After prolonged stress, some people still have the capacity for enjoyment, but others find no pleasure in activities that used to be rewarding — one of the main symptoms of depression. Research on rats now links this loss of capacity for pleasure to the number of neurons that produce the brain signalling molecule serotonin.
Davide Dulcis and his colleagues at the University of California San Diego exposed male rats to aggressive attacks by other rats. The researchers also trained the animals to deliver electrical pulses to regions of their own brains that produce feelings of pleasure.
The team found that in response to stress, some rodents needed more stimulation than others to feel pleasure. Compared with resilient rats, those susceptible to stress had more serotonin-making neurons in an area of the brain involved in regulating stress and reward.
The authors also found that turning on a set of neurons in a separate brain region suppressed the effects of stress, making the animals more resilient. The findings could help to inspire new treatments for depression, the scientists say.