Synergistic antiemetic interactions between serotonergic 5-HT3 and tachykininergic NK1-receptor antagonists in the least shrew (Cryptotis parva)

Significant electrophysiological and biochemical findings suggest that receptor cross-talk occurs between serotonergic 5-HT3- and tachykininergic NK1-receptors in which co-activation of either receptor by ineffective doses of their corresponding agonists (serotonin (5-HT) or substance P (SP), respectively) potentiates the activity of the other receptor to produce a response. In contrast, selective blockade of any one of these receptors attenuates the increase in abdominal vagal afferent activity caused by either 5-HT or SP. This interaction has important implications in chemotherapy-induced nausea and vomiting (CINV) since 5-HT3- and NK1-receptor antagonists are the major classes of antiemetics used in cancer patients receiving chemotherapy. The purpose of this study was to demonstrate whether the discussed interaction produces effects at the behavioral level in a vomit-competent species, the least shrew. Our results demonstrate that pretreatment with either a 5-HT3 (tropisetron)- or an NK1 (CP99,994)-receptor specific antagonist, attenuates vomiting caused by a selective agonist (2-methyl 5-HT or GR73632, respectively) of both emetic receptors. In addition, relative to each antagonist alone, their combined doses were 4–20 times more potent against vomiting caused by each emetogen. Moreover, combined sub-maximal doses of the agonists 2-methyl 5-HT and GR73632, produced 8–12 times greater number of vomits relative to each emetogen tested alone. However, due to large variability in vomiting caused by the combination doses, the differences failed to attain significance. The antiemetic dose–response curves of tropisetron against both emetogens were U-shaped probably because larger doses of this antagonist behave as a partial agonist. The data demonstrate that 5-HT3- and NK1-receptors cross-talk to produce vomiting, and that synergistic antiemetic effects occur when both corresponding antagonists are concurrently used against emesis caused by each specific emetogen. ► Blockade of either 5-HT3- or NK1-receptors reduce emesis caused by an agonist of each receptor. ► Relative to each antagonist tested alone, their combination is more potent against each emetogen. ► Combined doses of each agonist induce greater number of vomits relative to each emetogen alone. ► Emetic cross-talk occurs between serotonergic 5-HT3 and tachykininergic NK1 receptors. ► Synergistic antiemetic effects occur when combination antagonists are used against each emetogen.