Modulation of the crayfish escape reflex--physiology and neuroethology

We review here factors that control the excitability of the giant neuron-mediated tail-flip escape behavior in crayfish, focusing especially on recent findings concerning serotonergic modulation. Serotonin can either facilitate or inhibit escape depending on concentration and pattern of application. Low concentrations facilitate while high ones inhibit; however, if high concentrations arise gradually they facilitate instead of inhibiting. The effects of serotonin can also be altered by social experience, with application regimens that cause facilitation in social isolates coming to produce inhibition after an extended period of living as a subordinate. Attempts to understand both the possible physiological basis of some of these complexities and their possible function are discussed. Neuroethological investigations indicate that giant neuron-mediated escape is inhibited during the initial fights that establish social relationships and is facilitated in their immediate aftermath. Once the relationship of a pair is well-established, the presence of the dominant tends to suppress giant neuron-mediated escape (but not tail-flip escape mediated by non-giant circuitry) in the subordinate, but the presence of the subordinate has relatively little effect on the dominant. These patterns of modulation can be seen as consistent with the known variations in serotonin's effect as a function of concentration and social experience and may provide a biological reason for these variations.