Duke University Medical System researchers have reached a step closer to developing potential new smoking-cessation products by discovering that there are differing taste pathways for nicotine.
"We learned some of nicotine's secrets. This is the first study to explore both the peripheral taste pathways activated by nicotine, and how these pathways are integrated in sensory areas of the brain," said Dr. Albino Oliveira-Maia, a postdoctoral fellow of the Duke Department of Neurobiology.
Describing the peripheral nervous system, the researchers said that it refers to nerves that are outside of the brain and spinal cord.
The team revealed that they used genetic engineering and measurements of nervous system activity in mice, which showed them that nicotine sends signals directly to the brain's sensory systems by several pathways, similar to the way taste is perceived.
According to them, their findings complement whatever is known about the effects of nicotine in the dopamine pathway, the classic pleasure pathway in the brain that has been much studied by addiction experts.
"Our study in no way contradicts prior findings about nicotine and dopamine. Our findings add to what is known and suggest new approaches for further study," Oliveira-Maia said.
"One reason that our findings are interesting is because they relate to previous work that looked at humans with lesions in the insula region of the brain - they had an easier time giving up cigarettes than most people.
"We found that a part of the insula, the gustatory cortex, has robust responses to nicotine and a capacity to integrate diverse peripheral information to create a unique sensory representation for nicotine," Oliveira-Maia said.
In their study report, the Duke researchers say that one taste pathway they have uncovered involves nicotinic acetylcholine receptors (nAChR), which scientists previously proposed were taste receptors for nicotine.
They claim to have found a previously unknown link between these receptors and activity in the taste region of the insula.
The researchers have also found a second pathway, the peripheral Trpm5 protein pathway -that helps animals sense a bitter taste.
During the study, said the group, the mice with their Trpm5 pathway deleted were unresponsive to several different tastes, including bitterness, but they could still sense the presence of nicotine.
"The mice preferred plain water to the nicotine solution, suggesting that there would be a second taste pathway in play, besides the one that had been knocked out," Oliveira-Maia said.
Upon measuring nerve activity in the chorda tympani (CT), a branch of the facial nerve that serves the taste buds in the front of the tongue, the researchers found that activity in CT nerve fibres increased when nicotine was put on the mice's tongues.
Oliveira-Maia said that drugs that block the nAChR receptors are presently being used in the treatment of tobacco addiction mainly because of their effects on the central nervous system, "but it is possible they could also modify the sensory effects of cigarette smoke."
The findings have been revealed in the PNAS Early Edition.