Serotonergic regulation of blood glucose levels in the crayfish, Procambarus clarkii: Site of action and receptor characterization

The present study investigated the site of action of 5-hydroxytryptamine (5 -HT) and pharmacologically characterized the receptors involved in regulati ng blood glucose levels in the crayfish, Procambarus clarkii. Injection of 5-HT into intact animals increased glucose levels in a dose-dependent manne r. In contrast, 5-HT failed to elicit a hyperglycemic response in eyestalk- ablated animals. Effects of several 5-HT receptor agonists and antagonists were examined. 5-CT, oxymetazoline (both 5-HT1 receptor agonists) and alpha -methyl-5-HT (a 5-HT2 receptor agonist), but not 1-phenylbiguanide, m-CPBG (both 5-HT3 receptor agonists), or RS 67333 (a 5-HT4 receptor agonist), ind uced hyperglycemic responses in a dose-dependent manner. In addition, 8-OH- DPAT (a 5-HT1A receptor agonist), L-694,247 (a 5-HT1B/1D receptor agonist), and DOI (a 5-HT2A receptor agonist) were effective in significantly increa sing the glucose levels, whereas both BW 723C86 (a 5-HT2B receptor agonist) and m-CPP (a 5-HT2C receptor agonist) were ineffective. Finally, ketanseri n (a 5-HT2A receptor antagonist), but not p-MPPF (a 5-HT1A receptor antagon ist), GR 55562 (a 5-HT1B/1D receptor antagonist), SE 206553 (a 5-HT2B/2C re ceptor antagonist), or tropisetron (a 5-HT3 receptor antagonist), was able to block 5-HT-induced hyperglycemia. The combined results support the hypot hesis that 5-HT exerts its hyperglycemic effect by enhancing the release of hyperglycemic factor(s) from the eyestalks, and suggest that 5 HT-induced hyperglycemia is mediated by 5-HT1- and 5-HT2-like receptors. J. Exp. Zool. 286:596-605, 2000. (C) 2000 Wiley-Liss, Inc.