Medullary visceral reflex circuits: local afferents to nucleus tractus solitarii synthesize catecholamines and project to thoracic spinal cord

Altered 5-HT2A/C receptor binding in the medulla oblongata in the sudden infant death syndrome (SIDS): Part I. Tissue-based evidence for serotonin receptor signaling abnormalities in cardiorespiratory- and arousal-related circuits

The sudden infant death syndrome (SIDS), the leading cause of postneonatal infant mortality in the United States, is typically associated with a sleep period. Previously, we showed evidence of serotonergic abnormalities in the medulla (e.g. altered serotonin (5-HT)1A receptor binding), in SIDS cases. In rodents, 5-HT2A/C receptor signaling contributes to arousal and autoresuscitation, protecting brain oxygen status during sleep. Nonetheless, the role of 5-HT2A/C receptors in the pathophysiology of SIDS is unclear. We hypothesize that in SIDS, 5-HT2A/C receptor binding is altered in medullary nuclei that are key for arousal and autoresuscitation. Here, we report altered 5-HT2A/C binding in several key medullary nuclei in SIDS cases (n = 58) compared to controls (n = 12). In some nuclei the reduced 5-HT2A/C and 5-HT1A binding overlapped, suggesting abnormal 5-HT receptor interactions. The data presented here (Part 1) suggest that a subset of SIDS is due in part to abnormal 5-HT2A/C and 5-HT1A signaling across multiple medullary nuclei vital for arousal and autoresuscitation. In Part II to follow, we highlight 8 medullary subnetworks with altered 5-HT receptor binding in SIDS. We propose the existence of an integrative brainstem network that fails to facilitate arousal and/or autoresuscitation in SIDS cases.

Brain-wide analysis of the supraspinal connectome reveals anatomical correlates to functional recovery after spinal injury

The supraspinal connectome is essential for normal behavior and homeostasis and consists of numerous sensory, motor, and autonomic projections from brain to spinal cord. Study of supraspinal control and its restoration after damage has focused mostly on a handful of major populations that carry motor commands, with only limited consideration of dozens more that provide autonomic or crucial motor modulation. Here, we assemble an experimental workflow to rapidly profile the entire supraspinal mesoconnectome in adult mice and disseminate the output in a web-based resource. Optimized viral labeling, 3D imaging, and registration to a mouse digital neuroanatomical atlas assigned tens of thousands of supraspinal neurons to 69 identified regions. We demonstrate the ability of this approach to clarify essential points of topographic mapping between spinal levels, measure population-specific sensitivity to spinal injury, and test the relationships between region-specific neuronal sparing and variability in functional recovery. This work will spur progress by broadening understanding of essential but understudied supraspinal populations.

The Solitary Nucleus Connectivity to Key Autonomic Regions in Humans MRI and Literature based Considerations

The nucleus tractus solitarius (NTS), is a key brainstem structure relaying interoceptive peripheral information to the interrelated brain centers for eliciting rapid autonomic responses and for shaping longer-term neuroendocrine and motor patterns. Structural and functional NTS' connectivity has been extensively investigated in laboratory animals. But there is limited information about NTS' connectome in humans. Using MRI, we examined diffusion and resting state data from 20 healthy participants in the Human Connectome Project. The regions within the brainstem (n=8), subcortical (n=6), cerebellar (n=2) and cortical (n=5) parts of the brain were selected via a systematic review of the literature and their white matter NTS connections were evaluated via probabilistic tractography along with functional and directional (i.e., Granger-causality) analyses. The underlying study confirms previous results from animal models and provides novel aspects on NTS integration in humans. Two key findings can be summarized: (i) the NTS predominantly processes afferent input and (ii) a lateralization towards a predominantly left-sided NTS processing. Our results lay the foundations for future investigations into the NTS' tripartite role comprised of interoreceptors' input integration, the resultant neurochemical outflow and cognitive/affective processing. The implications of these data add to the understanding of NTS' role in specific aspects of autonomic functions.

Afferents of the mouse linear nucleus

The linear nucleus (Li) was identified in 1978 from its projections to the cerebellum. However, there is no systematic study of its connections with other areas of the central nervous system possibly due to the challenge of injecting retrograde tracers into this nucleus. The present study examines its afferents from some nuclei involved in motor and cardiovascular control with anterograde tracer injections. BDA injections into the central amygdaloid nucleus result in labeled fibers to the ipsilateral Li. Bilateral projections with an ipsilateral dominance were observed after injections in a) jointly the paralemniscal nucleus, the noradrenergic group 7/ Köllike -Fuse nucleus/subcoeruleus nucleus, b) the gigantocellular reticular nucleus, c) and the solitary nucleus/the parvicellular/intermediate reticular nucleus. Retrogradely labeled neurons were observed in Li after BDA injections into all these nuclei except the central amygdaloid and the paralemniscal nuclei. Our results suggest that Li is involved in a variety of physiological functions apart from motor and balance control it may exert via its cerebellar projections.

Cervical vagotomy increased the distal colon distention to urinary bladder inhibitory reflex in male rats

PURPOSE

Many studies have demonstrated the convergence of vagal inputs into brainstem centers with inputs from the urinary bladder and colon, as well as the convergence of vagal inputs into other centers controlling the urinary bladder and colon reflexes. However, the effect of the vagal inputs on the interaction between the urinary bladder and other pelvic organs has not been studied. In this study, the effect of bilateral cervical vagotomy on the distal colon to urinary bladder reflex was examined.

METHODS

Changes to cystometry parameters in response to increased distal colon distensions (1, 2, and 3 ml) were tested in urethane-anesthetized male rats with or without bilateral cervical vagotomy.

RESULTS

In animals with intact vagus nerves, 1 and 2 ml distal colon distentions had no significant effects on micturition frequency; however, 3 ml distal colon distention significantly decreased the frequency of micturition cycles. Also, 3 ml distal colon distention inhibited micturition cycles in 37.5 % of these animals. On the other hand, following cervical vagotomy, 1 ml distal colon distention was enough to significantly decrease the frequency of micturition cycles and to inhibit the cycles in 75 % of the animals.

CONCLUSION

These results demonstrate the presence of supraspinal inhibitory regulation, via the vagus nerve, over the distal colon to urinary bladder inhibitory reflex.

Afferent and efferent connections of C1 cells with spinal cord or hypothalamic projections in mice

The axonal projections and synaptic input of the C1 adrenergic neurons of the rostral ventrolateral medulla (VLM) were examined using transgenic dopamine-beta hydroxylase Cre mice and modified rabies virus. Cre-dependent viral vectors expressing TVA (receptor for envelopeA) and rabies glycoprotein were injected into the left VLM. EnvelopeA-pseudotyped rabies-EGFP glycoprotein-deficient virus (rabies-EGFP) was injected 4-6 weeks later in either thoracic spinal cord (SC) or hypothalamus. TVA immunoreactivity was detected almost exclusively (95 %) in VLM C1 neurons. In mice with SC injections of rabies-EGFP, starter cells (expressing TVA + EGFP) were found at the rostral end of the VLM; in mice with hypothalamic injections starter C1 cells were located more caudally. C1 neurons innervating SC or hypothalamus had other terminal fields in common (e.g., dorsal vagal complex, locus coeruleus, raphe pallidus and periaqueductal gray matter). Putative inputs to C1 cells with SC or hypothalamic projections originated from the same brain regions, especially the lower brainstem reticular core from spinomedullary border to rostral pons. Putative input neurons to C1 cells were also observed in the nucleus of the solitary tract, caudal VLM, caudal spinal trigeminal nucleus, cerebellum, periaqueductal gray matter and inferior and superior colliculi. In sum, regardless of whether they innervate SC or hypothalamus, VLM C1 neurons receive input from the same general brain regions. One interpretation is that many types of somatic or internal stimuli recruit these neurons en bloc to produce a stereotyped acute stress response with sympathetic, parasympathetic, vigilance and neuroendocrine components.

Hindbrain noradrenergic A2 neurons: diverse roles in autonomic, endocrine, cognitive, and behavioral functions

Central noradrenergic (NA) signaling is broadly implicated in behavioral and physiological processes related to attention, arousal, motivation, learning and memory, and homeostasis. This review focuses on the A2 cell group of NA neurons, located within the hindbrain dorsal vagal complex (DVC). The intra-DVC location of A2 neurons supports their role in vagal sensory-motor reflex arcs and visceral motor outflow. A2 neurons also are reciprocally connected with multiple brain stem, hypothalamic, and limbic forebrain regions. The extra-DVC connections of A2 neurons provide a route through which emotional and cognitive events can modulate visceral motor outflow and also a route through which interoceptive feedback from the body can impact hypothalamic functions as well as emotional and cognitive processing. This review considers some of the hallmark anatomical and chemical features of A2 neurons, followed by presentation of evidence supporting a role for A2 neurons in modulating food intake, affective behavior, behavioral and physiological stress responses, emotional learning, and drug dependence. Increased knowledge about the organization and function of the A2 cell group and the neural circuits in which A2 neurons participate should contribute to a better understanding of how the brain orchestrates adaptive responses to the various threats and opportunities of life and should further reveal the central underpinnings of stress-related physiological and emotional dysregulation.

Opioid microinjection into raphe magnus modulates cardiorespiratory function in mice and rats

The raphe magnus (RM) participates in opioid analgesia and contains pain-modulatory neurons with respiration-related discharge. Here, we asked whether RM contributes to respiratory depression, the most prevalent lethal effect of opioids. To investigate whether opioidergic transmission in RM produces respiratory depression, we microinjected a mu-opioid receptor agonist, DAMGO, or morphine into the RM of awake rodents. In mice, opioid microinjection produced sustained decreases in respiratory rate (170 to 120 breaths/min), as well as heart rate (520 to 400 beats/min). Respiratory sinus arrhythmia, indicative of enhanced parasympathetic activity, was prevalent in mice receiving DAMGO microinjection. We performed similar experiments in rats but observed no changes in breathing rate or heart rate. Both rats and mice experienced significantly more episodes of bradypnea, indicative of impaired respiratory drive, after opioid microinjection. During spontaneous arousals, rats showed less tachycardia after opioid microinjection than before microinjection, suggestive of an attenuated sympathetic tone. Thus, activation of opioidergic signaling within RM produces effects beyond analgesia, including the unwanted destabilization of cardiorespiratory function. These adverse effects on homeostasis consequent to opioid microinjection imply a role for RM in regulating the balance of sympathetic and parasympathetic tone.

Visceral afferents directly activate catecholamine neurons in the solitary tract nucleus

Brainstem A2/C2 neurons are catecholamine (CA) neurons within the solitary tract nucleus (NTS) that influence many homeostatic functions, including cardiovascular reflexes, food intake, and stress. Because NTS is a major interface between sensory visceral afferents and the CNS, NTS CA neurons are ideally suited to coordinate complex responses by their projections to multiple brain regions. To test how NTS CA neurons process visceral afferent information carried by solitary tract (ST) afferents, we identified CA neurons using transgenic mice expressing TH-EGFP (enhanced green fluorescent protein under the control of the tyrosine hydroxylase promoter) and recorded synaptic responses to ST activation in horizontal slices. ST shocks evoked large-amplitude, short-latency, glutamatergic EPSCs (ST-EPSCs) in 90% of NTS CA neurons. Within neurons, ST-EPSCs had constant latency, rarely failed, and depressed substantially at high ST frequencies, indicating that NTS CA neurons receive direct monosynaptic connections from afferent terminals. NTS CA neurons received direct ST inputs from only one or two afferent fibers, with one-half also receiving smaller amplitude indirect inputs. Up to 90% of ST shocks evoked action potentials in NTS CA neurons. However, transmission of sensory afferent information through NTS CA neurons critically depended on the expression of an A-type potassium current (I(KA)), which when active attenuated ST-activated action potentials to a 37% success rate. The satiety peptide, cholecystokinin, presynaptically facilitated glutamate transmission in one-half of NTS CA neurons. Thus, NTS CA neurons are directly driven by visceral afferents with output being modulated by presynaptic peptide receptors and postsynaptic potassium channels.

Neuroanatomical basis of Sandifer's syndrome: a new vagal reflex?

Sandifer's syndrome is a gastrointestinal disorder with neurological features. It is characterized by reflex torticollis following deglutition in patients with gastroesophageal reflux and/or hiatal hernia. The authors believe that neurological manifestations of the syndrome are the consequence of vagal reflex with the reflex center in nucleus tractus solitarii (NTS). Three models for the neuroanatomical basis of the hypothetic reflex arc are presented. In the first one the hypothetic reflex arc is based on the classic hypothesis of two components nervus accessorius (n.XI) - radix cranialis (RC) and radix spinalis (RS) The nervous impulses are transmitted by nervus vagus (n.X) general visceral afferent (GVA) fibers to NTS situated in medulla oblongata, then by interneuronal connections on nucleus ambiguus (NA) and nucleus dorsalis nervi vagi (NDX). Special visceral efferent fibers (SVE) impulses from NA are in part transferred to n.XI ramus externus (RE) (carrying the majority of general somatic efferent (GSE) fibers) via hypothetic anastomoses in the region of foramen jugulare. This leads to contraction of trapezius and sternocleidomastoideus muscles, and the occurrence of intermittent torticollis. In the second suggested neuroanatomical model the hypothetic reflex arc is organized in the absence of n.XI RC, the efferent part of the reflex arc continues as NA, which is motor nucleus of nervus glossopharyngeus (n.IX) and n.X in this case while distal roots of n.XI that appear at the level of the olivary nucleus lower edge represent n.X roots. In the third presented model the hypothetic reflex arc includes no jugular transfer and could be realized via interneuronal connections directly from NTS to the spinal motoneurons within nucleus radicis spinalis nervi accessorii (NRS n.XI) or from NA to NRS n.XI. The afferent segment of the postulated reflex arc in all three models is mediated via n.X. We conclude that Sandifer's syndrome is a clinical manifestation of another vagal reflex that could be termed a "vagocervical" or "esophagocervical" reflex, based on the neuroanatomical hypotheses elaborated in this paper.

Identification of different types of respiratory neurones in the dorsal brainstem nucleus tractus solitarius of the rat

In Nembutal anaesthetised, spontaneously breathing rats, stereotaxic mapping of the nucleus tractus solitarius (NTS) for respiratory neuronal activity was undertaken. Eight different types of respiratory cells were found between 0.25 and 1.5 mm lateral to midline, extending 0.5 mm caudal to 1.5 mm rostral to obex, and 0.4-1.5 mm below the dorsal surface. A study of the respiratory motor (diaphragm EMG) and neuronal responses to excitatory amino acid (EAA) stimulation of the NTS areas was undertaken. Electrical stimulation of the vagus nerve was employed to study the NTS cellular responses to activation of pulmonary afferents. The effects of chemical activation of the midbrain periaqueductal grey (PAG) on NTS respiratory neuronal activity were investigated. EAA microinjections into the ventrolateral NTS rostral to the obex resulted in an increase in respiratory motor frequency along with increases to inspiratory cell discharge, whilst microinjections into the medial NTS caudal to the obex caused respiratory depression. EAA stimulation of calamus scriptorius produced apnea. NTS inspiratory neurones were inhibited following stimulation of ipsilateral vagus nerve, suggesting their involvement in the Hering-Breuer reflex pathway. PAG stimulation caused excitation of the NTS inspiratory cells indicating the presence of an excitatory respiratory pathway between the two nuclei. Following beta-adrenergic antagonist pre-treatment of ventrolateral NTS, EAA microinjections into PAG did not evoke a cardiorespiratory effect. Based on the various findings the role of NTS in organising respiration in the rat is discussed.

Divergent projections of catecholaminergic neurons in the nucleus of the solitary tract to limbic forebrain and medullary autonomic brain regions

The nucleus of the solitary tract (NTS) is a critical structure involved in coordinating autonomic and visceral activities. Previous independent studies have demonstrated efferent projections from the NTS to the nucleus paragigantocellularis (PGi) and the central nucleus of the amygdala (CNA) in rat brain. To further characterize the neural circuitry originating from the NTS with postsynaptic targets in the amygdala and medullary autonomic targets, distinct green or red fluorescent latex microspheres were injected into the PGi and the CNA, respectively, of the same rat. Thirty-micron thick tissue sections through the lower brainstem and forebrain were collected. Every fourth section through the NTS region was processed for immunocytochemical detection of tyrosine hydroxylase (TH), a marker of catecholaminergic neurons. Retrogradely labeled neurons from the PGi or CNA were distributed throughout the rostro-caudal segments of the NTS. However, the majority of neurons containing both retrograde tracers were distributed within the caudal third of the NTS. Cell counts revealed that approximately 27% of neurons projecting to the CNA in the NTS sent collateralized projections to the PGi while approximately 16% of neurons projecting to the PGi sent collateralized projections to the CNA. Interestingly, more than half of the PGi and CNA-projecting neurons in the NTS expressed TH immunoreactivity. These data indicate that catecholaminergic neurons in the NTS are poised to simultaneously coordinate activities in limbic and medullary autonomic brain regions.

Hindbrain catecholamine neurons control multiple glucoregulatory responses

Reduced brain glucose availability evokes an integrated constellation of responses that protect and restore the brain's glucose supply. These include increased food intake, adrenal medullary secretion, corticosterone secretion and suppression of estrous cycles. Our research has focused on mechanisms and neural circuitry underlying these systemic glucoregulatory responses. Using microinjection techniques, we found that localized glucoprivation of hindbrain but not hypothalamic sites, elicited key glucoregulatory responses, indicating that glucoreceptor cells controlling these responses are located in the hindbrain. Selective destruction of hindbrain catecholamine neurons using the retrogradely transported immunotoxin, anti-dopamine beta-hydroxylase conjugated to saporin (DSAP), revealed that spinally-projecting epinephrine (E) or norepinephrine (NE) neurons are required for the adrenal medullary response to glucoprivation, while E/NE neurons with hypothalamic projections are required for feeding, corticosterone and reproductive responses. We also found that E/NE neurons are required for both consummatory and appetitive phases of glucoprivic feeding, suggesting that multilevel collateral projections of these neurons coordinate various components of the behavioral response. Epinephrine or NE neurons co-expressing neuropeptide Y (NPY) may be the neuronal phenotype required for glucoprivic feeding: they increase NPY mRNA expression in response to glucoprivation and are nearly eliminated by DSAP injections that abolish glucoprivic feeding. In contrast, lesion of arcuate nucleus NPY neurons, using the toxin, NPY-saporin, does not impair glucoprivic feeding or hyperglycemic responses. Thus, hindbrain E/NE neurons orchestrate multiple concurrent glucoregulatory responses. Specific catecholamine phenotypes may mediate the individual components of the overall response. Glucoreceptive control of these neurons resides within the hindbrain.

Pattern differentiation of excitatory and inhibitory synaptic inputs on distinct neuronal types in the rat caudal nucleus of the tractus solitarius

Region- and size-specific neuronal organizations of the caudal nucleus of the tractus solitarius (cNTS) were investigated, followed by analyses of excitatory and inhibitory synaptic input patterns onto specific cell types by patch clamp recordings and immunoelectron microscopy. Cell size distribution and numerical density of cNTS neurons were examined in subregions at levels of the area postrema. In the subpostremal and dorsomedial subnuclei, characterized by the presence of dense glutamatergic and sparse GABAergic somata, small calbindin neurons constituted 42% of the total cells. The medial subnucleus contained large numbers of glutamatergic, GABAergic, and catecholaminergic somata and large tyrosine hydroxylase-containing cells constituted 13% in this region. In total, small neurons (<150 microm2) represented about 80% of the cell population in the cNTS. Predominant excitatory postsynaptic currents were observed in the adult small neurons, while inhibitory postsynaptic currents were more evident in larger neurons, irrespective of subnuclear location. This distinct differentiation of postsynaptic current patterns was not evident in neonates. GABAergic synapses were more frequently associated with dendrites of large catecholaminergic cells (73%) than with those of small calbindin-containing cells (10%) in adults. These results indicate that differential synaptic input patterns were developmentally established in distinct small and large neurons.

Local axonal arborization patterns of distinct neuronal types in the caudal nucleus of the tractus solitarius

Neurons in the caudal nucleus of the tractus solitarius (cNTS) are quite heterogeneous in cell size (50 to 450 microm(2) in somal area) and other morphologic characteristics. For a more objective classification of cNTS neurons, their morphologic features were analyzed quantitatively based on reconstructed biocytin-filled cells after whole-cell patch-clamp recordings. According to the patterns of axonal branching behaviors, cNTS cells could be classified into two groups: smaller cells (94.1 microm(2) in mean somal area, range 62-120 microm(2), n = 22) and larger cells (245 microm(2) in mean somal area, range 142-411 microm(2), n = 23). Extensive axonal arborization with numerous possible synaptic boutons was specifically associated with smaller neurons, while larger cells possessed no or few axon collaterals, suggesting their distinct roles as local circuit neurons (or interneurons) and projection neurons, respectively. With regard to somatodendritic characteristics, the following correlations with cell size were found: smaller cells had larger form factors than larger cells (P < 0.05). Larger neurons had more extensive dendritic arborization, expressed by total dendritic length (P < 0.01) and number of dendritic branching points (P < 0.01), than smaller cells. It was suggested that small cNTS neurons contribute specifically to an integration of input information generated in the local circuits, while large neurons convey the integrated information to other autonomic brain regions.

Ascending spinal pathways from sexual organs: effects of chronic spinal lesions

A recent survey of paraplegics indicates that regaining sexual function is of the highest priority for both males and females (Anderson, K.D. (2004) Targeting recovery: priorities of the spinal cord-injured population J. Newrotrauma, 21: 1371-1383). Our understanding of the neural pathways and mechanisms underlying sexual behavior and function is limited at the present time. More studies are obviously needed to direct experiments geared toward developing effective therapeutic interventions. In this chapter, a review of studies on the processing of sensory inputs from the male and female reproductive organs is presented with a review of what is known about the location of ascending spinal pathways conveying this information. The effect of spinal cord injury on sexual function and the problems that ensue are discussed.

Effects of Chronic Dorsal Column Lesions on Pelvic Viscerosomatic Convergent Medullary Reticular Formation Neurons

Single medullary reticular formation (MRF) neurons receive multiple somatovisceral convergent inputs originating from many different spinal and cranial nerves, including the pelvic nerve (PN), dorsal nerve of the penis (DNP), and the abdominal branches of the vagus. In a previous study, the input to MRF from the male genitalia was shown to be eliminated with chronic 30-day dorsal hemisection at the T8 spinal level. In this study, the effect of a smaller chronic lesion [dorsal column lesion (DCx)] on MRF neuronal responses was examined. Responses to bilateral electrical stimulation of the DNP remained. MRF neuronal responses to non-noxious (touch/stroke) levels of penile stimulation, however, were eliminated; only responses to noxious pinch remained. No differences were found for the number of neurons responding to noxious distention of the colon between the DCx and control groups. Although no differences were found across these groups for the percent MRF responses to vagal stimulation, the mean response latency for the DCx group was twice the sham-DCx/intact control group. Taken together, these results indicate that the MRF receives at least some of its input from the male genitalia via pathways located within the dorsal columns at the mid-thoracic spinal level.

Defining projections from the caudal pressor area of the caudal ventrolateral medulla

We previously defined a functional area in the caudal medulla oblongata that elicits an increase in arterial pressure when stimulated (Sun and Panneton [2002] Am. J. Physiol. 283:R768-R778). In the present study, anterograde and retrograde tracing techniques were used to investigate the projections of this caudal pressor area (CPA) to the medulla and pons. Injections of biotinylated dextran amine into the CPA resulted in numerous labeled fibers with varicosities in the ipsilateral subnucleus reticularis dorsalis, commissural subnucleus of the nucleus tractus solitarii, lateral medulla, medial facial nucleus, A5 area, lateral vestibular nucleus, and internal lateral subnucleus of the parabrachial complex. Sparser projections were found ipsilaterally in the pressor and depressor areas of the medulla and the spinal trigeminal nucleus and contralaterally in the CPA. Injections of the retrograde tracer Fluoro-Gold into these areas labeled neurons in the CPA as well as the nearby medullary dorsal horn and reticular formation. However, we conclude that the CPA projects preferentially to the subnucleus reticularis dorsalis, commissural nucleus tractus solitarii, lateral medulla, A5 area, and internal lateral parabrachial nucleus. Weaker projections were seen to the CVLM and RVLM and to the contralateral CPA. The projection to the facial nucleus arises from nearby reticular neurons, whereas projections to the vestibular nucleus arise from the lateral reticular nucleus. Labeled neurons in the CPA consisted mostly of small bipolar and some triangular neurons. The projection to the CVLM, or to A5 area, may provide for the increase in arterial pressure with CPA stimulation. However, most of the projections described herein are to nuclei implicated in the processing of noxious information. This implies a unique role for the CPA in somatoautonomic regulation.

Activation of brainstem catecholaminergic neurons during voluntary diving in rats

Underwater submergence produces a complex autonomic response that includes apnea, a parasympathetically-mediated bradycardia, and a sympathetically-mediated increase in total peripheral resistance (TPR). The present study was designed to identify brainstem catecholaminergic neurons that may be involved in producing the increased TPR during underwater submergence. Twelve male Sprague-Dawley rats were trained to voluntarily dive 5 m through an underwater maze. On the day of the experiment the rats were randomly separated into a Diving group that repetitively dived underwater, a Swimming group that repetitively swam on the surface of the water, and a Control group that remained in their cages. After the experiment the brainstems of the rats were immunohistologically processed for Fos as an indicator of neuronal activation, and for tyrosine hydroxylase (TH) as an indentifier of catecholaminergic neurons. Neurons labeled with both Fos and TH identified activated catecholaminergic neurons. In Diving rats there was increased Fos+TH labeling in A1, C1, A2, A5, and sub-coeruleus, as well as globosa neurons in the lateral A7 region compared with Control rats, and in A1, C1 and A5 compared with Swimming rats. In Swimming rats Fos+TH labeling was significantly increased in caudal A1, A5, sub-coeruleus and globosa neurons compared with Control rats. These data suggest that selective groups of catecholaminergic neurons within the brainstem are activated by voluntary underwater submergence, and some probably contribute to the sympathetically-mediated increase in vascular tone during diving.

Localization and neuronal response of RFamide related peptides in the rat central nervous system

RFamide related peptides (RFRP)-1 and RFRP-3 are neuropeptides derived from the same preproprotein. We have examined the distribution of RFRP-1 and RFRP-3 immunoreactivities (irs) in the rat central nervous system using specific antibodies. Neuronal cell bodies containing both RFRP-1 and RFRP-3 were detected within the caudal portion of the hypothalamus, the periventricular nucleus (PerVN), and the portion around or above the ventromedial nucleus of the hypothalamus. Both immunohistochemical and in situ hybridization analyses showed that neurons containing RFRP immunoreactivity and mRNA were distinct from those of neuropeptide FF, which contains the same structure at the C-terminus, Pro-Glu-Arg-Phe-NH2, as RFRP-3. Fibers containing both RFRP-1 and RFRP-3 were widely distributed in the brain: the lateral septal nucleus in the telencephalon, the paraventricular thalamic nucleus, various hypothalamic nuclei, the periaqueductal gray in the midbrain, the parabrachial nucleus in the pons, and the nucleus tractus solitarius (NTS) in the medulla oblongata. Only RFRP-1-ir was detected within the posterior gray horn in the spinal cord. Only RFRP-3-ir was detected in several thalamic nuclei and the spinal cord, especially at the posterior intermediate sulcus and within the anterior gray horn. Intracerebroventricular administration of RFRPs induced c-Fos expression in the anterior portion of the NTS, locus coeruleus, the nucleus of incertus, supraoptic nucleus, PerVN and the arcuate nucleus of the hypothalamus. These results show that RFRP-1 and RFRP-3 are widely distributed in the rat central nervous system and might be involved in various functions such as the neuroendocrine system or pain modulation.

cGMP/protein kinase G-dependent potentiation of glutamatergic transmission induced by nitric oxide in immature rat rostral ventrolateral medulla neurons in vitro

Although both nitric oxide (NO) and glutamate within the rostral ventrolateral medulla (RVLM) are important mediators of the central cardiovascular regulation, little is known about the functional interactions between these two mediators. Herein, we investigated the possible role of NO on the glutamatergic transmission of RVLM neurons. Whole-cell patch-clamp recordings were performed on visualized RVLM neurons in the brainstem slice preparation of rats. We found that bath application of l-arginine, the substrate for NO production, significantly increased the amplitude of excitatory postsynaptic currents (EPSCs). This enhancement was completely abolished by coadministration of the NO synthase inhibitor 7-nitroindazole and mimicked by the NO donors 3-morpholinylsydnoneimine and spermine NONOate. Bath application of a NO-sensitive guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, or a protein kinase G (PKG) inhibitor, Rp-8-bromo-guanosine 3',5'-cyclic monophosphorothioate, fully prevented the l-arginine-, 3-morpholinylsydnoneimine-, and N-[4-[1-(3-aminopropyl)-2-hydroxy-2-nitrosohydrazino]-butyl]-1,3-propanediamin (spermine NONOate)-induced synaptic potentiation. Direct activation of PKG with 8-(4-chlorophenylthio)-cGMP mimicked the action of NO donors. Furthermore, the augmentation by spermine NONOate of EPSC was accompanied by a reduction of the paired-pulse facilitation and synaptic failure rate of EPSCs. Spermine NONOate also significantly increased the frequency of both spontaneous and miniature EPSCs without altering their amplitude distribution. Pretreatment with the N-type Ca2+ channel blocker omega-conotoxin GVIA selectively blocked the spermine NONOate-induced synaptic potentiation. These results suggest that NO acts presynaptically to elicit a synaptic potentiation on the RVLM neurons through an enhancement of presynaptic N-type Ca2+ channel activity leading to facilitating glutamate release. The presynaptic action of NO is mediated by a cGMP/PKG-coupled signaling pathway.

Anatomic relationships of the human nucleus of the solitary tract in the medulla oblongata: a DiI labeling study

The nucleus of the solitary tract (nTS) is a major site of brainstem control of vital functions (e.g., cardiovascular reflexes and respiration). We examined anatomic relationships of the human nucleus of the solitary tract, using a bidirectional lipophilic fluorescent tracer 1-1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) in 10 postmortem human fetal midgestational medullae oblongatae. Labeling by diffusion of DiI from the nucleus of the solitary tract included: (1) neuropil of all future subdivisions of the nucleus of the solitary tract ipsilateral to the DiI crystal; (2) stellate cells in the caudal raphe at the junction of the nucleus raphe pallidus and the arcuate nucleus at the ventral medullary surface, as well as single fibers along the caudal raphe and the arcuate nucleus; (3) cells and fibers in other medullary areas related to autonomic and respiratory control, including the dorsal motor nucleus of the vagus, nucleus ambiguus complex/ventral respiratory group, rostral ventrolateral medulla (RVLM) and caudal ventrolateral medulla (CVLM), and medullary reticular formation. The pattern of connections of the nucleus of the solitary tract already established by midgestation in the human fetus is consistent with the pattern previously demonstrated in adult experimental animals. A major finding of the study is that of the stellate cells at the junction of nucleus raphe pallidus and the arcuate nucleus at the ventral medullary surface, which project to the nucleus of the solitary tract, and could be homologous to chemosensitive serotonergic neurons at the midline ventral medullary surface of experimental animals. This connection between the ventral caudal raphe and the nucleus of the solitary tract may participate in chemoreception and central regulation of cardiorespiratory reflexes during human perinatal development; it is, therefore, relevant to the study of sudden infant death syndrome (SIDS).

Acute changes in 3H-PAC and 125I-PYY binding in the nucleus tractus solitarii and hypothalamus after a hypertensive stimulus

Activation of alpha-2-adrenergic and neuropeptide Y (NPY) receptors in the nucleus tractus solitarii (NTS) induces hypotension and bradycardia. On the contrary, activation of angiotensin II (Ang II) receptors leads to hypertension. Acute changes in binding parameters of alpha-2-adrenergic, NPY and Ang II receptors were evaluated in the NTS and paraventricular hypothalamic nucleus (PVN) of rats after a hypertensive stimulus employing quantitative receptor autoradiography. Saturation experiments showed a decrease in the number (Bmax) of alpha-2-adrenergic binding sites in the NTS 6 hours after coarctation-induced hypertension. Furthermore, the affinity of NPY receptors was diminished as seen by the increase in the KD value of 125I-PYY. Tyrosine hydroxylase and NPY immunoreactivities were increased in the NTS and ventral medulla. Binding of 125I-Ang II was not changed in the NTS. Binding of all ligands analyzed was not altered in the PVN. The results suggest an acute down-regulation of alpha-2-adrenergic and NPY receptors involved with hypotension in response to hypertensive stimulus, which might be related to an increased availability of catecholamines and NPY in the NTS.

Anatomic relationships of the human nucleus paragigantocellularis lateralis: a DiI labeling study

The nucleus paragigantocellularis lateralis (PGL) is located in the rostral ventrolateral medulla (RVLM), a brainstem region that regulates homeostatic functions, such as blood pressure and cardiovascular reflexes, respiration. central chemosensitivity and pain. In the present study, we examined anatomic relationships of the human nucleus paragigantocellularis lateralis using a bidirectional lipophilic fluorescent tracer, 1,1'-dioctadecyl-3,3.3',3'-tetramethylindocarbocyanine perchlorate (DiI), in nine postmortem human fetal midgestational brainstems. The areas which were labeled by diffusion of DiI from the nucleus paragigantocellularis lateralis included the arcuate nucleus (ARC) of the medulla, caudal raphe (nucleus raphe obscurus and pallidus), hilum and amiculum of the inferior olive, bilateral "reticular formation" (including the nucleus paragigantocellularis lateralis, nucleus gigantocellular-is and the intermediate reticular zone (IRZ)). vestibular and cochlear nuclei, cells and fibers at the floor of the fourth ventricle with morphologic features of tanycytes, parabrachial nuclei (PBN), medial lemniscus, lateral lemniscus, inferior cerebellar peduncle and cerebellar white matter, central tegmental tract, and the capsule of the red nucleus. This pattern of DiI labeling bears many similarities with the pattern of connections of the nucleus paragigantocellularis lateralis previously demonstrated by tract-tracing methods in experimental animals, and is consistent with the role of the nucleus paragigantocellularis lateralis in central regulation of homeostatic functions. In contrast to the animal studies, however, we did not demonstrate connections of the nucleus paragigantocellularis lateralis with the nucleus of the tractus solitarius (nTS) (only connections with the rostral subdivision were examined), locus coeruleus, or the periaqueductal gray (PAG) in the human midgestational brainstem. In our previous studies, six medullary areas showed reduced serotonin receptor binding in a subset of victims of sudden infant death syndrome (SIDS). The present study demonstrated DiI labeling in all of these six areas, suggesting that they are interconnected.

The human nucleus of the solitary tract: visceral pathways revealed with an "in vitro" postmortem tracing method

Visceral relay neurons in the nucleus of the solitary tract (NTS) regulate behavior and autonomic reflex functions. NTS projections have been extensively characterized in animal studies but not in humans. For the first time, NTS fiber trajectories in the human medulla oblongata were revealed with an "in vitro" postmortem tracing method. Local intramedullary pathways were labeled by direct pressure injections of free horseradish peroxidase centered on the medial subnucleus at a level adjacent to true obex. Labeled elements were resolved by peroxidase histochemistry as a dark brown intracellular reaction product. A prominent transtegmental system of axons emerged from the NTS injection sites and entered the intermediate reticular zone, a region corresponding to an autonomic reflex center in other mammals. A medial system of axons arched across the dorsomedial reticular formation toward the dorsal medullary raphe and projected ventrally toward the nucleus gigantocellularis. A small lateral fiber trajectory coursed towards the dorsomedial zone of spinal trigeminal nucleus caudalis. Presumptive terminals appeared as dustings of fine punctate processes within the NTS, dorsomotor nucleus and reticular formation. NTS projections in humans resemble those identified in other mammals including primates. Axonal tracing studies predict that visceral impulses in humans may transmit over evolutionarily conserved pathways involved in autonomic feedback control and stress adaptation.

The pedunculopontine tegmental nucleus issues collaterals to the fastigial nucleus and rostral ventrolateral reticular nucleus in the rat

The pedunculopontine-laterodorsal tegmental nuclear complex was identified as a major source of brainstem afferents terminating in the fastigial cerebellar nucleus and/or ventrolateral reticular nucleus (n.Rvl). Collaterals from the pedunculopontine nucleus (Ch5 area) to rostral [vasopressor] regions of the fastigial nucleus and ventral reticular formation were revealed with a combined retrograde tracing technique. The data implicate acetylcholine as a transmitter and raise the hypothesis that the identified afferents may contribute to the autonomic and behavioral responses to midline cerebellar stimulation.

Direct synaptic projections to esophageal motoneurons in the nucleus ambiguus from the nucleus of the solitary tract of the rat

Neurons of the nucleus of the solitary tract (NTS) serve as interneurons in swallowing. We investigated the synaptology of the terminals of these neurons and whether they project directly to the esophageal motoneurons in the compact formation of the nucleus ambiguus (AmC). Following wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) injection into the NTS, many anterogradely labeled axodendritic terminals were found in the neuropil of the AmC. The majority of labeled axodendritic terminals (89%) contained round vesicles and made asymmetric synaptic contacts (Gray's type I), but a few (11%) contained pleomorphic vesicles and made symmetric synaptic contacts (Gray's type II). More than half of the labeled terminals contacted intermediate dendrites (1-2 microm diameter). There were no retrogradely labeled medium-sized motoneurons, but there were many retrogradely labeled small neurons having anterogradely labeled axosomatic terminals. A combined retrograde and anterograde transport technique was developed to verify the direct projection from the NTS to the esophageal motoneurons. After the esophageal motoneurons were retrogradely labeled by cholera toxin subunit B conjugated HRP, the injection of WGA-HRP into the NTS permitted ultrastructural recognition of anterogradely labeled axosomatic terminals contacting directly labeled esophageal motoneurons. Serial sections showed that less than 20% of the axosomatic terminals were labeled in the esophageal motoneurons. They were mostly Gray's type I, but a few were Gray's type II. In the small neurons, more than 30% of axosomatic terminals were labeled, which were exclusively Gray's type I. These results indicate that NTS neurons project directly not only to the esophageal motoneurons, but also to the small neurons which have bidirectional connections with the NTS.

Innervation of the lumbar facet joints. Origins and functions

STUDY DESIGN

The levels of dorsal root ganglia and paravertebral sympathetic ganglia innervating the lumbar facet joint were investigated in rats using the retrograde transport method. The pathways and functions of the nerve fibers supplying the lumbar facet joint were determined immunohistochemically.

OBJECTIVES

To study lumbar facet pain in relation to its innervation.

SUMMARY OF BACKGROUND DATA

The lumbar facet joints have been reported to be innervated segmentally. Little is known, however, about the origins and functions of the nerve fibers.

METHODS

Cholera toxin B subunit, a neural tracer, was placed in the L5-L6 facet joint, and the bilateral dorsal root ganglia and paravertebral sympathetic ganglia were examined immunohistochemically. The serial sections of lumbar vertebrae of newborn rats and the sections of the facet joint capsules, dorsal root ganglia, and paravertebral sympathetic ganglia of adult rats were investigated immunohistochemically. The pathways of the nerve fibers supplying the facet joint were reconstituted.

RESULTS

Labeled neurons existed in ipsilateral dorsal root ganglia from L1 to L5 and in paravertebral sympathetic ganglia from T12 to L6. The dorsal ramus of the spinal nerve and rami communicantes were connected to each other by calcitonin gene-related peptide immunoreactive fibers and dopamine beta-hydroxylase immunoreactive fibers.

CONCLUSIONS

The L5-L6 facet joint was innervated by ipsilateral dorsal root ganglia and paravertebral sympathetic ganglia, segmentally and nonsegmentally. Some of the sensory fibers from the facet joint may pass through the paravertebral sympathetic trunk, reaching L1 and/or L2 dorsal root ganglia. Inguinal and/or anterior thigh pain with lower lumbar facet joint lesions may be explained as referred pain.

Pontomedullary efferent projections of the ventral respiratory neuronal subsets of the rat

The pontomedullary trajectories of projections efferent from the ventral respiratory cell group were anterogradely labelled after discrete injections of Fluoro Ruby into three morphophysiologically identified subdivisions (Bötzinger complex, rostral inspiratory, and caudal expiratory cell groups). The anterogradely labelled varicosities were located in a variety of areas involved in cardiorespiratory function: other subdivisions of the ventral respiratory cell group, the parabrachial (medial, central, and external lateral), Kölliker-Fuse, and lateral paragigantocellular nuclei, A5, and perifacial areas. Although the target areas were similar for the three studied subdivisions, some differences of the location and densities of labelled varicosities were found. Anterogradely labelled fibre bundles were found bilaterally after all of the tracer injections. Three caudally efferent bundles passed through the ventral respiratory cell group, dorsal medullary, and paramedian reticular nuclei. A labelled fibre bundle also took an ascending route through the ventral respiratory cell group: it surrounded the facial nucleus, and then followed two different pathways, one coursing towards forebrain areas and the other to the parabrachial and Kölliker-Fuse complex. Bundles of efferent axons decussated mainly at medullary levels and to a lesser extent in the pons. In the contralateral medulla and pons these labelled fibre bundles followed pathways similar to those observed ipsilaterally. The three ventral respiratory neuronal subsets sent axonal projections through similar tracts, but within them they were topographically organized. The present data are discussed with respect to the circuitry involved in the mechanisms of cardiorespiratory and other visceral functions.

Differential c-fos expression in the nucleus of the solitary tract and spinal cord following noxious gastric distention in the rat

c-Fos has been used as a marker for activity in the spinal cord following noxious somatic or visceral stimulation. Although the viscera receive dual afferent innervation, distention of hollow organs (i.e. esophagus, stomach, descending colon and rectum) induces significantly more c-Fos in second order neurons in the nucleus of the solitary tract and lumbosacral spinal cord, which receive parasympathetic afferent input (vagus, pelvic nerves), than the thoracolumbar spinal cord, which receives sympathetic afferent input (splanchnic nerves). The purpose of this study was to determine the contribution of sympathetic and parasympathetic afferent input to c-Fos expression in the nucleus of the solitary tract and spinal cord, and the influence of supraspinal pathways on Fos induction in the thoracolumbar spinal cord. Noxious gastric distention to 80 mmHg (gastric distension/80) was produced by repetitive inflation of a chronically implanted gastric balloon. Gastric distension/80 induced c-Fos throughout the nucleus of the solitary tract, with the densest labeling observed within 300 microns of the rostral pole of the area postrema. This area was analysed quantitatively following several manipulations. Gastric distension/80 induced a mean of 724 c-Fos-immunoreactive nuclei per section. Following subdiaphragmatic vagotomy plus distention (vagotomy/80), the induction of c-Fos-immunoreactive nuclei was reduced to 293 per section, while spinal transection at T2 plus distention (spinal transection/80) induced a mean of 581 nuclei per nucleus of the solitary tract section. Gastric distension/80 and vagotomy/80 induced minimal c-Fos in the T8-T10 spinal cord (50 nuclei/section), but spinal transection/80 induced 200 nuclei per section. Repetitive bolus injections of norepinephrine produced transient pressor responses mimicking the pressor response produced by gastric distension/80. This manipulation induced minimal c-Fos in the nucleus of the solitary tract and none in the spinal cord. It is concluded that noxious visceral input via parasympathetic vagal afferents, and to a lesser extent sympathetic afferents and the spinosolitary tract, contribute to gastric distention-induced c-Fos in the nucleus of the solitary tract. The induction of c-Fos in the nucleus of the solitary tract is significantly greater than in the viscerotopic segments of the spinal cord, which is partially under tonic descending inhibition, but is not subject to modulation by vagal gastric afferents. Distention pressures produced by noxious gastric distention are much greater than those produced during feeding, suggesting that c-Fos induction in the nucleus of the solitary tract to noxious distention is not associated with physiological mechanisms of feeding and satiety. The large vagal nerve-mediated induction of c-Fos in the nucleus of the solitary tract following gastric distension suggests that parasympathetic afferents contribute to the processing of noxious visceral stimuli, perhaps by contributing to the affective-emotional component of visceral pain.

Organization of excitatory and inhibitory local networks in the caudal nucleus of tractus solitarius of rats revealed in in vitro slice preparation

Morphological and physiological properties of neurons in the caudal nucleus of tractus solitarius (NTS) of rats were studied in vitro by whole-cell recording and intracellular staining with biocytin. Synaptic responses following the solitary tract stimulation were also investigated to elucidate anatomical substrates of the underlying local circuits. Biocytin-filled NTS cells were divided into three groups according to the pattern of their axonal arborization: (1) local circuit neurons whose axon collaterals were extensively distributed within the NTS with the main axons leaving the NTS; (2) presumed interneurons whose axon collaterals seemed to be restricted within the NTS; and (3) projection neurons whose axons had few, if any, collaterals. Both local circuit neurons and presumed interneurons had small cell bodies (< 150 microns2 in somal area) and exhibited tonic regular spiking at depolarized membrane potentials. Polysynaptic excitatory background activity was increased and lasted for 300-1000 msec in these neurons following solitary tract stimulation. The projection neurons had medium to large cell bodies (> 150 microns2 in somal area). Inhibitory postsynaptic responses produced by an increased CI-conductance were recorded in these projection neurons. These findings suggest that excitatory local networks are organized by an assembly of the local circuit neurons in the caudal NTS, and that the interneurons are arranged to connect the excitatory local network with medium to large projection neurons via inhibitory synapses. Visceral afferent information is probably processed in the highly organized excitatory and inhibitory local networks within the caudal NTS and conveyed to other brain regions.

Ultrastructural characterization of pharyngeal and esophageal motoneurons in the nucleus ambiguus of the rat

The viscerotopic organization of the upper alimentary tract has been established in the nucleus ambiguus, but there is little information about the morphology of the individual neurons innervating the pharynx and esophagus. We studied the ultrastructure of pharyngeal (PH), cervical esophageal (CE), and subdiaphragmatic esophageal (SDE) motoneurons labeled by retrogradely transported wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) in the compact formation of the nucleus ambiguus. WGA-HRP was injected into the lower pharynx, or the cervical and subdiaphragmatic esophagus of male rats. The retrogradely labeled PH neurons in the rostral portion of the compact formation were large (26.1 x 50.1 microns, 906.7 microns2), polygonal, and contained well-developed cell organelles with a round nucleus. Subsurface cisterns connected with rough endoplastic reticulum were often present near the postsynaptic membrane. Both CE and SDE neurons in the compact formation were medium-sized, round or oval, and contained well-developed cell organelles, although the SDE neuron was significantly larger than the CE neuron (24.9 x 33.6 microns, 593.0 microns2 in the SDE neuron, and 19.5 x 30.2 microns, 440.3 microns2 in the CE neuron). The average number of axosomatic terminals in a sectional plane was largest in PH neurons (29.0), smaller in CE neurons (7.9), and smallest in SDE neurons (4.2). The number of axosomatic terminals containing round vesicles (Gray's type I) was almost equal to that of terminals containing pleomorphic vesicles (Gray's type II) in PH and CE neurons, but there were few Gray's type II axosomatic terminals in SDE neurons. Desmosome-like junctions at somato-somatic or somato-dendritic apposition were often present in the area surrounding SDE neurons. There were also small unlabeled neurons (9.5 x 18.1 microns, 131.8 microns2) in the compact formation of the nucleus ambiguus. The small neurons contained poorly developed cell organelles and an irregular shaped nucleus with invaginated nuclear membrane, and had no Nissl bodies. These results indicate that PH neurons have the characteristics of somatic motoneurons, and that CE and SDE neurons are similar to visceral motoneurons.

Hypotension-induced expression of the c-fos gene in the medulla oblongata of piglets

Neural networks that mediate the reflex response to baroreceptor withdrawal were explored in Sus scrofa. Induction of c-fos was used as a monitor of synaptic activity in response to hypotension sustained by systemic administration of a peripheral vasodilator, sodium nitroprusside. Patterns of c-fos gene expression were compared between Saffan-anesthetized experimental animals and age-matched normotensive controls administered vehicle. Effects of other variables were controlled including 1 h preoperative accommodation to the novel environment, anesthesia, blood gases and pH. Identical post-stimulus survival periods were allowed for accumulation of transcript. The c-fos protein, Fos, was identified immunocytochemically with two rabbit antisera raised against amino acids 1-131 of Fos or residues 4-17 of synthetic human transcript. Fos was identified in catecholaminergic neurons labeled with an antiserum to tyrosine hydroxylase (TH). Fos was induced in the nucleus tractus solitarii (NTS) of hypotensive piglets. Neurons encoding Fos matched projection patterns of first order visceral afferents. Induction was prominent in the dorsolateral nucleus coinciding with the baroreceptor field. Indices of increased neuronal activity were evident in other baroreceptor terminal sites, e.g., medial subnucleus, the medial commissural field, the intermediate subnucleus and a ventral A2 noradrenergic area. In reticular formation c-fos protein was induced in circumscribed columns in the lateral tegmental field (LTF) extending from facial nucleus to calamus scriptorius. Catecholaminergic (TH-positive) neurons expressed Fos in the porcine C1 and A1 areas of ventrolateral medulla. Fos was also induced in a dorsal intermediate reticular zone of LTF. Minor or inconsistent differences between experimental and control were observed in nucleus raphe pallidus, rostral paramedian reticular formation, upper thoracic intermediolateral cell column, and stellate ganglia. In conclusion, baroreceptor withdrawal in young animals induced patterns of neuronal response along established cardiovascular reflex pathways.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.