Tuesday December 7 12:10 AM ET
WASHINGTON (Reuters) - Marijuana and hashish may affect vision by plugging into tailor-made receptors in the eye, researchers said on Monday.
They said they found the receptors -- which are chemical doorways into cells -- in animals ranging from chicks to salamanders to monkeys.
This suggests that chemicals similar to the cannabinoids in hashish and marijuana, known to occur naturally in the human body, are ancient and highly important in eye function, the researchers said.
``The fact that this system is so highly conserved in species separated by hundreds of millions of years of evolution suggests that it's important,'' Alex Straiker of the University of California, San Diego, who led the research, said in a statement.
``Nature likes to tinker, so any time you see something this consistent, it raises eyebrows.''
Working with colleagues at the Neurosciences Institute in San Diego and the University of Washington in Seattle, Straiker found cannabinoid receptors known as CB1 receptors in the retinal cells of rhesus monkeys, chicks, salamanders, goldfish, mice and rats.
They also occurred in both the rods and cones, which are the eye structures that respond to light, the researchers wrote in a report in the Proceedings of the National Academy of Sciences.
``We understand very little about how the retina works. By demonstrating that this receptor system is present, we add another piece to the puzzle, opening one more window into how the eye works,'' Straiker said.
``It also suggests that marijuana affects vision because it plugs into an existing signaling system that is abundant in the retina.''
Cannabinoids naturally occur in vertebrates. For instance, pain triggers the release of one class of cannabinoids known as anandamides. Anandamides are neurotransmitters, or message-carrying chemicals.
The chemical THC found in marijuana also is a cannabinoid.
Cannabis has been used for centuries to help relieve pain, and some research suggests it can affect vision and also may be able to help relieve symptoms of the eye disease glaucoma.
Researchers think THC must plug into the body's natural system for using anandamide and other neurotransmitters.