Concentrations of Essential Trace Metals in the Brain of Animal Species-A Comparative Study

Abstract

The essential trace metals iron, zinc, and copper have a significant physiological role in healthy brain development and function. Especially zinc is important for neurogenesis, synaptogenesis, synaptic transmission and plasticity, and neurite outgrowth. Given the key role of trace metals in many cellular processes, it is important to maintain adequate levels in the brain. However, the physiological concentration of trace metals, and in particular zinc, in the human and animal brain is not well described so far. For example, little is known about the trace metal content of the brain of animals outside the class of mammals. Here, we report the concentration of iron, zinc, and copper in fresh brain tissue of different model-species of the phyla Chordata (vertebrates (mammals, fish)), Annelida, Arthropoda (insects), and Mollusca (snails), using inductively coupled plasma mass-spectrometry (ICP-MS). Our results show that the trace metals are present in the nervous system of all species and that significant differences can be detected between species of different phyla. We further show that a region-specific distribution of metals within the nervous system already exists in earthworms, hinting at a tightly controlled metal distribution. In line with this, the trace metal content of the brain of different species does not simply correlate with brain size. We conclude that although the functional consequences of the controlled metal homeostasis within the brain of many species remains elusive, trace metal biology may not only play an important role in the nervous system of mammals but across the whole animal kingdom.

Keywords: CNS; ICP-MS; biometals; copper; earthworm; herring; iron; locust; mouse; pig; rat; selenium; snail; zinc.

Figure 1

The concentrations of essential trace metals in the cerebral cortex (ctx) from rat using fixed and fresh frozen tissue. (a) The comparison of zinc concentrations within the fixed cortex vs. fresh Figure 3.4 ± 0.45 vs. 14 ± 1.9 mg/kg; n = 2; p = 0.0165); (b) The iron concentrations within the fixed cortex vs. fresh frozen cortex tissue revealed higher levels in fresh frozen tissue that are not statistically significant (16 ± 1.6 vs. 11 ± 1.2 mg/kg; n = 2; p = 0.0715); (c) A significantly higher copper concentration in fresh frozen cortex tissue (1.4 ± 0.14 vs. 2.3 ± 0.22 mg/kg; n = 2; p = 0.0395) were found. Significances are stated with p values < 0.05 *.

Figure 2

Concentrations of zinc, iron, copper, and selenium in fresh cortex brain tissues of the Vertebrates (Chordata). (a) The comparison of zinc concentrations within the brains of pig (12 ± 1.4 mg/kg; n = 2), rat (14 ± 1.9 mg/kg; n = 2), mouse (11.67 ± 1.155 mg/kg; n = 3), and fish (12 ± 0.76 mg/kg; n = 6) did not reveal any significant differences (Welch’s ANOVA test p = 0.7054). (b) Iron levels in brain tissue of pig (16 ± 2.8 mg/kg; n = 2), rat (16 ± 1.6 mg/kg; n = 2), mouse (13.67 ± 3.786 mg/kg; n = 3), and fish (18 ± 2.6 mg/kg; n = 6) were not significantly different between species (Welch’s ANOVA test p = 0.5312). (c) Significant differences were found in the distribution of copper levels: pig brain (3.5 ± 0.61 mg/kg; n = 2), rat (2.3 ± 0.22 mg/kg; n = 2), mouse (3.4 ± 0.2 mg/kg; n = 3), and fish (1.6 ± 0.2 mg/kg; n = 6), (Welch’s ANOVA test p = 0.0130). Post-hoc tests show a significant difference between mouse and fish (p = 0.0007). (d) Selenium levels in the brain of pig (0.38 ± 0.11 mg/kg; n = 2), rat (0.46 ± 0.048 mg/kg; n = 2), mouse (0.21 ± 0.05 mg/kg; n = 3), and in the brain of the fish (0.84 ± 0.12 mg/kg; n = 6) show significant differences (Welch’s ANOVA test p = 0.0093). Post-hoc reveal statistically relevant differences between rat and fish (p = 0.0050) and mouse and fish (p < 0.0001). Significances are stated with p values < 0.01 **; <0.001 ***.

Figure 3

Concentrations of trace metals in the nervous system of the earthworm. (a) Zinc concentrations were higher in the head ganglion of earthworms (74 ± 1.4 mg/kg; n = 15) compared to the zinc levels in the nerve cord of the worms (39 ± 10 mg/kg; n = 15) (t-test, p < 0.0001). (b) Iron levels in the head ganglion (170 ± 13 mg/kg; n = 15) were significantly different compared to the nerve cord (140 ± 0.2 mg/kg; n = 15) (t-test, p < 0.0001). (c) Copper concentrations were significantly different between nerve cord (2.2 ± 0.054 mg/kg; n = 15) and earthworm head ganglion (3.5 ± 0.26 mg/kg; n = 15) (t-test, p < 0.0001). Significances are stated with p values < 0.001 ***.

Figure 4

Concentrations of trace metals in earthworm, locust, and snail. (a) The comparison of zinc concentrations within the head ganglion of earthworm, locust, and snail resulted in statistically significant differences (Welch’s ANOVA test p < 0.0001). Post-hoc analysis: earthworm vs. locust (p < 0.0001), earthworm vs. snail (p < 0.0001), and locust vs. snail (p < 0.0001). (b) Iron levels were significantly different in earthworm, locust, and snail (Welch’s ANOVA test p < 0.0001). Post-hoc analysis: earthworm vs. locust (p < 0.0001), earthworm vs. snail (p < 0.0001), and locust vs. snail (p < 0.0001). (c) Copper levels in the “brain” of earthworm, locust, and snail are different (Welch’s ANOVA test p < 0.0001). Post-hoc analysis: earthworm vs. locust (p < 0.0001), earthworm vs. snail (p < 0.0001), and locust vs. snail (p < 0.0001). Significances are stated with p values < 0.001 ***.

Figure 5

Correlation between total zinc (μg) (a), iron (b), and copper (c), and brain weight (g). A Pearson correlation analysis shows the following data: zinc: r = −0.1866, p = 0.7234, and the iron: r = −0.1704, p = 0.7469, copper: r = −0.0973, p = 0.8545. Thus, no significant correlation between brain weight and zinc, iron, and copper content has been found. (d) The ratio between zinc and iron (mg/kg brain weight) is relatively constant across all species (average 0.86 ± 0.3 SD), while the brain zinc/copper (9.34 ± 7.85 SD) and iron/copper (14.62 ± 17.46 SD) ratio is much more variable across different species.