Role of neuronal nicotinic-acetylcholine receptors in the activation of neurons in trigeminal subnucleus caudalis by nicotine delivered to the oral mucosa

Migraine signaling pathways: amino acid metabolites that regulate migraine and predispose migraineurs to headache

Migraine is a common, debilitating disorder for which attacks typically result in a throbbing, pulsating headache. Although much is known about migraine, its complexity renders understanding the complete etiology currently out of reach. However, two important facts are clear, the brain and the metabolism of the migraineur differ from that of the non-migraineur. This review centers on the altered amino acid metabolism in migraineurs and how it helps define the pathology of migraine.

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.

Sensory Effects of Nicotine and Tobacco

INTRODUCTION

Ingestion of nicotine by smoking, vaping, or other means elicits various effects including reward, antinociception, and aversion due to irritation, bitter taste, and unpleasant side effects such as nausea and dizziness.

AIMS AND METHODS

Here we review the sensory effects of nicotine and the underlying neurobiological processes.

RESULTS AND CONCLUSIONS

Nicotine elicits oral irritation and pain via the activation of neuronal nicotinic acetylcholine receptors (nAChRs) expressed by trigeminal nociceptors. These nociceptors excite neurons in the trigeminal subnucleus caudalis (Vc) and other brainstem regions in a manner that is significantly reduced by the nAChR antagonist mecamylamine. Vc neurons are excited by lingual application of nicotine and exhibit a progressive decline in firing to subsequent applications, consistent with desensitization of peripheral sensory neurons and progressively declining ratings of oral irritation in human psychophysical experiments. Nicotine also elicits a nAChR-mediated bitter taste via excitation of gustatory afferents. Nicotine solutions are avoided even when sweeteners are added. Studies employing oral self-administration have yielded mixed results: Some studies show avoidance of nicotine while others report increased nicotine intake over time, particularly in adolescents and females. Nicotine is consistently reported to increase human pain threshold and tolerance levels. In animal studies, nicotine is antinociceptive when delivered by inhalation of tobacco smoke or systemic infusion, intrathecally, and by intracranial microinjection in the pedunculopontine tegmentum, ventrolateral periaqueductal gray, and rostral ventromedial medulla. The antinociception is thought to be mediated by descending inhibition of spinal nociceptive transmission. Menthol cross-desensitizes nicotine-evoked oral irritation, reducing harshness that may account for its popularity as a flavor additive to tobacco products.

IMPLICATIONS

Nicotine activates brain systems underlying reward and antinociception, but at the same time elicits aversive sensory effects including oral irritation and pain, bitter taste, and other unpleasant side effects mediated largely by nicotinic acetylcholine receptors (nAChRs). This review discusses the competing aversive and antinociceptive effects of nicotine and exposure to tobacco smoke, and the underlying neurobiology. An improved understanding of the interacting effects of nicotine will hopefully inform novel approaches to mitigate nicotine and tobacco use.

Animal Models to Investigate the Impact of Flavors on Nicotine Addiction and Dependence

BACKGROUND

Tobacco use in humans is a long-standing public health concern. Flavors are common additives in tobacco and alternative tobacco products, added to mask nicotine's harsh orosensory effects and increase the appeal of these products. Animal models are integral for investigating nicotine use and addiction and are helpful for understanding the effects of flavor additives on the use of nicotine delivery products.

OBJECTIVE

This review focuses on preclinical models to evaluate the contribution of flavor additives to nicotine addiction.

MATERIALS AND METHODS

An electronic literature search was conducted by authors up to May 2022. Original articles were selected.

RESULTS

The behavioral models of rodents described here capture multiple dimensions of human flavored nicotine use behaviors, including advantages and disadvantages.

CONCLUSION

The consensus of the literature search was that human research on nicotine use behavior has not caught up with fast-changing product innovations, marketing practices, and federal regulations. Animal models are therefore needed to investigate mechanisms underlying nicotine use and addiction. This review provides a comprehensive overvie.

Menthol decreases oral nicotine aversion in C57BL/6 mice through a TRPM8-dependent mechanism

BACKGROUND

Nicotine is a major oral irritant in smokeless tobacco products and has an aversive taste. Mentholated smokeless tobacco products are highly popular, suggesting that menthol increases their palatability and may facilitate initiation of product use. While menthol is known to reduce respiratory irritation by tobacco smoke irritants, it is not known whether this activity extends to oral nicotine and its aversive effects.

STUDY DESIGN

The two-bottle choice drinking assay was used to characterise aversion and preference in C57BL/6 mice to a range of menthol concentrations (10-200 µg/mL). Then, effects of menthol on oral nicotine aversion were determined. Responses were compared with those in mice deficient in the cold/menthol receptor, TRPM8, expressed in trigeminal sensory neurons innervating the oral cavity.

RESULTS

Mice showed aversion to menthol concentrations of 100 µg/mL and above. When presented with a highly aversive concentration of nicotine (200 µg/mL), mice preferred solutions with 50 or 100 µg/mL menthol added over nicotine alone. In contrast to wild-type mice, Trpm8-/- showed a strong aversion to mentholated (100 µg/mL) nicotine (200 µg/mL) and preferred nicotine alone. Trpm8-/- mice show aversion to lower concentrations of menthol than wild-type mice.

CONCLUSIONS

Oral menthol can reduce the aversive effects of oral nicotine and, at higher concentrations, acts as an irritant by itself. Menthol's effects in relation to nicotine require TRPM8, the cool temperature sensing ion channel that activates analgesic and counterirritant mechanisms. These mechanisms may underlie preference for menthol-containing smokeless tobacco products and may facilitate initiation of product use.

Effects of nicotine on spinal cord injury pain: a randomized, double-blind, placebo controlled crossover trial

BACKGROUND

One factor affecting spinal cord injury (SCI)-related pain may be nicotine. Case reports have described a worsening of neuropathic pain from smoking and relief from abstinence. Neurobiological correlates also implicate the potential effect of nicotine on SCI-related pain.

METHOD

The current study employed a randomized, placebo-controlled crossover design to examine the effect of nicotine exposure on subtypes of SCI-related pain among smokers and nonsmokers.

RESULTS

Whereas nonsmokers with SCI showed a reduction in mixed forms of pain following nicotine exposure, smokers with SCI showed a converse increase in pain with regard to both mixed and neuropathic forms of pain. The exacerbation of pain in chronic nicotine or tobacco users may not only elucidate possible pain mechanisms but may also be of use in smoking cessation counseling among those with SCI.

Impaired somatosensation in tongue mucosa of smokers

Smoking has been indicated as a risk factor for oral diseases and can lead to altered sense of taste. So far, the effects of sensory changes on the tongue are not investigated. In this study, quantitative sensory testing was used to evaluate somatosensory function in the lingual region. Eighty healthy volunteers were investigated (20 smokers, 20 non-smokers). Subjects were bilaterally tested in innervation areas of lingual nerves. Thresholds of cold and warm detection, cold and heat pain, and mechanical detection were determined. As control for systemic, extraoral effects of smoking, tests were additionally performed in 40 volunteers (20 smokers, 20 non-smokers) on the skin of the chin innervated by the mental branch of the trigeminal nerve. Cold (p < 0.001), warm detection thresholds (p < 0.001), and thermal sensory limen (p < 0.001) showed higher sensitivity in non-smokers as compared to smokers. Heat pain and mechanical detection, as well as all tests in the skin of the chin, showed no significant differences. The impaired temperature perception in smokers indicates a reduction of somatosensory functions in the tongue, possibly caused by nerve degeneration associated with smoking. Possible systemic effects of smoking do not seem to affect extraoral trigeminal branches.

Rat brain CYP2B induction by nicotine is persistent and does not involve nicotinic acetylcholine receptors

CYP2B is a drug-metabolizing enzyme expressed in the liver and brain that metabolizes a variety of centrally acting drugs (e.g. propofol, bupropion and nicotine), endogenous neurochemicals (e.g. serotonin and testosterone) and toxins (e.g. chlorpyrifos). Human CYP2B6 is found at higher levels in the brains of smokers, and 7-day nicotine treatment induces rat brain CYP2B while not altering hepatic CYP2B. We characterized the time course of rat brain CYP2B induction by nicotine and determined if nicotinic acetylcholine receptors (nAChRs) mediated this induction. Rats were treated once daily with 1mg/kg nicotine base or saline s.c. for 1 or 7days and sacrificed from 30minutes to 7days after the last injection. One-day nicotine treatment did not induce brain CYP2B, whereas 7-day nicotine treatment significantly increased CYP2B expression for up to 24hours in the frontal cortex and brainstem; these levels returned to baseline by 7days post-treatment. CYP2B activity was also higher at 24hours in these regions. No change was seen in the cerebellar CYP2B levels or in vivo activity following nicotine treatment. Time of day of treatment and sacrifice altered the magnitude of brain CYP2B induction while chlorisondamine, an irreversible nAChR blocker, pre-treatment did not block CYP2B induction. Seven-day nicotine treatment resulted in an induction of rat brain CYP2B protein and in vivo activity for up to 24hours, which would suggest greater local drug metabolism by brain CYP2B. Humans or animals exposed to nicotine may have altered therapeutic drug response, brain levels of neurotransmitters and/or neurotoxicity.

The cholinomimetic agent carbachol induces headache in healthy subjects

The parasympathetic nervous system is likely to be involved in migraine pathogenesis. We hypothesized that the cholinomimetic agonist carbachol would induce headache and vasodilation of cephalic and radial arteries. Carbachol (3 microg/kg) or placebo was randomly infused into 12 healthy subjects in a double-blind crossover study. Headache was scored on a verbal rating scale from 0-10. Velocity in the middle cerebral artery (V(MCA)) and diameter of the superficial temporal artery (STA) and radial artery (RA) were recorded. Nine participants developed headache after carbachol compared with three after placebo. The area under the curve for headache was increased after carbachol compared with placebo both during infusion (0-30 min) (P = 0.042) and in the postinfusion period (30-90 min) (P = 0.027). Carbachol infusion caused a drop in V(MCA) (P = 0.003) and an increase in STA diameter (P = 0.006), but no increase in the RA diameter (P = 0.200). In conclusion, the study demonstrated that carbachol caused headache and dilation of cephalic arteries in healthy subjects.

Time course of self-desensitization of oral irritation by nicotine and capsaicin

Nicotine contacting mucous membranes elicits irritation that decreases with repeated exposures (self-desensitization). We investigated the time course of nicotine self-desensitization and compared it with that of capsaicin. Nicotine (300 mM, 10 microl) was applied to one-half of the dorsal tongue and vehicle to the other. Following a rest period ranging from 0.5 to 48 h, nicotine (5 microl) was reapplied to each side of the tongue and subjects indicated on which side they experienced stronger irritation and separately rated the intensity of the sensation on each side. After intervals of 0.5, 1, and 24 h, a significant majority of subjects chose the vehicle-treated side as having stronger irritation and assigned significantly higher intensity ratings to that side, indicating self-desensitization. The effect was not present after 48 h. By comparison, 10 parts per million (ppm) (33 microM) capsaicin induced significant self-desensitization at 1 but not 24 h, whereas a higher concentration of capsaicin (100 ppm, 330 microM) induced significant self-desensitization at intervals of 1, 24, and 48 h. These results indicate that initial exposure to nicotine or capsaicin can markedly attenuate irritant sensations elicited by subsequent exposure to these irritants hours to days later.

Guidelines on nicotine dose selection for in vivo research

RATIONALE

This review provides insight for the judicious selection of nicotine dose ranges and routes of administration for in vivo studies. The literature is replete with reports in which a dosaging regimen chosen for a specific nicotine-mediated response was suboptimal for the species used. In many cases, such discrepancies could be attributed to the complex variables comprising species-specific in vivo responses to acute or chronic nicotine exposure.

OBJECTIVES

This review capitalizes on the authors' collective decades of in vivo nicotine experimentation to clarify the issues and to identify the variables to be considered in choosing a dosaging regimen. Nicotine dose ranges tolerated by humans and their animal models provide guidelines for experiments intended to extrapolate to human tobacco exposure through cigarette smoking or nicotine replacement therapies. Just as important are the nicotine dosaging regimens used to provide a mechanistic framework for acquisition of drug-taking behavior, dependence, tolerance, or withdrawal in animal models.

RESULTS

Seven species are addressed: humans, nonhuman primates, rats, mice, Drosophila, Caenorhabditis elegans, and zebrafish. After an overview on nicotine metabolism, each section focuses on an individual species, addressing issues related to genetic background, age, acute vs chronic exposure, route of administration, and behavioral responses.

CONCLUSIONS

The selected examples of successful dosaging ranges are provided, while emphasizing the necessity of empirically determined dose-response relationships based on the precise parameters and conditions inherent to a specific hypothesis. This review provides a new, experimentally based compilation of species-specific dose selection for studies on the in vivo effects of nicotine.

Nicotine suppression of gustatory responses of neurons in the nucleus of the solitary tract

This study investigated effects of nicotine applied to the tongue surface on responses of gustatory neurons in the nucleus of the solitary tract (NTS) in rats. In pentobarbital-anesthetized rats, single-unit recordings were made from NTS units responsive to one or more tastants (sucrose, NaCl, citric acid, monosodium glutamate, quinine). Application of nicotine (0.87, 8.7, or 600 mM) excited gustatory NTS units and significantly attenuated NTS unit responses to their preferred tastant in a dose-dependent manner. The depressant effect of nicotine was equivalent regardless of which tastant best excited the NTS unit. Nicotinic excitation of NTS units and depression of their tastant-evoked responses were both significantly attenuated by the nicotinic antagonist mecamylamine, which itself did not excite NTS units. In rats with bilateral trigeminal ganglionectomy, nicotine still excited nearly all NTS units but no longer depressed tastant-evoked responses. Nicotine did not elicit plasma extravasation when applied to the tongue. The results indicate that nicotine directly excites NTS units by gustatory nerves and inhibits their tastant-evoked responses by a nicotinic acetylcholine receptor-mediated excitation of trigeminal afferents that inhibit NTS units centrally.

Mediation of the Effect of Nicotine on Kir6.1 Channels by Superoxide Anion Production

KATP channels are a complex of regulatory sulfonylurea receptor subunits and the pore-forming inward rectifiers such as Kir6.1. Using the whole-cell patch-clamp technique, we investigated the interaction of nicotine with the Kir6.1 subunit as well as the underlying mechanism. Stable expression of Kir6.1 in HEK-293 cells yielded a detectable inward rectifier KATP current. This inward current was significantly inhibited by PNU-37883A and by a specific anti-Kir6.1 antibody. Nicotine at 30 and 100 microM increased Kir6.1 currents by 42 +/- 11.8% and 26.2 +/- 14.6%, respectively (n = 4-6, P < 0.05). In contrast, nicotine at 1-3 mM inhibited Kir6.1 currents (P < 0.05). Nicotine at 100 microM increased the production of superoxide anion (O2) by 20.3 +/- 5.7%, whereas at 1 mM it significantly decreased the production of O2 by 37.7 +/- 4.3%. Coapplication of hypoxanthine (HX) and xanthine oxidase (XO) to the transfected HEK-293 cells resulted in a significant and reproducible increase in Kir6.1 currents (P < 0.05). The stimulatory effect of HX/XO on Kir6.1 current was abolished by tempol, a scavenger of O2. Tempol also abolished the stimulatory effect of 30 muM nicotine on Kir6.1 currents. In conclusion, nicotine stimulates Kir6.1 channel at least in part through the production of O2.

Modulation of nicotine but not ethanol preference by the mouse Chrna4 A529T polymorphism

Available evidence indicates that common genes influence alcohol and tobacco abuse in humans. The studies reported here used mouse models to evaluate the hypothesis that genetically determined variability in the alpha4beta2* nicotinic receptor modulates genetically determined variability in the intake of both nicotine and alcohol. Data obtained with inbred mouse strains suggested an association between a polymorphism in the mouse alpha4 nAChR subunit gene, Chrna4, and variability in nicotine and ethanol preference. These associations were assessed in F2 animals derived by crossing C57BL/6-super(beta2-/-) mice and A/J mice. The results obtained by the authors indicate that the polymorphism in Chrna4 plays an important role in modulating variability in oral nicotine intake but is linked to a gene that regulates alcohol intake.

Nicotinic AChR in subclassified capsaicin-sensitive and -insensitive nociceptors of the rat DRG

Nociceptive cells of the dorsal root ganglion (DRG) were subclassified, in vitro, according to patterns of voltage-activated currents. The distribution and form of nicotinic ACh receptors (nAChRs) were determined. nAChRs were present on both capsaicin-sensitive and -insensitive nociceptors but were not universally present in unmyelinated nociceptors. In contrast, all A delta nociceptors (types 4, 6, and 9) expressed slowly decaying nAChR. Three major forms of nicotinic currents were identified. Specific agonists and antagonists were used to demonstrate the presence of alpha7 in two classes of capsaicin-sensitive, unmyelinated nociceptors (types 2 and 8). In type 2 cells, alpha7-mediated currents were found in isolation. Whereas alpha7 was co-expressed with other nAChR in type 8 cells. These were the only classes in which alpha7 was identified. Other nociceptive classes expressed slowly decaying currents with beta4 pharmacology. Based on concentration response curves formed by nicotinic agonists [ACh, nicotine, dimethyl phenyl piperazinium (DMPP), cytisine] evidence emerged of two distinct nAChR differentially expressed in type 4 (alpha3beta4) and types 5 and 8 (alpha3beta4 alpha5). Although identification could not be made with absolute certainty, patterns of potency (type 4: DMPP > cytisine > nicotine = ACh; type 5 and type 8: DMPP = cytisine > nicotine = ACh) and efficacy provided strong support for the presence of two distinct channels based on an alpha3beta4 platform. Studies conducted on one nonnociceptive class (type 3) failed to reveal any nAChR. After multiple injections of Di-I (1,1'-dilinoleyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) into the hairy skin of the hindlimb, we identified cell types 2, 4, 6, 8, and 9 as skin nociceptors that expressed nicotinic receptors. We conclude that at least three nicotinic AChR are diversely distributed into discrete subclasses of nociceptors that innervate hairy skin.

Mecamylamine reduces nicotine cross-desensitization of trigeminal caudalis neuronal responses to oral chemical irritation

We investigated the role of neuronal nicotinic acetylcholine receptors (nAChRs) in nicotine cross-desensitization of chemonociceptive responses of trigeminal subnucleus caudalis (Vc) neurons in rats. Vc responses to lingually applied pentanoic acid were significantly reduced following nicotine, and this was prevented when the nAChR antagonist mecamylamine was applied before or after nicotine. A peripheral site of nicotine cross-desensitization is suggested via a nAChR-mediated reduction in acidic excitation of lingual nociceptors that project to Vc.

Activation of neurons in trigeminal caudalis by noxious oral acidic or salt stimuli is not reduced by amiloride

We investigated the possible role of amiloride-sensitive ion channels of the ENaC/DEGenerin superfamily in the activation of trigeminal nociceptive neurons elicited by noxious chemical stimulation of the oral mucosa using two methodologies, single-unit recording and c-fos immunohistochemistry. In pentobarbital-anesthetized rats, single-unit recordings were made from neurons in superficial laminae of dorsomedial trigeminal subnucleus caudalis (Vc) that responded to noxious thermal and chemical stimuli applied to the dorsal tongue. Successive application of each of three chemicals (250 mM pentanoic acid, n=6 units; 250 mM citric acid, n=8; 5 M NaCl, n=6) evoked responses that were not affected following topical application of amiloride (1 mM). In separate experiments, pentobarbital-anesthetized rats received one of the following stimuli delivered to the dorsal tongue: 250 mM pentanoic acid (n=6); 1 mM amiloride followed by 250 mM pentanoic (N=6); 5 M NaCl (n=5); or 1 mM amiloride followed by 5 M NaCl (n=5). Two hours later they were perfused with 4% paraformaldehyde and the brain stems processed for c-fos immunoreactivity. Both pentanoic acid and 5 M NaCl evoked similar numbers and patterns of fos-like immunoreactivity (FLI) in dorsomedial Vc and other brain stem regions, with no significant difference in counts of FLI in animals pretreated with amiloride. These results suggest that amiloride-sensitive Na(+) channels are not essential in mediating the activation of intraoral trigeminal nociceptors.

Sensitization, desensitization and stimulus-induced recovery of trigeminal neuronal responses to oral capsaicin and nicotine

Repeated application of capsaicin at a 1-min interstimulus interval (ISI) to the tongue induces a progressively increasing irritant sensation (sensitization), followed after a rest period by reduced sensitivity to further capsaicin (desensitization). Sequential reapplication of capsaicin induces irritation that eventually increases to initial levels: stimulus-induced recovery (SIR). In contrast, repeated application of nicotine elicits a declining irritant sensation across trials. To investigate possible neural correlates of these phenomena, we recorded from single units in superficial laminae of the dorsomedial trigeminal subnucleus caudalis (Vc) that responded to noxious thermal (54 degrees C) and chemical (1 M pentanoic acid) stimulation of the tongue of anesthetized rats. We then recorded responses to either capsaicin (330 microM) or nicotine (0.6 M), delivered either once, repeatedly at 1-min ISI, or continually by constant flow. After the initial capsaicin application and a rest period, the capsaicin was reapplied in the identical manner to test for SIR. The mean response of 14 Vc units to sequential application of pentanoic acid did not vary significantly across trials, indicating lack of tachyphylaxis or sensitization. The averaged response of 11 Vc units to repeated capsaicin increased significantly across the first eight trials and then plateaued. Following the rest period, spontaneous firing had returned to the precapsaicin level. With capsaicin reapplication, the averaged response increased again after a significant delay (due to desensitization), but did not reattain the peak firing rate achieved in the initial series (partial SIR). Constant-flow application of capsaicin induced an identical sensitization followed by nearly complete SIR. A single application of capsaicin induced a significant rise in firing in eight other units, but the rate of rise and maximal firing rate were both much lower compared with repetitive or constant-flow capsaicin. When capsaicin was reapplied once after the rest period, there was no change in firing rate indicating absence of SIR. These results indicate that maintenance of the capsaicin concentration induces a progressive increase in neuronal response that parallels sensitization. With recurrent capsaicin application, desensitization can be overcome to result in a delayed recovery of Vc responses similar to SIR. In contrast, the averaged response of 17 Vc units to repeated or constant-flow application of nicotine increased only over the first 3 min, and then decreased to spontaneous levels even as nicotine was still being applied. These results are consistent with the decrease in the perceived irritation elicited by sequential application of nicotine in humans.