Histaminergic and cholinergic neuron systems in the impairment of human thermoregulation during motion sickness

Pharmacological torpor prolongs rat survival in lethal normobaric hypoxia

Resistance to hypoxia is one of the most prominent features of natural hibernation and is expected to be present in the pharmacological torpor (PT) that simulates hibernation. We studied resistance to lethal hypoxia (3.5% oxygen content) in rats under PT. To initiate PT, we used the previously developed pharmacological composition (PC) which, after a single intravenous injection, can induce a daily decrease in Tb by 7 °C-8 °C at the environmental temperature of 22 °C-23 °C. Half-survival (median) time of rats in lethal hypoxia was found to increase from 5 ± 0.8 min in anesthetized control rats to 150 ± 12 min in rats injected with PC, which is a 30-fold increase. Behavioral tests after PT and hypoxia, including the traveling distance, the number of rearing and grooming episodes, revealed that animal responses are significantly restored within a week. It is assumed that the discovered unprecedented resistance of artificially torpid rats to lethal hypoxia may open up broad prospects for the therapeutic use of PT for preconditioning to various damaging factors, treatment of diseases, and extend the so-called "golden hour" for lifesaving interventions.

Thermoregulation and nausea

The major symptoms of motion sickness are well known and include facial pallor, nausea and vomiting, and sweating, but it is poorly recognized that they actually reflect severely perturbed thermoregulation. Thus, the purpose of this chapter is to present and discuss existing data related to this subject. While hypothermia during seasickness was first noted nearly 150 years ago, detailed studies of this phenomenon were conducted only during the last two decades. Our own research confirmed that motion sickness-induced hypothermia is quite broadly expressed phylogenetically as, besides humans, it could be provoked in several other animals (rats, musk shrews, and mice). Evidence from human and animal experiments indicates that the physiologic mechanisms responsible for the motion sickness-induced hypothermia include cutaneous vasodilation and sweating (leading to an increase of heat loss) and reduced thermogenesis. Together, these results suggest that motion sickness triggers a highly coordinated physiologic response aiming to reduce body temperature. The chapter is concluded by presenting hypotheses of how and why motion sickness evokes this hypothermic response.

Cybersickness provoked by head-mounted display affects cutaneous vascular tone, heart rate and reaction time

Evidence from studies of provocative motion indicates that motion sickness is tightly linked to the disturbances of thermoregulation. The major aim of the current study was to determine whether provocative visual stimuli (immersion into the virtual reality simulating rides on a rollercoaster) affect skin temperature that reflects thermoregulatory cutaneous responses, and to test whether such stimuli alter cognitive functions. In 26 healthy young volunteers wearing head-mounted display (Oculus Rift), simulated rides consistently provoked vection and nausea, with a significant difference between the two versions of simulation software (Parrot Coaster and Helix). Basal finger temperature had bimodal distribution, with low-temperature group (n=8) having values of 23-29 °C, and high-temperature group (n=18) having values of 32-36 °C. Effects of cybersickness on finger temperature depended on the basal level of this variable: in subjects from former group it raised by 3-4 °C, while in most subjects from the latter group it either did not change or transiently reduced by 1.5-2 °C. There was no correlation between the magnitude of changes in the finger temperature and nausea score at the end of simulated ride. Provocative visual stimulation caused prolongation of simple reaction time by 20-50 ms; this increase closely correlated with the subjective rating of nausea. Lastly, in subjects who experienced pronounced nausea, heart rate was elevated. We conclude that cybersickness is associated with changes in cutaneous thermoregulatory vascular tone; this further supports the idea of a tight link between motion sickness and thermoregulation. Cybersickness-induced prolongation of reaction time raises obvious concerns regarding the safety of this technology.

Mice with conditional deletion of Cx26 exhibit no vestibular phenotype despite secondary loss of Cx30 in the vestibular end organs

Connexins are components of gap junctions which facilitate transfer of small molecules between cells. One member of the connexin family, Connexin 26 (Cx26), is prevalent in gap junctions in sensory epithelia of the inner ear. Mutations of GJB2, the gene encoding Cx26, cause significant hearing loss in humans. The vestibular system, however, does not usually show significant functional deficits in humans with this mutation. Mouse models for loss of Cx26 function demonstrate hearing loss and cochlear pathology but the extent of vestibular dysfunction and organ pathology are less well characterized. To understand the vestibular effects of Cx26 mutations, we evaluated vestibular function and histology of the vestibular sensory epithelia in a conditional knockout (CKO) mouse with Cx26 loss of function. Transgenic C57BL/6 mice, in which cre-Sox10 drives excision of the Cx26 gene from non-sensory cells flanking the sensory epithelium of the inner ear (Gjb2-CKO), were compared to age-matched wild types. Animals were sacrificed at ages between 4 and 40 weeks and their cochlear and vestibular sensory organs harvested for histological examination. Cx26 immunoreactivity was prominent in the peripheral vestibular system and the cochlea of wild type mice, but absent in the Gjb2-CKO specimens. The hair cell population in the cochleae of the Gjb2-CKO mice was severely depleted but in the vestibular organs it was intact, despite absence of Cx26 expression. The vestibular organs appeared normal at the latest time point examined, 40 weeks. To determine whether compensation by another connexin explains survival of the normal vestibular sensory epithelium, we evaluated the presence of Cx30 in the Gjb2-CKO mouse. We found that Cx30 labeling was normal in the cochlea, but it was decreased or absent in the vestibular system. The vestibular phenotype of the mutants was not different from wild-types as determined by time on the rotarod, head stability tests and physiological responses to vestibular stimulation. Thus presence of Cx30 in the cochlea does not compensate for Cx26 loss, and the absence of both connexins from vestibular sensory epithelia is no more injurious than the absence of one of them. Further studies to uncover the physiological foundation for this difference between the cochlea and the vestibular organs may help in designing treatments for GJB2 mutations.

Motion sickness, nausea and thermoregulation: The "toxic" hypothesis

Principal symptoms of motion sickness in humans include facial pallor, nausea and vomiting, and sweating. It is less known that motion sickness also affects thermoregulation, and the purpose of this review is to present and discuss existing data related to this subject. Hypothermia during seasickness was firstly noted nearly 150 years ago, but detailed studies of this phenomenon were conducted only during the last 2 decades. Motion sickness-induced hypothermia is philogenetically quite broadly expressed as besides humans, it has been reported in rats, musk shrews and mice. Evidence from human and animal experiments indicates that the physiological mechanisms responsible for the motion sickness-induced hypothermia include cutaneous vasodilation and sweating (leading to an increase of heat loss) and reduced thermogenesis. Together, these results suggest that motion sickness triggers highly coordinated physiological response aiming to reduce body temperature. Finally, we describe potential adaptive role of this response, and describe the benefits of using it as an objective measure of motion sickness-induced nausea.

Effects of motion sickness on thermoregulatory responses in a thermoneutral air environment

Motion sickness (MS) has been identified as a non-thermal factor that can moderate autonomic thermoregulatory responses. It has been shown that MS exaggerates core cooling during immersion in cold (15 °C) and luke-warm (28 °C) water by attenuating cold-induced vasoconstriction. The aim of the present study was to investigate whether MS affects thermal balance in a thermoneutral air environment. Eleven subjects were exposed to rotation in two conditions, control (CN) and MS. In the CN condition subjects refrained from head movements, whereas in the MS condition they performed a sequence of maximal head movements (left, right, up, down) at 15-s intervals until they were very nauseous. Sweating rate, rectal temperature (T(re)), the difference in temperature between the right forearm and tip of the second finger (T(ff)) as an index of cutaneous vasomotor tone, perceived MS, thermal comfort and temperature perception were recorded before and during rotation, and during 90-min post-rotation. During the post-rotation period, T(re) dropped and sweating rate increased in the MS but not in the CN condition. The T(ff) response suggests that MS-induced peripheral vasodilatation which, together with the sweating resulted in increased heat loss. During rotation, subjects perceived temperature to be uncomfortably high, suggesting that MS may also affect thermoregulatory behaviour. It thus appears that also in a thermoneutral air environment MS may substantially affect thermal balance.

Scopolamine (hyoscine) for preventing and treating motion sickness

BACKGROUND

This is an update of a Cochrane Review first published in The Cochrane Library in Issue 3, 2004 and previously updated in 2007 and 2009.Motion sickness, the discomfort experienced when perceived motion disturbs the organs of balance, may include symptoms such as nausea, vomiting, pallor, cold sweats, hypersalivation, hyperventilation and headaches. The control and prevention of these symptoms has included pharmacological, behavioural and complementary therapies. Although scopolamine (hyoscine) has been used in the treatment and prevention of motion sickness for decades, there have been no systematic reviews of its effectiveness.

OBJECTIVES

To assess the effectiveness of scopolamine versus no therapy, placebo, other drugs, behavioural and complementary therapy or two or more of the above therapies in combination for motion sickness in persons (both adults and children) without known vestibular, visual or central nervous system pathology.

SEARCH STRATEGY

We searched the Cochrane Ear, Nose and Throat Disorders Group Trials Register; the Cochrane Central Register of Controlled Trials (CENTRAL); PubMed; EMBASE; CINAHL; Web of Science; BIOSIS Previews; Cambridge Scientific Abstracts; ICTRP and additional sources for published and unpublished trials. The date of the most recent search was 14 April 2011.

SELECTION CRITERIA

All parallel-arm, randomised controlled trials (RCTs) focusing on scopolamine versus no therapy, placebo, other drugs, behavioural and complementary therapy or two or more of the above therapies in combination. We considered outcomes relating to the prevention of onset or treatment of clinically-defined motion sickness, task ability and psychological tests, changes in physiological parameters and adverse effects.

DATA COLLECTION AND ANALYSIS

Two authors independently extracted data from the studies using standardised forms. We assessed study quality. We expressed dichotomous data as odds ratio (OR) and calculated a pooled OR using the random-effects model.

MAIN RESULTS

Of 35 studies considered potentially relevant, 14 studies enrolling 1025 subjects met the entry criteria. Scopolamine was administered via transdermal patches, tablets or capsules, oral solutions or intravenously. Scopolamine was compared against placebo, calcium channel antagonists, antihistamine, methscopolamine or a combination of scopolamine and ephedrine. Studies were generally small in size and of varying quality.Scopolamine was more effective than placebo in the prevention of symptoms. Comparisons between scopolamine and other agents were few and suggested that scopolamine was superior (versus methscopolamine) or equivalent (versus antihistamines) as a preventative agent. Evidence comparing scopolamine to cinnarizine or combinations of scopolamine and ephedrine is equivocal or minimal.Although sample sizes were small, scopolamine was no more likely to induce drowsiness, blurring of vision or dizziness compared to other agents. Dry mouth was more likely with scopolamine than with methscopolamine or cinnarizine.No studies were available relating to the therapeutic effectiveness of scopolamine in the management of established symptoms of motion sickness.

AUTHORS' CONCLUSIONS

The use of scopolamine versus placebo in preventing motion sickness has been shown to be effective. No conclusions can be made on the comparative effectiveness of scopolamine and other agents such as antihistamines and calcium channel antagonists. In addition, we identified no randomised controlled trials that examined the effectiveness of scopolamine in the treatment of established symptoms of motion sickness.