To continue the theme of “how painkillers kill pain” I’m going to take a tour of the opiate drugs, which include morphine, codeine and, unnervingly, heroin. So, we know from earlier posts that NSAIDs (nonsteroidal anti-inflammatory drugs, for those of you who haven’t read that post, or just don’t remember silly long acronyms) act by inhibiting inflammation, which often causes pain. Opiates, on the other hand, don’t act at the source of pain; they simply stop the messages that tell your brain to feel pain from getting through. To get your head around this, we might need to have a closer look at the concept of pain.
What we think of as pain is actually a behavioural action of the brain; the response to a “harmful stimulus” (i.e., big hot sharp spiky thing) itself is known as nociception. This means that the big hot sharp spiky thing is noticed by the receptors on your toe, fingers, knee or other miscellaneous body part and a message is sent to your brain, which then makes you experience pain, to make sure that you get your relevant body part away from the big hot sharp spiky thing (and also shout ouch, and any number of other four-letter words, because that always helps). This is all, obviously, quite a handy process, big hot sharp spiky things can have detrimental effects on… most things actually, they’re not often popular. However, sometimes we may prefer not to feel the pain, because unfortunately it doesn’t necessarily go away as soon as the big hot sharp spiky thing goes away (usually because said big hot sharp spiky thing has caused some damage to your body part). Opiate drugs stop you feeling pain by stopping your brain from receiving the nociceptive messages.
They do this by (you guessed it) binding to a receptor, specifically an opioid receptor (I know, original naming, indeed). There are actually three types of opioid receptor, the µ-receptor, the d-receptor and the k-receptor (these not-at-all-scary symbols are pronounced mu, delta and kappa, respectively, and they are Greek letters, which scientists LOVE). Opiate drugs bind to the µ-receptor, which is found in the brain and spinal cord. When bound to by an opiate (like morphine) the µ‑receptor acts to inhibit the transmission of impulses along a nerve in your body. It does this in a number of ways; particularly, within your cell it blocks the opening of calcium channels and encourages the opening of potassium channels. This has the effect of stopping nerve impulses being transmitted. The reason for this is that transmission of a nerve impulse requires these channels to open and close in sequence along the nerve, passing a wave of changes in the positivity or negativity down the length of the nerve (this is otherwise known as an action potential, something I shall elaborate on at a later date, so as not to frighten you all away with the world’s longest blog post!).
To summarise, then, for anyone that got a bit lost up there in the chemistry, nociceptors (receptors that sense big hot sharp spiky things) send messages to your brain to make it tell your body to quickly shift it and then shout ouch. Morphine, codeine and other opiate drugs bind to opioid receptors, which act to stop this message getting through, thus preventing you from feeling the pain. Interestingly, there are also some natural opioids in the body, which act by binding to opioid receptors, and these include endorphins, which I’m sure you’ve all heard of as being the mediators of good things (they’re famously released when you exercise and/or eat chocolate). These natural opioids can reduce pain, but not necessarily on command. Theoretically this means that if you’re in pain you could try going for a run, or eating a bar of chocolate. A good excuse, if ever I heard one!
Of course, painkilling isn’t all that opiates do, but I shall leave the side effects for a different post, because they are rather extensive, including as they do a joyous cocktail of euphoria, addiction, constipation, cough suppression, withdrawal symptoms and potential coma. Sounds fun, right? It will be.
Finally, while we’re on the subject of opiates, I literally can’t resist giving you a bit of background and leading you into one of my favourite nerdy science jokes. In addition to their actions above, opiates block an enzyme called adenylate cyclase, which reduces levels of another protein, cyclic adenosine monophosphate (or cAMP, and if you remember this acronym by picturing a protein in pink hotpants, you wouldn’t be the first). The joke, therefore, is… how do you know morphine is homophobic? It keeps trying to repress cAMP levels!
I know, brilliant. I like to leave you on a chuckle.
As with all my posts, but particularly this one as it is such an early one, please please let me know if anything is unclear to you or appears inaccurate, confusing or downright wrong. I’m extremely receptive to feedback, and I’d love to hear you thoughts. I’d also love to know if there are any particular drugs you’d be interested in understanding, or indeed anything else you’d like to see me blogging about. I’m always up for a bit of research. 🙂