Inhibitory effect of hypoxic condition on acetylcholine release is partly due to the effect of adenosine released from the tissue

Effects of acute exposure to hypoxia on sleep structure in healthy adults: A systematic review

The sleep quality of lowlanders in hypoxic environments has become increasingly important with an increase in highland and alpine activities. This study aimed to identify the effects of acute exposure to hypoxia on the sleep structure of lowlanders and to analyze the changes in sleep indicators at varying levels of hypoxia. This review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Twenty-three studies were screened and included in the quantitative analysis. The results showed that acute exposure to hypoxia reduced sleep quality in lowlanders. Post-sleep arousal events and the percentage of N1 were significantly increased, whereas total sleep time, sleep efficiency, and the percentage of N3 and rapid eye movement sleep were significantly decreased in hypoxic environments. Acute exposure to hypoxia had the greatest negative impact on wakefulness after sleep onset (WASO). In addition, a larger decrease in sleep efficiency and higher increase in the percentages of N1 and WASO were observed when lowlanders were exposed to higher levels of hypoxia. This study clarifies the quantitative effects of acute hypoxic exposure on sleep in lowlanders based on original studies and explains the sleep disorders faced by lowlanders in hypoxic environments.

Nootropics as Cognitive Enhancers: Types, Dosage and Side Effects of Smart Drugs

Nootropics, also known as "smart drugs" are a diverse group of medicinal substances whose action improves human thinking, learning, and memory, especially in cases where these functions are impaired. This review provides an up-to-date overview of the potential effectiveness and importance of nootropics. Based on their nature and their effects, this heterogeneous group of drugs has been divided into four subgroups: classical nootropic compounds, substances increasing brain metabolism, cholinergic, and plants and their extracts with nootropic effects. Each subgroup of nootropics contains several main representatives, and for each one, its uses, indications, experimental treatments, dosage, and possible side effects and contraindications are discussed. For the nootropic plant extracts, there is also a brief description of each plant representative, its occurrence, history, and chemical composition of the medicinal part. Lastly, specific recommendations regarding the use of nootropics by both ill and healthy individuals are summarized.

An Integrative Model of Effortful Control

This article presents an integrative model of effortful control, a resource-limited top-down control mechanism involved in mental tasks and physical exercises. Based on recent findings in the fields of neuroscience, social psychology and cognitive psychology, this model posits the intrinsic costs related to a weakening of the connectivity of neural networks underpinning effortful control as the main cause of mental fatigue in long and high-demanding tasks. In this framework, effort reflects three different inter-related aspects of the same construct. First, effort is a mechanism comprising a limited number of interconnected processing units that integrate information regarding the task constraints and subject’s state. Second, effort is the main output of this mechanism, namely, the effort signal that modulates neuronal activity in brain regions involved in the current task to select pertinent information. Third, effort is a feeling that emerges in awareness during effortful tasks and reflects the costs associated with goal-directed behavior. Finally, the model opens new avenues for research investigating effortful control at the behavioral and neurophysiological levels.

Post-inflammatory Ileitis Induces Non-neuronal Purinergic Signaling Adjustments of Cholinergic Neurotransmission in the Myenteric Plexus

Uncoupling between ATP overflow and extracellular adenosine formation changes purinergic signaling in post-inflammatory ileitis. Adenosine neuromodulation deficits were ascribed to feed-forward inhibition of ecto-5′-nucleotidase/CD73 by high extracellular adenine nucleotides in the inflamed ileum. Here, we hypothesized that inflammation-induced changes in cellular density may also account to unbalance the release of purines and their influence on [³H]acetylcholine release from longitudinal muscle-myenteric plexus preparations of the ileum of 2,4,6-trinitrobenzenesulfonic acid (TNBS)-treated rats. The population of S100β-positive glial cells increase, whereas Ano-1-positive interstitial cells of Cajal (ICCs) diminished, in the ileum 7-days after the inflammatory insult. In the absence of changes in the density of VAChT-positive cholinergic nerves detected by immunofluorescence confocal microscopy, the inflamed myenteric plexus released smaller amounts of [³H]acetylcholine which also became less sensitive to neuronal blockade by tetrodotoxin (1 μM). Instead, [³H]acetylcholine release was attenuated by sodium fluoroacetate (5 mM), carbenoxolone (10 μM) and A438079 (3 μM), which prevent activation of glial cells, pannexin-1 hemichannels and P2X7 receptors, respectively. Sodium fluoroacetate also decreased ATP overflow without significantly affecting the extracellular adenosine levels, thus indicating that surplus ATP release parallels reactive gliosis in post-inflammatory ileitis. Conversely, loss of ICCs may explain the lower amounts of adenosine detected in TNBS-treated preparations, since blockade of Ca_v3 (T-type) channels existing in ICCs with mibefradil (3 μM) or inhibition of the equilibrative nucleoside transporter 1 with dipyridamole (0.5 μM), both decreased extracellular adenosine. Data indicate that post-inflammatory ileitis operates a shift on purinergic neuromodulation reflecting the upregulation of ATP-releasing enteric glial cells and the depletion of ICCs accounting for decreased adenosine overflow via equilibrative nucleoside transporters.

Impact of ectonucleotidases in autonomic nervous functions

Adenine and uracil nucleotides play key functions in the autonomic nervous system (ANS). For instance, ATP acts as a neurotransmitter, co-transmitter and neuromodulator in the ANS. The purinergic system encompasses (1) receptors that respond to extracellular purines, which are designated as P1 and P2 purinoceptors, (2) purine release and uptake, and (3) a cascade of enzymes that regulate the concentration of purines near the cell surface. Ectonucleotidases and adenosine deaminase (ADA) are enzymes responsible for the hydrolysis of ATP (and other nucleotides such as ADP, UTP, UDP, AMP) and adenosine, respectively. Accordingly, these enzymes are expected to play an important role in the control of neuro-effector transmission in tissues innervated by both the sympathetic and parasympathetic divisions of the ANS. Indeed, ectonucleotidases have the ability to either terminate P2 receptor responses initiated by nucleoside triphosphates (ATP and UTP), and/or to favor the activation of ADP (e.g. P2Y1,12,13) and UDP (e.g. P2Y6) and/or adenosine (P1) specific receptors. In addition, ectonucleotidases can also importantly protect some P2 receptors from desensitization (e.g. P2X1, P2Y1). In this review, we present the (putative) roles of ectonucleotidases and ADA in the ANS with a focus on their regulatory activity at neuro-effector junctions in the following tissues: heart, vas deferens, urinary bladder, salivary glands, blood vessels and the intestine. We also present their implication in nociceptive transmission.

Feed-forward inhibition of CD73 and upregulation of adenosine deaminase contribute to the loss of adenosine neuromodulation in postinflammatory ileitis

Purinergic signalling is remarkably plastic during gastrointestinal inflammation. Thus, selective drugs targeting the “purinome” may be helpful for inflammatory gastrointestinal diseases. The myenteric neuromuscular transmission of healthy individuals is fine-tuned and controlled by adenosine acting on A_2A excitatory receptors. Here, we investigated the neuromodulatory role of adenosine in TNBS-inflamed longitudinal muscle-myenteric plexus of the rat ileum. Seven-day postinflammation ileitis lacks adenosine neuromodulation, which may contribute to acceleration of gastrointestinal transit. The loss of adenosine neuromodulation results from deficient accumulation of the nucleoside at the myenteric synapse despite the fact that the increases in ATP release were observed. Disparity between ATP outflow and adenosine deficit in postinflammatory ileitis is ascribed to feed-forward inhibition of ecto-5′-nucleotidase/CD73 by high extracellular ATP and/or ADP. Redistribution of NTPDase2, but not of NTPDase3, from ganglion cell bodies to myenteric nerve terminals leads to preferential ADP accumulation from released ATP, thus contributing to the prolonged inhibition of muscle-bound ecto-5′-nucleotidase/CD73 and to the delay of adenosine formation at the inflamed neuromuscular synapse. On the other hand, depression of endogenous adenosine accumulation may also occur due to enhancement of adenosine deaminase activity. Both membrane-bound and soluble forms of ecto-5′-nucleotidase/CD73 and adenosine deaminase were detected in the inflamed myenteric plexus. These findings provide novel therapeutic targets for inflammatory gut motility disorders.

Transferred inter-cell ischemic preconditioning-induced neuroprotection may be mediated by adenosine A1 receptors

Ischemic preconditioning-induced neuroprotection is a well-known phenomenon. We hypothesize that this form of neuroprotection is transferable among the same type of cells. To test this hypothesis, human neuroblastoma SH-SY5Y cells were induced to become neuron-like cells. Primary rat cortical neuronal cultures were also used. These cells were subjected to various lengths of short oxygen-glucose deprivation (OGD, an in vitro simulation of ischemia) and then 1-h OGD. Some cells that were not exposed to a short episode of ischemia were incubated with culture medium from the cells that had 3- or 5-min OGD. Those cells were subjected to OGD for 1h at 1 or 24h after they were exposed to the medium. Cell injury was evaluated at 24h after the 1-h OGD by lactate dehydrogenase release assay. In another experiment, cells subjected to a 3-min OGD or exposed to the medium from cells that had a 3-min OGD were harvested at 30min after the OGD or the medium exposure for Western blotting of Akt, a prosurvival protein. Our study showed that a prior episode of ischemia lasting from 3 to 10min significantly reduced the 1-h OGD-induced cell injury. Medium from cells subjected to a 3-min OGD also induced acute and delayed phases of neuroprotection in OGD-naïve human neuron-like cells and primary rat cortical neuronal cultures. Cells subjected to a 3-min OGD or incubated with the medium from cells exposed to a 3-min OGD had increased phosphorylated/activated Akt. The increased phosphorylated Akt and neuroprotection induced by medium transferring were inhibited by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), an adenosine A1 receptor inhibitor. The 3-min OGD-induced neuroprotection was inhibited by LY294002, an Akt activation inhibitor. These results suggest that ischemic preconditioning-induced neuroprotection is transferable among the cells. Small molecules, such as adenosine, may mediate this effect.

Postoperative complications: delirium

Delirium is a common feature of the postoperative period, leading to increased morbidity and mortality and significant costs. Multiple factors predispose a patient to delirium in its hypoactive, hyperactive, or mixed forms. Tools have been validated for its quick and accurate identification to ensure timely and effective multidisciplinary intervention and treatment. A significant percentage of patients may require placement in skilled nursing facilities or similar care environments because of the long-lasting effects. The physician must be vigilant in the search for and identification of all forms of delirium and must effectively treat the underlying medical condition and symptoms.

Non-synaptic receptors and transporters involved in brain functions and targets of drug treatment

Beyond direct synaptic communication, neurons are able to talk to each other without making synapses. They are able to send chemical messages by means of diffusion to target cells via the extracellular space, provided that the target neurons are equipped with high-affinity receptors. While synaptic transmission is responsible for the 'what' of brain function, the 'how' of brain function (mood, attention, level of arousal, general excitability, etc.) is mainly controlled non-synaptically using the extracellular space as communication channel. It is principally the 'how' that can be modulated by medicine. In this paper, we discuss different forms of non-synaptic transmission, localized spillover of synaptic transmitters, local presynaptic modulation and tonic influence of ambient transmitter levels on the activity of vast neuronal populations. We consider different aspects of non-synaptic transmission, such as synaptic-extrasynaptic receptor trafficking, neuron-glia communication and retrograde signalling. We review structural and functional aspects of non-synaptic transmission, including (i) anatomical arrangement of non-synaptic release sites, receptors and transporters, (ii) intravesicular, intra- and extracellular concentrations of neurotransmitters, as well as the spatiotemporal pattern of transmitter diffusion. We propose that an effective general strategy for efficient pharmacological intervention could include the identification of specific non-synaptic targets and the subsequent development of selective pharmacological tools to influence them.

Muscarinic M(3) facilitation of acetylcholine release from rat myenteric neurons depends on adenosine outflow leading to activation of excitatory A(2A) receptors

Acetylcholine (ACh) is a major excitatory neurotransmitter in the myenteric plexus, and it regulates its own release acting via muscarinic autoreceptors. Adenosine released from stimulated myenteric neurons modulates ACh release preferentially via facilitatory A(2A) receptors. In this study, we investigated how muscarinic and adenosine receptors interplay to regulate ACh from the longitudinal muscle-myenteric plexus of the rat ileum. Blockade of the muscarinic M(2) receptor with 11-[[2-1[(diethylamino) methyl-1-piperidinyl]- acetyl]]-5,11-dihydro-6H-pyrido [2,3-b][1,4] benzodiazepine-6-one (AF-DX 116), 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) and atropine facilitated [3H]ACh release evoked by short stimulation trains (5 Hz, 200 pulses). Prolonging stimulus train length (>750 pulses) shifted muscarinic autoinhibition towards facilitatory M(3) receptors activation, as predicted by blockade with J104129 (a selective M(3) antagonist), 4-DAMP and atropine, whereas the selective M(2) antagonist, AF-DX 116, was without of effect. Blockade of A(2A) receptors with ZM 241385, inhibition of adenosine transport with dipyridamole, and inhibition of ecto-5'-nucleotidase with concanavalin A, all attenuated release inhibition caused by 4-DAMP. J104129 and 4-DAMP, but not AF-DX 116, decreased ( approximately 60%) evoked adenosine outflow (5 Hz, 3000 pulses). Oxotremorine (300 micromol L(-1)) facilitated the release of [3H]ACh (34 +/- 4%, n = 5) and adenosine (57 +/- 3%, n = 6) from stimulated myenteric neurons. 4-DAMP, dipyridamole and concanavalin A prevented oxotremorine-induced facilitation. ZM 241385 blocked oxotremorine facilitation of [3H]ACh release, but kept adenosine outflow unchanged. Thus, ACh modulates its own release from myenteric neurons by activating inhibitory M(2) and facilitatory M(3) autoreceptors. While the M(2) inhibition is prevalent during brief stimulation periods, muscarinic M(3) facilitation is highlighted during sustained nerve activity as it depends on extracellular adenosine accumulation leading to activation of facilitatory A(2A) receptors.

Relative contribution of ecto-ATPase and ecto-ATPDase pathways to the biphasic effect of ATP on acetylcholine release from myenteric motoneurons

BACKGROUND AND PURPOSE

The relative contribution of distinct ecto-nucleotidases to the modulation of purinergic signalling may depend on differential tissue distribution and substrate preference.

EXPERIMENTAL APPROACH

Extracellular ATP catabolism (assessed by high-performance liquid chromatography) and its influence on [(3)H]acetylcholine ([(3)H]ACh) release were investigated in the myenteric plexus of rat ileum in vitro.

KEY RESULTS

ATP was primarily metabolized via ecto-ATPDase (adenosine 5'-triphosphate diphosphohydrolase) into AMP, which was then dephosphorylated into adenosine by ecto-5'-nucleotidase. Alternative conversion of ATP into ADP by ecto-ATPase (adenosine 5'-triphosphatase) was more relevant at high ATP concentrations. ATP transiently increased basal [(3)H]ACh outflow in a 2',3'-O-(2,4,6-trinitrophenyl)adenosine-5'-triphosphate (TNP-ATP)-dependent, tetrodotoxin-independent manner. ATP and ATPgammaS (adenosine 5'-[gamma-thio]triphosphate), but not alpha,beta-methyleneATP, decreased [(3)H]ACh release induced by electrical stimulation. ADP and ADPbetaS (adenosine 5'[beta-thio]diphosphate) only decreased evoked [(3)H]ACh release. Inhibition by ADPbetaS was prevented by MRS 2179 (2'-deoxy-N(6)-methyl adenosine 3',5'-diphosphate diammonium salt, a selective P2Y(1) antagonist); blockade of ADP inhibition required co-application of MRS 2179 plus adenosine deaminase (which inactivates endogenous adenosine). Blockade of adenosine A(1) receptors with 1,3-dipropyl-8-cyclopentyl xanthine enhanced ADPbetaS inhibition, indicating that P2Y(1) stimulation is cut short by tonic adenosine A(1) receptor activation. MRS 2179 facilitated evoked [(3)H]ACh release, an effect reversed by the ecto-ATPase inhibitor, ARL67156, which delayed ATP conversion into ADP without affecting adenosine levels.

CONCLUSIONS AND IMPLICATIONS

ATP transiently facilitated [(3)H]ACh release from non-stimulated nerve terminals via prejunctional P2X (probably P2X(2)) receptors. Hydrolysis of ATP directly into AMP by ecto-ATPDase and subsequent formation of adenosine by ecto-5'-nucleotidase reduced [(3)H]ACh release via inhibitory adenosine A(1) receptors. Stimulation of inhibitory P2Y(1) receptors by ADP generated alternatively via ecto-ATPase might be relevant in restraining ACh exocytosis when ATP saturates ecto-ATPDase activity.

Pathoetiological model of delirium: a comprehensive understanding of the neurobiology of delirium and an evidence-based approach to prevention and treatment

Delirium is the most common complication found in the general hospital setting. Yet, we know relatively little about its actual pathophysiology. This article contains a summary of what we know to date and how different proposed intrinsic and external factors may work together or by themselves to elicit the cascade of neurochemical events that leads to the development delirium. Given how devastating delirium can be, it is imperative that we better understand the causes and underlying pathophysiology. Elaborating a pathoetiology-based cohesive model to better grasp the basic mechanisms that mediate this syndrome will serve clinicians well in aspiring to find ways to correct these cascades, instituting rational treatment modalities, and developing effective preventive techniques.

Delirium in the acute care setting: characteristics, diagnosis and treatment

Delirium is a neurobehavioral syndrome caused by the transient disruption of normal neuronal activity secondary to systemic disturbances. It is also the most common psychiatric syndrome found in the general hospital setting, its prevalence surpassing better known psychiatric disorders. This article reviews the published literature on delirium and addresses the epidemiology, known etiologic factors, presentation and characteristics of delirium, while emphasizing what is known about treatment strategies and prevention. Given increasing evidence that delirium is not always reversible and the many sequelae associated with its development, physicians must do everything possible to prevent its occurrence or shorten its duration, by recognizing its symptoms early, correcting underlying contributing causes, and using treatment strategies proven to help recover functional status.

Fine-tuning modulation of myenteric motoneurons by endogenous adenosine: on the role of secreted adenosine deaminase

Besides the well-characterized inhibitory effect of adenosine in the gastrointestinal tract mediated by A1 receptors, we recently demonstrated that endogenously generated adenosine facilitates [3H]acetylcholine release from myenteric neurons through preferential activation of prejunctional A2A receptors. The co-existence of both receptor subtypes on cholinergic neurons prompted the question of how does adenosine discriminate between these receptors to regulate synaptic transmission in the longitudinal muscle-myenteric plexus (LM-MP) of the rat ileum. Electrical stimulation of the LM-MP increased the outflow of adenosine, inosine and hypoxanthine. Myenteric neurons seem to be the main source of endogenous adenosine, since blockade of action potentials with tetrodotoxin (1 microM) or omission of Ca2+ (plus EGTA, 1 mM) in the buffer essentially abolished nucleosides release, while adenosine outflow remained unchanged when smooth muscle contractions were prevented by nifedipine (1 microM). Inhibition of ecto-5'-nucleotidase by concanavalin A (0.1 mg ml-1) produced only a moderate decrease (approximately 25%) on adenosine accumulation in the LM-MP, indicating that the extracellular catabolism of released ATP might not be a major source of the nucleoside. Data using the acetylcholinesterase inhibitor, physiostigmine (10 microM), and several subtype-specific muscarinic receptor antagonists, 4-DAMP (100 nM), AF-DX 116 (10 microM) and muscarinic toxin-7 (1 nM), suggest that cholinergic motoneurons are endowed with muscarinic M3 autoreceptors facilitating the outflow of adenosine. Surprisingly, bath samples collected after stimulating the LM-MP exhibited a relatively high adenosine deaminase (ADA) activity (0.60+/-0.07 U ml-1), which increased in parallel with the accumulation of adenosine and its deamination products. Our findings are in keeping with the hypothesis that ADA secretion, along with a less-efficient dipyridamole-sensitive nucleoside transport system, may restrict endogenous adenosine actions to the synaptic region channelling to facilitatory A2A receptors activation. Such a local environment may also limit diffusion of exogenously added adenosine towards the active zones, as we showed that this constrain may be overcome by inhibiting ADA activity with erythro-9(2-hydroxy-3-nonyl) adenine (50 microM).

Glutamate stimulation of acetylcholine release from myenteric plexus is mediated by endogenous nitric oxide

Glutamate was found to be an excitatory neurotransmitter in the enteric nervous system. Although several lines of evidence indicate a role of glutamate in the regulation of gut motility and secretion the physiological significance of glutamatergic transmission is not clear yet. We studied the effect of glutamate on [3H]acetylcholine release and nicotinamide adenine dinucleotide phosphate-diaphorase staining in longitudinal muscle strips with attached myenteric plexus of guinea pig ileum. L-glutamate (100 microM) significantly enhanced both the evoked [3H]acetylcholine release and the optical density of nicotinamide adenine dinucleotide phosphate-diaphorase positive neurones, i.e. the intensity of staining. The non-competitive N-methyl-D-aspartate receptor antagonist MK-801 (3 microM) abolished the stimulatory effect of L-glutamate on acetylcholine efflux. Similarly, the nitric oxide synthase inhibitor N(omega)-nitro-L-arginine (100 microM) significantly reduced the effect of L-glutamate on [3H]acetylcholine release and nicotinamide adenine dinucleotide phosphate-diaphorase staining. Our data suggest that endogenous nitric oxide seems to mediate the stimulatory effect of glutamate on acetylcholine release from guinea pig myenteric neurons.

Dual effects of adenosine on acetylcholine release from myenteric motoneurons are mediated by junctional facilitatory A(2A) and extrajunctional inhibitory A(1) receptors

1. The coexistence of both inhibitory A(1) and facilitatory A(2) adenosine receptors in the rat myenteric plexus prompted the question of how adenosine activates each receptor subtype to regulate cholinergic neurotransmission. 2. Exogenously applied adenosine (0.3-300 microm) decreased electrically evoked [(3)H]acetylcholine ([(3)H]ACh) release. Blocking A(1) receptors with 1,3-dipropyl-8-cyclopentylxanthine (10 nm) transformed the inhibitory action of adenosine into a facilitatory effect. Adenosine-induced inhibition was mimicked by the A(1) receptor agonist R-N(6)-phenylisopropyladenosine (0.3 microm), but the A(2A) agonist CGS 21680C (0.003 microm) produced a contrasting facilitatory effect. 3. Increasing endogenous adenosine levels, by the addition of (1) the adenosine precursor AMP (30-100 microm), (2) the adenosine kinase inhibitor 5'-iodotubercidin (10 microm) or (3) inhibitors of adenosine uptake (dipyridamole, 0.5 microm) and of deamination (erythro-9(2-hydroxy-3-nonyl)adenine, 50 microm), enhanced electrically evoked [(3)H]ACh release (5 Hz for 40 s). Release facilitation was prevented by adenosine deaminase (ADA, 0.5 U ml(-1)) and by the A(2A) receptor antagonist ZM 241385 (50 nm); these compounds decreased [(3)H]ACh release by 31+/-6% (n=7) and 37+/-10% (n=6), respectively. 4. Although inhibition of ecto-5'-nucleotidase by alpha,beta-methylene ADP (200 microm) or by concanavalin A (0.1 mg ml(-1)) attenuated endogenous adenosine formation from AMP, analysed by HPLC, the corresponding reduction in [(3)H]ACh release only became evident when stimulation of the myenteric plexus was prolonged to over 250 s. 5. In summary, we found that endogenously generated adenosine plays a predominantly tonic facilitatory effect mediated by prejunctional A(2A) receptors. Extracellular deamination and cellular uptake may restrict endogenous adenosine actions to the neuro-effector region near the release/production sites.

Cytokine-purine interactions in behavioral depression in rats

This paper reviews recent findings from our laboratories concerning metabolic and immune mediators of behavioral depression in rats. Specifically, a single injection of 6 mg/kg of reserpine substantially increases behavioral depression, as evidenced by an increase in the amount of time spent floating by independent groups of rats tested for swim performance at various times during the next week. The behavioral impairment consists of two components. An early component emerges one hour after reserpine treatment and persists for about 24 hours. The deficit is not reversed by intracranial ventricular infusion of the receptor antagonist for interleukin-1beta (IL-1beta). A second, late-component deficit appears approximately 48 hours after reserpine treatment and recovers within a week. Late-component depression is reversed by central infusion of the IL-1beta receptor antagonist, and is mimicked by central infusion of the proinflammatory cytokine. Importantly, both early and late components of reserpine-induced depression and IL-1beta induced depression are reversed by a systemic injection of the highly selective A2A adenosine receptor antagonist 8-(3-Chlorostyryl) caffeine. These data are discussed in terms of the overlap in the conservation-withdrawal reaction during sickness, traumatic stress, and major depression and the regional contribution of purines and cytokines to the organization of this reaction in the brain.

Stressor controllability and learned helplessness research in the United States: sensitization and fatigue processes

Recent work in the learned helplessness paradigm suggests that neuronal sensitization and fatigue processes are critical to producing the behavioral impairment that follows prolonged exposure to an unsignaled inescapable stressor such as a series of electric tail shocks. Here we discuss how an interaction between serotonin (5-HT) and corticosterone (CORT) sensitizes GABA neurons early in the pretreatment session with inescapable shock. We propose that this process eventually depletes GABA, thus removing an important form of inhibition on excitatory glutamate transmission in the amygdala, hippocampus, and frontal cortex. When rats are re-exposed to shock during shuttle-escape testing 24 hrs later, the loss of inhibition (as well as other excitatory effects) results in unregulated excitation of glutamate neurons. This state of neuronal over-excitation rapidly compromises metabolic homeostasis. Metabolic fatigue results in compensatory inhibition by the nucleoside adenosine, which regulates neuronal excitation with respect to energy availability. The exceptionally potent form of inhibition associated with adenosine receptor activation yields important neuroprotective benefits under conditions of metabolic failure, but also precludes the processing of information in fatigued neurons. The substrates of adaptive behavior are removed; performance deficits ensue.

Differential gene expression of adenosine A1, A2a, A2b, and A3 receptors in the human enteric nervous system

Adenosine receptors (ADORs) in the enteric nervous system may be of importance in the control of motor and secretomotor functions. Gene expression and distribution of neural adenosine A1, A2a, A2b, or A3 receptors (Rs) in the human intestine was investigated using immunochemical, Western blotting, RT-PCR, and short-circuit current (I(sc)) studies. Adenosine A1R, A2aR, A2bR, or A3R mRNAs were differentially expressed in neural and nonneural layers of the jejunum, ileum, colon, and cecum and in HT-29, T-84, T98G, and Bon cell lines. A1R, A2aR, A2bR, and A3R immunoreactivities (IRs) were differentially expressed in PGP 9.5-immunoreactive neurons. A2bR IR occurs exclusively in 50% of submucosal vasoactive intestinal peptide (VIP) neurons (interneurons, secretomotor or motor neurons) in jejunum, but not colon; A2aR is also found in other neurons. A3R IR occurs in 57% of substance P-positive jejunal submucosal neurons (putative intrinsic primary afferent neurons) and less than 10% of VIP neurons. Western blots revealed bands for A3R at 44 kDa, 52 kDa, and 66 kDa. A2aR and A2bR are coexpressed in enteric neurons and epithelial cells. 5'-N-methylcarboxamidoadenosine or carbachol evoked an increase in I(sc). A2bR IR is more prominent than A2aR IR in myenteric neurons, nerve fibers, or glia. A1R is expressed in jejunal myenteric neurons and colonic submucosal neurons. Regional differences also exist in smooth muscle expression of ADOR IR(s). It is concluded that neural and nonneural A1, A2a, A2b, and A3Rs may participate in the regulation of neural reflexes in the human gut. Clear cell and regional differences exist in ADOR gene expression, distribution, localization, and coexpression.

The nootropic drug vinpocetine inhibits veratridine-induced [Ca2+]i increase in rat hippocampal CA1 pyramidal cells

The alkaloid derivative vinpocetine (14-ethoxycarbonyl-(3alpha,16alpha-ethyl)-14,15-eburnamine; Cavinton) has a well known beneficial effect on brain function in hypoxic and ischemic conditions. While it increases CNS blood flow and improves cellular metabolism, relatively little is known about vinpocetine's underlying molecular mechanisms on the single cell level. Since apoptotic and necrotic cell damage is always preceded by an increase in [Ca2+]i, this study investigated the effect of vinpocetine on [Ca2+]i increases in acute brain slices. Sodium influx is an early event in the biochemical cascade that takes place during ischemia. The alkaloid veratridine can activate this Na+ influx, causing depolarization and increasing [Ca2+]i in the cells. Therefore, it can be used to simulate an ischemic attack in brain cells. Using a cooled CCD camera-based ratio imaging system and cell loading with fura 2/AM, the effect of vinpocetine on [Ca2+]i changes in single pyramidal neurons in the vulnerable CA1 region of rat hippocampal slices was investigated. Preperfusion and continuous administration of vinpocetine (10 microM) significantly inhibited the elevation in [Ca2+]i induced by veratridine (10 microM). When the drug was administered after veratridine, it could accelerate the recovery of cellular calcium levels. Piracetam, another nootropic used in clinical practice, could attenuate the elevation of [Ca2+]i only at a high, 1 mM, concentration. We have concluded that vinpocetine, at a pharmacologically relevant concentration, can decrease pathologically high [Ca2+]i levels in individual rat hippocampal CA1 pyramidal neurons; this effect might contribute to the neuroprotective property of the drug.

Participation of endogenous nitric oxide in the effect of hypoxia in vitro on neuro-effector transmission in guinea-pig ileum

The implication of endogenous nitric oxide in the effect of hypoxia on the neurotransmission in the enteric nervous system of guinea-pig ileum was studied in vitro. Three methodological approaches have been used: (i) Stretch-induced phases of peristaltic reflex in ileal segments; (ii) twitch contractions of longitudinal segments, evoked by electrical field stimulation; and (iii) release of [3H]acetylcholine from longitudinal muscle-myenteric plexus preparations, measured by liquid spectrophotometry. The effect of nitric oxide synthase inhibitor N(omega)-nitro-L-arginine (L-NNA, 100 microM) was studied under normoxic conditions. L-NNA did not change significantly the ascending contraction phase of peristaltic reflex and the amplitude of twitch contractions. However, the same concentration of L-NNA increased the stimulation-evoked acetylcholine release. The descending relaxation phase decreased in the presence of L-NNA. In another set of experiments, hypoxia was mimicked by replacement of oxygen from the perfusion medium with nitrogen for a period of 30 min. Hypoxia significantly decreased the ascending contraction phase, the twitch contractions, and the release of acetylcholine from the myenteric plexus. Under hypoxic conditions, pretreatment with L-NNA did not change either the contractile responses, nor the release of acetylcholine. Our results suggest that under conditions of oxygen deprivation, endogenous nitric oxide seems to be inefficient in modulating the cholinergic neurotransmission in guinea-pig ileum.

Escape deficits induced by inescapable shock and metabolic stress are reversed by adenosine receptor antagonists

We examined the relationship between metabolic stress, brain adenosine regulation, and the learned helplessness effect in four experiments in rats. Glucoprivation and metabolic inhibition were induced by treating previously restrained (nonshocked) rats with 2-deoxy-D-glucose (2DG) shortly before escape testing. Experiment 1 demonstrated that 2-deoxy-D-glucose impairs escape performance in a dose-dependent manner. Experiment 2 showed that 2-deoxy-D-glucose and shock induced escape deficits are completely reversed by peripheral administration of the adenosine receptor antagonist caffeine. This result indicates that both inescapable shock and 2-deoxy-D-glucose result in compensatory adenosine regulation which, in turn, mediates the behavioral impairment. Experiment 3 determined that 8-[p-sulfophenyl]-theophylline, a peripheral adenosine receptor antagonist, fails to reverse the escape deficit resulting from metabolic stress, whereas centrally acting theophylline does. Experiment 4 showed that the behavioral impairments from both 2-deoxy-D-glucose and inescapable shock are reversed by intracranial ventricular (icv) caffeine treatment. The results of Experiments 3 and 4 indicate that the enhanced adenosine regulation and the ensuing performance deficit resulting from 2-deoxy-D-glucose treatment occurred in the central nervous system. These data are discussed in terms of the metabolic demands of neuronal over-activation during escape testing in inescapably shocked rats and the loss of normal behavioral function due to compensatory adenosine regulation in the brain.

Activation of presynaptic A1-receptors by endogenous adenosine inhibits acetylcholine release in the guinea-pig ileum

1. It is well established that presynaptic adenosine A1-receptor activation inhibits acetylcholine (ACh) release in the guinea-pig ileum. The present study extends this observation and examines a possible role for endogenous adenosine in modulating cholinergic nerve function. 2. The actions of the adenosine uptake blocker, dipyridamole, the adenosine deaminase inhibitor, erythro-9(2-hydroxy-3-nonyl)adenine (EHNA) and the A1-receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) were examined on electrically evoked neurogenic, cholinergic twitch contractions of the guinea-pig ileum. Some additional studies measuring [3H]-ACh release were also performed. 3. Adenosine and the selective A1-receptor agonist, 2-chloroadenosine (2-CA), inhibited electrically evoked contractions and, in the case of 2-CA, [3H]-ACh release. The actions were antagonized by DPCPX. At low concentrations, dipyridamole and EHNA enhanced the effect of adenosine causing a leftward shift of the concentration-response curve. In contrast, inhibition induced by 2-CA was unaffected by either dipyridamole or EHNA. 4. When applied alone at higher concentrations, EHNA and dipyridamole produced a concentration-dependent suppression of cholinergic neurotransmission. In both cases, the effect could be reversed by DPCPX. At the same concentration, DPCPX alone produced a small but consistent increase in twitch height and [3H]-ACh release. 5. The data confirm the existence of inhibitory presynaptic adenosine A1-receptors modulating cholinergic nerve function in the guinea-pig ileum and suggests that these receptors can be activated by endogenous adenosine released either as adenosine itself or as an ATP metabolite.

Role of sodium channel inhibition in neuroprotection: effect of vinpocetine

Vinpocetine (ethyl apovincaminate) discovered during the late 1960s has successfully been used in the treatment of central nervous system disorders of cerebrovascular origin for decades. The increase in the regional cerebral blood flow in response to vinpocetine administration is well established and strengthened by new diagnostical techniques (transcranial Doppler, near infrared spectroscopy, positron emission tomography). The latest in vitro studies have revealed the effect of the compound on Ca(2+)/calmodulin dependent cyclic guanosine monophosphate-phosphodiesterase 1, voltage-operated Ca(2+) channels, glutamate receptors and voltage dependent Na(+)-channels; the latest being especially relevant to the neuroprotective action of vinpocetine. The good brain penetration profile and heterogenous brain distribution pattern (mainly in the thalamus, basal ganglia and visual cortex) of labelled vinpocetin were demonstrated by positron emission tomography in primates and man. Multicentric, randomized, placebo-controlled clinical studies proved the efficacy of orally administered vinpocetin in patients with organic psychosyndrome. Recently positron emission tomography studies have proved that vinpocetine is able to redistribute regional cerebral blood flow and enhance glucose supply of brain tissue in ischemic post-stroke patients.

Increase in plasma adenosine during brain ischemia in man: a study during transient ischemic attacks, and stroke

Adenosine is a "retaliatory metabolite" which accumulates during experimental brain ischemia and has vasodilatory and putative neuroprotective effects. The aim of this study was to assess whether human cerebral ischemia and necrosis-evaluated in the clinical models of transient ischemic attack (TIA) and stroke, respectively-acutely raise plasma adenosine levels. We studied 20 patients: 10 with TIA and 10 with stroke. In all, blood was serially sampled for assessment of plasma adenosine by an high-performance liquid chromatography method. Sampling occurred on peripheral blood during TIA and stroke upon admission, and serially thereafter every day up to 7 days and every other day up to 20 days. We found that in TIA and stroke patients, peripheral adenosine levels were increased to a similar extent upon admission (TIA = 264 +/- 53 vs. stroke = 257 +/- 60 nM, p = ns), peaked on the day 2 for TIA (300 +/- 60) and on day 3 for stroke (289 +/- 43) patients, and steadily decreased towards the normal range, reached by all TIA patients by day 5 and by stroke patients by day 15. Stroke and TIA are associated with a rapid increase in circulating plasma adenosine concentration in man, detectable in peripheral vein. The adenosine surge likely mirrors an increased production from the ischemic brain, and it lasts days (for TIA) and weeks (for stroke) after the acute event.

Endogenous interstitial adenosine in isolated myenteric neural networks varies inversely with prevailing PO2

Isolated myenteric ganglion networks were used in a perifusion protocol to characterize the response of interstitial adenosine levels to changes in prevailing PO2. The biological activity of such adenosine was assessed using inhibition of release of substance P (SP) as a functional measure of adenosine activity, and the effect of altered O2 tension on both spontaneous and elevated extracellular K+ concentration-evoked SP release from networks was determined over a range of PO2 values from hypoxic (PO2 = 54 mmHg) to hyperoxic (PO2 = 566 mmHg). Release of SP was found to be sensitive to PO2, and a linear graded relationship was obtained. Perifusion in the additional presence of the adenosine A1-receptor-selective antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) revealed considerable adenosinergic inhibition with an inverse exponential relationship and hyperoxic threshold PO2. Disinhibition of evoked SP release by DPCPX in the absence of TTX was double that observed in its presence, indicating a neural source for some of the adenosine released during hypoxia. A postulated neuroprotective role for adenosine is consistent with the demonstrated relationship between interstitial adenosine and prevailing O2 tension.

Hypoxia and brain development

Hypoxia threatens brain function during the entire life-span starting from early fetal age up to senescence. This review compares the short-term, long-term and life-spanning effects of fetal chronic hypoxia and neonatal anoxia on several behavioural paradigms including novelty-induced spontaneous and learning behaviours. Furthermore, it reveals that perinatal hypoxia is an additional threat to neurodegeneration and decline of cognitive and other behaviours during the aging process. Prenatal hypoxia evokes a temporary delay of ingrowth of cholinergic and serotonergic fibres into the hippocampus and neocortex, and causes an enhanced neurodegeneration of 5-HT-ir axons during aging. Neonatal anoxia suppresses hippocampal ChAT activity and up-regulates muscarinic receptor sites for 3H-QNB and 3H-pirenzepine binding in the hippocampus in the early postnatal age. The altered development of axonal arborization and pre- and postsynaptic cholinergic functions may be an important underlying mechanism to explain the behavioural deficits. As far as the cellular mechanisms of perinatal hypoxia is concerned, our primary aim was to study the putative importance of Ca2+ homeostasis of developing neurons by means of pharmacological interventions and by measuring the development of immunoexpression of Ca(2+)-binding proteins. We assessed that nimodipine, an L-type calcium channel blocker, prevented or attenuated the adverse behavioural and neurochemical effects of perinatal hypoxias, while it enhanced the early postnatal development of ir-Ca(2+)-binding proteins. The results are discussed in the context of different related research areas on brain development and hypoxia and ischaemia.

A1-receptor-mediated effect of adenosine on the release of acetylcholine from the myenteric plexus: role and localization of ecto-ATPase and 5'-nucleotidase

No attempt has been made so far to classify the subtypes of presynaptic inhibitory adenosine receptors located in the myenteric plexus and to localize ecto-ATPase and 5'-nucleotidase in the intestine. The release of [3H]acetylcholine and smooth muscle responses to acetylcholine were measured and the effect of selective adenosine receptor ligands was studied using field-stimulated isolated longitudinal muscle strips of guinea-pig ileum. Release of ATP and its hydrolysis rate were also measured using the luciferin-luciferase technique. A histochemical method combined with electron microscopy was used for localization of ecto-ATPase and 5'-nucleotidase, enzymes responsible for destruction of extracellular ATP, ADP and AMP. Subtype-selective A1-receptor agonists and antagonists inhibited and enhanced, respectively, the release of acetylcholine associated with neuronal activity. A significant amount of ATP was released in response to electrical stimulation and administration of carbamylcholine. The release of ATP was inhibited by atropine and 4-diphenylacetoxy-N-methylpiperidine methiodide, an M3-receptor antagonist. Hydrolysis of ATP was rapid and resulted in an accumulation of extracellular adenosine involved in presynaptic A1-receptor-mediated inhibition of acetylcholine release. While the inhibitory effect of adenosine and ATP was significantly potentiated by dipyridamol, an adenosine uptake blocker, that of 2-ms ATP was not. The effect of ATP was not competitively antagonized by 8-cyclopentyl-1,3-dipropylxanthine, a selective A1-receptor antagonist. In conclusion, axon terminals of cholinergic interneurons are equipped with inhibitory A1- and P2 gamma-receptors. Therefore, both adenosine and ATP control the release of acetylcholine through these receptors. ATP is mainly released from the smooth muscle in response to stimulation of M3-muscarinic receptors by endogenous acetylcholine (cascade transmission [Vizi E. S. et al. (1992) Neuroscience 50, 455-465]) and is rapidly hydrolysed by ecto-ATPase localized on the surface of the smooth muscle and axon terminals producing ADP and AMP, and by 5'-nucleotidase present only on the surface of smooth muscle cells producing adenosine.

Postnatal development of hippocampal and neocortical cholinergic and serotonergic innervation in rat: effects of nitrite-induced prenatal hypoxia and nimodipine treatment

Postnatal development of ingrowing cholinergic and serotonergic fiber patterns were studied in the rat hippocampus and parietal cortex employing a histochemical procedure for acetylcholinesterase as a cholinergic fiber marker, and immunocytochemistry of serotonin for serotonergic fiber staining. The rat pups were killed at postnatal days 1, 3, 5, 7, 10, and 20. The development of cholinergic and serotonergic innervation was described and the fiber density quantified under normal conditions and after long-term prenatal anemic hypoxia induced by chronic exposure to sodium nitrite. Furthermore, a third group was studied in which the nitrite hypoxia was combined with a simultaneous treatment with the Ca(2+)-entry blocker nimodipine to test the neuroprotective potential of this drug. Quantitative measurement of fiber density from postnatal day 1 to day 20 yielded the following results: (i) both neurotransmitter systems revealed an age-dependent and an anatomically-organized developmental pattern; (ii) the serotonergic innervation of the dorsal hippocampus preceded that of cholinergic afferentation in postnatal days 1-3; (iii) prenatal hypoxia induced a transient delay in the innervation of parietal neocortex and dentate gyrus for both neurotransmitter systems, but left the innervation of the cornu ammonis unaffected; and (iv) the hypoxia-induced retardation of cholinergic and serotonergic fiber development was prevented by concomitant application of the Ca(2+)-antagonist nimodipine during the hypoxia. The results indicate that prenatal hypoxia evokes a temporary delay in the cholinergic and serotonergic fiber outgrowth in cortical target areas in a region-specific manner. The hypoxia-induced growth inhibition is prevented by the calcium antagonist nimodipine, which supports the importance of the intracellular Ca2+ homeostasis of cells and growth cones in regulating axonal proliferation.

The effects of centrally administered adenosine on fetal sheep heart rate accelerations

OBJECTIVE

We examined the effect of administering a long-acting adenosine analog, L-2-N6-(phenylisopropyl) adenosine, into the cerebrospinal fluid of the fourth ventricle on fetal sheep heart rate accelerations.

STUDY DESIGN

Pregnant ewes between 123 and 130 days' gestation were anesthetized, and the fetal head was exteriorized. Catheters were placed in the fourth cerebral ventricle through the foramen magnum and in the brachial artery to record fetal heart rate. Studies were performed in unanesthetized fetuses 4 to 7 days after surgery. Accelerations were defined as a 10 beats/min rise in heart rate for at least 5 seconds.

RESULTS

The mean number of accelerations before administration of L-2-N6- (phenylisopropyl) adenosine was 3.9 +/- 0.7 (mean +/- SE) per 10-minute epoch, decreasing to 2.0 +/- 0.7 and 1.4 +/- 0.8 after instillation of 0.2 microgram and 0.5 micrograms of L-2-N6-(phenylisopropyl) adenosine, respectively (p < 0.05). Increasing the L-2-N6-(phenylisopropyl) adenosine dose to 10.0 micrograms resulted in loss of heart rate accelerations. Accelerations were not reduced when theophylline, an adenosine receptor blocker, was given before L-2-N6-(phenylisopropyl) adenosine.

CONCLUSION

Increasing amounts of a centrally administered adenosine analog progressively decreased the number of fetal sheep heart rate accelerations, most probably by suppression of brainstem sympathetic outflow.

Presynaptic inhibition by adenosine A1 receptors on guinea pig small intestinal myenteric neurons

BACKGROUND

Adenosine acts at A1 receptors to inhibit the release of most neurotransmitters. This study tested the hypothesis that both exogenous adenosine (ADO) and tonic release of endogenous ADO act at presynaptic A1 receptors to suppress excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) in myenteric neurons.

METHODS

Intracellular microelectrodes were used to study actions of ADO, the agonists 2-chloro-N6-cyclopentyl ADO, its 1-deaza derivative, 5'-N-ethylcarboxamido ADO, and CGS 21680 or the antagonists 8-cyclopentyl-1,3-dimethylxanthine, its 1,3-dipropyl analog, and 1,3-dipropyl-8-p-sulfophenylxanthine on synaptic behavior in myenteric neurons.

RESULTS

Each of the agonists suppressed slow EPSPs in all 35 AH/type 2, 8 of 10 S/type 1, and 7 of 7 nonspiking neurons. ADO also decreased neuronal excitability (n = 63) in AH/type 2 neurons. Agonists suppressed fast nicotinic EPSPs in all 20 S/type 1, 10 nonspiking, and 3 AH/type 2 neurons without having any effect on postsynaptic responses to nicotinic agonists. CCPA was more potent than CGS 21680 in suppressing EPSPs. In 30% of neurons, the only action of antagonists was to block the effect of A1 or A2 agonists on EPSPs. Agonists did not inhibit IPSPs, but unmasked robust slow IPSPs by preventing slow EPSPs. Antagonists acted alone to enhance EPSPs in 70% of neurons.

CONCLUSIONS

(1) ADO acts at presynaptic A1 sites to suppress EPSPs in all neurons, (2) IPSPs are revealed by ADO, and (3) ongoing release of endogenous ADO inhibits synaptic transmission.