Rats Love Their Friends More Than Sugar But Less Than Heroin

A 'saver' rat rescuing its cage-mate from a plastic tube. Tomek et al./Addiction Biology

Rats will free another rat from a tight squeeze, and they're willing to give up a feed of sugar to do it. However, rats addicted to heroin are less noble, leaving their species-mate trapped if they have to pass up a hit to provide assistance. The discovery could help pave the way to understanding, and eventually tackling, the way social isolation and drug addiction interact in humans as well as rodents.

Three years ago, a study revealed that rats will give up a bite of chocolate to rescue a fellow rat from an unpleasant, though not dangerous, situation, and even share the chocolate with the rat they saved afterward. IFLScience reported the paper, and we're glad we did because our coverage inspired a new round of research.

Seven Tomek, a graduate student at Arizona State University, read the IFLScience article and told us: “I saw the potential of extending it within addiction research.” Although addiction research relies heavily on animal models, these, Tomek said, “are lacking because one thing that is incredibly important in the human condition and a reoccurring theme of humans acquiring and maintaining addiction is the social and relationship aspects we experience.”

To see a testable model for measuring prosocial behavior in animals excited Tomek and she "couldn't wait to incorporate it with drugs of abuse.”

In Tomek's version of the study, 64 rats were housed in pairs to bond. For two weeks, one rat from each pair would be placed in an uncomfortably small plastic tube, while its mate, dubbed the “saver” rat, was trained in how to free the trapped rat. Seven of the savers failed to live up to their name and were dumped from the study.

Once the pattern was learned, and each saver’s propensity to assist established, the saver rats were removed each day for 30-60 minute sessions where they could obtain a pleasure hit, either of sugar or heroin, depending on which group the rat was assigned to. After the saver rats were hooked on their assigned drug, they were again given the opportunity to save their cage-mate. However, to do so, savers needed to take time away from pressing levers that would deliver either sugar or a heroin injection.

The rats assigned to the sugar group overwhelmingly decided friendship was sweeter than sugar and rescued their fellow rat almost as quickly as at baseline, Tomek reports in Addiction Biology. The heroin-receiving rats, however, prioritized their addiction over helping their fellow rat, with a 100 percent failure to rescue within the 30 minutes allowed. The authors observed the rats and confirmed that those on heroin were actually not trying to save their fellow rat, rather than making an effort but being too drugged up to succeed.

A famous study of rat addiction found that, while isolated rats took to drugs easily, those in social environments were uninterested in drugs. However, Tomek told IFLScience more recent work has found that even rats with company will come to use opiates if given enough time. Her own rats “learned very quickly to self-administer”.

Tomek added that the evidence in both humans and animals is that the social environment has a complex effect, since social isolation can often encourage harmful drug-taking, but so can a peer group of drug users. Either way, the antisocial effects of heroin use are both damaging to society and reduce one's prospects for recovery. Consequently, the paper expresses the hope that rat studies will assist in developing therapies or medications that will break the cycle by restoring prosocial behavior, as well as assessing whether prescription opioids have a similar antisocial effect

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