Evidence of rapid recovery from perceptual odor adaptation using a new stimulus paradigm

Increase in diastolic blood pressure induced by fragrance inhalation of grapefruit essential oil is positively correlated with muscle sympathetic nerve activity

Fragrance inhalation of essential oils is widely used in aromatherapy, and it is known to affect blood pressure (BP) and heart rate (HR) via autonomic control of circulation. In this study, we aimed to test the hypothesis that the changes in hemodynamics with fragrance inhalation were observed along with changes in muscle sympathetic nerve activity (MSNA). In study 1, thirteen healthy men were exposed to fragrance stimulation of grapefruit essential oil for 10 min, and BP, HR, and MSNA were continuously measured. In study 2, another nine healthy men were exposed to the same fragrance stimulation; responses in BP and HR were continuously measured, and plasma noradrenaline and cortisol concentrations were determined. We found that diastolic BP increased significantly during fragrance inhalation, while the other variables remained unchanged in both studies. Although MSNA burst frequency, burst incidence, and total activity remained unchanged during fragrance inhalation, we found a significant linear correlation between changes in diastolic BP in the last 5 min of fragrance inhalation and changes in MSNA burst frequency. The plasma cortisol concentration decreased significantly at 10 min of fragrance inhalation, though the noradrenaline concentration remained unchanged. These results suggest, for the first time, that changes in BP with fragrance inhalation of essential oil are associated with changes in MSNA even with decreased stress hormone.

Habituation and adaptation to odors in humans

Habituation, or decreased behavioral response, to odors is created by repeated exposure and several detailed characteristics, whereas adaptation relates to the neural processes that constitute this decrease in a behavioral response. As with all senses, the olfactory system continually encounters an enormous variety of odorants which is why mechanisms must exist to segment them and respond to changes. Although most olfactory habitation studies have focused on animal models, this non-systematic review provides an overview of olfactory habituation and adaptation in humans, and techniques that have been used to measure them. Thus far, psychophysics in combination with modern techniques of neural measurement indicate that habituation to odors, or decrease of intensity, is relatively fast with adaptation occurring more quickly at higher cerebral processes than peripheral adaptation. Similarly, it has been demonstrated that many of the characteristics of habitation apply to human olfaction; yet, evidence for some characteristics such as potentiation of habituation or habituation of dishabituation need more support. Additionally, standard experimental designs should be used to minimize variance across studies, and more research is needed to define peripheral-cerebral feedback loops involved in decreased responsiveness to environmental stimuli.

A technique for characterizing the time course of odor adaptation in mice

Although numerous studies have analyzed the temporal characteristics underlying olfactory adaptation at the level of the olfactory receptor neuron, to date, there have been no comparable behavioral measures in an animal model. In this study, odor adaptation was estimated in a group of mice employing a psychophysical technique recently developed for use in humans. The premise of this technique is that extended presentation of an odorant will produce odor adaptation, decreasing the sensitivity of the receptors and increasing thresholds for a brief, simultaneous target odorant presented at different time points on the adaptation contour; adaptation is estimated as the increase in threshold for a target odorant presented simultaneously with an adapting odorant, across varying adapting-to-target odorant onset delays. Previous research from our laboratory suggests that this method provides a reliable estimate of the onset time course of rapid adaptation in human subjects. Consistent with physiological and behavioral data from human subjects, the present findings demonstrate that measurable olfactory adaptive effects can be observed for odorant exposures as brief as 50-100ms, with asymptotic levels evident 400-600ms following adapting odorant onset. When compared with the adaptation contour in humans using the same odorant and stimulus paradigm, some differences in the onset characteristics are evident and may be related to sniffing behavior and to relative differences in thresholds. These data show that this psychophysical paradigm can be adapted for use in animal models, where experimental and genetic manipulations can be used to characterize the different mechanisms underlying odor adaptation.