Insula and Olfaction: A Literature Review and Case Report

Characterization of pathological changes in the olfactory system of mice exposed to methylmercury

Methylmercury (MeHg) is a well-known environmental neurotoxicant that causes severe brain disorders such as Minamata disease. Although some patients with Minamata disease develop olfactory dysfunction, the underlying pathomechanism is largely unknown. We examined the effects of MeHg on the olfactory system using a model of MeHg poisoning in which mice were administered 30 ppm MeHg in drinking water for 8 weeks. Mice exposed to MeHg displayed significant mercury accumulation in the olfactory pathway, including the nasal mucosa, olfactory bulb, and olfactory cortex. The olfactory epithelium was partially atrophied, and olfactory sensory neurons were diminished. The olfactory bulb exhibited an increase in apoptotic cells, hypertrophic astrocytes, and amoeboid microglia, mainly in the granular cell layer. Neuronal cell death was observed in the olfactory cortex, particularly in the ventral tenia tecta. Neuronal cell death was also remarkable in higher-order areas such as the orbitofrontal cortex. Correlation analysis showed that neuronal loss in the olfactory cortex was strongly correlated with the plasma mercury concentration. Our results indicate that MeHg is an olfactory toxicant that damages the central regions involved in odor perception. The model described herein is useful for analyzing the mechanisms and treatments of olfactory dysfunction in MeHg-intoxicated patients.

The anterior insular cortex processes social recognition memory

Impaired social abilities are characteristics of a variety of psychiatric disorders such as schizophrenia, autism spectrum disorder, and bipolar disorder. Studies consistently implicated the relationship between the anterior insular cortex (aIC) and social ability, however, how the aIC involves in processing specific subtypes of social ability was uninvestigated. We, therefore, investigated whether the absence or presence of the aIC affects the social behaviors of mice. We found that electrolytic lesions of the aIC specifically impaired mice's ability to recognize a novel stranger mouse, while the sociability of the aIC-lesioned mice was intact. Interestingly, the aIC-lesioned mice were still distinguished between a mouse that had been housed together before the aIC lesion and a novel mouse, supporting that retrieval of social recognition memory may not involve the aIC. Additional behavioral tests revealed that this specific social ability impairment induced by the aIC lesion was not due to impairment in olfaction, learning and memory, locomotion, or anxiety levels. Together our data suggest that the aIC is specifically involved in processing social recognition memory, but not necessarily involved in retrieving it.

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.

White Matter Microstructural Alterations in Newly Diagnosed Parkinson’s Disease: A Whole-Brain Analysis Using dMRI

Parkinson’s disease (PD) is a neurodegenerative disorder characterized by cardinal motor symptoms and other non-motor symptoms. Studies have investigated various brain areas in PD by detecting white matter alterations using diffusion magnetic resonance imaging processing techniques, which can produce diffusion metrics such as fractional anisotropy and quantitative anisotropy. In this study, we compared the quantitative anisotropy of whole brain regions throughout the subcortical and cortical areas between newly diagnosed PD patients and healthy controls. Additionally, we evaluated the correlations between the quantitative anisotropy of each region and respective neuropsychological test scores to identify the areas most affected by each neuropsychological dysfunction in PD. We found significant quantitative anisotropy differences in several subcortical structures such as the basal ganglia, limbic system, and brain stem as well as in cortical structures such as the temporal lobe, occipital lobe, and insular lobe. Additionally, we found that quantitative anisotropy of some subcortical structures such as the basal ganglia, cerebellum, and brain stem showed the highest correlations with motor dysfunction, whereas cortical structures such as the temporal lobe and occipital lobe showed the highest correlations with olfactory dysfunction in PD. Our study also showed evidence regarding potential neural compensation by revealing higher diffusion metric values in early-stage PD than in healthy controls. We anticipate that our results will improve our understanding of PD’s pathophysiology.

The role of the piriform cortex in temporal lobe epilepsy: A current literature review

Temporal lobe epilepsy is the most common form of focal epilepsy and can have various detrimental consequences within many neurologic domains. Recent evidence suggests that the piriform cortex may also be implicated in seizure physiology. The piriform cortex is a primary component of the olfactory network and is located at the junction of the frontal and temporal lobes, wrapping around the entorhinal sulcus. Similar to the hippocampus, it is a tri-layered allocortical structure, with connections to many adjacent regions including the orbitofrontal cortex, amygdala, peri- and entorhinal cortices, and insula. Both animal and human studies have implicated the piriform cortex as a critical node in the temporal lobe epilepsy network. It has additionally been shown that resection of greater than half of the piriform cortex may significantly increase the odds of achieving seizure freedom. Laser interstitial thermal therapy has also been shown to be an effective treatment strategy with recent evidence hinting that ablation of the piriform cortex may be important for seizure control as well. We propose that sampling piriform cortex in intracranial stereoelectroencephalography (sEEG) procedures with the use of a temporal pole or amygdalar electrode would be beneficial for further understanding the role of the piriform cortex in temporal lobe epilepsy.