Adenosine In Rat Hippocampus Research Articles

Acute ethanol exposure has bidirectional actions on the endogenous neuromodulator adenosine in rat hippocampus.

Background and PurposeEthanol is a widely used recreational drug with complex effects on physiological and pathological brain function. In epileptic patients, the use of ethanol can modify seizure initiation and subsequent seizure activity with reports of ethanol being both pro‐ and anticonvulsant. One proposed target of ethanol's actions is the neuromodulator adenosine, which is released during epileptic seizures to feedback and inhibit the occurrence of subsequent seizures. Here, we investigated the actions of acute ethanol exposure on adenosine signalling in rat hippocampus.Experimental ApproachWe have combined electrophysiology with direct measurements of extracellular adenosine using microelectrode biosensors in rat hippocampal slices.Key ResultsWe found that ethanol has bidirectional actions on adenosine signalling: depressant concentrations of ethanol (50 mM) increased the basal extracellular concentration of adenosine under baseline conditions, leading to the inhibition of synaptic transmission, but it inhibited adenosine release during evoked seizure activity in brain slices. The reduction in activity‐dependent adenosine release was in part produced by effects on NMDA receptors, although other mechanisms also appeared to be involved. Low concentrations of ethanol (10–15 mM) enhanced pathological network activity by selectively blocking activity‐dependent adenosine release.Conclusions and ImplicationsThe complex dose‐dependent actions of ethanol on adenosine signalling could in part explain the mixture of pro‐convulsant and anticonvulsant actions of ethanol that have previously been reported.

Adenosine A 2 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 μM 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 μM tetrodotoxin (tetrodotoxin). In the presence of 5 μM 8-( p-sulphophenyl)theophylline (8-SPT) adenosine (10 μM) enhanced the amplitude of evoked EPSPs by 20%±3.6 ( n=5) in all neurons tested. Chloroadenosine (chloroadenosine; a stable analog of adenosine; 20 μM) 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 μM) 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 A 2 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 A 2 receptor antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX, 10 μM). In the presence of 1 μM 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 μM) 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 μM 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 A 2 receptors at both presynaptic and postsynaptic sites in the hippocampal CA1 region.

Presynaptic inhibition of excitatory synaptic transmission by adenosine in rat hippocampus: analysis of unitary EPSP variance measured by whole- cell recording

We have utilized the favorable signal-to-noise ratios provided by whole-cell recording, combined with variance analysis, to determine the pre- or postsynaptic actions of a variety of manipulations on unitary EPSPs evoked by low-intensity stimulation of afferents to CA1 pyramidal neurons in slices of hippocampus. Estimates of quantal content (mcv) were determined by calculating the ratio of the squared average unitary EPSP amplitude (determined from 150-275 responses) to the variance of these responses (M2/sigma 2), while quantal amplitudes (qcv) were estimated by calculating the ratio of the response variance to average EPSP size (sigma 2/M). Estimates of mcv were highly correlated with those determined using the method of failures (mf). With paired stimulation (50 msec interpulse interval) there was a significant facilitation of the second unitary EPSP, accompanied by an increase in mcv, but not qcv, suggesting that this facilitation was of presynaptic origin. Superfusion of hippocampal slices with various concentrations of adenosine, the A1-selective adenosine receptor agonist cyclohexyladenosine, or the Ca2+ channel blocker cadmium significantly reduced average unitary EPSP amplitudes and mcv, without significantly altering qcv, suggesting a presynaptic locus for this inhibition. The 50% effective concentration for the apparent presynaptic action of adenosine on mcv in the present study (5.7 microM; 95% confidence limits = 4.2-7.7 microM) was significantly lower than its EC50 for reducing conventional, large EPSPs (33 microM; recorded with high-resistance microelectrodes), or extracellular field EPSPs (29 microM), as previously reported by this laboratory. The glutamate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) reduced average unitary EPSP amplitudes; in contrast to the above manipulations, it had no effect on mcv, but significantly altered qcv, which is consistent with its presumed postsynaptic mechanism of action. We conclude from these data that adenosine presynaptically reduces synaptic strength at Schaffer collateral-commissural synapses in the hippocampus by diminishing the number of quanta released, not by reducing the size of these individual quanta or postsynaptic sensitivity to excitatory neurotransmitter. These results suggest that the mechanism by which adenosine inhibits synaptic transmission in the hippocampus is similar, if not identical, to the mechanism by which it inhibits synaptic transmission at the neuromuscular junction.

Arachidonic acid metabolites do not mediate modulation of neurotransmitter release by adenosine in rat hippocampus or striatum

The possible involvement of arachidonic acid metabolites as mediators of the modulation of neurotransmitter release by adenosine, acetylcholine, and GABA was examined in brain slices of rat hippocampus and striatum. The synaptic modulatory effects of these 3 agents on excitatory transmission in the CA1 region of hippocampus were completele unaffected by a phospholipase inhibitor (p-bromophenacyl bromide, BPB; 10–50 μM), a lipoxygenase inhibitor (nordihydroguaiaretic acid; 5–50 μM), the cyclooxygenase inhibitor indomethacin (10–20 μM), and a cyclooxygenase/lipoxygenase inhibitor (U53059; 5–10 μM). BPB was also found to be ineffective in altering the modulation of transmission by adenosine in the perforant path, and the adenosine inhibition of electrically stimulated release of endogenous dopamine from striatal slices. Arachidonic acid itself also had no effect on synaptic transmission. While these experiments do not rule out such a role for arachidonic acid or its metabolites in mammalian brain, they suggest that in a number of systems the inhibition of transmitter release must occur through an entirely independent mechanism.

Inhibition of Purkinje cell firing by systematic administration of phenylisopropyl adenosine: Effect of central noradrenaline depletion by DSP

The effect of the metabolically stable adenosine analog (−)-N 6(R-phenyl-isopropyl)-adenosine (PIA) on the rate of spontaneous Purkinje cell firing was studied in anesthetized rats. In control animals, systematically administered PIA elicited only small and inconsistent changes in firing rate. However, in animals previously treated with DSP4 (50 mg/kg i.p.), which selectively lesions central noradrenergic afferents, or with the adrenergic antagonist sotalol (15 mg/kg), PIA elicited consistent decreases in firing rate. These effects were antagonized by the systematic administration of the adenosine receptor antagonist aminophylline (50–150 umol/kg). Local administration of adenosine by pressure ejection caused a dose-dependent depression of Purkinje cell firing that was likewise inhibited by the methylxanthine. In DSP4 treated rats the depression of synaptic transmission by adenosine in rat hippocampus in vitro was unaltered, and theophylline did not cause any marked rise in Purkinje cell firing, suggesting that DSP4 does not sensitize neurons to the depressant effects of adenosine derivatives. PIA also caused a dose-dependent decrease in arterial blood pressure and a decrease in heart rate that was of equal magnitude in control and DSP4 treated rats. The results show that the central effects of systemically administered adenosine analogs are altered by procedures that disrupt the normal depressant effect of tonic noradrenergic input.