Age-Dependent Neurogenesis and Neuron Numbers within the Olfactory Bulb and Hippocampus of Homing Pigeons

A comparative assessment of age-related nicotinamide adenine dinucleotide phosphate-diaphorase positivity in the spinal cord and medulla oblongata of pigeons, rats, and mice

Nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase (N-d) positive neurons have been extensively studied across various animals, and N-d neurodegenerative neurites have been detected in some aged animal models. However, detailed knowledge on N-d positivity and aging-related alterations in the spinal cord and medulla oblongata of pigeons is limited. In this study, we investigated N-d positivity and age-related changes in the pigeon's spinal cord and medulla oblongata and compared them to those in rats and mice. Pigeons, had more N-d neurons in the dorsal horn, around the central canal, and in the column of Terni in the thoracic and lumbar segments, with scattered neurons found in the ventral horn of the spinal segments. N-d neurons were also present in the white matter of the spinal cord. Morphometric analysis revealed that the size of N-d soma in the lumbosacral, cervical, and thoracic regions was substantially altered in aged pigeons compared to young birds. Furthermore, the lumbar to sacral segments underwent significant morphological alterations. The main findings of this study were the presence of age-related N-d positive bodies (ANB) in aged pigeons, predominantly in the external cuneate nucleus (CuE) and occasionally in the gracilis and CuEs. ANBs were also identified in the gracile nuclei and spinal cord in the aged rats and mice, whereas in aged rats, ANBs were detected in the CuE spinal nucleus. Immunohistochemistry showed that the age-related alterations occurred in the cell types and neuropeptides in old animals. The results suggest weak inflammatory response and neuronal dysfunction in the spinal cord in aged pigeons. Our results suggested that the ANB could be a potential aging marker for the central nervous system.

Adult Hippocampal Neurogenesis in the Human Brain: Updates, Challenges, and Perspectives

The existence of neurogenesis in the adult human hippocampus has been under considerable debate within the past three decades due to the diverging conclusions originating mostly from immunohistochemistry studies. While some of these reports conclude that hippocampal neurogenesis in humans occurs throughout physiologic aging, others indicate that this phenomenon ends by early childhood. More recently, some groups have adopted next-generation sequencing technologies to characterize with more acuity the extent of this phenomenon in humans. Here, we review the current state of research on adult hippocampal neurogenesis in the human brain with an emphasis on the challenges and limitations of using immunohistochemistry and next-generation sequencing technologies for its study.

Age-related reductions in whole brain mass and telencephalon volume in very old white Carneau pigeons (Columba livia)

While studies have identified age-related cognitive impairment in pigeons (Columba livia), no study has detected the brain atrophy which typically accompanies cognitive impairment in older mammals. Instead, Coppola and Bingman (Aging is associated with larger brain mass and volume in homing pigeons (Columba livia), Neurosci. Letters 698 (2019) 39-43) reported increased whole brain mass and telencephalon volume in older, compared to younger, homing pigeons. One reason for this unexpected finding might be that the older pigeons studied were not old enough to display age-related brain atrophy. Therefore, the current study repeated Coppola and Bingman, but with a sample of older white Carneau pigeons that were on average 5.34 years older. Brains from young and old homing pigeons were weighed and orthogonal measurements of the telencephalon, cerebellum, and optic tectum were obtained. Despite having a heavier body mass than younger pigeons, older pigeons had a significant reduction in whole brain mass and telencephalon volume, but not cerebellum or optic tectum volume. This study is therefore the first to find that pigeons experience age-related brain atrophy.

Neuronal circuits within the homing pigeon hippocampal formation

The current study aimed to reveal in detail patterns of intrahippocampal connectivity in homing pigeons (Columba livia). In light of recent physiological evidence suggesting differences between dorsomedial and ventrolateral hippocampal regions and a hitherto unknown laminar organization along the transverse axis, we also aimed to gain a higher-resolution understanding of the proposed pathway segregation. Both in vivo and high-resolution in vitro tracing techniques were employed and revealed a complex connectivity pattern along the subdivisions of the avian hippocampus. We uncovered connectivity pathways along the transverse axis that started in the dorsolateral hippocampus and continued to the dorsomedial subdivision, from where information was relayed to the triangular region either directly or indirectly via the V-shaped layers. The often-reciprocal connectivity along these subdivisions displayed an intriguing topographical arrangement such that two parallel pathways could be discerned along the ventrolateral (deep) and dorsomedial (superficial) aspects of the avian hippocampus. The segregation along the transverse axis was further supported by expression patterns of the glial fibrillary acidic protein and calbindin. Moreover, we found strong expression of Ca²⁺ /calmodulin-dependent kinase IIα and doublecortin in the lateral but not medial V-shape layer, indicating a difference between the two V-shaped layers. Overall, our findings provide an unprecedented, detailed description of avian intrahippocampal pathway connectivity, and confirm the recently proposed segregation of the avian hippocampus along the transverse axis. We also provide further support for the hypothesized homology of the lateral V-shape layer and the dorsomedial hippocampus with the dentate gyrus and Ammon's horn of mammals, respectively.

Regional Patterning of Adult Neurogenesis in the Homing Pigeon’s Brain

In the avian brain, adult neurogenesis has been reported in the telencephalon of several species, but the functional significance of this trait is still ambiguous. Homing pigeons (Columba livia f.d.) are well-known for their navigational skills. Their brains are functionally adapted to homing with, e.g., larger hippocampi. So far, no comprehensive mapping of adult neuro- and gliogenesis or studies of different developmental neuronal stages in the telencephalon of homing pigeons exists, although comprehensive analyses in various species surely will result in a higher understanding of the functional significance of adult neurogenesis. Here, adult, free flying homing pigeons were treated with 5-bromo-deoxyuridine (BrdU) to label adult newborn cells. Brains were dissected and immunohistochemically processed with several markers (GFAP, Sox2, S100ß, Tbr2, DCX, Prox1, Ki67, NeuN, Calbindin, Calretinin) to study different stages of adult neurogenesis in a quantitative and qualitative way. Therefore, immature and adult newborn neurons and glial cells were analyzed along the anterior–posterior axis. The analysis proved the existence of different neuronal maturation stages and showed that immature cells, migrating neurons and adult newborn neurons and glia were widely and regionally unequally distributed. Double- and triple-labelling with developmental markers allowed a stage classification of adult neurogenesis in the pigeon brain (1: continuity of stem cells/proliferation, 2: fate specification, 3: differentiation/maturation, 4: integration). The most adult newborn neurons and glia were found in the intercalated hyperpallium (HI) and the hippocampal formation (HF). The highest numbers of immature (DCX+) cells were detected in the nidopallium (N). Generally, the number of newborn glial cells exceeded the number of newborn neurons. Individual structures (e.g., HI, N, and HF) showed further variations along the anterior–posterior axis. Our qualitative classification and the distribution of maturing cells in the forebrain support the idea that there is a functional specialization, respectively, that there is a link between brain-structure and function, species-specific requirements and adult neurogenesis. The high number of immature neurons also suggests a high level of plasticity, which points to the ability for rapid adaption to environmental changes through additive mechanisms. Furthermore, we discuss a possible influence of adult neurogenesis on spatial cognition.

SARS-CoV-2-Mediated Neuropathogenesis, Deterioration of Hippocampal Neurogenesis and Dementia

A significant portion of COVID-19 patients and survivors display marked clinical signs of neurocognitive impairments. SARS-CoV-2-mediated peripheral cytokine storm and its neurotropism appear to elicit the activation of glial cells in the brain proceeding to neuroinflammation. While adult neurogenesis has been identified as a key cellular basis of cognitive functions, neuroinflammation-induced aberrant neuroregenerative plasticity in the hippocampus has been implicated in progressive memory loss in ageing and brain disorders. Notably, recent histological studies of post-mortem human and experimental animal brains indicate that SARS-CoV-2 infection impairs neurogenic process in the hippocampus of the brain due to neuroinflammation. Considering the facts, this article describes the prominent neuropathogenic characteristics and neurocognitive impairments in COVID-19 and emphasizes a viewpoint that neuroinflammation-mediated deterioration of hippocampal neurogenesis could contribute to the onset and progression of dementia in COVID-19. Thus, it necessitates the unmet need for regenerative medicine for the effective management of neurocognitive deficits in COVID-19.

Unilateral vocal nerve resection alters neurogenesis in the avian song system in a region-specific manner

New neurons born in the adult brain undergo a critical period soon after migration to their site of incorporation. During this time, the behavior of the animal may influence the survival or culling of these cells. In the songbird song system, earlier work suggested that adult-born neurons may be retained in the song motor pathway nucleus HVC with respect to motor progression toward a target song during juvenile song learning, seasonal song restructuring, and experimentally manipulated song variability. However, it is not known whether the quality of song per se, without progressive improvement, may also influence new neuron survival. To test this idea, we experimentally altered song acoustic structure by unilateral denervation of the syrinx, causing a poor quality song. We found no effect of aberrant song on numbers of new neurons in HVC, suggesting that song quality does not influence new neuron culling in this region. However, aberrant song resulted in the loss of left-side dominance in new neurons in the auditory region caudomedial nidopallium (NCM), and a bilateral decrease in new neurons in the basal ganglia nucleus Area X. Thus new neuron culling may be influenced by behavioral feedback in accordance with the function of new neurons within that region. We propose that studying the effects of singing behaviors on new neurons across multiple brain regions that differentially subserve singing may give rise to general rules underlying the regulation of new neuron survival across taxa and brain regions more broadly.

New Perspectives on Avian Models for Studies of Basic Aging Processes

Avian models have the potential to elucidate basic cellular and molecular mechanisms underlying the slow aging rates and exceptional longevity typical of this group of vertebrates. To date, most studies of avian aging have focused on relatively few of the phenomena now thought to be intrinsic to the aging process, but primarily on responses to oxidative stress and telomere dynamics. But a variety of whole-animal and cell-based approaches to avian aging and stress resistance have been developed-especially the use of primary cell lines and isolated erythrocytes-which permit other processes to be investigated. In this review, we highlight newer studies using these approaches. We also discuss recent research on age-related changes in neural function in birds in the context of sensory changes relevant to homing and navigation, as well as the maintenance of song. More recently, with the advent of "-omic" methodologies, including whole-genome studies, new approaches have gained momentum for investigating the mechanistic basis of aging in birds. Overall, current research suggests that birds exhibit an enhanced resistance to the detrimental effects of oxidative damage and maintain higher than expected levels of cellular function as they age. There is also evidence that genetic signatures associated with cellular defenses, as well as metabolic and immune function, are enhanced in birds but data are still lacking relative to that available from more conventional model organisms. We are optimistic that continued development of avian models in geroscience, especially under controlled laboratory conditions, will provide novel insights into the exceptional longevity of this animal taxon.

Age-associated decline in septum neuronal activation during spatial learning in homing pigeons (Columba livia)

The relationship between hippocampal aging and spatial-cognitive decline in birds has recently been investigated. However, like its mammalian counterpart, the avian hippocampus does not work in isolation and its relationship to the septum is of particular interest. The current study aimed to investigate the effects of age on septum (medial and lateral) and associated nucleus of the diagonal band (NDB) neuronal activation (as indicated by c-Fos expression) during learning of a spatial, delayed non-match-to-sample task conducted in a modified radial arm maze. The results indicated significantly reduced septum, but not NDB, activation during spatial learning in older pigeons. We also preliminarily investigated the effect of age on the number of cholinergic septum and NDB neurons (as indicated by expression of choline acetyltransferase; ChAT). Although underpowered to reveal a statistical effect, the data suggest that older pigeons have substantially fewer ChAT-expressing cells in the septum compared to younger pigeons. The data support the hypothesis that reduced activation of the septum contributes to the age-related, spatial cognitive impairment in pigeons.

Early deficits in olfaction are associated with structural and molecular alterations in the olfactory system of a Huntington disease mouse model

Olfactory dysfunction and altered neurogenesis are observed in several neurodegenerative disorders including Huntington disease (HD). These deficits occur early and correlate with a decline in global cognitive performance, depression and structural abnormalities of the olfactory system including the olfactory epithelium, bulb and cortices. However, the role of olfactory system dysfunction in the pathogenesis of HD remains poorly understood and the mechanisms underlying this dysfunction are unknown. We show that deficits in odour identification, discrimination and memory occur in HD individuals. Assessment of the olfactory system in an HD murine model demonstrates structural abnormalities in the olfactory bulb (OB) and piriform cortex, the primary cortical recipient of OB projections. Furthermore, a decrease in piriform neuronal counts and altered expression levels of neuronal nuclei and tyrosine hydroxylase in the OB are observed in the YAC128 HD model. Similar to the human HD condition, olfactory dysfunction is an early phenotype in the YAC128 mice and concurrent with caspase activation in the murine HD OB. These data provide a link between the structural olfactory brain region atrophy and olfactory dysfunction in HD and suggest that cell proliferation and cell death pathways are compromised and may contribute to the olfactory deficits in HD.

c-Fos revealed lower hippocampal participation in older homing pigeons when challenged with a spatial memory task

Homing pigeons experience age-related spatial-cognitive decline similar to that seen in mammals. In contrast to mammals, however, previous studies have shown the hippocampal formation (HF) of old, cognitively impaired pigeons to be greater in volume and neuron number compared with young pigeons. As a partial explanation of the cognitive decline in older birds, it was hypothesized that older pigeons have reduced HF activation during spatial learning. The present study compared HF activation (via the activity-dependent expression of the immediate early gene c-Fos) between younger and older pigeons during learning of a spatial, delayed nonmatch-to-sample task. On the last day of training, c-Fos activation significantly correlated with behavioral performance in the young, but not old, pigeons suggesting more HF engagement by the young pigeons in solving the task. The behavioral correlation was additionally associated with consistently higher, but insignificant c-Fos activation across practically every HF subdivision in the young compared with the old pigeons. In sum, the results of the present study are consistent with the hypothesis that age-related decline in the spatial cognitive ability of homing pigeons is in part a result of an older HF being less responsive to the processing of spatial information. However, alternative interpretations of the data are discussed.

Quantitative analysis of age and life-history stage related changes in DCX expression in the male Japanese quail (Cortunix japonica) telencephalon

Most avian neurogenesis studies focused on the song control system and little attention has been given to non-song birds such as the Japanese quail. However, the only few neurogenesis studies in quails mainly focused on the sex steroid sensitive areas of the brain such as the medial preoptic and lateral septal nuclei. Despite the important role the quail telencephalon plays in filial imprinting and passive avoidance learning, neurogenesis in this structure has been completely overlooked. The aim of this study was therefore to quantitatively determine how DCX expression in the Japanese quail telencephalon changes with post hatching age (3-12 weeks) and life history stage. In this study, DCX was used as a proxy for neuronal incorporation. Bipolar and multipolar DCX immunoreactive cells were observed in the entire telencephalon except for the entopallium and arcopallium. In addition, DCX expression in all the eight telencephalic areas quantified was strongly negatively correlated with post-hatching age. Furthermore, numbers of bipolar and multipolar DCX immunoreactive cells were higher in the juvenile compared to subadult and adult quails. In conclusion, neuronal incorporation in the quail telencephalon is widespread but it declines with post hatching age. In addition, the most dramatic decline in neuronal incorporation in the telencephalic areas quantified takes place just after the birds have attained sexual maturity.

Adult Hippocampal Neurogenesis in Different Taxonomic Groups: Possible Functional Similarities and Striking Controversies

Adult neurogenesis occurs in many species, from fish to mammals, with an apparent reduction in the number of both neurogenic zones and new neurons inserted into established circuits with increasing brain complexity. Although the absolute number of new neurons is high in some species, the ratio of these cells to those already existing in the circuit is low. Continuous replacement/addition plays a role in spatial navigation (migration) and other cognitive processes in birds and rodents, but none of the literature relates adult neurogenesis to spatial navigation and memory in primates and humans. Some models developed by computational neuroscience attribute a high weight to hippocampal adult neurogenesis in learning and memory processes, with greater relevance to pattern separation. In contrast to theories involving neurogenesis in cognitive processes, absence/rarity of neurogenesis in the hippocampus of primates and adult humans was recently suggested and is under intense debate. Although the learning process is supported by plasticity, the retention of memories requires a certain degree of consolidated circuitry structures, otherwise the consolidation process would be hampered. Here, we compare and discuss hippocampal adult neurogenesis in different species and the inherent paradoxical aspects.

Functional and Anatomic Correlates of Neural Aging in Birds

Avian species show variation in longevity, habitat, physiologic characteristics, and lifetime endocrine patterns. Lifetime reproductive and metabolic function vary. Much is known about the neurobiology of the song system in many altricial birds. Little is known about aging in neural systems in birds. Captive birds often survive beyond the age they would in the wild, providing an opportunity to gain an understanding of the physiologic and neural changes. This paper reviews the available information with the goal of capturing areas of potential investigation into gaps in our understanding of neural aging as reflected in physiologic, endocrine, and cognitive aging.