Distribution of Adiponectin Receptors 1 and 2 in the Rat Olfactory Bulb and the Effect of Adiponectin Injection on Insulin Receptor Expression

The olfactory bulb: A neuroendocrine spotlight on feeding and metabolism

Olfaction is the most ancient sense and is needed for food-seeking, danger protection, mating and survival. It is often the first sensory modality to perceive changes in the external environment, before sight, taste or sound. Odour molecules activate olfactory sensory neurons that reside on the olfactory epithelium in the nasal cavity, which transmits this odour-specific information to the olfactory bulb (OB), where it is relayed to higher brain regions involved in olfactory perception and behaviour. Besides odour processing, recent studies suggest that the OB extends its function into the regulation of food intake and energy balance. Furthermore, numerous hormone receptors associated with appetite and metabolism are expressed within the OB, suggesting a neuroendocrine role outside the hypothalamus. Olfactory cues are important to promote food preparatory behaviours and consumption, such as enhancing appetite and salivation. In addition, altered metabolism or energy state (fasting, satiety and overnutrition) can change olfactory processing and perception. Similarly, various animal models and human pathologies indicate a strong link between olfactory impairment and metabolic dysfunction. Therefore, understanding the nature of this reciprocal relationship is critical to understand how olfactory or metabolic disorders arise. This present review elaborates on the connection between olfaction, feeding behaviour and metabolism and will shed light on the neuroendocrine role of the OB as an interface between the external and internal environments. Elucidating the specific mechanisms by which olfactory signals are integrated and translated into metabolic responses holds promise for the development of targeted therapeutic strategies and interventions aimed at modulating appetite and promoting metabolic health.

Regulation of Metabolic Health by an "Olfactory-Hypothalamic Axis" and Its Possible Implications for the Development of Therapeutic Approaches for Obesity and T2D

The olfactory system is responsible for the reception, integration and interpretation of odors. However, in the last years, it has been discovered that the olfactory perception of food can rapidly modulate the activity of hypothalamic neurons involved in the regulation of energy balance. Conversely, the hormonal signals derived from changes in the metabolic status of the body can also change the sensitivity of the olfactory system, suggesting that the bidirectional relationship established between the olfactory and the hypothalamic systems is key for the maintenance of metabolic homeostasis. In the first part of this review, we describe the possible mechanisms and anatomical pathways involved in the modulation of energy balance regulated by the olfactory system. Hence, we propose a model to explain its implication in the maintenance of the metabolic homeostasis of the organism. In the second part, we discuss how the olfactory system could be involved in the development of metabolic diseases such as obesity and type two diabetes and, finally, we propose the use of intranasal therapies aimed to regulate and improve the activity of the olfactory system that in turn will be able to control the neuronal activity of hypothalamic centers to prevent or ameliorate metabolic diseases.

Protective effects of intracerebroventricular adiponectin against olfactory impairments in an amyloid β1-42 rat model

BACKGROUND

Alzheimer's disease (AD) is characterized by cognitive impairment that eventually develops into dementia. Amyloid-beta (Aβ) accumulation is a widely described hallmark in AD, and has been reported to cause olfactory dysfunction, a condition considered an early marker of the disease associated with injuries in the olfactory bulb (OB), the hippocampus (HIPP) and other odor-related cortexes. Adiponectin (APN) is an adipokine with neuroprotective effects. Studies have demonstrated that APN administration decreases Aβ neurotoxicity and Tau hyperphosphorylation in the HIPP, reducing cognitive impairment. However, there are no studies regarding the neuroprotective effects of APN in the olfactory dysfunction observed in the Aβ rat model. The aim of the present study is to determine whether the intracerebroventricular (i.c.v) administration of APN prevents the early olfactory dysfunction in an i.c.v Amyloid-beta1-42 (Aβ1-42) rat model. Hence, we evaluated olfactory function by using a battery of olfactory tests aimed to assess olfactory memory, discrimination and detection in the Aβ rat model treated with APN. In addition, we determined the number of cells expressing the neuronal nuclei (NeuN), as well as the number of microglial cells by using the ionized calcium-binding adapter molecule 1 (Iba-1) marker in the OB and, CA1, CA3, hilus and dentate gyrus (DG) in the HIPP. Finally, we determined Arginase-1 expression in both nuclei through Western blot.

RESULTS

We observed that the i.c.v injection of Aβ decreased olfactory function, which was prevented by the i.c.v administration of APN. In accordance with the olfactory impairment observed in i.c.v Aβ-treated rats, we observed a decrease in NeuN expressing cells in the glomerular layer of the OB, which was also prevented with the i.c.v APN. Furthermore, we observed an increase of Iba-1 cells in CA1, and DG in the HIPP of the Aβ rats, which was prevented by the APN treatment.

CONCLUSION

The present study describes the olfactory impairment of Aβ treated rats and evidences the protective role that APN plays in the brain, by preventing the olfactory impairment induced by Aβ1-42. These results may lead to APN-based pharmacological therapies aimed to ameliorate AD neurotoxic effects.

Extrinsic neuromodulation in the rodent olfactory bulb

Evolutionarily, olfaction is one of the oldest senses and pivotal for an individual's health and survival. The olfactory bulb (OB), as the first olfactory relay station in the brain, is known to heavily process sensory information. To adapt to an animal's needs, OB activity can be influenced by many factors either from within (intrinsic neuromodulation) or outside (extrinsic neuromodulation) the OB which include neurotransmitters, neuromodulators, hormones, and neuropeptides. Extrinsic sources seem to be of special importance as the OB receives massive efferent input from numerous brain centers even outweighing the sensory input from the nose. Here, we review neuromodulatory processes in the rodent OB from such extrinsic sources. We will discuss extrinsic neuromodulation according to points of origin, receptors involved, affected circuits, and changes in behavior. In the end, we give a brief outlook on potential future directions in research on neuromodulation in the OB.