Olfactory and trigeminal interaction of menthol and nicotine in humans

Post-traumatic olfactory dysfunction: a scoping review of assessment and rehabilitation approaches

Post-traumatic Olfactory Dysfunction (PTOD) consists of a complete or partial loss of olfactory function that may occur after a traumatic brain injury (TBI). PTOD may be linked to some neuropsychiatric features, such as social, cognitive and executive dysfunction, as well as behavioral symptoms, especially when TBI involves the orbito-frontal cortex. The diagnosis of PTOD is based on medical history and clinical data and it is supported by psychometric tests (i.e., subjective tools) as well as electrophysiological and neuroimaging measures (i.e., objective methods). The assessment methods allow monitoring the changes in olfactory function over time and help to establish the right therapeutic and rehabilitative approach. In this context, the use of the olfactory training (OT), which is a non-pharmacological and non-invasive treatment option, could promote olfactory function through top-down (central) and bottom-up (peripheral) processes. To better manage patients with TBI, PTOD should be detected early and properly treated using the various therapeutic rehabilitative possibilities, both conventional and advanced, also taking into consideration the emerging neuromodulation approach.

Impact of trigeminal and/or olfactory nerve stimulation on measures of inspiratory neural drive: Implications for breathlessness

The perception of breathlessness is mechanistically linked to the awareness of increased inspiratory neural drive (IND). Stimulation of upper airway cold receptors on the trigeminal nerve (TGN) with TGN agonists such as menthol or cool air to the face/nose has been hypothesized to reduce breathlessness by decreasing IND. The aim of this systematic scoping review was to identify and summarize the results of studies in animals and humans reporting on the impact of TGN stimulation or blockade on measures of IND. Thirty-one studies were identified, including 19 in laboratory animals and 12 in human participants. Studies in laboratory animals consistently reported that as TGN activity increased, measures of IND decreased (e.g., phrenic nerve activity). In humans, stimulation of the TGN with a stream of cool air to the face/nose decreased the sensitivity of the ventilatory chemoreflex response to hypercapnia. Otherwise, TGN stimulation with menthol or cool air to the face/note had no effect on measures of IND in humans. This review provides new insight into a potential neural mechanism of breathlessness relief with selected TGN agonists.

Trigeminal Stimulus Menthol Masks Bitter Off-Flavor of Artificial Sweetener Acesulfame-K

Consumer health concerns and regulatory policies lead to a growing demand for sugar-sweetened beverage alternatives. A reduced energy content can be achieved using artificial sweeteners, which often also convey a metallic or bitter off-flavor. Therefore, the alteration of sweetness perception and masking of potential off-flavors are paramount for improving sweet beverages. Trigeminal stimuli, such as capsaicin (spicy) or menthol (cooling), have been used to influence taste perception in food items, although their use in beverages has not yet been systematically investigated. Here, the influence of menthol on sweetness perception in an aqueous solution is examined both on the sensory and psychophysiological level. The addition of menthol had no sensory effect on sweetness perception; however, psychophysiological measurements suggest a boost in the physiological response to cold perception through the addition of sugar. Moreover, menthol addition shifted the recognition threshold of unpleasant bitterness of the sweetener acesulfame-K from 21.35 to 36.93 mg/L, masking the off-flavor. These findings illuminate the complexity of trigeminal perception influences on taste. Further investigation of these effects can render trigeminal stimuli an effective tool to enhance beverage aroma and flavor.

Chemosensory Contributions of E-Cigarette Additives on Nicotine Use

While rates of smoking combustible cigarettes in the United States have trended down in recent years, use of electronic cigarettes (e-cigarettes) has dramatically increased, especially among adolescents. The vast majority of e-cigarette users consume "flavored" products that contain a variety of chemosensory-rich additives, and recent literature suggests that these additives have led to the current "teen vaping epidemic." This review, covering research from both human and rodent models, provides a comprehensive overview of the sensory implications of e-cigarette additives and what is currently known about their impact on nicotine use. In doing so, we specifically address the oronasal sensory contributions of e-cigarette additives. Finally, we summarize the existing gaps in the field and highlight future directions needed to better understand the powerful influence of these additives on nicotine use.

The Role of TRP Channels in Nicotinic Provoked Pain and Irritation from the Oral Cavity and Throat: Translating Animal Data to Humans

Tobacco smoking-related diseases are estimated to kill more than 8 million people/year and most smokers are willing to stop smoking. The pharmacological approach to aid smoking cessation comprises nicotine replacement therapy (NRT) and inhibitors of the nicotinic acetylcholine receptor, which is activated by nicotine. Common side effects of oral NRT products include hiccoughs, gastrointestinal disturbances and, most notably, irritation, burning and pain in the mouth and throat, which are the most common reasons for premature discontinuation of NRT and termination of cessation efforts. Attempts to reduce the unwanted sensory side effects are warranted, and research discovering the most optimal masking procedures is urgently needed. This requires a firm mechanistic understanding of the neurobiology behind the activation of sensory nerves and their receptors by nicotine. The sensory nerves in the oral cavity and throat express the so-called transient receptor potential (TRP) channels, which are responsible for mediating the nicotine-evoked irritation, burning and pain sensations. Targeting the TRP channels is one way to modulate the unwanted sensory side effects. A variety of natural (Generally Recognized As Safe [GRAS]) compounds interact with the TRP channels, thus making them interesting candidates as safe additives to oral NRT products. The present narrative review will discuss (1) current evidence on how nicotine contributes to irritation, burning and pain in the oral cavity and throat, and (2) options to modulate these unwanted side-effects with the purpose of increasing adherence to NRT. Nicotine provokes irritation, burning and pain in the oral cavity and throat. Managing these side effects will ensure better compliance to oral NRT products and hence increase the success of smoking cessation. A specific class of sensory receptors (TRP channels) are involved in mediating nicotine's sensory side effects, making them to potential treatment targets. Many natural (Generally Recognized As Safe [GRAS]) compounds are potentially beneficial modulators of TRP channels.

Measurement of the Free-Base Nicotine Fraction (αfb) in Electronic Cigarette Liquids by Headspace Solid-Phase Microextraction

A method for determining the fraction of free-base nicotine (α_fb) in electronic cigarette liquids ("e-liquids") based on headspace solid-phase microextraction (h-SPME) is described. The free-base concentration c_e,fb = α_fbc_e,T, where c_e,T is the total (free-base + protonated) nicotine in the liquid. For gas/liquid equilibrium of the volatile free-base form, the headspace nicotine concentration is proportional to c_e,fb and thus also to α_fb. Headspace nicotine is proportionally absorbed with an SPME fiber. The fiber is thermally desorbed in the heated inlet of a gas chromatograph coupled to a mass spectrometer: the desorbed nicotine is measured by gas chromatography-mass spectrometry. For a second h-SPME measurement, an adequate base is added to the sample vial to convert essentially all protonated nicotine to the free-base form (α_fb → 1.0). The ratio of the first h-SPME measurement to the second h-SPME measurement gives α_fb in the initial sample. Using gaseous ammonia as the added base, the method was (1) verified using lab-prepared e-liquid solutions with known α_fb values and (2) used to determine the α_fb values for 18 commercial e-liquids. The measured α_fb values ranged from 0.0 to 1.0. Increasing measurement error with decreasing α_fb caused modestly lower method precision at small α_fb. Adding a liquid organic base may be more convenient than adding gaseous ammonia: one of the samples was examined using triethylamine as the added base; the measurements agreed well (with ammonia, 0.27 ± 0.01; with triethylamine, 0.26 ± 0.04). Other workers have proposed examining the nicotine protonation state in e-liquids using three steps: (1) 1:10 dilution with CO₂-free water; (2) measurement of pH; and (3) calculation of the resulting values for α_fb,w,1:10, the free-base fraction in the diluted mostly aqueous phase. As expected and verified here, because of the generally greater abilities of organic acids to protonate nicotine in water versus in an e-liquid phase, α_fb,w,1:10 values can be significantly less than actual e-liquid α_fb values when α_fb is not close to either 0 or 1.

Pain, Smell, and Taste in Adults: A Narrative Review of Multisensory Perception and Interaction

Every day our sensory systems perceive and integrate a variety of stimuli containing information vital for our survival. Pain acts as a protective warning system, eliciting a response to remove harmful stimuli; it may also be a symptom of an illness or present as a disease itself. There is a growing need for additional pain-relieving therapies involving the multisensory integration of smell and taste in pain modulation, an approach that may provide new strategies for the treatment and management of pain. While pain, smell, and taste share common features and are strongly linked to emotion and cognition, their interaction has been poorly explored. In this review, we provide an overview of the literature on pain modulation by olfactory and gustatory substances. It includes adult human studies investigating measures of pain threshold, tolerance, intensity, and/or unpleasantness. Due to the limited number of studies currently available, we have structured this review as a narrative in which we comment on experimentally induced and clinical pain separately on pain–smell and pain–taste interaction. Inconsistent study findings notwithstanding, pain, smell, and taste seem to interact at both the behavioral and the neural levels. Pain intensity and unpleasantness seem to be affected more by olfactory substances, whereas pain threshold and tolerance are influenced by gustatory substances. Few pilot studies to date have investigated these effects in clinical populations. While the current results are promising for the future, more evidence is needed to elucidate the link between the chemical senses and pain. Doing so has the potential to improve and develop novel options for pain treatment.

Subjective and Objective Assessments of Post-traumatic Olfactory Dysfunction

Introduction: Traumatic brain injuries are the most common cause of olfactory dysfunction. Deficits in olfaction may be conductive or neurosensory in nature, with varying degrees of impairment resulting in a diminished quality of life and an increased risk for personal injury among patients. The aim of this research is to evaluate the results of the subjective and objective quantitative examinations of olfactory function in a group of patients with post-traumatic anosmia in order to predict its value in identifying olfactory deficits in clinical practice.

Materials and Methods: The present study included 38 patients who reported anosmia or hyposmia caused by a traumatic head injury, and a group of 31 age- and sex-matched controls without olfactory dysfunction or prior history of head injury. The comparison of odor perception and identification of two oils (mint and anise) was assessed with the use of blast olfactometry with cortical olfactory event-related potentials.

Results: Subjective olfactory tests revealed anosmia or hyposmia in 94% of patients with head injury-related olfactory dysfunction. Objective tests revealed olfactory event-related potentials from cranial nerve I produced by the stimulation with both mint and anise in 20 patients (52.6%). Olfactory event-related potentials from cranial nerve V produced by the stimulation with mint were registered in 26 patients (68.4%). The lack of any responses, from both cranial nerve I and V, was found in 12 patients (32% of cases).

Conclusions: Findings from our study indicate the application of both subjective and objective examinations in the evaluation of patients with olfactory impairment. In the diagnosis of post-traumatic anosmia or hyposmia, objective examinations are particularly useful when the patients' level of cognition may be impaired or when subjects may be exaggerating their olfactory defects for a secondary gain. The diagnosis of damage to the olfactory system, specifically in the receptive part of the olfactory pathway, can be established in patients who showed reduced amplitudes or absent cortical responses in addition to absent odor identification and perception threshold in the subjective examination.

Menthol additives to tobacco products. Reasons for withdrawing mentholated cigarettes in European Union on 20th may 2020 according to tobacco products directive (2014/40/EU)

Menthol, which is a natural cyclic monoterpene alcohol with a minty smell, is one of the main constituents of essential oils that naturally occur in some aromatic plants, such as Mentha × piperita L. This natural compound shows many biological properties, such as anesthetic, analgesic, antibacterial and antifungal, immunomodulating, and skin penetration-enhancing. It is added to a variety of goods, such as food, oral-care products, OTC products, cosmetics, and tobacco products. Menthol is not just a simple flavoring agent, especially when it comes to tobacco products. Its ability to 'mask' the negative effects of nicotine and its additional positive sensory effects makes it the most common additive in such products. For the customers, mentholated tobacco products may be mistakenly perceived as less harmful for health, which may increase their consumption. However, as the evidence shows, menthol cigarettes are no safer than conventional cigarettes and may lead to more frequent disease exacerbation during prolonged exposure to smoke from such products. In addition, because of its complex interactions with nicotine, menthol may affect smoking behavior and may increase addiction to nicotine. For those reasons, the European Union banned flavored cigarettes (whose sale size reached more than 3% of the total tobacco product market) by implementing the Tobacco Products Directive (2014/40/EU) on 20th May 2020. While the menthol ban was based on health concerns, the ultimate effect on consumers, regarding potential quitting, is yet to be determined.

Why We Must Continue to Investigate Menthol's Role in the African American Smoking Paradox

BACKGROUND

The disproportionate burden of tobacco use among African Americans is largely unexplained. The unexplained disparities, referred to as the African American smoking paradox, includes several phenomena. Despite their social disadvantage, African American youth have lower smoking prevalence rates, initiate smoking at older ages, and during adulthood, smoking rates are comparable to whites. Smoking frequency and intensity among African American youth and adults are lower compared to whites and American Indian and Alaska Natives, but tobacco-caused morbidity and mortality rates are disproportionately higher. Disease prediction models have not explained disease causal pathways in African Americans. It has been hypothesized that menthol cigarette smoking, which is disproportionately high among African Americans, may help to explain several components of the African American smoking paradox.

PURPOSE

This article provides an overview of the potential role that menthol plays in the African American smoking paradox. We also discuss the research needed to better understand this unresolved puzzle.

METHODS

We examined prior synthesis reports and reviewed the literature in PubMed on the menthol compound and menthol cigarette smoking in African Americans.

RESULTS

The pharmacological and physiological effects of menthol and their interaction with biological and genetic factors may indirectly contribute to the disproportionate burden of cigarette use and diseases among African Americans.

CONCLUSIONS

Future studies that examine taste sensitivity, the menthol compound, and their effects on smoking and chronic disease would provide valuable information on how to reduce the tobacco burden among African Americans.

IMPLICATIONS

Our study highlights four counterintuitive observations related to the smoking risk profiles and chronic disease outcomes among African Americans. The extant literature provides strong evidence of their existence and shows that long-standing paradoxes have been largely unaffected by changes in the social environment. African Americans smoke menthols disproportionately, and menthol's role in the African American smoking paradox has not been thoroughly explored. We propose discrete hypotheses that will help to explain the phenomena and encourage researchers to empirically test menthol's role in smoking initiation, transitions to regular smoking and chronic disease outcomes in African Americans.

The distinctive significance of analgesic drugs and olfactory stimulants on learned pain in mice

Chronic pain is often intractable to analgesics, and in animals it involves a conditioned nociceptive response (CR) - learned pain. The neural pathways of nociception and olfactory function in the brain overlap. The influence of olfactory stimuli on acute pain has been studied in some depth in animal and human models, but the influence of olfactory stimuli on learned pain has not been understood. We examined the effects of analgesic drugs and olfactory stimulants (preferred or repellent odor) on acute pain, the unconditioned nociceptive response (UCR) and the CR in mice. The CR was provoked by repeated injection of formalin into the hind-paw in animals in the same context, which elicited the typical pain behaviors of paw licking (including biting). The analgesic drugs acetaminophen, fentanyl, gabapentin and fluvoxamine diminished the UCR but did not affect the CR. In contrast, the preferred odor reduced both the UCR and the CR. Our findings suggest that, like chronic pain, the CR is resistant to analgesic drugs and that preferred odor suppress the neural pathways that mediate the CR of pain perception.