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MODULATION OF THE CALLINECTES SAPIDUS CARDIAC SYSTEM BY SEROTONIN**

Abstract

The heart of the blue crab, Callinectes sapidus, is a simple central pattern generator-effector system. The nine neuron cardiac ganglion, embedded within the cardiac muscle, sets the basic rhythm of the heart. The activity of the cardiac ganglion is modulated by both neuronal (via cardioregulatory fibers) and hormonal signals (via the pericardial organs). Gamma aminobutyric acid (GABA) and dopamine have been previously identified as the neurotransmitters of two of the three cardioregulatory fibers and are also both present in the pericardial organs. 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 was observed in cells in the brain and thoracic ganglion and in multiple fibers in the brain-thoracic connectives. Several of these fibers terminated in varicosities within the commissural ganglia. Serotonin-like immunoreactive fibers, arising from cells in the posterior of the thoracic ganglion, project via segmental nerve 1 towards the pericardial organs. Multiple branching fine caliber fibers and varicosities expressed serotonin-like immunoreactivity within the pericardial organs. Immunoreactivity was not observed in the dorsal nerve or cardiac ganglion. Serotonin induced both positive chronotropic and inotropic effects in isolated heart preparations at concentrations consistent with hormonal signaling. The serotonin-like immunoreactivity distribution and cardiac responsiveness to serotonin both suggest a possible cardiomodulatory role for serotonin in the heart of the blue crab.

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