Neuropeptide changes in aging and Alzheimer's disease

Innate Receptors Expression by Lung Nociceptors: Impact on COVID-19 and Aging

The lungs are constantly exposed to non-sterile air which carries harmful threats, such as particles and pathogens. Nonetheless, this organ is equipped with fast and efficient mechanisms to eliminate these threats from the airways as well as prevent pathogen invasion. The respiratory tract is densely innervated by sensory neurons, also known as nociceptors, which are responsible for the detection of external stimuli and initiation of physiological and immunological responses. Furthermore, expression of functional innate receptors by nociceptors have been reported; however, the influence of these receptors to the lung function and local immune response is poorly described. The COVID-19 pandemic has shown the importance of coordinated and competent pulmonary immunity for the prevention of pathogen spread as well as prevention of excessive tissue injury. New findings suggest that lung nociceptors can be a target of SARS-CoV-2 infection; what remains unclear is whether innate receptor trigger sensory neuron activation during SARS-CoV-2 infection and what is the relevance for the outcomes. Moreover, elderly individuals often present with respiratory, neurological and immunological dysfunction. Whether aging in the context of sensory nerve function and innate receptors contributes to the disorders of these systems is currently unknown. Here we discuss the expression of innate receptors by nociceptors, particularly in the lungs, and the possible impact of their activation on pulmonary immunity. We then demonstrate recent evidence that suggests lung sensory neurons as reservoirs for SARS-CoV-2 and possible viral recognition via innate receptors. Lastly, we explore the mechanisms by which lung nociceptors might contribute to disturbance in respiratory and immunological responses during the aging process.

Distinct modulation of microglial amyloid β phagocytosis and migration by neuropeptides (i)

Microglial activation plays an integral role in the development and course of neurodegeneration. Although neuropeptides such as bradykinin (BK), somatostatin (SST), and endothelin (ET) are known to be important mediators of inflammation in the periphery, evidence of a similar function in brain is scarce. Using immunocytochemistry, we demonstrate the expression of receptors for BK (B1, B2 subtypes), ET (ETA, ETB subtypes) and SST (SST 2, 3, 4 subtypes) in primary microglia and microglial cell lines. Exposure of BV2 and N9, as well as primary microglial cells to BK or SST increased Aβ uptake in a concentration-dependent manner, whereas endothelin decreased Aβ uptake. This was caused by increased phagocytosis of Aβ since the rate of intracellular Aβ degradation remained unaffected. All neuropeptides increased chemotactic activity of microglia. In addition, BK reduced Aβ-induced expression of proinflammatory genes including iNOS and COX-2. ET decreased the Aβ-induced expression of monocyte chemoattractant protein 1 and interleukin-6. These results suggest that neuropeptides play an important role in chemotaxis and Aβ clearance and modulate the brain's response to neuroinflammatory processes.

Alzheimer's disease, neuropeptides, neuropeptidase, and amyloid-beta peptide metabolism

Amyloid-beta peptide (Abeta), the pathogenic agent of Alzheimer's disease (AD), is a physiological metabolite in the brain. We have focused our attention and effort on elucidating the unresolved aspect of Abeta metabolism: proteolytic degradation. Among a number of Abeta-degrading enzyme candidates, we used a novel in vivo paradigm to identify a member of the neutral endopeptidase family, neprilysin, as the major Abeta catabolic enzyme. Neprilysin deficiency results in defects in the metabolism of endogenous Abeta 40 and 42 in a gene dose-dependent manner. Our observations suggest that even partial down-regulation of neprilysin activity, which could be caused by aging, can contribute to AD development by promoting Abeta accumulation. Moreover, we discuss the fact that an aging-dependent decline of neprilysin activity, which leads to elevation of Abeta concentrations in the brain, is a natural process that precedes AD pathology. In this Perspective, we hypothesize that neprilysin down-regulation has a role in sporadic AD (SAD) pathogenesis, and we propose that this knowledge be used for developing preventive and therapeutic strategies through use of a G protein-coupled receptor (GPCR).

Activation and degeneration during aging: a morphometric study of the human hypothalamus

During the course of aging both activation and degenerative changes are found in the human hypothalamus. Degeneration may start around middle-age in some neurotransmitter- or neuromodulator-containing neurons. For instance, a decreased number of vasoactive intestinal polypeptide (VIP) neurons was observed in the suprachiasmatic nucleus (SCN) of middle-aged males. The normal circadian fluctuations seen in the number of vasopressin (AVP) neurons in the SCN of young subjects diminished in subjects older than 50 years. Moreover, a sharp decline in cell number was found in the sexually dimorphic nucleus (SDN) after 50 years in males. On the other hand, many hypothalamic systems remain perfectly intact during aging like the oxytocin (OXT) neurons in the paraventricular nucleus (PVN). The AVP neurons in the PVN are activated during aging as appears from their increasing cell number. Also the corticotrophin-releasing hormone (CRH) neurons of the PVN are activated in the course of aging, as indicated by their increased number and their increased AVP coexpression. Part of the infundibular nucleus, the subventricular nucleus, contains hypertrophic neurokinin B neurons in postmenopausal women. It can be concluded that a multitude of changes in the various hypothalamic nuclei may be the biological basis for many functional changes in aging, i.e., both endocrine and central alterations, and that only a minority of the possible human hypothalamic changes have so far been studied.

Are sleep disorders hereditary? A questionnaire survey of elderly persons about themselves and their parents

The aim of the present study was to investigate the occurrence of sleep complaints in an elderly population in relation to the sleep pattern in their parents. The study is based on a postal questionnaire, which was sent to 10,216 elderly persons in northern Sweden. The mean (s.d.) ages of the participating men and women were 73.0 (6.0) and 72.6 (6.7) years, respectively. Among the men whose parents had no sleep problems, a good night's sleep was reported by 11.1%, and among those whose parents both had such problems, this proportion was 18.9%. The corresponding figures for women were 20.6% and 60.0%, respectively. Restless sleep, frequent awakening, difficulty in getting to sleep again after waking up in the night as well as tiredness at night but with inability to get to sleep was more common in both sexes if either of the parents had difficulty in sleeping, and even more common if such problems had occurred in both parents. Also visits to doctors because of sleep problems and treatment with sleeping pills were more common in both men and women whose parents had had sleep problems, compared with those whose parents had been good sleepers. Thus, a poor sleep was more common in elderly persons if their parents had sleep problems, compared with those whose parents had been good sleepers. The results suggest that hereditary factors could be an underestimated cause of sleep disorders in the elderly.

Light and electron microscopic analysis of two divisions of the suprachiasmatic nucleus in the young and aged rat

The suprachiasmatic nucleus (SCN) is a principal controller of mammalian circadian rhythms. However, in spite of documented disturbance of biological rhythms in old animals, few significant age-related changes have been observed in this nucleus. This study examined age-related differences in SCN volume, neuronal number, density, and ultrastructural features in the entire rat SCN and in its two divisions, the denser ventromedial (compacta) and less dense dorsolateral (dissipata). Light and electron microscopic morphometric techniques were utilized in weanlings (21-28 days), young adults (3-6 mo), and aged (30-36 mo) animals. The total SCN volume, as well as volumes of the compacta region, were significantly greater in young adult and aged rats than in weanlings. Thus, as the rat ages the SCN increases in total size. However, the dissipata region appears to decrease in volume while the compacta increases. Even though the total number of SCN neurons was quite constant in the three age groups, the number of neurons in the dissipata region was decreased significantly in the young adult and aged groups as compared to the weanling. Neurons in the compacta region were usually spindled-shaped with two dendritic processes, while oval to spheroidal cells with 3-4 processes predominated in the dissipata. Nuclei of SCN cells were often invaginated. In weanlings, more SCN neuronal nuclei had invaginated nuclei in the dissipata region (66%) compared to the compacta (37%). In the two older age groups of rats, a higher percentage of invaginated neuronal nuclei were found in both regions. However, more were still found in the dissipata (90%) compared to the compacta (72%), even though the number of these cells in the compacta doubled. Thus, there was a large increase in the number of invaginated nuclei, as well as the number of invaginations, in the young adult rats compared to the weanling group, and this increase persisted in aged rats. SCN neurons usually had nuclei surrounded by a thin perimeter of cytoplasm containing sparse mitochondria and granular endoplasmic reticulum, multiple Golgi regions, and a moderate number of free ribosomes. In weanlings, mitochondria contained dense cristae and the granular endoplasmic reticulum was relatively prominent. Degenerative ultrastructural changes which included mitochondrial enlargement/vacuolation, Golgi vacuolation, lysosome, and lipofuscin development occurred in less than 10% of young adult SCN cells, and were more frequently found in the dissipata. In aged, rats 30% of the neurons showed degenerative changes in the dissipata compared with 18% in the compacta. Degenerative changes appeared highly correlated with the degree of membrane folding.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuropeptides as psychotropic drugs

Neuropeptides are endogenous substances present in nerve cells and involved in nervous system functions. Neuropeptides are synthetized in large precursor proteins and several are formed in the same precursor. Neuropeptides affect learning and memory processes, social, sexual and maternal behavior, pain and addiction, body temperature, food and water intake e.a. In addition, neuropeptides possess trophic influences on the nervous system, neuroleptic-like andpsychostimulant-like activities. Disturbances in classical neurotransmitter activity as found in Parkinson's disease, psychoses, and dementia, may also be caused by disturbances in neuropeptide activity. In fact, alterations in the concentration of a number of neuropeptides in schizophrenia, depression, and dementia have been found. Much work has been done during the last decade on the influence of neuropeptides in schizophrenia, autism, depression, and in various disorders associated with memory disturbances. These studies concern neuropeptides related to adrenocorticotropic hormone (ACTH) and melanocyte stimulating hormone (MSH), vasopressin- and endorphin-type neuropeptides, thyrotropic releasing hormone (TRH), and the C-terminal part of oxytocin Pro-Leu-Gly-NH2 (PLG). Several of these exert positive effects but in not more than 25% the response is clinically relevant. This may have to do with the severity of the disease and its chronicity. The modest effects may also be caused by the poor bioavailability of peptides and insufficient pharmacotherapeutic experience regarding dose, and duration of treatment.

Neuronal plasticity and astrocytic reaction in Down syndrome and Alzheimer disease

Proteins relatively enriched in neurons (neural cell adhesion molecule (NCAM) and D3-protein) or in glia (glutamine synthetase, glial fibrillary acidic protein (GFAP) and S100) were measured by quantitative immunochemical methods in autopsy samples of the cerebral cortex of subjects with Alzheimer disease (AD) and adults with Down syndrome (DS), the latter also presenting manifest signs of Alzheimer type of neuropathology. The trend of changes was similar in AD and DS, but more marked in the latter. The biochemical make-up of astrocytes was differentially affected: in both the frontal and DS temporal cortex the specific concentration of glutamine synthetase was unaltered, while that of S100 and the soluble form of GFAP was markedly elevated (about 260% and 690% of control values, respectively). In the AD frontal cortex the estimates for glutamine synthetase were normal, while S100 and GFAP were about 180% and 230% of control. The observations (normal GS and elevated levels of the other markers) might suggest that the pathological changes involve a differentiated astrocytic reaction and that the astrocytic reaction is more marked in DS than in AD. In DS the increase in S100 could be explained, in part, by a gene dosage effect and in part by reactive gliosis. The neuronal markers were also differentially affected. In comparison with appropriate controls, the concentration of D3-protein in frontal cortex was decreased by 24% in DS and by 14% in AD, whereas NCAM levels were not significantly affected. The ratio of NCAM to D3-protein was significantly increased by 32% and 8.5% in DS and AD, respectively. These observations are consistent with the view that the destruction of mature neuronal structures (as marked by the D-3 protein) coincides with the formation of new neuronal membranes (as indicated by NCAM), i.e. in these degenerative disorders plastic changes are taking place involving cerebral cortex neurons in which trophic substances may be instrumental.

Vasopressin response to dehydration in Alzheimer's disease

Alzheimer's disease is a progressive deterioration of neuropsychological functioning. One of the main neuropathological correlates of the disease is a drop-out of cholinergic neurons within the central nervous system. The neuropeptide that is responsible for water homeostasis and defense against dehydration, vasopressin, is also under direct cholinergic control. Several studies have suggested that in Alzheimer's disease there has been a trend toward lower vasopressin levels than in age-matched controls. In order to improve discrimination of normal from diminished vasopressin levels, nine subjects with Alzheimer's disease (mean age 65 +/- 2 years) and nine age- and sex-matched controls (68 +/- 3 years) underwent a mild provocative challenge of overnight fluid restriction. Individuals with Alzheimer's disease had a greater degree of dehydration, with overnight serum osmolality of 313 +/- 4 vs 300 +/- 3 Mosmol/kg, P = .01, and diminished "thirst" as measured by water ingested in one hour of ad libitum water intake. Eight of the nine with Alzheimer's disease had levels of vasopressin which, by extrapolation, appear to be subnormal for their serum osmolalities, whereas seven of the nine control subjects has vasopressin levels within or above the reference range (P less than .05). Elderly individuals with Alzheimer's disease may be at increased risk of dehydration during periods of fluid restriction due to the loss of normal physiological responses of "thirst" and vasopressin secretion.