Moderate iron deficiency in infancy: Biology and behavior in young rats

Abstract

Iron deficiency anemia in early childhood is associated with developmental delays and perhaps, irreversible alterations in neurological functioning. The goals were to determine if dietary induced gestational and lactational iron deficiency alters brain monoamine metabolism and behaviors dependent on that neurotransmitter system. Young pregnant rats were provided iron deficient or control diets from early in gestation through to weaning of pups and brain iron concentration, regional monoamine variables and achievement of specific developmental milestones were determined throughout lactation. Despite anemia during lactation, most brain iron concentrations did not fall significantly until P25, and well after significant changes in monoamine levels, transporter levels, and D₂R density changed in terminal fields. The changes in D₂R density were far smaller than previously observed models that utilized severe dietary restriction during lactation or after weaning. Iron deficient pups had normal birth weight, but were delayed in the attainment of a number of milestones (bar holding, vibrissae-evoked forelimb placing). This approach of iron deficiency in utero and during lactation sufficient to cause moderate anemia but not stunt growth demonstrates that monaminergic metabolism changes occur prior to profound declines in brain iron concentration and is associated with developmental delays. Similar developmental delays in iron deficient human infants suggest to us that alterations in iron status during this developmental period likely affects developing brain monaminergic systems in these infants.

Introduction

Iron deficiency and anemia during infancy have been associated with poorer performance on mental and motor measures and altered social–emotional behavior [18], [49]. Despite iron treatment, developmental alterations persisted in infancy [29], [36], [45] and may continue into adolescence [28]. The neurobiological alterations that account for these findings are not fully delineated or clearly identified in a causal model [5].

Investigators have explored central nervous system (CNS) effects of iron deficiency anemia (IDA) in rodent models for several decades [18], [50] utilizing research designs that incorporate both the pre-weaning and post-weaning effects of iron deficiency [37], [47], [50]. Brain regions particularly iron rich in adulthood (striatum, substantia nigra and deep cerebellar nuclei) are not particularly rich in iron in early development, as the process of regional acquisition of iron appears to be developmentally bound [35]. Thus, dietary iron restriction during early periods of growth and development results in a very different profile of regional brain iron deficits than does iron deficiency during later periods of life. Indeed, our working hypothesis is that a significant portion of the consequences of iron restriction to the brain is likely to be related directly to the developmental biology occurring at the time of this nutrient deficiency.

The direct and indirect effects of iron deficiency are not clear, but likely include effects on cell growth and differentiation [22], cellular bioenergetics [9] and biochemistry [5], [37]. Previous studies identified manifestations of iron deficiency in neurotransmitter systems [8], [11], [12], [30], [50], [51], myelin biology [6], [31], [52], and behavior [6], [10], [11], [13], [34]. Recently, new observations regarding metabolism and dendritic arborization in the hippocampus document lasting effects of pre- and post-natal iron deficiency on brain morphology [22], [37]. Changes in monoamine metabolism can be used as a marker of developmental brain pathology or compensatory reorganization in the forebrain involving other neurotransmitters and cellular and molecular events [32], [38], [41]. Recently published studies indicate that relatively mild early iron deficiency in human infants result in delays in achievement of both short term and long-term developmental milestones [3]. The observations of a paucity of motor movements while iron deficient as infants and poorer executive functioning years later as adolescents may both be explained, in part, by alterations in the monoamergic systems.

Most previous rat animal model studies that examined brain iron, DA, and generally utilized a post-natal model of iron deficiency [4], [11], [12], [30], [50], [51]. Some studies commenced an iron deficient diet at P4 [11], [12], others at P10 [34], [35], others at P21 [4], [6], [50], [51] and all used a severe dietary restriction to produce animals that generally had brain iron losses >50% in many brain regions. Other rodent studies utilized designs of pre-natal iron deficiency to examine the influences of intrauterine iron deficiency on brain development and subsequent functioning [9], [13], [22], [25], [42].

Many of the cited rodent studies utilized designs in which the brain iron deficiency was quite severe and concentrations were reduced by more than 50% in many brain regions. In some studies, the severity was chosen to match the degree of iron deficiency that occurs in human conditions such as intrauterine growth retardation and diabetes mellitus during pregnancy where fetal (but not post-natal) iron balance is compromised [17], [32], [33], [41]. For instance, some infants of diabetic mothers may have a greater than 40% reduction in brain iron concentration and a 60% reduction in liver iron. Studying severe iron deficiency in rodent models has been very useful in defining changes in biology in the extreme. However, such situations fail to answer questions regarding less severe degrees of iron deficiency anemia like that commonly occurring throughout the period of human pregnancy and infancy that result in biologically meaningful alterations in CNS functioning.

The objective of this study was to define the effects of a modest degree of brain iron deficiency during intrauterine life and lactation on regional iron concentrations, monoamine metabolism, and developmental milestones in a rat model. In order to avoid the teratology and profound growth failures of some earlier studies of iron deficiency during gestation in rodents, we utilized a dietary protocol designed to reduce brain iron by 10–20%. The hypothesis was that moderate iron deficiency that persisted through gestation and lactation would alter developmental progress and monoamine metabolism The model was chosen to be relevant to common conditions in the developing world where maternal ID is widespread, infants are likely born with proportionately reduced iron stores, and the iron deficiency continues for the maternal-infant dyadic through the period of lactation.

To test this hypothesis, we used a developmental model of iron deficiency in rodents designed (by dietary means) to maintain a moderate level of anemia in the pups during lactation.