New Approaches to Quantifying Sympathetic Nerve Activity

Effects of Postural Resonance on Skin Sympathetic Nerve Activity and Blood Pressure: A Pilot Study Evaluating Vascular Tone Baroreflex Stimulation Through Biofeedback

Heart rate and vascular tension baroreflex exhibit resonance characteristics at approximately 0.1 and 0.03 Hz. In this study, we aimed to induce postural resonance (PR) through rhythmic postural adjustments. To assess the viability of this technique, we investigated the acute impacts of postural resonance on blood pressure, sympathetic nerve activity, and mood. Fifteen healthy study participants, consisting of 8 males and 7 females, were selected for this self-controlled study. Skin sympathetic nerve activity was continuously monitored during both the intervention and stress test on the experimental day. After PR intervention, the diastolic blood pressure and mean arterial pressure in the PR group exhibited significant reductions compared to the CON group (P = 0.032, CON = 71.67 ± 2.348, PR = 64.08 ± 2.35; P = 0.041, CON = 75.00 ± 2.17, PR = 81.67 ± 2.17). After PR intervention both left brachial ankle pulse wave velocity and right brachial ankle pulse wave velocity exhibited a significant reduction compared to pre-intervention levels (from 1115.86 ± 150.08 to 1048.43 ± 127.40 cm/s, p < 0.001; 1103.86 ± 144.35 to 1060.43 ± 121.35 cm/s, p = 0.018). PR intervention also led to a significant decrease in burst frequency and duration (P = 0.049; CON = 8.96 ± 1.17, PR = 5.51 ± 1.17) and a noteworthy decrease in burst amplitude and burst threshold during the cold-pressor test (P = 0.002; P = 0.002). Additionally, VAS scores exhibited a substantial increase following PR (P = 0.035, CON = 28.4 ± 4.49, PR = 42.17 ± 4.10). PR can induce resonance effects within the cardiovascular system, resulting in the effective reduction of blood pressure, skin sympathetic nerve activity and pulse wave velocity, and decreased burst amplitude and burst threshold of the sympathetic nerve during the cold-pressor test.

Influence of sex and sedentary conditions on sympathetic burst characteristics in prepubertal, postpubertal, and young adult rats

Recent evidence indicates that sex-based differences in cardiovascular disease (CVD) begin early in life, particularly when associated with risk factors such as a sedentary lifestyle. CVD is associated with elevated sympathetic nerve activity (SNA), quantified as increased SNA burst activity in humans. Whether burst characteristics are influenced by sex or sedentary conditions at younger ages is unknown. The purpose of our study is to compare SNA bursts in active and sedentary female and male rats at ages including prepuberty and young adulthood. We hypothesized that burst characteristics and blood pressure are higher under sedentary conditions and lower in female rats compared with males. We analyzed splanchnic SNA (SpSNA) recordings from Inactin-anesthetized male and female rats at 4-, 8-, and 16-wk of age. Physically active and sedentary rats were each housed in separate, environmentally controlled chambers where physically active rats had free access to an in-cage running wheel. Sympathetic bursts were obtained by rectifying and integrating the raw SpSNA signal. Burst frequency, burst height, and burst width were calculated using the Peak Parameters extension in LabChart. Our results showed that sedentary conditions produced a greater burst width in 8- and 16-wk-old rats compared with 4-wk-old rats in both males and females (P < 0.001 for both). Burst frequency and incidence were both higher in 16-wk-old males compared with 16-wk-old females (P < 0.001 for both). Our results suggest that there are sedentary lifestyle- and sex-related mechanisms that impact sympathetic regulation of blood pressure at ages that range from prepuberty into young adulthood.NEW & NOTEWORTHY The mechanisms of decreased incidence of cardiovascular disease (CVD) in reproductive-age women compared with age-matched men are unknown. The strong association between elevated sympathetic activity and CVD led us to characterize splanchnic sympathetic bursts in female and male rats. Prepubescent males and females exhibited narrower sympathetic bursts, whereas young adult males had higher resting burst frequency compared with age-matched females. Sex-based regulation of sympathetic activity suggests a need for sex-dependent therapeutic strategies to combat CVD.

Sex-dependent development of enhanced sympathoexcitation in sedentary versus physically active rats

KEY POINTS

The rostral ventrolateral medulla (RVLM) may contribute to sex-based differences in cardiovascular disease (CVD) based on overactivation of the sympathetic nervous system observed in sedentary male rats; however, the added influence of the reproductive cycle in females is currently unknown. To our knowledge this is the first study to demonstrate greater increases in sympathetic nerve activity in response to direct activation of the RVLM in female versus male rats prior to the onset of the reproductive cycle, which persisted after the onset of the reproductive cycle. Lower resting blood pressures in females also suggest peripheral adaptations contribute to sex-based differences in CVD. Sedentary versus physically active conditions appear to promote higher resting sympathetic outflow independent of age and sex. Our results demonstrate the importance of examining sedentary conditions in the context of sex differences and the reproductive cycle in contributing to sympathetic overactivity associated with cardiovascular disease.

ABSTRACT

Female reproductive hormones are considered cardioprotective based on higher risks of cardiovascular disease (CVD) in post- versus pre-menopausal women. Similarly, based on epidemiological studies, a sedentary lifestyle is also a major risk factor for CVD. The mechanisms by which sedentary conditions contribute to CVD, and their influences in the presence and absence of female reproductive hormones are unknown. We hypothesized that sexually immature male and female rats would have similar centrally mediated regulation of blood pressure, but upon sexual maturation, female rats would have lower resting blood pressure and centrally-mediated sympathoexcitation compared to age-matched males. We also predicted resting sympathetic activity would increase upon exposure to sedentary versus active conditions (voluntary wheel running) in males but not in females. We recorded splanchnic sympathetic nerve activity (SSNA) and blood pressure in 4-, 8- and 16-week-old male and female rats under Inactin anaesthesia before and during microinjections of glutamate (1-100 mM) into the rostral ventrolateral medulla (RVLM). Four-week-old female rats had lower resting blood pressure and greater sympathoexcitation following activation of the RVLM, as did 8- and 16-week-old female rats, independent of age or activity condition. Sedentary animals had higher baseline SSNA compared to active animals, independent of sex or age. Our results reveal a complex influence of the interactions between the female reproductive cycle and sedentary conditions. They also demonstrate the importance of examining sedentary conditions in the context of sex- and female reproductive cycle-dependent incidences of cardiovascular disease.

Contribution of the Renal Nerves to Hypertension in a Rabbit Model of Chronic Kidney Disease

Overactivity of the sympathetic nervous system and high blood pressure are implicated in the development and progression of chronic kidney disease (CKD) and independently predict cardiovascular events in end-stage renal disease. To assess the role of renal nerves, we determined whether renal denervation (RDN) altered the hypertension and sympathoexcitation associated with a rabbit model of CKD. The model involves glomerular layer lesioning and uninephrectomy, resulting in renal function reduced by one-third and diuresis. After 3-week CKD, blood pressure was 13±2 mm Hg higher than at baseline (P<0.001), and compared with sham control rabbits, renal sympathetic nerve activity was 1.2±0.5 normalized units greater (P=0.01). The depressor response to ganglion blockade was also +8.0±3 mm Hg greater, but total norepinephrine spillover was 8.7±3.7 ng/min lower (bothP<0.05). RDN CKD rabbits only increased blood pressure by 8.0±1.5 mm Hg. Renal sympathetic activity, the response to ganglion blockade and diuresis were similar to sham denervated rabbits (non-CKD). CKD rabbits had intact renal sympathetic baroreflex gain and range, as well as normal sympathetic responses to airjet stress. However, hypoxia-induced sympathoexcitation was reduced by −9±0.4 normalized units. RDN did not alter the sympathetic response to hypoxia or airjet stress. CKD increased oxidative stress markers Nox5 and MCP-1 (monocyte chemoattractant protein-1) in the kidney, but RDN had no effect on these measures. Thus, RDN is an effective treatment for hypertension in this model of CKD without further impairing renal function or altering the normal sympathetic reflex responses to various environmental stimuli.

Novel Approach for Simultaneous Recording of Renal Sympathetic Nerve Activity and Blood Pressure with Intravenous Infusion in Conscious, Unrestrained Mice

Renal sympathetic nerves contribute significantly to both physiological and pathophysiological phenomena. Evaluating renal sympathetic nerve activity (RSNA) is of great interest in many areas of research such as chronic kidney disease, hypertension, heart failure, diabetes and obesity. Unequivocal assessment of the role of the sympathetic nervous system is thus imperative for proper interpretation of experimental results and understanding of disease processes. RSNA has been traditionally measured in anesthetized rodents, including mice. However, mice usually exhibit very low systemic blood pressure and hemodynamic instability for several hours during anesthesia and surgery. Meaningful interpretation of RSNA is confounded by this non-physiological state, given the intimate relationship between sympathetic nervous tone and cardiovascular status. To address this limitation of traditional approaches, we developed a new method for measuring RSNA in conscious, freely-moving mice. Mice were chronically instrumented with radio-telemeters for continuous monitoring of blood pressure as well as a jugular venous infusion catheter and custom-designed bipolar electrode for direct recording of RSNA. Following a 48-72 hour recovery period, survival rate was 100% and all mice behaved normally. At this time-point, RSNA was successfully recorded in 80% of mice, with viable signals acquired up to 4 and 5 days post-surgery in 70% and 50% of mice, respectively. Physiological blood pressures were recorded in all mice (116±2 mmHg; n=10). Recorded RSNA increased with eating and grooming, as well-established in the literature. Furthermore, RSNA was validated by ganglionic blockade and modulation of blood pressure with pharmacological agents. Herein, an effective and manageable method for clear recording of RSNA in conscious, freely-moving mice is described.

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/.

Central proopiomelanocortin but not neuropeptide Y mediates sympathoexcitation and hypertension in fat fed conscious rabbits

OBJECTIVE

High-fat diet (HFD)-induced hypertension in rabbits is neurogenic because of the central sympathoexcitatory actions of leptin. Hypothalamic melanocortin and neuropeptide Y (NPY) neurons are recognized as the major signalling pathways through which leptin exerts its central effects. In this study, we assessed the effects of specific antagonists and agonists to melanocortin and NPY receptors on HFD-induced sympathoexcitation and hypertension.

METHODS

Rabbits were instrumented with intracerebroventricular cannula, renal sympathetic nerve activity (RSNA) electrode, and blood pressure telemetry transmitter.

RESULTS

After 3 weeks HFD (13.5% fat, n = 12) conscious rabbits had higher RSNA (+3.8  nu, P = 0.02), blood pressure (+8.6  mmHg, P < 0.001) and heart rate (+15  b/min, P = 0.01), and brain-derived neurotrophic factor levels in the hypothalamus compared with rabbits fed a control diet (4.2% fat, n = 11). Intracerebroventricular administration of the melanocortin receptor antagonist SHU9119 reduced RSNA (-2.7  nu) and blood pressure (-8.5  mmHg) in HFD but not control rabbits, thus reversing 100% of the hypertension and 70% of the sympathoexcitation induced by a HFD. By contrast, blocking central NPY Y1 receptors with BVD10 increased RSNA only in HFD rabbits. Intracerebroventricular α-melanocortin stimulating hormone increased RSNA and heart rate (P < 0.001) in HFD rabbits but had no effect in control rabbits.

CONCLUSION

These findings suggest that obesity-induced hypertension and increased RSNA are dependent on the balance between greater activation of melanocortin signalling through melanocortin receptors and lesser activation of NPY sympathoinhibitory signalling. The amplification of the sympathoexcitatory effects of α-melanocortin stimulating hormone also indicates that the underlying mechanism is related to facilitation of leptin-melanocortin signalling, possibly involving chronic activation of brain-derived neurotrophic factor.

Current Approaches to Quantifying Tonic and Reflex Autonomic Outflows Controlling Cardiovascular Function in Humans and Experimental Animals

The role of the autonomic nervous system in the pathophysiology of human and experimental models of cardiovascular disease is well established. In the recent years, there have been some rapid developments in the diagnostic approaches used to assess and monitor autonomic functions. Although most of these methods are devoted for research purposes in laboratory animals, many have still found their way to routine clinical practice. To name a few, direct long-term telemetry recording of sympathetic nerve activity (SNA) in rodents, single-unit SNA recording using microneurography in human subjects and spectral analysis of blood pressure and heart rate in both humans and animals have recently received an overwhelming attention. In this article, we therefore provide an overview of the methods and techniques used to assess tonic and reflex autonomic functions in humans and experimental animals, highlighting current advances available and procedure description, limitations and usefulness for diagnostic purposes.

Effects of leptin on sympathetic nerve activity in conscious mice

The adipocyte-derived hormone, leptin, has emerged as an important regulator of regional sympathetic nerve activity (SNA) with pathophysiological implications in obesity. Genetically engineered mice are useful to understand the molecular pathways underlying the SNA responses evoked by leptin. However, so far the effect of leptin on direct SNA in mice has been studied under general anesthesia. Here, we examined the sympathetic responses evoked by leptin in conscious mice. Mice were instrumented, under ketamine/xylazine anesthesia, with renal or lumbar SNA recordings using a thin (40 gauge) bipolar platinum-iridium wire. The electrodes were exteriorized at the nape of the neck and mice were allowed (5 h) to recover from anesthesia. Interestingly, the reflex increases in renal and lumbar SNA caused by sodium nitroprusside (SNP)-induced hypotension was higher in the conscious phase versus the anesthetized state, whereas the increase in both renal and lumbar SNA evoked by leptin did not differ between anesthetized or conscious mice. Next, we assessed whether isoflurane anesthesia would yield a better outcome. Again, the SNP-induced increase in renal SNA and baroreceptor-renal SNA reflex were significantly elevated in the conscious states relative to isoflurane-anesthetized phase, but the renal SNA response induced by leptin in the conscious states were qualitatively comparable to those evoked above. Thus, despite improvement in sympathetic reflexes in conscious mice the sympathetic responses evoked by leptin mimic those induced during anesthesia.

Direct conscious telemetry recordings demonstrate increased renal sympathetic nerve activity in rats with chronic kidney disease

Chronic kidney disease (CKD) is associated with sympathetic hyperactivity and impaired blood pressure control reflex responses, yet direct evidence demonstrating these features of autonomic dysfunction in conscious animals is still lacking. Here we measured renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP) using telemetry-based recordings in a rat model of CKD, the Lewis Polycystic Kidney (LPK) rat, and assessed responses to chemoreflex activation and acute stress. Male LPK and Lewis control animals (total n = 16) were instrumented for telemetric recording of RSNA and MAP. At 12–13 weeks-of-age, resting RSNA and MAP, sympathetic and haemodynamic responses to both peripheral (hypoxia: 10% O₂) and central chemoreflex (hypercapnia: 7% CO₂) activation and acute stress (open-field exposure), were measured. As indicators of renal function, urinary protein (U_Pro) and creatinine (U_Cr) levels were assessed. LPK rats had higher resting RSNA (1.2 ± 0.1 vs. 0.6 ± 0.1 μV, p < 0.05) and MAP (151 ± 8 vs. 97 ± 2 mmHg, p < 0.05) compared to Lewis. MAP was negatively correlated with U_Cr (r = −0.80, p = 0.002) and positively correlated with RSNA (r = 0.66, p = 0.014), with multiple linear regression modeling indicating the strongest correlation was with U_cr. RSNA and MAP responses to activation of the central chemoreflex and open-field stress were reduced in the LPK relative to the Lewis (all p < 0.05). This is the first description of dual conscious telemetry recording of RSNA and MAP in a genetic rodent model of CKD. Elevated RSNA is likely a key contributor to the marked hypertension in this model, while attenuated RSNA and MAP responses to central chemoreflex activation and acute stress in the LPK indicate possible deficits in the neural processing of autonomic outflows evoked by these sympathoexcitatory pathways.

[Position paper on the results of Symplicity HTN-3 trial. Grupo de estudio de la hipertensión arterial resistente]

Renal artery denervation has shown to be an effective treatment for resistant hypertension. Symplicity HTN 1 and 2 trials showed in small and uncontrolled groups, significant systolic blood pressure reductions down to 30 mm Hg. Symplicity HTN-3, a double blind, randomized, placebo controlled clinical trial shaded this initial enthusiasm. Surprisingly, their results showed that renal denervation has a similar effect to placebo. Pre-specified subgroup analysis showed that non-black race individuals, younger than 65 years and with normal renal function, had a statistically significant systolic blood pressure decrease. This manuscript critically appraises the Symplicity HTN-3 trial, proposing possible explanations for the results. Also declares our group position and future actions regarding renal denervation.

Recording sympathetic nerve activity chronically in rats: surgery techniques, assessment of nerve activity, and quantification

The sympathetic nervous system plays a pivotal role in homeostasis through its direct innervation and functional impact on a variety of end organs. In rats, a number of methods are available to assess sympathetic nervous system function. Traditionally, direct recording of sympathetic nerve activity (SNA) has been restricted to acute, anesthetized preparations or conscious animals within a few days after electrode implantation. However, these approaches provide short-term data in studies designed to investigate changes in SNA during chronic disease states. Over the last several years, chronic SNA recording has been pioneered in rabbits and more recently in rats. The purpose of this article is to provide insights and a "how to" guide for chronic SNA recordings in rats based on experiences from two independent laboratories. We will present common methodologies used to chronically record SNA, characteristics and methods to distinguish sympathetic bursts versus electrical artifacts (and provide corresponding audio clips when available), and provide suggestions for analysis and presentation of data. In many instances, these same guidelines are applicable to acute SNA recordings. Using the surgical approaches described herein, both laboratories have been able to chronically record SNA in >50% of rats for a duration >3 wk. The ability to record SNA over the time course of several weeks will, undoubtedly, greatly impact the field of autonomic and cardiovascular physiology.

Translational physiology and SND recordings in humans and rats: a glimpse of the recent past with an eye on the future

The sympathetic nervous system (SNS) plays an important role in cardiovascular function, and based on the critical mechanistic relationship between altered sympathetic neural mechanisms and cardiovascular disease, it is important that the autonomic research community identifies deficiencies in the translational exchange of information and strives for a more thorough understanding of the translational significance of findings from studies involving sympathetic nerve discharge (SND) regulation in human and animal subjects. The present review assesses the state of the literature regarding studies that have used direct recordings of SND during the past three decades in humans and rats, focusing on; 1) identifying the number of studies reporting SND recordings in humans and rats, 2) briefly describing the translational exchange of SND regulation information from these studies, 3) contrasting the number of studies completed in anesthetized and conscious rats, and 4) assessing the prevalence of long-term SND recording studies in conscious rats. The majority of SND recordings in rats have been completed using anesthetized preparations, although a substantial number of studies have been completed in conscious rats. However, few studies have completed long-term (>5 days) SND recordings in freely-behaving rats, and even fewer studies have used experimental preparations that combine long-term nerve recordings with the capacity for completing central neural microinjections, or have been completed in animal models of cardiovascular disease. The wide-spread implementation of long-term SND recordings in rodent models of cardiovascular disease would be expected to enhance the translational exchange of clinically-relevant information between animals and humans.

Selective enhancement of glutamate-mediated pressor responses after GABA(A) receptor blockade in the RVLM of sedentary versus spontaneous wheel running rats

Overactivity of the sympathetic nervous system (SNS) is a hallmark of many cardiovascular diseases. It is also well-known that physical inactivity independently contributes to cardiovascular diseases, likely in part via increased SNS activity. Recent work from our laboratory has demonstrated increased SNS responses in sedentary animals following either direct activation or disinhibition of the rostral ventrolateral medulla (RVLM), an integral cardiovascular brainstem region. These data led us to hypothesize that the interaction between excitation and inhibition of the RVLM is altered in sedentary versus physically active animals. To test this hypothesis, we recorded mean arterial pressure (MAP) and lumbar sympathetic nerve activity (LSNA) in Inactin anesthetized rats that were housed for 8-12 weeks with or without access to a running wheel. Pressor responses to direct activation of the RVLM with glutamate were similar between groups under intact conditions. However, blockade of γ-aminobutyric acid (GABA)(A) receptors with bicuculline selectively enhanced pressor responses to glutamate in sedentary animals. Interestingly, LSNA responses to glutamate were not enhanced in sedentary versus active animals in the presence or absence of tonic GABAergic tone. These results suggest that sedentary compared to active conditions enhance GABAergic inhibition of glutamate-sensitive neurons in the RVLM that are involved in blood pressure regulation, and by mechanisms that do not involve LSNA. We also speculate that regular physical activity has differential effects on SNS activity to specific vascular beds and may reduce the risk of developing cardiovascular diseases via changes occurring in the RVLM.

Advantage of recording single-unit muscle sympathetic nerve activity in heart failure

Elevated sympathetic activation is a characteristic feature of heart failure (HF). Excessive sympathetic activation under resting conditions has been shown to increase from the early stages of the disease, and is related to prognosis. Direct recording of multiunit efferent muscle sympathetic nerve activity (MSNA) by microneurography is the best method for quantifying sympathetic nerve activity in humans. To date, this technique has been used to evaluate the actual central sympathetic outflow to the periphery in HF patients at rest and during exercise; however, because the firing occurrence of sympathetic activation is mainly synchronized by pulse pressure, multiunit MSNA, expressed as burst frequency (bursts/min) and burst incidence (bursts/100 heartbeats), may have limitations for the quantification of sympathetic nerve activity. In HF, multiunit MSNA is near the maximum level, and cannot increase further than the heartbeat. Single-unit MSNA analysis in humans is technically demanding, but provides more detailed information regarding central sympathetic firing. Although a great deal is known about the response of multiunit MSNA to stress, little information is available regarding the responses of single-unit MSNA to physiological stress and disease. The purposes of this review are to describe the differences between multiunit and single-unit MSNA during stress and to discuss the advantages of single-unit MSNA recording in improving our understanding the pathology of increased sympathetic activity in HF.

(In)activity-dependent alterations in resting and reflex control of splanchnic sympathetic nerve activity

The negative effects of sympathetic overactivity on long-term cardiovascular health are becoming increasingly clear. Moreover, recent work done in animal models of cardiovascular disease suggests that sympathetic tone to the splanchnic vasculature may play an important role in the development and maintenance of these disease states. Work from our laboratory and others led us to hypothesize that a lack of chronic physical activity increases resting and reflex-mediated splanchnic sympathetic nerve activity, possibly through changes occurring in a key brain stem center involved in sympathetic regulation, the rostral ventrolateral medulla (RVLM). To address this hypothesis, we recorded mean arterial pressure (MAP) and splanchnic sympathetic nerve activity (SSNA) in a group of active and sedentary animals that had been housed for 10-13 wk with or without running wheels, respectively. In experiments performed under Inactin anesthesia, we tested responses to RVLM microinjections of glutamate, responses to baroreceptor unloading, and vascular reactivity, the latter of which was performed under conditions of autonomic blockade. Sedentary animals exhibited enhanced resting SSNA and MAP, augmented increases in SSNA to RVLM activation and baroreceptor unloading, and enhanced vascular reactivity to α(1)-receptor mediated vasoconstriction. Our results suggest that a sedentary lifestyle increases the risk of cardiovascular disease by augmenting resting and reflex-mediated sympathetic output to the splanchnic circulation and also by increasing vascular sensitivity to adrenergic stimulation. We speculate that regular physical exercise offsets or reverses the progression of these disease processes via similar or disparate mechanisms and warrant further examination into physical (in)activity-induced sympathetic nervous system plasticity.