Serotonergic nuclei of the raphe are not affected in human ageing

Mitochondrial Interaction with Serotonin in Neurobiology and Its Implication in Alzheimer's Disease

Alzheimer's disease (AD) is a lethal neurodegenerative disorder characterized by severe brain pathologies and progressive cognitive decline. While the exact cause of this disease remains unknown, emerging evidence suggests that dysregulation of neurotransmitters contributes to the development of AD pathology and symptoms. Serotonin, a critical neurotransmitter in the brain, plays a pivotal role in regulating various brain processes and is implicated in neurological and psychiatric disorders, including AD. Recent studies have shed light on the interplay between mitochondrial function and serotonin regulation in brain physiology. In AD, there is a deficiency of serotonin, along with impairments in mitochondrial function, particularly in serotoninergic neurons. Additionally, altered activity of mitochondrial enzymes, such as monoamine oxidase, may contribute to serotonin dysregulation in AD. Understanding the intricate relationship between mitochondria and serotonin provides valuable insights into the underlying mechanisms of AD and identifies potential therapeutic targets to restore serotonin homeostasis and alleviate AD symptoms. This review summarizes the recent advancements in unraveling the connection between brain mitochondria and serotonin, emphasizing their significance in AD pathogenesis and underscoring the importance of further research in this area. Elucidating the role of mitochondria in serotonin dysfunction will promote the development of therapeutic strategies for the treatment and prevention of this neurodegenerative disorder.

Age-Related Changes in Central Nervous System 5-Hydroxytryptamine Signalling and Its Potential Effects on the Regulation of Lifespan

Serotonin or 5-hydroxytryptamine (5-HT) is an important neurotransmitter in the central nervous system and the periphery. Most 5-HT (~99%) is found in the periphery where it regulates the function of the gastrointestinal (GI) tract and is an important regulator of platelet aggregation. However, the remaining 1% that is found in the central nervous system (CNS) can regulate a range of physiological processes such as learning and memory formation, mood, food intake, sleep, temperature and pain perception. More recent work on the CNS of invertebrate model systems has shown that 5-HT can directly regulate lifespan.This chapter will focus on detailing how CNS 5-HT signalling is altered with increasing age and the potential consequences this has on its ability to regulate lifespan.

Sleep and Human Aging

Older adults do not sleep as well as younger adults. Why? What alterations in sleep quantity and quality occur as we age, and are there functional consequences? What are the underlying neural mechanisms that explain age-related sleep disruption? This review tackles these questions. First, we describe canonical changes in human sleep quantity and quality in cognitively normal older adults. Second, we explore the underlying neurobiological mechanisms that may account for these human sleep alterations. Third, we consider the functional consequences of age-related sleep disruption, focusing on memory impairment as an exemplar. We conclude with a discussion of a still-debated question: do older adults simply need less sleep, or rather, are they unable to generate the sleep that they still need?

Wake-active neurons across aging and neurodegeneration: a potential role for sleep disturbances in promoting disease

Sleep/wake disturbance is a feature of almost all common age-related neurodegenerative diseases. Although the reason for this is unknown, it is likely that this inability to maintain sleep and wake states is in large part due to declines in the number and function of wake-active neurons, populations of cells that fire only during waking and are silent during sleep. Consistent with this, many of the brain regions that are most susceptible to neurodegeneration are those that are necessary for wake maintenance and alertness. In the present review, these wake-active populations are systematically assessed in terms of their observed pathology across aging and several neurodegenerative diseases, with implications for future research relating sleep and wake disturbances to aging and age-related neurodegeneration.

Beta-amyloid cortical deposits are accompanied by the loss of serotonergic neurons in the dog

Dogs may naturally suffer an age-related cognitive impairment that has aroused a great deal of interest, even beyond the field of the veterinary clinic. This canine senile dementia reproduces several key aspects of Alzheimer's disease (AD), including the presence of beta-amyloid (A beta) deposits in the cerebral cortex, neurodegeneration, and learning and memory impairments. In the present study, we have used unbiased stereological procedures to estimate the number of the dorsal and median raphe nuclei (DRN and MRN, respectively) serotonergic neurons immunolabeled with an anti-tryptophan hydroxylase (TrH) monoclonal antibody in young and aged dogs without A beta cortical deposits and in aged dogs with A beta cortical deposits. The estimated total number of TrH-labeled neurons (mean +/- SD) was 94,790 +/- 26,341 for the DRN and 40,404 +/- 8,692 for the MRN. The statistical analyses revealed that aged dogs with A beta cortical pathology had 33% fewer serotonergic neurons in the DRN and MRN than aged dogs without A beta cortical deposits (108,043 +/- 18,800 vs. 162,242 +/- 39,942, respectively; P = 0.01). In contrast, no significant variations were found between young and aged dogs without A beta cortical deposits. These results suggest that degeneration of the serotonergic neurons could be involved in the cognitive damage that accompanies A beta cortical pathology in the dog and reinforce the use of the canine model for exploring the potential mechanisms linking the cortical A beta pathology and serotonergic neurodegeneration that occurs during the course of AD.

Brain regional α-[11C]methyl-L-tryptophan trapping, used as an index of 5-HT synthesis, in healthy adults: absence of an age effect

PURPOSE

Previous functional neuroimaging studies suggest that selective aspects of the brain serotonin (5-HT) system change during the aging process. Here, we assessed the effects of aging on the brain regional alpha-[(11)C]methyl-L: -tryptophan (alpha-[(11)C]MTrp) trapping rate constant (K*; microl.g(-1).min(-1)), which, with certain assumptions, could be taken as a proxy of 5-HT synthesis.

METHODS

Thirty-six healthy right-handed subjects had positron emission tomography (PET) scans following injection with alpha-[(11)C]MTrp [18 males aged 46.6 +/- 22.2 years (range 20-80 years) and 18 females aged 33.0 +/- 15.5 years (range 20-80 years)]. The trapping rate constant, K*, was calculated with the graphical method for irreversible ligands using the sinus-venous input function. A priori selected volumes of interest (VOIs) were defined using an automatic algorithm.

RESULTS

VOI analysis showed no correlation between age and brain regional K* values. As reported by others, significant age-related reductions of gray matter were observed in the thalamus and frontal and cingulate cortices; even with partial volume correction there was still no significant relationship between K* and age. Further exploratory SPM voxelwise correlation between age and alpha-[(11)C]MTrp trapping, using p = 0.05 (uncorrected), as well as voxel-based morphometry, was in agreement with the VOI analysis.

CONCLUSION

The dissociation between age-related changes in brain anatomy and this index of serotonin synthesis suggests independent mechanisms underlying the normal aging process.

Damage of serotonergic axons and immunolocalization of Hsp27, Hsp72, and Hsp90 molecular chaperones after a single dose of MDMA administration in Dark Agouti rat: Temporal, spatial, and cellular patterns

3,4-Methylenedioxymethamphetamine (MDMA, "ecstasy") causes long-term disturbance of the serotonergic system. We examined the temporal, spatial, and cellular distribution of three molecular chaperones, Hsp27, Hsp72, and Hsp90, 3 and 7 days after treatment with 7.5, 15, and 30 mg/kg single intraperitoneal (i.p.) doses of MDMA in Dark Agouti rat brains. Furthermore, we compared the immunostaining patterns of molecular chaperones with serotonergic axonal-vulnerability evaluated by tryptophan-hydroxylase (TryOH) immunoreactivity and with astroglial-activation detected by GFAP-immunostaining. There was a marked reduction in TryOH-immunoreactive axon density after MDMA treatment in all examined areas at both time points. Three days after treatment, a significant dose-dependent increase in Hsp27-immunoreactive protoplasmic astrocytes was found in the cingulate, frontal, occipital, and pyriform cortex, and in the hippocampus CA1. However, there was no increase in astroglial Hsp27-immunoreactivity in the caudate putamen, lateral septal nucleus, or anterior hypothalamus. A significant increase in the GFAP immunostaining density of protoplasmic astrocytes was found only in the hippocampus CA1. In addition, numerous strong Hsp72-immunopositive neurons were found in some brain areas only 3 days after treatment with 30 mg/kg MDMA. Increased Hsp27-immunoreactivity exclusively in the examined cortical areas reveals that Hsp27 is a sensitive marker of astroglial response to the effects of MDMA in these regions of Dark Agouti rat brain and suggests differential responses in astroglial Hsp27-expression between distinct brain areas. The co-occurrence of Hsp27 and GFAP response exclusively in the hippocampus CA1 may suggest the particular vulnerability of this region. The presence of strong Hsp72-immunopositive neurons in certain brain areas may reflect additional effects of MDMA on nonserotonergic neurons.

Aging is accompanied by a subfield-specific reduction of serotonergic fibers in the tree shrew hippocampal formation

The hippocampal formation is a crucial structure for learning and memory, and serotonin together with other neurotransmitters is essential in these processes. Although the effects of aging on various neurotransmitter systems in the hippocampus have been extensively investigated, it is not entirely clear whether or how the hippocampal serotonergic innervation changes during aging. Rat studies, which have mostly focused on aging-related changes in the dentate gyrus, have implied a loss of hippocampal serotonergic fibers. We used the tree shrew (Tupaia belangeri), an intermediate between insectivores and primates, as a model of aging. We applied immunocytochemistry with an antibody against serotonin to assess serotonergic fiber densities in the various hippocampal subfields of adult (0.9-1.3 years) and old (5-7 years) tree shrews. Our results have revealed a reduction of serotonergic fiber densities in the stratum radiatum of CA1 and CA3, and in the stratum oriens of CA3. A partial depletion of serotonin in the hippocampal formation, as can be expected from our current observations, will probably have an impact on the functioning of hippocampal principal neurons. Our findings also indicate that the rat and the tree shrew hippocampal serotonergic innervation show some variations that seem to be differentially affected during aging.

The dorsal raphe nucleus in schizophrenia: a post mortem study of 5-hydroxytryptamine neurones

The 5-hydroxytryptamine (5-HT, serotonin) system has been implicated in the pathophysiology and treatment of schizophrenia. In this study, we addressed the hypothesis that a deficit of 5-HT neurones, either inherited or acquired, is central to the developmental pathology of the disorder. We examined putative 5-HT neurones of the dorsal raphe nucleus (DRN) in post mortem, formalin-fixed tissue from 15 schizophrenic patients and 20 control subjects matched for age and gender. No significant difference was detected between these groups in the number or size (cross-sectional area or diameter) of tryptophan-hydroxylase-immunoreactive cell profiles viewed in transverse sections collected from the level of the trochlear decussation to the emergence of the trigeminal nerve. Profile number was not affected by age, gender, side of the brainstem (left or right) or post mortem interval; however, time in formalin correlated negatively with the number of neurones counted. Moreover, a significant negative correlation was detected between time in formalin and the levels of immunoreaction product (optical density), which in turn correlated positively with our profile counts. A positive correlation was found between the age of subjects and our estimates of cell size. Our results do not support the proposal that an abnormality in the number and/or size of DRN 5-HT neurones is central to the aetiopathology of schizophrenia.

Age-associated changes in the monoaminergic innervation of rat lumbosacral spinal cord

The effects of ageing on the innervation patterns of lumbosacral spinal nuclei involved in controlling lower urinary tract functions, including micturition, were studied using immunohistochemistry for serotonin (5-HT) and tyrosine hydroxylase (TH) in male Wistar rats of 3 and 24 months. Quantitative image analysis revealed significant age-associated declines in the innervation of most regions including the intermediolateral cell nucleus, sacral parasympathetic nucleus, dorsal grey commissure and in the ventral horn including the dorsolateral nucleus which in the rat is one of the component nuclei homologous to Onuf's nucleus in man. Notable exceptions to this generalised decline were observed in the 5-HT innervation of the sacral parasympathetic nucleus, which was maintained, and in the region of the dorsolateral motor nucleus where TH-like immunoreactivity did not significantly decline. These results suggest that the changes in micturition characteristics observed in aged rats may in part be a consequence of the alterations in, and decline of, aminergic inputs to both autonomic and somatic spinal nuclei associated with bladder function.

Nucleus-specific alteration of raphe neurons in human neurodegenerative disorders

Neurodegenerative diseases share symptoms suggested to be related to the serotonergic system. To evaluate the involvement of serotonergic raphe nuclei, we compared the percentage of neurons synthesizing serotonin in the nucleus centralis superior (NCS), raphe obscurus and pallidus (NROP) in Alzheimer's disease (AD), progressive supranuclear palsy (PSP), Parkinson's disease (PD), multiple system atrophy (MSA), and control brains. We used immunohistochemistry for tryptophan hydroxylase (TpOH), phosphorylated tau, and alpha-synuclein. We observed a significant decrease in the NCS in the NROP in AD, but a significant increase in PSP and MSA. Cytoskeletal pathology was present in the NCS and NROP to a variable degree. We conclude that there is disease- and nucleus-specific alteration of serotonin synthesis in the raphe.

Life-long stability of neurons: a century of research on neurogenesis, neuronal death and neuron quantification in adult CNS

In this chapter we provide an extensive review of 100 years of research on the stability of neurons in the mammalian brain, with special emphasis on humans. Although Cajal formulated the Neuronal Doctrine, he was wrong in his beliefs that adult neurogenesis did not occur and adult neurons are dying throughout life. These two beliefs became accepted "common knowledge" and have shaped much of neuroscience research and provided much of the basis for clinical treatment of age-related brain diseases. In this review, we consider adult neurogenesis from a historical and evolutionary perspective. It is concluded, that while adult neurogenesis is a factor in the dynamics of the dentate gyrus and olfactory bulb, it is probably not a major factor during the life-span in most brain areas. Likewise, the acceptance of neuronal death as an explanation for normal age-related senility is challenged with evidence collected over the last fifty years. Much of the problem in changing this common belief of dying neurons was the inadequacies of neuronal counting methods. In this review we discuss in detail implications of recent improvements in neuronal quantification. We conclude: First, age-related neuronal atrophy is the major factor in functional deterioration of existing neurons and could be slowed down, or even reversed by various pharmacological interventions. Second, in most cases neuronal degeneration during aging is a pathology that in principle may be avoided. Third, loss of myelin and of the white matter is more frequent and important than the limited neuronal death in normal aging.