Red pigment concentrating hormone is a modulator of the crayfish swimmeret system.

The crustacean red pigment concentrating hormone (RPCH) has been localized in neurons of the crayfish abdominal nerve cord and modulates the crayfish swimmeret rhythm. An antibody to RPCH labels a small set of cell bodies and axons in each abdominal ganglion. Physiological experiments in which RPCH was perfused into the ganglia of isolated nerve cords showed that RPCH modulated the swimmeret rhythm. In nerve cords that were spontaneously producing the swimmeret rhythm, RPCH lengthened both the period and the duration of bursts of action potentials, but did not alter the phase relationships between bursts in different segments. RPCH did not initiate the swimmeret rhythm in preparations that showed intermittent or no bursting activity. We believe that RPCH is released as a neurotransmitter in the lateral neuropil, where it exerts its effects on the local swimmeret circuits.