Celia Garau

Neuropeptide S enhances memory during the consolidation phase and interacts with noradrenergic systems in the brain.

Neuropeptide S (NPS) has been shown to promote arousal and anxiolytic-like effects, as well as facilitation of fear extinction. In rodents, NPS receptors (NPSR) are prominently expressed in brain structures involved in learning and memory. Here, we investigate whether exogenous or endogenous NPS signaling can modulate acquisition, consolidation, or recall of emotional, spatial, and contextual memory traces, using two common behavioral paradigms, inhibitory avoidance (IA) and novel object recognition. In the IA paradigm, immediate and delayed post-training central NPS administration dose dependently enhanced memory retention in mice, indicating that NPS may act during the consolidation phase to enhance long-term memory. In contrast, pre-training or pre-test NPS injections were ineffective, suggesting that NPS had no effect on IA memory acquisition or recall. Peripheral administration of a synthetic NPSR antagonist attenuated NPS-induced IA memory enhancement, showing pharmacological specificity. NPS also enhanced hippocampal-dependent non-aversive memory in the novel object recognition task. In contrast, NPSR knockout mice displayed deficits in IA memory, novel object recognition, and novel place or context recognition, suggesting that activity of the endogenous NPS system is required for memory formation. Blockade of adrenergic signaling by propranolol attenuated NPS-induced memory enhancement in the IA task, indicating involvement of central noradrenergic systems. These results provide evidence for a facilitatory role of NPS in long-term memory, independent of memory content, possibly by acting as a salience signal or as an arousal-promoting factor.

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.

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.

Age-related changes in circadian rhythm of serotonin synthesis in ring doves: Effects of increased tryptophan ingestion

Alterations in the function of the hypothalamic suprachiasmatic nucleus (SCN) with age have been reported. As serotonin is an important regulator of the circadian clock located in SCN, this work studied the changes produced in the synthesis of serotonin with age using the accumulation of 5-HTP after decarboxylase inhibition as a measure of serotonin synthesis in the brain in vivo, in young and old ring doves at the onset of lights-on and lights-off. A diurnal cycle in tryptophan hydroxylation was observed in young animals, with an increased daylight synthesis and metabolism of 5-HT in hippocampus, neostriatum and hypothalamus. A single dose of melatonin (1 mg/kg, i.p., 1 h) at lighttime produced an inhibitory effect on the synthesis of 5-HT. In contrast, differences in 5-HT synthesis and metabolism between day and night disappeared in old animals indicating an absence of a circadian rhythm in 5-HT synthesis and metabolism. The administration of L-tryptophan (240 mg/kg, i.p.) strongly increased the 5-HT synthesis in young animals only during lights-off time while it increased in old ones irrespective of the administration time. These results suggest that the supplemental administration of tryptophan might aid to improve the descent in 5-HT that normally occurs, as animals get old.

Chrononutrition: Use of dissociated day/night infant milk formulas to improve the development of the wake–sleep rhythms. Effects of tryptophan

Abstract Three different lactation experiments have been tested in a double blind procedure for 3 weeks, to improve sleep–wake patterns in infants. In a control experiment, standard infant commercial milk (1.5% tryptophan) was administered without changes during the day. In a second control (inverse), enriched milk (3.4% tryptophan) was given during light-time (06.00–18.00 h), and standard commercial milk during night-time (18.00–06.00 h). During the experimental week, the infants received standard milk during light-time and tryptophan enriched milk during night-time. The infants receiving the enriched formula during dark time showed improvements in the sleep parameters studied, and no statistical differences were found between the two control lactations. The urinary metabolites of serotonin suggest that the observed improvements were due to an increased use of serotonin to melatonin synthesis. In conclusion, the chronobiological changes in the normal components of the diet can improve infantile development of sleep/wake rhythms.

Cognitive improvement by acute growth hormone is mediated by NMDA and AMPA receptors and MEK pathway.

It has been reported that Growth hormone (GH) has an immediate effect enhancing excitatory postsynaptic potentials mediated by AMPA and NMDA receptors in hippocampal area CA1. As GH plays a role in adult memory processing, this work aims to study the acute effects of GH on working memory tasks in rodents and the possible involvement of NMDA and AMPA receptors and also the MEK/ERK signalling pathway. To evaluate memory processes, two different tests were used, the spatial working memory 8-arm radial maze, and the novel object recognition as a form of non-spatial working memory test. Acute GH treatment (1mg/kg i.p., 1h) improved spatial learning in the radial maze respect to the control group either in young rats (reduction of 46% in the performance trial time and 61% in the number of errors), old rats (reduction of 38% in trial time and 48% in the number of errors), and adult mice (reduction of 32% in the performance time and 34% in the number of errors). GH treatment also increased the time spent exploring the novel object respect to the familiar object compared to the control group in young rats (from 63% to 79%), old rats (from 53% to 70%), and adult mice (from 61 to 68%). The improving effects of GH on working memory tests were blocked by the NMDA antagonist MK801 dizocilpine (0.025 mg/kg i.p.) injected 10 min before the administration of GH, in both young and old rats. In addition, the AMPA antagonist DNQX (1mg/kg i.p.) injected 10 min before the administration of GH to young rats, blocked the positive effect of GH. Moreover, in mice, the MEK inhibitor SL 327 (20mg/kg i.p.) injected 30 min before the administration of GH, blocked the positive effect of GH on radial maze and the novel object recognition. In conclusion, GH improved working memory processes through both glutamatergic receptors NMDA and AMPA and it required the activation of extracellular MEK/ERK signalling pathway. These effects could be related to the enhancement of excitatory synaptic transmission in the hippocampus reported by GH.

Age related changes in the activity-rest circadian rhythms and c-fos expression of ring doves with aging. Effects of tryptophan intake

Age related changes in the circadian rhythms and sleep quality has been linked with impairment in the function of the suprachiasmatic nucleus (SCN) and melatonin secretion. The precursor of melatonin, serotonin (5-HT) is a neurotransmitter involved in the synchronisation of the circadian clock located in SCN, which shows decreased levels with age. The present work studied the effects of L-tryptophan, the precursor of 5-HT, on the circadian activity-rest rhythm and c-fos expression in the SCN of young and old ring doves, animals diurnal and monocyclic as humans. Two hours before the onset of dark phase, animals housed in cages equipped for activity recording and maintained under 12/12 L/D conditions, received orally L-tryptophan (100 and 240 mg/kg) and, for comparative purposes, melatonin (2.5 and 5 mg/kg). The administration of both L-tryptophan and melatonin reduced the nocturnal activity of all ring doves although only the highest doses were effective in old ones. A reduced amplitude in the activity-rest rhythm was observed in old animals in comparison to youngest, but it was increased after the treatments. Sleep parameters, calculated from the activity data, indicated a worsened sleep quality in old animals but it was improved with the treatments. In addition, the expression of c-fos in the SCN was reduced after both mentioned treatments. The results point to the SCN as a target for the observed nocturnal effects of L-tryptophan and melatonin, and support the supplemental administration of the essential amino acid L-tryptophan to reverse the disturbances of the circadian activity-rest cycle related with ageing.

Improving effects of long-term growth hormone treatment on monoaminergic neurotransmission and related behavioral tests in aged rats.

An age-related decline in cognitive functions and physical performance has been associated with reductions in growth hormone (GH) secretion and brain neurotransmitter function. In vivo experiments were performed to study the long-term effects of exogenously administered GH on the central monoaminergic neurotransmitters serotonin, dopamine, and noradrenaline and behavioral tests in old Wistar rats. The accumulation of 5-hydroxytryptophan (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 vivo. Also, the content of the neurotransmitters serotonin, dopamine, and noradrenaline and some metabolites was measured by high-pressure liquid chromatography (HPLC) in the hippocampus and striatum, brain regions involved in adult memory processing and motor coordination. The age-related decline observed in all the neurochemical parameters in control rats was significantly reversed after repeated subcutaneous administration of GH (2 mg/kg per day, 4 weeks). Thus, GH treatment exerted a long-term effect on serotonin, dopamine, and noradrenaline neurotransmission by enhancing neurotransmitter synthesis and metabolism in aged rats. The results obtained after examining working memory tasks in the eight-radial maze and motor ability in the Rotarod treadmill in aged rats were consistent with these neurochemical data; both tests were significantly improved after chronic GH treatment. Overall, these in vivo findings suggest that the positive effects induced by GH on serotonin, dopamine, and noradrenaline neurotransmitters might explain, at least in part, the effects of chronic GH treatment in improving cognitive and motor ability in aged rats, and could aid in preventing or delaying deficits in monoamines associated with learning or motor disabilities.

Chronic treatment with selective I2-imidazoline receptor ligands decreases the content of pro-apoptotic markers in rat brain

Selective I2-imidazoline receptor ligands induce neuroprotection through various molecular mechanisms including blockade of N-methyl-D-aspartate (NMDA) receptors. To investigate new neuroprotective mechanisms associated with I2-imidazoline receptors, the effects of selective (2-styryl-2-imidazoline (LSL 61122), 2-(2-benzofuranyl)-2-imidazoline (2-BFI), 2-(4,5-dihydroimidazol-2-yl) quinoline hydrochloride (BU-224)) and non-selective (idazoxan) I2-drugs on canonical apoptotic pathways were assessed in rat brain cortex. The acute treatment with LSL 61122 (10 mg/kg) reduced the content of mitochondrial (pro-apoptotic) Bax (-33%) and cytochrome c (-31%), which was prevented by idazoxan, an I2-receptor antagonist. The sustained stimulation of I2-imidazoline receptors with selective drugs (10 mg/kg, every 12 h for seven days) was associated with down-regulation of key components of the extrinsic (Fas receptor: -20%; Fas associated protein with death domain (FADD) adaptor: -47–54%) and/or intrinsic (Bax: -20–23%; cytochrome c: -22–28%) apoptotic signalling and/or up-regulation of survival anti-apoptotic factors (p-Ser194 FADD/FADD ratio: +1.6-2.5-fold; and/or Bcl-2/Bax ratio: +1.5-fold), which in the long-term could dampen cell death in the brain. Similar chronic treatments with LSL 60101 (the imidazole analogue of 2-BFI) and idazoxan (a mixed I2/α2-ligand) did not induce significant alterations of pro- or anti-apoptotic proteins. The disclosed anti-apoptotic mechanisms of selective I2-imidazoline drugs may work in concert with other molecular mechanisms of neuroprotection (e.g. blockade of NMDA receptors) that are engaged by I2-ligands.

Intake of melatonin increases tryptophan hydroxylase type 1 activity in aged rats: Preliminary study

Pineal melatonin is important not only for synchronization of biological rhythms, but also in the ageing process as a potential drug to relieve oxidative damage. During ageing, the nocturnal melatonin production decreases resulting in an increased incidence of disorders. Present in vivo experiments were performed to study the effects of exogenous melatonin chronically administered to old rats on the pineal biosynthesis of melatonin and the precursor serotonin (5-HT) mediated by tryptophan hydroxylase type 1 (TPH-1). Accumulation of 5-hydroxytryptophan (5-HTP) after decarboxylase inhibition was used as a measure of the TPH-1 activity. 5-HT and its metabolite 5-HIAA were also quantified by HPLC-ED. As expected, ageing resulted in worsening of different neurochemical parameters. However, chronic intake of melatonin (1mg/kg/day, diluted in drinking water, 4 weeks) increased TPH-1 activity and significantly improved the age-induced deficits in nocturnal melatonin content in the pineal gland. Results suggest that melatonin intake (or melatonin rich foods) may contribute to recover the pineal function preventing the nocturnal descent of 5-HT and melatonin biosynthesis that normally occur in pineal gland as a consequence of ageing.