Large GWAS in 836 outbred HS rats identifies six loci linked to cocaine self-administration traits, highlighting Ces1 carboxylesterase genes and other loci overlapping human substance-use genetics.
0:00Welcome to Base by Base, the paper cast that brings genomics to you wherever you are. Thanks for listening, and don't forget to follow and rate us in your podcast app. So, um, I imagine you're at a party.
0:12Two friends, right? Similar backgrounds, similar stress levels, similar upbringings. And they both decide to try cocaine for the very 1st time. They had the exact same initial experience. But the next morning, their paths just completely diverge.
0:27One of them wakes up, goes about their day and and never touches the drug again. But the other person takes that exact same drug and almost immediately, like, a switch flips in their brain. They begin this devastating lifelong spiral into addiction.
0:40It's a tragic reality for so many. Yeah, and we've spent decades trying to figure out why this happens. You know, we look at their childhoods, their friend groups. Uh, their willpower. But the data points somewhere else entirely.
0:53Up to 70% of a person's vulnerability to cocaine dependence is actually hardwired into their genetics. Yeah, which is just a number that completely stops you in your tracks, honestly. It really does. Because when you look at cocaine use disorder.
1:05or CUD. I mean, we are dealing with a massive global health crisis. In the US alone, Recent data shows around 5000000 people using cocaine annually. Wow, 5 million. Yeah, and about 140000000 of those meet the strict clinical criteria for a severe use disorder.
1:22The scientific community has known for a long time that this susceptibility is hiding in our DNA. The frustration has been that the genome is vast. Finding the specific genetic machinery driving this behavior has been like, like looking for a microscopic needle in a global haystack.
1:39Yeah, and that brings us to the core of today's deep dive. Today we celebrate the work of Laura and colleagues who have advanced our understanding of this with their landmark 2026 paper published in nature communications.
1:50An incredible piece of work. Truly. This team conducted the largest genetic study of cocaine self-administration ever attempted in rats. The goal here is monumental. They wanted to map out the specific genes driving addiction, trace what those genes are doing to the liver in the brain, and you know, figure out if these findings can fundamentally change how we treat human addiction.
2:12Yeah. But before we get to the DNA, I just have to ask the obvious question. If this is a human crisis, why are we looking at a rat study to solve it? Well, it really comes down to the sheer chaos of human life.
2:24Chaos. Yeah, the standard way we find genes linked to diseases is through something called a genome wide association study or a gigobase. But running a G ways on human addiction is just, it's a logistical nightmare.
2:36Right, because we're just so messy. Right. Precisely. It's because of all that environmental noise, imagine trying to isolate a genetic signal for addiction in a massive population of people, every single person in your study has a different diet, a different level of childhood trauma, different daily strip.
2:52access to drugs. Exactly. And on top of that, human beings are just notoriously unreliable when reporting their own substance use to researchers. Oh for sure. So to find a clear, undeniable genetic signal through all that static, you'd need an astronomical sample size.
3:08But with rats, researchers can just strip away the noise entirely. Right, they can control everything. Every single animal in this study had the exact same housing, the same diet, and the exact same exposure to the drug.
3:20But they didn't just use standard, you know, identical white lab rats for this, right? I mean, if the goal is to map genetics using clones wouldn't help you find any variations. Right. And that is the crucial design element of this study.
3:32They utilized 836 NNIH heterogeneous stock wraps. Heterogeneous stock, so HS rats. Yep, we call them HS rats. And these animals are intentionally outbred. They are incredibly genetically diverse, specifically bred over many generations, to basically mimic the wild, messy genetic variation you see in a human population.
3:53That's brilliant. So the researchers created this perfect experimental paradox. You have totally uniform environmental conditions combined with massive genetic diversity. Exactly. So any difference in how these rats behave with cocaine can be confidently traced back to their DNA, not their upbringing.
4:11Okay, so how do you actually measure addiction in Iraq? The behavioral setup they used? It's called the extended access intravenous self-administration model. And I found the mechanics of this just fascinating.
4:23Very rigorous. Yeah. So the rats have a catheter, and they quickly learn that pressing a lever delivers a dose of cocaine. But the researchers didn't just let them press it whenever they broke it down into these specific phases.
4:35First, there's short access, or SHA, which is limited to 2 hours a day. Right. And the short access phase is all about baseline learning. It sort of mimics initial casual drug use. The rat learns the association between the lever and the reward.
4:48But the real data comes in the next phase, which is the long access or LGA. Here, the window is expanded to 6 hours a day. Six hours. Wow. Yeah, this simulates a binge environment. In human addiction, the hallmark is not just the act of taking the drug, but the eventual loss of control and the escalation of intake over time.
5:07Right. And beyond just seeing how much they took, the researchers threw some serious hurdles at these rats. They use the progressive ratio test, which is essentially a motivation test. Yeah, I was trying to picture this.
5:21It's, um, it's basically like paying surge pricing for a rideshare app, right? That's a good way to put it Like, at first, one lever press gets you one hit. But then the price goes up. You have to press it 2 times for the same hit, then 4 times, then 20, then 50, then over a 100 times.
5:38It's a really helpful way to think about it, but the currency they are paying with makes it much darker. They aren't paying with money, they are paying with sheer physical exertion and endurance. The progressive ratio is designed to find the rat's breaking point.
5:51Like, how much will they take? Exactly. How much physical exhaustion are you willing to endure for one single infusion? And they took it a step further with a compulsivity test involving foot shocks. In this phase, pressing the lever for cocaine randomly resulted in a painful electric shock.
6:09Ouch. So it measures if you will continue to use the drug, even when it literally guarantees negative consequences. Yes. And because these are HS rats with all that diverse DNA, the results across these tests were incredibly varied.
6:24Under the exact same laboratory conditions. Some rats kept their intake perfectly stable. They would, you know, press the lever a few times, get their dose and walk away. Just like the first guy at the party.
6:34Exactly. But others completely spiraled. By the end of the 6 hour long access phase, Some of these animals were taking over 150 infusions in a single session. 150. That's insane. Yeah, they would work themselves to the point of collapse on the progressive ratio lever, and they would consistently endure painful shocks just to get another dose.
6:53Which perfectly mirrors that human spectrum when we talked about at the beginning. You have the people who can walk away and the people who are just totally consumed. So we have the behavior mapped out.
7:02Now we need the blueprint. When they took the DNA from the heavy using rats and compared it to the light using rats. What did they actually find? So they scanned 5.4 million genetic markers across the rat's genomes, and the most significant breakthrough was a locust, a specific region on chromosome 19.
7:22Okay, chromosome 19. Yeah. This genetic real estate was strongly associated with one very precise behavioral metric, which they call the post-infusion interval. The post-infusion interval. That's, um, that's just the exact amount of time the rap pauses between hits, right?
7:37That's it. The waiting period. Yeah. And sitting right there on chromosome 19, driving this behavior, were variations in 2 specific genes called Cess1C and Cess1T. Okay. Now, these are the RAD equivalents of the human CES one gene.
7:51And these genes are responsible for producing enzymes called carboxylisteruses. Carboxylustruses. Let's break that down for a 2nd because it's a mouthful. If I have these enzymes floating around, What is their actual job?
8:03Think of them as the body's cleanup crew? Carbox illustrious are enzymes primarily produced in the liver, and their specific job in this context is to metabolize or break down cocaine in your bloodstream.
8:15The researchers found that rats possessing a specific genetic variation, which they call the GG genotype, produced highly active versions of these enzymes. Oh, interesting. Yeah, and because of this, these rats had significantly shorter post-infusion intervals.
8:31They literally could not wait very long between doses. Because their liver is clearing the drug out too fast. Okay, this is making sense now. It's, imagine the rat's body is a bucket and the cocaine is water.
8:42Right. If your genetics give you highly active liver enzymes, your bucket essentially has a massive hole in it. The cocaine leaks out of your bloodstream rapidly. So to maintain that high. You are forced to keep going back to the lever to refill the bucket over and over again.
8:57Your own metabolism is driving you into a compulsive cycle. The leaky bucket analogy captures the mechanics perfectly. And this leads us to a fascinating pharmacological concept called the compulsion zone theory.
9:10The compulsion zone. Yeah, addiction isn't just about chasing a high. It's heavily driven by maintenance. An addicted user is psychologically compelled to maintain a very specific concentration of the drug in their body.
9:21So they don't crash. Exactly. They need it high enough to stave off the crash of withdrawal, but low enough to avoid the toxic jittery effects of taking too much at once. It's a tight rope walk. But wait, if this whole chromosome 19 discovery is about liver enzymes, breaking down a chemical, does that mean addiction is primarily a metabolic issue, rather than a psychological or brain issue?
9:46Well, it is an intricate feedback loop between the two. The brain is the organ setting the target. It demands that specific concentration of dopamine to feel normal. But the liver dictates how much physical work the animal has to do to hit that target.
10:00If you have the genetic variant that rapidly metabolizes the drug, your behavior has to radically change to compensate. You just have to work harder. Yeah, you become hyperfocused, your intervals shorten, and you ignore negative consequences like foot shocks purely because your liver keeps pulling the rug out from under your brain's reward system.
10:16That reframes addiction so clearly. It's a full body system failure. The compulsion is the outward symptom of a metabolic engine running way too fast, but obviously, as you said, the brain is setting the target.
10:28The reward system gets hijacked. So when they scan those 5.400000 genetic markers, did they find variations that specifically altered the brain's hardware alongside the liver's metabolism? They did find brain specific genetic signals, yes.
10:42But to isolate them. The researchers had to be very clever with their data. Rather than just looking at isolated behaviors like how many times a rat pressed a lever or how long they paused. They used a statistical method called principal component analysis or PCA.
10:57Oh, PCA. I've seen PCA mentioned in data science before. How does it apply to studying behavior in rats? Well, think of PCA as a way to find the hidden architecture behind complex data. Addiction isn't just one trait, right?
11:10It's an amalgamation of escalating use, high motivation and ignoring consequences. PCA mathematically combines all those separate behavioral tests into one single overarching composite score. It's a master score for addiction-like behavior.
11:26Oh, I see. So instead of asking, like, did the rat pull the lever a lot? The math asks, how severe is the total addiction profile of this specific rat? Exactly. And when they ran that master score against the genetics, they found it was highly heritable, about 16% of the variants in this total addiction profile was purely genetic.
11:47Wow, 16%. Yeah. Scanning the genome for this master trait led them to several other critical genes, with RazD2 and Nas being the major standouts. Right. Right. And what makes these 2 genes so vital is where they operate in the body.
12:00They show major expression changes in the nucleus accumbence. Nucleus incumbents. Okay, let's make sure we're grounded here. What role does that specific structure play in the brain? The nucleus accumbence is the central hub of the brain's rewards circuitry.
12:14When an animal or a human experiences something necessary for survival like eating high calorie food or reproducing, the nucleus accumbents lights up with a neurotransmitter called dopamine. It's the feel good chemical.
12:26Yes. It's the brain's way of saying, that was good. Remember how you did that and do it again. Cocaine completely floods this hub with dopamine, hijacking the survival circuitry. And the genes, RazD2, and Gatus are sitting right in the middle of that hub.
12:41What are they doing to the dopamine? They are the architects of the dopamine receptors and the signaling pathways themselves. Razdy 2 modulates how dopamine transmission actually flows, and it dictates the physical motor responses after the brain is exposed to cocaine.
12:56Okay, and gayness. Gainous codes for a protein that regulates how the dopamine receptors communicate with the rest of the cell. If a rat inherits variations in these specific genes, the physical structure of how their brain processes reward, craving, and reinforcement is fundamentally different from a rat without those variations.
13:13So we have this multi-layer genetic trap. You might inherit a liver that burns through the drug so fast it forces you to binge while simultaneously inheriting a nucleus incumbents that is structurally hypersensitive to the reward of the drug.
13:27Yes. That is just a devastating combination, but there's another discovery in this paper that I want to spend some time on because it feels like a massive evolutionary revelation. They found a gene called track 2 in these rats.
13:40Why did the researchers highlight that specific finding so prominently? Because it provides the ultimate validation for the entire study. If you look at human geOS data for cocaine used disorder, which, as we discussed, is incredibly difficult to gather and verify, track 2 is one of the very few genes that has been positively identified.
14:01Wow, to run a completely independent, highly controlled study in rats, and pull the exact same gene out of the data is extraordinary. I mean, we're talking about two entirely different species. The evolutionary paths of humans and rodents diverged roughly 80 million years ago, right?
14:15Roughly, yeah. If the exact same genetic code is driving the exact same compulsive behavior across 1000000s of years of evolutionary separation, that gene must be doing something so fundamental. It implies that the underlying biological machinery of reward, and our vulnerability to having that machinery hijacked is deeply conserved in mammalian evolution.
14:37Yeah, that makes sense. The reward circuitry wasn't built to process highly refined modern chemical compounds. It was built to keep us alive. When you introduce a substance like cocaine, it exploits these ancient conserved genetic pathways. Finding tree I 2 in both species proves without a doubt that the rat model isn't just an approximation, it is a direct window into the human condition.
15:00And cocaine wasn't the only substance this genetic web touched. The paper mentions other genes, like SLC, 10 A7, and SAB B2. Yes. Now, in this specific experiment, these genes were linked to how the rats handled cocaine, but the research point out that human studies have linked these exact same genes to completely different substances. Yeah, in human genetic data, variations in SLC, 10A7, and set B2 have been strongly associated with alcohol consumption and severe nicotine dependence.
15:28Which is fascinating because those drugs do very different things. Cocaine is a powerful stimulant, while alcohol is a depressant. How can the same gene make someone vulnerable to both? It reinforces the idea of a universal vulnerability?
15:42We tend to think of addiction in silos, right? Like someone is an alcoholic or someone has a cocaine addiction? But biologically, these genes aren't cocaine genes or alcohol genes. They are structural variations in the reward and metabolic pathways.
15:56If you possess these variations, your baseline circuitry is simply more vulnerable to any artificial reward that floods the system. Ah, I see. The specific drug a person ends up addicted to might be determined by their environment or what they have access to, but the underlying structural vulnerability is exactly the same.
16:13This research completely changes the landscape of how we understand this disorder. But I want to bring this out of the laboratory and into the real world. If I'm a doctor working a recovery clinic or, you know, if I have a family member who is currently in the grip of cocaine use disorder.
16:27How does knowing about a liver gene, like CES1 or a brain gene, like Turkey 2, actually change things. Does this point to a new way to help people? It points directly toward the future of precision medicine.
16:40Right now, treating cocaine use disorder is incredibly frustrating because there are no FDA approved targeted pharmacological treatments. None. We rely heavily on behavioral therapies, which are vital, but we are fighting a massive biological headwind.
16:56Because you're trying to use talk therapy to fix a leaky bucket. Exactly. But let's take the CES one liver enzyme as a theoretical target. If we know that highly active CDS1 enzymes cause that rapid metabolism, which in turn drives the frantic compulsive binging.
17:10What if we could design a medication that temporarily slows that enzyme down? You plug the hole in the bucket. You plug the hole. By artificially modulating how long the cocaine stays in the system, you prevent the rapid crash.
17:23The brain's required threshold is met without the patient feeling the immediate overwhelming panic to find more of the drug. That's incredible. You aren't necessarily curing the addiction in one step, but you are breaking the frantic physiological cycle of compulsion.
17:38You give the patient's brain the breathing room it needs to actually engage in behavioral therapy and begin the slow process of recovery. That is a total paradigm shift in treatment. Instead of just trying to dull the craving in the brain, you treat the metabolic engine in the body that is forcing the craving to happen.
17:55It's brilliant. Although, um, to be scientifically rigorous, we should acknowledge that the researchers did note some limitations to their work. This isn't the final puzzle piece. No, science is always an ongoing process of refinement.
18:07The authors were careful to note that they didn't measure the actual concentration of cocaine in the rat's blood directly. No, they inferred the metabolic rate based on the genetic markers and the behavioral timing.
18:18Furthermore, the rats were only exposed to intravenous cocaine. Humans consume cocaine in a variety of ways, most commonly intranasally, which alters how quickly it hits the bloodstream and how the liver processes it.
18:30But even keeping those limitations in mind. This paper represents just an undeniable leap forward. It paints such a vivid mechanical picture of what addiction actually is. It really does. We are looking at a multi-layered biological vulnerability.
18:44Susceptibility to addiction is not a character flaw. It's a highly complex web of genetics. You have inherited code in your liver dictating how fast a chemical disappears, interacting with inherited code in your nucleate incumbents, dictating how intensely your brain cares that it's gone.
19:00It firmly anchors cocaine use disorder in the realm of complex biological disease, right alongside conditions like asthma or diabetes, which operate on the exact same principles of genetic susceptibility and environmental triggers.
19:12Which leaves us with a pretty wild final thought to ponder. If our susceptibility to compulsive behavior, how fast our metabolic bucket leaks, and how intensely our brain processes reward, is this deeply hardwired into our genes, it opens up a massive question for the future of medicine.
19:29Should we be implementing mandatory genetic screening before doctors prescribe any highly addictive medications? Wow, yeah. Imagine a near future, where a simple cheek swab tells your doctor, hey, this patient carries the highly active CES one variant, and the trake 2 vulnerability.
19:45If you prescribe them this specific class of painkillers after their surgery, their biology almost guarantees a spiral into dependence. We might soon have the data to stop the cycle of addiction before the very 1st doses ever taken.
19:58It's a really profound thought. This episode was based on an open access article under the CCBY 4.0 license. You can find a direct link to the paper and the license in our episode description. If you enjoyed this, follow or subscribe in your podcast app and leave a 5 star rating.
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