Abstract
The peptide hormone adiponectin is secreted by adipose tissue and the circulating concentration is reversely correlated with body fat mass; it is considered as starvation signal. The observation that mature sensory neurons of the main olfactory epithelium express the adiponectin receptor 1 has led to the concept that adiponectin may affect the responsiveness of the olfactory system. In fact, electroolfactogram recordings from olfactory epithelium incubated with exogenous adiponectin resulted in large amplitudes upon odor stimulation. To determine whether the responsiveness of the olfactory sensory neurons was enhanced, we have monitored the odorant-induced expression of the immediate early gene Egr1. It was found that in an olfactory epithelium incubated with nasally applied adiponectin the number of Egr1 positive cells was significantly higher compared to controls, suggesting that adiponectin rendered the olfactory neurons more responsive to an odorant stimulus. To analyze whether the augmented responsiveness of sensory neurons was strong enough to elicit a higher neuronal activity in the olfactory bulb, the number of activated periglomerular cells of a distinct glomerulus was determined by monitoring the stimulus-induced expression of c-fos. The studies were performed using the transgenic mOR256-17-IRES-tauGFP mice which allowed to visualize the corresponding glomerulus and to stimulate with a known ligand. The data indicate that upon exposure to 2,3-hexanedione in adiponectin-treated mice the number of activated periglomerular neurons was significantly increased compared to controls. The results of this study indicate that adiponectin increases the responsiveness of the olfactory system, probably due to a higher responsiveness of olfactory sensory neurons.
Highlights
The necessity to find suitable food sources is desperately needed when the level of internal energy resources, the body fat mass, is running low
As a first step to scrutinize the hypothesis that adiponectin may affect the responsiveness of olfactory sensory neurons (OSNs), we have investigated the electrical response of the olfactory epithelium (OE) to odorants by recording electroolfactograms (EOG)
The discovery that mature olfactory sensory neurons in the main olfactory epithelium express the adiponectin receptor subtype adiponectin receptor 1 (adipoR1), has led to the hypothesis that the hormone adiponectin, which is considered as an important starvation signal, may affect the responsiveness of the olfactory system [9]
Summary
The necessity to find suitable food sources is desperately needed when the level of internal energy resources, the body fat mass, is running low. The effect is supposed to be mediated by neurons in hypothalamic nuclei which express receptors for adiponectin and respond to adiponectin with a change in their electrical excitability [7,8]. These neurons are supposed to govern the activities which correlate with hunger and satiety, including the control of food intake and the search for food sources. In a recent study it has been found that all mature olfactory sensory neurons (OSNs) in the olfactory epithelium express one of the adiponectin receptor subtypes, namely adiponectin receptor 1 (adipoR1) [9]. The results indicate that pretreatment with adiponectin resulted in a higher number of OSNs responding to an odorant and a higher neuronal activity in the olfactory bulb corresponding glomeruli
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