Role of serotonergic input to the ventrolateral medulla in expression of the 10-Hz sympathetic nerve rhythm

2019 Ludwig Lecture: Rhythms in sympathetic nerve activity are a key to understanding neural control of the cardiovascular system

This review is based on the Carl Ludwig Distinguished Lecture, presented at the 2019 Experimental Biology Meeting in Orlando, FL, and provides a snapshot of >40 years of work done in collaboration with the late Gerard L. Gebber and colleagues to highlight the importance of considering the rhythmic properties of sympathetic nerve activity (SNA) and brain stem neurons when studying the neural control of autonomic regulation. After first providing some basic information about rhythms, I describe the patterns and potential functions of rhythmic activity recorded from sympathetic nerves under various physiological conditions. I review the evidence that these rhythms reflect the properties of central sympathetic neural networks that include neurons in the caudal medullary raphe, caudal ventrolateral medulla, caudal ventrolateral pons, medullary lateral tegmental field, rostral dorsolateral pons, and rostral ventrolateral medulla. The role of these brain stem areas in mediating steady-state and reflex-induced changes in SNA and blood pressure is discussed. Despite the common appearance of rhythms in SNA, these oscillatory characteristics are often ignored; instead, it is common to simply quantify changes in the amount of SNA to make conclusions about the function of the sympathetic nervous system in mediating responses to a variety of stimuli. This review summarizes work that highlights the need to include an assessment of the changes in the frequency components of SNA in evaluating the cardiovascular responses to various manipulations as well as in determining the role of different brain regions in the neural control of the cardiovascular system.

Deciphering the Neural Control of Sympathetic Nerve Activity: Status Report and Directions for Future Research

Sympathetic nerve activity (SNA) contributes appreciably to the control of physiological function, such that pathological alterations in SNA can lead to a variety of diseases. The goal of this review is to discuss the characteristics of SNA, briefly review the methodology that has been used to assess SNA and its control, and to describe the essential role of neurophysiological studies in conscious animals to provide additional insights into the regulation of SNA. Studies in both humans and animals have shown that SNA is rhythmic or organized into bursts whose frequency varies depending on experimental conditions and the species. These rhythms are generated by brainstem neurons, and conveyed to sympathetic preganglionic neurons through several pathways, including those emanating from the rostral ventrolateral medulla. Although rhythmic SNA is present in decerebrate animals (indicating that neurons in the brainstem and spinal cord are adequate to generate this activity), there is considerable evidence that a variety of supratentorial structures including the insular and prefrontal cortices, amygdala, and hypothalamic subnuclei provide inputs to the brainstem regions that regulate SNA. It is also known that the characteristics of SNA are altered during stress and particular behaviors such as the defense response and exercise. While it is a certainty that supratentorial structures contribute to changes in SNA during these behaviors, the neural underpinnings of the responses are yet to be established. Understanding how SNA is modified during affective responses and particular behaviors will require neurophysiological studies in awake, behaving animals, including those that entail recording activity from neurons that generate SNA. Recent studies have shown that responses of neurons in the central nervous system to most sensory inputs are context-specific. Future neurophysiological studies in conscious animals should also ascertain whether this general rule also applies to sensory signals that modify SNA.

What can we learn about neural control of the cardiovascular system by studying rhythms in sympathetic nerve activity?

Since the first recordings of sympathetic nerve activity in the 1930s, it was very clear that the activity was organized into bursts synchronized to the respiratory and cardiac cycles. Since the early studies, evidence has accumulated showing that sympathetic neural networks are quite complex and generate a variety of periodicities that range between ~0.04 and 10Hz, depending on the physiological state, type of nerve being analyzed, age of the subject, and the species. Despite the ubiquity of sympathetic rhythms, many investigators have failed to consider this oscillatory characteristic of sympathetic nerve activity and instead rely on simply quantifying changes in the level of activity to make decisions about the role of the sympathetic nervous system in mediating certain behaviors. This review highlights work that shows the importance of including an assessment of the frequency characteristics of sympathetic nerve activity.

Computational solution of spike overlapping using data-based subtraction algorithms to resolve synchronous sympathetic nerve discharge

Sympathetic nerves conveying central commands to regulate visceral functions often display activities in synchronous bursts. To understand how individual fibers fire synchronously, we establish "oligofiber recording techniques" to record "several" nerve fiber activities simultaneously, using in vitro splanchnic sympathetic nerve-thoracic spinal cord preparations of neonatal rats as experimental models. While distinct spike potentials were easily recorded from collagenase-dissociated sympathetic fibers, a problem arising from synchronous nerve discharges is a higher incidence of complex waveforms resulted from spike overlapping. Because commercial softwares do not provide an explicit solution for spike overlapping, a series of custom-made LabVIEW programs incorporated with MATLAB scripts was therefore written for spike sorting. Spikes were represented as data points after waveform feature extraction and automatically grouped by k-means clustering followed by principal component analysis (PCA) to verify their waveform homogeneity. For dissimilar waveforms with exceeding Hotelling's T(2) distances from the cluster centroids, a unique data-based subtraction algorithm (SA) was used to determine if they were the complex waveforms resulted from superimposing a spike pattern close to the cluster centroid with the other signals that could be observed in original recordings. In comparisons with commercial software, higher accuracy was achieved by analyses using our algorithms for the synthetic data that contained synchronous spiking and complex waveforms. Moreover, both T(2)-selected and SA-retrieved spikes were combined as unit activities. Quantitative analyses were performed to evaluate if unit activities truly originated from single fibers. We conclude that applications of our programs can help to resolve synchronous sympathetic nerve discharges (SND).

The influence of PC6 on cardiovascular disorders: a review of central neural mechanisms

PC6 is a classic acupuncture point in traditional Chinese medicine. It is considered to be effective when treating cardiovascular disorders. In the present review the authors have focused on the neurophysiological bases of the effects of PC6 stimulation on cardiovascular mechanisms. Experimental studies have shown that the hypothalamic rostral ventrolateral medulla, arcuate nucleus and ventrolateral periaqueductal gray are involved in acupuncture attenuation of sympathoexcitatory cardiovascular reflex responses. This long-loop pathway also appears to contribute to the long-lasting, acupuncture-mediated attenuation of sympathetic premotor outflow and excitatory cardiovascular reflex responses. Acupuncture of PC6 modulates the activity in the cardiovascular system, an effect that may be attributed to attenuation of sympathoexcitatory cardiovascular reflex responses.

Responses of opioid and serotonin containing medullary raphe neurons to electroacupuncture

The midline medulla oblongata, which includes the nucleus raphe obscurus, raphe magnus and raphe pallidus (NRP), is involved in regulation of cardiovascular responses. Opioids and serotonin (5-HT) are thought to function as important neurotransmitters in this region. We previously have demonstrated that electroacupuncture (EA) at the Neiguan-Jianshi acupoints (P5-P6, overlying the median nerves) attenuates sympathoexcitatory blood pressure reflexes through its influence on several brain regions. However, the role of these three raphe nuclei in the acupuncture responses is unknown. In baroreceptor denervated and vagotomized cats, the present study evaluated c-Fos activation in the raphe nuclei induced by EA and examined its relationship to enkephalin and 5-HT. To enhance detection of perikarya containing enkephalin, colchicine (90-100 microg/kg) was administered into the subarachnoid space in anesthetized cats 28-30 h before the placement of acupuncture needles at P5-P6 acupoints with or without electrical stimulation for 30 min. Perikarya containing the opioid and 5-HT were found in the raphe nuclei of all animals following application of colchicine. Compared to controls without electrical stimulation (n=5), c-Fos immunoreactivity and neurons double-labeled with c-Fos and either enkephalin or 5-HT were found more frequently in all three midline medullary nuclei, especially in NRP (n=6, all P<0.05) of EA-treated cats. Moreover, neurons triple-labeled with c-Fos, enkephalin and 5-HT were noted frequently in the NRP following EA stimulation. These results suggest that the medullary raphe nuclei, particularly the NRP, process somatic signals during EA and participate in EA-related modulation of cardiovascular function through an opioid or serotonergic mechanism.