The Aging Brain

Immune and behavioral consequences of microglial reactivity in the aged brain

Bidirectional communication between the immune system and the brain is essential for mounting the appropriate immunological, physiological, and behavioral responses to immune activation. Aging, however, may impair this important bi-directional interaction. In support of this notion, peripheral infection in the elderly is associated with an increased frequency of behavioral and cognitive complications. Recent findings in animal models of aging and neurodegenerative disease indicate that microglia, innate immune cells of the brain, become primed or reactive. Understanding age- and disease-associated alterations in microglia is important because glia (microglia and astrocytes) play an integral role in propagating inflammatory signals that are initiated in the periphery. In this capacity, brain glia produce inflammatory cytokines that target neuronal substrates and elicit a sickness-behavior syndrome that is normally beneficial to the host organism. Increased reactivity of microglia sets the stage for an exaggerated neuroinflammatory cytokine response following activation of the peripheral innate immune system, which may underlie subsequent long-lasting behavioral and cognitive deficits. In support of this premise, recent findings indicate that stimulation of the peripheral immune system in aged rodents causes exaggerated neuroinflammation that is paralleled by cognitive impairment, prolonged sickness, and depressive-like complications. Therefore, the purpose of this review is to discuss the new evidence that age-associated priming of microglia could play a pathophysiological role in exaggerated behavioral and cognitive sequelae to peripheral infection.

Globus pallidus glial pigment and its changes with age and chronic illness in childhood

Glial lipopigment appears in the globus pallidus without accumulating in neurons (except for late adolescence) in multiple chronic childhood diseases. In this observational study, we compared the age-related development of glial pigmentation in children with the chronic illness (cystic fibrosis) and children dying acutely. A secondary goal was to search for pallidal neuronal lipopigment in childhood. We recorded pigmentation in the brains of 37 consecutive cystic fibrosis children ranging in age from 0-23 yr and in 17 controls ranging in age from 0-18 yr. We characterized the lipofuscin histochemically and used several regression models to describe the mode of deposition. We observed that in the controls, intraglial pallidal pigment accumulated in 2 forms (relatively large globules and, separately, as clusters of fine granules) at a slow rate during childhood. In cystic fibrosis, both forms of pallidal glial pigment started accumulating at a younger age and were deposited far more rapidly. There was a further increase in the rate of accumulation between 8 and 10 yr of age. We did not encounter pallidal neuronal lipofuscin at any age. These observations are consistent with 2 propositions: 1) that globus pallidus glial cells are unique in their ability to accumulate lipofuscin before it accumulates in nearby neurons; and 2) that they are particularly susceptible to some systemic effect of this chronic illness.

Dizocilpine binding to cerebral cortical membranes from developing and ageing mice

The binding of [3H]dizocilpine (MK-801) to the N-methyl-D-aspartate (NMDA)-gated ion channel was characterized in cerebral cortical membranes during the major portion of the mouse life-span (from 7-day- to 22-month-olds). The binding was saturable, consisting of only one component at all ages studied. The maximal binding capacity Bmax was very substantial in 14-day-old mice when compared to adults (3-month-olds), decreasing thereafter during ageing. The binding constant KD remained unchanged during development and increased only slightly in aged mice. Glutamate and glycine potentiated dizocilpine binding concentration-dependently. Their efficacy varied markedly with age. Both glutamate and glycine had considerably less effect on the immature cerebral cortex and in the oldest group of mice (22-month-old) than in young adults. The marked increase in dizocilpine binding sites at the age of 2 weeks coincides with the previously reported transient increase in NMDA binding sites in the cerebral cortex. The weak potentiation of dizocilpine binding by glutamate and glycine in the immature brain could be a factor which protects neurons during this period from excitotoxicity and increased susceptibility to seizures induced by acidic amino acids. The decrease in the number of dizocilpine binding sites during ageing could result partly from the loss of cortical neurons.

The vestibular system in the elderly: clinical implications

Cell loss, changes in synapse morphology, electrophysiologic alterations, and changes in the supporting microenvironment have all been noted in portions of the vestibular systems of aged animals and humans. Increased variability with age is also a prominent finding in many studies. Quantitative vestibular testing in humans has shown alterations with age as well, but decline with aging is not a prominent feature of all measures, and many reported studies are methodogically flawed. The connection between these observed aging changes and the increased incidence of dizziness and falls in the elderly is unproven, however, and clinicians should search carefully for specific disease processes in their elderly patients who present with vestibular symptoms.