Inhibitory effect of adenosine on transmission in sympathetic ganglia

Adenosine A1 receptors reduce release from excitatory but not inhibitory synaptic inputs onto lateral horn neurons

Although adenosine is an important neuromodulator in the CNS, its role in modulating sympathetic outflow at the level of the spinal cord has not been studied. Because very little is known about adenosine A1 receptors (A1Rs) in the spinal cord, we determined their location and role with particular reference to the control of sympathetic preganglionic activity and interneuronal activity in the rat. High levels of immunoreactivity for A1Rs were observed throughout the spinal cord. Immunostaining was dense in the intermediolateral cell column (IML) and intercalated nucleus, regions containing retrogradely labeled sympathetic preganglionic neurons (SPNs). Electron microscopy revealed A1R immunoreactivity (A1R-IR) within presynaptic terminals and (to a lesser extent) postsynaptic structures in the IML, as well as the luminal membrane of endothelial cells lining capillaries. Using double-labeling techniques, some presynaptic terminals were observed to synapse onto SPNs. To investigate the effects of activating these A1Rs, visualized whole-cell patch-clamp recordings were made from electrophysiologically and morphologically identified SPNs and interneurons. Applications of the A1R agonist cyclopentyladenosine (CPA) reduced the amplitude of EPSPs elicited by stimulation of the lateral funiculus, an effect blocked by the A1R antagonist 8-cyclopentyl-1,3-dipropylxanthine. These effects were attributable to adenosine acting at a presynaptic site because CPA application increased the paired-pulse ratio. CPA did not affect evoked IPSPs. These data show that activating A1Rs reduces fast excitatory, but not inhibitory, transmission onto SPNs and interneurons in the IML and that A1Rs may play a protective role on neurons involved in the control of sympathetic outflow.

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.

Calcium in sympathetic boutons of rat superior cervical ganglion during facilitation, augmentation and potentiation

The sympathetic preganglionic nerve terminals of the rat superior cervical ganglion were loaded with the calcium indicator oregon green 488 BAPTA-1 to measure the change in calcium concentration in the terminal boutons, (delta[Ca2+]b) following short (1 or 5 impulses) and long (200 impulses) trains at 30 Hz. The delta[Ca2+]b after a single action potential or a short train declined in two phases: a fast phase with a time constant of 530+/-30 ms and a moderate phase with a time constant of 4.0+/-0.2 s. The delta[Ca2+]b following a long train eventually declined with a time constant of 127+/-34 s (slow phase). The addition of either omega-agatoxin TK (100 nM), omega-conotoxin GVIA (100 nM) or nifedipine (20 microM) to block P-type, N-type or L-type calcium channels respectively showed that the rise in delta[Ca2+ ]b in boutons was predominantly mediated by an influx of calcium through P-type (53+/-7%) and N-type (46+/-4%) calcium channels. Experiments with caffeine, ryanodine and thapsigargin indicate that intracellular caffeine-sensitive calcium stores have a small but statistically significant effect on the fast and moderate phases. The mitochondrial uncoupler carbonyl cyanide m-chlorophenyl hydrazone (CCCP; 2 microM) significantly decreased the amplitude of the slow phase of delta[Ca2+]b relaxation, and sped its time course, suggesting that mitochondria normally dump calcium during this phase. Adenosine reduced the amplitude of delta[Ca2+]b in response to single action potentials by 30+/-6%, suggesting that adenosine-mediated autoinhibition in these boutons reduces Ca2+ influx. Spontaneous increases in delta[Ca2+]b demonstrated Ca2+ coupling between adjacent boutons. The delta[Ca2+]b kinetics are compared with F2 facilitation, augmentation and post-tetanic potentiation.

Cyclic AMP antagonizes adenosine-induced inhibition of ganglionic transmission

The mechanism of adenosine-induced inhibition of ganglionic transmission was investigated in the isolated superior cervical ganglion (SCG) of the rat. The inhibitory effect of adenosine on the postganglionic compound action potential (CAP) was antagonized by pretreatment of ganglia with forskolin, isoproterenol (IPNE), arginine vasopressin (AVP), or papaverine, all of which are known to increase tissue cAMP level by different mechanisms. Furthermore, pretreatment of ganglia with the adenylate cyclase inhibitor SQ 22, 536, or the phosphodiesterase activator imidazole reversed the effects of IPNE and forskolin. Pretreatment with 8-bromo-cAMP, resulted in a marked antagonism of the adenosine-induced inhibition. By themselves, none of these drugs had any significant effect on the CAP. Adenosine slightly but significantly decreased the basal level of cAMP in untreated ganglia. Formation of cAMP induced by IPNE was markedly reduced by adenosine. This was largely reversed in the presence of the adenosine A1 receptor antagonist 8-cyclopentyl-1, 3-dipropylxanthine (DPCPX) but not the A2 receptor antagonist 3, 7-dimethyl-1-propargylxanthine (DPMX). We conclude that the inhibition of ganglionic transmission by adenosine involves reduction of cAMP formation through activation of A1 receptors.

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.

Evidence for extracellular deamination of adenosine in the rat heart

1. In rat heart perfused with adenosine (10(-6) M), dilazep (10(-4) M) inhibited incorporation of adenosine into nucleotides (an index of nucleoside transport and phosphorylation) to a greater extent (70%) than metabolism to inosine and uric acid (40%) and actually increased the recovery of inosine to 30% of the adenosine infused. 2. Extrapolating for complete inhibition of transport suggested that 60% of adenosine metabolism was intracellular and 40% extracellular. 3. Static incubations of atria also gave an estimate for extracellular metabolism of 40%. 4. Adenosine deaminase was localised by immunocytochemistry to the extracellular surface of endothelial cells of small coronary arteries. 5. Extracellular deamination may explain the lack of effect of nucleoside transport inhibitors on responses to adenosine in rat heart.

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.

Effect of adenosine, adenosine derivatives, and caffeine on acetylcholine release from brain synaptosomes: interaction with muscarinic autoregulatory mechanisms

Synaptosomes, prepared from rat cerebral cortex and hippocampus, were preincubated with [methyl-3H]choline. The effect of adenosine, cyclohexyladenosine, N-ethylcarboxamide adenosine, 2'-deoxyadenosine, and oxotremorine on K+-evoked 3H efflux was investigated. High-voltage electrophoretic separation showed that in the presence of physostigmine, the K+-evoked 3H efflux from hippocampal synaptosomes was 90% [3H]acetylcholine and 10% [3H]choline. Adenosine (30 microM) and oxotremorine (100 microM) both decreased [3H]acetylcholine release from hippocampal synaptosomes. The effect was inversely proportional to the KCl concentration and disappeared at a KCl concentration of 50 mM. Cyclohexyladenosine was approximately 3,000 times more active than adenosine, whereas N-ethylcarboxamide adenosine and 2'-deoxyadenosine were inactive. This indicates that A1 adenosine receptors were involved in the inhibitory effect. Caffeine antagonized the adenosine effect, and at a concentration of 100 microM, it stimulated [3H]acetylcholine efflux. The inhibitory effect of oxotremorine was as great in cortical as in hippocampal synaptosomes. In contrast, adenosine was much less active in cortical than in hippocampal synaptosomes. When inhibitory concentrations of adenosine and oxotremorine were added together into the incubation medium, the effect of adenosine on [3H]acetylcholine release was consistently reduced. An interaction between muscarinic and A1 adenosine presynaptic receptors at a common site modulating acetylcholine release can be assumed.