NeuroLove

DOPAMINE!

Alright, moving along to dopamine (DA), which is probably my favorite neurotransmitter (and likely yours too- even though you don’t realize it!). Dopamine is the neurotransmitter involved in addiction (shown above for cocaine- since cocaine blocks the dopamine transporter, it causes dopamine to stay in the synapse longer), and basically anything rewarding.  When you get a surprise or win some money, dopamine is what causes that feel-good rush! It’s the transmitter involved in reward, but it also has other functions as well- it helps with prefrontal concentration, accurate movements, control of thoughts (too much can cause hallucinations), etc. It’s really an incredible molecule!

Dopamine is produced in the Ventral Tegmental Area (VTA) and Substantia Nigra (SN) in the midbrain and has widespread effects throughout the brain- in the basal ganglia circuitry and throughout cortex. I’ll be talking more about this one in coming posts since it’s quite fascinating to me!

[Image Source]

Another neurotransmitter is serotonin (5HT is the abbreviation- derived from the chemical name, 5-hydroxytryptamine). 5HT is largely found in the gastrointestinal tract where it regulates the muscles there. However, I think its functions in the brain are even more interesting.  Serotonin is largely produced by the Raphe Nuclei in the brain and have functions throughout cortex. The context you have most likely heard about 5HT in the past is because most anti-depressants target serotonin. Selective Serotonin Reuptake Inhibitors (SSRIs) block the reuptake of serotonin and thus keep serotonin in the synapse for longer allowing for greater/prolonged activation of serotonin receptors. Other anti-psychotics also target the serotonin system.

[Image Source]

Nicotine acts as an acetylcholinergic agonist, meaning it increases the activity of the ACh receptors. Ach acts on muscles- and is found in the brain. For people who don’t know, nicotine is the addictive chemical in cigarettes. In some parts of the brain, the activity on these receptors can increase release of dopamine (which is what is thought to be responsible for the addictive properties). Nicotinic promotion of these receptors also increases activity in the sympathetic nervous system, which regulates the fight or flight type response (increased heart rate, decreased digestion, etc.).

[Image Source]

Acetylcholine (ACh) is another neurotransmitter, best known for being the neurotransmitter that bridges the gaps between neurons and muscles. Unlike neurotransmitters acting upon other neurons, ACh has a 1:1 action on muscles. One action potential causes a muscle contraction, no need for build up (like with EPSPs and IPSPs).  The amount of ACh released and the pattern in which it is released will determine how much the muscle contracts and for how long.

I found this great infographic online to show the process of ACh on muscle contraction from here.  Nicotine, the addictive substance in cigarettes, acts upon ACh receptors. ACh also has its own actions in the brain, which I will talk about another time.

Image showing GABAergic neurons in the mouse brain (left). GABA is widespread through the human brain as well, and plays an important role in inhibition especially.  See the previous post for more information.

[Image Source]

GABA!

The next neurotransmitter I will talk about is GABA (gamma-aminobutyric acid). GABA is the primary inhibitory neurotransmitter in the brain, but depending on the receptor type, it can be inhibitory or excitatory. We mainly talk about it’s actions as an inhibitory neurotransmitter, but it’s important to note that it can act as an excitatory one as well, depending on the receptor that it acts upon.

There are two main receptor subtypes for GABA, which are known as GABA-A and GABA-B receptors.  GABA-A receptors are ligand-gated chloride channels.  Hopefully, without further explanation, from my past entries, this makes sense.  If you need a refresher, this means that when GABA interacts with these GABA-A receptors, they undergo a conformational change that opens their “pore” to allow chloride (Cl-) ions to flow through.  Since Cl- is negative, it can hyperpolarize the cell (make it more negative) and make it less likely to fire, in the simplest explanation. 

GABA-B receptors are more complex, as they are those G-protein coupled receptors.  Downstream effects can be to open Cl- channels or K+ channels (since there is more potassium inside the cell, K+ might flow out or even if it does not move, shunt an excitatory signal if it arrives while the channels are open), amongst other things.

Inhibitory actions can be very complex on their own and really help to fine-tune the rest of the brain’s activity.  The image above shows a presynaptic cell below and a postsynaptic neuron above it, as GABA is involved in the hypothalamus/feeding behavior- found in this paper (Richards & Berthoud, 2006).

This is to give a pictoral representation of the process of LTP (long-term potentiation), which seems to be related to the glutamatergic signaling I spoke about previously. (A) would be before LTP and (b) would be after.  You can see that Ca2+ entering through NMDA receptors acts through various downstream molecules to insert more AMPA receptors into the membrane (“AMPAfication”).

[See here for image source and more in depth information]

Alright, we can start talking about neurotransmitters! The first one up is the most abundant in the brain (chemical structure pictured above). It’s largely excitatory and generally acts on ion channels. Have you guessed it yet? It’s glutamate!

Glutamate is implicated in synaptic plasticity and long-term potentiation. Two of the main receptors it acts upon are the AMPA and NMDA receptors.  AMPA receptors let Na+ into the neuron, depolarizing it and potentially causing an action potential. NMDA receptors, which are voltage-dependent (they have a Magnesium block that only moves when the inside of the cell is depolarized enough) AND ligand-gated (they also need glutamate to open), let in both Na+ AND Ca2+ (calcium)- they are less ion specific.  This calcium seems to have effects for long-term potentiation (LTP- which is the core of learning and memory, whereby the same activation will make a cell more likely to fire after LTP… so we increase the EPSP after LTP, thus, making it closer to the threshold for an action potential - see here).

The calcium that enters essentially (through downstream effectors) causes more AMPA receptors to be inserted into the postsynaptic membrane.  A process sometimes called “AMPAfication” (amplification- ampafication- get it?).  This means that when glutamate is released the next time by the presynaptic neuron, more AMPA receptors will open and more sodium will enter the postsynaptic neuron, meaning there will be more depolarization! 

[Image Source]

Neurotransmitter action

I mentioned in a previous post that neurotransmitters can act by opening a ligand-gated ion channel, which can let a specific ion into the cell/neuron.  Neurotransmitters can have more complicated actions by acting on G-protein coupled receptors (shown in the image above).

When a neutrotransmitter binds to a G-protein coupled receptor, ions do not pass through- instead, the receptor undergoes a conformational (shape) change that “activates” the G-protein. Depending on the type of G-protein that is associated with that receptor, it can have different downstream effects.  Essentially, once that G-protein has been activated, it can activate or shut down different molecules inside the cell, which will then have their own actions on other molecules, through a whole cascade of events.

Compared to a ligand-gated ion channel, the actions through a G-protein coupled receptor are slow, but they can be much further reaching.  For instance, through that cascade, different molecules can be activated that may activate certain genes that change the way the cell/neuron will act in the future.  Note that of course, like many other things I talk about, this is all a simplified explanation of what is going on, but will help you understand the basic principles.

To make it a bit more complicated, for example… One major effector of G-protein coupled signaling is the cAMP (cyclic adenosine monophosphate- often said as cyclic A-M-P), which can have big effects with calcium pathways and protein kinase A (PKA). For an example of a faster effect, one thing that this pathway does is open HCN and KCNQ channels, which cause a current leak in a dendrite, by letting out potassium.  This can then “shunt” a signal coming in with sodium, such that the neuron would be less likely to build to a action potential.  I’ll mention here that at Yale, Amy Arnsten’s lab has done some fabulous work to look at how this process may act in prefrontal cortex (http://www.ncbi.nlm.nih.gov/pubmed/17448997).

[Image Source]

The process I mentioned before with action potentials is much more complicated than I described.  For instance, though I described sodium coming in through ligand-gated ion channels, it is not so simple.  It is not always a 1:1 presynaptic neuron has one action potential causing one action potential in the postsynaptic neuron.  It can take many action potentials in the presynaptic neuron to cause an action potential in the postsynaptic neuron (or sometimes just one).  There are also other ion channels (such as chloride Cl-) which can cause hyperpolarization or negative current in the neuron. It really depends on which neurons are involved and which neurons are giving them input.

I am going to try to put it in simple terms, but when we record from the postsynaptic neuron, we can see “post-synaptic potentials” (PSPs), which are just changes in the membrane potential, akin to perhaps mini-action potentials in the dendrites of the neuron (but not carrying a message on to the next cell). Simply, these PSPs are voltage changes in the postsynaptic cell/neuron.  These post-synaptic potentials can be excitatory (EPSPs) or inhibitory (IPSPs). You can see examples of these in the image above.

An EPSP could be caused by ligand-gated sodium ion channels opening in the postsynaptic neuron and causing a positive influx.  It can take a few EPSPs (so a few action potentials from a presynaptic neuron) to build up to a “threshold” at which point the action potential will fire.  This threshold is determined by the voltage at which the voltage-gated sodium channels will open, as I described before.

An IPSP could be caused by a different neurotransmitter opening ligand-gated chloride channels for instance, which could cause Cl- to enter the neuron and decrease the membrane’s voltage.  You could imagine that if this happens at the same time as an EPSP, this could prevent the postsynaptic neuron from reaching that threshold for an action potential, and thus, inhibit the signal.

The height of the EPSP can be determined by the number of ion channels in the postsynaptic membrane, where more ion channels will let in more sodium and depolarize the membrane more (make it more positive). You may wish to note that IPSPs are not always negative- they really just make the cell less likely to fire.  This can get really complicated though, so I will not say much more than that at this time.

[Image Source]

[Image Source]

So, now that we know what an action potential looks like, how does it begin?

The action potential begins with ligand-gated Na+ channels.  These “ligands” are small molecules, often called neurotransmitters.  Neurotransmitters are packaged into little containers called synaptic vesicles. Note that pre-synaptic means the neuron before the synapse (space between two neurons) and post-synaptic means the neuron after the synapse.  The pre-synaptic neuron is communicating the message to the post-synaptic neuron, through the release of neurotransmitter.

The synaptic vesicles will fuse with the pre-synaptic membrane and pour out their contents into the synapse, the space between two neurons that they use to communicate.  How do they know when to do this?  Well, the action potential comes down the axon from the pre-synaptic neuron and causes voltage-gated Ca2+ (calcium) channels to open.  Ca2+ causes the vesicles to fuse to the pre-synaptic membrane and the neurotransmitters to be released into the synapse.  These neurotransmitters can then cause the ligand-gated Na+ channels to open on the next neuron and begin an action potential there.

[Image Source]

This all leads us into talking about the observed action potential.  If you put in some electrodes to measure voltage across the membrane, this is what you would see.  In phase 1, “leak” channels are open and the Na-K pump is maintaining the negative potential.

Phase 2 is when the ligand-gated Na+ channels have opened causing the voltage-gated Na+ channels to open and sodium to rush into the cell.  The ball closes the pore and phase 3 is then caused by the slow to open voltage-gated K+ channels opening and K+ leaving the neuron.

Phase 4 is caused by the slow K+ VG channels being slow to close.  Too much K+ has left!  Resting state (labeled phase 1 here) is restored when those channels have closed and the Na-K pump sets to work!

There are two main ions involved in generating the action potential- Na+ and K+ (sodium and potassium, respectively).  As I mentioned in the last post, there is more Na+ outside relative to inside and K+ has the reverse concentration gradient (meaning concentrations are different outside and inside- where for K+, it is greater inside the cell).  There are three types of channels I will mention here- ligand-gated Na+ and voltage-gated Na+ and K+ channels.

The first thing that happens in an action potential is that Na+ (sodium) enters the neuron.  It usually does so through ligand-gated Na+ channels, which is a fancy way of saying something- a ligand (often a neurotransmitter)- opens the channels and Na+ rushes into the cell.

Na+ is positive; therefore, it stands to reason that when Na+ enters, the cell would become more positive.  When the cell is more positive, voltage-gated Na+ channels open.  Voltage-gated (VG) simply means that when the positivity inside the cell reaches a certain level, they can open and let more sodium into the cell.

The cool thing about these channels is that you can see in the picture, they have a type of “ball” on a chain.  This ball will swing in and close the pore after a little while, stopping sodium from entering the cell, even though it is still positive due to all the sodium inside the cell. We call this the “inactivated” state.

Voltage-gated K+ (potassium) channels will also open, but they are very slow.  Slow to open and slow to close.  When they finally open, the ball is closing the Na+ voltage-gated channels and K+ will rush outside the cell to make it more negative, or hyperpolarize, the neuron.

The action potential only travels one way down the axon because of the “ball” on the Na+ voltage-gated channels that prevent Na+ from entering again along that section of axon.  Instead, the Na+ VG channels further down the axon will open and the signal will travel that way.

[Image Source]

Recently, someone asked me to talk about Neurotransmitters, so I am going to go back to the basics as to how neurons work, what action potentials are, how they happen, and then what neurotransmitters are and what different ones may do! 

Let’s begin to talk action potentials.  Action potentials are how messages are carried from one neuron to the next.  Neurons are specialised for this kind of signal because they have excitable membranes.  This can be pretty hard to grasp, but essentially, they use specialized ion channels (things small molecules can move through) to send an electrical current down the length of the axon.

Action potentials are able to happen because the cell is kept more negative than the outside by having a larger amount of potassium (K+) inside the cell and larger amounts of sodium (Na+) outside the cell.  This gradient, or difference in the concentrations inside and outside the cell, are created by the Na-K pump.  These are like cell membrane club bouncers that push the K+ (hot girls) inside the cell (club) and push the Na+ (wasted people) outside the cell.  If this analogy doesn’t work for you, then just try to get it from the basic science standpoint.

So you have the Na-K pump that keeps more K+ inside and more Na+ outside.  To do this, it uses energy, known as ATP.  Therefore, you might sometimes hear this referred to as a Na-K ATPase pump, simply meaning it uses ATP to work.

[Image Source]

Apologies for the radio silence as of late.  I’ve been finishing up my thesis, which I will defend this week (and then hopefully have my official doctorate)! It’s been a lot of work, so I haven’t had time/energy to work on posts, but hopefully starting next week or the week after, I should be back to the usual frequency! Please bear with me!  In the meantime, here are some lovely neurons!! Enjoy!

[Image Source: Dr Jonathan Clarke. Wellcome Images]