Role of the medullary lateral tegmental field in reflex-mediated sympathoexcitation in cats

Responses of Neurons in the Medullary Lateral Tegmental Field and Nucleus Tractus Solitarius to Vestibular Stimuli in Conscious Felines

Considerable evidence shows that the vestibular system contributes to adjusting sympathetic nervous system activity to maintain adequate blood pressure during movement and changes in posture. However, only a few prior experiments entailed recordings in conscious animals from brainstem neurons presumed to convey baroreceptor and vestibular inputs to neurons in the rostral ventrolateral medulla (RVLM) that provide inputs to sympathetic preganglionic neurons in the spinal cord. In this study, recordings were made in conscious felines from neurons in the medullary lateral tegmental field (LTF) and nucleus tractus solitarius (NTS) identified as regulating sympathetic nervous system activity by exhibiting changes in firing rate related to the cardiac cycle, or cardiac-related activity (CRA). Approximately 38% of LTF and NTS neurons responded to static 40° head up tilts with a change in firing rate (increase for 60% of the neurons, decrease for 40%) of ~50%. However, few of these neurons responded to 10° sinusoidal rotations in the pitch plane, in contrast to prior findings in decerebrate animals that the firing rates of both NTS and LTF neurons are modulated by small-amplitude body rotations. Thus, as previously demonstrated for RVLM neurons, in conscious animals NTS and LTF neurons only respond to large rotations that lead to changes in sympathetic nervous system activity. The similar responses to head-up rotations of LTF and NTS neurons with those documented for RVLM neurons suggest that LTF and NTS neurons are components of the vestibulo-sympathetic reflex pathway. However, a difference between NTS/LTF and RVLM neurons was variability in CRA over time. This variability was significantly greater for RVLM neurons, raising the hypothesis that the responsiveness of these neurons to baroreceptor input is adjusted based on the animal's vigilance and alertness.

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.

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.

Processing of vestibular inputs by the medullary lateral tegmental field of conscious cats: implications for generation of motion sickness

The dorsolateral reticular formation of the caudal medulla, the lateral tegmental field (LTF), participates in generating vomiting. LTF neurons exhibited complex responses to vestibular stimulation in decerebrate cats, indicating that they received converging inputs from a variety of labyrinthine receptors. Such a convergence pattern of vestibular inputs is appropriate for a brain region that participates in generating motion sickness. Since responses of brainstem neurons to vestibular stimulation can differ between decerebrate and conscious animals, the current study examined the effects of whole-body rotations in vertical planes on the activity of LTF neurons in conscious felines. Wobble stimuli, fixed-amplitude tilts, the direction of which moves around the animal at a constant speed, were used to determine the response vector orientation, and also to ascertain whether neurons had spatial-temporal convergence (STC) behavior (which is due to the convergence of vestibular inputs with different spatial and temporal properties). The proportion of LTF neurons with STC behavior in conscious animals (25 %) was similar to that in decerebrate cats. Far fewer neurons in other regions of the feline brainstem had STC behavior, confirming findings that many LTF neurons receive converging inputs from a variety of labyrinthine receptors. However, responses to vertical plane vestibular stimulation were considerably different in decerebrate and conscious felines for LTF neurons lacking STC behavior. In decerebrate cats, most LTF neurons had graviceptive responses to rotations, similar to those of otolith organ afferents. However, in conscious animals, the response properties were similar to those of semicircular canal afferents. These differences show that higher centers of the brain that are removed during decerebration regulate the labyrinthine inputs relayed to the LTF, either by gating connections in the brainstem or by conveying vestibular inputs directly to the region.

Responses of neurons in the caudal medullary lateral tegmental field to visceral inputs and vestibular stimulation in vertical planes

The dorsolateral reticular formation of the caudal medulla, or the lateral tegmental field (LTF), has been classified as the brain's "vomiting center", as well as an important region in regulating sympathetic outflow. We examined the responses of LTF neurons in cats to rotations of the body that activate vestibular receptors, as well as to stimulation of baroreceptors (through mechanical stretch of the carotid sinus) and gastrointestinal receptors (through the intragastric administration of the emetic compound copper sulfate). Approximately half of the LTF neurons exhibited graviceptive responses to vestibular stimulation, similar to primary afferents innervating otolith organs. The other half of the neurons had complex responses, including spatiotemporal convergence behavior, suggesting that they received convergent inputs from a variety of vestibular receptors. Neurons that received gastrointestinal and baroreceptor inputs had similar complex responses to vestibular stimulation; such responses are expected for neurons that contribute to the generation of motion sickness. LTF units with convergent baroreceptor and vestibular inputs may participate in producing the cardiovascular system components of motion sickness, such as the changes in skin blood flow that result in pallor. The administration of copper sulfate often modulated the gain of responses of LTF neurons to vestibular stimulation, particularly for units whose spontaneous firing rate was altered by infusion of drug (median of 459%). The present results raise the prospect that emetic signals from the gastrointestinal tract modify the processing of vestibular inputs by LTF neurons, thereby affecting the probability that vomiting will occur as a consequence of motion sickness.

Ionotropic glutamate receptors in the external lateral parabrachial nucleus participate in processing cardiac sympathoexcitatory reflexes

Stimulation of cardiac sympathetic afferents during myocardial ischemia with metabolites such as bradykinin (BK) evokes sympathoexcitatory reflex responses and activates neurons in the external lateral parabrachial nucleus (elPBN). The present study tested the hypothesis that this region in the pons processes sympathoexcitatory cardiac reflexes through an ionotropic glutamate receptor mechanism. The ischemic metabolite BK (0.1-1 μg) was injected into the pericardial space of anesthetized and bilaterally vagotomized or intact cats. Hemodynamic and renal sympathetic nerve activity (RSNA) responses to repeated administration of BK before and after unilateral 50-nl microinjections of kynurenic acid (Kyn; 25 mM), 2-amino-5-phosphonopentanoic acid (AP5; 25 mM), and 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzol(F)quinoxaline (NBQX; 10 mM) into the elPBN were recorded. Intrapericardial BK evoked significant increases in mean arterial pressure (MAP) and RSNA in seven vagotomized cats. After blockade of glutamate receptors with the nonselective glutamate receptor antagonist Kyn, the BK-evoked reflex increases in MAP (50 ± 6 vs. 29 ± 2 mmHg) and RSNA (59 ± 8.6 vs. 29 ± 4.7%, before vs. after) were significantly attenuated. The BK-evoked responses returned to pre-Kyn levels 85 min after the application of Kyn. Similarly, BK-evoked reflex responses were reversibly attenuated by blockade of glutamate N-methyl-d-aspartate (NMDA) receptors with AP5 (n = 5) and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors with NBQX (n = 5). In contrast, we observed that the repetitive administration of BK evoked consistent reflex responses including MAP and RSNA before and after microinjection of 50 nl of the artificial cerebrospinal fluid vehicle into the elPBN in five animals. Microinjection of glutamate receptor antagonists into regions outside the elPBN did not alter BK-induced reflex responses. Microinjection of Kyn into the elPBN reversibly attenuated BK-induced reflex responses in four vagus intact animals. These data are the first to show that NMDA and AMPA ionotropic glutamate receptors in the elPBN play an important role in processing cardiac excitatory reflex responses.

Rostral ventrolateral medullary but not medullary lateral tegmental field neurons mediate sympatho-sympathetic reflexes in cats

This study was designed to build on past work from this laboratory by testing the hypothesis that medullary lateral tegmental field (LTF) neurons play a critical role in mediating sympathoexcitatory responses to activation of sympathetic afferent fibers. We studied the effects of microinjection of N-methyl-d-aspartate (NMDA) or non-NMDA receptor antagonists or muscimol bilaterally into the LTF on the area under the curve of the computer-averaged sympathoexcitatory potential in the right inferior cardiac nerve elicited by short trains of stimuli applied to afferent fibers in the left inferior cardiac or left splanchnic nerve (CN, SN) of baroreceptor-denervated and vagotomized cats anesthetized with a mixture of diallylbarbiturate and urethane. In contrast to our hypothesis, sympathoexcitatory responses to stimulation of CN (n = 5-7) or SN (n = 4-7) afferent fibers were not significantly affected by these procedures. We then determined whether the rostral and caudal ventrolateral medulla (RVLM, CVLM) and nucleus tractus solitarius (NTS) were involved in mediating these reflexes. Blockade of non-NMDA, but not NMDA, receptors in the RVLM significantly reduced the area under the curve of the sympathoexcitatory responses to electrical stimulation of either CN (P = 0.0110; n = 6) or SN (P = 0.0131; n = 5) afferent fibers. Neither blockade of excitatory amino acid receptors nor chemical inactivation of CVLM or NTS significantly affected the responses. These data show that activation of non-NMDA receptors in the RVLM is a critical step in mediating the sympatho-sympathetic reflex.

Role of the medulla oblongata in normal and high arterial blood pressure regulation: the contribution of Escola Paulista de Medicina - UNIFESP

Several forms of experimental evidence gathered in the last 37 years have unequivocally established that the medulla oblongata harbors the main neural circuits responsible for generating the vasomotor tone and regulating arterial blood pressure. Our current understanding of this circuitry derives mainly from the studies of Pedro Guertzenstein, a former student who became Professor of Physiology at UNIFESP later, and his colleagues. In this review, we have summarized the main findings as well as our collaboration to a further understanding of the ventrolateral medulla and the control of arterial blood pressure under normal and pathological conditions.

New insights into the electrophysiology of brainstem circuits controlling blood pressure

The brainstem contains the necessary circuitry for the maintenance and regulation of arterial blood pressure. It has become increasingly clear in the past few years that the characteristics of the neurons that constitute these circuits are not static, but can be altered in the face of chronic changes in physiological state. Alterations in voltage-gated and ligand-gated ion channels have been reported in neurons located within the nucleus of the solitary tract and the nucleus ambiguus in response to hypertension and exposures to hypoxia and environmental pollutants (eg, ozone and cigarette smoke). A discussion of these neuronal adaptations, the mechanisms that might initiate and sustain the adaptations, and their potential significance is the focus of this brief review.

Medullary lateral tegmental field neurons influence the timing and pattern of phrenic nerve activity in cats

In an effort to characterize the role of the medullary lateral tegmental field (LTF) in regulating respiration, we tested the effects of selective blockade of excitatory (EAA) and inhibitory amino acid (IAA) receptors in this region on phrenic nerve activity (PNA) of vagus-intact and vagotomized cats anesthetized with dial-urethane. We found distinct patterns of changes in central respiratory rate, duration of inspiratory and expiratory phases of PNA (Ti and Te, respectively), and I-burst amplitude after selective blockade of EAA and IAA receptors in the LTF. First, blockade of N-methyl-D-aspartate (NMDA) receptors significantly (P < 0.05) decreased central respiratory rate primarily by increasing Ti but did not alter I-burst amplitude. Second, blockade of non-NMDA receptors significantly reduced I-burst amplitude without affecting central respiratory rate. Third, blockade of GABAA receptors significantly decreased central respiratory rate by increasing Te and significantly reduced I-burst amplitude. Fourth, blockade of glycine receptors significantly decreased central respiratory rate by causing proportional increases in Ti and Te and significantly reduced I-burst amplitude. These changes in PNA were markedly different from those produced by blockade of EAA or IAA receptors in the pre-Bötzinger complex. We propose that a proper balance of excitatory and inhibitory inputs to several functionally distinct pools of LTF neurons is essential for maintaining the normal pattern of PNA in anesthetized cats.

Medullary lateral tegmental field mediates the cardiovascular but not respiratory component of the Bezold-Jarisch reflex in the cat

We determined the effects of bilateral microinjection of muscimol and excitatory amino acid receptor antagonists into the medullary lateral tegmental field (LTF) on changes in sympathetic nerve discharge (SND), mean arterial pressure (MAP), and phrenic nerve activity (PNA; artificially ventilated cats) or intratracheal pressure (spontaneously breathing cats) elicited by right atrial administration of phenylbiguanide (PBG; i.e., the Bezold-Jarisch reflex) in dial-urethane anesthetized cats. The PBG-induced depressor response (−66 ± 8 mmHg; mean ± SE) was converted to a pressor response after muscimol microinjection in two of three spontaneously breathing cats and was markedly reduced in the other cat; however, the duration of apnea (20 ± 3 vs. 17 ± 7 s) was essentially unchanged. In seven paralyzed, artificially ventilated cats, muscimol microinjection significantly ( P < 0.05) attenuated the PBG-induced fall in MAP (−39 ± 7 vs. −4 ± 4 mmHg) and the magnitude (−98 ± 1 vs. −35 ± 13%) and duration (15 ± 2 vs. 3 ± 2 s) of the sympathoinhibitory response. In contrast, the PBG-induced inhibition of PNA was unaffected (3 cats). Similar results were obtained by microinjection of an N-methyl-d-aspartate (NMDA) receptor antagonist, d(-)-2-amino-5-phosphonopentanoic acid, into the LTF. In contrast, neither the cardiovascular nor respiratory responses to PBG were altered by blockade of non-NMDA receptors with 1,2,3,4-tetrahydro-6-nitro-2,3-dioxobenzo[ f]quinoxaline-7-sulfonamide. We conclude that the LTF subserves a critical role in mediating the sympathetic and cardiovascular components of the Bezold-Jarisch reflex. Moreover, these data show separation of the pathways mediating the respiratory and cardiovascular responses of this reflex at a level central to bulbospinal outflows to phrenic motoneurons and preganglionic sympathetic neurons.

Medullary lateral tegmental field: control of respiratory rate and vagal lung inflation afferent influences on sympathetic nerve discharge

We used spectral analysis and event-triggered averaging to determine the effects of chemical inactivation of the medullary lateral tegmental field (LTF) on 1) the relationship of intratracheal pressure (ITP, an index of vagal lung inflation afferent activity) to sympathetic nerve discharge (SND) and phrenic nerve activity (PNA) and 2) central respiratory rate in paralyzed, artificially ventilated dial-urethane-anesthetized cats. ITP-SND coherence value at the frequency of artificial ventilation was significantly ( P < 0.05; n = 18) reduced from 0.73 ± 0.04 (mean ± SE) to 0.24 ± 0.04 after bilateral microinjection of muscimol into the LTF. Central respiratory rate was unexpectedly increased in 12 of these experiments (0.28 ± 0.03 vs. 0.95 ± 0.25 Hz). The ITP-PNA coherence value was variably affected by chemical inactivation of the LTF. It was unchanged when central respiratory rate was also not altered, decreased when respiratory rate was increased above the rate of artificial ventilation, and increased when respiratory rate was raised from a value below the rate of artificial ventilation to the same frequency as the ventilator. Chemical inactivation of the LTF increased central respiratory rate in four of six vagotomized cats but did not significantly affect the PNA-SND coherence value. These data demonstrate that the LTF 1) plays a critical role in mediating the effects of vagal lung inflation afferents on SND but not PNA, 2) helps maintain central respiratory rate in the physiological range, but 3) is not involved in the coupling of central respiratory and sympathetic circuits.