From this column, recently dug out of Cecil’s archives:
Does this mean I shouldn’t give apple cores to my pet rats? Whenever I give them one, they decend upon it like the ravenous little beasties they are and tear it apart. They love them apple seeds.
Sorry, forgot the link.
http://www.straightdope.com/classics/a1_212.html
I’ve eaten plenty of apple seed without any harm, so if they contain cyanide it can’t be very much.
As the article indicates, the rats should be OK if you don’t feed them to many cores in too short of a time. Rats may also be more resistant to some toxins than humans, given the diet they eat.
Apple seeds contain amygdalin which is a cyanogenic glycoside, which can release cyanide. Cyanide is not really that toxic, in relative terms.
"Apple seeds average around 0.6 mg hydrogen cyanide (HCN) per gram of dry seed. Since the lethal dose of HCN is estimated to be about 50 mg, you need around 85 grams (3 ounces) of dry seeds. This is around half a cup, which requires a lot of apples. The HCN must be liberated from the sugar it’s chemically attached to. This occurs when the moistened seed is crushed,releasing an enzyme, emulsion, which does the job. Apparently this also occurs in the stomach, due to the hydrochloric acid there. "
http://www.spaink.net/suicide/suicide_poison_natural.html#rose