Endogenous adenosine inhibits hippocampal CA1 neurones: further evidence from extra- and intracellular recording

TMS and drugs revisited 2014

The combination of pharmacology and transcranial magnetic stimulation to study the effects of drugs on TMS-evoked EMG responses (pharmaco-TMS-EMG) has considerably improved our understanding of the effects of TMS on the human brain. Ten years have elapsed since an influential review on this topic has been published in this journal (Ziemann, 2004). Since then, several major developments have taken place: TMS has been combined with EEG to measure TMS evoked responses directly from brain activity rather than by motor evoked potentials in a muscle, and pharmacological characterization of the TMS-evoked EEG potentials, although still in its infancy, has started (pharmaco-TMS-EEG). Furthermore, the knowledge from pharmaco-TMS-EMG that has been primarily obtained in healthy subjects is now applied to clinical settings, for instance, to monitor or even predict clinical drug responses in neurological or psychiatric patients. Finally, pharmaco-TMS-EMG has been applied to understand the effects of CNS active drugs on non-invasive brain stimulation induced long-term potentiation-like and long-term depression-like plasticity. This is a new field that may help to develop rationales of pharmacological treatment for enhancement of recovery and re-learning after CNS lesions. This up-dated review will highlight important knowledge and recent advances in the contribution of pharmaco-TMS-EMG and pharmaco-TMS-EEG to our understanding of normal and dysfunctional excitability, connectivity and plasticity of the human brain.

Different regional neuroinhibitory effects of adenosine on stimulus-induced patterns of bioelectric activity of rat hippocampal and neocortical tissues

Adenosine is an inhibitory modulator of brain activity with neuroprotective and anticonvulsant properties. To investigate the distribution of bioelectric activities under application of adenosine, rat hippocampal and neocortical slices were incubated with the voltage-sensitive dye RH795 and neuronal activity was monitored using a fast-imaging photodiode array combined with standard field potential recordings. The effects of adenosine (1-50 micromol/l) on the spatial distribution of stimulus-induced activities were studied in non-epileptiform as well as epileptiform conditions. Epileptiform activity was induced by omission of Mg(2+) from the bath medium. The adenosine's inhibitory effects on the amplitude and spatial extent of stimulus-induced bioelectric activity in the hippocampus were most prominent in strata radiatum and pyramidale in both control and epileptic mediums. Adenosine's inhibitory actions were different on various layers of neocortical tissues in non-epileptiform and epileptiform conditions. Layers II and III showed the most inhibition by application of adenosine in control slices. In epileptiform medium, however, adenosine exerts significant suppressive effects only in layer I of neocortical slices. The data demonstrate a region-specific modulatory potential of adenosine on neuronal network excitability in the hippocampus and neocortex. This may be important in local adenosine therapy in epilepsy.

Enhancement of wind-up by the combined administration of adenosine A1 receptor ligands on spinalized rats with carrageenan-induced inflammation

The adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) is very effective in reducing wind-up in intact but not in spinalized adult rats with carrageenan-induced inflammation, suggesting an adenosine-mediated supraspinal modulation. Since wind-up is a spinal cord mediated phenomenon but highly influenced by descending modulatory systems, especially in situations of sensitization, we assessed the possible involvement of adenosine in the modulation of wind-up. We studied the effect of the adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT) in the presence and in the absence of the adenosine A(1) receptor agonist CPA. The experiments were carried out in spinalized male Wistar rats under alpha-chloralose anaesthesia. Withdrawal reflexes, studied as single motor units, were activated by noxious mechanical and high-intensity repetitive electrical stimulation (wind-up). While CPA and CPT were not able to induce any change on wind-up when injected alone, the combination of the two drugs, in any order, lead to an important enhancement of wind-up. This enhancement not always paralleled an increase of responses to noxious mechanical stimulation, indicating that the effect is mainly located in the spinal cord. In addition, the enhancement of wind-up was not further increased by the administration of the opioid receptor antagonist naloxone. We conclude that the depression of the wind-up phenomenon observed in spinalized animals is, at least in part, dependent of adenosine systems and can be relieved by the combined administration of CPA and CPT.

The A3 adenosine receptor agonist 2-Cl-IB-MECA facilitates epileptiform discharges in the CA3 area of immature rat hippocampal slices

The effects of the A(3) adenosine receptor agonist 2-Cl-IB-MECA were tested on epileptiform field potentials recorded in the CA3 area of postnatal days 10-20 immature hippocampal slices, during perfusion with the GABA(A) receptor antagonist bicuculline (10 microM). Evoked potentials: 2-Cl-IB-MECA (1-50 microM, n = 17) had consistently excitatory effects, blocked by the A(3) receptor antagonist MRS 1220 (1 microM, n = 7), but not occluded in the presence of the A(1) antagonist DPCPX (1 microM, n = 12) or the A(2A) antagonist ZM-241385 (0.1 microM, n = 12). 2-Cl-IB-MECA reversed the inhibitory effects (n = 5) of the adenosine uptake blocker nitrobenzylthioinosine (NBTI, 50 microM), but did not increase its excitatory effects (n = 19). Spontaneous discharges: 2-Cl-IB-MECA (1 microM) induced them or increased their frequency in 14/30 slices, an effect reversed by MRS 1220 (n = 3), and observed also following pre-perfusion with DPCPX (n = 11), ZM-241385 (n = 11) or both (n = 10). In the presence of the A(1) antagonist DPCPX, NBTI increased the frequency of spontaneous discharges, an effect partially reversed by MRS 1220 (n = 8), thus suggesting that a rise in endogenous adenosine during disinhibition may activate A(3) receptors. In conclusion, these findings suggest strongly that activation of A(3) receptors, following a rise in endogenous adenosine (i.e. during seizures, hypoxia), facilitates excitation, thus limiting the known inhibitory and/or neuroprotective effects of adenosine in immature brain.

Enhanced release of adenosine under cell-damaging conditions in the developing and adult mouse hippocampus

The inhibitory neuromodulator adenosine has been thought to act as an endogenous neuroprotectant against cerebral ischemia and neuronal damage. The release of preloaded [3H]adenosine from hippocampal slices from developing (7-day-old) and adult (3-month-old) mice was characterized using a superfusion system under various cell-damaging conditions, including hypoxia, hypoglycemia, ischemia, oxidative stress, and the presence of free radicals and metabolic poisons. The release of adenosine was greatly potentiated under the above conditions at both ages, with free radicals, metabolic poisons, and ischemia generally having the strongest stimulatory effects. Depolarization by K+ ions (50 mM) could then evoke more release of adenosine only in the immature hippocampus. Omission of Ca2+ from the superfusion media had no effect on the ischemia-induced release in the adults, indicating that it occurs by a Ca2+-independent system. In contrast, the release in the immature hippocampus was partially dependent on extracellular Ca2+. Furthermore, the ischemia-induced adenosine release was reduced in Na+-deficient media and enhanced by ouabain at both ages, pointing to the involvement of Na+-dependent transporters. The release was also reduced by Cl- channel blockers, thus indicating that a part of the evoked release occurs through anion channels. Another inhibitory neuromodulator and cell volume regulator, taurine, was seen to enhance adenosine release in ischemia at both ages. The simultaneous release of taurine and adenosine under cell-damaging conditions could constitute an important protective mechanism against excessive amounts of excitatory amino acids, counteracting their harmful effects and preventing excitation from reaching neurotoxic levels.

Opposing facilitatory and inhibitory modulation of glutamate release elicited by cAMP production in cerebrocortical nerve terminals (synaptosomes)

Activation of cAMP-protein kinase A (PKA) is widely reported to facilitate synaptic transmission. Here, we examined the presynaptic loci of PKA action using isolated nerve terminals (synaptosoms). The adenylyl cyclase (AC) activator, forskolin, failed to have any effect on 4-aminopyridine (4-AP)-evoked glutamate release, when added alone. However, in the presence of the alkylxanthine, IBMX, forskolin strongly facilitated glutamate release. This potentiation of release was blocked by the PKA inhibitors Rp-cAMPS and H7. Given that IBMX has dual activity, antagonizing adenosine receptors as well as inhibiting cAMP phosphodiesterase, we examined the effect of a selective adenosine A(1) receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and RO20-1724, a specific phosphodiesterase inhibitor. Both unmasked the forskolin-mediated modulation of glutamate release. Conversely, the adenosine analogue, N(6)-cyclohexyladenosine (CHA), reversed the facilitation induced by forskolin+RO20-1724. Adenosine A(1) receptor activation, therefore, appears to curtail cAMP/PKA-induced potentiation of glutamate release. Looking at the targets for cAMP/PKA-mediated potentiation of glutamate release, while synaptosomal excitability was only marginally increased, basal and 4-AP-evoked-increases in [Ca(2+)](c) were substantially enhanced by forskolin+IBMX. Moreover, glutamate release elicited by Ca(2+)-ionophore (ionomycin)-induced Ca(2+)-entry was facilitated by forskolin+IBMX. cAMP/PKA-mediated facilitation of glutamate release may therefore involve modulation of Ca(2+)-entry, as well as downstream events controlling synaptic vesicle recruitment and exocytosis.

Modulatory role of adenosine receptors in insect motor nerve terminals

The effects of adenosine and ATP were studied on blowfly larvae Calliphora vicina neuromuscular preparation. Adenosine diminished (IC50 = 40 +/- 3 microM) the amplitude of nerve-evoked postsynaptic currents (EPSCs) and slightly decreased the frequency of spontaneous currents without affecting their amplitude. EPSCs were slightly reduced by ATP, and this effect was prevented by concanavalin A. Presynaptic inhibition by adenosine was temperature-dependent and insensitive to pertussis toxin. A1 agonists of vertebrate adenosine receptor CPA and NECA failed to reproduce the effect of adenosine, and 2-CADO enhanced the EPSCs. A1 antagonist DPCPX competitively inhibited adenosine action. A2 agonist DPMA potentiated EPSCs, and its effect was abolished by A2 antagonist DMPX. Adenosine and ATP failed to affect the nonquantal release of glutamate. The results show for the first time the presence of presynaptic adenosine receptors regulating transmitter release at insect motor nerve terminals and point to differences in pharmacological properties of adenosine receptor subtypes in insects and vertebrates.

Local energy depletion in the basal forebrain increases sleep

Sleep saves energy, but can brain energy depletion induce sleep? We used 2,4-dinitrophenol (DNP), a molecule which prevents the synthesis of ATP, to induce local energy depletion in the basal forebrain of rats. Three-hour DNP infusions induced elevations in extracellular concentrations of lactate, pyruvate and adenosine, as well as increases in non-REM sleep during the following night. Sleep was not affected when DNP was administered to adjacent brain areas, although the metabolic changes were similar. The amount and the timing of the increase in non-REM sleep, as well as in the concentrations of lactate, pyruvate and adenosine with 0.5-1.0 mM DNP infusion, were comparable to those induced by 3 h of sleep deprivation. Here we show that energy depletion in localized brain areas can generate sleep. The energy depletion model of sleep induction could be applied to in vitro research into the cellular mechanisms of prolonged wakefulness.

Mechanisms of adenosine release in the developing and adult mouse hippocampus

Adenosine is a neuromodulator known to inhibit the synaptic release of neurotransmitters, e.g., glutamate, and to hyperpolarize postsynaptic neurons. The release of adenosine is markedly enhanced under ischemic conditions. It may then act as an endogenous neuroprotectant against cerebral ischemia and excitotoxic neuronal damage. The mechanisms by which adenosine is released from nervous tissue are not fully known, particularly in the immature brain. We now characterized the release of [3H]adenosine from hippocampal slices from developing (7-day-old) and adult (3-month-old) mice using a superfusion system. The properties of the release differed only partially in the immature and mature hippocampus. The K(+)-evoked release was Ca2+ and Na+ dependent. Anion channels were also involved. Ionotropic glutamate receptor agonists potentiated the release in a receptor-mediated manner. Activation of metabotropic glutamate receptors enhanced the release in developing mice, with group II receptors alone being effective. The evoked adenosine release apparently provides neuroprotective effects against excitotoxicity under cell-damaging conditions. Taurine had no effect on adenosine release in adult mice, but depressed the release concentration dependently in the immature hippocampus.

Interaction of Noradrenergic and Cholinergic Agonists with Ligands Increasing K-conductance of Guinea Pig Hippocampal Neurons, in vitro

Single electrode current clamp and voltage clamp recordings were employed to study the effects of noradrenergic agonists and a cholinergic agonist (carbachol, Cch) on the resting membrane potential of CA3 neurons in guinea pig hippocampal slices. Stimulation of muscarinic and beta-adrenergic receptors depolarized, and stimulation of alpha1-adrenergic receptor hyperpolarized, CA3 neurons but the membrane potential changes were small. Hyperpolarizations or outward currents induced by baclofen, adenosine or serotonin (5-HT) were strongly potentiated by alpha-noradrenergic agonists and suppressed by Cch at concentrations ten times lower than those having any direct effects on membrane potential. Both the enhancement of the baclofen-induced hyperpolarization by phenylephrine and its suppression by Cch were pronounced at low concentrations of baclofen, but diminished at higher concentrations. The modulatory effects persisted after blockade of sodium spikes by tetrodotoxin and after blockade of fast inhibitory and excitatory synaptic transmission by picrotoxin and 6-cyano-7-nitroquinoxaline-2,3-dione. Our data suggest that, through the postsynaptic interaction with ligands activating potassium conductance, noradrenergic and muscarinic receptor stimulation can exert a stronger inhibitory and excitatory effect on CA3 pyramidal neurons at their resting membrane potential than would be expected from the changes in membrane potential induced by these neuromodulators on their own.

Phasic and tonic attenuation of EPSPs by inward rectifier K+ channels in rat hippocampal pyramidal cells

We made whole-cell recordings from CA1 pyramidal cells of hippocampal slices in combination with brief dendritic glutamate pulses to study the role of constitutive inwardly rectifying K+ channels (IRK, Kir2.0) and G-protein-activated inwardly rectifying K+ channels (GIRK, Kir3.0) in the processing of excitatory inputs. Phasic activation of GIRK channels by baclofen (20 microM) produced a reversible reduction of glutamate-evoked postsynaptic potentials (GPSPs), our equivalent of EPSPs, by about one-third. Conversely, tertiapin (30 nM), a selective inhibitor of GIRK channels, and Ba2+ (200 microM), a non-selective blocker of inwardly rectifying K+ channels, enhanced GPSPs and, in voltage-clamp experiments, reduced the underlying K+ conductances, indicating a functionally significant background GIRK conductance, in addition to constitutive IRK channel activity. When examined after suppression of endogenous adenosinergic inhibition, using either adenosine deaminase or the selective A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine, tertiapin failed to influence either the GPSPs or the inwardly rectifying K+ conductance. Voltage-clamp recordings from acutely isolated CA1 pyramidal cells not exposed to ambient adenosine exhibited no response to tertiapin, whereas Ba2+ was still capable of reducing hyperpolarizing inward rectification. Our data indicate that in hippocampal pyramidal cells, two components of the inwardly rectifying K+ conductance can be identified, which together exert a tonic modulation of excitatory synaptic input: one arises from constitutive putative IRK channels, the other is mediated by the background activity of GIRK channels that results from the tonic activation of A1 receptors by ambient adenosine.

Adenosine-mediated presynaptic modulation of glutamatergic transmission in the laterodorsal tegmentum

The laterodorsal tegmentum (LDT) neurons supply most of the cholinergic tone to the brainstem and diencephalon necessary for physiological arousal. It is known that application of adenosine in the LDT nucleus increases sleep in vivo (Portas et al., 1997) and directly inhibits LDT neurons in vitro by activating postsynaptic adenosine A(1) receptors (Rainnie et al., 1994). However, adenosine effects on synaptic inputs to LDT neurons has not been previously reported. We found that both evoked glutamatergic EPSCs and GABAergic IPSCs were reduced by adenosine (50 micrometer). A presynaptic site of action for adenosine A(1) receptors on glutamatergic afferents was suggested by the following: (1) adenosine did not affect exogenous glutamate-mediated current, (2) adenosine reduced glutamatergic miniature EPSC (mEPSC) frequency, without affecting the amplitude, and (3) inhibition of the evoked EPSC was mimicked by the A(1) agonist N6-cyclohexyladenosine (100 nm) but not by the A(2) agonist N6-[2-(3,5-dimethoxyphenyl)-2-(methylphenyl)-ethyl]-adenosine (10 nm). The A(1) receptor antagonist 8-cyclopentyltheophylline (CPT; 200 nm) potentiated the evoked EPSCs, suggesting the presence of a tonic activation of presynaptic A(1) receptors by endogenous adenosine. The adenosine kinase inhibitor, 5-iodotubercidin (10 micrometer), mimicked adenosine presynaptic and postsynaptic effects. These effects were antagonized by CPT or adenosine deaminase (0.8 IU/ml), suggesting mediation by increased extracellular endogenous adenosine. Together, these data suggest that the activity of LDT neurons is under inhibitory tone by endogenous adenosine through the activation of both presynaptic A(1) receptors on excitatory terminals and postsynaptic A(1) receptors. Furthermore, an alteration of adenosine kinase activity modifies the degree of this inhibitory tone.

Update on the pathophysiology of the epilepsies

The pathophysiology of convulsive and non-convulsive epilepsies is discussed in its primary generalised forms. Focal, clinical and experimental epilepsies, with emphasis placed on the temporal lobe epilepsies (TLE) and their pathophysiologies are also reviewed. Neurotransmitters and neuromodulators and between them, the second messenger systems are considered in the generation, maintenance or inhibition of the epileptic discharge. Action mechanisms of the more classic antiepileptic drugs are briefly summarized along with the therapeutic strategies that might achieve the final control of abnormal discharges, including genetic control as a promising alternative in the current state of research. We emphasized the study of all type of glutamate and GABA receptors and their relation with mRNA editing in the brain. Some of the genetic studies which have been so fruitful during the last ten years and which have brought new insights regarding the understanding of epileptic syndromes are summarized in this article.

Age-dependent changes in adenosine A1 receptor and uptake site binding in the mouse brain: an autoradiographic study

Ageing is a multifactorial, inevitable event of life span, which affects neurotransmission in the CNS. Since adenosine is a major neuromodulator of the synaptic activity, it was of interest to investigate the possible modification of the adenosinergic system in the brain during ageing. Using "in vitro" quantitative autoradiography and the radioactive ligands [(3)H]Cyclohexyladenosine and [(3)H]Nitrobenzylthioinosine, we have studied the distribution of A1 adenosine receptors and adenosine uptake sites in the aged mice (26 months) compared to the young ones (3 months). Our results showed a widespread reduction in A1 receptor binding in the aged animals, which was brain area-specific, occurring in areas where adenosine plays a significant neuromodulatory role such as the hippocampus, cortex, basal ganglia, and thalamus. Interestingly, the significant reduction in NBI-sensitive adenosine uptake sites was restricted to few areas of the aged brain, mainly in thalamic nuclei. Since the alterations in the density of A1 receptors and adenosine uptake sites showed no regional correlation and since no significant changes in either neuronal or glial cell number are observed, at least in hippocampus and cortex in this mouse strain during ageing, our findings could be explained by a selective age-dependent reduction of these adenosinergic components rather than by a general neuronal cell degeneration. As adenosine depresses electrical activity in hippocampus, a downregulation of adenosinergic function could probably be related to enhanced excitability seen in hippocampal neurons of the CA1 subregion and dentate gyrus of aged animals.

Investigation of the role of intracellular Ca(2+) stores in generation of the muscarinic agonist-induced slow afterdepolarization (sADP) in guinea-pig olfactory cortical neurones in vitro

1. Intracellular recordings were made from guinea-pig olfactory cortical brain slice neurones to assess the possible role of intracellular Ca(2+) stores in the generation of the slow post-stimulus afterdepolarization (sADP) and its underlying tail current (I(ADP)), induced by muscarinic receptor activation. 2. Caffeine or theophylline (0.5 - 3 mM) reduced the amplitude of the I(ADP) (measured under 'hybrid' voltage clamp) induced in the presence of the muscarinic agonist oxotremorine-M (OXO-M, 10 microM) by up to 96%, without affecting membrane properties or muscarinic depolarization of these neurones. 3. The L-type Ca(2+) channel blocker nifedipine (1, 10 microM) also inhibited I(ADP) (by up to 46%), while ryanodine (10 microM) (a blocker of Ca(2+) release from internal stores) produced a small ( approximately 10%) reduction in I(ADP) amplitude; however, neither 10 microM dantrolene (another internal Ca(2+) release blocker) nor the intracellular Ca(2+) store re-uptake inhibitors thapsigargin (3 microM) or cyclopiazonic acid (CPA, 15 microM) affected I(ADP) amplitude. 4. IBMX (100 microM), a phosphodiesterase inhibitor, also had no effect on I(ADP). Furthermore, inhibition of I(ADP) by caffeine was not reversed by co-application of 100 microM adenosine. 5. Caffeine (3 mM) or nifedipine (10 microM) reduced the duration of presumed Ca(2+) spikes revealed by intracellular Cs(+) loading. When applied in combination, nifedipine and caffeine effects were occlusive, rather than additive, suggesting a common site of action on L-type calcium channels. 6. We conclude that Ca(2+)-induced Ca(2+) release (CICR) from internal stores does not contribute significantly to muscarinic I(ADP) generation in olfactory cortical neurones. However caffeine and theophylline, which enhance CICR in other systems, blocked I(ADP) induction. We suggest that this action might involve a combination of L-type voltage-gated Ca(2+) channel blockade, and a direct inhibitory action on the putative I(ADP) K(+) conductance.

Trophic effects of purines in neurons and glial cells

In addition to their well known roles within cells, purine nucleotides such as adenosine 5' triphosphate (ATP) and guanosine 5' triphosphate (GTP), nucleosides such as adenosine and guanosine and bases, such as adenine and guanine and their metabolic products xanthine and hypoxanthine are released into the extracellular space where they act as intercellular signaling molecules. In the nervous system they mediate both immediate effects, such as neurotransmission, and trophic effects which induce changes in cell metabolism, structure and function and therefore have a longer time course. Some trophic effects of purines are mediated via purinergic cell surface receptors, whereas others require uptake of purines by the target cells. Purine nucleosides and nucleotides, especially guanosine, ATP and GTP stimulate incorporation of [3H]thymidine into DNA of astrocytes and microglia and concomitant mitosis in vitro. High concentrations of adenosine also induce apoptosis, through both activation of cell-surface A3 receptors and through a mechanism requiring uptake into the cells. Extracellular purines also stimulate the synthesis and release of protein trophic factors by astrocytes, including bFGF (basic fibroblast growth factor), nerve growth factor (NGF), neurotrophin-3, ciliary neurotrophic factor and S-100beta protein. In vivo infusion into brain of adenosine analogs stimulates reactive gliosis. Purine nucleosides and nucleotides also stimulate the differentiation and process outgrowth from various neurons including primary cultures of hippocampal neurons and pheochromocytoma cells. A tonic release of ATP from neurons, its hydrolysis by ecto-nucleotidases and subsequent re-uptake by axons appears crucial for normal axonal growth. Guanosine and GTP, through apparently different mechanisms, are also potent stimulators of axonal growth in vitro. In vivo the extracellular concentration of purines depends on a balance between the release of purines from cells and their re-uptake and extracellular metabolism. Purine nucleosides and nucleotides are released from neurons by exocytosis and from both neurons and glia by non-exocytotic mechanisms. Nucleosides are principally released through the equilibratory nucleoside transmembrane transporters whereas nucleotides may be transported through the ATP binding cassette family of proteins, including the multidrug resistance protein. The extracellular purine nucleotides are rapidly metabolized by ectonucleotidases. Adenosine is deaminated by adenosine deaminase (ADA) and guanosine is converted to guanine and deaminated by guanase. Nucleosides are also removed from the extracellular space into neurons and glia by transporter systems. Large quantities of purines, particularly guanosine and, to a lesser extent adenosine, are released extracellularly following ischemia or trauma. Thus purines are likely to exert trophic effects in vivo following trauma. The extracellular purine nucleotide GTP enhances the tonic release of adenine nucleotides, whereas the nucleoside guanosine stimulates tonic release of adenosine and its metabolic products. The trophic effects of guanosine and GTP may depend on this process. Guanosine is likely to be an important trophic effector in vivo because high concentrations remain extracellularly for up to a week after focal brain injury. Purine derivatives are now in clinical trials in humans as memory-enhancing agents in Alzheimer's disease. Two of these, propentofylline and AIT-082, are trophic effectors in animals, increasing production of neurotrophic factors in brain and spinal cord. Likely more clinical uses for purine derivatives will be found; purines interact at the level of signal-transduction pathways with other transmitters, for example, glutamate. They can beneficially modify the actions of these other transmitters.

The anticonvulsant BW534U87 depresses epileptiform activity in rat hippocampal slices by an adenosine-dependent mechanism and through inhibition of voltage-gated Na+ channels

1. The cellular and molecular actions of BW534U87 were studied using intracellular and extracellular recordings from the CA1 region of rat hippocampal slices and whole-cell voltage-clamp recordings of recombinant human brain type IIA Na+ channels expressed in Chinese hamster ovary (CHO) cells. 2. Normal excitatory and inhibitory postsynaptic potentials evoked in hippocampal slices were unaffected by BW534U87 or the adenosine deaminase inhibitor EHNA. However, epileptiform activity was depressed by BW534U87 (50 micronM) and this inhibition was reversed by the adenosine receptor antagonist 8-phenyl theophylline (8-PT, 30 micronM). EHNA (10 micronM) mimicked the effects of BW534U87. Furthermore, BW534U87 enhanced the inhibitory effects of exogenous adenosine on evoked synaptic potentials. BW534U87 (50 micronM) also voltage- and use-dependently inhibited action potentials elicited by current injection, independent of the adenosine system, since it was not affected by 8-PT. 3. In CHO cells expressing the recombinant human brain Na+ channel, BW534U87 produced a concentration- and voltage-dependent inhibition of Na+ currents with a half-maximal inhibitory concentration of 10 micronM at a Vh of -60 mV. Use-dependent inhibition was evident at high-frequencies (20x20 ms pulse train at 10 Hz). 4 In conclusion, BW534U87 blocks hippocampal epileptiform activity by a dual mechanism. The first action is similar to that produced by EHNA and is dependent on endogenous adenosine probably by inhibition of adenosine deaminase. Secondly, BW534U87 directly inhibits voltage-gated Na+ channels in a voltage- and frequency-dependent manner. Both actions of BW534U87 are activity-dependent and may synergistically contribute to its overall anticonvulsant effects in animal models of epilepsy.

Pharmacological characterization of adenosine A1 receptors and its functional role in brown trout (Salmo trutta) brain

The adenosine receptor agonist N(6)-cyclohexyl[(3)H]adenosine ([(3)H]CHA) was used to identify and pharmacologically characterize adenosine A1 receptors in brown trout (Salmo trutta) brain. In membranes prepared from trout whole brain, the A1 receptor agonist [(3)H]CHA bound saturably, reversibly and with high affinity (K(d)=0. 69+/-0.04 nM; B(max)=0.624+/-0.012 pmol/mg protein) to a single class of binding sites. In equilibrium competition experiments, the adenosine agonists and antagonists all displaced [(3)H]CHA from high-affinity binding sites with the rank order of potency characteristic for an adenosine A1 receptors. A1 receptor density appeared not age-related (from 3 months until 4 years), and was similar in different brain areas. The specific binding was inhibited by guanosine 5'-triphosphate (IC(50)=0.778+/-0.067 microM). GTP (5 microM) induced a low affinity state of A1 receptors. In superfused trout cerebral synaptosomes, 30 mM K(+) stimulated the release of glutamate in a calcium dependent manner. Glutamate-evoked release was dose-dependently reduced by CHA, and the inhibition was reversed by the A1 antagonist 8-cyclopentyltheophylline (CPT). In the same synaptosomal preparation, 30 mM K(+) as well as 1 mM glutamate stimulated the release of adenosine in a Ca(2+)-independent manner and tetrodotoxin insensitive. These findings show that in trout brain adenosine A1 receptors are present which are involved in the modulation of glutamate transmitter release. Moreover, the stimulation of adenosine release by K(+) depolarisation or glutamate support the hypothesis that, as in mammalian brain, a cross-talk between adenosine and glutamate systems exists also in trout brain.

Distribution of equilibrative, nitrobenzylthioinosine-sensitive nucleoside transporters (ENT1) in brain

Nucleoside transport processes may play a role in regulating endogenous levels of the inhibitory neuromodulator adenosine in brain. The cDNAs encoding species homologues of one member of the equilibrative nucleoside transporter (ENT) gene family have recently been isolated from rat (rENT1) and human (hENT1) tissues. The current study used RT-PCR, northern blot, in situ hybridization, and [3H]nitrobenzylthioinosine autoradiography to determine the distribution of mRNA and protein for ENT1 in rat and human brain. Northern blot analysis indicated that hENT1 mRNA is widely distributed in adult human brain. 35S-labeled sense and antisense riboprobes, transcribed from a 153-bp segment of rENT1, were hybridized to fresh frozen coronal sections from adult rat brain and revealed widespread rENT1 mRNA in pyramidal neurons of the hippocampus, granule neurons of the dentate gyrus, Purkinje and granule neurons of the cerebellum, and cortical and striatal neurons. Regional localization in rat brain was confirmed by RT-PCR. Thus, ENT1 mRNA has a wide cellular and regional distribution in brain, indicating that this nucleoside transporter subtype may be important in regulating intra- and extracellular levels of adenosine in brain.

Involvement of the adenosine neuromodulatory system in the benzodiazepine-induced depression of excitatory synaptic transmissions in rat hippocampal neurons in vitro

We investigated whether adenosine neuromodulation is involved in a benzodiazepine (midazolam)-induced depression of excitatory synaptic transmissions in the CA1 and dentate gyrus (DG) regions in rat hippocampal slices. Field excitatory postsynaptic potentials (fEPSPs), evoked by electrical stimulation of the CA1-Schaffer collateral or the DG-perforant path, were recorded with extracellular microelectrodes from CA1-stratum radiatum or DG-stratum moleculare in oxygenated ACSF. The initial slope of the fEPSPs was analyzed for assessing the drug effects. Midazolam (1 microM) transiently depressed CA1- and DG-fEPSPs. The fEPSPs were depressed to approximately 75% of the control values, and then gradually recovered. The depression was not affected by bicuculline, a GABAA receptor antagonist, although it was completely antagonized by aminophylline, an adenosine receptor antagonist. Dipyridamole (5 microM), an adenosine uptake inhibitor, depressed the fEPSPs in a similar manner to midazolam. An adenosine deaminase inhibitor, EHNA, also transiently depressed the fEPSPs, but in a different manner. Exogenous adenosine persistently depressed the fEPSPs. The effects of the drugs were not significantly different in the CA1 and DG regions. The results suggest that midazolam (1 microM) depresses excitatory synaptic transmissions through the adenosine neuromodulatory system by inhibiting adenosine uptake in the CA1 and DG regions of the hippocampus.

Masking of forskolin-induced long-term potentiation by adenosine accumulation in area CA1 of the rat hippocampus

At hippocampal Schaffer collateral-CA1 synapses, activation of beta-adrenergic receptors and adenylyl cyclase increases transmitter release. However, this effect is transient, which is in contrast to that seen at mossy fiber-CA3 synapses, where activation of cyclic-AMP-dependent protein kinase results in long-lasting facilitation of transmitter release, a phenomenon known as a presynaptic form of long-term potentiation. The present study was aimed at investigating whether forskolin, an adenylyl cyclase activator, could produce long-term effects at the Schaffer collateral-CA1 synapses using extracellular recording techniques. As has been reported previously, forskolin persistently increased the amplitude of evoked population spikes without having a long-term effect on the field excitatory postsynaptic potentials. However, under the conditions where adenosine A1 receptors are inhibited, cyclic-AMP metabolism is disrupted or the transport of cyclic-AMP is blocked, forskolin induces long-term potentiation. Forskolin-induced potentiation is associated with a decrease in paired-pulse facilitation and is blocked by the cyclic-AMP-dependent protein kinase inhibitor Rp-adenosine-3',5'-cyclic monophosphorothioate. Activation of N-methyl-D-aspartate receptors is not required for forskolin-induced long-term potentiation, because pretreatment of slices with the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovalerate did not prevent forskolin-induced potentiation. These results suggest that blockade of adenosine A1 receptors unmasks forskolin-induced long-term potentiation, and activation of cyclic-AMP-dependent protein kinase induces a form of long-term potentiation which is different from that induced by tetanic stimulation.

Neurotransmitter activation of inwardly rectifying potassium current in dissociated hippocampal CA3 neurons: interactions among multiple receptors

We characterized potassium current activated by G-protein-coupled receptors in acutely dissociated hippocampal CA3 neurons. Agonists for serotonin, adenosine, and somatostatin receptors reliably activated a potassium-selective conductance that was inwardly rectifying and that was blocked by 1 mM external Ba2+. The conductance had identical properties to that activated by GABAB receptors in the same cells. In one-half of the CA3 neurons that were tested, the metabotropic glutamate agonist 1S,3R-ACPD also activated inwardly rectifying Ba2+-sensitive potassium current. Activation of the current by serotonin and adenosine agonists occurred with a time constant of 200-700 msec after a lag of 50-100 msec; on removal of agonist the current deactivated with a time constant of 1-2 sec after a lag of 200-400 msec. These kinetics are similar to GABAB-activated current and consistent with a direct action of G-protein on the channels. For somatostatin, both activation and deactivation were approximately fourfold slower, probably limited by agonist binding and unbinding. The half-maximally effective agonist concentrations were approximately 75 nM for somatostatin, approximately 100 nM for serotonin, and approximately 400 nM for 2-chloroadenosine. Dose-response relationships had Hill coefficients of 1.2-1.9, suggesting cooperativity in the receptor-to-channel coupling mechanism. At saturating concentrations of agonists, the combined application of baclofen and either somatostatin, serotonin, or 2-chloroadenosine produced effects that were subadditive and often completely occlusive. However, at subsaturating concentrations the effects of baclofen and 2-chloroadenosine were supra-additive. Thus, low levels of different transmitters can act synergistically in activating inwardly rectifying potassium current.

Adenosine A2 receptors modulate hippocampal synaptic transmission via a cyclic-AMP-dependent pathway

Blockade of adenosine A2 receptors has been shown to significantly reduce the level of tetanus-induced long-term potentiation in area CA1 of rat hippocampus [Kessey K. et al. (1997) Brain Res. 756, 184-190; Sekino Y. et al. (1991) Biochem. biophys. Res. Commun. 181, 1010-1014]. In the present study, the effects of A2 receptor activation and blockade on the modulation of normal synaptic transmission and tetanus-induced long-term potentiation were examined at the Schaffer-CA1 synapse in rat hippocampal slices. A2 receptor activation reversibly enhanced synaptic transmission evoked by low-frequency test pulses as measured by the dendritic field excitatory postsynaptic potential. In the presence of A1 receptor blockade, A2 activation further enhanced the excitatory postsynaptic potential, while A2 receptor blockade resulted in a reversible decrease of the excitatory postsynaptic potential. The A2a receptor agonist, CGS21680, had no effect on the excitatory postsynaptic potential, suggesting that tonic activation of A2b receptors contributes to synaptic transmission under normal physiological conditions. Furthermore, we investigated the contribution of A2 receptors to the level of tetanus-induced long-term potentiation. Under control conditions, a single tetanus potentiated the excitatory postsynaptic potential by 63.5% relative to baseline 30 min post-tetanus. In contrast, tetanus-induced long-term potentiation during A2 blockade was 21.3%. A2 receptor activation increased the level of tetanus-induced long-term potentiation to 90.2%. Because A2 receptors are known to stimulate cyclic-AMP accumulation, the possible involvement of cyclic-AMP was examined. Forskolin, a direct adenylate cyclase activator, and 8-bromo-cyclic-AMP, a membrane-permeable analog of cyclic-AMP, were able to reconstitute tetanus-induced long-term potentiation during A2 receptor blockade; however, the inactive analog 1,9-dideoxyforskolin had no effect, indicating that the effects of A2 activation on synaptic transmission were mediated largely through the regulation of intracellular cyclic-AMP. Because A1 receptors exert an opposing effect on synaptic transmission relative to A2 receptors, these results suggest that the stoichiometry of A1 versus A2 receptor activation appears to play an important role in the modulation of normal synaptic transmission and long-term potentiation in the CA1 region of the hippocampus.

Adenosine: a mediator of the sleep-inducing effects of prolonged wakefulness

Both subjective and electroencephalographic arousal diminish as a function of the duration of prior wakefulness. Data reported here suggest that the major criteria for a neural sleep factor mediating the somnogenic effects of prolonged wakefulness are satisfied by adenosine, a neuromodulator whose extracellular concentration increases with brain metabolism and which, in vitro, inhibits basal forebrain cholinergic neurons. In vivo microdialysis measurements in freely behaving cats showed that adenosine extracellular concentrations in the basal forebrain cholinergic region increased during spontaneous wakefulness as contrasted with slow wave sleep; exhibited progressive increases during sustained, prolonged wakefulness; and declined slowly during recovery sleep. Furthermore, the sleep-wakefulness profile occurring after prolonged wakefulness was mimicked by increased extracellular adenosine induced by microdialysis perfusion of an adenosine transport inhibitor in the cholinergic basal forebrain but not by perfusion in a control noncholinergic region.

A role for adenosine in the regulation of long-term depression in the adult rat hippocampus in vitro

We describe how endogenous adenosine can prevent the induction of homosynaptic long-term depression (LTD) in the CA1 region of slices of adult rat hippocampus. Neither of two consecutive periods of prolonged low frequency stimulation (LFS; 1 Hz, 900 stimuli) of the Schaffer collateral-commissural fibres resulted in the induction of LTD in the CA1 region of hippocampal slices from adult (8-30 week) animals. However, in the presence of adenosine deaminase or the selective adenosine A1 receptor antagonist, 1,3-dipropyl-8-cyclopentyl-xanthine (DPCPX), LTD was induced by each of the first and second of two periods of LFS. The first period of LFS did not, but the second period of LFS did, induce LTD in the presence of DPCPX and the NMDA receptor antagonist, D-2-amino-5-phosphonopentanoate (AP5). The present results show that A1 receptor activation by endogenous adenosine can prevent the induction of LTD in the adult hippocampus.

Endogenous adenosine on membrane properties of CA1 neurons in rat hippocampal slices during normoxia and hypoxia

The effects of endogenous adenosine release on CA1 neurons in hippocampal slices were studied under normoxic and hypoxic conditions, by using extra-/intracellular and whole-cell recordings. During normoxia, the adenosine antagonist, 8-(p-sulphophenyl) theophylline (8-SPT) or adenosine deaminase (ADA) potentiated both evoked CA1 EPSPs and spontaneous synaptic activity, but not monosynaptic IPSPs; there was a minimal depolarization (by 1 mV), probably caused by the enhanced synaptic activity, but no increase in input conductance. Under voltage-clamp with KCl electrodes (with holding potential (VH) near -70 mV), hypoxia (4-5 min) elicited a rise in input conductance and an outward current that reversed near -90 mV, in keeping with the activation of K conductance. These effects of hypoxia were partly attenuated by 8-SPT (10 microM). The hypoxia-induced outward current and conductance increase were abolished by 1 mM Ba, being replaced by a small inward current and a conductance decrease. These data indicate that adenosine tonically inhibits excitatory, but not inhibitory, synaptic transmission, has no direct effect on input conductance, and contributes to the hyperpolarization and fall in input resistance induced by hypoxia.

The contribution of adenosine to paired-pulse inhibition in the normal and disinhibited hippocampal slice

The effects of the adenosine receptor antagonist 1,3-dimethyl-8-cyclopentylxanthine (cyclopentyltheophylline) and the enzyme adenosine deaminase have been examined on paired-pulse inhibition between orthodromic evoked field potentials in the CA1 region of the normal and disinhibited hippocampal slice. In the presence of the GABAA receptor antagonist (-)-bicuculline methobromide, cyclopentyltheophylline suppressed homosynaptic paired-pulse inhibition between stimuli 300 ms apart. Slices treated with (-)-bicuculline and cyclopentyltheophylline together tended to develop spontaneous burst potentials. In slices in which a surgical cut isolated the CA1 and CA3 areas, thereby preventing the development of bursts in CA1, the effect on paired-pulse inhibition was lessened but was still apparent. Adenosine deaminase, in the presence of (-)-bicuculline showed the same effect as cyclopentyltheophylline, decreasing substantially the amount of paired-pulse inhibition. These results suggest that adenosine may contribute to homosynaptic paired-pulse inhibition in disinhibited slices. For comparison, we also examined the effect of cyclopentyltheophylline in normal ((-)-bicuculline-free) slices. At 100 nM, cyclopentyltheophylline increased reversibly the size of orthodromically evoked synaptic population potentials in the CA1 region of the slices and also reduced reversibly the degree of homosynaptic paired-pulse inhibition between two stimuli delivered only 30 ms apart. This suggests that adenosine may also contribute to shorter latency paired-pulse inhibition in the normal hippocampal slice.

Modulation of excitatory synaptic transmission by adenosine released from single hippocampal pyramidal neurons

Adenosine is a potent neuromodulator in the CNS, but the mechanisms that regulate adenosine concentrations in the extracellular space remain unclear. The present study demonstrates that increasing the intracellular concentration of adenosine in a single hippocampal CA1 pyramidal neuron selectively inhibits the excitatory postsynaptic potentials in that cell. Loading neurons with high concentrations of adenosine via the whole-cell patch-clamp technique did not affect the GABAA-mediated inhibitory postsynaptic potentials, the membrane resistance, or the holding current, whereas it significantly increased the adenosine receptor-mediated depression of excitatory postsynaptic currents. The effects of adenosine could not be mimicked by an agonist at the intracellular adenosine P-site, but the effects could be antagonized by a charged adenosine receptor antagonist and by adenosine deaminase, demonstrating that the effect was mediated via adenosine acting at extracellular adenosine receptors. The effect of adenosine loading was not blocked by BaCl2 and therefore was not caused by an adenosine-activated postsynaptic potassium conductance. Adenosine loading increased the paired-pulse facilitation ratio, demonstrating that the effect was mediated by presynaptic adenosine receptors. Finally, simultaneous extracellular field recordings demonstrated that the increase in extracellular adenosine was confined to excitatory synaptic inputs to the loaded cell. These data demonstrate that elevating the intracellular concentration of adenosine in a single CA1 pyramidal neuron induces the release of adenosine into the extracellular space in such a way that it selectively inhibits the excitatory inputs to that cell, and the data support the general conclusion that adenosine is a retrograde messenger used by pyramidal neurons to regulate their excitatory input.

Extracellular adenosine levels in neostriatum and hippocampus during rest and activity periods of rats

Adenosine is an inhibitory modulator in the mammalian brain with a possible role in sleep regulation, which is mainly indicated by pharmacological studies showing that adenosine or its analogs can induce sedation and sleep, whereas adenosine antagonists, like caffeine and theophylline, are potent behavioral and neuronal stimulants. In contrast to these pharmacological findings, data on endogenous adenosine in relation to sleep and waking are sparse. Therefore, we have now used in vivo microdialysis to investigate the extracellular levels of adenosine in the neostriatum and hippocampus of freely moving rats. Adenosine was monitored over a time course of 24 h, during which the animals were exposed to a 12 h day/night rhythm with lights-off from 19.00 to 07.00. In this lights-off period, i.e. the rats' active period, the maximal levels of neostriatal and hippocampal extracellular adenosine were higher than during the lights-on period. In contrast to the neostriatum, extracellular levels of hippocampal adenosine tended to increase towards the end of the lights-off period, reaching its maximal level at 07.00, and decreasing again within the following hour. The changes of hippocampal adenosine levels were related to behavior, since significant increases in "sleep-like" behavior, as well as decreases in overall movements and consummatory behavior, were observed when adenosine levels had reached their maxima in the hippocampus; no such relationship was found with respect to the neostriatum. These results are in keeping with a role of endogenous adenosine in the regulation of sleep and wakefulness, and point to a specific role of adenosine in the hippocampus. They also raise the possibility that adenosine may be involved in different behavioral processes dependent on the area of the brain, as well as the type of adenosine receptor involved. Finally, given the known evidence for neuroprotective actions of adenosine, its accumulation in the hippocampus as a function of behavioral activity may serve to prevent or repair the neural degenerative consequences of such activity. It is proposed that adenosine's sleep-promoting effects result from its signalling to cease behavioral activity in order to prevent excessive activity-related changes, and thus allow other restorative sleep-related processes to take over.

Mechanisms of long-lasting hyperpolarizations underlying slow sleep oscillations in cat corticothalamic networks

1. To explore the nature of the long-lasting hyperpolarizations that characterize slow oscillations in corticothalamic circuits in vivo, intracellular recordings were obtained under ketamine-xylazine anaesthesia from cortical (Cx) cells of the cat precruciate motor cortex, thalamic reticular (RE) cells from the rostrolateral sector, and thalamocortical (TC) cells from the ventrolateral (VL) nucleus. 2. Measurements in the three cell types showed input resistance (Rin) to be highest during the long-lasting hyperpolarizations that correspond to depth-positive waves of the cortical EEG. Rin was lowest during the early phase of high-amplitude depth-negative EEG waves and increased thereafter until the next cycle of the slow oscillation. 3. Spontaneous long-lasting hyperpolarizations were compared with those evoked by dorsal thalamic stimulation. Voltage versus current (V-I) plots showed similar membrane potential (Vm) ranges and slopes for spontaneous and evoked hyperpolarizations in both Cx and RE cells. V-I plots from TC cells had similar slopes, but Vm during evoked hyperpolarizations was displaced towards more negative values. 4. Intracellular injection of constant hyperpolarizing current in Cx cells increased the amplitude of the initial part of the depolarizing plateau of the slow oscillation, but decreased the amplitude of the last part. 5. These results suggest disfacilitation to be the dominant mechanism in the membrane of cortical and thalamic cells during the spontaneous long-lasting hyperpolarizations, which shape and synchronize slow oscillations in corticothalamic networks. In Cx and RE cells, the same mechanism underlies thalamically evoked long-lasting hyperpolarizations. By contrast, evoked responses in TC cells show a strong additional hyperpolarizing factor. We propose that GABAB processes are stronger in TC than in Cx neurones, thus rendering the thalamus an easier target for absence-type epileptic phenomena through potentiation of thalamic rebound capabilities.

Effects of adenosine on synaptic transmission in hippocampal slices from hibernating and warm-acclimated Turkish hamsters and rats

The effects of adenosine on synaptic transmission were studied with extracellular recordings of stimulus-induced population action potentials in area CA1 of hippocampal slices prepared from hibernating (HH) and warm-acclimated Turkish hamsters (WH) and rats. In HH as well as WH and rat slices, adenosine generally reduced the population spike amplitude and the slope of the field EPSP. The sensitivity for adenosine was significantly lower in HH slices than in WH and rat slices. The results are discussed with regard to the involvement of endogenous adenosine in the regulation of neuronal activity during entrance into and arousal from hibernation.

Electrophysiological subtypes of inhibitory P1 purinoceptors on myenteric neurones of guinea-pig small bowel

1. Conventional intracellular microelectrode techniques were used to subclassify P1 purinoceptors linked to reduction of cell input resistance, steady-state hyperpolarization of the membrane potential, or inhibition of fast e.p.s.ps, in neurones of microdissected myenteric plexus preparations from guinea-pig ileum. The potencies of P1 purinoceptor agonists were estimated in neurones that were current clamped to a fixed membrane potential. 2. In AH/Type 2 neurones, the A2 agonist, CGS 21680, the A1 agonist, CCPA or the mixed A1-A2 agonist, NECA, suppressed excitability by reducing input resistance (40-50% max.) and causing hyperpolarization (20-25 mV max.). CGS 21680 (0.1-1 microM) enhanced the after-hyperpolarizing potential. 3. From cumulative dose-response data, the potency order for reducing input resistance was CCPA (IC50 = 5.1 +/- 2.2 nM) >>> CGS 21680 (IC50 = 5.6 +/- 2.5 microM). This effect was reversed by the A1 antagonist, CPT (EC50 = 65 +/- 11 nM). 4. In contrast, the potency order for membrane hyperpolarization was CCPA (IC50 = 61 +/- 23 nM) = CGS 21680 (IC50 = 290 +/- 90 nM) > or = NECA (IC50 = 450 +/- 100 nM). Hyperpolarization elicited by CCPA was sensitive to the A1-A2 antagonist, DPSPX. 5. Agonists suppressed fast e.p.s.ps, but not DMPP responses, with an order of CCPA (IC50 = 8.1 +/- 3.0 nM) >>> CGS 21680 (IC30 = 10 +/- 2.9 microM). 6. In conclusion, the excitability of AH/Type 2 neurones is suppressed by activation of high affinity A l receptors that may be linked to a cyclic AMP-dependent pathway, leading to increase in calcium dependent potassium conductance and enhancement of the after-hyperpolarizing potential. Activation of lower affinity non A1 receptors linked to a cyclic AMP-independent pathway reduces excitability and leads mainly to a steady-state hyperpolarization. Adenosine also suppresses nicotinic cholinergic transmission by activating presynaptic high affinity Al receptors.

Endogenous adenosine deaminase does not modulate synaptic transmission in rat hippocampal slices under normoxic or hypoxic conditions

Field and intracellular potentials were recorded from CA1 pyramidal stratum in submerged slices (at 33 degrees). During "normal" oxygenation (95% O2 + 5% CO2), tonic depression of population spikes and field excitatory postsynaptic potentials by endogenous adenosine was demonstrated by (i) the marked enhancement by the adenosine antagonists 8-(p-sulfophenyl)theophylline (10 microM) and caffeine (0.2 mM), (ii) depression by the transport blocker dipyridamole (5 microM), and (iii) enhancement by exogenous adenosine deaminase (all tested by bath application). Thus, adenosine deaminase (0.5 units/ml) reduced by 10.7 +/- 3.0% (S.E.) the half-maximal stimulus intensity (for population spikes). The effects of adenosine deaminase were prevented by the specific inhibitor, deoxycoformycin (30 microM). In intracellular recordings, excitatory postsynaptic potentials were enhanced in a comparable manner by adenosine deaminase. By contrast, neither deoxycoformycin (5 and 30 microM) nor erythro-9-(2-hydroxy-3-nonyl)adenine (another adenosine deaminase inhibitor; 10 and 50 microM) had significant effects on population spikes. Superfusion with anoxic medium (saturated with 95% N2 + 5% CO2) for 2-3 min suppressed population spikes reversibly, by a mechanism involving adenosine, because 8-(p-sulfophenyl)theophylline (10 microM) and caffeine (0.2 mM) delayed the onset of anoxic block and accelerated the subsequent recovery, and the recovery was much slower or incomplete in the presence of dipyramidole (0.5 microM). However, the anoxic suppression of population spikes was not affected by deoxycoformycin (30 microM) or erythro-9-(2-hydroxy-3-nonyl)adenine (10 microM); the corresponding 50% postanoxic recovery times were also unchanged (e.g. 4.0 +/- 0.2 min for controls and 4.1 +/- 0.3 min in deoxycoformycin).(ABSTRACT TRUNCATED AT 250 WORDS)

Direct excitatory opiate effects mediated by non-synaptic actions on rat medial vestibular neurons

Opiates increase firing of rat medial vestibular nucleus neurons. We have attempted to determine the mechanism of these excitatory opiate actions by extracellular recording of neuronal activity with ionophoretic application of opiate agonists and bath application of antagonists. The spontaneous activity of approximately 30% of medial vestibular neurons, scattered throughout the nucleus, was increased by ionophoretic application of either morphine or [D-Ala2]leucine enkephalin, implicating the presence of both mu and delta opiate receptors. The responses to both were blocked by the opiate receptor antagonist, naloxone. In only a few neurons opiates decreased firing. Most previous reports of direct opiate excitation have proven to be due to disinhibition. This is not the case here, as indicated by three observations: 1) the excitatory opiate response was sustained when gamma-aminobutyric acid (GABA) receptors were blocked by bicuculline; 2) perfusion of a solution containing 0.1 mM Ca2+ and 6.3 mM Mg2+ blocks synaptic transmission, but does not block the excitatory responses to both opiates and 3) the opiate-induced depolarization and action potential generation was evident in neurons whose spontaneous firing was almost totally depressed by adenosine. These results indicate that the excitation is neither due to disinhibition nor to a presynaptic opiate action. We conclude that medial vestibular neurons have postsynaptic opiate receptors that mediate direct neuronal excitation.

Endogenous adenosine modulates long-term potentiation in the hippocampus

The effect of endogenous adenosine on frequency-induced long-term potentiation of the responses evoked by stimulation of the Schaffer fibres and recorded in the CA1 area was studied in hippocampal slices of the rat. Long-term potentiation was facilitated in the presence of the selective A1 adenosine receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (10-20 nM), and was reduced in the presence of the adenosine uptake blocker, nitrobenzylthioinosine (5 microM), suggesting that endogenous adenosine exerted a tonic inhibitory role on long-term potentiation, which was mediated through adenosine A1 receptors. We also found that long-term potentiation was increased in the presence of the selective A2 receptor agonist, CGS 21680 (30 nM), suggesting that the activation of adenosine A2 receptors may have excitatory effects on long-term potentiation. We suggest that, endogenous adenosine is able to modulate mechanisms of synaptic plasticity, such as long-term potentiation, in the hippocampus.

Inhibition of adenosine kinase increases endogenous adenosine and depresses neuronal activity in hippocampal slices

Endogenous adenosine in the extracellular space inhibits neuronal activity. The roles of adenosine kinase, S-adenosylhomocysteine-hydrolase and adenosine deaminase activities in the regulation of the adenosine levels were investigated in rat hippocampal slices. Iodotubercidin, an inhibitor of adenosine kinase, added to the perfusion fluid at 5 microM increased the release of adenosine from the slices more than 2-fold. Iodotubercidin treatment caused inhibition of population spike discharges and hyperpolarization of pyramidal cells, mimicking the effects of exogenously applied adenosine. Adenosine dialdehyde, an inhibitor of S-adenosylhomocysteine hydrolase, and erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA), an inhibitor of adenosine deaminase had little or no effect on the parameters tested. The action of iodotubercidin was greater during deaminase inhibition. The A1-receptor antagonist DPCPX had actions opposite to those of adenosine and blocked the electrophysiological effects of exogenous adenosine and of iodotubercidin. Thus adenosine kinase activity is a significant factor in the regulation of adenosine levels in the hippocampus.

Synchronized sleep oscillations and their paroxysmal developments

The state of resting sleep is associated with a series of oscillations generated in cortical and thalamic networks. A newly discovered rhythm groups the spindle and delta sleep oscillations within slowly recurring (< 1 Hz) sequences. Multi-site, extra- and intracellular recordings provide evidence for synchronization of various classes of cell in the neocortex and thalamus during sleep oscillations that might reach paroxysmal levels similar to epileptic states. Sleep oscillations and the underlying synchronizing processes are disrupted during transition to brain arousal.

Multivesicular release from excitatory synapses of cultured hippocampal neurons

Release of neurotransmitter from presynaptic terminals occurs by exocytosis of vesicular contents into the synaptic cleft. We find that more than one quantum of transmitter can interact with the same population of postsynaptic NMDA receptors in conditions which increase the probability of transmitter release. Increasing release probability also results in proportional increases in both AMPA and NMDA receptor components of the synaptic current. These results suggest that the fraction of AMPA and NMDA receptors occupied by transmitter following the release of a single quantum is similar. Based on AMPA and NMDA receptor responses of outside-out patches to short applications of glutamate, we suggest that both receptor types may be saturated normally by synaptic release.

Disinhibition of hippocampal CA3 neurons induced by suppression of an adenosine A1 receptor-mediated inhibitory tonus: pre- and postsynaptic components

Intracellular recordings were performed on hippocampal CA3 neurons in vitro to investigate the inhibitory tonus generated by endogenously produced adenosine in this brain region. Bath application of the highly selective adenosine A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine at concentrations up to 100 nM induced both spontaneous and stimulus-evoked epileptiform burst discharges. Once induced, the 1,3-dipropyl-8-cyclopentylxanthine-evoked epileptiform activity was apparently irreversible even after prolonged superfusion with drug-free solution. The blockade of glutamatergic excitatory synaptic transmission by preincubation of the slices with the amino-3-hydroxy-5-methyl-4-isoxazolpropionic acid receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM), but not with the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovaleric acid (50 microM), prevented the induction of epileptiform activity by 1,3-dipropyl-8-cyclopentylxanthine. The generation of the burst discharges was independent of the membrane potential, and the amplitude of the slow component of the paroxysmal depolarization shift increased with hyperpolarization, indicating that the 1,3-dipropyl-8-cyclopentylxanthine-induced bursts were synaptically mediated events. Recordings from tetrodotoxin-treated CA3 neurons revealed a strong postsynaptic component of endogenous adenosinergic inhibition. Both 1,3-dipropyl-8-cyclopentylxanthine and the adenosine-degrading enzyme adenosine deaminase produced an apparently irreversible depolarization of the membrane potential by about 20 mV. Sometimes, this depolarization attained the threshold for the generation of putative calcium spikes, but no potential changes resembling paroxysmal depolarization shift-like events were observed. At the concentrations used in electrophysiological experiments (30-100 nM), 1,3-dipropyl-8-cyclopentylxanthine displayed only a negligible inhibitory action on total cyclic nucleotide phosphodiesterase activity measured by means of a radiochemical assay in a homogenate of the rat cerebral cortex. Furthermore, even high concentrations of the selective phosphodiesterase inhibitor rolipram (10 microM), which displays no affinity to adenosine receptors, did not mimic the electrophysiological actions of 1,3-dipropyl-8-cyclopentylxanthine, thus excluding the possibility that the effects of the A1 receptor antagonist on neuronal discharge behavior can be ascribed to an inhibition of phosphodiesterases. The present data demonstrate that endogenously released adenosine exerts a vigorous control on the excitability of hippocampal CA3 neurons on both the pre- and postsynaptic sites. The long-lasting disinhibition following a transient suppression of adenosinergic inhibition strongly suggests that, besides its well-known short-term effects on neuronal activity, adenosine might also contribute to the long-term control of hippocampal excitability.

The adenosine binding enhancer, PD 81,723, inhibits epileptiform bursting in the hippocampal brain slice

The effects of adenosine and the adenosine binding enhancer, PD 81,723, on low magnesium-induced bursting in the in vitro hippocampal slice were examined. Extracellular recordings were obtained from the CA3 pyramidal cell layer while electrically stimulating in the stratum radiatum under low magnesium perfusion. Adenosine (6-100 microM) reduced the duration of epileptiform bursting in a dose-related manner, which was reversible upon washout of adenosine. Application of PD 81,723 (50-100 microM) also resulted in a dose-dependent reduction in the duration of the triggered burst, which was irreversible. These results demonstrate anticonvulsant activity of adenosine and the adenosine binding enhancer, PD 81,723, in the low magnesium model of epilepsy.

The influence of caffeine on human EEG under resting conditions and during mental loads

The effect of caffeine (single oral doses of 200 mg and 400 mg) on the CNS was tested under resting conditions and while performing a concentration performance test in a placebo-controlled pilot study on ten healthy males. The EEG was evaluated quantitatively by spectral analysis with a Computer Aided Topographical ElectroEncephaloMetry system. Comparison of the averaged frequency content revealed a clear difference between the change in the functional state of the brain due to the mental arithmetics, on the one hand, and the caffeine effect, on the other. Both states of altered brain activity were reflected in a particular topographical distribution of the frequency change with respect to a frontal-occipital accentuation. Comparison of the two periods of mental arithmetics in the absence or presence of caffeine showed a tendency to concentration-dependent differences from each other. Administration of 200 mg and 400 mg caffeine in the relaxed state effected the decrease in spectral power in the theta and alpha ranges. The concentration performance test without caffeine effected decreases in power in the alpha range in frontal to parietal cortex and enhanced theta power in frontal and occipital regions and the alpha power in occipital cortex. The caffeine-dependent decrease in theta power and the decrease in delta power seen under relaxation conditions after 400 mg are not observed during the concentration performance test in the presence of caffeine.

The effects of the adenosine reuptake inhibitor soluflazine on synaptic potentials and population hypoxic depolarizations in area CA1 of rat hippocampus in vitro

Adenosine has recently been shown to play a potentially important role in the regulation of synaptic excitability during experimental hypoxia in the hippocampus of the rat. Endogenous adenosine, rapidly released at the initiation of a hypoxic episode, produced synaptic depression, which could protect sensitive neurons. In the present experiments, an inhibitor of the reuptake of adenosine, soluflazine (R64719) was employed to increase the levels of endogenous adenosine under normoxic and hypoxic conditions in slices of the hippocampus of the rat. Soluflazine produced a slow-onset, concentration-dependent depression of population excitatory postsynaptic potentials, which was reversed by the specific A1 adenosine receptor antagonist, 8-cyclopentyltheophylline. During severe N2-induced hypoxia, soluflazine significantly delayed hypoxic depolarization. These results suggest that inhibition of the reuptake of adenosine may have therapeutic potential in the amelioration of hypoxic/ischemic neuronal damage, particularly in the hippocampus.

Release of endogenous adenosine and its metabolites by the activation of NMDA receptors in the rat hippocampus in vivo

1. The effects of N-methyl-D-aspartate (NMDA), KCl, and veratridine on the release of endogenous adenosine and its metabolites, inosine and hypoxanthine, from the rat hippocampus have been studied by in vivo microdialysis. 2. In the hippocampus of rats anaesthetized with urethane the adenosine level reached a stable state estimated at 0.93 microM during the first 2 h after the implantation of the dialysis probe. NMDA (50 microM to 25 mM) in the perfusate evoked a concentration-dependent release of adenosine, inosine and hypoxanthine with an EC50 of 180 microM. The release was reduced by 93% by the specific NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (2-AP5) at 200 microM, indicating an NMDA receptor-mediated process. In addition, the 100 mM KCl-evoked release of adenosine was also substantially reduced by 77% by 2-AP5, suggesting that a large component of the K(+)-evoked release is NMDA-receptor-mediated. 3. Perfusion with zero-Ca2+ artificial cerebrospinal fluid attenuated the NMDA-evoked release of adenosine only by 16% (not significant) but depressed the K(+)-evoked release by 62%, indicating that most of the NMDA-evoked release is directly receptor-mediated, whereas a large component of the K(+)-evoked release could be via the release of an excitatory amino acid acting at the NMDA receptors.

Inhibition of quantal transmitter release in the absence of calcium influx by a G protein-linked adenosine receptor at hippocampal synapses

Spontaneous miniature excitatory postsynaptic currents (MEPSCs) were recorded by whole-cell voltage-clamp techniques in cultured rat hippocampal pyramidal neurons. The specific adenosine A1 receptor agonist cyclopentyladenosine (CPA) reduced the frequency of MEPSCs without affecting their amplitude distribution or kinetic properties. This action was blocked by pretreatment of the cells with pertussis toxin. In the presence of divalent cation Ca2+ channel blockers, CPA was still effective in reducing the frequency of MEPSCs. It was shown that this effect cannot be explained by changes in basal Ca2+ influx. These results suggest that neurotransmitters that produce presynaptic inhibition at hippocampal synapses utilize several mechanisms, one of which may involve inhibition of some component of the quantal release apparatus that occurs independently of inhibition of Ca2+ influx.

Hypoxia produces cell death in the rat hippocampus in the presence of an A1 adenosine receptor antagonist: an anatomical and behavioral study

Endogenous adenosine depresses synaptic transmission in rat hippocampal slices during periods of hypoxia, a potentially neuroprotective cellular response that is attenuated by the adenosine antagonist 8-cyclopentyltheophylline. In this study, rats were exposed to moderate hypoxic conditions (5% O2- 95% N2, 40 min x three days) in the presence or absence of 8-cyclopentyltheophylline, and the effects of reducing adenosinergic inhibition during hypoxia were assessed histologically and behaviorally. Half the rats exposed to low levels of oxygen in the presence of 8-cyclopentyltheophylline (10 mg/kg) suffered unilateral or bilateral hippocampal damage largely limited to the CA1 subfield. Animals which had suffered hippocampal damage were also impaired in their performance of a working memory version of the Morris Water Maze, but not a passive avoidance task (step-through). Hypoxia alone did not result in neuronal damage or behavioral impairment. These results provide further evidence that endogenous adenosine provides an important level of neuronal protection during even prolonged periods of hypoxia.

Comparison of the actions of adenosine at pre- and postsynaptic receptors in the rat hippocampus in vitro

1. Intracellular microelectrode recordings were used to study the cellular location, the receptor pharmacology, and the mechanism of action of adenosine on pyramidal cells and presynaptic axonal endings in area CA3 of organotypic hippocampal slice cultures. 2. Adenosine (bath applied at 50 microM) caused a 10-15 mV hyperpolarization of CA3 cells, as well as a 75-100% decrease in the amplitude of excitatory and polysynaptic inhibitory postsynaptic potentials (EPSPs and IPSPs). Adenosine had no effect on the amplitude of monosynaptic IPSPs elicited in the presence of excitatory amino acid receptor antagonists, but did reduce the amplitude of isolated EPSPs, elicited after blocking GABAA receptors and reducing subsequent epileptic bursts with excitatory amino acid receptor antagonists. These data indicate that adenosine receptors are located on excitatory, but not inhibitory, presynaptic elements. 3. The A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, bath applied at 200 nM) blocked the pre- and postsynaptic actions of adenosine. DPCPX had no effect on the amplitude of control synaptic responses, suggesting that there is no tonic activation of adenosine receptors in hippocampal slice cultures under control conditions. The A1 receptor agonists R-N6-phenylisopropyladenosine (R-PIA) mimicked all pre- and postsynaptic actions of adenosine. 4. Pertussis toxin pretreatment (500 ng/ml for 48 h) prevented adenosine from activating postsynaptic K+ conductance, but not from inhibiting EPSPs. In contrast, stimulation of protein kinase C with phorbol ester (phorbol 12, 13-dibutyrate, 1 microM for 10 min) reduced the presynaptic, but not the postsynaptic, actions of adenosine. 5. Barium (bath applied at 1 mM) blocked the adenosine-activated K+ conductance, but not the inhibition of isolated EPSPs by adenosine. 6. Adenosine at 0.03-1 microM reduced the frequency of, or blocked, spontaneous epileptiform bursting produced by bicuculline. DPCPX (200 nM) increased the rate of spontaneous bursting, consistent with a tonic activation of adenosine receptors during hyperactivity, and led to the development of prolonged ictal-like bursts, suggesting that the endogenous release of adenosine may contribute to the termination of epileptic bursts. 7. We conclude that adenosine acts at pre- and postsynaptic receptors which are pharmacologically indistinguishable. Postsynaptically, adenosine increases a barium-sensitive K+ conductance via a pertussis toxin-sensitive GTP-binding protein. The presynaptic action of adenosine must, however, be mediated by some other mechanism.