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.
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-HT(1crust)ir) ranging from preparations with hundreds of labeled cells per ganglion to some containing only a handful of 5-HT(1crust)ir cells in the entire nerve cord. The interanimal variability in 5-HT(1crust)ir 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-HT(1crust)ir 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.
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.
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.
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