Anna Salleh for ABC Science Online
Thursday, 20 October 2005
Cannabis is well known for increasing appetite. Now scientists know why.
Scientists have found the part of the brain that makes cannabis users crave pizza, chocolate and chips.
The discovery of the part of the brain that creates the 'munchies' could help to develop pharmaceuticals for anorexia or obesity, with minimal side-effects.
Australian researcher Dr Paul Mallet of the University of New England in Armidale and team will report their rat study in the journal Neuropharmacology.
"Because smoking cannabis increases appetite, it was believed that this was somehow related to the effects of cannabis on some brain centre but that was until now not identified," says Mallet, a biological psychologist.
"We've actually identified which part of the brain is responsible for THC's [the active substance in cannabis] effect on the stimulation of appetite."
They injected THC, or tetrahydrocannabinol, into a specific region of the brain's hypothalamus, known to control feeding behaviour, called the paraventricular hypothalamic nucleus (PVN).
And they found it stimulated the laboratory animals' appetite.
"What we find is that these rats get the munchies," says Mallet.
The cannabinoid system
The brain has long been known to have a cannabinoid system involving receptors that react with both THC and a naturally occurring substance called anandamide.
Mallet and team are part of a research effort focused on trying to understand the reason why the brain has evolved this system.
"It's very unlikely we evolved a brain cannabinoid system so that we can get stoned at parties," says Mallet.
So far the cannabinoid system has been found to play a role in perceiving pain, motor control, storing memories and feeling hungry.
And it's this last role that has led to the development of new pharmaceuticals that could either stimulate appetite or suppress it by either blocking or stimulating cannabinoid receptors.
The problem is that cannabinoid receptors exist throughout the brain and play different roles in different parts of the brain.
This means that many of the cannabinoid pharmaceuticals being developed have unwelcome side-effects.
"For some individuals they experience pleasure, about four out of five individuals. But for one out of five individuals cannabis is quite anxiety provoking," says Mallet.
Now that his team have pinpointed a specific part of the cannabinoid system that is involved in hunger, this may pave the way for drugs that are more specific and have fewer side-effects.
"We've given the pharmaceutical industry ... a bit more evidence," he says.
He says future drugs could perhaps be designed to target cannabinoid receptors that are found specifically within the PVN of the hypothalamus.
Mallet says there may be other areas of the cannabinoid system that control hunger.