Post-inhibitory excitation of adenosine on neurotransmission in guinea pig hippocampal slices

Using auditory and visual stimuli to investigate the behavioral and neuronal consequences of reflexive covert orienting

Reflexively orienting toward a peripheral cue can influence subsequent responses to a target, depending on when and where the cue and target appear relative to each other. At short delays between the cue and target [cue-target onset asynchrony (CTOA)], subjects are faster to respond when they appear at the same location, an effect referred to as reflexive attentional capture. At longer CTOAs, subjects are slower to respond when the two appear at the same location, an effect referred to as inhibition of return (IOR). Recent evidence suggests that these phenomena originate from sensory interactions between the cue- and target-related responses. The capture of attention originates from a strong target-related response, derived from the overlap of the cue- and target-related activities, whereas IOR corresponds to a weaker target-aligned response. If such interactions are responsible, then modifying their nature should impact the neuronal and behavioral outcome. Monkeys performed a cue-target saccade task featuring visual and auditory cues while neural activity was recorded from the superior colliculus (SC). Compared with visual stimuli, auditory responses are weaker and occur earlier, thereby decreasing the likelihood of interactions between these signals. Similar to previous studies, visual stimuli evoked reflexive attentional capture at a short CTOA (60 ms) and IOR at longer CTOAs (160 and 610 ms) with corresponding changes in the target-aligned activity in the SC. Auditory cues used in this study failed to elicit either a behavioral effect or modification of SC activity at any CTOA, supporting the hypothesis that reflexive orienting is mediated by sensory interactions between the cue and target stimuli.

Excitatory effects of adenosine are not mediated by inhibition of GABAergic system in slices of superior colliculus and hippocampus from guinea pig

Neurotransmission in the superior colliculus (SC) and the hippocampus is enhanced by application of adenosine. To investigate the possible involvement of disinhibition by adenosine on gamma-aminobutyric acid (GABA) -ergic action, the effects of adenosine and picrotoxin, a GABA antagonist, were studied using slices of SC and hippocampus of the guinea pig. Application of adenosine on SC slices (concentration at 100 microM) and hippocampus (concentration at 0.1 microM) enhanced the amplitude of population spikes (PS) in the superficial gray layer (SGL) of SC and in the granule cell layer of dentate gyrus of hippocampus. The application of picrotoxin at 100 microM increased the amplitude of both slices even after previous treatment with adenosine which once increased the amplitude. This was also true for adenosine with the previous treatment of picrotoxin. In SC slices, the increased amplitude by adenosine was reduced to the original amplitude after application of 3-[1-[3-(Dimetylamino) propyl]-1 H-indol-3-yl-4-(1H-indol-3-yl)-1H-pyrrole-2,5-dione (GF109203X) a protein kinase C inhibitor, the previous treatment by the drug counteracted the excitatory action of adenosine. However GF109203X failed to counteract the excitatory effects of picrotoxin. These results indicate that the excitatory mechanism of adenosine is different from that of picrotoxin and is not mediated by disinhibition of the GABAergic system through inhibitory action of adenosine on GABAergic interneurons.

Adenosine A2 receptor mediation of pre- and postsynaptic excitatory effects of adenosine in rat hippocampus in vitro

Excitatory effects of adenosine in the rat hippocampus were studied by intracellular recording from CA1 pyramidal cells in vitro. Application of 100 microM adenosine induced a rapid hyperpolarization and a decrease in input resistance, and depressed the excitatory postsynaptic potentials (EPSPs) evoked by stimulation of Schaffer collaterals in all neurons tested. In 55% of the neurons this was followed by an excitation. This excitation consisted of a slow depolarization lasting 9 +/- 4.7 min, an increase in input resistance and an increase in the amplitude of the evoked EPSPs. This excitation could also be observed when synaptic transmission was prevented by 1 microM tetrodotoxin (tetrodotoxin). In the presence of 5 microM 8-(p-sulphophenyl)theophylline (8-SPT) adenosine (10 microM) enhanced the amplitude of evoked EPSPs by 20% +/- 3.6 (n = 5) in all neurons tested. Chloroadenosine (chloroadenosine; a stable analog of adenosine; 20 microM) induced a similar hyperpolarization associated with decrease in input resistance; this was followed by a similar excitation as seen with adenosine in 22 of the 27 neurons tested. L-baclofen (20 microM) induced a hyperpolarization associated with decrease in input resistance in all six neurons tested but an ensuing excitation was not observed. CGS 21680 (30 nM), a selective adenosine A2 receptor agonist, induced a slow depolarization associated with an increase in input resistance and, in 11 of 36 neurons, enhanced the amplitude of evoked EPSPs. These excitatory effects of CGS 21680 were blocked by the selective adenosine A2 receptor antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX, 10 microM). In the presence of 1 microM tetrodotoxin 30 nM CGS 21680 still produced a slow depolarization and an increase in input resistance. In addition, high doses of CGS 21680 (10 and 20 microM) depressed the amplitude of EPSPs evoked by stimulation of Schaffer collateral afferents, yet there was little effect on the resting membrane potential or input resistance despite the fact that 20 microM chloroadenosine caused a pronounced hyperpolarization associated with a decrease in input resistance in the same cell. These results indicate that the excitatory effects of adenosine may be mediated via activation of adenosine A2 receptors at both presynaptic and postsynaptic sites in the hippocampal CA1 region.

The effects of simulated ischemia on the levels of adenosine and its metabolites in slices of cerebellum, superior colliculus and hippocampus in the guinea pig-in vitro study

The purpose of this present study is to clarify adenosine (ADO) metabolism in guinea pig brain slices during simulated ischemia. In slice preparations after decapitation, ADO levels were lowest in slices of the cerebellum (1.2 nmol/mg protein), followed by the superior colliculus (3.4) and highest in the hippocampus (6.4), and the combined concentrations of inosine (Ino) and hypoxanthine (HX) were highest in the cerebellum (5.5), followed by the superior colliculus (3.5) and the hippocampus (1.5). After preincubation with standard medium with oxygen and glucose for 30 min, total ADO levels (tissue ADO plus ADO lost into medium during incubation) decreased to 0.3 in the cerebellum, to 1.3 in the superior colliculus and to 2. 9 in the hippocampus. On the other hand, levels of total Ino and HX increased to 21.1 in the cerebellum, to 14.3 in the hippocampus and to 12.0 in the superior colliculus. To investigate the effect of simulated ischemia on ADO metabolism, preincubated slices were exposed for 10 min in medium deprived of oxygen and glucose. The increases of ADO content after 10 min ischemia were 0.2 in the cerebellum, 1.0 in the superior colliculus and 1.3 in the hippocampus. In contrast, the increases of both Ino and HX concentrations were 2.9 in the cerebellum, 2.2 in the superior colliculus and 1.4 in the hippocampus. The total amount of the increase in ADO, Ino and HX was approximately 3 in all three regions. These results indicate that there are significant differences in the metabolic rate to degrade ADO into Ino and HX in various areas of brain possibly due to differences in adenosine deaminase activity in those areas.

Activation of hippocampal adenosine A3 receptors produces a desensitization of A1 receptor-mediated responses in rat hippocampus

The adenosine A3 receptor is expressed in brain, but the consequences of activation of this receptor on electrophysiological activity are unknown. We have characterized the actions of a selective adenosine A3 receptor agonist, 2-chloro-N6-(3-lodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA), and a selective A3 receptor antagonist, 3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1, 4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS 1191), in brain slices from rat hippocampus. In the CA1 region, activation of A3 receptors had no direct effects on synaptically evoked excitatory responses, long-term potentiation, or synaptic facilitation. However, activation of A3 receptors with Cl-IB-MECA antagonized the adenosine A1 receptor-mediated inhibition of excitatory neurotransmission. The effects of Cl-IB-MECA were blocked by pretreatment with MRS 1191, which by itself had no effect on A1 receptor-mediated responses. The presynaptic inhibitory effects of baclofen and carbachol, mediated via GABA(B) and muscarinic receptors, respectively, were unaffected by Cl-IB-MECA. The maximal response to adenosine was unchanged, suggesting that the primary effect of Cl-IB-MECA was to reduce the affinity of adenosine for the receptor rather than to uncouple it. Similar effects could be demonstrated after brief superfusion with high concentrations of adenosine itself. Under normal conditions, endogenous adenosine in brain is unlikely to affect the sensitivity of A1 receptors via this mechanism. However, when brain concentrations of adenosine are elevated (e.g., during hypoxia, ischemia, or seizures), activation of A3 receptors and subsequent heterologous desensitization of A1 receptors could occur, which might limit the cerebroprotective effects of adenosine under these conditions.

Hyperexcitability in CA1 of the rat hippocampal slice following hypoxia or adenosine

Participation of adenosine receptors in the depression of synaptic transmission during hypoxia, and the production of multiple populations spikes in the pyramidal neurons following hypoxia, has been investigated in the CA1 area of the rat hippocampal slice. A method is presented for analysing such hyperexcitability, using input/output curves of the second population spike. This method provides evidence that rebound hyperexcitability following hypoxia or prolonged adenosine-mediated inhibition results from an increase in excitability of the CA1 pyramidal neurons rather than from an increase in excitatory neurotransmitter release. Hypoxia-induced depression of the synaptic components of evoked field potentials was blocked in a concentration dependent manner by the selective A1 receptor antagonist 8-cyclopentyltheophylline (8-CPT), demonstrating extracellular accumulation of adenosine during hypoxia. Upon reoxygenation of slices following 30 min hypoxia, multiple population spikes were evoked by a single orthodromic stimulus in slices that exhibited only a single population spike prior to hypoxia. Such post-hypoxic hyperexcitability was not prevented by superfusion of slices with 8-CPT during hypoxia. Depression of synaptic transmission by 30 min superfusion of slices with 50 microM adenosine was also followed, upon washout, by the appearance of multiple population spikes. However, such hyperexcitability could not be produced by superfusion with adenosine analogues selective for A1 receptors, cyclopentyladenosine, selective for A2a receptors, 2-p-(2-carboxyethyl)phenetheylamino-5'-ethylcarboxamidoadenosine (CGS 21680), or active at A2a and A2b receptors, N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)ethyl]adenosine, suggesting that adenosine receptors other than the A1, A2a or A2b subtypes are involved in its generation.

Evidence for functionally important adenosine A2a receptors in the rat hippocampus

Adenosine A2a receptors are not confined to dopamine-rich areas of the brain, since thermocycling analysis shows that adenosine A2a receptor mRNA is expressed also in the hippocampus (CA1, CA3 and dentate gyrus) and cerebral cortex. The expression of A2a mRNA in three main areas of the hippocampus was confirmed by in situ hybridization; A2a mRNA expression was mainly localized in the pyramidal and granular cells, the same hippocampal regions that showed adenosine A1 receptor mRNA expression. Receptor autoradiographic studies with [3H]CGS 21680 (30 nM), a selective adenosine A2a receptor agonist, showed specific binding sites in the hippocampus. The density of [3H]CGS 21680 binding was greatest in the stratum radiatum of the CA1 area, followed by the stratum oriens of the cornu Ammonis, stratum radiatum of the CA3 are and supra-granular layer of the dentate gyrus. This anatomical distribution of [3H]CGS 21680 binding was similar to the pattern of [3H]CHA binding in the hippocampus. Electrophysiological studies in the Schaffer fibers/CA1 pyramids showed that upon activation of the A2a receptors with CGS 21680 (10 nM) the ability of the adenosine A1 receptor agonist, CPA, to inhibit neuronal activity was significantly attenuated. These results show functionally important co-expression and co-localization of adenosine A2a and A1 receptors in the hippocampus. The results also suggest that adenosine A2a receptor-mediated neuromodulation is not confined to the basal ganglia, but is more widespread throughout the nervous system.

Use of HPLC to measure circulating adenosine levels in migrainous patients

Adenosine is a powerful natural vasodilator that could be involved in migraine. It is difficult to assay this nucleoside, however, because it has a short half-life. We have used HPLC to compare the concentrations of blood adenosine sampled in crisis-free intervals and during crisis periods in ten patients with common migraine and have compared these levels to those noted in a control population. Our sampling technique uses vacuum suction and enables rapid mixing of the blocking solution and whole venous blood. This results in reproducible HPLC assays. We also show that, during a migraine crisis, mean blood adenosine levels increase by 47%. However, the origin of this adenosine release is difficult to define.

Excitatory effects of adenosine receptor agonists and antagonists on neurotransmission in guinea pig superior collicular slices

Adenosine has excitatory actions on neurotransmission in the superior colliculus. To investigate whether adenosine A1 or A2 receptors are involved in mediating these excitatory actions, the effect of A1 and A2 receptor agonists and antagonists on the evoked postsynaptic potentials (PSP) in the superficial grey layer were tested using slices of the superior colliculus. Application of both A1 agonists, such as CHA, R-PIA, and the A2 agonist, CGS-21680 increased the amplitude of the PSP. The increase in PSP amplitude occurred gradually over 20-30 min after application of these adenosine agonists. Application of the A1 antagonist 8-CPT, and the A2 antagonists, DMPX and CGS-15943, increased the amplitude of the PSP and could not antagonize the excitatory effect of adenosine. These results suggest that the mechanism of the excitatory effect of adenosine cannot be explained by the classical concept of A1 and A2 adenosine receptor subtypes which were identified by their effect on adenylate cyclase activity.

Adenosine facilitates glutamate release in a protein kinase-dependent manner in superior colliculus slices

Experiments were performed to clarify the excitatory mechanism of adenosine in superior colliculus slices. Postsynaptic field potential was recorded in the superficial gray layer after stimulation to the optic layer. Application of adenosine to the perfusion medium at a concentration of 0.2 microM or 100 microM enhanced the amplitude of the postsynaptic field potential and the excitatory effect of adenosine was counteracted by application of 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7) (100 microM), a non-selective protein kinase inhibitor or N-(2-guanidinoethyl)-5-isoquinolinesulfonamide (HA1004) (100 microM), a selective protein kinase A inhibitor. The presence of adenosine at a concentration of 0.2 microM or 100 microM during electrical stimulation to slices of the superficial gray layer increased the release of glutamate 2.5 and 2.3 times, respectively, and this increase was suppressed in Ca(2+)-free medium or by applying tetrodotoxin (1 microM) to the medium. Application of H-7 or HA1004 to the incubation medium inhibited the enhancement of the glutamate release. These results suggest that the excitatory effect of adenosine in the superior colliculus is due to an increase in glutamate release and involves the protein kinase system.

Possible functional role of diadenosine polyphosphates: negative feedback for excitation in hippocampus

Diadenosine polyphosphates (Ap4A and Ap5A) are present in secretory granules of chromaffin cells as well as in the rat brain synaptic terminals. Their contribution to the exocytosis of the total synaptosomal content is considerable, ranging from 7% to 12%. Ap4A and Ap5A are released from synaptosomes in a Ca(2+)-dependent manner. There are indications on the high affinity of diadenosine polyphosphates to P2 receptors, but their action on P1 receptors remains unclear. Here we report that both substances induce a blocking action on excitatory synaptic transmission in the rat hippocampus. This action is elicited via the A1 (subclass of P1) receptors and differs in some respects from the action of adenosine.