Similarly, previous studies have shown attenuation of naloxone-precipitated withdrawal symptoms by inhibition of enkephalinase activity (suggesting an increase in endogenous enkephalin levels; (Haffmans and Dzoljic, 1987; Haffmans et al., 1987) and induced heroin reinstatement during naloxone-precipitated WD by a priming injection of heroin (Shaham et al., 1996). This evidence concerns the gene MME and Wilson disease.