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.