Susana Esteban

Caloric restriction, resveratrol and melatonin: Role of SIRT1 and implications for aging and related-diseases.

Aging is an inevitable and multifactorial biological process. Free radicals have been implicated in aging processes; it is hypothesized that they cause cumulative oxidative damage to crucial macromolecules and are responsible for failure of multiple physiological mechanisms. However, recent investigations have also suggested that free radicals can act as modulators of several signaling pathways such as those related to sirtuins. Caloric restriction is a non-genetic manipulation that extends lifespan of several species and improves healthspan; the belief that many of these benefits are due to the induction of sirtuins has led to the search for sirtuin activators, especially sirtuin 1, the most studied. Resveratrol, a polyphenol found in red grapes, was first known for its antioxidant and antifungal properties, and subsequently has been reported several biological effects, including the activation of sirtuins. Endogenously-produced melatonin, a powerful free radical scavenger, declines with age and its loss contributes to degenerative conditions of aging. Recently, it was reported that melatonin also activates sirtuins, in addition to other functions, such as regulator of circadian rhythms or anti-inflammatory properties. The fact that melatonin and resveratrol are present in various foods, exhibiting possible synergistic effects, suggests the use of dietary ingredients to promote health and longevity.

Chronic melatonin treatment and its precursor L-tryptophan improve the monoaminergic neurotransmission and related behavior in the aged rat brain

Abstract:  Melatonin has an important role in the aging process as a potential drug to relieve oxidative damage, a likely cause of age‐associated brain dysfunction. As age advances, the nocturnal production of melatonin decreases potentially causing physiological alterations. The present experiments were performed to study in vivo the effects of exogenously administered melatonin chronically on monoaminergic central neurotransmitters serotonin (5‐HT), dopamine (DA) and norepinephrine (NE) and behavioral tests in old rats. The accumulation of 5‐hydroxy‐tryptophan (5‐HTP) and L‐3,4‐dihydroxyphenylalanine (DOPA) after decarboxylase inhibition was used as a measure of the rate of tryptophan and tyrosine hydroxylation in rat brain. Also neurotransmitters 5‐HT, DA and NE and some metabolites were quantified by HPLC. In control rats, an age‐related decline was observed in neurochemical parameters. However, chronic administration of melatonin (1 mg/kg/day, diluted in drinking water, 4 wk) significantly reversed the age‐induced deficits in all the monoaminergic neurotransmitters studied. Also, neurochemical parameters were analyzed after administration of melatonin biosynthesis precursor L‐tryptophan (240 mg/kg/day, i.p., at night for 4 wk) revealing similar improvement effects to those induced by melatonin. Behavioral data corresponded well with the neurochemical findings since spatial memory test in radial‐maze and motor coordination in rota‐rod were significantly improved after chronic melatonin treatment. In conclusion, these in vivo findings suggest that melatonin and L‐tryptophan treatments exert a long‐term effect on the 5‐HT, DA and NE neurotransmission by enhancing monoamine synthesis in aged rats, which might improve the age‐dependent deficits in cognition and motor coordination.

The α2-adrenoceptor antagonist idazoxan is an agonist at 5-HT1A autoreceptors modulating serotonin synthesis in the rat brain in vivo ☆

The in vivo effects of the alpha 2-adrenoceptor idazoxan, rauwolscine and phentolamine on alpha 2-auto/heteroreceptors and 5-HT1A autoreceptors modulating the synthesis of dopa/noradrenaline and 5-HTP/serotonin were assessed in rats, using the accumulation of dopa and 5-HTP after decarboxylase inhibition as a measure of the rate of tyrosine and tryptophan hydroxylation. The acute administration of idazoxan (0.1-40 mg/kg) induced a pronounced dose-dependent increase in the synthesis of dopa in the cerebral cortex (22-86%) and hippocampus (8-80%), as a consequence of the powerful blockade of alpha 2-autoreceptors. However, idazoxan did not increase the synthesis of 5-HTP in these brain regions, as it would have been expected by the concurrent blockade of alpha 2-heteroreceptors on serotonergic terminals. Instead, idazoxan decreased the synthesis of 5-HTP in the cerebral cortex (13-33%) and hippocampus (25-48%), suggesting that these inhibitory effects were mediated through activation of 5-HT1A autoreceptors. Similar results were obtained for rauwolscine. Pre-treatment of rats with the selective 5-HT1A receptor antagonist WAY100135 (10 mg/kg) fully antagonized the inhibitory effects of idazoxan (10 mg/kg) on 5-HTP synthesis, but it did not prevent the stimulatory effects of idazoxan on dopa synthesis. The results indicate that idazoxan is a potent and specific agonist at 5-HT1A autoreceptors modulating brain serotonin synthesis in vivo.

Alpha 2-autoreceptors and alpha 2-heteroreceptors modulating tyrosine and tryptophan hydroxylase activity in the rat brain in vivo: an investigation into the alpha 2-adrenoceptor subtypes.

The subtype determination of auto- and hetero-α2-adrenoceptors modulating the synthesis of noradrenaline (NA) and serotonin (5-HT), respectively, was assessed using the accumulation of 3,4-dihydroxyphenylalanine (dopa) and 5-hydroxytryptophan (5-HTP) after decarboxylase inhibition as a measure of the rate of tyrosine and tryptophan hydroxylation in the rat brain in vivo.In the cerebral cortex and hippocampus, Org 3770 (non-selective a2-adrenoceptor antagonist, 0.5–10 mg/kg, i.p.) increased (43%–58%) and clonidine (non-selective α2-adrenoceptor agonist, 1 mg/kg) decreased (37%–49%) the synthesis of dopa. Also the antagonist ARC 239 (α2B/C selective, 5–40 mg/kg) increased the synthesis of dopa in cortex (39%–46%) and hippocampus (17%–85%). In contrast, the antagonist BRL 44408 (α2D selective, 1–10 mg/kg) did not increase the synthesis of dopa in cortex, and increased it modestly in hippocampus only. The agonist guanoxabenz (α2B/C selective, 0.03–3 mg/kg) decreased the synthesis of dopa in both brain regions (20%–65%), whereas the agonist oxymetazoline (α2D selective, 0.1–3 mg/kg) failed to do so. These results indicated that the α2-autoreceptors that modulate the synthesis of dopa/NA are probably associated with the α2B/C-subtypes.In cortex and hippocampus, clonidine decreased (35%–53%) the synthesis of 5-HTP but Org 3770 failed to induce the opposite effect (except the 2 mg/kg dose in cortex). BRL 44408 markedly increased the synthesis of 5-HTP in cortex (113%–148%) but not in hippocampus. Similarly, also ARC 239 increased the formation of 5-HTP in cortex (36%–48%) but not in hippocampus, where it was decreased (30%–55%). Oxymetazoline decreased the synthesis of 5-HTP in hippocampus (28%–30%) but failed to do so in cortex. Guanoxabenz in the low dose range (0.03–0.3 mg/kg) did not decrease the synthesis of 5-HTP in any brain region. These results indicated that the α2-heteroreceptors that modulate the synthesis of 5-HTP/5-HT may well be different from the proposed α2B/C-autoreceptors modulating the synthesis of dopa/NA. These α2-heteroreceptors appear to be associated with the α2D-subtype.

Evolution of wakefulness, sleep and hibernation: From reptiles to mammals

Thus far, most hypotheses on the evolutionary origin of sleep only addressed the probable origin of its main states, REM and NREM. Our article presents the origin of the whole continuum of mammalian vigilance states including waking, sleep and hibernation and the causes of the alternation NREM-REM in a sleeping episode. We propose: (1) the active state of reptiles is a form of subcortical waking, without homology with the cortical waking of mammals; (2) reptilian waking gave origin to mammalian sleep; (3) reptilian basking behaviour evolved into NREM; (4) post-basking risk assessment behaviour, with motor suspension, head dipping movements, eye scanning and stretch attending postures, evolved into phasic REM; (5) post-basking, goal directed behaviour evolved into tonic REM and (6) nocturnal rest evolved to shallow torpor. A small number of changes from previous reptilian stages explain these transformations.

Acute, chronic and withdrawal effects of the cannabinoid receptor agonist WIN55212-2 on the sequential activation of MAPK/Raf-MEK-ERK signaling in the rat cerebral frontal cortex: short-term regulation by intrinsic and extrinsic pathways.

The cannabinoids (CB) modulate the extracellular signal‐regulated kinase (ERK), leading to various forms of plasticity in the brain. Little is known, however, on the in vivo short‐ and long‐term activation and regulation of the components of mitogen‐activated protein kinase (MAPK)/ERK signaling by CB. The CB agonist WIN55212‐2 (8 mg/kg) increased the immunodensities of phosphorylated c‐Raf‐1 (42%), MEK1/2 (63%), ERK1 (24%), and ERK2 (28%) in the rat cerebral frontal cortex. These effects were antagonized by SR141716A (rimonabant, 10 mg/kg), a selective CB1 receptor antagonist. Repeated WIN55212‐2 treatment (2–8 mg/kg for 5 days) resulted in tachyphylaxis to the acute activation of Raf‐MEK‐ERK signaling. Acute WIN55212‐2 also induced a hypothermic effect in rats, which was reduced after repeated administration (tolerance). Treatment with SR141716A after chronic WIN55212‐2 resulted in the expected cannabinoid withdrawal syndrome, without concomitant alterations in the phosphorylation state of c‐Raf‐1, MEK1/2, or ERK1/2. Pretreatment with SL327 (20 mg/kg, a MEK1/2 inhibitor) increased the basal phosphorylation of c‐Raf‐1 (40%) and MEK1/2 (74%; feedback regulation) and fully prevented the up‐regulation of ERK1/2 (23–31%) induced by WIN55212‐2. Pretreatment with MK801 (1 mg/kg, a NMDA receptor antagonist) effectively blocked the up‐regulation c‐Raf‐1 (41%), MEK1/2 (57%) and ERK1/2 (25–30%) induced by the CB agonist. The main findings demonstrate that the acute stimulation of CB1 receptors in the frontal cortex results in the sequential phosphorylation of Raf‐MEK‐ERK cascade, in which c‐Raf‐1 activation (rate‐limiting process) plays a crucial role. Moreover, the in vivo stimulating effect of WIN55212‐2 on Raf‐MEK‐ERK signaling is under the extrinsic regulation of an excitatory glutamatergic mechanism.

The time course of unconditioned morphine-induced psychomotor sensitization mirrors the phosphorylation of FADD and MEK/ERK in rat striatum: role of PEA-15 as a FADD-ERK binding partner in striatal plasticity.

Drugs of abuse induce behavioral neuroadaptations whose molecular mechanisms, partly known, are crucial to understanding drug addictions. The multifunctional adaptor Fas-associated protein with death domain (FADD) was recently associated with the induction of neuroplasticity. This study investigated the modulation of FADD and MAP kinase signaling, as well as their interactions with PEA-15 (phosphoprotein enriched in astrocytes-15 kDa) and Akt1 pathways, during the expression of unconditioned morphine-induced psychomotor sensitization. In morphine-pretreated rats (10mg/kg during 5 days), a challenge dose of the opiate induced a robust psychomotor sensitization at early withdrawal (3 days, SW 3), but not after a prolonged abstinence period (14 days), which was coincident with an accelerated dopamine turnover in the striatum. Marked concomitant increases in the content of p-FADD (48%) and the activation of MEK-ERK (46-79%) were quantified during the short-term expression of morphine sensitization (SW 3, in the absence of morphine challenge). At SW 3, p-PEA-15, a FADD-ERK binding partner, was also upregulated (51%) as well as the activation of its phosphorylating Akt1 kinase (49%). Notably, the MEK inhibitor SL 327 attenuated (58%) the expression of morphine-induced psychomotor sensitization (SW 3) and fully prevented the upregulation of p-FADD, p-PEA-15 and p-Akt1 at SW 3. The results indicate that the activation of MEK/ERK, the upregulation of p-FADD and that of the linking partners PEA-15/Akt1 have a major role in mediating the short-lasting expression of unconditioned psychomotor sensitization induced by morphine in rats.

Effects induced by cannabinoids on monoaminergic systems in the brain and their implications for psychiatric disorders

The endocannabinoid system and CB(1) receptors participate in the control of emotional behavior and mood through a functional coupling with the classic monoaminergic systems. In general, the acute stimulation of CB(1) receptors increases the activity (spontaneous firing rate) of noradrenergic (NE), serotonergic (5-HT) and dopaminergic (DA) neurons as well as the synthesis and/or release of the corresponding neurotransmitter in specific brain regions. Notably, the antagonist/inverse agonist rimonabant (SR141617A) can decrease the basal activity of NE and 5-HT neurons, suggesting a tonic/constitutive regulation of these neuronal systems by endocannabinoids acting at CB(1) receptors. Monoaminergic systems are modulated via CB(1) receptors by direct or indirect effects depending on the localization of this inhibitory receptor, which can be present on monoaminergic neurons themselves and/or inhibitory (GABAergic) and/or excitatory (glutamatergic) regulatory neurons. The repeated stimulation of CB(1) receptors is not associated with the induction of tolerance (receptor desensitization) on the activity of NE, 5-HT and DA neurons, in contrast to chronic agonist effects on neurotransmitter synthesis and/or release in some brain regions. CB(1) receptor desensitization may alter the direct and/or indirect effects of cannabinoid drugs modulating the functionality of monoaminergic systems. The sustained activation of monoaminergic neurons by cannabinoid drugs can also be related to changes in the function of presynaptic inhibitory α(2)-adrenoceptors or 5-HT(1A) receptors (autoreceptors and heteroreceptors), whose sensitivity is downregulated or upregulated upon chronic CB(1) agonist exposure. The functional interactions between endocannabinoids and monoaminergic systems in the brain indicate a potential role for CB(1) receptor signaling in the neurobiology of various psychiatric disorders, including major depression and schizophrenia as the major syndromes.

Ethanol desensitizes cannabinoid CB1 receptors modulating monoamine synthesis in the rat brain in vivo.

The endocannabinoid system and the cannabinoid CB(1) receptors are involved in the development of ethanol tolerance and dependence. This study aimed to investigate the in vivo sensitivity of a CB(1) receptor agonist (WIN 55,212-2) modulating the synthesis of 3,4-dihydroxy-phenylalanine/dopamine/noradrenaline (DOPA/DA/NA) and that of 5-hydroxy-tryptophan/serotonin (5-HTP/5-HT) in rat brain after ethanol treatment and withdrawal. In control rats, WIN 55,212-2 (4 mg/kg, i.p., for 1h), through a mechanism sensible to the CB(1) antagonist SR 141716A, increased the synthesis of DOPA/NA in a slice of brainstem containing the locus ceruleus (250%) and in the hippocampus (64%), and it reduced DOPA/DA synthesis in the striatum (47%). WIN 55,212-2 also decreased the synthesis of 5-HTP/5-HT in the locus ceruleus (43%), hippocampus (35%) and striatum (35%). In the locus ceruleus of ethanol-treated rats, the stimulatory effect of WIN 55,212-2 on DOPA/NA synthesis was abolished (acute treatment) or markedly attenuated (53-55%, chronic treatment and withdrawal), whereas in the hippocampus this effect was reduced only in chronic ethanol-withdrawn rats (33%). In the striatum of ethanol-treated rats (acute, chronic and withdrawal), the inhibitory effect of WIN 55,212-2 on DOPA/DA synthesis was completely blunted or markedly reduced. Similarly, the inhibitory effect of WIN 55,212-2 on 5-HTP/5-HT synthesis was reduced or abolished in the three brain regions after chronic ethanol and during withdrawal. These results indicate that treatment with ethanol in rats induces a functional desensitization of CB(1) receptors modulating the synthesis of brain monoamines.

Withdrawal from chronic ethanol increases the sensitivity of presynaptic 5-HT1A receptors modulating serotonin and dopamine synthesis in rat brain in vivo

The in vivo sensitivity of presynaptic 5-HT(1A) receptors (autoreceptors and heteroreceptors) modulating the synthesis of 5-hydroxytryptophan/serotonin (5-HTP/5-HT) and 3,4-dihydroxyphenylalanine/dopamine (DOPA/DA) in rat brain was investigated after ethanol treatment and withdrawal. In saline-treated rats as well as in acute ethanol (2 g/kg, intraperitoneally (i.p.), 2 h)- and chronic ethanol (2 g/kg for 7 days)-treated rats, a low dose of the 5-HT(1A) receptor agonist 8-hydroxy-2-di-n-propylamino-tetralin (8-OH-DPAT; 0.1 mg/kg, i.p., 1 h) did not decrease the synthesis of 5-HTP in brain (except modestly in striatum; 20% after the chronic treatment) or that of DOPA in striatum. In contrast, in chronic ethanol-withdrawn rats (24 h), 8-OH-DPAT significantly decreased the synthesis of 5-HTP in the hippocampus (29%), cerebral cortex (41%) and striatum (33%) and that of DOPA in the striatum (28%). Similar effects were induced by the mixed 5-HT(1A) agonist/D(2) antagonist buspirone (1 mg/kg, i.p., 1 h) which also decreased 5-HTP synthesis in the hippocampus (24%), cerebral cortex (36%) and striatum (35%) of chronic ethanol-withdrawn rats. These results indicate that chronic ethanol and more clearly the spontaneous withdrawal from chronic ethanol induce supersensitivity of 5-HT(1A)-auto/heteroreceptors modulating the synthesis of 5-HT and DA in rat brain.