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Lateral excitation is a mechanism for amplifying coordinated input to postsynaptic neurons that has been described recently in several species. Here, we describe how a postsynaptic neuron, the lateral giant (LG) escape command neuron, enhances lateral excitation among its presynaptic mechanosensory afferents in the crayfish tailfan. A lateral excitatory network exists among electrically coupled tailfan primary afferents, mediated through central electrical synapses. EPSPs elicited in LG dendrites as a result of mechanosensory stimulation spread antidromically back through electrical junctions to unstimulated afferents, summate with EPSPs elicited through direct afferent-to-afferent connections, and contribute to recruitment of these afferents. Antidromic potentials are larger if the afferent is closer to the initial input on LG dendrites, which could create a spatial filtering mechanism within the network. This pathway also broadens the temporal window over which lateral excitation can occur, because of the delay required for EPSPs to spread through the large LG dendrites. The delay allows subthreshold inputs to the LG to have a priming effect on the lateral excitatory network and lowers the threshold of the network in response to a second, short-latency stimulus. Retrograde communication within neuronal pathways has been described in a number of vertebrate and invertebrate species. A mechanism of antidromic passage of depolarizing current from a neuron to its presynaptic afferents, similar to that described here in an invertebrate, is also present in a vertebrate (fish). This raises the possibility that short-term retrograde modulation of presynaptic elements through electrical junctions may be common.

Serotonin is an important neurotransmitter that is involved in modulation of sensory, motor, and higher functions in many species. In the crayfish, which has been developed as a model for nervous system function for over a century, serotonin modulates several identified circuits. Although the cellular and circuit effects of serotonin have been extensively studied, little is known about the receptors that mediate these signals. Physiological data indicate that identified crustacean cells and circuits are modulated via several different serotonin receptors. We describe the detailed immunocytochemical localization of the crustacean type 1 serotonin receptor, 5-HT1crust, throughout the crayfish nerve cord and on abdominal superficial flexor muscles. 5-HT1crust is widely distributed in somata, including those of several identified neurons, and neuropil, suggesting both synaptic and neurohormonal roles. Individual animals show very different levels of 5-HT1crust immunoreactivity (5-HT1crustir) ranging from preparations with hundreds of labeled cells per ganglion to some containing only a handful of 5-HT1crustir cells in the entire nerve cord. The interanimal variability in 5-HT1crustir is great, but individual nerve cords show a consistent level of labeling between ganglia. Quantitative RT-PCR shows that 5-HT1crust mRNA levels between animals are also variable but do not directly correlate with 5-HT1crustir levels. Although there is no correlation of 5-HT1crust expression with gender, social status, molting or feeding, dominant animals show significantly greater variability than subordinates. Functional analysis of 5-HT1crust in combination with this immunocytochemical map will aid further understanding of this receptor's role in the actions of serotonin on identified circuits and cells. J. Comp. Neurol. 484:261–282, 2005. © 2005 Wiley-Liss, Inc.

The lateral giant (LG) escape circuit of crayfish mediates a coordinated escape triggered by strong attack to the abdomen. The LG circuit is one of the best understood of small systems, but models of the circuit have mostly been limited to simple ball-and-stick representations, which ignore anatomical details of contacts between circuit elements. Many of the these contacts are electrical; here we use differential dye coupling, a technique which could help reveal connection patterns in many neural circuits, to reveal in detail the circuit within the terminal abdominal ganglion. Sensory input from the tailfan forms a somatotopic map on the projecting LG dendrites, which together with interafferent coupling mediates a lateral excitatory network that selectively amplifies strong, phasic, converging input to LG. Mechanosensory interneurons contact LG at sites distinct from the primary afferents and so maximize their summated effect on LG. Motor neurons and premotor interneurons are excited near the initial segments of the LGs and innervate muscles for generating uropod flaring and telson flexion. Previous research has shown that spatial patterns of input are important for signal integration in LG; this map of electrical contact points will help us to understand synaptic processing in this system.

Immunocytochemical mapping of serotonergic and octopaminergic neurons in the central nervous system of the squat lobster Munida quadrispina reveal approximately 120 serotoninimmunoreactive cell bodies (distributed throughout the neuromeres except in abdominal ganglion 5) and 48 octopamine-immunoreactive cell bodies (in brain and thoracic neuromeres but none in the circumesophageal or abdominal ganglia). Immunopositive neuropils for both amines are distributed in multiple areas in each neuromere and overlap extensively. Serotonergic and octopaminergic neurons have extensive bilateral projections in abdominal ganglia, whereas the majority of projections in thoracic and subesophageal ganglia are unilateral (contralateral to soma). This difference correlates with typical differences between abdominal and thoracic motor system coordination. Processes of immunoreactive cells for both amines form extensive, peripheral, neurosecretory-like structures. Serotonin seems to be released peripherally in more segments, and from more nerves per segment, than octopamine. M. quadrispina has fewer serotonergic and octopaminergic immunoreactive cells, in particular, fewer segmentally repeated cells, than other species studied to date. Nevertheless, the general organization of the aminergic systems is similar, and several aminergic cells have locations and morphologies that strongly suggest homology with identified aminergic cells in other crustaceans. Among these are segmentally repeated neurons that, in M. quadrispina, form serotonin-immunopositive tubular structures in the thoracic hemiganglia innervating pereiopods 1‐3 that are unlike anything reported previously for any species. Comparisons of immunocytochemical maps within one species and between species exhibiting different behaviors provide insights into possible sites of action, functional differences between, and evolution of biogenic aminergic systems. J. Comp. Neurol.

The lateral giant (LG) command neuron of crayfish responds to an attack directed at the abdomen by triggering a single highly stereotyped escape tail flip. Experimentally applied serotonin (5-hydroxytrptamine, 5-HT) can increase or decrease LG's excitability, depending on the concentration, rate, and duration of 5-HT application. Here we describe three physiological mechanisms that mediate serotonergic facilitation of LG. Two processes strengthen electrical coupling between the primary mechanosensory afferent neurons and LG: first, an early increase in the conductance of electrical synapses between primary afferent neurons and LG dendrites and second, an early increase in the membrane resistance of LG dendrites. The increased coupling facilitates LG's synaptic response and it promotes recruitment of weakly excited afferent neurons to contribute to the response. Third, a delayed increase in the membrane resistance of proximal regions of LG increases the cell's input resistance near the initial segment. Together these mechanisms contribute to serotonergic facilitation of LG's response.

Neuromodulation provides a means of changing the excitability of neurons or the effect of synapses, and so extends the performance range of neural circuits. Metamodulation occurs when the neuromodulatory effect is itself modulated, often in response to a change in the behavioral state of the animal. The well-studied neural circuit that mediates escape in the crayfish is modulated by serotonin, and this modulation is subject to two forms of metamodulation. First, the serotonergic modulation of the Lateral Giant (LG) command neuron for escape depends on the pattern of exposure of the cell to serotonin. High and low concentrations, and rapid and slow exposures each produce opposite modulatory effects on sensory-evoked EPSPs in LG. In addition, brief exposures produce transient modulatory effects, whereas longer exposures produce long-term facilitation. These different patterns of exposure may result from serotonin neurotransmission, paracrine transmission, and hormonal release, all of which occur in the vicinity of LG. The second form of metamodulation enables serotonergic modulation to track slow changes in the social status of the crayfish. Slowly applied serotonin facilitates LG’s response in socially isolated crayfish and in new dominant and subordinate animals. Facilitation is retained in the dominant animal during two weeks of continuous pairing of the animals, but facilitation gradually changes to inhibition in the subordinate crayfish. These and related changes in serotonin modulation appear to result from changes in the population of serotonin receptors that mediate the modulatory effects in LG. Whereas the exposure-dependent metamodulation enables rapid changes in serotonergic modulation of LG to occur, the status-dependent metamodulation enables serotonergic modulation of LG to track the slow maturation of social relationships.

A phasic stimulus directed to the rear of a crayfish (Procambarus clarkii) creates mechanosensory input to the lateral giant (LG) interneuron, a command neuron for escape. A single LG spike is necessary and sufficient to produce a highly stereotyped tail flip that thrusts the animal away from the source of stimulation. Here we describe a lateral excitatory network among primary afferent axons in the last abdominal ganglion of crayfish that produces nonlinear amplification of the sensory input to the command circuitry for escape. The lateral excitation is mediated by electrical synapses between central terminals of primary mechanosensory afferents. The network enables stimulated afferents to recruit unstimulated afferents that contribute additional input to LG and to mechanosensory interneurons that also converge on LG. When depolarized, the LG neuron increases its own inputs from primary afferents and primary interneurons by facilitating the recruitment of both. Conversely, hyperpolarization of LG reduces the excitability of primary afferents and primary interneurons. The crayfish's decision to escape, previously thought to lie exclusively in the synaptic integrative properties of LG, is now seen to depend on the interactions between LG dendritic postsynaptic potentials and the responses of primary afferent terminals in the lateral excitatory network.

Evolutionary experimentation (Bullock 1984) has given rise to a large repertoire of locomotor, and other, behaviors in malacostracan crustaceans. Just within the Decapoda (Fig.1), behavioral diversity exceeds the considerable morphological diversity. More than half a century of research on the neurobiology of decapods has led to the insight that neurobehavioural circuits are often fractionated into subsets of neurons (e.g.,central pattern generators: Ikeda and Wiersma 1964; Mendelson 1971; Skorupski et al. 1984; Sillar et al. 1987; Murchison et al. 1993; groups of motor units operating muscles at single joints or even single muscles: Ayers and Clarac 1978) which are responsible for discrete aspects of behavior and that separate mechanisms exist to link the activities of these units. These latter coordinating mechanisms, which include coordinating interneurons (Wiersma and Ikeda 1964; Stein 1971; Namba and Mulloney 1999; Mulloney, this Vol.) and neuromodulatory neurons (Bartos et al. 1999; Blitz et al. 1999; Marder this vol), provide much of the adaptive flexibility of motor behavior as a whole. This organization suggests that evolutionary flexibility of motor systems might also be greatest in the coordinating mechanisms linking subsets of neurons or motor modules which themselves are more evolutionarily conserved. We summarize evidence from decapod locomotion for this hypothesis, then briefly address evolution of motor modules themselves.

The use of model animals or model systems for studying biological problems is increasingly pervasive. Proponents of consolidation of research effort on a few species argue that using many different animals to study the same system or problem results in unfruitful duplication and unjustifiable wastefulness. Of course, thoughtlessly repeating experiments over and in a wide array of animals is pointless and wasteful. However, we note a fundamental problem with limiting research to a few model organisms: it ignores the fact that biological evolution has tinkered with the genes and ontogenies of each species (that is why we distinguish them!). Model species, often initially chosen for no reason other than being easily accessible or maintainable, may be representative of a larger taxonomic group with respect to some features but prove to be very unusual with respect to the particular features under study, so that findings have only limited application to other animals. In addition to providing insight into evolutionary relationships, comparative approaches can give us clues about the structure and function of systems in extant organisms that might not be apparent from study of single species. This approach has lead to important discoveries in both vertebrate and invertebrate neuroscience (Paul 1991; Bullock 1993; Paul and Wilson 1994; Striedter 1998). What we as researchers must do is to choose carefully when comparative studies are merited, and to use them to try to investigate specific problems in the species most amenable to the particular research.