Mental Stress Elicits Sustained and Reproducible Increases in Skin Sympathetic Nerve Activity

Use of Infrared Thermography and Heart Rate Variability to Evaluate Autonomic Activity in Domestic Animals

Most of the responses present in animals when exposed to stressors are mediated by the autonomic nervous system. The sympathetic nervous system, known as the one responsible for the "fight or flight" reaction, triggers cardiovascular changes such as tachycardia or vasomotor alterations to restore homeostasis. Increase in body temperature in stressed animals also activates peripheral compensatory mechanisms such as cutaneous vasodilation to increase heat exchange. Since changes in skin blood flow influence the amount of heat dissipation, infrared thermography is suggested as a tool that can detect said changes. The present review aims to analyze the application of infrared thermography as a method to assess stress-related autonomic activity, and their association with the cardiovascular and heart rate variability in domestic animals.

Differential control of sympathetic outflow to muscle and skin during physical and cognitive stressors

PURPOSE

Sympathetic nerve activity towards muscle (MSNA) and skin (SSNA) regulates various physiological parameters. MSNA primarily functions in blood pressure and flow, while SSNA operates in thermoregulation. Physical and cognitive stressors have been shown to have effects on both types of sympathetic activity, but there are inconsistencies as to what these effects are. This article aims to address the discrepancies in the literature and compare MSNA and SSNA responses.

METHODS

Microelectrode recordings were taken from the common peroneal nerve in 29 participants: MSNA (n = 21), SSNA (n = 16) and both MSNA and SSNA (n = 8). Participants were subjected to four different 2-min stressors: two physical (isometric handgrip task, cold pressor test) and two cognitive (mental arithmetic task, Stroop colour-word conflict test), the latter of which saw participants separated into responders and non-responders to the stressors. It was hypothesised that the physical stressors would have a greater effect on MSNA than SSNA, while the cognitive stressors would operate conversely.

RESULTS

Peristimulus time histogram (PSTH) analysis showed the mental arithmetic task to significantly increase both MSNA and SSNA; the isometric handgrip task and cold pressor test to increase MSNA, but not SSNA; and Stroop test to have no significant effects on changing MSNA or SSNA from baseline. Additionally, stress responses did not differ between MSNA and SSNA in participants who had both sets of data recorded.

CONCLUSIONS

This study has provided evidence to support the literature which claims cognitive stressors increase sympathetic activity, and provides much needed SSNA data in response to stressors.

Mitigating Initial Orthostatic Hypotension: Mechanistic Roles of Muscle Contraction Versus Sympathetic Activation

BACKGROUND

Initial orthostatic hypotension (IOH) is defined by a large drop in blood pressure (BP) within 15 s of standing. IOH often presents during an active stand, but not with a passive tilt, suggesting that a muscle activation reflex involving lower body muscles plays an important role. To our knowledge, there is no literature exploring how sympathetic activation affects IOH. We hypothesized involuntary muscle contractions before standing would significantly reduce the drop in BP seen in IOH while increasing sympathetic activity would not.

METHODS

Study participants performed 4 sit-to-stand maneuvers including a mental stress test (serial 7 mental arithmetic stress test), cold pressor test, electrical stimulation, and no intervention. Continuous heart rate and beat-to-beat BP were measured. Cardiac output and systemic vascular resistance were estimated from these waveforms. Data are presented as mean±SD.

RESULTS

A total of 23 female IOH participants (31±8 years) completed the study. The drops in systolic BP following the serial 7 mental arithmetic stress test (-26±12 mm Hg; P=0.004), cold pressor test (-20±15 mm Hg; P<0.001), and electrical stimulation (-28±12 mm Hg; P=0.01) were significantly reduced compared with no intervention (-34±11 mm Hg). The drops in systemic vascular resistance following the serial 7 mental arithmetic stress test (-391±206 dyne×s/cm⁵; P=0.006) and cold pressor test (-386±179 dyne×s/cm⁵; P=0.011) were significantly reduced compared with no intervention (-488±173 dyne×s/cm⁵). Cardiac output was significantly increased upon standing (7±2 L/min) compared with during the sit (6±1 L/min; P<0.001) for electrical stimulation.

CONCLUSION

Sympathetic activation mitigates the BP response in IOH, while involuntary muscle contraction mitigates the BP response and reduces symptoms. Active muscle contractions may induce both of these mechanisms of action in their pretreatment of IOH. Registration: URL: https://www.clinicaltrials.gov; Unique identifier: NCT03970551.

The AC conductivity of human sweat ducts as the dominant factor in the sub-THz reflection coefficient of skin

The helical nature of human sweat ducts, combined with the morphological and dielectric properties of skin, suggests electromagnetic activity in the sub-THz frequency band. A detailed electromagnetic simulation model of the skin, with embedded sweat ducts, was created. The model includes realistic dielectric properties based on the measured water content of each layer of skin, derived from Raman Spectroscopy. The model was verified by comparing it to measurements of the reflection coefficient of the palms of 13 volunteers in the frequency band 350-410 GHz. They were subjected to a measurement protocol intended to induce mental stress, thereby also activating the sweat glands. The Galvanic Skin Response was concurrently measured. Using the simulation model the optimal ac-conductivity for each measurement was found. The range of variation for all subjects was found to be from 100 S/m to a maximum value of 6000 S/m with averages of 1000 S/m. These are one order of magnitude increase from the accepted values for water at these frequencies (~100 s/m at 100 GHz). Considering the known biochemical mechanism for inducing perspiration, we conclude that these ac-conductivity levels are probably valid, even though the real time measurements of sweat ac-conductivity levels inside the duct are inaccessible.

Parasympathetic-Sympathetic Causal Interactions Assessed by Time-Varying Multivariate Autoregressive Modeling of Electrodermal Activity and Heart-Rate-Variability

OBJECTIVE

Most of the bodily functions are regulated by multiple interactions between the parasympathetic (PNS) and sympathetic (SNS) nervous system. In this study, we propose a novel framework to quantify the causal flow of information between PNS and SNS through the analysis of heart rate variability (HRV) and electrodermal activity (EDA) signals.

METHODS

Our method is based on a time-varying (TV) multivariate autoregressive model of EDA and HRV time-series and incorporates physiologically inspired assumptions by estimating the Directed Coherence in a specific frequency range. The statistical significance of the observed interactions is assessed by a bootstrap procedure purposely developed to infer causalities in the presence of both TV model coefficients and TV model residuals (i.e., heteroskedasticity). We tested our method on two different experiments designed to trigger a sympathetic response, i.e., a hand-grip task (HG) and a mental-computation task (MC).

RESULTS

Our results show a parasympathetic driven interaction in the resting state, which is consistent across different studies. The onset of the stressful stimulation triggers a cascade of events characterized by the presence or absence of the PNS-SNS interaction and changes in the directionality. Despite similarities between the results related to the two tasks, we reveal differences in the dynamics of the PNS-SNS interaction, which might reflect different regulatory mechanisms associated with different stressors.

CONCLUSION

We estimate causal coupling between PNS and SNS through MVAR modeling of EDA and HRV time-series.

SIGNIFICANCE

Our results suggest promising future applicability to investigate more complex contexts such as affective and pathological scenarios.

Game Face expressions and performance on competitive tasks

Facial expressions influence both affective and cardiovascular responses to stress. However, previous research focuses primarily on positive expressions and is limited regarding additional facial expressions utilized on a day-to-day basis. This study examined an expression that is colloquially called a "Game Face": which refers to a serious, focused or determined facial expression. The current study examined whether Game Face expressions would influence psychophysiological response (e.g., heart rate and skin conductance) and performance. In an investigation of physical performance (Study 1), participants (N = 62) were asked to complete the cold-pressor task. Study 2 tested cognitive performance utilizing a puzzle task. Participants (N = 62) were divided into two groups and were asked to complete a puzzle. In both studies, one group was asked to make a Game Face, and the other was given no instruction related to facial expression. Results show no significant differences in performance on the physical task. In terms of cognitive performance, results reveal significantly better performance in the Game Face group. Additionally, assessments of skin conductance show that participants, who employed the Game Face during the cognitive task, displayed significant decreases from baseline following the puzzle manipulation. These results are promising regarding performance on a cognitive task and sympathetic nervous system activation, in concert with making a Game Face.

Effect of examination stress on intraocular pressure in university students

Intraocular pressure (IOP) has been investigated as a possible objective index of mental stressors. Here, we assessed the effect of examination stress on IOP in 33 university students. A repeated-measures design was used with two experimental conditions (examination and control) and two points of measurements (pre- and post-sessions). Also, the cardiovascular response, subjective perceived stress, as well as calculated ocular perfusion pressure and blood-pulse pressure were determined. A Bayesian statistical analysis showed higher IOP values in the examination in comparison to the control condition (BF₀₁ < 0.001). A similar pattern was found for the cardiovascular indices (diastolic and systolic blood pressure, and heart rate), and these findings were corroborated by subjective reports (BF₀₁ < 0.001 in all cases). Our data incorporates evidence in relation to the utility of IOP as an objective marker of examination stress, and it may help in the assessment and management of stress in applied scenarios.

Measuring and quantifying skin sympathetic nervous system activity in humans

Development of the technique of microneurography has substantially increased our understanding of the function of the sympathetic nervous system (SNS) in health and in disease. The ability to directly record signals from peripheral autonomic nerves in conscious humans allows for qualitative and quantitative characterization of SNS responses to specific stimuli and over time. Furthermore, distinct neural outflow to muscle (MSNA) and skin (SSNA) can be delineated. However, there are limitations and caveats to the use of microneurography, measurement criteria, and signal analysis and interpretation. MSNA recordings have a longer history and are considered relatively more straightforward from a measurement and analysis perspective. This brief review provides an overview of the development of the technique as used to measure SSNA. The focus is on the utility of measuring sympathetic activity directed to the skin, the unique issues related to analyzing and quantifying multiunit SSNA, and the challenges related to its interpretation.

Recording sympathetic nerve activity in conscious humans and other mammals: guidelines and the road to standardization

Over the past several decades, studies of the sympathetic nervous system in humans, sheep, rabbits, rats, and mice have substantially increased mechanistic understanding of cardiovascular function and dysfunction. Recently, interest in sympathetic neural mechanisms contributing to blood pressure control has grown, in part because of the development of devices or surgical procedures that treat hypertension by manipulating sympathetic outflow. Studies in animal models have provided important insights into physiological and pathophysiological mechanisms that are not accessible in human studies. Across species and among laboratories, various approaches have been developed to record, quantify, analyze, and interpret sympathetic nerve activity (SNA). In general, SNA demonstrates "bursting" behavior, where groups of action potentials are synchronized and linked to the cardiac cycle via the arterial baroreflex. In humans, it is common to quantify SNA as bursts per minute or bursts per 100 heart beats. This type of quantification can be done in other species but is only commonly reported in sheep, which have heart rates similar to humans. In rabbits, rats, and mice, SNA is often recorded relative to a maximal level elicited in the laboratory to control for differences in electrode position among animals or on different study days. SNA in humans can also be presented as total activity, where normalization to the largest burst is a common approach. The goal of the present paper is to put together a summary of "best practices" in several of the most common experimental models and to discuss opportunities and challenges relative to the optimal measurement of SNA across species.Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/guidelines-for-measuring-sympathetic-nerve-activity/.

Folic acid supplementation increases cutaneous vasodilator sensitivity to sympathetic nerve activity in older adults

During heat stress, blunted increases in skin sympathetic nervous system activity (SSNA) and reductions in end-organ vascular responsiveness contribute to the age-related reduction in reflex cutaneous vasodilation. In older adults, folic acid supplementation improves the cutaneous vascular conductance (CVC) response to passive heating; however, the influence of folic acid supplementation on SSNA:CVC transduction is unknown. Fourteen older adults (66 ± 1 yr, 8 male/6 female) ingested folic acid (5 mg/day) or placebo for 6 wk in a randomized, double-blind, crossover design. In protocol 1, esophageal temperature (T_es) was increased by 1.0°C (water-perfused suit) while SSNA (peroneal microneurography) and red cell flux in the innervated dermatome (laser Doppler flowmetry; dorsum of the foot) were continuously measured. In protocol 2, two intradermal microdialysis fibers were placed in the skin of the lateral calf for graded infusions of acetylcholine (ACh; 10^-10 to 10^-1 M) with and without nitric oxide synthase (NOS) blockade (20 mM nitro-l-arginine methyl ester). Folic acid improved reflex vasodilation (46 ± 4% vs. 31 ± 3% CVC_max for placebo; P < 0.001) without affecting the increase in SSNA (Δ506 ± 104% vs. Δ415 ± 73% for placebo; NS). Folic acid increased the slope of the SSNA-to-CVC relation (0.08 ± 0.02 vs. 0.05 ± 0.01 for placebo; P < 0.05) and extended the response range. Folic acid augmented ACh-induced vasodilation (83 ± 3% vs. 66 ± 4% CVC_max for placebo; P = 0.002); however, there was no difference between treatments at the NOS-inhibited site (53 ± 4% vs. 52 ± 4% CVC_max for placebo; NS). These data demonstrate that folic acid supplementation enhances reflex vasodilation by increasing the sensitivity of skin arterioles to central sympathetic nerve outflow during hyperthermia in aged human subjects.

Neurovascular mechanisms underlying augmented cold-induced reflex cutaneous vasoconstriction in human hypertension

KEY POINTS

In hypertensive adults (HTN), cardiovascular risk increases disproportionately during environmental cold exposure. Despite ample evidence of dysregulated sympathetic control of the peripheral vasculature in hypertension, no studies have examined integrated neurovascular function during cold stress in HTN. The findings of the present study show that whole-body cold stress elicits greater increases in sympathetic outflow directed to the cutaneous vasculature and, correspondingly, greater reductions in skin blood flow in HTN. We further demonstrate an important role for non-adrenergic sympathetic co-transmitters in mediating the vasoconstrictor response to cold stress in hypertension. In the context of thermoregulation and the maintenance of core temperature, sympathetically-mediated control of the cutaneous vasculature is not only preserved, but also exaggerated in hypertension. Given the increasing prevalence of hypertension, clarifying the mechanistic underpinnings of hypertension-induced alterations in neurovascular function during cold exposure is clinically relevant.

ABSTRACT

Despite ample evidence of dysregulated sympathetic control of the peripheral vasculature in hypertension, no studies have examined integrated neurovascular function during cold stress in hypertensive adults (HTN). We hypothesized that (i) whole-body cooling would elicit greater cutaneous vasoconstriction and greater increases in skin sympathetic nervous system activity (SSNA) in HTN (n = 14; 56 ± 2 years) compared to age-matched normotensive adults (NTN; n = 14; 55 ± 2 years) and (ii) augmented reflex vasoconstriction in HTN would be mediated by an increase in cutaneous vascular adrenergic sensitivity and a greater contribution of non-adrenergic sympathetic co-transmitters. SSNA (peroneal microneurography) and red cell flux (laser Doppler flowmetry; dorsum of foot) were measured during whole-body cooling (water-perfused suit). Sympathetic adrenergic- and non-adrenergic-dependent contributions to reflex cutaneous vasoconstriction and vascular adrenergic sensitivity were assessed pharmacologically using intradermal microdialysis. Cooling elicited greater increases in SSNA (NTN: +64 ± 13%_baseline  vs. HTN: +194 ± 26%_baseline ; P < 0.01) and greater reductions in skin blood flow (NTN: -16 ± 2%_baseline  vs. HTN: -28 ± 3%_baseline ; P < 0.01) in HTN compared to NTN, reflecting an increased response range for sympathetic reflex control of cutaneous vasoconstriction in HTN. Norepinephrine dose-response curves showed no HTN-related difference in cutaneous adrenergic sensitivity (logEC₅₀ ; NTN: -7.4 ± 0.3 log M vs. HTN: -7.5 ± 0.3 log M; P = 0.84); however, non-adrenergic sympathetic co-transmitters mediated a significant portion of the vasoconstrictor response to cold stress in HTN. Collectively, these findings indicate that hypertension increases the peripheral cutaneous vasoconstrictor response to cold via greater increases in skin sympathetic outflow coupled with an increased reliance on non-adrenergic neurotransmitters.

Blunted increases in skin sympathetic nerve activity are related to attenuated reflex vasodilation in aged human skin

Reflex cutaneous vasodilation in response to passive heating is attenuated in human aging. This diminished response is mediated, in part, by age-associated reductions in endothelial function; however, the contribution of altered skin sympathetic nervous system activity (SSNA) is unknown. We hypothesized that 1) healthy older adults would demonstrate blunted SSNA responses to increased core temperature compared with young adults and 2) the decreased SSNA response would be associated with attenuated cutaneous vasodilation. Reflex vasodilation was elicited in 13 young [23 ± 1 (SE) yr] and 13 older (67 ± 2 yr) adults using a water-perfused suit to elevate esophageal temperature by 1.0°C. SSNA (peroneal microneurography) and red cell flux (laser Doppler flowmetry) in the innervated dermatome (the dorsum of foot) were continuously measured. SSNA was normalized to, and expressed as, a percentage of baseline. Cutaneous vascular conductance (CVC) was calculated as flux/mean arterial pressure and expressed as a percentage of maximal CVC (local heating, 43°C). Reflex vasodilation was attenuated in older adults (P < 0.001). During heating, SSNA increased in both groups (P < 0.05); however, the response was significantly blunted in older adults (P = 0.01). The increase in SSNA during heating was linearly related to cutaneous vasodilation in both young (R² = 0.87 ± 0.02, P < 0.01) and older (R² = 0.76 ± 0.05, P < 0.01) adults; however, slope of the linear regression between ΔSSNA and ΔCVC was reduced in older compared with young (older: 0.05 ± 0.01 vs. young: 0.08 ± 0.01; P < 0.05). These data demonstrate that age-related impairments in reflex cutaneous vasodilation are mediated, in part, by blunted efferent SSNA during hyperthermia.

Rosacea: Molecular Mechanisms and Management of a Chronic Cutaneous Inflammatory Condition

Rosacea is a chronic cutaneous inflammatory disease that affects the facial skin. Clinically, rosacea can be categorized into papulopustular, erythematotelangiectatic, ocular, and phymatous rosacea. However, the phenotypic presentations of rosacea are more heterogeneous. Although the pathophysiology of rosacea remains to be elucidated, immunologic alterations and neurovascular dysregulation are thought to have important roles in initiating and strengthening the clinical manifestations of rosacea. In this article, we present the possible molecular mechanisms of rosacea based on recent laboratory and clinical studies. We describe the genetic predisposition for rosacea along with its associated diseases, triggering factors, and suggested management options in detail based on the underlying molecular biology. Understanding the molecular pathomechanisms of rosacea will likely aid toward better comprehending its complex pathogenesis.

Sympathetic regulation during thermal stress in human aging and disease

Humans control their core temperature within a narrow range via precise adjustments of the autonomic nervous system. In response to changing core and/or skin temperature, several critical thermoregulatory reflex effector responses are initiated and include shivering, sweating, and changes in cutaneous blood flow. Cutaneous vasomotor adjustments, mediated by modulations in sympathetic nerve activity (SNA), aid in the maintenance of thermal homeostasis during cold and heat stress since (1) they serve as the first line of defense of body temperature and are initiated before other thermoregulatory effectors, and (2) they are on the efferent arm of non-thermoregulatory reflex systems, aiding in the maintenance of blood pressure and organ perfusion. This review article highlights the sympathetic responses of humans to thermal stress, with a specific focus on primary aging as well as impairments that occur in both heart disease and type 2 diabetes mellitus. Age- and pathology-related changes in efferent muscle and skin SNA during cold and heat stress, measured directly in humans using microneurography, are discussed.

Sympathetic neural reactivity to mental stress in humans: test-retest reproducibility

Mental stress consistently increases arterial blood pressure, but this reliable pressor response is often associated with highly variable muscle sympathetic nerve activity (MSNA) responsiveness between individuals. Although MSNA has been shown to be reproducible within individuals at rest and during the cold pressor test (CPT), intraindividual reproducibility of MSNA responsiveness to mental stress has not been adequately explored. The purpose of this study was to examine MSNA reactivity to mental stress across three experimental sessions. Sixteen men and women (age 21 ± 1 yr) performed two experimental sessions within a single laboratory visit and a third experimental session 1 mo later. Each experimental session consisted of a mental stress trial via mental arithmetic and a CPT trial. Blood pressure, heart rate (HR), and MSNA were measured, and the consistencies of these variables were determined using intraclass correlation (Cronbach's α coefficient). MSNA, mean arterial pressure (MAP), and HR were highly reproducible across the baselines preceding mental stress (Cronbach's α ≥ 0.816, P ≤ 0.001) and CPT (Cronbach's α ≥ 0.782, P ≤ 0.001). Across the three mental stress trials, changes in MSNA (Cronbach's α = 0.875; P = 0.001), MAP (Cronbach's α = 0.749; P < 0.001), and HR (Cronbach's α = 0.919; P < 0.001) were reproducible. During CPT, changes in MSNA (Cronbach's α = 0.805; P = 0.008), MAP (Cronbach's α = 0.878; P < 0.001), and HR (Cronbach's α = 0.927; P < 0.001) remained consistent across the three sessions. In conclusion, our findings demonstrate that MSNA reactivity to mental stress is consistent within a single laboratory visit and across laboratory sessions conducted on separate days.

Sympathoneural and adrenomedullary responses to mental stress

This concept-based review provides historical perspectives and updates about sympathetic noradrenergic and sympathetic adrenergic responses to mental stress. The topic of this review has incited perennial debate, because of disagreements over definitions, controversial inferences, and limited availability of relevant measurement tools. The discussion begins appropriately with Cannon's "homeostasis" and his pioneering work in the area. This is followed by mental stress as a scientific idea and the relatively new notions of allostasis and allostatic load. Experimental models of mental stress in rodents and humans are discussed, with particular attention to ethical constraints in humans. Sections follow on sympathoneural responses to mental stress, reactivity of catecholamine systems, clinical pathophysiologic states, and the cardiovascular reactivity hypothesis. Future advancement of the field will require integrative approaches and coordinated efforts between physiologists and psychologists on this interdisciplinary topic.

Augmented supraorbital skin sympathetic nerve activity responses to symptom trigger events in rosacea patients

Facial flushing in rosacea is often induced by trigger events. However, trigger causation mechanisms are currently unclear. This study tested the central hypothesis that rosacea causes sympathetic and axon reflex-mediated alterations resulting in trigger-induced symptomatology. Twenty rosacea patients and age/sex-matched controls participated in one or a combination of symptom triggering stressors. In protocol 1, forehead skin sympathetic nerve activity (SSNA; supraorbital microneurography) was measured during sympathoexcitatory mental (2-min serial subtraction of novel numbers) and physical (2-min isometric handgrip) stress. In protocol 2, forehead skin blood flow (laser-Doppler flowmetry) and transepithelial water loss/sweat rate (capacitance hygrometry) were measured during sympathoexcitatory heat stress (whole body heating by perfusing 50°C water through a tube-lined suit). In protocol 3, cheek, forehead, forearm, and palm skin blood flow were measured during nonpainful local heating to induce axon reflex vasodilation. Heart rate (HR) and mean arterial pressure (MAP) were recorded via finger photoplethysmography to calculate cutaneous vascular conductance (CVC; flux·100/MAP). Higher patient transepithelial water loss was observed (rosacea 0.20 ± 0.02 vs. control 0.10 ± 0.01 mg·cm(-2)·min(-1), P < 0.05). HR and MAP changes were not different between groups during sympathoexcitatory stressors or local heating. SSNA during early mental (32 ± 9 and 9 ± 4% increase) and physical (25 ± 4 and 5 ± 1% increase, rosacea and controls, respectively) stress was augmented in rosacea (both P < 0.05). Heat stress induced more rapid sweating and cutaneous vasodilation onset in rosacea compared with controls. No axon reflex vasodilation differences were observed between groups. These data indicate that rosacea affects SSNA and that hyperresponsiveness to trigger events appears to have a sympathetic component.

Two dopamine receptors play different roles in phase change of the migratory locust

The migratory locust, Locusta migratoria, shows remarkable phenotypic plasticity at behavioral, physiological, and morphological levels in response to fluctuation in population density. Our previous studies demonstrated that dopamine (DA) and the genes in the dopamine metabolic pathway mediate phase change in Locusta. However, the functions of different dopamine receptors in modulating locust phase change have not been fully explored. In the present study, DA concentration in the brain increased during crowding and decreased during isolation. The expression level of dopamine receptor 1 (Dop1) increased from 1 to 4 h of crowding, but remained unchanged during isolation. Injection of Dop1 agonist SKF38393 into the brains of solitary locusts promoted gregarization, induced conspecific attraction-response and increased locomotion. RNAi knockdown of Dop1 and injection of antagonist SCH23390 in gregarious locusts induced solitary behavior, promoted the shift to repulsion-response and reduced locomotion. By contrast, the expression level of dopamine receptor 2 (Dop2) gradually increased during isolation, but remained stable during crowding. During the isolation of gregarious locusts, injection of Dop2 antagonist S(–)-sulpiride or RNAi knockdown of Dop2 inhibited solitarization, maintained conspecific attraction-response and increased locomotion; by comparison, the isolated controls displayed conspecific repulsion-response and weaker motility. Activation of Dop2 in solitary locusts through injection of agonist, R(-)-TNPA, did not affect their behavioral state. Thus, DA-Dop1 signaling in the brain of Locusta induced the gregariousness, whereas DA-Dop2 signaling mediated the solitariness. Our study demonstrated that Dop1 and Dop2 modulated locust phase change in two different directions. Further investigation of Locusta Dop1 and Dop2 functions in modulating phase change will improve our understanding of the molecular mechanism underlying phenotypic plasticity in locusts.

Impaired increases in skin sympathetic nerve activity contribute to age-related decrements in reflex cutaneous vasoconstriction

KEY POINTS

The reduction in skin blood flow during whole-body cooling is impaired in healthy older adults. However, the relative contributions of altered skin sympathetic nerve activity (SSNA), transduction of this efferent neural outflow to the cutaneous vasculature, and peripheral vascular responsiveness to adrenergic stimuli to the impaired reflex vasoconstrictor response to whole-body cooling in human ageing remain unclear. We report that the SSNA response to whole-body cooling is blunted in healthy older adults, and this attenuated sympathetic response is related to a marked impairment in reflex cutaneous vasoconstriction. Further, the reflex SSNA response to a non-thermoregulatory stimulus was preserved in older adults during cooling. We additionally show that cutaneous vascular responsiveness to adrenergic stimuli is not reduced in older adults. These results further our understanding of the physiological mechanisms underlying impaired thermal-cardiovascular integration in healthy ageing.

ABSTRACT

Reflex cutaneous vasoconstriction is impaired in older adults; however, the relative roles of altered skin sympathetic nerve activity (SSNA) and end-organ peripheral vascular responsiveness are unclear. We hypothesized that in older adults whole-body cooling would elicit a blunted SSNA response and cutaneous adrenergic responsiveness would be reduced. Twelve young adults (Y; 24 ± 1 years) and 12 older adults (O; 57 ± 2 years) participated in two protocols. In Protocol 1, SSNA (peroneal microneurography) and red cell flux in the affected dermatome (laser Doppler flowmetry; dorsum of foot) were measured during whole-body cooling (mean skin temperature (Tsk ) 30.5°C; water-perfused suit). Mental stress was performed at mean Tsk 34.0°C (thermoneutral) and at 30.5°C. In Protocol 2, an intradermal microdialysis fibre was placed in the skin of the lateral calf for graded infusions of noradrenaline (norepinephrine) (NA; 10(-12) to 10(-2)  m). Cutaneous vascular conductance (CVC = flux/mean arterial pressure) was expressed as a change from baseline (ΔCVCbase ). Vasoconstriction was attenuated in O. SSNA increased significantly during cooling in Y (+184 ± 37%; P < 0.05) but not O (+51 ± 12%; P > 0.05). Mental stress at Tsk 30.5°C further increased SSNA in both groups. There was no age-related difference in adrenergic responsiveness to exogenous NA (logEC50 : -6.41 ± 0.24 in Y, -6.37 ± 0.25 in O; P > 0.05). While the SSNA response to whole-body cooling is impaired with ageing, SSNA can be further increased by a non-thermoregulatory stimulus. Cutaneous adrenergic sensitivity is not reduced in O. These findings suggest that alterations in afferent signalling or central processing likely contribute to blunted SSNA responses to cooling and subsequent impairments in reflex cutaneous vasoconstriction in ageing.

Ascorbic acid does not enhance hypoxia-induced vasodilation in healthy older men

In response to hypoxia, a net vasodilation occurs in the limb vasculature in young healthy humans and this is referred to as “hypoxia‐induced vasodilation”. We performed two separate experiments to determine (1) if hypoxia‐induced forearm vasodilation is impaired in older men (n = 8) compared to young men (n = 7) and (2) if acute systemic infusion of ascorbic acid would enhance hypoxia‐induced vasodilation in older men (n = 8). Heart rate, mean arterial pressure, oxygen saturation, minute ventilation, forearm vascular conductance (FVC, Doppler ultrasound), and cutaneous vascular conductance (CVC, laser Doppler flowmetry) were recorded continuously while subjects breathed 10% oxygen for 5 min. Changes from baseline were compared between groups and between treatments. The older adults had a significantly attenuated increase in FBF (13 ± 4 vs. 30 ± 7%) and FVC (16 ± 4 vs. 30 ± 7%) in response to 5 min of hypoxia. However, skin blood flow responses were comparable between groups (young: 35 ± 9, older: 30 ± 6%). In Experiment 2, FVC responses to 5 min of breathing 10% oxygen were not significantly different following saline (3 ± 10%) and ascorbic acid (8 ± 10%) in the older men. Ascorbic acid also had no physiological effects in the young men. These findings advance our basic understanding of how aging influences vascular responses to hypoxia and suggest that, in healthy humans, hypoxia‐induced vasodilation is not restrained by reactive oxygen species.

e12091

In Experiment 1, heart rate (HR), mean arterial pressure (MAP), forearm vascular conductance (FVC), and forearm cutaneous vascular conductance (CVC) were measured in response to 5 min of continuous hypoxia (10% oxygen) in young men (black diamonds) and older men (white squares). In Experiment 2, older men breathed continuous hypoxia after receiving normal saline solution (NSS, white squares) and ascorbic acid (Vit C, black squares). These findings advance our basic understanding of how aging influences vascular responses to hypoxia and suggest that, in healthy humans, hypoxia‐induced vasodilation is not restrained by reactive oxygen species.

Sex differences in forearm vasoconstrictor response to voluntary apnea

Clinical evidence indicates that obstructive sleep apnea is more common and more severe in men compared with women. Sex differences in the vasoconstrictor response to hypoxemia-induced sympathetic activation might contribute to this clinical observation. In the current laboratory study, we determined sex differences in the acute physiological responses to maximal voluntary end-expiratory apnea (MVEEA) during wakefulness in healthy young men and women (26 ± 1 yr) as well as healthy older men and women (64 ± 2 yr). Mean arterial pressure (MAP), heart rate (HR), brachial artery blood flow velocity (BBFV, Doppler ultrasound), and cutaneous vascular conductance (CVC, laser Doppler flowmetry) were measured, and changes in physiological parameters from baseline were compared between groups. The breath-hold duration and oxygen-saturation nadir were similar between groups. In response to MVEEA, young women had significantly less forearm vasoconstriction compared with young men (ΔBBFV: 2 ± 7 vs. -25 ± 6% and ΔCVC: -5 ± 4 vs. -31 ± 4%), whereas ΔMAP (12 ± 2 vs. 16 ± 3 mmHg) and ΔHR (4 ± 2 vs. 6 ± 3 bpm) were comparable between groups. The attenuated forearm vasoconstriction in young women was not observed in postmenopausal women (ΔBBFV -21 ± 5%). We concluded that young women have blunted forearm vasoconstriction in response to MVEEA compared with young men, and this effect is not evident in older postmenopausal women. These data suggest that female sex hormones dampen neurogenic vasoconstriction in response to apnea-induced hypoxemia.

Forearm vascular responses to mental stress in healthy older adults

Forearm vascular conductance (FVC) increases in response to mental stress (verbal mental arithmetic) in young people. However, the effect of healthy aging and mental stress on FVC is unknown. In this study, we tested the hypothesis that FVC and cutaneous vascular conductance (CVC) would be attenuated in older adults compared to young adults. In 13 young (27 ± 1 year) and 11 older (62 ± 1 year) subjects, we quantified heart rate (HR), mean arterial pressure (MAP), FVC (Doppler ultrasound), and CVC (laser Doppler flowmetry) in response to a 3‐min bout of mental stress in the supine posture. Changes from baseline were compared between groups and physiological variables were also correlated. Older adults had a blunted HR response to mental stress (Δ = 7 ± 2 vs. 14 ± 2 beats/min) but ΔMAP was comparable between groups (Δ = 11 ± 2 mmHg vs. 9 ± 1). During the third minute of mental stress, the %ΔFVC (−2 ± 5 vs. 31 ± 12%) and %ΔCVC (2 ± 6 vs. 31 ± 15%) were both impaired in older adults compared to young subjects. There was no relationship between ΔHR and %ΔCVC in either group, but there was a positive relationship between ΔHR and %ΔFVC in both young subjects (R = 0.610, P < 0.027) and older subjects (R = 0.615, P < 0.044), such that larger tachycardia was associated with higher forearm vasodilation. These data indicate that older adults have impaired forearm vasodilation in response to mental stress.

Forearm vascular conductance (FVC) increases in response to mental stress (verbal mental arithmetic) in young people. However, the effect of healthy aging and mental stress on FVC is unknown. The current data indicate that older adults have impaired forearm vasodilation in response to mental stress due in part to an attenuated heart rate response.

Melatonin attenuates the skin sympathetic nerve response to mental stress

Melatonin attenuates muscle sympathetic nerve responses to sympathoexcitatory stimuli, but it is unknown whether melatonin similarly attenuates reflex changes in skin sympathetic nerve activity (SSNA). In this double-blind, placebo-controlled, crossover study, we tested the hypothesis that melatonin (3 mg) would attenuate the SSNA response to mental stress (mental arithmetic). Twelve healthy subjects underwent experimental testing on two separate days. Three minutes of mental stress occurred before and 45 min after ingestion of melatonin (3 mg) or placebo. Skin temperature was maintained at 34°C. Reflex increases in SSNA (peroneal nerve), mean arterial pressure, and heart rate (HR) to mental stress before and after melatonin were determined. Melatonin lowered HR (pre, 66 ± 3 beats/min; and post, 62 ± 3 beats/min, P = 0.046) and SSNA (pre, 14,282 ± 3,706 arbitrary units; and post, 9,571 ± 2,609 arbitrary units, P = 0.034) at rest. In response to mental stress, SSNA increases were significantly attenuated following melatonin ingestion (second minute, 114 ± 30 vs. 74 ± 14%; and third minute, 111 ± 29 vs. 54 ± 12%, both P < 0.05). The mean arterial pressure increase to mental stress was blunted in the third minute (20 ± 2 vs. 17 ± 2 mmHg, P = 0.032), and the HR increase was blunted in the first minute (33 ± 3 vs. 29 ± 3 beats/min, P = 0.034) after melatonin. In summary, exogenous melatonin attenuates the SSNA response to mental stress.