Illuminating the locus coeruleus: control of posture and arousal

Resistance exercising on unstable surface leads to Pupil Dilation

BACKGROUND

Chronic resistance training and acute resistance exercises improve physical performance and can enhance cognitive performance. However, there is still uncertainty about the mechanism(s) responsible for cognitive improvement following resistance training and exercise. Recent findings suggest that resistance exercise has metabolic as well as cognitive demands, which potentially activate similar neural circuitry associated with higher-order cognitive function tasks. Exercising on unstable devices increases the coordinative and metabolic demands and thus may further increase cognitive activation during resistance exercise. The measurement of pupil diameter could provide indications of cognitive activation and arousal during resistance exercise. Pupil dilation is linked to the activity in multiple neuromodulatory systems (e.g., activation of the locus coeruleus and the release of the neurotransmitter norepinephrine (LC-NE system)), which are involved in supporting processes for executive control. Therefore, the purpose of this study was to compare the cognitive activation measured by pupil diameter during an acute bout of resistance exercise on stable and unstable surfaces.

METHODS

18 participants (23.5 ± 1.5 years; 10 females) performed ten kettlebell squats in a preferred repetition velocity in stable and unstable (BOSU® Balance Trainer) ground conditions. Pupil diameter was recorded with eye tracking glasses (SMI ETG) during standing (baseline) and during squatting. Raw pupil data were cleaned of artifacts (missing values were linearly interpolated) and subjected to a subtractive baseline correction. A student t-test was used to compare mean pupil diameter between ground conditions.

RESULTS

The mean pupil diameter was significantly greater during squats in the unstable condition than in the stable condition, t (17) = -2.63, p =.018, Cohen's dZ = -0.62; stable: 0.49 ± 0.32 mm; unstable: 0.61 ± 0.25 mm).

CONCLUSION

As indicated by pupil dilation, the use of unstable devices can increase the cognitive activation and effort during acute bouts of resistance exercise. Since pupil dilation is only an indirect method, further investigations are necessary to describe causes and effects of neuromodulatory system activity during resistance exercise. Resistance training with and without surface instability can be recommended to people of all ages as a physically and cognitively challenging training program contributing to the preservation of both physical and cognitive functioning.

Age Differences in Diffusivity in the Locus Coeruleus and its Ascending Noradrenergic Tract

The noradrenergic locus coeruleus (LC) is a small brainstem nucleus that promotes arousal and attention. Recent studies have examined the microstructural properties of the LC using diffusion-weighted magnetic resonance imaging and found unexpected age-related differences in fractional anisotropy - a measure of white matter integrity. Here, we used two datasets (Berlin Aging Study-II, N = 301, the Leipzig Study for Mind-Body-Emotion Interactions, N = 220), to replicate published findings and expand them by investigating diffusivity in the LC’s ascending noradrenergic bundle. In younger adults, LC fractional anisotropy was significantly lower, compared to older adults. However, in the LC’s ascending noradrenergic bundle, we observed significantly higher fractional anisotropy in younger adults, relative to older adults. These findings indicate that diffusivity in the LC versus the ascending noradrenergic bundle are both susceptible to structural changes in aging that have opposing effects on fractional anisotropy.

Influence of posture on blink reflex prepulse inhibition induced by somatosensory inputs from upper and lower limbs

BACKGROUND

Prepulse inhibition (PPI) is a neurophysiological phenomenon whereby a weak stimulus modulates the reflex response to a subsequent strong stimulus. Its physiological purpose is to avoid interruption of sensory processing by subsequent disturbing stimuli at the subcortical level, thereby preventing undesired motor reactions. An important hub in the PPI circuit is the pedunculopontine nucleus, which is also involved in the control of posture and sleep/wakefulness.

OBJECTIVE

To study the effect of posture (supine versus standing) on PPI, induced by somatosensory prepulses to either upper or lower limb. PPI was measured as the percentage inhibition of the blink reflex response to electrical supraorbital nerve (SON) stimulation.

METHODS

Sixteen healthy volunteers underwent bilateral blink reflex recordings following SON stimulation either alone (baseline) or preceded by an electrical prepulse to the median nerve (MN) or sural nerve (SN), both in supine and standing. Stimulus intensity was 8 times sensory threshold for SON, and 2 times sensory threshold for MN and SN, respectively. Eight stimuli were applied in each condition.

RESULTS

Baseline blink reflex parameters did not differ significantly between the two postures. Prepulse stimulation to MN and SN caused significant inhibition of R2. In supine but not in standing, R2 was significantly more inhibited by MN than by SN prepulses. In standing, SN stimulation caused significantly more inhibition of R2 than in supine, while the inhibition caused by MN prepulses did not differ significantly between postures.

SIGNIFICANCE

PPI induced by lower limb afferent input may contribute to postural control while standing.

Increased Postural Demand Is Associated With Greater Cognitive Workload in Healthy Young Adults: A Pupillometry Study

Introduction: Balance tasks require cognitive resources to ensure postural stability. Pupillometry has been used to quantify cognitive workload of various cognitive tasks, but has not been studied in postural control. The current investigation utilized pupillometry to quantify the cognitive workload of postural control in healthy young adults. We hypothesized that cognitive workload, indexed by pupil size, will increase with challenging postural control conditions including visual occlusion and cognitive dual tasking.

Methods: Twenty-one young healthy adults (mean ± standard error of the mean), (age = 23.2 ± 0.49 years; 12 females) were recruited for this study. Participants completed four tasks: (1) standing with eyes open; (2) standing with eyes occluded (3) standing with eyes open while performing an auditory Stroop task; and (4) standing with eyes occluded while performing an auditory Stroop task. Participants wore eye tracking glasses while standing on a force platform. The eye tracking glasses recorded changes in pupil size that in turn were converted into the Index of Cognitive Activity (ICA). ICA values were averaged for each eye and condition. A two-way Analysis of Variance with post-hoc Sidak correction for pairwise comparisons was run to examine the effect of visual occlusion and dual tasking on ICA values as well on Center of Pressure (CoP) sway velocity in anterior–posterior (AP) and medio-lateral (ML) directions. A Pearson’s correlation coefficient was utilized to determine the relationship between ICA values and CoP sway velocity.

Results: Significant within-condition effect was observed with visual occlusion for the right eye ICA values (p = 0.008). Right eye ICA increased from eyes open to eyes occluded conditions (p = 0.008). In addition, a significant inverse correlation was observed between right eye ICA values and CoP sway velocity in the ML direction across all the conditions (r = -0.25, p = 0.02).

Conclusion: This study demonstrated support for increased cognitive workload, measured by pupillometry, as a result of changes in postural control in healthy young adults. Further research is warranted to investigate the clinical application of pupillometry in balance assessment.

Sleep and Sedative States Induced by Targeting the Histamine and Noradrenergic Systems

Sedatives target just a handful of receptors and ion channels. But we have no satisfying explanation for how activating these receptors produces sedation. In particular, do sedatives act at restricted brain locations and circuitries or more widely? Two prominent sedative drugs in clinical use are zolpidem, a GABA_A receptor positive allosteric modulator, and dexmedetomidine (DEX), a selective α2 adrenergic receptor agonist. By targeting hypothalamic neuromodulatory systems both drugs induce a sleep-like state, but in different ways: zolpidem primarily reduces the latency to NREM sleep, and is a controlled substance taken by many people to help them sleep; DEX produces prominent slow wave activity in the electroencephalogram (EEG) resembling stage 2 NREM sleep, but with complications of hypothermia and lowered blood pressure—it is used for long term sedation in hospital intensive care units—under DEX-induced sedation patients are arousable and responsive, and this drug reduces the risk of delirium. DEX, and another α2 adrenergic agonist xylazine, are also widely used in veterinary clinics to sedate animals. Here we review how these two different classes of sedatives, zolpidem and dexmedetomideine, can selectively interact with some nodal points of the circuitry that promote wakefulness allowing the transition to NREM sleep. Zolpidem enhances GABAergic transmission onto histamine neurons in the hypothalamic tuberomammillary nucleus (TMN) to hasten the transition to NREM sleep, and DEX interacts with neurons in the preoptic hypothalamic area that induce sleep and body cooling. This knowledge may aid the design of more precise acting sedatives, and at the same time, reveal more about the natural sleep-wake circuitry.

Genetic, Epigenetic, and Environmental Factors Influencing Neurovisceral Integration of Cardiovascular Modulation: Focus on Multiple Sclerosis

Thought to be an autoimmune inflammatory CNS disease, multiple sclerosis (MS) involves multiple pathologies with heterogeneous clinical presentations. An impaired neurovisceral integration of cardiovascular modulation, indicated by sympathetic and parasympathetic autonomic nervous system (ANS) dysfunction, is among common MS clinical presentations. ANS dysfunction could not only enhance MS inflammatory and neurodegenerative processes, but can also lead to clinical symptoms such as depression, fatigue, sleep disorder, migraine, osteoporosis, and cerebral hemodynamic impairments. Therefore, factors influencing ANS functional activities, in one way or another, will have a significant impact on MS disease course. This review describes the genetic and epigenetic factors, and their interactions with a number of environmental factors contributing to the neurovisceral integration of cardiovascular modulation, with a focus on MS. Future studies should investigate the improvement in cardiovascular ANS function, as a strategy for preventing and minimizing MS-related morbidities, and improving patients' quality of life.

GHB for cataplexy: Possible mode of action

The sleep disorder narcolepsy is caused by the loss of orexinergic neurones in the lateral hypothalamus. A troublesome symptom of narcolepsy is cataplexy, the sudden loss of muscle tone in response to strong emotions. It can be alleviated by antidepressants and sodium oxybate (γ-hydroxybutyric acid (GHB)). It is likely that the noradrenergic nucleus locus coeruleus (LC) is involved since it is essential for the maintenance of muscle tone, and ceases to fire during cataplectic attacks. Furthermore, alpha-2 adrenoceptors proliferate in the LC in cataplexy, probably due to 'heterologous denervation supersensitivity' resulting from the loss/weakening of the orexinergic input to the LC. This would lead to the sensitization of the autoinhibition mechanism of LC neurones mediated by inhibitory alpha-2 adrenoceptors ('autoreceptors'). Thus the excitatory input from the amygdala to the LC, activated by an emotional stimulus, would lead to the 'switching off' of LC activity via the supersensitive auto-inhibition mechanism. GHB is an agonist at both γ-aminobutyric acid (GABA) GABA (B) and GHB receptors that may be a subtype of an extrasynaptic GABA(A) receptor. GHB may prevent a cataplectic attack by dampening the tone of LC neurones via the stimulation of inhibitory extrasynaptic GABA receptors in the LC, and thus increasing the threshold for autoinhibition.

Neuronal ensembles sufficient for recovery sleep and the sedative actions of α2 adrenergic agonists

Do sedatives engage natural sleep pathways? It is usually assumed that anesthetic-induced sedation and loss-of-righting-reflex (LORR) arise by influencing the same circuitry to lesser or greater extents. For the α2 adrenergic receptor agonist dexmedetomidine, we find that sedation and LORR are in fact distinct states, requiring different brain areas, the preoptic hypothalamic area and locus coeruleus (LC) respectively. Selective knockdown of α2A adrenergic receptors from the LC abolished dexmedetomidine-induced LORR, but not sedation. Instead, we found that dexmedetomidine-induced sedation resembles the deep recovery sleep that follows sleep deprivation. We used TetTag-pharmacogenetics in mice to functionally mark neurons activated in the preoptic hypothalamus during dexmedetomidine-induced sedation or recovery sleep. The neuronal ensembles could then be selectively reactivated. In both cases NREM sleep, with the accompanying drop in body temperature, was recapitulated. Thus α2 adrenergic receptor-induced sedation and recovery sleep share hypothalamic circuitry sufficient for producing these behavioral states.

Desensitization of the Mechanoreceptors in Müller's Muscle Reduces the Increased Reflex Contraction of the Orbicularis Oculi Slow-Twitch Fibers in Blepharospasm

OBJECTIVE

Although the mixed orbicularis oculi muscle lacks the muscle spindles required to induce reflex contraction of its slow-twitch fibers, the mechanoreceptors in Müller's muscle function as extrinsic mechanoreceptors to induce reflex contraction. We hypothesize that strong stretching of these mechanoreceptors increases reflex contraction of the orbicularis oculi slow-twitch muscle fibers, resulting in blepharospasm.

METHODS

We examined a 71-year-old man with right blepharospasm and bilateral aponeurosis-disinserted blepharoptosis to determine whether the patient's blepharospasm was worsened by increased trigeminal proprioceptive evocation via stretching of the mechanoreceptors in Müller's muscle owing to a 60° upward gaze and serrated eyelid closure, and whether local anesthesia of the mechanoreceptors via lidocaine administration to the upper fornix as well as surgical disinsertion of Müller's muscle from the tarsus and fixation of the disinserted aponeurosis to the tarsus decreased trigeminal proprioceptive evocation and improved patient's blepharospasm.

RESULTS

Before pharmacological desensitization, 60° upward gaze and serrated eyelid closure exacerbated the patient's blepharospasm. In contrast, these maneuvers did not worsen his blepharospasm following lidocaine administration. One year after surgical desensitization, the blepharospasm had disappeared and a 60° upward gaze did not induce blepharospasm.

CONCLUSIONS

Strong stretching of the mechanoreceptors in Müller's muscle appeared to increase reflex contraction of the orbicularis oculi slow-twitch muscle fibers, resulting in blepharospasm. In addition to botulinum neurotoxin injections into the involuntarily contracted orbicularis oculi muscle and myectomy, surgical desensitization of the mechanoreceptors in Müller's muscle may represent an additional procedure to reduce blepharospasm.

Trigeminal Proprioception Evoked by Strong Stretching of the Mechanoreceptors in Müller's Muscle Induces Reflex Contraction of the Orbital Orbicularis Oculi Slow-Twitch Muscle Fibers

OBJECTIVE

The mixed orbicularis oculi muscle lacks an intramuscular proprioceptive system such as muscle spindles, to induce reflex contraction of its slow-twitch fibers. We evaluated whether the mechanoreceptors in Müller's muscle function as extrinsic mechanoreceptors to induce reflex contraction of the slow-twitch fibers of the orbicularis oculi in addition to those of the levator and frontalis muscles.

METHODS

We evaluated in patients with aponeurosis-disinserted blepharoptosis whether strong stretching of the mechanoreceptors in Müller's muscle from upgaze with unilateral lid load induced reflex contraction of the orbicularis oculi slow-twitch fibers and whether anesthesia of Müller's muscle precluded the contraction. We compared the electromyographic responses of the bilateral orbicularis oculi muscles to unilateral intraoperative direct stimulation of the trigeminal proprioceptive nerve with those to unilateral transcutaneous electrical stimulation of the supraorbital nerve.

RESULTS

Upgaze with a unilateral 3-g lid load induced reflex contraction of the bilateral orbicularis oculi muscles with ipsilateral dominance. Anesthesia of Müller's muscle precluded the reflex contraction. The orbicularis oculi reflex evoked by stimulation of the trigeminal proprioceptive nerve differed from that by electrical stimulation of the supraorbital nerve in terms of the intensity of current required to induce the reflex, the absence of R1, and duration.

CONCLUSIONS

The mechanoreceptors in Müller's muscle functions as an extramuscular proprioceptive system to induce reflex contraction of the orbital orbicularis oculi slow-twitch fibers. Whereas reflex contraction of the pretarsal orbicularis fast-twitch fibers functions in spontaneous or reflex blinking, that of the orbital orbicularis oculi slow-twitch fibers may factor in grimacing and blepharospasm.

Role of the hypocretin (orexin) receptor 2 (Hcrt-r2) in the regulation of hypocretin level and cataplexy

Hypocretin receptor-2 (Hcrt-r2)-mutated dogs exhibit all the major symptoms of human narcolepsy and respond to drugs that increase or decrease cataplexy as do narcoleptic humans; yet, unlike narcoleptic humans, the narcoleptic dogs have normal hypocretin levels. We find that drugs that reduce or increase cataplexy in the narcoleptic dogs, greatly increase and decrease, respectively, hypocretin levels in normal dogs. The effects of these drugs on heart rate and blood pressure, which were considerable, were not correlated with their effects on cataplexy. Administration of these drugs to Hcrt-r2-mutated dogs produced indistinguishable changes in heart rate and blood pressure, indicating that neither central nor peripheral Hcrt-r2 is required for these cardiovascular effects. However, in contrast to the marked Hcrt level changes in the normal dogs, these drugs did not alter hypocretin levels in the Hcrt-r2 mutants. We conclude that Hcrt-r2 is a vital element in a feedback loop integrating Hcrt, acetylcholine, and norepinephrine function. In the absence of functional Hcrt-r2, Hcrt levels are not affected by monoaminergic and cholinergic drugs, despite the strong modulation of cataplexy by these drugs. Conversely, strong transient reductions of Hcrt level by these drugs do not produce episodes of cataplexy in normal dogs. The Hcrt-r2 mutation causes drug-induced cataplexy by virtue of its long-term effect on the functioning of other brain systems, rather than by increasing the magnitude of phasic changes in Hcrt level. A similar mechanism may be operative in spontaneous cataplexy in narcoleptic dogs as well as in narcoleptic humans.