MARIJUANA and heroin may be frowned on by parents and the police, but they do have a beneficial side for scientists. Recent breakthroughs in understanding how these two addictive drugs act on the brain could result in a non-addictive opiate that will help drug addicts cope better with withdrawal symptoms.

Marijuana and heroin have long been known to have therapeutic qualities, but their use was discouraged because of their mood-altering and addictive properties. Besides analgesic, antihypertensive and antinausea qualities, marijuana also lowers eye pressure in glaucoma; heroin is an effective painkiller.

Marijuana and heroin affect the brain differently. Unlike cocaine, which interrupts brain processes, the active ingredients in marijuana and heroin release specific chemicals called receptors in the brain. The role receptors play and the nature of the chemicals that bind with them during ordinary brain functions, have long eluded scientists.

In order to understand and control the properties of these drugs, researchers must identify the gene for the receptor associated with each drug and also the natural brain chemical, called an endogenous ligand that binds the receptor. The endogenous ligand is important because it can be regarded as the internal equivalent for the drug and is expected to simulate its effects.

The receptor for marijuana was cloned two years ago and the endogenous ligand has now been identified by William Devane and Raphael Mechoulam of the Hebrew University of Jerusalem (Science, Vol 258, No 5090). In the case of heroin, researchers have at last identified the gene for the opiate receptor, though the endogenous ligands for it were discovered in the 1970s. The identification was accomplished by teams, researching independently, from the University of California at Los Angeles and the Ecole Superiere de Biotechnologie in Strasbourg.

Researchers hope to prepare a complete inventory of receptors for the two drugs and establish their role in brain functions. This knowledge is expected to facilitate development of drugs that will target the specific receptors involved in pain-killing.