HOW CANNABIS TURNS YOUNG RATS INTO DOPE-HEADS

YOUNG rats high on cannabis suffer far greater memory loss, and more
lasting changes in brain function than doped-up adult rodents.

Research by Sydney scientists has also found the drug is a turn-off
for grown-up rats. However, the wisdom of age comes too late.

"Cannabis produces much greater long-term changes in adolescent than
adult rat brains," said Iain McGregor, professor of psychopharmacology
at the University of Sydney.

His research team used sophisticated cages - more like three-room
houses - for the experiments. Each room was given different wallpaper
and smells so the rats could tell them apart.

Adolescent and adult rats were locked in one room and injected 12
times over two weeks with tetrahydrocannabinol, or THC, the drug's
active ingredient.

When the doors were opened, allowing the rats to roam from room to
room, the researchers noticed something unusual.

"The adults avoided the room where they had been under the influence
of THC," Professor McGregor said. But the adolescents had no fear of
the room.

Professor McGregor said the results reflected cannabis use patterns in
humans. "Studies show that most adult [humans] have given it away by
the time they are 30 or 35."

The rats were then left drug-free for two weeks, given blood checks to
ensure they no longer had THC in their bodies, and then had their
memory tested. "The 'teenagers' that had received cannabis were still
memory impaired two weeks later, while the adults were fine,"
Professor McGregor said. "The adolescents showed poor memory for what
had happened an hour ago."

Finally the researchers examined the brains of the rats, looking for
tell-tale protein changes in the hippocampus, the area responsible for
memory.

The adolescents that had been given cannabis had three times as many
changed proteins in this brain region as the adults given the same
dose.

"It was a double whammy," Professor McGregor said. "The adolescents
liked cannabis more, but it was worse for their brains and their memory."

Although Professor McGregor did not want to sound as if he was
preaching that cannabis was "a drug of doom", he said it was "another
cautionary study about heavy use" by young people whose brains were
still developing.

The findings appear in the journal Neuropsychopharmacology this month.