Olczak M, Duszczyk M, Mierzejewski P, Majewska MD. 2009 Neonatal administration of a vaccine preservative, thimerosal, produces lasting impairment of nociception and apparent activation of opioid system in rats. Brain Res. 2009 Dec 8;1301:143-51. doi: 10.1016/j.brainres.2009.09.003. Epub 2009 Sep 9.
Rat study, thimerosal (no longer used in U.S. childhood vaccines except for some influenza vaccines), concentrations higher than formerly found in vaccines.
Note: lead researcher M Dorota Majewska seems to have specialized in rat studies that show thimerosal damage.