The distribution of red pigment concentrating hormone (RPCH)-like immunoreactivity (RPLI) in the stomatogastric nervous system of the crab, Cancer borealis, was studied using whole-mount immunocytochemistry. RPLI was seen in neuropilar processes in the stomatogastric ganglion (STG), and in somata in the oesophageal ganglion and commissural ganglia. Staining was blocked by preincubating the antiserum with RPCH, as well as with a number of adipokinetic hormones (AKHs) and related peptides.

Synthetic RPCH had strong actions on the pyloric rhythm of the isolated STG. Bath applications of RPCH (10−9-10−6moll−1) increased the cycle frequency in preparations displaying slow pyloric rhythms, and initiated rhythmic pyloric activity in silent preparations. In the presence of tetrodotoxin (TTX), RPCH evoked rhythmic non-impulse-mediated alternations in membrane potential in the lateral pyloric and pyloric dilator motor neurones. The effects of RPCH were compared to those of a series of AKHs which resemble RPCH structurally.

The immunocytochemical and physiological data together suggest that RPCH or a similar molecule is a neurally released modulator of the STG.

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