Identifying the threshold of iron deficiency in the central nervous system of the rat by the auditory brainstem response

Abstract

The deleterious effects of anemia on auditory nerve (AN) development have been well investigated; however, we have previously reported that significant functional consequences in the auditory brainstem response (ABR) can also occur as a consequence of marginal iron deficiency (ID). As the ABR has widespread clinical use, we evaluated the ability of this electrophysiological method to characterize the threshold of tissue ID in rats by examining the relationship between markers of tissue ID and severity of ABR latency defects. To generate various levels of ID, female Long-Evans rats were exposed to diets containing sufficient, borderline, or deficient iron (Fe) concentrations throughout gestation and offspring lifetime. We measured hematological indices of whole body iron stores in dams and offspring to assess the degree of ID. Progression of AN ID in the offspring was measured as ferritin protein levels at different times during postnatal development to complement ABR functional measurements. The severity of ABR deficits correlated with the level of Fe restriction in each diet. The sufficient Fe diet did not induce AN ID and consequently did not show an impaired ABR latency response. The borderline Fe diet, which depleted AN Fe stores but did not cause systemic anemia resulted in significantly increased ABR latency isolated to Peak I.The low Fe diet, which induced anemia and growth retardation, significantly increased ABR latencies of Peaks I to IV. Our findings indicate that changes in the ABR could be related to various degrees of ID experienced throughout development.

Figure 1.

Postnatal growth and hematological parameters in offspring fed an IDD. (a) Weight of offspring at P14, 21, and 40. Data are mean ± SEM for n = IS: 12–17, IDD-30: 10–11, IDD-20: 11–18, and IDD-6: 4–8 rats per group, per age. Hematocrit (b) and hemoglobin (c) values in offspring at P14, 21, and 40. *p < .05 versus age-matched 240-IS group, ^#p < .05 versus all other diet groups within each age. Data are mean ± SEM for n = IS: 8–21, IDD-30: 6–9, IDD-20: 10–11, and IDD-6: 3–7 rats per group, per age. IS = iron sufficient; IDD = iron-deficient diet.

Figure 2.

Iron store mobilization in AN tissue of offspring fed an IDD compared with the IS group. Ferritin protein expression in the IDD-30 group (a), IDD-20 group (b), and IDD-6 group (c). Data are mean ± SEM, n = 4 offspring per age and diet group. Representative Western blot bands of IS tissue (left band) and deficient tissue (right band) are shown below each age for each respective diet. Data are represented as a ratio to age-matched IS controls. *p < .05 versus age-matched IS control. IS = iron sufficient; IDD = iron-deficient diet; FTH = anti-ferritin heavy chain.

Figure 3.

Peak 1 neuronal conduction velocity by ABR analysis in P40 offspring fed variable IDDs. (a) Representative aligned ABR potentials evoked at 16 kHz stimulus frequency and 70 dB intensity level. The dotted line demarcates the relative latency of Peak 1 (P1) in IS controls compared with each IDD group. (b) Quantified latencies from all three test frequencies in each diet group. Data are mean ± SEM, n = IS: 11, IDD-30: 9, IDD-20: 13, IDD-6: 5 rats per group. *p < .05 versus frequency-matched IS controls, ^#p < .05 versus all diet groups within that frequency. IS = iron sufficient; IDD = iron-deficient diet.