Depolarization and stimulation of neurons in nucleus tractus solitarii by carbon dioxide does not require chemical synaptic input

Separation of multiple functions in ventilatory control of pneumotaxic mechanisms

Multiple functions have been ascribed to the pontile pneumotaxic center. We hypothesized that these functions might be separable among neurons in different regions. In decerebrate, vagotomized, paralyzed and ventilated cats, activities of the phrenic and triangularis sterni nerves were recorded. Microinjections of kainic acid were used to destroy neurons. Neurons in the rostrolateral tegmentum at the ponto-mesencephalic border controlled the duration of neural inspiration. Expiratory duration was controlled by neurons in the more caudal nucleus parabrachialis medialis and Kolliker-Fuse nucleus. Ventilatory responses to hypercapnia were depressed following injections of kainic acid into regions controlling either inspiratory or expiratory durations. The phases of expiration were regulated by two groups of neurons, located medial and lateral in the rostral pons. We conclude that rostral pontile and mesencephalic mechanisms control multiple aspects of the eupneic ventilatory cycle. There mechanisms are served by neurons in separable anatomical regions.

c-fos-like immunoreactivity in the cat's neuraxis following moderate hypoxia or hypercapnia

The overall pattern of c-fos immunoreactivity was studied in the brainstem and spinal cord of cats subjected to moderate hypoxia or hypercapnia. In control cats (normoxic, normocapnic), c-fos was expressed mainly in pontine and periaqueductal grey but not in brainstem structures engaged in respiratory control nor in the spinal cord. Both hypoxia and hypercapnia induced c-fos expression in the parabrachial area (pneumotaxic center). In the retrotrapezoid nucleus, a structure involved in respiratory rhythmogenesis and chemoreception, immunoreactivity was detected in hypoxic but not in hypercapnic cats. Neurons in the nucleus raphe pallidus preferentially expressed c-fos in response to hypercapnia. Labelled neurons were concentrated in the dorsal and gelatinosus subnuclei of the solitary tract following hypoxia and hypercapnia, respectively. Our data suggest that some neurons that express c-fos in hypoxic or hypercapnic cats may be involved in coordination of cardiovascular and respiratory function.

In vivo and in vitro responses of neurons in the ventrolateral medulla to hypoxia

Neurons in the ventrolateral medulla (VLM) are known to be involved in several cardiorespiratory reflexes and to provide tonic drive to sympathetic preganglionic neurons. Recent studies have suggested that VLM neurons modulate the respiratory responses to hypoxia and to hypercapnia. The purpose of the present study was to determine with electrophysiological techniques if the discharge of these neurons is altered by hypoxia and/or by hypercapnia both in vivo and in vitro. Extracellular single-unit activity of VLM neurons (n = 39) was recorded during inhalation of a hypoxic gas (10% O2) and during inhalation of a hypercapnic gas (5% CO2) in anesthetized, spontaneously breathing rats (n = 16). Hypoxia elicited an increase in the discharge frequency in 64% of the VLM neurons studied; hypercapnia stimulated 42% of the neurons. Fifty-two percent of the neurons were stimulated by both hypoxia and hypercapnia. Signal averaging revealed that 76% of the hypoxia-stimulated neurons had a resting discharge related to the cardiac and/or respiratory cycle. Similar percentages of VLM neurons (35/54) were stimulated by hypoxia in a second group of animals (n = 14) that were studied after sinoaortic denervation. A rat brain slice preparation was then used to determine if hypoxia exerts a direct effect upon neurons in the VLM. Perfusing a hypoxic gas over the surface of medullary slices evoked an increase in the discharge frequency in the majority (39/49) of VLM neurons studied; responses were graded in relation to the magnitude of the hypoxic stimulus. Similar responses to hypoxia were observed in VLM neurons studied during perfusion with a synaptic blockade medium. Retrograde labeling of VLM neurons with rhodamine tagged microspheres injected into the thoracic intermediolateral cell column demonstrated that the hypoxia sensitive neurons were located in a region of the VLM that projects to the thoracic spinal cord. These results demonstrate that neurons in the ventrolateral medulla are excited by a direct effect of hypoxia; these neurons may play a critical role in the cardiorespiratory responses to hypoxia.

CO2 chemoreception in the pulmonate snail, Helix aspersa

We have studied the response of the pneumostome to CO2, O2 and combined CO2 and O2 in intact snails. We found that pneumostomal opening increases in response to both hypercapnia and mild hypoxia. We determined which neural structures were essential for the pneumostomal response to CO2 by eliminating parts of the nervous system: the subesophageal ganglia and an intact anal nerve were necessary and sufficient elements for the CO2 response. Within the subesophageal ganglia, we identified a discrete region on the medial margin of the visceral ganglion that was capable of increasing pneumostomal area when focally stimulated with 6% CO2. Ion substitution experiments indicated that pneumostomal responses to hypercapnia were not mediated by the pneumostomal motor neurons themselves, but rather by interneurons connected polysynaptically to the motor neurons controlling pneumostomal function. In conclusion, intact H. aspersa have a ventilatory response to CO2, and this response is mediated by CO2 sensitive cells located in a small area of the central nervous system.

In vitro responses of caudal hypothalamic neurons to hypoxia and hypercapnia

Results from previous studies have suggested that the hypothalamus modulates cardiorespiratory responses to hypoxia and/or hypercapnia. Many neurons in the caudal hypothalamus are stimulated by hypercapnia and hypoxia in vivo; however, it is not known if these responses are dependent upon input from other areas. Whole-cell patch and extracellular recordings from a brain slice preparation were used in the present study to determine the direct effects of hypoxia (5% CO2/95% N2 or 10% O2/5% CO2/85% N2) and hypercapnia (7% CO2/93% O2) on caudal hypothalamic neurons in vitro. Coronal sections (400-500 microns) were obtained from young Sprague-Dawley rats and placed in a recording chamber that was perfused with nutrient media equilibrated with 95% O2/5% CO2. Extracellular recordings demonstrated that hypoxia stimulated over 80% of the neurons tested; the magnitude of the response was dependent upon the degree of hypoxia. In addition, over 80% of cells that were excited by hypoxia retained this response during synaptic blockade. Hypercapnia increased the discharge frequency of 22% of the caudal hypothalamic neurons that were studied. A second set of caudal hypothalamic neurons were studied with whole-cell patch recordings; the mean resting membrane potential of these neurons was -51.8 +/- 1.0 mV with an average input resistance of 399 +/- 49 M omega. Hypoxia produced a depolarization in 76% of these neurons; a poststimulus hyperpolarization often occurred. A depolarization and/or increase in discharge rate during hypercapnia was observed in 35% of the neurons tested. Only 10% of all neurons studied were excited by both hypoxia and hypercapnia. These findings suggest that separate subpopulations of caudal hypothalamic neurons are sensitive to hypoxia and hypercapnia. Thus, this hypothalamic area may be a site of central hypoxic and hypercapnic chemoreception.

In search of the central respiratory neurons: II. Electrophysiologic studies of medullary fetal cells inherently sensitive to CO2 and low pH

Although extensively pursued, the central respiratory neurons have remained elusive. We departed from the more conventional physiologic and morphologic methods of system and tissue examination and cultured dissociated fetal rat cells (Fitzgerald et al., J Neurosci Res 33:579-589, 1992) from the area of the nucleus ambiguus and the nucleus tractus solitarius located within the 2 mm rostral to the obex. Pacemaker-like cells, with a regular single or bursting activity, studied at 3-5 weeks of age, responded to very small pulses of CO2 (50 ms) and low pH with an increase in spike frequency and a decrease in spike amplitude. Other irregularly beating or silent cells did not respond or else required very large pulses (> 200 ms) to do so. The pacemaker cells also responded to hypoxia induced by administration of sodium hydrosulfite with an increase in spike frequency and amplitude; high oxygen (> 600 torr) and adenosine produced a decrease in electrical activity. Most of these cells were multipolar after staining with antibodies to neuron-specific enolase (NSE) and Fragment C of tetanus toxin. They did not stain for choline acetyltransferase (ChAT). The results suggest that these cultured cells, expressing a phenotype inherently responsive to CO2 and low pH, have the characteristics of central respiratory chemoreceptors, and may be involved in the generation of the respiratory rhythm.

In search of the central respiratory neurons: I. Dissociated cell cultures of respiratory areas from the upper medulla

Dissociated cells from the areas of the nucleus ambiguus and the nucleus tractus solitarius obtained by tissue punch or block dissection from coronal slices of the medulla at the level of the obex were cultured from fetal rats at 18 to 21 days gestation. The dissociated neurons were plated either directly in vitrogen-coated 35 mm tissue culture dishes or in such dishes which had been seeded with subcultures of cortex- or medulla-derived astrocytes. After the astrocytes reached confluency and were treated with an antimitotic agent, dissociated nucleus ambiguus or nucleus tractus solitarius was plated at 0.5-1.0 x 10(6) cells per dish. Neurons grew well on monolayers of medullary or cortical astrocytes, but survived poorly on vitrogen-coated dishes without a cellular substrate. Rat medulla was preferred as the source of astrocytes. Tissue dissociation with papain rather than trypsin produced less cellular debris, and the neuronal yield from the tissue was higher. The neuronal population was heterogenous in morphology including small and large bipolar, pyramidal, and multipolar cells. Neurons sensitive to CO2 and/or low pH (Rigatto et al., J Neurosci Res 33:590-597, 1992) did not appear to have any definitive morphologic characteristics, but most were multipolar. These neurons stained well with antibodies to neuron-specific enolase and Fragment C of tetanus toxin, but not to choline acetyltransferase (ChAT). These findings suggest that neurons possibly responsible for the central regulation of respiration can be maintained for several weeks in dissociated cell culture, providing a system for neurotransmitter, electrophysiological, and morphological studies.

Acetazolamide on the ventral medulla of the cat increases phrenic output and delays the ventilatory response to CO2

1. Acetazolamide (0.1 mM) applied to the surface of the rostral ventrolateral medulla or microinjected beneath the medullary surface in chloralose-urethane-anaesthetized, vagotomized, carotid-denervated, paralysed, servo-ventilated cats produced a long-lasting increase in integrated phrenic nerve activity. 2. Extracellular pH measured beneath the rostral ventrolateral medulla exhibited a long-lasting decrease after surface acetazolamide but was not a good predictor, in each individual animal, of changes in phrenic activity. 3. Medullary carbonic anhydrase inhibition reduced the slope and the half-time of the phrenic response to rapid step CO2 increases. Conversely, acetazolamide did not affect the phrenic response to steady-state CO2 increases. 4. These data indicate that localized inhibition of medullary carbonic anhydrase causes a centrally mediated increase in ventilation that we attribute to medullary tissue hypercapnia and acidosis. In addition, these data indicate that medullary carbonic anhydrase may play a role in central CO2 chemotransduction.

Chemosensitivity of medullary neurons in explant tissue cultures

To determine whether cultured medulla contains chemosensitive neurons which are excited by CO2 and fixed acid and whether this function is specific to the ventral medulla, tissue explants of ventral and dorsal medulla were prepared from neonatal rats and incubated for two to three weeks. Cultures were superfused with artificial cerebrospinal fluid, maintained at 37 degrees C, and pH of the superfusate was varied either with PCO2 (14-71 Torr) at constant HCO3- (22 mM) or HCO3- (10-30 mM) at constant PCO2 (35 Torr). Spontaneous action potentials were recorded extracellularly in 51 ventral and 23 dorsal medullary neurons. Ventral medullary neurons exhibited a steady baseline firing frequency of 4 +/- 0.8 Hz. In contrast, dorsal medullary neurons exhibited two different patterns of spontaneous activity: 11 fired continuously (7.2 +/- 1.4 Hz) while 12 fired with a bursting pattern. Burst duration was 0.80 +/- 0.14 min and cycle time was 1.74 +/- 0.43 min. Decreasing pH with CO2 caused an increase in the activity of 10 of 27 ventral medullary neurons and two of six dorsal medullary neurons with a mean response of 7.5 Hz/pH unit. Varying pH by changing HCO3- had no effect on firing frequency. These results demonstrate that: (i) chemosensitive neurons are present in both ventral and dorsal medullary explant cultures; (ii) these cells only respond to changes in pH induced with CO2; and (iii) about half of the dorsal medullary neurons fire spontaneously with a regular bursting pattern of activity.