IT'S A hallucinogenic drug that was once hailed as a promising psychiatric therapy before being banned amid concerns over recreational use - now scientists believe LSD could hold the key to new drug treatments for illnesses such as schizophrenia.

A team of American researchers has discovered a biological link between LSD "acid trips" and psychotic delusions in the brain. Both states can cause hallucinations and feelings of disassociation from reality, and now scientists have uncovered what they share at a biochemical level. They say this could open the door to promising new drug treatments for psychotic illnesses such as schizophrenia, and bipolar disorder, which is also known as manic depression.

LSD was first produced by Swiss chemist Albert Hofmann in 1938 and quickly became a therapeutic agent that appeared to show great promise. Between 1950 and 1965, the drug and other hallucinogens generated hundreds of scientific papers and were prescribed to more than 40,000 patients.

But it also became a popular recreational drug and by the late 1960s was outlawed around the world as a threat to public safety. In recent years, however, there has been renewed interest in the drug's therapeutic potential and in 2006, the Royal College of Psychiatrists discussed its use at its annual meeting for the first time in more than 30 years.

The new research hinges around a pair of receptors in the brain that bind together to form a functional complex.

Receptors are protein "locks" that trigger biological responses when the right shaped molecular "keys" latch on to them.

The receptor complex identified in the research switches on responses to drugs such as LSD, and psilocybin - the active ingredient in "magic mushrooms". It is also involved in schizophrenia.

One of the two receptors, mGluR2, reacts to glutamate and the other, 2AR, responds to serotonin. Both chemicals are neurotransmitters, which help to pass messages between nerves.

The receptors normally work together in a balanced way, said the scientists, who reported their findings in the online edition of the journal "Nature".

In brains removed from untreated schizophrenia patients, 2AR is over-active and mGluR2 under-active. The researchers believe this is a pattern that could predispose people to psychosis. Schizophrenic patients treated with newer anti-psychotic drugs such as Clozapine had brains in which the 2AR receptor was downregulated.

Hallucinations and delusions associated with psychosis usually reduce with age, which may be a reflection of reduced 2AR-mGluR2 activity as people get older, said the scientists.

The team, led by Dr Stuart Sealfon from New York's Mount Sinai School of Medicine, wrote: "These studies identify the 2AR-mGluR2 complex as a possible site of action of hallucinogenic drugs. The glutamate and serotonin systems have both been implicated in psychotic disorders, and the components of this complex are found to be differentially regulated in cortex from individuals with schizophrenia.

"Our results are consistent with the hypothesis that the 2AR-mGluR2 complex integrates serotonin and glutamate signalling to regulate the sensory gating functions of the cortex - a process that is disrupted in psychosis."

The research has been welcomed by mental health campaigners. Marjorie Wallace, chief executive of charity SANE, said: "We hope these findings will create a better understanding of precisely how medications work on the brain and lead to direct improvements in the kinds of drug treatments available for people with psychosis.

"Currently, limitations in knowledge of how the drugs work mean that people given medication do not respond, doses may be too high, and the side effects can outweigh the benefits.

"Finding new drugs tailored to a person's individual biochemistry would mean that thousands of people with severe mental illness could look forward to a brighter future."