Comparison of blood pressure and sympathetic activity of rabbits in their home cage and the laboratory environment

Method of measuring effects of study procedures in single and pair housed New Zealand White rabbits (Oryctolagus cuniculus)

Social housing of laboratory rabbits is encouraged and thought to improve animal welfare due to the social nature of this species. However, there is limited published information comparing the physiologic and cardiovascular (CV) effects of paired and single housed adult female rabbits in commonly used laboratory caging. This study describes measurement of heart rate, systolic blood pressure, activity level, body temperature and pairing methods in four female New Zealand White rabbits that were previously implanted with M10 cardiovascular telemetry devices. Data was collected in single housed rabbits having no history of social housing while they were undisturbed in the home cage, during restraint, intramuscular injections and intravenous blood collection. The same animals were then placed in compatible pairs and housed in conventional Allentown caging. As expected, we found increased activity in paired rabbits but no significant differences in body temperatures, and CV parameters in single and paired rabbits undergoing the same procedures. These data suggest that paired rabbits can be used for safety pharmacology studies with minimal impact to data, while supporting improved animal welfare.

Fluid Resuscitation for Refractory Hypotension

Hypotension is a common occurrence, especially in anesthetized patients and in critical patients suffering from hypovolemia due to shock and sepsis. Hypotension can also occur in normovolemic animals, anesthetized or conscious, under conditions of vasodilation or decreased cardiac function. The main consequence of hypotension is decreased organ perfusion and tissue injury/dysfunction. In the human literature there is no consensus on what is the threshold value for hypotension, and ranges from < 80 to < 100 mmHg for systolic blood pressure and from < 50 to < 70 mmHg for mean arterial blood pressure have been referenced for intraoperative hypotension. In veterinary medicine, similar values are referenced, despite marked differences in normal arterial blood pressure between species and with respect to humans. Therapeutic intervention involves fluid therapy to normalize volemia and use of sympathomimetics to enhance cardiac function and regulate peripheral vascular resistance. Despite these therapeutic measures, there is a subset of patients that are seemingly refractory and exhibit persistent hypotension. This review covers the physiological aspects that govern arterial blood pressure control and blood flow to tissues/organs, the pathophysiological mechanisms involved in hypotension and refractory hypotension, and therapeutic considerations and expectations that include proper interpretation of cardiovascular parameters, fluid recommendations and therapy rates, use of sympathomimetics and vasopressors, and newer approaches derived from the human literature.

Cardiovascular effects of increasing dosages of norepinephrine in healthy isoflurane-anesthetized New Zealand White rabbits

OBJECTIVE

To characterize the cardiovascular effects of increasing dosages of norepinephrine (NE) in healthy isoflurane-anesthetized rabbits.

STUDY DESIGN

Prospective experimental study.

ANIMALS

A total of nine female ovariohysterectomized New Zealand White rabbits weighing 3.4 ± 0.2 kg (mean ± standard deviation).

METHODS

Rabbits were premedicated intramuscularly with buprenorphine (0.05 mg kg^-1) and midazolam (0.5 mg kg^-1). Anesthesia was induced with intravenous propofol and maintained with a 1.1 × minimum alveolar concentration of isoflurane for this species to induce hypotension. Rabbits were administered NE infusions at three doses: low, 0.1 μg kg^-1 minute^-1; medium, 0.5 μg kg^-1 minute^-1; and high doses, 1 μg kg^-1 minute^-1 for 10 minutes each in that order. Cardiovascular variables including heart rate (HR), cardiac output (CO) by lithium dilution technique and systolic (SAP), mean (MAP) and diastolic (DAP) invasive arterial blood pressures measured in the auricular artery were recorded at baseline, 10 minutes after the start of the infusion of each NE treatment and 10 minutes after NE was discontinued. A linear mixed model and a type III anova with Tukey's post hoc comparison was performed (p < 0.05).

RESULTS

Significant increases in SAP (28% and 90%), MAP (27% and 90%) and DAP (33% and 97%) were measured with medium and high dose treatments, respectively (p < 0.001), with no changes in CO. HR decreased and stroke volume increased significantly with high dose treatment (by 17% and 15%, respectively; p < 0.05). No arrhythmias were noticed with NE treatments.

CONCLUSIONS AND CLINICAL RELEVANCE

The infusion of NE at 0.5-1.0 μg kg^-1 minute^-1 is a potentially effective treatment for hypotension in healthy isoflurane-anesthetized New Zealand White rabbits.

Sympathomimetics in veterinary species under anesthesia

Sympathomimetic drugs mimic the physiological action of the sympathetic nervous system through interaction with adrenergic receptors. These drugs are commonly used to provide cardiovascular support in many veterinary species. Despite their common use, the literature evaluating their effectiveness can be somewhat limited depending on the species. This review details the mechanism of action of various sympathomimetic drugs and summarizes the literature that is available describing the efficacy of these drugs and their use in anesthetized veterinary species.

Continuous Rate Infusion of Ketamine Hydrochloride and Dexmedetomidine for Maintenance of Anesthesia during Laryngotracheal Surgery in New Zealand White Rabbits (Oryctolagus cuniculus)

New Zealand white rabbits (Oryctolagus cuniculus) are an established in vivo model for the study of structural and functional consequences of vocal-fold vibration. Research design requires invasive laryngotracheal procedures, and the presence of laryngospasms or pain responses (or both) hinder phonation-related data collection. Published anesthesia regimens report respiratory depression and muscle tone changes and have been unsuccessful in mitigating autonomic laryngeal responses in our protocol. Infusion of ketamine hydrochloride and dexmedetomidine hydrochloride in pediatric medicine provides effective analgesia and sedation for laryngotracheal procedures including intubation and bronchoscopy; however, data evaluating the use of ketamine-dexmedetomidine infusion in rabbits are unavailable. This study reports a new infusion regimen, which was used in 58 male New Zealand white rabbits that underwent a nonsurvival laryngotracheal procedure to induce phonotraumatic vocal-fold injury. Animals were sedated by using ketamine hydrochloride (20 mg/kg IM) and dexmedetomidine (0.125 mg/kg IM). Maintenance anesthesia was provided by using continuous rate intravenous infusion of ketamine hydrochloride (343 μg/kg/min) and dexmedetomidine (1.60 μg/kg/min). A stable plane of anesthesia with no autonomic laryngeal response (laryngospasm) was achieved in 32 of the 58 rabbits (55%). Laryngospasms occurred in 25 of 58 animals (43%) and were controlled in 20 cases (80%) by providing 0.33 mL 2% topical lidocaine, incremental increase in infusion rate, or both. Continuous rate infusion of ketamine hydrochloride-dexmedetomidine with prophylactic topical lidocaine provides a predictable and adjustable surgical plane of anesthesia, with minimal confounding respiratory and autonomic laryngeal responses, during extended-duration laryngotracheal surgery in rabbits. This regimen should be considered as an alternative to injection maintenance for prolonged, invasive procedures.

Autonomic nerves and circadian control of renal function

Cardiovascular and renal physiology follow strong circadian rhythms. For instance, renal excretion of solutes and water is higher during the active period compared to the inactive period, and blood pressure peaks early in the beginning of the active period of both diurnal and nocturnal animals. The control of these rhythms is largely dependent on the expression of clock genes both in the central nervous system and within peripheral organs themselves. Although it is understood that the central and peripheral clocks interact and communicate, few studies have explored the specific mechanism by which various organ systems within the body are coordinated to control physiological processes. The renal sympathetic nervous innervation has long been known to have profound effects on renal function, and because the sympathetic nervous system follows strong circadian rhythms, it is likely that autonomic control of the kidney plays an integral role in modulating renal circadian function. This review highlights studies that provide insight into this interaction, discusses areas lacking clarity, and suggests the potential for future work to explore the role of renal autonomics in areas such as blood pressure control and chronic kidney disease.

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.

Comparison of sympathetic nerve activity normalization procedures in conscious rabbits

One of the main constraints associated with recording sympathetic nerve activity (SNA) in both humans and experimental animals is that microvolt values reflect characteristics of the recording conditions and limit comparisons between different experimental groups. The nasopharyngeal response has been validated for normalizing renal SNA (RSNA) in conscious rabbits, and in humans muscle SNA is normalized to the maximum burst in the resting period. We compared these two methods of normalization to determine whether either could detect elevated RSNA in hypertensive rabbits compared with normotensive controls. We also tested whether either method eliminated differences based only on different recording conditions by separating RSNA of control (sham) rabbits into two groups with low or high microvolts. Hypertension was induced by 5 wk of renal clipping (2K1C), 3 wk of high-fat diet (HFD), or 3 mo infusion of a low dose of angiotensin (ANG II). Normalization to the nasopharyngeal response revealed RSNA that was 88, 51, and 34% greater in 2K1C, HFD, and ANG II rabbits, respectively, than shams (P < 0.05), but normalization to the maximum burst showed no differences. The RSNA baroreflex followed a similar pattern whether RSNA was expressed in microvolts or normalized. Both methods abolished the difference between low and high microvolt RSNA. These results suggest that maximum burst amplitude is a useful technique for minimizing differences between recording conditions but is unable to detect real differences between groups. We conclude that the nasopharyngeal reflex is the superior method for normalizing sympathetic recordings in conscious rabbits.

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.