The role of cyclic nucleotides in the CNS

From cAMP to adenosine: an illuminating shift of focus

In a remarkable career, straddling five decades, John Phillis pursued with fierce determination and exceptional energy the main goal of his scientific life, to throw light on the chemical agents that control brain function. Starting in Australia, he settled in North America, first in Canada, then in the USA, where his long tenure at Wayne State brought his career to its culmination.

Methods for studying neurotransmitter transduction mechanisms

This review describes the methodologies used to study the transduction mechanisms that are activated in excitable cells by G-protein-coupled agonists. In view of the complexity of second-messenger systems, it is no longer relevant to ask, "What is the transduction mechanism involved in the action of a given neuromodulator?" because, in many cases, a variety of transduction mechanisms and physiological responses are invoked following receptor activation. This means that a single aspect of the physiological response must be selected for study in order to address the question of transduction mechanism. This review is therefore concerned with a description the use of patch- and voltage-clamp procedures to study transduction mechanism because they are designed to isolate one aspect of the physiological response: the change in activity of a single type of membrane ion channel.

Age-dependent changes in second messenger and rolipram receptor systems in the gerbil brain

Age-related alterations in binding sites of major second messengers and a selective adenosine 3',5'-cyclic monophosphate (cyclic-AMP) phosphodiesterase (PDE) in the gerbil brain were analysed by receptor autoradiography. [3H]Phorbol 12,13-dibutyrate (PDBu), [3H]inositol 1,4,5-trisphosphate (IP3), [3H]forskolin, [3H]cyclic-AMP, and [3H]rolipram were used to label protein kinase C (PKC), IP3 receptor, adenylate cyclase, cyclic-AMP dependent protein kinase (PKA), and Ca2+/calmodulin-independent cyclic-AMP PDE, respectively. In middle-aged gerbils (16 months old), [3H]PDBu binding was significantly reduced in the hippocampal CA1 sector, thalamus, substantia nigra, and cerebellum, compared with young animals (1 month old). [3H]IP3 binding revealed significant elevations in the nucleus accumbens, hippocampal CA1 sector, dentate gyrus, and a significant reduction in cerebellum of middle-aged gerbils. [3H]Forskolin binding in middle-aged animals was significantly increased in the nucleus accumbens and hilus of dentate gyrus, but was diminished in the substantia nigra and cerebellum. On the other hand, in middle-aged animals, [3H]cyclic-AMP binding revealed a significant elevation only in the hippocampal CA3 sector, whereas [3H]rolipram binding showed a significant reduction in the thalamus and cerebellum. Thus, the age-related alteration in these binding sites showed different patterns among various brain regions in middle-aged gerbils indicating that the binding sites of PKC, IP3, and adenylate cyclase are more markedly affected by aging than those of PKA and cyclic-AMP PDE and that the hippocampus and cerebellum are more susceptible to these aging processes than other brain regions. The findings suggest that intracellular signal transduction is affected at an early stage of senescence and this may lead to neurological deficits.

Cyclic adenosine 3'5'-monophosphate potentiates excitatory amino acid and synaptic responses of rat spinal dorsal horn neurons

Intracellular recordings were made from rat dorsal horn neurons in the in vitro slice preparation to study the actions of cyclic adenosine 3',5'-monophosphate (cyclic AMP). In the presence of TTX, bath application of the membrane permeable analogue of cyclic AMP, 8-Br cyclic AMP (25-100 microM) caused a small depolarization of the resting membrane potential accompanied by a variable change in membrane input resistance. In addition, 8-Br cyclic AMP caused a long-lasting increase in the spontaneous synaptic activity and the amplitude of presumed monosynaptic excitatory postsynaptic potentials evoked in the substantia gelatinosa neurons by orthodromic stimulation of a lumbar dorsal root. When the fast voltage-sensitive Na conductance was blocked by TTX, 8-Br cyclic AMP enhanced in a reversible manner, the depolarizing responses of a proportion of dorsal horn neurons to N-methyl-D-aspartic acid (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), quisqualic acid (QA) and kainic acid (KA). The effects of 8-Br cyclic AMP on the resting membrane potential and the NMDA response of dorsal horn neurons were mimicked by reducing phosphodiesterase activity with bath application of 3-isobutyl-1-methylxanthine, but not by cyclic AMP applied extracellularly. Moreover, we have found that intracellular application of a protein inhibitor of cyclic AMP-dependent protein kinase (PKI) into dorsal horn neurons prevents the 8-Br cyclic AMP-induced potentiation of the NMDA response of these cells. These results suggest that in the rat spinal dorsal horn the activation of the adenylate cyclase-cyclic AMP-dependent protein kinase system may be involved in the enhancement of the sensitivity of postsynaptic excitatory amino acid (NMDA, AMPA, KA) receptors and modulation of primary afferent neurotransmission, including nociception.

Regional difference in responsiveness of adenosine-sensitive cyclic AMP-generating systems in chronic epileptic cerebral cortex of the rat

Cyclic AMP accumulation in brain slices incubated with adenosine or the adenosine analogue 2-chloroadenosine was examined in different areas of rat cerebral cortex following a unilateral injection of FeCl2 solution into the sensorimotor cortex to induce chronic epileptic activity. In the epileptic cortex, cyclic AMP accumulation in cortical slices was elicited three- to 11-fold by adenosine. The elicitation by adenosine of cyclic AMP accumulation was markedly inhibited by the adenosine antagonist 8-phenyltheophylline. In anterior cortical areas of rats in which the appearance of electrographic isolated spikes was dominant either ipsilateral or contralateral to the injection site 8 days or more after the injection, the adenosine-elicited accumulation of cyclic AMP was greater on the side of dominant spike activity than on the other. In anterior cortical areas of rats showing nearly equal spike activity on the two sides 19 days or more after the injection, the cyclic AMP accumulation was greater on the side ipsilateral to the injection site than on the other. In anterior and posterior cortical areas of rats showing spike-and-wave complexes and isolated spikes 1 month or more after the injection, the cyclic AMP accumulation was greater on the ipsilateral side than on the other. Similar regional differences in the adenosine-elicited accumulation of cyclic AMP were detected in the presence of the adenosine uptake inhibitor dipyridamole or the phosphodiesterase inhibitor DL-4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (Ro 20-1724). The cyclic AMP accumulation was elicited five- to 17-fold by 2-chloroadenosine, in which case the elicitation was markedly inhibited by 8-phenyltheophylline. Regional differences in the 2-chloroadenosine-elicited accumulation of cyclic AMP were similar to those with adenosine and were detected in the presence of Ro 20-1724 or adenosine deaminase. The regional differences which correlated with the electrographic discharge patterns were due mainly to persistent changes in cyclic AMP accumulation on the primary epileptic side. These results suggest that alterations in adenosine-sensitive cyclic AMP generation in the cortex are associated with the neurochemical process leading to chronic iron-induced epilepsy.

Comparative assessment of enprofylline and theophylline for chronic obstructive airways disease in the elderly

Enprofylline, a recently developed xanthine derivative, is a more potent bronchodilator than theophylline. This study compares the efficacy and safety of enprofylline with theophylline for chronic obstructive airways disease (COAD) in elderly subjects. The study was of a randomized double-blind parallel design and commenced with a 1-week reference period when oral bronchodilators were withdrawn. Patients were then treated with either enprofylline or theophylline 150 mg bd for 2 weeks (period 1) followed by 300 mg bd for a further 3 weeks (period 2). Patients recorded peak expiratory flow rate (PEFR) and adverse experiences, if any, in a diary, daily. Of 111 patients recruited for the study, 85 entered active treatment (theophylline, n = 44; enprofylline, n = 41). Mean age was 72 years and mean bronchodilator reversibility was 22%. Enprofylline increased mean morning PEFR by 11% (period 1) and 19% (period 2) whereas theophylline increased PEFR by 13% and 19%, respectively. From the enprofylline group 29% were withdrawn from the study due mainly to headache and nausea/vomiting and from the theophylline group 7% were withdrawn due mainly to nausea/vomiting. Mean plasma concentrations of enprofylline were 2.0 mg l-1 and 3.4 mg l-1, and with theophylline 5.4 mg l-1 and 10.0 mg l-1 at the end of periods 1 and 2, respectively. Enprofylline and theophylline produced similar improvements in lung functions and symptoms of chronic obstructive airways disease, but enprofylline was less well tolerated than theophylline.

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)

Characteristics of analgesia induced by adenosine triphosphate

Adenosine triphosphate (ATP) injected intravenously in mice was found to have dose-dependent analgesic activity in the hot plate and phenylquinone-induced stretching assays. ATP prolonged the hot plate latency (ED50 value of 1 (0.7-1.4) mg/kg) and inhibited phenylquinone-induced writhing (ED50 value of 0.4 (0.31-0.52) mg/kg). Low doses of ATP produced a potent antinociceptive effect without any significant depression of locomotor activity. Treatment of mice for either 4 days or 14 days with ATP did not result in development of physical dependence on or tolerance to ATP. The analgesic action of ATP was not antagonized by naloxone at 1 and 5 mg/kg. ATP analgesia was antagonized, in a dose-related fashion, by Ca++ ion injected intracerbroventricularly which may indicate that Ca++ plays a role in ATP-induced antinociception.

Actions of substance P on rat spinal dorsal horn neurones

The membrane actions of substance P (SP) and the effects on the Ca-dependent action potential of dorsal horn neurones have been investigated by means of intracellular recording techniques in the immature rat in vitro spinal cord slice preparation. Bath application of SP (2 X 10(-6) to 1 X 10(-5) M) induced a biphasic membrane response consisting of an initial hyperpolarization followed by a depolarization in about one-third of the cells examined. Initial hyperpolarization was not observed when synaptic activity was blocked by perfusing the slice with a tetrodotoxin-containing or low Ca, high Mg Ringer solution. This result is consistent with a presynaptic action of SP mediated through excitation of inhibitory interneurones. This interpretation was supported by recording of repetitive spontaneous inhibitory post-synaptic potential (i.p.s.p.)-like hyperpolarizing potentials during the initial hyperpolarization. When Co ions were used to block voltage-dependent Ca conductance and possible indirect presynaptic actions, SP induced only a small depolarization of membrane potential. It seems, therefore, that Ca conductance may have contributed to the depolarizing phase of the SP response, either through its mediation of synaptic transmission or through direct effects as a charge carrier for inward current. When tetrodotoxin was used, the SP-induced increase in neuronal input resistance was not modified, although depolarization was slightly diminished. In contrast, in medium containing tetrodotoxin and tetraethylammonium, the SP-depolarizing response was enhanced and accompanied by a small decrease in input resistance and firing of Ca spikes. These results suggest that SP-induced depolarization might be a consequence of a reduction in a voltage-dependent K conductance allowing Na and/or Ca conductances to dominate. SP modified the duration of Ca-dependent action potentials of dorsal horn neurones, the most consistent change being an initial dose-dependent and reversible decrease in the spike duration. The decrease in Ca spike duration was associated with a small reduction in the rate of rise and peak amplitude, and a significant parallel increase in dV/dt of the falling phase of the Ca spike. Our data indicate that the initial decrease in Ca spike duration was not due to the depolarizing action of SP, although shunting of the membrane resistance, either through presynaptic or post-synaptic mechanisms, has not been ruled out. Alternatively, these data are consistent with the possibility that SP shortens the duration of the Ca spike by decreasing a voltage-sensitive inward Ca current and/or augmenting an outward K current.(ABSTRACT TRUNCATED AT 400 WORDS)

Cyclic nucleotides and seizures in a hereditary model of epilepsy

The high seizure susceptibility in epileptic fowl is due to an autosomal recessive mutation. Cyclic AMP and cyclic GMP concentrations were determined in brains from two day old epileptic chicks (homozygotes) during an inter-ictal period as well as during and following a seizure evoked by stroboscopic stimulation. The data were compared to values obtained from non-epileptic carrier chicks (heterozygotes) sacrificed in an unstimulated state or subjected to the seizure evoking stimulus. During the inter-ictal state in epileptics no abnormalities were found in cyclic nucleotide concentrations indicating that the high seizure susceptibility is not related to abnormalities of these nucleotides. Although seizure activity in epileptics was associated with reduced cyclic AMP in the optic lobes this also occurred in carrier chicks subjected to the seizure evoking stimulus. The only significant changes in cyclic GMP levels, occurring as a result of seizures in epileptics, were an increase in cyclic GMP in the cerebral hemispheres during the seizure and a decrease in the optic lobes during the postictal period.

The effect of some drugs with purported antianoxic effect on veratridine-induced purine release from isolated rat hypothalamic synaptosomes

Several drugs claimed to protect against anoxic damage to the brain and/or to aleviate symptoms of senility were tested for their ability to influence purine overflow from hypothalamic synaptosomes. Theophylline did not influence the total purine release but tended to decrease the nucleoside and increase the nucleotide release. Cyclic nucleotide phosphodiesterase inhibitors were ineffective, whereas drugs that inhibit carrier mediated nucleoside transport inhibited veratridine-induced purine release. Some ergot derivatives, some vinca-analogues and the vasodilator ifenprodil also decreased stimulated purine release, but their effect could not be attributed to adenosine transport inhibition. The results suggest that drugs are reported to aleviate symptoms of anoxic damage or senility may inhibit the release of adenosine and related compounds in the central nervous system. Some possible reasons why a decreased purine release may protect against ischaemic brain damage are discussed.

Structure-activity studies on the potentiation of benzodiazepine receptor binding by ethylenediamine analogues and derivatives

The effect of ethylenediamine analogues on in vitro binding of [3H]-diazepam to crude cerebral cortical synaptosomal membranes in the rat was studied. Ethylenediamine significantly increased [3H]-diazepam binding to a maximum potentiation of 154% control (EC50 = 1.8 X 10(-4) M) and was the most active compound studied in terms of both potency and the maximum potentiation observed. Potentiation of [3H]-diazepam binding by ethylenediamine analogues is dependent on carbon-chain length, appears to require two terminal amino groups, and is not observed in the rigid analogues studied. Potentiation of [3H]-diazepam binding by ethylenediamine analogues is mediated largely by a change in receptor number and not receptor affinity. Results are discussed in terms of the possible nature of the ethylenediamine binding site.

Effect of intraventricular adenosine on food intake in rats

Previous studies have shown that peripherally administered purines suppress food intake in rats. In this study we show that central administration of adenosine, adenine and AMP potently suppressed food intake in rats. Intraperitoneal adenosine suppressed feeding at the 100 and 50 mg/kg dose whereas 100, 50 and 10 micrograms of intraventricular adenosine suppressed feeding after intracerebroventricular injection at 30 minutes and up to 120 minutes at the high doses. Inosine, 2-deoxyinosine, 7-methyl-inosine and 2-deoxyguanosine all failed to suppress food intake when given intraventricularly at the same doses used for adenosine, adenine and AMP. Adenosine, 10 micrograms ICV, also decreases water uptake. The effect of adenosine was specific for ingestive behaviors as it did not significantly decrease spontaneous movement or grooming. These results suggest that adenosine suppresses feeding via a central mechanism and that this suppressive effect is not dependent on deamination of adenosine to inosine. The central adenosine effect appears to work by a different mechanism to the satiety effect of peripherally administered inosine.

Antinociception following microinjection of dibutyryl cyclic nucleotides into the caudal reticular formation and periaqueductal gray of the rat brain

The tail flick, paw pinch, and hot plate tests were used to assess changes in nociceptive threshold following microinjection of dibutyryl derivatives of cyclic nucleotides into areas of the central nervous system previously shown to be involved in modulation of nociceptive threshold and mediation of morphine analgesia. An elevation in the nociceptive threshold was observed on all three tests following administration of 10 micrograms dibutyryl cyclic 3':5' adenosine monophosphate (db cAMP) into the caudal brainstem reticular formation (CRF) and periaqueductal gray (PAG). Two micrograms db cAMP produced the same magnitude of analgesia but had a shorter duration of action. Twenty micrograms dibutyryl cyclic 3':5' guanosine monophosphate (db cGMP) produced analgesia on all three tests following microinjection at CRF sites but not at PAG sites. These data indicate that morphine analgesia and the antinociception produced by cyclic nucleotides may involve, at least in part, common neuronal substrates. However, the observed capacity of db cAMP to elevate nociceptive threshold does not support the hypothesis that the mechanism of morphine's analgesic action involves inhibition of adenylate cyclase.

Adenosine metabolism in a rat hippocampal slice preparation: incorporation into S-adenosylhomocysteine

The incorporation of [14C]adenosine into various metabolites was studied in a hippocampal slice preparation in order to assess the extent of adenosine metabolism via synthesis of S-adenosylhomocysteine, a potent inhibitor of transmethylation reactions. Highest incorporation of 14C occurred into nucleotides, with only a few percent being recovered in inosine + hypoxanthine, S-adenosylhomocysteine, and the free adenosine pool. Labeling of S-adenosylhomocysteine did not significantly increase with higher concentrations of added adenosine despite greater accumulation of free [14C]adenosine in the tissue. Addition of L-homocysteine significantly increased the labelling of S-adenosylhomocysteine. The results indicate that S-adenosylhomocysteine synthesis is a minor pathway of adenosine metabolism in brain tissue under steady-state conditions. Further, changes in adenosine concentration, without a concomitant change in L-homocysteine availability, are unlikely to lead to a significant accumulation of S-adenosylhomocysteine. S-Adenosylhomocysteine is therefore not likely to play a significant role in mediating the biological effects of adenosine in the CNS via inhibition of transmethylations.

Evidence for a cyclic GMP mechanism in the mediation of hippocampal post-tetanic potentiation

Correlative electrophysiological and biochemical techniques were used to study hippocampal post-tetanic potentiation in acutely prepared rabbits following stimulation of the medial septal region and contralateral hippocampal field CA3. The results indicate that calcium ions, guanosine-3':5'-monophosphate, and phosphodiesterase inhibitors selectively enhanced the duration of post-tetanic potentiation. Potassium ions selectively enhanced tetanic potentiation. Adenosine-3':5'-cyclic monophosphate suppressed both tetanic and post-tetanic potentiation. The electrophysiological findings were supported by biochemical observations that guanosine-3':5'-monophosphate levels show marked increases following tetanic stimulation of either the medial septal region or contralateral hippocampal field CA3 pathways. The data suggest that a calcium-dependent process in the presence of a guanosine-3':5'-monophosphate mechanism promotes periods of hippocampal pyramidal cell hyperexcitability. The mechanism by which the cyclic nucleotide alters potentiation does not appear to be coupled to a single receptor variety.

Ethylenediamine and GABA potentiation of [3H]diazepam binding to benzodiazepine receptors in rat cerebral cortex

Specific binding of [3H]diazepam at a free concentration of 2 nM was found to be maximally potentiated by 117% in Tris-HCl buffer and 160% in Tris-citrate buffer by ethylenediamine (EDA), but only at relatively high concentrations of EDA (ED50 = 5 X 10(-5) M), although this potentiation was susceptible to a low dose (6 microM) of bicuculline. Dose-response curves show that EDA differs from GABA with respect to both potency and efficacy. In additivity experiments no evidence was found that EDA could act as a partial agonist at GABA receptors, and it was concluded that EDA and GABA apparently do not potentiate [3H]diazepam binding by acting on the same receptor. Scatchard analysis lends support to this hypothesis, indicating that the potentiation of [3H]diazepam binding by 3.16 X 10(-3) M EDA is due to an increase in receptor number (from 930 to 1170 fmol/mg protein) and not receptor affinity (remaining constant about 20 nM). Subsequent studies showed the potentiation to be reversible. It is concluded that EDA can act on the GABA-benzodiazepine receptor ionophore complex but that this is probably not a direct action on the GABA receptor. It is suggested that EDA can be used to differentiate GABA receptors linked to benzodiazepine receptors from those not so linked.

Ionic mechanisms of cholinergic excitation in mammalian hippocampal pyramidal cells

Intracellular recordings from CA1 hippocampal pyramidal neurons were obtained using the in vitro hippocampal slice preparation. Responses to ACh were monitored in the presence of blockers of voltage-dependent conductances including Mn2+, TTX and Ba2+. When Mn2+ was used to block voltage-dependent Ca conductance and possible indirect presynaptic cholinergic actions, ACh still induced a significant voltage-sensitive increase in apparent input resistance (Ra) (29%), but only an insignificant depolarization of membrane potential (Vm). When both voltage-dependent Ca and Na conductances were blocked by application of Mn2+ and TTX, respectively, ACh produced voltage-dependent increases in Ra (31%) without significant depolarization. In solutions containing TTX alone, ACh produced voltage-sensitive increases in Ra (32%) as well as a significant depolarization (6.2 +/- 3.1 mV (S.D.)). ACh transiently blocked the conductance increase which followed presumed Ca spikes, suggesting an action on the Ca-activated K-dependent conductance. The effects of Ba2+ application (100-200 microM) on Ra mimicked those of ACh. When ACh was applied to neurons in the presence of Ba2+, Ra remained unchanged, although Vm depolarization of 5-15 mV was still seen. The data indicate that ACh decreases both a voltage-dependent K conductance (distinct from that of the delayed rectifier) and a Ca-activated K conductance. Muscarinic cholinergic depolarization occurs as a result of blockade of K conductance, and is mediated by voltage-dependent Ca and Na conductances, and perhaps by presynaptic actions.

Release of norepinephrine and dopamine from brain vesicular preparations: effects of adenosine analogues

1. Adenosine analogues inhibit calcium-dependent K+-evoked release of [3H]norepinephrine from guinea pig cerebral cortical and hippocampal vesicular preparations. Inhibition requires high concentrations (100 microM) of the adenosine analogues and is abolished in the presence of high concentrations (2 mM) of calcium ions. The inhibitory effect of 2-chloroadenosine is blocked by theophylline. The structure activity profile (N6-D-phenylisopropyladenosine greater than or equal to N6-L-phenylisopropyladenosine greater than or equal to 2-chloroadenosine greater than N6-cyclohexyladenosine, adenosine 5'-cyclopropylcarboxamide) is not that expected of either A1 (high-affinity) or A2 (low-affinity) adenosine receptors. 2. Calcium-dependent K+-evoked release of [3H]dopamine from guinea pig striatal vesicular preparations is inhibited by apomorphine. However, only 2-chloroadenosine causes an inhibition of K+-evoked release of [3H]dopamine. Other adenosine analogues such as D- and L-phenylisopropyladenosine and adenosine 5'-cyclopropylcarboxamide cause a facilitation of K+-evoked release. The facilitation is abolished or reduced in the presence of high concentrations (2 mM) of calcium ions. The sites of action of adenosine analogues do not appear to have structural requirements identical to those expected of A1 (high-affinity) or A2 (low-affinity) adenosine receptors. 3. The results indicate that adenosine analogues can have either inhibitory or facilitory effects on K+-evoked release of catecholamines from central synaptic terminals.

Adenosine receptors mediating cyclic AMP production in the rat hippocampus

In the transversely cut rat hippocampus, adenosine caused a dose-dependent increase in the accumulation of [3H]cyclic AMP from [3H]ATP. Adenosine breakdown products were inactive. AMP was somewhat less effective than adenosine, and its effect could be partially, but not completely, abolished by alpha, beta-methylene-ADP and GMP, which inhibited its metabolism by 5'-nucleotidase. The effect of adenosine was unaffected by inhibitors of adenosine deaminase, but enhanced by several inhibitors of adenosine uptake. Some analogues of adenosine, including N6-phenylisopropyladenosine (PIA), 2-chloroadenosine and adenosine 5'-ethylcarboxamide (NECA), were more active than adenosine, whereas others such as 2-deoxyadenosine and 9-(tetrahydro-2-furyl)adenine (SQ 22536) actually inhibited the response. The effect of PIA was highly stereospecific. The action of adenosine was inhibited by several alkylxanthines, the most potent of which was 8-phenyltheophylline. [3H]Cyclohexyladenosine (CHA) bound specifically to cell membranes from the rat hippocampus. The extent of binding was similar to that found in other cortical areas. The relative potency of some adenosine analogues and alkylxanthines to displace labelled CHA was essentially similar to their potency as effectors of the cyclic AMP system. Adenosine contributed to the cyclic AMP-elevating effect of alpha-adrenoceptor-stimulating drugs and several amino acids, but not to that seen with isoprenaline. The cyclic AMP increase seen following depolarization was only partially adenosine-dependent. The present results demonstrate that the rat hippocampus contains adenosine receptors mediating cyclic AMP accumulation and that these receptors have similar characteristics to those mediating pyramidal cell depression. Adenosine-induced cyclic AMP accumulation may be used as a biochemical correlate to electrophysiology and as a convenient parameter to assess the influence of drugs on adenosine mechanisms in the rat hippocampus.

Universal adenosine receptor antagonism is neither necessary nor desirable with xanthine antiasthmatics

The great diversity of pharmacological effects of xanthines may well reflect different cellular mechanisms of action. Major attention is presently devoted to adenosine receptor antagonism that, in contrast to phosphodiesterase inhibition, is clearly produced by therapeutic concentrations of theophylline. The ubiquitous adenosine effects together with the universal and potent blocking action of methylxanthines have led investigators to believe that most pharmacological actions including antiasthmatic effects of theophylline reflect adenosine antagonism. The present hypothesis proposes that universal adenosine receptor antagonism is neither necessary nor desirable with xanthine antiasthmatics. Supporting the hypothesis a xanthine derivative that seems to be devoid of functional effects at important adenosine receptor sites has been shown to be a potent bronchodilator drug that lacks theophylline-like diuretic and CNS-stimulant behavioural effects.

Purine levels in the intact rat brain. Studies with an implanted perfused hollow fibre

A thin dialysis tube was implanted stereotaxically under halothane anesthesia in the caudate nucleus of Sprague-Dawley rats and perfused with Ringer solution at a rate of 2 microliters/min. Initially there was a high rate of purine outflow but after 1-2 h of perfusion the rate was essentially constant (anesthetized - adenosine 0.4 +/- 0.04 microM, inosine 0.8 +/- 0.2 microM; non-anesthetized - adenosine 0.33 +/- 0.03 microM, inosine 0.21 +/- 0.07 microM). Hypoxia (9% O2) increased the levels more than 3-fold. The adenosine deaminase inhibitor erythro-2-(2-hydroxy-3-nonyl)adenine (EHNA) increased the adenosine level and decreased the inosine level. In vitro recovery of adenosine was about 30%. Therefore, we conclude that the free exchangable concentration of adenosine in the rat brain is likely to be 102 micro M. This level is high enough to potentially affect central nervous function.

The effect of alpha-, beta adrenergic receptor agonists and antagonists on the efflux of 22Na and uptake of 42K by rat brain cortical slices

The effects of norepinephrine on ion fluxes in rat brain cortical slices have now been ascertained. 22Na efflux and 42K influx are enhanced by norepinephrine. The increase in ion fluxes can be blocked by ouabain, phentolamine and propranolol, suggesting that the catecholamine activates a membrane sodium pump by a receptor-mediated step. The facilitation of 22Na efflux is stereospecific as demonstrated by the very weak action of D-norepinephrine at 10(-5) M concentration. Various alpha-adrenergic and beta-adrenergic receptor agonists, including oxymetazoline, naphazoline, clonidine, tramazoline, methoxamine, phenylephrine, L-isoproterenol and methoxyphenamine are potent stimulants of the sodium pump as demonstrated by their enhancement of ion fluxes in rat brain cortical slices. Our results are consistent with the hypothesis that norepinephrine hyperpolarizes central neurons by activating an ouabain-sensitive, receptor-mediated sodium pump.

An A1-adenosine receptor, characterized by [3H] cyclohexyladenosine binding, mediates the depression of evoked potentials in a rat hippocampal slice preparation

In slices of rat hippocampus, adenosine and several adenosine derivatives depressed evoked neuronal responses to afferent stimulation. The nanomolar potency of adenosine derivatives and their relative effectiveness indicate that the depression of evoked potentials is mediated via an A1-adenosine receptor. A remarkable similarity was found between the relative potencies of nucleoside derivatives with respect to their electrophysiological effects and to their inhibition of high affinity [3H] cyclohexyladenosine ([3H]CHA) binding to rat brain membranes. We conclude that the [3H] CHA binding site in rat brain membranes represents a physiological receptor of the A1-type.

Metabolic changes in cerebral cortex, hippocampus, and cerebellum during sustained bicuculline-induced seizures

The objective of the present experiments was to study metabolic correlates to the localization of neuronal lesions during sustained seizures. To that end, status epilepticus was induced by i.v. administration of bicuculline in immobilized and artificially ventilated rats, since this model is known to cause neuronal cell damage in cerebral cortex and hippocampus but not in the cerebellum. After 20 or 120 min of continuous seizure activity, brain tissue was frozen in situ through the skull bone, and samples of cerebral cortex, hippocampus, and cerebellum were collected for analysis of glycolytic metabolites, phosphocreatine (PCr), ATP, ADP, AMP, and cyclic nucleotides. After 20 min of seizure activity, the two "vulnerable" structures (cerebral cortex and hippocampus) and the "resistant" one (cerebellum) showed similar changes in cerebral metabolic state, characterized by decreased tissue concentrations of PCr, ATP, and glycogen, and increased lactate concentrations and lactate/pyruvate ratios. In all structures, though, the adenylate energy charge remained close to control. At the end of a 2-h period of status epilepticus, a clear deterioration of the energy state was observed in the cerebral cortex and the hippocampus, but not in the cerebellum. The reduction in adenylate energy charge in the cortex and hippocampus was associated with a seemingly paradoxical decrease in tissue lactate levels and with failure of glycogen resynthesis (cerebral cortex). Experiments with infusion of glucose during the second hour of a 2-h period of status epilepticus verified that the deterioration of tissue energy state was partly due to reduced substrate supply; however, even in animals with adequate tissue glucose concentrations, the energy charge of the two structures was significantly lowered. The cyclic nucleotides (cAMP and cGMP) behaved differently. Thus, whereas cAMP concentrations were either close to control (hippocampus and cerebellum) or moderately increased (cerebral cortex), the cGMP concentrations remained markedly elevated throughout the seizure period, the largest change being observed in the cerebellum. It is concluded that although the localization of neuronal damage and perturbation of cerebral energy state seem to correlate, the results cannot be taken as evidence that cellular energy failure is the cause of the damage. Thus, it appears equally probable that the pathologically enhanced neuronal activity (and metabolic rate) underlies both the cell damage and the perturbed metabolic state. The observed changes in cyclic nucleotides do not appear to bear a causal relationship to the mechanisms of damage.