Serotonergic control of the heart and pericardium in the chiton Acanthopleura japonica

Serotonergic control of the heart and pericardium of the chiton, Acanthople ura japonica, was examined during this combined histological and physiologi cal study. Glyoxylic acid (2%) induced fluorescence in neural elements of f ibers and varicosities in the heart and pericardium. Serotonin exerted posi tive chronotropic and inotropic actions on the heart and pericardium in a d ose-dependent manner. The threshold dose of serotonin that stimulated excit atory effects on the heart and pericardium was less than 10(-9) M. The hist ological and pharmacological results suggest that serotonin may be a neurot ransmitter present in excitatory nerves innervating the heart and pericardi um. We conclude that serotonergic excitatory control of the heart appeared early during molluscan phylogeny with the evolution of the chitons. The per icardium of the chiton may receive serotonergic excitatory neural control, which has not been reported in higher molluscan taxa. (C) 1999 Elsevier Sci ence Inc. All rights reserved.