of abuse produces a variety of stereotyped behaviors. Moreover, if presented in the context of novelty, drugs directly stimulate exploration and appetitive motor patterns along with molecular processes for drug conditioned reward. Findings from these studies indicate the existence of drug sensitive circuitry in crayfish that facilitates exploratory behavior and appetitive motor patterns via increased incentive salience of environmental stimuli or by increasing exploratory motor patterns. This work demonstrates the potential of crayfish as a model system for research into the neural mechanisms of addiction, by contributing an evolutionary, comparative context to our understanding of natural reward as an important life-sustaining process.

Functional organization and adaptability of a decision-making network in aplysia

Whereas major insights into the neuronal basis of adaptive behavior have been gained from the study of automatic behaviors, including reflexive and rhythmic motor acts, the neural substrates for goal-directed behaviors in which decision-making about action selection and initiation are crucial, remain poorly understood. However, the mollusk Aplysia is proving to be increasingly relevant to redressing this issue. The functional properties of the central circuits that govern this animal’s goal-directed feeding behavior and particularly the neural processes underlying the selection and initiation of specific feeding actions are becoming understood. In addition to relying on the intrinsic operation of central networks, goal-directed behaviors depend on external sensory inputs that through associative learning are able to shape decision-making strategies. Here, we will review recent findings on the functional design of the central network that generates Aplysia’s feeding-related movements and the sensory-derived plasticity that through learning can modify the selection and initiation of appropriate action. The animal’s feeding behavior and the implications of decision-making will be briefly described. The functional design of the underlying buccal network will then be used to illustrate how cellular diversity and the coordination of neuronal burst activity provide substrates for decision-making. The contribution of specific synaptic and neuronal membrane properties within the buccal circuit will also be discussed in terms of their role in motor pattern selection and initiation. The ability of learning to “rigidify” these synaptic and cellular properties so as to regularize network operation and lead to the expression of stereotyped rhythmic behavior will then be described. Finally, these aspects will be drawn into a conceptual framework of how Aplysia’s goal-directed circuitry compares to the central pattern generating networks for invertebrate rhythmic behaviors.

Neural circuit reconfiguration by social status

The social rank of an animal is distinguished by its behavior relative to others in its community. Although social-status-dependent differences in behavior must arise because of differences in neural function, status-dependent differences in the underlying neural circuitry have only begun to be described. We report that dominant and subordinate crayfish differ in their behavioral orienting response to an unexpected unilateral touch, and that these differences correlate with functional differences in local neural circuits that mediate the responses. The behavioral differences correlate with simultaneously recorded differences in leg depressor muscle EMGs and with differences in the responses of depressor motor neurons recorded in reduced, in vitro preparations from the same animals. The responses of local serotonergic interneurons to unilateral stimuli displayed the same status-dependent differences as the depressor motor neurons. These results indicate that the circuits and their intrinsic serotonergic modulatory components are configured differently according to social status, and that these differences do not depend on a continuous descending signal from higher centers.

Dominance hierarchy-dependent behavioural plasticity of crayfish avoidance reactions

Crayfish showed avoidance reactions when mechanical stimulation was applied to their tailfan. The response pattern of the avoidance reaction was dependent on crayfish size. Small crayfish showed an escape-like dart response while larger crayfish displayed a defensive-like turn response. We show that the response pattern to the same sensory stimulus changed with social status in the small crayfish. Crayfish are territorial animals and a dominant–subordinate relationship was determined when two previously unacquainted animals were paired. This winner–loser hierarchy was observed in pairs of small crayfish. Before fighting, all crayfish showed a dart response following mechanical stimulation of the tailfan. However, the probability of occurrence of a turn response increased significantly in the crayfish that won a fight. This behavioural transition from dart to turn response occurred immediately and was maintained for at least 3 days even if a dominant crayfish was isolated again with no opponent.

Serotonergic modulation of startle-escape plasticity in an African cichlid fish: a single-cell molecular and physiological analysis of a vital neural circuit

Social life affects brain function at all levels, including gene expression, neurochemical balance, and neural circuits. We have previously shown that in the cichlid fish Astatotilapia burtoni brightly colored, socially dominant (DOM) males face a trade-off between reproductive opportunities and increased predation risk. Compared with camouflaged subordinate (SUB) males, DOMs exposed to a loud sound pip display higher startle responsiveness and increased excitability of the Mauthner cell (M-cell) circuit that governs this behavior. Using behavioral tests, intracellular recordings, and single-cell molecular analysis, we show here that serotonin (5-HT) modulates this socially regulated plasticity via the 5-HT receptor subtype 2 (5-HTR2). Specifically, SUBs display increased sensitivity to pharmacological manipulation of 5-HTR2 compared with DOMs in both startle-escape behavior and electrophysiological properties of the M-cell. Immunohistochemistry showed serotonergic varicosities around the M-cells, further suggesting that 5-HT impinges directly onto the startle-escape circuitry. To determine whether the effects of 5-HTR2 are pre- or postsynaptic, and whether other 5-HTR subtypes are involved, we harvested the mRNA from single M-cells via cytoplasmic aspiration and found that 5-HTR subtypes 5A and 6 are expressed in the M-cell. 5-HTR2, however, was absent, suggesting that it affects M-cell excitability through a presynaptic mechanism. These results are consistent with a role for 5-HT in modulating startle plasticity and increase our understanding of the neural and molecular basis of a trade-off between reproduction and predation.

Drug-sensitive reward in crayfish: an invertebrate model system for the study of SEEKING, reward, addiction, and withdrawal

In mammals, rewarding properties of drugs depend on their capacity to activate appetitive motivational states. With the underlying mechanisms strongly conserved in evolution, invertebrates have recently emerged as a powerful new model in addiction research. In crayfish natural reward has proven surprisingly sensitive to human drugs of abuse, opening an unlikely avenue of research into the basic biological mechanisms of drug addiction. In a series of studies we first examined the presence of natural reward systems in crayfish, then characterized its sensitivity to a wide range of human drugs of abuse. A conditioned place preference (CPP) paradigm was used to demonstrate that crayfish seek out those environments that had previously been paired with the psychostimulants cocaine and amphetamine, and the opioid morphine. The administration of amphetamine exerted its effects at a number of sites, including the stimulation of circuits for active exploratory behaviors (i.e., SEEKING). A further study examined morphine-induced reward, extinction and reinstatement in crayfish. Repeated intra-circulatory infusions of morphine served as a reward when paired with distinct visual or tactile cues. Morphine-induced CPP was extinguished after repeated saline injections. Following this extinction phase, morphine-experienced crayfish were once again challenged with the drug. The priming injections of morphine reinstated CPP at all tested doses, suggesting that morphine-induced CPP is unrelenting. In an exploration of drug-associated behavioral sensitization in crayfish we concurrently mapped measures of locomotion and rewarding properties of morphine. Single and repeated intra-circulatory infusions of morphine resulted in persistent locomotory sensitization, even 5 days following the infusion. Moreover, a single dose of morphine was sufficient to induce long-term behavioral sensitization. CPP for morphine and context-dependent cues could not be disrupted over a drug free period of 5 days. This work demonstrates that crayfish offer a comparative and complementary approach in addiction research. Serving as an invertebrate animal model for the exposure to mammalian drugs of abuse, modularly organized and experimentally accessible nervous systems render crayfish uniquely suited for studying (1) the basic biological mechanisms of drug effects, (2) to explore how the appetitive/seeking disposition is implemented in a simple neural system, and (3) how such a disposition is related to the rewarding action of drugs of abuse. This work aimed to contribute an evolutionary, comparative context to our understanding of a key component in learning, and of natural reward as an important life-sustaining process.

Social and Ecological Regulation of a Decision-Making Circuit

Ecological context, sensory inputs, and the internal physiological state are all factors that need to be integrated for an animal to make appropriate behavioral decisions. However, these factors have rarely been studied in the same system. In the African cichlid fish Astatotilapia burtoni, males alternate between two phenotypes based on position in a social hierarchy. When dominant (DOM), fish display bright body coloration and a wealth of aggressive and reproductive behavioral patterns that make them conspicuous to predators. Subordinate (SUB) males, on the other hand, decrease predation risk by adopting cryptic coloration and schooling behavior. We therefore hypothesized that DOMs would show enhanced startle-escape responsiveness to compensate for their increased predation risk. Indeed, behavioral responses to sound clicks of various intensities showed a significantly higher mean startle rate in DOMs compared with SUBs. Electrophysiological recordings from the Mauthner cells (M-cells), the neurons triggering startle, were performed in anesthetized animals and showed larger synaptic responses to sound clicks in DOMs, consistent with the behavioral results. In addition, the inhibitory drive mediated by interneurons (passive hyperpolarizing potential [PHP] cells) presynaptic to the M-cell was significantly reduced in DOMs. Taken together, the results suggest that the likelihood for an escape to occur for a given auditory stimulus is higher in DOMs because of a more excitable M-cell. More broadly, this study provides an integrative explanation of an ecological and social trade-off at the level of an identifiable decision-making neural circuit.

Cloning and distribution of a putative octopamine/tyramine receptor in the central nervous system of the freshwater prawn Macrobrachium rosenbergii

There is ample evidence linking octopamine (OA) and tyramine (TA) to several neurophysiological functions in arthropods. In our laboratory we use the freshwater prawn Macrobrachium rosenbergii to study the neural basis of aggressive behavior. As a first step towards understanding the possible role of these amines and their receptors in the modulation of interactive behaviors, we have cloned a putative octopamine/tyramine receptor. The predicted sequence of the cloned OA/TA(Mac) receptor consists of 1,579 base pairs (bp), with an open reading frame of 1,350bp that encodes a 450 amino acid protein. This putative protein displays sequence identities of 70% to an Aedes aegypti mosquito TA receptor, followed by 60% to a Stegomyia aegypti mosquito OA receptor, 59% and 58% to the migratory locust TA-1 and -2 receptors respectively, and 57% with the silkworm OA receptor. We also mapped the OA/TA(Mac) receptor distribution by in-situ hybridization to the receptor's mRNA, and by immunohistochemistry to its protein. We observed stained cell bodies for the receptor's mRNA, mainly in the midline region of the thoracic and in the abdominal ganglia, as well as diffuse staining in the brain ganglia. For the receptor's protein, we observed extensive punctate staining within the neuropil and on the membrane of specific groups of neurons in all ganglia throughout the CNS, including the brain, the midline region and neuropiles of the thoracic ganglia, and ventral part and neuropiles of the abdominal ganglia. The same pattern of stained cells was observed on the thoracic and abdominal ganglia in both in-situ hybridization and immunohistochemistry experiments. Diffuse staining observed with in-situ hybridization also coincides with punctate staining observed in brain, SEG, thoracic, and abdominal ganglia in immunohistochemical preparations. This work provides the first step towards characterizing the neural networks that mediate octopaminergic signaling in prawn.

Social interactions determine postural network sensitivity to 5-HT

The excitability of the leg postural circuit and its response to serotonin (5-HT) were studied in vitro in thoracic nervous system preparations of dominant and subordinate male crayfishes. We demonstrate that the level of spontaneous tonic activity of depressor and levator motoneurons (MNs) (which control downward and upward movements of the leg, respectively) and the amplitude of their resistance reflex are larger in dominants than in subordinates. Moreover, we show that serotonergic neuromodulation of the postural circuit also depends on social status. Depressor and levator MN tonic firing rates and resistance reflex amplitudes were significantly modified in the presence of 10 mum 5-HT in dominants but not in subordinates. Using intracellular recording from depressor MNs, we show that their input resistance was not significantly different in dominants and subordinates in control conditions. However, 5-HT produced a marked depolarization in dominants and a significantly weaker depolarization in subordinates. Moreover, in the presence of 5-HT, the amplitude of the resistance reflex and the input resistance of MNs increased in dominants and decreased in subordinates. The peak amplitude and the decay phase of unitary EPSPs triggered by sensory spikes were significantly increased by 5-HT in dominants but not in subordinates. These observations suggest that neural networks are more reactive in dominants than in subordinates, and this divergence is even reinforced by 5-HT modulation.

Circadian Regulation of Agonistic Behavior in Groups of Parthenogenetic Marbled Crayfish, Procambarus sp

Crustaceans have frequently been used to study the neuroethology of both agonistic behavior and circadian rhythms, but whether their highly stereotyped and quantifiable agonistic activity is controlled by circadian pacemakers has, so far, not been investigated. Isolated marbled crayfish ( Procambarus spec.) displayed rhythmic locomotor activity under 12-h light:12-h darkness (LD12:12) and rhythmicity persisted after switching to constant darkness (DD) for 8 days, suggesting the presence of endogenous circadian pacemakers. Isogenetic females of parthenogenetic marbled crayfish displayed all behavioral elements known from agonistic interactions of previously studied decapod species including the formation of hierarchies. Groups of marbled crafish displayed high frequencies of agonistic encounters during the 1st hour of their cohabitation, but with the formation of hierarchies agonistic activities were subsequently reduced to low levels. Group agonistic activity was entrained to periods of exactly 24 h under LD12:12, and peaks of agonistic activity coincided with light-to-dark and dark-to-light transitions. After switching to DD, enhanced agonistic activity was dispersed over periods of 8-to 10-h duration that were centered around the times corresponding with light-to-dark transitions during the preceding 3 days in LD12:12. During 4 days under DD agonistic activity remained rhythmic with an average circadian period of 24.83 ± 1.22 h in all crayfish groups tested. Only the most dominant crayfish that participated in more than half of all agonistic encounters within the group revealed clear endogenous rhythmicity in their agonistic behavior, whereas subordinate individuals, depending on their social rank, initiated only between 19.4% and 0.03% of all encounters in constant darkness and displayed no statistically significant rhythmicity. The results indicate that both locomotion and agonistic social interactions are rhythmic behaviors of marbled crayfish that are controlled by light-entrained endogenous pacemakers.

Duration of socialization influences responses to a mirror: responses of dominant and subordinate crayfish diverge with time of pairing

Reflective surfaces have been shown previously to modify behaviour in socialized crayfish. Socializing crayfish by pairing them for two weeks established a hierarchy with one dominant and one subordinate crayfish per pair. Dominant crayfish exhibited specific behaviours, such as cornering, turning and crossing, more frequently in a reflective environment than in a non-reflective environment. After 2 weeks of pairing, subordinate crayfish did not respond in this manner but, instead, performed more reverse walking in a reflective environment. The present study investigated how the length of social pairing affects the response to mirrors. Crayfish from a communal tank were paired for 30 min or for 3 days, and their activity was videotaped for 20 min in a test aquarium lined with mirrors on one half and a non-reflective matte lining on the other half. Crayfish housed in the communal tank were used as a comparison group. After 30 min of pairing, dominant and subordinate crayfish responded similarly to the reflection, showing essentially the same pattern for seven of nine behaviours examined. After 3 days of pairing, dominant crayfish continued to respond to the reflection in essentially the same way, but subordinate crayfish behaved differently, showing differences in seven out of nine behaviours. Thus, the pattern of responses of dominant and subordinate crayfish to reflection diverged with time of pairing.

Social domination increases neuronal survival in the brain of juvenile crayfish Procambarus clarkii

Olfactory cues are among the sensory inputs that crayfish use in establishing dominance hierarchies. Throughout their lives, new neurons are continuously added into brain cell clusters 9 and 10, which contain somata of olfactory local and projection interneurons, respectively. Using markers for DNA synthesis (bromodeoxyuridine) and mitosis (phospho-histone-3), we tested juvenile crayfish (Procambarus clarkii) to examine effects of pairwise social experience on proliferation and survival of cells in these brain regions. Proliferating and mitotic cells appeared within restricted neurogenic areas in both clusters and in ;tails' extending from them. These tails, embedded in tubulin-positive strands, are linked by a patch of cells. Neither cell proliferation nor mitotic activity was affected by social dominance. Cell survival of neuronal precursors was affected by dominance: compared to dominants, subordinates had fewer newborn cells surviving in cluster 9 after 14 days of social experience. Social experience also affected body growth rate, but the effect of social experience on neurogenesis remained when differences in body growth rate were statistically controlled. We conclude that social domination enhances survival of new olfactory interneuronal precursors compared to social subordination but not compared to social isolation.

Responses of crayfish to a reflective environment depend on dominance status

Previous work has shown that the reflection from aquarium walls can modify several aspects of crayfish behaviour, but only if the crayfish are socialized. Such socialization is known to generate a dominance hierarchy composed of dominant and subordinate animals. The present study was conducted to determine whether or not behavioural responses to reflection depend on dominance status. Procambarus clarkii (Girard, 1852) were maintained in pairs for 2 weeks to establish dominance ranks and were subsequently observed in an aquarium in which half of the walls were lined with mirrors and the other half were lined with nonreflective plastic. Dominant crayfish spent more time on the reflective side of the aquarium than on the nonreflective side, and they spent more time in reflective corners than in nonreflective corners. Dominant crayfish also turned more frequently on the reflective side and crossed the tank more frequently toward reflective walls. Such differences in behaviour were not observed for subordinate crayfish or for crayfish that had been isolated for 2 weeks. Subordinate crayfish exhibited more reverse walking on the reflective side of the aquarium than on the nonreflective side, but crayfish in the other groups did not. Thus, responses of crayfish to reflection depend on dominance status.

Ethological analyses of crayfish behavior: a new invertebrate system for measuring the rewarding properties of psychostimulants

Recent investigations in invertebrate neurobiology have opened up a new line of research into the basic behavioral, neurochemical and genomic alterations that accompany psychostimulant drug exposure. However, the extent to which such findings relate to changes in motivational and learning processes, such as those that typify drug addictions, remains unclear. The present study addressed this issue in the crayfish, Orconectes rusticus. The first set of experiments demonstrated that intramuscular injections of cocaine and amphetamine have robust and distinguishable effects on crayfish behavior. In the second part of the study, the reinforcing properties of psychostimulants were tested in a series of conditioned place preference experiments. Amphetamine and, to a lesser extent, cocaine were both found to serve as rewards when their intra-circulatory infusion was coupled to a distinct visual environment. The monoaminergic regulation of behavior has been extensively studied in decapod crustaceans and the present experiments demonstrated that (mammalian) drugs of abuse, capable of interfering with monoamine chemistry, are similarly rewarding to crayfish. Behavioral studies in crayfish can provide a complementary approach to using other invertebrate species in addiction research.

Aggression in invertebrates

Invertebrates are outstanding model systems for the study of aggression. Recent advances and promising new research approaches are bringing investigators closer to the goal of integrating behavioral findings with those from other disciplines of the neurosciences. The presence of highly structured, easily evoked behavioral systems offer unique opportunities to quantify the aggressive state of individuals, to explore the mechanisms underlying the formation and maintenance of dominance relationships, to investigate the dynamic properties of hierarchy formation, and to explore the significance of neural, neurochemical and genetic mechanisms in these behavioral phenomena.