About as far away from human trials as possible, but the title says it all:
"Sodium benzoate exposure downregulates the expression of tyrosine hydroxylase and dopamine transporter in dopaminergic neurons in developing zebrafish."
Chen Q1, Huang NN, Huang JT, Chen S, Fan J, Li C, Xie FK.
Birth Defects Res B Dev Reprod Toxicol. 2009 Apr;86(2):85-91. doi: 10.1002/bdrb.20187.
http://www.ncbi.nlm.nih.gov/pubmed/19294673
The significance of tyrosine hydroxylase is that it catalyses the conversion of tyrosine to levodopa.
The reported result will most likely apply to cinnamon too.
John