I'm no chemist, but according to wikipedia, cuscohygrine is found in belladona plants and it metabolizes into hygrine. So that could be what he's referring to?
I read the same wikipedia page and that statement is confusing if not incorrect. Hygrine is a not a metabolite of cuscohygrine, it's in fact the other way round: hygrine is the precursor and cuscohygrine is the metabolite.
The first reference on that page is "The role of hygrine in the biosynthesis of cuscohygrine and hyoscyamine"
The first reference on that page is "The role of hygrine in the biosynthesis of cuscohygrine and hyoscyamine"