Histological structures in the chemosensitive regions on the ventral surface of the cat's medulla oblongata

Development of the human medullary arcuate nucleus from mid-gestation to the perinatal period: A morphometric study

INTRODUCTION

Development of the human medullary arcuate nucleus (AN) has not been sufficiently investigated. The present study provides morphometric data by examining the brains from preterm and perinatal infants.

MATERIALS AND METHODS

Nine brains were obtained from infants aged 21-43 postmenstrual weeks (PW). Serial celloidin sections were cut and stained using the Klüver-Barrera method. After microscopic observations, morphometric parameters [AN volume, numerical density (Nv) and total number (Nt) of neurons, and neuronal profile area (PA)] were analyzed.

RESULTS

The AN was found as a pair of neuronal masses on the ventral medullary surface at 21 PW. Caudally, it was ventrolateral to the pyramidal tract (PT), and rostrally, medial to the PT. In the middle, it was diminished in size or interrupted. The AN neurons were gradually enlarged with age, showing multiplicity in size and shape. The following findings had a marked asymmetry and individual variability: (1) complete or partial inclusion of the AN in the PT; (2) connection between the rostral AN and the pontine nuclei; (3) coexistence of pyknotic neurons. The AN volume increased exponentially with age, while the Nv decreased exponentially. The Nt changed along two phases (decrease-increase) after mid-gestation. The mean PA increased linearly with age. Asymmetry and/or individual variability were demonstrated in the AN volume, Nt, and mean PA.

CONCLUSIONS

Asymmetry and individual variability in the AN morphology are present in fetal period. The AN may undergo neuron death and neuroblasts production in tandem after mid-gestation.

Connexin26 mediates CO2-dependent regulation of breathing via glial cells of the medulla oblongata

Breathing is highly sensitive to the PCO2 of arterial blood. Although CO2 is detected via the proxy of pH, CO2 acting directly via Cx26 may also contribute to the regulation of breathing. Here we exploit our knowledge of the structural motif of CO2-binding to Cx26 to devise a dominant negative subunit (Cx26DN) that removes the CO2-sensitivity from endogenously expressed wild type Cx26. Expression of Cx26DN in glial cells of a circumscribed region of the mouse medulla - the caudal parapyramidal area - reduced the adaptive change in tidal volume and minute ventilation by approximately 30% at 6% inspired CO2. As central chemosensors mediate about 70% of the total response to hypercapnia, CO2-sensing via Cx26 in the caudal parapyramidal area contributed about 45% of the centrally-mediated ventilatory response to CO2. Our data unequivocally link the direct sensing of CO2 to the chemosensory control of breathing and demonstrates that CO2-binding to Cx26 is a key transduction step in this fundamental process.

Cardiorespiratory and neural consequences of rats brought past their aerobic dive limit

The mammalian diving response is a dramatic autonomic adjustment to underwater submersion affecting heart rate, arterial blood pressure, and ventilation. The bradycardia is known to be modulated by the parasympathetic nervous system, arterial blood pressure is modulated via the sympathetic system, and still other circuits modulate the respiratory changes. In the present study, we investigate the submergence of rats brought past their aerobic dive limit, defined as the diving duration beyond which blood lactate concentration increases above resting levels. Hemodynamic measurements were made during underwater submergence with biotelemetric transmitters, and blood was drawn from cannulas previously implanted in the rats' carotid arteries. Such prolonged submersion induces radical changes in blood chemistry; mean arterial PCO(2) rose to 62.4 Torr, while mean arterial PO(2) and pH reached nadirs of 21.8 Torr and 7.18, respectively. Despite these radical changes in blood chemistry, the rats neither attempted to gasp nor breathe while underwater. Immunohistochemistry for Fos protein done on their brains revealed numerous Fos-positive profiles. Especially noteworthy were the large number of immunopositive profiles in loci where presumptive chemoreceptors are found. Despite the activation of these presumptive chemoreceptors, the rats did not attempt to breathe. Injections of biotinylated dextran amine were made into ventral parts of the medullary dorsal horn, where central fibers of the anterior ethmoidal nerve terminate. Labeled fibers coursed caudal, ventral, and medial from the injection to neurons on the ventral surface of the medulla, where numerous Fos-labeled profiles were seen in the rats brought past their aerobic dive limit. We propose that this projection inhibits the homeostatic chemoreceptor reflex, despite the gross activation of chemoreceptors.

Maps of cardiovascular and respiratory regions of rat ventral medulla: focus on the caudal medulla

The ventral medulla oblongata is critical for cardiorespiratory regulation. Here we review previous literature relating to sites within the ventral medulla that have been identified as having a 'cardiovascular' or 'respiratory' function. Together with the maps generated here, of sites from which cardiovascular and respiratory responses were evoked by glutamate microinjection, specific 'cardiovascular' regions have been defined and delineated. Commonly investigated regions, including the vasopressor rostral ventrolateral medulla (RVLM) and vasodepressor caudal ventrolateral medulla (CVLM), or areas only described by others, such as the medullary cerebral vasodilator area, are included for completeness. Emphasis is given to the caudal medulla, where three pressor regions, the caudal pressor area (CPA), the intermediate pressor area (IPA) and the medullo-cervical pressor area (MCPA), caudal to the vasodepressor CVLM were defined in the original data provided. The IPA is most responsive under pentobarbitone rather than urethane anaesthesia clearly delineating it from both the rostrally located CPA and the caudally located MCPA. The description of these multiple pressor areas appears to clarify the confusion that surrounds the identification of the 'CPA'. Also noted is a vasopressor region adjacent to the vasodepressor CVLM. Apart from the well described ventral respiratory column, a region medial to the pre-Bötzinger is described, from which increases in both phrenic nerve frequency and amplitude were evoked. Limitations associated with the technique of glutamate microinjection to define functionally specific regions are discussed. Particular effort has been made to define and delineate the regions with respect to ventrally located anatomical landmarks rather than the commonly used ventral surface or dorsal landmarks such as the obex or calamus scriptorius that may vary with the brain orientation or histological processing. This should ensure that a region can easily be defined by all investigators. Study of defined regions will help expedite the identification of the role of the multiple cell groups with diverse neurotransmitter complements that exist even within each of the regions described, in coordinating the delivery of oxygenated blood to the tissues.

Primary sensory neurons in the central nervous system

Published data and our own results relating to exteroceptor and a variety of interceptor neurons in the brain and spinal cord, such as intraspinal Hesse ocelli and light-sensitive epiphyseal and ependymal neurons, are presented. Light-sensitive ganglion neurons in invertebrates are also described, along with intrinsic spinal cord bipolar sensory neurons within the spinal cord, primary chemo-and thermosensitive neurons, and sensory unipolar neurons associated with the three fine "central nerves" of Motavkin, which perforate the sheath of the spinal cord and ending with bush-like receptors close to vessels or near the ependyma of the central canal. Data on all known intracortical interoceptors in vertebrates are generalized into a single scheme. It is hypothesized that the brains of animals and humans have an intrinsic sensory innervation comparable with the innervation of other organs and containing local primary sensory neurons and their asynaptic dendrites, which can be divided into two groups: interceptor and exteroceptor.

Late-developing rostral ventrolateral medullary surface responses to cardiovascular challenges during sleep

Pressor and depressor manipulations are usually followed by compensatory autonomic, respiratory, somatomotor or arousal responses that limit the extent of blood pressure change. Of neural sites participating in blood pressure control, the rostral ventrolateral medullary surface (RVLMS) contributes significantly, and exhibits rapid-onset overall activity declines and increases to pressor and depressor challenges, respectively. In addition, longer-latency physiological responses develop that further compensate for the homeostatic challenge; some of these later influences are associated with arousal. Late-developing RVLMS activity changes accompanying physiologic responses that normalize a cardiovascular manipulation may provide insights into compensatory neural mechanisms during sleep following sustained or extreme blood pressure changes. We used intrinsic optical imaging procedures in seven unanesthetized adult cats to examine RVLMS and control site responses to pressor and depressor challenges during sleep that resulted in somatomotor, respiratory, heart rate or electroencephalographic indications of late-developing (post-baroreflex) compensatory responses. Although initial RVLMS responses differed in direction between pressor and depressor challenges, neural activity increased later in both manipulations, coincident with overt physiological manifestations indicative of compensatory responses, including arousal. Arousal occurred in 44% of blood pressure challenges. Comparable late-developing neural activity increases were not apparent in control sites. Latencies of late RVLMS responses during rapid eye movement sleep were significantly longer than in quiet sleep for pressor challenges. The pattern of the late RVLMS responses was not dependent on arousal, and suggests that the RVLMS participates in both the early baroreflex response and the late-developing compensatory actions.

CO(2)-induced c-Fos expression in brainstem preprotachykinin mRNA containing neurons

Tachykinin peptides are found in brainstem regions involved in central chemoreception and they may play a modulatory role in ventilatory response to hypercapnia. We determined whether tachykinin peptide containing neurons are activated by CO(2) by combining in situ hybridization and immunohistochemistry (IHH). Experiments were performed in 21-day-old rats exposed to 12% CO(2) for 1 h. c-Fos expression was identified by IHH on free floating sections (40 microm) that were mounted and then hybridized with anti-sense 35S labeled ribonucleotide probe of the rat preprotachykinin A (PPT-A) gene. Sections were analyzed for expression of the PPT-A gene, c-Fos protein and colocalization of PPT-A gene with c-Fos protein. Within the chemosensory region of the nucleus tractus solitarius (nTS), 19% of c-Fos positive cells expressed PPT-A mRNA after hypercapnic loading. In medullary raphe nuclei, 64% of c-Fos positive cells expressed the PPT-A gene after exposure to CO(2), while 21% of c-Fos labeled neurons in parapyramidal nuclei also expressed PPT-A mRNA. These results indicate that a subpopulation of CO(2) activated neurons within the nTS and in the parapyramidal and midline regions of the ventral aspect of the medulla oblongata express the PPT-A gene, suggesting that these are substance P- or neurokinin A-containing neurons. Furthermore, these peptides may play a role in modulation of respiratory and cardiovascular responses to changes in CO(2)/H(+) content of the extracellular fluid.

Cytoarchitecture of central chemoreceptors in the mammalian ventral medulla

We reviewed the previous reports on the fine anatomy of the mammalian ventral medulla with special attention to the cytoarchitecture of the superficial chemosensitive regions to summarize what is known, what is not yet known, and what should be studied in the future. We also reviewed studies on anatomical relationship between neurons and vessels, and morphological studies on dendrites of respiratory or chemosensitive neurons. When we compared the morphological reports on the ventral and dorsal putative chemosensitive regions, similarities were found as follows. Chemosensitive cells were often found not only near the ventral surface but near the dorsal surface of the brainstem. Dendritic projection towards the surface was a common characteristic in the ventral and dorsal chemosensitive neurons. Morphological abnormality in the brainstem of sudden infant death syndrome victims was also summarized. On the basis of the previous reports we discussed the perspective on the future study on central chemoreception. Among various unanswered questions in central chemosensitivity studies, physiological significance of surface cells and surface extending dendrites is the most important topic, and must be thoroughly investigated.

Monoaminergic neurons, chemosensation and arousal

In recent years, immense progress has been made in understanding central chemosensitivity at the cellular and functional levels. Combining molecular biological techniques (early gene expression as an index of cell activation) with neurotransmitter immunohistochemistry, new information has been generated related to neurochemical coding in chemosensory cells. We found that CO(2) exposure leads to activation of discrete cell groups along the neuraxis, including subsets of cells belonging to monoaminergic cells, noradrenaline-, serotonin-, and histamine-containing neurons. In part, they may play a modulatory role in the respiratory response to hypercapnia that could be related to their behavioral state control function. Activation of monoaminergic neurons by an increase in CO(2)/H(+) could facilitate respiratory related motor discharge, particularly activity of upper airway dilating muscles. In addition, these neurons coordinate sympathetic and parasympathetic tone to visceral organs, and participate in adjustments of blood flow with the level of motor activity. Any deficit in CO(2) chemosensitivity of a network composed of inter-related monoaminergic nuclei might lead to disfacilitation of motor outputs and to failure of neuroendocrine and homeostatic responses to life-threatening challenges (e.g. asphyxia) during sleep.

Chronic prenatal exposure to carbon monoxide results in a reduction in tyrosine hydroxylase-immunoreactivity and an increase in choline acetyltransferase-immunoreactivity in the fetal medulla: implications for Sudden Infant Death Syndrome

Maternal cigarette smoking during pregnancy is associated with a significantly increased risk of Sudden Infant Death Syndrome (SIDS). This study investigated the effects of prenatal exposure to carbon monoxide (CO), a major component of cigarette smoke, on the neuroglial and neurochemical development of the medulla in the fetal guinea pig. Pregnant guinea pigs were exposed to 200 p.p.m CO for 10 h per day from day 23-25 of gestation (term = 68 days) until day 61-63, at which time fetuses were removed and brains collected for analysis. Using immunohistochemistry and quantitative image analysis, examination of the medulla of CO-exposed fetuses revealed a significant decrease in tyrosine hydroxylase-immunoreactivity (TH-IR) in the nucleus tractus solitarius, dorsal motor nucleus of the vagus (DMV), area postrema, intermediate reticular nucleus, and the ventrolateral medulla (VLM), and a significant increase in choline acetyltransferase-immunoreactivity (ChAT-IR) in the DMV and hypoglossal nucleus compared with controls. There was no difference between groups in immunoreactivity for the m2 muscarinic acetylcholine receptor, substance P- or met-enkephalin in any of the medullary nuclei examined, nor was there evidence of reactive astrogliosis. The results show that prenatal exposure to CO affects cholinergic and catecholaminergic pathways in the medulla of the guinea pig fetus, particularly in cardiorespiratory centers, regions thought to be compromised in SIDS.

CO2-induced c-fos expression in medullary neurons during early development

In this study, we characterized the responses of brainstem neurons to hypercapnic loading at 5, 15, and 40 postnatal days, using c-fos gene encoded protein (Fos), as a marker of neuronal activity. At any of these studied ages exposure to 10% CO2 for 1 h produced a significant increase in the number of activated neurons within the ventral and the dorsal aspects of the brainstem. In the ventrolateral aspect of the medulla oblongata, Fos positive cells were observed within the ventrolateral medulla, extending from the pontomedullary border to the decussation of the pyramids. In the most rostral regions, within the retrotrapezoid field, the number of Fos positive cells was lower than in caudal ventral medullary regions at the levels of the area postrema and the caudal to it. No age related differences were observed in the number of neurons exhibiting CO2-induced Fos expression. Fos positive cells were additionally observed in the lateral paragigantocellular and gigantocellular reticular nuclei, in the medullary midline complex, in the raphe pallidus and in the raphe obscurus. The number of activated cells in the midline neurons was higher at 5 than at 40 days of age. In the dorsal aspect of the medulla oblongata Fos positive neurons were observed mainly within the caudal nucleus tractus solitarius (nTS). Postnatal age had no effect on the distribution and number of nTS cells activated by hypercapnic loading. These findings indicate that neurons activated by increases in CO2/H+ concentrations appear to be well developed from the first days of postnatal life in maturing rat pups.

In vitro study of H+-sensitive neurons in the ventral medullary surface of neonate rats

We hypothesized that the direct stimulus of the central chemoreceptor neurons is the CO2/H+-induced change in intracellular pH (pHi). If it is true, pHi responses during hypercapnic stimulation should be exhibited in the central chemoreceptor neurons in the ventral medullary surface (VMS) and some neurons in the CO2/H+ sensitive regions such as the nucleus tractus solitarii of the medial dorsal medulla (MDM). To test this hypothesis, the cultured VMS and MDM neurons (control) derived from one day-old neonate rats were labeled with H+-sensitive fluorescent indicator 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF), and were exposed to perfusate of various pHs. The H+-sensitive neurons were determined by a rapid decrease in the intracellular BCECF fluorescence intensity. In almost all the MDM neurons (99.6%) and 94% of the VMS neurons, the intracellular BCECF fluorescence intensity remained unchanged when the extracellular pH (pHo) was decreased. In contrast, in 0.4% of the MDM neurons (8/1800) and in 6% of the VMS neurons (111/1800), the intracellular BCECF fluorescence intensity decreased when the pHo was decreased from 7.4 to 7.2. This subpopulation of MDM and VMS neurons were considered to be H+-sensitive neurons. The H+-sensitive neurons in the VMS showed positive immunoreactivity to glutamate (57%, 17/30) and glutamic acid decarboxylase (23%, 7/30), but no immunoreactivity to choline acetyltransferase, tyrosine hydroxylase, phenylethanolamine N-methyltransferase, somatostatin, serotonin and substance P. These results indicate that the H+-sensitive neurons are present specifically in the VMS, and are mainly glutamatergic and GABAergic.

Central cocaine neurotoxicity at brainstem cardiorespiratory control sites

Cocaine hydrochloride was applied topically to the ventrolateral medullary surface (VMS) where chemosensitive respiratory and vasomotor control sites are colocalized. Cats (n = 16) were anesthetized with urethane (2.0 g/kg, 80 percent of dose titrated over 60 min). The trachea of each animal was cannulated and the VMS was surgically exposed. Tidal volume (VT), frequency of breathing (f), systolic and diastolic blood pressure (SBP and DBP, respectively), and heart rate (HR) were measured. Cocaine (62.5 micrograms per site) administered at the VMS control sites decreased f, SBP, and DBP significantly (p < 0.05), without changing HR or VT values. This cocaine-induced hypoventilation was associated with brief intervals of inspiratory cramp (apneusis). Central cocaine neurotoxicity may result from interaction of cocaine with VMS sites, producing increased inspiratory drive and decreased vasomotor tone.

Rete mirabile medullae: vascular specialization of the avian brain stem

A circumscribed vascular plexus is located between the conjoint proximal ganglion of cranial nerves IX-X and the lateral surface of the avian brain stem. Gross, light, and electron microscopic analyses reveal that this structure is a rete mirabile. This rete is unique in that it resides in the subarachnoid space where it is bathed by cerebrospinal fluid. Microscopic and ultrastructural studies document that the rete is composed of arterioles with wide luminal diameters, marginal folds project from the endothelial cells into the lumen of the blood vessel. The endothelial cells contain numerous pinocytotic vesicles. The underlying basal lamina is penetrated by myoendothelial junctions. One to three layers of smooth muscle cells comprise the tunica media. Bundles of unmyelinated nerve fibers course through the loose connective tissue encompassing the blood vessels.

Candidate cell populations for respiratory chemosensitive fields in the human infant medulla

The histology and location of human respiratory chemosensitive fields are not known. In contrast, the physiology of respiratory chemosensory areas in the ventral medulla of cats has been studied extensively, and their anatomy has been partially described. Using basic principles of comparative cytoarchitecture and computer-aided reconstructions of serial-sectioned medullae, we describe the histology and three-dimensional distribution of putative respiratory chemosensors in the feline and human infant medulla. We found that ventrolateral neurons of the human nucleus conterminalis are homologous to neurons identified in the feline L chemosensitive field by Trouth and others, and that ventrolaterally situated neurons in the human arcuate nucleus correspond to neurons predominating in the feline S and M fields. In addition, there are foci of thickened marginal glia along the feline ventral medullary surface that colocalize with chemosensitive fields identified by physiologic studies reported by others; we identify similar foci in the infant medulla. Thickened marginal glia are intermixed with neuronal fibers, often adjacent to neurons of the feline chemosensitive fields and their human counterparts, suggesting that they constitute a chemosensory neuropil at the medullary surface. Computer-aided reconstructions provide insight into the three-dimensional topography of putative respiratory chemosensors and their relationships to other brainstem structures in ways not obvious in single or even multiple sections. This delineation of candidate human respiratory chemosensors is a first step toward their postmortem analysis in patients with central ventilatory control disorders where finding histological abnormalities in these sites would support their role in human ventilation.

Substance P-containing medullary projections to the intermediolateral cell column: identification with retrogradely transported rhodamine-labeled latex microspheres and immunohistochemistry

Substance P (SP)-containing medullary neurons that project to the intermediolateral cell column (IML) of the rat were studied. Neurons were retrogradely labeled with rhodamine-labeled latex microspheres (RITC-M) injected into the T-3 IML, and SP-immunoreactive neurons were identified with immunocytochemistry. RITC-M labeled cells occurred in the nucleus reticularis paragigantocellular lateralis (RPgcl), adjacent and lateral to the pyramidal tract at the level of the rostral inferior olivary nucleus and extended to the mid-facial nucleus in the medulla. Cells were also labeled caudal to the RPgcl, in the nucleus reticularis ventralis, pars alpha (RVa), rostral to the RVa, in the nucleus reticularis gigantocellularis (RGc), and in the raphe nuclei. SP immunoreactivity (SP-IR) was seen in cells that were also retrogradely labeled. These double-labeled cells were observed in the RPgcl, RVa and the raphe pallidus. These data show that the IML receives SP-neuronal projections from multiple locations in the medulla. The SP-neuronal projections from the RPgcl of the ventral medulla to the IML likely represent one component of the ventral medullary region that influences cardiovascular functions.

Target sites for anticholinesterases on the ventral surface of the medulla oblongata: hypotension elicited by organophosphorus agents

Sensitivity of the ventral surface of the medulla oblongata to organophosphorus agents, oxime reactivators, and muscarinic antagonists was examined in order to delineate sites of cholinergic activity in the central nervous system. The exposed ventral surface of the medulla oblongata in anaesthetized cats was treated with the organophosphorus anticholinesterase agents soman and (7-nitro-2-oxa-1,3-diazole) aminopentyl methylphosphonofluoridate (NBD-AP-MFP), a fluorescent active centre-selective probe of acetylcholinesterase. Topical application of soman (1-5 micrograms) or NBD-AP-MPF (5-120 micrograms) elicited a profound (80-90 mm Hg), long-lasting (0.5-3 h), dose-dependent vasodepression with only minor changes in heart rate and respiration. The vasodepression was rapidly reversed (7-10 min) upon topical application of muscarinic antagonists (atropine methylnitrate, atropine sulphate) and the bisquaternary oxime HI-6; systemic administration was without effect. Reversal of the hypotension by HI-6 occurred irrespective of whether the organophosphorus agent was NBD-AP-MPF, which forms conjugates with acetylcholinesterase that undergo no aging, or soman, which forms conjugates that undergo extensive aging rendering the enzyme refractory to oxime reactivation. Hence, oxime efficacy for reversal of the physiologic hypotension was not dependent solely on the fraction reactivatable enzyme. By virtue of the fluorescence distribution of NBD-AP-MPF the chemosensitive sites were estimated to reside no deeper than 50 microns into the medulla oblongata, providing a direct indication for localization of the chemosensitive cells on the superficial surface.

Electrophysiological study of sympathoexcitatory structures of the bulbar ventrolateral surface as related to vasomotor regulation

The responses in T3-4, T10-11 and L2-3 white rami to stimulation of different zones of the bulbar ventrolateral surface were maximal when the region of about 4 mm laterally to the midline was stimulated. A weak surface stimulation of all these zones elicited only a long latency response consisting of three waves. A short latency response appeared when supramaximal stimuli were applied only to the intermediate zone--the region up to 6 mm rostrally to the hypoglossal nerve root level (zone S and caudal part of zone M). The data presented show that long and short latency responses are conducted from the intermediate zone to the spinal cord via dorsolateral funiculus fibres with a conduction velocity of about 5.6 +/- 0.6 m/s. In addition, a special descending sympathoexcitatory pathway oriented to T2 preganglionic neurons with a conduction velocity of about 12.3 +/- 3.2 m/s was demonstrated. Antidromic discharges of the output sympathoexcitatory neurons elicited by dorsolateral funiculus stimulation were found in the intermediate zone only at a depth of about 400-2000 microM. Stimuli applied to different regions of the ipsilateral bulbar ventrolateral surface activate at least two groups of surface fibres (conduction velocities 6.7-8.0 and 2-3.2 m/s) which, in turn, activate the output neurons with a rather constant delay of about 20 ms equal to a difference between the latencies of long and short latency white rami responses. The mechanism of delay formation seems to be concentrated in the intermediate zone and formed probably by a chain of interneurons. A possible scheme of neuronal organization of the bulbar ventrolateral sympathoexcitatory structures is presented and discussed. The descending tonic activation of spinal vasomotor neurons is formed by spontaneous discharges of antidromically identified output neurons with a mean firing rate of about 14.4 imp./s. Some neurons are reflexly activated within the time limits of the late somatosympathetic reflex response. Coagulation of the intermediate zone resulted in a profound fall of blood pressure, disappearance of pressor and late somatosympathetic reflexes, whereas the spinobulbospinal somatosomatic reflex remains unchanged. The baroreceptor inhibition is partly realized through the elements of sympathoexcitatory intermediate zone because the preferable inhibition of the long latency white rami response was demonstrated in the middle of R-R interval and during a sharp increase in the arterial pressure induced by vasoconstrictor drugs. Thus, the structures of the intermediate zone seem to play a key role in supporting of blood pressure level and organization of pressure reflexes.(ABSTRACT TRUNCATED AT 400 WORDS)

Histochemical study of the distribution of carbonic anhydrase in the cat brain

The distribution of carbonic anhydrase (CA) in cerebrum, cerebellum and medulla oblongata of the cat brain has been examined by a histochemical method. Neuron cell bodies and dendrites are stained in some locations. Many axons are distinctly stained and different intensities of staining can be seen even in adjacent axons. One of the most intensely stained structures is the capillary endothelium and stained capillaries are found in all parts examined. Glial cells are intensely stained in agreement with biochemical and earlier histochemical works. Myelin sheaths are never stained, possibly due to enzyme loss during embedding. The localization of the enzyme shows regional differences. In this respect, the medulla oblongata has been examined in more detail. A small area close to the ventral surface, medial to the roots of the hypoglossal nerve, is characterized by a high CA staining of the neuropil. The cell membrane of some large neurones and the capillary endothelium in this area were also stained. With regard to position and CA content, this area corresponds well to the characteristics of the medullary chemosensitive area as defined by previous experimental studies.

Monoamine distribution on the ventral surface of the rat medulla oblongata

The distribution of monoamine transmitters in the area near the ventral surface of the rat medulla oblongata was studied using the Falck-Hillarp histofluorescence method. Histological examination and scanning electron microscopy of these regions were also performed. It was found that there is a wide area dense with catecholamine terminals in the external layer of the ventral medulla oblongata. 5-Hydroxytryptamine-containing terminals and nerve cell bodies on and near the surface were also found. Due to their superficial localization these monoamines may influence the content of cerebrospinal fluid and in this way have effects on cardiovascular and other physiological functions.

Central chemosensitivity and the reaction theory

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Morphological observations on superficial medullary CO2--chemosensitive areas

Physiological investigations have indicated that the ventrolateral surface of the medulla oblongata is involved in the chemical drive to respiration. In this investigation, light and electron microscopic investigations of the 3 chemosensitive regions reveal the following. (1) Evaginations of the ventral surface abut the overlying pia mater thereby delimiting discrete compartments; invaginations of the surface delimit wide cisternae lined with basement membrane. Neuronal elements with numerous synapses, were found scattered among astrocytic processes of the marginal glia in intermediate and caudal chemosensitive areas Microvasculature are conspicuously absent from the marginal glia. Intramedullary vessels are surrounded by perivascular spaces and the endothelium shows zonulae occludentes at cell junctions. (2) Horseradish peroxidase (HRP) applied to the ventral surface diffused throughout the interstitial and perivascular compartments, into synaptic clefts and neuronal soma. Diffusion of HRP into blood vessels was blocked at zonulae occludentes. Following intravenous injection of HRP, no reaction product was found outside cerebral vasculature in chemosensitive areas. (3) In spontaneously breathing cats, 2% procaine applied to the caudal chemosensitive area resulted in respiratory depression which began with the second breath. It is proposed, that substances which stimulate or depress respiration, when applied to the ventral medullary surface, produce their effects on superficial neurons located in the intermediate and caudal chemosensitive areas after diffusion through interstitial spaces.

Morphology of the medullary chemosensitive fields. 1. Mapping of the neuronal matrix by a horseradish peroxidase technique

A modified horseradish peroxidase labelling technique was used to study the distribution pattern of neurons in the central chemosensitive fields of the medulla oblongata of cats. In several cryosectioned medullae a mapping of superficially located HRP-labelled neurons was achieved. The distribution and configuration of the labelled neurons indicate that most of them belong to the nucleus paragigantocellularis lateralis. However, by varying the time of incubation it was possible to identify different types of neurons. On the basis of certain aspects of the HRP incorporation mode and neuronal topography a specific type of small-sized neurons has been identified. Some functional implications of these small neurons with respect to their possible chemosensitive activity are discussed.

Projections to the spinal cord from neurons close to the ventral surface of the hindbrain in the rat

Neurons located within 100 micron of the ventral surface of the hindbrain were labeled following injections of horseradish peroxidase into the thoracic and lumbar spinal cord in the rat. Labeled neurons were distributed into two groups, one ventrolateral to the inferior olive and ventral to the facial nucleus, and one ventral to the raphe pallidus. The location of the ventrolateral subpial group is very similar to that of the brain stem chemosensitive zones, and suggests that the neurons in this group may be important in cardiovascular regulation.

Projections to the ventral surface of the cat brainstem demonstrated by horseradish peroxidase

Topical application of horseradish peroxidase (HRP) to the ventral surface of the medulla oblongata revealed various cells with axons terminating near this region. Fibers traced from the ventral surface originated in the nucleus of the tractus solitarius and the dorsal nucleus of the vagus, both near the dorsal surface of the medulla. Projections from the cochlear nucleus were also observed. Using short HRP contact time, fibers were shown which terminate at, or near, the ventral surface. The possible involvement of these projections in various responses mediated from the ventral surface is discussed.

Influence of the CSF bicarbonate concentration on the ventilatory response to CO2 in relation to the location of the central chemoreceptors

In anaesthetized cats, in which the cerebrospinal fluid bicarbonate concentration was varied by a ventriculocisternal perfusion technique, the ventilatory response to CO2 during hyperoxia could be satisfactorily described by VE = S(PCSFCO2 -B). Both the slope S and the intercept B were positively and linearly related to the CSF bicarbonate concentration. Assuming that the PCSFCO2 is equal to the PCO2 in extracellular fluid, it can be shown that VE is a linear, but not a unique function of the [H+] at the site of the chemoreceptors; the slope of this relation varies with the bicarbonate concentration at that site, possibly due to chemical complex formation between HCO-3 and Ca2+ or Mg2+. Changes in the B-value were related to the location of the central chemoreceptors with the models of Pappenheimer and Berndt aand their coworkers. It was found that changes in the CSF bicarbonate concentration are reflected for 60 per cent at the site of the central chemoreceptors, and that this was independent of the cerebral perfusion. Using Berndt's model a distance between CSF and central chemoreceptors of approximately 100 micron was found; this calculated distance is relatively insensitive to relationship (logarithmic or not) between ventilation and H+ concentration and to changes in cerebral perfusion, owing to the approximate nature of the diffusion model.

Neurones within the "chemosensitive area" on the ventral surface of the brainstem which project to the intermediolateral column

With the method of retrograde transport of horseradish peroxidase it has been demonstrated that neurones within the "chemosensitive area" of the brainstem project to the thoracic intermediolateral column. The function of these neurones is discussed in regard to the regulation of blood pressure.

The paratrigeminal nucleus. II. Identification and inter-relations of catecholamine axons, indoleamine axons, and substance P immunoreactive cells in the neuropil

The localization of three different putative neurotransmitters -- indoleamine, catecholamine, and Substance P -- was studied in the paratrigeminal nucleus of the rat and rhesus monkey at the light and electron microscope level by autoradiography following administration of [3H]5-hydroxytryptamine, or [3H]norepinephrine, and by immunocytochemistry using the unlabelled anti-Substance P antiserum peroxidase--antiperoxidase technique. The paratrigeminal neurons are not monoaminergic but certain cells exhibit Substance P-like immunoreactivity. These cells receive a rich plexus of indoleamine afferents, a sparse catecholamine input, and a rich plexus of fibres with Substance P-like immunoreactivity. Of the entire monoaminergic population of labelled axons, more than 60% are synaptic and less then 40% nonsynaptic, and this proportion is the same for indoleamines as for catecholamines. Indoleamine axons form a heterogeneous population with at least four different morphological types that are synaptic and three that are nonsynaptic. They bear distinctive collections of small, clear, tubular or large granular vesicles, which distinguish one category of axon from another. These axons engage in numerous axo--somatic, axo--spinous, axo--dendritic, and possibly axo--axonic relations with paratrigeminal neurons. The catecholamine axons are also heterogeneous in axoplasmic morphology but their terminal contacts are distributed to more peripheral portions of dendrites. The significance of the inter-relations between the monaminergic and peptidergic elements in the paratrigeminal nucleus is discussed in relation to the possible functions of this nucleus as a nociceptive, chemosensitive, or pressure-sensitive centre on the lateral medullary surface.

Carbon dioxide sensitivity of the central chemosensitive mechanisms. An exploration by direct stimulation in rats

In urethane-anaesthetised adult albino rats ventral surface of the brainstem was stimulated chemically by increasing the local CO2 concentration and electrically. Two areas were demarcated on the ventral surface of the brainstem, one which showed an increase in pulmonary ventilation on chemical and electrical stimulation, and another which showed a decrease in pulmonary ventilation and sometimes even respiratory arrest. EEG activity recorded from the area from where increased pulmonary ventilation was obtained showed a synchronous slow wave activity during chemical stimulation and inhalation of a CO2-air mixture. This area is situated 0.5--1 mm lateral to the mid-line extending up to the rootlets of the VIIth to IXth cranial nerves. The response increased proportionately on increasing the strength of the chemical stimulus, till it reached a plateau. In carotid body denervated and chronic hypoxic animals, the magnitude of the responses was shown to be increased, probably due to increased sensitivity of the central chemosensitive mechanisms.

The catecholamine pontine cellular groups locus coeruleus, A4, subcoeruleus in the primate Cebus apella

The distrubution of CA neurons of areas A6 and A4 was delineated in Cebus apella monkey using the fluorescent histochemical technique of Falck and Hillarp. Cytospectroscopy was utilized for CA differentiation. The noradrenergic cellular regions A6, A4, and subcoeruleus have extensively increased in size in the Cebus as compared to the rat and appear to be separate nuclear regions. Area A4 is made up of two cellular subgroups: a more abundant lateral magnocellular area with cells as large as 45 mum and a smaller medial parvocellular group where the neurons are spindle-shaped and lie within 10-100 mum of the ependyma. The neuronal processes of A4 tend to be directed towards the flocculus and paraflocculus of the cerebellum. Some processes seem to enter the ependyma and others end subependymally. The functional significance of the pontine CA neurons is discussed.

'Exponential peeling' of ventilatory transients following inhalation of 5, 6 and 7% CO2

The 'exponential peeling' technique has been applied to minute ventilation and tidal volume transients occurring after the abrupt removal of 7, 6 and 5% CO2 in inspired air. These transients, in many cases, were found to be composed of three exponential components, each contributing to the total ventilatory response and each having individual time responses. Gelfand and Lambertsen (1973) have attributed these components to the peripheral chemoreceptors as a group and to two central chemoreceptors. Statistical analysis to determine the constancy of the contribution of the three components over the range of CO2 values studied showed that, although the values for each at the different stimulus levels were not significantly different, the great subject-to-subject variation in the data precluded a firm conclusion about the constancy of the components. Because of a number of considerations it was concluded that exponential peeling of respiratory transients following abrupt removal of CO2 inhalation is not a satisfactory way to approach the problem of the numbers, relative contributions and time responses of the various receptor groups comprising the respiratory controller.

pH sensitivity of cells located at the ventrolateral surface of the cat medulla oblongata in vitro

pH sensitivity of cells located at the ventral surface of the medulla oblongata was examined in a thin brain slice of the cat in vitro and the following results were obtained: (1) Transmembrane potential of the surface cells located in the area medial to the XIIth cranial nerve was reduced slightly by application of low pH solution; (2) In the rostral part of the area medial to the XIIth cranial nerve regular neuronal discharges could be observed extracellularly. The rate of firing of these cells was increased by lowering the external pH. These results were considered to support the idea the H+ receptor cells may exist in the surface layer of the ventral medulla.

Vasopressin release by nicotine: the site of action

1. In cats anaesthetized with chloralose the release of neurohypophysial hormones was examined after injection of nicotine into the cerebral ventricles or cisterna magna or its topical application through perspex rings to the ventral surface of the brain stem. The release was measured by assaying the hormones in samples of venous blood. 2. Injected into a lateral or the third cerebral ventricle, nicotine (0.5 to 1 mg) produced release of vasopressin without oxytocin. When the aqueduct was cannulated, preventing access to the fourth ventricle and to the subarachnoid space, this release did not occur. 3. Vasopressin was also released without oxytocin when nicotine (0.25 to 2 mg) was injected into the subarachnoid space through the cisterna magna. With this route of administration the nicotine did not enter any part of the ventricular system. 4. Applied through paired perspex rings placed across the ventral surface of the brain stem, nicotine again produced release of vasopressin without ocytocin. The amount of nicotine placed in each ring was usually 80 mug, but a release was obtained with 10 mug and in one experiment with as little as 5 mug. 5. The bilateral region on the ventral surface of the brain stem where nicotine acts when producing release of vasopressin lies lateral to the pyramids and in a longitudinal direction, 6 to 9 mm caudal to the trapezoid bodies. 6. The vasopressin release by nicotine injected intraventricularly or intracisternally, or applied topically to the ventral surface of the brain stem was not due to absorption of nicotine into the blood stream, nor to blood pressure effects. 7. It is concluded that nicotine acts on the ventral surface of the brain stem probably by activating the central projection to the supra-optic and possibly also the paraventricular nuclei of afferent pathways in the sinus and vagus nerves which control the release of vasopressin in response to changes in blood volume or distribution.

Fall in blood pressure produced from discrete regions of the ventral surface of the medulla by glycine and lesions

1. In cats anaesthetized with pentobarbitone sodium, atropinized by i.v. atropine methyl nitrate and artificially ventilated, experiments were carried out (a) to localize the site where glycine acts on the ventral surface of the medulla when, on topical application through paired Perspex rings caudal to the trapezoid bodies, it produces a fall in arterial blood pressure, (b) to compare the effects of uni- and bilateral application, and (c) to study the blood pressure effects produced by electrolytic lesions of the glycine-sensitive areas.2. Blood pressure fell only a little on unilateral application of glycine through one of the Perspex rings, but a pronounced fall occurred on its bilateral application. The fall was too large to be explained by two minimal responses added together. Thus the application of glycine to one side potentiated the depressor effect of glycine applied to the other side.3. By moving the paired Perspex rings rostrally or caudally to different positions on the medulla and determining for each position the effectiveness of glycine, the glycine-sensitive areas were found to be restricted to a 1.5 mm wide strip situated 1-2.5 mm caudal to the trapezoid bodies. By making electrolytic lesions within the limits of this strip the glycine-sensitive areas were found to be not wider than 1.5 mm in the mediolateral direction and to be situated about 4 mm lateral to the mid line. Histologically, such lesions involved the cells of the parvicellular part of the lateral reticular nucleus.4. Placing an electrode, with a diameter of 1 mm, under light pressure on the glycine-sensitive area produced a short-lasting steep rise in blood pressure. The same effect was produced when a current was passed through the electrode to destroy the underlying tissue, but after its destruction the passage of current no longer produced the pressor effect.5. Once the glycine-sensitive area of one side was destroyed, glycine applied to the destroyed area through one of the Perspex rings no longer produced its small depressor effect, but when applied to the intact area of the other side, a pronounced fall in blood pressure occurred. Thus unilateral destruction had the same effect as unilateral glycine application. It potentiated the depressor effect of glycine applied to the other side.6. Following unilateral destruction of the glycine-sensitive area there was only a small fall in blood pressure; following its bilateral destruction blood pressure fell to a low level.7. It is concluded that the cells in the small circumscribed glycine-sensitive areas on the ventral surface of the medulla may play a key role in the maintenance of arterial blood pressure and that the cells of one side are sufficient for this function.