Pigment Translocation in Caridean Shrimp Chromatophores: Receptor Type, Signal Transduction, Second Messengers, and Cross Talk Among Multiple Signaling Cascades.

Pigment aggregation in shrimp chromatophores is triggered by red pigment concentrating hormone (RPCH), a neurosecretory peptide whose plasma membrane receptor may be a G-protein coupled receptor (GPCR). While RPCH binding activates the Ca2+ /cGMP signaling cascades, a role for cyclic AMP (cAMP) in pigment aggregation is obscure, as are the steps governing Ca2+ release from the smooth endoplasmic reticulum (SER). A role for the antagonistic neuropeptide, pigment dispersing homone (α-PDH) is also unclear. In red, ovarian chromatophores from the freshwater shrimp Macrobrachium olfersi, we show that a G-protein antagonist (AntPG) strongly inhibits RPCH-triggered pigment aggregation, suggesting that RPCH binds to a GPCR, activating an inhibitory G-protein. Decreasing cAMP levels may cue pigment aggregation, since cytosolic cAMP titers, when augmented by cholera toxin, forskolin or vinpocentine, completely or partially impair pigment aggregation. Triggering opposing Ca2+ /cGMP and cAMP cascades by simultaneous perfusion with lipid-soluble cyclic nucleotide analogs induces a "tug-of-war" response, pigments aggregating in some chromatosomes with unpredictable, oscillatory movements in others. Inhibition of cAMP-dependent protein kinase accelerates aggregation and reduces dispersion velocities, suggesting a role in phosphorylation events, possibly regulating SER Ca2+ release and pigment aggregation. The second messengers IP3 and cADPR do not stimulate SER Ca2+ release. α-PDH does not sustain pigment dispersion, suggesting that pigment translocation in caridean chromatophores may be regulated solely by RPCH, since PDH is not required. We propose a working hypothesis to further unravel key steps in the mechanisms of pigment translocation within crustacean chromatophores that have remained obscure for nearly a century.