The ionic basis of adenosine receptor actions on post-ganglionic neurones in the rat

Satellite glial cells in sympathetic and parasympathetic ganglia: in search of function

Glial cells are established as essential for many functions of the central nervous system, and this seems to hold also for glial cells in the peripheral nervous system. The main type of glial cells in most types of peripheral ganglia - sensory, sympathetic, and parasympathetic - is satellite glial cells (SGCs). These cells usually form envelopes around single neurons, which create a distinct functional unit consisting of a neuron and its attending SGCs. This review presents the knowledge on the morphology of SGCs in sympathetic and parasympathetic ganglia, and the (limited) available information on their physiology and pharmacology. It appears that SGCs carry receptors for ATP and can thus respond to the release of this neurotransmitter by the neurons. There is evidence that SGCs have an uptake mechanism for GABA, and possibly other neurotransmitters, which enables them to control the neuronal microenvironment. Damage to post- or preganglionic nerve fibers influences both the ganglionic neurons and the SGCs. One major consequence of postganglionic nerve section is the detachment of preganglionic nerve terminals, resulting in decline of synaptic transmission. It appears that, at least in sympathetic ganglia, SGCs participate in the detachment process, and possibly in the subsequent recovery of the synaptic connections. Unlike sensory neurons, neurons in autonomic ganglia receive synaptic inputs, and SGCs are in very close contact with synaptic boutons. This places the SGCs in a position to influence synaptic transmission and information processing in autonomic ganglia, but this topic requires much further work.

Adenosine A1 receptor activation inhibits LTP in sympathetic ganglia

The effects of adenosine on long-term potentiation of sympathetic ganglia was studied in the isolated superior cervical ganglion of the rat, using extracellularly recorded compound action potential as an index of synaptic transmission. Adenosine in a small concentration (2 microM) blocked the post-tetanic potentiation without affecting long-term potentiation. Higher concentrations blocked both responses with no significant effect on basal transmission. The inhibitory effect appears to be due to activation of adenosine A1 receptors. This was indicated by results from experiments with the A1 agonist N6-cyclopentyladenosine (1 microM) which caused inhibition of the basal transmission as well as long-term potentiation and post-tetanic potentiation. This inhibition was readily antagonized by 8-phenyltheophylline (1 microM), an A1 receptor antagonist. A small enhancement of basal transmission was seen on treatment with 8-phenyltheophylline. The inhibitory effect of N6-cyclopentyladenosine on long-term potentiation was totally prevented when the Ca2+ concentration in the superfusate was doubled (from 2.2 to 4.4 mM). The adenosine A2 receptor agonist 5'-(N-cyclopropyl)-carboxamidoadenosine (1 microM), although caused a slight potentiation of basal transmission, had no significant effect on the post-tetanic potentiation or long-term potentiation. The adenosine transport inhibitors, dipyridamole (2 microM) and S-(4-nitorobenzyl)-6-thioinosine (2 microM) caused significant inhibition of the basal ganglionic transmission without affecting post-tetanic potentiation or long-term potentiation. The effect of dipyradimole on basal transmission was not antagonized in the presence of 8-phenyltheophylline suggesting a non-specific action. The results suggest that exogenous adenosine can inhibit both post-tetanic potentiation and long-term potentiation in sympathetic ganglia, probably by activation of presynaptic A1 receptors. The results also suggest that endogenous adenosine, which is probably released in minute amounts, may only modulate basal transmission without influencing induction or maintenance of long-term potentiation in the superior cervical ganglion.

Adenosine A3 receptors potentiate hippocampal calcium current by a PKA-dependent/PKC-independent pathway

The modulation of high-threshold Ca current (I(Ca)) by adenosine receptors was studied using the voltage clamp method on acutely dissociated guinea pig hippocampal CA3 pyramidal neurons. When these neurons were exposed to adenosine in the presence of A1, A2a and A2b receptor antagonists, I(Ca) potentiation occurred at test potentials of -10 mV, but not at -40 mV. Similar potentiation also occurred using the A3 agonist N6-2-(4-aminophenyl)ethyl-adenosine (APNEA), either alone or in the presence of A1 and A2 antagonists. The putative A4 agonist 2-phenylaminoadenosine (CV-1808; Cornfield et al., 1992) did not potentiate I(Ca) at four concentrations tested between 25 nM and 2500 nM. K0.5 for the APNEA-induced potentiation was 25.4 nM, comparable to that determined in binding studies for the cloned receptor (15.5 nM; Zhou et al., 1992). I(Ca) potentiation by APNEA was blocked by intracellular application of WIPTIDE, a PKA inhibitor (p < 0.001), but was not affected by protein kinase C (PKC) inhibitor peptide (19-36). These results indicate that: (1) A3 receptor activation can significantly potentiate I(Ca), and (2) because the A3 receptor has been linked to down-regulation of adenylyl cyclase (Zhou et al., 1992), PKA appears to be negatively coupled to I(Ca).

Adenosine analogs inhibit acetylcholine release and cyclic AMP synthesis in the guinea-pig superior cervical ganglion

The ability of adenosine agonists to modulate the electrically evoked release of acetylcholine (ACh) from [3H]choline preloaded guinea-pig superior cervical ganglia (SCG) was investigated. The adenosine A1-receptor selective agonist N6-cyclohexyladenosine (CHA) and 2-chloroadenosine (2-CADO) inhibited the evoked transmitter release, the effect being reversed by the A1-receptor selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), and by sulmazole (SUL), which blocks both the A1-receptor and the adenylate cyclase inhibitory regulator Gi. In whole ganglia, CHA decreased both the basal and the forskolin (FSK)-stimulated cyclic AMP synthesis. The latter effect was again prevented by the A1 antagonist DPCPX. These results are compatible with the existence, in the guinea-pig SCG, of adenosine A1-receptors, part of which are located on the presynaptic nerve terminals mediating an inhibition of ACh release.

Action of purine and pyrimidine nucleotides on the rat superior cervical ganglion

1. Using a grease-gap technique, we have investigated the effects of purine and pyrimidine nucleotides on the d.c. potential of the rat isolated superior cervical ganglion (SCG). 2. Of the purines tested, adenosine, adenosine 5'-triphosphate (ATP), beta,gamma-methylene-adenosine 5'-triphosphate (beta,gamma-MeATP) at up to 300 microM produced concentration-dependent hyperpolarizations, whereas 2-methyl-thio-ATP (2-Me.S.ATP) and alpha,beta-methylene-ATP (alpha,beta-MeATP) depolarized ganglia. Of the pyrimidines tested, uridine 5'-triphosphate (UTP) produced concentration-dependent depolarizations and cytosine 5'-triphosphate (CTP) at 1000 microM produced considerably smaller but significant depolarizations. In contrast uridine 5'-monophosphate (UMP) at 1000 microM hyperpolarized ganglia. The relative order of potency of purines and pyrimidines to depolarize ganglia was: UTP > alpha,beta-MeATP >> CTP > 2-Me.S.ATP and to hyperpolarize ganglia was: adenosine = beta,gamma-MeATP > ATP > UMP. 3. The ability of purines and pyrimidines to alter the depolarizing response caused by muscarine and of purines to alter depolarization induced by gamma-aminobutyric acid (GABA) was determined. The relative order of potency of nucleotides in depressing submaximal depolarization caused by muscarine (100 nM) was: adenosine = ATP > beta,gamma-MeATP whereas 2-Me.S.ATP, alpha,beta-MeATP and UTP did not significantly alter depolarization caused by muscarine. At 100 microM beta,gamma-MeATP and adenosine but not ATP potentiated GABA-induced depolarizations. 4. Hyperpolarizations caused by adenosine, ATP, beta,gamma-MeATP and UMP and depolarizations caused by alpha,beta-MeATP were enhanced in medium containing reduced concentrations of calcium (0.1 mM) and potassium (2 mM). In this medium 8-phenyltheophylline abolished hyperpolarizations caused by adenosine and reversed hyperpolarizations caused by ATP into depolarizations. Suramin (300 microM), a P2-purinoceptor antagonist, significantly reduced the depolarizing response caused by alpha,beta-MeATP and significantly increased hyperpolarizations caused by ATP and Beta,gamma-MeATP. Suramin (300 microM) did not significantly alter depolarizations caused by l,l-dimethyl-4-phenylpiperazinium (10 microM), potassium(3 mM) or muscarine (100 nM) and significantly potentiated depolarizations caused by UTP (100 microM).5.It is concluded that the rat SCG contains PI-purinoceptors that hyperpolarize the ganglion and diminish sensitivity to muscarine, and P2X-purinoceptors that depolarize the SCG. There is also some evidence to suggest the presence of receptors for UTP, i.e., pyrimidinoceptors, which depolarize SCG neurones.

Ionic mechanism of action of adenosine on the rat superior cervical ganglion

1. The ionic mechanism responsible for hyperpolarization of the rat superior cervical ganglion (SCG) and depression of the depolarizing response to muscarine by adenosine was studied using an extracellular grease-gap recording technique. 2. Both the hyperpolarizations to adenosine and 2-chloroadenosine and the depression of the response to muscarine by adenosine were potentiated in reduced external calcium (Ca2+). Hyperpolarizations to adenosine were either unaltered or potentiated in the presence of the dihydropyridine Ca2+ channel antagonists, nitrendipine or (+)PN200 110 respectively. Hyperpolarizations to adenosine were unaltered by inorganic Ca2+ channel antagonists except for cobalt, which also antagonized hyperpolarizations to carbachol and depolarizations to muscarine. 3. Hyperpolarizations to adenosine were unaltered in nominally magnesium (Mg2+)-free or in reduced external chloride (Cl-) media. When sodium ions (Na+) were replaced by lithium ions (Li+) maximal responses to adenosine were initially enhanced, returning to pretreatment levels and subsequently reduced in their duration. In contrast, responses to adenosine were significantly enhanced in nominally potassium (K+)-free medium and reduced upon doubling the extracellular K+. 4. Hyperpolarizations were enhanced in the presence of the K+ channel antagonists, 4-aminopyridine and 3,4-diaminopyridine, and reduced by a low concentration (2 mM) of tetraethylammonium (TEA), but not in 10 mM TEA. 5. The results support the hypothesis that adenosine-mediated hyperpolarization of postganglionic neurones of the rat SCG is by a Ca(2+)-independent mechanism and is probably mediated via an increase of a K+ current. The results also indicate that adenosine-induced hyperpolarizations of the rat SCG are independent of the presence of extracellular magnesium.

On the site of action and inactivation of adenosine by the rat superior cervical ganglion

1. Using an extracellular recording technique, we have investigated the site of action of adenosine and muscarine on the rat superior cervical ganglion (SCG). The adenosine-induced hyperpolarization and muscarine-induced depolarization of ganglia were localized to the cell bodies of the ganglia. Responses to muscarine and adenosine were larger when recorded via the internal carotid nerve (ICN) compared with the external carotid nerve. Depression of the response to muscarine by adenosine was similar for both nerve trunks. 2. The effects of adenosine and cyclic nucleotides on the d.c. potential and the depolarization to muscarine were examined by recording via the ICN. Adenosine at concentrations up to 1 mM produced concentration-dependent hyperpolarizations. Hyperpolarization induced by 100 microM adenosine was unaffected by 1 microM tetrodotoxin or the muscarinic M1-receptor antagonist pirenzepine (0.3 microM). In contrast, hyperpolarizations to 100 microM adenosine were significantly reduced by 10 microM 8-phenytheophylline (55 +/- 7 microV vs 15 +/- 9 microV, P < 0.01, n = 4). Two agents known to increase intracellular cAMP, i.e. 8-bromo-cyclic-adenosine-3'-5' monophosphate (8BrcAMP) and isoprenaline, depolarized ganglia. Depolarizations to 100 nM mucarine were significantly depressed by adenosine (100 microM) by 26 +/- 2% (n = 61), but unaltered by 8BrcAMP or cyclic guanosine-3'-5' monophosphate. 3. Dipyridamole and hydroxy-nitro-benzylthioguanosine (inhibitors of adenosine transport) and erythro-6-amino-9-(2-hydroxy-3-nonyl)adenine (EHNA, an inhibitor of adenosine deaminase), potentiated the depression by adenosine of the response to muscarine, and the hyperpolarization to adenosine respectively. However, there was no evidence to support the hypothesis that there was spontaneous release of endogenous adenosine under the conditions of study, as dipyridamole or EHNA did not alter the control d.c. potential or the depolarization to muscarine. 4. It is concluded that the ability of adenosine to hyperpolarize and depress the response of the rat SCG to muscarine is due to the direct activation of postsynaptic somatodendritic P1-purinoceptors and unlikely to be mediated by an increase in intracellular cAMP. In addition the rat SCG has mechanisms for both the uptake and inactivation of adenosine.

Differential activation of adenosine receptors decreases N-type but potentiates P-type Ca2+ current in hippocampal CA3 neurons

Adenosine is released in the brain in significant quantities in response to increased cellular activity. Adenosine has been shown either to decrease synaptic transmission or to produce an excitatory response in hippocampal synapses, resulting in increased glutamate release. Previous reports have shown that adenosine or its analogs reduced Ca2+ current in dorsal root ganglion and hippocampal neurons. Here we show that the selective activation of adenosine receptor subtypes has different effects on Ca2+ channels from acutely isolated pyramidal neurons from the CA3 region of guinea pig hippocampus. Activation of A1 receptors inhibited primarily N-type Ca2+ current. In contrast, activation of A2b receptors resulted in significant potentiation of P-type but not N-type Ca2+ current. This potentiation could be inhibited by blocking the cAMP-dependent protein kinase. Because of the ubiquity of adenosine, the differential effects on Ca2+ channels of adenosine receptor subtype activation may have significant implications for neuronal excitability.

Contrasting excitatory and inhibitory effects of adenosine in blood pressure regulation

Administration of adenosine results in profound hypotension without the expected activation of reflex sympathetic and renin mechanisms in most animal models. This action can be explained by the vasodilatory and neuroinhibitory effects of adenosine. It is generally considered an inhibitory neuromodulator because it inhibits the release of virtually all neurotransmitters studied and produces hyperpolarization of neurons. In contrast, adenosine produces vasoconstriction of some vascular beds, including the renal and pulmonary circulations. Renal vasoconstriction is caused by activation of A1 receptors and involves an interaction with angiotensin II. In other vascular beds adenosine releases eicosanoids, including thromboxane, also resulting in vasoconstriction. Adenosine-induced vasoconstriction is transient and species dependent. Neither the receptor type, the molecular mechanisms of these actions, nor their significance to pathophysiological processes have been defined. Adenosine also has an apparent excitatory effect in the nucleus tractus solitarii. Microinjections of adenosine into this brain stem nucleus lead to decreased sympathetic tone and hypotension similar to those produced by the excitatory amino acid glutamate. The mechanism that explains this action has recently been explored and involves the release of glutamate by adenosine. Adenosine also stimulates afferent fibers mediating sympathetic activity, including renal and myocardial afferent nerves, and carotid and aortic chemoreceptors. Afferent nerve activation seems to be more pronounced in humans and may explain most of the cardiovascular and respiratory actions of adenosine in this species. Finally, animal studies suggest that endogenous adenosine plays a role in the regulation of the baroreceptor reflex and restrains the full expression of renin-dependent hypertension.

Adenosine-mediated synaptic inhibition: partial blockade by barium does not prevent anti-epileptiform activity

Adenosine-induced inhibition of evoked postsynaptic potentials (PSPs) and epileptiform burst firing in the CA1 subfield of rat hippocampal slices was studied with intracellular recordings in vitro. Adenosine (50 microM) caused a membrane hyperpolarization which was abolished during superfusion with 2 mM Ba2+. The adenosine-induced inhibition of the PSPs was still evident, although the magnitude of the effect was significantly reduced. Adenosine also reduced Ba(2+)-induced burst firing, but less effectively than it did bursts evoked by TEA (5 mM). The results suggest that adenosine inhibits synaptic transmission and epileptiform activity by at least 2 mechanisms: a postsynaptic barium-sensitive increase in gK and a presynaptic effect independent of this adenosine-evoked outward potassium conductance.

Adenosine modulation of calcium currents in postganglionic neurones of avian cultured ciliary ganglia

1. Calcium currents in postganglionic neurones of cultured 7- to 10-day embryonic avian ciliary ganglia were analyzed under whole-cell voltage-clamp and their modulation by 2-chloroadenosine determined. 2. In the presence of tetrodotoxin (200 nM) in the medium to block the Na+ current and CsCl (105 mM) in the patch-clamp electrode to block the K+ current, two different components of the calcium currents (transient and sustained) were identified on the basis of their voltage-dependent kinetics as well as their sensitivity to the dihydropyridine agonist Bay K 8644 and antagonist nifedipine. 3. The sustained current inactivated very slowly (tau greater than 1000 ms; for test potentials from -20 mV to +40 mV) but was reactivated at a holding potential (Vh) of -40 mV. The current was increased on average over 50% by 1 microM of Bay K 8644 at a test potential of 0 mV and decreased over 35% by 1 microM of nifedipine. 4. The transient current inactivated slowly (tau less than 200 ms; for test potentials from -20 mV to +40 mV), and could be completely reactivated at a Vh of -80 mV. This current was unaffected by Bay K 8644 (1 microM) but reduced on average by 8% with nifedipine (1 microM). 5. The sustained and transient currents were decreased more than 70% by 5 microM of omega-conotoxin and decreased more than 50% by 250 microM verapamil. 6. 2-Chloroadenosine (1 microM) decreased the transient current by over 50% and the sustained current by less than 10%. In the presence of nifedipine (1 microM), 2-chloroadenosine decreased the transient current by over 30% and the remaining sustained current by 35%.In the presence of 8-phenyltheophylline (10 microM), 2-chloroadenosine no longer decreased either the transient or sustained currents but did have a slight potentiating effect on both the transient and sustained currents.7. These observations of the effects of 2-chloroadenosine on the transient and sustained currents are discussed in relation to the different calcium channel types at preganglionic nerve terminals.

Excitatory effects of adenosine 5'-triphosphate on rat locus coeruleus neurones

Pontine slices of the rat brain were used for extracellular recording of the frequency of spontaneous action potentials of locus coeruleus (LC) neurones. In the absence of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), alpha,beta-methyleneadenosine 5'-triphosphate (alpha,beta-meATP; 0.3-30 mumol/l) and 2-methylthio ATP (0.3-100 mumol/l), but not ATP (1-100 mumol/l) increased the firing rate. In the presence of DPCPX 0.1 mumol/l, all three purinoceptor agonists were active, the potency order being alpha,beta-meATP greater than 2-methylthio ATP = ATP. Preincubation of the slices with tetrodotoxin (TTX) 0.5 mumol/l decreased the spike discharge but did not alter the percent facilitatory effect of alpha,beta-meATP 30 mumol/l. There was no desensitization to alpha,beta-meATP 10 mumol/l on repeated or continuous application. Suramin 100 mumol/l selectively depressed the effect of alpha,beta-meATP 30 mumol/l without interfering with the effect of equiactive concentrations (10-100 mumol/l) of glutamic acid. The concentration-response curve of alpha,beta-meATP was shifted in a parallel manner to the right by suramin 10 mumol/l. While DPCPX 0.1 mumol/l facilitated firing, suramin 100 mumol/l did not change it. In conclusion, LC neurones may possess P2-purinoceptors of an unidentified type, which share some P2x characteristics.

Probabilistic secretion of quanta from nerve terminals in toad (Bufo marinus) muscle modulated by adenosine

1. A study has been made of the effect of blocking endogenous adenosine on the statistics of quantal secretion at nerve terminals in toad (Bufo marinus) muscle during summer and winter. 2. Exogenous adenosine (10-50 microM) reduces the mean quantal content of the endplate potential (EPP) recorded with an intracellular microelectrode (m) by 36 +/- 6% (mean +/- S.E.M.), independent of the control value of m in both summer and winter. The variance of the EPP (S2) was reduced by adenosine in proportion to m, so that the probability of quantal secretion (p) remained relatively constant. 3. Exogenous adenosine reduces the mean quantal content of secretion recorded with an extracellular electrode (m(e)) to a similar extent (47 +/- 6%) at different relatively high secreting sites along nerve terminal branches in both summer and winter. 4. Both theophylline (20-100 microM) and adenosine deaminase (2.5 i.u./ml) increase the amplitude of the EPP in summer in a [Ca2+]o of 0.5 mM or greater; a maximum increase of about 40% is reached at a [Ca2+]o of 1.2 mM. The amplitude of the EPP in winter is usually reduced by theophylline in a [Ca2+]o of 0.5 mM, but is always increased in a [Ca2+]o greater than 0.9 mM to reach a maximum increase of about 40% at high [Ca2+]o. 5. The variance of the EPP (S2) was always increased by theophylline to a greater extent than m in summer, so that p decreased and Poisson rather than binomial statistics could be used to describe the distribution of EPP amplitudes. In winter, theophylline generally increased m and S2 to about the same extent, so that p did not change much. 6. An autocorrelation analysis of the amplitude of successive EPPs in a long train at 0.5 Hz in high [Ca2+]o showed that these are likely to be independently distributed. Adenosine secreted by one impulse is then unlikely to affect secretion by a subsequent impulse in the train. 7. These observations are discussed in terms of the hypothesis that endogenously secreted adenosine at a release site inhibits secretion by nearby release sites in summer; this has the effect of reducing m and to a greater extent S2 so that binomial rather than Poisson statistics can describe the frequency distribution of EPP amplitudes.

Probabilistic secretion of quanta from nerve terminals in avian ciliary ganglia modulated by adenosine

1. The effects of adenosine on the probability of secretion of acetylcholine quanta and on presynaptic and postsynaptic action potentials was examined in the post-hatched avian ciliary ganglion. 2. Adenosine (20 microM) reduced the average size of the excitatory postsynaptic potential (EPSP) by 33%. This was due to a decrease in quantal content of the EPSP (m). The effect was blocked by theophylline (50 microM). 3. Adenosine deaminase (2.5 i.u./ml) increased the size of the EPSP by 70%, suggesting that endogenous adenosine modulates synaptic transmission in the ciliary ganglion. However, theophylline (20-100 microM) did not affect the EPSP in a low [Ca2+]o of 1 mM and high [Mg2+]o of 6 mM. 4. Plateau-type action potentials with a large calcium component were generated in the ciliary neurones by bathing the ganglion in tetraethylammonium ions (TEA, 10 mM). Adenosine (20 microM) reduced the duration of these action potentials on short exposures (less than 20 min) but increased the duration on longer exposure (greater than 30 min). Adenosine did not affect the normal action potential recorded in the absence of TEA. 5. Adenosine (20 microM) hyperpolarized the nerve terminal and as a consequence increased the size of the presynaptic action potential and reduced its after-hyperpolarization. 6. Plateau-type action potentials with a large calcium component were generated in the nerve terminals using TEA (10 mM). The duration of these action potentials was significantly reduced by adenosine (20 microM). 7. Adenosines action on nerve terminals, to hyperpolarize the membrane and reduce calcium influx, may contribute to its effect in reducing m of the EPSP.

Effects of calcium and potassium extracellular ionic concentration changes on the hippocampal CA1 activity of purinergic drugs

1. The influence of the change of potassium and calcium concentrations in the medium bathing hippocampal slices has been tested on the purinergic drug effects. 2. The depressive effect of purinergic agonists were antagonized by 50% by doubling the potassium concentration, while they were not affected by doubling calcium concentrations. 3. The data demonstrate a direct intrinsic activity of potassium ions, and suggest a possible direct interaction between adenosine receptors and potassium transneuronal fluxes.

Inhibitory adenosine A1-receptors on rat locus coeruleus neurones. An intracellular electrophysiological study

Intracellular recordings were performed in a pontine slice preparation of the rat brain containing the locus coeruleus (LC). Adenosine (100, 300 mumol/l) and its structural analogues, namely (-)-N6-(R-phenylisopropyl)-adenosine (R-PIA; 3-30 mumol/l) and S-PIA (10, 30 mumol/l), as well as 5'-N-ethylcarboxamido-adenosine (NECA; 3-30 mumol/l) inhibited the firing rate of spontaneous action potentials and produced hyperpolarization; their rank order of potency was R-PIA congruent to NECA greater than S-PIA greater than adenosine. When applied by superfusion, all agonists strongly desensitized the LC cells; the hyperpolarization never surmounted 6 mV. Upon pressure ejection of adenosine 10 mmol/l from a micropipette positioned close to an LC neurone, the membrane potential was raised by 14 mV and the apparent input resistance decreased by 20%. When the membrane potential was hyperpolarized by current injection to a similar extent as adenosine did, the fall in input resistance was only 7%. The adenosine uptake inhibitor S-(p-nitrobenzyl)-6-thioguanosine (NBTG) 30 mumol/l decreased the frequency of action potentials alone; on simultaneous bath-application with adenosine 300 mumol/l it potentiated the hyperpolarization caused by the purine derivative. 8-Cyclopentyl-1,3-dipropylxanthine (CPDPX) 0.1 mumol/l had no effect on its own, but it antagonized both R-PIA 30 mumol/l and NBTG 30 mumol/l. A higher concentration of CPDPX (1 mumol/l) facilitated the spontaneous firing. In conclusion, both exogenous and endogenous adenosine activates somatic and/or dendritic A1-receptors of LC neurones leading to an enhancement of potassium conductance and thereby to a decreased firing rate and a hyperpolarization.

Effects of N6,2'-O-dibutyryladenosine 3',5'-cyclic monophosphate, adenosine, and of oxotremorine and 3-isobutyl-1-methylxanthine on the electrically evoked [3H]acetylcholine secretion in the guinea-pig ileum myenteric plexus

The guinea-pig ileum longitudinal muscle-myenteric plexus preparation, pre-incubated with [3H]choline, was mounted in an organ bath and superfused with Tyrode's solution. [3H]Acetylcholine secretion was evoked by 150 electrical shocks at 0.5 Hz. N6,2'-O-Dibutyryladenosine 3',5'-cyclic monophosphate (dibutyryl cyclic AMP) enhanced the [3H]acetylcholine secretion in the presence of eserine and the adenosine receptor antagonist 8-phenyltheophylline (10 mumol l-1). Conversely, in the absence of 8-phenyltheophylline the [3H]acetylcholine secretion was reduced by dibutyryl cyclic AMP. In the absence and presence of 8-phenyltheophylline (apparent KD = 12 mumol l-1), adenosine reduced the [3H]acetylcholine secretion to 33% of control (IC50 = 8 mumol l-1) and to 48% of control (IC50 = 14 mumol l-1) respectively. Neither butyrate, dibutyryl cyclic GMP nor guanosine altered the [3H]acetylcholine secretion. Interaction experiments with 3-isobutyl-1-methylxanthine and oxotremorine were done in the absence of eserine, i.e. when oxotremorine is effective. Oxotremorine depressed the fractional secretion of [3H]acetylcholine with a 'maximal inhibition' of 13% of control (IC50 = 10 nmol l-1). In the presence of 3-isobutyl-1-methylxanthine (5 mmol l-1) oxotremorine depressed the secretion to 2% of control with an apparent IC50 value of 0.9 mumol l-1. 3-Isobutyl-I-methylxanthine (0.01-4 mmol l-1) enhanced the fractional secretion of [3H]acetylcholine with a 'maximal enhancement' value of 232% of control (EC50 = 0.19 mmol l-1). The presence of oxotremorine (30 nmol l-1) counteracted, and higher concentrations reversed, the enhancement caused by 3-isobutyl-1-methylxanthine.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of purine nucleotides on transmission in vesical parasympathetic ganglia of the cat

1. In adult cats, postganglionic nerve fibres on the surface of the bladder were isolated and multiunit activity of these fibres was recorded. In these cats, the urinary bladder was cannulated and intravesical pressure was also recorded. 2. ATP, APPCP, ADP, AMP and adenosine depress transmission in vesical parasympathetic ganglia equipotently; however, 2-chloroadenosine was 10-fold more potent than ATP. 3. 2-chloroadenosine, ATP, ADP, AMP, adenosine and APPCP inhibit neurally evoked bladder contractions in the same order or potency with which they depress pelvic ganglionic transmission; however, adenine, inosine, IMP and ITP were ineffective. 4. 3',5'-cyclic AMP and dibutyryl cAMP produced little or no effect on bladder activity. 5. ATP and APPCP produced a transient rise in intravesical pressure at doses 2 to 50 times the dose needed for inhibition, presumably through ATP (P2) receptors. APPCP was 10 to 20 times more potent in exciting the bladder than ATP. 6. Theophylline and caffeine effectively antagonized purinergic effects mediated through adenosine (P1) receptors on both pelvic ganglia and bladder smooth muscle. 7. ATP inhibition of TMA-evoked bladder contractions and postganglionic nerve firing suggests that purinergic inhibition occurs, at least in part, at a postsynaptic site in the ganglia.

2-Chloroadenosine reduces the N calcium current of cultured mouse sensory neurones in a pertussis toxin-sensitive manner

1. The adenosine analogue 2-chloroadenosine (CADO) reduced the duration of calcium-dependent action potentials (CAPs) in mouse dorsal root ganglion (DRG) neurones in culture, by reducing voltage-activated calcium conductance (Macdonald, Skerritt & Werz, 1986). Using the single-electrode voltage clamp technique, we recorded three calcium current components in these neurones, the transient low-threshold (T), transient high-threshold (N) and slowly inactivating high-threshold (L) currents, as described previously (Nowycky, Fox & Tsien, 1985; Gross & Macdonald, 1987). CADO (100 microM) had no effect on the isolated T and L currents. In contrast, CADO reduced calcium currents evoked at clamp potentials positive to -20 mV from holding potentials (Vh) near the resting membrane potential; under these conditions, the calcium current consisted primarily of N and L calcium current components. 2. This effect of CADO was not voltage dependent. CADO reduced the magnitude of the calcium current without affecting the voltage dependence of the calcium current-voltage relation. In addition, similar reductions of calcium current were observed when currents were evoked from Vh of -60 or -80 mV. 3. In order to determine if a guanine nucleotide-binding (G) protein was involved in the CADO effect on calcium current, cultures were pre-treated with pertussis toxin (PT) for at least four hours. PT (100 ng/ml) reduced or abolished the CADO-induced reduction of CAP duration and calcium current. 4. Since CADO inhibits adenylate cyclase through the PT-sensitive G protein, Gi, we compared the effects of CADO and 8-Br-adenosine 3',5'-cyclic-monophosphate (8-Br-cyclic AMP) on calcium current. The effect of 8-Br-cyclic AMP was voltage dependent, unlike that of CADO. 8-Br-cyclic AMP reduced calcium currents evoked from Vh = -65 mV, but had no effect on currents evoked from Vh = -85 mV. 5. We conclude that the adenosine agonist CADO reduced CAP duration in mouse DRG neurones by selectively reducing the N current component, and that the coupling between the adenosine receptor and the calcium channel required a PT-sensitive G protein. The CADO effect was unlikely, however, to be due to modulation of adenylate cyclase activity.

Somatostatin blocks a calcium current in rat sympathetic ganglion neurones

1. The effects of somatostatin and somatostatin analogues on a Ca2+ current from acutely isolated and short-term (24-48 h) cultured adult rat superior cervical ganglion (SCG) neurones were studied using the whole-cell variant of the patch-clamp technique. 2. [D-Trp8]Somatostatin (SOM) produced a rapid, reversible and concentration-dependent reduction of the Ca2+ current. Ca2+ current amplitude was reduced over the voltage range -15 to +40 mV with the greatest reduction occurring where the amplitude was maximal (ca +10 mV). In the presence of SOM, the Ca2+ current rising phase was slower and biphasic at potentials between 0 and +40 mV. 3. Application of 0.1 microM-SOM for greater than 10 s resulted in a desensitization of the response. During a 4 min application of 0.1 microM-SOM, Ca2+ current amplitude returned to about 90% of control. A second application of 0.1 microM-SOM produced less block than the initial application. 4. Concentration-response curves for SOM, somatostatin-14 (SOM-14) and somatostatin-28 (SOM-28) were fitted to a single-site binding isotherm. The concentrations producing half-maximal block and the maximal attainable blocks of the Ca2+ current for SOM, SOM-14 and SOM-28 were 3.3, 5.4 and 35 nM, respectively and 55, 51 and 54%, respectively. SOM-14 and SOM-28 slowed the Ca2+ current rising phase in a manner similar to that of SOM. Somatostatin-28 had no effect on the Ca2+ current at 1 microM. 5. The magnitude of the Ca2+ current block produced by 0.1 microM-SOM was not significantly altered in the presence of 1 microM-idazoxan, atropine, naloxone or the somatostatin antagonist aminoheptanoyl-Phe-D-Trp-Lys-O-benzyl-Thr. 6. Internal dialysis with solutions containing 500 microM-guanylyl-imidodiphosphate (Gpp(NH)p) or guanosine-5'-O-(3-thiotriphosphate)(GTP-gamma-S) decreased the Ca2+ current amplitude by 36 and 41%, respectively, and induced a biphasic rising phase in the Ca2+ current. Under these conditions, application of 0.1 microM-SOM produced significantly less block of Ca2+ current amplitude (7.1 and 14.7%, respectively) when compared with controls. 7. Internal dialysis with solutions containing 500 microM-guanosine-5'-O-(2-thiodiphosphate)(GDP-beta-S) had no significant effect on either the Ca2+ current amplitude or block produced by 0.1 microM-SOM. 8. Internal dialysis with solutions containing 500 microM-cyclic adenosine 3',5'-monophosphate (cyclic AMP) and 3-isobutyl-1-methylxanthine had no significant effect on either the Ca2+ current block produced by 0.1 microM-SOM or the Ca2+ current amplitude.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of Ca-spikes in rat preganglionic cervical sympathetic nerves by sympathomimetic amines

1. Propagated Ca-spikes were recorded from isolated cervical sympathetic nerve trunks of the rat when bathed in a solution containing 5 mM Ca2+, 0.5 or 1 microM tetrodotoxin (to block Na currents) and 1 mM 4-aminopyridine (to reduce K currents). 2. Spikes persisted when external Ca2+ was replaced with Sr2+ or Ba2+, but were blocked by the addition of the following inorganic Ca-channel blockers (in descending order of potency): Cd2+ greater than La3+ greater than Ni2+ greater than Co2+ greater than Mn2+ greater than Mg2+. 3. Ca-spike amplitude was reduced by up to 90% by (-)-noradrenaline (IC50 1.5 microM). The following sympathomimetic amines imitated this effect (in descending order of potency): clonidine greater than or equal to (-)-adrenaline greater than or equal to [(-)-noradrenaline] greater than or equal to dopamine greater than (-)-phenylephrine greater than or equal to (+/-)-amidephrine. 4. Ca-spike inhibition by (-)-noradrenaline was antagonized by phentolamine (pA2 6.5). Yohimbine was about 10 times weaker than phentolamine; (+/-)-propranolol (1 microM) and prazosin (10 microM) had no clear effect. 5. (-)-Noradrenaline reduced the amplitude of the compound action potential recorded from the superior cervical sympathetic ganglion following supramaximal preganglionic trunk stimulation when recorded in normal Krebs solution and hyperpolarized the ganglion with respect to the post-ganglionic trunk. Depression of the transmitted ganglionic action potential was antagonized by phentolamine (5 microM) but not by yohimbine (1 microM); in contrast 1 microM yohimbine completely prevented the ganglionic hyperpolarization. (-)-Noradrenaline did not hyperpolarize the preganglionic cervical sympathetic nerve trunk under these recording conditions. 6. It is suggested that inhibition of transmitter release from sympathetic preganglionic fibres produced by noradrenaline results from a depression of the voltage-gated Ca current in the fibres and/or their terminals, and that this action is mediated by an alpha-adrenoceptor which does not fully conform to either alpha 1 or alpha 2 subtypes.

A study of the actions of P1-purinoceptor agonists and antagonists in the mouse vas deferens in vitro

1. We have examined the effects of purinoceptor agonists and antagonists on the mechanical 'twitch' response, excitatory junction potential (e.j.p.) amplitude and [3H]-noradrenaline overflow in the mouse vas deferens. 2. The agonist profile for inhibition of the mechanical response was N6-([R]-2-phenylisopropyl)adenosine (L-PIA) congruent to N6-cyclohexyladenosine (CHA) greater than 5' N-ethylcarboxamido-adenosine (NECA) greater than 2-chloroadenosine (2ClA) congruent N6-([S]-2-phenylisopropyl)adenosine (D-PIA). 3. The P1-purinoceptor agonists inhibited the e.j.p. with an agonist profile of CHA greater than L-PIA congruent to NECA greater than 2ClA. 4. 2ClA inhibited [3H]-noradrenaline overflow with an EC50 of 1.2 microM which was not significantly different from the values for inhibition of the e.j.p. and the mechanical response. 5. The inhibitory action of 2ClA on the mechanical response was antagonized by 5 microM 8-phenyltheophylline (8-PT). However, neither blockade of P1-purinoceptors by 8-PT nor increasing the rate of degradation of endogenous adenosine by addition of adenosine deaminase had any effect on the mechanical response per se. 8-PT (5 microM) also failed to alter the e.j.p. amplitude or [3H]-noradrenaline overflow. 6. These results indicate that there are P1-purinoceptors present on sympathetic nerve terminals of the mouse vas deferens which are more like A1- than A2-receptors, but may be better classified as being of the A3-subtype (Ribeiro & Sebastiao, 1986). These receptors are not normally involved in the feedback regulation of transmitter release in this tissue.

Long-term regulation of synaptic acetylcholine release and nicotinic transmission: the role of cyclic AMP

1. Using the rat superior cervical ganglion in vitro, the relative efficacy of nicotinic synaptic transmission was estimated by recording the postganglionic compound action potential and the amount of endogenous acetylcholine (ACh) released. These two parameters were correlated in individual ganglia by sampling the bathing medium for the assay of ACh while simultaneously recording the postganglionic response. 2. The beta-adrenoceptor agonist isoprenaline potentiated both the evoked release of ACh and the postganglionic response by about 20% during preganglionic stimulation at 0.2 Hz. 3. The adenosine receptor agonist 2-chloroadenosine inhibited ACh release and the postganglionic response by about 35%. 4. Tetanic preganglionic stimulation for a few seconds induced a long-term potentiation of nicotinic responses and of ACh release. Both of these potentiations were dependent upon extracellular Ca2+ during the tetani. 5. Forskolin and analogues of cyclic AMP also caused a long-lasting potentiation of both the evoked release of ACh and the postganglionic response, indicating that cyclic AMP may regulate transmission by a presynaptic mechanism. The specificity of the cyclic AMP analogues was tested using various butyryl- and bromo-purine nucleotides. 6. The effects of forskolin and 8-bromo-cyclic AMP did not appear to be dependent upon extracellular Ca2+. 7. The potentiation caused by forskolin was consistently augmented by three phosphodiesterase inhibitors--AH 21-132, papaverine and SQ 20-006. However, the effect of forskolin was not consistently enhanced by theophylline, nor was it reduced by the adenylate cyclase inhibitor SQ 22-536. 8. The neurogenic long-term potentiation was augmented by two of the phosphodiesterase inhibitors that also augmented the forskolin-induced potentiation--papaverine and SQ 20-006. 9. It was concluded that cyclic AMP can enhance nicotinic transmission, and can do so by increasing the evoked release of ACh. However, it was not possible to prove that cyclic AMP mediates the long-term potentiation induced by tetanic preganglionic stimulation.

Electrophysiology of benzodiazepine receptor ligands: multiple mechanisms and sites of action

Electrophysiology of BZR ligands has been reviewed from different points of view. A great effort was made to critically discuss the arguments for and against the temporarily leading hypothesis of the mechanism of action of BZR ligands, the GABA hypothesis. As has been discussed at length in the present article, an impressive body of electrophysiological and biochemical evidence suggests an enhancement of GABAergic inhibition in CNS as a mechanism of action of BZR agonists. Biochemical data even indicate a physical coupling between GABA recognition sites and BZR which, together with the effector site build-up by Cl- channels, form a supramolecular GABAA/BZR complex. By binding to a specific site on this complex, BZR agonists allosterically increase and BZR inverse agonists decrease the gating of GABA-linked Cl- channels, whereas BZR antagonists bind to the same site without an appreciable intrinsic activity and block the binding and action of both agonists as well as inverse agonists. While this model is supported by many electrophysiological experiments performed with BZR ligands in higher nanomolar and lower micromolar concentrations, it does not explain much controversial data from animal behavior and, more importantly, is not in line with electrophysiological effects obtained with low nanomolar BZ concentrations. The latter actions of BZR ligands in brain slices occur within a concentration range compatible with concentrations of BZ observed in CSF fluid, which would be expected to be found in the biophase (receptor level) during anxiolytic therapy in man. Enhanced K+ conductance seems to be a suitable candidate for this effect of BZR ligands. This direct action on neuronal membrane properties may underlie the many electrophysiological observations with extremely low systemic doses of BZR ligands in vivo which demonstrated a depressant effect on spontaneous neuronal firing in various CNS regions. Skeletomuscular spasticity and epilepsy are two neurological disorders, where both the enhanced GABAergic inhibition and increased K+ conductance may contribute to the therapeutic effect of BZR agonists, since electrophysiological and behavioral studies strongly support GABA-dependent as well as GABA-independent action of BZR ligands elicited by low to intermediate doses of BZ necessary to evoke anticonvulsant and muscle relaxant effects. Somewhat higher doses of BZR ligands, inducing sedation and sleep, lead perhaps to the only pharmacologically relevant CNS concentrations (ca. 1 microM) which might be due entirely to increased GABAergic inhibition.(ABSTRACT TRUNCATED AT 400 WORDS)

Presynaptic K-channel blockade counteracts the depressant effect of adenosine in olfactory cortex

Slices of isolated olfactory cortex from guinea-pig have been used to study the action of adenosine at synapses between axons of the lateral olfactory tract and neurons in the olfactory cortex. Adenosine depressed the excitatory postsynaptic potential, and, with paired or multiple stimuli, the reduced excitatory postsynaptic potentials in adenosine showed more synaptic facilitation. Very small excitatory postsynaptic potentials which were estimated not to be affected by postsynaptic membrane conductance changes were highly sensitive to adenosine. Both observations indicate a presynaptic action of adenosine. To test whether a conductance increase to potassium ions mediated adenosine action, the K-channel blockers, 3,4-diaminopyridine (1-100 mumol/l) or 4-aminopyridine (100-500 mumol/l) were applied or Cs partially substituted for K. These substances reduced or prevented adenosine from having its depressant effect on synaptic transmission. These particular K-channel blockers also prolonged the action potential propagating along the lateral olfactory tract. When the increased excitability was counteracted by high Mg or low concentrations of tetrodotoxin, 3,4-diaminopyridine still blocked adenosine action. UO2 ions prolonged the lateral olfactory tract action potential without blockade of K-conductance, but still supported an adenosine depression of the excitatory postsynaptic potential. Veratridine also supported the adenosine depression. These observations suggest that the action of 3,4-diaminopyridine on adenosine was not solely the result of increased tissue excitability. In contrast, tetraethylammonium (20 mmol/l), Ba (0.5-4 mmol/l) or Rb replacement for K had a negligible effect on the duration of the presynaptic action potential and had no effect on the depressant action of adenosine. These data are compatible with the idea that adenosine enhances an aminopyridine-sensitive potassium conductance in nerve terminals and changes in Ca influx are consequential to this.

Role of adenine compounds in autonomic neurotransmission

Clearly adenine compounds exert numerous effects throughout the autonomic nervous system. The responses of various peripheral tissues to purines are summarized in Table 2. The evidence supporting a possible excitatory neurotransmitter function for ATP is very good in the vas deferens and good in both the bladder detrusor and certain blood vessels. ATP may also be an excitatory neurotransmitter in the colon, hepatocytes and frog atrium. These responses appear to be mediated by P2x-purinoceptors. There is good evidence supporting a role for ATP as an inhibitory neurotransmitter in the taenia coli and duodenum, and some support in the anal sphincter and possibly the rabbit portal vein; these responses appear to be mediated by P2y-purinoceptors. There is good evidence against ATP being an inhibitory neurotransmitter in the stomach fundic muscle and ileum. ATP (or more likely its metabolite adenosine) may act as an inhibitory neurotransmitter by interacting with postsynaptic P1-purinoceptors in cultured sympathetic neurones and also in the parasympathetic vesicle ganglion of the cat. It seems likely that ATP released from heart, platelets or vascular endothelium could be an endogenous relaxant of blood vessels through its actions on the endothelium. Although the addition of exogenous adenosine affects many tissues, evidence supporting modulatory functions for endogenous extracellular adenosine has only been clearly demonstrated in the ileum, gallbladder, vas deferens, fallopian tubes, kidney, blood vessels, carotid sinus, heart and adipose tissue. Both ATP and adenosine, released during periods of hypoxia or ischemia, could exert negative inotropic, chronotropic and dromotropic actions in the heart. In many cases, the potential sources of extracellular purines have not been established. This is particularly important when attempting to establish a neurotransmitter function for ATP in a tissue. For instance, the one outstanding piece of evidence required to confirm that ATP is an excitatory neurotransmitter released from sympathetic nerves in blood vessels is the unequivocal demonstration that it is, in fact, released from the sympathetic nerves when they are stimulated. To date, only the release of radiolabeled metabolites of ATP, possibly from post- rather than presynaptic sites, has been detected. Studies of the release of ATP are complicated by its rapid degradation extracellularly by ecto-ATPase. Unfortunately, there are no specific inhibitors of ecto-ATPase available at present, but one hopes that a suitable inhibitor will be developed shortly.(ABSTRACT TRUNCATED AT 400 WORDS)

A subpopulation of preganglionic parasympathetic neurons in the rat contain adenosine deaminase

Immunohistochemical staining and retrograde fluorescent tracing techniques were used to demonstrate the presence of adenosine deaminase in preganglionic parasympathetic neurons. Both brainstem and sacral spinal cord parasympathetic nuclei were found to contain a subpopulation of neurons immunoreactive for adenosine deaminase. Immunostaining of preganglionic neurons in brainstem was restricted to a group of cells which were shown by retrograde tracing with Fast Blue to project exclusively to the sphenopalatine ganglion. This group was defined as the lacrimo-nasopalatine parasympathetic nucleus. Neurons in all other cranial preganglionic centers were devoid of adenosine deaminase immunoreactivity. In spinal cord adenosine deaminase-immunoreactive neurons were found in the intermediolateral gray matter in the region of the sacral parasympathetic nucleus. Injections of Fast Blue into the pelvic ganglion labeled large numbers of neurons in this nucleus, only some of which contained adenosine deaminase. The majority of neurons immunoreactive for adenosine deaminase were also shown to be immunoreactive for choline acetyltransferase in both brainstem and sacral parasympathetic nuclei. The present results show that a subclass of preganglionic parasympathetic neurons are among the few structures in the central nervous system that express what appear to be high levels of adenosine deaminase. This observation together with evidence suggesting that purines serve as neurotransmitters in some sacral parasympathetic neurons supports the notion that adenosine deaminase may constitute a marker for adenine nucleoside and/or nucleotide neurotransmission.

An analysis of the adenosine receptors responsible for modulation of an excitatory acetylcholine response on an identified Helix neuron

1. Electrophysiological recordings were made from an identified neuron, F1, in the isolated suboesophageal ganglionic mass of Helix aspersa. 2. Bath applied adenosine (AD) (60-600 nM) depressed the depolarisation induced in the cell F1 by bath or iontophoretically applied acetylcholine (ACh). L-Phenylisopropyladenosine (L-PIA) also depressed the ACh response but NECA had no depressive effect. This effect of AD or L-PIA is inhibited by 12 microM 8-phenyltheophylline and is believed to be mediated by an A1 receptor. 3. If the proportion of the excitatory ACh response that was carried by calcium ions was increased, the percentage depression of this modified response by AD was significantly greater. 4. There was a residual current evoked by ACh in high calcium/sodium free Ringer. This ACh induced current was antagonized by 3 mM cobalt or 50 microM verapamil suggesting that it was calcium mediated. This residual current was also completely abolished by 0.6 microM AD. 5. Lower bath concentrations of AD (0.6-6 nM) and L-PIA than caused the depression of the ACh response and also adenosine triphosphate (ATP) (0.7 microM) and alpha,beta-methylene ATP (0.6 microM), enhanced the ACh D response. The relative potencies of AD and its two analogues 5'-N-ethylcarboxamideadenosine (NECA) and L-PIA in causing this enhancement of the ACh response were: NECA greater than AD greater than L-PIA. This is the potency ranking described for an A2 receptor.

Effects of synaptic activity on the metabolism and release of purines in the rat superior cervical ganglion

The release of radioactive metabolites from isolated rat superior cervical ganglia was measured under various conditions following preloading with 3H-adenosine. The 3H label was recovered primarily in the adenosine metabolites, ATP, ADP, AMP, IMP, and inosine, rather than in adenosine itself. Increased release was evoked by preganglionic stimulation or by exposure to a high-K+ medium, whereas in a low-Ca2+-high-Mg2+ medium, both spontaneous release and evoked release of most metabolites were inhibited. Exposure of the ganglion to an atmosphere of N2 also increased the release of most labeled metabolites, but this release was not substantially affected by a low-Ca2+ medium. The fluorescent derivatives of the endogenous adenine-containing compounds present in the ganglion were prepared from homogenates and separated by high-performance liquid chromatography (HPLC). By the end of the testing period (6 hr), levels of ATP in the isolated ganglia had dropped to 10-20% of the initial values, while levels of ADP, AMP, and adenosine increased. There was little difference in these values between nonstimulated ganglia and those exposed to N2 or to a high-K+ medium.

Uptake and metabolism of [3H]adenosine by Aplysia ganglia and by individual neurons

[3H]Adenosine was taken up and metabolized by isolated ganglia of the marine mollusc Aplysia californica. After 2 h, most of the radioactivity was recovered as metabolites, including ATP, ADP, and AMP, as well as the deaminated products, inosine, hypoxanthine, and uric acid. Little remained in the form of adenosine. These pathways were not uniformly distributed among various tissue elements. In most individual neurons, inosine and its breakdown products were the principal metabolites of [3H]adenosine, whereas ATP and other nucleotides predominated in the connective tissue sheath. Endogenous levels of ATP, ADP, AMP, and adenosine in ganglia, sheath, and individual neurons were also determined using a fluorimetric-HPLC assay. The concentrations of the nucleotides were quite uniform in sheath and among the individual neurons assayed (1-5 pmol/microgram of protein); however, concentrations of adenosine were considerably higher in neurons than in the sheath.

Calcium-dependent currents in cultured rat dorsal root ganglion neurones are inhibited by an adenosine analogue

Dorsal root ganglion neurones from 2-day-old rats were grown in dissociated culture. The effect of the adenosine analogue 2-chloroadenosine (2-CA) was investigated on action potential duration and on Ca2+ current (ICa) activation. 2-CA (0.5 microM) shortened both control action potentials and those prolonged by tetraethylammonium (TEA), Ba2+, or intracellular Cs+. This effect was prevented by two adenosine antagonists isobutylmethylxanthine (IBMX, 1-2 mM) and 8-phenyltheophylline (8-PT, 2.5 microM). The inward current, ICa, recorded using the 'whole-cell' patch-clamp technique in medium containing 2.5 microM-tetrodotoxin, 25 mM-TEA and 2.5 mM-Ba2+ was reduced by 2-CA (0.05 microM). The activation of ICa was decreased, but its reversal potential was unchanged. The effect of 2-CA was antagonized by IBMX (1 mM) and 8-PT (1 microM). 2-CA also reduced the large inward tail currents which occurred at the termination of the depolarizing voltage step command in a proportion of neurones. Brief application of 2-CA (0.05 microM) did not affect the inward holding current required to maintain the cells at -80 mV. In the presence of TTX (2.5 microM) and Ca2+ (5 mM), 2-CA decreased the activation of outward K+ currents caused by 5 s depolarizing voltage commands from -80 mV or -40 mV. The GABAB agonist (-)-baclofen (50-100 microM) also shortened the action potential duration and reduced ICa. 8-PT (1 microM) did not prevent the effect of baclofen on ICa. It is concluded that in cultured rat dorsal root ganglion neurones 2-CA reduces ICa activation by a direct effect on an A1 adenosine receptor.

A new class of adenosine receptors in brain. Characterization by 2-chloro[3H]adenosine binding

Micromolar concentrations of adenosine and its analogs have profound depressant effects on neuronal firing and synaptic transmission in many brain areas. Using the adenosine agonist 2-chloro[3H]adenosine (Cl[3H]Ado), we have identified a distinct class of micromolar-affinity adenosine binding sites in rat forebrain membranes. Specific Cl[3H]Ado binding was reversible and saturable with an apparent KD of 9.1 microM and a Bmax of 61 pmoles/mg protein. The present studies were conducted using washed brain membrane fractions not treated with adenosine deaminase. Specific Cl[3H]Ado binding under these conditions was insensitive to (-)-N6-(R-phenylisopropyl)adenosine ((-)PIA) and treatment with 3 mM N-ethylmaleimide, unlike high-affinity A1 adenosine receptor binding. Treatment of membranes with adenosine deaminase revealed an additional population of binding sites sensitive to (-)PIA. Inhibition of Cl[3H]Ado binding by adenosine analogs exhibited an order of potency ClAdo greater than 5'-N-ethylcarboxamide adenosine (NECA) greater than (-)PIA which differs from that of both A1 and A2 adenosine receptors. The potent A1 and A2 receptor antagonist 8-phenyltheophylline had no significant effect on binding up to 10 microM. Specific binding, however, was inhibited by the adenosine antagonists 8(p-sulfophenyl)theophylline, isobutylmethylxanthine, theophylline, and caffeine. Micromolar Cl[3H]Ado binding was highly selective for adenosine agonists and antagonists. These results suggest that the micromolar-affinity Cl[3H]Ado binding sites may represent a novel central purinergic receptor, distinct from the A1 and A2 adenosine receptors involved in the regulation of adenylate cyclase.

Cyclic adenosine 3',5'-monophosphate mediates beta-receptor actions of noradrenaline in rat hippocampal pyramidal cells

Intracellular recordings were made from rat hippocampal CA1 pyramidal neurones in the in vitro slice preparation to study the actions of cyclic adenosine 3',5'-monophosphate (cyclic AMP). Application of the membrane permeant analogue of cyclic AMP, 8-Br cyclic AMP caused a small depolarization of the resting membrane potential accompanied by an increase in membrane input resistance and also reduced the amplitude of depolarization-evoked calcium-activated potassium after-hyperpolarizations (a.h.p.s.). 8-Br cyclic AMP reduced calcium-activated a.h.p.s but did not reduce calcium action potentials in these cells. 8-Br cyclic AMP also reduced action potential frequency accommodation. The effects of 8-Br cyclic AMP were not mimicked by cyclic AMP applied extracellularly but were imitated by intracellular injections of cyclic AMP. Activation of the endogenous adenylate cyclase of pyramidal cells either by intracellular injection of the stable guanosine 5'-triphosphate (GTP) analogue guanylyl-imidodiphosphate, or by extracellular application of forskolin, reduced the a.h.p. and accommodation. Reducing phosphodiesterase activity with application of either 3-isobutyl-1-methylxanthine or Ro20-1724 reduced the amplitude of the a.h.p. and potentiated the a.h.p.-blocking action of noradrenaline. Reducing adenylate cyclase activity by application of SQ22,536 slightly increased the amplitude of the (a.h.p.) and reduced the a.h.p.-blocking action of noradrenaline. We conclude that the beta-receptor actions of NA on hippocampal CA1 pyramidal cells are mediated by intracellularly produced cyclic AMP.

Adenosine inhibits locus coeruleus neurons: an intracellular study in a rat brain slice preparation

Intracellular recording was used to study the effect of adenosine (3-100 microM) on rat locus coeruleus (LC) neurons in a brain slice preparation. Bath application of adenosine (100 microM) reduced the rate of spontaneous firing in 88% of LC neurons. In some LC neurons adenosine also caused membrane hyperpolarization (2-10 mV) and reductions in input resistance of 9-24%. Adenosine effects were dose-dependent and antagonized by the adenosine receptor antagonist theophylline.

Adenosine agonists reduce voltage-dependent calcium conductance of mouse sensory neurones in cell culture

Adenosine and several of its analogues produced a concentration-dependent shortening of calcium-dependent action potential (c.a.p.) duration of mouse dorsal root ganglion (d.r.g.) neurones in dissociated cell culture. The following rank order of potency was obtained: N6-(L-phenylisopropyl)adenosine greater than N6-(D-phenylisopropyl)adenosine greater than N6-cyclohexyladenosine greater than 2-chloroadenosine much greater than 1-methylisoguanosine greater than adenosine. Effects of adenosine agonists on c.a.p. duration were blocked by methylxanthine adenosine antagonists. Adenosine monophosphate (AMP) and cyclic AMP shortened c.a.p.s in d.r.g. neurones, while ATP also depolarized cells. Voltage-clamp analysis revealed that the effect arose from reduction of a voltage-dependent calcium conductance. Adenosine agonists reduced depolarization-evoked inward currents but did not alter membrane conductance following blockade of calcium channels by cadmium. Additionally, adenosine reduced the instantaneous current-voltage slope (chord conductance) during step commands that produced maximal activation of voltage-dependent calcium conductance. If effects of adenosine on neuronal somata and synaptic terminals are similar, adenosine agonists may inhibit neurotransmitter release in the central nervous system by inhibiting a voltage-dependent calcium conductance. Since effects of adenosine agonists did not correspond with their relative potencies as modulators of adenylate cyclase activity or inhibitors of neurotransmitter release in peripheral tissues, a novel adenosine receptor may be involved in regulation of this conductance.

Effects of aminophylline and 2-chloroadenosine on seizures produced by pilocarpine in rats: morphological and electroencephalographic correlates

The effects of 2-chloroadenosine, aminophylline, bicuculline, beta-carboline-3-carboxylic acid methylester and Ro 15-1788 on seizures produced by pilocarpine were examined in rats. In animals pretreated with aminophylline at doses of 25-100 mg/kg, non-convulsant dose of pilocarpine, 100 mg/kg, resulted in severe motor limbic seizures, which rapidly developed into the status epilepticus. Electroencephalographic monitoring showed progressive evolution of seizure activity with initial high-voltage fast activity followed by high-voltage spiking and electrographic seizures. Morphological analysis of frontal forebrain sections with light microscopy demonstrated widespread damage to the hippocampal formation, thalamus, amygdala, olfactory cortex, substantia nigra and neocortex. Bicuculline, 2 mg/kg, beta-carboline-3-carboxylic acid methylester, 5 mg/kg, and Ro 15-1788, 50 mg/kg, did not augment seizures produced by pilocarpine, 100 mg/kg. 2-Chloroadenosine, 5 and 10 mg/kg, blocked the appearance of behavioral and electrographic seizures produced by pilocarpine, 380 mg/kg, and prevented the occurrence of brain damage. The results indicate that purinergic mechanisms are involved in the buildup of pilocarpine-induced convulsions and seizure-related brain damage in rats.

Cobalt ion enhancement of 2-chloro[3H]adenosine binding to a novel class of adenosine receptors in brain: antagonism by calcium

We have recently reported the identification of a novel class of micromolar-affinity adenosine binding sites in rat brain membranes using the adenosine agonist 2-chloro[3H]adenosine (C1[3H]Ado). These binding sites are distinguishable from the A1 and A2 adenosine receptors by a number of pharmacological criteria, and we have designated this new class of binding sites as the A3 adenosine binding sites. In the present study, the effects of a wide range of divalent and trivalent cations on micromolar C1[3H]Ado binding to brain membranes were examined. Co2+, Ni2+ and La3+ markedly stimulated specific C1[3H]Ado binding by 45-150% above control when tested at concentrations of 1-10 mM. Ca2+ had no significant effect on binding except at high concentrations where it depressed binding slightly. Ca2+, however, completely prevented the stimulation of C1[3H]Ado binding by Co2+. These findings further distinguish the A3 class of adenosine binding sites from the previously characterized adenosine receptors and suggest that the A3 binding sites are associated with calcium systems in brain.

Adenosine-activated potassium conductance in cultured striatal neurons

We have examined the effect of adenosine on the membrane properties of cultured embryonic mouse striatal neurons using patch electrode techniques. Adenosine at 50 microM effectively blocked spontaneous action potential activity. Adenosine or 2-chloroadenosine caused a slow hyperpolarization of the membrane potential and, under voltage clamp, an outward current that was blocked by 1 mM theophylline. ATP also caused a hyperpolarization that was slower and weaker than the adenosine response and could be blocked by 1 mM theophylline. The current induced by adenosine appears to be carried by potassium since (i) an inward current was generated by adenosine when the cells were internally perfused with cesium salts and (ii) the reversal potential of the outward current shifted 57 mV with a 10-fold change in extracellular potassium concentration. The adenosine response is voltage dependent in that the current evoked by adenosine is reduced at holding potentials more positive than -55 mV, despite a larger driving force. Though calcium influx is not required for adenosine to activate the potassium conductance, some components of the cytosol may be essential, since the response is lost during intracellular perfusion.