Modulation of the Callinectes sapidus Cardiac System by Serotonin

The heart of the blue crab, Callinectes sapidus, is a simple central pattern generator-effector system. The 9 neuron cardiac ganglion (CG), embedded within the cardiac muscle, sets the basic rhythm of the heart. The activity of the CG is modulated by both neuronal (via cardioregulatory fibers) and hormonal signals (via the pericardial organs). Gamma aminobutyric acid (GABA) and dopamine have previously been identified as the neurotransmitters of two of the three cardioregulatory fibers and are both present in the pericardial organs (PO). In this study, we used immunohistochemistry to determine the distribution of the biogenic amine, Serotonin (5-Hydroxytryptamine) in the central nervous system of the blue crab. Serotonin-like immunoreactivity (5-HT-li) was observed in cells in the brain and thoracic ganglion and in fibers within the brain-thoracic connectives. 5-HT-li fibers projected via segmental nerve 1 towards the PO. Multiple branching fine caliber fibers and varicosities expressed 5-HT-li within the PO. 5-HT-li was not observed in the dorsal nerve or CG. Serotonin induced both positive chronotropic and inotropic effects in isolated heart preparations at concentrations consistent with hormonal signaling. The 5-HT-li distribution and cardiac responsiveness to serotonin both suggest a possible cardiomodulatory role for serotonin in the heart of the blue crab