[MUSIC] Okay so, so far we've talked about receptors, all of one type. The type where when a neurotransmitter binds to the receptor, the receptor molecule actually, or complex actually opens up. And there's a pore through which ions can pass. And these receptors are called ionatropic receptors and they are all ion channels. They are channels through which ions can pass. So ionotropic receptors and ion channels are synonyms. There are some ion channels that are not ionotropic receptors they are instead gated by voltage rather than by a neurotransmitter. But we're talking about receptors now and what's important is that there's a huge class of receptors that don't form a pore and these are called metabotropic receptors. So metabotropic receptors like ionotropic receptors will bind neurotransmitter. But they do not directly lead to an electrical change because this molecule this complex cannot form a pore. Instead this complex is attached to a G protein. So, this is a G-Protein and these are also these are metabotropic receptors are also called G-Protein Coupled Receptors. And so you'll see this abbreviation GPCR a lot, they're G-Protein Coupled Receptors. Now so what's the deal with these metabotropic receptors. Well, why are they important? Well, first let's talk about how they're different. First of all because these G-proteins are now once this is bound to neurotransmitters, these G-proteins are now activated. And they're going to go off and they're going to do things like simulate enzymatic reactions or they're going to bind to other molecules that are in turn going to simulate enzymatic reactions. So these G-proteins are going to go and do things, but that means it takes time. This ionotropic receptor binds, opens, done. Very quick, less than a millisecond. This takes milliseconds, okay, so it has to go off and do stuff. And there's okay, so first, it takes more time. The second thing is that what's the effect of this? Well the effect could be to open a channel, an ion channel somewhere, it could be to close an ion channel. It could have no effect on the ion channels. It could just go off and actually elicit a change in the genetic transcription. So it could have an electrically silent effect on the cell. So there is a huge variation in effect. Another thing that is different about this metabotropic receptors is that they're incredibly numerous and varied. There are more than 1000 types and there are less than 10 types of ionotropic receptors. So one final way to think about this is not only does it take time to activate to see the effect of metabotropic receptor activation. But the metabotropic receptor amplifies it goes through many rounds of activating these G-proteins and so it can have a lasting effect that is pretty difficult to turn off. And so one way to think about this is to contrast something such as the knee jerk reflex that the stretch reflex that a physician evokes when you're sitting in a doctor's office. That's all based on ionotropic receptors versus something like the release of adrenaline when you get really scared, okay. So, you feel like you're about to crash, and you have this incredible rush of adrenaline and that leads to an increase in blood pressure and heart rate, and you're breathing faster. Well, can you turn that off? Can you turn that off on a dime? Not so much, that's really hard to turn off. And one of the things is that not only does it takes time for that to happen, doesn't feel like it takes that much time. But it does take longer than it takes for the ionotropic receptors, but it takes then seconds to minutes for that to turn off. You just simply can't turn off the metabotropic receptors very quickly. Okay, so are these important in your lives? Well, I looked up what are the most commonly prescribed drugs In the United States and the most commonly prescribed drug is the drug called Vicodin which is a mix of Hydrocodone and Acetaminophen. Acetaminophen a different deal but Hydrocodone which is the big deal in Vicodin is acting on G-protein-coupled receptor on the mu opioid receptor. So similarly, all of the autonomic target organs are using metabotropic receptors. Consequently all of those drugs that we use, not all of them but many of the drugs that we use to treat Glaucoma, motion sickness, arrhythmias, hypertension, asthma, irritable bowl syndrome and so on. Those drugs, many of those drugs are acting on G protein coupled receptors. So this is a very common target for drug development. These G protein coupled receptors. Okay. In the next segment we'll wrap up neurotransmission. [MUSIC]
