Where are you getting that hygrine is found in nightshades? The authors of the paper specifically say it's only found in Erythroxylum and I'm not finding any references in my own research to that specific chemical being present in nightshades.
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"
