Maladaptive responses to stress are a hallmark of alcohol use disorder, but the mechanisms that underlie this are not well characterized. Here, we show that kappa opioid receptor signaling in the bed nucleus of the stria terminalis (BNST) is a critical molecular substrate underlying abnormal stress responses to predator odor following heavy alcohol drinking. Exposure to predator odor during protracted withdrawal from intermittent alcohol drinking resulted in enhanced prefrontal cortex (PFC)-driven excitation of prodynorphin-containing neurons in the BNST. Furthermore, deletion of prodynorphin in the BNST and chemogenetic inhibition of the PFC-BNST pathway restored abnormal responses to predator odor in alcohol-exposed mice. These findings suggest that increased corticolimbic drive may promote abnormal stress behavioral responses to predator odor during protracted withdrawal. Various nodes of this PFC-BNST dynorphin-related circuit may serve as potential targets for potential therapeutic mediation as well as biomarkers of negative responses to stress following heavy alcohol drinking.
Keywords: alcohol; dynorphin; kappa opioid receptor; mouse; neuroscience; norBNI; predator odor; stress.
The connection between stress and alcohol use is highly complex. On one hand, there is the idea of having a drink to ‘steady the nerves’. On the other hand, in alcoholics, abnormal responses to stress often accompany heavy drinking. In this case, it remains unknown whether stress cause excessive drinking, or vice versa. Areas of the brain that normally help respond to stress work differently in long-term, heavy drinkers. One example is a structure called the bed nucleus of the stria terminalis (BNST), which is over-active in anxiety disorders and is also associated with some of the symptoms of alcohol withdrawal. The mechanism behind both problems is thought to be a specific ‘signaling system’ that is activated by a small molecule called dynorphin. Previous research into the effects of dynorphin was performed either in the context of alcoholism or of anxiety disorders, but it was not known if there was a connection between the two. Therefore, Hwa et al. wanted to determine how prolonged alcohol use might affect responses to stress, and whether dynorphin signaling plays a role. To model long-term alcohol use in the laboratory, a group of mice was given free access to alcohol every other day, ensuring that they developed the mouse equivalent of a drinking habit. After six weeks, these ‘heavy drinkers’ went through a period of abstinence, mimicking alcohol withdrawal. Then, the mice were stressed by exposing them to a chemical that smelled like a fox, one of the mice’s predators in the wild. When mice smell predators, they normally respond by fleeing from the area and digging up debris to defend itself. As expected, the control mice in this study, which did not drink alcohol, did just that. In contrast, the heavy drinkers largely ignored the predator scent by not digging and even spent time hanging around the area that smelled like the predator. Blocking dynorphin-induced signaling in the alcoholic mice, either using a drug or by deleting the gene that codes for dynorphin, reset the stress response to normal, allowing these mice to avoid the predator and dig as normal. Furthermore, measuring the electrical activity in the brain revealed that the BNST was abnormally active in alcohol-drinking mice, driven by signals from another part of the brain, the prefrontal cortex. This reveals part of the circuitry in the brain responsible for the connection between alcohol withdrawal and the stress response. These results shed new light on the biological mechanisms underpinning the relationship between alcohol use and stress. In the future, these could be used to determine why heavy drinking can overlap with anxiety disorders, or to develop new treatments that would help recovering alcoholics cope better with stress.
© 2020, Hwa et al.