From sniffing out diseases and explosives, to some clever animal and dinosaur adaptations for life, the nose and its broad olfactory functions are an important, if somewhat underappreciated, part of many animals' sensory systems.
Now, scientists believe it could be wielding more power over our behavior than previously thought, with researchers at the University of Colorado Anschutz Medical Campus (CU Anschutz) uncovering that certain odors stimulate specific brain cells that motivate a fast, physical response to them.
In a mouse study, the researchers were focused on the hippocampus area of the brain, which is best known as the region that plays a critical role in learning, memory and emotion. In particular, they were interested in time cells, the neurons responsible for the temporal organization of memory, and what kind of role they potentially played in associated learning.
They gave the animals the option of reacting to a fruity smell, which presented them with a spout of sugary water to lick, or a mineral odor, which offered nothing from the tap.
"They have to associate the odor with the outcome of what they are doing so that's why they learn decision-making," said first author Ming Ma, senior instructor in cell and developmental biology in the School of Medicine. "When it's a fruit odor, they lick and get a reward. When it's mineral oil, they stop licking."
While this kind of reward-stimuli behavioral experiment is not particularly new, the way the mice responded increasingly faster to the favorable stimuli piqued the interest of the researchers. And what they found was that a specific, previously unknown group of time cells spring into action when a quick decision is needed based on a smell that likely offers a reward or a positive outcome.
"The more they learned, the more the cells were stimulated, leading to more rapid decoding of the odors and allowing the mice to quickly become proficient at choosing the fruity smell," said co-first author Fabio Simoes de Souza, assistant research professor in the School of Medicine.
The scientists found that when they looked at how individual cells in the stratum pyramidale (SP), or principal cellular layer in the hippocampus, were responding differently to stimuli, they discovered that rapid decision-making was taking place at different times following odor exposure.
Essentially, when a smell travels up the nose, neural signals are fired along to the olfactory bulb and hippocampus, and "decision-predicting" time cells switch on to hasten a 'go/do not go' choice based on previous memories thought to be stored more passively in the region. Interestingly, the cells don't fire up with every odor, which the team believes could be a discretionary function to avoid overloading the entire sensory system.
“These are cells that would remind you to make a decision – do this or do that,” said the study’s senior author Diego Restrepo, a neuroscientist and professor in the School of Medicine. “The hippocampus turns on decision-predicting time cells, which would give you a hint of what to remember."
Prior to this discovery, researchers were not aware of any decision-making cells in the hippocampus. While found in mice, the team expects physiology and function to be replicated in humans. As such, it's a fascinating insight into how multifaceted olfaction is, and how the phrase "to follow one's nose" may be more applicable to neurons than to instincts.
“In the past, time cells were thought to only remind you of events and time," Restrepo said. "Here we see memory encoded in the neurons and then retrieved instantly when making a decision."
"The hippocampus is multitasking," he added.
The study was published in the journal Current Biology.
Source: University of Colorado Anschutz
