Effects of chronic administration of caffeine on adenosine A1 and A2 receptors in rat brain

The Effect of Post-Activation Potentiation Enhancement Alone or in Combination with Caffeine on Anaerobic Performance in Boxers: A Double-Blind, Randomized Crossover Study

Post-activation performance enhancement (PAPE) is a physiological phenomenon that refers to an acute excitation of the neuromuscular system following intense exercise that ends in enhanced physical performance in a subsequent bout of exercise. The scientific literature has primarily examined the effectiveness of PAPE alone or combined with caffeine (CAF) intake in all-out tests lasting ≤10 s, as the effect of PAPE is transitory. The aim of the present study was to determine the effect of a protocol to induce PAPE alone or in combination with caffeine intake on the 30 s Wingate Anaerobic Test in highly trained boxers. Twenty-five male and highly trained boxers (mean age: 20 ± 1 years) participated in a double-blind, randomized crossover study consisting of three different experimental conditions: (i) control (CON), with no substance intake and no PAPE protocol before the Wingate Anaerobic Test; (ii) PAPE + PLA, involving the intake of a placebo 60 min before and a PAPE protocol comprising a 10 s cycling sprint overloaded with 8.5% of the participants' body weight 10 min before the Wingate Anaerobic Test; and (iii) PAPE + CAF, involving the intake of 3 mg/kg of caffeine 60 min before and the same PAPE protocol used in the (ii) protocol before the Wingate Anaerobic Test. In all conditions, the participants performed the 30 s version of the Wingate Anaerobic Test with a load equivalent to 7.5% of their body weight, while the cycle ergometer setting was replicated. Immediately following the Wingate test, heart rate (HR), the rating of perceived exertion (RPE), and blood lactate concentration (Bla) were measured. In comparison to CON, PAPE + PLA enhanced mean power (p = 0.024; Effect size [ES] = 0.37) and total work (p = 0.022; ES = 0.38) during the Wingate test, accompanied by an increase in post-test blood lactate concentration (p < 0.01; ES = 0.83). In comparison to CON, PAPE + CAF enhanced mean power (p = 0.001; ES = 0.57), peak power (p = 0.013; ES = 0.57), total work (p = 0.001; ES = 0.53), post-test blood lactate concentration (p < 0.001; ES = 1.43) and participants' subjective perception of power (p = 0.041). There were no differences in any variable between PAPE + PLA and PAPE + CAF. In summary, a PAPE protocol that involves a 10 s all-out sprint 10 min before the Wingate Anaerobic Test was effective in enhancing Wingate mean power in highly trained boxers. The addition of 3 mg/kg of caffeine to the PAPE protocol produced an effect on mean power of a higher magnitude than PAPE alone, and it enhanced peak power along with participants' subjective perception of power. From a practical point of view, PAPE before exercise seems to be an effective approach for increasing Wingate performance in highly trained boxers, while the addition of caffeine can increase some benefits, especially peak power.

Effects of Preinjury and Postinjury Exposure to Caffeine in a Rat Model of Traumatic Brain Injury

Background: Lethal apnea is a significant cause of acute mortality following a severe traumatic brain injury (TBI). TBI is associated with a surge of adenosine, which also suppresses respiratory function in the brainstem. Methods and Materials: This study examined the acute and chronic effects of caffeine, an adenosine receptor antagonist, on acute mortality and morbidity after fluid percussion injury. Results: We demonstrate that, regardless of preinjury caffeine exposure, an acute bolus of caffeine given immediately following the injury dosedependently prevented lethal apnea and has no detrimental effects on motor performance following sublethal injuries. Finally, we demonstrate that chronic caffeine treatment after injury, but not caffeine withdrawal, impairs recovery of motor function. Conclusions: Preexposure of the injured brain to caffeine does not have a major impact on acute and delayed outcome parameters; more importantly, a single acute dose of caffeine after the injury can prevent lethal apnea regardless of chronic caffeine preexposure.

Effects of Long-Term Caffeine Consumption on the Adenosine A1 Receptor in the Rat Brain: an In Vivo PET Study with [18F]CPFPX

PURPOSE

Caffeine, a nonselective antagonist of adenosine receptors, is the most popular psychostimulant worldwide. Recently, a protective role of moderate chronic caffeine consumption against neurodegenerative diseases such as Alzheimer's and Parkinson's disease has been discussed. Thus, aim of the present study was an in vivo investigation of effects of long-term caffeine consumption on the adenosine A₁ receptor (A₁AR) in the rat brain.

PROCEDURES

Sixteen adult, male rats underwent five positron emission tomography (PET) scans with the highly selective A₁AR radioligand [¹⁸F]CPFPX in order to determine A₁AR availability. After the first baseline PET scan, the animals were assigned to two groups: Caffeine treatment and control group. The caffeine-treated animals received caffeinated tap water (30 mg/kg bodyweight/day, corresponding to 4-5 cups of coffee per day in humans) for 12 weeks. Subsequently, caffeine was withdrawn and repeated PET measurements were performed on day 1, 2, 4, and 7 of caffeine withdrawal. The control animals were measured according to the same time schedule.

RESULTS

At day 1, after 4.4 h of caffeine withdrawal, a significant decrease (- 34.5%, p < 0.001) of whole brain A₁AR availability was observed. Unlike all other investigated brain regions in caffeine-treated rats, the hypothalamus and nucleus accumbens showed no significant intraindividual differences between baseline and first withdrawal PET scan. After approximately 27 h of caffeine withdrawal, the region- and group-specific effects disappeared and A₁AR availability settled around baseline.

CONCLUSIONS

The present study provides evidence that chronic caffeine consumption does not lead to persistent changes in functional availability of cerebral A₁ARs which have previously been associated with neuroprotective effects of caffeine. The acute and region-specific decrease in cerebral A₁AR availability directly after caffeine withdrawal is most likely caused by residual amounts of caffeine metabolites disguising an unchanged A₁AR expression at this early time-point.

The acute effects of plyometric and sled towing stimuli with and without caffeine ingestion on vertical jump performance in professional soccer players

Background

Post-activation potentiation (PAP) is the phenomenon by which muscular performance is enhanced in response to a conditioning stimulus. PAP has typically been evidenced via improved counter movement jump (CMJ) performance. This study examined the effects of PAP, with and without prior caffeine ingestion, on CMJ performance.

Methods

Twelve male professional soccer players (23 ± 5 years) performed two trials of plyometric exercises and sled towing 60 min after placebo or caffeine ingestion (5 mg.kg^− 1) in a randomized, counterbalanced and double-blinded design. CMJ performance was assessed at baseline and 1, 3 and 5 min after the conditioning stimulus (T1, T3 and T5, respectively).

Results

Two way ANOVA main effects indicated a significant difference in jump height after the PAP protocol (F[3, 11] = 14.99, P < 0.001, partial η2 = 0.577). Analysis also indicated a significant difference in CMJ performance across conditions, with caffeine eliciting a greater response (F[1, 11] = 10.12, P = 0.009, partial η2 = 0.479). CMJ height was increased at T1, T3 and T5 in caffeine condition (5.07%, 5.75% and 5.40%, respectively; P < 0.01) compared to baseline. In the placebo condition, jump performance was increased at T3 (4.94%; P < 0.01) only. Jump height was higher in caffeine condition on T1, T3 and T5 (P < 0.05) but not on baseline (P > 0.05) compared to placebo.

Conclusions

The results of this study suggest that acute plyometric and sled towing stimuli enhances jump performance and that this potentiation is augmented by caffeine ingestion in male soccer players.

Caffeine exposure alters adenosine system and neurochemical markers during retinal development

Evidence points to beneficial properties of caffeine in the adult central nervous system, but teratogenic effects have also been reported. Caffeine exerts most of its effects by antagonizing adenosine receptors, especially A1 and A2A subtypes. In this study, we evaluated the role of caffeine on the expression of components of the adenosinergic system in the developing avian retina and the impact of caffeine exposure upon specific markers for classical neurotransmitter systems. Caffeine exposure (5-30 mg/kg by in ovo injection) to 14-day-old chick embryos increased the expression of A1 receptors and concomitantly decreased A2A adenosine receptors expression after 48 h. Accordingly, caffeine (30 mg/kg) increased [(3) H]-8-cyclopentyl-1,3-dipropylxanthine (A1 antagonist) binding and reduced [(3) H]-ZM241385 (A2A antagonist) binding. The caffeine time-response curve demonstrated a reduction in A1 receptors 6 h after injection, but an increase after 18 and 24 h. In contrast, caffeine exposure increased the expression of A2A receptors from 18 and 24 h. Kinetic assays of [(3) H]-S-(4-nitrobenzyl)-6-thioinosine binding to the equilibrative adenosine transporter ENT1 revealed an increase in Bmax with no changes in Kd , an effect accompanied by an increase in adenosine uptake. Immunohistochemical analysis showed a decrease in retinal content of tyrosine hydroxylase, calbindin and choline acetyltransferase, but not Brn3a, after 48 h of caffeine injection. Furthermore, retinas exposed to caffeine had increased levels of phosphorylated extracellular signal-regulated kinase and cAMP-response element binding protein. Overall, we show an in vivo regulation of the adenosine system, extracellular signal-regulated kinase and cAMP-response element binding protein function and protein expression of specific neurotransmitter systems by caffeine in the developing retina. The beneficial or maleficent effects of caffeine have been demonstrated by the work of different studies. It is known that during animal development, caffeine can exert harmful effects, impairing the correct formation of CNS structures. In this study, we demonstrated cellular and tissue effects of caffeine's administration on developing chick embryo retinas. Those effects include modulation of adenosine receptors (A1 , A2 ) content, increasing in cAMP response element-binding protein (pCREB) and extracellular signal-regulated kinase phosphorylation (pERK), augment of adenosine equilibrative transporter content/activity, and a reduction of some specific cell subpopulations. ENT1, Equilibrative nucleoside transporter 1.

Chronic sleep restriction induces long-lasting changes in adenosine and noradrenaline receptor density in the rat brain

Although chronic sleep restriction frequently produces long-lasting behavioural and physiological impairments in humans, the underlying neural mechanisms are unknown. Here we used a rat model of chronic sleep restriction to investigate the role of brain adenosine and noradrenaline systems, known to regulate sleep and wakefulness, respectively. The density of adenosine A1 and A2a receptors and β-adrenergic receptors before, during and following 5 days of sleep restriction was assessed with autoradiography. Rats (n = 48) were sleep-deprived for 18 h day(-1) for 5 consecutive days (SR1-SR5), followed by 3 unrestricted recovery sleep days (R1-R3). Brains were collected at the beginning of the light period, which was immediately after the end of sleep deprivation on sleep restriction days. Chronic sleep restriction increased adenosine A1 receptor density significantly in nine of the 13 brain areas analysed with elevations also observed on R3 (+18 to +32%). In contrast, chronic sleep restriction reduced adenosine A2a receptor density significantly in one of the three brain areas analysed (olfactory tubercle which declined 26-31% from SR1 to R1). A decrease in β-adrenergic receptors density was seen in substantia innominata and ventral pallidum which remained reduced on R3, but no changes were found in the anterior cingulate cortex. These data suggest that chronic sleep restriction can induce long-term changes in the brain adenosine and noradrenaline receptors, which may underlie the long-lasting neurocognitive impairments observed in chronic sleep restriction.

Involvement of the central melanocortin system in the effects of caffeine on anxiety-like behavior in mice

AIMS

To investigate the role of the melanocortin (MC) system in the framework of the central nucleus of the amygdala (CeA) in the differential effects of the adenosine receptor blocker caffeine on anxiety-like behavior, using the social interaction (SI) test.

MAIN METHODS

Caffeine was injected intraperitoneally, alone or in combination with alpha-melanocyte stimulating hormone (α-MSH), the MC4 receptor agonist RO27-3225 or the antagonist HS014 via the intra-CeA route. The effects of chronic (21 days) caffeine, given alone or concurrently with α-MSH, or RO27-3225, were investigated. The effects of withdrawal of these treatments on SI time were also evaluated. Furthermore, the acute effects of HS014 were investigated in different sets of caffeine-withdrawn mice.

KEY FINDINGS

Acute injection of caffeine, RO27-3225, or α-MSH produced anxiety-like behavior. Prior treatment with α-MSH, or RO27-3225 potentiated the caffeine-induced anxiety-like behavior. Subchronic treatment with HS014 increased the SI time, which was attenuated by caffeine. Chronic administration of caffeine resulted in tolerance to caffeine's anxiogenic effect, while abrupt discontinuation of the treatment produced peak anxiety-like behavior at 72 h post-withdrawal. Concurrent administration of α-MSH, or RO27-3225 with chronic caffeine delayed the development of tolerance and prevented withdrawal-induced anxiety-like behavior. Moreover, acute treatment with HS014 at 72 h post-withdrawal attenuated the anxiety-like behavior.

SIGNIFICANCE

α-MSH, possibly via MC4 receptor in the neuroanatomical framework of the CeA, may contribute to the acute, chronic and withdrawal actions of caffeine associated with anxiety-like behavior in the neuroanatomical framework of the CeA.

Caffeinated Alcoholic Beverages - An Emerging Trend in Alcohol Abuse

Alcohol use disorders are pervasive in society and their impact affects quality of life, morbidity and mortality, as well as individual productivity. Alcohol has detrimental effects on an individual’s physiology and nervous system, and is associated with disorders of many organ and endocrine systems impacting an individual’s health, behavior, and ability to interact with others. Youth are particularly affected. Unfortunately, adolescent usage also increases the probability for a progression to dependence. Several areas of research indicate that the deleterious effects of alcohol abuse may be exacerbated by mixing caffeine with alcohol. Some behavioral evidence suggests that caffeine increases alcohol drinking and binge drinking episodes, which in turn can foster the development of alcohol dependence. As a relatively new public health concern, the epidemiological focus has been to establish a need for investigating the effects of caffeinated alcohol. While the trend of co-consuming these substances is growing, knowledge of the central mechanisms associated with caffeinated ethanol has been lacking. Research suggests that caffeine and ethanol can have additive or synergistic pharmacological actions and neuroadaptations, with the adenosine and dopamine systems in particular implicated. However, the limited literature on the central effects of caffeinated ethanol provides an impetus to increase our knowledge of the neuroadaptive effects of this combination and their impact on cognition and behavior. Research from our laboratories indicates that an established rodent animal model of alcoholism can be extended to investigate the acute and chronic effects of caffeinated ethanol.

Mice heterozygous for both A1 and A(2A) adenosine receptor genes show similarities to mice given long-term caffeine

Caffeine is believed to exert its stimulant effects by blocking A(2A) and A(1) adenosine receptors (A(2A)R and A(1)R). Although a genetic knockout of A(2A)R eliminates effects of caffeine, the phenotype of the knockout animal does not resemble that of caffeine treatment. In this study we explored the possibility that a mere reduction of the number of A(1)Rs and A(2A)Rs, achieved by deleting one of the two copies of the A(1)R and A(2A)R genes, would mimic some aspects of long-term caffeine ingestion. The A(1)R and A(2A)R double heterozygous (A(1)R-A(2A)R dHz) mice indeed had approximately one-half the number of A(1)R and A(2A)R, and there were little compensatory changes in A(2B) or A(3) adenosine receptor (A(2B)R or A(3)R) expression. The ability of a stable adenosine analog to activate receptors was shifted to the right by caffeine and in A(1)R-A(2A)R dHz tissue. Caffeine (0.3 g/l in drinking water for 7-10 days) and A(1)R-A(2A)R dHz genotype increased locomotor activity (LA) and decreased heart rate without significantly influencing body temperature. The acute stimulatory effect of a single injection of caffeine was reduced in A(1)R-A(2A)R dHz mice and in mice treated long term with oral caffeine. Thus at least some aspects of long-term caffeine use can be mimicked by genetic manipulation of the A(1)R and A(2A)R.

Reversal of caffeine-induced anxiety by neurosteroid 3-alpha-hydroxy-5-alpha-pregnane-20-one in rats

Caffeine has been shown to increase brain and plasma content of neurosteroid 3alpha-hydroxy-5alpha-pregnan-20-one (3alpha,5alpha-THP) that allosterically modulates GABA(A) receptors. The present study evaluated the role of neurosteroid 3alpha,5alpha-THP in the caffeine-induced anxiogenic-like effect using the elevated plus-maze (EPM) test in rats. Acute administration of caffeine (50 or 100mg/kg, i.p.) produced anxiogenic-like activity that was reversed by pretreatment with the neurosteroid 3alpha,5alpha-THP or progesterone, the GABA(A) agonist muscimol, or the benzodiazepine receptor agonist diazepam. On the contrary, caffeine produced higher anxiety in animals previously treated with the GABA(A) receptor antagonist, bicuculline or either of the various neurosteroid biosynthesis enzyme inhibitors viz. trilostane, finasteride or indomethacin. Furthermore, pretreatment with DHEAS, a neurosteroid that negatively modulates GABA(A) receptors also enhanced the caffeine-induced anxiety. Moreover, adrenalectomy potentiated the anxiogenic-like response of caffeine indicating the contributory role of peripheral steroidogenesis. Thus, it is speculated that neurosteroid 3alpha,5alpha-THP through positive modulation of GABA(A) receptor activity may serve as a counter-regulatory mechanism against caffeine-induced anxiety.

Postnatal caffeine exposure: effects on motor skills and locomotor activity during ontogenesis

It has been shown that long-term postnatal caffeine treatment results in biochemical and behavioral changes persisting into adulthood. The effects of postnatal chronic exposure to this methylxanthine on motor skills and locomotor activity were studied with a battery of motor and behavioral tests (negative geotaxis test, wire mesh ascending test, jumping down with a choice test, bar holding test, rotarod test and open field test) in rats during development. Pup rats were daily injected with caffeine (10 mg/kg or 20 mg/kg, s.c.) during postnatal days 7-11 (the P7 group) and/or 13-17 (the P13 group). Motor impairments occurred in 15-, 18- and 21-day-old rats treated with caffeine at P7-P11, whereas, pups exposed to caffeine from P13 to P17 exhibited worse performance only in the bar holding test at the age of 18 days. The P7 group was hyperactive in an open field during the whole period of testing. In contrast, the P13 group showed a decrease of motor activity at postnatal days 25 and 32. In conclusion, postnatal treatment of rats with caffeine leads to impairments of motor skills and changes in locomotor activity. The results are dependent at the developmental stages at caffeine administration as well as at age when tests are performed.

Effects of caffeine as an adjuvant to morphine in advanced cancer patients. A randomized, double-blind, placebo-controlled, crossover study

Psychomotor abnormalities are one of the complications of opioid therapy in advanced cancer patients. Caffeine has potential properties to counteract the central effects of morphine. Twelve patients receiving stable doses of slow release morphine with adequate pain relief were scheduled for this double-blind placebo-controlled crossover trial. The treatment consisted of an intravenous dose of 1/6 of the daily morphine dose, using an intravenous/oral conversion ratio of 1:3. The dose calculated was administered in 5 minutes. Patients were randomly divided to received in a double-blind manner an infusion of 200 mg of caffeine or saline solution intravenously over one hour. A crossover took place after 2-3 days. Patients were assessed immediately before the infusion and once at the end (one hour after). Each assessment included pain, nausea, confusion, and drowsiness intensity. Psychomotor tests, including tapping speed with 10-30 seconds trials, arithmetic tests, memory for digits, and visual memory were also performed. Caffeine infusion induced a significant decrease in pain intensity (from 25.3 to 16.3, p =0.003), but this was no different from the placebo. Caffeine increased both tapping speed tests (p = 0.041 and 0.010, respectively) in comparison with placebo treatment. No other significant differences were found in the other parameters examined. Caffeine showed a partial effect on the cognitive performance of advanced cancer patients on chronic morphine treatment who received a bolus of intravenous morphine. Further studies are necessary to evaluate whether higher doses of caffeine may be more effective and to establish the role of tolerance to caffeine in this group of patients.

Effects of sub-chronic combined treatment with pergolide and caffeine on contralateral rotational behavior in unilateral 6-hydroxydopamine-denervated rats

We studied the synergistic effects of pergolide and bromocriptine with caffeine on turning behavior in 6-OHDA denervated rats. Both pergolide and bromocriptine were synergistic with caffeine, and prevented tolerance to caffeine-induced turning. When caffeine was removed, tolerance to bromocriptine effects was observed for 1 day only, while no tolerance was observed to pergolide. These results suggest that caffeine could be useful in the treatment of Parkinson's disease, preferentially as an adjuvant of mixed dopaminergic agonists like pergolide.

Lack of synergism between caffeine and SKF 38393 on rotational behavior in 6-hydroxydopamine-denervated rats

We have recently shown a synergistic effect between caffeine and the dopamine D(2) receptor agonist, bromocriptine, on contralateral rotational behavior in unilaterally 6-hydroxydopamine-denervated rats. In addition, we found that bromocriptine prevented caffeine-induced tolerance to this behavior following repeated treatment. In the present study, we investigated whether or not the dopamine D(1) receptor agonist, (+/-)-phenyl-2,3,4, 5-tetrahydro-(1H)-3-benzazepine-7,8-diol (SKF 38393), presented similar characteristics. Different groups of rats received simultaneous injections of either vehicle plus vehicle, caffeine (40 mg/kg) plus vehicle, SKF 38393 (0.5, 1, 2, and 4 mg/kg) plus vehicle, or caffeine plus SKF 38393 (0.5, 1, 2, and 4 mg/kg) for 5 consecutive days, and both ipsilateral and contralateral rotational behavior was measured. Results showed that, on the first day of treatment, caffeine produced significantly more rotational behavior than did a low dose of SKF 38393 (0.5 mg/kg), and significantly less turning than at higher doses (2 and 4 mg/kg). Combined treatment with caffeine and a high dose of SKF 38393 (4 mg/kg) produced significantly more rotational behavior than did caffeine plus vehicle. With repeated administration, caffeine produced sustained tolerance to its effects on rotational behavior, whereas SKF 38393 did not. In the groups treated with low doses of SKF 38393 (0.5, and 1 mg/kg) plus caffeine, tolerance was observed while in the groups that received high doses of SKF 38393 (2 and 4 mg/kg) plus caffeine, no tolerance was observed to rotational behavior. These results suggest that maximal stimulation of dopamine D(1) receptors may be needed to prevent the tolerance effects of caffeine in this animal model. This finding may have clinical relevance to the therapeutic treatment of Parkinson's disease.

Alteration of the behavioral effects of nicotine by chronic caffeine exposure

The prevalence of tobacco smoking and coffee drinking place nicotine and caffeine among the most used licit drugs in many societies and their consumption is often characterised by concurrent use. The pharmacological basis for any putative interaction between these drugs remains unclear. Some epidemiological reports support anecdotal evidence, which suggests that smokers consume caffeine to enhance the effects of nicotine. This paper reviews various aspects of the pharmacology of caffeine and nicotine, in humans and experimental animals, important for the understanding of the interactions between these drugs. In particular, recent experiments are reviewed in which chronic exposure to caffeine in the drinking water of rats facilitated acquisition of self-adminstration behavior, enhanced nicotine-induced increases in dopamine levels in the shell of the nucleus accumbens and altered the dopaminergic component of a nicotine discrimination. These studies provide evidence that the rewarding and subjective properties of nicotine can be changed by chronic caffeine exposure and indicate that caffeine exposure may be an important environmental factor in shaping and maintaining tobacco smoking.

Chronic effects of xanthines on levels of central receptors in mice

1. Chronic ingestion of caffeine causes a significant increase in levels of A1-adenosine, nicotinic and muscarinic receptors, serotonergic receptors, GABAA receptors and L-type calcium channels in cerebral cortical membranes from mice NIH Swiss strain mice. 2. Chronic theophylline and paraxanthine had effects similar to those of caffeine except that levels of L-type channels were unchanged. Chronic theobromine, a weak adenosine antagonist, and 1-isobutyl-3-methylxanthine (IBMX), a potent adenosine antagonist and phosphodiesterase inhibitor, caused only an increase in levels of A1-adenosine receptors. A combination of chronic caffeine and IBMX had the same effects on receptors as caffeine alone. Chronic 3,7-dimethyl-1-propargylxanthine (DMPX), a somewhat selective A2A-antagonist, caused only an increase in levels of A1-adenosine receptors. Pentoxifylline, an adenosine-uptake inhibitor inactive at adenosine receptors, had no effect on receptor levels or calcium channels. 3. A comparison of plasma and brain levels of xanthines indicated that caffeine penetrated more readily and attained somewhat higher brain levels than theophylline or theobromine. Penetration and levels were even lower for IBMX, paraxanthine, DMPX, and pentoxyfylline. 4. The results suggest that effective blockade of both A1 and A2A-adenosine receptors is necessary for the full spectrum of biochemical changes elicited by chronic ingestion of xanthines, such as caffeine, theophylline, and paraxanthine.

Adenosine A2A receptor interactions with receptors for other neurotransmitters and neuromodulators

Adenosine, by activating adenosine A2A receptors, seems to have a crucial function in regulating the activation of multiple receptors that affect neurotransmitter release and/or synaptic transmission, in particular receptors for neuropeptides (calcitonin gene related peptide (CGRP) and vasoactive intestinal peptide (VIP)), and NMDA receptors, metabotropic glutamate receptors, nicotinic autofacilitatory receptors, dopamine receptors and adenosine A1 receptors. The manner in which these A2A receptors are involved in interactions with the receptors for other neurotransmitters and or neuromodulators opens novel avenues for the action of this 'omnipresent' nucleoside. Either by direct receptor-receptor modulation or by post-receptor mechanisms, adenosine, in its 'obsession' to protect cells from insults, uses as many receptor systems as possible to synchronize synaptic transmission, in order to exert what seems to be the 'destiny' of this nucleoside--protection of the nervous system.

Can Prenatal Caffeine Exposure Affect Behavioral Inhibition?

Given the widespread use of caffeine and its potential nonoptimal effects during pregnancy, it is remarkable and distressing that almost nothing is known about the long-term effects of prenatal caffeine exposure in humans. Research on nonhumans reveals that prenatal caffeine exposure is associated with an enduring tendency toward a profile of fearfulness, hypoactivity, and exaggerated stress in uncertain situations. A similar pattern of behavioral inhibition has been observed in humans and has been attributed to various causes such as inborn temperament or environmental influences. The author's conjecture is that prenatal caffeine exposure could be an additional factor contributing to the tendency toward behavioral inhibition. One mechanism for this effect is that caffeine causes an upregulation of adenosine receptors, thus creating a vulnerability to adenosine-induced inhibition of neuronal firing and neurotransmitter release.

Scopolamine prevents tolerance to the effects of caffeine on rotational behavior in 6-hydroxydopamine-denervated rats

Continuous administration of caffeine has been shown to induce tolerance to its psychostimulant effects. In this study, using unilateral 6-hydroxydopamine nigrostriatal denervated rats, we tested the hypothesis that the muscarinic receptor antagonist, scopolamine, would prevent the tolerance to caffeine-induced contralateral rotational behavior. For that purpose we administered either caffeine (40 mg/kg) plus saline or scopolamine (5, 10 and 20 mg/kg) plus saline, as well as caffeine in combination with the various doses of scopolamine for 7 consecutive days, and measured ipsilateral and contralateral rotational behavior. The results showed that acute injections of scopolamine plus saline produced similar levels of both ipsilateral and contralateral turning, while caffeine produced more contralateral than ipsilateral turning. Tolerance to caffeine-induced contralateral turning was observed as of the second administration, while scopolamine plus saline injections did not produce significant changes in rotational behavior with repeated treatment. Scopolamine co-administered with caffeine significantly attenuated the increased contralateral turning produced by acute injections of caffeine plus saline, but significantly prevented the tolerance effects with repeated administration. These findings strongly suggest that muscarinic cholinergic processes may be involved in tolerance to caffeine-induced contralateral turning. The results are interpreted in terms of the possible interactions between dopamine, adenosine and acetylcholine neurotransmitter systems within the basal ganglia circuitry involved in motor behavior.

Is GABA involved in the development of caffeine tolerance?

Caffeine (10 or 20 mg/kg per day, po)-induced stimulation of locomotor activity (LA) reached its peak following 4 consecutive days of caffeine administration. Caffeine-induced stimulation of LA was restored to the control values following caffeine tolerance after 16 or 12 consecutive days of caffeine treatment at a dose of 10 or 20 mg/kg per day, po. Biochemical studies showed that caffeine in the nontolerant condition reduced GABAergic activity in cerebral cortex, corpus striatum, cerebellum, hypothalamus and pons-medulla; but tolerance to caffeine (10 or 20 mg/kg per day, po) pushed up the GABAergic activity to the control value in all these regions of brain. Further, it was found that muscimol reduced the LA while bicuculline stimulated LA in the caffeine tolerant condition. Thus, from the present study it may be concluded that: (a) caffeine-induced stimulation of LA is dependent on dose and duration of caffeine treatment, (b) development of tolerance to caffeine is dependent on the dosage of caffeine, and (c) the reduction of central GABAergic activity in the caffeine-nontolerant condition pushed up and restored the LA to the control level on the development of tolerance to caffeine.

A1 and A2A adenosine receptors and A1 mRNA in mouse brain: effect of long-term caffeine treatment

The effect of oral treatment with caffeine, in doses that are known to produce marked adaptive effects, was investigated on A1 and A2A receptors in the mouse brain. Caffeine (0.1, 0.3 or 1 g/l) was added to the drinking water and the animals were sacrificed after a 14-day treatment period. Ligand binding to A1 receptors was studied, using quantitative autoradiography, with the agonist [3H]cyclohexyladenosine (CHA) and the antagonist [3H]1,3-dipropyl-8-cyclopentyl xanthine (DPCPX). Caffeine did not remain in the sections during the autoradiography experiments. Caffeine treatment (1 g/l, but not 0.1 or 0.3 g/l) tended to increase [3H]CHA binding to the CA3 subfield of the hippocampus, but in no other region studied. There was no change in the number of A1 receptors since [3H]DPCPX binding to the CA3, cerebral and cerebellar cortex was not influenced by caffeine treatment. There was similarly no change in the ability of CHA to displace [3H]DPCPX binding, suggesting that there are no major changes in the proportion of A1 receptors that are coupled to G-proteins. mRNA for the A1 receptor, measured by in situ hybridization, did not differ significantly between caffeine-treated and control mice in the structures examined. Thus, higher doses of caffeine can cause an increase in A1 agonist binding without a corresponding change in A1 mRNA or in A1 antagonist binding, suggesting that the adaptive changes seen upon prolonged caffeine treatment may be in sites different from A1 receptors. Caffeine (1 g/l) increased A2A receptors in the striatum measured as binding of the agonist [3H]CGS 21680 suggesting that up-regulation of A2A receptors may be an adaptive effect of caffeine intake.

Dose-dependent surmountability of locomotor activity in caffeine tolerance

For two chronic intraperitoneal caffeine dose regimens (10 and 80 mg/kg per day), tolerance developed rapidly (2-3 days) to the stimulatory effects of caffeine on locomotor activity. However, surmountability of the tolerant activity rate levels by caffeine administration was dose dependent: Activity rate was restored fully by acute caffeine administration for the 10 mg/kg per day series, but not for the 80 mg/kg per day series. The extent of tolerance was also dose-dependent: Tolerance was incomplete for the low-dose daily caffeine series but complete for the high-dose series. Upon discontinuation of daily caffeine dosing, activity rate decreased to the original baseline levels for both chronic series. Caffeine tolerance and the quantification of its surmountability may be explained by the pharmacokinetics of caffeine and the upregulation of adenosine receptors.

Reduced proconvulsant activity of caffeine in rats after a series of electroconvulsive seizures

A variety of neurotransmitter receptor changes occur after a course of electroconvulsive seizures (ECS) in rats, including an increased density of adenosine A1 sites. Adenosine antagonism has been related to the proconvulsant action of methylxanthines such as caffeine. We determined tonic-clonic seizure duration in rats given ECS with caffeine (0-175 mg/kg, IP) after a course of one or six daily ECS. A single day of ECS did not affect the dose-dependent proconvulsant action of caffeine. After six daily ECS, the proconvulsant action of caffeine was reduced. After nine daily ECS, an A1 antagonist (8-cyclopentyl-1,3-dipropylxanthine) and an A2A antagonist (1-allyl-3,7-dimethyl-8-p-sulfophenylxanthine) showed reduced proconvulsant activity. The results suggest that the reduced proconvulsant action of caffeine after chronic ECS depends on adenosine antagonism.

Changes in neurotransmitter sensitivity in the mouse neocortical slice following propranolol and theophylline administration

1. The mouse neocortical slice has been used to examine the sensitivity of neurones to isoprenaline, 5-hydroxytryptamine (5-HT) and adenosine acutely and following chronic treatment of animals with propranolol or theophylline. 2. While having little effect alone, all three agonists enhanced the d.c. depolarizing potential produced by N-methyl-D-aspartate (NMDA). The effect of (-)-isoprenaline (0.2 microM) was shared by (+)-isoprenaline at the much higher concentration of 10 microM. 3. Superfusion of slices with theophylline or 8-phenyltheophylline blocked responses to adenosine with evidence of selectivity. A single injection of theophylline 24 h before slice preparation did not alter agonist sensitivity, but when administered daily at 100 mg kg-1 for 14 days, the xanthine caused a loss of sensitivity to adenosine and (-)-isoprenaline but not 5-HT. The lower dose of theophylline, 10 mg kg-1 daily, also led to a loss of adenosine responses but no change of sensitivity to the amines. 4. Following the 14 day treatment with theophylline at 100 mg kg-1 daily in two groups of mice, responses to adenosine recovered to control levels after 20 days. 5. Propranolol superfusion blocked responses to both isomers of isoprenaline and 5-HT but did not affect sensitivity to adenosine. 6. Chronic treatment with propranolol at 25 mg kg-1 daily for 14 days induced a loss of sensitivity to (-)-isoprenaline and 5-HT but not adenosine. A lower dose of 5 mg kg-1 daily caused no change in responses to adenosine or 5-HT, but yielded an increased sensitivity to (-)-isoprenaline. 7. The results are discussed with respect to reports of receptor up-regulation in binding studies; caution is clearly required in extrapolating from such work to receptor activity in a functional system, especially in the case of theophylline and adenosine.

Caffeine cross-tolerance to selective dopamine D1 and D2 receptor agonists but not to their synergistic interaction

This study examined the role of dopamine systems in tolerance to caffeine-induced stimulation of motor activity in the rat. Selective dopamine D1 and D2 receptor agonists were tested alone and combined in rats receiving caffeine chronically by a method of scheduled access to a caffeine solution or in control rats receiving no caffeine. Rats treated chronically with oral caffeine were tolerant to the motor stimulant effects of caffeine (3.0-100 mg/kg, i.p.). The partial dopamine D1 receptor agonist SKF-77434 (1.0-30 mg/kg, s.c.) and the dopamine D2 receptor agonists quinpirole (0.03-10 mg/kg, s.c.) and R(-)-pro-pylnorapomorphine (NPA, 0.03-1.0 mg/kg, s.c.) dose-dependently increased activity in control rats. Caffeine-treated rats were cross-tolerant to the locomotor stimulant effect of these selective dopamine D1 and D2 receptor agonist. The concurrent administration of the partial dopamine D1 receptor agonist SKF-38393 (10 mg/kg, s.c.) with the dopamine D2 receptor agonists quinpirole and R(-)-propylnorapomorphine had a synergistic effect on locomotor activity in both control and caffeine-treated rats. This synergistic effect was also seen when the partial dopamine D1 receptor agonist SKF-77434 (1.0 and 10 mg/kg, s.c.) was given in combination with quinpirole. There was little or no cross-tolerance to the concurrent administration of selective dopamine D1 and D2 receptor agonists. It appears that tolerance to the locomotor stimulant effect of caffeine is mediated by both dopamine D1 and D2 receptor sub-types. Cross-tolerance occurs to drugs that activate a single type of dopamine receptor but not to drug combinations that concurrently activate more than one type of receptor.

A2a adenosine receptor gene expression in developing rat brain

Adenosine is a neuromodulator in the adult central nervous system. Membrane-bound receptors for adenosine have been identified and cDNAs for A1, A2a, A2b, and A3 adenosine receptor subtypes have been cloned recently. The present study documents the developmental appearance of A2a adenosine receptor gene expression in the rat brain. In situ hybridization using 35S-labeled RNA probes generated from the rat A2a adenosine receptor cDNA revealed receptor gene expression in the striatum on gestational day (GD) 14. Developmental alterations in the pattern of receptor gene expression within the striatum suggest that this receptor mRNA is expressed by striatal neurons soon after they complete neurogenesis. Transient expression of the A2a adenosine receptor mRNA was observed in cerebral cortex, subiculum, parafascicularis nucleus of the thalamus, facial nucleus, trigeminal nucleus, locus coeruleus, area postrema, anterior pituitary gland and in the fetal cerebral vasculature. The ganglia of cranial nerves V, VII, VIII, IX and X expressed A2a adenosine receptor mRNA in fetuses; adults have not been examined. A2a adenosine receptor mRNA was expressed in the carotid body and intermediate lobe of the pituitary during development and also in adult rats. Northern blot analysis revealed that the A2a adenosine receptor transcript is consistent in size (ca 2.5 kb) across the developmental period examined (GD 14 through adult). Previous studies in adult rats have reported that A2a adenosine receptor gene expression is limited to a population of striatal medium spiny neurons. This study documents early developmental expression of the A2a adenosine receptor gene in the striatum and its transient expression elsewhere in the brain and cerebral vasculature. If the A2a adenosine receptor mRNA is translated into receptor protein shortly after the mRNA is expressed, adenosine could influence neuronal differentiation, migration, synaptogenesis, and angiogenesis. Expression of A2a adenosine receptor mRNA in cranial ganglia, carotid body, and intermediate lobe of the pituitary gland similarly suggests novel sites of adenosine action during development and in the adult.

Effects of Combinations of Methylxanthines and Adenosine Analogs on Locomotor Activity in Control and Chronic Caffeine-Treated Mice

Chronic caffeine ingestion (CCI) by male NIH Swiss strain mice results in a prolonged reduction in locomotor activity and alterations in response to caffeine, other xanthines, and adenosine analogs. Caffeine, the A1 selective 8-cyclopentyltheophylline (CPT), and the A2-selective 3,7-dimethyl-1-propargylxanthine (DMPX) remain stimulatory and the bell-shaped locomotor dose-response curves are left-shifted after CCI. The depressant effects of methylxanthines that are potent phosphodiesterase inhibitors remain after CCI. After CCI, mice became more sensitive to depressant effects of A1, mixed A1/A2, and A2 agonists. In the presence of caffeine the A1-selective agonist N6-cyclohexyladenosine (CHA), the mixed A1/A2 agonist 5'-N-ethylcarboxamidoadenosine and the A2-selective agonist 2-[(2-aminoethylamino)-carbonylethylphenylethylamino]-5'-N-ethylcarboxamidoadenosine (APEC) all have dose-response curves, appearing to consist of initial depressant effects, then stimulatory effects, and finally pronounced depressant effects. The phasic character is reduced or absent after CCI. Synergistic depressant effects of combinations of CHA and APEC also appear reduced after CCI.

Chronic caffeine alters the density of adenosine, adrenergic, cholinergic, GABA, and serotonin receptors and calcium channels in mouse brain

1. Chronic ingestion of caffeine by male NIH strain mice alters the density of a variety of central receptors. 2. The density of cortical A1 adenosine receptors is increased by 20%, while the density of striatal A2A adenosine receptors is unaltered. 3. The densities of cortical beta 1 and cerebellar beta 2 adrenergic receptors are reduced by ca. 25%, while the densities of cortical alpha 1 and alpha 2 adrenergic receptors are not significantly altered. Densities of striatal D1 and D2 dopaminergic receptors are unaltered. The densities of cortical 5 HT1 and 5 HT2 serotonergic receptors are increased by 26-30%. Densities of cortical muscarinic and nicotinic receptors are increased by 40-50%. The density of cortical benzodiazepine-binding sites associated with GABAA receptors is increased by 65%, and the affinity appears slightly decreased. The density of cortical MK-801 sites associated with NMDA-glutaminergic receptors appear unaltered. 4. The density of cortical nitrendipine-binding sites associated with calcium channels is increased by 18%. 5. The results indicate that chronic ingestion of caffeine equivalent to about 100 mg/kg/day in mice causes a wide range of biochemical alterations in the central nervous system.

Increased sensitivity to adenosine in the rat dentate gyrus molecular layer two weeks after partial entorhinal lesions

The molecular layer of the dentate gyrus exhibits extensive circuit and receptor reorganization after entorhinal lesions and in Alzheimer's disease, including decreased adenosine (A1) receptor binding in the terminal zone of damaged perforant path fibers. We examined the adenosine-sensitivity of evoked synaptic activity recorded from the rat dentate gyrus molecular layer in hippocampal slices prepared after electrolytic lesions were placed in approximately the middle third of the entorhinal cortex. Extracellular field potentials (EFPs) recorded in slices prepared from animals two days post-lesion were small, upward-going, and exhibited paired-pulse potentiation, but by two weeks post-lesion EFPs had recovered to large, downward-going responses that exhibited paired-pulsed depression. EFPs recorded from two week post-lesion slices were about 2-fold more sensitive (P < or = 0.05) to exposure to adenosine when compared to EFPs recorded from slices from unlesioned animals. Adenosine-induced reduction of paired-pulse depression was similar between unlesioned and post-lesion slices. AChE histochemistry performed after recording revealed dense staining in the dentate gyrus molecular layer of post-lesion slices as compared to slices from unlesioned animals, confirming that sprouting of cholinergic fibers occurred as expected from previous entorhinal lesion studies. Autoradiography performed on adjacent slices showed a decrease in binding to A1-adenosine receptors in the dentate gyrus molecular layer in post-lesion slices as compared to slices from unlesioned animals, indicating that there was a loss of presynaptically located A1-adenosine receptors on damaged perforant pathway terminals. These results indicate that, in addition to the recovery of the major excitatory signal to the hippocampus after entorhinal cell loss, this signal is more sensitive to modulation by adenosine, suggesting an increase in A1-adenosine receptor efficacy in the reinnervated region.

Locomotor activity in mice during chronic treatment with caffeine and withdrawal

Chronic ingestion of caffeine by mice caused a marked reduction in locomotor exploratory activity. At least 4 days of withdrawal were required to restore activity to normal levels. Stimulatory effects of injected caffeine were lower in chronically treated mice and the biphasic dose-response (stimulatory followed by depressant) curve for injected caffeine was left shifted. Seven days of withdrawal were required before the dose-response curve to caffeine was identical to that of control mice. The depressant effects of a potent xanthine phosphodiesterase inhibitor, 1,3-dipropyl-7-methylxanthine, were blunted in caffeine-treated mice. The depressant effects of A1- and A2-selective adenosine analogs were enhanced after chronic caffeine. There was little or no effect of chronic caffeine on the stimulatory effects of dopaminergic agents (amphetamine, caffeine), while both depressant and stimulatory effects of cholinergic agents (nicotine, oxotremorine, scopolamine) were reduced. The results indicate that chronic caffeine affects functions of adenosine and cholinergic receptors related to regulation of locomotor exploratory activity.

Caffeine and the central nervous system: mechanisms of action, biochemical, metabolic and psychostimulant effects

Caffeine is the most widely consumed central-nervous-system stimulant. Three main mechanisms of action of caffeine on the central nervous system have been described. Mobilization of intracellular calcium and inhibition of specific phosphodiesterases only occur at high non-physiological concentrations of caffeine. The only likely mechanism of action of the methylxanthine is the antagonism at the level of adenosine receptors. Caffeine increases energy metabolism throughout the brain but decreases at the same time cerebral blood flow, inducing a relative brain hypoperfusion. Caffeine activates noradrenaline neurons and seems to affect the local release of dopamine. Many of the alerting effects of caffeine may be related to the action of the methylxanthine on serotonin neurons. The methylxanthine induces dose-response increases in locomotor activity in animals. Its psychostimulant action on man is, however, often subtle and not very easy to detect. The effects of caffeine on learning, memory, performance and coordination are rather related to the methylxanthine action on arousal, vigilance and fatigue. Caffeine exerts obvious effects on anxiety and sleep which vary according to individual sensitivity to the methylxanthine. However, children in general do not appear more sensitive to methylxanthine effects than adults. The central nervous system does not seem to develop a great tolerance to the effects of caffeine although dependence and withdrawal symptoms are reported.

Neonatal exposure to therapeutic caffeine alters the ontogeny of adenosine A1 receptors in brain of rats

Caffeine is a methylxanthine, commonly used in the premature neonate to treat apnea of prematurity. It is efficacious and appears to have few short-term side effects. An animal model, designed to mimic the developmental period in brain and level and duration of exposure in humans, was used to investigate possible long-term effects of early developmental exposure to caffeine on the ontogeny of the adenosine receptor to which caffeine binds. Specific binding at the adenosine A1 receptor, in five distinct regions of the brain was determined in rats, 14-90 days old, as a function of early postnatal exposure to caffeine, over days 2-6. In cortex, cerebellum and hippocampus but not in the brain stem or hypothalamus, there was an increase in specific binding, following neonatal exposure to caffeine, compared to specific binding in control animals. Kinetic analysis of binding to the A1 site in cortical tissue suggests that this increase was due to an increased maximum binding density (Bmax); binding affinity (Kd) did not change. Thus, limited exposure to caffeine, in the early neonatal period, may result in up-regulation of the adenosine A1 receptor that persists to young adulthood in the rat.

Biochemical mechanism of caffeine tolerance

Most of the biological actions of caffeine are possibly mediated through its antagonistic effects to adenosine. Adenosine activates an inhibitory GTP-binding protein (Gi). One of the physiological actions of Gi is the inhibition of cAMP formation. Caffeine overcomes this action thus leading to elevation of cAMP. Firing of neurons and the release of neurotransmitters is also inhibited by adenosine. Caffeine overcomes this effect, thus producing increased CNS-activity. During long term administration of caffeine many functions of the organism develop tolerance including cardiovascular and central nervous systems. Present evidence suggests that caffeine tolerance following continuous severe coffee ingestion is the response of the body against caffeine through the upregulation of adenosine receptors.

Chronic caffeine exposure reduces the excitant action of acetylcholine on cerebral cortical neurons

Chronic administration of caffeine (s.c. for a period of 14 days in escalating doses of 10-70 mg/kg) decreased the sensitivity of rat cerebral cortical neurons to the excitant action of microiontophoretically applied acetylcholine. The sensitivity of spontaneously firing rat cerebral cortical neurons in caffeine-treated animals was compared with that of saline-treated controls using the same multiple barrel micropipettes tested on the same day. Acetylcholine sensitivity was determined by the E.T50 method. The E.T50 for 71 neurons in the caffeine-treated rats of 224.0 +/- 11.3 (S.E.M.) was significantly (P less than 0.001) greater than that of 65 neurons in the saline-treated control rats (153.8 +/- 6.9), indicating a reduction in the excitant action of acetylcholine on neurons which had been chronically exposed to caffeine. The level of spontaneous activity was also reduced in the caffeine-treated animals. A down-regulation of acetylcholine receptors is a possible cause for these effects.

Caffeine-induced behavioural stimulation is dose- and concentration-dependent

1. The relationship between plasma and brain caffeine and metabolite concentrations and behavioural stimulation was investigated over a 4 h time course. 2. CD-1 mice receiving single intraperitoneal doses of caffeine-sodium benzoate solution (caffeine doses: 0, 20, and 40 mg kg-1) were evaluated in an activity monitor, and their plasma and brain caffeine and metabolite concentrations were determined by high performance liquid chromatography (h.p.l.c.). 3. Kinetic variables for caffeine at low and high caffeine doses were: volume of distribution (Vd), 1.16 and 0.88 l kg-1; plasma elimination half-life (t1/2), 1.25 and 1.62 h; brain t1/2, 0.93 and 1.30 h; clearance, 0.64 and 0.38 l h-1 kg-1, respectively, with Vd and brain t1/2 differing significantly between the two caffeine doses. 4. Low-dose caffeine stimulated vertical behaviours significantly more than high-dose, during the first 150 min post-dosage; both doses stimulated vertical behaviours significantly more than vehicle treatment. 5. Low-dose and high-dose caffeine stimulated horizontal and stereotypic behaviours equivalently, during the first 150 min post-dosage; both doses stimulated these behaviours significantly more than vehicle. 6. Only later, 150 min post-dosage, did high-dose caffeine stimulate all behaviours significantly more than both low-dose and vehicle treatment; this occurred when caffeine concentrations had fallen to approximately 10 micrograms g-1 in the high-dose group. 7. The maximal stimulant effects of caffeine occurred in an intermediate concentration range, between 10-20 micrograms g-1, while lower and higher concentrations produced either no additional stimulation or decrements in activity.

Chronic caffeine exposure enhances adenosinergic inhibition of cerebral cortical neurons

Chronic administration of caffeine (s.c. for a period of 14 days in escalating doses of 10-70 mg/kg) increased the sensitivity of rat cerebral cortical neurons to the inhibitory action of microiontophoretically applied adenosine. The sensitivity of spontaneously firing rat cerebral cortical neurons in caffeine-treated animals was compared with that of saline-treated controls using the same multiple-barrel micropipettes tested on the same day. Adenosine sensitivity was determined by the I.T50 method. The I.T50 value for 134 neurons in the caffeine-treated rats of 130.77 +/- 4.33 (S.E.M.) was significantly (P less than 0.001) different to that of 136 neurons in the saline-treated control rats (222.16 +/- 6.68), indicating a supersensitivity to adenosine in neurons which had been chronically exposed to caffeine.

Theophylline down-regulates adenosine receptor function

Chronic treatment of animals with caffeine or theophylline has been reported to increase the number of adenosine receptors in the CNS detected by ligand binding, but few functional studies have been performed. In the present study adenosine enhanced depolarising responses to the excitatory amino acid N-methyl-D-aspartate (NMDA) in slices of mouse cerebral cortex. This effect was blocked acutely by theophylline but not by theophylline given 24 h previously. Twenty-four hours after 2 weeks of treatment with this antagonist, however (10 or 100 mg/kg/day) adenosine was no longer effective, sensitivity recovering subsequently. Theophylline treatment also reduced sensitivity to (-)-isoprenaline but not 5-hydroxytryptamine. It is concluded that changes of adenosine receptor number in the brain do not necessarily reflect the induced changes of receptor function.