From receptors to the brain: Psychophysical clues to taste physiology

[Mechanisms and Management of COVID-19-Associated Taste Disorders]

The taste buds in the human tongue contain specialized cells that generate taste signals when they are stimulated. These signals are then transmitted to the central nervous system, allowing the human body to distinguish nutritious substances from toxic or harmful ones. This process is critical to the survival of humans and other mammals. A number of studies have shown that dysgeusia, or taste disorder, is a common complication of coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, which can severely affect patients' nutritional intake and quality of life. Based on the physiological process of taste perception, the direct causes of dysgeusia include dysfunction of taste receptors and damage to the taste nervous system, while indirect causes include genetic factors, aging-related changes, bacterial and viral infections, and cancer treatments such as radiotherapy and chemotherapy. The pathogenic factors of dysgeusia are complicated, further research is needed to fully understand the underlying mechanisms, and some of the reported findings and conclusions still need further validation. All these form a great challenge for clinical diagnosis of the cause and targeted treatment of dysgeusia. Herein, we reviewed published research on the physiological process of taste perception, the potential mechanisms of taste disorders related to SARS-CoV-2 infection, and strategies for prevention and treatment, providing theoretical support for establishing and improving the comprehensive management of COVID-19 complicated by taste disorders.

Activation of the Nucleus Taeniae of the Amygdala by Umami Taste in Domestic Chicks (Gallus gallus)

In chickens, the sense of taste plays an important role in detecting nutrients and choosing feed. The molecular mechanisms underlying the taste-sensing system of chickens are well studied, but the neural mechanisms underlying taste reactivity have received less attention. Here we report the short-term taste behaviour of chickens towards umami and bitter (quinine) taste solutions and the associated neural activity in the nucleus taeniae of the amygdala, nucleus accumbens and lateral septum. We found that chickens had more contact with and drank greater volumes of umami than bitter or a water control, and that chicks displayed increased head shaking in response to bitter compared to the other tastes. We found that there was a higher neural activity, measured as c-Fos activation, in response to umami taste in the right hemisphere of the nucleus taeniae of the amygdala. In the left hemisphere, there was a higher c-Fos activation of the nucleus taeniae of the amygdala in response to bitter than in the right hemisphere. Our findings provide clear evidence that chickens respond differently to umami and bitter tastes, that there is a lateralised response to tastes at the neural level, and reveals a new function of the avian nucleus taeniae of the amygdala as a region processing reward information.

Tasting temperature: neural and behavioral responses to thermal stimulation of oral mucosa

Temperature sensation contributes to human enjoyment of foods and beverages. The mouthfeel of warmed foods or drinking ice-cold water on a hot day are respectively pleasant and refreshing. Although historically under-studied for a role in food preference, new data have shed light on how oral temperature sensing and thermoreceptor mechanisms inside the mouth influence ingestive acceptance behaviors in rodent models used in flavor neurobiology. Moreover, recent functional data have uncovered a broad diversity of thermosensory neurons in primary afferents and brain pathways that signal oral temperature. This review will discuss some of the progress made in these areas. Ultimately, unraveling the biological basis of oral temperature sensing will be critical to reveal how thermosensory factors interact with other orosensory modalities to shape ingestive preferences. Elucidating oral thermal processing will also be key for establishing general principles of temperature coding by the nervous system.