ABSENCE OF EMETIC EFFECTS OF MORPHINE AND LOPERAMIDE IN SUNCUS-MURINUS

The house musk shrew Suncus murinus recently has been introduced for t he study of emesis. We investigated the emetic effects of the opioids morphine (0.1-21.5 mg/kg i.p.) and loperamide (0.01-10 mg/kg i.p.) and found a complete lack of emetogenic potential. Nicotine, however, dos e dependently induced vomiting in the Suncus with an ED(50) of 8.8 mg/ kg s.c. and a 100% incidence at 20 mg/kg. This drug-induced vomiting w as reduced by morphine or loperamide: ED(50) values obtained were 1.2 mg/kg i.p. for morphine and 0.7 mg/kg i.p. for loperamide. Naloxone (2 mg/kg s.c.) antagonised the inhibitory effect of morphine (2 mg/kg i. p.) or loperamide (10 mg/kg i.p.). Serotonin (20 mg/kg s.c.) had less reliable emetogenic potency than nicotine in the Suncus with incidence s between 50 and 100%. However, the serotonin-induced vomiting was abo lished by morphine and loperamide and this inhibition was antagonised by naloxone. These results suggest that systemically administered opio ids are pure antiemetics in Suncus murinus in contrast to other animal models and man. Naloxone antagonism indicates that this antiemetic ef fect is mediated by opioid receptors.