Modulation of the crayfish escape reflex--physiology and neuroethology

We review here factors that control the excitability of thegiant neuron-mediated tail-flip escape behavior in crayfish,focusing especially on recent findings concerning serotonergicmodulation. Serotonin can either facilitate or inhibit escapedepending on concentration and pattern of application. Low concentrationsfacilitate while high ones inhibit; however, if high concentrationsarise gradually they facilitate instead of inhibiting. The effectsof serotonin can also be altered by social experience, withapplication regimens that cause facilitation in social isolatescoming to produce inhibition after an extended period of livingas a subordinate. Attempts to understand both the possible physiologicalbasis of some of these complexities and their possible functionare discussed. Neuroethological investigations indicate thatgiant neuron-mediated escape is inhibited during the initialfights that establish social relationships and is facilitatedin their immediate aftermath. Once the relationship of a pairis well-established, the presence of the dominant tends to suppressgiant neuron-mediated escape (but not tail-flip escape mediatedby non-giant circuitry) in the subordinate, but the presenceof the subordinate has relatively little effect on the dominant.These patterns of modulation can be seen as consistent withthe known variations in serotonin's effect as a function ofconcentration and social experience and may provide a biologicalreason for these variations.