Age-related change of neuropeptide Y-immunoreactive neurons in the rat anterior olfactory nucleus

Spread of aggregates after olfactory bulb injection of α-synuclein fibrils is associated with early neuronal loss and is reduced long term

Parkinson’s disease is characterized by degeneration of substantia nigra dopamine neurons and by intraneuronal aggregates, primarily composed of misfolded α-synuclein. The α-synuclein aggregates in Parkinson’s patients are suggested to first appear in the olfactory bulb and enteric nerves and then propagate, following a stereotypic pattern, via neural pathways to numerous regions across the brain. We recently demonstrated that after injection of either mouse or human α-synuclein fibrils into the olfactory bulb of wild-type mice, α-synuclein fibrils recruited endogenous α-synuclein into pathological aggregates that spread transneuronally to over 40 other brain regions and subregions, over 12 months. We previously reported the progressive spreading of α-synuclein aggregates, between 1 and 12 months following α-synuclein fibril injections, and now report how far the pathology has spread 18- and 23-month post-injection in this model. Our data show that between 12 and 18 months, there is a further increase in the number of brain regions exhibiting pathology after human, and to a lesser extent mouse, α-synuclein fibril injections. At both 18 and 23 months after injection of mouse and human α-synuclein fibrils, we observed a reduction in the density of α-synuclein aggregates in some brain regions compared to others at 12 months. At 23 months, no additional brain regions exhibited α-synuclein aggregates compared to earlier time points. In addition, we also demonstrate that the induced α-synucleinopathy triggered a significant early neuron loss in the anterior olfactory nucleus. By contrast, there was no loss of mitral neurons in the olfactory bulb, even at 18 month post-injection. We speculate that the lack of continued progression of α-synuclein pathology is due to compromise of the neural circuitry, consequential to neuron loss and possibly to the activation of proteolytic mechanisms in resilient neurons of wild-type mice that counterbalances the spread and seeding by degrading pathogenic α-synuclein.

Neuropeptide Y: An Anti-Aging Player?

Accumulating evidence suggests that neuropeptide Y (NPY) has a role in aging and lifespan determination. In this review, we critically discuss age-related changes in NPY levels in the brain, together with recent findings concerning the contribution of NPY to, and impact on, six hallmarks of aging, specifically: loss of proteostasis, stem cell exhaustion, altered intercellular communication, deregulated nutrient sensing, cellular senescence, and mitochondrial dysfunction. Understanding how NPY contributes to, and counteracts, these hallmarks of aging will open new avenues of research on limiting damage related to aging.

Aging in the olfactory system

With advancing age, the ability of humans to detect and discriminate odors declines. In light of the rapid progress in analyzing molecular and structural correlates of developing and adult olfactory systems, the paucity of information available on the aged olfactory system is startling. A rich literature documents the decline of olfactory acuity in aged humans, but the underlying cellular and molecular mechanisms are largely unknown. Using animal models, preliminary work is beginning to uncover differences between young and aged rodents that may help address the deficits seen in humans, but many questions remain unanswered. Recent studies of odorant receptor (OR) expression, synaptic organization, adult neurogenesis, and the contribution of cortical representation during aging suggest possible underlying mechanisms and new research directions.

New insights into brain BDNF function in normal aging and Alzheimer disease

The decline observed during aging involves multiple factors that influence several systems. It is the case for learning and memory processes which are severely reduced with aging. It is admitted that these cognitive effects result from impaired neuronal plasticity, which is altered in normal aging but mainly in Alzheimer disease. Neurotrophins and their receptors, notably BDNF, are expressed in brain areas exhibiting a high degree of plasticity (i.e. the hippocampus, cerebral cortex) and are considered as genuine molecular mediators of functional and morphological synaptic plasticity. Modification of BDNF and/or the expression of its receptors (TrkB.FL, TrkB.T1 and TrkB.T2) have been described during normal aging and Alzheimer disease. Interestingly, recent findings show that some physiologic or pathologic age-associated changes in the central nervous system could be offset by administration of exogenous BDNF and/or by stimulating its receptor expression. These molecules may thus represent a physiological reserve which could determine physiological or pathological aging. These data suggest that boosting the expression or activity of these endogenous protective systems may be a promising therapeutic alternative to enhance healthy aging.

Reproduction phase-related variations in neuropeptide Y immunoreactivity in the olfactory system, forebrain, and pituitary of the female catfish,Clarias batrachus (Linn.)

The aim of this study was to determine whether neuropeptide Y (NPY) immunoreactivity in the cells and fibers in the forebrain and pituitary of Clarias batrachus is linked to the annual reproductive cycle. A steady rise in luteinizing hormone (LH) immunoreactivity was seen in the pituitary through preparatory (February-April) and prespawning (May-June) phases; it was greatly reduced during spawning (July-August; P < 0.001) and partially replenished during postspawning (September-November; P < 0.01) through resting (December-January) phases. Although NPY immunoreactivity in olfactory receptor neurons and olfactory nerve layer in olfactory bulb was gradually augmented during resting through prespawning phases (P < 0.001), attaining a peak in spawning phase (P < 0.001), a dramatic decline was encountered during postspawning phase (P < 0.001). A similar pattern was also observed in NPY-containing fibers of the medial olfactory tract (MOT) and pituitary. However, a different pattern of NPY immunoreactivity was observed in the neurons of nucleus entopeduncularis (NE) and nucleus preopticus periventricularis (NPP). Whereas these neurons and fibers in the forebrain showed significant augmentation during the resting through prespawning phases (P < 0.001), the immunoreactivity dramatically declined during spawning (P < 0.001) and was partially replenished in the postspawning phase. Testosterone injection of juveniles significantly augmented (P < 0.001) NPY immunoreactivity in NE neurons. We suggest that NPY cells of NE and NPP, and related fiber systems, might be involved in processing of sex steroid-borne information and regulation of the gonadotropin-releasing hormone-LH axis.

A field guide to the anterior olfactory nucleus (cortex)

While portions of the mammalian olfactory system have been studied extensively, the anterior olfactory nucleus (AON) has been relatively ignored. Furthermore, the existing research is dispersed and obscured by many different nomenclatures and approaches. The present review collects and assembles the relatively sparse literature regarding the portion of the brain situated between the olfactory bulb and primary olfactory (piriform) cortex. Included is an overview of the area's organization, the functional, morphological and neurochemical characteristics of its cells and a comprehensive appraisal of its efferent and afferent fiber systems. Available evidence suggests the existence of subdivisions within the AON and demonstrates that the structure influences ongoing activity in many other olfactory areas. We conclude with a discussion of the AON's mysterious but complex role in olfactory information processing.

Age-related changes of γ-aminobutyric acid transaminase immunoreactivity in the hippocampus and dentate gyrus of the Mongolian gerbil

We investigated the age-related changes of gamma-aminobutyric acid transaminase (GABA-T, a GABA degradation enzyme) in the hippocampus and dentate gyrus of the gerbil at postnatal month 1 (PM 1), PM 3, PM 6, PM 12, and PM 24. Age-related changes of GABA-T immunoreactivity were distinct in the hippocampal CA1 region and in the dentate gyrus. GABA-T immunoreactivity was weak at PM 1, but at PM 3, it had increased significantly, and then increased further. Between PM 6 and PM 12, strong GABA-T immunoreactivity was found in nonpyramidal cells (GABAergic) in the stratum pyramidale of the CA1 region, and at PM 6, strong GABA-T immunoreactivity was found in neurons of the dentate gyrus subgranular zone. At PM 24, CA1 pyramidal cells showed strong GABA-T immunoreactivity. Western blot analysis showed a pattern of GABA-T expression similar to that shown by immunohistochemistry at various ages. In conclusion, our results suggest that the age-related changes of GABA-T provide important information about the aged brain with GABA dysfunction.

Age-related change of somatostatin-immunoreactive neurones in the main olfactory bulb of the rat

Somatostatin is found in the olfactory system, including the main olfactory bulb (MOB), and is thought to be one of the neuroactive substances for olfaction. However, somatostatin immunoreactivity in the olfactory system has not been determined during ageing. Hence, we examined the age-related changes of somatostatin-immunoreactive (IR) neurones in the rat MOB over a period of 2 years, at the following various ageing stages: post-natal month 1 (PM 1), PM 3, PM 6, PM 12 and PM 24. In PM 1 group, a few somatostatin-IR neurones were detected in the granule cell layer (GCL), and had slender or oval somata and short processes. At PM 3, somatostatin-IR neurones were observed in the glomerular, external plexiform and GCL. The size of somatostatin-IR somata was larger than that at PM 1. In PM 6 group, the number and size of somatostatin-IR neurones increased, and their processes became longer while running in various directions. At PM 12, somatostatin-IR neurones increased in number, and their processes became markedly longer than those at PM 6. At this stage, somatostatin-IR neurones had multipolar somata, and were the largest in size. In PM 24 group, somatostatin-IR neurones were most numerous. However, the processes of somatostatin-IR neurones were shorter than those at PM 12. This study suggests that the increased number of somatostatin-IR neurones in the MOB of aged rats may play a role to compensate for any decrease of olfactory function.