The pineal gland as ontogenetic scanner of reproduction, immunity, and aging. The aging clock

Age-related memory decline is accelerated by pinealectomy in young adult and middle-aged rats via BDNF / ERK / CREB signalling

Memory decline is considered a normal part of aging, while the relationship between melatonin deficiency and cognitive function is complex and not fully understood. The present study investigated the role of melatonin deficiency at different ages on working and short-term recognition and spatial memory in rats. An age-related decline in memory function was tested using the Y-maze, the object recognition test, and the radial arm maze. The brain-derived neurotrophic factor (BDNF), TrkB receptor, the extracellular signal-regulated kinase (ERK)1/2 and pERK1/2 expression in the hippocampus was assessed by immunohistochemistry. The pCREB/CREB ratio in the frontal cortex (FC) and hippocampus was evaluated by ELISA. Young adult and middle-aged rats with pinealectomy had memory impairment whereas old melatonin-deficient rats were unaffected. Aging was associated with reduced expression of BDNF and its receptor throughout the hippocampus and reduced ratio of pCREB/CREB in the FC and hippocampus, whereas pinealectomy exacerbated this process in 3- and 14-month-old rats. The region-specific reduced expression of the ERK1/2 and pERK1/2 was observed in young adult rats with pinealectomy. However, in middle-aged rats, the expression of these signaling molecules was either downregulated or upregulated in different regions of the hippocampus. Our study provides insights into the molecular pathways involved in age-related memory changes associated with melatonin deficiency, highlighting the importance of the BDNF/ERK1/2/CREB pathway in the hippocampus and suggesting a critical period for intervention.

Pinealectomy-Induced Melatonin Deficiency Exerts Age-Specific Effects on Sphingolipid Turnover in Rats

The existing body of literature, in conjunction with our recent studies, shows that melatonin dysfunction can accelerate the aging process, with this effect depending on the specific age of the subject. The present study aims to ascertain the impact of pinealectomy on sphingolipid (SL) turnover in young adult (3-month-old), middle-aged (14-month-old), and old (18-month-old) rats. Ceramide (Cer) levels, neutral (NSMase) and acid sphingomyelinase (ASMase), acid ceramidase (ASAH1), and sphingosine-1-phosphate (S1P) levels in hippocampus and/or plasma, were evaluated by enzyme-linked immunosorbent assay. The accumulation of Cer and its metabolite second messenger S1P in the hippocampus and plasma was associated with increased levels and activity of hippocampal NSMase in the hippocampus and plasma. However, no such association was observed for hippocampal ASMase, whose levels and activity were reduced in middle-aged and old rats compared to young adult rats. Pinealectomy-induced melatonin deficiency in young adult rats showed an increase in hippocampal Cer content compared to the sham group. However, in contrast to young adult rats, pinealectomy had an inverse effect on age-related changes in hippocampal Cer, NSMase, and ASMase in middle-aged rats. Furthermore, pinealectomy exacerbated the age-related increase in S1P in the hippocampus of 18-month-old rats. Collectively, the results of the present study suggest that melatonin deficiency may influence the aging process by modulating SL turnover in an age-specific manner.

In Vitro Effects of Fentanyl on Aortic Viscoelasticity in a Rat Model of Melatonin Deficiency

Melatonin influences arterial biomechanics, and its absence could cause remodeling of the arterial wall, leading to increased stiffness. Direct effects of fentanyl on the aortic wall have also been observed previously. This study aimed to evaluate in vitro the effects of fentanyl on aortic viscoelasticity in a rat model of melatonin deficiency and to test the hypothesis that melatonin deficiency leads to increased arterial wall stiffness. The viscoelasticity was estimated in strip preparations from pinealectomized (pin, melatonin deficiency) and sham-operated (sham, normal melatonin) adult rats using the forced oscillations method. In the untreated aortic wall pin, the viscoelasticity was not significantly altered. However, combined with 10-9 M fentanyl, the pin increased the natural frequency (f0) and modulus of elasticity (E') compared to the sham-operated. Independently, fentanyl treatment decreased f0 and E' compared separately to untreated sham and pin preparations. The effects of fentanyl were neither dose-dependent nor affected by naloxone, suggesting a non-opioid mechanism. Furthermore, an independent effect of naloxone was also detected in the normal rat aortic wall, resulting in reduced E'. Additional studies are needed that may improve the clinical decisions for pain management and anesthesia for certain patients with co-occurring chronic low levels of blood plasma melatonin and some diseases.

"If I Were in Nature's Place, I Would Do It Like This..." Life and Hypotheses of Alexey Olovnikov

In this article, we commemorate the life and scientific journey of the brilliant gerontologist-theorist Alexey Olovnikov (1936-2022). In 1971, he published his famous "marginotomy" hypothesis, in which he predicted the replicative shortening of telomeres and its role as a counter of cell divisions and biological age of an organism. This work put forth several remarkable assumptions, including the existence of telomerase, which were confirmed two decades later. Despite this, Alexey Olovnikov moved further in his theoretical studies of aging and proposed a series of new hypotheses that seem no less exotic than the marginotomy hypothesis once appeared. Alexey Olovnikov had an extraordinary way of looking at biological problems and, in addition to aging, authored striking concepts about development, biorhythms, and evolution.

Metabolic footprint in young, middle-aged and elderly rats with melatonin deficit

The pineal gland is suggested to be an essential area involved in the programming of fertility, growth, aging, and death of mammals via the released hormone melatonin.The present study aimed to ascertain the effect of melatonin deficit on several physiological and metabolic parameters, closely associated with the aging process, at certain stages of ontogenesis. Sham and rats with pinealectomy, operated at ages 3, 14, and 18-months, respectively, were tested two months later. Sham rats demonstrated an age-related decline of muscle strength, exercise endurance, motor activity, food intake, calorimetric parameters, and impaired lipid profile. Pinealectomy reduced the maximal time to exhaustion and body weight gain while diminished motor activity, food intake, O₂ consumption, CO₂ production, and energy expenditure during the Dark phase in the youngest rat group. In addition, melatonin deficit elevated arterial blood pressure (systolic, diastolic, and mean arterial pressure) and increased serum glucose and triglyceride level in 3-month-old rats while decreased the liver enzyme activity in 14-month-old rats. In conclusion, the present study brought new insights confirming the complex impact of melatonin deficit on important physiological, metabolic and biochemical markers related to aging and demonstrated for the first time that the lack of melatonin hormone is harmful in young adult rats.

Impact of Melatonin Deficit on Emotional Status and Oxidative Stress-Induced Changes in Sphingomyelin and Cholesterol Level in Young Adult, Mature, and Aged Rats

The pineal gland regulates the aging process via the hormone melatonin. The present report aims to evaluate the effect of pinealectomy (pin) on behavioral and oxidative stress-induced alterations in cholesterol and sphingomyelin (SM) levels in young adult, mature and aging rats. Sham and pin rats aged 3, 14 and 18 months were tested in behavioral tests for motor activity, anxiety, and depression. The ELISA test explored oxidative stress parameters and SM in the hippocampus, while total cholesterol was measured in serum via a commercial autoanalyzer. Mature and aged sham rats showed low motor activity and increased anxiety compared to the youngest rats. Pinealectomy affected emotional responses, induced depressive-like behavior, and elevated cholesterol levels in the youngest rats. However, removal of the pineal gland enhanced oxidative stress by diminishing antioxidant capacity and increasing the MDA level, and decreased SM level in the hippocampus of 14-month-old rats. Our findings suggest that young adult rats are vulnerable to emotional disturbance and changes in cholesterol levels resulting from melatonin deficiency. In contrast, mature rats with pinealectomy are exposed to an oxidative stress-induced decrease in SM levels in the hippocampus.

Effects of melatonin administration on plasma leptin concentration and adipose tissue leptin secretion in mice

Both melatonin and leptin show a circadian variation in circulating levels and participate in energy metabolism. An interrelationship between these two hormones has thus been proposed. In addition, melatonin has been shown to be capable of influencing circulating leptin concentration. However, whether melatonin will increase or decrease leptin production is still uncertain. This study was undertaken to examine the effect of melatonin on leptin production using male C57BL/6 adult mice treated with or without daily melatonin supplements (10 mug/mL) in drinking water for 1 month. In addition, in vitro experiments using adipose tissue fragments derived from epididymal fat pads of adult mice incubated with or without melatonin (1 nM) administration were also conducted. The results showed that melatonin-supplemented mice had significantly higher plasma leptin levels than control mice. However, melatonin incubation did not cause any marked changes in the amount of leptin secreted from adipose tissue fragments. Our findings from this study indicate that melatonin does not affect leptin secretion via mouse adipose tissue. Nevertheless, melatonin could still influence leptinemia indirectly via regulatory effects in intact animals.

The reproductive system at the neuroendocrine-immune interface: focus on LHRH, estrogens and growth factors in LHRH neuron-glial interactions

Bidirectional communication between the neuroendocrine and immune systems plays a pivotal role in health and disease. Signals generated by the hypothalamic-pituitary-gonadal (HPG) axis (i.e. luteinizing hormone-releasing hormone, LHRH, and sex steroids) are major players coordinating the development immune system function. Conversely, products generated by immune system activation exert powerful and longlasting effects on HPG axis activity. In the central nervous system (CNS), one chief neuroendocrine-immune (NEI) compartment is represented by the astroglial cell population and its mediators. Of special interest, the major supporting cells of the brain and the thymus, astrocytes and thymic epithelial cells, share a similar origin and a similar set of peptides, transmitters, hormones and cytokines functioning as paracrine/autocrine regulators. This may explain some fundamental analogies in LHRH regulation of both cell types during ontogeny and in adult life. Hence, the neuropeptide LHRH significantly modulates astrocyte and thymic cell development and function. Here we focus this work on LHRH neuron-glial signaling cascades which dictate major changes during LHRH neuronal differentiation and growth as well as in response to hormonal manipulations and pro-inflammatory challenges. The interplay between LHRH, growth factors, estrogens and pro-inflammatory mediators will be discussed, and the potential physiopathological implications of these findings summarized. The overall study highlights the plasticity of this intersystem cross-talk and emphasize neuron-glial interactions as a key regulatory level of neuroendocrine axes activity.

Neuroendocrine-immune (NEI) circuitry from neuron-glial interactions to function: Focus on gender and HPA-HPG interactions on early programming of the NEI system

Bidirectional communication between the neuroendocrine and immune systems during ontogeny plays a pivotal role in programming the development of neuroendocrine and immune responses in adult life. Signals generated by the hypothalamic-pituitary-gonadal axis (i.e. luteinizing hormone-releasing hormone, LHRH, and sex steroids), and by the hypothalamic-pituitary-adrenocortical axis (glucocorticoids (GC)), are major players coordinating the development of immune system function. Conversely, products generated by immune system activation exert a powerful and long-lasting regulation on neuroendocrine axes activity. The neuroendocrine-immune system is very sensitive to preperinatal experiences, including hormonal manipulations and immune challenges, which may influence the future predisposition to several disease entities. We review our work on the ongoing mutual regulation of neuroendocrine and immune cell activities, both at a cellular and molecular level. In the central nervous system, one chief compartment is represented by the astroglial cell and its mediators. Hence, neuron-glial signalling cascades dictate major changes in response to hormonal manipulations and pro-inflammatory triggers. The interplay between LHRH, sex steroids, GC and pro-inflammatory mediators in some physiological and pathological states, together with the potential clinical implications of these findings, are summarized. The overall study highlights the plasticity of this intersystem cross-talk for pharmacological targeting with drugs acting at the neuroendocrine-immune interface.

Gender, neuroendocrine-immune interactions and neuron-glial plasticity. Role of luteinizing hormone-releasing hormone (LHRH)

Signals generated by the hypothalamic-pitutary-gonadal (HPG) axis powerfully modulate immune system function. This article summarizes some aspects of the impact of gender in neuroendocrine immunomodulation. Emphasis is given to the astroglial cell compartment, defined as a key actor in neuroendocrine immune communications. In the brain, the principal hormones of the HPG axis directly interact with astroglial cells. Thus, luteinizing hormone releasing hormone, LHRH, influences hypothalamic astrocyte development and growth, and hypothalamic astrocytes direct LHRH neuron differentiation. Hormonally induced changes in neuron-glial plasticity may dictate major changes in CNS output, and thus actively participate in sex dimorphic immune responses. The impact of gender in neuroimmunomodulation is further underlined by the sex dimorphism in the expression of genes encoding for neuroendocrine hormones and their receptors within the thymus, and by the potent modulation exerted by circulating sex steroids during development and immunization. The central role of glucocorticoids in the interactive communication between neuroendocrine and immune systems, and the impact of gender on hypothalamic-pituitary-adrenocortical (HPA) axis modulation is underscored in transgenic mice expressing a glucocorticoid receptor antisense RNA.

Zinc coadministration attenuates melatonin's effect on nitric oxide production in mice

Both melatonin (MEL) and zinc (Zn) are considered beneficial for anti-immunosenescence. MEL's effects on immune functions are partly attributed to an interaction with Zn. However, the augmentation of or interference with MEL's effects by coadministration of Zn remains unclear. In this study, adult older mice received either MEL (10 microg/mL), Zn (22 microg/mL), MEL+Zn, or null supplementation from drinking water for 3 mo. The results showed that treated mice, irrespective of the type of added chemicals, had higher body-weight gain and body-fat content than control mice. MEL- and Zn-treated mice also had increased serum free fatty acid levels. In addition, the MEL group had decreased serum NOx (nitrite+nitrate) values. Serum tumor necrosis factor-alpha levels were increased, although nonsignificantly, in mice that received either MEL or Zn supplementation. However, the differences described were not retained in the mice that received MEL+Zn treatment. We conclude that a high-dose Zn coadministration might exert negative influences on MEL's regulatory effects, at least on nitric oxide production.

Luteinizing hormone-releasing hormone is a primary signaling molecule in the neuroimmune network

The brain-pituitary-reproductive axis and the brain thymus-lymphoid axis are linked by an array of internal mechanisms of communication that use similar signals (neurotransmitters, peptides, growth factors, hormones) acting on similar recognition targets. Moreover, such communication networks form the basis and control each step and every level of reproductive physiology. This presentation highlights the extent to which endocrine, neural, glial, or immunologically competent cells may achieve their specific functions using common mechanisms, but employing them to different degrees. In particular, this work will focus on LHRH, the chief hormone orchestrating reproductive events. Within the thymus LHRH plays a unique role of immunomodulator, contributing to the sex-dependent changes in immune responsiveness during the estrous-menstrual cycle as well as pregnancy. From the recent cloning and sequencing of lymphocyte LHRH, the expression of LHRH receptor mRNA in lymphocyte, the transduction mechanisms involved, and the steroidogenic sensitivity of the intralymphocyte LHRH system. It would appear that this peptide may act as an immunological response modifier in the brain-pituitary-lymphoid-gonadal axis. The interplay between neuronal, endocrine, and immune compartments is further emphasized in the study of LHRH-astroglial interactions. Astrocytes are able to manufacture a wide variety of signaling agents and can secrete immunoregulatory molecules that influence immune cells, as well as the glial cells themselves. Astroglia and the immortalized hypothalamic LHRH (GT1-1) neurons communicate with an array of mechanisms, via soluble mediators as well as cell-to-cell contacts. Manipulation of astroglial-derived cytokines and nitric oxide (NO) in GT1-1 neuron-astroglia cocultures, underscores a potential cross-talk between different intra/inter-cellular mediators in the dynamic control of LHRH release. Further studies aimed to disclose at a biochemical and a molecular level such bidirectional, informative network will give us new insights into more general issues concerned with the malfunction of the neuroendocrine-immune axis.

Neuroimmunomodulation of aging. A program in the pineal gland

We have investigated for 35 years the relationship between the neuroendocrine and the thymo-lymphatic, immune system. In the last decade we have shown that the pineal gland is a main adapter and fine synchronizer of environmental variables and endogenous messages into physiological modifications of basic functions. In particular the pineal gland itself seems to regulate, via circadian, night secretion of melatonin, all basic hormonal functions and also immunity. We have shown with several in vivo models that this fundamental role of the pineal gland decays during aging. Aging itself seems to be a strictly pineal-programmed event similar to growth and puberty. The continuation of our interventions with melatonin against the typical degenerative diseases of aging must be based on an accurate evaluation of its mechanisms of action. Melatonin being a ubiquitous molecule in nature, we suggest that it has acquired during evolution of the species numerous levels of activities. In fact, melatonin can be found in a large variety of cells and tissues, and bindings sites and "receptors" have been identified in many tissues and cells of the neuroendocrine and immune system. Therefore, the progressive understanding of the aging-programming role of the pineal gland also depends on studies of melatonin and its basic regulatory function. Our present studies will be described.

Circadian melatonin and young-to-old pineal grafting postpone aging and maintain juvenile conditions of reproductive functions in mice and rats

Chronic, night administration of melatonin to aging mice and transplantation of a young pineal gland into the thymic rudiment of older mice and rats have been studied with the aim of evaluating their effects on aging of gonadal, sexual, and reproductive functions. Both melatonin administration and young-to-old pineal grafting positively affect size and function of testes and maintenance of juvenile hippocampal and testicular LHRH-receptors and beta-adrenergic receptors in the tests of old rats and mice. These results demonstrate that a pineal-directed circadian function and cyclicity is fundamental for the regulation of sexual, reproductive physiology, and that proper intervention with melatonin may potentially postpone aging of both neural and gonadal sexual function.

Neuroendocrineimmunology (NEI) at the turn of the century: towards a molecular understanding of basic mechanisms and implications for reproductive physiopathology

The interactions between the nervous, endocrine and immune systems require a complex communication network. The central nervous system (CNS) affects the immune system through endocrine, paracrine and neuronal mechanisms. Evidence that this bidirectional communication plays a vital role in the regulation of physiological homeostatic mechanisms while a disfunction of the neuroendocrineimmune balance favors the susceptibility to a number of diseases is derived largely by animal models but also by an increasing number of clinical studies in different fields, including endocrinology, reproductive physiology, pediatrics, oncology, neurology and psychiatry. An increasing number of endocrine hormones, neurotransmitters and neuropeptides are expressed in immune tissues and cells and are actively involved in the physiological regulation of immunity. Conversely, the endocrine and nervous systems harbor receptors for a wide variety of immunologically-derived substances, suggesting potential regulatory feedback loops between the three major integrative bodily systems. Major implications for the reproductive endocrinology field are that psychoneuroendocrine processes may alter fertility via immunomodulation, and that events that occur as part of immune responses influence the neuroendocrine axes, which in turn counter-regulate immune function. In the present article, some features of reproductive-immune interactions will be described, and the neuroendocrineimmune dialogue via the chief reproductive hormone, luteinizing hormone-releasing hormone (LHRH), will be summarized as prototype of intersystem crosstalk. A particular emphasis will be given to the cytokine-LHRH interrelationships both at central (i.e. especially with the astroglial compartment) and peripheral levels. The surprisingly similar communication network systems used by the gonads and the thymus will be summarized, and the sexually-driven dimorphisms dictating female versus male reproductive and immunological capacities reviewed. Evidence that neural, endocrine and immune systems work together as a single unit are emphasized in animal models and human pathologies where interruption of NEI feedback loops results in long lasting pathological consequences for the nervous, endocrine and immune functions.