In vivo assessment of antiemetic drugs and mechanism of lycorine-induced nausea and emesis.

Lycorine is the main alkaloid of many Amaryllidaceae and known to cause poisoning with still unknown mechanisms. Longer lasting toxicological core symptoms of nausea and emesis may become a burden for human and animal patients and may result in substantial loss of water and electrolytes. To optimise the only empirical symptomatic antiemetic drug treatment at present, it is important to elucidate the causative involved targets of lycorine-induced emesis. Therefore, in the current study, we have tested the actions of a various antiemetic drugs with selective receptor affinities on lycorine-induced nausea and emesis in vivo in dogs. Beagle dogs were pre-treated in a saline vehicle-controlled crossover and random design with diphenhydramine, maropitant, metoclopramide, ondansetron or scopolamine prior lycorine administration (2 mg/kg subcutaneously). In vivo effects were assessed by a scoring system for nausea and emesis as well as by the number and lag time of emetic events for at least 3 h. Moreover, plasma pharmacokinetic analysis was carried out for ondansetron before and after lycorine injection. The data show that histaminergic (H₁), muscarinic and dopaminergic (D₂) receptors are presumably not involved in lycorine-induced emetic effects. While ondansetron significantly reduced the number of emetic events, lycorine-induced emesis was completely blocked by maropitant. Only ondansetron also significantly decreased the level of nausea and was able to prolong the lag time until onset of emesis suggesting a preferential participation of 5-HT₃ receptors in lycorine-induced nausea. Thus, it is the first in vivo report evidencing that predominantly neurokinin-1 (NK₁) and to a lesser extent 5-hydroxytryptamine 3 (5-HT₃) receptors are involved in lycorine-induced emesis facilitating a target-oriented therapy.