Melatonin attenuates isoflurane-induced acute memory impairments in aged rats

Progress in Research on the Effect of Melatonin on Postoperative Cognitive Dysfunction in Older Patients

Postoperative cognitive dysfunction (POCD) is a common complication of the central nervous system in elderly patients after operation. It will prolong the length of stay, reduce the independence and quality of daily life, and increase the risk of death. However, at present, there is a lack of safe and effective ideal drugs for the prevention and treatment of POCD. Melatonin is one of the hormones secreted by the pineal gland of the brain, which has the functions of regulating circadian rhythm, anti-inflammation, anti-oxidation, anti-apoptosis, and so on. Some recent studies have shown that MT can prevent and treat POCD by adjusting circadian rhythm, restoring cholinergic system function, neuroprotection, and so on. This article will introduce POCD, melatonin and the mechanism of melatonin on POCD, respectively, to provide a basis for clinical prevention and treatment of POCD in the elderly.

Melatonin and Rapamycin Attenuate Isoflurane-Induced Cognitive Impairment Through Inhibition of Neuroinflammation by Suppressing the mTOR Signaling in the Hippocampus of Aged Mice

Melatonin exerts neuroprotective effects on isoflurane-induced cognitive impairment. However, the underlying mechanism has yet to be elucidated. The present study sought to determine if melatonin confers its beneficial effects by acting on mammalian target of rapamycin (mTOR) and attenuates the neuroinflammation in the hippocampus of aged mice. A total of 72 male C57BL/6 mice, 16-month-old, were randomly and equally divided into six groups: (1) the control group (CON); (2) the rapamycin group (RAP); (3) the melatonin group (MEL); (4) the isoflurane group (ISO); (5) the rapamycin + isoflurane group (RAP + ISO); and (6) the melatonin + isoflurane group (MEL + ISO). RAP, RAP + ISO, MEL, MEL + ISO groups received 1 mg/kg/day mTOR inhibitor rapamycin solution or 10 mg/kg/day melatonin solution, respectively, intraperitoneally at 5:00 p.m. for 14 days consecutively. Mice in the CON and ISO groups were administered an equivalent volume of saline. Subsequently, ISO, RAP + ISO, and MEL + ISO groups were exposed to inhale 2% isoflurane for 4 h; the CON, RAP, and MEL mice received only the vehicle gas. Then, the memory function and spatial learning of the mice were examined via the Morris water maze (MWM) test. mTOR expression was detected via Western blot, whereas the concentration of inflammatory cytokines tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-6 and that of melatonin was quantified with enzyme-linked immunosorbent assay (ELISA). Melatonin and rapamycin significantly ameliorated the isoflurane-induced cognitive impairment and also led to a decrease in the melatonin levels as well as the expression levels of TNF-α, IL-1β, IL-6, and p-mTOR in the hippocampus. In conclusion, these results showed that melatonin and rapamycin attenuates mTOR expression while affecting the downstream proinflammatory cytokines. Thus, these molecular findings could be associated with an improved cognitive function in mice exposed to isoflurane.

Melatonin as a Hormone: New Physiological and Clinical Insights

Melatonin is a ubiquitous molecule present in almost every live being from bacteria to humans. In vertebrates, besides being produced in peripheral tissues and acting as an autocrine and paracrine signal, melatonin is centrally synthetized by a neuroendocrine organ, the pineal gland. Independently of the considered species, pineal hormone melatonin is always produced during the night and its production and secretory episode duration are directly dependent on the length of the night. As its production is tightly linked to the light/dark cycle, melatonin main hormonal systemic integrative action is to coordinate behavioral and physiological adaptations to the environmental geophysical day and season. The circadian signal is dependent on its daily production regularity, on the contrast between day and night concentrations, and on specially developed ways of action. During its daily secretory episode, melatonin coordinates the night adaptive physiology through immediate effects and primes the day adaptive responses through prospective effects that will only appear at daytime, when melatonin is absent. Similarly, the annual history of the daily melatonin secretory episode duration primes the central nervous/endocrine system to the seasons to come. Remarkably, maternal melatonin programs the fetuses' behavior and physiology to cope with the environmental light/dark cycle and season after birth. These unique ways of action turn melatonin into a biological time-domain-acting molecule. The present review focuses on the above considerations, proposes a putative classification of clinical melatonin dysfunctions, and discusses general guidelines to the therapeutic use of melatonin.

The Role of Neuroinflammation in Postoperative Cognitive Dysfunction: Moving From Hypothesis to Treatment

Postoperative cognitive dysfunction (POCD) is a common complication of the surgical experience and is common in the elderly and patients with preexisting neurocognitive disorders. Animal and human studies suggest that neuroinflammation from either surgery or anesthesia is a major contributor to the development of POCD. Moreover, a large and growing body of literature has focused on identifying potential risk factors for the development of POCD, as well as identifying candidate treatments based on the neuroinflammatory hypothesis. However, variability in animal models and clinical cohorts makes it difficult to interpret the results of such studies, and represents a barrier for the development of treatment options for POCD. Here, we present a broad topical review of the literature supporting the role of neuroinflammation in POCD. We provide an overview of the cellular and molecular mechanisms underlying the pathogenesis of POCD from pre-clinical and human studies. We offer a brief discussion of the ongoing debate on the root cause of POCD. We conclude with a list of current and hypothesized treatments for POCD, with a focus on recent and current human randomized clinical trials.

Epigenetic regulation of melatonin receptors in neuropsychiatric disorders

Melatonin, the primary indoleamine hormone of the mammalian pineal gland, is known to have a plethora of neuroregulatory, neuroprotective and other properties. Melatonergic signalling is mediated by its two GPCRs, MT₁ and MT₂ , which are widely expressed in the mammalian CNS. Melatonin levels and receptor expression often show a decrease during normal ageing, and this reduction may be accelerated in some disease states. Depleted melatonergic signalling has been associated with neuropsychiatric dysfunction and impairments in cognition, memory, neurogenesis and neurorestorative processes. The anticonvulsant and mood stabilizer, valproic acid (VPA), up-regulates melatonin MT₁ and/or MT₂ receptor expression in cultured cells and in the rat brain. VPA is known to affect gene expression through several mechanisms, including the modulation of intracellular kinase pathways and transcription factors, as well as the inhibition of histone deacetylase (HDAC) activity. Interestingly, other HDAC inhibitors, such as trichostatin A, which are structurally distinct from VPA, can also up-regulate melatonin receptor expression, unlike a VPA analogue, valpromide, which lacks HDAC inhibitory activity. Moreover, VPA increases histone H3 acetylation along the length of the MT₁ gene promoter in rat C6 cells. These findings indicate that an epigenetic mechanism, linked to histone hyperacetylation/chromatin remodelling and associated changes in gene transcription, is involved in the up-regulation of melatonin receptors by VPA. Epigenetic induction of MT₁ and/or MT₂ receptor expression, in areas where these receptors are lost because of ageing, injury or disease, may be a promising therapeutic avenue for the management of CNS dysfunction and other disorders. LINKED ARTICLES: This article is part of a themed section on Recent Developments in Research of Melatonin and its Potential Therapeutic Applications. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.16/issuetoc.

Epigenetic induction of melatonin MT1 receptors by valproate: Neurotherapeutic implications

We have reported that the anticonvulsant/mood stabilizer and histone deacetylase (HDAC) inhibitor valproate (VPA) induces expression of melatonin receptors both in vitro and in vivo, but the mechanisms involved were not known. Here we show that pharmacological inhibition of CREB, PKC, PI3K, or GSK3β signaling pathways, which are known targets for VPA, do not prevent its upregulation of melatonin MT₁ receptors in rat C6 glioma cells. M344, an HDAC inhibitor unrelated to VPA, mimics the effects of VPA on MT₁ expression, whereas valpromide, a VPA derivative lacking HDAC inhibitory activity, does not. Furthermore, VPA, at a concentration which upregulates the MT₁ receptor, induces histone H3 hyperacetylation along the length of the MT₁ receptor promoter. These results show that an epigenetic mechanism involving histone acetylation underlies induction of MT₁ receptor expression by VPA. Given the neuropsychiatric effects of melatonin coupled with evidence that VPA upregulates melatonin receptors in the rat brain, these findings suggest that the melatonergic system contributes to the psychotropic effects of VPA.

Isoflurane anesthesia exacerbates learning and memory impairment in zinc-deficient APP/PS1 transgenic mice

Zinc (Zn) is known to play crucial roles in numerous brain functions including learning and memory. Zn deficiency is believed to be widespread throughout the world, particularly in patients with Alzheimer's disease (AD). A number of studies have shown that volatile anesthetics, such as isoflurane, might be potential risk factors for the development of AD. However, whether isoflurane exposure accelerates the process of AD and cognitive impairment in AD patients with Zn deficiency is yet to be documented. The aim of the present study was to explore the effects of 1.4% isoflurane exposure for 2 h on learning and memory function, and neuropathogenesis in 10-month-old Zn-adequate, Zn-deficient, and Zn-treated APP/PS1 mice with the following parameters: behavioral tests, neuronal apoptosis, Aβ, and tau pathology. The results demonstrated that isoflurane exposure showed no impact on learning and memory function, but induced transient elevation of neuroapoptosis in Zn-adequate APP/PS1 mice. Exposure of isoflurane exhibited significant neuroapoptosis, Aβ generation, tau phosphorylation, and learning and memory impairment in APP/PS1 mice in the presence of Zn deficiency. Appropriate Zn treatment improved learning and memory function, and prevented isoflurane-induced neuroapoptosis in APP/PS1 mice. Isoflurane exposure may cause potential neurotoxicity, which is tolerated to some extent in Zn-adequate APP/PS1 mice. When this tolerance is limited, like in AD with Zn deficiency, isoflurane exposure markedly exacerbated learning and memory impairment, and neuropathology, indicating that AD patients with certain conditions such as Zn deficiency may be vulnerable to volatile anesthetic isoflurane.

Neuroprotective Effects Against POCD by Photobiomodulation: Evidence from Assembly/Disassembly of the Cytoskeleton

Postoperative cognitive dysfunction (POCD) is a decline in memory following anaesthesia and surgery in elderly patients. While often reversible, it consumes medical resources, compromises patient well-being, and possibly accelerates progression into Alzheimer's disease. Anesthetics have been implicated in POCD, as has neuroinflammation, as indicated by cytokine inflammatory markers. Photobiomodulation (PBM) is an effective treatment for a number of conditions, including inflammation. PBM also has a direct effect on microtubule disassembly in neurons with the formation of small, reversible varicosities, which cause neural blockade and alleviation of pain symptoms. This mimics endogenously formed varicosities that are neuroprotective against damage, toxins, and the formation of larger, destructive varicosities and focal swellings. It is proposed that PBM may be effective as a preconditioning treatment against POCD; similar to the PBM treatment, protective and abscopal effects that have been demonstrated in experimental models of macular degeneration, neurological, and cardiac conditions.

Ozone exposure of Flinders Sensitive Line rats is a rodent translational model of neurobiological oxidative stress with relevance for depression and antidepressant response

RATIONALE

Major depression has been associated with higher levels of air pollution that in turn leads to neurodegeneration via increased oxidative stress. There is a need for suitable translational animal models to study the role of oxidative stress in depression and antidepressant action.

OBJECTIVE

Considering the gene X environment hypothesis of depression, the present study investigated the effect of chronic ozone inhalation on depression and anxiety-related behavior, cognition, and brain markers of oxidative stress in the Flinders Sensitive Line (FSL) rat. In addition, response to the antioxidant melatonin, and the antidepressants desipramine or escitalopram, was assessed.

METHODS

Rats were exposed to ozone (0.0 or 0.3 parts per million (ppm)) per inhalation for 4 h daily for a period of 15 days, while simultaneously receiving saline or the above-mentioned drugs.

RESULTS

The data indicate that chronic ozone inhalation induced memory impairment, anxiety and depression-like effects, reduced cortical and hippocampal superoxide dismutase and catalase activity, and compromised central monoamine levels similar to that noted in depression. Moreover, the behavioral and neurochemical effects of melatonin, desipramine, and escitalopram were mostly attenuated in the presence of ozone.

CONCLUSION

Thus, genetically susceptible individuals exposed to high levels of oxidative stress are at higher risk of developing mood and/or an anxiety disorders, showing greater redox imbalance and altered behavior. These animals are also more resistant to contemporary antidepressant treatment. The presented model provides robust face, construct, and predictive validity, suitable for studying neuronal oxidative stress in depression, antidepressant action and mechanisms to prevent neuronal oxidative stress.

Acute melatonin treatment alters dendritic morphology and circadian clock gene expression in the hippocampus of Siberian hamsters

In the hippocampus of Siberian hamsters, dendritic length and dendritic complexity increase in the CA1 region whereas dendritic spine density decreases in the dentate gyrus region at night. However, the underlying mechanism of the diurnal rhythmicity in hippocampal neuronal remodeling is unknown. In mammals, most daily rhythms in physiology and behaviors are regulated by a network of circadian clocks. The central clock, located in the hypothalamus, controls melatonin secretion at night and melatonin modifies peripheral clocks by altering expression of circadian clock genes. In this study, we examined the effects of acute melatonin treatment on the circadian clock system as well as on morphological changes of hippocampal neurons. Male Siberian hamsters were injected with melatonin in the afternoon; 4 h later, mRNA levels of hypothalamic and hippocampal circadian clock genes and hippocampal neuron dendritic morphology were assessed. In the hypothalamus, melatonin treatment did not alter Period1 and Bmal1 expression. However, melatonin treatment increased both Period1 and Bmal1 expression in the hippocampus, suggesting that melatonin affected molecular oscillations in the hippocampus. Melatonin treatment also induced rapid remodeling of hippocampal neurons; melatonin increased apical dendritic length and dendritic complexity in the CA1 region and reduced the dendritic spine density in the dentate gyrus region. These data suggest that structural changes in hippocampal neurons are regulated by a circadian clock and that melatonin functions as a nighttime signal to coordinate the diurnal rhythm in neuronal remodeling.

Isoflurane preconditioning ameliorates renal ischemia-reperfusion injury through antiinflammatory and antiapoptotic actions in rats

Renal ischemia-reperfusion (I/R) injury is a major cause of acute kidney injury via inflammation and cell apoptosis. Volatile anesthetics have been shown to exert organ-protective effects against kidney damage in vivo and in vitro. In the present study, we investigated the effects of isoflurane, a commonly used volatile anesthetic, on renal I/R injury and the underlying mechanisms. Rats subjected to renal I/R displayed higher serum creatinine and blood urea nitrogen levels than sham rats as well as severe histopathological damage. Renal I/R also resulted in a nuclear factor-κB (NF-κB)-mediated inflammatory response and dysfunction of the p53-Bax-caspase-3 apoptotic pathway. Rats preconditioned with 1.5% isoflurane for 2 h had better renal function and less tubular apoptosis 24 h after I/R injury than control rats. Pretreatment with isoflurane suppressed renal NF-κB activation, leading to a reduction in proinflammatory molecules (high-mobility group box 1, interleukin-1β, and tumor necrosis factor-α) both in the kidneys and circulation. In addition, rats subjected to isoflurane preconditioning had a higher Bcl-2/Bax ratio and less cleaved caspase-3. Our findings suggest that preconditioning with a clinically relevant concentration of isoflurane attenuates renal I/R injury, based at least in part on its ability to modulate renal inflammation and apoptosis.

Activation of the canonical nuclear factor-κB pathway is involved in isoflurane-induced hippocampal interleukin-1β elevation and the resultant cognitive deficits in aged rats

Although much recent evidence has demonstrated that neuroinflammation contributes to volatile anesthetic-induced cognitive deficits, there are few existing mechanistic explanations for this inflammatory process. This study was conducted to investigate the effects of the volatile anesthetic isoflurane on canonical nuclear factor (NF)-κB signaling, and to explore its association with hippocampal interleukin (IL)-1β levels and anesthetic-related cognitive changes in aged rats. After a 4-h exposure to 1.5% isoflurane in 20-month-old rats, increases in IκB kinase and IκB phosphorylation, as well as a reduction in the NF-κB inhibitory protein (IκBα), were observed in the hippocampi of isoflurane-exposed rats compared with control rats. These events were accompanied by an increase in NF-κB p65 nuclear translocation at 6h after isoflurane exposure and hippocampal IL-1β elevation from 1 to 6h after isoflurane exposure. Nevertheless, no significant neuroglia activation was observed. Pharmacological inhibition of NF-κB activation by pyrrolidine dithiocarbamate markedly suppressed the IL-1β increase and NF-κB signaling, and also mitigated the severity of cognitive deficits in the Morris water maze task. Overall, our results demonstrate that isoflurane-induced cognitive deficits may stem from upregulation of hippocampal IL-1β, partially via activation of the canonical NF-κB pathway, in aged rats.