Morphine reward in dopamine-deficient mice

The pleasure principle People take drugs of abuse, at least initially, because of their pleasurable effects; later on, chronic use can lead to addiction. There is a large literature supporting the idea that dopamine release is responsible for the pleasurable effects of morphine, and a general assumption that most drugs of abuse release dopamine, as do other stimuli including food and sex. Dopamine has even been called the ‘pleasure transmitter’. So it is a surprise to find that mice that cannot make dopamine show essentially the same pleasure response as mice that can. This result, based on a behaviour called conditioned place preference as a measure of murine ‘pleasure’, follows on from a finding that dopamine-deficient mice have an intact reward system as measured by a liking for sugar. The link between dopamine and pleasure may not be the ‘given’ it once seemed.