Decrease in neuron size in docosahexaenoic acid-deficient brain

Abstract

Docosahexaenoic acid is an important fatty acid for neuronal function because its deficiency leads to many behavioral and functional deficits. In a previous study, we reported that docosahexaenoic acid deficiency caused a reduction in the size of neurons of the CA1 region in the hippocampus. To extend these results to other regions of the brain, the present study entailed a morphologic analysis of neuronal size in hippocampus, hypothalamus, piriform cortex, and parietal cortex in rats that were raised on docosahexaenoic acid-deficient and supplemented diets for three generations. Neuron size in these regions was measured both at weaning (21 days) and maturity (68 days), and docosahexaenoic acid content in the brain was measured on a separate set of sibling rats using fatty acid analysis. Neuron size in hippocampus, hypothalamus, and parietal cortex decreased in weanling and in piriform cortex in mature rats raised on the docosahexaenoic acid-deficient diet. The brains of these rats exhibited a nearly 90% decrease of docosahexaenoic acid. Decrease of neuron size has been linked to a loss of optimal function in neurons. In the United States, human infant-milk formulas use vegetable oils as fat sources that lack docosahexaenoic acid. If docosahexaenoic acid deficiency reduces neuron size, then human infants raised on these formulas may also have smaller neurons relative to breast-fed infants.

Introduction

Docosahexaenoic acid (DHA, 22:6n-3) is a major component of membrane phospholipids in nerve cells [1], [2], [3], [4]. DHA rapidly accumulates in the brain through in utero nutrition, mother’s milk, and diet during perinatal and early postnatal life [5], [6], [7]. When n-3 fatty acid-deficient diets are fed to animals for two to three generations, a marked decrease in nervous system DHA results [8], [9], [10], [11], [12]. Loss of brain DHA results in the loss of many sensory, behavioral, and cognitive functions both in animals and humans. These include loss in visual sensitivity [10], [13], [14], visual acuity [15], [16], [17], visual attention and visual recognition memory [18], dark adaptation responses [19], two-odor discriminations [20], [21], and spatial navigation memory [12], [20], [22], [23], [24], [25]. Supplementation of DHA is known to slowly restore the levels of brain docosahexaenoic acid in animals that were previously raised on DHA-deficient diets [26], [27], [28], [29]. DHA-repleted animals can recover from deficits in spatial tasks [29]; however, electroretinographic parameters and mean arterial blood pressure do not normalize even after DHA is repleted [30], [31].

The present understanding of the impact of a loss of brain DHA on brain anatomy and morphology is minimal. Apart from two previous morphometric studies involving DHA-deficient hippocampus [32] and DHA-supplemented cortical neurons in culture [33], insights into morphologic changes in the DHA-deficient brain comes from ultrastructural studies in which synaptic vesicle number [34], [35] or receptor density [36] were analyzed after n-3 deficiency. Our previous study also examined a number of other morphologic parameters in the hippocampus, including layer volume, density, and neuron number. Although no differences were detected in these parameters [32], further studies with larger sample sizes are required to demonstrate that conclusively.

This study was conducted to investigate whether changes in the neuronal size were limited to the hippocampus or were a more general feature of brain. To address this question, the hippocampus, hypothalamus, piriform cortex (endopiriform cortex), and parietal cortex were selected for neuron size analysis. In addition, the number of animals in both DHA-deficient and -supplemented diets were increased so that statistical power is augmented relative to the initial study.