Endocannabinoid-Mediated Neuromodulation in the Olfactory Bulb: Functional and Therapeutic Significance

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.

The gut microbiota in neurodegenerative diseases: revisiting possible therapeutic targets for cannabidiol

Understanding the pathophysiology of Alzheimer's disease (AD) is essential to improve the efficacy of treatments and, consequently, patients' lives. Unfortunately, traditional therapeutic strategies have not been effective. There is therefore an urgent need to discover or develop alternative treatment strategies. Recently, some pieces of the puzzle appear to emerge: on a hand, the gut microbiota (GM) has gained attention since intestinal dysbiosis aggravates and generates some of the pathological processes of AD; on the other hand, cannabidiol (CBD), a phytocannabinoid, attenuates intestinal inflammation and possesses neuroprotective properties. Intestinal dysbiosis (increased population of proinflammatory bacteria) in AD increases plasma lipopolysaccharide and Aβ peptide levels, both responsible for increasing the permeability of the blood-brain barrier (BBB). A leaky BBB may facilitate the entry of peripheral inflammatory mediators into the central nervous system and ultimately aggravate neuroinflammation and neuronal death due to chronic activation of glial cells. Studies investigating the GM reported a strong relationship between intestinal dysbiosis and AD. In this review we conjecture that the GM is a promising therapeutic target for CBD in the context of AD.

Possible role of endocannabinoids in olfactory and taste dysfunctions in Alzheimer's and Parkinson's patients and volumetric changes in the brain

The purpose of this study is to determine the volumes of primary brain regions associated with smell and taste in Alzheimer's and Parkinson's patients and healthy controls using MR imaging and examine volumetric changes in comparison to smell/taste questionnaire and test results and endocannabinoid (EC) levels. The study included 15 AD patients with mild cognitive dysfunction scored as 18 ≤ MMSE ≤ 23, 15 PD patients with scores of 18 < MoCA < 26 and 18 ≤ MMSE ≤ 23, and 15 healthy controls. A taste and smell questionnaire was given to the participants, and their taste and smell statuses were examined using the Sniffin' Sticks smell identification test and Burghart Taste Strips. EC levels were analyzed in the blood serum samples of the participants using the ELISA method. The volumes of the left olfactory bulb (p = 0.001), left amygdala (p = 0.004), left hippocampus (p = 0.008), and bilateral insula (left p = 0.000, right p = 0.000) were significantly smaller in the Alzheimer's patients than the healthy controls. The volumes of the left olfactory bulb (p = 0.001) and left hippocampus (p = 0.009) were significantly smaller in the Parkinson's patients than the healthy controls. A significant correlation was determined between volume reduction in the left Rolandic operculum cortical region and taste dysfunction. EC levels were significantly higher in both AD (p = 0.000) and PD (p = 0.006) in comparison to the controls. Our results showed that volumetric changes occur in the brain regions associated with smell and taste in Alzheimer's and Parkinson's patients. It was observed that ECs played a role in these volumetric changes and the olfactory and taste dysfunctions of the patients.

Possible Role of Endocannabinoids in Olfactory and Taste Dysfunctions in COVID-19 Patients and Volumetric Changes in the Brain

Introduction

COVID-19 infection develops neurologic symptoms such as smell and taste loss. We aimed to determine the volumetric changes in the brain and correlation of possible related biochemical parameters and endocannabinoid levels after COVID-19 recovery.

Methods

Brain magnetic resonance images of recovered COVID-19 patients and healthy volunteers, whose olfactory and gustatory scores were obtained through a questionnaire, were taken, and the volumes of the brain regions associated with taste and smell were measured by automatic and semiautomatic methods. Endocannabinoids (EC), which are critical in the olfactory system, and vitamin B12, zinc, iron, ferritin, thyroid-stimulating hormone (TSH), and thyroxine (T4) levels, which are reported to have possible roles in olfactory disorders, were measured in peripheral blood.

Results

Taste and smell disorder scores and EC levels were found to be higher in recovered COVID-19 patients compared to controls. EC levels were negatively correlated with bilateral entorhinal cortex (ENT) volumes in the COVID-19 group. Subgenual anterior cingulate cortex volumes showed correlations with gustatory complaints and ferritin in recovered COVID-19 patients.

Conclusions

The critical finding of our study is the high EC levels and negative correlation between EC levels and left ENT volumes in recovered COVID-19 patients.

Implications

It is possible that ECs are potential neuromodulators in many conditions leading to olfactory disorders, including COVID-19.

Olfaction in the context of obesity and diabetes: Insights from animal models to humans

The olfactory system is at the crossroad between sensory processing and metabolic sensing. In addition to being the center of detection and identification of food odors, it is a sensor for most of the hormones and nutrients responsible for feeding behavior regulation. The consequences of modifications in body homeostasis, nutrient overload and alteration of this brain network in the pathological condition of food-induced obesity and type 2 diabetes are still not elucidated. The aim of this review was first to use both humans and animal studies to report on the current knowledge of the consequences of obesity and type 2 diabetes on odorant threshold and olfactory perception including identification discrimination and memory. We then discuss how olfactory processing can be modified by an alteration of the metabolic homeostasis of the organism and available elements on pharmacological treatments that regulate olfaction. We focus on data within the olfactory system but also on the interactions between the olfactory system and other brain networks impacted by metabolic diseases.

The Olfactory System as Marker of Neurodegeneration in Aging, Neurological and Neuropsychiatric Disorders

Research studies that focus on understanding the onset of neurodegenerative pathology and therapeutic interventions to inhibit its causative factors, have shown a crucial role of olfactory bulb neurons as they transmit and propagate nerve impulses to higher cortical and limbic structures. In rodent models, removal of the olfactory bulb results in pathology of the frontal cortex that shows striking similarity with frontal cortex features of patients diagnosed with neurodegenerative disorders. Widely different approaches involving behavioral symptom analysis, histopathological and molecular alterations, genetic and environmental influences, along with age-related alterations in cellular pathways, indicate a strong correlation of olfactory dysfunction and neurodegeneration. Indeed, declining olfactory acuity and olfactory deficits emerge either as the very first symptoms or as prodromal symptoms of progressing neurodegeneration of classical conditions. Olfactory dysfunction has been associated with most neurodegenerative, neuropsychiatric, and communication disorders. Evidence revealing the dual molecular function of the olfactory receptor neurons at dendritic and axonal ends indicates the significance of olfactory processing pathways that come under environmental pressure right from the onset. Here, we review findings that olfactory bulb neuronal processing serves as a marker of neuropsychiatric and neurodegenerative disorders.

Endocannabinoid-mediated neuromodulation in the main olfactory bulb at the interface of environmental stimuli and central neural processing

The olfactory system has become an important functional gateway to understand and analyze neuromodulation since olfactory dysfunction and deficits have emerged as prodromal and, at other times, as first symptoms of many of neurodegenerative, neuropsychiatric and communication disorders. Considering olfactory dysfunction as outcome of altered, damaged and/or inefficient olfactory processing, in the current review, we analyze how olfactory processing interacts with the endocannabinoid signaling system. In the human body, endocannabinoid synthesis is a natural and on-demand response to a wide range of physiological and environmental stimuli. Our current understanding of the response dynamics of the endocannabinoid system is based in large part on research advances in limbic system areas, such as the hippocampus and the amygdala. Functional interactions of this signaling system with olfactory processing and associated pathways are just emerging but appear to grow rapidly with multidimensional approaches. Recent work analyzing the crystal structure of endocannabinoid receptors bound to their agonists in a signaling complex has opened avenues for developing specific therapeutic drugs that could help with neuroinflammation, neurodegeneration, and alleviation/reduction of pain. We discuss the role of endocannabinoids as signaling molecules in the olfactory system and the relevance of the endocannabinoid system for synaptic plasticity.