John Ciriello

Function of the ventrolateral medulla in the control of the circulation

The CNS control of the cardiovascular system involves the coordination of a series of complex neural mechanisms which integrate afferent information from a variety of peripheral receptors and produce control signals to effector organs for appropriate physiological responses. Although it is generally thought that these control signals are generated by a network of neural circuits that are widely distributed in the CNS, over the last two decades a considerable body of experimental evidence has accumulated suggesting that several of these circuits involve neurons found on or near the ventral surface of the medulla oblongata. Neurons in the VLM have been shown to be involved in the maintenance of vasomotor tone, in baroreceptor and chemoreceptor (central and peripheral) reflex mechanisms, in mediating the CIR and somatosympathetic reflexes and in the control of the secretion of vasopressin. These physiological functions of VLM neurons have been supported by neuroanatomical and electrophysiological studies demonstrating direct connections with a number of central structures previously implicated in the control of the circulation, including the IML, the site of origin of sympathetic preganglionic axons, and the SON and PVH, the site of origin of neurohypophyseal projecting axons containing AVP. Considerable suggestive evidence has also been obtained regarding the chemical messengers involved in transmitting information from VLM neurons to other central structures. There have been developments suggesting a role for monoamines and neuropeptides in mediating the neural and humoral control of SAP by neurons in the VLM. This review presents a synthesis of the literature suggesting a main role for VLM neurons in the control of the circulation.

Brainstem projections of aortic baroreceptor afferent fibers in the rat.

Brainstem projections of the aortic nerve in the rat were studied using the transganglionic transport of horseradish peroxidase. Labeled axons were found to project predominantly to the ipsilateral interstitial nucleus and to the ipsilateral dorsolateral aspect of the nucleus of the solitary tract near the level of the obex. Lighter bilateral projections were also found to the medial, ventrolateral and dorsolateral aspects of the solitary complex, and to the commissural nucleus. These data provide evidence of direct aortic baroreceptor afferent projections to restricted regions of the solitary complex and indicate that these specific areas function in the integration of the baroreceptor reflex.

Central projections of afferent renal fibers in the rat: an anterograde transport study of horseradish peroxidase.

The projections of afferent renal fibers (ARN) to dorsal root ganglia and into the spinal cord of the rat were studied using the anterograde transport of horseradish peroxidase (HRP). Crystalline HRP was applied to the proximal cut ends of renal nerves or injected as a concentrated solution into the kidney, on either the right or left side. After a survival time of 40-120 h, sections of thoraco-lumbar dorsal root ganglia (DRG) and spinal cord were cut and processed according to the tetramethyl benzidine method. HRP applied either to the ARN or to the kidney on the left labeled neurons in the DRG from T8 to L2. On the other hand, HRP application on the right side resulted in labeling of neurons in DRG T6 to T13. No labeled neurons were found in the contralateral DRG. Labeled neurons in the DRG were of the small (11-20 micron) and medium (30-42 micron) size and were distributed in all portions of the DRG. In the spinal cord the greatest concentration of labeled ARN from the left were found in segments T10-L1, whereas projections from the right ARN were concentrated primarily in segments T7-T10. Labeled fibers entered along the medial aspect of the ipsilateral dorsal horn and projected both rostrally and caudally in the medial portion of Lissauers tract, sending some collaterals into lamina I. The majority of labeled fibers coursed ventrally along the medial aspect of the dorsal horn towards the midline where they terminated in the region of the dorsal gray commissure, just dorsal to the central canal. Additionally, labeled fibers from the medial projection passed into laminae III-V. No labeled fibers or terminals were observed in the contralateral spinal cord. These data show that ARN enter the spinal cord through several DRG and provide the first anatomical demonstration of central sites of termination of ARN. These spinal sites of projection of sensory information from the kidney are likely to be central sites of integration of reno-renal and visceral reflexes.

Horseradish peroxidase study of brain stem projections of carotid sinus and aortic depressor nerves in the cat.

Abstract The connections of carotid sinus (CSN) and aortic depressor (ADN) afferent fibers in the brain stem of the cat were studied using horseradish peroxidase (HRP). Crystalline HRP was applied to the proximal cut end of either the CSN or ADN for 4–10.5 h and after a survival period of 24–120 h the animals were perfused and frozen sections of brain stem, nodose and petrosal ganglia were processed according to the tetramethyl-benzidine method. CSN labeled axons were found to project ipsilaterally to several nuclei of the solitary complex: the dorsal aspect of the medial (Sm), the lateral (Slt), the ventrolateral (Svl), the commissural (Com), the intermediate (Int) and the dorsomedial aspect of the parvocellular (Spc) solitary nuclei. These nuclei, except for the Svl, also received a less intense contralateral projection. Additional terminal labeling was observed on the ipsilateral side along the dorsal border of the dorsal motor nucleus of the vagus (DMV), in the reticular formation ventral to the solitary complex, and in the ventrolateral external cuneate nucleus, and labeled fibers were found in the dorsolateral spinal trigeminal tract. On the other hand, ADN labeled fibers were found to project only to the solitary complex, bilaterally. Terminal labeling was found primarily in the Sm, Slt, Com and dorsal Spc. After HRP application to the CSN few cells were found labeled in the petrosal ganglion; in addition clusters of labelled neurons were found bilaterally in the region of the rostral nucleus ambiguus, retrofacial and facial nuclei, and in the ipsilateral rostral dorsomedial reticular formation. ADN labeled ganglion cells were identified in clusters in the medial aspect of the nodose ganglion primarily near the entry of the superior laryngeal nerve; additional clusters of labeled smaller neurons were found in the ventromedial portion of the ganglion and intermingled with vagal fibers just caudal to the ganglion. No HRP-positive cells were identified in the brain stem after ADN labeling. These data demonstrate that different regions of the solitary complex receive direct inputs from either one or both buffer nerves, suggesting a degree of separation of central pathways carrying CSN and ADN afferent information. Furthermore, the finding of labeled cell bodies in the medulla after CSN labeling suggests a possible route by which the central nervous system may alter activity of receptors in the carotid sinus and body.

Direct pathway from cardiovascular neurons in the ventrolateral medulla to the region of the intermediolateral nucleus of the upper thoracic cord: an anatomical and electrophysiological investigation in the cat.

Horseradish peroxidase (HRP) and single unit recording experiments were done in cats to identify neurons in the ventrolateral medulla (VLM) projecting directly to the intermediolateral nucleus (IML) of the thoracic cord and relaying cardiovascular afferent information from the buffer nerves and hypothalamus. In the first series, HRP was allowed to diffuse from a micropipette into the region of the IML at the level of T2. After a survival period of 30-138 h, transverse and horizontal sections of the brainstem were processed according to the tetramethyl benzidine method. Labeled neurons were found in the VLM 1-5 mm rostral to the obex, bilaterally, but with an ipsilateral predominance. The majority were observed in sections 2-4 mm rostral to the obex, clustered in an area lateral to the inferior olivary nucleus around the intramedullary rootlets of the hypoglossal nerve. Additional labeled neurons were found scattered along the ventral surface of the medulla; most of these neurons were oval in shape, 15-30 micron in diameter, and had dendritic processes which lay parallel to the ventral surface. In the second series, the region of the VLM shown to contain labeled neurons was systematically explored for single units antidromically activated by electrical stimulation of the IML in chloralosed, paralyzed and artificially ventilated animals. These antidromically identified units were then tested for their responses to electrical stimulation of the carotid sinus (CSN) and aortic depressor (ADN) nerves, and the paraventricular nucleus (PVH). Ninety-four single units in the VLM were antidromically activated with latencies corresponding to a mean conduction velocity of 19.1 +/- 1.5 m/s. Of these units 52% (49/94) were orthodromically excited by stimulation of buffer nerves; 12 by stimulation of the CSN only (mean latency, 16.0 +/- 3.6 ms), 5 by stimulation of the ADN only (mean latency, 9.5 +/- 2.0 ms), 7 by both buffer nerves, and the remaining 25 units responded to at least one of the buffer nerves and to PVH. Stimulation of PVH excited orthodromically 42 of the 94 units (45%), of which 17 responded only to stimulation of PVH (mean latency, 17.9 +/- 3.5 ms). These experiments provide anatomical and electrophysiological evidence for the existence of a direct cardiovascular pathway from the VLM to the region of the IML and suggest that neurons in the VLM are involved in the integration of cardiovascular afferent inputs from buffer nerves and the hypothalamus to provide an excitatory input to vasoconstrictor neurons in the IML.

Renal afferent nerves affect discharge rate of medullary and hypothalamic single units in the cat

Abstract Electrical activity of spontaneously firing single units in the medulla and hypothalamus of 22 cats anesthetized with chloralose was monitored for changes in firing frequency during electrical stimulation of afferent renal (ARN) and carotid sinus (CSN) nerves. Stimulation of ARN altered the firing frequency of 214 out of 540 units studied in the ipsi- and contralateral medulla; the majority of the responses were excitatory but a few units (8%) were inhibited by stimulation. Of the units responding to ARN stimulation, 57% were found to respond in the same manner to stimulation of the CSN. Responsive units were found primarily in 3 regions: the lateral tegmental field, the area of the paramedian reticular nucleus and the region of the dorsal vagal complex around the obex. In the hypothalamus stimulation of ARN affected the activity of 197 of the 407 units studied ipsi- and contralaterally; the majority of the units were excited but 8% were found to be inhibited. Of the units responding to ARN 75% also responded to stimulation of the CSN. Responsive units were found in most areas but were concentrated in 3 anterior regions: lateral preoptic area, lateral hypothalamic area and the region of the paraventricular nucleus. This is the first demonstration that stimulation of afferent renal nerves can influence the electrical activity of medullary and hypothalamic neurons bilaterally. Because of the demonstrated physiological role of the structures where these responsive units were found these results suggest that sensory receptors in the kidney convey important information to central sites involved in physiological responses related to cardiovascular adjustments and fluid balance. Furthermore it has been demonstrated that the majority of medullary and hypothalamic neurons responding to stimulation of ARN also receive an input from the CSN suggesting that certain regions of both medulla and hypothalamus can integrate peripheral information from the kidney and from cardiovascular receptors to bring about appropriate homeostatic responses.

Glossopharyngeal and vagal afferent projections to the brain stem of the cat: A horseradish peroxidase study

Brain stem projections of the glossopharyngeal and vagus nerves in the cat were studied using the anterograde transport of horseradish peroxidase (HRP). Crystalline HRP was applied to the proximal cut ends of the nerves for a period of 4-10.5 h, and after a survival time of 24-120 h, transverse and horizontal sections of the brain stem were processed according to the tetramethylbenzidine method. Labeled fibers from both nerves were found to project bilaterally to the solitary complex, and ipsilaterally to the ventral region of the external cuneate nucleus and to the medial region of the nucleus praepositus hypoglossi, just dorsolateral to the medial longitudinal fasciculus. Within the solitary complex terminal labeling was found in the parvocellular, ventrolateral, lateral, medial and commissural solitary nuclei. Exclusive glossopharyngeal nerve projections were found ipsilaterally in the rostral dorsal motor nucleus of the vagus, the ventrolateral portion of the medial cuneate nucleus, the dorsal part of the nuclei caudalis and interpolaris of the trigeminal complex, the nuclei insulae cuneati lateralis, and the dorsolateral aspect of the nucleus medullae oblongata centralis. Finally, in the area postrema a bilateral projection of vagal and an ipsilateral projection of glossopharyngeal fibers were found. These findings demonstrate that the glossopharyngeal nerve has more widely distributed brain stem projections that the vagus nerve and provide essential information on projection sites of visceral and taste inputs to the central nervous system.

Lesions of the paraventricular nucleus alter the development of spontaneous hypertension in the rat.

The role of the paraventricular nucleus of the hypothalamus (PVH) in the development of hypertension was determined after bilateral electrolytic or sham lesions of this structure in 4-5-week-old male spontaneously hypertensive rats (SHR). The average arterial pressure in the PVH-lesioned group was significantly lower compared to sham-lesioned animals during the first 3 weeks after the PVH lesions. At 9 weeks of age the arterial pressures of the PVH-lesioned animals increased, but remained significantly lower than those of the sham-operated animals of the same age. This difference in arterial pressures was observed to 16 weeks of age. Heart rate was significantly reduced by PVH lesions up to 5 weeks after the lesions, at which point the heart rate tended towards the control values of the sham-lesioned animals. These data have demonstrated that the region of the PVH is important in the initial phase of the development of hypertension and in the full expression of the hypertension in the SHR, and provide evidence of a central mechanism in the hypertensive process in the SHR.

Ventral pallidum projections to mediodorsal nucleus of the thalamus: an anatomical and electrophysiological investigation in the rat.

Horseradish peroxidase (HRP) and single unit recording experiments were done in rats to investigate neural connections from the ventral pallidal region to the mediodorsal nucleus of the thalamus (MD). In the first series, following the diffusion or iontophoretic injection of HRP into the MD, retrogradely labeled neurons were observed throughout the rostrocaudal extent of the ipsilateral ventral pallidum. Most of the labeled neurons were found in an area between the nucleus of the diagonal band and the ventral aspect of the substantia innominata subcommissuralis. Additional labeled neurons were found in the ventral aspect of the globus pallidus and substantia innominata sublenticularis. In the second series, the region shown to contain labeled neurons was explored for single units antidromically activated by single pulse stimulation of the MD in urethane anesthetized rats. One hundred and fifty-nine single units in the subpallidal area were antidromically activated with latencies corresponding to conduction velocities of 0.2-3.9 m/s. A greater percentage of units in the subcommissural region (50.3%) were activated antidromically as compared to the sublenticular region (27.4%). In the third series, the MD was explored for single units which responded orthodromically to stimulation of the ventral pallidum. Fifty-eight percent (40/69) of MD units responded to stimulation of the subcommissural substantia innominata, whereas 90% (72/80) MD units responded to stimulation of the sublenticular substantia innominata. The most frequent type of orthodromic response observed in MD neurons was inhibition with short onset latencies (less than 10 ms). These data provide anatomical and electrophysiological evidence for the existence of direct pathways from the ventral pallidum to the MD and suggest that this projection is part of a corticosubcortical loop through which the frontal cortex with the ventral striatum and pallidum may contribute to motor function.

Medullary origin of vagal preganglionic axons to the heart of the cat

It is apparent from the literature that a controversy exists concerning the site of origin of cardiac vagal preganglionic axons. Physiological studies have suggested that the location of these neurons may be different in different species and there has been disagreement between physiological and anatomical findings in the same species. We now present anatomical and neurophysiological studies suggesting that in the cat cardiac vagal preganglionic neurons are located in two medullary regions: the areas of the dorsal motor nucleus of the vagus (DMV) and of the nucleus ambiguus (AMB). This suggestion is based on the following observations. Firstly, after application of horseradish peroxidase to the right cardiac branches of the vagus nerve, labeled neurons were found primarily in the regions of te DMV and AMB. Additional scattered neurons were found in the reticular formation between these two nuclei. Secondly, following injections of tritiated amino acids into either the DMV or AMB, labeled vagal fibers were found in the atrial myocardium. Finally, electrical stimulation of the right cardiac branches of the vagus nerve antidromically activated DMV or AMB, labeled vagal fibers were found in the atrial myocardium. Finally, neurons in the DMV and AMB regions with latencies corresponding to conduction velocities of B-fibers. In addition, these neurons were orthodromically excited by electrical stimulation of the carotid sinus and aortic depressor nerves.