Cellular and synaptic alterations in the aging brain

Chronic Sustained Hypoxia Leads to Brainstem Tauopathy and Declines the Power of Rhythms in the Ventrolateral Medulla: Shedding Light on a Possible Mechanism

Hypoxia, especially the chronic type, leads to disruptive results in the brain that may contribute to the pathogenesis of some neurodegenerative diseases such as Alzheimer's disease (AD). The ventrolateral medulla (VLM) contains clusters of interneurons, such as the pre-Bötzinger complex (preBötC), that generate the main respiratory rhythm drive. We hypothesized that exposing animals to chronic sustained hypoxia (CSH) might develop tauopathy in the brainstem, consequently changing the rhythmic manifestations of respiratory neurons. In this study, old (20-22 months) and young (2-3 months) male rats were subjected to CSH (10 ± 0.5% O2) for ten consecutive days. Western blotting and immunofluorescence (IF) staining were used to evaluate phosphorylated tau. Mitochondrial membrane potential (MMP or ∆ψm) and reactive oxygen species (ROS) production were measured to assess mitochondrial function. In vivo diaphragm's electromyography (dEMG) and local field potential (LFP) recordings from preBötC were employed to assess the respiratory factors and rhythmic representation of preBötC, respectively. Findings showed that ROS production increased significantly in hypoxic groups, associated with a significant decline in ∆ψm. In addition, tau phosphorylation elevated in the brainstem of hypoxic groups. On the other hand, the power of rhythms declined significantly in the preBötC of hypoxic rats, parallel with changes in the respiratory rate, total respiration time, and expiration time. Moreover, there was a positive and statistically significant correlation between LFP rhythm's power and inspiration time. Our data showed that besides CSH, aging also contributed to mitochondrial dysfunction, tau hyperphosphorylation, LFP rhythms' power decline, and changes in respiratory factors.

Age-related alteration of PKC, a key enzyme in memory processes: physiological and pathological examples

Brain aging is characterized by a progressive decline of the cognitive and memory functions. It is becoming increasingly clear that protein phosphorylation and, in particular, the activity of the calcium-phospholipid-dependent protein kinase C (PKC) may be one of the fundamental cellular changes associated with memory function. PKC is a multigene family of enzymes highly expressed in brain tissues. The activation of kinase C is coupled with its translocation from the cytosol to different intracellular sites and recent studies have demonstrated the key role played by several anchoring proteins in this mechanism. PKC-phosphorylating activity appears to be impaired during senescence at brain level in a strain-dependent fashion in rodents. Whereas the levels of the various isoforms do not show age-related alterations, the enzyme translocation upon phorbol-ester treatment is deficitary among all strains investigated. Anchoring proteins may contribute to this activation deficit. We discuss also modifications of the PKC system in Alzheimer's disease that may be related to pathological alterations in neurotransmission. A better insight of the different factors controlling brain-PKC activation may be important not only for elucidating the molecular basis of neuronal transmission, but also for identifying new approaches for correcting or even preventing age-dependent changes in brain function.

Passive temperature lability in the elderly

Thermoregulatory responses of nine healthy elderly [seven men and two women; mean age (SD) 73.9 (4.8) years] were compared to those of nine young adult men [26.6 (5.2) years]. They exercised on a cycle ergometer for 20 min at an intensity inducing a heart rate equivalent to 65% of their predicted maximum, and were thereafter immersed in 28 degrees C water. The exercise was conducted to elevate tympanic temperature (Tty) and initiate a steady rate of sweating. The post-exercise immersion period induced gradual cooling of Tty, and changes in Tty relative to resting levels (delta Tty) at which sweating abated and shivering commenced were defined as the delta Tty thresholds for the cessation of sweating (Tsw) and onset of shivering (Tsh), respectively. In addition to Tty, oxygen uptake (VO2; l.min-1), sweating rate (g.m-2.min-1), and forehead skin blood perfusion were also measured during the trials. The mean (SD) Tsw occurred at a significantly (P < 0.005) higher delta Tty [0.48 (0.18) degrees C] in the elderly than in the young adults [0.21 (0.06) degrees C], while the Tsh occurred at significantly (P < 0.005) lower delta Tty in the elderly [-0.64 (0.34) degrees C] than in young adults [-0.22 (0.10) degrees C]. Decreases in delta Tty below the shivering threshold were met with a significantly (P < 0.01) reduced VO2. The range of temperature lability between Tsw and Tsh, defined as the null-zone, was significantly greater in the elderly [1.12 (0.39) degrees C] than in the young adults [0.43 (0.12) degrees C], and the slope of the vasoconstrictor response in the null-zone was significantly (P < 0.001) lower in the elderly subjects. The present study demonstrates a greater passive core temperature lability in older individuals, since the effector responses of sweating and shivering were initiated at higher and lower levels of Tty, respectively. The magnitudes of the effector responses beyond the thresholds were also significantly reduced, suggesting that the elderly may be more susceptible to hypo-/hyperthermia during periods of endogenous and/or exogenous thermal stress.

Circadian secretion of melatonin and thyrotropin in hospitalized aged patients

Alterations in periodical functions are known to occur in aging and may be regarded as markers of the aging process itself. Melatonin and Thyroid Stimulating Hormone (TSH) circadian periodicities were studied in 22 aged subjects and in 13 adult controls. The study of rhythmicity was performed by the Cosinor analysis. Elderly subjects were hospitalized because of various concomitant diseases. Circadian periodicity of both hormones was disrupted in the aged group, and the deterioration of melatonin periodicity was significantly correlated with the decay in cognitive functions, quantified by the Mini Mental State evaluation. Diabetes was also found to affect, though not significantly, melatonin, but not TSH, periodicity. Melatonin and TSH nocturnal peaks were decreased in aged people. TSH oscillation amplitudes were inversely correlated with age. No correlation was found between melatonin and TSH secretory features both in adult and in aged subjects.