Ghrelin Helps Us to Sniff Out Food

Summary
The sense of smell is essential for searching for food in rodents and humans. In both rodents and humans, Ghrelin is an appetite-stimulating hormone produced by the stomach. It is reported that olfactory circuits in the brain can respond to ghrelin via receptor molecules.This study noted that ghrelin functions by increasing the sensitivity to smells and boosting the sniffing capacity for locating food. Reports in rats and humans show that ghrelin increased the smell sensitivity by decreasing the levels of odors that can be detected by the brain. This enhanced sniffing probably increased the chances of detecting, recognizing, and selecting the food.

Introduction
The sense of smell helps animals to decide their responses to their surroundings, which includes the ability to find, identify, and select food. This is the case in rodents and humans too. The feeling of starvation intensifies the sensitivity to smells. Sniffing is the first step in olfactory processing in the brain. It is a necessary behavior required for searching food. Ghrelin, produced by the stomach, increases in the blood before meals and reduces afterwards. It is believed to make a person feel hungry, induce ingestion and increase the feeling of enjoyment of meals. The molecular sensor or receptor for ghrelin, known as growth hormone secretagogue receptor, is responsible for the sniffing behavior. The current study investigated to see whether ghrelin stimulates its receptor to increase sniffing and sensitivity to smells when the animal is starved.

Methods

  • In this study, 2.5 nmol per day of ghrelin was injected into rat brains for eight days. The brain sections were treated with ghrelin tagged with a dye to see where it binds.
  • The gene for ghrelin receptor in the mice was replaced by a reporter gene to mark its normal localization.
  • A video-based analysis system recorded sniffing frequency. Saline was injected into three well-fed rats and ghrelin was injected into three other rats for the analysis.
  • In this study, 19 rats were fed only for 4 hours a day for 7 days. Ghrelin levels were recorded before and after meals. These rats were conditioned to avoid water with isoamyl acetate at 1 in 100,000 (10-5) dilution, detectable only by smell. The conditioned saline-injected and ghrelin-injected rats were given water with 10-10, 10-9, 10-8 and 10-7 dilutions of the chemical on sequential days to observe their sensitivity.
  • A similar test was performed using human subjects and the data were statistically analyzed.

Results

  • Ghrelin and ghrelin receptor were detected in the smell sensing parts of brain, suggesting that ghrelin may function during smell analysis.
  • The results showed that ghrelin injections increased sniffing. “These data are consistent with the hypothesis that ghrelin acts in the brain to modulate sniffing behavior.”
  • Unlike the saline-injected rats, ghrelin-injected well-fed rats avoided chemical-mixed water at much lower concentrations, showing increased sensitivity to smells. In fasting rats, no difference was recorded.
  • Humans also showed similar responses to ghrelin injections.

Shortcomings/Next steps
Previous research work indicated that nutritional states that modulate responses of brain to odorants have been identified have been identified, but the exact mechanism that controls the olfactory perception by nutritional status is yet to be discovered. The authors suggest that future studies need to unravel the phenomenon of how hunger or increased ghrelin levels affect sniffing in humans.

Conclusion
Ghrelin impacts the processing of smell in human and rat brains by increasing the sensitivity and sniffing action, both of which are necessary for food acquisition. This study proves that ghrelin receptors are present in the smell sensing regions of the brain, and that ghrelin binds to them directly. This study provides a physiological basis to the functioning of ghrelin, suggesting that ghrelin directly modulates responses of brain upon odorant stimulation. While ghrelin increases sensitivity to smell, it does not bias the brain towards the quality (pleasantness) of the smell. Hence, ghrelin is necessary for exploration of food, but not in making it more enjoyable.

For More Information:
Ghrelin Enhances Olfactory Sensitivity and Exploratory Sniffing in Rodents and Humans
Publication Journal: The Journal of Neuroscience, April 2011
By Jenny Tong; Erica Mannea
From the University of Cincinnati, Cincinnati, Ohio

*FYI Living Lab Reports Are Summaries of the Original Research.

 

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