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

Modulation of the centrally-evoked visceral alerting/defence response by changes in CSF pH at the ventral surface of the medulla oblongata and by systemic hypercapnia

In the present study on nine cats, repeated tests were made of the effects of superfusion of the ventral surface of the medulla oblongata with acid or alkaline CSF. Only two animals showed slight hyperventilation, tachycardia, mesenteric vasoconstriction and variable changes in hindlimb vascular conductance when the ventral surface was superfused with acid CSF; alkaline CSF produced opposite effects. These changes are qualitatively similar to, but much smaller than, published results which support the idea that the central chemoreceptor areas for CO2 are near the surface of the ventral medulla. But, in accord with those who have disputed this idea, the remaining 7 animals showed no response to superfusion with acid or alkaline CSF. Yet, all 9 animals showed marked hyperventilation in response to inhalation of 5% or 8% CO2. These findings accord with the view that chemosensitive structures on the ventral medulla represent part, but not all of the central chemosensitive mechanism for CO2. Inhalation of CO2 also induced bradycardia, mesenteric vasodilatation and either vasodilatation or vasoconstriction in hindlimb, attributable to a predominance of the direct myocardial depressant and local vasodilator effects of CO2, over the increase in sympathetic activity produced by central hypercapnia. But, despite the different effects of acid CSF and inhaled CO2 on baselines, they produced comparable effects on the visceral altering/defence response evoked by electrical stimulation in the ventral amygdalo-hypothalamic pathway viz, the magnitude of the characteristic hindlimb dilatation was reduced while that of the mesenteric constriction was increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Intracisternal diethylpyrocarbonate inhibits central chemosensitivity in conscious rabbits

As a direct chemical test of the alpha-imidazole hypothesis for the function of mammalian central chemoreceptors (CCR), diethylpyrocarbonate (DEPC) a relatively specific reactant with imidazole groups in vitro has been administered in vivo via intracisterna magna (ICM) infusion in conscious rabbits using each rabbit as its own control. DEPC, in a dose-dependent fashion, induced resting hypoventilation and inhibited (1) the ventilatory response to CO2 in peripherally chemodenervated animals, and (2) both the PaCO2 and minute ventilation responses to ICM infusion of an acidic mock cerebrospinal fluid (CSF). DEPC had no effect on the hypoxic ventilatory response and had small non-dose-dependent effects on body temperature. ICM administration of hydroxylamine (HDA), a substance that reverses the DEPC-imidazole binding in vitro, prevented DEPC induced inhibition of CCR function. These data support but do not prove the alpha-imidazole hypothesis for mammalian central chemoreceptor function and demonstrate a potentially useful chemical tool for the study of central chemoreception.

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)

Substance P-immunoreactive neurons in the brainstem of the cat related to cardiovascular centers

The distribution of substance P-immunoreactivity (SP-IR) in the brainstem and spinal cord of normal and colchicine-pretreated cats was analysed using the peroxidase-antiperoxidase (PAP) technique. Numerous SP-IR fibers are present in the nucleus solitarius, nucleus dorsalis nervi vagi and nucleus spinalis nervi trigemini, various parts of the formatio reticularis, substantia grisea centralis mesencephali, locus coeruleus and nucleus parabrachialis. SP-IR perikarya occur in the substantiae gelatinosa and intermedia of the spinal cord, the nucleus spinalis nervi trigemini-pars caudalis, the nucleus dorsalis nervi vagi, and the nucleus solitarius, as well as in the adjacent formatio reticularis and the medullary nuclei of the raphe. In addition, SP-IR cell bodies are located in the nuclei raphe magnus and incertus, ventral and dorsal to the nucleus tegmentalis dorsalis (Gudden), nucleus raphe dorsalis, substantia grisea centralis mensencephali, locus coeruleus, nucleus parabrachialis and colliculus superior. The results indicate that SP-IR neurons may be involved in the regulation of cardiovascular functions both at the central and peripheral level. A peripheral afferent portion seems to terminate in the nucleus solitarius and an efferent part is postulated to originate from the nucleus dorsalis nervi vagi and from the area of the nuclei retroambiguus, ambiguus and retrofacialis.

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 respiratory CO2 sensitivity at extreme hypocapnia

In 7 cats anaesthetized with chloralose-urethane the ponto-medullary region was artificially perfused with blood having PaCO2 values (central PaCO2) in the range of 0.3-4.5 kPa. The ventilatory response to changes in central PaCO2 was measured at constant hypercapnic and hypoxic conditions in the systemic circulation. Ventilation decreased upon lowering the central PaCO2 down to values of 0.5 kPa. There was no threshold for the effect of the central PaCO2 on ventilation. The CO2 sensitivity was undiminished at extreme hypocapnia compared to eucapnia. Under extreme central hypocapnic conditions the breathing pattern became irregular. It is concluded that there is still central CO2 sensitivity related to ventilation at extreme hypocapnia. Our findings suggest that central chemosensitive structures have a neural threshold below a PaCO2 of 0.5 kPa.

Distribution of NT-IR perikarya in the brain of the guinea pig with special reference to cardiovascular centers in the medulla oblongata

The occurrence and distribution of neurotensin-immunoreactive (NT-IR) perikarya was studied in the central nervous system of the guinea pig using a newly raised antibody (KN 1). Numerous NT-IR perikarya were found in the nuclei amygdaloidei, nuclei septi interventriculare, hypothalamus, nucleus parafascicularis thalami, substantia grisea centralis mesencephali, ventral medulla oblongata, nucleus solitarius and spinal cord. The distribution of NT-IR perikarya was similar to that previously described in the rat and monkey. In the gyrus cinguli, hippocampus and nucleus olfactorius, though, no NT-IR neurons were detected in this investigation. Additional immunoreactive perikarya, however, were observed in areas of the ventral medulla oblongata, namely in the nucleus paragigantocellularis, nucleus retrofacialis and nucleus raphe obscurus. The relevance of the NT-IR perikarya within the ventral medulla oblongata is discussed with respect to other neuropeptides, which are found in this area, and to cardiovascular regulation.

Central chemosensitivity and the reaction theory

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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.

H+-sensitivity and pattern of discharge of neurons in the chemosensitive areas of the ventral medulla oblongata of rats in vitro

Effect of H+ on neuronal activity in medullary chemosensitive structures was analyzed in brain slices of the rat in vitro. Spontaneous discharges of spikes were recorded from two populations of neurons in the ventral surface layer of the medulla oblongata. Neurons located in the rostro-lateral part of the hypoglossal nerve root (area II) fired irregular phasic and/or continuous tonic discharges. More rostral and medial to it, a population of 'H+-sensitive' tonic neurons was found previously (area I). The phasic activity of neurons in area II with variable bursting and silent periods (0.1--120 s) was either increased or decreased by H+. Some neurons firing tonically in area II started to show phasic burst under low pH conditions. After cutting the slice between area I and II, the number of neurons firing phasically in area II was reduced, and the activity of area II neurons was mostly depressed by H+. An increase in activity of area I neurons by H+ was kept intact even after section. The results suggest that neurons in area I play an important role not only in the excitatory response to H+ of area II neurons but also in the initiation of phasic neuronal discharges in area II.

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.