Adenosine A1-receptor-mediated tonic inhibition of glutamate release at rat hippocampal CA3–CA1 synapses is primarily due to inhibition of N-type Ca2+ channels

Anticonvulsant effect of equilibrative nucleoside transporters 1 inhibitor in a mouse model of Dravet syndrome

There are limited therapeutic options for patients with Dravet syndrome (DS). The equilibrative nucleoside transporters 1 (ENT1) mediate both the influx and efflux of adenosine across the cell membrane exerted beneficial effects in the treatment of epilepsy. This study aimed to evaluate the anticonvulsant effect of the ENT1 inhibitor in an animal model of DS (Scn1aE1099X/+ mice). J7 (5 mg/kg) treatment was efficacious in elevating seizure threshold in Scn1aE1099X/+ mice after hyperthermia exposure. Moreover, the J7 treatment significantly reduced the frequency of spontaneous excitatory post-synaptic currents (sEPSCs, ~35% reduction) without affecting the amplitude in dentate gyrus (DG) granule cells. Pretreatment with the adenosine A1 receptor (A1R) antagonist, DPCPX, abolished the J7 effects on sEPSCs. These observations suggest that the J7 shows an anticonvulsant effect in hyperthermia-induced seizures in Scn1aE1099X/+ mice. This effect possibly acts on presynaptic A1R-mediated signaling modulation in granule cells.

Theophylline reverses oxycodone's but not fentanyl's respiratory depression in mice while caffeine is ineffective against both opioids

RATIONALE

The opioid epidemic remains a pressing public health crisis in the United States. Most of these overdose deaths are a result of lethal respiratory depression. In recent years the increasing incidence of opioid-involved overdose deaths has been driven by fentanyl, which is more resistant to adequate reversal by naloxone (NARCAN ®) than semi-synthetic or classical morphinan predecessors like oxycodone and heroin. For this and other reasons (e.g., precipitating withdrawal) non-opioidergic pharmacotherapies to reverse opioid-depressed respiration are needed. Methylxanthines are a class of stimulant drugs including caffeine and theophylline which exert their effects primarily via adenosine receptor antagonism. Evidence suggests methylxanthines can stimulate respiration by enhancing neural activity in respiratory nuclei in the pons and medulla independent of opioid receptors. This study aimed to determine whether caffeine and theophylline can stimulate respiration in mice when depressed by fentanyl and oxycodone.

METHODS

Whole-body plethysmography was used to characterize fentanyl and oxycodone's effects on respiration and their reversal by naloxone in male Swiss Webster mice. Next, caffeine and theophylline were tested for their effects on basal respiration. Finally, each methylxanthine was evaluated for its ability to reverse similar levels of respiratory depression induced by fentanyl or oxycodone.

RESULTS AND CONCLUSIONS

Oxycodone and fentanyl dose-dependently reduced respiratory minute volume (ml/min; MVb) that was reversible by naloxone. Caffeine and theophylline each significantly increased basal MVb. Theophylline, but not caffeine, completely reversed oxycodone-depressed respiration. In contrast, neither methylxanthine elevated fentanyl-depressed respiration at the doses tested. Despite their limited efficacy for reversing opioid-depressed respiration when administered alone, the methylxanthines safety, duration, and mechanism of action supports further evaluation in combination with naloxone to augment its reversal of opioid-depressed respiration.

Cholinergic and Adenosinergic Modulation of Synaptic Release

In this review we will discuss the effect of two neuromodulatory transmitters, acetylcholine (ACh) and adenosine, on the synaptic release probability and short-term synaptic plasticity. ACh and adenosine differ fundamentally in the way they are released into the extracellular space. ACh is released mostly from synaptic terminals and axonal bouton of cholinergic neurons in the basal forebrain (BF). Its mode of action on synaptic release probability is complex because it activate both ligand-gated ion channels, so-called nicotinic ACh receptors and G-protein coupled muscarinic ACh receptors. In contrast, adenosine is released from both neurons and glia via nucleoside transporters or diffusion over the cell membrane in a non-vesicular, non-synaptic fashion; its receptors are exclusively G-protein coupled receptors. We show that ACh and adenosine effects are highly specific for an identified synaptic connection and depend mostly on the presynaptic but also on the postsynaptic receptor type and discuss the functional implications of these differences.

Genetic and molecular basis of epilepsy-related cognitive dysfunction

Epilepsy is a common neurological disease characterized by recurrent seizures. About 70 million people were affected by epilepsy or epileptic seizures. Epilepsy is a complicated complex or symptomatic syndromes induced by structural, functional, and genetic causes. Meanwhile, several comorbidities are accompanied by epileptic seizures. Cognitive dysfunction is a long-standing complication associated with epileptic seizures, which severely impairs quality of life. Although the definitive pathogenic mechanisms underlying epilepsy-related cognitive dysfunction remain unclear, accumulating evidence indicates that multiple risk factors are probably involved in the development and progression of cognitive dysfunction in patients with epilepsy. These factors include the underlying etiology, recurrent seizures or status epilepticus, structural damage that induced secondary epilepsy, genetic variants, and molecular alterations. In this review, we summarize several theories that may explain the genetic and molecular basis of epilepsy-related cognitive dysfunction.

Neuroprotection of cordycepin in NMDA-induced excitotoxicity by modulating adenosine A1 receptors

Cerebral ischemia impairs physiological form of synaptic plasticity such as long-term potentiation (LTP). Clinical symptoms of cognitive dysfunction resulting from cerebral ischemia are associated with neuron loss and synaptic function impairment in hippocampus. It has been widely reported that cordycepin displays neuroprotective effect on ameliorating cognitive dysfunction induced by cerebral ischemia. Therefore, it is necessary to study whether cordycepin recovers cognitive function after brain ischemia through improving LTP induction. However, there has been very little discussion about the effects of cordycepin on LTP of cerebral ischemia so far. In the present study, we investigated the effects of cordycepin on LTP impairment and neuron loss induced by cerebral ischemia and excitotoxicity, using electrophysiological recording and Nissl staining techniques. The models were obtained by bilateral common carotid artery occlusion (BCCAO) and intrahippocampal NMDA microinjection. We also explored whether adenosine A₁ receptors involve in the neuroprotection of cordycepin by using western blot. We found that cordycepin remarkably alleviated LTP impairment and protected pyramidal cell of hippocampal CA1 region against cerebral ischemia and excitotoxicity. Meanwhile, cordycepin prevented the reduction on adenosine A₁ receptor level caused by ischemia but did not alter the adenosine A_2A receptor level in hippocampal CA1 area. The improvement of LTP in the excitotoxic rats after cordycepin treatment could be blocked by DPCPX, a selective antagonist of adenosine A₁ receptor. In summary, our findings provided new insights into the mechanisms of cordycepin neuroprotection in excitotoxic diseases, which is through regulating adenosine A₁ receptor to improve LTP formation and neuronal survival.

Expression, pharmacology and functional activity of adenosine A1 receptors in genetic models of Huntington's disease

Adenosine A1 receptor (A1R) stimulation exerts beneficial effects in response to various insults to the brain and, although it was found neuroprotective in a lesional model of Huntington's disease (HD), the features of this receptor in genetic models of HD have never been explored. In the present study we characterized the expression, affinity and functional effects of A1Rs in R6/2 mice (the most widely used transgenic model of HD) and in a cellular model of HD. Binding studies revealed that the density of A1Rs was significantly reduced in the cortex and the striatum of R6/2 mice compared to age-matched wild-type (WT), while receptor affinity was unchanged. The selective A1R agonist cyclopentyladenosine (CPA, 300nM) was significantly more effective in reducing synaptic transmission in corticostriatal slices from symptomatic R6/2 than in age-matched WT mice. Such an effect was due to a stronger inhibition of glutamate release from the pre-synaptic terminal. The different functional activities of A1Rs in HD mice were associated also to a different intracellular signaling pathway involved in the synaptic effect of CPA. In fact, while the PKA pathway was involved in both genotypes, p38 MAPK inhibitor SB203580 partially prevented synaptic effects of CPA in R6/2, but not in WT, mice; moreover, CPA differently modulated the phosphorylation status of p38 in the two genotypes. In vitro studies confirmed a different behavior of A1Rs in HD: CPA (100 nM for 5h) modulated cell viability in STHdh(Q111/Q111) (mhttHD cells), without affecting the viability of STHdh(Q7/Q7) (wthtt cells). This effect was prevented by the application of SB203580. Our results demonstrate that in the presence of the HD mutation A1Rs undergo profound changes in terms of expression, pharmacology and functional activity. These changes have to be taken in due account when considering A1Rs as a potential therapeutic target for this disease.

Purinergic control of hippocampal circuit hyperexcitability in Dravet syndrome

OBJECTIVE

Severe myoclonic epilepsy in infancy (SMEI) or Dravet syndrome is one of the most devastating childhood epilepsies. Children with SMEI have febrile and afebrile seizures (FS and aFS), ataxia, and social and cognitive dysfunctions. SMEI is pharmacologically intractable and can be fatal in 10-20% of patients. It remains to be elucidated how channelopathies that cause SMEI impact synaptic activities in key neural circuits, and there is an ongoing critical need for alternative methods of controlling seizures in SMEI. Using the SCN1A gene knock-in mouse model of SMEI (mSMEI), we studied hippocampal cell and circuit excitability, particularly during hyperthermia, and tested whether an adenosine A1 receptor (A1R) agonist can reliably control hippocampal circuit hyperexcitability.

METHODS

Using a combination of electrophysiology (extracellular and whole-cell voltage clamp) and fast voltage-sensitive dye imaging (VSDI), we quantified synaptic excitation and inhibition, spatiotemporal characteristics of neural circuit activity, and hyperthermia-induced febrile seizure-like events (FSLEs) in juvenile mouse hippocampal slices. We used hyperthermia to elicit FSLEs in hippocampal slices, while making use of adenosine A1R agonist N6-cyclopentyladenosine (CPA) to control abnormally widespread neural activity and FSLEs.

RESULTS

We discovered a significant excitation/inhibition (E/I) imbalance in mSMEI hippocampi, in which inhibition was decreased and excitation increased. This imbalance was associated with an increased spatial extent of evoked neural circuit activation and a lowered FSLE threshold. We found that a low concentration (50 nm) of CPA blocked FSLEs and reduced the spatial extent of abnormal neural activity spread while preserving basal levels of excitatory synaptic transmission.

SIGNIFICANCE

Our study reveals significant hippocampal synapse and circuit dysfunctions in mSMEI and demonstrates that the A1R agonist CPA can reliably control hippocampal hyperexcitability and FSLEs in vitro. These findings may warrant further investigations of purinergic agonists as part of the development of new therapeutic approaches for Dravet syndrome.

Modulation of synaptic transmission by adenosine in layer 2/3 of the rat visual cortex in vitro

Adenosine is a wide-spread endogenous neuromodulator. In the central nervous system it activates A1 and A2A receptors (A1Rs and A2ARs) which have differential distributions, different affinities to adenosine, are coupled to different G-proteins, and have opposite effects on synaptic transmission. Although effects of adenosine are studied in detail in several brain areas, such as the hippocampus and striatum, the heterogeneity of the effects of A1R and A2AR activation and their differential distribution preclude generalization over brain areas and cell types. Here we study adenosine's effects on excitatory synaptic transmission to layer 2/3 pyramidal neurons in slices of the rat visual cortex. We measured effects of bath application of adenosine receptor ligands on evoked excitatory postsynaptic potentials (EPSPs), miniature excitatory postsynaptic potentials (mEPSPs), and membrane properties. Adenosine reduced the amplitude of evoked EPSPs and excitatory postsynaptic currents (EPSCs), and reduced frequency of mEPSPs in a concentration-dependent and reversible manner. Concurrent with EPSP/C amplitude reduction was an increase in the paired-pulse ratio. These effects were blocked by application of the selective A1R antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine), suggesting that activation of presynaptic A1Rs suppresses excitatory transmission by reducing release probability. Adenosine (20μM) hyperpolarized the cell membrane from -65.3±1.5 to -67.7±1.8mV, and reduced input resistance from 396.5±44.4 to 314.0±36.3MOhm (∼20%). These effects were also abolished by DPCPX, suggesting postsynaptic A1Rs. Application of the selective A2AR antagonist SCH-58261 (2-(2-furanyl)-7-(2-phenylethyl)-7H-pyrazolo[4,3-e][1,2,4]triazolo[1,5-c]pyrimidin-5-a-mine) on the background of high adenosine concentrations revealed an additional decrease in EPSP amplitude. Moreover, application of the A2AR agonist CGS-21680 (4-[2-[[6-amino-9-(N-ethyl-β-d-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid hydrochloride) led to an A1R-dependent increase in mEPSP frequency. Dependence of the A2AR effects on the A1R availability suggests interaction between these receptors, whereby A2ARs exert their facilitatory effect on synaptic transmission by inhibiting the A1R-mediated suppression. Our results demonstrate functional pre and postsynaptic A1Rs and presynaptic A2ARs in layer 2/3 of the visual cortex, and suggest interaction between presynaptic A2ARs and A1Rs.

Blockers of adenosine A1, but not muscarinic acetylcholine, receptors improve excessive extracellular glutamate-induced synaptic depression

We investigated adenosinergic and cholinergic effects on excessive glutamate-induced depressions of central excitatory synaptic transmissions in vitro. From the CA1 region in rat hippocampal slices, orthodromically elicited population spikes (PSs) and field excitatory postsynaptic potentials (fEPSPs) at 0.1Hz were simultaneously recorded. ANOVA was used for statistics, and p<0.05 was accepted as significant. Glutamate (10mM for 10min) completely depressed PSs and fEPSPs, which were partially recovered by the following washout for 40min (67.5±15.7% and 65.4±13.9% of the control, respectively, p<0.01, n=12). The recoveries in PSs and fEPSPs were exacerbated by edrophonium and carbamoylcholine but improved by non- and A1-selective adenosine receptor antagonists (p<0.01, n=6). The recovery in PSs, not that in fEPSPs, was exacerbated by adenosine, adenosine A1-receptor agonist and A2a-receptor antagonist (p<0.01, n=6). The effects of edrophonium were blocked by non-, M2- and M4-selective muscarinic acetylcholine receptor antagonists (p<0.01, n=6). Excessive glutamate depresses glutamatergic excitatory synaptic transmissions, which are exacerbated by muscarinic acetylcholine receptor stimulation but improved by adenosine A1 receptor block. Somatic excitability is impaired by excessive glutamate with adenosine A1 receptor stimulation.

Control of cannabinoid CB1 receptor function on glutamate axon terminals by endogenous adenosine acting at A1 receptors

Marijuana is a widely used drug that impairs memory through interaction between its psychoactive constituent, Delta-9-tetrahydrocannabinol (Delta(9)-THC), and CB(1) receptors (CB1Rs) in the hippocampus. CB1Rs are located on Schaffer collateral (Sc) axon terminals in the hippocampus, where they inhibit glutamate release onto CA1 pyramidal neurons. This action is shared by adenosine A(1) receptors (A1Rs), which are also located on Sc terminals. Furthermore, A1Rs are tonically activated by endogenous adenosine (eADO), leading to suppressed glutamate release under basal conditions. Colocalization of A1Rs and CB1Rs, and their coupling to shared components of signal transduction, suggest that these receptors may interact. We examined the roles of A1Rs and eADO in regulating CB1R inhibition of glutamatergic synaptic transmission in the rodent hippocampus. We found that A1R activation by basal or experimentally increased levels of eADO reduced or eliminated CB1R inhibition of glutamate release, and that blockade of A1Rs with caffeine or other antagonists reversed this effect. The CB1R-A1R interaction was observed with the agonists WIN55,212-2 and Delta(9)-THC and during endocannabinoid-mediated depolarization-induced suppression of excitation. A1R control of CB1Rs was stronger in the C57BL/6J mouse hippocampus, in which eADO levels were higher than in Sprague Dawley rats, and the eADO modulation of CB1R effects was absent in A1R knock-out mice. Since eADO levels and A1R activation are regulated by homeostatic, metabolic, and pathological factors, these data identify a mechanism in which CB1R function can be controlled by the brain adenosine system. Additionally, our data imply that caffeine may potentiate the effects of marijuana on hippocampal function.

Adenosine A 2A receptor deficiency reduces striatal glutamate outflow and attenuates brain injury induced by transient focal cerebral ischemia in mice

Recent studies have demonstrated that adenosine A(2A) receptor (A(2A)R) inactivation protects against brain injury caused by cerebral ischemia in various animal models. However, the underlying mechanisms remain to be fully elucidated. We examined the effect A(2A)R genetic inactivation on extracellular glutamate in the striatum and its relationship to the neuroprotection afforded by A(2A)R inactivation following transient middle cerebral artery occlusion (MCAo) in mice. Extracellular glutamate in the striatum was collected by in vivo microdialysis during cerebral ischemia and after reperfusion, and then determined with high-performance liquid chromatography. We found that the glutamate level was indistinguishable between A(2A)R knock-out (A(2A)R-KO) mice and their wild type (A(2A)R-WT) littermates before MCAo. After MCAo a remarkable increase in the glutamate level was observed in the A(2A)R-WT mice, but the increase in glutamate level was significantly attenuated in the A(2A)R-KO mice. The cerebral reperfusion induced a second wave of increase of the glutamate level in the A(2A)R-WT mice, and again this increase was largely attenuated in the A(2A)R-KO mice. Correlating with attenuated glutamate level, the neurological deficits and the cerebral infarct volume were also significantly reduced in the A(2A)R-KO mice compared with their WT littermates. These results demonstrate that the genetic inactivation of A(2A)R inhibits the glutamate outflow and ameliorates the brain injury in both ischemic and reperfusion phases in the transient focal cerebral ischemia model. It suggests that the protection of A(2A)R inactivation against ischemic brain injury is associated with the suppression of glutamate-dependent toxicity.

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

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

mGluR7 inhibits glutamate release through a PKC-independent decrease in the activity of P/Q-type Ca2+ channels and by diminishing cAMP in hippocampal nerve terminals

The modulation of calcium channels by metabotropic glutamate receptors (mGluRs) is a key event in the fine-tuning of neurotransmitter release. Here we report that, in hippocampal nerve terminals from adult rats, the inhibition of glutamate release by the group III mGluR agonist L-2-amino-4-phosphonobutyrate (L-AP4) is largely mediated by mGluR7. In this preparation, P/Q-type Ca(2+) channels support the major component of glutamate release while the remaining release is supported by N-type Ca(2+) channels. The release associated with P/Q channels was modulated by mGluR7, either in the presence of omega-conotoxin-GVIA or after decreasing the extracellular Ca(2+) concentration [Ca(2+)](o) to abolish the contribution of N-type Ca(2+) channels. Under these conditions, L-AP4 (1 mm) reduced the evoked glutamate release by 35 +/- 2%. This inhibition was largely prevented by pertussis toxin, but it was insensitive to inhibitors of protein kinase C (bisindolylmaleimide) and protein kinase A (H-89). Furthermore, this inhibition was associated with a reduction in the Ca(2+) influx mediated by P/Q channels in the absence of any detectable change in cAMP levels. However, L-AP4 decreased the levels of cAMP in the presence of forskolin. The activation of this additional signalling pathway was very efficient in counteracting the facilitation of glutamate release induced by forskolin. Thus, mGluR7 mediates the inhibition of glutamate release at hippocampal nerve terminals primarily by inhibiting P/Q-type Ca(2+) channels, although augmenting the levels of cAMP reveals the ability of the receptor to decrease cAMP.

C-Jun N-terminal kinase regulates adenosine A1 receptor-mediated synaptic depression in the rat hippocampus

Adenosine A1 receptors are ubiquitous mediators of presynaptic inhibition of neurotransmission in the central nervous system, yet the signalling pathway linking A1 receptor activation and decreased neurotransmitter release remains poorly resolved. We tested the contribution of c-Jun N-terminal kinase (JNK) to adenosine A1 receptor-mediated depression of field excitatory postsynaptic potentials (fEPSPs) in area CA1 of the rat hippocampus. We found that inhibition of JNK with SP600125 or JNK inhibitor V, but not an inactive analogue, attenuated the depression of fEPSPs induced by adenosine, hypoxia, and the A1 receptor agonist N(6)-cyclopentyladenosine (CPA). In contrast, the JNK inhibitor SP600125 did not inhibit GABA(B)-mediated synaptic depression. In support of our electrophysiological findings, Western blot analysis showed that A1 receptor stimulation resulted in a transient increase in JNK phosphorylation in the membrane fraction of hippocampal lysates. The total amount of JNK in the membrane fraction was unchanged by CPA treatment. The increase in phosphorylated JNK induced by A1 receptor stimulation was blocked by the A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), indicating that A1 receptors specifically activate JNK in the hippocampus. Together with functional data indicating that JNK inhibition decreased CPA-induced paired pulse facilitation, these results suggest that JNK activation is necessary for adenosine A1 receptor-mediated synaptic depression occurring at a presynaptic locus The adenosine A1 receptor-JNK signalling pathway may represent a novel mechanism underlying inhibition of neurotransmitter release in the CNS.

Paired-pulse ratio of synaptically induced transporter currents at hippocampal CA1 synapses is not related to release probability

When a synapse is stimulated in rapid succession, the second post-synaptic response can be larger than the first and termed paired-pulse facilitation. It has been reported that the paired-pulse ratio (PPR), which is the ratio of the amplitude of the second response to that of the first, depends on the probability of vesicular release at the synapse, and PPR has been used as an easy measure of the release probability. To re-examine the relation of PPR with transmitter release probability, we made whole-cell recordings from astrocytes and pyramidal neurons in the CA1 area of rat hippocampal slices, and studied responses evoked by paired-pulse stimulus of the Schaffer collaterals. In a control condition in which blockers for ionotropic glutamate receptors were added to the artificial cerebrospinal fluid, synaptically induced transporter currents (STCs) recorded from astrocytes showed PPF with similar dependency on stimulus interval as the AMPA-receptor-mediated excitatory post-synaptic currents (AMPA-EPSCs) recorded from pyramidal neurons. When the transmitter release was enhanced by raising Ca2+ concentration in the bathing medium or by applying 8-CPT, an adenosine A1 receptor antagonist, the PPR of the neuronal AMPA-EPSCs decreased significantly. In the same condition, although the amplitude of STCs was significantly increased, the PPR of STCs did not show significant change. The PPR of AMPA-EPSCs, however, recovered by lowering the stimulus intensity or by applying low concentration of NBQX, a competitive antagonist for AMPA-receptor. These results imply that the PPR of transmitter release at Schaffer collateral synapses stays constant as the release probability was altered.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Presynaptic adenosine A1 receptors modulate excitatory synaptic transmission in the posterior piriform cortex in rats

The effect of adenosine on the fEPSP was examined in the lateral olfactory tract (Ia input) and associative tract (Ib input) of the rat piriform cortex. The fEPSP evoked in the Ia input showed paired-pulse facilitation, while that in the Ib input showed paired-pulse depression, suggesting a lower resting release probability in the Ia input. This was supported by results showing that MK801 blocked the NMDA receptor-induced fEPSP more rapidly in the Ib input than the Ia input. Adenosine caused dose-dependent inhibition of the fEPSP in both inputs, the sensitivity being higher in the Ib input. This effect was mimicked by the A(1) receptor agonist, CHA, and antagonized by co-application of the A(1) receptor antagonist, DPCPX, showing that adenosine was acting at A(1) receptors. Application of DPCPX alone caused an increase in the fEPSP, the increase being larger in the Ia input. DPCPX also caused paired-pulse depression in both inputs, and the paired-pulse ratio measured in its presence was very similar in both inputs. These results suggest there is a lower endogenous concentration of adenosine in the Ib sublayer than the Ia sublayer, which might account for the native difference in the resting release probability of the two inputs. The adenosine-induced inhibition of the fEPSP in both inputs was associated with a significant reduction in the rate at which MK801 blocked NMDA receptor-mediated fEPSP activity, suggesting a presynaptic location of the A(1) receptors. Blocking of N-, P/Q-type calcium channels occluded the inhibition by adenosine, indicating that they are downstream effectors of presynaptic A(1) receptor activation.

p38 mitogen-activated protein kinase contributes to adenosine A1 receptor-mediated synaptic depression in area CA1 of the rat hippocampus

Adenosine is arguably the most potent and widespread presynaptic modulator in the CNS, yet adenosine receptor signal transduction pathways remain unresolved. Here, we demonstrate a novel mechanism in which adenosine A1 receptor stimulation leads to p38 mitogen-activated protein kinase (MAPK) activation and contributes to the inhibition of synaptic transmission. Western blot analysis indicated that selective A1 receptor activation [with N6-cyclopentyladenosine (CPA)] resulted in rapid increases in phosphorylated p38 (phospho-p38) MAPK immunoreactivity in membrane fractions, and decreases in phospho-p38 MAPK in cytosolic fractions. Immunoprecipitation with a phospho-p38 MAPK antibody revealed constitutive association of this phosphoprotein with adenosine A1 receptors. Phospho-p38 MAPK activation by A1 receptor stimulation induced translocation of PP2a (protein phosphatase 2a) to the membrane. We then examined the actions of p38 MAPK activation in A1 receptor-mediated synaptic inhibition. Excitatory postsynaptic field potentials evoked in area CA1 of the rat hippocampus markedly decreased in response to adenosine (10 microM), the A1 receptor agonist CPA (40 nM), or a 5 min exposure to hypoxia. These inhibitory responses were mediated by A1 receptor activation because the selective antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine) (100 nM) prevented them. In agreement with the biochemical analysis, the selective p38 MAPK inhibitor SB203580 [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole] (25 microM) blocked the inhibitory actions of A1 receptor activation, whereas both the inactive analog SB202474 [4-ethyl-2-(p-methoxyphenyl)-5-(4'-pyridyl)-1H-imidazole] (25 microM) and the ERK 1/2 (extracellular signal-regulated kinase 1/2) MAPK inhibitor PD98059 [2'-amino-3'-methoxyflavone] (50 microM) were ineffective. In contrast, the p38 MAPK inhibitors did not inhibit GABA(B)-mediated synaptic depression. These data suggest A1 receptor-mediated p38 MAPK activation is a crucial step underlying the presynaptic inhibitory effect of adenosine on CA3-CA1 synaptic transmission.

Depletion of ATP and release of presynaptic inhibition in the CA1 region of hippocampal slices during hypoglycemic hypoxia

Transient recovery (TR) of evoked synaptic potentials and ATP depletion during the late stage of hypoxic hypoglycemic insults were investigated in rat hippocampal slices. TR was observed not only in the late stage of insult, but also during recovery. The concentration of ATP corresponded to the appearance (27% of control) and disappearance (15% of control) of TR. Paired pulse studies showed the presynaptic nature of the release of inhibition of synaptic transmission during TR. Both N- and P/Q-type voltage-dependent calcium channels were involved in the appearance of TR. This evidence suggests that underlying mechanisms of TR appearance during hypoxic hypoglycemic insult might be related to ATP depletion and release of A1 adenosine receptor mediated inhibition of presynaptic voltage-dependent calcium channels.

Transient recovery of synaptic transmission is related to rapid energy depletion during hypoxia

Transient recovery (TR) of evoked synaptic potential during the late stage of hypoxic hypoglycemia (HH) insult was investigated in rat hippocampal slices using extracellular recording methods. TR was observed in association with a rapid deterioration of antidromic population spikes (aPSs) following HH insult. TR was not elicited in normoglycemic hypoxia (NH), in which a gradual and delayed deterioration of aPSs was noted. TR was not modulated by either Ca(2+)- or PKC-dependent processes. When a glycolytic inhibitor was added, NH resulted in a rapid deterioration of aPSs and prompted appearance of TR. TR was also seen in slices using lactate to generate energy via oxidative phosphorylation, when hypoxic conditions were subsequently created. Other pharmacological interventions that aimed to cause rapid deterioration of aPSs without depleting energy stores failed to reproduce TR. The evidence thus suggests that the underlying mechanisms of TR appearance during HH insult are highly correlated with rapid energy depletion.

Adenosine A2A receptors are expressed by GABAergic neurons of medulla oblongata in developing rat

During early development, adenosine contributes to the occurrence of respiratory depression and recurrent apneas. Recent physiological studies indicate that GABAergic mechanisms may be involved in this inhibitory action of adenosine, via their A(2A) receptors. In the present study, in situ hybridization with ribonucleotide probes for A(2A) receptor (A(2A)R) mRNA was combined with the immunolabeling technique for parvalbumin and transneuronal retrograde tracing method using green fluorescent protein expressing pseudorabies virus (GFP-PRV) to (1) characterize age-dependent changes in the expression of adenosine A(2A)Rs mRNA in brain stem regions where GABAergic neurons are located; (2) determine whether GABA-containing neurons express A(2A)R mRNA traits, and (3) identify whether bulbospinal GABAergic neurons projecting to phrenic nuclei contain A(2A)R mRNA. Results revealed expression of A(2A) receptors in regions of medulla oblongata containing GABAergic neurons, namely in the ventral aspect of the medulla, within the Bötzinger region and caudal to it, the gigantocellular reticular nucleus, midline neurons and the caudal ventrolateral medulla oblongata. Furthermore, a subpopulation of identified GABAergic neurons, projecting to the phrenic motor nuclei, possess A(2A)R mRNA. It is concluded that adenosine A(2A)Rs expressed by GABAergic neurons are likely to play a role in mediating adenosine-induced respiratory depression.