Transient and selective blockade of adenosine A1-receptors by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) causes sustained epileptiform activity in hippocampal CA3 neurons of guinea pigs

Effect of trans-resveratrol on dexamethasone-induced changes in the expression of MMPs by human trabecular meshwork cells: Involvement of adenosine A1 receptors and NFkB

Intraocular pressure (IOP) lowering in glaucomatous eyes is currently achieved mainly by improved aqueous outflow via alternate drainage pathways. However, the focus is now shifting to trabecular meshwork (TM), the site or major pathological changes including increased extracellular matrix (ECM) deposition and reduced matrix metalloproteinases (MMPs) secretion by TM cells. Trans-resveratrol was previously shown to lower IOP and reduce ECM deposition; however, the mechanisms of action remain unclear. Therefore, we determined the effect of trans-resveratrol on MMP-2 and -9 expression by human TM cells (HTMCs) in the presence of dexamethasone and whether it also affects adenosine A₁ receptors (A₁AR) expression and nuclear factor kappa B (NFkB) activation. We observed that trans-resveratrol, 12.5 μM, increased MMP-2 and -9 protein expression by HTMCs despite exposure to dexamethasone (1.89- and 1.53-fold, respectively; P < 0.001). Further it was observed that trans-resveratrol increases A₁AR expression in HTMC in the presence of dexamethasone (1.55-fold; P < 0.01). Trans-resveratrol also increased NFkB activation in the presence of dexamethasone and A₁AR antagonist (P < 0.01 versus dexamethasone group). These effects of trans-resveratrol were associated with increased MMP -2 and -9 expression. It could be concluded that trans-resveratrol prevents dexamethasone-induced reduction in MMP-2 and -9 secretion by NFkB activation in HTMCs. This effect of trans-resveratrol is likely to involve increased A₁AR expression.

Purinergic signaling orchestrating neuron-glia communication

This review discusses the evidence supporting a role for ATP signaling (operated by P₂X and P₂Y receptors) and adenosine signaling (mainly operated by A₁ and A_2A receptors) in the crosstalk between neurons, astrocytes, microglia and oligodendrocytes. An initial emphasis will be given to the cooperation between adenosine receptors to sharpen information salience encoding across synapses. The interplay between ATP and adenosine signaling in the communication between astrocytes and neurons will then be presented in context of the integrative properties of the astrocytic syncytium, allowing to implement heterosynaptic depression processes in neuronal networks. The process of microglia 'activation' and its control by astrocytes and neurons will then be analyzed under the perspective of an interplay between different P₂ receptors and adenosine A_2A receptors. In spite of these indications of a prominent role of purinergic signaling in the bidirectional communication between neurons and glia, its therapeutical exploitation still awaits obtaining an integrated view of the spatio-temporal action of ATP signaling and adenosine signaling, clearly distinguishing the involvement of both purinergic signaling systems in the regulation of physiological processes and in the control of pathogenic-like responses upon brain dysfunction or damage.

Activity- and pH-dependent adenosine shifts at the end of a focal seizure in the entorhinal cortex

Adenosine (ADO) is an endogenous modulator of neuronal excitability, with anticonvulsant and neuroprotective effects. It has been proposed that the activity-dependent release of ADO promoted by the extracellular acidification occurring during seizures contributes to seizure termination. To verify this hypothesis, we recorded field potentials, pH and ADO changes measured with enzymatic biosensors during acute focal seizures in the medial entorhinal cortex (mEC) of the isolated guinea-pig brain maintained in vitro. The effect of ADO on seizure-like events (SLEs) induced by GABAa receptor antagonism with bicuculline methiodide (BMI; 50 μM) was assessed by arterial applications of 1 mM ADO. ADO either reduced or prevented epileptiform activity. The A1 receptor antagonist DPCPX (100-500 μM) prolonged BMI-induced seizures and was able to precipitate SLEs in the absence of proconvulsant. Simultaneous recordings of brain activity, extracellular ADO and pH shifts demonstrated that ADO decreases at the onset and progressively rises toward the end of SLEs induced by either BMI or 4-aminopyridine (4AP; 50 μM), reaching maximal values 1-5 min after SLE termination. ADO changes were preceded by a SLE-dependent extracellular acid shift. Both pH acidification and ADO changes were abolished by 22 mM HEPES in the arterial perfusate. In these conditions, SLE duration was prolonged. Our data confirm that ADO plays a role in regulating brain excitability. Its increase depends on seizure-induced acid pH shift and it is maximal after the end of the SLE. These findings strongly suggest that ADO contributes to termination of focal seizures and to the establishment of the postictal depression.

Adenosine A1 receptor-mediated suppression of carbamazepine-resistant seizure-like events in human neocortical slices

OBJECTIVE

The need for alternative pharmacologic strategies in treatment of epilepsies is pressing for about 30% of patients with epilepsy who do not experience satisfactory seizure control with present treatments. In temporal lobe epilepsy (TLE) even up to 80% of patients are pharmacoresistant, and surgical resection of the ictogenic tissue is only possible for a minority of TLE patients. In this study we investigate purinergic modulation of drug-resistant seizure-like events (SLEs) in human temporal cortex slices.

METHODS

Layer V/VI field potentials from a total of 77 neocortical slices from 17 pharmacoresistant patients were recorded to monitor SLEs induced by application of 8 mM [K(+) ] and 50 μm bicuculline.

RESULTS

Activating A1 receptors with a specific agonist completely suppressed SLEs in 73% of human temporal cortex slices. In the remaining slices, incidence of SLEs was markedly reduced. Because a subportion of slices can be pharmacosensitive, we tested effects of an A1 agonist, in slices insensitive to a high dose of carbamazepine (50 μm). Also in these cases the A1 agonist was equally efficient. Moreover, ATP and adenosine blocked or modulated SLEs, an effect mediated not by P2 receptors but rather by adenosine A1 receptors.

SIGNIFICANCE

Selective activation of A1 receptors mediates a strong anticonvulsant action in human neocortical slices from pharmacoresistant patients. We propose that our human slice model of seizure-like activity is a feasible option for future studies investigating new antiepileptic drug (AED) candidates.

A1 adenosine receptor-mediated GIRK channels contribute to the resting conductance of CA1 neurons in the dorsal hippocampus

The dorsal and ventral hippocampi are functionally and anatomically distinct. Recently, we reported that dorsal Cornu Ammonis area 1 (CA1) neurons have a more hyperpolarized resting membrane potential and a lower input resistance and fire fewer action potentials for a given current injection than ventral CA1 neurons. Differences in the hyperpolarization-activated cyclic nucleotide-gated cation conductance between dorsal and ventral neurons have been reported, but these differences cannot fully account for the different resting properties of these neurons. Here, we show that coupling of A1 adenosine receptors (A1ARs) to G-protein-coupled inwardly rectifying potassium (GIRK) conductance contributes to the intrinsic membrane properties of dorsal CA1 neurons but not ventral CA1 neurons. The block of GIRKs with either barium or the more specific blocker Tertiapin-Q revealed that there is more resting GIRK conductance in dorsal CA1 neurons compared with ventral CA1 neurons. We found that the higher resting GIRK conductance in dorsal CA1 neurons was mediated by tonic A1AR activation. These results demonstrate that the different resting membrane properties between dorsal and ventral CA1 neurons are due, in part, to higher A1AR-mediated GIRK activity in dorsal CA1 neurons.

Decreased purinergic inhibition of synaptic activity in a mouse model of Niemann-Pick disease type C

Niemann-Pick disease type C (NPC) is a progressive neurodegenerative disorder characterized by accumulation of free cholesterol in lysosomes, mainly due to a mutation in the NPC1 gene. The pathophysiological basis of the neural disorders in NPC, however, is not well understood. We found that the hippocampal field excitatory postsynaptic potential (fEPSP) was enhanced in NPC1 mutant mice. A1-receptor antagonist or adenosine degrading enzyme enhanced the fEPSP in both types of mice, but had a much weaker effect in the mutant mice, suggesting less tonic inhibition of synaptic transmission by endogenous adenosine in the mutant. Further evidence showed impaired hippocampal long term potentiation (LTP) in mutant mice. Supplement of A1 agonist N6-Cyclopentyladenosine (CPA) partially rescued the impaired LTP in mutant mice. Moreover, adenosine release from hippocampal slices was significantly decreased in the mutant. The enhanced excitatory synaptic transmission and the decreased synaptic plasticity due to the decreased adenosine release in NPC brain may partially contribute to the neural disorders of NPC disease, such as seizures, neurodegeneration, and dementia.

Neurons that fire together also conspire together: is normal sleep circuitry hijacked to generate epilepsy?

Brain circuits oscillate during sleep. The same circuits appear to generate pathological oscillations. In this review, we discuss recent advances in our understanding of how epilepsy co-opts normal, sleep-related circuits to generate seizures.

Adenosine as an endogenous regulating factor of hippocampal sharp waves

The rodent hippocampus exhibits population activities called sharp waves (SPWs) during slow wave sleep and wake immobility. SPWs are important for hippocampal-cortical communication and memory consolidation, and abnormal sharp wave-ripple complexes are closely related to epileptic seizures. Although the SPWs are known to arise from the CA3 circuit, the local mechanisms underlying their generation are not fully understood. We hypothesize that endogenous adenosine is a local regulator of hippocampal SPWs. We tested this hypothesis in thick mouse hippocampal slices that encompass a relatively large hippocampal circuit and have a high propensity of generating spontaneous in vitro SPWs. We found that application of adenosine A1 receptor antagonists induced in vitro SPWs and that such induction was sensitive to blockade by NMDA receptor antagonists. By contrast, an increase in endogenous adenosine via pharmacological inhibition of adenosine transporters or adenosine degrading enzymes suppressed spontaneous in vitro SPWs. We thus suggest that the initiation and incidence of sharp wave-like population events are under tight control by the activity of endogenously stimulated A1 receptors.

Long-term potentiation of high-frequency oscillation and synaptic transmission characterize in vitro NMDA receptor-dependent epileptogenesis in the hippocampus

The implication of high-frequency network oscillations (HFOs) in brain pathology resides in as yet unclear mechanisms. Employing field recordings from ventral hippocampal slices and two models of epileptogenesis (i.e. establishment of interictal-like persistent bursts), we found that HFOs associated with epileptiform bursts and excitatory synaptic transmission were co-modulated during epileptogenesis. NMDA receptor-dependent epileptogenesis in CA3 was consistently accompanied by long-lasting strengthening in synaptic transmission (by 94+/-17%, n=5) and HFOs (frequency, power and duration increased by 24+/-8%, 57+/-18% and 33+/-10%, respectively). Co-modulation of synaptic transmission and HFOs was also observed in NMDA receptor-independent epileptogenesis, although in individual experiments either enhancement or depression of both phenomena was observed. Pathological HFOs >200 Hz were unequivocally present in persistent bursts induced by NMDA receptor-dependent but not NMDA receptor-independent mechanisms. The duration of pathological HFOs associated with persistent bursts but not of HFOs associated with bursts before the establishment of epileptogenesis was linearly and strongly correlated with the duration of bursts (r=0.58, P<0.0001). We propose that interplay between spontaneous synchronous bursting and long-lasting synaptic potentiation accompanying certain forms of epileptogenesis may underlie long-lasting potentiation of HFOs, whose quantitative aspects may reliably signal the degree of network changes involved in epileptogenesis.

Caffeine-dependent stimulus-triggered oscillations in the CA3 region of hippocampal slices from rats chronically exposed to lead

Yoshimura et al. [Yoshimura, H., Sugai, T., Onoda, N., Segami, N., Kato, N., 2002. Age-dependent occurrence of synchronized population oscillation suggestive of a developing functional coupling between NMDA and ryanodine receptors in the neocortex. Dev. Brain Res., 136, 63-68.] have shown that caffeine can elicit synchronized oscillations (10-12 Hz) dependent on calcium-induced calcium release in rat neocortex neurons. In the present work, synchronized oscillations in the CA3 region of rat hippocampus were studied by recording field excitatory postsynaptic potentials (fEPSPs) in vitro. In the presence of 0.1 mM caffeine, in CA3 of 44 of 45 (97.8%) slices from chronic lead-exposed rats, single electrical stimuli triggered a burst of high-frequency oscillations (approximately 230 Hz), whereas in CA3 of caffeine-treated slices from control rats, such oscillations could be elicited in only 2 of 24 (8.3%) slices. The complete (but fully reversible) block of caffeine-dependent oscillations by 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX; 20 microM) indicates that alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors are necessary for the high-frequency synchronized oscillations. 2-Amino-5-phosphonopentanoate (AP-5; 50 micoM) partially reduced the amplitude of caffeine-dependent oscillations without significantly altering their frequency. Caffeine-dependent oscillations could be abolished by application of AP-5 and 3 mM Mg2+ during the initial period of bursting, indicating that N-methyl-D-aspartate (NMDA) receptors play an important role in the generation of oscillations. The Ca2+ chelator ethylene glycol bis-(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA; 5 mM) added in standard artificial cerebrospinal fluid (ACSF) containing 0.1 mM caffeine fully blocked the oscillations. Caffeine-dependent oscillations are insensitive to an antagonist of gamma-aminobutyric acid (GABAA) receptors (10 microM bicuculline), L-type Ca2+ channels (10 muM nicardipine), L-type and N-type voltage-dependent calcium channels (100 microM Cd2)), and T-type Ca2+ channels (100 microM Ni2+). Previous studies have demonstrated that expression and function of NMDA and AMPA receptors are altered in the hippocampus of chronic lead-exposed rats. We propose that caffeine-dependent stimulus-induced oscillations in CA3 area of hippocampus from chronic lead-exposed rats are mainly mediated by the entry of extracellular Ca2+ through NMDA and non-NMDA receptors, without participation of GABAA receptors. Additionally, the underlying mechanisms are also discussed.

A guinea pig hippocampal slice model of organophosphate-induced seizure activity

Extracellular recording techniques have been used in the guinea pig hippocampal slice preparation to investigate the electrophysiological actions of the organophosphate (OP) anticholinesterase soman. When applied at a concentration of 100 nM, soman induced epileptiform activity in the CA1 region in approximately 75% of slices. This effect was mimicked by the anticholinesterases paraoxon (1 and 3 microM), physostigmine (30 microM), and neostigmine (30 microM), thus providing indirect evidence that the epileptiform response was mediated by elevated acetylcholine levels. Soman-induced bursting was inhibited by the muscarinic receptor antagonists atropine (concentrations tested, 0.1-10 microM), telenzepine (0.03-3 microM), AF-DX116 [11-(2-[(diethylamino)methyl]-1-piperidinyl acetyl)-5,11-dihydro-6H-pyrido 92.b-b) (1,4)-benzodiazepin-6-one] (0.3-300 microM), and biperiden (0.1-10 microM) and by the benzodiazepine anticonvulsants diazepam (3-30 microM) and midazolam (3-30 microM), but it was not inhibited by the nicotinic antagonists mecamylamine (30 microM) and methyllycaconitine (300 nM). In contrast to soman-induced epileptiform activity, bursting induced by the K(+) channel blocker 4-aminopyridine (30 microM), the adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (30 nM) or perfusion with low Mg(2+) buffer was insensitive to atropine (10 microM). The ability of muscarinic antagonists and benzodiazepines to inhibit soman-induced epileptiform activity is in accordance with the in vivo pharmacology of soman-induced seizures and suggests that the guinea pig hippocampal slice preparation may provide a useful tool for the evaluation of novel anticonvulsant therapies for the treatment of seizures related to OP poisoning.

Periodic orbit analysis reveals subtle effects of atropine on epileptiform activity in the guinea-pig hippocampal slice

Epileptiform activity is a state often induced in vitro in order to study seizures and antiepileptic/anticonvulsant drugs. Traditional methods of evaluating drug effects have commonly relied upon measuring changes in the frequency and duration of such events. We have used a recently developed mathematical technique based on periodic orbit analysis to investigate the effect of atropine (a muscarinic antagonist) on epileptiform activity induced by soman (an irreversible acetylcholinesterase inhibitor), 4-aminopyridine (a K+ channel blocker) and 8-cyclopentyl-1,3-dipropylxanthine (an adenosine A1 receptor antagonist) in the guinea-pig hippocampal slice. This technique showed that significant changes in periodic orbits can occur without an accompanying change in burst rate. These results suggest that periodic orbit analysis may be useful in detecting and predicting novel actions of anticonvulsant drugs.

Partial adenosine A(1) receptor agonists inhibit sarin-induced epileptiform activity in the hippocampal slice

Organophosphate poisoning can result in seizures and subsequent neuropathology. One possible therapeutic approach would be to employ adenosine A(1) receptor agonists, which have already been shown to have protective effects against organophosphate poisoning. Using an in vitro model of organophosphate-induced seizures, we have investigated the ability of several adenosine A(1) receptor agonists to inhibit epileptiform activity induced by the organophosphate sarin, in the CA1 stratum pyramidale of the guinea pig hippocampal slice. Application of the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) or the partial adenosine A(1) receptor agonists 2-deoxy-N(6)-cyclopentyladenosine (2-deoxy-CPA) and 8-butylamino-N(6)-cyclopentyladenosine (8-butylamino-CPA) abolished epileptiform activity in a concentration-related manner. The rank order of potency was CPA (IC(50) 4-5 nM) >2-deoxy-CPA (IC(50) 113-119 nM)=8-butylamino-CPA (IC(50) 90-115 nM). These data suggest that partial adenosine A(1) receptor agonists, which have fewer cardiovascular effects, should be further evaluated in vivo as potential treatments for organophosphate poisoning.

Adenosine A1 receptor-mediated presynaptic inhibition of GABAergic transmission in immature rat hippocampal CA1 neurons

In the mechanically dissociated rat hippocampal CA1 neurons with native presynaptic nerve endings, namely "synaptic bouton" preparation, the purinergic modulation of spontaneous GABAergic miniature inhibitory postsynaptic currents (mIPSCs) was investigated using whole-cell recording mode under the voltage-clamp conditions. In immature neurons, adenosine (10 microM) reversibly decreased GABAergic mIPSC frequency without affecting the mean current amplitude. The inhibitory effect of adenosine transmission was completely blocked by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 100 nM), a selective Alpha(1) receptor antagonist, and was mimicked by N(6)-cyclopentyladenosine (CPA, 1 microM), a selective Alpha(1) receptor agonist. However, CPA had no effect on GABAergic mIPSC frequency in postnatal 30 day neurons. N-ethylmaleimide (10 microM), a guanosine 5'-triphosphate binding protein uncoupler, and Ca(2+)-free external solution removed the CPA-induced inhibition of mIPSC frequency. K(+) channel blockers, 4-aminopyridine (100 microM) and Ba(2+) (1 mM), had no effect on the inhibitory effect of CPA on GABAergic mIPSC frequency. Stimulation of adenylyl cyclase with forskolin (10 microM) prevented the CPA action on GABAergic mIPSC frequency. Rp-cAMPS (100 microM), a selective PKA inhibitor, also blocked the CPA action. It was concluded that the activation of presynaptic Alpha(1) receptors modulates the probability of spontaneous GABA release via cAMP- and protein kinase A dependent pathway. This Alpha(1) receptor-mediated modulation of GABAergic transmission may play an important role in the regulation of excitability of immature hippocampal CA1 neurons.

Adenosine acting on A1 receptors protects NO-triggered rebound potentiation and LTP in rat hippocampal slices

Exposure of hippocampal slices to nitric oxide (NO) results in a depression of CA1 synaptic transmission. Under 0.2-Hz stimulation, washout of NO leads to a persistent potentiation that depends on N-methyl-D-aspartate (NMDA) receptors and endogenous NO formation and that occludes tetanus-induced long-term potentiation (LTP). The experiments were initially aimed at determining the relationship between the NO-induced synaptic depression and rebound potentiation. The adenosine A1 antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) partially inhibited the depression produced by the NO donor diethylamine NONOate (300 microM). It also led to a complete block of both the rebound potentiation and the subsequent tetanus-induced LTP. LTP was preserved in the presence of DPCPX if the stimulation frequency was reduced to 0.033 Hz or if the NO application was omitted. The NO-triggered rebound potentiation was restored if the experiment (DPCPX followed by exogenous NO) was conducted in the presence of an NMDA antagonist. The restored potentiation was completely blocked by the NO synthase inhibitor, L-nitroarginine. It is concluded that the NO-induced depression is partially mediated by increased release of endogenous adenosine acting on A1 receptors. Moreover, tonic A1 receptor activation by adenosine protects LTP and the rebound potentiation from being disabled by untimely NMDA receptor activity. Hence, the NO-induced depression and rebound potentiation are linked in the sense that the depression helps to preserve the capacity of the synapses to undergo potentiation. Finally, the results give the first example of exogenous NO eliciting an enduring potentiation of hippocampal synaptic transmission that is dependent on endogenous NO formation, but not on NMDA receptors.

Effects of adenosine receptor agonists and antagonists on pentylenetetrazole-induced amnesia

The effect of adenosine agents on amnesia induced by pentylenetetrazole was examined in mice. Post-training administration of pentylenetetrazole (50 and 60 mg/kg) disrupted 24-h retention of a single-trial passive avoidance task. The adenosine receptor antagonists, theophylline (2.5-25 mg/kg) and 8-phenyltheophylline (0.5-2 mg/kg), administered 30 min before and just after training at doses which did not affect retention, reduced the amnestic effect of pentylenetetrazole in a dose-dependent manner. Post-training administration of the adenosine A(1) receptor agonists, N(6)-cyclohexyladenosine (CHA, 0.1 and 0.5 mg/kg) and N(6)-phenylisopropyladenosine (R-PIA, 0.03 and 0.1 mg/kg), but not the adenosine A(2) receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA, 0.01 and 0.001 mg/kg), impaired retention. Nonamnestic doses of CHA and R-PIA potentiated the disruption induced by a lower dose of pentylenetetrazole (40 mg/kg). NECA did not induce any response in this respect. It is suggested that an adenosine A(1) receptor mechanism is involved in amnesia induced by pentylenetetrazole.

The role and regulation of adenosine in the central nervous system

Adenosine is a modulator that has a pervasive and generally inhibitory effect on neuronal activity. Tonic activation of adenosine receptors by adenosine that is normally present in the extracellular space in brain tissue leads to inhibitory effects that appear to be mediated by both adenosine A1 and A2A receptors. Relief from this tonic inhibition by receptor antagonists such as caffeine accounts for the excitatory actions of these agents. Characterization of the effects of adenosine receptor agonists and antagonists has led to numerous hypotheses concerning the role of this nucleoside. Previous work has established a role for adenosine in a diverse array of neural phenomena, which include regulation of sleep and the level of arousal, neuroprotection, regulation of seizure susceptibility, locomotor effects, analgesia, mediation of the effects of ethanol, and chronic drug use.

Anticonvulsant A(1) receptor-mediated adenosine action on neuronal networks in the brainstem-spinal cord of newborn rats

Membrane potential of ventral respiratory group neurons as well as inspiratory-related cranial (hypoglossal) and spinal (C(1)-Th(4)) nerve activities were analysed in brainstem-spinal cord preparations from neonatal rats. Block of Cl(-)-mediated inhibition with bicuculline (plus strychnine) affected neither rhythmic depolarizations nor spike discharge in 23 of 30 ventral respiratory group cells. In the other seven neurons, block of inhibitory postsynaptic potentials evoked pronounced depolarizations and spike discharge that was synchronous with seizure-like spinal nerve activity. Respiratory hypoglossal nerve activity persisted after transection at the spinomedullary junction, whereas spinal rhythm was blocked. After transection, the moderate bicuculline-evoked seizure-like perturbation of hypoglossal nerve activity was abolished and rhythmic ventral respiratory group neuron activity was not disturbed, whereas epileptiform discharge persisted in spinal nerves. The seizure-like nerve activity and depolarization of the minor subpopulation of perturbed ventral respiratory group neurons were reversed by either adenosine or the A(1) adenosine receptor agonist 2-chloro-N(6)-cyclopentyladenosine. The A(2) receptor agonist CGS 21860 had no effect. In control preparations, inspiratory nerve activity and membrane potential fluctuations (29 of 35 cells) were not changed by adenosine, 2-chloro-N(6)-cyclopentyladenosine or CGS 21860. In the other six cells, adenosine evoked a hyperpolarization (<10 mV) with no major change in input resistance. The anticonvulsant effects of adenosine and 2-chloro-N(6)-cyclopentyladenosine were antagonized by the A(1) adenosine receptor blocker 8-cyclopentyl-1,3-dipropylxanthine. After pre-incubation with 8-cyclopentyl-1,3-dipropylxanthine, bicuculline also evoked seizure-like discharge in the hypoglossal nerve. The results indicate that seizure-like spinal motor output of the respiratory network upon block of Cl(-)-mediated inhibition is caused by disinhibition of spinal neuronal networks with afferent connections to the ventral respiratory group. Presynaptic A(1) adenosine receptors exert an anticonvulsant action on the disinhibited spinal motor network, but have no depressing effect per se on the isolated medullary respiratory network.

Adenosine receptor antagonists induce persistent bursting in the rat hippocampal CA3 region via an NMDA receptor-dependent mechanism

Adenosine receptor antagonists initiate repetitive bursting activity in the CA3 region of hippocampal slices. Although some studies have suggested that this effect is irreversible, this has been difficult to establish because many adenosine antagonists wash out of brain slices extremely slowly. Furthermore the cellular mechanism that underlies persistent bursting is unknown. To resolve these issues, we studied the effects of nonselective (8-p-sulfophenyltheophylline, 8SPT, 50-100 microM), A(l)-selective (8-cyclopentyl-1, 3-dipropylxanthine, 100 nM; xanthine carboxylic acid congener, 200 nM), and A(2A)-selective (chlorostyryl-caffeine; 200 nM) adenosine antagonists in the CA3 region of rat hippocampal slices using extracellular recording. Superfusion with all of the adenosine antagonists except chlorostyryl-caffeine induced bursting, and the burst frequency after 30 min drug superfusion did not differ for the different antagonists. Most slices showed a period of rapid initial bursting, followed either by stable bursting at a lower frequency or a pattern of oscillating burst frequency. In either case, the bursting continued after drug washout. Virtually identical patterns of long-term bursting activity were observed when 8SPT was washed out or applied continuously. Control experiments using exogenous adenosine to characterize the persistence of 8SPT in tissue demonstrated >95% washout at 60 min, a time when nearly all slices still showed regular bursting activity. When the N-methyl-D-aspartate (NMDA) antagonists DL-2-amino-5-phosphonovaleric acid (AP5; 50 microM) or dizocilpine (10 microM) were applied before and during 8SPT superfusion, bursting occurred in the presence of the NMDA antagonists but did not persist once the 8SPT was washed out. AP5 had no effect on persistent bursting when applied after the initiation of spiking. The selective calcium/calmodulin-dependent protein kinase inhibitor 1-[N, O-bis-(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine (KN-62; 3 microM), which has been shown to block NMDA receptor-dependent synaptic plasticity in the CA1 region, also significantly decreased the long-term effect of 8SPT. Thus adenosine antagonists initiate persistent spiking in the CA3 region; this activity does not depend on continued occupation of adenosine receptors by antagonists, and can be blocked by treatments that prevent NMDA receptor-dependent plasticity.

Regulation of extracellular adenosine in rat hippocampal slices is temperature dependent: role of adenosine transporters

While a great deal is known about stimuli that can induce the release of adenosine from brain tissue, relatively little is known about the regulation of the basal extracellular concentration of adenosine that is present in the absence of stimulation. Under normal conditions, enough adenosine is present to tonically activate a significant portion of the high-affinity adenosine A1 receptors. The present experiments demonstrated that the estimated basal concentration of extracellular adenosine in rat hippocampal slices maintained at 21 degrees C (430 nM) is approximately twice that at 32 degrees C (220 nM). The sensitivity of presynaptic modulatory adenosine A1 receptors was not significantly different at 21 degrees C or at 32 degrees C. Slices maintained at 21 degrees C also showed a reduced ability to inactivate extracellular adenosine, which reflects a reduction in adenosine transport across cell membranes. This effect appears to be primarily due to a reduction in the function of the equilibrative, dipyridamole-sensitive (ei) adenosine transporter; the nitrobenzylthioinosine-sensitive equilibrative transporter (es transporter) appears to be relatively less affected by temperature than is the ei transporter. These experiments demonstrate that extracellular concentrations of adenosine in the brain are sensitive to temperature, and suggest that some of the neurological effects of hypothermia might be mediated via increased concentrations of adenosine in the extracellular space.

Calcium and increase excitability promote tolerance against anoxia in hippocampal slices

We have previously demonstrated that anoxic preconditioning (APC) protects against a subsequent otherwise 'lethal' anoxic insult in hippocampal slices. Tested here are two hypotheses: (a) APC requires calcium to improve electrical recovery in hippocampal slices; and (b) mild excitation promotes preconditioning neuroprotection. Control hippocampal slices were given a single 'test' anoxic insult followed by reoxygenation. Experimental slices were preconditioned by three short anoxic insults of 1 min separated by 10 min of reoxygenation. At 30 min after the third 'conditioning' insult, slices underwent a 'test' anoxic insult [1 min of anoxic depolarization (AD)], and then slices were reoxygenated. Evoked potentials (EPs) were recorded throughout the experiment. In other slices, APC was emulated by inducing spreading depression (as determined by a negative DC shift) with KCL or by inducing increased neuronal excitability with the excitatory agent 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX) (an adenosine A1 receptor blocker). 'Test' anoxic insults lasted 2 min of AD in these groups. To determine the role of calcium during APC, extracellular CaCl2 was decreased to 0.5 mM but only during the APC episodes ('test' anoxia, 1 min of AD). EP amplitudes recovered significantly better after anoxia in preconditioned slices, and in KCl- and DPCPX-treated slices (147.2+/-33.3, n=8, **p<0.01, 71.7+/-13.5, n=7, **p<0.01, and 117.8+/-37.3, n=5, ***p<0.001, respectively) compared to controls. Decreases in extracellular CaCl2 during APC blocked the recovery of EPs after 'test' anoxia (80.6+/-23.0, n=8). These data confirm that increases in excitability can emulate APC. These data also demonstrate that calcium influx during preconditioning is required for the induction of tolerance during APC.

A comparison of the adenosine-mediated synaptic inhibition in the CA3 area of immature and adult rat hippocampus

We compared the effects of the adenosine A1 receptor activation on the postsynaptic potentials (psps) recorded from the CA3 area of immature (postnatal days 10-20) and adult rat hippocampal neurons in vitro. The adenosine A1 receptor agonist 2-phenyl-isopropyl-adenosine (PIA, 1 microM) depressed the stimulus-induced psps less in immature and more in adult neurons. In the presence of the GABAA receptor antagonist bicuculline methiodide (BMI, 10 microM), PIA reduced the duration and number of action potentials of the stimulus-induced paroxysmal depolarizations (PDs) in immature neurons, while it blocked PDs in adult neurons. Spontaneous BMI-induced PDs, were blocked by PIA in less than half (5/12) immature and all (6/6) adult neurons. The adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 1 microM) enhanced the stimulus-induced psps in immature and adult neurons alike; this effect did not lead to stimulus-induced bursting in immature neurons. DPCPX induced spontaneous bursts (proconvulsant effect) in only 2/16 immature but in all adult (12/12) neurons. In BMI, DPCPX increased the duration and number of action potentials of the stimulus-induced PDs in immature and adult neurons alike (by about 30%), but it increased the rates of occurrence of spontaneous PDs in significantly more adult neurons. In conclusion, our results suggest that adenosine, acting via A1 receptors, is a more effective endogenous anti-epileptic in adult than in immature hippocampus, a fact which may contribute to the susceptibility of the latter to epileptogenesis.

Alkylxanthine adenosine antagonists and epileptiform activity in rat hippocampal slices in vitro

Despite its potent proconvulsant effects in vitro, the adenosine A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) does not induce seizures when administered in vivo. This contrasts with the effects of less selective adenosine antagonists such as theophylline or cyclopentlytheophylline, and led us to reexamine the nature of DPCPX-induced epileptiform activity. In the present study, we report that proconvulsant effects of bath-applied DPCPX in rat hippocampal slices are only observed after a preceding stimulus such as NMDA receptor activation or brief tetanic stimulation. While this may be due to the absence of a basal "purinergic tone", the relatively high interstitial concentrations of adenosine present in the slice suggest that access of the drug to A1 receptors may instead be prevented by tightly coupled endogenous adenosine, with the ternary adenosine-A1 receptor-G protein complex stabilised in the high-affinity conformation by a coupling cofactor. This implies that a substantial percentage of adenosine A1 receptors are inactive under physiological conditions, but that access of adenosine A1 receptor antagonists may be facilitated under pathological conditions. Once induced, DPCPX-evoked spiking persists for long periods of time. A "kindling" effect of A1 receptor blockade is unlikely, since persistent spiking is not usually observed with less selective A1 antagonists even after prolonged application. Alternatively, endogenous adenosine released during increased neuronal activity may activate A2 receptors during selective A1 blockade. The most important factor determining the duration of DPCPX-induced spiking, however, may be a persistence of the drug in the tissue and subsequent access to the A1 receptor via a membrane-delineated pathway, since DPCPX-induced spiking could be shown to decrease markedly after a transient superfusion of theophylline. This hypothesis, which implies that the apparent affinity of adenosine antagonists for the A1 receptor is in part a function of their membrane partitioning coefficient, is supported by a close correlation between alkylxanthine logP values obtained from the literature and their Ki value at A1 receptors, but not at the enzyme phosphodiesterase, whose xanthine binding site is presented to the cytosol. The implications for the therapeutic value of purinergic drugs are discussed.

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.

Effects of glutamate application on the rhythm of low magnesium-induced epileptiform activity in hippocampal slices of guinea-pigs

The extracellular concentration of glutamate has previously been reported to increase to more than 10-fold the basal level during seizure activity. In the present study, we tested whether localized increases in extracellular glutamate concentration influence the rhythm of epileptiform discharges in the low-magnesium epilepsy model. In hippocampal slices of guinea-pigs, epileptiform activity was induced by omission of magnesium from the bath fluid. Glutamate and its subreceptor agonists N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were ejected into different strata of the CA3 and CA1 regions using microiontophoretic and micropressure application. Glutamate, NMDA and AMPA applied to the CA3 region, but not to the CA1 region, induced a short-lasting increase in epileptiform discharge frequency, often followed by a transient reduction. The effect was most pronounced with application into the stratum lacunosum-moleculare of the CA3 region and could only be evoked in slices exceeding 400 microns in thickness. The effects on the rhythm of epileptiform discharges induced by NMDA and AMPA were blocked by their specific receptor antagonists. They were not influenced by application of GABAA and GABAB receptor antagonists. Changes in somatic membrane potential of CA3 pyramidal neurons did not correlate with changes in the rhythm of epileptiform discharges elicited in this region. The transient suppression of epileptiform discharges that followed the increase in discharge frequency was abolished by an adenosine A1 receptor antagonist. We propose that localized increases in extracellular glutamate concentration modify the rhythm of epileptiform discharges due to changes in neuronal network activity.

Action of adenosine receptor antagonists on hypoxia-induced effects in the rat hippocampus in vitro

1. We have studied three hypoxia-induced phenomena in the CA1 stratum pyramidale of the rat hippocampal slice: (a) the increase in extracellular potassium ion concentration ([K+]e) measured with ion-sensitive microelectrodes, (b) the intracellularly-recorded pyramidal cell hyperpolarization and (c) the extracellularly-recorded depression of the synaptically-evoked field potential recorded in stratum pyramidale. 2. The extracellular potassium ion concentration ([K+]e) rose from 3 mM to 4.1-4.4 mM at a time when the pyramidal cells hyperpolarized by about 6 mV and neurotransmission was virtually abolished. 3. Presumed glial cells depolarized in response to hypoxia. The shape and time course of this response was remarkably similar to the rise in [K+]e so induced. This is consistent with findings that glial cell membrane potential is dependent on transmembrane K+ gradient. 4. We investigated the effects of theophylline (100 microM) and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 0.1 microM) on these effects. We have found that these compounds attenuated by about half the hypoxia-induced increase in [K+]e; however, they did not reduce the hypoxia-induced hyperpolarization. We have confirmed that they dramatically reduced the suppression of excitatory transmission caused by the hypoxia. We conclude that adenosine A1 receptors may be involved in the alteration of K+ homeostasis in the hippocampal slice during hypoxia.

Strategies for the development of drugs for pharmacoresistant epilepsies

Presently, most strategies for development of antiepileptic drugs (AEDs) center around seizure models that are known to respond to presently marketed AEDs. These strategies do not take into account that epilepsy can be a progressive disease. Moreover, region-specific aspects of epileptogenesis are rarely considered when new AEDs are developed. Seizures in the temporal lobe are often difficult to treat. Animal studies on various seizure models in the hippocampus and the entorhinal cortex (EC) suggest that these structures do not a priori produce seizures that are difficult to treat. However, seizure-like events in the EC tend to progress to a state of status epilepticus-like activity that cannot be suppressed by presently marketed AEDs. Loss of gamma-aminobutyric acid (GABA)ergic neurotransmission and increased excitatory synaptic coupling seem to cooperate for induction of this state. Epilepsy induced alterations in the interaction between the EC and the hippocampus may lead to alterations that facilitate precipitation of seizures. Because of the recurrent interaction between the hippocampus and the EC, these seizures may reach an intensity that is no longer controllable by presently available AEDs. Ontogenetic alterations of the circuitry between the EC and the hippocampus, seizure-induced stabilization of synaptic connections overexpressed during ontogenesis, seizure-induced lesions and subsequent rearrangements of internal cell properties, and synaptic arrangements and kindling-like alterations of nerve cell and glial behavior may all be involved in the generation of a neuronal aggregate whose balance between inhibitory and excitatory processes becomes readily disturbed. Strategies for the development of AEDs treating such seizures should suppress hyperactivity and prevent progression of epileptogenesis. AEDs directed against seizures may be effective if they can be given in sufficient concentrations to suppress very intense local seizures.

A paradoxical inhibitory effect of xanthines on hippocampal excitability in calcium-free media

In calcium-free media, neurones in the rat hippocampal slice develop bursts of population potentials and lose their sensitivity to adenosine. The present paper reports the unexpected and paradoxical finding that the xanthines theophylline and cyclopentyltheophylline, the latter of which is selective for A1 purine receptors, depressed the excitability of hippocampal pyramidal neurones in calcium-free media. Chelating residual calcium with EGTA reduced excitability which was additive with the xanthine effect, while 100 microM calcium depressed the response to theophylline. The inhibition by xanthines was prevented by adenosine, which had no effect by itself, but was not reproduced or modified by adenosine deaminase. The xanthine effects were also prevented by baclofen and carbamazepine. A common feature of adenosine, baclofen and carbamazepine which may account for their antagonism of the xanthines is the blockade of calcium fluxes. It is proposed that in the presence of low external concentrations of calcium xanthines can reduce excitability by promoting the mobilisation and trans-membrane movement of residual calcium in the medium or neuronal membranes.

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.

Adenosine inhibits evoked synaptic transmission primarily by reducing presynaptic calcium influx in area CA1 of hippocampus

Adenosine is a potent modulator of neuronal activity throughout the nervous system. Previous evidence suggests that adenosine presynaptically inhibits synaptic transmission. The mechanism of presynaptic inhibition is uncertain. Adenosine may inhibit transmitter release by reducing voltage-dependent Ca2+ currents, activating K+ currents, or by mechanisms downstream to Ca2+ influx. By simultaneously recording the presynaptic Ca2+ transient ([Ca]t) and the field excitatory postsynaptic potential (fEPSP) at CA3-CA1 synapses of hippocampal slices, we found that adenosine, through activation of presynaptic A1 receptors, inhibits the fEPSP primarily by reducing the [Ca]t. Reduced [Ca]t was due to inhibition of omega-conotoxin GVIA-sensitive and some unidentified Ca2+ channels, probably including Q-type, but not to omega-agatoxin-IVA-sensitive Ca2+ channels. The adenosine A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine enhanced both the [Ca]t and the fEPSP, suggesting a tonic inhibition of the [Ca]t by endogenous adenosine in the hippocampal slice.

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.

Epileptogenic actions of xanthines in relation to their affinities for adenosine A1 receptors in CA3 neurons of hippocampal slices (guinea pig)

In order to analyze the epileptogenic mechanisms of caffeine and related xanthines, putative effects of these drugs were studied on adenosine receptors of CA3 neurons in hippocampal slices. Epileptogenic concentrations of different xanthine derivatives strongly correlated with their affinities for the inhibitory A1 adenosine receptor subtype. The A1 receptor agonists adenosine and R-PIA reversibly depressed xanthine-induced epileptic activity without effects on the resting membrane potential or on spontaneously occurring action potentials. These findings suggest that the epileptogenic potency of xanthines is primarily due to the blockade of the A1 receptors through an abnormal rise of intracellular cAMP and to the excessive transmembrane calcium fluxes underlying paroxysmal depolarization shifts.

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.

Effects of chronic administration of adenosine A1 receptor agonist and antagonist on spatial learning and memory

Spatial memory acquisition in Morris water maze was tested in C57BL/6 mice. Animals were injected once daily with different doses of either N6-cyclopentyladenosine (CPA) or 8-cyclopentyl-1,3-dipropylxanthine (CPX). Drugs were administered for 9 days either concurrently with water maze testing (drugs injected 1 h after each trial), or prior to the entire block of trials. In the latter case, 1 day without injections preceded water maze experiments. Chronic administration of CPA resulted in a significant, dose-dependent reduction of target latencies, rapid development of spatial preference, and the absence of animals unable to perform the task. CPX treated animals did not show significant performance changes, and failed to develop spatial preference. Locomotor disturbances were not the cause of the observed effects. Our results indicate that chronic treatment with agents acting at adenosine A1 receptors results in behavioral effects that are significantly different from those observed following their acute administration. Therefore, particular caution is required in development of adenosine-based strategies targeted at neurodegenerative or cognitive disorders in which chronic treatment is advocated.

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.

Adenosinergic modulation of the EEG and locomotor effects of the A2 agonist, CGS 21680

The present study in rats was designed to identify the respective roles of A1 and A2 adenosine receptor activation in the anticonvulsant and behavioral actions of adenosine. Intracaudate injections of the highly selective A2 agonist, CGS 21680, did not affect caudate seizures. However, seizure threshold was increased in the presence of CGS 21680 after blockade of the A1 receptor with CPX, or following activation of the A1 receptor with R-PIA or NECA. Additionally, CGS 21680 led to a dose-related inhibition of locomotor activity when injected into the caudate. These results implicate the involvement of the A2 adenosine receptor in the locomotor depressant actions of adenosine and also suggest possible A2 anticonvulsant effects may depend upon the activation of the A1 receptor.

Role of adenosine in heterosynaptic, posttetanic depression in area CA1 of hippocampus

Conditioning stimulation of afferent fibers in hippocampal area CA1 produced heterosynaptic, posttetanic depression (PTD) of responses evoked by stimulation of an independent set of afferent fibers. PTD was present within 5 s of conditioning stimulation, amounted to a 60-80% reduction of excitatory postsynaptic potentials (EPSPs), and required a period of 3-5 min for recovery. Antagonists of A1 adenosine receptors substantially reduced PTD. Adenosine released into, or formed in, the extracellular space during conditioning stimulation may diffuse within the slice to depress evoked release of glutamate.

Quantal analysis of modulating action of adenosine on the mossy fiber synapse in hippocampal slices

Although adenosine has been considered to depress release of neurotransmitter in central synapses, evidence for this notion is circumstantial. To obtain more direct and quantitative evidence, the suppressing action of adenosine on transmission through the mossy fiber synapse was examined by a quantal analysis in thin transverse sections of the guinea pig hippocampus. Adenosine suppressed excitatory postsynaptic potentials (EPSPs) induced in CA3 neurons by activation of single mossy fibers to 57 +/- 19% at 10 microM (6 neurons). The input resistance of the postsynaptic neurons was reduced to 84 +/- 5%. The suppression of the unitary EPSPs was accompanied by a reduction of the mean quantal content (m) to 51 +/- 13% with no significant decrease in mean quantal amplitude (q). These findings indicate that adenosine at 10 microM suppresses transmission through mossy fiber synapses mainly by depressing release of transmitter. This study provides direct evidence for the presynaptic action of adenosine on central synapses for the first time.

A1 adenosine-receptor antagonists activate chloride efflux from cystic fibrosis cells

A1 adenosine-receptor-antagonist drugs such as 8-cyclopentyl-1,3-dipropylxanthine (CPX) and xanthine amine congener (XAC) are found to activate the efflux of 36Cl- from CFPAC cells. These cells are a pancreatic adenocarcinoma cell line derived from a cystic fibrosis (CF) patient homozygous for the common mutation, deletion of Phe-508. The active concentrations for these compounds are in the low nanomolar range, consistent with action on A1 adenosine receptors. In addition, drug action can be blocked by exogenous agonists such as 2-chloroadenosine and also can be antagonized by removal of endogenous agonists by treatment with adenosine deaminase. Cells lacking the CF genotype and phenotype, such as HT-29 and T84 colon carcinoma cell lines, appear to be resistant to activation of chloride efflux by either drug. CFPAC cells transfected with the CF transmembrane regulator gene, CFTR, are also resistant to activation by CPX. We conclude that, since these antagonists are of relatively low toxicity and appear to act somewhat selectively, they might be considered as promising therapeutic candidates for CF.

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.

Postsynaptic inhibition by adenosine in hippocampal CA3 neurons: Co(2+)-sensitive activation of an inwardly rectifying K+ conductance

The properties of the current underlying the membrane hyperpolarization evoked by adenosine (50-100 microM) were investigated in hippocampal CA3 neurons in vitro using current-clamp and single-electrode voltage-clamp techniques. In voltage-clamp measurements, the adenosine-induced current (IAdo) was outward at rest and reversed at membrane potentials close to the equilibrium potential of K+ (EK), indicating that IAdo was carried by K+ ions. Determination of IAdo at several membrane potentials revealed a nonlinear current/voltage (I/V) relationship of the current displaying inward rectification in the hyperpolarizing direction. Similarly, adenosine increased the membrane slope conductance only at membrane potentials negative to rest, whereas the slope of the neuronal I/V curve remained unchanged when determined at potentials positive to rest. Since the electrophysiological properties of IAdo were very similar to those described for K+ conductances activated by other neuroactive substances like serotonin, opioid peptides and gamma-aminobutyric acid B receptor (GABAB) agonists, we conclude that IAdo belongs to a family of ligand-operated, inwardly rectifying K+ currents which apparently share a common mechanism to reduce postsynaptic excitability. As an additional feature, the postsynaptic adenosine response was reduced by bath application of Co2+ or Ni2+. The adenosine-induced membrane hyperpolarization was not affected by low-Ca2+ or low-Mg2+ solutions, nor by buffering of intracellular Ca2+, but a gradual decline of IAdo was observed following superfusion with Co2+ or Ni2+. In contrast, Mn2+ caused only a weak attenuation of the adenosine response.

N6-cyclopentyladenosine impairs passive avoidance retention by selective action at A1 receptors

The effects of N6-cyclopentyladenosine (CPA), a highly selective agonist for adenosine A1 receptors, on retention of one-trial inhibitory avoidance behavior were examined in mice. Water-deprived animals were trained to avoid drinking by pairing foot-shock with licks from a water spout. Retention was measured as the suppression of drinking (latency to drink) 48 h following training. Administration of CPA (0.15-2.25 mumol/kg) 30 min prior to training produced a dose-dependent impairment in memory of the original avoidance task. The CPA-elicited deficits in retention performance were blocked by pretreatment with 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective A1 receptor antagonist; DPCPX (15 mumol/kg) administration alone had no effect on retention performance. These findings suggest that selective activation of a presumably central population of A1 receptors may impair retention performance and influence information processing.

Adenosinergic inhibition in hippocampus is mediated by adenosine A1 receptors very similar to those of peripheral tissues

The amplitude of the orthodromically evoked population spike (PS) of CA1 neurons was used to investigate quantitatively adenosine receptor antagonism in guinea pig hippocampal slices. Increasing concentrations of the highly selective adenosine A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 3-100 nM) produced parallel, rightward shifts of the dose-response curve for the N6-cyclopentyladenosine (CPA)-induced decrease in PS amplitude. Schild plot analyses of the respective antagonism data obtained in both the presence and virtual absence of endogenous adenosine yielded apparent dissociation constants (KD) of DPCPX at the hippocampal A1 receptor of 3.3 and 3.6 nM, respectively. This indicates that the inhibitory tonus generated by endogenously produced adenosine is due to tonic activation of A1 receptors. The KD values agree well with the binding affinity of DPCPX to A1 receptors determined in brain tissue sections. Since, in our preparation, Schild plot analyses of DPCPX antagonism revealed KD values close to those reported for other tissues, it is concluded that the central A1 receptor mediating adenosinergic inhibition is pharmacologically not distinct from A1 receptors identified in peripheral tissues.

Adenosine depresses induction of LTP at the mossy fiber-CA3 synapse in vitro

Using the hippocampal slice preparation, extracellular recordings of CA3 field excitatory postsynaptic potentials (fEPSPs) were performed to assess the effects of adenosine on the induction of mossy fiber long-term potentiation (LTP). When present during tetanization, adenosine (50 microM) significantly suppressed mossy fiber LTP whereas it failed to inhibit LTP when applied immediately after high-frequency stimulation. Based on the hypothesis that mossy fiber LTP is presynaptic in origin, our data thus provide first evidence that adenosine's presynaptic action alone is sufficiently powerful to interfere with synaptic plasticity.