## 4 Discussion
### Heads: Impact of Age on Cr(VI) Neurotoxicity
These data suggest shorter Cr(VI) exposure can induce anxiolytic effects in young and middle-aged females, but anxiogenic effects in geriatric females, whereas longer Cr(VI) exposures may induce an anxiogenic effect in middle-aged females. Our previous report is the only Cr(VI) neurotoxicity study that includes Cr(VI) effects on anxiety, and we observed some sex differences. Similar to females, young male rats also exhibited an anxiogenic effect after 2 weeks of exposure to Cr(VI), however, this was apparent in the 0.05 mg/L male group vs. the 0.1 mg/L group in females; middle-aged males exhibited the opposite effect as females, with an anxiogenic effect apparent in the 0.05 mg/L males vs. anxiolytic in females. We further assessed anxiety using the elevated plus maze, where Cr(VI) exhibited anxiogenic effects in geriatric females after 9 weeks but no effect in young or middle-aged females. In contrast, we reported anxiogenic effects in young and middle-aged male rats after 9 weeks. In sum, these data suggest Cr(VI) exhibits variable effects on anxiety that are age-, sex-, time-, and context-specific. Our spatial memory assessment in the Y-maze is potentially the most interesting result of this study. We observed no Cr(VI) effects in females after 4 or 10 weeks of exposure, but males exhibited a concentration-associated increase in non-alternations after 4 weeks. Considering age groups at week 4, we observed no effects across all female age groups, while the male Cr(VI) effect was primarily driven by the young and geriatric male groups. After 10 weeks Cr(VI) exposure, we observed middle-aged females exhibited a concentration-associated decrease in non-alternations after 10 weeks Cr(VI) exposure, which directly contrasted with a concentration-associated increase in middle-aged males. These data suggest Cr(VI) exposure improved spatial memory in middle-aged females but impaired spatial memory in middle-aged males. Instead, we posit middle-aged females developed a turning side preference from Cr(VI)-induced hippocampal damage, a phenomenon where severe hippocampal damage results in decreased spontaneous non-alternations in the Y-maze [58; 59]. We observed some age- and sex-differences in rat activity, as measured by the number of arm entries per minute during the Y-maze. Females were generally more active than males when comparing across age groups. After 4 weeks of Cr(VI) exposure, middle-aged females became slightly hypovactive with no apparent effects in young or geriatric females, whereas young and geriatric males were slightly hypovactive after Cr(VI) exposure, and middle-aged males were slightly hypoactive. After 10 weeks of Cr(VI) exposure, young females became hyperactive, and there was no apparent effect in older females. In males, we observed middle-aged and geriatric rats became hypoactive after 10 weeks, whereas young rats were still slightly hyperactive. Other studies reported Cr(VI) exposure decreased motor activity in the motor activity cage assay [42; 51; 52; 53]. Hyperactivity in rodents is an autism spectrum disorder phenotype, which is linked to Cr(VI) exposure in human populations [33; 60]. Evidence also suggests metal exposure induces hyperactivity in rats, which is likely due to neuropathology in the cerebellum and striatum [61]. Cr(VI) induced neuropathology in the rat, guinea pig, and chicken cerebellum, suggesting cerebellar damage may induce hyperactivity after Cr(VI) exposure [40; 62; 63]. Cr(VI)-induced neuropathology in the striatum has not been reported. Cr(VI) altered sociability and social novelty preference distinctly across age groups in the 3-chamber assay. Cr(VI) reduced sociability in middle-aged rats (both sexes) after 5 weeks; after 11 weeks, we observed decreased sociability in all Cr(VI)-exposed female rats, while effects in males were more variable. During the social novelty preference trial, we observed females generally had a stronger preference for social novelty than males.