We will first summarize behavioral responses of null mutant mice to opiates, then overview reports investigating the effects of other drugs of abuse, including cannabinoids, psychostimulants (cocaine, MDMA, amphetamine), nicotine and alcohol in these mice, and finally conclude on the respective roles of opioid peptides and receptors, and perspectives of opioid research in the area of drug abuse. Whereas data from receptor KO mice have unambiguously clarified receptor roles in vivo, data from peptide KO mice are by essence more complex (low receptor selectivity) and the latter mutants still deserve further investigations.
