Motion sickness: a modulatory role for the central cholinergic nervous system

Transcutaneous Auricular Vagus Nerve Stimulation for Visually Induced Motion Sickness: An eLORETA Study

Transcutaneous auricular vagus nerve stimulation (taVNS), a non-invasive form of electrical brain stimulation, has shown potent therapeutic potential for a wide spectrum of conditions. How taVNS influences the characterization of motion sickness - a long mysterious syndrome with a polysymptomatic onset - remains unclear. Here, to examine taVNS-induced effects on brain function in response to motion-induced nausea, 64-channel electroencephalography (EEG) recordings from 42 healthy participants were analyzed; collected during nauseogenic visual stimulation concurrent with taVNS administration, in a crossover randomized sham-controlled study. Cortical neuronal generators were estimated from the obtained EEG using exact low-resolution brain electromagnetic tomography (eLORETA). While both sham and taVNS increased insula activation during electrical stimulation, compared to baseline, taVNS additionally augmented middle frontal gyrus neuronal activity. Following taVNS, brain regions including the supramarginal, parahippocampal, and precentral gyri were activated. Contrasting sham, taVNS markedly increased activity in the middle occipital gyrus during stimulation. A repeated-measures ANOVA showed that taVNS reduced motion sickness symptoms. This reduction in symptoms correlated with taVNS-induced neural activation. Our findings provide new insights into taVNS-induced brain changes, during and after nauseogenic stimuli exposure, including accompanying behavioral response. Together, these findings suggest that taVNS has promise as an effective neurostimulation tool for motion sickness management.

Ameliorative effect of vanillic acid against scopolamine-induced learning and memory impairment in rat via attenuation of oxidative stress and dysfunctional synaptic plasticity

Alzheimer's disease (AD) is characterized by cognitive impairment, loss of learning and memory, and abnormal behaviors. Scopolamine (SCOP) is a non-selective antagonist of muscarinic acetylcholine receptors that exhibits the behavioral and molecular hallmarks of AD. Vanillic acid (VA), a phenolic compound, is obtained from the roots of a traditional plant called Angelica sinensis, and has several pharmacologic effects, including antimicrobial, anti-inflammatory, anti-angiogenic, anti-metastatic, and antioxidant properties. Nevertheless, VA's neuroprotective potential associated with the memory has not been thoroughly investigated. Therefore, this study investigated whether VA treatment has an ameliorative effect on the learning and memory impairment induced by SCOP in rats. Behavioral experiments were utilized to assess the learning and memory performance associated with the hippocampus. Using western blotting analysis and assay kits, the neuronal damage, oxidative stress, and acetylcholinesterase activity responses of hippocampus were evaluated. Additionally, the measurement of long-term potentiation was used to determine the function of synaptic plasticity in organotypic hippocampal slice cultures. In addition, the synaptic vesicles' density and the length and width of the postsynaptic density were evaluated using electron microscopy. Consequently, the behavioral, biochemical, electrophysiological, and ultrastructural analyses revealed that VA treatment prevents learning and memory impairments caused by SCOP in rats. The study's findings suggest that VA has a neuroprotective effect on SCOP-induced learning and memory impairment linked to the hippocampal cholinergic system, oxidative damage, and synaptic plasticity. Therefore, VA may be a prospective therapeutic agent for treating AD.

Biochemical analysis reveals the systematic response of motion sickness mice to ginger (Zingiber officinale) extract's amelioration effect

ETHNOPHARMACOLOGICAL RELEVANCE

As a common medicinal and edible plant, Zingiber officinale Roscoe (ginger) is often used for the prevention of motion sickness. However, the mechanism of its anti-motion sickness remains to be elucidated.

AIM OF THE STUDY

To explore novel treatment for motion sickness with less side effects, anti-motion sickness effect of ginger (Zingiber officinale) extract (GE) and the possible molecular mechanisms were investigated.

MATERIALS AND METHODS

The anti-motion sickness effect of ginger was evaluated through mice animal experimental models. Components of ginger that might contribute to the anti-motion sickness effect were analyzed by LC-MS/MS. Subsequently, biochemical analysis integrated with serum metabolomic profiling were performed to reveal the systematic response of motion sickness mice to ginger extract's amelioration effect.

RESULTS

Exhaustive swimming time of mice in the GE group reached 8.9 min, which was 52.2% longer than that in the model group. Motion sickness index scores and time taken traversing balance beam of mice in the GE group were decreased by 53.2% and 38.5%, respectively. LC-MS/MS analysis suggested that various active ingredients in GE, such as gingerol, ginger oil and terpenoids, might contribute to its appealing anti-motion sickness activity. Biochemical analysis revealed that GE can relieve motion sickness through reducing histamine and acetylcholine release in vestibular system, regulating fatty acid oxidation, sugar metabolism and bile acid metabolism in mice.

CONCLUSION

Gavage of mice with GE can effectively relieve the symptoms of autonomic nervous system dysfunction, improve the balance and coordination ability and ameliorate the ability to complete complex work after rotation stimulation. GE has attractive potential for development and utilization as novel anti-motion sickness food or drugs.

Na(+) -Ca(2+) Exchanger, Leak K(+) Channel and Hyperpolarization-Activated Cyclic Nucleotide-Gated Channel Comediate the Histamine-Induced Excitation on Rat Inferior Vestibular Nucleus Neurons

AIMS

Antihistaminergic drugs have traditionally been used to treat vestibular disorders in the clinic. As a potential central target for antihistaminergic drugs, the inferior vestibular nucleus (IVN) is the largest subnucleus of the central vestibular nuclear complex and is considered responsible for vestibular-autonomic responses and integration of vestibular, cerebellar, and multisensory signals. However, the role of histamine on the IVN, particularly the underlying mechanisms, is still not clear.

METHODS

Using whole-cell patch-clamp recordings on rat brain slices, histamine-induced effect on IVN neurons and the underlying receptor and ionic mechanisms were investigated.

RESULTS

We found that histamine remarkably depolarized both spontaneous firing neurons and silent neurons in IVN via both histamine H1 and histamine H2 receptors. Furthermore, Na(+) -Ca(2+) exchangers (NCXs) and background leak K(+) channels linked to H1 receptors and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels coupled to H2 receptors comediate the histamine-induced depolarization on IVN neurons.

CONCLUSION

These results demonstrate the multiple ionic mechanisms underlying the excitatory modulation of histamine/central histaminergic system on IVN neurons and the related vestibular reflexes and functions. The findings also suggest potential targets for the treatment of vestibular disorders in the clinic, at the level of ionic channels in central vestibular nuclei.

Pharmaceutical countermeasures have opposite effects on the utricles and semicircular canals in man

INTRODUCTION

Sensory conflicts in the vestibular system lead to motion sickness of which space motion sickness (SMS) is a special case. SMS affects up to 70% of the astronauts during the first 3 days in space. The search for effective countermeasures has led to several nonpharmacological and pharmacological approaches. The current study focuses on the effects of lorazepam (1 mg), meclizine (25 mg), promethazine (25 mg), and scopolamine (0.4 mg) on the vestibular system, with special focus on the canal and otolith functions separately.

METHODS

The study had a placebo-controlled, single blind, repeated measures design. Sixteen healthy volunteers were subjected to a total of 7 test sessions, the first and last being without intake of medication. Semicircular canal function was evaluated by means of electronystagmography and otolith function with unilateral centrifugation. The horizontal semicircular canal function was characterized by the vestibulo-ocular reflex (VOR) gain measured during earth vertical axis rotation as well as the total caloric response. The function of the utricles was represented by the utricular sensitivity, reflecting the ocular counter roll relative to the virtual induced head tilt.

RESULTS

Promethazine significantly decreased the semicircular canal and utricular parameters. Both scopolamine and lorazepam caused only a decrease in the utricular sensitivity, whereas meclizine only decreased the semicircular canal-induced VOR gain.

DISCUSSION

The results show that the drugs affected different areas of the vestibular system and that the effects can thus be attributed to the specific pharmacological properties of each drug. Meclizine, as an antihistaminergic and weak anticholinergic drug, only affected the VOR gain, suggesting a central action on the medial vestibular nucleus. The same site of action is suggested for the anticholinergic scopolamine since acetylcholine receptors are present and utricular fibers terminate here. The global vestibular suppression caused by promethazine is probably a consequence of its anticholinergic, antihistaminergic, and antidopaminergic properties. Based on the fact that lorazepam increased the affinity of gamma-aminobutyric acid (GABA) for the GABA(A)-receptor and its effects on the utriculi, the site of action seems to be the lateral vestibular nucleus.

CONCLUSION

Meclizine, scopolamine, and lorazepam selectively suppress specific parts of the vestibular system. Selective suppression of different parts of the vestibular system may be more beneficial for alleviating (space) motion sickness than general suppressive agents. Additionally, this knowledge may help the clinician in his therapeutic management of patients with either semicircular canal or otolith dysfunction.

Characteristic changes in the physiological components of cybersickness

We investigated the characteristic changes in the physiology of cybersickness when subjects were exposed to virtual reality. Sixty-one participants experienced a virtual navigation for a total of 9.5 min, and were required to detect specific virtual objects. Three questionnaires for sickness susceptibility and immersive tendency were obtained before the navigation. Sixteen electrophysiological signals were recorded before, during, and after the navigation. The severity of cybersickness experienced by participants was reported from a simulator sickness questionnaire after the navigation. The total severity of cybersickness had a significant positive correlation with gastric tachyarrhythmia, eyeblink rate, heart period, and EEG delta wave and a negative correlation with EEG beta wave. These results suggest that cybersickness accompanies the pattern changes in the activities of the central and the autonomic nervous systems.

Pharmacological profiles of an anticholinergic agent, phencynonate hydrochloride, and its optical isomers

AIM

To comparatively study the pharmacological profiles of 3-methyl-3-azabicyclo(3,3,1)nonanyl-9-alpha-yl-alpha-cyclopentyl-alpha-phenyl-alpha-glycolate (phencynonate hydrochloride, CPG), an anticholinergic agent, and its enantiomers [R(-)-and S(+)-CPG].

METHODS

The affinity and relative efficacy were tested using radioligand-binding assay with muscarinic acetylcholine receptors from rat cerebral cortex. The pharmacological activities were assessed in three individual experiments: (1) potentiating the effect of subthreshold hypnotic dose of sodium pentobarbital; (2) inhibiting oxotremorine-induced salivation; and (3) inhibiting the contractile response to carbachol.

RESULTS

The order of potency of phencynonate hydrochloride and its optical isomers to inhibit the binding of [3H]quinuclidinyl benzilate ([3H]QNB) was R(-)-CPG (K(i)=46.49+/-1.27 nmol/L)>CPG(K(i)=271.37+/-72.30nmol/L)>S(+)-CPG(K(i)=1263.12+/-131.64 nmol/L). The results showed that R(-)-CPG had the highest affinity to central muscarinic receptors among the three compounds, but did not show any central depressant effects at dose from 10.00 to 29.15 mg/kg. CPG increased the effects of subthreshold hypnotic dose of sodium pentobarbital induced-sleeping [the ED50+/-95% LC value was 21.06+/-3.04 mg/kg]. CPG and R(-)-CPG displayed nearly equipotent effect in depressing oxotremorine-induced salivation [the ED50 +/-95% LC for R(-) and CPG were 1.10+/-0.28 and 1.07+/-0.15 mg/kg, respectively], and the contractile response to carbachol (pA(2) values for R (-) and CPG were 6.84 and 6.80, respectively). S(+)-CPG presented the lowest anticholinergic profiles, but could potentate effects of its enantiomers in some manner.

CONCLUSIONS

These data suggested that R(-)-CPG acted as an eutomer in racemate and a competitive antagonist to acetylcholine muscarinic receptors, but S(+)-CPG was less active in comparison to R(-)-CPG and its racemate. The central depressant effects of R(-)-CPG and S(+)-CPG were lower in comparison to its racemate.

Relationship between temporal changes in cardiac parasympathetic activity and motion sickness severity

Chemotherapy-induced nausea has been associated with a time-related decrease in cardiac parasympathetic activity. We tested the hypothesis that a time-related decrease in cardiac parasympathetic activity would also be associated with nausea and other motion sickness symptoms during illusory self-motion (vection). Fifty-nine participants (aged 18-34 years: 25 male) were exposed to a rotating optokinetic drum to induce vection. Symptoms of motion sickness and an estimate of cardiac parasympathetic activity (respiratory sinus arrhythmia; RSA) were obtained at baseline and throughout a drum-rotation period. As expected, motion sickness symptoms increased and RSA decreased over time during drum rotation. Moreover, greater decreases in RSA over time correlated with greater motion sickness severity. These results suggest that a time-related decrease in cardiac parasympathetic activity may be an important correlate of nausea and motion sickness across different evocative contexts.

Comparison of efficacy of ginger with various antimotion sickness drugs

Ginger and several other medications were compared with scopolamine and d-amphetamine for effectiveness in prevention of motion sickness.

METHODS

Double-blind techniques were used. The subjects were given the medications two hours before they were rotated in a chair making head movements until a symptom total short of vomiting was reached. Standardized N.A.S.A. techniques were used for speed of rotation and end-point of motion sickness.

RESULTS

The three doses of ginger were all at the placebo level of efficacy. Amitriptyline, ethopropazine and trihexyphenidyl increased the tolerated head movements but the increase was not statistically significant. Significant levels of protection were produced by dimenhydrinate, promethazine, scopolamine and d-amphetamine. Protection was further increased by combination of these latter drugs with d-amphetamine. Efficacy was greatest as the dose was increased.

CONCLUSIONS

The medication of choice in this study was scopolamine 0.6 mg with d-amphetamine 10 mg. This combination provided good protection with acceptable side effects.

Distribution of choline acetyltransferase immunoreactivity in the vestibular nuclei of normal and unilateral vestibular neurectomized cats

Post-lesion recovery of vestibular functions is a suitable model for studying adult central nervous system plasticity. The vestibular nuclei complex (VN) plays a major role in the recovery process and neurochemical reorganizations have been described at this brainstem level. The cholinergic system should be involved because administration of cholinergic agonists and antagonists modify the recovery time course. This study was aimed at analysing the postlesion changes in choline acetyltransferase immunoreactivity (ChAT-Ir) in the VN of cats killed 1 week, 3 weeks or 1 year following unilateral vestibular neurectomy. ChAT-positive neurons and varicosities were immunohistochemically labelled and quantified (cell count and surface measurement, respectively) by means of an image analysing system. The spatial distribution of ChAT-Ir within the VN of control cats showed darkly stained neurons and varicosities mainly located in the caudal parts of the medial (MVN) and inferior (IVN) VN, the nucleus prepositus hypoglossi (PH) and, to a lesser extent, in the medial part of the superior vestibular nucleus (SVN). Lesion-induced changes consisted in a significant increase in both the number of ChAT-positive neurons (IVN, SVN) and the surface of ChAT-positive varicosities (IVN, SVN, PH). They were observed bilaterally in the acute (1 year and 3 weeks) and compensated (1 year) cats for the SVN and PH, while they persisted only in the IVN on the lesioned side in the compensated cats. These findings demonstrate vestibular lesion-induced reorganization of the cholinergic system in the IVN, SVN and PH which could contribute to postural and oculomotor function recovery.

Psychophysiological aspects of motion sickness

Motion sickness may occur during travel by sea, automobile, airplane, and space. Susceptibility changes with age and may be influenced by psychological factors. Susceptibility can be reduced in most people by medications that involve histamine or neurotransmitters acetylcholine and noradrenaline, and influence the vestibular system.

Cardiovascular responses to KC-135 hyper-gravity

The present study was designed with two intentions; Are the effects of angular velocity detectable in the cardiovascular responses during the hyper-G? Another is object to examine how the otolith signal could modify the cardiovascular responses provoked by the exposure to the hyper-G. NASA/KC-135 hyper-gravity flight was used to generate high gravito-inertial forces to exclude a possible effect of angular velocity. Six healthy subjects was indicated to make dorsal flexion of the neck to reduce the otolith input. An exposure to +l.8Gz stress resulted in a remarkable increase of systolic and diastolic blood pressure, thereby pulse pressure became a little bit narrower. R-R interval revealed a tachycardia during the hyper-G except one subject. The present experiment bore the similar cardiovascular responses as those observed in the previous studies with a short rotating radius, suggesting that almost no effect of angular velocity acts on their responses. A weaker otolith input could possibly work on them. However a systematical observation can not recognize among the subjects for the vestibular effect on the cardiovascular responses. This fact of vestibular qualification leads us to speculate that it would depend on the subjects or other factors.

Do the organs of the labyrinth differentially influence the sympathetic and parasympathetic systems?

It has long been recognized that the vestibular system plays a major role in autonomic control. The nature of this control remains in dispute, however, as some evidence points to a vestibularly mediated parasympathetic activation, whereas other evidence points to a sympatho-excitatory role for labyrinthine outputs. A theoretical explanation is offered that attempts to resolve this issue by postulating that the utricles exert a predominantly sympatho-excitatory influence via their interactions with brain noradrenergic pathways, while the semicircular canals (and possibly saccules) increase parasympathetic tone via their cholinergic brain stem and cerebellar projections. This explanation is relevant for understanding the vestibular role in orthostatic regulation, motion sickness, oculomotor control, and in many disorders or situations associated with neurochemical or autonomic imbalances.

Body volume changes during simulated microgravity: auditory changes, segmental fluid redistribution, and regional hemodynamics

Space adaptation syndrome (SAS), manifested by cephalad fluid shifts, spacial disorientation, nausea, and vomiting, is of varied expression and uncertain etiology. One theory is that fluid shift to the upper body alters the function of the vestibular apparatus to create an entity similar to Meniere's disease. Since clinical vestibular dysfunction syndromes are mirrored by altered cochlear function, this experiment was undertaken to study the relation between fluid redistribution and the auditory effects of initial antiorthostatic bed rest. Manual and bone audiometry, impedance tympanometry, and brain-stem evoked potentials were used to monitor auditory changes prior to, during, and following short term exposure to -6 degrees head down tilt. Impedance plethysmography was performed to assess the segmental and intracranial fluid redistribution and hemodynamic changes during short-term head down tilt simulated microgravity. Even though significant cephalad fluid shift produced marked intracranial congestion and the subjects exhibited SAS symptoms, no clinically significant changes in the auditory system could be detected.

Abnormal motoric laterality in strabismus and a hypothesis concerning its neurological origins

A survey of the literature concerning motoric laterality in strabismus was undertaken. The assessment of manual and ocular dominance was based on a total of eleven studies conducted between 1934 and 1986. The average percentage of right-handedness in strabismics was 73.8%, whereas the average percentage of right-eyedness was 46.9%. Both figures are significantly lower than those obtained for the normal population. It is hypothesized that reduced right dominance in strabismics results from dysfunction of the otoliths and/or their higher brainstem pathways.

Cholinergic innervation of the cerebellum of the rat by secondary vestibular afferents

The cholinergic innervation of the cerebellar cortex of the rat was studied by immunohistochemical localization of choline acetyltransferase, radiochemical measurement of ChAT activity, and double labeling of ChAT-positive neurons with HRP injected into the cerebellum. ChAT immunohistochemistry revealed large mossy fiber rosettes as well as finely beaded terminals with different morphological characterization, laminar distribution within the cerebellar cortex, and regional differences within the cerebellum. Large "grapelike" ChAT-positive mossy fiber rosettes that were distributed primarily in the granule cell layer were concentrated, but not exclusively located, in three separate regions of the cerebellum: (1) the uvula-nodulus (lobules 9 and 10); (2) the flocculus, and (3) the anterior lobe vermis (lobules 1 and 2). Regional differences in ChAT-positive afferent terminations in the cerebellar cortex demonstrated by immunohistochemistry were confirmed by regional biochemical measurements of ChAT activity. Using ChAT immunohistochemistry in combination with HRP injections into the uvula-nodulus, we have studied the origin of the cholinergic projection. The caudal medial vestibular nucleus and to a lesser extent the nucleus prepositus hypglossus contain ChAT-positive neurons that were double labeled following HRP injections into the uvula-nodulus. We conclude that (1) there is a prominent cholinergic mossy fiber pathway to the vestibulocerebellum, (2) this pathway originates primarily in the caudal third of the medial vestibular nucleus, and (3) this cholinergic pathway likely mediates secondary vestibular information related to postural adjustment.

Direct muscarinic and nicotinic receptor-mediated excitation of rat medial vestibular nucleus neurons in vitro

We have utilized intracellular recording techniques to investigate the cholinoceptivity of rat medial vestibular nucleus (MVN) neurons in a submerged brain slice preparation. Exogenous application of the mixed cholinergic agonists, acetylcholine (ACh) or carbachol (CCh), produced predominantly membrane depolarization, induction of action potential firing, and decreased input resistance. Application of the selective muscarinic receptor agonist muscarine (MUSC), or the selective nicotinic receptor agonists nicotine (NIC) or 1,1-dimethyl-4-phenylpiperazinium (DMPP) also produced membrane depolarizations. The MUSC-induced depolarization was accompanied by decreased conductance, while an increase in conductance appeared to underlie the NIC- and DMPP-induced depolarizations. The muscarinic and nicotinic receptor mediated depolarizations persisted in tetrodotoxin and/or low Ca2+/high Mg2+ containing media, suggesting direct postsynaptic receptor activation. The MUSC-induced depolarization could be reversibly blocked by the selective muscarinic-receptor antagonist, atropine, while the DMPP-induced depolarization could be reversibly suppressed by the selective ganglionic nicotinic-receptor antagonist, mecamylamine. Some neurons exhibited a transient membrane hyperpolarization during the depolarizing response to CCh or MUSC application. This transient inhibition could be reversibly blocked by the gamma-aminobutyric acid (GABA) antagonist, bicuculline, suggesting that the underlying hyperpolarization results indirectly from the endogenous release of GABA acting at GABA receptors. This study confirms the cholinoceptivity of MVN neurons and establishes that individual MVN cells possess muscarinic as well as nicotinic receptors. The data provide support for a prominent role of cholinergic mechanisms in the direct and indirect regulation of the excitability of MVN neurons.

Secondary vestibular cholinergic projection to the cerebellum of rabbit and rat as revealed by choline acetyltransferase immunohistochemistry, retrograde and orthograde tracers

Previously we have shown that four regions of the cerebellum, the uvula-nodulus, flocculus, ventral paraflocculus, and anterior lobe 1, receive extensive, but not exclusive, cholinergic mossy fiber projections. In the present experiment we have studied the origin of three of these projections in the rat and rabbit (uvula-nodulus, flocculus, ventral paraflocculus), using choline acetyltransferase (ChAT) immunohistochemistry in combination with a double label, retrogradely transported horseradish peroxidase (HRP). We have demonstrated that in both the rat and rabbit the caudal medial vestibular nucleus (MVN) and to a lesser extent the nucleus prepositus hypoglossus (NPH) contain ChAT-positive neurons. Neurons of the caudal MVN are double-labeled following HRP injections into the uvula-nodulus. HRP injections into the uvula-nodulus also labeled less than 5% of the neurons in the cholinergic vestibular efferent complex. Fewer ChAT-positive neurons in the MVN and some ChAT-positive neurons in the NPH are double-labeled following HRP injections into the flocculus. Almost no ChAT-positive neurons in the MVN and some ChAT-positive neurons in the NPH are double-labeled following HRP injections into the ventral paraflocculus. Injections of Phaseolus leucoagglutinin (PHA-L) into the caudal MVN of both the rat and rabbit demonstrated projection patterns to the uvula-nodulus and flocculus that were qualitatively similar to those observed using ChAT immunohistochemistry. We conclude that the cholinergic mossy fiber pathway to the cerebellum in general and the uvula-nodulus in particular is likely to mediate secondary vestibular information related to postural adjustments.

New pharmacologic approaches to the prevention of space/motion sickness

Fundamental approaches in selection of new agents for evaluation in prevention of space/motion sickness (SMS) are reviewed. The discussion centers on drugs under investigation at the Johnson Space Center. Methodology that employs the rotating chair for measuring SMS symptomatology and susceptibility is described. The most obvious approach to the development of new agents relies on selection of agents from drug classes that possess pharmacologic properties of established anti-motion sickness agents. A second approach selects drugs that are used to prevent emesis caused by means other than exposure to motion. The third approach relies on basic research that characterizes individual differences in susceptibility. The hypothesis is: detection of individual differences leads to identification of specific drugs, which target physiologic systems that show individual differences. These physiologic systems are targets for therapy and may play a role in the etiology of SMS. Two drugs that reduce susceptibility to SMS include dexamethasone and d(CH2)5Tyr(Me)AVP, a vasopressin (AVP)V1 antagonist. The latter peptide has demonstrated complete blockade of emesis and other significant symptoms in squirrel monkeys. These studies were predicated on observations that subjects who were more resistant to SMS had higher plasma AVP after severe nausea than subjects with lower resistances. Investigations are underway to test a 0.5-mg intravenous dose in humans. Kappa opioid agonists inhibit AVP release and offer new therapeutic possibilities and advantages over AVP peptides. This review details the experimental data collected on AVP and adrenocorticotropin. The literature supports interrelated roles for AVP and opioid peptides in SMS. Experimental testing of kappa agonists is warranted because specific opioid agonists act at neuroanatomical sites causing nausea and vomiting. It is argued opioid receptors in the chemoreceptor trigger zone and vomiting center stimulate and inhibit the emetic response, respectively. The evidence suggests kappa and/or mu receptors at VC are involved in inhibition of emesis, whereas delta opioid receptors at CTZ are involved in stimulation of emesis.

Central and peripheral effects of arecoline in patients with autonomic failure

Increased plasma adrenalin (A) levels following arecoline in normal subjects and patients with multiple system atrophy (MSA) may result from nicotinic adrenal stimulation. Lack of this response in patients with pure autonomic failure (PAF) is consistent with peripheral sympathetic dysfunction. The mechanisms underlying diminished plasma corticotropin (ACTH) responses to arecoline may differ in patients with autonomic failure. Hypothalamic, cholinergic degeneration could prevent the response in MSA whereas patients with PAF do not manifest the normal increase in A which may be required to elicit an ACTH response. The appearance and exacerbation of tremor, vertigo, and pathological affect in the MSA group suggest that some central cholinergic receptors remain functional.

Effects of anti-cholinergic and cholinergic drugs on habituation to motion in rats

The effects of the anti-cholinergic drug scopolamine, an anti-motion sickness drug, and of the cholinergics physostigmine and neostigmine on habituation to motion sickness in rats were examined using pica, measured as eating of kaolin, as a behavioural index of motion sickness in rats. Rats were rotated around two axes for 1 h once a day for 10 or 11 days. Rotation-induced kaolin intake of control rats gradually decreased from day 9 of daily rotation. Test rats were not treated for the first 3 days, given drugs on days 4-7 of rotation and then again given no drugs for the next 3 or 4 days. Rotation-induced kaolin intake of test rats was compared to that of controls. Results showed that TTS (Transdermal Therapeutic System)-scopolamine administration facilitated habituation to rotation, whereas physostigmine, a centrally acting cholinesterase inhibitor, suppressed it, and neostigmine, a peripherally active cholinesterase inhibitor, had no effect on habituation at all. These findings suggest that the central cholinergic neuron system plays an important role in the neural mechanism of habituation to motion in rats.

Therapeutic management of nausea and vomiting

1. The aim of this study is to review the mechanisms implicated in nausea and vomiting and the treatment of these symptoms. 2. Metoclopramide, a benzamide, is the drug most frequently used to alleviate or abolish the majority of nausea and vomiting of different origin. Domperidone, which scarcely penetrates the central nervous system (CNS), is less used. 3. The treatment of vomiting induced by cytotoxic drugs is necessary to use a combination (two or more) of antiemetic drugs (metoclopramide, glucocorticoids, antihistamines, butyrophenones, anticholinergics, cannabinoids). Recently, antagonists of serotonergic (5-HT) receptors of the subtype 5-HT3 appear to possess interesting antiemetic properties and they have a promising future in this field. 4. Antagonists of dopamine receptors (benzamides, phenotiazines, butyrophenones and domperidone) induce adverse reactions in CNS (mainly extrapyramidal disorders), which are scarce with metoclopramide and practically absent with domperidone. These disorders must not suppress antiemetic therapy when it is needed.

Primary afferent excitatory transmission recorded intracellularly in vitro from rat medial vestibular neurons

Intracellular recordings were made from rat medial vestibular nucleus (MVN) neurons in transverse brain slices containing the root of the vestibular nerve (N. VIII). Electrical stimuli applied to the N. VIII tract evoked an orthodromic excitatory postsynaptic potential (EPSP) that lasted about 50 ms following a 0.5 to 1.5 ms delay between the stimulus artifact and synaptic potential. These orthodromic EPSPs were insensitive to the following antagonists: atropine, hexamethonium, diphenhydramine, and caffeine. Based on these results we conclude that the primary afferent excitatory transmitter is not acetylcholine, histamine, or adenosine, respectively. However, kynurenic acid, a general excitatory amino acid receptor antagonist, blocked the orthodromic EPSP while having no effect on the resting membrane potential, input resistance, or action potential configuration of MVN neurons. Our data suggest that an excitatory amino acid, or amino acid-like substance, is responsible for primary afferent excitatory transmission in the rat medial vestibular nucleus.

Rostral hypothalamus: a new neuroanatomical site of neurochemically-induced emesis in the cat

The localized effect of noradrenergic agonists administered directly in the anterior hypothalamic preoptic area (AH/POA) in inducing emesis in the cat was investigated. Of the noradrenergic agonists tested, which included norepinephrine, clonidine, phenylephrine and methoxamine, only clonidine in doses of 5.0-50.0 micrograms was found to evoke emesis consistently when micro-injected in a volume of 1.0 microliter into AH/POA of the unrestrained cat. The emetic response to clonidine was short-lasting, generally dose-dependent in terms of latency and frequency, and occurred in bouts of one to three episodes. The sequence of the vomiting response, beginning with licking and retching, functionally resembled a normal pattern of an emetic response. The clonidine-induced emesis was not antagonized by the following antagonists micro-injected in AH/POA just prior to clonidine: alpha-adrenergic blocking agents, yohimbine, RX 781094 and phentolamine; the antimuscarinic drug, atropine; the serotonin antagonist, methysergide; the opioid antagonist, naloxone; and the dopamine antagonist, chlorpromazine. Therefore, it would appear that clonidine-induced emesis is not mediated by alpha noradrenergic, serotonergic, dopaminergic, muscarinic and opiate receptor systems within the AH/POA of the cat. Finally, the obtained results show that apart from the area postrema and a circumscribed zone of the brain-stem reticular formation, the hypothalamus is now implicated as a neuroanatomical site in the central nervous system mechanism underlying neurochemically-induced emesis.

Autonomic effects on R-R variations of the heart rate in the squirrel monkey: an indicator of autonomic imbalance in conflict sickness

To assess the contribution of the autonomic nervous system (ANS) to beat-to-beat (R-R interval) variations in the squirrel monkey, heart rate was analyzed after atropine, propranolol, carbachol, or isoproterenol administration, using a computer analyzing system. Heart rate variations were expressed as coefficient of variance (CV) of mean R-R interval. CV of R-R interval clearly was reduced by atropine but not by propranolol, indicating that the effect could be mediated through the parasympathetic nerve. CV of R-R interval was measured also under vestibulo-visual conflict (VVC) situation in pitch. Although CV of R-R interval increased during VVC, this variation almost was abolished after atropine administration. Monkeys with unilateral labyrinthectomy showed smaller variations than those without operation during exposure to VVC. CV of R-R interval is a useful parameter for the objective and quantitative evaluation of ANS function and may be a good indicator to demonstrate the severity of motion sickness without invasive methods.

Dexamethasone mimicks the antimotion sickness effects of amphetamine and scopolamine

Based on preliminary suggestions that individual differences in susceptibility to stressful motion might be related to physiological differences in responses of the hypothalamic-pituitary-adrenal axis, we tested the efficacy of dexamethasone and metyrapone in subjects exposed to cross-coupled accelerative semicircular canal stimulation on a rotating chair. Subjects given 0.5 mg of dexamethasone every 6 h for 48 h could endure 80% more stressful motion (P = 0.03) in a within-subjects design study, whereas, no improvement followed treatment with 750 mg of metyrapone[correction of metryapone] every 4 h for 24 h. The efficacy of dexamethasone might be explained in terms of its neurochemical actions on several neurotransmitter systems which are also modulated by such classical antimotion sickness drugs as amphetamine and scopolamine. Because dexamethasone induces adaptive changes within the central nervous system it may prove superior to scopolamine and amphetamine which possess significant side effects, are short acting, and rapidly tolerated.

The effects of transdermal scopolamine and four dose levels of oral scopolamine (0.15, 0.3, 0.6, and 1.2 mg) upon psychological performance

Four dose levels of oral scopolamine (0.15 mg, 0.3 mg, 0.6 mg, 1,2 mg), transdermal scopolamine, and placebo, were investigated for their effects upon a battery of psychological performance measures in normal subjects. Oral scopolamine produced significant linear dose-related decrements on tasks involving continuous attention, continuous performance, memory storage for new information, and on self-rated feelings of alertness and sociability. Transdermal scopolamine produced significant performance impairments on these same assessment measures. Resting heart rate levels were significantly reduced by all scopolamine conditions. Side effects (dry mouth, dizziness) were frequent following transdermal scopolamine and the higher oral dose conditions. The overall effects of the transdermal scopolamine patch were broadly equivalent to the effects of 0.8 mg oral scopolamine. This oral dose equivalence for transdermal scopolamine is higher than expected, and possible reasons for this are discussed.

The relative selectivity of anticholinergic drugs for the M1 and M2 muscarinic receptor subtypes

Anticholinergic drugs are used to treat a number of neurologic disorders, including parkinsonism, vestibular disturbances, and dystonia. Traditionally, these drugs have been thought to act in similar fashion, as competitive antagonists at a single class of muscarinic receptors, and not to differ significantly in their therapeutic efficacy. Recently, however, pharmacologic studies have shown that the novel antagonist pirenzepine is capable of recognizing heterogeneity among muscarinic receptors; high-affinity pirenzepine sites have been classified as M1 sites and low-affinity sites as M2. This study examined whether the anticholinergics currently available for treatment of neurologic symptoms have selectivity for these subtypes and whether they differ in their degree of selectivity; the study showed that these drugs do demonstrate selectivity. All had greater affinity for the M1 site, indicated by higher affinity for rat forebrain membranes, where M1 predominates, than hindbrain preparations, where M2 predominates. The degree of selectivity varied greatly; some compounds, such as ethopropazine, had little M1 selectivity, whereas others, such as scopolamine, trihexyphenidyl, and biperiden, were quite selective, like pirenzepine. It is unknown whether these differences in selectivity have any immediate therapeutic implications. However, these results support the emerging concept of muscarinic receptor subtypes and the prospect of developing more selective agents, with enhanced therapeutic efficacy.

Pituitary hormone response to cigarette smoking

Nausea was induced by having subjects smoke two high nicotine cigarettes in quick succession. Plasma levels of prolactin, adrenocorticotropic hormone, beta-endorphin/beta-lipotropin, growth hormone, arginine vasopressin, and neurophysin I increased without changes in thyroid stimulating hormone, luteinizing hormone, or follicle stimulating hormone. Nausea and pituitary hormone release correlated with high nicotine intake (smoking 2.87 mg nicotine cigarettes) but did not occur during lower nicotine intake (smoking 0.48 mg nicotine cigarettes). Individual differences in nausea and related hormonal responses may provide an objective method for predicting receptivity to smoking.

Muscarinic cholinergic receptors in area postrema and brainstem areas regulating emesis

Central cholinergic pathways modulate both the perception of excessive motion stimuli and the expression of motion sickness symptoms, such as nausea and vomiting. Specific brainstem areas which mediate motion-induced emesis include the area postrema (AP), vagal nuclear complex (VNC), reticular formation (RF) at the site of the vomiting center, and the vestibular complex (VC). In this report, histological studies indicated the cellular organization of brainstem structures mediating emesis was similar in bovine and squirrel monkey brain. The objective of this study was to characterize biochemical and pharmacological properties of muscarinic cholinergic receptors assayed by 3H-QNB binding in these regions of bovine brainstem. Scatchard analyses of specific 3H-QNB binding showed an uneven distribution of muscarinic receptors, with high densities of sites in VNC and AP, intermediate levels in RF and lowest receptor concentrations in VC. Dissociation constants for 3H-QNB, measured in saturation and kinetic experiments, were similar in all brainstem regions. The pharmacological potency of cholinergic agonists and antagonists was the same as reported for muscarinic receptors labeled in other brain areas or peripheral organs. Several drugs which potently inhibited 3H-QNB binding in bovine brainstem also exhibited antiemetic activity in a squirrel monkey model of motion-induced emesis. The antimotion sickness effects of these drugs may be due, in part, to their antagonism of muscarinic receptors in brainstem areas regulating emesis.