Neuromodulation of rhythmic motor patterns in the blue crab Callinectes sapidus by amines and the peptide proctolin.

The blue crab Callinectes sapidus, provides an opportunity to study neuromodulation of three variations of rhythmic behavior produced by the same appendages. These behaviors are sideways swimming, backward swimming and courtship display (CD). Each behavior has a different context, and despite similarities among them, each is quantifiably distinguishable. CD behavior occurs in males, is stimulated naturally by pheromone, and elements of the behavior are evoked by proctolin and dopamine. Sideways and backward swimming do not share these characteristics. Bath-applied proctolin, combined with either electrical or pheromonal stimulation, was used to search for interneurons influencing motor outflows from the fifth legs. Interneurons were found which, when stimulated electrically, initiated rhythmic behavior. At least one of these neurons responded to pheromonal stimulation. Application of proctolin combined with stimulation of descending 'trigger' cells resulted in changes from a backward swimming motor pattern toward a CD pattern. Dopamine applied with proctolin lowered the concentration threshold for proctolin-evoked changes in motor outflows. Octopamine co-applied with proctolin extinguished the proctolin effect unless dopamine was co-applied. Combinations of modulators appear to play critical roles in shaping patterns of rhythmic motor activity of the fifth legs.