Age-associated changes in the serotonergic system in rat superior colliculus and pretectum

Past, present, and future of serotonin-targeting therapeutics for Alzheimer's disease: Perspectives from DNA methylation

With population aging, Alzheimer's disease (AD) is becoming increasingly prevalent, causing great health and economic burdens worldwide. Despite decades of research, there are still no effective disease-modifying treatments for AD, highlighting the urgent need for more in-depth understanding of the disease-causing mechanisms. The brain serotonin (5-HT) neurotransmission system undergoes structural and functional changes in aging and AD, which contributes to cognitive decline and comorbid mood disturbances. This review discusses the critical involvement of the brain 5-HT system in aging and AD. Existing findings on the changes in projection fiber innervation and receptor/transporter expression in AD are reviewed. Preclinical and clinical progress on the development of 5-HT-modulating drugs for AD and the obstacles faced by these development efforts are discussed. Epigenetic control of the brain 5-HT system and the potential of modulating 5-HT transmission via DNA methylation are also examined.

Prolonged intermittent hypoxia differentially regulates phrenic motor neuron serotonin receptor expression in rats following chronic cervical spinal cord injury

Low-dose (< 2 h/day), acute intermittent hypoxia (AIH) elicits multiple forms of serotonin-dependent phrenic motor plasticity and is emerging as a promising therapeutic strategy to restore respiratory and non-respiratory motor function after spinal cord injury (SCI). In contrast, high-dose (> 8 h/day), chronic intermittent hypoxia (CIH) undermines some forms of serotonin-dependent phrenic motor plasticity and elicits pathology. CIH is a hallmark of sleep disordered breathing, which is highly prevalent in individuals with cervical SCI. Interestingly, AIH and CIH preconditioning differentially impact phrenic motor plasticity. Although mechanisms of AIH-induced plasticity in the phrenic motor system are well-described in naïve rats, we know little concerning how these mechanisms are affected by chronic SCI or intermittent hypoxia preconditioning. Thus, in a rat model of chronic, incomplete cervical SCI (lateral spinal hemisection at C2 (C2Hx), we assessed serotonin type 2A, 2B and 7 receptor expression in and near phrenic motor neurons and compared: 1) intact vs. chronically injured rats; and 2) the impact of preconditioning with varied "doses" of intermittent hypoxia (IH). While there were no effects of chronic injury or intermittent hypoxia alone, CIH affected multiple receptors in rats with chronic C2Hx. Specifically, CIH preconditioning (8 h/day; 28 days) increased serotonin 2A and 7 receptor expression exclusively in rats with chronic C2Hx. Understanding the complex, context-specific interactions between chronic SCI and CIH and how this ultimately impacts phrenic motor plasticity is important as we leverage AIH-induced motor plasticity to restore breathing and other non-respiratory motor functions in people with chronic SCI.

Neuronal Stability, Volumetric Changes, and Decrease in GFAP Expression of Marmoset (Callithrix jacchus) Subcortical Visual Nuclei During Aging

The physiological aging process is well known for functional decline in visual abilities. Among the components of the visual system, the dorsal lateral geniculate nucleus (DLG) and superior colliculus (SC) provide a good model for aging investigations, as these structures constitute the main visual pathways for retinal inputs reaching the visual cortex. However, there are limited data available on quantitative morphological and neurochemical aspects in DLG and SC across lifespan. Here, we used optical density to determine immunoexpression of glial fibrillary acidic protein (GFAP) and design-based stereological probes to estimate the neuronal number, total volume, and layer volume of the DLG and SC in marmosets (Callithrix jacchus), ranging from 36 to 143 months of age. Our results revealed an age-related increase in total volume and layer volume of the DLG, with an overall stability in SC volume. Furthermore, a stable neuronal number was demonstrated in DLG and superficial layers of SC (SCv). A decrease in GFAP immunoexpression was observed in both visual centers. The results indicate region-specific variability in volumetric parameter, possibly attributed to structural plastic events in response to inflammation and compensatory mechanisms at the cellular and subcellular level. Additionally, the DLG and SCv seem to be less vulnerable to aging effects in terms of neuronal number. The neuropeptidergic data suggest that reduced GFAP expression may reflect morphological atrophy in the astroglial cells. This study contributes to updating the current understanding of aging effects in the visual system and stablishes a crucial foundation for future research on visual perception throughout the aging process.

Suitability of [18F]altanserin and PET to determine 5-HT2A receptor availability in the rat brain: in vivo and in vitro validation of invasive and non-invasive kinetic models

PURPOSE

While the selective 5-hydroxytryptamine type 2a receptor (5-HT2AR) radiotracer [18F]altanserin is well established in humans, the present study evaluated its suitability for quantifying cerebral 5-HT2ARs with positron emission tomography (PET) in albino rats.

PROCEDURES

Ten Sprague Dawley rats underwent 180 min PET scans with arterial blood sampling. Reference tissue methods were evaluated on the basis of invasive kinetic models with metabolite-corrected arterial input functions. In vivo 5-HT2AR quantification with PET was validated by in vitro autoradiographic saturation experiments in the same animals.

RESULT

Overall brain uptake of [18F]altanserin was reliably quantified by invasive and non-invasive models with the cerebellum as reference region shown by linear correlation of outcome parameters. Unlike in humans, no lipophilic metabolites occurred so that brain activity derived solely from parent compound. PET data correlated very well with in vitro autoradiographic data of the same animals.

CONCLUSION

[18F]Altanserin PET is a reliable tool for in vivo quantification of 5-HT2AR availability in albino rats. Models based on both blood input and reference tissue describe radiotracer kinetics adequately. Low cerebral tracer uptake might, however, cause restrictions in experimental usage.

Head-twitch response in rodents induced by the hallucinogen 2,5-dimethoxy-4-iodoamphetamine: a comprehensive history, a re-evaluation of mechanisms, and its utility as a model

Two primary animal models persist for assessing hallucinogenic potential of novel compounds and for examining the pharmacological and neurobiological substrates underlying the actions of classical hallucinogens, the two-lever drug discrimination procedure and the drug-induced head-twitch response (HTR) in rodents. The substituted amphetamine hallucinogen, serotonin 2 (5-HT(2) ) receptor agonist, 2,5-dimethoxy-4-iodoamphetamine (DOI) has emerged as the most popular pharmacological tool used in HTR studies of hallucinogens. Synthesizing classic, recent, and relatively overlooked findings, addressing ostensibly conflicting observations, and considering contemporary theories in receptor and behavioural pharmacology, this review provides an up-to-date and comprehensive synopsis of DOI and the HTR model, from neural mechanisms to utility for understanding psychiatric diseases. Also presented is support for the argument that, although both the two-lever drug discrimination and the HTR models in rodents are useful for uncovering receptors, interacting proteins, intracellular signalling pathways, and neurochemical processes affected by DOI and related classical hallucinogens, results from both models suggest they are not reporting hallucinogenic experiences in animals.

Axonal projections originating from raphe serotonergic neurons in the developing and adult zebrafish, Danio rerio, using transgenics to visualize raphe-specific pet1 expression

Serotonin is a major central nervous modulator of physiology and behavior and plays fundamental roles during development and plasticity of the vertebrate central nervous system (CNS). Understanding the developmental control and functions of serotonergic neurons is therefore an important task. In all vertebrates, prominent serotonergic neurons are found in the superior and inferior raphe nuclei in the hindbrain innervating most CNS regions. In addition, all vertebrates except for mammals harbor other serotonergic centers, including several populations in the diencephalon. This, in combination with the intricate and wide distribution of serotonergic fibers, makes it difficult to sort out serotonergic innervation originating from the raphe from that of other serotonergic cell populations. To resolve this issue, we isolated the regulatory elements of the zebrafish raphe-specific gene pet1 and used them to drive expression of an eGFP transgene in the raphe population of serotonergic neurons. With this approach together with retrograde tracing we 1) describe in detail the development, anatomical organization, and projection pattern of zebrafish pet1-positive neurons compared with their mammalian counterparts, 2) identify a new serotonergic population in the ventrolateral zebrafish hindbrain, and 3) reveal some extent of functional subdivisions within the zebrafish superior raphe complex. Together, our results reveal for the first time the specific innervation pattern of the zebrafish raphe and, thus, provide a new model and various tools to investigate further the role of raphe serotonergic neurons in vertebrates.

Autoradiographic analyses of 5-HT1A and 5-HT2A receptors after social isolation in mice

Social isolation of rodents is used to model human psychopathological processes. In the present study, the effects of intermediate and long term isolation housing on postsynaptic 5-HT(1A) and 5-HT(2A) receptors were analyzed in male mice housed in groups or isolation for 4 and 12 weeks. [3H]8-OH-DPAT and [3H]ketanserin were used to label 5-HT(1A) and 5-HT(2A) receptors. Four representative sagittal sections (planes 1-4) were scored by in vitro autoradiography. Whereas after 4 weeks of housing both receptor densities were lowered significantly in isolated mice, after 12 weeks of housing only marginal isolation effects were seen. Intermediate isolation reduced 5-HT(1A) receptors especially in the lateral frontal, parietal and entorhinal cortex (-63%), in the lateral CA1-3 and dentate gyrus region of the hippocampus (-68%), in the basolateral, basomedial, central and medial amygdaloid nuclei (between -38 and -66%), and in the hypothalamus (-28%). 5-HT(2A) receptors were strongly reduced in the frontal cortex (between -47 and -74%), in the hippocampus (between -47 and -95%), in the striatum (between -66 and -76%), and in the accumbens nucleus (between -59 and -73%) in comparison to group housed control mice. After 12 weeks of isolation in the hippocampus continuously decreased 5-HT(1A) receptor densities were demonstrated (between -24 and -61%). But increased 5-HT(2A) receptor densities were seen in the lateral striatum (+86%) compared to control mice. Age-dependent effects were also found. After 12 weeks of group housing the 5-HT(1A) and 5-HT(2A) receptor densities were decreased (between -28 and -54%) in all analyzed brain regions in comparison to 4 weeks of group housing. Isolated animals showed diminished 5-HT(1A) receptor densities in the cortex (-14%) and hippocampus (-15%), but increased 5-HT(1A) receptor densities in the amygdala (+33%) after 12 weeks. The 5-HT(2A) receptor densities were increased in all analyzed regions (between +31 and +96%) after 12 weeks of isolation compared to 4 weeks. To explain these dynamic, time-dependent pattern of isolation-induced changes different regulation processes are supposed regarding 5-HT(1A) and 5-HT(2A) receptors. Besides metabolism-related adaptation processes also neurotransmitter and hormonal (e.g., glucocorticoid) interactions especially in limbic regions have to be considered.