Adenosine antagonists have differential effects on induction of long-term potentiation in hippocampal slices

How adenosine leakage and tetanic release might affect long-term potentiation (LTP) was investigated by applying adenosine antagonists 8(p-sulfophenyl)theophylline (8SPT) or 8-cyclopentyl-3, 7-dihydro-1, 3-dipropyl-1H-purine-2, 6-dione (DPCPX) to slices, while recording CA1 field EPSPs and population spikes. In the first series of experiments, we applied weak double tetani (at 100 Hz, for 1 s) that were subliminal for evoking LTP in initial control runs. In the presence of 8SPT—at concentrations (10–50 μM) which block both A₁ and A₂ receptors—the same tetani consistently evoked LTP of population spikes but not of excitatory postsynaptic potentials (EPSPs), whereas DPCPX (50 nM), which blocks only A₁ receptors, facilitated LTP of both EPSPs and population spikes. These results are consistent with previous evidence that tetanic adenosine release on the one hand depresses LTP via A₁ receptors but on the other facilitates LTP via A₂ receptors. In a second set of experiments, 8SPT (50–100 μM) did not prevent the induction of LTP of both EPSPs and population spikes by stronger tetanic stimulation. Therefore A₂ receptor activation is not essential for the induction of LTP when stronger tetani are applied. Overall, the main effect of endogenous adenosine release is to oppose LTP induction. © 1995 Wiley-Liss, Inc.     

Caffeine prevents antihyperalgesic effect of gabapentin in an animal model of CRPS‐I: evidence for the involvement of spinal adenosine A1 receptor

This study was designed to determine whether 3 weeks of gabapentin treatment is effective in alleviating neuropathic pain‐like behavior in animal models of complex regional pain syndrome type‐I and partial sciatic nerve ligation (PSNL). We investigated the contribution of adenosine subtypes to the antihyperalgesic effect of gabapentin by examining the effect of caffeine, a non‐selective adenosine A₁ and A₂ receptor antagonist or 1,3‐dipropyl‐8‐cyclopentylxanthine (DPCPX), a selective adenosine A₁ subtype receptor antagonist on this effect. Neuropathic pain was produced by unilateral prolonged hind paw ischemia and reperfusion (I/R) or PSNL procedures which resulted in stimulus‐evoked mechanical hyperalgesia. After procedures, animals received gabapentin (10, 30, or 100 mg/kg intraperitoneal, respectively), caffeine (10 mg/kg intraperitoneal or 150 nmol intrathecally) or DPCPX (3 µg intrathecally) alone or in combination. Mice were tested for tactile mechanical hyperalgesia at 1, 2, and 3 weeks following procedures. Gabapentin produced dose‐related inhibition of mechanical hyperalgesia over a 3‐week period, and this effect was blocked by concomitant caffeine or DPCPX administration 1 week after injuries. The results of this study demonstrated that the mechanism through which gabapentin produces its effect may involve the activation of adenosine A₁ subtype receptor.     

Involvement of the cerebellar adenosine A1 receptor in cannabinoid-induced motor incoordination in the acute and tolerant state in mice

Cannabinoids are known to impair motor function in humans and laboratory animals. We have demonstrated an accentuation of cannabinoid (CP55,940)-induced motor incoordination in mice by the adenosine A₁ receptor-selective agonist N⁶-cyclohexyladenosine (CHA) (4 ng) using an intracerebellar (ICB) microinjection method. This effect was mediated by the A₁ receptor because pre-treatment with ICB 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) (100 ng), an adenosine A₁ receptor selective antagonist, completely abolished the accentuation. Furthermore, ICB pre-treatment with DPCPX (100 ng) before ICB CP55,940 (15 μg) attenuated the motor incoordination suggesting a modulation by an endogenous adenosine A₁ system. ICB microinjection of CHA or DPCPX prior to ICB vehicle had no effect on normal motor coordination. ICB microinjection of dipyridamole (25 μg), an adenosine transport inhibitor, significantly accentuated the motor incoordination by ICB CP55,940 (15 μg), providing further support for the involvement of endogenous adenosine in the action of CP55,940. Tolerance to the motor incoordinating effect of ICB CP55,940 was demonstrated following 3 days of i.p. CP55,940 (0.1, 1 or 2 mg/kg every 12 or 24 h; total of six or three injections, respectively). Interestingly, animals which exhibited tolerance to ICB CP55,940 also demonstrated tolerance to the accentuating effect of ICB CHA suggesting cross-tolerance between adenosine agonists and cannabinoids. Cross-tolerance was also demonstrated following 3 days of i.p. CHA (0.25 or 1 mg/kg every 24 h; total of three injections) as further evidence of the modulatory role of the cerebellar adenosine system in the acute manifestation of CP55,940-induced motor incoordination. The involvement of cerebellar adenosine and the A₁ receptor in cannabinoid actions is circumstantially supported by previous evidence that CB₁ receptors and A₁ receptors are both localized on cerebellar granule cell parallel fiber terminals and basket cell neurons where they serve to inhibit the release of neurotransmitters.     

Deep brain stimulation suppresses epileptic seizures in rats via inhibition of adenosine kinase and activation of adenosine A1 receptors

Aims

Deep brain stimulation (DBS) of the anterior nucleus of the thalamus, is an effective therapy for patients with drug-resistant epilepsy, yet, its mechanism of action remains elusive. Adenosine kinase (ADK), a key negative regulator of adenosine, is a potential modulator of epileptogenesis. DBS has been shown to increase adenosine levels, which may suppress seizures via A1 receptors (A₁Rs). We investigated whether DBS could halt disease progression and the potential involvement of adenosine mechanisms.

Methods

Control group, SE (status epilepticus) group, SE-DBS group, and SE-sham-DBS group were included in this study. One week after a pilocarpine-induced status epilepticus, rats in the SE-DBS group were treated with DBS for 4 weeks. The rats were monitored by video-EEG. ADK and A₁Rs were tested with histochemistry and western blot, respectively.

Results

Compared with the SE group and SE-sham-DBS group, DBS could reduce the frequency of spontaneous recurrent seizures (SRS) and the number of interictal epileptic discharges. The DPCPX, an A₁R antagonist, reversed the effect of DBS on interictal epileptic discharges. In addition, DBS inhibited the overexpression of ADK and the downregulation of A₁Rs.

Conclusion

The findings indicate that DBS can reduce SRS in epileptic rats via inhibition of ADK and activation of A₁Rs. A₁Rs might be a potential target of DBS for the treatment of epilepsy.     

Endogenous adenosine facilitates neurotransmission via A2A adenosine receptors in the rat superior colliculus in vivo

The concentration of endogenous adenosine in the cerebrospinal fluid increased 2–3-fold of the original level in the area of rat superior colliculus after the intraperitoneal administration of an adenosine deaminase inhibitor, EHNA (erythro-9-(2-hydroxy-3-nonyl)adenosine, 10 mg/kg). Potentials evoked in the superior colliculus by optic tract stimulation were also facilitated by 120–160% of their initial amplitudes. A selective A₁ adenosine receptor antagonist, DPCPX (8-cyclopentyl-1,3-dipropylxanthine), failed to reduce such EHNA-induced facilitation. However, a selective A_2A adenosine receptor antagonist, KF17837 (8(3,4-dimethoxystyryl)-1,3-dipropyl-7-methylxanthine) completely eliminated the facilitatory effects of EHNA. Northern blot analysis demonstrated abundant expression of A₁ adenosine receptor mRNA in the superior colliculus. RT-PCR analysis was able to detect the concomitant expression of A_2A adenosine receptor mRNA, but at levels lower than one-tenth of the striatal expression. In the superior colliculus, A_2A adenosine receptors function predominantly on the facilitatory effects of adenosine, irrespective of the ubiquitous expression of A₁ adenosine receptors.     

Potential use of DPCPX as probe for in vivo localization of brain A1 adenosine receptors

The suitability of (³H)DPCPX (8-cyclopentyl-1,3-dipropylxanthine), a xanthine derivative, as an vivo probe for labelling adenosine A₁ receptors was studied in rats. [³H]DPCPX (nM) penetrated largely into the brain (0.8% of the injected dose per gram of brain tissue 5 min after injection). Brain concentrations stayed at a plateau level from 5 to 15 min after the injection. The distribution in the different brain regions was heterogeneous with the highest amount of [³H]DPCPX in cerebellum and hippocampus and the lowest concentrations in hypothalamus and brain stem. Displacement (45–70% of total radioactivity) was obtained by the injection of 250 nM of cold DPCPX or cyclopentylxanthine, an analog of DPCPX. The ex vivo autoradiographic distribution of [³H]DPCPX was similar to the in vitro autoradiographic distribution of tritiated A₁ adenosine receptor ligand as [³H]CHA. These results suggest the potential use of DPCPX for further in vivo investigation of A₁ adenosine receptors with techniques such as positron emission tomography.     

Thyroid function in clinical subtypes of major depression: an exploratory study

Background

Concentrations of monoamine metabolites in human cerebrospinal fluid (CSF) have been used extensively as indirect estimates of monoamine turnover in the brain. CSF monoamine metabolite concentrations are partly determined by genetic influences.

Methods

We investigated possible relationships between DNA polymorphisms in the serotonin 2C receptor (HTR2C), the serotonin 3A receptor (HTR3A), the dopamine D₄ receptor (DRD4), and the dopamine β-hydroxylase (DBH) genes and CSF concentrations of 5-hydroxyindolacetic acid (5-HIAA), homovanillic acid (HVA), and 3-methoxy-4-hydroxyphenylglycol (MHPG) in healthy volunteers (n = 90).

Results

The HTR3A 178 C/T variant was associated with 5-HIAA levels (p = 0.02). The DBH-1021 heterozygote genotype was associated with 5-HIAA (p = 0.0005) and HVA (p = 0.009) concentrations. Neither the HTR2C Cys23Ser variant, nor the DRD4 -521 C/T variant were significantly associated with any of the monoamine metabolites.

Conclusions

The present results suggest that the HTR3A and DBH genes may participate in the regulation of dopamine and serotonin turnover rates in the central nervous system.     

Adenosine modulates synaptic plasticity in hippocampal slices from aged rats

Adenosine is known to modulate synaptic plasticity in the hippocampus of young animals through activation of adenosine A1 receptors. The objective of the present study is to investigate whether the modulatory role of adenosine on phenomena of synaptic plasticity is maintained or modified in the hippocampus of aged animals. We compared the effects of the selective adenosine A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX, 50 nM), on paired-pulse facilitation (PPF), long-term depression (LTD), long-term potentiation (LTP) and depotentiation elicited in hippocampal slices taken from young adult (5-6 weeks) and old (2 years old) male Wistar rats. DPCPX attenuated PPF both in young (1.64 +/- 0.05 vs. 1.76 +/- 0.05%, n = 6) and in old rats (1.33 +/- 0.05 vs. 1.55 +/- 0.1%, n = 6). LTD was only observed in the presence of DPCPX in both young (21.3 +/- 0.6%, n = 4) and old rats (14.4 +/- 0.9%, n = 6). LTP induced by high-frequency stimulation (HFS) was not significantly different in young and old animals, in the presence or in the absence of DPCPX. A larger depotentiation was observed in the presence of DPCPX in young rats (27.6 +/- 4.4% vs. 16.8 +/- 4.7%, n = 7) as well as in old rats (41.3 +/- 5.1% vs. 16.1 +/- 2.7%, n = 6). LTP induced by theta-burst stimulation was observed only in the presence of DPCPX (53.9 +/- 4.9%, n = 5) in young rats, but could be obtained either in the control solution (81.8 +/- 17.9%, n = 7) or in the presence of DPCPX (98.5 +/- 24.2%, n = 7) in old rats. The modulatory role of endogenous adenosine on synaptic plasticity is generally maintained in aged animals. Drugs interfering with adenosine A1 receptor effects could then be used in old animals to modify synaptic plasticity with relevant behavioural consequences.     

Mouse cerebellar GABAB participation in the expression of acute ethanol-induced ataxia and in its modulation by the cerebellar adenosinergic A1 system

The possible modulation and of co-modulation by the cerebellar GABA_B and adenosine A₁ receptors of ethanol-induced motor impairment were investigated in the mice using rotorod performance as the test response. Direct cerebellar microinfusion of GABA_B agonist, baclofen, and antagonist, phaclofen, into the permanently cannulated mice, produced a dose-dependent accentuation and attenuation, respectively, of ethanol-induced motor impairment. The baclofen and phaclofen exhibited accentuation and attenuation, respectively, via GABA_B receptors linked to pertussis toxin-sensitive G protein. A co-modulation by the cerebellar adenosine A₁ receptors was also observed because intracerebellar microinfusion of adenosine agonists N⁶-cyclohexyladenosine (CHA), 5′-N-ethylcarboxamidoadenosine (NECA), and 2-p-(2-carboxyethyl)-phenyl-ethylamino-5′-N-ethylcarbox-amidoadenosine (CGS-21680), and antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), also accentuated and attenuated, respectively, ethanol-induced motor impairment. The accentuation of ethanol-induced motor impairment by baclofen was further enhanced after the intracerebellar microinfusion of CHA, suggesting a co-modulation by the co-localized adenosine A₁ receptors. A similar response was observed after the intracerebellar microinfusion of adenosine A₁ = A₂ agonist NECA and the several-fold higher dose of adenosine A₂-selective agonist CGS-21680. Ethanol-induced motor impairment was markedly blocked by intracerebellar A₁-selective antagonist, DPCPX, as well as by the intracerebellar pertussis toxin pretreatment suggesting again a co-modulation by the adenosine A₁ receptors and the involvement of pertussis toxin-sensitive G protein, respectively. The almost 25-fold higher dose of CGS-21680 to accentuate and DPCPX to attenuate, respectively, ethanol-induced motor impairment together with the reported cerebellar localization of adenosine A₁ subtype only, suggested A₁ receptor activation by NECA and CGS-21680. The functional similarity between GABA_B and adenosine A₁, receptors associated with their anatomical co-localization on the cerebellar granule cells, mainly axons and axonal terminals, may suggest a possible common adenylate cyclase catalytic unit as the basis of modulation of ethanol's motor impairment by these two receptor mechanisms.     

Adenosine modulates the excitability of layer II stellate neurons in entorhinal cortex through A1 receptors

Stellate neurons in layer II entorhinal cortex (EC) provide the main output from the EC to the hippocampus. It is believed that adenosine plays a crucial role in neuronal excitability and synaptic transmission in the CNS, however, the function of adenosine in the EC is still elusive. Here, the data reported showed that adenosine hyperpolarized stellate neurons in a concentration‐dependent manner, accompanied by a decrease in firing frequency. This effect corresponded to the inhibition of the hyperpolarization‐activated, cation nonselective (HCN) channels. Surprisingly, the adenosine‐induced inhibition was blocked by 3 μM 8‐cyclopentyl‐1,3‐dipropylxanthine (DPCPX), a selective A₁ receptor antagonists, but not by 10 μM 3,7‐dimethyl‐1‐propargylxanthine (DMPX), a selective A₂ receptor antagonists, indicating that activation of adenosine A₁ receptors were responsible for the direct inhibition. In addition, adenosine reduced the frequency but not the amplitude of miniature EPSCs and IPSCs, suggesting that the global depression of glutamatergic and GABAergic transmission is mediated by a decrease in glutamate and GABA release, respectively. Again the presynaptic site of action was mediated by adenosine A₁ receptors. Furthermore, inhibition of spontaneous glutamate and GABA release by adenosine A₁ receptor activation was mediated by voltage‐dependent Ca²⁺ channels and extracellular Ca²⁺. Therefore, these findings revealed direct and indirect mechanisms by which activation of adenosine A₁ receptors on the cell bodies of stellate neurons and on the presynaptic terminals could regulate the excitability of these neurons. © 2010 Wiley‐Liss, Inc.     

Adenosine A<Subscript>1</Subscript> receptor distribution in the nucleus tractus solitarii of normotensive and spontaneously hypertensive rats

Summary. Adenosine acts at many sites to modulate neuronal activity. The nucleus tractus solitarii (NTS) is a major brain site in cardiovascular control. The present study was undertaken for a detailed analysis of the distribution of A₁ adenosine receptor (A₁R) in the NTS of spontaneously hypertensive rats (SHR) and Wistar Kyoto (WKY), using in vitro autoradiography with [³H]DPCPX. The density of [³H]DPCPX in the whole NTS decreased according to the rostral-caudal levels. This high level of [³H]DPCPX binding at rostral sites is due to an specific label of the dorsomedial/dorsolateral subnuclei. On the other hand, analysis of subpostremal subnucleus, showed opposite results. The density of [³H]DPCPX binding in the subpostremal NTS increased according to the rostral-caudal levels. Furthermore, it was observed an increased [³H]DPCPX binding in the SHR compared with WKY. The results show a complex pattern of A₁R distribution in the NTS, which highlight the powerful modulatory actions mediated by adenosine in the NTS barosensitive neurons.     

Agonists of A1 and A2A adenosine receptors attenuate methamphetamine-induced overflow of dopamine in rat striatum

The effect of adenosine receptor agonists on the release of striatal dopamine (DA), induced by repeated doses of methamphetamine (MTH), was evaluated. Rats received three injections of MTH (5 mg/kg i.p.) at 2-h intervals. The release of DA in the striatum was measured by a microdialysis in freely moving animals. The agonist of adenosine A₁ receptor, N⁶-cyclopentyladenosine (CPA) and the agonist of adenosine A_2A receptor, 2-[p-(carboxy-ethyl)phenylethylamino]-5′-N-ethylcarboxyamidoadenosine (CGS 21680), either of them being infused locally into the striatum at concentrations of 50 and 100 μM, produced decreases in the extracellular DA level during exposure to MTH, and a weaker effect on the levels of DOPAC and HVA. The above effects were reversed by the specific antagonists of adenosine A₁ and A_2A receptors, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and 3,7-dimethyl-1-propargylxanthine (DMPX), respectively. Our results indicate that both the A₁ and A_2A adenosine receptors appear to be involved in reducing the excessive release of DA in the striatum; furthermore, they suggest a neuroprotective role of adenosine in MTH neurotoxicity.     

Modulation of ethanol-induced motor incoordination by mouse striatal A1 adenosinergic receptor

We have demonstrated that ethanol-induced motor incoordination is modulated by cerebellar adenosine A₁ receptor. This study represents an extension into another important brain motor area, the striatum that, unlike cerebellum, has high density of both A₁ and A_2A receptors. Direct intra-striatal micro-infusion of Ro15-4513 (0.05, 0.5, 1 ng), a partial inverse-agonist of benzodiazepine, significantly and nearly dose-dependently attenuated ethanol-induced motor incoordination indicating mediation of ethanol’s motor incoordination by striatum. Intra-striatal A₁-selective agonist N⁶-cyclohexyladenosine (CHA; 1, 2, 4 ng), A₁ = A_2A non-selective agonist, 5′-N-ethylcarboxamidoadenosine (NECA; 1.5, 3, 6 ng), and A₁-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 25, 50, 100 ng) dose-dependently accentuated and attenuated, respectively, ethanol-induced motor incoordination, strongly suggesting modulation by striatal adenosine A₁ receptor. Intra-striatal DPCPX significantly antagonized not only ethanol-induced motor incoordination but also its potentiation by intra-striatal CHA, R-(+)-N⁶-(2-phenylisopropyladenosine) (R-PIA), or NECA. No change in motor coordination occurred after the highest dose of CHA, R-PIA, or NECA followed by saline. Similarly, the highest intra-striatal dose of Ro15-4513 or DPCPX neither altered motor coordination or locomotor activity indicating relative selectivity of interaction with ethanol. Nearly 25-fold higher dose of A_2A-selective agonist, CGS-21680, compared to CHA was necessary to produce a comparable potentiation of ethanol’s motor incoordination perhaps suggesting a lack of or less significant striatal A_2A involvement. Intra-striatal pertussis toxin (0.5 μg) pre-treatment markedly attenuated ethanol-induced motor incoordination as well as its potentiation by intra-striatal CHA. These results support that striatum is one of the brain motor areas mediating the motor impairing effects of acute ethanol and that the latter’s modulation occurs via A₁-selective receptors coupled to pertussis toxin-sensitive G proteins.     

Early adenosine release contributes to hypoxia‐induced disruption of stimulus‐induced sharp wave‐ripple complexes in rat hippocampal area CA3

We investigated the effects of hypoxia on sharp wave‐ripple complex (SPW‐R) activity and recurrent epileptiform discharges in rat hippocampal slices, and the mechanisms underlying block of this activity. Oxygen levels were measured using Clark‐style oxygen sensor microelectrodes. In contrast to recurrent epileptiform discharges, oxygen consumption was negligible during SPW‐R activity. These network activities were reversibly blocked when oxygen levels were reduced to 20% or less for 3 min. The prolongation of hypoxic periods to 6 min caused reversible block of SPW‐Rs during 20% oxygen and irreversible block when 0% oxygen (anoxia) was applied. In contrast, recurrent epileptiform discharges were more resistant to prolonged anoxia and almost fully recovered after 6 min of anoxia. SPW‐Rs were unaffected by the application of 1‐butyl‐3‐(4‐methylphenylsulfonyl) urea, a blocker of K_ATP channels, but they were blocked by activation of adenosine A₁ receptors. In support of a modulatory function of adenosine, the amplitude and incidence of SPW‐Rs were increased during application of the A₁ receptor antagonist 8‐cyclopentyl‐1,3‐dipropylxanthine (DPCPX). Interestingly, hypoxia decreased the frequency of miniature excitatory post‐synaptic currents in CA3 pyramidal cells, an effect that was converted into increased frequency by the adenosine A₁ agonist DPCPX. In addition, DPCPX also delayed the onset of hypoxia‐mediated block of SPW‐Rs. Our data suggest that early adenosine release during hypoxia induces a decrease in pre‐synaptic glutamate release and that both might contribute to transient block of SPW‐Rs during hypoxia/anoxia in area CA3.     

Adenosine A1 receptors inhibit Ca channels coupled to the release of ACh, but not of GABA, in cultured retina cells

We investigated the effect of adenosine A₁ receptors on the release of acetylcholine (ACh) and GABA, and on the intracellular calcium concentration ([Ca²⁺]_i) response in cultured chick amacrine-like neurons, stimulated by KCl depolarization. The KCl-induced release of [³H]ACh, but not the release of [¹⁴C]GABA, was potentiated when adenosine A₁ receptor activation was prevented by perfusing the cells with adenosine deaminase (ADA) or with 1,3-dipropyl-8-cycloentylxanthine (DPCPX). The changes in the [Ca²⁺]_i induced by KCl depolarization, measured in neurite segments of single cultured cells, were also modulated by endogenous adenosine, acting on adenosine A₁ receptors. Our results show that adenosine A₁ receptors inhibit Ca²⁺ entry coupled to ACh release, but not to the release of GABA, suggesting that the synaptic vesicles containing each neurotransmitter are located in different zones of the neurites, containing different VSCC and/or different densities of adenosine A₁ receptors.     

Effects of A1 receptor agonist/antagonist on spontaneous seizures in pilocarpine-induced epileptic rats

Adenosine is an endogenous anticonvulsant that activates pre- and postsynaptic adenosine A₁ receptors. A₁ receptor agonists increase the latency for the development of seizures and status epilepticus following pilocarpine administration. Although hippocampal adenosine is increased in the chronic phase of the pilocarpine model, it is not known whether the modulation of A₁ receptors may influence the frequency of spontaneous recurrent seizures (SRS). Here, we tested the hypothesis that the A1 receptor agonist RPia ([R]-N-phenylisopropyladenosine) and the A1 antagonist DPCPX (8-Cyclopentyl-1,3-dipropylxanthine) administered to chronic pilocarpine epileptic rats would respectively decrease and increase the frequency of SRS and hippocampal excitability. Four months after Pilo-induced SE, chronic epileptic rats were video-monitored for the recording of SRS before (basal) and after a 2-week treatment with RPia (25 μg/kg) or DPCPX (50 μg/kg). Following sacrifice, brain slices were studied with electrophysiology. We found that rats given RPia had a 93% nonsignificant reduction in the frequency of seizures compared with their own pretreatment baseline. In contrast, the administration of DPCPX resulted in an 87% significant increase in seizure rate. Nontreated epileptic rats had a similar frequency of seizures along the study. Corroborating our behavioral data, in vitro recordings showed that slices from animals previously given DPCPX had a shorter latency to develop epileptiform activity, longer and higher DC shifts, and higher spike amplitude compared with slices from nontreated Pilo controls. In contrast, smaller spike amplitude was recorded in slices from animals given RPia. In summary, the administration of A1 agonists reduced hippocampal excitability but not the frequency of spontaneous recurrent seizures in chronic epileptic rats, whereas A1 receptor antagonists increased both.     

Duplicate preconditioning with sevoflurane in vitro improves neuroprotection in rat brain via activating the extracellular signal-regulated protein kinase

Objective

Sevoflurane preconditioning has been demonstrated to reduce cerebral ischemia-reperfusion (IR) injury, but the underlying mechanisms have not been fully elucidated. Besides, different protocols would usually lead to different results. The objective of this study was to determine whether dual exposure to sevoflurane improves the effect of anesthetic preconditioning against oxygen and glucose deprivation (OGD) injury in vitro.

Methods

Rat hippocampal slices under normoxic conditions (95% O₂/5% CO₂) were pre-exposed to sevoflurane 1, 2 and 3 minimum alveolar concentration (MAC) for 30 min, once or twice, with 15-min washout after each exposure. The slices were then subjected to 13-min OGD treatment (95% N₂/5% CO₂, glucose-free), followed by 30-min reoxygenation. The population spikes (PSs) were recorded in the CA1 region of rat hippocampus. The percentage of PS amplitude at the end of 30-min reoxygenation to that before OGD treatment was calculated, since it could indicate the recovery degree of neuronal function. In addition, to assess the role of mitogen-activated protein kinases (MAPKs) in preconditioning, U0126, an inhibitor of extracellular signal-regulated protein kinase (MEK-ERK1/2, ERK1/2 MAPK), and SB203580, an inhibitor of p38 MAPK, were separately added 10 min before sevoflurane exposure.

Results

Preconditioning once with sevoflurane 1, 2, and 3 MAC increased the percentage of PS amplitude at the end of 30-min reoxygenation to that before OGD treatment, from (15.13±3.79)% (control) to (31.88±5.36)%, (44.00±5.01)%, and (49.50±6.25)%, respectively, and twice preconditioning with sevoflurane 1, 2, and 3 MAC increased the percentage to (38.53±4.36)%, (50.74±7.05)% and (55.86±6.23)%, respectively. The effect of duplicate preconditioning with sevoflurane 3 MAC was blocked by U0126 [(16.23±4.62)%].

Conclusion

Sevoflurane preconditioning can induce neuroprotection against OGD injury in vitro, and preconditioning twice enhances this effect. Besides, the activation of extracellular signal-regulated protein kinase (MEK-ERK1/2, ERK1/2 MAPK) may be involved in this process.     

Effect of Adenosine A2A Receptor Antagonists on l-DOPA-Induced Hydroxyl Radical Formation in Rat Striatum

A_2A adenosine receptor antagonists have been proposed as a new therapy for Parkinson’s disease (PD). Since oxidative stress plays an important role in the pathogenesis of PD, we studied the effect of the selective A_2A adenosine receptor antagonists 8-(3-chlorostyryl)caffeine (CSC) and 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385) on l-3,4-dihydroxyphenylalanine (l-DOPA)-induced hydroxyl radical generation using in vivo microdialysis in the striatum of freely moving rats. l-DOPA (100 mg/kg; in the presence of benserazide, 50 mg/kg) given acutely or repeatedly for 14 days generated a high level of hydroxyl radicals, measured by HPLC with electrochemical detection, as the product of their reaction with p-hydroxybenzoic acid (PBA). CSC (1 mg/kg) and ZM 241385 (3 mg/kg) decreased haloperidol (0.5 mg/kg)-induced catalepsy, while at low doses of 0.1 and 0.3 mg/kg, respectively, they did not display an effect. CSC (1 and 5 mg/kg) and ZM 241385 (3 and 9 mg/kg) given acutely, or CSC (1 mg/kg) and ZM 241385 (3 mg/kg) given repeatedly, increased the production of hydroxyl radicals in dialysates from rat striatum. Both acute and repeated administration of CSC (0.1 and 1 mg/kg) and ZM 241385 (3 mg/kg) decreased l-DOPA-induced generation of hydroxyl radicals. However, a high single dose of either CSC (5 mg/kg) and ZM 241385 (9 mg/kg) markedly potentiated the effect of l-DOPA on hydroxyl radical production. The increase in hydroxyl radical production by acute and chronic injection of CSC and ZM 241385 may be related to the increased release of dopamine (DA) and its metabolism in striatal dialysates. Similarly, increased DA release following a single high dose of CSC or ZM 241385 appears to be responsible for augmentation of l-DOPA-induced hydroxyl radical formation. Conversely, the inhibition of l-DOPA-induced production of hydroxyl radical by single and repeated low doses of CSC or repeated low doses of ZM 241385 may be related to reduced DA metabolism. Summing up, A_2A antagonists, used as a supplement of l-DOPA therapy, depending on the dose used, may have a beneficial or adverse effect on ongoing neurodegenerative processes and accompanying oxidative stress.     

Effects of selective adenosine A1 and A2a agonists on amphetamine-induced locomotion and c-Fos in striatum and nucleus accumbens

Low to moderate doses of amphetamine produce locomotion which is dependent on release of dopamine in the anteromedial striatum and nucleus accumbens. The effects of selective adenosine A₁ and A_2a receptor agonists on locomotion and c-Fos induction following a moderate dose of amphetamine was assessed in rats. Pretreatment with the adenosine A₁ receptor agonist N-6-cyclohexyladenosine (CHA) or the adenosine A_2a receptor agonist 2-[(2-aminoethylamino)carbonylethylphenylethylamino]-5′-N-ethylcarboxamidoadenosine (APEC) inhibited locomotion following an injection of amphetamine (1.5 mg/kg). This dose of amphetamine induced Fos-like immunoreactivity in an antero-dorsomedial distribution in the caudate-putamen and uniformly in the core and shell of the nucleus accumbens. Pretreatment with the adenosine A_2a receptor agonist APEC, but not the adenosine A₁ receptor agonist CHA, attenuated c-Fos induction in caudate-putamen and nucleus accumbens by amphetamine. These findings indicate that amphetamine-induced behavior is subject to modulation by adenosine receptors through mechanisms which are both related to and independent of c-Fos induction.