Significant electrophysiological and biochemical findings suggest that receptor cross-talk occurs between serotonergic 5-HT(3)- and tachykininergic NK(1)-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-HT(3)- and NK(1)-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-HT(3) (
tropisetron)- or an NK(1) (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-HT(3)- and NK(1)-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.