Lighting up the Brain: Optogenetics for Addiction ⚡️
Using Algae DNA to revolutionize neuroscience and deconstruct mysteries of the brain
“Wrong Number!”
Imagine you’re trying to call your friend about something personal, and instead of calling that friend, you call everyone on your contact list — at the same time. Or, maybe you want to send an email to a colleague about an important issue, but they aren’t responding. What do you do? You end up having to talk to everyone else just to reach them. You might call their home phone, other colleagues, and anyone else that might be able to reach them with this critically important issue.
This is similar to the issue we’re facing right now in neuroscience.
Techniques that are currently used, such as Deep-Brain stimulation (DBS), stimulate large areas of the brain and are based on the physical position of large electrodes. Not very precise, but DBS has been used to treat Parkinson’s, depression, and potentially, addiction. Just as the scenarios above, you may reach your goal, but there could be unintended consequences, the effectiveness of what you’re trying to do is extremely limited, and the ability to study benefits is dismal… particularly so when compared to what Francis Crick proposed.
Who is Francis Crick?
He is the guy that discovered that very special molecule called DNA. This British biologist discovered one of the most foundational pieces in science that propelled human beings understanding of ourselves and the world around us to an entirely different level. Francis Crick won the Nobel Prize in 1962 (along with James Dewey Watson and Maurice Hugh Frederick Wilkins) for “their discoveries concerning the molecular structure of nucleic acids and its significance for information transfer in living material.”
However, it is a specific branch within Mr. Crick’s research that he studied after his Nobel prize which is of interest to this particular topic.
“In 1979, Francis Crick suggested that controlling all cells from one type in the brain while leaving the others more or less unaltered is a real challenge for neuroscience. Electrical stimulation allows for temporal precision, but it lacks spatial precision. In other words, it stimulates/inhibits all cell types (excitatory or inhibitory cells) located around the tip of the electrode. On the other hand, genetic and pharmacologic manipulations may have spatial precision, but they are slow and lack temporal resolution on the timeframe of neural activity and signaling. Francis Crick speculated that a technology using light might be useful to control neuronal activity with temporal and spatial precision, but at the time there was no technique to make neurons responsive to light.” — NCBI Optogenetics Overview
Simply, optogenetics is an emerging field in science which uses a combination of gene therapy and light, to study the brain (primarily) by specefically introducing key genes into some neurons and not others.
Optogenetics gives us a chance to call that friend directly, instead of all of our contacts, or phone a colleague's personal number when they don’t respond to emails. This technology gives a chance to get to the direct source of many illnesses and target these sources with unbelievable precision.
Thanks to the remarkable control optogenetics gives us over parts of the brain, optogenetics gives us a chance to:
- Restore vision to the blind
- Restore nerve functions in patients with spinal cord injuries
- Control fear, motivation, and reward for addiction treatment
Above are just a few applications of this phenomenal technology.
A Brain Breakdown
It is something out of science fiction that we’re using light to control the brain. However, it is important to understand how the brain works before discussing how optogenetics, which utilizes this function, works on the brain.
The Neuron
The neuron is the most critical part of the brain because it is the functional unit. Other parts of the brain like glial cells (microglia, astrocytes, oligodendrocytes, and ependymal cells) are still important, but neurons are what actually relay signals throughout the brain. Neurons are made up of three main components:
- Soma (Cell body) — Contains genetic information, produces peptide neurotransmitters, and creates vesicles which carry neurotransmitters
- Dendrite — The receiving end of a neuron that is typically characterized with many short branches
- Axon — The relaying end of a neuron that is typically characterized by a long branch coming from the cell body
Neurotransmitters
Neurotransmitters (chemical messengers) are instrumental in the process of dendrites receiving and axons relaying information.
There are junctions between neurons that allow for space between the two cells. When a signal needs to be propagated to the proceeding neuron(s) neurotransmitters are released from the pre-synaptic axon → across the synaptic cleft → bind to receptors on post-synaptic dendrite → potential response.
Action Potentials
There are two main types of neurotransmitters:
- Inhibitory — Hyperpolarize (decrease) the membrane potential of the postsynaptic neuron which prevents it from firing
- Excitatory — Depolarize (increase) the membrane potential of the postsynaptic neuron which may lead to the neuron firing
Inhibitory neurotransmitters produce “Inhibitory postsynaptic potentials” or “IPSP”. Excitatory neurotransmitters produce “Excitatory postsynaptic potentials” or “EPSP”.
When the summation of EPSP’s is great enough, an action potential (firing of the neuron) occurs.
In the illustration above, there are numbered stages. Stages 2 and 3 are when the neuron is depolarized and then repolarized. This happens when ion channels along the cell membrane allow sodium ions into the neuron and potassium ions out of the neuron. The opening and closing of these ion channels are critical to optogenetics.
Optogenetics Breakdown
The video below shows a test of optogenetics on a mice test subject which is acting “predatory” when stimulation is delivered, and more passive when it is not.
The mouse from the video above has had the standard fiber optic cable placed, but there are new experiments with wireless implants which take away the limitations of this standard technique such as:
- The size of the cable creates damage during insertion
- Irritation as the animal moves
- Animals have to be physically tied to the light source
There is also work being done on non-invasive optogenetics where an implant does not even need to be placed.
The Science behind Optogenetics
If neurons normally reacted to light, we’d end up acting very differently. Neurons currently only react to electrical and chemical stimulation. To get neurons to react to light, we have to genetically engineer neurons to express proteins called opsins.
In our body, our DNA leads to RNA strands which lead to the formation of polypeptide chains that make proteins. The process from DNA to RNA is called transcription, and the process of going from RNA to peptide chains is called translation.
When we insert the genes from specific algae into the nucleus of target cells, and they begin to express these opsins which are light-sensitive, they can depolarize some neurons and hyperpolarize others based on exposure to different color light.
Once these genes are inserted, these light-sensitive proteins can be expressed in certain neurons.
There are a few popular opsins that are used in optogenetic experiments:
- Channelrhodopsin (CHR2) — Sensitive to blue light and causes depolarization by focusing on the transport of cations into the cell
- Halorhodopsin (HR) — Sensitive to green/yellow light which causes hyperpolarization by focusing on the transport of chloride ions into the cell
- Archrhodopsin (ARCH) — Sensitive to green/yellow light which causes hyperpolarization by focusing on hydrogen ions out of the cell
How are genes “inserted”?
There are two primary methods for gene insertion for opsin expression: Viral Delivery with Adeno-Associated Virus and Electroporation.
The latter works by applying an electrical field to cells to increase the permeability of the cell membrane. This means that it is easier for something to pass into the cell such as chemicals, drugs, or even DNA.
The former works with extrachromosomal DNA within a cell that is physically separated from other chromosomal DNA and also has the ability to replicate independently (called Plasmids). There are three types of plasmids involved with AAV’s:
- Packaging Plasmid
- Helper Plasmid
- Transfer Plasmid
The Transfer plasmid encodes the AAV vector genome and AAV transfer plasmid has two major components:
- Transgene — The gene that will be delivered by the AAV and is going to be inserted in the genome of the targeted organism
- Promoter — Controls the tissue the transgene should be exposed in. Ensures that only certain neurons (in this case) express the gene for opsin expression
Viral Delivery with AAV’s are common in studying optogenetics and allow for the expression of opsins in specific neurons which allows scientists to make discoveries that we were NEVER able to before the development of this technology. These new advances include revolutionizing treatment for addiction.
Why Addiction?
If you’ve read my previous work, you know I feel strongly about addiction and the poor treatment those who suffer from it receive.
Over 100 people die every day from opioid overdose and of the patients which end up receiving treatment, only 30% complete rehabilitation. On top of that, $700 billion is spent annually in the USA towards addiction and related costs.
This is a deadly illness that takes lives and rips apart families.
Dirty Dopamine
The use of optogenetics for addiction has been an exciting potential treatment in humans since this technology started to be tested. Dopamine is key in understanding how addiction affects those who suffer from it, and it’s pathways are targets for optogenetic therapy when trying to treat addiction.
The reward pathway is also called the mesolimbic pathway, which runs from the Ventral Tegmental Area (VTA) to the Nucleus Accumbens (NAc). The Mesolimbic pathway is one of four major dopaminergic pathways which also include the Nigrostriatal, Tuberoinfundibular, and Mesocortical pathways.
Contrary to popular belief, dopamine is not just about pleasure. Dopamine is involved in memory, movement, learning, and reward. Dopamine has also been implicated with different neurological disorders like Parkinson’s Disease which is caused by the depletion of dopaminergic neurons in the substantia nigra.
Dopamine is fundamental to understanding the reward system, emotion, and addiction. Dopamine is a monoamine neurotransmitter which means it is derived from amino acids (the monomer of proteins) — specifically phenylalanine (programmed with codons UUU, and UUC) tyrosine (programmed with codons UAU, and UAC).
Addictive drugs have in common that they target the mesocorticolimbic dopamine pathways. This system projects from the VTA to NAc and the prefrontal cortex (PFC). These substances affect glutamatergic (Producing glutamate — Excitatory) and GABAergic (Producing GABA — Inhibitory) synaptic transmission in these three brain areas. These changes are referred to as “drug-evoked synaptic plasticity” which results in the alteration of neural circuits.
Light Bulb Moments
Over the past decade, there have been many breakthroughs in this field which could forever change the way we deal with this deadly illness along with many others.
Using Lasers to treat Addiction to Alcohol
In 2016, scientists identified a neuronal “ensemble”, or group of connected cells in the central region of the amygdala (CeA). This critical finding illustrates major progress in mapping the brain. However, it is still necessary to characterize the identity of neurons in this group.
It turns out that 80% of these neurons are corticotropin-releasing factor (“CRF” is a hormone) neurons. Using optogenetics, researchers targeted these CRF neurons through a surgically implanted fiber-optic cable. The light from these implants would cause the inhibition of CRF neurons.
The rats which this experiment (which was in 2019) used, tested how much the rats drank before they became dependent on alcohol (they drank very little), and then the researchers spend several months increasing this dependency through increased alcohol consumption.
Once the scientists stopped providing alcohol, withdrawal began, and alcohol was given to the rats again. However, in the rats in which had inactivated CRF neurons, immediately returned to their pre-dependent drinking levels. Their motivation to drink had dissipated. Interestingly, the inactivation of these neurons also limited physical symptoms of withdrawal such as abnormal gait and shaking.
It is known that the extended amygdala is directly involved in the second stage of addiction (withdrawal), and the ability to manipulate its response to treat the patient fights a tortuous feedback loop: Conditioned negative response to withdrawal mediated by the amygdala and hippocampus → avoidance of going through withdrawal and an increase in tolerance → increase in drug-seeking behavior → addiction reinstated and withdrawal symptoms removed.
Curing Cocaine Addiction in Mice
In 2013, research was published by researchers from a team from the NIH and University of California San Francisco tested whether the prelimbic region of the prefrontal cortex impacts addiction. To do this, the team used gene therapy to cause the expression of opsins in this region of the brain. Using optogenetics, researchers “activated” neurons with this protein expression which subsequently stopped compulsive behavior. Doing the opposite, cause compulsive behavior.
“The research team’s work demonstrated the central role that the prefrontal cortex plays in compulsive cocaine addiction. It also suggests a new therapy that could be tested immediately in humans” — Billy Chen of NIDA, who led the study
Proceeding human trials but would most likely rely on Transcranial Magnetic Stimulation (TMS). TMS uses magnetic fields to stimulate nerve cells and is non-invasive which makes it ideal for testing beginning stages for optogenetic-generated hypotheses.
Wrapping it Up 🎁
This article would be highly technical for someone who has no background in biology, and I’m sure that many of those who are reading this piece have forgotten some of the earlier information. So, to sum everything up, here are your key takeaways:
- Optogenetics is the science of using light to activate certain cells in the body (however, the primary use is in the brain)
- Optogenetics uses gene therapy to choose the selective cells which express opsins (the light-sensitive ion channels)
- Neurons are the functional unit of the brain and are made up of the cell body (soma), dendrites, and it’s axon
- Neurotransmitters are chemical messengers which allow neurons to send signals to each other
- An action potential is triggered once the membrane potential of a neuron reaches a certain threshold (typically around -55mV)
- During an action potential, sodium ions move into the neuron while potassium ions move out which causes depolarization and repolarization of the neuron respectively
- When ions move in and out of neurons, the go through voltage-gated ion channels
- Optogenetics uses gene therapy to introduce the DNA necessary for specific neurons to express these light-sensitive channels and allows specific neurons to be controlled
- Optogenetics is being used to treat blindness, spinal cord injury, and addiction
That is still a lot of information, but if there is ONE thing you take away it should be that optogenetics is forever changing the way we view the brain and the illnesses which ail it.
