Nobel Prize winner Venkatraman Ramakrishnan would certainly approve of this piece of research that comes from a lab straddling the Arabian Sea in Mumbai. Ramakrishnan, currently in Calcutta for the 100th edition of Indian Science Congress, during a previous visit had lamented Indian scientists’ penchant for what he called “derivative research” — finding second and third examples of scientific problems that had been resolved.
A team of researchers at the Tata Institute of Fundamental Research (TIFR) in Mumbai has just come up with a research finding that is as original as it can get. The TIFR team, led by neuroscientist Shubha Tole, in a study published yesterday in the journal Nature Neuroscience, shows why a mammalian brain reads chemical cues that help it distinguish a threat and possible sex differently.
This evolutionary insight comes exactly 200 years after Danish surgeon Ludwig Jacobson first unravelled the role of an organ called vomeronasal organ (VNO) found inside the nose or mouth of higher order animals such as reptiles and mammals. Also called Jacobson organ, VNO is the first outpost of the brain that receives chemical stimuli such as pheromones — substances that a fertile body excretes to attract the attention of the opposite sex.
The ability to perceive smell (olfaction) appeared nearly 300 million years ago when species emerged from aquatic environments to terrestrial, air-breathing environments. The standard “smell” circuit in the brain (MOB) that helps us process the smell of food or perfume is kept separate in the brain in many animals from a circuit that processes smells of emotional significance.
For instance, pheromones do not have a discernable smell and only members of the same species can detect them. This segregation is not very clear in humans but is evident in species such as rats that rely greatly on smell.
“It plays a critical role in the mammals that do possess it. In rodents the pheromonal system is beautifully set apart from the standard olfactory system,” Tole told KnowHow.
This signalling system is vital for building and maintaining animal societies. It is crucial in mother-child bonding, finding a mate and reacting to threats, she says.
Till recently, scientists didn’t have a clue how chemical stimuli sensed by the neurons in the VNO were processed in a brain part called the accessory olfactory bulb (AOB). Interestingly, AOB has two distinct components: a front portion that signals reproductive behaviour and a rear part that processes emotional cues relating to fight or flight.
In the new work, which is a doctoral thesis of Tole’s student Dhananjay Huilgol, the scientists studied how these halves of AOB formed in a mouse embryo.
Much to their surprise, they found that stem cells that give rise to neurons in these two components reside in two regions of the brain that are far from each other and they follow very different genetic recipes for their formation. This could probably explain why the two halves of the AOB have distinctly different functions, the scientists explain.
The stem cells that give rise to the part that detects mating opportunities are created in a nearby region in the brain. But those found in the section dealing with the “fear or fight” sensation is produced far away and shipped to the AOB, says Tole.
“Neurons in the AOB that have different functions originate in two different locations and come together,” says Huilgol, the first author of the study. “It’s as if the components of the fear or fight circuit are created in one place and then shipped out to their final locations (the AOB, the amygdala, and the hypothalamus) from where they somehow connect to each other,” says Tole.
While amphibians such as frogs have no demarcation in the AOB, the divisions are all too evident in species that originated subsequently, such as mammals.
The study is important because circuit formation — or malformation — controls our behaviour, sensation, perception, and emotions. New discoveries in how particular circuits are formed and how they evolved provide a framework to understand what happens when things go wrong, says Tole. Apart from the TIFR scientists, their counterparts in Japan, Spain and the US also contributed to the study.
“The paper is interesting,” says Upinder Bhalla, a researcher at The National Centre for Biological Sciences, in Bangalore.
What is remarkable is that the scientists traced the role of the AOB region back to its ancestral form as seen in the frog. These cell types may have been, long ago in evolution, part of the same neural complex, but now they reside in quite separate parts of the brain even though they retain their old functions and connections, observes Bhalla, who has been studying the olfactory system in animals for a long time.
The research finding has opened up new vistas. The study on the scent of emotions, clearly, is not to be sniffed at.
which does what
VNO: The vomeronasal organ is found inside the nose or mouth of reptiles and mammals. It receives chemical stimuli such as pheromones
MOB: The main olfactory bulb helps us process the smell of food or perfume. It is usually separate from the circuit that processes smells of emotional significance
AOB: The accessory olfactory bulb processes chemical stimuli sensed by the VNO. It has a front portion that signals reproductive behaviour and a rear part that processes emotional cues relating to danger