Ex­pert: How Ibupro­fen Turns Off Headaches

SAYS ANTIDEPRESSANTS STOP THE NERVE CELLS SUCK­ING UP EX­TRA SEROTONIN Painkillers are ‘ag­o­nists’ - drugs that bind to re­cep­tors in body’s cells

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This trig­gers a re­ac­tion - stop­ping the sig­nals that cause us to feel pain

Whether a drug is pre­scribed by the doc­tor, bought over the counter or ob­tained il­le­gally, we mostly take their mech­a­nism of ac­tion for granted and trust they will do what they’re sup­posed to. But how does the ibupro­fen pill turn off your headache? And what does the an­tide­pres­sant do to help bal­ance your brain chem­istry? For some­thing that seems so in­cred­i­ble, drug me­chan­ics are won­der­fully sim­ple. It’s mostly about re­cep­tors and the mol­e­cules that activate them.

RE­CEP­TORS

Re­cep­tors are large pro­tein mol­e­cules em­bed­ded in the cell wall, or mem­brane. They re­ceive (hence be­ing called ‘re­cep­tors’) chem­i­cal in­for­ma­tion from other mol­e­cules – such as drugs, hor­mones or neu­ro­trans­mit­ters – out­side the cell. These out­side mol­e­cules bind to re­cep­tors on the cell, ac­ti­vat­ing the re­cep­tor and gen­er­at­ing a bio­chem­i­cal or elec­tric sig­nal in­side the cell. This sig­nal then makes the cell do cer­tain things such as mak­ing us feel pain.

AGONIST DRUGS

Those mol­e­cules that bind to spe­cific re­cep­tors and cause a process in the cell to be­come more ac­tive are called ag­o­nists. An agonist is some­thing that causes a spe­cific phys­i­o­log­i­cal re­sponse in the cell. They can be nat­u­ral or ar­ti­fi­cial. For in­stance, en­dor­phins are nat­u­ral ag­o­nists of opi­oid re­cep­tors. But mor­phine – or heroin that turns into mor­phine in the body – is an ar­ti­fi­cial agonist of the main opi­oid re­cep­tor.

An ar­ti­fi­cial agonist is so struc­turally sim­i­lar to a re­cep­tor’s nat­u­ral agonist that it can have the same ef­fect on the re­cep­tor. Many drugs are made to mimic nat­u­ral ag­o­nists so they can bind to their re­cep­tors and elicit the same – or much stronger – re­ac­tion. Sim­ply put, an agonist is like the key that fits in the lock (the re­cep­tor) and turns it to open the door - or to send a bio­chem­i­cal or elec­tri­cal sig­nal to ex­ert an ef­fect. The nat­u­ral agonist is the master key but it is pos­si­ble to de­sign other keys (agonist drugs) that do the same job.

Mor­phine, for in­stance, wasn’t de­signed by the body but can be found nat­u­rally in opium pop­pies. By luck it mim­ics the shape of the nat­u­ral opi­oid ag­o­nists, the en­dor­phins, that are nat­u­ral pain re­liev­ers re­spon­si­ble for the ‘en­dor­phin high’.

Spe­cific ef­fects such as pain re­lief or eu­pho­ria hap­pen be­cause opi­oid re­cep­tors are only present in some parts of the brain and body that af­fect those ar­eas. The main ac­tive in­gre­di­ent in cannabis, THC, is an agonist of the cannabi­noid re­cep­tor. And hal­lu­cino­genic drug LSD is a syn­thetic mol­e­cule mim­ick­ing the agonist ac­tions of the neu­ro­trans­mit­ter serotonin at one of its many re­cep­tors – the 5HT2A re­cep­tor.

MEM­BRANE TRANS­PORT IN­HIBITORS

Mem­brane trans­porters are large pro­teins em­bed­ded in a cell’s mem­brane that shut­tle smaller mol­e­cules – such as neu­ro­trans­mit­ters – from out­side of the cell that re­leases them, back to the in­side. Some drugs act to in­hibit their ac­tion.

Se­lec­tive serotonin re­up­take in­hibitors (SSRIs) – such as the an­tide­pres­sant flu­ox­e­tine (mar­keted as Prozac) – work like this. Serotonin is a brain neu­ro­trans­mit­ter that reg­u­lates mood, sleep and other func­tions such as body tem­per­a­ture.

It’s re­leased from nerves, bind­ing to serotonin re­cep­tors on nearby cells in the brain. For the process to work smoothly, the brain must quickly turn off the sig­nals com­ing from the serotonin soon af­ter the chem­i­cals are re­leased from the nerves. Oth­er­wise mo­ment-to-mo­ment con­trol of brain and body func­tion would be im­pos­si­ble. The brain does so with the help of serotonin trans­porters in the nerve’s mem­brane. Like a vac­uum cleaner, the trans­porters scoop serotonin mol­e­cules that haven’t bound to re­cep­tors and trans­port them back to the in­side of the nerve for later use. SSRI drugs work by get­ting stuck in­side the vac­uum hose so un­bound serotonin mol­e­cules can’t be trans­ported back into the nerve. Be­cause more serotonin mol­e­cules are then hang­ing around re­cep­tors for longer, they con­tinue to stim­u­late them.

Like a vac­uum cleaner, the trans­porters scoop serotonin mol­e­cules that haven’t bound to re­cep­tors and trans­port them back to the in­side of the nerve for later use.

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