Awake Subjects Research Articles

OP 9. Tailoring transcranial current stimulation to modulate cortical oscillations in computer simulations, ferrets, and humans

Non-invasive transcranial current stimulation (TCS) has emerged as a highly promising therapeutic neuromodulation approach for a broad range of neurological and psychiatric illnesses. The majority of TCS studies are performed with direct current (tDCs). TCS with sine-wave stimulation waveforms (tACs) represents an attractive alternative to enhance cortical oscillations ( Marshall et al., 2006 ). Yet, little is known about how weak sine-wave electric fields interact with endogenous network dynamics ( Frohlich and McCormick, 2010 ). We hypothesize that more closely matching stimulation waveforms to ongoing cortical activity may increase and focus the effect of electrical stimulation. We utilized a multi-technique approach to determine the interaction dynamics between endogenous cortical activity and applied sine-wave stimulation (large-scale computer simulations, in vivo ferret animal model, awake healthy human subjects). Large-scale computer simulations of cortical networks (Izhikevich neuron models, pyramidal cells and fast-spiking inhibitory interneurons) were performed with CUDA-enhanced, custom-written C. TCS was modeled by injection of electric current equal to that caused by application of the electric field. tACs of varying frequencies and tDCs were used to elucidate the underlying mechanisms of how global weak pertubations of the membrane voltage interact with macroscopic network dynamics. An animal model with a gyrencephalic cortex was used to test tACs in vivo. Isoflurane/xylazine anesthesia induced slow endogenous cortical oscillations. tACs matching intrinsic activity from 0.5 to 2 Hz (in steps of 0.5 Hz) was applied transcranially. Extracellular electrophysiology was conducted to measure modulation of oscillation structure. A pre-clinical study in awake, healthy human subjects assessed differential effects of 0.75 and 40 Hz tACs stimulation. Subjects ( N = 16) were fitted with EEG electrodes and two 5 × 7 cm TCS electrodes. Each subject underwent three sessions of tACs (active sham, 0.75 Hz: frequency did not match intrinsic activity of awake subjects, 40 Hz: frequency matched ongoing activity). Five blocks of stimulation (five minutes each) were interleaved with 1-min blocks of EEG recordings. The computational simulations revealed tACs frequencies matched to intrinsic oscillations require the lowest amplitude to achieve entrainment. tACs was more effective than tDCs at entrainment. Entrainment of endogenous oscillations by weak global perturbations is mediated by a non-linear threshold effect where stimulation increases the number of “hotspots” that give rise to network-wide activation patterns. tACs in ferrets increased oscillation power at the frequency matched to stimulation. In our human study, we found that 40 Hz tACs targets ongoing oscillations in the awake cortex more specifically than 0.75 Hz tACs. 40 Hz tACs amplifies the balance between resting and activated states mediated by alpha and gamma oscillations (ratio of normalized stimulation effect to sham: alpha = .944, p = .027; gamma = 1.08, p = .022). Our computer simulations, in vivo animal experiments, and pre-clinical human trial demonstrate that tACs has the potential to become an effective modulator of cortical oscillations, particularly when stimulation frequency is matched to ongoing endogenous cortical oscillations.

IS 24. TMS-EEG to measure excitability and connectivity of human cortical circuits

Recording TMS-evoked EEG potentials (TEPs) that are unconfounded by electromagnetic and biological artifacts is challenging. Neuronavigation guided TMS-EEG recordings performed in brain-injured patients showed that, when confounding factors are appropriately controlled, TMS elicits a significant EEG response only when is targeted on a scalp region overlying healthy cortical tissue and no response when is targeted over scalp areas overlying dead portions of the cortex while, in healthy subjects TEPs are highly reproducible over time. Thus, TEPs seem to purely reflect the response of the cortical tissue to TMS and can be used reliably to detect changes of excitability and connectivity in physiological, psychiatric and neurological conditions. For instance, in awake healthy subjects each cortical region tends to generate TEPs characterized by a dominant frequency (natural frequency) either when directly or indirectly activated by a TMS pulse. On the contrary, the tendency of different cortical circuits to oscillate at a specific frequency when directly perturbed is lost in schizophrenic patients. Most importantly, TMS-EEG measurements showed that effective connectivity is reduced during loss of consciousness in deep sleep, anesthesia and vegetative state. Interestingly, effective cortical connectivity recovered during REM sleep, and in acute brain injured patients before they could reestablish a functional communication with the environment. Overall, these observations suggest that TMS-EEG may represent a valuable tool to probe directly and non-invasively cortical excitability and connectivity in humans.

Reconstructing integration in sympathetic ganglia

This talk introduces a symposium focusing on current efforts to dissect nicotinic and non-nicotinic mechanisms of synaptic transmission in autonomic ganglia and their integrative consequences in health and disease. We begin with a general theory of ganglionic integration. This framework incorporates basic features of synaptic divergence and convergence that operate in most paravertebral sympathetic ganglia. The goal is to reconstruct the integrative properties of ganglia from the bottom up beginning with nicotinic synapses and using information from the literature describing natural patterns of activity observed in experimental animals and in awake human subjects.

A method for conducting functional MRI studies in alert nonhuman primates: Initial results with opioid agonists in male cynomolgus monkeys.

Functional MRI (fMRI) has emerged as a powerful technique for assessing neural effects of psychoactive drugs and other stimuli. Several experimental approaches have been developed to use fMRI in anesthetized and awake animal subjects, each of which has its advantages and complexities. We sought to assess whether one particular method to scan alert postanesthetized animals can be used to assess fMRI effects of opioid agonists. To date, the use of fMRI as a method to compare pharmacological effects of opioid drugs has been limited. Such studies are important because mu and kappa opioid receptor agonists produce distinct profiles of behavioral effects related both to clinically desirable endpoints (e.g., analgesia) and to undesirable effects (e.g., abuse potential). This study sought to determine whether we could use our fMRI approach to compare acute effects of behaviorally equipotent (3.2 μg/kg) intravenous doses of fentanyl and U69,593 (doses that do not affect cardiorespiratory parameters). Scans were acquired in alert male cynomolgus macaques acclimated to undergo fMRI scans under restraint, absent excessive stress hormone increases. These opioid agonists activated bilateral striatal and nucleus accumbens regions of interest. At the dose tested, U69,593 induced greater left nucleus accumbens BOLD activation than fentanyl, while fentanyl activated left dorsal caudate nucleus more than U69,593. Our results suggest that our fMRI approach could be informative for comparing effects of opioid agonists.

Modulation of muscle sympathetic nerve activity by low-frequency physiological activation of the vestibular utricle in awake humans

We recently showed that selective stimulation of one set of otolithic organs-those located in the utricle, sensitive to displacement in the horizontal axis-causes a marked entrainment of skin sympathetic nerve activity (SSNA). Here, we assessed whether muscle sympathetic nerve activity (MSNA) is similarly modulated. MSNA was recorded via tungsten microelectrodes inserted into cutaneous fascicles of the common peroneal nerve in 12 awake subjects, seated (head vertical, eyes closed) on a motorised platform. Slow sinusoidal accelerations-decelerations (±4 mG) were applied in the X (antero-posterior) or Y (medio-lateral) direction at 0.08 Hz. Cross-correlation analysis revealed partial entrainment of MSNA: vestibular modulation was 32 ± 3 % for displacements in the X-axis and 29 ± 3 % in the Y-axis; these were significantly smaller than those evoked in SSNA (97 ± 3 and 91 ± 5 %, respectively). For each sinusoidal cycle, there were two peaks of modulation-one associated with acceleration as the platform moved forward or to the side and one associated with acceleration in the opposite direction. We believe the two peaks reflect inertial displacement of the stereocilia within the utricle during sinusoidal acceleration, which evokes vestibulosympathetic reflexes that are expressed as vestibular modulation of MSNA as well as of SSNA. The smaller vestibular modulation of MSNA can be explained by the dominant modulation of MSNA by the arterial baroreceptors.

Polyvinylidene fluoride film based nasal sensor to monitor human respiration pattern: An initial clinical study

Design and development of a piezoelectric polyvinylidene fluoride (PVDF) thin film based nasal sensor to monitor human respiration pattern (RP) from each nostril simultaneously is presented in this paper. Thin film based PVDF nasal sensor is designed in a cantilever beam configuration. Two cantilevers are mounted on a spectacle frame in such a way that the air flow from each nostril impinges on this sensor causing bending of the cantilever beams. Voltage signal produced due to air flow induced dynamic piezoelectric effect produce a respective RP. A group of 23 healthy awake human subjects are studied. The RP in terms of respiratory rate (RR) and Respiratory air-flow changes/alterations obtained from the developed PVDF nasal sensor are compared with RP obtained from respiratory inductance plethysmograph (RIP) device. The mean RR of the developed nasal sensor (19.65±4.1) and the RIP (19.57±4.1) are found to be almost same (difference not significant, p>0.05) with the correlation coefficient 0.96, p<0.0001. It was observed that any change/alterations in the pattern of RIP is followed by same amount of change/alterations in the pattern of PVDF nasal sensor with k=0.815 indicating strong agreement between the PVDF nasal sensor and RIP respiratory air-flow pattern. The developed sensor is simple in design, non-invasive, patient friendly and hence shows promising routine clinical usage. The preliminary result shows that this new method can have various applications in respiratory monitoring and diagnosis.

Cardiorespiratory coupling of sympathetic outflow in humans: a comparison of respiratory and cardiac modulation of sympathetic nerve activity to skin and muscle

What is the central question of this study?Muscle sympathetic nerve activity (MSNA) is well known to be modulated by the arterial baroreceptors and respiration, but what are the magnitudes of cardiac and respiratory modulation of skin sympathetic nerve activity (SSNA), which primarily subserves thermoregulation?What is the main finding and what is its importance?Using direct microelectrode recordings of MSNA and SSNA in awake humans, we show that the magnitude of respiratory modulation of SSNA is identical to that of MSNA, the primary difference between the two sources of sympathetic outflow being the greater cardiac modulation of MSNA. This emphasises the role of the baroreceptors in entraining sympathetic outflow to muscle. It is well known that microelectrode recordings of skin sympathetic nerve activity (SSNA) in awake human subjects reveal spontaneous bursts of activity with no overt modulation by changes in blood pressure or respiration, in contrast to the clear cardiac and respiratory modulation of muscle sympathetic nerve activity (MSNA). However, cross-correlation analysis has revealed that, like individual muscle vasoconstrictor neurones, the firing of individual cutaneous vasoconstrictor neurones is temporally coupled to both the cardiac and respiratory rhythms during cold-induced cutaneous vasoconstriction, and the same is true of single sudomotor neurones during heat-induced sweating. Here, we used cross-correlation analysis to determine whether SSNA exhibits cardiac and respiratory modulation in thermoneutral conditions and to compare respiratory and cardiac modulation of SSNA with that of MSNA. Oligounitary recordings of spontaneous SSNA (n = 20) and MSNA (n = 18) were obtained during quiet, unrestrained breathing. Respiration was recorded by a strain-gauge transducer around the chest and ECG recorded by surface electrodes. Respiratory and cardiac modulation of SSNA and MSNA were quantified by fitting polynomial equations to the cross-correlation histograms constructed between the sympathetic spikes and respiration or ECG. The amplitude of the respiratory modulation (52.5 ± 3.4%) of SSNA was not significantly different from the amplitude of the cardiac modulation (46.6 ± 3.2%). Both were comparable to the respiratory modulation of MSNA (47.7 ± 4.2%), while cardiac modulation of MSNA was significantly higher (89.8 ± 1.5%). We conclude that SSNA and MSNA share similar levels of respiratory modulation, the primary difference between the two sources of sympathetic outflow being the marked cardiac modulation of MSNA provided by the baroreceptors.

The vestibular system does not modulate fusimotor drive to muscle spindles in contracting leg muscles of seated subjects

We previously showed that sinusoidal galvanic vestibular stimulation (GVS) does not modulate the firing of spontaneously active muscle spindles in relaxed human leg muscles. However, given that there is little, if any, fusimotor drive to relaxed human muscles, we tested the hypothesis that vestibular modulation of muscle spindles becomes apparent during volitional contractions at levels that engage the fusimotor system. Unitary recordings were made from 28 muscle spindle afferents via tungsten microelectrodes inserted percutaneously into the common peroneal nerve of seated awake human subjects. Twenty-one of the spindle afferents were spontaneously active at rest and each increased its firing rate during a weak static contraction; seven were silent at rest and were recruited during the contraction. Sinusoidal bipolar binaural galvanic vestibular stimulation (±2mA, 100 cycles) was applied to the mastoid processes at 0.8Hz. This continuous stimulation produced a sustained illusion of "rocking in a boat" or "swinging in a hammock" but no entrainment of EMG. Despite these robust vestibular illusions, none of the fusimotor-driven muscle spindles exhibited phase-locked modulation of firing during sinusoidal GVS. We conclude that this dynamic vestibular input was not sufficient to modulate the firing of fusimotor neurones recruited during a voluntary steady-state contraction, arguing against a significant role of the vestibular system in adjusting the sensitivity of muscle spindles via fusimotor neurones.

Effects of Different Sleeping Postures on Intraocular Pressure and Ocular Perfusion Pressure in Healthy Young Subjects

To investigate the effects of different sleeping positions of head and body on intraocular pressure (IOP) and ocular perfusion pressure (OPP) in healthy, young subjects. Prospective, comparative case series. Twenty healthy young Korean subjects. We measured IOP and blood pressure (BP) with the subjects seated and recumbent, including supine, right lateral decubitus, left lateral decubitus, prone with right head turn, and prone with left head turn positions. We measured IOP using an Icare tonometer in both eyes 5 minutes after assuming each position in a randomized sequence. We calculated the OPP using the formulas based on the mean BP adjusted for the height of the eye over the heart. The eye on the lower side in the lateral decubitus or prone with head turn position was termed the dependent eye. Difference in IOP and OPP of the dependent and nondependent eyes during changes of sleeping positions of body and head. Mean IOP of right and left eyes while sitting was significantly lower than that measured in each recumbent position (all P<0.001). The OPPs in both eyes were significantly higher in all recumbent positions than in a sitting position (all P<0.001). Mean IOP of the dependent eyes was higher than that of the nondependent eyes in the lateral decubitus positions and in the prone positions with head turns (all P<0.001). No significant intereye difference in OPP was found for any position. Among IOPs measured in the recumbent positions, mean IOP of the dependent eye in the lateral decubitus position or in the prone position with head turn was significantly higher than that of the ipsilateral eye in the supine position (all P<0.0001). All sleeping positions of head and body resulted in an elevation of IOP and an increase in the calculated OPP compared with the sitting position in healthy, young subjects. The postural change from supine to lateral decubitus or prone with head turn position increased the IOP of the dependent eyes without significant alteration in OPP in healthy awake subjects. Further research is needed under nocturnal conditions in a sleep laboratory. The authors have no proprietary or commercial interest in any of the materials discussed in this article.

Awake Intubation with Video Laryngoscope and Fiberoptic Bronchoscope in Difficult Airway Patients

In a randomized clinical trial, Rosenstock et al.1 showed no significant difference in time to awake intubation by experienced investigators using the McGrath video laryngoscope (MVL) compared with the fiberoptic bronchoscope (FOB) in difficult airway patients. Accordingly, the authors conclude that awake MVL intubation seems to be a potential alternative to awake fiberoptic intubation. However, an important issue ignored by them is that awake intubation actually includes two parts: airway topical anesthesia and subsequent intubation.2 Moreover, effective airway topical anesthesia is a prerequisite to successfully perform awake intubation.3 When adequate airway topical anesthesia is obtained, subsequent intubation is usually easy. To obtain a uniform airway topical anesthesia in the two groups, transtracheal injection of lidocaine was used in this study. This method is invasive and carries more potential risk than other topical anesthesia methods do. More importantly, it can be difficult or even impossible to perform if the patient’s neck anatomy is troublesome to locate.4 In this study, a total of seven patients were excluded because transtracheal injection was impossible.In our view, a limitation of this study design is lack of assessment on the performance of airway topical anesthesia provided by the two devices. As a “gold standard” tool in managing difficult airway, FOB is not only a common choice for awake intubation, but can also provide flexibility in selectively anesthetizing the airway by a “spray as you go” technique.5 That is, two parts of the awake intubation can be completed with an FOB. In the Discussion section, the authors claim, “Awake MVL intubation may not prove as easy in using the ‘spray as you go’ technique, because insertion of the MVL blade causes pressure on the tongue and on the laryngeal structures, thereby probably creating a greater degree of patient discomfort compared with introducing the FOB.” It would be interesting to know whether there is any evidence to support the above comments. Had the authors performed airway topical anesthesia with the MVL?The MVL has an anatomically shaped blade with an extra curve, and oropharyngeal tissues do not need to be retracted and compressed to achieve a straight line of sight during laryngoscopy with the MVL.6 Thus, there is usually no need for significant lifting force to visualize the glottis. It has been shown that the use of Glidescope video laryngoscope with an anatomically shaped blade creates less pressure on the tongue when compared with the Macintosh blade.7,8 After topical anesthesia of the tongue and pharynx with lidocaine spray, patients can well tolerate the MVL with minimal discomfort.9 In our experience, once the oropharyngeal mucosa is anesthetized by the method described in this study, the MVL can be advanced easily to a position in the hypopharynx where the epiglottis and larynx can be clearly visualized. At this point, aliquots of lidocaine can be sprayed using a MADgic® atomizer (Wolfe Tory Medical Inc., Salt Lake City, UT). The MADgic® atomizer is then advanced through the glottis into the larynx and trachea to spray further aliquots of lidocaine in the remaining airway. This modified spray-as-you-go technique with the video laryngoscope can provide excellent airway topical anesthesia and is less affected by secretions or blood compared with fibreoptic technique. It has been used successfully in difficult airway patients who undergo awake intubation with Glidescope video laryngoscope.10 All of these suggest that performing airway topical anesthesia under superior vision of the airway with a video laryngoscope on awake subjects is feasible. Unfortunately, there has been no randomized clinical study comparing video laryngoscopic and fiberoptic techniques of airway topical anesthesia. Before we have enough evidence to make a conclusion that the video laryngoscope is a useful alternative to the FOB for awake intubation, therefore, further studies are needed to evaluate and compare performances of both airway topical anesthesia and awake intubation in difficult airway patients. In such a study, other than the intubation time and success rate, the observed variables should also include the patient’s comfort during airway topical anesthesia and awake intubation, time required for airway topical anesthesia, awake intubating condition, possible difficulties and so forth.2,5

The effect of inhaled menthol on upper airway resistance in humans: A randomized controlled crossover study

Menthol (l-menthol) is a naturally-occurring cold receptor agonist commonly used to provide symptomatic relief for upper airway congestion. Menthol can also reduce the sensation of dyspnea. It is unclear whether the physiological action of menthol in dyspnea reduction is through its cold receptor agonist effect or whether associated mechanical changes occur in the upper airway. To determine whether menthol inhalation alters upper airway resistance in humans. A randomized, sham-controlled, single-blinded crossover study of inhaled menthol on upper airway resistance during semirecumbent quiet breathing in healthy subjects was conducted. Ten healthy participants (eight female) with a mean (± SD) age of 21±1.6 years completed the study. Nasal resistance before testing was similar on both occasions. No differences were found in respiratory frequency (mean ± SEM) (menthol 17.0±1.1 cmH2O⁄L⁄s; sham 16.9±0.9 cmH2O⁄L⁄s), minute ventilation (menthol 7.7±0.5 cmH2O⁄L⁄s; sham 7.9±0.5 cmH2O⁄L⁄s) or total inspiratory time⁄total breath time (menthol 0.4±0.1 cmH2O⁄L⁄s; sham 0.4±0.1 cmH2O⁄L⁄s). The upper airway resistance was similar during menthol (3.47±0.32 cmH2O⁄L⁄s) and sham (3.27±0.28 cmH2O⁄L⁄s) (P=0.33) inhalation. Inhalation of menthol does not alter upper airway resistance in awake human subjects.

Sleep to implement an intention.

Sleep supports the consolidation of new memories. However, this effect has mainly been shown for memories of past events. Here we investigated the role of sleep for the implementation of intentions for the future. Subjects were instructed on a plan that had to be executed after a delay of 2 days. After plan instruction, subjects were either allowed to sleep or stayed awake for one night (Exp. 1) or had a 3-h sleep period either during the early night (SWS-rich sleep) or late night (REM-rich sleep; Exp. 2). In both experiments, retesting took place 2 days later after one recovery night. Sleep laboratory. A total of 56 healthy young adults participated in the study. N/A. All of the subjects who were allowed to sleep after plan instruction executed the intention 2 days later, whereas only 61% of wake subjects did so (P = 0.004; Exp. 1). Also after early SWS-rich sleep all of the subjects remembered to execute the intention, but only 55% did so after late REM-rich sleep (P = 0.015; Exp. 2). Sleep, especially SWS, plays an important role for the successful implementation of delayed intentions.

Real-time imaging of cortical and subcortical control of muscle sympathetic nerve activity in awake human subjects

Blood pressure is controlled on a beat-to-beat basis through fluctuations in heart rate and the degree of sympathetically-mediated vasoconstriction in skeletal muscles. By recording muscle sympathetic nerve activity (MSNA) at the same time as performing functional magnetic resonance imaging (fMRI) of the brain, we aimed to identify cortical structures involved in central cardiovascular control in awake human subjects. Spontaneous bursts of MSNA were recorded via a tungsten microelectrode inserted percutaneously into the peroneal nerve of 14 healthy subjects in a 3T MRI scanner. Blood Oxygen Level Dependent (BOLD) contrast - gradient echo, echo-planar - images were continuously collected in a 4s ON, 4s OFF sampling protocol. MSNA burst amplitudes were measured during the OFF periods and BOLD signal intensity was measured during the subsequent 4s period to allow for neurovascular coupling and nerve conduction delays. Group analysis demonstrated regions showing fluctuations in BOLD signal intensity that covaried with the intensity of the concurrently recorded bursts of MSNA. Signal intensity and MSNA were positively correlated in the left mid-insula, bilateral dorsolateral prefrontal cortex, bilateral posterior cingulate cortex and bilateral precuneus. In addition, MSNA covaried with signal intensity in the left dorsomedial hypothalamus and bilateral ventromedial hypothalamus (VMH). Construction of a functional connectivity map revealed coupling between activity in VMH and the insula, dorsolateral prefrontal cortex, precuneus, and in the region of the left and right rostroventrolateral medulla (RVLM). This suggests that activity within suprabulbar regions may regulate resting MSNA by projections to the premotor sympathetic neurons in the rostroventrolateral medulla.

Elevated gamma band power in humans receiving naloxone suggests dorsal periaqueductal and periventricular gray deep brain stimulation produced analgesia is opioid mediated

BackgroundThe midbrain periaqueductal and periventricular gray (PAVG) region is important for pain and autonomic modulation. We have previously described changes in blood pressure dependent upon dorsal or ventral electrode positioning with PAVG deep brain stimulation (DBS), yet controversy exists about whether DBS acts via endogenous opioid release. Method and resultsWe combined local field potential (LFP) recording from PAVG DBS electrodes in humans with naloxone and saline infusions to determine whether dorsal and ventral PAVG DBS act through opioidergic or other mechanisms. Four awake human subjects were investigated. DBS were implanted contralateral to the painful body part. Electrode contact positions were mapped using MRI and brain atlas information. Naloxone then saline were randomly administered to the blinded subjects and pain rated using a numeric pain rating scale at 30s intervals for 3min. Two subjects received dorsal DBS electrodes and two had ventral placements. Significantly elevated gamma frequency band (30–90Hz) power concomitant with pain exacerbation was found with naloxone versus both saline and rest in dorsal not ventral PAVG LFPs (p<0.005). Significantly elevated delta frequency band (0–4Hz) power (p=0.001) was seen in one ventral PAVG subject with both naloxone and saline infusions. ConclusionsDorsal PAVG DBS may reduce pain by augmenting endogenous opioid release. Elevated gamma oscillations enhance awareness of worsening pain with opioid blockade. Ventral PAVG DBS may act by separate possibly autonomic mechanisms. Increased delta oscillations indicate a survival rhythm involved in the initiation of passive coping responses to homeostatic changes.

Inhibition of augmented muscle vasoconstrictor drive following asphyxic apnoea in awake human subjects is not affected by relief of chemical drive

Progressive asphyxia, produced by a prolonged voluntary breath hold (end-expiratory apnoea), evokes large bursts of muscle sympathetic nerve activity (MSNA). These bursts increase in amplitude until the asphyxic break point is reached, at which point the bursts are inhibited. We tested the hypothesis that lung inflation, rather than relief from hypoxia and hypercapnia, is responsible for the inhibition of MSNA. Multiunit MSNA was recorded from motor fascicles of the common peroneal nerve in 11 subjects. Following a period of quiet breathing, subjects were instructed to behave as follows: (i) to hold their breath in expiration for as long as they could (mean duration 32.3 ± 1.9 s); (ii) to take a single breath of room air, 100% N(2) or 10% CO(2) + 90% N(2) at the asphyxic break point; (iii) to exhale and continue the apnoea until the next break point; and then (iv) to resume breathing. All subjects reported relief during inhalation of any gas, and could continue holding their breath for a further 30.7 ± 2.8 s with room air, 18.6 ± 1.7 s with N(2) and 11.8 ± 1.8 s with 10% CO(2) + 90% N(2). Despite the exaggerated chemoreceptor drive in the latter two conditions (hence the significantly shorter latencies to the subsequent asphyxic break point), the inhibition still occurred; moreover, there was no significant difference in duration of the inhibition of MSNA following the single breath of room air (7.6 ± 0.7 s), N(2) (6.2 ± 0.6 s) or 10% CO(2) + 90% N(2) (5.5 ± 0.4 s). Following the resumption of breathing, however, the duration of MSNA inhibition (11.0 ± 1.0 s) was significantly longer than that following a single breath. To investigate the involvement of chemoreceptors in the respiratory modulation of MSNA further, the same gases were used during an inspiratory-capacity apnoea, which causes a brief inhibition of MSNA during the inflation phase and a sustained increase during the hold phase. The duration of the apnoea was shortest after a breath of 10% O(2) + 90% N(2), but the latency until the bursts resumed after the inspiratory breath hold were similar for all gases, which suggests that there is no chemoreceptor involvement during the sympathetic silence seen during the inflation phase of inspiratory-capacity apnoeas. We conclude that neither peripheral nor central chemoreceptors are responsible for the inhibition of muscle vasoconstrictor drive following an end-expiratory apnoea or an end-inspiratory apnoea. Rather, we suggest that the inhibition is evoked by stretch receptors in the lungs and/or chest wall, which may also contribute to the longer inhibition associated with the hyperventilation following the subsequent resumption of rhythmic breathing.

Identification of Sites of Sympathetic Outflow During Concurrent Recordings of Sympathetic Nerve Activity and fMRI

The sympathetic division of the nervous system is critical for maintaining both resting arterial pressure and for producing changes in regional perfusion required by behavioral state changes. A primary determinant of arterial pressure is the level of vasoconstriction within skeletal muscle. It is well established that there is a tight relationship between dynamic changes in arterial pressure and muscle sympathetic nerve activity (MSNA) through the workings of the baroreflex. While the central circuitry underlying the baroreflex has been extensively investigated in anesthetized experimental animals, few studies have investigated the central circuitry responsible for the baroreflex in awake human subjects. Recently we were the first to record concurrently MSNA (using microneurography) and brain activity (using functional magnetic resonance imaging) in awake humans in a series of experiments designed to determine the central circuitry underlying the baroreflex in humans. We confirmed that the baroreflex involves activity changes within the nucleus tractus solitarius, caudal ventrolateral, and rostral ventrolateral medulla. Because conditions such as essential hypertension, obesity, and obstructive sleep apnea are all characterized by significant increases in resting MSNA, it is important to understand both brainstem and cortical sites involved in regulating resting levels of MSNA. Future investigations which define cortical sites involved in generating and modulating MSNA are important if we are to understand the underlying mechanisms of many conditions characterized by hypertension.

The Serotonin 2C Receptor Antagonist SB 242084 Exhibits Abuse-Related Effects Typical of Stimulants in Squirrel Monkeys

Antagonists of the serotonin (5-hydroxytryptamine; 5-HT) type 2C receptor (5-HT(2C)R) are being considered as potential pharmacotherapeutics for various affective disorders, but evidence suggests that these compounds enhance the effects of cocaine and related psychostimulants in rodents. However, the effects of selective 5-HT(2C)R antagonists have not been evaluated in nonhuman primates. The present studies used operant-behavioral and in vivo microdialysis techniques to assess the impact of 5-HT(2C)R antagonism on the behavioral and neurochemical effects of cocaine in squirrel monkeys. In subjects trained to lever-press on a fixed-interval schedule of stimulus termination, pretreatment with the highly selective 5-HT(2C)R antagonist 6-chloro-2,3-dihydro-5-methyl-N-[6-[(2-methyl-3-pyridinyl)oxy]-3-pyridinyl]-1H-indole-1-carboxyamide dihydrochloride (SB 242084) (vehicle, 0.01-0.1 mg/kg) produced behavioral-stimulant effects alone and interacted with cocaine in an apparently additive manner. In monkeys trained to self-administer intravenous cocaine according to a second-order schedule of drug delivery, SB 242084 (vehicle, 0.03-0.1 mg/kg) modulated cocaine-induced reinstatement of previously extinguished responding and maintained self-administration behavior when substituted for cocaine availability. These studies are the first to assess the direct reinforcing effects of a 5-HT(2C)R-selective antagonist in any species. Finally, in vivo microdialysis studies revealed that pretreatment with SB 242084 (0.1 mg/kg) modulated cocaine-induced dopamine increases within the nucleus accumbens, but not the caudate nucleus, of awake subjects. Taken together, the results suggest that SB 242084 exhibits a behavioral profile that is qualitatively similar to other psychostimulants, although its efficacy is modest compared with cocaine. The observed interactions with cocaine and the substitution for cocaine self-administration may be indicative of some degree of abuse potential in humans.

Low-frequency physiological activation of the vestibular utricle causes biphasic modulation of skin sympathetic nerve activity in humans

We have previously shown that sinusoidal galvanic vestibular stimulation, a means of selectively modulating vestibular afferent activity, can cause partial entrainment of sympathetic outflow to muscle and skin in human subjects. However, it influences the firing of afferents from the entire vestibular apparatus, including the semicircular canals. Here, we tested the hypothesis that selective stimulation of one set of otolithic organs-those located in the utricle, which are sensitive to displacement in the horizontal axis-could entrain sympathetic nerve activity. Skin sympathetic nerve activity (SSNA) was recorded via tungsten microelectrodes inserted into cutaneous fascicles of the common peroneal nerve in 10 awake subjects, seated (head vertical, eyes closed) on a motorised platform. Slow sinusoidal accelerations-decelerations (~4 mG) were applied in the X (antero-posterior) or Y (medio-lateral) direction at 0.08 Hz; composite movements in both directions were also applied. Subjects either reported feeling a vague sense of movement (with no sense of direction) or no movement at all. Nevertheless, cross-correlation analysis revealed a marked entrainment of SSNA for all types of movements: vestibular modulation was 97 ± 3 % for movements in the X axis and 91 ± 5 % for displacements in the Y axis. For each sinusoidal cycle, there were two major peaks of modulation-one associated with acceleration as the platform moved forward or to the side, and one associated with acceleration in the opposite direction. We interpret these observations as reflecting inertial displacement of the stereocilia within the utricle during acceleration, which causes a robust vestibulosympathetic reflex.

Surface Laplacian of Central Scalp Electrical Signals is Insensitive to Muscle Contamination

The objective of this paper was to investigate the effects of surface Laplacian processing on gross and persistent electromyographic (EMG) contamination of electroencephalographic (EEG) signals in electrical scalp recordings. We made scalp recordings during passive and active tasks, on awake subjects in the absence and in the presence of complete neuromuscular blockade. Three scalp surface Laplacian estimators were compared to left ear and common average reference (CAR). Contamination was quantified by comparing power after paralysis (brain signal, B) with power before paralysis (brain plus muscle signal, B+M). Brain:Muscle (B:M) ratios for the methods were calculated using B and differences in power after paralysis to represent muscle (M). There were very small power differences after paralysis up to 600Hz using surface Laplacian transforms (B:M > 6 above 30Hz in central scalp leads). Scalp surface Laplacian transforms reduce muscle power in central and pericentral leads to less than one sixth of the brain signal, two to three times better signal detection than CAR. Scalp surface Laplacian transformations provide robust estimates for detecting high-frequency (gamma) activity, for assessing electrophysiological correlates of disease, and also for providing a measure of brain electrical activity for use as a standard in the development of brain/muscle signal separation methods.

Intracortical modulation of somatosensory evoked fields during movement: Evidence for selective suppression of postsynaptic inhibition

As accurate finger movements depend on guidance by afferent sensory feedback information, it is of interest to examine how the cortical processing of afferent signals is altered during movement states compared with rest. In the present study we evaluated afferent input to the primary somatosensory cortex (SI) in human subjects performing a finger opposition task. We recorded somatosensory evoked magnetic fields (SEFs) in 6 healthy subjects to stimulation of left and right median nerves in a resting condition and during active right-sided finger movements. At the left SI, the SEFs to right (moving hand) median nerve stimulation showed a selective and robust reduction of the P35m deflection during movement compared with rest, while there were only minor non-significant changes in the other SEF deflections, including N20m, which represents the 1st excitatory cortical event after stimulation. In contrast, at the right SI the SEFs to left (non-moving hand) median nerve stimulation were modified in the opposite direction: the P35m deflection was slightly enhanced during right-sided movement, there being no significant changes in the other deflections. The results thus show that the P35m SEF deflection can be selectively reduced during finger movements of the stimulated hand, and selectively enhanced if the movement is being performed with the fingers of the opposite hand. Because N20m was not changed, the modulation took place at the cortical level rather than in the afferent pathways. As the P35m SEF deflection likely represents postsynaptic IPSPs at SI, the results suggest that postsynaptic inhibition to somatosensory impulses from the moving part of the body is suppressed. Comparison of the present results with recent intracellular studies in behaving mice suggests that the P35m reduction specifically corresponds to a reduction in the activity of parvalbumin-containing fast-spiking inhibitory interneurons during movement. The results provide evidence that precision movements can be executed without this type of cortical postsynaptic inhibition.