Age-related alterations of isoproterenol-stimulated adenylyl-cyclase activity are partially corrected by thymic graft

Age-related circadian disorganization caused by sympathetic dysfunction in peripheral clock regulation

The ability of the circadian clock to adapt to environmental changes is critical for maintaining homeostasis, preventing disease, and limiting the detrimental effects of aging. To date, little is known about age-related changes in the entrainment of peripheral clocks to external cues. We therefore evaluated the ability of the peripheral clocks of the kidney, liver, and submandibular gland to be entrained by external stimuli including light, food, stress, and exercise in young versus aged mice using in vivo bioluminescence monitoring. Despite a decline in locomotor activity, peripheral clocks in aged mice exhibited normal oscillation amplitudes under light–dark, constant darkness, and simulated jet lag conditions, with some abnormal phase alterations. However, age-related impairments were observed in peripheral clock entrainment to stress and exercise stimuli. Conversely, age-related enhancements were observed in peripheral clock entrainment to food stimuli and in the display of food anticipatory behaviors. Finally, we evaluated the hypothesis that deficits in sympathetic input from the central clock located in the suprachiasmatic nucleus of the hypothalamus were in part responsible for age-related differences in the entrainment. Aged animals showed an attenuated entrainment response to noradrenergic stimulation as well as decreased adrenergic receptor mRNA expression in target peripheral organs. Taken together, the present findings indicate that age-related circadian disorganization in entrainment to light, stress, and exercise is due to sympathetic dysfunctions in peripheral organs, while meal timing produces effective entrainment of aged peripheral circadian clocks.

Insulin receptors in mouse brain: Reversibility of age-related impairments by a thymic extract

Recently, we have shown that insulin receptors (InsRs) in the brain undergo impairments with aging. Interestingly, age-related alterations of brain InsRs, are not irreparable as thymus grafts are able to recover them. With the present study we verified the possibility that an aqueous extract from calf thymus (TME) can mimic the restoring action of age-related impairments induced by thymus graft. InsR characteristics were assayed in a group of 25 months old BALB/c-nu mice treated with TME: 2μg/g body weight every third day, for total five subcutaneous injections. The last dose was injected the day before animals were killed. Other two groups of young (4 months) and old (25 months) mice received saline solution with the same schedule. A two-sites model analysis of receptor data confirms the age-dependent decrease of InsR number and kd previously observed in the high affinity population. Furthermore, a statistically significant recovery of number impairment is shown in TME-treated animals. On the contrary, the characteristics of the low affinity receptor subset show no statistically significant differences among the three animal models studied. TME induced recovery of the age-related changes found in brain InsRs, together with previously observed regulatory action of the same thymic extract on the adrenergic system, suggest that thymic gland does not necessarily have to mutually interact with other controlling systems for maintaining or recoving homeostasis of the complex neuroendocrine network during development and aging.

Ageing as upregulation failure

This paper investigates similarities in physiological senescent changes in various mammalian organ systems and relates these to postulated mechanisms of age modulation. It proposes that the observed decline in homeostasis with age is due to an inability of target cells, of neural, endocrine and other forms of control, to upregulate after their downregulation. The well-documented flattening of circadian rhythms with age, particularly the loss of the sustained diurnal hypothermic phase, is described as integral to such change, and maintenance or induction of certain youthful circadian patterns is proposed as a means of restoring homeostasis in senescence and other conditions.

Parallel decrease of adenylyl cyclase activity and β1-adrenoceptor density in brain cortex of aging mice

Beta-adrenergic receptors (betaARs) and adenylyl cyclase (AC) activity have been found to undergo progressive alterations in the brain cortex of aging mice. In particular, betaAR changes are in charge of the beta1 subpopulation (beta1AR), beta2-adrenoceptors (beta2ARs) showing no age-related impairments. On these bases, the question arises whether AC alterations can be accounted for by beta1AR changes or, alternatively, whether there are also receptor independent AC modifications. Experiments have thus been performed on the brain cortex from young and old mice assaying beta1AR and beta2AR characteristics and isoproterenol (IPR) and forskolin stimulated AC activity in the same membrane preparations. Other membranes were previously treated with the reducing agent dithiothreitol (DTT) which is known to specifically destroy the binding capacity of beta1ARs. No statistically significant differences in basal AC activity have been found among the groups studied. Data on IPR-stimulated AC activity in old animals confirm previous findings demonstrating impaired responsiveness when compared with that of young mice. DTT treated membrane preparations from both young and old mice show decreased AC activities. The entity of the decrease is lower in aged animals due to the lower initial level of beta1ARs. Forskolin stimulation, which is assumed to directly activate AC, has been found impaired in aged mice when compared with young animals, suggesting that also post-receptor alterations occur with advancing age.