Serotonergic modulation of social status-dependent behavioural plasticity of the crayfish avoidance reaction.

Small crayfish usually showed escape-like dart responses to mechanical stimulation of the tailfan. Following agonistic bouts with conspecifics, dominant crayfish showed a defensive-like turn response to the same sensory stimulus. During the dart response, both uropods closed and animals walked forwards with the abdomen extended, while during the turn response the uropod on the stimulated side opened and animals turned towards the stimulus source with the abdomen frequently flexed. Using an isolated nerve cord preparation, we found that the spike activities of both the abdominal postural and uropod motor neurones in response to sensory stimulation of the exopodite reversed in dominant animals. In naive and subordinate animals, extensor motor neurones were excited and antagonistic flexor motor neurones inhibited in response to sensory stimulation. Furthermore, the spike frequency of uropod closer motor neurones increased while that of the antagonistic opener motor neurones decreased. By contrast, in more than half of the dominant animals, flexor and opener motor neurones showed excitatory responses while extensor and closer motor neurones showed inhibitory responses to the same sensory stimulation. This reverse of activity of the abdominal postural and uropod motor neurones was also observed when serotonin of more than 12.5 μM in concentration was applied.