Neuro-hormonal Regulation Is a Better Indicator of Human Cognitive Abilities Than Brain Anatomy: The Need for a New Paradigm

Emerging epigenetic dynamics in gut-microglia brain axis: experimental and clinical implications for accelerated brain aging in schizophrenia

Brain aging, which involves a progressive loss of neuronal functions, has been reported to be premature in probands affected by schizophrenia (SCZ). Evidence shows that SCZ and accelerated aging are linked to changes in epigenetic clocks. Recent cross-sectional magnetic resonance imaging analyses have uncovered reduced brain reserves and connectivity in patients with SCZ compared to typically aging individuals. These data may indicate early abnormalities of neuronal function following cyto-architectural alterations in SCZ. The current mechanistic knowledge on brain aging, epigenetic changes, and their neuropsychiatric disease association remains incomplete. With this review, we explore and summarize evidence that the dynamics of gut-resident bacteria can modulate molecular brain function and contribute to age-related neurodegenerative disorders. It is known that environmental factors such as mode of birth, dietary habits, stress, pollution, and infections can modulate the microbiota system to regulate intrinsic neuronal activity and brain reserves through the vagus nerve and enteric nervous system. Microbiota-derived molecules can trigger continuous activation of the microglial sensome, groups of receptors and proteins that permit microglia to remodel the brain neurochemistry based on complex environmental activities. This remodeling causes aberrant brain plasticity as early as fetal developmental stages, and after the onset of first-episode psychosis. In the central nervous system, microglia, the resident immune surveillance cells, are involved in neurogenesis, phagocytosis of synapses and neurological dysfunction. Here, we review recent emerging experimental and clinical evidence regarding the gut-brain microglia axis involvement in SCZ pathology and etiology, the hypothesis of brain reserve and accelerated aging induced by dietary habits, stress, pollution, infections, and other factors. We also include in our review the possibilities and consequences of gut dysbiosis activities on microglial function and dysfunction, together with the effects of antipsychotics on the gut microbiome: therapeutic and adverse effects, role of fecal microbiota transplant and psychobiotics on microglial sensomes, brain reserves and SCZ-derived accelerated aging. We end the review with suggestions that may be applicable to the clinical setting. For example, we propose that psychobiotics might contribute to antipsychotic-induced therapeutic benefits or adverse effects, as well as reduce the aging process through the gut-brain microglia axis. Overall, we hope that this review will help increase the understanding of SCZ pathogenesis as related to chronobiology and the gut microbiome, as well as reveal new concepts that will serve as novel treatment targets for SCZ.

Comparative Study of Brain Size Ontogeny: Marsupials and Placental Mammals

There exists a negative allometry between vertebrate brain size and body size. It has been well studied among placental mammals but less is known regarding marsupials. Consequently, this study explores brain/body ontogenetic growth in marsupials and compares it with placental mammals. Pouch young samples of 43 koalas (Phascolarctos cinereus), 28 possums (Trichosurus vulpecula), and 36 tammar wallabies (Macropus eugenii) preserved in a solution of 10% buffered formalin, as well as fresh juveniles and adults of 43 koalas and 40 possums, were studied. Their brain size/body size allometry was compared to that among humans, rhesus monkeys, dogs, cats, rats, guinea pigs, rabbits, wild pigs, and mice. Two patterns of allometric curves were found: a logarithmic one (marsupials, rabbits, wild pigs, and guinea pigs) and a logistic one (the rest of mammals).

Thyroid hormone regulation of adult neural stem cell fate: A comparative analysis between rodents and primates

Thyroid hormone (TH) signaling, a highly conserved pathway across vertebrates, is crucial for brain development and function throughout life. In the adult mammalian brain, including that of humans, multipotent neural stem cells (NSCs) proliferate and generate neuronal and glial progenitors. The role of TH has been intensively investigated in the two main neurogenic niches of the adult mouse brain, the subventricular and the subgranular zone. A key finding is that T3, the biologically active form of THs, promotes NSC commitment toward a neuronal fate. In this review, we first discuss the roles of THs in the regulation of adult rodent neurogenesis, as well as how it relates to functional behavior, notably olfaction and cognition. Most research uncovering these roles of TH in adult neurogenesis was conducted in rodents, whose genetic background, brain structure and rate of neurogenesis are considerably different from that of humans. To bridge the phylogenetic gap, we also explore the similarities and divergences of TH-dependent adult neurogenesis in non-human primate models. Lastly, we examine how photoperiodic length changes TH homeostasis, and how that might affect adult neurogenesis in seasonal species to increase fitness. Several aspects by which TH acts on adult NSCs seem to be conserved among mammals, while we only start to uncover the molecular pathways, as well as how other in- and extrinsic factors are intertwined. A multispecies approach delivering more insights in the matter will pave the way for novel NSC-based therapies to combat neurological disorders.

Association of Peripheral Plasma Neurotransmitters with Cognitive Performance in Chronic High-altitude Exposure

Long-term living at high altitude causes significant impairment of cognitive function. Central neurotransmitters are potential mediators of cognitive performance. We aimed to determine whether there were significant associations between select peripheral plasma neurotransmitters and cognitive performance in humans with chronic high-altitude (HA) exposure and to determine the association between peripheral plasma neurotransmitters and brain neurotransmitters in rats after chronic hypobaric hypoxia (HH) exposure. We demonstrated that 3,4-dihydroxy-L-phenylalanine (DOPA), dopamine, serotonin, 5-hydroxyindole-3-acetic acid (5-HIAA) and GABA in the peripheral plasma were associated with cognitive performance in humans with HA exposure. Consistent with this result, peripheral plasma DOPA, dopamine, serotonin, 5-HIAA and glutamate were associated with brain neurotransmitter levels after chronic HH exposure in rats. These results provide experimental data indicating that neurotransmitter levels and cognitive performance are modified in chronic high-altitude exposure, with a possible causal effect.