Atropine: Muscarinic Acetylcholine Receptor

 

 Marneros, A.; Gutmann, P.; Uhlmann, F. Eur Arch Psychiatry Clin Neurosci 2006, 256, 458–459.

Holy Molly! That's what atropine and scopolamine can do to you. Atropine poisoning produces a unique set of symptoms known as the anticholinergic toxidrome. There's even a medical mnemonic for it: ‘‘Red as a beet, dry as a bone, blind as bat, hot as hare, mad as hatter, seizing as a squirrel.’’ Today, we will examine how atropine works on a molecular level, but before that, let’s look at the basics of how poison interacts with human body. 

Drugs and poisons bind to cellular target to elicit their action. Drug targets are often proteins or nucleic acids (DNA), which may or may not be specific to certain organs. When a drug binds to its target, it induces a change of shape (conformation), or activation/inhibition of certain genes that would lead to diminished, amplified or halted downstream response. If the response is overwhelming and harmful, that's poisoning. In analogy, a drug (poison) and its target are like lock and key. When a key is inserted into a lock, the ‘unlocking’ action changes the inner shape of the lock, allowing it to open or close. On a molecular level, the ‘unlocking’ mechanism is induced by different chemical substituents of a drug, which can interact with its target (induced-fit model). For example, a negatively charged 'lock' can be attracted and opened by a key with opposite charge. The binding of ‘lock to a key’ is usually very specific, but the binding itself may or may not be permanent. Some 'keys' get stuck and can only be used once, others can be reused or recycled indefinitely.

The human nervous system in particular relies on many locks and keys to ensure proper functioning. We call the locks receptors,  the keys are hormones or neurotransmitters. It is the neurotransmitters that will be our focus. In general, the human nervous system is comprised of two parts, the central nervous system (brain and spinal cord) and peripheral nervous system (motor division and sensory division). Central nervous system processes thoughts and memories, as well as spinal cord mediated reflexes. In contrast, peripheral nervous system takes charge of sensory perceptions and movement (motor response). The human motor response can be further classified into two types, i.e., voluntary (somatic) and involuntary (autonomic). Within the autonomic nervous system, there are two opposing divisions, namely, sympathetic (accelerator: fight or flight) Vs parasympathetic (decelerator: rest, and digest). Both work together to regulate heart rate, bowel movement, pupil dilation and matters humans can’t mind over. The parasympathetic nervous system is the drug target of atropine, scopolamine and related tropane alkaloids. Figure 1 is an executive summary of the human nervous transmission. We will refer to it many times in the future.

Figure 1: Basic outline of the human nervous system.

The most important neurotransmitter to unlock receptors of the peripheral nervous system is called acetylcholine (ACh). There are two types of ACh receptors, namely, the nicotinic ACh receptor (nAChR) and muscarinic ACh receptor (mAChR). Both are proteins found on cell membrane, but they are very different. nAChR is an ion channel, it is mainly responsible for voluntary control of skeletal muscle. Hence, nAChRs are very fast in action, often counted by milliseconds. The moment you wish to click ‘back’, your finger is already here thanks to nAChR. Many plant toxins like curare and nicotine target nAChRs, we will examine them in the future.

In contrast, mAChR acts slower (seconds or minutes) because it relies on a second messenger to carry out signaling. Think of it as a business which requires a middleman. If a black out happens now, your pupils can't dilate as fast as you wish, it takes a minute or two for your vision to adjust. Not that you can do anything about it, thanks to mAChR. On a molecular level, mAChR is comprised of seven protein chains (helices) which loop in and out of cell membrane. This is called a G-Protein Coupled Receptor (GPCR). The “G” stands for a special protein called G-protein, evidently biologists aren’t very creative! The G-protein is made up of three components, namely, alpha, beta and gamma sub-units. The three live happily together but think of their relationship as a love triangle. The alpha subunit is engaged to a molecule called GDP (Guanosine diphosphate). However, when ACh binds to mAChR, it convinces alpha subunit to break up with GDP. Now, Alpha hooks up  GTP (guanosine triphosphate), making Beta and Gamma very unhappy, they kick Alpha-GTP out of the house. The new couple travels across the cell membrane to find a settlement for their honeymoon, which happens to be certain enzymes. Depending on the subtypes of mAChRs, it may be an enzyme called adenylate cyclase or phospholipase C. In either case, it results in the slowing down of adenylate cyclase, or activation of phospholipase C. This generates two important middlemen (second messenger) called cAMP (reduced upon activation of mAChR) or IP3 and DAG (increased upon activation of mAChR), which regulate the parasympathetic nervous system. Ironically, both enzymes have intrinsic ability to ruin honeymoon, which causes the Alpha to dump GTP (hydrolysis) and reconciles with GDP. At last, the reconciled couple goes home and embraces their old mates (Beta and Gamma subunits). The whole cycle goes all over again when another ACh molecule comes visit!  See Figure 2 for pictorial explanation.

Figure 2: Cartoon representation of a GPCR activation pathway. Rang, H. P.; Ritter, J. M.; Flower, R. J.; Henderson, G. Rang & Dales’s Pharmacology; Elsevier: Edinburgh, 2012; pp 31.

There are five subtypes of mAChRs, i.e., M1, M2, M3, M4 and M5. Subtypes M1, M3 and M5 hire IP3 (and DAG, Gq-GPCR) as their middlemen, subtypes M2 and M4 hire cAMP (Gi-GPCR) instead. M1 and M3 regulate digestion, sweating, excretion, airway constriction and pupil dilation. M2 slows down the heart rate, but we are not  sure what M4 and M5 do just yet. mAChRs are also present in the central nervous system, they collectively modulate cognitive behavior, memory, and locomotion. Regardless of where they are, GPCR signalling pathways remain the same. Atropine and scopolamine antagonise all subtypes of mAChRs (non-selective antagoniost), they compete with ACh and prevent it from binding to target. The net effect mAChR antagonism is the dysregulation of all aforementioned processes. This produces the broad spectrum symptoms of anticholinergic toxidrome.

With these in mind, we can explore the structure activity relationship of atropine in the next article. Before I end, let me leave you with two questions. How can acetylcholine escape its embrace with the muscarinic receptor? What does ‘muscarinic’ mean? We will unlock these in the next post. There’s no chemistry this round, but I guarantee a lot to come. In case you are curious about the fate of the unfortunate boy who chomped his own tongue and penis, find the full story below.

Marneros, A.; Gutmann, P.; Uhlmann, F. Eur Arch Psychiatry Clin Neurosci 2006, 256, 458–459.