Total reflection X-ray spectroscopy (TXRF)

Tissue samples were collected and weighed in 1.5 ml Eppendorf tubes. The weight of the tissue was directly used to calculate element concentrations in µg/kg units. Each tissue sample was dissolved in 100 µL of nitric acid (Sigma: NX0408) with 2 µL of a gallium standard (conc. 1000 ppm). Each sample was assessed in duplicate for TXRF elemental analysis using an S2 Picofox (Bruker, Billerica, MA). This instrument exposes the sample to an X-ray beam and measures fluorescence radiation specific to the element(s) of interest. Human samples were prepared from postmortem tissue collected from the anterior caudate. Wild-type (WT) or ZnT3 KO mice received either saline, a single cocaine injection (20 mg/kg, i.p), or 8 repeated daily cocaine (20 mg/kg, i.p) injections and were euthanized 24 h after the last injection. Mice fed the 30 ppm and 5 ppm Zn²⁺ diets were exposed to each respective diet for 35 days and then euthanized. Mouse samples were prepared by slicing flash-frozen tissue on a cryostat (100 µm sections from Bregma 1.00 to 0.00 mm) or dissecting the Ctx and striatum from each section.