One of the biggest problems with drug addiction is that it is a disorder that is characterized by relapse. You just CAN’T QUIT. It’d be one thing if you got hooked, got sober, there were some initial bad reactions, and then you were ok. But drug addiction isn’t like that. Drug addicts relapse, even when they are completely and totally sure they never want to do the drug again, when they know the drug isn’t worth it. They relapse anyway. And this is one of the biggest problems with trying to treat drug addicts. Scientists have been working for years to determine what triggers relapse to drug taking behavior, what connections in the brain are involved, and how permanent they are. Answers so far: a lot of triggers, lots of connections, and pretty long-lasting.
So when Sci saw this paper in PLoS ONE, she got excited:
Root et al. “Evidence for Habitual and Goal-Directed Behavior Following Devaluation of Cocaine: A Multifaceted Interpretation of Relapse” PLoS ONE, 2009.
As of right now, there are two major hypothesis of drug relapse: goal-related and habit-related. It is Sci’s personal opinion that the reality probably involves some of both of these hypotheses. The habit hypothesis is that drug-related cues (like drug taking needles, the crack house, or the friend who you do drugs with) cause the start of a habit cycle of behavior. Once you have the cue, habit kicks in, and drug-taking just happens, regardless of the negative consequences and all promises you made to the contrary. On the other hand, there is the goal-related hypothesis, which is the idea that the drug-related cues make you anticipate the positive effects of the drug, and this makes you seek them out, overwhelming all negative consequences and promises you made that you wouldn’t do this anymore.
So is it habit? Or is it goal-directed? In this study, the authors wanted to find out. Using some rats, and some cocaine, and a little compound we like to call lithium chloride.
So how do you do this? Well, the scientists reasoned that, if cocaine taking had become habit, rats that had been on coke, and then abstinent, would relapse when a lever and light were present, showing that cocaine was available. To determine this, they gave a bunch of rats the opportunity to self-administer cocaine. All rats were trained on how to self-administer cocaine using a lever and a cue light.
Once the animals had all learned to self-administer, half of them got cocaine alone every time they pressed the lever. The other half got cocaine paired with lithium chloride (LiCl), a compound that isn’t bad for you, but tastes AWFUL and makes for one very queasy rat. I should note that the coke alone rats also got LiCl, only on a different day and not paired with a cue light, just to control who got what.
Once the animals had all been through training, they were taken off the cocaine for a month, which is a pretty long time for a rat. They then put them back in the chamber with the lever and cue light present (but no cocaine). The coke trained animals of course started banging on the lever for their hit, while the coke-LiCl paired guys banged less. Interestingly, though, when the cocaine WAS present along with the cues, the LiCl rats hit just as hard. The authors concluded that relapse to cocaine-seeking behavior is BOTH habitual and goal-directed (told you so!).
So, here’s what we like:
I like how this study has all animals receiving equal amounts of cocaine, and equal amounts of LiCl. With LiCl, cocaine responding can be reduced, so I like how they controlled for that.
And here’s what we don’t like:
This study is very simple, and does imply possible goal-directed behavior. But honestly, Sci finds this study to be incomplete. It seems like there’s one main set of data, one experiment, which they analyzed the living heck out of. Which is fine, but this is coke self-administration, and there’s a lot more they could have done with what they had.
1) First off, the animals ALL learned to self-administer cocaine before the LiCl group got treated to be queasy. So they could well have formed some habit behavior for the lever and cues right off, which then got erased by the LiCl. This wasn’t tested. It definitely could have been. I want to see them relapse, and then pair with LiCl and “stop” relapse by stopping the goal-directed behavior. And I want to see it more than once, over a long period of time.
2) I want to SEE that LiCl paired with cocaine is decreasing the reward value of the coke stimulus. For that, I’d like to see some conditioned place preference testing, which is considered to be a measure of the rewarding value of stimuli in rodents (basically, give a rat two chambers, with cues to tell them apart, like different smells or something. Pair one side with cocaine, and one with saline. Do this for a few times. Then place the rat in the box and see which chamber they like more. If LiCl has reduced the rewarding value of cocaine, cocaine place preference should be reduced in these rats, and I’d like to see them do this following abstinence), and though they mention that LiCl doesn’t block cocaine conditioned place preference, it doesn’t block EXPRESSION, but giving it concurrently to training, or prior to training, may have a different effect).
Or perhaps, instead of conditioned place preference, some progressive ratio studies (which is a lever pressing task in which the animal has to press once for coke, then twice, then four times, then 8, then 16…etc, until the animal decides its just not worth it. If cocaine is “worth” less to the LiCl treated rats, their progressive ratio values would be lower, because they aren’t willing to “pay”).
3) They talk about both “habitual” and “goal-directed” responding, but it’s very confusing to tell which is which (Sci reads stuff like this all the time, and was still confused). They mention that habitual responding may have been taking place at the beginning of the session, and goal-directed later, but they don’t really go into it. They don’t seem able to really separate the habitual and goal-directed responding in the results or discussion, and I don’t think the experimental set-up can really even tell the difference. Honestly, the DID have results, but were unable to really interpret them without further studies. A few more studies in the rats really could have given them some better ideas. And note that, when the coke was present, all the animals were self-administering, regardless of LiCl, implying that the LiCl treatment wasn’t long-term effective at all, and that the animals could pick up the coke self-administration right where they left off.
4) They conclude that reducing the rewarding effects of cocaine in addicts (by giving them something nasty in conjunction or something) could help prevent relapse. And this is where I need to get a little bit snarky. Oh REALLY?! Tell me something I didn’t know. Pairing chocolate cake with pepper will indeed make me want less chocolate cake. But there’s a big problem with this idea. When I want chocolate cake bad enough, I’m not going to shake pepper on it. And I’m going to have my cake and eat it, too.
We’ve been trying this technique with alcoholics for YEARS. And it doesn’t work well at all. There is a drug in use, called Antabuse, which is the drug disulfuram. This drug prevents elimination of acetylaldehyde from the body. Acetylaldehyde is a breakdown product of ethanol metabolism, and causes some NASTY feelings. Like the worst hangover you’ve ever had. While drinking. So you think, hey yeah, then people won’t want to drink. Well, sure. But try and get them to stay on their medication. Doesn’t happen. When they get the craving for alcohol, they WILL skip their medication, throw it down the drain, and relapse. I seriously doubt that a similar treatment for cocaine would be any different. You have to FORCE them to take it every day. And even then, sometimes they’ll get around it. (Best study had a 50{9f43b4361d9a125bc126dd2a2d1949be02545ec69880430bc4fed2272fd72da3} effectiveness under constant supervision, but I’d love to see where those addicts are now.)
5) Can I see a bar graph of total responses over the extinction session? Any differences?
So there you have it. Perhaps Sci interpreted this paper incorrectly, but it seems the authors were just as confused. When in doubt, a little more data supporting your point is never amiss.
And don’t eat your cake with pepper on it. All it does is make you hate pepper.
Root DH, Fabbricatore AT, Barker DJ, Ma S, Pawlak AP, & West MO (2009). Evidence for habitual and goal-directed behavior following devaluation of cocaine: a multifaceted interpretation of relapse. PloS one, 4 (9) PMID: 19779607
