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Serotonergic Modulation of Crayfish Hindgut

Neuroscience Institute, Georgia State University, Atlanta, Georgia 30302-5030
¹ Current address: Department of Natural Science, Clayton State University, Morrow, GA 30260
² Current address: Marshall University, Department of Biology, Huntington, WV 25755

^* To whom correspondence should be addressed. E-mail: dedwards{at}gsu.edu

The crayfish hindgut is a morphologically differentiated tube that varies along its length in the distribution of muscles and glands, contractile properties, serotonergic innervation, patterns of 5-HT receptor expression, and sensitivity to serotonin (5-HT). Anatomical differences divide the hindgut into five distinct segments along its length. Spontaneous pulsatile contractions produced by the isolated hindgut decrease in force and increase in frequency along the anterior-posterior axis. Central input to the hindgut comes from a large cluster of 5-HT-immunoreactive neurons in the terminal abdominal ganglion that form a large nerve plexus on the hindgut. 5-HT₁ and 5-HT_2β receptors vary in their distribution along the hindgut, and are associated with longitudinal and circular muscles and with axon collaterals of the 5-HT-immunoreactive neurons. Application of 30 nmol l^–1 to 1 µmol l^–1 5-HT to rostral, middle, or caudal sections of hindgut produced tension changes that varied with the concentration and section. 5-HT also initiated antiperistaltic waves in the posterior hindgut. These results indicate that 5-HT is an important neuromodulator for initiating contractions and coordinating activity in the different functional compartments along the rostral-to-caudal axis of the hindgut.

Abbreviations: A6, abdominal ganglion 6 • AAHG (anterior abdominal hindgut • AIN, anteriorly projecting intestinal nerve • CM, circular muscle • HGN, hindgut neurons • 5-HT-ir, 5-HT-immunoreactivity • ICC, immunocytochemistry • LM, longitudinal muscle • N7, seventh nerve of abdominal ganglion six • PAHG, posterior abdominal hindgut • PIN, posteriorly projecting intestinal nerve • THG, thoracic hindgut