Ghrelin enhances olfactory sensitivity and exploratory sniffing in rodents and humans.

Olfaction is an integral part of feeding providing predictive cues that anticipate ingestion. Although olfactory function is modulated by factors such as prolonged fasting, the underlying neural mechanisms remain poorly understood. We recently identified ghrelin receptors in olfactory circuits in the brain. We therefore investigated the role of the appetite-stimulating hormone ghrelin in olfactory processing in rodents and humans, testing the hypothesis that ghrelin lowers olfactory detection thresholds and enhances exploratory sniffing, both being related to food seeking. In rats, intracerebroventricular ghrelin decreased odor detection thresholds and increased sniffing frequency. In humans, systemic ghrelin infusions significantly enhanced sniff magnitudes in response to both food and nonfood odorants and air in comparison to control saline infusions but did not affect the pleasantness ratings of odors. This is consistent with a specific effect on odor detection and not the hedonic value of odors. Collectively, our findings indicate that ghrelin stimulates exploratory sniffing and increases olfactory sensitivity, presumably enhancing the ability to locate, identify, and select foods. This novel role is consistent with ghrelin's overall function as a signal amplifier at the molecular interface between environmental and nutritional cues and neuroendocrine circuits controlling energy homeostasis.

Odor recognition memory as a function of odor-naming performance.

A series of experiments sought to clarify the relationship between odor naming and memory by manipulating odor label availability during a dual naming-memory task. Experiment 1 demonstrated that recognition memory and odor naming were both better when the naming task provided participants with odor label alternatives. Consistent and correct odor naming was associated with nearly perfect memory, whereas inconsistent or incorrect naming was associated with very weak memory if any at all. Experiment 2 showed that the availability of odor labels was effective at improving memory only if labels were available at both memory encoding and retrieval, suggesting that the labels were aiding memory by improving the identification of the odors. Odor naming was manipulated in Experiment 3 by varying the number of available labels from 4 to 16 during each odor-naming trial. As found in the previous experiments, naming and memory were strongly related in each of the labeling conditions. Experiment 4 showed that corrective naming feedback produced better memory performance but only when the feedback led to correct odor naming. It was concluded that perceptual processes related to matching olfactory input to acquired, multidimensional representations of odors play a critical role in both odor naming and episodic memory.