CYP2D6 variants are an interesting possible contributing factor because
cyp2d6 has fairly common polymorphisms with wide degrees of relative activity, from poor metabolizers to extremely rapid metabolizers
cyp2d6 metabolizes tricyclics and SSRIs
cyp2d6 is involved in conversion of allopregnanolone to THDOC, and other neurosteroid conversions in the brain
cyp2d6 provides an alternative pathway than tyrosine hydroxylase for dopamine production
cyp2d6 status is testable - either through genetic testing or particular drug challenges
I seem to be a poor metabolizer of cyp2d6, according to my 23andme results fed through promethease at snpedia.
A couple of papers on CYP2D6 and neurosteroids.
http://www.jssx.org/dmpk/d186/dmpk186337.pdf
http://endo.endojournals.org/cgi/reprint/145/2/699.pdf
Anyway, I thought I’d throw this out as another possible mechanism.