Analysis of adenosine actions on Ca2+ currents and synaptic transmission in cultured rat hippocampal pyramidal neurones

The Role of the Adenosine System on Emotional and Cognitive Disturbances Induced by Ethanol Binge Drinking in the Immature Brain and the Beneficial Effects of Caffeine

Binge drinking intake is the most common pattern of ethanol consumption by adolescents, which elicits emotional disturbances, mainly anxiety and depressive symptoms, as well as cognitive alterations. Ethanol exposure may act on the adenosine neuromodulation system by increasing adenosine levels, consequently increasing the activation of adenosine receptors in the brain. The adenosine modulation system is involved in the control of mood and memory behavior. However, there is a gap in the knowledge about the exact mechanisms related to ethanol exposure’s hazardous effects on the immature brain (i.e., during adolescence) and the role of the adenosine system thereupon. The present review attempts to provide a comprehensive picture of the role of the adenosinergic system on emotional and cognitive disturbances induced by ethanol during adolescence, exploring the potential benefits of caffeine administration in view of its action as a non-selective antagonist of adenosine receptors.

Therapeutic potential of targeting G protein-gated inwardly rectifying potassium (GIRK) channels in the central nervous system

G protein-gated inwardly rectifying potassium channels (Kir3/GirK) are important for maintaining resting membrane potential, cell excitability and inhibitory neurotransmission. Coupled to numerous G protein-coupled receptors (GPCRs), they mediate the effects of many neurotransmitters, neuromodulators and hormones contributing to the general homeostasis and particular synaptic plasticity processes, learning, memory and pain signaling. A growing number of behavioral and genetic studies suggest a critical role for the appropriate functioning of the central nervous system, as well as their involvement in many neurologic and psychiatric conditions, such as neurodegenerative diseases, mood disorders, attention deficit hyperactivity disorder, schizophrenia, epilepsy, alcoholism and drug addiction. Hence, GirK channels emerge as a very promising tool to be targeted in the current scenario where these conditions already are or will become a global public health problem. This review examines recent findings on the physiology, function, dysfunction, and pharmacology of GirK channels in the central nervous system and highlights the relevance of GirK channels as a worthful potential target to improve therapies for related diseases.

Low-frequency Stimulation Decreases Hyperexcitability Through Adenosine A1 Receptors in the Hippocampus of Kindled Rats

INTRODUCTION

In this study, the role of A1 adenosine receptors in improving the effect of Low-Frequency Electrical Stimulation (LFS) on seizure-induced hyperexcitability of hippocampal CA1 pyramidal neurons was investigated.

METHODS

A semi-rapid hippocampal kindling model was used to induce seizures in male Wistar rats. Examination of the electrophysiological properties of CA1 pyramidal neurons of the hippocampus using whole-cell patch-clamp recording 48 h after the last kindling stimulation revealed that the application of LFS as two packages of stimulations at a time interval of 6 h for two consecutive days could significantly restore the excitability CA1 pyramidal neurons evidenced by a decreased in the of the number of evoked action potentials and enhancement of amplitude, maximum rise slope and decay slope of the first evoked action potential, rheobase, utilization time, adaptation index, first-spike latency, and post-AHP amplitude. Selective locked of A1 receptors by the administration of 8-Cyclopentyl-1,3-dimethylxanthine (1 μM, 1 μl, i.c.v.) before applying each LFS package, significantly reduced LFS effectiveness in recovering these parameters.

RESULTS

On the other hand, selective activation of A1 receptors by an injection of N6-cyclohexyladenosine (10 μM, 1 μl, i.c.v.), instead of LFS application, could imitate LFS function in improving these parameters.

CONCLUSION

It is suggested that LFS exerts its efficacy on reducing the neuronal excitability, partially by activating the adenosine system and activating its A1 receptors.

Synaptic energy use and supply

Neuronal computation is energetically expensive. Consequently, the brain's limited energy supply imposes constraints on its information processing capability. Most brain energy is used on synaptic transmission, making it important to understand how energy is provided to and used by synapses. We describe how information transmission through presynaptic terminals and postsynaptic spines is related to their energy consumption, assess which mechanisms normally ensure an adequate supply of ATP to these structures, consider the influence of synaptic plasticity and changing brain state on synaptic energy use, and explain how disruption of the energy supply to synapses leads to neuropathology.

Genetically controlled upregulation of adenosine A(1) receptor expression enhances the survival of primary cortical neurons

Adenosine has a key endogenous neuroprotective role in the brain, predominantly mediated by the adenosine A(1) receptor (A(1)R). This has been mainly explored using pharmacological tools and/or receptor knockout mice strains. It has long been suggested that the neuroprotective effects of A(1)R are increased following receptor upregulation, thus attenuating neuronal damage in pathological conditions. We have previously shown that the neuroprotective and neuromodulatory actions of the cytokines IL-6 and oncostatin M are mediated by induction of neuronal A(1)R expression. In order to investigate the direct effects of A(1)R upregulation in neurons, we have generated a tetracycline-regulated expression system with a bidirectional promoter, directing the simultaneous expression of the mouse A(1)R and GFP/mCherry reporter genes. In a first step, we tested the efficacy of the system in transiently transfected human embryonic kidney 293 cells. In addition, we confirmed the functional integrity of the expressed A(1)R by whole-cell patch clamp recordings. We demonstrated that A(1)R-transfected primary neurons show enhanced survival against N-methyl-D-aspartate-induced excitotoxicity. Pretreatment with an A(1)R-selective agonist additionally strongly decreased neuronal cell death, while an A(1)R antagonist completely abolished the neuroprotective effects of A(1)R upregulation. The presented data provide for the first time direct evidence that the upregulation of A(1)R enhances neuronal survival.

Cellular Links between Neuronal Activity and Energy Homeostasis

Neuronal activity, astrocytic responses to this activity, and energy homeostasis are linked together during baseline, conscious conditions, and short-term rapid activation (as occurs with sensory or motor function). Nervous system energy homeostasis also varies during long-term physiological conditions (i.e., development and aging) and with adaptation to pathological conditions, such as ischemia or low glucose. Neuronal activation requires increased metabolism (i.e., ATP generation) which leads initially to substrate depletion, induction of a variety of signals for enhanced astrocytic function, and increased local blood flow and substrate delivery. Energy generation (particularly in mitochondria) and use during ATP hydrolysis also lead to considerable heat generation. The local increases in blood flow noted following neuronal activation can both enhance local substrate delivery but also provides a heat sink to help cool the brain and removal of waste by-products. In this review we highlight the interactions between short-term neuronal activity and energy metabolism with an emphasis on signals and factors regulating astrocyte function and substrate supply.

Enhanced actions of adenosine in medial entorhinal cortex layer II stellate neurons in temporal lobe epilepsy are mediated via A(1)-receptor activation

PURPOSE

The adenosinergic system is known to exert an inhibitory affect in the brain, and as such adenosine has been considered an endogenous anticonvulsant. Entorhinal cortex (EC) layer II neurons, which serve as the primary input to the hippocampus, are spared in temporal lobe epilepsy (TLE) and become hyperexcitable. Because these neurons also express adenosine receptors, the activity of these neurons may be controlled by adenosine, specifically during seizure activity when adenosine levels are thought to rise. In light of this, we determined if the actions of adenosine on medial EC (mEC) layer II stellate neurons are augmented in TLE and by which receptor subtype.

METHODS

Horizontal brain slices were prepared from rats exhibiting spontaneous seizures (TLE) induced by electrical stimulation and compared with age-matched control rats. mEC layer II stellate neurons were visually identified, and action potentials (APs) were evoked either by a series of depolarizing current injection steps or via presynaptic stimulation of mEC deep layers. The effects of adenosine were compared with actions of adenosine A(1) and A(2A) receptor-specific agonists (CPA and CGS-21680) and antagonists (DPCPX and ZM-241385), respectively. Immunohistochemical and qPCR techniques were also employed to assess relative adenosine A(1)-receptor message and expression.

KEY FINDINGS

mEC layer II stellate neurons were hyperexcitable in TLE, evoking a higher frequency of APs when depolarized and generating bursts of APs when synaptically stimulated. Adenosine reduced AP frequency and synaptically evoked APs in a dose-dependent manner (500 nM-100 μM); however, in TLE, the inhibitory actions of adenosine occurred at concentrations that were without affect in control neurons. In both cases, the inhibitory actions of adenosine were mediated via activation of the A(1)- and not the A(2A)-receptor subtype. Quantitative polymerase chain reaction (qPCR) and immunohistochemical experiments revealed an upregulation of the adenosine A(1) mRNA and an increase in A(1)-receptor staining in TLE neurons compared to control.

SIGNIFICANCE

Our data indicate that the actions of adenosine on mEC layer II stellate neurons is accentuated in TLE due to an upregulation of adenosine A(1)-receptors. Because adenosine levels are thought to rise during seizure activity, activation of adenosine A(1)-receptors could provide a possible endogenous mechanism to suppress seizure activity and spread within the temporal lobe.

Neuroadaptations in adenosine receptor signaling following long-term ethanol exposure and withdrawal

Ethanol affects the function of neurotransmitter systems, resulting in neuroadaptations that alter neural excitability. Adenosine is one such receptor system that is changed by ethanol exposure. The current review is focused on the A(1) and the A(2A) receptor subtypes in the context of ethanol-related neuroadaptations and ethanol withdrawal because these subtypes (i) are activated by basal levels of adenosine, (ii) have been most well-studied for their role in neuroprotection and ethanol-related phenomena, and (iii) are the primary site of action for caffeine in the brain, a substance commonly ingested with ethanol. It is clear that alterations in adenosinergic signaling mediate many of the effects of acute ethanol administration, particularly with regard to motor function and sedation. Further, prolonged ethanol exposure has been shown to produce adaptations in the cell surface expression or function of both A(1) and the A(2A) receptor subtypes, effects that likely promote neuronal excitability during ethanol withdrawal. As a whole, these findings demonstrate a significant role for ethanol-induced adaptations in adenosine receptor signaling that likely influence neuronal function, viability, and relapse to ethanol intake following abstinence.

Dynamic modulation of phasic and asynchronous glutamate release in hippocampal synapses

Although frequency-dependent short-term presynaptic plasticity has been of long-standing interest, most studies have emphasized modulation of the synchronous, phasic component of transmitter release, most evident with a single or a few presynaptic stimuli. Asynchronous transmitter release, vesicle fusion not closely time locked to presynaptic action potentials, can also be prominent under certain conditions, including repetitive stimulation. Asynchrony has often been attributed to residual Ca(2+) buildup in the presynaptic terminal. We verified that a number of manipulations of Ca(2+) handling and influx selectively alter asynchronous release relative to phasic transmitter release during action potential trains in cultured excitatory autaptic hippocampal neurons. To determine whether other manipulations of vesicle release probability also selectively modulate asynchrony, we probed the actions of one thoroughly studied modulator class whose actions on phasic versus asynchronous release have not been investigated. We examined the effects of the phorbol ester PDBu, which has protein kinase C (PKC) dependent and independent actions on presynaptic transmitter release. PDBu increased phasic and asynchronous release in parallel. However, while PKC inhibition had relatively minor inhibitory effects on PDBu potentiation of phasic and total release during action potential trains, PKC inhibition strongly reduced phorbol-potentiated asynchrony, through actions most evident late during stimulus trains. These results lend new insight into PKC-dependent and -independent effects on transmitter release and suggest the possibility of differential control of synchronous versus asynchronous vesicle release.

Adenosine inhibits activity of hypocretin/orexin neurons by the A1 receptor in the lateral hypothalamus: a possible sleep-promoting effect

Neurons in the lateral hypothalamus (LH) that contain hypocretin/orexin have been established as important promoters of arousal. Deficiencies in the hypocretin/orexin system lead to narcolepsy. The inhibition of hypocretin/orexin neurons by sleep-promoting neurotransmitters has been suggested as one part of the sleep regulation machinery. Adenosine has been identified as a sleep promoter and its role in sleep regulation in the basal forebrain has been well documented. However, the effect of adenosine on arousal-promoting hypocretin/orexin neurons has not been addressed, despite recent evidence that immunocytochemical visualization of adenosine receptors was detected in these neurons. In this study, we examined the hypothesis that adenosine inhibits the activity of hypocretin/orexin neurons by using electrophysiological methods in brain slices from mice expressing green fluorescent protein in hypocretin/orexin neurons. We found that adenosine significantly attenuated the frequency of action potentials without a change in membrane potential in hypocretin/orexin neurons. The adenosine-mediated inhibition arises from depression of excitatory synaptic transmission to hypocretin/orexin neurons because adenosine depresses the amplitude of evoked excitatory postsynaptic potential and the frequency of spontaneous and miniature excitatory postsynaptic currents in these neurons. At the cell body of the hypocretin/orexin neurons, adenosine inhibits voltage-dependent calcium currents without the induction of GIRK current. The inhibitory effect of adenosine is dose dependent, pertussis toxin sensitive, and mediated by A1 receptors. In summary, our data suggest that in addition to its effect in the basal forebrain, adenosine exerts its sleep-promoting effect in the LH by inhibition of hypocretin/orexin neurons.

Different roles of adenosine A1, A2A and A3 receptors in controlling kainate-induced toxicity in cortical cultured neurons

Adenosine is a neuromodulator that can control brain damage through activation of A(1), A(2A) and A(3) receptors, which are located in both neurons and other brain cells. We took advantage of cultured neurons to investigate the role of neuronal adenosine receptors in the control of neurotoxicity caused by kainate and cyclothiazide. Both A(1), A(2A) and A(3) receptors were immunocytochemically identified in cortical neurons. Activation of A(1) receptors with 100 nM CPA did not modify the extent of neuronal death whereas the A(1) receptor antagonist, DPCPX (50 nM), attenuated neurotoxicity by 28 +/- 5%, and effect similar to that resulting from the removal of endogenous adenosine with 2U/ml of adenosine deaminase (27 +/- 3% attenuation of neurotoxicity). In the presence of adenosine deaminase, DPCPX had no further effect and CPA now exacerbated neurotoxicity by 42 +/- 4%. Activation of A(2A) receptor with 30 nM CGS21680 attenuated neurotoxicity by 40 +/- 8%, an effect prevented by the A(2A) receptor antagonists, SCH58261 (50 nM) or ZM241385 (50 nM), which by themselves were devoid of effect. Finally, neither A(3) receptor activation with Cl-IB-MECA (100-500 nM) nor blockade with MRS1191 (5 microM) modified neurotoxicity. These results show that A(1) receptor activation enhances and A(2A) receptor activation attenuates neurotoxicity in cultured cortical neurons, indicating that these two neuronal adenosine receptors directly control neurodegeneration. Interestingly, the control by adenosine of neurotoxicity in cultured neurons is similar to that observed in vivo in newborn animals and is the opposite of what is observed in adult brain preparations where A(1) receptor activation and A(2A) receptor blockade are neuroprotective.

Neuroprotection by adenosine in the brain: From A(1) receptor activation to A (2A) receptor blockade

Adenosine is a neuromodulator that operates via the most abundant inhibitory adenosine A(1) receptors (A(1)Rs) and the less abundant, but widespread, facilitatory A(2A)Rs. It is commonly assumed that A(1)Rs play a key role in neuroprotection since they decrease glutamate release and hyperpolarize neurons. In fact, A(1)R activation at the onset of neuronal injury attenuates brain damage, whereas its blockade exacerbates damage in adult animals. However, there is a down-regulation of central A(1)Rs in chronic noxious situations. In contrast, A(2A)Rs are up-regulated in noxious brain conditions and their blockade confers robust brain neuroprotection in adult animals. The brain neuroprotective effect of A(2A)R antagonists is maintained in chronic noxious brain conditions without observable peripheral effects, thus justifying the interest of A(2A)R antagonists as novel protective agents in neurodegenerative diseases such as Parkinson's and Alzheimer's disease, ischemic brain damage and epilepsy. The greater interest of A(2A)R blockade compared to A(1)R activation does not mean that A(1)R activation is irrelevant for a neuroprotective strategy. In fact, it is proposed that coupling A(2A)R antagonists with strategies aimed at bursting the levels of extracellular adenosine (by inhibiting adenosine kinase) to activate A(1)Rs might constitute the more robust brain neuroprotective strategy based on the adenosine neuromodulatory system. This strategy should be useful in adult animals and especially in the elderly (where brain pathologies are prevalent) but is not valid for fetus or newborns where the impact of adenosine receptors on brain damage is different.

The effects of tiazofurin on basal and amphetamine-induced motor activity in rats

The effects of tiazofurin (TR; 2-beta-d-ribofuranosylthiazole-4-carboxamide), a purine nucleoside analogue on basal and amphetamine (AMPH)-induced locomotor and stereotypic activity of adult Wistar rat males were studied. The animals were injected with low (3.75, 7.5, and 15 mg/kg ip) and high (62.5, 125, and 250 mg/kg ip) TR doses. Neither low nor high TR doses influenced basal locomotor and stereotypic activity in comparison with the corresponding controls treated with saline only. However, pretreatment with TR at any dose applied, except for the lowest one, significantly decreased AMPH-induced (1.5 mg/kg ip) locomotor activity, while AMPH-induced stereotypic activity was inhibited with the two highest TR doses. In addition, TR was detected in the brain by HPLC already 15 min after the injection (125 mg/kg ip) to reach a maximum 2 h after the administration and was detectable in this tissue during the next 4 h. Our results indicate that TR modifies central regulation of the motor activity, possibly by influencing dopaminergic (DA-ergic) transmission.

Signalling from adenosine receptors to mitogen-activated protein kinases

The purine nucleoside adenosine acts via four distinct adenosine receptor subtypes: the adenosine A(1), A(2A), A(2B), and A(3) receptor. They are all G protein-coupled receptors (GPCR) coupling to classical second messenger pathways such as modulation of cAMP production or the phospholipase C (PLC) pathway. In addition, they couple to mitogen-activated protein kinases (MAPK), which could give them a role in cell growth, survival, death and differentiation. Although each of the adenosine receptors can activate one or more of the MAPKs, the mechanisms appear to differ substantially, both between receptor subtypes in the same cell type and between the same receptor in different cell types.

The adenosine inhibition of glutamate exocytosis in synaptosomes is removed by the collapse of the vesicle-cytosol deltapH plus the opening of farnesol-sensitive Ca(2+) channels

Adenosine inhibits synaptosomal exocytosis of glutamate, triggered by KCl or by the K(+) channel inhibitor, 4-aminopyridine (4-AP), without affecting Ca(2+) influx. Its effect is removed by the activation of protein kinase C (PKC). We show that in the presence of the protein kinase inhibitor, staurosporine, the adenosine inhibition is removed also by collapsing deltapH between secretory vesicle and the cytosol with methylamine (MA), provided that exocytosis is triggered by KCl (which activates an initial transient spike of Ca(2+) influx) but not by 4-AP. If KCl is supplied prior to Ca(2+), the spike of Ca(2+) influx is absent and the adenosine inhibition is maintained. MA can remove the adenosine inhibition also with 4-AP, provided that tetraethylammonium (TEA), an inhibitor of a different class of K(+) channels, is supplied together with 4-AP. TEA promotes a further increase of cytosolic free Ca(2+) concentration ([Ca(2+)](i)), which adds to the 4-AP-induced Ca(2+) influx. Farnesol (5-10 microM), a physiological derivative of farnesyl pyrophosphate of the sterol biosynthetic pathway, specifically inhibits the Ca(2+) spike after KCl as well as the TEA-promoted Ca(2+) increase. At the same time, it prevents the removal of the adenosine inhibition by MA. We conclude that the adenosine inhibition is removed by the coincidence of two signals, the alkalinization of secretory vesicles and the opening of a particular class of Ca(2+) channels associated to the TEA-sensitive K(+) channels, equivalent to the Ca(2+) spike after KCl, and sensitive to farnesol.

Theophylline-induced respiratory recovery following cervical spinal cord hemisection is augmented by serotonin 2 receptor stimulation

Cervical spinal cord hemisection leads to a disruption of bulbospinal innervation of phrenic motoneurons resulting in paralysis of the ipsilateral hemidiaphragm. We have previously demonstrated separate therapeutic roles for theophylline, and more recently serotonin (5-HT) as modulators to phrenic nerve motor recovery; mechanisms that likely occur via adenosine A1 and 5-HT2 receptors, respectively. The present study was designed to specifically determine if concurrent stimulation of 5-HT2 receptors may enhance motor recovery induced by theophylline alone. Adult female rats (250-350 g; n=7 per group) received a left cervical (C2) hemisection that resulted in paralysis of the ipsilateral hemidiaphragm. Twenty-four hours later rats were given systemic theophylline (15 mg/kg, i.v.), resulting in burst recovery in the ipsilateral phrenic nerve. Theophylline-induced recovery was enhanced with the 5-HT2A/2C receptor agonist, (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI; 1.0 mg/kg). DOI-evoked augmentation of theophylline-induced recovery was attenuated following subsequent injection of the 5-HT2 receptor antagonist, ketanserin (2.0 mg/kg). In a separate group, rats were pretreated with ketanserin, which did not prevent subsequent theophylline-induced respiratory recovery. However, pretreatment with ketanserin did prevent DOI-induced augmentation of the theophylline-evoked phrenic nerve burst recovery. Lastly, using immunocytochemistry and in situ hybridization, we showed for the first time a positive co-localization of adenosine A1 receptor mRNA and immunoreactivity with phrenic motoneurons of the cervical ventral horns. Taken together, the results of the present study suggest that theophylline may induce motor recovery likely at adenosine A1 receptors located at the level of the spinal cord, and the concurrent stimulation of converging 5-HT2 receptors may augment the response.

Ca2+-independent feedback inhibition of acetylcholine release in frog neuromuscular junction

The effect of membrane potential on feedback inhibition of acetylcholine (ACh) release was studied using the frog neuromuscular junction. It was found that membrane potential affects the functional affinity (K(i)) of the presynaptic M2 muscarinic receptor. The K(i) for muscarine shifts from approximately 0.23 microm (at resting potential) to approximately 8 microm (at a high depolarization). Measurements of Ca2+ currents in axon terminals showed that the depolarization-mediated shift in K(i) does not stem from depolarization-dependent changes in Ca2+ influx. Pretreatments with pertussis toxin (PTX) abolished the depolarization-dependent shift in K(i); at all depolarizations K(i) was the same and higher (approximately 32 microm) than before PTX treatment. The inhibitory effect of muscarine on ACh release is produced by two independent mechanisms: a slow, PTX-sensitive process, which prevails at low to medium depolarizations and operates already at low muscarine concentrations, and a fast, PTX-insensitive and voltage-independent process, which requires higher muscarine concentrations. Neither of the two processes involves a reduction in Ca2+ influx.

G-protein alpha subunit isoforms couple differentially to receptors that mediate presynaptic inhibition at rat hippocampal synapses

Presynaptic receptors that are coupled to heterotrimeric G-proteins are found throughout the brain and are responsible for modulating synaptic transmission. At least 10 G-protein-coupled receptors (GPCRs) reduce transmission in hippocampal neurons. Additionally, hippocampal neurons express up to 17 different Galpha, Gbeta, and Ggamma subunits, making for a striking array of possible heterotrimer compositions and GPCR-heterotrimer interactions. The identity of the Galpha subunit is likely a critical determinant in coupling specificity between GPCRs and their molecular effectors mediating presynaptic inhibition. We studied the role of four Galpha(i/o) subunits (Galpha(o1), Galpha(i1,) Galpha(i2), and Galpha(i3)) in mediating presynaptic inhibition in hippocampal neurons by expressing pertussis toxin-insensitive (PTx-ins) Galpha(i/o) mutants. PTx treatment of these cells disrupts coupling of endogenous subunits, leaving only the mutant Galpha subunits to couple with native GPCRs and betagamma subunits. Successful rescue of presynaptic inhibition indicates that the expressed mutant Galpha subunit can couple to the GPCR of interest. All four PTx-ins Galpha subunits rescued presynaptic inhibition by adenosine A1 receptors. A PTx-ins Galpha subunit also rescued adenosine A1-mediated inhibition of spontaneous vesicle fusion frequency. Of the remaining GPCRs tested, cannabinoid CB1, somatostatin, and GABA(B) receptors displayed an alpha subunit-dependent selectivity in binding to G-protein heterotrimers, whereas group III metabotropic glutamate receptor-mediated inhibition was not rescued by expression of any of the four PTx-ins Galpha subunits. Differential coupling of G-protein alpha subunits may be a means of achieving specificity between different GPCRs and their molecular targets for mediating presynaptic inhibition.

Adenosine depresses a Ca(2+)-independent step in transmitter exocytosis at frog motor nerve terminals

The depressant action of adenosine on acetylcholine release at frog motor nerve terminals was studied by intracellular recording and Ca(2+)-imaging techniques. Adenosine (200 microm) quickly and reversibly decreased the amplitude and quantal content of end-plate potentials (EPPs) with no change in quantal size in a low-Ca(2+), high-Mg(2+) solution, and EPP amplitude in normal Ringer containing d-tubocurarine. Likewise, adenosine (200 microm) reduced miniature EPP (MEPP) frequency, but not amplitude, in a high-K(+) (6 mm) solution. Adenosine (40-200 microm), however, did not affect single or repetitive impulse(s)-induced rises in Ca(2+) in the nerve terminals or its basal level. Adenosine (100-200 microm) reduced the Ca(2+)-independent enhancement of MEPP frequency caused by hypertonicity. EPPs induced by tetanic stimulation (33 Hz) in Ringer with d-tubocurarine initially increased in amplitude within 10 stimuli and then declined to the minimum. Adenosine (200 microm) decreased EPP amplitude in the initial phase of the tetanus, but enhanced it in the middle phase, thus prolonging the decay of EPP amplitude. The total sum of these EPPs, reflecting the readily releasable pool of vesicles and its refilling, however, was not changed. The results suggest that adenosine inhibits a Ca(2+)-independent step of transmitter exocytosis at frog motor nerve terminals.

Postsynaptic induction and presynaptic expression of group 1 mGluR-dependent LTD in the hippocampal CA1 region

Activation of metabotropic glutamate receptors (mGluRs) with the group I mGluR selective agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) induces a long-term depression (LTD) of excitatory synaptic transmission in the CA1 region of the hippocampus. Here we investigated the potential roles of pre- and postsynaptic processes in the DHPG-induced LTD at excitatory synapses onto hippocampal pyramidal cells in the mouse hippocampus. Activation of mGluRs with DHPG, but not ACPD, induced LTD at both Schaffer collateral/commissural fiber synapses onto CA1 pyramidal cells and at associational/commissural fiber synapses onto CA3 pyramidal cells. DHPG-induced LTD was blocked when the G-protein inhibitor guanosine-5'-O-(2-thiodiphosphate) was selectively delivered into postsynaptic CA1 pyramidal cells via an intracellular recording electrode, suggesting that DHPG depresses synaptic transmission through a postsynaptic, GTP-dependent signaling pathway. The effects of DHPG were also strongly modulated, however, by experimental manipulations that altered presynaptic calcium influx. In these experiments, we found that elevating extracellular Ca(2+) concentrations ([Ca(2+)](o)) to 6 mM almost completely blocked the effects of DHPG, whereas lowering [Ca(2+)](o) to 1 mM significantly enhanced the ability of DHPG to depress synaptic transmission. Enhancing Ca(2+) influx by prolonging action potential duration with bath applications of the K(+) channel blocker 4-aminopyridine (4-AP) also strongly reduced the effects of DHPG in the presence of normal [Ca(2+)](o) (2 mM). Although these findings indicate that alterations in Ca(2+)-dependent signaling processes strongly regulate the effects of DHPG on synaptic transmission, they do not distinguish between potential pre- versus postsynaptic sites of action. We found, however, that while inhibiting both pre- and postsynaptic K(+) channels with bath-applied 4-AP blocked the effects of DHPG; inhibition of postsynaptic K(+) channels alone with intracellular Cs(+) and TEA had no effect on the ability of DHPG to inhibit synaptic transmission. This suggests that presynaptic changes in transmitter release contribute to the depression of synaptic transmission by DHPG. Consistent with this, DHPG induced a persistent depression of both AMPA and N-methyl-D-aspartate receptor-mediated components of excitatory postsynaptic currents in voltage-clamped pyramidal cells. Together our results suggest that activation of postsynaptic mGluRs suppresses transmission at excitatory synapses onto CA1 pyramidal cells through presynaptic effects on transmitter release.

Differential desensitization of responses mediated by presynaptic and postsynaptic A1 adenosine receptors

G-protein-coupled receptors (GPCRs) often desensitize during continuous activation, but it is not known whether desensitization is influenced by subcellular location. In hippocampal neurons, activation of adenosine A1 receptors (A1Rs) or GABA(B) receptors on synaptic terminals inhibits neurotransmitter release, whereas activation of the same receptors on cell bodies and dendrites decreases excitability by activating inwardly rectifying potassium (GIRK) channels. Here we report that responses mediated by presynaptic A1Rs desensitize more slowly than responses mediated by postsynaptic (somatodendritic) A1Rs in cultured neurons. Agonist treatment for 2 hr has no effect on adenosine-induced presynaptic inhibition, whereas such treatment nearly abolishes adenosine-induced activation of postsynaptic GIRK channels. Agonist treatment for longer periods (>12 hr) eventually desensitizes A1R-mediated presynaptic inhibition. Presynaptic and postsynaptic responses both recover from desensitization after agonist removal, but recovery of presynaptic inhibition requires more time. Desensitization of postsynaptic responses apparently occurs at the level of the receptor, because postsynaptic G-proteins and GIRK channels appear to be fully functional. Inhibition of voltage-gated calcium channels by postsynaptic A1Rs also desensitizes rapidly, although this desensitization is less complete than is observed for activation of postsynaptic GIRK channels. Comparison of concentration-response curves for presynaptic and postsynaptic responses suggests that a receptor reserve exists for presynaptic inhibition, but that the magnitude of this reserve is insufficient to account for the absence of presynaptic desensitization after brief agonist exposure. These results suggest that agonist-induced desensitization of responses mediated by neuronal GPCRs may depend on the subcellular location of the receptors.

Differential desensitization of responses mediated by presynaptic and postsynaptic A1 adenosine receptors

G-protein-coupled receptors (GPCRs) often desensitize during continuous activation, but it is not known whether desensitization is influenced by subcellular location. In hippocampal neurons, activation of adenosine A1 receptors (A1Rs) or GABA(B) receptors on synaptic terminals inhibits neurotransmitter release, whereas activation of the same receptors on cell bodies and dendrites decreases excitability by activating inwardly rectifying potassium (GIRK) channels. Here we report that responses mediated by presynaptic A1Rs desensitize more slowly than responses mediated by postsynaptic (somatodendritic) A1Rs in cultured neurons. Agonist treatment for 2 hr has no effect on adenosine-induced presynaptic inhibition, whereas such treatment nearly abolishes adenosine-induced activation of postsynaptic GIRK channels. Agonist treatment for longer periods (>12 hr) eventually desensitizes A1R-mediated presynaptic inhibition. Presynaptic and postsynaptic responses both recover from desensitization after agonist removal, but recovery of presynaptic inhibition requires more time. Desensitization of postsynaptic responses apparently occurs at the level of the receptor, because postsynaptic G-proteins and GIRK channels appear to be fully functional. Inhibition of voltage-gated calcium channels by postsynaptic A1Rs also desensitizes rapidly, although this desensitization is less complete than is observed for activation of postsynaptic GIRK channels. Comparison of concentration-response curves for presynaptic and postsynaptic responses suggests that a receptor reserve exists for presynaptic inhibition, but that the magnitude of this reserve is insufficient to account for the absence of presynaptic desensitization after brief agonist exposure. These results suggest that agonist-induced desensitization of responses mediated by neuronal GPCRs may depend on the subcellular location of the receptors.

Differential effects of PACAP-38 on synaptic responses in rat hippocampal CA1 region

Pituitary adenylate cyclase-activating polypeptide (PACAP-38) is a member of the vasointestinal polypeptide (VIP)/secretin/glucagon family of neuropeptides for which neuroregulatory functions have been postulated. PACAP-38 receptors are expressed in different brain regions, including hippocampus. In this study, we examined the dose-dependent effects of PACAP-38 on the excitatory postsynaptic field potential (fEPSP) evoked at the Schaffer collateral-CA1 synapse in rat hippocampal slices. Bath application of low dose (0.05 nM) of PACAP-38 induced long-lasting facilitation of the fEPSP. This enhancement was blocked by the cholinergic receptor antagonist atropine and partially by the NMDA receptor antagonist 2-amino-5-phosphonovalerate (APV) and therefore, shares a common mechanism with LTP. In contrast, a high dose (1 microM) of PACAP-38 induced a persistent depression of the fEPSP that was not blocked by antagonists of cholinergic receptors (i.e., atropine and mecamylamine), adenosine receptors (i.e., DCPCX), or glutamatergic NMDA receptors (APV). Intermediate doses (0.1-0.5 microM) of PACAP-38 produced an initial decrease of the fEPSP followed by an enhancement. This decrease was not blocked by atropine whereas the facilitation was. These results show that PACAP-38 modulates CA1 synaptic transmission in a dose-dependent manner and that the peptide interacts with cholinergic and glutamatergic systems.

Adenosine inhibits glutamate exocytosis largely without interfering with Ca2+ influx in rat cerebrocortical synaptosomes

Adenosine is an inhibitor of glutamate release in synaptosomes. The inhibition is removed by the A(1) adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). We monitored the variations of cytoplasmic free Ca(2+) concentrations ([Ca(2+)](i)) in KCl or 4-aminopyridine-stimulated synaptosomes, in the presence of adenosine or adenosine plus DPCPX. The increment of [Ca(2+)](i) upon stimulation was unmodified by adenosine (up to 400-500 microM) while it was strongly decreased when exocytosis was decreased to a similar extent by lowering KCl or 4-aminopyridine. Adenosine also inhibited glutamate release induced by the Ca(2+) ionophore ionomycin. Increasing adenosine to 1.5 mM resulted in a decrease of the stimulus-induced increase of [Ca(2+)](i) and in the further potentiation of the adenosine inhibition of exocytosis from 41+/-3 to 51+/-4%. We conclude that adenosine affects glutamate exocytosis mostly in a Ca(2+) independent mode.

Adenosine-mediated presynaptic modulation of glutamatergic transmission in the laterodorsal tegmentum

The laterodorsal tegmentum (LDT) neurons supply most of the cholinergic tone to the brainstem and diencephalon necessary for physiological arousal. It is known that application of adenosine in the LDT nucleus increases sleep in vivo (Portas et al., 1997) and directly inhibits LDT neurons in vitro by activating postsynaptic adenosine A(1) receptors (Rainnie et al., 1994). However, adenosine effects on synaptic inputs to LDT neurons has not been previously reported. We found that both evoked glutamatergic EPSCs and GABAergic IPSCs were reduced by adenosine (50 micrometer). A presynaptic site of action for adenosine A(1) receptors on glutamatergic afferents was suggested by the following: (1) adenosine did not affect exogenous glutamate-mediated current, (2) adenosine reduced glutamatergic miniature EPSC (mEPSC) frequency, without affecting the amplitude, and (3) inhibition of the evoked EPSC was mimicked by the A(1) agonist N6-cyclohexyladenosine (100 nm) but not by the A(2) agonist N6-[2-(3,5-dimethoxyphenyl)-2-(methylphenyl)-ethyl]-adenosine (10 nm). The A(1) receptor antagonist 8-cyclopentyltheophylline (CPT; 200 nm) potentiated the evoked EPSCs, suggesting the presence of a tonic activation of presynaptic A(1) receptors by endogenous adenosine. The adenosine kinase inhibitor, 5-iodotubercidin (10 micrometer), mimicked adenosine presynaptic and postsynaptic effects. These effects were antagonized by CPT or adenosine deaminase (0.8 IU/ml), suggesting mediation by increased extracellular endogenous adenosine. Together, these data suggest that the activity of LDT neurons is under inhibitory tone by endogenous adenosine through the activation of both presynaptic A(1) receptors on excitatory terminals and postsynaptic A(1) receptors. Furthermore, an alteration of adenosine kinase activity modifies the degree of this inhibitory tone.

Long-term activation of adenosine A(2a) receptors blocks glutamate excitotoxicity in cultures of avian retinal neurons

Previous work showed the presence of adenosine receptors as well as adenosine uptake and release mechanisms in developing chick retinal neurons in culture. In the present work we show that exogenous glutamate or kainate promotes extensive cell death in these cultures which is blocked when the cultures are previously incubated with adenosine. Addition of glutamate or kainate to purified cultures of retinal neurons and photoreceptors induced massive death of cultured cells which was inhibited in both cases by preincubation with MK801, an NMDA antagonist, or DNQX, an AMPA/kainate antagonist. Cell death was also greatly attenuated by preincubation with adenosine plus EHNA, an adenosine deaminase inhibitor, NBI, an adenosine uptake blocker, the permeable cAMP analogs 8-Br cAMP and Sp cAMP and the A(2a) agonists CGS 21680 and DPMA, but not with the A(1) receptor agonist CHA. Kinetic studies performed determining the intracellular LDH activity showed that maximal death was observed after 8 h and in concentrations of glutamate as low as 50 microM. We also observed a time-dependent protective effect of adenosine beginning after 1 h of preincubation and reaching a maximal effect after 24 h, indicating its association with changes in cellular metabolism induced by long-term exposure of cells to the nucleoside. The results show that adenosine inhibits glutamate toxicity in retinal neurons through a long-term activation of A(2a) receptors and elevation of intracellular cyclic AMP levels.

An activity-dependent switch from facilitation to inhibition in the control of excitotoxicity by group I metabotropic glutamate receptors

Activation of group I metabotropic glutamate receptors (mGlu1 or -5 receptors) is known to either enhance or attenuate excitotoxic neuronal death depending on the experimental conditions. We have examined the possibility that these receptors may switch between two different functional modes in regulating excitotoxicity. In mixed cultures of cortical cells, the selective mGlu1/5 agonist, 3,5-dihydroxyphenylglycine (DHPG), amplified neurodegeneration induced by a toxic pulse of NMDA. This effect was observed when DHPG was either combined with NMDA or transiently applied to the cultures prior to the NMDA pulse. However, two consecutive applications of DHPG consistently produced neuroprotection. Similar effects were observed with DHPG or quisqualate (a potent agonist of mGlu1/5 receptors) in pure cultures of cortical neurons virtually devoid of astrocytes. In cultures of hippocampal pyramidal neurons, however, only protective effects of DHPG were seen suggesting that, in these particular cultures, group I mGlu receptors were endogenously switched into a "neuroprotective mode". The characteristics of the activity-dependent switch from facilitation to inhibition were examined in mixed cultures of cortical cells. The switch in the response to DHPG was observed when the two applications of the drug were separated by an interval ranging from 1-45 min, but was lost when the interval was extended to 90 min. In addition, this phenomenon required the initial activation of mGlu5 receptors (as indicated by the use of subtype-selective antagonists) and was mediated by the activation of protein kinase C. We conclude that group I mGlu receptors are subjected to an activity-dependent switch in regulating excitotoxic neuronal death and, therefore, the recent "history" of these receptors is critical for the response to agonists or antagonists.

Effect of an adenosine A(1) receptor agonist and a novel pyrimidoindole on membrane properties and neurotransmitter release in rat cortical and hippocampal neurons

Activation of adenosine A(1) receptors by endogenous adenosine plays a neuroprotective role under various pathophysiological conditions including hypoxia. Intracellular recordings were made in rat pyramidal cells of the somatosensory cortex. Hypoxia (5 min) induced a membrane depolarization and a decrease of input resistance. The A(1) receptor agonist N(6)-cyclopentyladenosine (CPA, 100 microM) reversibly inhibited the hypoxic depolarization. The inhibition was also present after blockade of the A(2A), A(2B) and A(3) receptor subtypes by selective antagonists. CPA had no effect on the hypoxic decrease of input resistance. 1,3-Dipropyl-8-cyclopentylxanthine (DPCPX), a selective A(1) receptor antagonist, which did not alter hypoxic depolarization when given alone abolished the inhibitory effect of CPA. Neither CPA nor DPCPX influenced membrane potential or apparent input resistance under normoxic conditions. The novel pyrimidoindole (R)-9-(1-methylbenzyl)-2-(4'-pyridyl)-9H-pyrimido[4,5-b]indole-4-amine (APPPI, 1 and 10 microM) reversibly diminished hypoxic depolarization but had no significant effect on input resistance. The effect of APPPI at a concentration of 1 microM, but not at 10 microM, was blocked by DPCPX (0.1 microM). CPA (100 microM) inhibited [(3)H]-noradrenaline ([(3)H]-NA) release from rat hippocampal brain slices significantly only in the presence of rauwolscine (0.1 microM), an alpha(2)-adrenoceptor antagonist. APPPI (1 and 10 microM) exhibited an inhibitory effect similar to that observed with CPA. The effects of both CPA and APPPI were antagonized by DPCPX (0.1 microM). The present data suggest that mainly presynaptic mechanisms prevent neurons from hypoxic changes by an inhibition of transmitter release. However, in contrast to CPA, APPPI exhibited additional effects, which require further investigation.

Adenosine-mediated presynaptic modulation of glutamatergic transmission in the laterodorsal tegmentum

The laterodorsal tegmentum (LDT) neurons supply most of the cholinergic tone to the brainstem and diencephalon necessary for physiological arousal. It is known that application of adenosine in the LDT nucleus increases sleep in vivo (Portas et al., 1997) and directly inhibits LDT neurons in vitro by activating postsynaptic adenosine A(1) receptors (Rainnie et al., 1994). However, adenosine effects on synaptic inputs to LDT neurons has not been previously reported. We found that both evoked glutamatergic EPSCs and GABAergic IPSCs were reduced by adenosine (50 micrometer). A presynaptic site of action for adenosine A(1) receptors on glutamatergic afferents was suggested by the following: (1) adenosine did not affect exogenous glutamate-mediated current, (2) adenosine reduced glutamatergic miniature EPSC (mEPSC) frequency, without affecting the amplitude, and (3) inhibition of the evoked EPSC was mimicked by the A(1) agonist N6-cyclohexyladenosine (100 nm) but not by the A(2) agonist N6-[2-(3,5-dimethoxyphenyl)-2-(methylphenyl)-ethyl]-adenosine (10 nm). The A(1) receptor antagonist 8-cyclopentyltheophylline (CPT; 200 nm) potentiated the evoked EPSCs, suggesting the presence of a tonic activation of presynaptic A(1) receptors by endogenous adenosine. The adenosine kinase inhibitor, 5-iodotubercidin (10 micrometer), mimicked adenosine presynaptic and postsynaptic effects. These effects were antagonized by CPT or adenosine deaminase (0.8 IU/ml), suggesting mediation by increased extracellular endogenous adenosine. Together, these data suggest that the activity of LDT neurons is under inhibitory tone by endogenous adenosine through the activation of both presynaptic A(1) receptors on excitatory terminals and postsynaptic A(1) receptors. Furthermore, an alteration of adenosine kinase activity modifies the degree of this inhibitory tone.

Adenosine as a neuromodulator and as a homeostatic regulator in the nervous system: different roles, different sources and different receptors

Adenosine exerts two parallel modulatory roles in the CNS, acting as a homeostatic modulator and also as a neuromodulator at the synaptic level. We will present evidence to suggest that these two different modulatory roles are fulfilled by extracellular adenosine originated from different metabolic sources, and involve receptors with different sub-cellular localisation. It is widely accepted that adenosine is an inhibitory modulator in the CNS, a notion that stems from the preponderant role of inhibitory adenosine A(1) receptors in defining the homeostatic modulatory role of adenosine. However, we will review recent data that suggests that the synaptically localised neuromodulatory role of adenosine depend on a balanced activation of inhibitory A(1) receptors and mostly facilitatory A(2A) receptors. This balanced activation of A(1) and A(2A) adenosine receptors depends not only on the transient levels of extracellular adenosine, but also on the direct interaction between A(1) and A(2A) receptors, which control each other's action.

Endogenous regulators of G protein signaling proteins regulate presynaptic inhibition at rat hippocampal synapses

Presynaptic inhibition mediated by G protein-coupled receptors (GPCRs) can develop and decay in a few seconds. This time course is too rapid to be accounted for by the intrinsic GTPase activity of Galpha subunits alone. Here, we test the hypothesis that endogenous regulators of G protein signaling (RGS proteins) are required for rapid, brief presynaptic inhibition. Endogenous G protein alpha subunits were uncoupled from GPCRs by treating cultures with pertussis toxin (PTX). Adenoviral expression of mutant PTX-insensitive (PTX-i) Galpha(i1-3) or Galpha(o) subunits rescued adenosine-induced presynaptic inhibition in cultured hippocampal neurons. Expression of double mutant Galpha(i1) or Galpha(o) subunits that were both PTX-insensitive and unable to bind RGS proteins (PTX/RGS-i) also rescued presynaptic inhibition. Presynaptic inhibition mediated by PTX/RGS-i subunits decayed much more slowly after agonist removal than that mediated by PTX-i subunits or native G proteins. The onset of presynaptic inhibition mediated by PTX/RGS-i Galpha(o) was also slower than that mediated by PTX-i Galpha(o). In contrast, the onset of presynaptic inhibition mediated by PTX/RGS-i Galpha(i1) was similar to that mediated by PTX-i Galpha(i1). These results suggest that endogenous RGS proteins regulate the time course of G protein signaling in mammalian central nervous system presynaptic terminals.

Effects of adenosine agonists and an antagonist on excitatory transmitter release from the ischemic rabbit hippocampus

The purpose of this study was to determine the effects of adenosine agonists and an antagonist on ischemia-induced extracellular glutamate concentrations in an animal model of transient cerebral ischemia using in vivo cerebral microdialysis. Fifty New Zealand white rabbits were randomly assigned to one of five groups (normothermia, hypothermia, cyclopentyladenosine (CPA), theophylline, or propentofylline). Microdialysis probes were stereotactically placed in the dorsal hippocampus. Twenty minutes before the onset of ischemia, either 1 mg/kg CPA, 5 mg/kg propentofylline, or 20 mg/kg theophylline were administered intravenously. Esophageal temperature was maintained at 38 degrees C, except in the hypothermic animals, which were cooled to 30 degrees C throughout the entire experiment. Two 12-min periods of cerebral ischemia, separated by a 105-min interval of reperfusion, were produced by inflating a neck tourniquet. High-performance liquid chromatography was used to determine the glutamate concentration in the microdialysate. There were no significant increases in glutamate concentrations during the first ischemic period in any of the five groups. During the second ischemic episode, glutamate concentrations in the normothermic group peaked at levels approximately three times higher than the initial values. A similar pattern of changes in glutamate concentrations was observed in the CPA, propentofylline, and theophylline groups. In the hypothermic group, the concentrations of glutamate remained at baseline levels during the entire experiment. Contrary to expectations, neither the adenosine agonists (CPA, propentofylline) nor the antagonist (theophylline) had any effect on extracellular glutamate concentrations in the peri-ischemic period. Although adenosine and its analogs may be cerebroprotective agents, their mechanism of action is not fully understood. The data derived from this study indicates that the acute administration of such agents had no effect on ischemia-induced glutamate release within the hippocampus under these experimental conditions. Based on these results, further work is needed to compare in vivo versus in vitro experimental results in acute and long-term treatment studies with adenosine receptor agonists and antagonists.

Adenosine A1 receptors modulate high voltage-activated Ca2+ currents and motor pattern generation in the xenopus embryo

Adenosine causes voltage- and non-voltage-dependent inhibition of high voltage-activated (HVA) Ca2+ currents in Xenopus laevis embryo spinal neurons. As this inhibition can be blocked by 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX) and mimicked by N6-cyclopentyladenosine (CPA) it appears to be mediated by A1 receptors. Agents active at A2 receptors either were without effect or could be blocked by DPCPX. AMP had no agonist action on these receptors. By using omega-conotoxin GVIA we found that adenosine inhibited an N-type Ca2+ current as well as a further unidentified HVA current that was insensitive to dihydropyridines, omega-agatoxin TK and omega-conotoxin MVIIC. Both types of current were subject to voltage- and non-voltage-dependent inhibition. We used CPA and DPCPX to test whether A1 receptors regulated spinal motor pattern generation in spinalized Xenopus embryos. DPCPX caused a near doubling of, while CPA greatly shortened, the length of swimming episodes. In addition, DPCPX slowed, while CPA greatly speeded up, the rate of run-down of motor activity. Our results demonstrate a novel action of A1 receptors in modulating spinal motor activity. Furthermore they confirm that adenosine is produced continually throughout swimming episodes and acts to cause the eventual termination of activity.

Effect of adenosine on adenosine triphosphate-sensitive potassium channel during hypoxia in rat hippocampal neurons

Using the whole-cell patch clamp method, we explored the effect of adenosine on the K(ATP) current and its regulatory mechanisms in acutely dissociated rat hippocampal neurons. A chemical hypoxia model was made using 0.2 mmol/l 2,4dinitrophenol (2,4DNP). During hypoxia, the K(ATP) current was not raised significantly by adenosine alone, but was accelerated significantly by adenosine in combination with the selective A(2) receptor blocker 3, 7-dimethl-1-propargylxanth-ine. The selective A(1) receptor agonist N6-cyclopentyladenosine also accelerated the K(ATP) current. These results suggest that activation of the adenosine A(1) receptor can accelerate opening of the K(ATP) channel during hypoxia, and that the A(2) receptor may have an opposing effect to the A(1) receptor.

Adenosine A1 receptors inhibit Ca2+ channels coupled to the release of ACh, but not of GABA, in cultured retina cells

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

Adenosine receptor expression and modulation of Ca(2+) channels in rat striatal cholinergic interneurons

Adenosine is a potent regulator of acetylcholine release in the striatum, yet the mechanisms mediating this regulation are largely undefined. To begin to fill this gap, adenosine receptor expression and coupling to voltage-dependent Ca(2+) channels were studied in cholinergic interneurons by combined whole cell voltage-clamp recording and single-cell reverse transcription-polymerase chain reaction. Cholinergic interneurons were identified by the presence of choline acetyltransferase mRNA. Nearly all of these interneurons (90%, n = 28) expressed detectable levels of A(1) adenosine receptor mRNA. A(2a) and A(2b) receptor mRNAs were less frequently detected. A(3) receptor mRNA was undetectable. Adenosine rapidly and reversibly reduced N-type Ca(2+) currents in cholinergic interneurons. The A(1) receptor antagonist 8-cyclopentyl-1, 3-dimethylxanthine completely blocked the effect of adenosine. The IC(50) of the A(1) receptor selective agonist 2-chloro-N6-cyclopentyladenosine was 45 nM, whereas it was near 30 microM for the A(2a) receptor agonist CGS-21680. Dialysis with GDPbetaS or brief exposure to the G protein (G(i/o)) alkylating agent N-ethylmaleimide also blocked the adenosine modulation. The reduction in N-type currents was partially reversed by depolarizing prepulses. A membrane-delimited pathway mediated the modulation, because it was not seen in cell-attached patches when agonist was applied to the bath. Activation of protein kinase C attenuated the adenosine modulation. Taken together, our results argue that activation of A(1) adenosine receptors in cholinergic interneurons reduces N-type Ca(2+) currents via a membrane-delimited, G(i/o) class G-protein pathway that is regulated by protein kinase C. These observations establish a cellular mechanism by which adenosine may serve to reduce acetylcholine release.

Anticonvulsant action of 2-chloroadenosine injected focally into the perirhinal cortex in amygdaloid kindled rats

Possible anticonvulsant effects of 2-chloroadenosine injected focally into the perirhinal cortex of amygdala kindled rats were investigated over a 2 h period. Animals were microinfused (1 microl) with 2-chloroadenosine (2-CLA; 5, 10, 15, 25 and 100 nM) or artificial cerebrospinal fluid applied through a cannula located in the perirhinal cortex. At the doses employed, 2-CLA significantly reduced afterdischarge duration and stage 5 seizure duration. The latency to stage 4 seizure was increased only at the highest dose of 2-CLA (100 nM), while even at this dose no significant change in seizure stage could be seen. The maximum effect of 2-CLA was obtained 30 min after microinfusion of the drug. Pre-treatment (intraperirhinal cortex) of animals with the nonselective adenosine antagonist, caffeine (50 microM; 1 microl), blocked the anticonvulsant activity of 2-CLA. These results suggest that adenosine receptors located in the perirhinal cortex may play an important role in the suppression of seizure activity elicited from the amygdala.

High level of adenosine A1 receptor-like immunoreactivity in the CA2/CA3a region of the adult rat hippocampus

We describe the immunocytochemical distribution of adenosine A1 receptors in the rat hippocampus. Adenosine A1 receptor-like immunoreactivity was seen on the cell soma and dendrites of pyramidal cells and the cell soma and proximal part of dendrites of granule cells, but not on glial cells. Developmentally, adenosine A1 receptor-like immunoreactivity was diffuse on postnatal day 7 and increased in intensity in individual cells by day 21. In the CA2/CA3a region, the adult pattern of A1 receptor distribution was established by day 28. In the adult rat hippocampus, rostrocaudal inspection revealed that immunoreactivity in CA2/CA3a was greatest. Confocal microscopy revealed differences in the staining patterns for the adenosine A receptor and synaptophysin, a marker of presynaptic terminals. This result suggests that the adenosine A1 receptor might have postsynaptic physiological functions. Double-labeling of adenosine A1 receptors and anterogradely-labeled fibers from the supramammillary nucleus showed that the fibers from the supramammillary nucleus terminate directly on the cell soma of the A1 receptor-immunopositive neurons in CA2/CA3a and the dentate gyrus. These results indicate that the adenosine A 1 receptor in CA2/CA3a and the dentate gyrus are in a position to regulate hippocampal theta activity and that resultant strong synaptic depression in CA2/CA3a could play a role in regulating the intrinsic signal flow between CA3 and CA1.

Identification of different putative neuronal subtypes in cultures of the superior region of the hippocampus using electrophysiological parameters

Cultured neurons offer many advantages over a slice preparation for whole-cell patch-clamp studies, such as better control over the environment and space clamp control. However, heterogeneous cultures of neurons present problems in distinguishing the cell type from which recordings are made. The present study uses correlations with data obtained in the hippocampal slice preparation to determine the feasibility of "identifying" different neuronal subtypes in cultures obtained from the superior region of postnatal two- to 13-day-old rat hippocampus. Whole-cell patch-clamp recording in the current-clamp mode after 24-96 h in culture was used to determine if the action potential duration would be a useful criterion in distinguishing cell types. Single action potentials were elicited by a 0.1-0.2 ms, 2-4 nA depolarizing pulse. The average membrane potential and input resistance were -46.8+/-1.2 mV (n = 58) and 576+/-56 Mohms (n = 57), respectively. A frequency distribution of the action potential duration measured at half-maximal amplitude showed four distinct groups of neurons (group 1, 1.36+/-0.03 ms, n = 17; group 2, 2.19+/-0.05 ms, n = 20; group 3, 3.17+/-0.10 ms, n = 16; group 4, 4.36+/-0.13, n = 5). Based on correlations with previous studies using intracellular recording in identified cells in slices, the data suggest that group 1 represents basket cells, group 2 represents vertical cells, group 3 represents a combination of stellate cells and pyramidal cells, and group 4 represents another unidentified class of cells. Further analysis of the fast afterhyperpolarization allows distinction between pyramidal cells and stellate cells in group 3. In contrast to the interneurons in a slice preparation, these cells offer good voltage control and environmental control. Future studies will record from these cells in current-clamp mode to quickly characterize the action potential before switching to voltage-clamp recording to characterize the currents present in the different types of interneurons.

Inhibition by adenosine receptor agonists of synaptic transmission in rat periaqueductal grey neurons

1. The actions of selective adenosine A1 and A2 receptor agonists were examined on synaptic currents in periaqueductal grey (PAG) neurons using patch-clamp recordings in brain slices. 2. The A1 receptor agonist 2-chloro-N-cyclopentyladenosine (CCPA), but not the A2 agonist, 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS21680), inhibited both electrically evoked inhibitory (eIPSCs) and excitatory (eEPSCs) postsynaptic currents. The actions of CCPA were reversed by the A1 receptor antagonist 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX). 3. In the absence or presence of forskolin, DPCPX had no effect on eIPSCs, suggesting that concentrations of tonically released adenosine are not sufficient to inhibit synaptic transmission in the PAG. 4. CCPA decreased the frequency of spontaneous miniature action potential-independent IPSCs (mIPSCs) but had no effect on their amplitude distributions. Inhibition persisted in nominally Ca2+-free, high Mg2+ solutions and in 4-aminopyridine. 5. The CCPA-induced decrease in mIPSC frequency was partially blocked by the non-selective protein kinase inhibitor staurosporine, the specific protein kinase A inhibitor 8-para-chlorophenylthioadenosine-3',5'-cyclic monophosphorothioate (Rp-8-CPT-cAMPS), and by 8-bromoadenosine cyclic 3',5' monophosphate (8-Br-cAMP). 6. These results suggest that A1 adenosine receptor agonists inhibit both GABAergic and glutamatergic synaptic transmission in the PAG. Inhibition of GABAergic transmission is mediated by presynaptic mechanisms that partly involve protein kinase A.

Adenosine release mediates cyanide-induced suppression of CA1 neuronal activity

The rapid suppression of CNS function produced by cyanide (CN) was studied by field, intracellular, and whole-cell recording in hippocampal slices (at 33-34 degrees C). Population spikes and field EPSPs were depressed by 4-5 min bath applications of 50-100 microM CN (IC50 was 18 miroM for spikes and 72 microM for EPSPs). The actions of CN were reversibly suppressed by the adenosine antagonists 8-sulfophenyltheophylline (8-SPT; 10 microM) and 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 0.2 microM), potentiated by the adenosine transport inhibitor dipyridamole (0.5 microM), but unaffected by the KATP channel blocker glyburide (10 microM). Therefore the CN-induced reductions of synaptic efficacy and postsynaptic excitability-demonstrated by synaptic input:output plots-are mediated mainly by adenosine. In whole-cell or intracellular recordings, CN depressed EPSCs and elicited an increase in input conductance and an outward current, the reversal potential of which was approximately -90 mV (indicating that K+ was the major carrier). These effects also were attenuated by 8-SPT. In the presence of 1 mM Ba, CN had no significant postsynaptic action; Cs (2 mM) also prevented CN-induced outward currents but only partly blocked the increase in conductance. Another 8-SPT-sensitive action of CN was to depress hyperpolarization-activated slow inward relaxations (Q current). At room temperature (22-24 degrees C), although it did not change holding current and slow inward relaxations, CN raised the input conductance; this effect also was prevented by 8-SPT (10 microM), but not by glyburide (10 microM). Adenosine release thus appears to be the major link between acute CN poisoning and early depression of CNS synaptic function.

Adenosine inhibits L-type Ca2+ current and catecholamine release in the rabbit carotid body chemoreceptor cells

In an in vitro preparation of the intact carotid body (CB) of the rabbit, adenosine (100 microM) inhibited hypoxia-induced catecholamine release by 25%. The specific A1 antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 1 microM) prevented the inhibition and increased the response to hypoxia further. In isolated chemoreceptor cells from the same species, adenosine inhibited voltage-dependent Ca2+ currents by 29% at 1 microM (concentration producing half-maximal inhibition, IC50 = 50 nM). This inhibition was mimicked by R(-)N6-(2-phenylisopropyl)-adenosine and 2-chloroadenosine (1 microM), two purinergic agonists poorly active at the intracellular ('P') site, and persisted in the presence of dipyridamole (a blocker of adenosine uptake; 1 microM) and was fully inhibited by 8-phenyltheophylline (10 microM). The A1 antagonists DPCPX (10 microM) and 8-cyclopentyl-1,3-dimethylxantine (0.1 microM) inhibited the effect of adenosine by 93% (IC50 = 0.14 microM) and 59%, respectively. The inhibition of the Ca2+ current (I(Ca)) was reduced by nisoldipine (an L-type Ca2+ channel antagonist) by nearly 50%, and was unaltered by omega-conotoxin GVIA, a blocker of N-type Ca2+ channels. Adenosine did not affect the voltage-dependent Na+ current (I(Na)) or K+ current (I(K)). We conclude that adenosine A1 receptors are located in chemoreceptor cells and mediate the inhibition of L-type Ca2+ channels and thereby the release of catecholamines produced by hypoxia. The data also indicate that endogenous adenosine acts as a physiological negative modulator of the chemoreceptor cell function. The previously reported excitatory action of adenosine on the activity of the sensory nerve of the CB is discussed in terms of a balance between the inhibition mediated by A1 receptors and the excitation mediated by A2 receptors.

A1 adenosine receptors modulate respiratory activity of the neonatal mouse via the cAMP-mediated signaling pathway

The effects of adenosine and its analogs on the function of the respiratory center were studied in the spontaneously active rhythmic slice of neonatal and juvenile mice (4-14 days old). Whole cell, spontaneous postsynaptic currents (sPSCs) and single channel KATP currents were recorded in inspiratory neurons of the pre-Bötzinger complex. Adenosine (50-600 microM) inhibited the respiratory rhythm. This was accompanied by increase in the activity of KATP channels in cell-attached patches. The A1 adenosine receptor agonist, 2-chloro-N6-cyclopentyladenosine (CCPA, 0.3-2 microM), inhibited the respiratory rhythm, sPSCs, and enhanced activity of KATP channels. The A1 adenosine receptor antagonist, 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX, 1-3 microM), showed opposite effects and occluded the CCPA actions. Agents specific for A2 adenosine receptors (CGS 21860 and NECA, both applied at 1-10 microM) were without effect. Elevation of intracellular cAMP concentration ([cAMP]i) by 8-Br-cAMP (200-500 microM), forskolin (0.5-2 microM), or isobutylmethylxantine (IBMX, 30-90 microM) reinforced the rhythm, whereas NaF (100-800 microM) depressed it. The open probability of single KATP channels in cell-attached patches decreased after application of forskolin and increased in the presence of NaF. [cAMP]i elevation reversed the effects of A1 receptors both on the respiratory rhythm and KATP channels. A1 receptors and [cAMP]i modified the hypoxic respiratory response. In the presence of A1 agonists the duration of hypoxic augmentation shortened, and depression of the respiratory rhythm occurred earlier. Elevation of [cAMP]i prolonged augmentation and delayed the development of the depression. We conclude that A1 adenosine receptors modulate the respiratory rhythm via inhibition of intracellular cAMP production and concomitant activation of KATP channels.

[3H]acetylcholine release from rat amacrine-like neurons is inhibited by adenosine A1 receptor activation

We studied the effect of endogenous adenosine on the release of [3H]acetylcholine ([3H]ACh) in cultures enriched (96.4+/-0.4%) in rat cholinergic amacrine-like neurons, as determined by labeling with an antibody against choline acetyltransferase. A small population of these cells also contained GABA. Using these cultures we observed that both [3H]ACh release, which was largely Ca2+-dependent, and 45Ca2+ influx, evoked by depolarization with 50 mM KCl, were increased when adenosine A1 receptor activation was prevented by removal of endogenous adenosine with adenosine deaminase, or by application of the A1 receptor antagonist DPCPX. Our results indicate that, in cultured rat amacrine-like neurons, the activation of A1 receptors decreases calcium influx and, thereby, inhibits [3H]ACh release.

Adenosine and the nervous system: pharmacological data and therapeutic perspectives

1. Adenosine acts on a family of G-protein-coupled receptors called purinoreceptors. 2. Four subtypes have been cloned and pharmacologically characterized. 3. The principal pharmacological data and structure-function relations for agonist interactions with P1 receptors are presented. 4. We conclude that the potent role of adenosine in the nervous system may be interesting for the development of drugs targeted at purines and their receptors.

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

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

Rapid temperature changes induce adenosine-mediated depression of synaptic transmission in hippocampal slices from rats (non-hibernators) but not in slices from golden hamsters (hibernators)

Disturbances in neuronal communication induced by rapid temperature changes are a risk in the context of accidental hypothermia and would be fatal for hibernators during arousal from hibernation. Therefore, we investigated the effects of rapid temperature changes on synaptically induced CA1 population spikes in hippocampal slices from golden hamsters (hibernators) and rats (non-hibernators). Temperature was changed ramp-like by 0.3 degrees C/min, which corresponds to the rise of body temperature in golden hamsters during arousal from hibernation. During cooling from 35 to 10-15 degrees C, the population spike amplitude increased, reached maximal values at 25-30 degrees C and 20-25 degrees C in hamster and rat slices, respectively, and then decreased with further cooling. During rewarming, hamster slices displayed the same temperature dependence as during cooling. In contrast, in rat slices dynamic effects of the temperature change occurred. These were most obvious in a strong depression of the spike amplitude during rewarming as compared to cooling. Above 26-29 degrees C, the depression was superimposed by an excitatory effect. The depression was largely attenuated by theophylline (100-200 microM) and thus seems to be based on an increase of the concentration of endogenous adenosine, which in turn may result from an imbalance in energy metabolism during warming. The lack of warming-related depression in hamster slices can be explained by a lower sensitivity for adenosine as compared to rat slices. In addition, a better resistance of metabolic balance against rapid temperature changes may prevent large elevations of endogenous adenosine in the hamster hippocampus. For hibernators, the avoidance of temperature change-induced disturbances of neuronal communication may be a prerequisite for safe arousal from hibernation.