Researchers at Sweden's Karolinska Institute and at the National Institutes of Health are finding a connection between
tyrosine hydroxylase activity, thyroid hormone receptors, and depleted dopamine levels in the brain — particularly in the substantia nigra, a region associated with the movement difficulties characteristic of Parkinson's disease.11, 12,13
Abstract: Both D4 dopamine receptor (DRD4) exon 3 and
tyrosine hydroxylase (TH) intron 4 repeat polymorphisms have been linked to
activity and impulsivity in German Shepherd Dogs (GSDs).