Hydrogen sulfide production in the medullary respiratory center modulates the neural circuit for respiratory pattern and rhythm generations.

Hydrogen sulfide (H2S), which is synthesized in the brain, modulates the neural network. Recently, the importance of H2S in respiratory central pattern generation has been recognized, yet the function of H2S in the medullary respiratory network remains poorly understood. Here, to evaluate the functional roles of H2S in the medullary respiratory network, the Bötzinger complex (BötC), the pre-Bötzinger complex (preBötC), and the rostral ventral respiratory group (rVRG), we observed the effects of inhibition of H2S synthesis at each region on the respiratory pattern by using an in situ arterially perfused preparation of decerebrated male rats. After microinjection of an H2S synthase inhibitor, cystathionine ß-synthase, into the BötC or preBötC, the amplitude of the inspiratory burst decreased and the respiratory frequency increased according to shorter expiration and inspiration, respectively. These alterations were abolished or attenuated in the presence of a blocker of excitatory synaptic transmission. On the other hand, after microinjection of the H2S synthase inhibitor into the rVRG, the amplitude of the inspiratory burst was attenuated, and the respiratory frequency decreased, which was the opposite effect to those obtained by blockade of inhibitory synaptic transmission at the rVRG. These results suggest that H2S synthesized in the BötC and preBötC functions to limit respiratory frequency by sustaining the respiratory phase and to maintain the power of inspiration. In contrast, H2S synthesized in the rVRG functions to promote respiratory frequency by modulating the interval of inspiration and to maintain the power of inspiration. The underlying mechanism might facilitate excitatory synaptic transmission and/or attenuate inhibitory synaptic transmission.

Saccadic oscillations as a biomarker of clinical symptoms in amyotrophic lateral sclerosis.

BACKGROUND: Among eye movements in amyotrophic lateral sclerosis (ALS), we identified the characteristics of square-wave jerks (SWJs) seen during times without visual fixation (VF) and analyzed their relationships with clinical parameters. MATERIALS AND METHODS: Clinical symptoms were evaluated and eye movements were tested using electronystagmography in 15 patients with ALS (10 men, 5 women; mean age, 66.9 ± 10.5 years). SWJs with and without VF were recorded, and their characteristics were identified. Relationships between each SWJ parameter and clinical symptoms were evaluated. Results were compared with eye movement data from 18 healthy individuals. RESULTS: The frequency of SWJs without VF was significantly higher in the ALS group than in the healthy group (P < 0.001). When the condition was changed from VF to no-VF in the ALS group, the frequency of SWJs was significantly higher in healthy subjects (P = 0.004). A positive correlation was seen between frequency of SWJs and percentage predicted forced vital capacity (%FVC) (R = 0.546, P = 0.035). CONCLUSION: The frequency of SWJs was higher with VF in healthy people, and was suppressed without VF. In contrast, the frequency of SWJs was not suppressed without VF in ALS patients. This suggests that SWJs without VF have some clinical significance in ALS patients. Moreover, a relationship was noted between the parameters of SWJs without VF in ALS patients and results of pulmonary function tests, suggesting that SWJs during times without VF may offer a clinical parameter of ALS.

Lateral Habenula Regulates Cardiovascular Autonomic Responses via the Serotonergic System in Rats.

The lateral habenula (LHb) plays essential roles in behavioral responses to stressful events. Stress is tightly linked to autonomic responses such as cardiovascular responses, yet how the LHb regulates these responses is not well understood. To address this issue, we electrically stimulated the LHb in rats, measured its effects on heart rate (HR) and mean arterial pressure (MAP), and investigated the neural circuits that mediate these LHb-induced cardiovascular responses via the autonomic nervous system. We observed that stimulation of the LHb induced bradycardia and pressor responses, whereas stimulation of the adjacent areas changed neither the HR nor the MAP. Bilateral vagotomy and administration of a muscarinic receptor antagonist suppressed the LHb stimulation effect on the HR but not on the MAP, whereas administration of a ß-adrenoceptor antagonist partly attenuated the effect on the MAP but not on the HR. Thus, the LHb-induced cardiovascular responses of the HR and the MAP were likely caused by activations of the cardiac parasympathetic nerves and the cardiovascular sympathetic nerves, respectively. Furthermore, administration of a non-selective 5-HT receptor antagonist significantly attenuated the LHb stimulation effects on both the MAP and the HR. A 5-HT2 receptor antagonist also attenuated the LHb stimulation effects. A low dose of a 5-HT1A receptor antagonist enhanced the LHb stimulation effects, but a high dose of the drug attenuated them. 5-HT1B and 5-HT1D receptor antagonists as well as a 5-HT7 receptor antagonist did not affect the LHb stimulation effects. Taken together, our findings suggest that the LHb regulates autonomic cardiovascular responses at least partly through the serotonergic system, particularly via the 5-HT1A and 5-HT2 receptors.

Clinical Application of the Vestibular Stimulation Effect on Balance Disorders with Dementia.

BACKGROUND: In a previous study on Alzheimer's disease (AD), we showed that vestibular dysfunction derived from cerebral disorders contributes to balance disorders. No previous clinical study has attempted to prevent the progression of balance disorders in dementia patients through vestibular stimulation using an air caloric device. OBJECTIVE: The purpose of this pilot study was to delay the progression of balance disorders by inducing vestibular compensation, specifically by utilizing the effect of vestibular stimulation to activate the cerebrum. METHODS: Fifteen individuals were randomized and classified into a stimulation group or a nonstimulation group. Eight AD patients underwent vestibular stimulation every 2 weeks for 6 months in the stimulation group. Seven AD patients participated in the nonstimulation group (the control group). Both groups were subsequently evaluated using a Mini-Mental State Examination (MMSE), stepping test, caloric test, and smooth pursuit eye movement test just before starting the study and 6 months later. RESULTS: For balance parameters, the various tests did not show any significant differences between the two groups. However, in the stepping test, the decline rate tended to be higher in the nonstimulation group than in the stimulation group. The stimulation group's rate of decline in MMSE scores was lower than that of the nonstimulation group (p=0.015). No adverse events were tracked during the present study. CONCLUSION: Repeated vestibular stimulation might help patients retain greater balance and higher function. To prove these effects, the future clinical application will require an increased number of cases and longer periods of vestibular stimulation. This study showed that vestibular stimulation by air caloric device is safe and tolerable in patients with AD.

Endogenous hydrogen sulfide maintains eupnea in an in situ arterially perfused preparation of rats.

Hydrogen sulfide (H2S) is constitutively generated in the human body and works as a gasotransmitter in synaptic transmission. In this study, we aimed to evaluate the roles of endogenous H2S in generating eupnea at the respiratory center. We employed an in situ arterially perfused preparation of decerebrated rats and recorded the central respiratory outputs. When the H2S-producing enzyme cystathionine ß-synthase (CBS) was inhibited, respiration switched from the 3-phase eupneic pattern, which consists of inspiration, postinspiration, and expiration, to gasping-like respiration, which consists of inspiration only. On the other hand, when H2S synthesis was inhibited via cystathionine γ-lyase (CSE) or when H2S synthesis was activated via CBS, eupnea remained unchanged. These results suggest that H2S produced by CBS has crucial roles in maintaining the neuronal network to generate eupnea. The mechanism of respiratory pattern generation might be switched from a network-based system to a pacemaker cell-based system in low H2S conditions.

Abnormal Saccadic Intrusions with Alzheimer's Disease in Darkness.

BACKGROUND: Classified as saccadic intrusions, Square-Wave Jerks (SWJs) have been observed during Visual Fixation (VF) in Alzheimer's Disease (AD). However, the pathological significance of this phenomenon remains unclear. OBJECTIVE: The present study analyzed the characteristics of SWJs in patients with AD with their eyes open in the dark without VF. METHODS: Fifteen patients with AD and 15 healthy age- and sex-matched controls were investigated and compared. Saccadic intrusions with and without VF were detected as SWJs and measured using an electronystagmogram. RESULTS: No significant difference in the frequency of SWJs was observed between control and AD groups with VF, but significantly more SWJs were observed in the AD group than in the control group in the absence of VF (p<0.01). In the control group, the frequency of SWJs was significantly higher with VF as compared to without VF. Conversely, the frequency in the AD group was significantly higher without VF. Furthermore, a directly proportional relationship was observed between the frequency of SWJs and higher-order function (R>0.55) in the AD group. CONCLUSION: SWJs without VF may have pathological significance in AD. In healthy individuals, SWJs are generated by VF and suppressed without VF. Conversely, in AD, SWJs are generated rather than suppressed in the absence of VF. These pathognomonic SWJs without VF also appear to be correlated with higher-order dysfunction, reflecting AD-related cortical damage. These findings suggest that pathological SWJs without VF observed in AD derive from cortical damage and may constitute an important marker of a higher-order function.

Abnormal Pyramidal Decussation and Bilateral Projection of the Corticospinal Tract Axons in Mice Lacking the Heparan Sulfate Endosulfatases, Sulf1 and Sulf2.

The corticospinal tract (CST) plays an important role in controlling voluntary movement. Because the CST has a long trajectory throughout the brain toward the spinal cord, many axon guidance molecules are required to navigate the axons correctly during development. Previously, we found that double-knockout (DKO) mouse embryos lacking the heparan sulfate endosulfatases, Sulf1 and Sulf2, showed axon guidance defects of the CST owing to the abnormal accumulation of Slit2 protein on the brain surface. However, postnatal development of the CST, especially the pyramidal decussation and spinal cord projection, could not be assessed because DKO mice on a C57BL/6 background died soon after birth. We recently found that Sulf1/2 DKO mice on a mixed C57BL/6 and CD-1/ICR background can survive into adulthood and therefore investigated the anatomy and function of the CST in the adult DKO mice. In Sulf1/2 DKO mice, abnormal dorsal deviation of the CST fibers on the midbrain surface persisted after maturation of the CST. At the pyramidal decussation, some CST fibers located near the midline crossed the midline, whereas others located more laterally extended ipsilaterally. In the spinal cord, the crossed CST fibers descended in the dorsal funiculus on the contralateral side and entered the contralateral gray matter normally, whereas the uncrossed fibers descended in the lateral funiculus on the ipsilateral side and entered the ipsilateral gray matter. As a result, the CST fibers that originated from 1 side of the brain projected bilaterally in the DKO spinal cord. Consistently, microstimulation of 1 side of the motor cortex evoked electromyogram responses only in the contralateral forelimb muscles of the wild-type mice, whereas the same stimulation evoked bilateral responses in the DKO mice. The functional consequences of the CST defects in the Sulf1/2 DKO mice were examined using the grid-walking, staircase, and single pellet-reaching tests, which have been used to evaluate motor function in mice. Compared with the wild-type mice, the Sulf1/2 DKO mice showed impaired performance in these tests, indicating deficits in motor function. These findings suggest that disruption of Sulf1/2 genes leads to both anatomical and functional defects of the CST.

Vestibular dysfunction as cortical damage with amyotrophic lateral sclerosis.

BACKGROUND: Cortical damage in areas such as the frontal lobe is reported in amyotrophic lateral sclerosis (ALS). However, aside from executive dysfunction, the pathological significance of this cortical damage has yet to be clarified. The present study investigated the effects of cortical damage on vestibular function in ALS. METHODS: Subjects comprised 18 ALS patients and 18 age- and sex-matched healthy controls. Cold air caloric stimulation was performed in all subjects to induce vestibular nystagmus, which was analysed to evaluate vestibular function. Visual suppression testing to investigate the suppressive effects of visual stimuli on vestibular nystagmus was expressed as suppression rate (SR, %). Executive function was tested using the frontal assessment battery (FAB). RESULTS: Suppression rate and FAB score were significantly lower in the ALS group than in the control group (p < 0.01 each). A positive correlation was also observed between SR and FAB score (R = 0.65, p = 0.023). CONCLUSION: Visual suppression testing showed significant damage to the central nervous system vestibular control mechanisms, which utilize visual information in the ALS group and a positive correlation between SR and FAB score suggest a relationship between frontal lobe damage and impaired vestibular control. A simple vestibular function test may be useful as a tool to objectively monitor the progression of cerebral lesions in ALS.

Upbeat nystagmus is a useful sign in the regional diagnosis of trigeminal nerve disorder with multiple sclerosis.

Trigeminal nerve disorder is an important neurological sign that is often seen with multiple sclerosis (MS). We investigated eye movements in three MS patients with trigeminal disorder due to pontine lesions near the trigeminal root entry zone (REZ). Upbeat nystagmus was observed in all MS patients with trigeminal REZ lesions. We conjecture that trigeminal nerve disorder and upbeat nystagmus appeared due to simultaneous damage to both the trigeminal nerve and the vestibulo-ocular reflex pathway. If upbeat nystagmus appears in MS patients exhibiting a trigeminal nerve disorder, such as trigeminal neuralgia, and paralysis, pontine lesions near the trigeminal REZ should be considered. Upbeat nystagmus can be understood as a useful sign for the clinical regional diagnosis of trigeminal nerve disorder.

The Successful Application of Plasmapheresis in the Treatment of a Patient with Opsoclonus and Autoantibodies to Glutamate Receptor δ2.

Glutamate receptor δ2 (GluRδ2) is expressed in the neuronal postsynaptic densities at the junctions between the Purkinje cells and the parallel fibers. Recent reports have described patients with opsoclonus who possess anti-GluRδ2 antibodies. We report the case of a 53-year-old man with opsoclonus whose cerebrospinal fluid was positive for anti-GluRδ2 antibodies. Electronystagmography revealed abnormal sinusoidal eye movements, which were definitively identified as opsoclonus. The frequency and amplitude of saccadic oscillations diminished after plasmapheresis (PE). The patient's opsoclonus was altered after PE, suggesting that anti-GluRδ2 antibodies may act on the saccade generator in the brainstem via the cerebellum and that they may be involved in the onset of opsoclonus.

Vestibular Impairment in Frontotemporal Dementia Syndrome.

BACKGROUND: No studies to date have attempted to evaluate frontotemporal lobar degeneration from the perspective of the vestibular system. OBJECTIVE: The present study examined vestibular function in patients with frontotemporal dementia (FTD) clinical syndrome and evaluated whether vestibular disorders are involved in the clinical symptoms due to FTD. METHODS: Fourteen patients with FTD syndrome, as well as healthy elderly controls without dementia, were included in the present study. All subjects underwent vestibular function tests using electronystagmography, such as caloric tests and visual suppression (VS) tests, in which the induced caloric nystagmus was suppressed by visual stimuli. The association between clinical symptoms and vestibular function in the FTD syndrome group was further examined. RESULTS: In the FTD syndrome group, caloric nystagmus was not necessarily suppressed during VS tests. Furthermore, VS was observed to be significantly impaired in FTD syndrome patients with gait disturbance as compared to those without such disturbance. CONCLUSION: The present study revealed that impairment of VS in patients with FTD results in an inability to regulate vestibular function by means of visual perception, regardless of multiple presumed neuropathological backgrounds. This could also be associated with gait disturbance in patients with FTD syndrome.

Vestibular Function Impairment in Alzheimer's Disease.

BACKGROUND: Falls and fractures due to impaired balance in patients with Alzheimer's disease (AD) have an adverse effect on the clinical course of the disease. OBJECTIVE: To evaluate balance impairment in AD from the viewpoint of vestibular functional impairment. METHODS: The subjects were 12 patients with AD, 12 dementia-free elderly adults, and 12 younger adults. Vestibular function was assessed using a stepping test, caloric nystagmus, and a visual suppression (VS) test. RESULTS: The stepping test was abnormal in 9 of the 12 patients in the AD group. An abnormal stepping test was not associated with self-reported dizziness or tendency to fall. Significant VS abnormalities were present in the AD group. The suppression rate of VS was lower in AD patients with either a tendency to fall or constructional apraxia than in AD patients without either. The velocity of the rapid phase of caloric nystagmus before the VS test was similar in the AD group and the elderly control group. Significant abnormalities of both caloric nystagmus and VS were not present in either the elderly or the younger control groups. CONCLUSION: AD could involve impairments in the vestibular control of balance. The VS test is useful for assessing the tendency to fall in AD. Impairment of VS in AD might arise from cerebral vestibular cortex impairment rather than comorbid peripheral vestibular disorders.

Intrinsic chemosensitivity of rostral ventrolateral medullary sympathetic premotor neurons in the in situ arterially perfused preparation of rats.

Brainstem hypoperfusion is a major excitant of sympathetic activity triggering hypertension, but the exact mechanisms involved remain incompletely understood. A major source of excitatory drive to preganglionic sympathetic neurons originates from the ongoing activity of premotor neurons in the rostral ventrolateral medulla (RVLM sympathetic premotor neurons). The chemosensitivity profile of physiologically characterized RVLM sympathetic premotor neurons during hypoxia and hypercapnia remains unclear. We examined whether physiologically characterized RVLM sympathetic premotor neurons can sense brainstem ischaemia intrinsically. We addressed this issue in a unique in situ arterially perfused preparation before and after a complete blockade of fast excitatory and inhibitory synaptic transmission. During hypercapnic hypoxia, respiratory modulation of RVLM sympathetic premotor neurons was lost, but tonic firing of most RVLM sympathetic premotor neurons was elevated. After blockade of fast excitatory and inhibitory synaptic transmission, RVLM sympathetic premotor neurons continued to fire and exhibited an excitatory firing response to hypoxia but not hypercapnia. This study suggests that RVLM sympathetic premotor neurons can sustain high levels of neuronal discharge when oxygen is scarce. The intrinsic ability of RVLM sympathetic premotor neurons to maintain responsivity to brainstem hypoxia is an important mechanism ensuring adequate arterial pressure, essential for maintaining cerebral perfusion in the face of depressed ventilation and/or high cerebral vascular resistance.

Residual central nervous system damage due to organoarsenic poisoning.

BACKGROUND: Drinking well water contaminated with the organoarsenic compound diphenylarsinic acid (DPAA) causes central nervous system (CNS) disorders that improve within several years after last drinking such water. Subjective symptoms such as lightheadedness and dizziness appear to persist, however, suggesting CNS damage. We evaluated CNS damage due to DPAA by detecting abnormal eye movements. METHODS: Subjects comprised 29 victims of exposure to DPAA in whom this substance had been detected in the nails. Investigations were performed more than 3years following cessation of DPAA exposure. Abnormal eye movements were monitored using electronystagmography. We analysed unpaired t-test between exposure subjects who exhibited upbeat nystagmus and those who did not. Upbeat nystagmus parameters were measured, and mean values were calculated. Associations between the properties of upbeat nystagmus and maximum concentrations of DPAA among DPAA exposure were also investigated. RESULTS: Upbeat nystagmus was common among exposure victims, occurring in 23 of 29 subjects (79.0%). The subjects with upbeat nystagmus had significantly higher ratio than those without upbeat nystagmus in the points of subjective symptoms and DPAA concentration of drinking water (p<0.01). The slow-phase amplitude of upbeat nystagmus enlarged with increasing DPAA concentrations, showing a significant positive correlation (p<0.05). These findings suggest that the level of exposure to DPAA affects the properties of nystagmus. High-frequency pathological square-wave jerks (SWJ) were seen in 14 of 29 patients (48.0%), and mean SWJ frequency was 112.4±16.7/min. CONCLUSIONS: Detection of abnormal ocular movements may be useful in evaluating residual/persistent/chronic CNS damage due to organoarsenic poisoning.

Downbeat nystagmus associated with damage to the medial longitudinal fasciculus of the pons: a vestibular balance control mechanism via the lower brainstem paramedian tract neurons.

The paramedian tract (PMT) neurons, a group of neurons associated with eye movement that project into the cerebellar flocculus, are present in or near the medial longitudinal fasciculus (MLF) in the paramedian region of the lower brainstem. A 66-year-old man with multiple sclerosis in whom downbeat nystagmus appeared along with right MLF syndrome due to a unilateral pontomedullary lesion is described. In light of these findings, a possible schema for the vestibular balance control mechanism circuit of the PMT neurons via the flocculus is presented. Damage to the PMT neurons impaired the elective inhibitory control mechanism of the anterior semicircular canal neural pathway by the flocculus. This resulted in the appearance of anterior semicircular canal-dominant vestibular imbalance and the formation of downbeat nystagmus. From the pathogenesis of this vertical vestibular nystagmus, the action of the PMT neurons in the vestibular eye movement neuronal pathway to maintain vestibular balance was conjectured to be as follows. PMT neurons transmit vestibular information from the anterior semicircular canals to the cerebellum, forming a cerebellum/brainstem feedback loop. Vestibular information from that loop is integrated in the cerebellum, inhibiting only the anterior semicircular canal neuronal pathway via the flocculus and controlling vestibular balance.

Baroreceptor-mediated activation of sympathetic nerve activity to salivary glands.

Salivary gland function is regulated by both the sympathetic and parasympathetic nervous systems. Previously we showed that the basal sympathetic outflow to the salivary glands (SNA(SG)) was higher in hypertensive compared to normotensive rats and that diabetes reduced SNA(SG) discharge at both strains. In the present study we sought to investigate how SNA(SG) might be modulated by acute changes in the arterial pressure and whether baroreceptors play a functional role upon this modulation. To this end, we measured blood pressure and SNA(SG) discharge in Wistar-Kyoto rats (WKY-intact) and in WKY submitted to sinoaortic denervation (WKY-SAD). We made the following three major observations: (i) in WKY-intact rats, baroreceptor loading in response to intravenous infusion of the phenylephrine evoked an increase in SNA(SG) spike frequency (81%, p<0.01) accompanying the increase mean arterial pressure (ΔMAP: +77 ± 14 mmHg); (ii) baroreceptor unloading with sodium nitroprusside infusion elicited a decrease in SNA(SG) spike frequency (17%, p<0.01) in parallel with the fall in arterial blood pressure (ΔMAP: -30 ± 3 mmHg) in WKY-intact rats; iii) in the WKY-SAD rats, phenylephrine-evoked rises in the arterial pressure (ΔMAP: +56 ± 6 mmHg) failed to produce significant changes in the SNA(SG) spike frequency. Taken together, these data show that SNA(SG) increases in parallel with pharmacological-induced pressor response in a baroreceptor dependent way in anaesthetised rats. Considering the key role of SNA(SG) in salivary secretion, this mechanism, which differs from the classic cardiac baroreflex feedback loop, strongly suggests that baroreceptor signalling plays a decisive role in the regulation of salivary gland function.

Downbeat nystagmus due to a paramedian medullary lesion.

Cell groups of the paramedian tract, which are located in the paramedian region of the lower brainstem, are eye-movement-related neurons that project to the cerebellar flocculus. Their inactivation produces downbeat nystagmus, which resembles eye movement disorders resulting from lesions of the cerebellar flocculus in animal experiments. Therefore, paramedian tract cells are assumed to fulfill an important function in ocular movement control, such as gaze-holding and maintaining vestibular balance. This paper presents a 50-year-old female who manifested downbeat nystagmus due to damage to the paramedian tract cells caused by a localized ischemic lesion in the medulla oblongata. We found that a paramedian medullary lesion-induced nystagmus, similar to that observed following floccular lesions, clearly indicates that a subgroup of paramedian tract cells projecting to the flocculus was impaired. This finding has important implications in considering a brainstem-cerebellar feedback loop involved in vestibulo-oculomotor controls, such as vestibular balance. Although there have been a few reports of downbeat nystagmus caused by lesions in the midline region of the lower brainstem, to our knowledge none report the occurrence of nystagmus due to a strictly localized medullar lesion, such as the one described here.

Recurrent isolated abducens nerve paresis associated with persistent trigeminal artery variant.

We report a 74-year-old woman who presented with recurrent isolated abducens nerve paresis. Cranial magnetic resonance imaging revealed that the right abducens nerve was sandwiched between the right internal carotid artery and a persistent trigeminal artery (PTA) variant, which might have led to neurovascular compression of the abducens nerve, resulting in abducens nerve damage. Normal variants of PTA, which are cerebellar arteries originating from a precavernous portion of the internal carotid artery, must be carefully observed as such variants can potentially cause a neurovascular compression of the abducens nerve.

A µ-opioid receptor agonist DAMGO induces rapid breathing in the arterially perfused in situ preparation of rat.

Ventilatory responses to opioids are complex and not yet fully understood. We evaluated concentration-dependent effects of a selective µ-opioid receptor agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) on respiratory output in the arterially perfused in situ rat preparation, which preserves the integrity of the ponto-medullary respiratory network. DAMGO (300-3400 nM) was added accumulatively to the perfusate. DAMGO increased inspiratory time and diminished central vagal post-inspiratory activity. At 300-500 nM DAMGO caused rapid breathing with shortening of expiratory time. The change of breathing pattern occurred within a single breath. Bilateral vagotomy did not affect the change in respiratory pattern, suggesting that it was of central origin. Additional DAMGO up to 1800 nM did not affect the rapid breathing pattern, and further elevated concentrations (up to 3400 nM) caused inconsistent results. Since the rapid breathing pattern was associated with the obliteration of vagal post-inspiratory activity, we conclude that DAMGO reconfigures the respiratory output to an inspiratory phase-dominant, rapidly alternating inspiration-expiration pattern.