It is remarkable, however, that in mice, N
2 O blocks amphetamine -induced carrier-mediated dopamine release in the nucleus accumbens and behavioural sensitisation , abolishes the conditioned place preference (CPP) of cocaine and morphine , and does not produce reinforcing (or aversive) effects of its own.   Effects of CPP of N
2 O in rats are mixed, consisting of reinforcement, aversion, and no change.  In contrast, it is a positive reinforcer in squirrel monkeys,  and is well known as a drug of abuse in humans.  These discrepancies in response to N
2 O may reflect species variation or methodological differences.  In human clinical studies, N
2 O was found to produce mixed responses, similarly to rats, reflecting high subjective individual variability.