ber die Glia marginalis und oberfl�chliche Nervenzellen im Hirnstamm der Katze

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.

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.

Cardiorespiratory effects produced by microinjecting L-glutamic acid into medullary nuclei associated with the ventral surface of the feline medulla

The purpose of our study was to use microinjections of L-glutamic acid to better localize the cell bodies in the intermediate area of the ventral medullary surface that exert control over cardiorespiratory activity. L-glutamic acid (200 nl of a 1-M solution) was microinjected into the nucleus paragigantocellularis lateralis, lateral reticular nucleus and into an area which is part of the 'glycine-sensitive area', which lies in the center of the intermediate area. Normally, when L-glutamic acid is applied to the surface of the intermediate area, increases in arterial pressure and tidal volume are observed. Increases in tidal volume were never observed upon microinjection into the 3 sites associated with the intermediate area, suggesting that the tidal volume change elicited from surface application occurs because of L-glutamic acid interacting with cell bodies either on the surface or extremely close to the surface. Pressor responses were elicited with microinjection of L-glutamic acid into the lateral reticular nucleus and the 'glycine-sensitive area', but not the nucleus paragigantocellularis lateralis; indeed, microinjection of L-glutamic acid into the nucleus paragigantocellularis lateralis caused hypotension. Hence, cell bodies responsible for raising arterial pressure may reside in either the lateral reticular nucleus or the 'glycine-sensitive area'.

Inhibition of feline spinal cord dorsal horn neurons following electrical stimulation of nucleus paragigantocellularis lateralis. A comparison with nucleus raphe magnus

The effect of electrical stimulation applied to the nucleus paragigantocellularis lateralis (PGL) was assessed on the somatosensory responses of functionally identified spinal cord dorsal horn neurons in the cat. Neurons were classified as low threshold mechanoreceptive, wide dynamic range or nociceptive specific. The responses of over 95% of all neurons tested were inhibited by a conditioning stimulus to the PGL. For each cell the threshold current intensity necessary to produce inhibition from the PGL (inhibitory threshold) was determined. Analysis of the incidence of inhibition and the inhibitory thresholds showed that the PGL-induced inhibition was not selective for a particular class of neuron. Due to the many similarities between the PGL and the nucleus raphe magnus (NRM), a comparison was made between each region's potency in inhibiting the responses of spinal cord neurons. Based on an analysis of inhibitory thresholds, the PGL was found to be significantly more potent than the NRM. These results indicate the PGL to be an important site from which descending modulation of spinal cord somesthetic information emanates.

The nucleus paragigantocellularis lateralis in the rat. Demonstration of afferents by the retrograde transport of horseradish peroxidase

Injections of horseradish peroxidase (HRP) were placed in the middle or caudal portion of the nucleus paragigantocellularis lateralis (PGCL) and 24 h later the entire spinal cord and brain were processed and examined for labeled neurons. Spinal afferents arise from all levels of the cord. Rexed's lamination scheme was adapted to the spinal cord of the rat and labelled neurons were localized to laminae IV, V, VII, VIII and X mainly on the side contralateral to the injection. At cervical levels, labeled neurons were consistently found bilaterally. The medial reticular nuclei of the medulla and pons contained HRP-labelled perikarya, which were concentrated most heavily in the nuclei reticularis medullae oblongatae ventralis, gigantocellularis, and pontis caudalis predominantly ipsilateral to the injection. The medial vestibular nucleus was consistently labeled. HRP-labeled perikarya were found bilaterally within the commissural portion and in the medial part of the nucleus of the solitary tract on the side of the injection. The rostral portion of the PGCL receives afferents from some secondary auditory nuclei: the ipsilateral inferior colliculus and the posterior ventral cochlear nucleus bilaterally. Thus, the rostral PGCL may be involved in auditory feedback loops. The caudal raphe nuclei are a major source of afferents to the caudal PGCL. The lateral hypothalamic area, paraventricular nucleus, and zona incerta also contain labeled neurons when injections are centered in the caudal portion of the nucleus.

The nucleus paragigantocellularis lateralis in the rat. Conformation and cytology

The nucleus paragigantocellularis lateralis (PGCL) is located in the ventral portion of the rostral medulla. Serial sections of the rat brainstem were examined in the three cardinal planes and the boundaries of the PGCL were determined. In order to visualize the shape and extent of the nucleus, a three-dimensional reconstruction of the PGCL was made from a series of coronal sections. Measurements of neuronal areas, lengths, and widths indicate that a number of neuronal types are present. Small neurons measure less than 150 micron2 and large neurons greater than 250 micron2. Some neuronal types are distributed preferentially throughout the PGCL, and on this basis the nucleus may be divided into caudal and rostral subgroups. Most large neurons (greater than 250 micron2) are found in the caudal portion. Certain neurons contain intranuclear rods, and these neurons are often disposed in small groups, especially common the caudal PGCL. Two morphologically distinct neuronal types incorporate 3H-serotonin when this marker is infused into the ventricular system; the other neurons not marked by this method probably contain other, different transmitters. On the basis of neuronal measurements and staining qualities, it is ascertained that the PGCL is a parvocellular reticular nucleus characterized by many neuronal types.

The ventromedullary hypotensive effect of muscimol in the anaesthetized cat

Muscimol, a rigid analogue of GABA, was injected into the CNS of urethane-anaesthetized, normotensive cats. Bilateral microinjections of a small dose of muscimol (100 ng/kg, 0.5 microliter on each side) in the nucleus reticularis lateralis (NRL) induced hypotension. The marked fall in blood pressure obtained by injecting 1 microgram/kg of muscimol unilaterally into the NRL is completely reversed by subsequent local administration of bicuculline (5 microgram/kg, 0.5 microliter), a specific GABA antagonist. These data confirm that a GABAergic inhibitory system modulates the pressor tonic structures localized in the region of the NRL and that the anteroventral part of the medulla oblongata includes a trigger zone for the hypotensive action of muscimol.

Respiratory response to hypoxia and hypercapnia after elimination of central chemosensitivity

Central respiratory drive responding to pH changes was eliminated by bilateral coagulation or cold block of area S (intermediate area) on the ventral medullary surface in 7 anaesthetized cats. Arterial pH, PCO2, and PO2 (4 cats) and the respiratory response to hypoxia and hypercapnia (6 cats) were observed before and after coagulation. After coagulation in hyperoxia the arterial pH dropped from 7.30 to 7.09, the arterial PCO2 was elevated from 4.80 kPa to 8.17 kPa (6 cats). Ventilation increased by 477 ml at a PCO2a of 6.58 kPa when PO2a was reduced from 39.5 kPa to 8.5 kPa before coagulation, after coagulation ventilation increased by 241 ml (4 cats). The peripheral chemoreceptors guaranteed spontaneous breathing even in hyperoxia. The data reveal that the loss of respiratory homeostasis by elimination of the S areas is due to the loss of central chemosensitive drive with concomitant reduction of peripheral chemoreceptor effect.

Elimination of central chemosensitivity by coagulation of a bilateral area on the ventral medullary surface in awake cats

Breathing and respiratory response to CO2 were observed in 6 awake cats and 1 control before and after bilateral coagulation of the formerly described area S (Schläfke and Loeschcke, 1967) on the ventral medullary surface under hyperoxic conditions. Ventilation decreased, PCO2 rose and CO2 response was almost or completely abolished in 4 cats, and moderately reduced in 2 cats. Inhalation of CO2 had an inhibitory effect on ventilation in two cases. In some instances the respiratory frequency was increased by CO2. Periodic breathing as well as spontaneous hyperventilation elicited by 'arousal' indicate parallels to the Pickwickian or Ondine's curse syndrome. No respiratory changes were produced by a lesion on the pyramidal tract medial to the area S. It is concluded that central chemosensitivity can be eliminated within the superficial layer of the area S. The loss of CO2 response seems to be correlated with complete destruction of the superficial nerve cells located within the area S (Petrovický, 1968) and degeneration within the ventral part of the nucleus paragigantocellularis.

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.

Relationship between the ventromedullary clonidine-sensitive area and the posterior hypothalamus

The connections between the areas 'S' which have been previously described as the ventromedullary sites of the action of clonidine and the posterior hypothalamus have been investigated. Superficial electrocoagulation of the left area 'S' suppresses the pressor response to electrical stimulation of the homolateral part of the posterior hypothalamus. Although such medullary lesions cause a significant reduction of the mean arterial pressure, the contralateral hypothalamic stimulation can still increase blood pressure. Clonidine it self applied topically (8 micrograms/kg) to the ventral face of the brain stem decreases the blood pressure response to liminal hypothalamic stimulation. It is concluded that efferent pathways, which are involved in vasomotor regulation, originate in the posterior hypothalamus and run through the ventrolateral part of the brain stem. The mechanism of the blocking effect of clonidine on these pathways is discussed.

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.

Role of the ventral surface of the brain stem in the hypotensive action of clonidine

The areas S of the ventral surface of the brain stem and the immediately surrounding zone were superficially destroyed by the means of electro-coagulation, in 14 cats. This destruction produced a drop in blood pressure, which was transient in 9 and definitive in 4 animals; in one cat only the arterial pressure did not change after the destruction. In 6 animals which have been sham-operated, clonidine (15 mug/kg, i.v.) always induced a marked fall in blood pressure whereas in 10 animals which had maintained or recovered a normal blood pressure after the destruction of the area S, clonidine (15 mug/kg) injected intravenously no longer produced any decrease of the arterial pressure. These results suggest that the integrity of the areas S is necessary for the development of the hypotensive action of clonidine. This hypotensive drug may act, at least at the level of the ventral surface of the brain stem, through inhibition of a vasopressive structure.

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.