Immunohistochemical localization of adenosine A2A receptors in the rat central nervous system

An adenosine A2A receptor agonist induces sleep by increasing GABA release in the tuberomammillary nucleus to inhibit histaminergic systems in rats

The adenosine A(2A) receptor (A(2A)R) has been demonstrated to play a crucial role in the regulation of the sleep process. However, the molecular mechanism of the A(2A)R-mediated sleep remains to be elucidated. Here we used electroencephalogram and electromyogram recordings coupled with in vivo microdialysis to investigate the effects of an A(2A)R agonist, CGS21680, on sleep and on the release of histamine and GABA in the brain. In freely moving rats, CGS21680 applied to the subarachnoid space underlying the rostral basal forebrain significantly promoted sleep and inhibited histamine release in the frontal cortex. The histamine release was negatively correlated with the amount of non-rapid eye movement sleep (r = - 0.652). In urethane-anesthetized rats, CGS21680 inhibited histamine release in both the frontal cortex and medial pre-optic area in a dose-dependent manner, and increased GABA release specifically in the histaminergic tuberomammillary nucleus but not in the frontal cortex. Moreover, the CGS21680-induced inhibition of histamine release was antagonized by perfusion of the tuberomammillary nucleus with a GABA(A) antagonist, picrotoxin. These results suggest that the A(2A)R agonist induced sleep by inhibiting the histaminergic system through increasing GABA release in the tuberomammillary nucleus.

Small conductance calcium-activated K+ channels, SkCa, but not voltage-gated K+ (Kv) channels, are implicated in the antinociception induced by CGS21680, a A2A adenosine receptor agonist

It has been shown that A2A adenosine receptors are implicated in pain modulation. The precise mechanism by which activation of A2A receptors produces analgesic effects, however, remains unclear. The aim of this study was to investigate the possible involvement of apamin-sensitive calcium-activated potassium channels (SKCa) and voltage-gated potassium (Kv) channels in A2A receptor activation-induced analgesic effects. Using mice, we evaluated the influence of apamin, a non specific blocker of SKCa channels, Lei-Dab7 (an analog of scorpion Leiurotoxin), a selective blocker of SKCa2 channels, and kaliotoxin (KTX) a Kv channel blocker, on the CGS 21680 (A2A adenosine receptor agonist)-induced increases in hot plate and tail pinch latencies. All drugs were injected in mice via the intracerebroventricular route. We found that apamin and Lei-Dab7, but not KTX, reduced antinociception produced by CGS21680 on the hot plate and tail pinch tests in a dose dependent manner. Lei-Dab 7 was more potent than apamin in this regard. We conclude that SKCa but not Kv channels are implicated in CGS 21680-induced antinociception.

Selective inactivation or reconstitution of adenosine A2A receptors in bone marrow cells reveals their significant contribution to the development of ischemic brain injury

Inactivation of the adenosine A(2A) receptor (A(2A)R) consistently protects against ischemic brain injury and other neural insults, but the relative contribution of A(2A)Rs on peripheral inflammatory cells versus A(2A)Rs expressed on neurons and glia is unknown. We created a chimeric mouse model in which A(2A)Rs on bone marrow-derived cells (BMDCs) were selectively inactivated or reconstituted by bone marrow transplantation. Selective reconstitution of A(2A)Rs on BMDCs (A(2A)R knockout mice transplanted with wild-type bone marrow cells) largely reinstates ischemic brain injury in global A(2A)R knockout mice. Conversely, selective inactivation of A(2A)Rs on BMDCs (wild-type mice transplanted with A(2A)R knockout bone marrow cells) attenuates infarct volumes and ischemia-induced expression of several proinflammatory cytokines in the brain, but exacerbates ischemic liver injury. These results indicate that the A(2A)R-stimulated cascade in BMDCs is an important modulator of ischemic brain injury and that ischemic brain and liver injuries are regulated distinctly by A(2A)Rs on BMDCs.

Lack of adenosine A1 and dopamine D2 receptor-mediated modulation of the cardiovascular effects of the adenosine A2A receptor agonist CGS 21680

Some behavioral and biochemical effects of the systemically administered adenosine A(2A) receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680) in rats are potentiated by adenosine A(1) receptor agonists and counteracted by dopamine D2 receptor agonists. In the present study we compared potentiating and antagonistic interactions between CGS 21680 and adenosine A(1) and dopamine D2 receptor agonists on motor activity and on cardiovascular responses (arterial blood pressure and heart rate). The motor-depressant effects produced by CGS 21680 (0.5 mg/kg, i.p.) were potentiated by the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA, 0.3 mg/kg, i.p.) and counteracted by the dopamine D2 receptor agonist quinpirole (0.5 mg/kg, i.p.). In contrast, neither CPA nor quinpirole significantly modified the decrease in arterial pressure or the increase in heart rate induced by CGS 21680. However, the adenosine A(2A) receptor antagonist 3-(3-hydroxypropyl)-8-(m-methoxystyryl)-7-methyl-1-propargylxanthine phosphate disodium salt (MSX-3, 3 mg/kg, i.p.) counteracted both the motor-depressant and cardiovascular effects of CGS 21680. Therefore, the effects of the systemically administered adenosine A(2A) receptor agonist CGS 21680 on cardiovascular function, in contrast to its effects on motor behavior, appear to be independent of the effects of adenosine A(1) and dopamine D2 receptor activity.

Adenosine postsynaptically modulates supraoptic neuronal excitability

Effects of adenosine on the excitability of supraoptic nucleus neurons were investigated in whole cell patch-clamp experiments conducted in horizontal slices of rat hypothalamus. Adenosine (10-100 muM) inhibited all neurons tested by reducing or abolishing spontaneous or evoked discharge. Large hyperpolarizations were seen, averaging -6.08 +/- 0.83 mV below resting membrane potential, and action potential durations were significantly reduced by 134 +/- 41 mus in the presence of 100 muM adenosine. The A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 1 muM) blocked these effects, whereas the A(1) agonists N(6)-cyclopentyladenosine (CPA) and N(6)-cyclohexyladenosine (CHA) mimicked the actions of adenosine. A(2) receptor contributions to excitability were assessed by application of an A(2) agonist, carboxamidoadenosine (CPCA). This resulted in membrane depolarizations (3.56 +/- 0.65 mV) and maintenance of firing. The presence of endogenous adenosine in the slice was revealed by both the application of the adenosine uptake inhibitor dilazep (1-100 muM), which resulted in a strong inhibition of firing activity, and the application of DPCPX, which induced firing in cells silenced by negative current injection. We tested for postsynaptic actions of adenosine by blocking G protein activation via GDP-beta-S infusion into recorded neurons. Under these conditions, the adenosinergic inhibition of firing and reduction of spike duration were blocked, suggesting the effects were mediated by postsynaptic adenosine receptors. That the effects on excitability could be due to direct activation of adenosine A(1) receptors on supraoptic neurons was further explored immunocytochemically via the co-labeling of magnocellular neurons with polyclonal antibodies raised against the A(1) receptors. It is concluded that adenosine, acting at postsynaptic A(1) receptors, exhibits a powerful inhibitory influence on supraoptic magnocellular activity and is an important endogenous regulator of magnocellular neuroendocrine function.

Neuroprotective effect of L-DOPA co-administered with the adenosine A2A receptor agonist CGS 21680 in an animal model of Parkinson’s disease

Adenosine A2A receptors are a new target for drug development in Parkinson's disease. Some experimental and clinical data suggest that A2A receptor antagonists can provide symptomatic improvement by potentiating the effects of L-DOPA as well as a decrease in secondary effects such as L-DOPA-induced dyskinesia. L-DOPA-induced behavioral sensitization in unilateral 6-hydroxydopamine-lesioned rats is frequently used as an experimental model of L-DOPA-induced dyskinesia. In the present work this model was used to evaluate the effect of the A2A receptor agonist CGS 21680 and the A2A receptor antagonist MSX-3 on L-DOPA-induced behavioral sensitization and 6-hydroxydopamine-induced striatal dopamine denervation. L-DOPA-induced behavioral sensitization was determined as an increase in L-DOPA-induced abnormal involuntary movements and enhancement of apomorphine-induced turning behavior. Striatal dopamine innervation was determined by measuring tyrosine-hydroxylase immunoreactivity. Chronic administration of MSX-3 was not found to be effective at counteracting L-DOPA-induced behavioral sensitization. On the other hand, CGS 21680 completely avoided the development of L-DOPA-induced behavioral sensitization. The analysis of the striatal dopamine innervation showed that L-DOPA-CGS 21680 co-treatment conferred neuroprotection to the toxic effects of 6-hydroxydopamine. This neuroprotective effect was dependent on A2A and D2 receptor stimulation, since it was counteracted by MSX-3 and by the D2 receptor antagonist haloperidol. These results open new therapeutic avenues in early events in Parkinson's disease.

Recent advances in the neurobiology of alcoholism: the role of adenosine

Neuronal responses to alcohol involve several hormone- and neurotransmitter-activated signal transduction pathways. Recent studies suggest that the adenosine A2 receptor (A2) mediates important actions of alcohol. Ethanol inhibits adenosine reuptake, increases extracellular adenosine, and promotes activation of A2. This leads to enhanced cAMP/protein kinase A (PKA) signaling ranging from increases in cAMP to stimulation of cAMP-dependent cAMP response element (CRE)-mediated gene expression. Medium spiny neurons in the striatum/nucleus accumbens (NAc) express A2 and dopamine D2 receptor (D2) on the same cells. Studies in model neuronal cell lines and primary neurons in culture expressing A2 and D2 provide evidence for synergy between ethanol/A2 and D2. Subthreshold concentrations of ethanol or a D2 agonist, without effect separately, synergistically activate cAMP/PKA signaling. Thus, neurons expressing A2 and D2 on the same cells, like in the NAc, are characterized by hypersensitivity to ethanol with a simultaneous activation of dopaminergic signaling. Synergy requires adenosine and appears to be mediated by the release of free betagamma dimers from G(i/o) via D2 activation. The release of free betagamma has pathophysiological significance in the drinking animal because specific blockade of betagamma signaling in the NAc strikingly reduces voluntary alcohol consumption. These findings suggest that signaling pathways, which regulate synergy between A2 and D2, might contain molecular targets for the prevention and treatment of alcoholism and alcohol abuse.

Reverse microdialysis of a dopamine D2 receptor antagonist alters extracellular adenosine levels in the rat nucleus accumbens

Recent evidence suggests that modulation of dopaminergic transmission alters striatal levels of extracellular adenosine. The present study used reverse microdialysis of the selective dopamine D(2) receptor antagonist raclopride to investigate whether a blockade of dopamine D(2) receptors modifies extracellular adenosine concentrations in the nucleus accumbens. Results reveal that perfusion of raclopride produced an increase of dialysate adenosine which was significant with a high (10 mM) and intermediate (1 mM) drug concentration, but not with lower drug concentrations (10 and 100 microM). Thus, the present study demonstrates that a selective blockade of dopamine D(2) receptors in the nucleus accumbens produced a pronounced increase of extracellular adenosine. The cellular mechanisms underlying this effect are yet unknown. It is suggested that the increase of extracellular adenosine might be related to a homeostatic modulatory mechanism proposed to be a key function of adenosine in response to neuronal metabolic challenges.

CVT-510: a selective A1 adenosine receptor agonist

Adenosine is an endogenous nucleoside that has potent antiarrhythmic effects on paroxysmal supraventricular tachycardia (PSVT) due to its negative dromotropic effects on the atrioventricular node. In addition to its electrophysiologic effects, adenosine has important effects on vascular smooth muscle cells, inflammatory cells, the central nervous system, and the kidney. Four known adenosine receptor subtypes (A1, A2A, A2B, and A3) mediate the pleiotropic effects of adenosine in humans. These receptors are coupled to a wide range of second messenger cascades. Activation of the A1 adenosine receptor accounts for the negative chronotropic and dromotropic effects of adenosine, whereas A2A, A2B and A3 adenosine receptor activation are responsible for such effects as coronary vasodilation, bronchospasm, inhibition of platelet aggregation, and neuronal stimulation. Elucidation of the specific properties of each of the adenosine receptor subtypes has led to the development of selective ligands as potential therapeutic agents. CVT-510, N-(3(R)-tetrahydrofuranyl)-6-aminopurine riboside, was developed as a selective A1 adenosine receptor agonist that specifically targets the atrioventricular node for termination of PSVT. Preliminary clinical trials have shown that CVT-510 is effective in terminating PSVT and eliminating many of the undesirable adverse effects of adenosine. CVT-510 is also being explored as a potential agent for controlling the ventricular rate of atrial fibrillation and flutter.

Adenosine modulates the striatal GABAergic inputs to the globus pallidus via adenosine A2A receptors in rats

Previous studies have shown presynaptic modulation of adenosine A(2A) receptors for GABAergic synaptic transmission in the globus pallidus (GP). The pallidal A(2A) receptor-mediated modulation is caused by an action on the terminals of striatopallidal medium spiny neurons (MSNs) and/or axon collaterals of GP neurons. Herein, we examined the precise target neurons of the A(2A) receptor-mediated modulation. Activation of A(2A) receptors enhanced striatopallidal GABAergic transmission onto GP neurons, accompanied by a reduction in the paired-pulse facilitation, indicating the presynaptic contribution of A(2A) receptors at terminals of striatopallidal MSNs in the GP. Besides, no A(2A) receptor mRNA was detected in GP neurons by single-cell reverse transcription-polymerase chain reaction analysis, implying no contribution of axon collaterals of GP neurons to the A(2A) receptor regulation. These results demonstrate that the target neurons of adenosinergic modulation via A(2A) receptors in the GP are the striatopallidal MSNs.

Stimulant doses of caffeine induce c-FOS activation in orexin/hypocretin-containing neurons in rat

Although caffeine is a commonly used CNS stimulant, neuronal mechanisms underlying its stimulatory effect are not fully understood. Orexin (hypocretin)-containing neurons play a critical role in arousal and might be activated by acute administration of caffeine. We examined this possibility by using dual-immunostaining for orexin B and c-Fos protein as a marker for neuronal activation. Rats were administered intraperitoneally with 10, 30 or 75 mg/kg caffeine, or saline. As previously reported, caffeine increased locomotion at 10 and 30 mg/kg, but not at 75 mg/kg. The numbers of orexin-immunoreactive and non-orexin-immunoreactive neurons expressing c-Fos were analysed using three counting boxes within the orexin field in the posterior hypothalamus. Compared with saline, all doses of caffeine increased the number of cells immunoreactive for both orexin and c-Fos. The average magnitude of this increase across doses in orexin neurons differed amongst regions; c-Fos expression increased by 343% in the perifornical area and by 158% in the more medial, dorsomedial nucleus. In the lateral hypothalamic area, c-Fos expression increased by 226% at 10 and 30 mg/kg but no change was seen at 75 mg/kg. In contrast, caffeine significantly increased the number of non-orexin-immunoreactive neurons expressing c-Fos only in the dorsomedial nucleus. These results indicate that systemically administered caffeine preferentially activates orexin neurons over non-orexin neurons in the same field, and that this activation is most pronounced in the perifornical region where orexin neurons are most concentrated. The activation of orexin neurons might play a role in the behavioural activation by caffeine.

A2A antagonist prevents dopamine agonist-induced motor complications in animal models of Parkinson’s disease

Adenosine A(2A) receptors, abundantly expressed on striatal medium spiny neurons, appear to activate signaling cascades implicated in the regulation of coexpressed ionotropic glutamatergic receptors. To evaluate the contribution of adenosinergic mechanisms to the pathogenesis of the response alterations induced by dopaminergic treatment, we studied the ability of the selective adenosine A(2A) receptor antagonist KW-6002 to prevent as well as palliate these syndromes in rodent and primate models of Parkinson's disease. In rats, KW-6002 reversed the shortened motor response produced by chronic levodopa treatment while reducing levodopa-induced hyperphosphorylation at S845 residues on AMPA receptor GluR1 subunits. In primates, KW-6002 evidenced modest antiparkinsonian activity when given alone. Once-daily coadministration of KW-6002 with apomorphine prevented the development of dyskinesias, which appeared in control animals 7-10 days after initiating apomorphine treatment. Animals initially given apomorphine plus KW-6002 for 3 weeks did not begin to manifest apomorphine-induced dyskinesias until 10-12 days after discontinuing the A(2A) antagonist. These results suggest that KW-6002 can attenuate the induction as well as the expression of motor response alterations to chronic dopaminergic stimulation in parkinsonian animals, possibly by blocking A(2A) receptor-stimulated signaling pathways. Our findings strengthen the rationale for developing A(2A) antagonists as an early treatment strategy for Parkinson's disease.

Th1 Cytokines Regulate Adenosine Receptors and Their Downstream Signaling Elements in Human Microvascular Endothelial Cells

We and others have shown that adenosine, acting at its receptors, is a potent modulator of inflammation and angiogenesis. To better understand the regulation of adenosine receptors during these processes we studied the effects of IL-1, TNF-alpha, and IFN-gamma on expression and function of adenosine receptors and select members of their coupling G proteins in human dermal microvascular endothelial cells (HMVEC). HMVEC expressed message and protein for A(2A) and A(2B), but not A(1) or A(3) receptors. IL-1 and TNF-alpha treatment increased message and protein expression of A(2A) and A(2B) receptor. IFN-gamma treatment also increased the expression of A(2B) receptors, but decreased expression of A(2A) receptors. Resting HMVEC and IFN-gamma-treated cells showed minimal cAMP response to the selective A(2A) receptor agonist 2-[2-(4-chlorophenyl)ethoxy]adenosine (MRE0094). In contrast, MRE0094 stimulated a dose-dependent increase in cAMP levels in TNF-alpha-treated cells that was almost completely blocked by the A(2A) receptor antagonist ZM-241385 (4-[2-[7-amino-2-(2-furyl)[1,2,4]triazolo-[2,3-a][1,3,5]triazin-5-ylamino]ethyl]phenol). The nonselective adenosine receptor agonist 5'-(N-ethylcarboxamido)adenosine increased cAMP levels in both TNF-alpha- and IFN-gamma-treated cells, but not control cells, and its effect was only partially reversed by ZM-241385 in TNF-alpha-treated cells and not affected in IFN-gamma-treated cells. HMVEC expressed a higher level of G protein beta1 isoform than beta4 isoform. Although none of the cytokines tested affected G(beta1) expression, both IL-1 and TNF-alpha significantly up-regulated G(beta4) expression. These findings indicate that inflammatory cytokines modulate adenosine receptor expression and function on HMVECs and suggest that the interaction between proinflammatory cytokines and adenosine receptors may affect therapeutic responses to anti-inflammatory drugs that act via adenosine-dependent mechanisms.

Effects of an adenosine A2A receptor blockade in the nucleus accumbens on locomotion, feeding, and prepulse inhibition in rats

The nucleus accumbens (NAc) subserves behaviors governed by natural rewards, i.e., feeding or exploration, and has been implicated in control of prepulse inhibition (PPI), a measure of sensorimotor gating. The present study sought to determine whether a tonic stimulation of adenosine A(2A) receptors in the rat NAc is involved in control of spontaneous locomotor activity, feeding behavior, and PPI. To this end, bilateral microinfusions of a prodrug (MSX-3) (3 microg and 5 microg in 1 microl per side) of the selective A(2A) receptor antagonist MSX-2 or vehicle (1 microl per side) were administered into the NAc. Results show that blockade of intra-NAc adenosine A(2A) receptors by a high (5 microg), but not by a low (3 microg), dose of MSX-3 increased locomotor activity in an open field, reduced food intake, and delayed intake onset in food-deprived rats examined in a test cage with standard laboratory chow. Furthermore, PPI was significantly disrupted after intra-NAc infusion of 5 microg, but not 3 microg, MSX-3. These findings suggest that locomotor activity as well as intact PPI and feeding behavior rely on tonic activation of intra-NAc A(2A) receptors. The data add further support to the view that adenosine is a tonically active modulator of striatal function through actions on A(2A) receptors.

Renal protection from ischemia mediated by A2A adenosine receptors on bone marrow-derived cells

Activation of A2A adenosine receptors (A2ARs) protects kidneys from ischemia-reperfusion injury (IRI). A2ARs are expressed on bone marrow-derived (BM-derived) cells and renal smooth muscle, epithelial, and endothelial cells. To measure the contribution of A2ARs on BM-derived cells in suppressing renal IRI, we examined the effects of a selective agonist of A2ARs, ATL146e, in chimeric mice in which BM was ablated by lethal radiation and reconstituted with donor BM cells derived from GFP, A2AR-KO, or WT mice to produce GFP-->WT, A2A-KO-->WT, or WT-->WT mouse chimera. We found little or no repopulation of renal vascular endothelial cells by donor BM with or without renal IRI. ATL146e had no effect on IRI in A2A-KO mice or A2A-KO-->WT chimera, but reduced the rise in plasma creatinine from IRI by 75% in WT mice and by 60% in WT-->WT chimera. ATL146e reduced the induction of IL-6, IL-1beta, IL-1ra, and TGF-alpha mRNA in WT-->WT mice but not in A2A-KO-->WT mice. Plasma creatinine was significantly greater in A2A-KO than in WT mice after IRI, suggesting some renal protection by endogenous adenosine. We conclude that protection from renal IRI by A2AR agonists or endogenous adenosine requires activation of receptors expressed on BM-derived cells.

Molecular mechanisms and therapeutical implications of intramembrane receptor/receptor interactions among heptahelical receptors with examples from the striatopallidal GABA neurons

The molecular basis for the known intramembrane receptor/receptor interactions among G protein-coupled receptors was postulated to be heteromerization based on receptor subtype-specific interactions between different types of receptor homomers. The discovery of GABAB heterodimers started this field rapidly followed by the discovery of heteromerization among isoreceptors of several G protein-coupled receptors such as delta/kappa opioid receptors. Heteromerization was also discovered among distinct types of G protein-coupled receptors with the initial demonstration of somatostatin SSTR5/dopamine D2 and adenosine A1/dopamine D1 heteromeric receptor complexes. The functional meaning of these heteromeric complexes is to achieve direct or indirect (via adapter proteins) intramembrane receptor/receptor interactions in the complex. G protein-coupled receptors also form heteromeric complexes involving direct interactions with ion channel receptors, the best example being the GABAA/dopamine D5 receptor heteromerization, as well as with receptor tyrosine kinases and with receptor activity modulating proteins. As an example, adenosine, dopamine, and glutamate metabotropic receptor/receptor interactions in the striatopallidal GABA neurons are discussed as well as their relevance for Parkinson's disease, schizophrenia, and drug dependence. The heterodimer is only one type of heteromeric complex, and the evidence is equally compatible with the existence of higher order heteromeric complexes, where also adapter proteins such as homer proteins and scaffolding proteins can exist. These complexes may assist in the process of linking G protein-coupled receptors and ion channel receptors together in a receptor mosaic that may have special integrative value and may constitute the molecular basis for some forms of learning and memory.

SCH 58261, a selective adenosine A2A receptor antagonist, decreases the haloperidol-enhanced proenkephalin mRNA expression in the rat striatum

In the striatum, dopamine D(2) receptors are co-localized with adenosine A(2A) receptors on the GABAergic neurons of the striopallidal pathway. Moreover, blockade of A(2A) receptors has been previously shown to suppress parkinsonian-like symptoms (catalepsy, akinesia, muscle rigidity) in rodent and primate models of Parkinson's disease (PD). Since it is believed that main motor symptoms of PD are due to the overactivity of the GABAergic striopallidal pathway, the aim of the present study was to find out whether SCH 58261, a selective antagonist of the adenosine A(2A) receptors, is capable of counteracting both the catalepsy and the enhancement of proenkephalin (PENK) mRNA expression in the rat striatum, induced by haloperidol administered at 1.5 mg/kg s.c. 3 times, every 3 h. Systemic administration of SCH 58261 (5 mg/kg i.p., 3 times, every 3 h, 10 min before haloperidol), partially decreased the haloperidol-induced catalepsy and the increase in the PENK mRNA expression in both dorsolateral and ventrolateral parts of the striatum at all three examined levels. No such changes were seen in the medial striatum and in the nucleus accumbens. Moreover, SCH 58261 given alone did not influence the level of PENK mRNA in any examined part of the striatum. The present results suggest that similarly to other A(2A) receptor antagonists, SCH 58261 normalizes activity of the striopallidal pathway, enhanced by blockade of dopamine D(2) receptors with haloperidol, which may result in recovery of motor functions.

Alkylation of sulfhydryl groups on Galpha(s/olf) subunits by N-ethylmaleimide: regulation by guanine nucleotides

In rat striatum A(2A) adenosine receptors activate adenylyl cyclase through coupling to G(s)-like proteins, mainly G(olf) that is expressed at high levels in this brain region. In this study we report that the sulfhydryl alkylating reagent, N-ethylmaleimide (NEM), causes a concentration- and time-dependent inhibition of [3H] 2-p-(2-carboxyethyl)phenylethylamino)-5'-N-ethylcarboxamido adenosine ([3H]CGS21680) binding to rat striatal membranes. Membrane treatment with [14C]N-ethylmaleimide ([14C]NEM) labels numerous proteins while addition of 5'-guanylylimidodiphosphate (Gpp(NH)p) reduces labeling of only three protein bands that migrate in SDS-polyacrylamide gel electrophoresis with apparent molecular masses of approximately 52, 45 and 39 kDa, respectively. The 52- and 45-kDa labeled bands show electrophoretic motilities as Galpha(s)-long and Galpha(s)-short/Galpha(olf) subunits. An anti-Galpha(s/olf) antiserum immunoprecipitates two 14C labeled bands of 44 and 39 kDa. The band density decreases by 21-26% when membranes are treated with NEM in the presence of Gpp(NH)p. An anti-A(2A) receptor antibody also immunoprecipitates two 14C labeled bands of 40 and 38 kDa, respectively. However, such protein bands do not show any decrease of their density upon membrane treatment with NEM plus Gpp(NH)p. These results indicate that in rat striatal membranes NEM alkylates sulfhydryl groups of both Galpha(s/olf) subunits and A(2A) adenosine receptors. In addition, cysteine residues of Galpha(s/olf) are easily accessible to modification when the subunit is in the GDP-bound form. The 39- and 38-kDa labeled proteins may represent proteolytic fragments of Galpha(s/olf) and A(2A) adenosine receptor, respectively.

Presynaptic inhibition of synaptic transmission by adenosine in rat subthalamic nucleus in vitro

Whole-cell patch clamp recordings were made from the subthalamic nucleus in rat brain slice preparations to examine the effect of adenosine on inhibitory and excitatory synaptic transmission. Adenosine reversibly inhibited both GABA-mediated inhibitory and glutamate-mediated excitatory postsynaptic currents. Adenosine at 100 microM reduced the amplitude of inhibitory and excitatory postsynaptic currents by 42+/-5% and 34+/-6%, respectively. Reductions in the amplitude of both inhibitory and excitatory postsynaptic currents were accompanied by increases in paired-pulse ratios. In addition, adenosine decreased the frequency of spontaneous miniature excitatory postsynaptic currents but had no effect on their amplitude. These results are consistent with a presynaptic site of action. The adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine completely reversed the adenosine-induced attenuation of inhibitory and excitatory postsynaptic currents, but 8-cyclopentyl-1,3-dipropylxanthine alone had no effect on synaptic currents evoked at 0.1 Hz. However, 8-cyclopentyl-1,3-dipropylxanthine inhibited a time-dependent depression of excitatory postsynaptic currents that was normally observed in response to a 5 Hz train of stimuli, suggesting that endogenous adenosine could be released during higher frequencies of stimulation. These results suggest that adenosine inhibits synaptic release of GABA and glutamate by stimulation of presynaptic A(1) receptors in the subthalamic nucleus.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Prolonged low-dose caffeine exposure protects against hippocampal damage but not against the occurrence of epilepsy in the lithium-pilocarpine model in the rat

PURPOSE

Acute caffeine exposure has proconvulsant effects and worsens epileptic and ischemic neuronal damage. Surprisingly, prolonged caffeine exposure decreases the susceptibility to seizures and the extent of ischemic damage. We explored whether the exposure to a low long-term dose of caffeine could protect the brain from neuronal damage and epileptogenesis in the lithium-pilocarpine model of temporal lobe epilepsy.

METHODS

Rats received either plain tap water or water containing caffeine (0.3 g/L) for 15 days before the induction of status epilepticus (SE) by lithium-pilocarpine and for 7 days after SE. The extent of neuronal damage was assessed in the hippocampus and piriform and entorhinal cortices in brain sections stained with thionine and obtained from animals killed 7 days after SE. The latency to spontaneous recurrent seizures was controlled by video monitoring.

RESULTS

Caffeine treatment induced a marked, almost total neuroprotection in CA1 and a very limited protection in the hilus of the dentate gyrus, whereas damage in layers III-IV of the piriform cortex was slightly worsened by the treatment. All rats, whether they received caffeine or plain tap water, became epileptic after the same latency (17-19 days).

CONCLUSIONS

Thus these data extend the neuroprotective effects of low long-term caffeine exposure to epileptic damage and confirm that the sole protection of the Ammon's horn has no influence on the genesis of spontaneous recurrent seizures in this model.

Extended amygdala and basal forebrain

The basal forebrain is a confluence of systems that are crucial to understanding some of the most important functions of the brain, including reward and punishment, learning and cognition, and feeding and reproduction. Basic to understanding this broad spectrum of behavior is untangling the interwoven functional systems in basal forebrain. This has been grounded by the appreciation that the major nearby structures, that is, amygdala and basal ganglia, provide a context for interpreting basal forebrain areas that are best viewed as extensions of either of these larger regions. The components of basal forebrain, the ventral striatopallidal system and the medial and central divisions of extended amygdala, are subcortical relays for information garnered from brain stem, thalamus, and cortical areas. With respect to the classically defined amygdala of the temporal lobe, the lateral-basolateral complex, and the superficial amygdaloid nuclei may tentatively be viewed as specialized cortical regions. Their output targets both the striatopallidal complex and the extended amygdala, with some of the most massive basal forebrain efferents originating in the basolateral amygdaloid complex. The subcortical projections of the basolateral nucleus, at least in the rat, appear to be dichotomous, with anterior (or magnocellular) portions of the nucleus preferentially targeting striatum and ventral striatum (including the core of the nucleus accumbens), while the posterior (small-celled) portions of the basolateral nucleus target the extended amygdala as well as the shell of the nucleus accumbens. This divergence represents a particular opportunity for behavioral neuroscientists analyzing basal forebrain functions. Studies exploiting the dual subcortical projection of basolateral amygdala indicate distinct functional roles for striatum versus extended amygdala. These reinforce the identification of extended amygdala as a functional-anatomical entity distinct from the striatopallidal system.

Synergistic interaction between adenosine A2A and glutamate mGlu5 receptors: implications for striatal neuronal function

The physiological meaning of the coexpression of adenosine A2A receptors and group I metabotropic glutamate receptors in gamma- aminobutyric acid (GABA)ergic striatal neurons is intriguing. Here we provide in vitro and in vivo evidence for a synergism between adenosine and glutamate based on subtype 5 metabotropic glutamate (mGluR5) and adenosine A2A (A2AR) receptor/receptor interactions. Colocalization of A2AR and mGluR5 at the membrane level was demonstrated in nonpermeabilized human embryonic kidney (HEK)-293 cells transiently cotransfected with both receptors by confocal laser microscopy. Complexes containing A2AR and mGluR5 were demonstrated by Western blotting of immunoprecipitates of either Flag-A2AR or hemagglutinin-mGluR5 in membrane preparations from cotransfected HEK-293 cells and of native A2AR and mGluR5 in rat striatal membrane preparations. In cotransfected HEK-293 cells a synergistic effect on extracellular signal-regulated kinase 1/2 phosphorylation and c-fos expression was demonstrated upon A2AR/mGluR5 costimulation. No synergistic effect was observed at the second messenger level (cAMP accumulation and intracellular calcium mobilization). Accordingly, a synergistic effect on c-fos expression in striatal sections and on counteracting phencyclidine-induced motor activation was also demonstrated after the central coadministration of A2AR and mGluR5 agonists to rats with intact dopaminergic innervation. The results suggest that a functional mGluR5/A2AR interaction is required to overcome the well-known strong tonic inhibitory effect of dopamine on striatal adenosine A2AR function.

SCH 58261 differentially influences quinolinic acid-induced effects in striatal and in hippocampal slices

The influence of the adenosine A(2A) receptor antagonist SCH 58261 (7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3-e]-1,2,4-trizolo[1,5-c] pyrimidine) (50, 200 nM, 1 microM) on quinolinic acid effects has been studied in rat striatal and hippocampal slices. Quinolinic acid induced disappearance of field potentials at concentrations of 500 microM and 2 mM in hippocampal and corticostriatal slices, respectively. We found that 1 microM SCH 58261 prevented quinolinic acid-induced field potential disappearance in corticostriatal but not in hippocampal slices. This finding demonstrates that the peculiar binding profile of SCH 58261 and the predominance in the hippocampus of "atypical" adenosine A(2A) receptor population (not recognized by SCH 58261) could have a functional relevance in the occurrence of region-specific neuroprotective effects.

A new neuromodulatory pathway with a glial contribution mediated via A(2a) adenosine receptors

A low concentration (10 nM) of adenosine potentiated hippocampal neuronal activity via A(2a) adenosine receptors without affecting presynaptic glutamate release or postsynaptic glutamatergic conductance. Adenosine inhibited glutamate uptake through the glial glutamate transporter, GLT-1, via A(2a) adenosine receptors. In addition, adenosine stimulated GLT-1-independent glutamate release from astrocytes, possibly in response to a rise in intracellular Ca(2+), via A(2a) adenosine receptors involving PKA activation. Those adenosine actions could lead to an increase in synaptic glutamate concentrations responsible for the potentiation of hippocampal neuronal activity. The results of the present study thus represent a novel neuromodulatory pathway with a glial contribution, bearing both inhibition of GLT-1 function and stimulation of glial glutamate release, as mediated via A(2a) adenosine receptors.

Expression of adenosine receptors in the preglomerular microcirculation

The purpose of this study was to systematically investigate the abundance of each of the adenosine receptor subtypes in the preglomerular microcirculation vs. other vascular segments and vs. the renal cortex and medulla. Rat preglomerular microvessels (PGMVs) were isolated by iron oxide loading followed by magnetic separation. For comparison, mesenteric microvessels, segments of the aorta (thoracic, middle abdominal, and lower abdominal), renal cortex, and renal medulla were obtained by dissection. Adenosine receptor protein and mRNA expression were examined by Western blotting, Northern blotting, and RT-PCR. Our results indicate that compared with other vascular segments and renal tissues, A1 and A2B receptor protein and mRNA are abundantly expressed in the preglomerular microcirculation, whereas A2A and A3 receptor protein and mRNA are barely detectable or undetectable in PGMVs. We conclude that, relative to other vascular and renal tissues, A1 and A2B receptors are well expressed in PGMVs, whereas A2A and A3 receptors are notably deficient. Thus A1 and A2B receptors, but not A2A or A3 receptors, may importantly regulate the preglomerular microcirculation.

Modification of adenosine extracellular levels and adenosine A(2A) receptor mRNA by dopamine denervation

Adenosine A(2A) receptor antagonists have been proposed as an effective therapy in the treatment of Parkinson's disease. To explore the possibility that dopamine denervation may produce modifications in adenosine A(2A) transmission, we measured the extracellular concentration of adenosine and adenosine A(2A) receptor mRNA in the striatum of rats infused unilaterally with 6-hydroxydopamine in the medial forebrain bundle. Fifteen days after 6-hydroxydopamine infusion, extracellular adenosine levels, measured by in vivo microdialysis, were significantly lower (-35%) in the dopamine-denervated striatum. At the time of the decrease in adenosine levels, an increase in striatal adenosine A(2A) receptor mRNA levels (+20%), measured by in situ hybridization, was observed. Modifications in adenosine A(2A) transmission, following nigrostriatal dopamine neuron degeneration, establish a potential neural basis for the effectiveness of adenosine A(2A) receptor antagonists in the treatment of Parkinson's disease.

An adenosine A2a agonist increases sleep and induces Fos in ventrolateral preoptic neurons

Considerable evidence indicates that adenosine may be an endogenous somnogen, yet the mechanism through which it promotes sleep is unknown. Adenosine may act via A1 receptors to promote sleep, but an A2a receptor antagonist can block the sleep induced by prostaglandin D(2). We previously reported that prostaglandin D(2) activates sleep-promoting neurons of the ventrolateral preoptic area, and we hypothesized that an A2a receptor agonist also should activate these neurons. Rats were instrumented for sleep recordings, and an injection cannula was placed in the subarachnoid space just anterior to the ventrolateral preoptic area. After an 8-10-day recovery period, the A2a receptor agonist CGS21680 (20 pmol/min) or saline was infused through the injection cannula, and the animals were killed 2 h later. The brains were stained using Fos immunohistochemistry, and the pattern of Fos expression was studied in the entire brain. CGS21680 increased non-rapid eye movement sleep and markedly increased the expression of Fos in the ventrolateral preoptic area and basal leptomeninges, but it reduced Fos expression in wake-active brain regions such as the tuberomammillary nucleus. CGS21680 also induced Fos in the shell and core of the nucleus accumbens and in the lateral subdivision of the central nucleus of the amygdala. To determine whether these effects may have been mediated through A1 receptors, an additional group of rats received subarachnoid infusion of the A1 receptor agonist N(6)-cyclopentyladenosine (2 pmol/min). In contrast to CGS21680, infusion of N(6)-cyclopentyladenosine into the subarachnoid space produced only a small decrease in rapid eye movement sleep, and the pattern of Fos expression induced by N(6)-cyclopentyladenosine was notable only for decreased Fos in regions near the infusion site. These findings suggest that an adenosine A2a receptor agonist may activate cells of the leptomeninges or nucleus accumbens that increase the activity of ventrolateral preoptic area neurons. These ventrolateral preoptic area neurons may then coordinate the inhibition of multiple wake-promoting regions, resulting in sleep.

Comparative analysis of the chemical neuroanatomy of the mammalian trigeminal ganglion and mesencephalic trigeminal nucleus

A characteristic peculiarity of the trigeminal sensory system is the presence of two distinct populations of primary afferent neurons. Most of their cell bodies are located in the trigeminal ganglion (TG) but part of them lie in the mesencephalic trigeminal nucleus (MTN). This review compares the neurochemical content of central versus peripheral trigeminal primary afferent neurons. In the TG, two subpopulations of primary sensory neurons, containing immunoreactive (IR) material, are identified: a number of glutamate (Glu)-, substance P (SP)-, neurokinin A (NKA)-, calcitonin gene-related peptide (CGRP)-, cholecystokinin (CCK)-, somatostatin (SOM)-, vasoactive intestinal polypeptide (VIP)- and galanin (GAL)-IR ganglion cells with small and medium-sized somata, and relatively less numerous larger-sized neuropeptide Y (NPY)- and peptide 19 (PEP 19)-IR trigeminal neurons. In addition, many nitric oxide synthase (NOS)- and parvalbumin (PV)-IR cells of all sizes as well as fewer, mostly large, calbindin D-28k (CB)-containing neurons are seen. The majority of the large ganglion cells are surrounded by SP-, CGRP-, SOM-, CCK-, VIP-, NOS- and serotonin (SER)-IR perisomatic networks. In the MTN, the main subpopulation of large-sized neurons display Glu-immunoreactivity. Additionally, numerous large MTN neurons exhibit PV- and CB-immunostaining. On the other hand, certain small MTN neurons, most likely interneurons, are found to be GABAergic. Furthermore, NOS-containing neurons can be detected in the caudal and the mesencephalic-pontine junction portions of the nucleus. Conversely, no immunoreactivity to any of the examined neuropeptides is observed in the cell bodies of MTN neurons but these are encircled by peptidergic, catecholaminergic, serotonergic and nitrergic perineuronal arborizations in a basket-like manner. Such a discrepancy in the neurochemical features suggests that the differently fated embryonic migration, synaptogenesis, and peripheral and central target field innervation can possibly affect the individual neurochemical phenotypes of trigeminal primary afferent neurons.

Purification and characterization of the human adenosine A(2a) receptor functionally expressed in Escherichia coli

The adenosine A(2a) receptor belongs to the seven transmembrane helix G-protein-coupled receptor family, is abundant in striatum, vasculature and platelets and is involved in several physiological processes such as blood pressure regulation and protection of cells during anoxia. For structural and biophysical studies we have expressed the human adenosine A(2a) receptor (hA2aR) at high levels inserted into the Escherichia coli inner membrane, and established a purification scheme. Expression was in fusion with the periplasmic maltose-binding protein to levels of 10-20 nmol of receptor per L of culture, as detected with the specific antagonist ligand [(3)H]ZM241385. As the receptor C-terminus was proteolyzed upon solubilization, a protease-resistant but still functional receptor was created by truncation to Ala316. Addition of the sterol, cholesteryl hemisuccinate, allowed a stable preparation of functional hA2aR solubilized in dodecylmaltoside to be obtained, and, increased the stability of the receptor solubilized in other alkylmaltosides. Purification to homogeneity was achieved in three steps, including ligand affinity chromatography based on the antagonist xanthine amine congener. The purified hA2aR fusion protein bound [(3)H]ZM241385 with a K(d) of 0.19 nm and an average B(max) of 13.7 nmol x mg(-1) that suggests 100% functionality. Agonist affinities for the purified solubilized receptor were higher than those for the membrane-bound form. Sufficient pure, functional hA2aR can now be prepared regularly for structural studies.

A(2A) adenosine receptor: a novel therapeutic target in renal disease

Present strategies in the treatment of inflammatory renal injury have focused on developing agents that specifically target individual mechanisms thought to contribute toward the pathogenesis of the disease. Such an approach is hindered by redundancies in the inflammatory cascade, rendering intervention suboptimal. The A(2A) adenosine receptor (A(2A)-AR) is a member of the family of guanine nucleotide binding proteins and has become a focus of major interest primarily because of its ability to broadly inactivate the inflammatory cascade. This review summarizes our present knowledge regarding the molecular biology and pharmacology of A(2A)-ARs as well as the physiological effects of activation of A(2A)-ARs in the kidney. We also review our recent experience in targeting this receptor subtype in abrogating the inflammatory cascade in ischemia-reperfusion injury.

alpha(2C)-Adrenergic receptors mediate spinal analgesia and adrenergic-opioid synergy

The alpha(2A)-adrenergic receptor (AR) subtype mediates antinociception induced by the alpha(2)AR agonists clonidine, dexmedetomidine, norepinephrine, and 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine (UK-14,304) as well as antinociceptive synergy of UK-14,304 with opioid agonists [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin and deltorphin II. Differential localization of alpha(2)-adrenergic (alpha(2A)-, alpha(2B)-(,) alpha(2C)-) and opioid (mu-, delta-, kappa-) subtypes suggests differential involvement of subtype pairs in opioid-adrenergic analgesic synergy. The present study applies a novel imidazoline(1)/alpha(2)-adrenergic receptor analgesic, moxonidine, to test for involvement of alpha(2B)- and alpha(2C)ARs in antinociception and antinociceptive synergy, because spinal antinociceptive activity of moxonidine shows minimal dependence on alpha(2A)AR. Intrathecal administration of moxonidine produced similar (2-3-fold) decreases in both mutant mice with a functional knockout of alpha(2A)AR (D79N-alpha(2A)AR) and alpha(2C)AR knockout (KO) mice. The potency of moxonidine was not altered in alpha(2B)KO mice, indicating that this subtype does not participate in moxonidine-induced spinal antinociception. Moxonidine-mediated antinociception was dose dependently inhibited by the selective alpha(2)-receptor antagonist SK&F 86466 in both D79N-alpha(2A) mice and alpha(2C)KO mice, indicating that alpha(2)AR activation is required in the absence of either alpha(2A)- or alpha(2C)AR. Spinal administration of antisense oligodeoxynucleotides directed against the alpha(2C)AR decreased both alpha(2C)AR immunoreactivity and the antinociceptive potency of moxonidine. Isobolographic analysis demonstrates that moxonidine-deltorphin antinociceptive synergy is present in the D79N-alpha(2A) mice but not in the alpha(2C)AR-KO mice. These results confirm that the alpha(2C)AR subtype contributes to spinal antinociception and synergy with opioids.

Adenosine A2A, 5-HT1A and 5-HT7 receptor in neonatally pregnenolone-treated rats

Steroid hormones synthesized in the brain, called 'neurosteroids', modulate neuronal activity. We treated neonatal rats with a main precursor of the neurosteroidogenesis, pregnenolone, and examined adenosine A2A receptor, 5- hydroxytryptamine (5-HT)1A and 5-HT7 receptor densities in the front-parietal cortex in juvenile and adult rats. In receptor binding assay using [3H]CGS21680 and [3H]8-OH-DPAT, it was shown that neonatal pregnenolone-treatment induced a significant decrease in the adenosine A2A receptor density with no significant effects on the 5-HT1A and 5-HT7 receptor densities.

Adenosine A(2a) receptor antagonists: potential therapeutic and neuroprotective effects in Parkinson's disease

The most effective treatment of Parkinson's disease (PD) is, at present, the dopamine precursor L-3,4-dihydroxyphenylalanine (L-DOPA), however a number of disadvantages such as a loss of drug efficacy and severe side-effects (psychoses, dyskinesias and on-off phenomena) limit long-term effective utilisation of this drug. Recent experimental studies in which selective antagonists of adenosine A(2A) receptors were used, have shown an improvement in motor disabilities in animal models of PD. The A(2A) antagonist [7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-(4,3-e)-1,2,4-triazolo(1,5-c) pyrimidine] (SCH 58261) potentiated the contralateral turning behavior induced by a threshold dose of L-DOPA or direct dopamine receptor agonists in unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats, an effect accompanied by an increase in Fos-like-immunoreactivity in neurons of the lesioned striatum. Likewise, other A(2A) receptor antagonists such as (3,7-dimethyl-1-propargylxanthine) (DMPX), [E-8-(3,4-dimethoxystyryl)-1,3-dipropyl-7-methylxanthine] (KF 17837) and [E-1,3-diethyl-8(3,4-dimethoxystyryl-7-methyl-3,7-dihydro-1H-purine-2,6-dione] (KW 6002) antagonized catalepsy induced by haloperidol or reserpine in the rat, whereas in non-human primate models of PD, KW 6002 reduced the rigidity and improved the disability score of MPTP-treated marmosets and cynomolgus monkeys. Moreover, in contrast to L-DOPA, selective A(2A) receptor antagonists administered chronically did not produce dyskinesias and did not evoke tolerance in 6-OHDA and MPTP models of PD. An additional therapeutic potential of adenosine A(2A) antagonists emerged from studies showing neuroprotective properties of these compounds in animal models of cerebral ischemia and excitotoxicity, as well as in the MPTP model of PD. Adenosine A(2A) receptor antagonists by reversing motor impairments in animal models of PD and by contrasting cell degeneration are some of the most promising compounds for the treatment of PD.

Striatal and central extended amygdala parts of the interstitial nucleus of the posterior limb of the anterior commissure: evidence from tract-tracing techniques in the rat

The interstitial nucleus of the posterior limb of the anterior commissure (IPAC) lies at the junction of the striatopallidal system and the lateral bed nucleus of the stria terminalis-central amygdaloid nucleus continuum (i.e., the central extended amygdala; EAc). Its efferent connections were investigated in the rat with anterograde (Phaseolus vulgaris leucoagglutinin) and retrograde (Fluoro-Gold and cholera toxin B subunit) tracers and compared with those of the central amygdaloid nucleus. Our anterograde tracing experiments reveal that the projections of the medial IPAC largely reciprocate its afferent connections (Shammah-Lagnado et al. [1999] Neuroscience 94:1097-1123) and are very similar to those of the medial part of the central amygdaloid nucleus. The lateral IPAC, on the other hand, innervates the pallidal complex, substantia nigra and retrorubral field. Local connections are found within medial IPAC and within lateral IPAC, but the two divisions are not interconnected. Our retrograde tracing experiments confirm that IPAC projections to EAc components, parabrachial area, and nucleus of the solitary tract originate chiefly from the medial division, whereas both medial and lateral divisions innervate the retrorubral field. Moreover, in sections processed for choline acetyltransferase, the strong projections from caudal IPACm to the posterior basolateral amygdaloid nucleus and the amygdalopiriform transition area were found to arise chiefly from cholinergic cells. Overall, our results suggest that the medial IPAC is intimately related to the EAc, whereas the lateral IPAC represents a striatal territory.

Catalepsy induced by a blockade of dopamine D1 or D2 receptors was reversed by a concomitant blockade of adenosine A(2A) receptors in the caudate-putamen of rats

The present study sought to determine, in more detail, the effects of an unselective and a selective adenosine A(2A) receptor blockade on catalepsy induced by a blockade of dopamine D1 or D2 receptors in rats. The results demonstrated that systemic administration of the unselective A1/A2 receptor antagonist, theophylline and the selective A(2A) receptor antagonist, CSC potently reversed catalepsy induced by a systemic D2 receptor blockade with raclopride or by a bilateral blockade of D2 receptors in the caudate-putamen (CPu) with S(-)sulpiride. Likewise, systemic administration of theophylline and CSC reversed catalepsy induced by a systemic D1 receptor blockade with SCH23390; theophylline also counteracted catalepsy after an intra-CPu D1 receptor blockade with SCH23390. Intracerebral co-microinfusions of the selective A(2A) receptor antagonist, MSX-3 together with a D1 (SCH23390) or D2 receptor [S(-) sulpiride] antagonist revealed that catalepsy due to intra-CPu D1 or D2 receptor blockade can be potently reversed by an intra-CPu A2A receptor blockade. In conclusion, our results with systemic and intra-CPu drug administration demonstrate that D1 and D2 receptor-mediated catalepsy can both be reversed by a concomitant blockade of A(2A) receptors. Our results implicate that the CPu is a critical neural substrate for antagonistic interactions of a D1/D2 receptor blockade and an A(2A) receptor blockade in control of motor activity. The present results provide further support for the view that A(2A) receptor antagonists may be potential therapeutics for the treatment of Parkinson's disease.

Inflammatory cytokines regulate function and expression of adenosine A(2A) receptors in human monocytic THP-1 cells

Adenosine, acting at its receptors, particularly A(2A) receptors, is a potent endogenous anti-inflammatory agent that modulates the functions and differentiation of inflammatory and immune cells. Because the inflammatory milieu abounds in proinflammatory cytokines, we investigated the effects of Th1-inflammatory cytokines on function and expression of adenosine A(2A) receptors in the human monocytic cell line THP-1. We found that, consistent with previous reports, adenosine and 2-[p-(2-carnonylethyl)phenylethylamino]-5'-N-ethylcarboxamidoadenosine (CGS-21680), a selective A(2A) receptor agonist, suppress IL-12 production but increase IL-10 production in LPS-activated THP-1 cells. These effects were blocked by the A(2A) receptor antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4-triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM-241385). More importantly, the suppressive effect of adenosine and CGS-21680 on IL-12 production was significantly enhanced in cells pretreated with either IL-1 (10 U/ml) or TNF-alpha (100 U/ml) but markedly attenuated in cells pretreated with IFN-gamma (100 U/ml). Similarly, IL-1 and TNF-alpha treatment potentiated the stimulatory effect of adenosine and CGS-21680 on IL-10 production, whereas IFN-gamma treatment almost completely abolished this effect. CGS-21680 stimulated an increase in intracellular cAMP in a time- and dose-dependent manner in IL-1- and TNF-alpha-treated cells but not in control or IFN-gamma-treated cells. Both IL-1 and TNF-alpha increased A(2A) receptor mRNA and protein. In parallel with its effect on A(2A) receptor function, IFN-gamma down-regulated A(2A) receptor message and protein. Because adenosine mediates many of the antiinflammatory effects of drugs such as methotrexate, these observations suggest that local changes in the cytokine milieu may influence the therapeutic response to those drugs by altering the expression and function of adenosine receptors on inflammatory cells.

Molecular approach to adenosine receptors: receptor-mediated mechanisms of tissue protection

Adenosine accumulation during ischemia and inflammation protects tissues from injury. In ischemic tissues adenosine accumulates due to inhibition of adenosine kinase, and in inflamed tissues adenosine is formed from adenine nucleotides that are released from many cells including platelets, mast cells, nerves, and endothelium. Nucleotides are rapidly converted to adenosine by a family of ecto-nucleotidases including CD39 and CD73. Activation of A(1) and possibly A(3) adenosine receptors (ARs) protects heart and other tissues by preconditioning through a pathway including protein kinase C and mitochondrial K(ATP) channels. Activation of A(2A) receptors limits reperfusion injury by inhibiting inflammatory processes in neutrophils, platelets, macrophages and T cells. Adenosine produces proinflammatory responses mediated by receptors that vary among species; A(3) and A(2B) receptors mediate degranulation of rodent and human or canine mast cells, respectively. Novel adenosine receptor subtype-selective ligands have recently been developed. These include MRS1754 (A(2B) blocker), MRS1220 (A(3) blocker), MRE 3008F20 (human A(3) blocker), MRS1523 (rat A(3) blocker), and ATL146e (A(2A) agonist). These new pharmacologic tools will help investigators to sort out how adenosine protects tissues from injury and to identify new therapeutic agents that hold promise for the treatment of inflammatory and ischemic diseases.

Adenosine extracellular brain concentrations and role of A2A receptors in ischemia

Various experimental approaches have been used to determine the concentration of adenosine in extracellular brain fluid. The cortical cup technique or the microdialysis technique, when adenosine concentrations are evaluated 24 hours after implantation of the microdialysis probe, are able to measure adenosine in the nM range under normoxic conditions and in the microM range under ischemia. In vitro estimation of adenosine show that it can reach 30 microM at the receptor level during ischemia, a concentration able to stimulate all adenosine receptor subtypes so far identified. Although the protective role of A1 receptors in ischemia seems consistent, the protective role of A2A receptors appears to be controversial. Both A2A agonists and antagonists have been shown to be neuroprotective in various in vivo ischemia models. Although A2A agonists may be protective, mainly through peripherally mediated effects, A2A antagonists may be protective through local brain mediated effects. It is possible that A2A receptors are tonically activated following a prolonged increase of adenosine concentration, such as occurs during ischemia. A2A receptor activation desensitizes A1 receptors and reduces A1 mediated effects. Under these conditions A2A receptor antagonists may be protective by potentiating all the neuroprotective A1 mediated effects, including decreased neurotoxicity due to reduced ischemia induced glutamate outflow.

Activation of protein kinase A and atypical protein kinase C by A(2A) adenosine receptors antagonizes apoptosis due to serum deprivation in PC12 cells

We found in the present study that stimulation of A(2A) adenosine receptors (A(2A)-R) prevents apoptosis in PC12 cells. This A(2A)-protective effect was blocked by protein kinase A (PKA) inhibitors and was not observed in a PKA-deficient PC12 variant. Stimulation of PKA also prevented apoptosis, suggesting that PKA is required for the protective effect of A(2A)-R. A general PKC inhibitor, but not down-regulation of conventional and novel PKCs, readily blocked the protective effect of A(2A)-R stimulation and PKA activation, suggesting that atypical PKCs (aPKCs) serve a critical role downstream of PKA. Consistent with this hypothesis, stimulation of A(2A)-R or PKA enhanced nuclear aPKC activity. In addition, the A(2A)-protective effect was blocked by a specific inhibitor of one aPKC, PKCzeta, whereas overexpression of a dominant-positive PKCzeta enhanced survival. In contrast, inhibitors of MAP kinase and phosphatidylinositol 3-kinase did not modulate the A(2A)-protective effect. Dominant-negative Akt also did not alter the A(2A)-protective effect, whereas it significantly reduced the protective action of nerve growth factor. Collectively, these data suggest that aPKCs can function downstream of PKA to mediate the A(2A)-R-promoted survival of PC12 cells. Furthermore, the results indicate that different extracellular stimuli can employ distinct signaling pathways to protect against apoptosis induced by the same insult.

Adenosine A(2A) receptor enhances GABA(A)-mediated IPSCs in the rat globus pallidus

1. The actions of adenosine A(2A) receptor agonists were examined on GABAergic synaptic transmission in the globus pallidus (GP) in rat brain slices using whole-cell patch-clamp recording. GP neurones were characterized into two major groups, type I and type II, according to the degree of time-dependent hyperpolarization-activated inward rectification and the size of input resistance. 2. The A(2A) receptor agonist 2-[p-(2-carboxyethyl)phenethylamino]-5'-N-ethylcarboxamido- adenosine (CGS21680; 0.3-3 microM) enhanced IPSCs evoked by stimulation within the GP. The actions of CGS21680 were blocked by the A(2A) antagonists (E)-8-(3,4-dimethoxystyryl)-1,3-dipropyl-7-methylxanthine (KF17837) and 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM241385). 3. The CGS21680-induced increase in IPSCs was associated with a reduction in paired-pulse facilitation. CGS21680 (0.3 microM) increased the frequency of miniature IPSCs (mIPSCs) without affecting mIPSC amplitude. These observations demonstrated that the enhancement of IPSCs in the GP was attributable to presynaptic, but not postsynaptic, A(2A) receptors. 4. The results suggest that A(2A) receptors in the GP serve to inhibit GP neuronal activity, thereby disinhibiting subthalamic nucleus neurone activity. Thus, the A(2A) receptor-mediated presynaptic regulation in the GP, together with the A(2A) receptor-mediated intrastriatal presynaptic control of GABAergic neurotransmission described previously, may play a crucial role in controlling the neuronal functions of basal ganglia. This A(2A) receptor-mediated presynaptic dual control in the striatopallidal pathway could also afford the mode of action of A(2A) antagonists for ameliorating the symptoms of Parkinson's disease in an animal model.

Activation of Trk neurotrophin receptors in the absence of neurotrophins

Neurotrophins regulate neuronal cell survival and synaptic plasticity through activation of Trk receptor tyrosine kinases. Binding of neurotrophins to Trk receptors results in receptor autophosphorylation and downstream phosphorylation cascades. Here, we describe an approach to use small molecule agonists to transactivate Trk neurotrophin receptors. Activation of TrkA receptors in PC12 cells and TrkB in hippocampal neurons was observed after treatment with adenosine, a neuromodulator that acts through G protein-coupled receptors. These effects were reproduced by using the adenosine agonist CGS 21680 and were counteracted with the antagonist ZM 241385, indicating that this transactivation event by adenosine involves adenosine 2A receptors. The increase in Trk activity could be inhibited by the use of the Src family-specific inhibitor, PP1, or K252a, an inhibitor of Trk receptors. In contrast to other G protein-coupled receptor transactivation events, adenosine used Trk receptor signaling with a longer time course. Moreover, adenosine activated phosphatidylinositol 3-kinase/Akt through a Trk-dependent mechanism that resulted in increased cell survival after nerve growth factor or brain-derived neurotrophic factor withdrawal. Therefore, adenosine acting through the A(2A) receptors exerts a trophic effect through the engagement of Trk receptors. These results provide an explanation for neuroprotective actions of adenosine through a unique signaling mechanism and raise the possibility that small molecules may be used to elicit neurotrophic effects for the treatment of neurodegenerative diseases.

Ultrastructural localization of adenosine A2A receptors suggests multiple cellular sites for modulation of GABAergic neurons in rat striatum

Activation of adenosine A2A receptors (A2AR) has been shown to antagonize the function of D2 dopaminergic regulation of striatal gamma-aminobutyric acid (GABA)-ergic output and, thus, locomotor activity. Adenosine A2A receptor immunoreactivity (A2A-LI) has been localized to rat striatum by light microscopy by using a previously characterized human A2AR monoclonal antibody. In this study, we evaluated the localization of A2A-LI and its colocalization with GABA immunoreactivity (GABA-LI) in dorsolateral rat striatum by immunoelectron microscopy to further characterize the potential mechanism of purinergic control of striatal output. Ultrastructural analysis demonstrated A2A-LI associated with the plasma membrane and cytoplasmic membranous structures of striatal neurons. A2A-LI was prevalent in dendrites and dendritic spines ( approximately 70% of total A2A-profiles counted) and less prevalent in axons and axon terminals (23%), soma (3%), and glia (3%). Cellular elements exhibiting both A2A-LI and GABA-LI comprised 23% of the total profiles counted; colabeling was most common in dendrites. A2A-LI was observed primarily at asymmetric synapses (n = 70) (both pre- and postsynaptically but predominantly in the postsynaptic element) and less frequently at symmetric synapses (n = 17). Of the 714 A2A-immunoreactive profiles examined, 37% were apposed to GABA-labeled profiles. The most common appositions were A2A-labeled dendrites apposed to GABA-immunoreactive dendrites (n = 132), axon terminals (n = 28), and somata (n = 22) and A2A-labeled axons apposed to GABA-labeled dendrites (n = 58), axon terminals (n = 14), and somata (n = 9). Our findings suggest that adenosine may play an important role in modulating excitatory input to striatal neurons and that A2AR may modulate GABAergic signaling at several cellular sites within the rat striatum.

Cyclic AMP-dependent inhibition of human neutrophil oxidative activity by substituted 2-propynylcyclohexyl adenosine A(2A) receptor agonists

Novel 2-propynylcyclohexyl-5'-N:-ehtylcarboxamidoadenosines, trans-substituted in the 4-position of the cyclohexyl ring, were evaluated in binding assays to the four subtypes of adenosine receptors (ARs). Two esters, 4-(3-[6-amino-9-(5-ethylcarbamoyl-3,4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl]-prop-2-ynyl)-cyclohexanecarboxylic acid methyl ester (ATL146e) and acetic acid 4-(3-[6-amino-9-(5-ethylcarbamoyl-3, 4-dihydroxy-tetrahydro-furan-2-yl)-9H-purin-2-yl] -prop-2-ynyl)-cyclohexylmethyl ester (ATL193) were >50 x more potent than 2-[4-(2-carboxyethyl)phenethylamino]-5'-N:-ethylcarboxamidoadenosine (CGS21680) for human A(2A) AR binding. Human A(2A) AR affinity for substituted cyclohexyl-propynyladenosine analogues was inversely correlated with the polarity of the cyclohexyl side chain. There was a comparable order of potency for A(2A) AR agonist stimulation of human neutrophil [cyclic AMP](i), and inhibition of the neutrophil oxidative burst. ATL146e and CGS21680 were approximately equipotent agonists of human A(3) ARs. We measured the effects of selective AR antagonists on agonist stimulated neutrophil [cyclic AMP](i) and the effect of PKA inhibition on A(2A) AR agonist activity. ATL193-stimulated neutrophil [cyclic AMP](i) was blocked by antagonists with the potency order: ZM241385 (A(2A)-selective)>MRS1220 (A(3)-selective)>>N-(4-Cyano-phenyl)-2-[4-(2,6-dioxo-1,3-dipropyl-2,3,4,5,6,7-hexahydro-1H-purin-8-yl)-phenoxy]-acetamide (MRS1754; A(2B)-selective) approximately 8-(N-methylisopropyl)amino-N(6)-(5'-endohydroxy-endonorbornyl)-9-methyladenine (WRC0571; A(1)-selective). The type IV phosphodiesterase inhibitor, rolipram (100 nM) potentiated ATL193 inhibition of the oxidative burst, and inhibition by ATL193 was counteracted by the PKA inhibitor H-89. The data indicate that activation of A(2A)ARs inhibits neutrophil oxidative activity by activating [cyclic AMP](i)/PKA.

Adenosine and neurotrauma: therapeutic perspectives

Cerebral ischemia studies demonstrating that stimulation of adenosine A1 receptors by either endogenously released adenosine or the administration of selective receptor agonists causes significant reductions in the morbidity and mortality associated with focal or global brain ischemias have triggered interest in the potential of purinergic therapies for the treatment of traumatic injuries to the brain and spinal cord. Preliminary findings indicate that activation of A1 adenosine receptors can ameliorate trauma-induced death of central neurons. Other avenues of approach include the administration of agents which elevate local concentrations of adenosine at injury sites by inhibiting its metabolism to inosine by adenosine deaminase, rephosphorylation to adenosine triphosphate by adenosine kinase; or re-uptake into adjacent cells. Amplification of the levels of endogenously released adenosine in such a 'site and event specific' fashion has the advantage of largely restricting the effect of such inhibitors to areas of injury-induced adenosine release. Another approach involving purinergic therapy has been applied to the problem of respiratory paralysis following high spinal cord injuries. In this instance, the adenosine antagonist theophylline has been used to enhance residual synaptic drive to spinal respiratory neurons by blocking adenosine A1 receptors. Theophylline induced, and maintained, hemidiaphragmatic recovery for prolonged periods after C2 spinal cord hemisection in rats and may prove to be beneficial in assisting respiration in spinal cord injury patients.