Adenosine Antagonist Caffeine Research Articles

Adenosine and caffeine modulate circadian rhythms in the Syrian hamster.

Extracellular adenosine accumulates in some brain areas during sleep deprivation. In Syrian hamsters, both sleep deprivation and adenosine A1 agonists can inhibit phase shifts of circadian rhythms to light at night. Sleep deprivation in the day (sleep period) can shift circadian phase. We examined whether the A1 agonist N-CHA mimics this effect. N-CHA (i.p. or i.c.) in the mid-sleep period induced dose-dependent shifts similar to those induced by 3 h sleep deprivation. The adenosine antagonist caffeine administered systemically at the mid-sleep period induced arousal without shifts, and dose-dependently attenuated shifts to a 3 h sleep deprivation procedure (running in a novel wheel). Adenosine may participate in resetting of the circadian clock by manipulations of behavioral state.

Adenosine agonists CGS 21680 and NECA inhibit the initiation of cocaine self-administration

Administration of the adenosine antagonist caffeine will facilitate the reinstatement of cocaine self-administration responding. This suggests that adenosine receptors may play a role in the motivational systems that regulate cocaine-seeking behaviors. If so then adenosine agonists may act to block cocaine self-administration. To test this hypothesis, the effects of the nonselective adenosine agonist NECA and of the A 2A selective agonist, CGS 21680 on the self-administration of cocaine were determined. In these experiments, rats were allowed to obtain intravenous cocaine infusions (0.6 mg/kg/infusion) delivered under a Fixed Ratio 5 schedule. Treatment with either NECA or CGS 21680 in comparison to vehicle administration reduced the number of infusions received per session. This, primarily, was due to a marked increase in the latency for delivery of the first cocaine infusion. Responding after drug-induced delays tended to be at control levels. Adenosine agonists are known to have sedative effects and these actions might play a role in NECA and CGS 21680-induced increases in latencies for cocaine delivery. These results indicate that the administration of adenosine agonists may inhibit cocaine-seeking behaviors. The degree to which these actions are on motivational systems as opposed to involving less specific effects remains to be fully elucidated.

The role of the D(2) dopamine receptor (D(2)R) in A(2A) adenosine receptor (A(2A)R)-mediated behavioral and cellular responses as revealed by A(2A) and D(2) receptor knockout mice.

The A(2A)R is largely coexpressed with D(2)Rs and enkephalin mRNA in the striatum where it modulates dopaminergic activity. Activation of the A(2A)R antagonizes D(2)R-mediated behavioral and neurochemical effects in the basal ganglia through a mechanism that may involve direct A(2A)R-D(2)R interaction. However, whether the D(2)R is required for the A(2A)R to exert its neural function is an open question. In this study, we examined the role of D(2)Rs in A(2A)R-induced behavioral and cellular responses, by using genetic knockout (KO) models (mice deficient in A(2A)Rs or D(2)Rs or both). Behavioral analysis shows that the A(2A)R agonist 2-4-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine reduced spontaneous as well as amphetamine-induced locomotion in both D(2) KO and wild-type mice. Conversely, the nonselective adenosine antagonist caffeine and the A(2A)R antagonist 8-(3-chlorostyryl)caffeine produced motor stimulation in mice lacking the D(2)R, although the stimulation was significantly attenuated. At the cellular level, A(2A)R inactivation counteracted the increase in enkephalin expression in striatopallidal neurons caused by D(2)R deficiency. Consistent with the D(2) KO phenotype, A(2A)R inactivation partially reversed both acute D(2)R antagonist (haloperidol)-induced catalepsy and chronic haloperidol-induced enkephalin mRNA expression. Together, these results demonstrate that A(2A)Rs elicit behavioral and cellular responses despite either the genetic deficiency or pharmacological blockade of D(2)Rs. Thus, A(2A)R-mediated neural functions are partially independent of D(2)Rs. Moreover, endogenous adenosine acting at striatal A(2A)Rs may be most accurately viewed as a facilitative modulator of striatal neuronal activity rather than simply as an inhibitory modulator of D(2)R neurotransmission.

Cross-sensitization between the motor activating effects of bromocriptine and caffeine: role of adenosine A 2A receptors

The acute motor response to caffeine was studied in rats repeatedly treated with vehicle or the dopamine D 2 agonist bromocriptine either in a novel cage or in the home cage. Rats receiving bromocriptine (5 mg/kg i.p.) in a novel cage were sensitized to the motor stimulating effects of bromocriptine itself and showed cross-sensitization to the acute administration of low (10 mg/kg s.c.) but not high (25 mg/kg s.c.) doses of caffeine, no matter if the novel cage was identical or different from the test cage. In contrast, caffeine (10 mg/kg i.p.) administered to rats which had received bromocriptine (5 mg/kg i.p.) in the home cage and which showed no sign of a sensitized response to bromocriptine, failed to show an increased locomotor and stereotyped response as compared to vehicle pretreated rats. Similarly to caffeine, the selective adenosine A 2A antagonist SCH 58261 (3 mg/kg i.p.) showed an increased motor response in bromocriptine sensitized rats. The sensitized response to caffeine or SCH 58261 did not appear to be due to an higher basal motor activity of bromocriptine sensitized rats since acute administration of vehicle induced a similar motor response in bromocriptine and vehicle pretreated rats. Dopamine D 2 and adenosine A 2A receptors are colocalized in striatal efferent neurons where they control in an opposite direction motor behavior. The results of the present study showed that changes in the sensitivity of D 2 receptors influenced the sensitivity of the adenosine antagonist caffeine through an action on A 2A receptors. D 2 and A 2A receptors, therefore, not only acutely interact in the mediation of motor behavior but long-term modification of the D 2 receptors, such as sensitization, affected the response of adenosine A 2A receptors.

Neuroanatomical targets of anxiogenic drugs in the hindbrain as revealed by Fos immunocytochemistry

It is speculated that specific hindbrain transmitter pathways centred on the periaqueductal gray and locus coeruleus are an important integrative neural substrate for the expression of anxiety and the somatic symptoms and cardiovascular changes that accompany severe anxiety states, such as in panic disorder. Here we investigated the effects of various drugs, known to induce panic in humans and to be anxiogenic in animals, on Fos expression in the periaqueductal gray, locus coeruleus and other parts of the rat hindbrain. The drugs tested were the benozodiazepine inverse agonist FG-7142, the α 2-adrenoceptor antagonist yohimbine, the non-selective 5-hydroxytryptamine 2C receptor agonist m-chlorophenyl piperazine, the adenosine antagonist caffeine and the cholecystokinin analogue BOC-CCK 4. A clear-cut finding was that administration of each anxiogenic drug caused a striking region-specific pattern of Fos expression within the hindbrain. In particular, the drugs commonly increased Fos-like immunoreactivity in the periaqueductal gray and locus coeruleus. Increased Fos expression in the periaqueductal gray was specific to the rostral dorsolateral and caudal ventrolateral regions. All the anxiogenic drugs also increased Fos-like immunoreactivity in the lateral parabrachial nucleus and nucleus of the solitary tract and all but one (BOC-CCK 4) increased Fos in the dorsal raphe nucleus. Rats habituated to the test environment and injected with saline vehicle displayed little or no Fos-like immunoreactivity in the hindbrain areas investigated. In summary, each of the anxiogenic drugs tested (FG-7142, yohimbine, m-chlorophenyl piperazine, caffeine and BOC-CCK 4) increased Fos expression in a restricted number of hindbrain regions, including the periaqueductal gray and locus coeruleus. Previous Fos studies have found that these same regions are activated by various fearful environmental stimuli. Therefore, a specific set of hindbrain circuits may be commonly involved in the processing of anxiety-related information evoked by pharmacological and environmental manipulation. The present findings also raise the possibility that measurement of the effect of anxiogenic drugs on Fos expression might be a useful way to model hindbrain pathways activated by anxiety and possibly panic.

Caffeine During Sleep Deprivation: Sleep Tendency and Dynamics of Recovery Sleep in Rats

The adenosine antagonist caffeine disrupts sleep, but whether caffeine promotes wakefulness by interfering with the expression of sleep or by attenuating sleepiness is unknown. The ability of caffeine to reduce sleep tendency in rats was directly tested by quantifying the number of stimuli needed to maintain wakefulness during sleep deprivation for 6 h after systemic caffeine treatment. In addition, the influence of caffeine on the dynamics between nonrapid-eye-movement (NREM) and rapid-eye-movement (REM) sleep was investigated by comparing the magnitude and time course of the compensatory sleep responses for 42 h postsleep deprivation. Caffeine significantly reduced the attempts to sleep during sleep deprivation, F(1, 9) 8.83, p = 0.0157; 44.9% of vehicle), but did not change compensatory slow-wave activity during recovery sleep. During the initial recovery phase, caffeine suppressed compensatory REM sleep and reduced, but did not block, compensatory NREM sleep duration and continuity. By 42 h postsleep deprivation, the amount of NREM recovered (70.0% of deficit) did not differ from vehicle. In contrast, the REM sleep deficit recovered after caffeine (100%) was more than after vehicle (43.9%). Thus, caffeine slowed the rate of compensatory sleep after sleep deprivation, as indexed by the duration of sleep states and sleep continuity.

Chronic exposure to adenosine receptor agonists and antagonists reciprocally regulates the A1 adenosine receptor-adenylyl cyclase system in cerebellar granule cells.

Chronic treatment with the adenosine receptor antagonist caffeine evokes an up-regulation of A1 adenosine receptors and increased coupling of the receptor to G proteins in rat brain membranes. However, chronic agonist exposure has not been explored. Primary cultures of cerebellar granule cells were exposed chronically to A1 adenosine receptor agonists and antagonists. Exposure to the A1 adenosine receptor agonist N6-cyclopentyladenosine resulted in (1) a time- and concentration-dependent reduction in the density of receptors labeled by 1,3-[3H]dipropyl-8-cyclopentylxanthine, (2) an enhanced ability of guanyl nucleotides to decrease the fraction of A1 adenosine receptor sites displaying high affinity for 2-chloroadenosine, and (3) a functional uncoupling of receptors from adenylyl cyclase (EC 4.6.1.1). The adenosine antagonists caffeine and 8-p-sulfophenyltheophylline produced alterations in A1 adenosine receptor homeostasis that were antipodal to those associated with agonist treatment. Antagonist exposure (1) increased the density of A1 adenosine receptors in cerebellar granule cell membranes, (2) blunted the effect of guanyl nucleotides on receptor coupling to G proteins, and (3) increased the functional coupling of receptors to adenylyl cyclase inhibition. Forskolin treatment of cerebellar granule cells did not affect receptor density, suggesting that cyclic AMP is not involved in the regulation of A1 adenosine receptor expression.

The differential role of A1 and A2 adenosine receptor subtypes in locomotor activity and place conditioning in rats

Previous studies have demonstrated that the non-specific adenosine antagonist caffeine possesses both motor activating and rewarding properties in a place-conditioning paradigm. The present experiments were designed to determine the relative contribution of A1 and A2 adenosine receptor subtypes to these effects. The A2 adenosine antagonist CGS 15943A (0.1-10.0mg/kg) dose-dependently produced both a place preference and enhanced locomotor activity. In contrast, the A1 antagonist CPX (0.01-40.0mg/kg) failed significantly to alter either behavioral measure. Both the A1 receptor agonist CPA (0.01-10.0mg/kg) and the A2 receptor agonist CGS 21680 (0.01-1.0mg/kg) reliably decreased activity but failed to produce significant place conditioning. The increased activity produced by the A2 antagonist CGS 15943A (1.0mg/kg) was attenuated by behaviourally active doses of either CPA or CGS 21680. The place preference produced by CGS 15943A (1.0mg/kg) was attenuated by CPA and CGS 21680, at agonist doses that failed to produce place conditioning when administered alone. In general, the results suggested that although it is the A2 receptor subtype that participates in the establishment of place conditioning and enhanced activity, both receptors participate in the diffuse depressant effects associated with adenosine.

Genetic variation in immediate and delayed postictal refractory period in rats

We determined postictal refractoriness in Sprague-Dawley rats by comparing lengths of two suprathreshold ECS seizures given 15 s to 24 h apart. A bimodal (immediate and delayed) decrease in seizure duration was found, suggesting ECS alters mechanisms of seizure termination. Since adenosine is implicated in seizure termination, we determined immediate (30 s) and delayed (24 h) postictal ECS refractoriness in Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats which vary in adenosine properties and initial ECS seizure length. At 30 s, the decrease in seizures did not differ between WKY (−44%) and SHR (−36%) rats. At 24 h, SHR rats showed no change while the WKY rats showed a 20% decrease in seizure length ( P < 0.01). These two strains also differed in the ability of the adenosine antagonist caffeine (50 mg/kg, i.p.) to prolong ECS seizures (no change for WKY, + 13% for SHR, P < 0.001). The results suggest immediate and delayed postictal refractoriness are subject to genetic variation and may depend on central adenosine mechanisms.

Caffeine reduces low-frequency delta activity in the human sleep EEG

In view of the hypothesis that adenosine is involved in sleep regulation, the effects of the adenosine antagonist caffeine on sleep and sleep EEG were investigated in eight young males. Compared to the placebo condition, caffeine (100 mg) administered at bedtime prolonged sleep latency and reduced sleep efficiency and stage 4 of non-rapid eye movement sleep (NREMS). Electroencephalographic slow-wave activity (SWA, spectral power density in the 1.75-4.5-Hz band) was reduced, whereas power density in the spindle frequency range was slightly enhanced. The suppression of SWA was limited to the first NREMS episode. Caffeine reduced the power density mainly in the lowest delta band, in contrast to the changes during physiological sleep that encompass both the delta and theta bands. Caffeine levels in saliva, assessed in a separate experiment, decreased from 7.5 mumol/l in the first hour of sleep to 3.5 mumol/l in the seventh hour. In the night following caffeine administration, stage 4 sleep had reverted to the baseline level, but sleep latency was still increased, and stage 2 sleep, as well as SWA in the first NREMS episode, were reduced. The data show that even a low dose of caffeine affects the sleep EEG. However, the effects of caffeine did not completely mimic the spectral changes observed during physiological sleep.

Partial sciatic nerve ligation results in an enlargement of the receptive field and enhancement of the response of dorsal horn neurons to noxious stimulation by an adenosine agonist

In this study we examined the effect of partial sciatic nerve ligation (PSNL) on the receptive field size, the baseline firing rate (BFR) and the response of spinal dorsal horn (DH) neurons to mechanical stimulation. In addition, we tested the effect of adenosine agonist, 5′- N-ethylcarboxamide-adenosine (NECA), and the adenosine antagonist caffeine on these parameters. Adult male Sprague-Dawley animals were used. One-third to one-half of the right sciatic nerve was tightly ligated. Unanesthetized animals were tested for their response to mechanical stimulation using Von Frey filaments and a blunt probe. The mean force that produced a paw withdrawal response in the operated animals was significantly less than the force that produced withdrawal in unoperated animals (median: 103.5 vs. 259.7; P < 0.001 for the paw ipsilteral to the ligation). Extracellular recordings were made from nociceptive-specific DH neurons located in laminai I–V of chloral hydrate-anesthetized rats. Recordings were made from 38 neurons in the right and 29 cells in the left DH of unoperated and 40 cells in right and 41 cell in the left DH of operated animals. The BFRs of neurons recorded in the operated animals were not significantly different from those recorded in normal animals. The mean receptive field size (RFS) of neurons (both ipsilateral and contralateral to the ligation) in the operated animals was significantly larger than the RFS of unoperated animals (right side: 180 ± 2.8 mm 2 compared to 66 ± 2.3 mm 2; left side: 93 ± 31 compared to 65 ± 21). Twenty-four percent of all neurons in the operated group had bilateral receptive fields; in contrast, only 3% of the neurons in the control animals showed bilateral receptive fields. To examine the effects of adenosine agonist and antagonist, NECA and caffeine were applied next to the recording electrode. The BFR and the RFS of neurons in control and operated animals were not affected by either NECA or caffeine. However, NECA significantly increased the duration of the response of neurons in the operated animals to noxious stimulation. These effects of NECA were blocked by caffeine. It is concluded that PSNL produces hyperalgesia in Sprague-Dawley rats that is associated with an increase in the RFS. In addition, in animals with PSNL, an adenosine agonist potentiates the response of nociceptive-specific neurons to noxious stimulation. Together these results suggest that PSNL alters the functional characteristics of DH neurons in part by changing purinergic transmission within the spinal cord.

Potentiation of 5‐methoxy‐n,n‐dimethyltryptamine‐induced head‐twitches by diazepam: Evidence for involvement of adenosine uptake inhibition

AbstractPrevious work shows that benzodiazepines potentiate head‐twitches induced by 5‐HT agonists and that this action is not mediated via the GABA receptor complex. In the present study the involvement of adenosinergic mechanisms in this effect has been examined, as in addition to their actions at the GABA receptor, benzodiazepines also inhibit adenosine uptake. The adenosine antagonists caffeine (0.3‐30 mg/kg ip) and 1,3‐dipropyl‐8‐(2‐amino‐4‐chlorophenyl)xanthine (0.03‐1 mg/kg ip) dose‐dependently inhibited the ability of diazepam (4 mg/kg ip) to potentiate head‐twitches induced by 5‐methoxy‐N,N‐dimethyltryptamine (5‐MeODMT; 2.5 mg/kg ip) without affecting head‐twitches induced by 5‐MeODMT alone at a higher dose (10 mg/kg), which induced a similar number of head‐twitches to the combination of 5‐MeODMT and diazepam. The adenosine uptake inhibitors papaverine, mioflazine, and dilazep all potentiated head‐twitches induced by 5‐MeODMT, but this effect was seen at only a single dose of each compound. The benzodiazepine antagonist flumazenil did not inhibit the potentiation of head‐twitches by diazepam but did itself potentiate head‐twitches at 30 mg/kg, consistent with its ability to inhibit adenosine uptake. In contrast, the adenosine uptake inhibitor dipyridamole and the peripheraltype benzodiazepine receptor antagonist Ro 5‐4864, which also inhibits adenosine uptake, failed to potentiate head‐twitches. The adenosine agonists N6‐cyclohexyladenosine, 5'‐(N‐ethylcarboxamido‐adenosine), and (–)‐N6‐(R‐phenylisopropyl)adenosine were similarly without effect. These results confirm previous findings that the potentiation of head‐twitches by benzodiazepines is not mediated via an action at benzodiazepine receptors and suggest that inhibition of adenosine uptake is an important component of the mechanism involved. © 1993 wiley‐Liss, Inc.

Dose-dependent inhibition of the hemodynamic response to dipyridamole by caffeine

We investigated the effects of the adenosine antagonist caffeine on the hemodynamic response to dipyridamole infusion (0.4 mg/kg for 4 minutes). According to a randomized, placebo-controlled double-blind protocol, eight normotensive volunteers each participated in five tests: placebo after placebo, dipyridamole after placebo, and dipyridamole after 1, 2, and 4 mg/kg caffeine. Infusion of caffeine alone (4 mg/kg) induced an increase in mean arterial pressure of 6.1 +/- 0.5 mm Hg versus 1.5 +/- 0.9 mm Hg after placebo (p less than 0.05). Infusion of dipyridamole alone exerted a characteristic hemodynamic response with an increase in systolic blood pressure (+8.4 +/- 2.4 mm Hg), pulse pressure (+7.0 +/- 2.4 mm Hg), heart rate (+25.7 +/- 3.8 beats/min) and calculated rate-pressure product (+3419 mm Hg x beats per minute), all being significantly different from the changes induced by placebo. Caffeine induced a dose-dependent attenuation of the response to dipyridamole, with a significant negative correlation between the dose of caffeine on the one hand (0, 1, 2, and 4 mg/kg) and the dipyridamole-induced increments in systolic blood pressure (r = -0.53), pulse pressure (r = -0.50), heart rate (r = -0.95), and rate-pressure product (r = -0.93) on the other hand. We conclude that caffeine attenuates the hemodynamic response to dipyridamole infusion in humans in a dose-dependent fashion. Because of the wide-spread use of caffeine, this pharmacologic interaction may be of clinical importance, for example, in the diagnostic use of dipyridamole in thallium-201 myocardial imaging.

Potentiation of adenosine-evoked depression of rat cerebral cortical neurons by triazolam

The triazolobenzodiazepine triazolam, applied iontophoretically onto rat cerebral cortical neurons, potentiated the magnitude and duration of adenosine-elicited depressions of spontaneous activity. Triazolam did not enhance the depressions evoked by adenosine 5′- N-ethylcarboxamide, an uptake resistant analof of adenosine, suggesting that potentiation of adenosine resulted from an inhibition of adenosine uptake. With larger application currents, triazolam depressed the firing of cortical neurons. This action was blocked by the adenosine antagonist caffeine (20 mg/kg, i.v.) implying that the depression resulted from an accumulation of endogenously released adenosine.

Modulatory action of purinergic drugs on high potassium-induced epileptiform bursting in rat hippocampal slices

Increase of the potassium concentration up to 8 mM in the superfused solution of rat hippocampal slices leads to the development of an epileptiform bursting. The derivative agonist L-phenyl isopropyl adenosine (L-PIA) (0.05–0.5 μM) is able to block the potassium induced epileptiform activity. The adenosine antagonist caffeine (100 μM) reverts the antiepileptic effect of L-PIA. Our data show a modulatory action of the purinergic transmission in a model of experimental in vitro epilepsy, and point out about a control of endogenous adenosine in the development of focal epileptiform activity.The relationships between the purinergic influence on the release of neurotransmitters, and the convulsant-anticonvulsant effects of the drugs are discussed.

Sleep improvement in dogs after oral administration of mioflazine, a nucleoside transport inhibitor.

Mioflazine, a nucleoside transport inhibitor, was given PO to dogs at doses of 0.04-10 mg/kg. Sixteen hour polygraphic sleep recordings were made and analysis and sleep stage classification was done by computer. Mioflazine decreased wakefulness and increased slow wave sleep, but did not affect the latencies of either REM sleep or slow wave sleep. This increased sleep was due to an increase in the number of light and deep slow wave sleep epochs. The effect lasted for about 8 h. The decreased wakefulness and increased slow wave sleep could be antagonized by the adenosine antagonist caffeine (2.5 and 10 mg/kg, PO); however, there was not a pure antagonistic effect. It might be that the enhancement of slow wave sleep is due to an activation of brain adenosine receptors. This is the first report of a drug acting on adenosine that given orally improves sleep. Mioflazine might be the prototype of substances worth considering for the treatment of a variety of sleep disorders.