Cannabinoid type 2 receptor inhibition enhances the antidepressant and proneurogenic effects of physical exercise after chronic stress.

Chronic stress is a major risk factor for neuropsychiatric conditions such as depression. Adult hippocampal neurogenesis (AHN) has emerged as a promising target to counteract stress-related disorders given the ability of newborn neurons to facilitate endogenous plasticity. Recent data sheds light on the interaction between cannabinoids and neurotrophic factors underlying the regulation of AHN, with important effects on cognitive plasticity and emotional flexibility. Since physical exercise (PE) is known to enhance neurotrophic factor levels, we hypothesised that PE could engage with cannabinoids to influence AHN and that this would result in beneficial effects under stressful conditions. We therefore investigated the actions of modulating cannabinoid type 2 receptors (CB2R), which are devoid of psychotropic effects, in combination with PE in chronically stressed animals. We found that CB2R inhibition, but not CB2R activation, in combination with PE significantly ameliorated stress-evoked emotional changes and cognitive deficits. Importantly, this combined strategy critically shaped stress-induced changes in AHN dynamics, leading to a significant increase in the rates of cell proliferation and differentiation of newborn neurons, overall reduction in neuroinflammation, and increased hippocampal levels of BDNF. Together, these results show that CB2Rs are crucial regulators of the beneficial effects of PE in countering the effects of chronic stress. Our work emphasises the importance of understanding the mechanisms behind the actions of cannabinoids and PE and provides a framework for future therapeutic strategies to treat stress-related disorders that capitalise on lifestyle interventions complemented with endocannabinoid pharmacomodulation.

Deep Brain Stimulation of the dorsal raphe abolishes serotonin 1A facilitation of AMPA receptor-mediated synaptic currents in the ventral hippocampus.

In a previous study we showed that Deep Brain Stimulation (DBS) of the rat dorsal subregion of the dorsal raphe (DRD), which sends serotonergic projections to forebrain areas, such as the ventral hippocampus, induces anxiolytic-like effects. The purpose of the present study was to investigate neurobiological alterations which might underline these behavioral effects. For that, we tested the influence of DBS upon the neuromodulatory action of serotonin on excitatory post-synaptic currents (EPSCs) in the ventral hippocampus. Male Wistar rats were submitted to high-frequency stimulation (100 µA, 100 Hz) of the DRD for 1 h during three consecutive days. On the third day, immediately after the DBS procedure, animals were euthanized. Slices of the ventral hippocampus were processed for whole cell patch clamp recordings of AMPA-receptor (AMPAR) mediated EPSCs in the CA1 area. As reported by others, we confirmed that in pre-weaning rats a high affinity 5-HT1A receptor agonist (8-OH-PIPAT, 0.5-5nM) inhibits EPSCs. However, in adult rats (non-operated or sham-operated), 8-OH-PIPAT (0.5-5 nM) increased EPSC amplitude, an effect blocked by the 5-HT1A antagonist WAY-100,635 (200 nM). Importantly, in adult rats exposed to DBS, the 5-HT1A agonist was devoid of effect. Taken together these results show that: 1) changes in 5-HT1A receptor-mediated hippocampal synaptic transmission occur with age; 2) these changes lead to a facilitatory effect of 5-HT1A receptors; 3) DBS blocks this serotonergic facilitatory action. These observations suggest that an alteration in serotonin modulation of limbic areas may underlie the psychotherapeutic effects of DBS.

Hippocampal synaptic dysfunction in the SOD1G93A mouse model of Amyotrophic Lateral Sclerosis: Reversal by adenosine A2AR blockade.

Amyotrophic Lateral Sclerosis (ALS) mostly affects motor neurons, but non-motor neural and cognitive alterations have been reported in ALS mouse models and patients. Here, we evaluated if time-dependent biphasic changes in synaptic transmission and plasticity occur in hippocampal synapses of ALS SOD1G93A mice. Recordings were performed in hippocampal slices of SOD1G93A and age-matched WT mice, in the pre-symptomatic and symptomatic stages. We found an enhancement of pre-synaptic function and increased adenosine A2A receptor levels in the hippocampus of pre-symptomatic mice. In contrast, in symptomatic mice, there was an impairment of long-term potentiation (LTP) and a decrease in NMDA receptor-mediated synaptic currents, with A2AR levels also being increased. Chronic treatment with the A2AR antagonist KW-6002, rescued LTP and A2AR values. Altogether, these findings suggest an increase in synaptic function during the pre-symptomatic stage, followed by a decrease in synaptic plasticity in the symptomatic stage, which involves over-activation of A2AR from early disease stages.

Homeostatic plasticity induced by brief activity deprivation enhances long-term potentiation in the mature rat hippocampus.

Different forms of plasticity occur concomitantly in the nervous system. Whereas homeostatic plasticity monitors and maintains neuronal activity within a functional range, Hebbian changes such as long-term potentiation (LTP) modify the relative strength of specific synapses after discrete changes in activity and are thought to provide the cellular basis for learning and memory. Here, we assessed whether homeostatic plasticity could influence subsequent LTP in acute hippocampal slices that had been briefly deprived of activity by blocking action potential generation and N-methyl-D-aspartate (NMDA) receptor activation for 3 h. Activity deprivation enhanced the frequency and the amplitude of spontaneous miniature excitatory postsynaptic currents and enhanced basal synaptic transmission in the absence of significant changes in intrinsic excitability. Changes in the threshold for Hebbian plasticity were evaluated by inducing LTP with stimulation protocols of increasing strength. We found that activity-deprived slices consistently showed higher LTP magnitude compared with control conditions even when using subthreshold theta-burst stimulation. Enhanced LTP in activity-deprived slices was also observed when picrotoxin was used to prevent the modulation of GABAergic transmission. Finally, we observed that consecutive LTP inductions attained a higher magnitude of facilitation in activity-deprived slices, suggesting that the homeostatic plasticity mechanisms triggered by a brief period of neuronal silencing can both lower the threshold and raise the ceiling for Hebbian modifications. We conclude that even brief periods of altered activity are able to shape subsequent synaptic transmission and Hebbian plasticity in fully developed hippocampal circuits.

Adenosine A(2A) receptor blockade reverts hippocampal stress-induced deficits and restores corticosterone circadian oscillation.

Maternal separation (MS) is an early life stress model that induces permanent changes in the central nervous system, impairing hippocampal long-term potentiation (LTP) and spatial working memory. There are compelling evidences for a role of hippocampal adenosine A(2A) receptors in stress-induced modifications related to cognition, thus opening a potential window for therapeutic intervention. Here, we submitted rats to MS and evaluated the long-lasting molecular, electrophysiological and behavioral impairments in adulthood. We then assessed the therapeutic potential of KW6002, a blocker of A(2A) receptors, in stress-impaired animals. We report that the blockade of A(2A) receptors was efficient in reverting the behavior, electrophysiological and morphological impairments induced by MS. In addition, this effect is associated with restoration of the hypothalamic-pituitary-adrenal axis (HPA-axis) activity, as both the plasma corticosterone levels and hippocampal glucocorticoid receptor expression pattern returned to physiological-like status after the treatment. These results reveal the involvement of A(2A) receptors in the stress-associated impairments and directly in the stress response system by showing that the dysfunction of the HPA-axis as well as the long-lasting synaptic and behavioral effects of MS can be reverted by targeting adenosine A(2A) receptors. These findings provide a novel evidence for the use of adenosine A(2A) receptor antagonists as potential therapy against psychopathologies.

Spintronic platforms for biomedical applications.

Since the fundamental discovery of the giant magnetoresistance many spintronic devices have been developed and implemented in our daily life (e.g. information storage and automotive industry). Lately, advances in the sensors technology (higher sensitivity, smaller size) have potentiated other applications, namely in the biological area, leading to the emergence of novel biomedical platforms. In particular the investigation of spintronics and its application to the development of magnetoresistive (MR) biomolecular and biomedical platforms are giving rise to a new class of biomedical diagnostic devices, suitable for bench top bioassays as well as point-of-care and point-of-use devices. Herein, integrated spintronic biochip platforms for diagnostic and cytometric applications, hybrid systems incorporating magnetoresistive sensors applied to neuroelectronic studies and biomedical imaging, namely magneto-encephalography and magneto-cardiography, are reviewed. Also lab-on-a-chip MR-based platforms to perform biological studies at the single molecule level are discussed. Overall the potential and main characteristics of such MR-based biomedical devices, comparing to the existing technologies while giving particular examples of targeted applications, are addressed.

Modulation and metamodulation of synapses by adenosine.

The presence of adenosine in all nervous system cells (neurones and glia) together with its intensive release following insults makes adenosine as a sort of 'regulator' of synaptic communication, leading to the homeostatic coordination of brain function. Besides the direct actions of adenosine on the neurosecretory mechanisms, to tune neurotransmitter release, adenosine receptors interact with other receptors as well as with transporters as part of its attempt to fine-tune synaptic transmission. This review will focus on examples of the different ways adenosine can use to modulate or metamodulate synapses, in other words, to trigger or brake the action of some neurotransmitters and neuromodulators, to cross-talk with other G protein-coupled receptors, with ionotropic receptors and with receptor kinases as well as with transporters. Most of these interactions occur through A2A receptors, which in spite of their low density in some brain areas, such as the hippocampus, may function as amplifiers of the signalling of other mediators at synapses.

Tuning and fine-tuning of synapses with adenosine.

The 'omnipresence' of adenosine in all nervous system cells (neurons and glia) together with the intensive release of adenosine following insults, makes adenosine as a sort of 'maestro' of synapses leading to the homeostatic coordination of brain function. Besides direct actions of adenosine on the neurosecretory mechanisms, where adenosine operates to tune neurotransmitter release, receptor-receptor interactions as well as interplays between adenosine receptors and transporters occur as part of the adenosine's attempt to fine tuning synaptic transmission. This review will focus on the different ways adenosine can use to trigger or brake the action of several neurotransmitters and neuromodulators. Adenosine receptors cross talk with other G protein coupled receptors (GPCRs), with ionotropic receptors and with receptor kinases. Most of these interactions occur through A2A receptors, which in spite their low density in some brain areas, such as the hippocampus, may function as metamodulators. Tonic adenosine A2A receptor activity is a required step to allow synaptic actions of neurotrophic factors, namely upon synaptic transmission at both pre- and post-synaptic level as well as upon synaptic plasticity and neuronal survival. The implications of these interactions in normal brain functioning and in neurologic and psychiatric dysfunction will be discussed.

Enhancement of long-term potentiation by brain-derived neurotrophic factor requires adenosine A2A receptor activation by endogenous adenosine.

The excitatory action of brain-derived neurotrophic factor (BDNF) on synaptic transmission is triggered by adenosine A2A receptor activation. Since high-frequency neuronal firing, such as that inducing long-term potentiation (LTP), favours both A2A receptor activation and BDNF effects on transmission, we now evaluated the influence of adenosine on the facilitatory action of BDNF upon CA1 hippocampal LTP. theta-Burst stimulation of the pyramidal inputs induced a significant and persistent increase in field EPSP slopes, and this potentiation was augmented in the presence of BDNF (20 ng/ml), an action prevented by the inhibitor of Trk receptor autophosphorylation, K252a (200 nM). Removal of endogenous extracellular adenosine with adenosine deaminase (ADA, 1 U/ml), as well as the antagonism of adenosine A2A receptors with SCH58261 (100 nM), prevented the excitatory action of BDNF upon LTP. In an adenosine depleted background (with ADA), activation of adenosine A2A receptors (with 10nM CGS21680) restored the facilitatory effect of BDNF on LTP; this was fully prevented by the protein kinase A inhibitor, H-89 (1 microM) and mimicked by the adenylate cyclase activator, forskolin (10 microM). In similar experiments, activation of adenosine inhibitory A1 receptors (with 5 nM CPA) did not affect the facilitatory effect of BDNF. In conclusion, the facilitatory action of BDNF upon hippocampal LTP is critically dependent on the presence of extracellular adenosine and A2A receptor activation through a cAMP/PKA-dependent mechanism. Since extracellular adenosine accumulates upon high-frequency neuronal firing, the present results reveal a key process to allow the influence of BDNF upon synaptic plasticity.

Interleukin-6 upregulates neuronal adenosine A1 receptors: implications for neuromodulation and neuroprotection.

The immunological response in the brain is crucial to overcome neuropathological events. Some inflammatory mediators, such as the immunoregulatory cytokine interleukin-6 (IL-6) affect neuromodulation and may also play protective roles against various noxious conditions. However, the fundamental mechanisms underlying the long-term effects of IL-6 in the brain remain unclear. We now report that IL-6 increases the expression and function of the neuronal adenosine A1 receptor, with relevant consequences to synaptic transmission and neuroprotection. IL-6-induced amplification of A1 receptor function enhances the responses to readily released adenosine during hypoxia, enables neuronal rescue from glutamate-induced death, and protects animals from chemically induced convulsing seizures. Taken together, these results suggest that IL-6 minimizes the consequences of excitotoxic episodes on brain function through the enhancement of endogenous adenosinergic signaling.

Tonic adenosine A1 and A2A receptor activation is required for the excitatory action of VIP on synaptic transmission in the CA1 area of the hippocampus.

Adenosine can regulate synaptic transmission through modulation of the action of other neurotransmitters. The influence of adenosine on VIP enhancement of synaptic transmission in hippocampal slices was investigated. Facilitation of fEPSP slope by 1 nM VIP (23.3+/-1.3%) was turned into an inhibition (-12.1+/-3.4%) when extracellular endogenous adenosine was removed using adenosine deaminase (ADA, 1U/ml). Blockade of adenosine A(1) receptors with 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 10 nM) or of A(2A) receptors with ZM241385 (20 nM) attenuated the effect of VIP. When both DPCPX and ZM241385 were present the effect of VIP was abolished. In the presence of ADA, selective A(1) receptor activation with N(6)-cyclopentyladenosine (CPA, 15 nM) or A(2A) receptor-activation with CGS21680 (10 nM) partially readmitted the excitatory effect of VIP on fEPSPs. In contrast, facilitation of PS amplitude by 1 nM VIP (19.1+/-1.2%) was attenuated in the presence of ADA or DPCPX but was not changed by ZM241385. CPA, in the presence of ADA, fully restored the effect of VIP on PS amplitude. In conclusion, VIP facilitation of synaptic transmission to hippocampal pyramidal cell dendrites is dependent on both A(1) and A(2A) receptor activation by endogenous adenosine. VIP effects on PS amplitude are only dependent on A(1) adenosine receptor activation. This differential sensitivity to adenosine modulation might be due to the different VIP circuits contributing to VIP effects on pyramidal cell dendrites and pyramidal cell bodies.

Participation of adenosine receptors in neuroprotection.

Adenosine is released from most cells, including neurons and glial cells. Once in the extracellular space, adenosine modifies cell functioning by operating G-protein-coupled receptors. Interaction between adenosine receptors and other receptors for neuromodulators might contribute to a fine tuning of neuronal function, and therefore, to neuroprotection. Manipulation of adenosine receptors may influence sleep and arousal, cognition and memory, neuronal damage and degeneration and neuronal maturation. The therapeutic implications for neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, epilepsy and multiple sclerosis are discussed.

Adenosine receptors in the nervous system: pathophysiological implications.

Adenosine is a ubiquitous homeostatic substance released from most cells, including neurones and glia. Once in the extracellular space, adenosine modifies cell functioning by operating G-protein-coupled receptors (GPCR; A(1), A(2A), A(2B), A(3)) that can inhibit (A(1)) or enhance (A(2)) neuronal communication. Interactions between adenosine receptors and other G-protein-coupled receptors, ionotropic receptors and receptors for neurotrophins also occur, and this might contribute to a fine-tuning of neuronal function. Manipulations of adenosine receptors influence sleep and arousal, cognition and memory, neuronal damage and degeneration, as well as neuronal maturation. These actions might have therapeutic implications for neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease, as well as for other neurological situations such as epilepsy, idiopathic pain or even drug addition. Peripheral side effects associated with adenosine receptor agonists limit their usefulness in therapeutics; in contrast, adenosine receptor antagonists appear to have less side effects as it is the case of the well-known non-selective antagonists theophylline (present in tea) or caffeine (abundant in coffee and tea), and their emerging beneficial actions in Parkinson's disease and Alzheimer's disease are encouraging. A(1) receptor antagonism may also be useful to enhance cognition and facilitate arousal, as well as in the periphery when deficits of neurotransmitter release occur (e.g. myasthenic syndromes). Enhancement of extracellular adenosine levels through drugs that influence its metabolism might prove useful approaches in situations such as neuropathic pain, where enhanced activation of inhibitory adenosine A(1) receptors is beneficial. One might then consider adenosine as a fine-tuning modulator of neuronal activity, which via subtle effects causes harmonic actions on neuronal activity. Whenever this homeostasis is disrupted, pathology may be installed and selective receptor antagonism or agonism required.

Activation of synaptic NMDA receptors by action potential-dependent release of transmitter during hypoxia impairs recovery of synaptic transmission on reoxygenation.

Increased levels of glutamate and the subsequent activation of NMDA receptors are responsible for neuronal damage that occurs after an ischemic or hypoxic episode. In the present work, we investigated the relative contribution of presynaptic and postsynaptic blockade of synaptic transmission, as well as of blockade of NMDA receptors, for the facilitation of recovery of synaptic transmission in the CA1 area of rat hippocampal slices exposed to prolonged (90 min) hypoxia. During hypoxia, there was a complete inhibition of field EPSPs, which was fully reversible if released adenosine was allowed to act. When adenosine A(1) receptors were blocked with the selective antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), recovery of synaptic transmission from hypoxia was significantly attenuated, and this impairment could be overcome by preventing synaptic transmission during hypoxia either with tetrodotoxin (TTX) or by switching off the afferent stimulation but not by postsynaptic blockade of transmission with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or selective blockade of adenosine A(2A) receptors. When synaptic transmission was allowed to occur during hypoxia, because of the presence of DPCPX, there was an NMDA receptor-mediated component of the EPSCs recorded in CA1 pyramidal neurons, and blockade of NMDA receptors with AP-5 restored recovery of synaptic transmission from hypoxia. It is concluded that impairment of recovery of synaptic transmission after an hypoxic insult results from activation of synaptic NMDA receptors by synaptically released glutamate and that adenosine by preventing this activation efficiently facilitates recovery.

Adenosine: does it have a neuroprotective role after all?

A neuroprotective role for adenosine is commonly assumed. Recent studies revealed that adenosine may unexpectedly, under certain circumstances, have the opposite effects contributing to neuronal damage and death. The basis for this duality may be the activation of distinct subtypes of adenosine receptors, interactions between these receptors, differential actions on neuronal and glial cells, and various time frames of adenosinergic compounds administration. If these aspects are understood, adenosine should remain an interesting target for therapeutical neuroprotective approaches after all.

Fine-tuning neuromodulation by adenosine.

In addition to its direct pre- and postsynaptic actions on neurones, adenosine is rich in nuances of priming, triggering and inhibiting the action of several neurotransmitters and neuromodulators. These actions are mediated by membrane adenosine receptors (A1, A2 and A3) and involve receptor-receptor interactions, which require, in most cases, the formation of an intermediate second messenger. The harmonic way adenosine builds its influence at synapses to control neuronal communication is operated through fine-tuning, 'synchronizing' or 'desynchronizing' receptor activation for neuropeptides such as calcitonin gene-related peptide and vasoactive intestinal peptide, nicotinic acetylcholine autofacilitatory receptors, NMDA receptors, metabotropic glutamate receptors, as well as its own adenosine receptors.

Tonic activation of A(2A) adenosine receptors unmasks, and of A(1) receptors prevents, a facilitatory action of calcitonin gene-related peptide in the rat hippocampus.

1. We investigated how manipulations of the degree of activation of adenosine A(1) and A(2A) receptors influences the action of the neuropeptide, calcitonin gene-related peptide (CGRP) on synaptic transmission in hippocampal slices. Field excitatory post-synaptic potentials (EPSPs) from the CA1 area were recorded. 2. When applied alone, CGRP (1 - 30 nM) was without effect on field EPSPs. However, CGRP (10 - 30 nM) significantly increased the field EPSP slope when applied to hippocampal slices in the presence of the A(1) receptor antagonist, 1,3-dipropyl-8-cyclopenthyl xanthine (DPCPX, 10 nM), or in the presence of the A(2A) adenosine receptor agonist CGS 21680 (10 nM). 3. The A(2A) receptor antagonist, ZM 241385 (10 nM) as well as adenosine deaminase (ADA, 2 U ml(-1)), prevented the enhancement of field EPSP slope caused by CGRP (30 nM) in the presence of DPCPX (10 nM), suggesting that this effect of CGRP requires the concomitant activation of A(2A) adenosine receptors by endogenous adenosine. 4. The protein kinase-A inhibitors, N-(2-guanidinoethyl)-5-isoquinolinesulfonamide (HA-1004, 10 microM) and adenosine 3',5'-cyclic monophosphorothioate, Rp-isomer (Rp-cAMPS, 50 microM), as well as the inhibitor of ATP-sensitive potassium (K(ATP)) channels, glibenclamide (30 microM), prevented the facilitation of synaptic transmission caused by CGRP (30 nM) in the presence of DPCPX (10 nM), suggesting that this effect of CGRP involves both K(ATP) channels and protein kinase-A. 5. It is concluded that the ability of CGRP to facilitate synaptic transmission in the CA1 area of the hippocampus is under tight control by adenosine, with tonic A(1) receptor activation by endogenous adenosine 'braking' the action of CGRP, and the A(2A) receptors triggering this action.

Modification of adenosine modulation of synaptic transmission in the hippocampus of aged rats.

We compared the modulation of synaptic transmission by adenosine A(1) receptors in the hippocampus of aged (24 months) and young adult rats (6 weeks). The adenosine A(1) receptor agonist, N(6)-cyclopentyladenosine, was less potent (P:<0.05) to inhibit synaptic transmission in aged (EC(50)=53 nM) than young adult (EC(50)=14 nM) hippocampal slices, these effects being prevented by the A(1) receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). In contrast with the lower effect of the A(1) receptor agonist, it was observed that blockade of A(1) receptors with DPCPX (50 nM), or removal of endogenous extracellular adenosine with adenosine deaminase (2 u ml(-1)), caused a more pronounced disinhibition of synaptic transmission in aged rats. Also consistent with a more intense A(1) receptor-mediated inhibitory tonus by endogenous adenosine in aged rats was the finding that to fully prevent the depression of synaptic transmission induced by 3 min hypoxia, a higher concentration of DPCPX was required in slices from aged (100 nM) than from young (50 nM) rats. It is concluded that in hippocampal slices of aged rats the efficiency of A(1) receptors to modulate synaptic transmission is reduced, but this may be compensated by an enhanced inhibitory tonus by endogenous adenosine.