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| John Williams, PhD |
John Williams, PhD, is a senior scientist at the Vollum Institute in Portland, Oregon. Williams investigates the actions of different endogenous neurotransmitters and exogenous drugs on the neurons that act in the "reward pathway," which comprise the cellular circuit that is responsible for drug addiction.
Give us a general overview of what a neurotransmitter is and what it does.
A neurotransmitter is a molecule that's released from the terminals of neurons, the very ends of the neurons, and it traverses a gap between neurons, and it activates receptors on the other side of the gap and signals either excitation on inhibition.
What is a neurotransmitter transporter?
There are a number of different ways that the actions of neurotransmitters are terminated. Sometimes they're metabolized into inactive molecules. Other times they're taken out of the extracellular solution through these neurotransmitter transporters.
Touch on what a receptor is.
A neurotransmitter binds to molecules called receptors. It's got a right fit for the transmitter, and very often, if we're talking about dopamine as being the neurotransmitter, the receptor is called a dopamine receptor. And there are a number of different kinds of dopamine receptors. They're all a little bit different, but they all recognize dopamine.
Once they recognize dopamine and that fit happens, what happens from there?
In the case of dopamine receptors, they change the conformation of the receptor so that another molecule that's on the inside of the cell is activated. These molecules are called G proteins. G proteins are made up of three subunits, and they're called G proteins because they bind another molecule called GTP (guanosine triphosphate). And once the receptor activates the G protein, then it goes on and has other effects in the cell.
Can you give us a definition of the reward pathway?
Yeah, the reward pathway is actually a pretty extensive pathway, but the heart and soul of the reward pathway is a group of cells in the mid-brain called the ventral tegmental area. These are dopamine cells, and they live very close to the neurons of the substantia nigra, which are also dopamine cells; those are the ones that are damaged in Parkinsonism. So the ventral tegmental area is, again, a group of dopamine cells that project to areas like the prefrontal cortex, the amygdala, and the nucleus accumbens. Those are the basic components probably of the reward pathway.
Can you talk about why the reward pathway is so named?
It's called the reward pathway because animals will work to have stimuli to their ventral tegmental area. So the activation of these dopamine cells is thought to be rewarding, because animals will do things to get those cells to be stimulated.
So is there a physically beneficial result of this activation?
Yeah. I mean, reward--endogenous reward--is really important for survival, right? People like to eat. Sex is very important. All the things, behaviors that people enjoy work through this pathway. So it's a real important survival area for everyone, basically.
What's an opioid?
An opioid is another molecule that could be considered a neurotransmitter. There are endogenous opioids, which are peptide molecules. And they're released from neurons, and they bind to opioid receptors. Just like dopamine's a transmitter and binds to dopamine receptors. So opiate drugs, like morphine, just mimic the endogenous opioids, and they bind to the opioid receptor just like the endogenous ones.
Can you give us a brief description of an ion channel?
There are a bunch of different kinds of ion channels. They're basically holes in the membrane, formed by protein molecules that span the plasma membrane. And ion channels allow certain ions to pass through them. There are potassium channels; those allow potassium ions through, but not sodium ions and not chloride ions. And there are sodium channels as well, which let only sodium through. The main ion channels that are found in neurons are sodium channels. They account for a lot of an action potential, and excitatory activity.On the other hand, potassium channels are the ones that are mostly active when the neuron is at rest, when the membrane is at its rest potential. Calcium channels are very important for neurotransmission. And chloride channels are very important for inhibition. Those are the main ion channels that are important in neuronal communication.
What metaphors are used to describe the function of the channels, in terms of the way they allow things to pass in and out of the cell?
There are voltage-dependent channels, like the sodium channels, that account for action potentials. There are other channels that are opened by neurotransmitters. Those we call ligand-gated channels, such as glutamate channels, and GABA channels. And there are channels that are opened by these G protein-linked receptors. So these would be G protein-activated or -inhibited ion channels.
Can you give us an overview of how drugs can interfere with neurotransmission?
Well, we've talked a little bit about opioids, right? And we also talked about how if you stimulate in the ventral tegmental area, that animals will work to receive that stimulation. And what opioids do is actually to increase the activity of dopamine cells.And they do it through a complicated way. Opioids inhibit neurons that in turn inhibit the dopamine cells. So if you inhibit those cells, you dis-inhibit dopamine cells, effectively exciting them, and they fire faster. So that's the way opioids act in the ventral tegmental area to increase the activity of dopamine cells.
Can you give us an example of a purely excitatory type of neurotransmission?
Well, we know there's another neurotransmitter called glutamate. Glutamate is an excitatory transmitter. It activates the glutamate receptor, which is a part of a ligand-gated ion channel. The glutamate receptor itself is the channel. And when that channel becomes activated, sodium ions move in, and that depolarizes the cell and increases the excitability.Areas like the prefrontal cortex have neurons that release glutamate as their neurotransmitter, and they innervate dopamine cells-they talk to dopamine cells. So when prefrontal cortex cells fire, they increase the excitability of dopamine cells.
Is there a different class of drugs that's involved in excitatory transmission versus dis-inhibition?
Well, if you think about drugs of abuse, all drugs of abuse seem to speed the activity of dopamine cells. Now glutamate is not a drug of abuse. It's just a neurotransmitter that speeds dopamine cells. There are not many drugs of abuse that directly activate dopamine cells; they mostly affect them indirectly.One example of one drug that does is nicotine. Nicotine activates dopamine cells directly.
So different classes of drugs have either a disinhibitory or an excitatory result?
So opioids cause the disinhibition. Nicotine causes a direct excitation. Things like cocaine and amphetamine, if anything, they cause an inhibition of dopamine cells, and the way they do that is by blocking the reuptake molecule that takes dopamine out of the extracellular space. And that would cause an inhibition of dopamine cells.But the key thing is that these things do the same thing at the terminal areas of the dopamine cells. So in the nucleus accumbens and the prefrontal cortex, dopamine hangs around longer, and therefore the dopamine cells fire faster-because there's more dopamine in the projection areas, where the dopamine cells' terminals are.
On the topic of drugs, do you have anything to say about Ritalin?
Well, Ritalin is a reuptake blocker, a dopamine reuptake blocker. And that's its mechanism of action. How it produces positive effects for kids, we really don't know. But there's some sort of pathology in the kids that the Ritalin corrects.So for us to study the effects of Ritalin in a normal rat doesn't really make a whole lot of sense. We can talk about mechanism, but since we don't know the pathology, it's really hard to predict why it works.
Can you give us an overview of your general methodology for assessing drug effects, what your area of study involves?
We study rat brains. We use the brains from animals that have developed normally. They're generally adult animals, and we take thin sections of the brain - we call them brain slices - and we can actually identify the cells that we're recording from. And we look at the excitability of the cells using recording electrodes. We can stimulate neurotransmitter release. We can also apply drugs exogenously, so that we can really test the effects of the endogenous compounds, the neurotransmitters, as well as exogenously apply the neurotransmitters as well as the drugs of abuse.
Can you give us an idea of what patch clumping is?
Patch clumping is just a method to make recordings from cells. It involves taking a glass pipette and making a very tight seal between the glass and the neuron that we're studying.You make a tight seal between a glass pipette and the plasma membrane of a cell. The glass pipette is like a little straw. So once you make the seal between the straw and the cell, then you can disrupt the membrane, the plasma membrane that's on the inside of the pipette, and have direct access to the inside of the cell.So now you've got very good electrical continuity between your pipette and the inside of the cell. And when ion channels open and close in the cell membrane, you're able to record the ion currents that pass through those ion channels.
And what kind of things can you conclude about the cell by measuring the currents?
By recording the activity of cells, you can say, "does this transmitter excite the cell or inhibit the cell? What does the drug of abuse do to the excitability of the cell? And then you can determine which ion channels are opened or closed by the neurotransmitter or the drug. You can also study the molecules that are controlling the channels inside the cell. So it's a really powerful way to look at the insides of cells.
Do you have anything to say about the idea that pleasurable feelings are maybe sometimes greater with an exogenously introduced drug versus a natural occurrence?
So like we talked about before, the reward pathway is very important, and the regulation of dopamine cells through the endogenous reward pathways is regulated by things like glutamate and GABA and dopamine. And what drugs of abuse do is just take over that pathway, so that perhaps even without any kind of endogenous reward, the drug takes over that pathway. And that's the problem. It basically floods the reward system with no real reward happening. And then it makes it seem like the normally rewarding things are less rewarding. And then you run into trouble. That's terrible.
Does your work ever get to questions of addiction?
Well, we don't really study addiction. We'll study the adaptive mechanisms that happen after treating animals chronically with morphine or cocaine. So we'll take a normal animal and give them cocaine and see how neurotransmission changes in these dopamine cells.We and many other people are also doing experiments where they ask the animal to take cocaine himself. So we'll allow the animal to self-administer drugs and then see what's changed in the neurotransmission around these dopamine cells. And the hope is that by learning about these adaptive changes, we'll be able to make some comments about what really happens during addiction.
Would you agree that it's not an overstatement to say that the functioning of the brain is fundamentally altered by certain drug abuse?
It certainly is altered acutely, and with repeated treatments, there are changes that can endure for many, many years. Even in rats. You can cause changes that last for years.
Can you give us an example of dopamine as a neurotransmitter, and how drugs influence its actions?
So dopamine is made only by dopamine cells, right? It starts with an enzyme called tyrosine hydroxylase-tyrosine is an amino acid, and through a series of steps, it becomes dopamine. There are probably five different subtypes of dopamine receptors, and they do a number of different things. All of these receptors are G protein-linked receptors, so they have complex cascades of events that they set off in the cell.And it is really interesting because dopamine is known to be very important, but there are very, very few studies where the dopamine has been studied as a neurotransmitter, right? There are very few studies where people have been able to stimulate dopamine cells and then record the effect of the endogenous dopamine on the neuron down the line. And I think that's one of the challenges for us to be able to understand a little bit more about what dopamine is doing.So I think, in general, the effects of dopamine are pretty subtle. And that's why we've had difficulty discovering really what it does at a cellular level. At the behavioral level, it's clearly important. People study it all the time. Neurochemically, people study it with dialysis and with voltammetry. And we know that in diseases like Parkinsonism, if dopamine is gone, then the animal is sick. So it's clearly important, but as for the cellular studies of it, I think there needs to be a lot more work at the cellular level on dopamine. Neurotransmitters like GABA and glutamate and acetylcholine, they cause very rapid and large changes in membrane potential in the excitability of the cells that they act on. Dopamine, on the other hand, causes slow and small changes, which makes it very hard to detect, and therefore there is a problem with being able to study it directly.
That leads into the next area, advances in technology and the ideas of the 1990's as the decade of the brain. Have there been technological improvements that have improved the study of dopamine at the cellular level?
Basically, in the 1980's, the brain slice technology was developed very rapidly. So people could take brain slices and record from neurons in areas that they never could before.In the '90's, what's happened is that with these brain slices, you can now put the slices under a high-power microscope and actually look at the cells in the living state. And that allows you to make these patch clamp recordings from cells and apply drugs directly onto the cells, and it's really revolutionized the way we've been able to look at brain cells.The other thing that's happened-that's actually happening now and will be very important-is that we can now, using genetic manipulations, mark specific cell groups and identify those cell groups with fluorescence microscopy. So for example, Malcolm Lowe here at the Vollum Institute has engineered an animal in which cells that produce beta-endorphin, one of the endogenous opioids, those cells now also express a green fluorescent protein so that we can identify those cells in a living slice.And, there are only a thousand or two thousand of these cells in an animal, and it would be very, very difficult to record from those guys if you had to just pick out randomly what they were. But now we can see them, we can record from them, and actually say something about specific cell types. And I think that's just beginning to happen, and it's going to change the way we understand the brain.
Can you talk about how technology has opened up these areas that you may not have been able to study before?
Yeah. One of the things that we've been interested in from my beginnings is the cellular effect of opioids. So opioids cause inhibition of cells directly. We've been able to identify the ion channels and the second messenger cascades that are activated by opioids. And we're currently looking at how, if you extend the application of opioids, chronic treatment of opioids, what are the other second messenger cascades that change how that opioid signaling happens. And with the development of this wholesale recording from identified cells and all the genetic manipulations that are possible now, I think we'll be able to dissect all these pathways that end up causing morphine to be less effective; that is, tolerance. You know, morphine is one of the compounds that are still very useful therapeutically, and one of the big problems with it is that people become tolerant to it.So it takes more morphine to cause the relief of pain. And if we can understand what causes this tolerance to morphine, maybe we can engineer ways to make the treatment more effective and more long lasting.
There are two kinds of neurotransmitters - neurotransmitters do two different things. The one is the G protein-linked receptor. We went through that. The other is a neurotransmitter that binds to a receptor molecule that is also an ion channel. So things like acetylcholine and glutamate and GABA bind to their receptors and they open an ion channel directly.So the receptor and the ion channel are one protein molecule. And that's a very good way for neurotransmitters to have point-to-point communication by opening this ion channel. It happens very rapidly, and the change in membrane potential or the change in excitability is very large.
So one way that people think about the excitability of neurons, rather than to think about channels opening and closing, is to think of them as resistors and batteries So if normally a channel is closed, the resistance through it would be very high, right? If the channel opens, the resistance decreases dramatically, and you see more of the battery, right? So that changes the electrical potential that you see across the cell.So the engineers think about neurons as a series of resistors and capacitors and little batteries.
Are there any other research questions that keep you going?
Well, I think that one of the areas that's also had a huge amount of study in the past 10 years or 20 years has been the whole issue of synaptic plasticity, neural plasticity. Memory. And I think one of the things that we've realized recently is that drugs of abuse basically act by altering plasticity. So that memories of drugs, although they don't affect the same circuits as normal memories of things, they probably act by similar mechanisms at the cellular level.So actually studying drugs of abuse is maybe a more simple way to investigate how memory occurs.
Do you have anything to say about synaptic plasticity, especially about the aging brain, the possibility of growing new connections?
We know that as we age, we lose dopamine cells. Some people lose them faster, and they become Parkinsonian. We think that probably one of the most important things that dopamine does is to help or facilitate memory, perhaps through this synaptic plasticity sort of thing at both the presynaptic level and the postsynaptic level.
Touch on the drug abuse connection.
Take, for example, the opioid system-there are endogenous opioids. They're found in many, many places throughout the brain, and probably some of the things that - that help us feel good, some of the rewarding things, are due to the release of those opioids. And those molecules-although they bind to the same receptors as drugs like morphine, they're released transiently. They give you a good feeling for a while.Morphine is a very different molecule. It stays around, it acts for a much longer period of time, and we're discovering now that it actually has very different effects that, again, overwhelm the reward system and require the brain to adapt in a very different way than it does to the endogenous compounds.And I think that's the problem with drugs of abuse like morphine, is that it overwhelms the system; it changes the plasticity of the brain in a way that's very difficult to recover from. It doesn't do anything acutely, that the endogenous compounds do. Morphine is a very, very effective therapeutic drug for the relief of pain and, unfortunately, it's also a drug of abuse.
Do you think there's a lot more work to be done in analyzing at the cellular level how both therapeutic drugs and drugs of abuse work?
In the case of opioids, people thought for a long time they could discover an opioid that would relieve pain, but not result in tolerance. And a huge amount of effort has gone into the development of many, many opioid compounds. And none of them relieves pain without causing tolerance.So I don't think the question of tolerance will be answered by finding a new drug. I think what we have to do is discover what about the drug causes tolerance, and understand tolerance more, and perhaps that way, we'll be able to develop therapies where pain relief can happen, but dependence doesn't.