Jonathan+Shamberg

= **Stay Posted....The Paper Below is Under Construction** =
 * Shamberg LOG** - Follow my research here.

= Caffeine, non-Selective Antagonist of Adenosine Receptors; A1 and A(2a) = Class: Chem 367 - Chemical Information Retrieval - Fall 2009 To be Submitted on: December 5th ,2009 Supplemental: Caffeine Properties - Assignment Number Three

Caffeine a widely used and enjoyed psycho-stimulant by the American populace for many years has been linked to many positive and negative side affects. Such effects as increased locomotion, anxiety, memory retention are of scientific interest. The mechanism through which caffeine has been found to work is through antagonism of adenosine receptors in the brain. More specifically it is through non-selective antagonism of the A1 and A(2a) adenosine receptors. Through experiments with use of receptor specific antagonist compounds data suggesting the antagonism of the A(2a) receptor has been found to be the dominate cause of these affects. Chronic use of caffeine has also been used to show an increase in A(2a) receptors has been linked to building up a tolerance to chronic caffeine use over an extended period of time.
 * [ Abstract] **

Caffeine, a methylxanthine (Figure 1), has been noted as the most widely consumed psychoactive drug on today's market for mass consumption [1 - 13]. Commonly referred to by the public as Caffeine; the IUPAC name for its chemical structure is 1,3,7-trimethyl- 1//H//-purine- 2,6(3//H//,7//H//)-dione. Statistics claim that between 80-90 percent of the population come in contact with caffeine on a regular basis[1, 2, 11]. This common psychoactive drug was initially isolated back in 1820 to reside in the beans used to make coffee[4]. Today caffeine is now; while not fully understood has at least a mapped structure and is commercially available for use. It is now known to reside not only in coffee, but in drinks such as teas, sodas and energy drinks; all of which are commonly consumed everyday in the United States by the adult population. Other uses for the compound have lead to their use in many over the counter pharmacy products for countering headaches and helping with dieting[8].
 * [Introduction]**



The reason for is popularity lies in the many different positive affects to consumption of caffeine. Some find it to be a mild stimulant that can help alleviate fatigue, to become more alert, and even overcome boredom; without any major side effects associated with harsher stimulants[6, 9]. The increase in the users’ locomotive function primarily associated with caffeine after consumption tends to arouse most of the interest in the daily ritual of its use. Experimental data backs this up by showing caffeine can enhance its users performance. This is especially true when the user is tired, not already energetic after a good nights sleep or a nap[6]. There are however documented side effects that would not be considered positive by its consumer. The negative symptoms experienced by acute caffeine users are arrhythmias, increased heart rate, blood pressure, and insomnia[14].

As with most chronically consumed drugs on a regular basis; which for caffeine is approximated to be a consumption of 200mg per adult daily, can experience a withdrawal syndrome upon ceasing use[14]. The result of suddenly halting caffeine use has the potential of showing itself through a variety of symptoms. The signs of going through caffeine withdrawal are migraine, cognitive deficits, fatigue or lethargy, depressed mood, nausea, vomiting, myalgias, and rhinorhoea[11, 14, 15]. The international classification of Headache Disorders have set a base criteria for caffeine withdrawal related headaches; at a minimum of 200mg of caffeine ingested daily over the course of a two week period of time[11]. Studies have been done to show that the positive effects that caffeine exhibits are to be attributed to a reversal of adverse effects[6, 11]. Under these conditions it is believed that the use of caffeine becomes detrimental to its consumer when it becomes a long-term used substance present in the system regularly at large quantities[6].

The experiments preformed to come to these conclusions have their share of hurdles. There many are difficulties in conducting the animal studies that are preformed to study the function and working mechanisms of caffeine. The primary problem is that when animal test subjects are used to study the effects caffeine has on the central nervous system; the metabolism of the compound varies from humans in every case making data extrapolation difficult to do. There are also other differences that need to be accounted for such as variances in age and gender as to how the compound will interact with its user; previous encounters with caffeine will also dictate the response and intensity experienced when used[8, 9]. This has not however stopped the experimental process.

Over the last 169 years and many experiments since its discovery, much has been learned and studied about this commonly used compound. The method in which caffeine interacts with its users system to create both its positive and detrimental effects, is now known more conclusively then before. With the new knowledge has come a possible source for the locomotive energy its users enjoy. It was already known that it affects the central nervous system and the user’s heart. The mechanism through which this happens has not been fully mapped out. However, a common belief is that caffeine acts as an active non-selective antagonist for the adenosine receptors in the brain [16]. While adenosine affects the central nervous system it however is not considered a neurotransmitter; this is because it is not accumulated into vesicles nor dependent on calcium to be released from nerve terminals[5]. Even so adenosine has been found to have a hand in the control of cyclic 3',5'-AMP levels present in the brain[17, 18]. During the caffeine antagonism; the adenosine receptors would reduce the level the cyclic 3',5'-AMP present due to the receptors being blockaded[8]. This action gives scientists a more visible way to monitor the affects and magnitude of caffeine's antagonism of adenosine in a subjects system.

In the striatum, a portion of the brain , adenonsine plays another role; this time as an intermediary for dopamine and glutamate neuro-transmission[16].The role of adenosine has yet to be fully mapped out and understood. What has been learned is that adenosine has been broken down into four separate receptors ,A1, A(2a), A(2b), and A3; each believed to play a different role and interact with different mechanisms[12, 13]. Caffeine has been shown to competitively antagonizes the adenosine receptors A1 and A(2a), specifically out of the four possible[2, 19].

While the mechanism of adenosine antagonism is generally agree upon. There is some discussion and debate over what happens when you interact with the receptors and which receptors cause what response. There is a spread of topics from locomotive energy alterations, development of anxiety, and building of tolerance. Each topic deals with caffeine's interactions with one, or both of the receptors. In this paper each topic will be reviewed encompassing different opinions on the topics.

Since caffeine is a non-selective antagonist of the adenosine receptors; it is difficult to analyze individual receptors through dosage of the stimulant. For studying caffeine's effects on the users system for symptoms of overdose, withdrawal, or building of tolerance knowing which receptor is being antagonized is not important because it is not the subject of the study. When it comes to studying; which receptor causes the desired response then having a receptor specific compound would be more beneficial in that kind of experiment. The way that this is done is that a compound with a known affinity for the desired adenosine receptors are found and categorized depending on origin and affinity. As is the case with many compounds it can be difficult to find a compound that only affects the desired receptor when other receptors may barely differ. When such is the case a compound with a higher affinity for one receptor over the other is used[4]. Some examples of such compounds are 8-Cyclopentyl-1,3-dipropylxanthine and DPCPX both specific for A1 and SCH 58261 which antagonizes A(2a)[19, 20].
 * [Using Specialized Antagonists]**

The use of specialized adenosine antagonists, has led to a more in-depth understanding of the stimulant properties that caffeine is well known for causing. For a while now it has already been determined that the adenosine receptors, A1 and A(2a), are antagonized by caffeine. This antagonism of these receptors have been experimentally linked to the stimulant properties of caffeine[5, 12, 13, 16]. Experimental evidence of this theory was found through various animal studies. Very few tests have been done on human test subjects[9]. Various breeds of mice and rats have been used as test subjects for the experiments. Their movements are monitored in a variety of ways to determine if the dosage administered to the test subject is showing any signs of increased locomotive activity. The monitoring does have a flaw in that small movements such as tail movement, reflexes, and tremors are however difficult to account for; which may play part in the results recorded[13].
 * [Locomotive Response]**

In one experiment run by Karc-Kubicha, 2003, mice were given one of three formulas. The control formula contained a predetermined quantity of Caffeine, a second formula contained quantities of CPT and MSX-3; which are respectfully A1 and A(2a) specific compounds. In the third formula a different mixture of receptor compounds. CPA and CGS-21680 were used in the same fashion as the second. The experiment was run so that over a period of time the test subjects locomotion was monitored as to see the response to the administered doses. The results of the experiment expressed two different findings; the first of which is that it experimentally confirms that adenosine receptors do play a role in creating the locomotive effects of caffeine, and that particularly the A1 and A(2a) receptors are indeed responsible. The second thing learned from the experimental data is that there was a small amount of evidence that gave support to a theory that the A(2a) adenonsine receptor was the specific source of the increased locomotion stimulation[16]. The only problem is that more research would need to be conducted to confirm whether or not this is the case.

To determine the exact mechanism responsible for the locomotion stimulation the use of receptor specific compounds were necessary. They were administered to test subjects one compound at a time in comparison to caffeine which antagonized both receptors at the same time. The theory was that if a similar response was seen from either of the receptor specific compounds an answer would be found. Dosages at levels of 7.5, 15, and 30 mg/kg were administered to the mice during the course of treatment. The compounds administered to the mice were one of three possibilities. Caffeine was used as the control, the A(2a) specific compound was SCH 58261, and the A1 specific compound DPCX[12, 19]. They were administered either in the form of a water solution orally or injected directly into the system[12, 13, 19]. If administered orally the quantity of the solution drank was monitored. The results witnessed were positive; the test subjects exhibited recordable increases in their locomotion as a result of coming in contact with either caffeine or SCH 58261[12, 19]. These results promote the theory that the A(2a) receptor is the starting point for the stimulant effects of caffeine[12, 13, 19]. Since most receptor specific compounds still share an affinity for other receptors there is still a possibility that the A1 receptor plays a small role.

A third experiment was run by Fisone, 2004. This was done building off of previous work and the findings that the A(2a) receptor was the source of psycho-stimulation. The experiment was repeated in a similar kind of design as was previously preformed. Dosages however were given this time at 15, 30, and 100 mg/kg to the test subjects in the experiment. Findings this time were however different from those found in the other experiments, in that there was a point in which the stimulant effects of caffeine stopped around 100 mg/kg dosages and began to give the reverse effect. depressing locomotive activity in the mice tested. It did however, confirm that the A(2a) receptor was responsible for the stimulant effects caused by caffeine[5]. This helps give the mechanism a strong backbone to work off of.

The second topic of interest concerning the antagonism of adenosine receptors is the process of building a tolerance to the usage of caffeine as a psycho-stimulant. As with most any drug a tolerance or immunity to its affects can form over time if the system is given a chance to slowly adapt to the drug's effects through usage of consistent non life threatening doses. The positive effects of building a tolerance to caffeine is the ability of a chronic user to continue use of the dug without detrimental side affects. Caffeine users whom consume drinks such as coffee, or sodas regularly are less likely to be plagued with a mild case of insomnia or just plain restlessness[15]. This is a typical result to chronic use over an long extended period of time.
 * [Tolerance]**

The cause of caffeine tolerance is not fully understood; however there are some theories and experiments to back up the concept behind it. The main belief that the quantity of adenosine receptors increase in the brain; which would make blockade by caffeine require a higher concentration in the user system. Another belief is that chronic consumption of caffeine produces a sensitization to the adenosine receptors in a whole, without a definitive increase in the quantity of the receptors. These theories for tolerance development comes from evidence for a development of tolerance to caffeine's effects on blood pressure and heart rate[1, 10]. In human caffeine studies, usually not focused on tolerance, there have been cases where the blood increase associated with caffeine use will decrease in intensity over a period of chronic usage[10].

In 1991 there was a human subject experiment preformed, by Suzette M. Evans, to see if a central nervous system tolerance to caffeine could be established. The study did not focus on the adenosine receptors for a cause to what creates a tolerance to caffeine, let alone to find any mechanism for tolerance development. While this study does not focus on that, it tells us that a central nervous system tolerance can be formed in human test subjects over a period of chronic usage of caffeine products[15]. As with most drugs it would still be possible to overdose on caffeine. The required dosage needed to hit the blood stream for this to take place is around 500 micro-molar [5]. Keeping usage under toxic levels would be advisable in any case for there is no evidence for a true development of tolerance to caffeine in the central nervous system[9].

A third topic of interest are other side effects that have been seen to emanate from antagonism of adenosine; such as alterations in ones memory capabilities and anxiety levels of the caffeine user. The stimulant affects of caffeine are the most well known, but these other affects are still prevalent to understanding the mechanisms behind the adenosine receptors.
 * [Anxiety / Memory]**

Anxiety is one known negative affect of caffeine usage for some people. Experimental data suggests that the anxiety onset by caffeine is a result of high doses of caffeine. There have also been studies on adenosine receptors that show that there are genetic differences in the receptors between people. What seems to be the case, is that in the genetic coding for the adenosine receptors have polymorphisms; which means two or more coexisting forms are present in the system. The alterations caused to the adenosine receptors by the polymorphism has been linked to the Panic DIsorder, PD; which is known for unexpected attacks of anxiety or fear. More precisely it is believed to be the alterations to the A(2a) receptor that has a larger effect on the appearance of anxiety side effects from caffeine usage. The anxiety appears because of the antagonism of the adenosine receptors but its effect increases in magnitude as a result from genetic variations in a users adenosine receptors[2].

The prevalent memory effects, unlike anxiety, seems to enhance memory retention. An experiment run by Kopf, in 1999, on adult albino Swiss male mice was to test the affects adenosine had on memory storage. Adenosine specific compounds, SCH 58261, DPCX were used. Caffeine was also administered in the study becuase of its known antagonism of adenosine receptors. The groups of mice were tested by being trained to complete an avoidance task. After being trained the mice were either immediately injected with a dose of a chosen compound or injected after a period of time. The results of the experiment expressed that caffeine and SCH 58261 compounds promoted memory retention when injected immediately after training. The results of this experiment promote the theory that the A(2a) receptor, when antagonized can promote memory storage in the user[19].

Anxiety, memory, tolerance, and locomotive stimulation are the four main topics that surround caffeine's antagonism of adenosine. While not all of the studies have been positive they have not been overly detrimental to the common user. The worst non-common side effect of chronic caffeine consumption can have on the user is amplify the affects of the Panic Disorder by adding an additional source of biological anxiety. For typical chronic users the worst effect caffeine can have is a total withdrawal.
 * [Conclusion]**

In the of case memory retention and locomotive stimulation it has been proven to have beneficial results when properly used. Under a consistent high dosage it has been suggested that a decrease in locomotion has been noticed in the test subjects studied. This however, is not a highly detrimental effect seen by acute users, but can be for the chronic users. For memory retention no majorly detrimental side effects have been noted to come from over use of caffeine. All together the best effect of chronic use of caffeine is that a tolerance to its affects does build over time.

All of these effects have also been linked to caffeine's antagonism of adenosine receptors. This was experimentally done using receptor specific compounds such as the SCH 58261 and DPCPX, against a caffeine control group. The results tended to promote the theory that the A(2a) receptor seems to play a larger part than the A1 receptor in the mechanisms that result in each of the four topics, and that a combination of the receptor specific compounds can be used to create similar reactions in a test subject that would be caused by caffeine use.

With the research done and preformed, it has answered many questions regarding the functionality of caffeine in the users system in regards to adenosine receptors. Questions and curiosities are also aroused by the information presented here. With some further studying into the mechanisms behind the adenosine A(2a) receptor the concept of a caffeine replacement comes to mind. While caffeine has not been shown to have any majorly detrimental properties, aside from a painful withdrawal process, the idea that a A(2a) receptor specific compound could be used in it place is of interest. It has already been experimentally documented that these compounds have been found and used to replicate the properties of caffeine. The main hindrance is that while many mechanisms have been proposed for the functionality of blockading the A(2a) receptor; there would need to be a definitive agreement on the full spectrum of effects caused solely by the single receptor antagonist. There may be no benefit to replacing caffeine, if some of the negative side effects of the whole caffeine molecule can not be discarded, by replacement through another compound.

[[|DOI]][1] Addicott, M. A., L. L. Yang, et al. (2009). "The effect of daily caffeine use on cerebral blood flow: How much caffeine can we tolerate?" Human Brain Mapping 30(10): 3102-3114. [[|DOI]][2] Alsene, K., J. Deckert, et al. (2003). "Association Between A2a Receptor Gene Polymorphisms and Caffeine-Induced Anxiety." Neuropsychopharmacology 28(9): 1694-1702. [[|DOI]][3] Cardinali, D. P. (1980). "Methylxanthines: possible mechanisms of action in brain." Trends in Pharmacological Sciences 1(2): 405-407. [[|DOI]][4] Daly, J. (2007). "Caffeine analogs: biomedical impact." Cellular and Molecular Life Sciences 64(16): 2153-2169. [[|DOI]][5] Fisone, G., A. Borgkvist, et al. (2004). "Caffeine as a psychomotor stimulant: mechanism of action." Cellular and Molecular Life Sciences 61(7): 857-872. [[|DOI]][6] James, J. and P. Rogers (2005). "Effects of caffeine on performance and mood: withdrawal reversal is the most plausible explanation." Psychopharmacology 182(1): 1-8. [[|DOI]][7] Lin, Y. and J. W. Phillis (1990). "Chronic caffeine exposure enhances adenosinergic inhibition of cerebral cortical neurons." Brain Research 520(1-2): 322-323. [[|DOI] ][8] Logan L., Seale T. W. and Carney J. M. (1986) Inherent differences in sensitivity to methylxanthines among inbred mice. Pharmacol. Biochem. Behav. 24: 1281–1286 [[|DOI]][9] Nehlig, A., J. L. Daval, et al. (1992). "CAFFEINE AND THE CENTRAL-NERVOUS-SYSTEM - MECHANISMS OF ACTION, BIOCHEMICAL, METABOLIC AND PSYCHOSTIMULANT EFFECTS." Brain Research Reviews 17(2): 139-169. [[|DOI]][10] Robertson, D., D. Wade, et al. (1981). "Tolerance to the humoral and hemodynamic effects of caffeine in man." The Journal of Clinical Investigation 67(4): 1111-1117. [[|DOI]][11] Shapiro, R. (2007). "Caffeine and headaches." Neurological Sciences 28(0): S179-S183. [[|DOI]][12] Svenningsson P., Nomikos G. G., Ongini E. and Fredholm B.B. (1997) Antagonism of adenosine A2A receptors Underlies the behavioural activating effect of caffeine and is associated with reduced expression of messenger RNA for NGFI-A and CMLS, Cell. Mol. Life Sci. Vol. 61, 2004 Review Article 869 NGFI-B in caudate-putamen and nucleus accumbens. Neuroscience 79: 753–764 [[|DOI]][13] Yang, J. N., O. Bjorklund, et al. (2009). "Mice heterozygous for both A(1) and A(2A) adenosine receptor genes show similarities to mice given long-term caffeine." Journal of Applied Physiology 106(2): 631-639. [[|DOI]][14] Green, R. M. and G. L. Stiles (1986). "Chronic caffeine ingestion sensitizes the A1 adenosine receptor-adenylate cyclase system in rat cerebral cortex." The Journal of Clinical Investigation 77(1): 222-227. [[|DOI]][15] Evans, S. and R. Griffiths (1992). "Caffeine tolerance and choice in humans." Psychopharmacology 108(1): 51-59. [[|DOI]][16] Karcz-Kubicha, M., K. Antoniou, et al. (2003). "Involvement of Adenosine A1 and A2A Receptors in the Motor Effects of Caffeine after its Acute and Chronic Administration." Neuropsychopharmacology 28(7): 1281-1291. [[|DOI]][17] Fredholm, B. B. (1979). "Are methylxanthine effects due to antagonism of endogenous adenosine?" Trends in Pharmacological Sciences 1(1): 129-132. [[|DOI]][18]SATTIN, A. and T. W. RALL (1970). "The Effect of Adenosine and Adenine Nucleotides on the Cyclic Adenosine 3',5'-Phosphate Content of Guinea Pig Cerebral Cortex Slices." Molecular Pharmacology 6(1): 13-23. [[|DOI]][19] Kopf, S. R., A. Melani, et al. (1999). "Adenosine and memory storage." Psychopharmacology 146(2): 214-219. [[|DOI]][20] Bruns R. F., Fergus J. H., Badger E. W., Bristol J. A., Santay L. A., Hartman J. D. et al. (1987) Binding of the A1-selective adenosine antagonist 8-cyclopentyl-1,3-dipropylxanthine to rat brain membranes. Naunyn-Schmiedeberg’s Arch. Pharmac.335: 59–63