Arterial baroreceptors in the management of systemic hypertension.

Hypertension is a multifactorial disease associated with significant morbidity. Increased sympathetic nervous system activity has been noted as an important etiologic factor and is, in part, regulated by afferent input arising from arterial and cardiopulmonary baroreceptors, activation of which causes inhibition of sympathetic output. It was thought for many years that baroreceptors control only short-term blood pressure changes, a conclusion stemming from observations in sinoaortic denervation (SAD) animal models and the phenomenon of rapid baroreceptor resetting, also seen in animal models. Newer observations, however, indicate that SAD is rather imperfect and resetting is rarely complete. Recent studies reveal that baroreceptors control sympathetic output on a more long-term basis and participate in fluid volume regulation by the kidney, and thus have the potential to adjust blood pressure chronically. Importantly, these findings are consistent with studies and observations in humans. Meanwhile, a model of electrical stimulation of the carotid sinus has been developed and successfully tested in animals. Following these encouraging results human trials to evaluate the clinical application of electrical carotid sinus manipulation in the treatment of systemic hypertension have commenced, and results so far indicate that this represents an exciting potential tool in the clinician's armament against chronic arterial hypertension.

Syncope and carotid artery stenosis.

The carotid sinus plays an important role in systemic blood pressure regulation. The baroreceptor mechanism that underlies this physiological process is affected by underlying disease processes and therapeutic procedures. Stenosis of the carotid artery is seldom implicated in causation of syncope. Syncope caused by pathology at the carotid sinus and iatrogenically during interventional procedures is more frequently seen. The consequences of carotid stenosis and carotid hypersensitivity leading to cerebral hypoperfusion with syncope are described, along with recommendations related to screening in these patients.

Interactions of carotid sinus or aortic input with emetic signals from gastric afferents and vestibular system.

This study investigated how baro- and chemoreceptor afferents interact with emetic signals from gastric afferents and the vestibular system, and how these interactions modulate emetic and prodromal responses. We performed splanchnic denervation and abdominal vagotomy in anesthetized shrews (Suncus murinus), and then induced emetic responses by gastric distension. Next, we investigated the effects of these gastric afferent sections on cardiovascular and emetic responses induced by electrical stimulation of the aortic depressor nerve (ADN) and the carotid sinus nerve (CSN) with or without gastric distension. Splanchnic denervation abolished the prodromal response before retching and aortic baroreflex inhibition caused by gastric distension, but had no effects on the emetic response. In contrast, abdominal vagotomy abolished the emetic response induced by gastric distension with or without CSN stimulation, but without affecting gastric distension-induced or CSN stimulation-induced vascular and respiratory responses. In conscious animals, CSN denervation significantly suppressed veratrine- and motion-induced emetic responses, whereas ADN denervation had no significant effects. These results suggest that aortic baroreflex inhibition via the activation of splanchnic afferents contributes to the prodromal response before retching and circulatory homeostasis. In contrast, carotid sinus inputs, which are usually non-emetic signals, interact with vagal and vestibular inputs, and modulate the development of retching and vomiting.

Electrophysiological and neuroanatomical evidence of sexual dimorphism in aortic baroreceptor and vagal afferents in rat.

Evidence for sexual dimorphism in autonomic control of cardiovascular function is both compelling and confounding. Across healthy and disease populations sex-associated differences in neurocirculatory hemodynamics are far too complex to be entirely related to sex hormones. As an initial step toward identifying additional physiological mechanisms, we investigated whether there is a sex bias in the relative expression of low-threshold-myelinated and high-threshold-unmyelinated aortic baroreceptor afferents in rats. These two types of afferent fibers have markedly different reflexogenic effects upon heart rate and blood pressure and thus the potential impact upon baroreflex dynamics could be substantial. Our results, using a combination of a patch-clamp study of fluorescently identified aortic baroreceptor neurons (ABN) and morphometric analysis of aortic baroreceptor nerve fibers, demonstrate that females exhibit a greater percentage of myelinated baroreceptor fibers (24.8% vs. 18.7% of total baroreceptor fiber population, P < 0.01) and express a functional subtype of myelinated ABN rarely found in age-matched males (11% vs. 2.3%, n = 107, P < 0.01). Interestingly, this neuronal phenotype is more prevalent in the general population of female vagal afferent neurons (17.7% vs. 3.8%, n = 169, P < 0.01), and ovariectomy does not alter its expression but does lessen neuronal excitability. These data suggest there are fundamental neuroanatomical and electrophysiological differences between aortic baroreceptor afferents of female and male rats. Possible explanations are presented as to how such a greater prevalence of low-threshold myelinated afferents could be a contributing factor to the altered baroreflex sensitivity and vagal tone of females compared with males.

Autonomic nervous system dysfunction after spinal cord injury.

The autonomic nervous system (ANS) plays a key role in the regulation of many physiologic processes, mediated by supraspinal control from centers in the central nervous system. The role of autonomic dysfunction in persons with spinal cord injuries is crucial to understand because many aspects of the altered physiology seen in these individuals are directly caused by ANS dysregulation.

Barosensory cells in the nucleus tractus solitarius receive convergent input from group III muscle afferents and central command.

Some neural mechanism must prevent the full expression of the baroreceptor reflex during static exercise because arterial blood pressure increases even though the baroreceptors are functioning. Two likely candidates are central command and input from the thin fiber muscle afferents evoking the exercise pressor reflex. Recently, activation of the mesencephalic locomotor region, an anatomical locus for central command, was found to inhibit the discharge of nucleus tractus solitarius cells that were stimulated by arterial baroreceptors in decerebrated cats. In contrast, the effect of thin fiber muscle afferent input on the discharge of nucleus tractus solitarius cells stimulated by baroreceptors is not known. Consequently in decerebrated unanesthetized cats, we examined the responses of barosensory nucleus tractus solitarius cells to stimulation of thin fiber muscle afferents and to stimulation of the mesencephalic locomotor region, a maneuver which evoked fictive locomotion. We found that electrical stimulation of either the mesencephalic locomotor region or the gastrocnemius nerve at current intensities that recruited group III afferents inhibited the discharge of nucleus tractus solitarius cells receiving baroreceptor input. We also found that the inhibitory effects of both gastrocnemius nerve stimulation and mesencephalic locomotor region stimulation converged onto the same barosensory nucleus tractus solitarius cells. We conclude that the nucleus tractus solitarius is probably the site whereby input from both central command and thin fiber muscle afferents function to reset the baroreceptor reflex during exercise.

Modulation of emetic response by carotid baro- and chemoreceptor activations.

We hypothesized that baroreceptor or chemoreceptor activation might be involved in the emetic, and prodromal cardiovascular and respiratory responses. To test this hypothesis, we induced the emetic responses by gastric distension in anesthetized Suncus murinus (house musk shrew), that had intact and absent baroreceptor and chemoreceptor afferents. Secondly, we stimulated the aortic depressor nerve (ADN) and the carotid sinus nerve (CSN) with or without gastric distension. Internal carotid artery ligation in the bifurcation area, which abolished reflex bradycardia by baroreceptor activation, and abolition of chemoreceptor reflex bradycardia and hyperventilation, by carotid body denervation, suppressed the emetic response but did not abolish it. ADN denervation, which produced no significant effects on the baroreceptor or chemoreceptor reflex bradycardia, had no effect on the emetic response, including the prodromal phase. CSN stimulation with gastric distension elicited retching accompanied by reflex bradycardia and hypotension during or just after stimulation, whereas ADN stimulation with gastric distension did not induce the cardiovascular reflex, and had no effects on the emetic response. These results indicate that carotid, rather than aortic, baroreceptor or chemoreceptor activation plays an important role in the augmentation of cardiac parasympathetic activity and the development of emetic response. In conclusion, carotid baroreceptor or chemoreceptor activation, which is non-emetic stimulation, acts as a modulator in the central mechanisms of emesis.

Morphological evidence of reinnervation of the baroreceptive regions in sinoaortic-denervated rats.

1. The arterial baroreflex (ABR) plays an important role in the maintenance of the stability of blood pressure. Sinoaortic denervation (SAD) destroys the integrity of the reflex arc and produces severe organ damage in rats. However, partial recovery of ABR function has been observed following chronic denervation. The aim of the present study was to determine whether there was morphological evidence of reinnervation of the aortic arch and carotid sinus following SAD. 2. A substantial body of physiological and morphological evidence suggests that substance P (SP) may be a neurotransmitter contained in first-order sensory baroreceptor afferents; therefore, the patterns of vascular SP and neurofilament (NF) immunoreactive (IR) innervation of the aortic arch and carotid sinus were investigated in the present study. 3. Ten-week-old male Sprague-Dawley rats underwent SAD or sham operation. Whole mounts of carotid bifurcation and aortic arch were prepared for immunohistochemical study at various time points (1, 9 and 16 weeks after operation). 4. The results of computerized image analysis show that the mean density of NF- and SP-IR nerves of SAD rats 9 and 16 weeks after operation increased gradually and significantly compared with that of rats 1 week after operation. 5. In conclusion, the results indicate that there is reinnervation of the aortic arch and carotid sinus by NF- and SP-IR fibres in SAD rats, which may be the morphological basis for the partial restoration of ABR function over time after SAD.

Baroreceptive and somatosensory convergent thalamic neurons project to the posterior insular cortex in the rat.

Connectivity between the rat posterior insula and the ventrobasal thalamus has been demonstrated anatomically. Neurons convergent for baroreceptor and nociceptive input have also been identified in the homologous anterior insula of the primate. Whether similar convergent cells exist in the ventrobasal thalamus was investigated in 30 urethane anesthetized male Sprague--Dawley rats. Six classes of cells were identified in the right ventrobasal thalamus: (a) 83/159 (52%) baroreceptive and nociceptive convergent units; (b) 2/159 (1%) convergent cells responding to baroreceptor activation and light touch; (c) 44/159 (28%) purely nociceptive units; (d)10/159 (6%) purely baroreceptive units; (e) 1/159 (0.6%) cells responding to brush alone and (f) 19/159 (12%) unresponsive units. Of the viscerosomatic convergent cells, 66/85 (78%) were situated in the ventroposterolateral nucleus (VPL), 6/85 (7%) in the ventroposterolateral parvicellular nucleus (VPLpc), and 13/85 (15%) in the ventroposteromedial nucleus (VPM). Fifteen right ventrobasal thalamic units were antidromically activated and 34 units orthodromically activated by right posterior insular microstimulation. Cobalt injection into the right ventrobasal thalamus blocked the right insular response to baroreceptor activation by >70%. These data indicate: (a) baroreceptive and somatosensory nociceptive convergent units exist in the ventrobasal thalamus; (b) thalamic convergent neurons project directly to the ipsilateral posterior insula and receive reciprocal insulothalamic projections; and (c) a significant proportion of baroreceptor input relays to the posterior insula through the ipsilateral ventrobasal thalamus.

Barosensitive neurons in the rostral ventrolateral medulla of the rat in vivo: morphological properties and relationship to C1 adrenergic neurons.

The aim of this study, conducted in anaesthetized rats, was to examine the morphology of barosensitive neurons in the rostral ventrolateral medulla and their immunoreactivity for a catecholamine synthesizing enzyme, tyrosine hydroxylase. Thirty neurons displaying inhibitory postsynaptic potentials following stimulation of the aortic depressor nerve were intracellularly labelled with Lucifer Yellow or Neurobiotin. Some of these neurons could be excited antidromically from the second thoracic segment of the spinal cord, with conduction velocities of spinal axons ranging from 1.9 to 7.2 m/s. The filled somas were found immediately caudal to the facial nucleus and ventral or ventromedial to compact formation of the nucleus ambiguus. Some dendrites reached the ventral medullary surface. Axons usually projected dorsomedially and then made a sharp rostral and/or caudal turn. The caudally projecting axon could, in some cases, be followed to the first cervical segment of the spinal cord. Seven cells issued fine axon collaterals on the ipsilateral side. These were identified mainly in two areas: in the rostral ventrolateral medulla (or immediately dorsomedial to that region), and within the dorsal vagal complex. Seven of 27 examined cells (26%) were tyrosine hydroxylase-immunoreactive and were classified as C1 adrenergic neurons. No clear relationship was found between the presence or absence of adrenergic phenotype and the morphology of filled cells. However, the amplitude of aortic nerve-evoked inhibitory postsynaptic potentials was significantly larger in tyrosine hydroxylase-positive neurons. Possible reasons for the low percentage of barosensitive cells with tyrosine hydroxylase immunoreactivity found in this study, in comparison with previously published estimates, are discussed. This is the first study describing the morphology of neurons in this part of the medulla identified as barosensitive in vivo, and directly demonstrating adrenergic phenotype in a subset of these neurons.

Functional morphology of the paratympanic organ in the middle ear of birds.

The paratympanic organ (PTO) is a small sense organ in the middle ear of birds. This luminal organ contains mechanoreceptors (hair cells) with afferent and efferent innervation and may function as a baroreceptor. The hypothesis that elastic ligaments of the middle ear may be involved in the transduction of barometric pressure was tested. Two elastic ligaments are shown to attach to the PTO. The columellar-squamosal ('Platner's') ligament inserts at its caudal pole; the superior portion of the superior drum-tubal ligament attaches at the rostral tip of the organ. To determine if pressure to the tympanic membrane and tension of ligaments may cause lumen changes in the PTO, the length of elastic ligaments, and the length, volume and configuration of the PTO were measured in normal animals and in animals following application of positive or negative pressure to the tympanic membrane. The rostral pole of the PTO changes its shape differentially as a function of the tension of the superior drum-tubal ligament, resulting in volume shifts of about 10% of the total volume in the PTO. With negative pressure, volume moves from the rostral pole to the caudal half of the PTO; with positive pressure, volume shifts rostrally. Displacement of fluid in the lumen of the PTO thus may stimulate the paratympanic hair cells. Tracing of efferent projections to the PTO with the fluorescent compound DiI reveals a cluster of about 40 labeled neurons at the caudal pole of the ventral facial motor nucleus adjacent to the superior olive. These findings support the notion that the PTO may be part of a neural circuit that controls the position of the tympanic membrane and may mediate barometric perception in birds.

Localization and retention in vitro of fluorescently labeled aortic baroreceptor terminals on neurons from the nucleus tractus solitarius.

The anterograde fluorescent tracer DiA was used to visualize baroreceptor fibers and synaptic terminals both in living and fixed tissue. Baroreceptor fibers labeled with DiA terminated as a dense synaptic field in the medial nucleus tractus solitarius (NTS), making synaptic contact on the soma, as well as processes of neurons that they innervated. A similar distribution and morphology was observed in baroreceptor fibers and terminals labeled with horseradish peroxidase. DiA also identified baroreceptor terminals and the neurons receiving these synaptic contacts in vitro. NTS neurons were dissociated from their surrounding tissue and identified by attached baroreceptor terminals that retained the fluorescent dye. These results will enable us to study the electrophysiological properties of dispersed neurons that receive identified baroreceptor synaptic terminals.

New technique to completely isolate carotid sinus baroreceptor regions in rats.

We developed a method by which we can completely isolate the carotid sinus baroreceptor regions in the rat. The carotid sinus baroreceptor region is exposed and, with the use of extra-fine forceps, a human hair is placed around and tied at the root of the bifurcation. This procedure occludes the external carotid artery and blood flow to the carotid body. An injector is then attached to a catheter in the common carotid artery. We introduce a cylindrical rubber plug into either the palentine or internal carotid artery. A second plug is introduced to occlude the other artery. In six of the eight rats studied, these procedures completely isolated the carotid sinus region. In those cases where a small leak persisted at a carotid sinus pressure of 180 mmHg, we introduced a small particle of the animal's own previously clotted blood. Carotid sinus pressure was either randomly changed between 40 and 180 mmHg in 20-mmHg increments or in sequential 20-mmHg steps from 40 to 180 mmHg while measuring the animal's pulsatile and mean blood pressures. Arterial pressure-carotid sinus pressure relationship indicates that there is a highly sigmoidal relationship between the two pressures. The peak gain of the carotid sinus reflex system had a range from 1.5 to 4.0 and a mean value of 2.07 +/- 0.08. Our data indicate that the rat exhibits a significant carotid sinus baroreceptor reflex response. This technique combined with other techniques will allow for the study of neural control of cardiovascular function in the rat.

Morphology of the carotid sinus wall in normotensive and spontaneously hypertensive rats.

The morphology of the carotid sinus region of the internal carotid artery was studied in spontaneously hypertensive rats (SHR) at 5, 8, 16, and 24 weeks of age. The carotid sinus region occupied the proximal millimeter of the internal carotid artery, and was easily recognizable by the presence of an extensive adventitial capillary plexus, which was absent on adjacent arteries (e.g., common and external carotid arteries). Methylene blue-stained whole-mount preparations showed the extent of baroreceptor nerves over the sinus. Baroreceptor fibers terminated in distinctive bulbous-like endings, which, at the ultrastructural level, were filled with mitochondria. No differences were noted in the sinus adventitial capillary network or baroreceptor distribution between SHR and age-matched Wistar-Kyoto (WKY) normotensive control animals. With the onset of a significant rise in SHR blood pressure, the carotid sinus wall increased in thickness and total vessel size. The wall/lumen ratios were significantly larger in the SHR than in age-matched WKY ratios in all age groups. SHR carotid sinus vessel enlargement was uniform throughout the vessel tunics, with no significant change in the proportion of the tunica media occupied by smooth muscle cells. The increase in the carotid sinus wall thickness associated with increasing hypertension could affect the ability of the sinus to distend and may play a secondary role in the maintenance of hypertension by compromising baroreceptor nerve ending sensitivity.

Brain stem projections of the glossopharyngeal nerve and its carotid sinus branch in the rat.

Transganglionic transport of horseradish peroxidase or lectin-conjugated horseradish peroxidase from an application site in the cervical trunk of the glossopharyngeal (IXth cranial) nerve of the rat produced extraperikaryal reaction product characteristic of axon terminal processes in three regions of the brain stem: (1) the nucleus of the tractus solitarius, from approximately 2.5 mm rostral to the obex to approximately 3 mm caudal to the obex; (2) the spinal trigeminal nucleus at the level of obex; (3) the cuneate fasciculus, approximately 3 mm caudal to the obex. In contrast, labelling of the carotid sinus nerve, a branch of the glossopharyngeal nerve which conveys chemoreceptor and baroreceptor afferent fibers from the carotid bifurcation, revealed a restricted central projection to within 1 mm of the obex and corresponding to the intermediate region of the glossopharyngeal nerve projection to the nucleus of the tractus solitarius. Two distinct aggregations of label were observed: (1) rostral to the obex, within the lateral and dorsomedial subnuclei of the nucleus of the tractus solitarius; (2) caudal to the obex, within the commissural and ventrolateral subnuclei of the nucleus of the tractus solitarius. Between these two sites the density of labelling was reduced. Retrogradely labelled neurons were demonstrated in the inferior salivatory nucleus and in the nucleus ambiguus after application of lectin-conjugated horseradish peroxidase to the glossopharyngeal nerve. Of the labelled neurons in the nucleus ambiguus (approximately 100), 25% contributed fibers to the carotid sinus nerve. The concentration of extraperikaryal reaction product located rostral to the obex after labelling of the carotid sinus nerve closely matches descriptions of the region of afferent terminations from carotid and aortic baroreceptors in the cat. The concentration of label caudal to the obex may therefore correspond to the region of afferent terminations from carotid chemoreceptors. This study may therefore provide some basis for a separation of the central synapses of primary afferent fibers from the carotid baroreceptors and chemoreceptors in the rat. The labelled neurons of the nucleus ambiguus provide the anatomical substrate for centrifugal control of carotid chemoreceptor activity.

Afferent connections and spinal projections of the pressor region in the rostral ventrolateral medulla of the cat.

Following microinjection of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the pressor region of the rostral ventrolateral medulla of the cat, the medulla, pons and hypothalamus were examined for retrogradely labelled cell bodies, while the thoracolumbar segments of the spinal cord were examined for anterogradely labelled axons. Dense groups of labelled cells were found in the following areas: (1) the nucleus of the solitary tract, particularly the medial, ventrolateral and commissural subnuclei; (2) the ambiguous complex and immediately surrounding area; (3) the Kölliker-Fuse nucleus in the pons; (4) the paraventricular nucleus and lateral hypothalamic area. In the spinal cord, labelled axons formed a band extending throughout the dorsolateral and ventrolateral funiculi at thoracic segments, while terminal labelling was observed in the intermediolateral nucleus and to a lesser extent the central autonomic area, but not in other parts of the grey matter. The findings are discussed in relation to the role of the rostral ventrolateral medulla in cardiovascular regulation, particularly the baroreceptor reflex.