Why are relapses more common during times of stress?
Brain changes that occurs in response to a stressful event make rats more vulnerable to addiction for several days afterward.
Daily stress increases the likelihood that people who take drugs will become addicted. A very early step in the development of addiction is learning that certain people, places, or paraphernalia are associated with obtaining drugs. These ‘cues’ — drug dealers, bars, cigarette advertisements, etc. — become powerful motivators to seek out drugs and can trigger relapse in recovering addicts. It is thought that learning happens when synapses (the connections between neurons in the brain) that relay information about particular cues become stronger. However, it is not clear how stress promotes the learning of cue-drug associations.
Claire Stelly and co-workers investigated whether repeated episodes of stress make it easier to strengthen synapses on dopamine neurons, which are involved in processing rewards and addiction. For the experiments, rats were repeatedly exposed to a stressful situation — an encounter with an unfamiliar aggressive rat — every day for five days. Stelly and co-workers found that these stressed rats formed stronger associations between the drug cocaine and the place where they were given the drug (the cue). Furthermore, a mechanism that strengthens synapses was more sensitive in the stressed rats than in unstressed rats. These changes persisted for 10–30 days after the stressful situation, suggesting that stress might begin a period of time during which the individual is more vulnerable to addiction.
The experiments also show that a hormone called corticosterone — which is released during stressful experiences — is necessary for stress to trigger the changes in the synapses and behavior of the rats. However, corticosterone must work with other factors because giving this hormone to unstressed rats was not sufficient to trigger the changes seen in the stressed rats. Future experiments will investigate what these other stress factors are and how they work together with corticosterone.
To find out more
Read the eLife research paper on which this eLife digest is based: “Repeated social defeat stress enhances glutamatergic synaptic plasticity in the VTA and cocaine place conditioning” (July 4, 2016).
