Puzzling over caffeine and adenosine-it's time to talk about receptors (please don't be antagonistic!)

Heading back to university for the start of the academic year it'll be inevitable that my coffee consumption will increase exponentially. So I'm back again and thinking about what happens in my body when I consume my daily caffeine fix.

After looking at caffeine metabolism and whether caffeine is addictive and causes insomnia in my previous posts. I thought it was time to start looking at HOW caffeine changes our body's functioning.

So time to start fitting the pieces together of the physiological effects of caffeine. This starts with receptors.

What are receptors?

Receptors are proteins which can be found on cell membranes. They bind to specific chemicals (these could be hormones, drugs, neurotransmitters etc). The binding of these chemicals can cause a change to occur in the cell.

It is natural when looking at the biological effect of caffeine in the body to start looking at what happens when caffeine binds to a receptor.

The caffeine-adenosine relationship

Everybody has two friends which are uncannily similar to each other. Caffeine and adenosine are those uncannily similar friends-they have a similar molecular structure.

Spot the difference for the chemists!

These similarities in structure mean that caffeine can bind to adenosine receptors and block their function. Adenosine on binding to its receptor generally inhibits physiological activity (it makes us feel more tired). Due to adenosine and its half brother caffeine having a similar molecular structure caffeine blocks adenosine receptors (mainly  A1 and A2 types of adenosine receptors) preventing their activation resulting in caffeine having a stimulant effect. Hence caffeine is an antagonist (It interferes with the binding and physiological affect of adenosine).

Still puzzled?

So if that’s perhaps gone over your head a little......

 Imagine you are putting together pieces of a puzzle. You are looking for a piece of the puzzle that joins to the piece you have in your hand (the adenosine receptor). You can see two similar looking pieces on the floor (an adenosine and caffeine molecule) that look like they'll both join onto the piece you are holding. You try piece A (the caffeine molecule) and find that it pretty much fits onto the piece you are holding (the receptor) but the fit is not perfect. Despite this you decide to continue putting the puzzle together. However you find that you cannot fit the rest of the puzzle together and complete it (receptor has not been activated so the physiological effect is inhibited-completing the puzzle) as the two pieces you first joined together are not the correct pieces to join to complete the puzzle (the physiological effect is inhibited).

You go back to the original piece you had in your hand (receptor) and this time you join piece B (the adenosine molecule) onto it and find that these two pieces fit perfectly together. This time you find you are easily able to complete the puzzle (the receptor is activated resulting in the full physiological effect-the puzzle is completed) as all the other pieces fit around the two you first joined together (the receptor and adenosine molecule).

Let’s put the puzzle together....

The binding of caffeine to adenosine receptors prevents adenosine from binding to the receptor causing a change in the cell's behaviour. Caffeine inhibits the effect of adenosine. This is how caffeine can make us feel more awake. As adenosine decreases neurotransmitter release, dilates blood vessel and inhibits lipolysis (break down of fats)-these effects make us feel more tired. Caffeine preventing this in turn makes us feel more awake.

Upcoming posts....

Caffeine and its effect on the physiological processes in our body- how this stimulant takes its effect on the nervous, gastrointestinal, respiratory systems and renal function.

Lets try and resolve some caffeine confusion

As I settled down to read the newspaper with (yes) a cup of coffee! An article about coffee caught my eye. "More than four cups of coffee a day increases risk of early death" (check it out and leave your comments).

Again the mysterious cup of coffee causes a stir. 

Drinking coffee is such a prominent part of the modern lifestyle with coffee shops clustering in every high street and our supermarket shelves stacked with all kind of coffee. It surprises me how variant
peoples views are on the effects of this seemingly everyday essential to our health is.


Caffeine is mainly found in coffee, tea, energy drinks, chocolate, cold and flu medicines and surprisingly some caffeine is still found in a cup of decaffeinated coffee (approximately 3.5% of the caffeine found in an non-decaffeinated coffee of the same strength and volume).

Sources of caffeine

Can caffeine be addictive?

Firstly I must define what I am referring to as an 'addictive' substance: a substance which an individual becomes physically and psychologically dependant and has the compulsion to increase the dose of said substance.
Scientists don't class caffeine as an abusive addictive drug. Regular caffeine users may experience a mild physical dependence. Withdrawal symptoms include: headache, fatigue and lack of concentration. However these symptoms may only last a day or so. These symptoms can be reduced is caffeine intake is reduced gradually.
But this still hasn't answered my question as to whether caffeine is addictive?
Well caffeine does not cause severe withdrawal symptoms or the user to exhibit compulsive behaviours as with other drugs. So caffeine is generally not considered addictive, certainly  medically speaking and according to my definition of 'addictive substance'.

Does caffeine cause insomnia? 

It's all about timings....

As mentioned in my previous post caffeine is rapidly absorbed and processed by the body (in about 45 minutes it is distributed to tissues in the body). It takes about 5-7 hours for half of the caffeine to be eliminated from the body (based on dosage from an average cup of coffee). It takes up to 10 hours for 3/4 of caffeine to be eliminated.

So what has this timing got to do with insomnia?

Caffeine is a stimulant so it keeps parts of your brain overactive preventing you from sleeping .

This means that a cup of coffee or two in the morning is unlikely to affect the average persons sleeping habits or consuming caffeine 6 hours before going to bed. However this depends of the individuals sensitivity to caffeine. People who are more sensitive to caffeine may experience insomnia, nervousness and gastrointestinal upset.

Caffeine is known to be a diuretic (it makes us need to pee), consuming a product containing caffeine late at night can mean we won't sleep properly as we keep needing the toilet.

What about increased risk of death?

Caffeine is just a small part of our diet and certainly in small doses it won't impact our health (although a large dose would mean drinking about 100 cups of coffee in a short space of time). Excessive coffee drinking is often linked to other unhealthy habits which may account for the statistics displayed in the article. But remain calm. You're not going to drop dead from drinking another cup of coffee.

Thanks for reading!

Next posts?

I'd like to investigate more of the biological effects of caffeine. And start asking how and why :)

Why does coffee make us pee? How does it mask symptoms of tiredness?