Neurons in the caudal ventrolateral medulla mediate the arterial baroreceptor reflex by inhibiting barosensitive reticulospinal neurons in the rostral ventrolateral medulla in rabbits.

Participation of the caudal ventrolateral medulla in the arterial baroreceptor reflex was examined in urethane-anesthetized, vagotomized and immobilized rabbits whose aortic nerve was cut bilaterally. The extent of the caudal ventrolateral medulla was mapped by decreases in the renal sympathetic nerve activity and arterial pressure following a local microinjection of a neuroexcitatory amino acid, sodium glutamate (0.075-1.5 nmol). It extended between the levels 1.3 mm rostral and 3.0 mm caudal to the obex. An injection of sodium glutamate into the caudal ventrolateral medulla also diminished spontaneous activity of barosensitive reticulospinal neurons in the rostral ventrolateral medulla. In the 'split medulla preparation' in which the medulla was split along the midsagittal plane to disrupt fiber connections associating both sides, a neurotoxic agent, kainic acid, was injected unilaterally into the rostral ventrolateral medulla. This treatment markedly attenuated responses of renal sympathetic nerve activity and arterial pressure induced by a sodium glutamate injection into the ipsilateral caudal ventrolateral medulla, whereas responses to an injection into the contralateral caudal ventrolateral medulla were totally preserved. In four separate experiments, three to five injections of kainic acid were made unilaterally to cover the whole extent of the caudal ventrolateral medulla. The sympathoinhibitory and depressor responses to stimulation of the ipsilateral aortic nerve were then totally abolished. Simultaneously, the cardiac cycle-related rhythmic fluctuation of renal sympathetic nerve activity, which represented activity of the carotid sinus baroreceptor reflex, was attenuated to the noise level. These results, together with our previous electrophysiological demonstration of barosensitive caudal ventrolateral medulla neurons with axonal projections to the rostral ventrolateral medulla, strongly support the hypothesis that neurons in the caudal ventrolateral medulla mediate the arterial baroreceptor-vasomotor reflex through inhibition of barosensitive reticulospinal neurons in the rostral ventrolateral medulla.     

Dorsal and ventral respiratory groups of neurons in the medulla of the rat

The aim of the present work was to identify and localize in rat the medullary neurons involved in respiration. Neural activity was recorded in ketamine-anesthetized, paralyzed and artificially ventilated rats. Active sites were marked by electrocoagulation. Neurons firing in relation to phrenic nerve activity were located between 0.5 and 2 mm lateral to the midline, extending from 0.5 mm caudal to 2 mm rostral to the posterior end of the area postrema. Two groups of respiratory neurons were found: a dorsal group located ventrolateral to the tractus solitarius and a ventral group located in the ventrolateral reticular formation close to the nucleus ambiguus. Neurons were classified as bulbospinal or laryngeal if stimulation of the spinal cord or the vagus nerve, respectively, elicited antidromic action potentials, or as propriobulbar if they were not activated. Neurons firing synchronously with lung inflation were termed pump (P) cells. The dorsal respiratory group includes inspiratory (I) bulbospinal and propriobulbar neurons, P cells, but few expiratory (E) propriobulbar neurons. The ventral respiratory group includes bulbospinal, laryngeal and propriobulbar I and E neurons. Laryngeal motoneurons project ipsilaterally whereas bulbospinal neurons project contralaterally. Cross-correlations between inspiratory bulbospinal neuronal activity and phrenic discharge suggest that bulbospinal I neurons of dorsal and ventral groups project monosynaptically to contralateral phrenic motoneurons. These results indicate a similarity of the medullary respiratory centers of rats and cats, suggesting that rats may profitably be used in studies of respiratory rhythmogenesis.     

Is the central inspiratory activity responsible for pCO2-dependent drive of the sympathetic discharge?

Out of 27 cats anesthetized with chloralose—urethane mixture, paralyzed, vagotomized and artificially ventilated, phrenic nerve response to systemic hypercapnia (7–8 vol.% CO₂/O₂ mixture) was accompanied by an increase in pressure and sympathetic discharge in 19 cats. Out of these 19 cats, 12 were totally debuffered and in the remaining 7 cats one carotid sinus nerve was left intact. Single unit activity in the sympathetic cervical nerve and spontaneous mass activity in the cervical, splanchnic, renal sympathetic and phrenic nerves were recorded. Evoked response in the phrenic nerve was produced by electrical stimulation of the descending bulbospinal inspiratory pathways in the midplane area of the medulla or in the ventrolateral cervical spinal cord. Starting from the control mean end-tidal CO₂ concentration of 4.7 vol.% (±1.0 S.D.) a progressing hypocapnia was induced by hyperventilation up to the end-tidal CO₂ concentration of 1.3—3.2 vol.% (mean 2.4 vol.% ± 0.5 S.D.) significantly below p_aCO₂ apneic threshold. In chemo- and baroreceptor denervated cats with a pressor and excitatory sympathetic response to hypercapnia, a hypocapnia resulted in a fall of the arterial blood pressure (mean 16.9 mm Hg ± 7.5 S.D., 2.2 kp_a ± S.D.). With the increasing p_aCO₂ over the period of hypocapnic apnea a pressor and excitatory sympathetic response preceded, in all experiments, the onset of the phrenic nerve rhythmic activity. The difference between p_aCO₂ threshold for the pressor and sympathetic response (35.7 mm Hg ± 3.6 S.D., 4.7 kp_a ± 0.5 S.D.) and p_aCO₂ threshold for the reappearance of the phrenic nerve rhythmic activity (43.6 mm Hg ± 2.6 S.D., 5.8 kp_a ± 0.3 S.D.) was highly significant. If apneic hypocapnia was combined with the continuous stimulation of the afferent fibers of the superior laryngeal nerve the CO₂ threshold for phrenic rhythmic activity was significantly increased whereas CO₂ threshold for the pressor and sympathetic excitatory response remained unchanged. CO₂ administration during hypocapnic apnea caused a progressing reduction of the magnitude of the evoked phrenic nerve response.From these findings it is concluded that the central excitatory effect of CO₂ on the sympathetic activity may be accomplished in the absence of the rhythmic respiratory activity and independently of the subthreshold tonic inspiratory activity. Pressor and sympathetic excitatory response to CO₂ observed during hypocapnic apnea is presumably caused by a neuronal pool different from that responsible for the central inspiratory activity. It is suggested that this CO₂ sensitive neuronal mechanism might be involved in the central generation of sympathetic tone.     

Functional reconnections established by central respiratory neurons regenerating axons into a nerve graft bridging the respiratory centers to the cervical spinal cord

The present work investigated, in adult rats, the long-term functional properties and terminal reconnections of central respiratory neurons regenerating axons within a peripheral nerve autograft bridging two separated central structures. A nerve graft was first inserted into the left medulla oblongata, in which the respiratory centers are located. Three months later, a C3 left hemisection was performed, and the distal tip of the graft was implanted into the C4 left spinal cord at the level of the phrenic nucleus, a natural central inspiratory target. Six to eight months after medullary implantation, the animals (n = 12) were electrophysiologically investigated to test 1) the phrenic target reinnervation by analyzing the phrenic responses elicited by bridge electrical stimulation and 2) the bridge innervation by unitary recordings of the spontaneous activity of regenerated axons within the nerve bridge. In the control group (n = 6), the medullary site of implantation corresponded to the dorsolateral medulla, a region known to be an unsuitable site for inducing respiratory axonal regrowth after nerve grafting. Stimulation of the nerve bridge never elicited phrenic nerve response, and no respiratory units were found within the nerve bridge. In the experimental group (n = 6), the proximal tip of the nerve bridge was implanted within the ventrolateral medulla at the level of the respiratory centers. Electrical stimulation of the nerve bridge induced phrenic nerve responses that reflected a postsynaptic activation of the phrenic target. Subsequent unitary recordings from teased fibers within the bridge revealed the presence of regenerated inspiratory fibers exhibiting discharge patterns typical of medullary inspiratory neurons, which normally make synaptic contacts with the inspiratory phrenic target. These results indicate that, when provided with an appropriate denervated target, central respiratory neurons with regenerated axons along a nerve bridge can remain functional for a long period and can make precise and specific functional reconnections with central homotypic target neurons.     

Respiratory rhythmicity in a split medulla preparation of the cat

Splitting the medulla in the cat resulted in the disappearance of inspiratory activity in the phrenic but not in the recurrent laryngeal nerves with minor changes in respiratory cycle duration. In one animal a complete desynchronization of respiratory rhythms was observed in the opposite recurrent laryngeal nerves. We conclude that each half of the cat's brain stem has its own respiratory phase-switching mechanism. Commissural connections are involved in the bilateral synchronization of activity and the mutual excitatory interactions between medullary respiratory neurons.     

Diffuse pathways convey efferent activity from rostral pontile pneumotaxic center to medullary respiratory regions

Studies were undertaken to evaluate the hypothesis that diffuse pathways serve to convey efferent activity from the rostral pontile pneumotaxic center to the respiratory regions of medulla. Activation of respiratory mechanisms within the pneumotaxic center was produced by electrical stimulation. In decerebrate, vagotomized, cerebellectomized, paralyzed and ventilated cats, stimulations of the pneumotaxic center produced a premature termination of activities of both phrenic nerves. Such a termination of neural activities was still obtained following hemisection of the brain stem at the pontomedullary junction either ipsilateral or contralateral to the site of stimulation. Likewise, both phrenic activities were terminated by stimulation of the pneumotaxic center after a midsagittal division of the brain stem from the intercollicular level to the medullary obex. These results demonstrate that efferent pathways from the pneumotaxic center must decussate in both the caudal pons and in the medulla. The finding of diffuse pathways from the pneumotaxic center to the medullary respiratory regions has implications for previous studies which reported a variety of respiratory patterns following a brain stem transection at the pontomedullary junction.     

An analysis of reflex changes in excitability of phrenic,laryngeal, and intercostal motoneurons

An investigation was carried out in anesthetized cats to ascertain whether self-excitation of phrenic motoneurons is a specific or generalized reflex mechanism for motoneurons allied to respiration. Whereas stimulation of only caudal intercostal nerves evoked discharge of phrenic motoneurons (intercostal-to-phrenic reflex), stimulation of all intercostal nerves elicited discharges in the recurrent laryngeal nerve (intercostal-to-recurrent laryngeal reflex). Weak superior laryngeal nerve stimulation provoked short-latency discharges in the recurrent laryngeal nerve but inhibited on-going inspiratory activity in phrenic and external intercostal motoneurons. In the presence of self-excitation of phrenic motoneurons (phrenophrenic system), there was concomitant excitation of laryngeal motoneurons. In contrast, when self-excitation of laryngeal motoneurons occurred (laryngolaryngeal system) there was concomitant inhibition of inspiratory activity (phrenic and external intercostal motoneurons). Paired shocks delivered to superior laryngeal and intercostal nerves while recording from phrenic, recurrent laryngeal, and intercostal nerves failed to reveal convergent interaction. It is concluded that self-excitation is a generalized reflex mechanism for certain motoneurons allied to respiration.     

Inspiratory neurons with decrementing firing pattern in raphe nuclei of feline medulla

Extracellular spikes of single inspiratory (I) neurons with decrementing firing pattern were recorded in the medullary raphe nuclei in decerebrated or Nembutal anesthetized cats. A total of 23 neurons with decrementing firing patterns during the I phase were recorded in the raphe obscurus and pallidus at the levels of 2.0-4.0 mm rostral to the obex. The respiratory neurons fired in the I phase during a brief stop of the ventilator, indicating that their respiratory-related activities were central in origin. The effect of electrical stimulation of the recording site of the respiratory neuron on diaphragm EMG was examined: both the diaphragm EMG activity and the respiratory frequency were increased. None of six neurons tested for projections to the cervical spinal cord was antidromically activated by electrical stimulation. The present results suggest that cat I-decrementing neurons in the medullary raphe nuclei receive inputs from the central respiratory rhythm generator and may modify the respiratory activity of supraspinal neural structures.     

The aortic nerve-sympathetic reflex in the rat

The effects of stimulation of aortic nerve A- and C-fibers on the renal and cardiac sympathetic nerve activities in anesthetized and immobilized Sprague-Dawley rats were investigated. A separate aortic nerve was found in 46 rats (90%) out of 51. Activation of A- and C-fiber groups, alone or in combination, resulted in an inhibition of renal and cardiac nerve activities. However, an excitatory component preceding the inhibitory component, representing the reflex response to stimulation of non-barosensory afferent fibers contained in the carotid sinus or aortic nerve, was never observed. This result provides electrophysiological evidence supporting the view that the rat's aortic nerve does not contain a significant amount of functionally active non-barosensory afferents. As with the aortic nerve reflex in the rabbit and cat, the sympatho-inhibitory action of C-fibers was more powerful and longer-lasting than that of A-fibers. Furthermore, the C-fiber reflex was elicited at stimulus frequencies as low as 2 Hz. No significant difference was found between the reflex response of cardiac and renal nerves. On the other hand, stimulation of the superior laryngeal nerve, which constitutes an important pathway carrying arterial baroreceptor fibers, caused a reflex sympathetic response typically consisting of excitatory and inhibitory components. Thus, the rat's aortic nerve provides a useful experimental means to activate selectively central neural structures associated with barosensory afferents and to elicit the reflex response homologous to that in the arterial baroreceptor reflex in rabbits and cats.     

Respiratory modulation of sympathetic activity

Sympathetic activity recorded from cardiac and renal nerves was correlated with phrenic and internal intercostal nerve activity under normocapnea and hypercapnea. Cats were anesthetized with halothane for surgery switching to chloralose for recording. Both vagal and carotid sinus nerves were cut, animals were paralyzed and artificially ventilated. We found that sympathetic activity followed the rhythmic pattern of phrenic nerve discharge fairly closely except in two important respects: first, sympathetic activity was significantly depressed during early inspiration and second, it reached a minimum during post inspiration while phrenic activity was decaying but still active. These effects were accentuated when PACO2 was raised. In one cat early inspiratory depression was the only manifestation of respiratory modulation of sympathetic activity superimposed on an otherwise tonic pattern. In 4 cats sympathetic activity increased in an augmenting fashion in parallel with the augmenting discharge of expiratory alpha motoneurones. We suggest that respiratory-related, excitatory and inhibitory inputs modulate sympathetic activity at the brainstem level. Inspiratory and possibly expiratory interneurones may be the source of activation, and inhibitory inputs may derive from early inspiratory and postinspiratory interneurones. The inhibitory effects may be the only manifestation of respiratory modulation during strong tonic drive of the sympathetic activity.     

Electro-acupuncture stimulation to a hindpaw and a hind leg produces different reflex responses in sympathoadrenal medullary function in anesthetized rats

The effects of electro-acupuncture stimulation (EAS) of two different areas of a hindlimb with different stimulus intensities on sympathoadrenal medullary functions were examined in anesthetized artificially ventilated rats. Two needles of 160 microm diameter and about 5 mm apart were inserted about 5 mm deep into a hindpaw (Chungyang, S42) or a hind leg (Tsusanli, S36) and current of various intensities passed to excite various afferent nerve fiber groups at a repetition rate of 20 Hz and pulse duration of 0.5 ms for 30-60 s. Fiber groups of afferent nerves stimulated in a hindlimb were monitored by recording evoked action potentials from the afferents innervating the areas stimulated. The sympathoadrenal medullary functions were monitored by recording adrenal sympathetic efferent nerve activity and secretion rates of catecholamines from the adrenal medulla. EAS of a hindpaw at a stimulus strength sufficient to excite the group III and IV somatic afferent fibers produced reflex increases in both adrenal sympathetic efferent nerve activity and the secretion rate of catecholamines. EAS of a hind leg at a stimulus strength sufficient to excite the group III and IV afferent fibers produced reflex responses of either increases or decreases in sympathoadrenal medullary functions. All responses of adrenal sympathetic efferent nerve activity were lost after cutting the afferent nerves ipsilateral to the stimulated areas, indicating that the responses are the reflexes whose afferents nerve pathway is composed of hindlimb somatic nerves. It is concluded that electro-acupuncture stimulation of a hindpaw causes an excitatory reflex, while that of a hind leg causes either excitatory or inhibitory reflex of sympathoadrenal medullary functions, even if both group III and IV somatic afferent fibers are stimulated.     

Spontaneous and reflexly evoked laryngeal abductor and adductor muscle activity of cat

A study of spontaneous and reflexly evoked activity of laryngeal abductor (posterior cricoarytenoid) and adductor (thyroarytenoid and lateral cricoarytenoid) muscles was carried out in cats anesthetized with chloraloseurethane or made decerebrate and supplemented with ketamine HCl. The posterior cricoarytenoid muscle was active largely during inspiration but showed tonic activity throughout the respiratory cycle; the thyroarytenoid and lateral cricoarytenoid muscles were rhythmically active during expiration. Anesthetic amounts of pentobarbital abolished adductor rhythmicity and enhanced cyclic inspiratory activity of the abductor muscle. Hyperventilation increased the tonic adductor muscle activity while diminishing abductor muscle activity prior to resolution of apnea. Glossopharyngeal (epipharyngeal branch) and superior laryngeal nerve stimulation evoked chiefly excitatory effects on adductors and largely and attenuating effect on the abductor during inspiration. Stimulation of caudal intercostal nerves caused similar effects but to a lesser degree. Peripheral phrenic nerve stimulation during inspiration facilitated reflex abductor muscle activity whereas such stimulation during expiration facilitated reflex adductor muscle activity. The collective evidence further supports the conclusion that the larynx has a dual function: that of a respiratory organ (widening of the glottis during inspiration and its narrowing during expiration) and of a guardian of the lower respiratory tract from invading foreign matter (reflex sphincter action of contracting adductor muscles with relaxation of the abductor).     

Effects of intravenous bicuculline and strychnine on inspiratory inhibitory responses in the cat

Single shock stimulation of the superior laryngeal nerve (SLN), intercostal nerve (ICN), phrenic nerve (PN) or within the medullary respiratory groups (DRG-VRG) produces a transient, short-latency attenuation of inspiratory motor activity. Trains of stimuli delivered to SLN and ICN cause premature termination of inspiration. This study examined involvement of glycine and GABAA receptors in these reflex inhibitions. Experiments were conducted in decerebrate, vagotomized, and paralyzed cats. Control responses of left PN activity to threshold single shock stimulation of SLN, PN, ICN and the DRG-VRG were recorded and the thresholds for SLN- and ICN-evoked inspiratory termination were determined. Five min after intravenous injection of bicuculline (1 mg/kg) or strychnine (50 micrograms/kg), the responses to stimulation were again recorded. This procedure was reiterated until the cumulative dose elicited marked convulsions. Neither drug affected the inspiratory terminating reflexes. Systemic bicuculline had no effect on transient inspiratory inhibition. However strychnine prolonged the onset latency and the duration of all four inhibitory responses. Since the degree of transient inhibition was not lessened (only delayed), it appears that these inspiratory inhibitory reflexes do not rely exclusively on actions of glycine or GABAA receptors.     

Behavior of inhibitory and excitatory propriobulbar respiratory neurons during fictive vomiting.

The behavior of propriobulbar respiratory neurons was studied during fictive vomiting in decerebrate, paralyzed, artificially ventilated cats. Fictive vomiting was identified by a characteristic series of synchronous phrenic and abdominal nerve bursts, induced by electrical stimulation of abdominal vagal afferents and/or i.v. infusion of emetic drugs. Data were obtained from inspiratory neurons having decrementing (I-DEC) or constant (I-CON) discharge patterns and expiratory decrementing (E-DEC) neurons located in the B?tzinger complex and adjacent rostral ventral respiratory group. These neurons are known to make excitatory (I-CON) and inhibitory (I-DEC, E-DEC) connections with a variety of medullary respiratory neurons. During fictive vomiting: 8 of 14 I-DEC neurons fired in phase with synchronous bursts of phrenic and abdominal nerve discharge; the other 6 were silent. Of 12 I-CON neurons, 5 fired in phase with phrenic and abdominal bursts; 7 were silent. All (6) E-DEC neurons were either silent or fired weakly between bursts of phrenic and abdominal discharges. The possible roles of I-DEC and I-CON neurons in actively reorganizing the behavior of other respiratory neurons during fictive vomiting are discussed. In particular, the firing of many I-DEC neurons was found to be appropriate to inhibit inspiratory, and two types of expiratory, bulbospinal neurons during fictive vomiting.     

Efferent activity in the phrenic nerve during the startle reflex in chloralose-anesthetized cats

Reflex activity in the phrenic nerve was studied in chloralose anesthetized cats during development of somatic startle reflexes in limb and lower intercostal nerves. It was shown that the main component of this activity during low-threshold reflexes evoked by acoustic, tactile and low-threshold somatic afferent stimulation was depression of phrenic inspiratory activity. The following reflex discharges were prevalent components of phrenic responses to high-threshold afferent stimulation: early, propriospinal (intercostal-to-phrenic reflex) and late, suprasegmental ones. The latter were of two types: inspiratory (observed mainly during inspiration in about 75% of experiments) and expiratory (observed during expiration in 25% of experiments) which could be classified as "phrenic startle reflexes". Modulation of all responses during the respiratory cycle was described. Structural characteristics of reflex responses evoked in the phrenic nerve by stimulation of various respiratory and nonrespiratory bulbar sites as well as their respiratory modulation have been analyzed. Organization of possible neurophysiological mechanisms of phrenic responses during startle reflexes is discussed.     

Absence of respiration modulation of carotid sinus nerve inputs to nucleus tractus solitarius neurons receiving arterial chemoreceptor inputs

The reflex responses to activation of the arterial chemoreceptors are dependent upon when in the respiratory cycle the chemoreceptor stimulus is given. To determine if the respiratory modulation of the chemoreflex occurs within the nucleus tractus solitarius (NTS), intracellular recordings were obtained in pentobarbital-anesthetized, paralyzed and mechanically ventilated cats, from 22 non-respiratory NTS cells which were depolarized following activation of the ipsilateral carotid body chemoreceptors (by close arterial injection of <100 μ1 CO₂ saturated bicarbonate). Activation of the ipsilateral carotid body chemoreceptors evoked depolarizations with amplitudes of 2.9–4.6 mV and durations of 2.1–5.9 s. Three of these cells also received a convergent excitatory input from the carotid sinus baroreceptors. Carotid sinus nerve (CSN) stimulation evoked either an excitatory post-synaptic potential (EPSPs) (n = 14, 8 monosynaptic) or an excitatory/inhibitory sequence (EPSP/IPSPs) (n = 8, 1 monosynaptic). CSN evoked PSPs were separately averaged (25–50 sweeps) during periods of phrenic nerve activity and phrenic nerve silence and during periods when the lungs were inflated and when the lungs were deflated. No parameter of the CSN evoked PSPs (latency, peak amplitude, duration) was altered during periods of phrenic nerve activity or lung inilation (all P values > 0.12, Wilxocon signed-rank test). The results suggest that there is no respiratory modulation of arterial chemoreceptor inputs by either central respiratory drive or lung stretch receptor afferent inputs at this early stage of the reflex arc.     

Renal afferent nerves affect discharge rate of medullary and hypothalamic single units in the cat

Electrical activity of spontaneously firing single units in the medulla and hypothalamus of 22 cats anesthetized with chloralose was monitored for changes in firing frequency during electrical stimulation of afferent renal (ARN) and carotid sinus (CSN) nerves. Stimulation of ARN altered the firing frequency of 214 out of 540 units studied in the ipsi- and contralateral medulla; the majority of the responses were excitatory but a few units (8%) were inhibited by stimulation. Of the units responding to ARN stimulation, 57% were found to respond in the same manner to stimulation of the CSN. Responsive units were found primarily in 3 regions: the lateral tegmental field, the area of the paramedian reticular nucleus and the region of the dorsal vagal complex around the obex. In the hypothalamus stimulation of ARN affected the activity of 197 of the 407 units studied ipsi- and contralaterally; the majority of the units were excited but 8% were found to be inhibited. Of the units responding to ARN 75% also responded to stimulation of the CSN. Responsive units were found in most areas but were concentrated in 3 anterior regions: lateral preoptic area, lateral hypothalamic area and the region of the paraventricular nucleus. This is the first demonstration that stimulation of afferent renal nerves can influence the electrical activity of medullary and hypothalamic neurons bilaterally. Because of the demonstrated physiological role of the structures where these responsive units were found these results suggest that sensory receptors in the kidney convey important information to central sites involved in physiological responses related to cardiovascular adjustments and fluid balance. Furthermore it has been demonstrated that the majority of medullary and hypothalamic neurons responding to stimulation of ARN also receive an input from the CSN suggesting that certain regions of both medulla and hypothalamus can integrate peripheral information from the kidney and from cardiovascular receptors to bring about appropriate homeostatic responses.     

Evidence for a tonic GABA-ergic inhibition of excitatory respiratory-related afferents to presympathetic neurons in the rostral ventrolateral medulla

The effect of blockade of ionotropic GABA and glutamate receptors in the rostral ventrolateral medulla (RVLM) on the relationship between phrenic nerve, splanchnic sympathetic nerve and lumbar sympathetic nerve activities was examined in urethane anesthetized, paralyzed and vagotomized Sprague-Dawley rats. Bilateral microinjection of the GABA-A receptor antagonist, bicuculline (4 mM, 100 nl), into the RVLM dramatically, and almost exclusively, increased the post-inspiratory related discharge in both splanchnic sympathetic nerve and lumbar sympathetic nerve activities and elicited hypertension with fluctuations of arterial pressure phase locked to the discharge of the phrenic nerve. Subsequent bilateral microinjection of kynurenate, a non-selective ionotropic excitatory amino acid receptor antagonist (50 mM, 100 nl), into the RVLM significantly attenuated the sympathoexcitation and hypertension evoked by injection of bicuculline. This was accompanied by an abolition of the post-inspiratory related burst discharge of splanchnic sympathetic nerve and lumbar sympathetic nerve activities. These data suggest that the GABAergic inputs to RVLM tonically inhibit glutamatergic inputs from central respiratory neurons that normally act to increase the firing of presympathetic neurons in the RVLM. Inputs from post-inspiratory neurons appear to be an especially potent excitatory synaptic drive to the presympathetic neurons in the absence of the GABAergic inhibition.     

Medullary respiratory neurons in the guinea pig: localization and firing patterns

The location and firing patterns of medullary respiratory neurons have been described in a small number of species. The cat has been the most widely studied species, but some potentially important differences have recently been noted in others. A more complete survey of species is required to determine the significance of these differences. We describe the location and firing patterns of respiratory neurons in the medulla of anesthetized, paralyzed and mechanically ventilated adult guinea pigs. Extracellular single-unit recordings were made from the medulla, their phase relationship with phrenic nerve activity used to define them as respiratory and their location marked with fast green. Respiratory units were concentrated ventrolateral to the nucleus tractus solitarius (NTS) and within and surrounding the nucleus ambiguus (NA), corresponding to the dorsal respiratory group (DRG) and ventral respiratory group (VRG) of the cat, respectively. Most DRG respiratory units were inspiratory, while the VRG contained equal numbers of inspiratory and expiratory units. The DRG and VRG both contained early, late and constant-frequency inspiratory and expiratory units. In general, these findings are similar to those in other mammalian species examined, consistent with these basic aspects of the respiratory network being highly conserved.     

Studies on the respiratory control mechanism of medullary raphe nuclei and their serotonergic system

Respiratory control mechanism of the medullary raphe nuclei were studied with some references to their serotonergic mechanisms. Anesthetized, paralyzed and artificially ventilated cats were used and their phrenic nerve efferent activity was always observed as an indicator of central respiratory activity. Following results were obtained. 1) Electrical stimulation of medullary raphe nuclei, namely, nucleus raphe magnus, obscurus and pallidus, produced dominantly inhibitory responses in the phrenic nerve activity, while raphe stimulation in the pons and more rostral portion did not produce any respiratory responses. The blood pressure was depressed by raphe stimulation, too, almost in parallel to the respiratory inhibition. These inhibitory responses in respiration and blood pressure were partially antagonized by cyproheptadine (0.3-0.5 mg/kg i.v.) and methysergide (0.3-0.5 mg/kg i.v.). 2) Raphe stimulation inhibited remarkably activities of the medullary inspiratory and expiratory neurons, similarly. 3) In the experiment, where single shot stimulus was added to the raphe nuclei at the various time point in the respiratory phase, raphe stimulation showed the retardative effect of inspiratory switching, in addition to the inhibitory effect of phrenic burst activity. 4) The mechanism of respiratory inhibition produced by raphe stimulation was analyzed by evoked potentials in the averaged phrenic nerve activity. The post-stimulus averaged potentials of the phrenic nerve consist of the depolarizing potentials of about 10 msec duration and the subsequent hyperpolarizing potentials of several 10 msec duration, the duration time depending on the stimulus intensity. When stimulation was given in high frequency, the post-stimulus averaged potential became flattened, and the phrenic burst activity was inhibited almost completely. But in the case of stimulation in ventral parts of the raphe nuclei, the initial depolarizing potential was comparatively more prominent, and when high frequency stimulation was given, continuous firing was observed in the phrenic nerve activity. At the time of the continuous firing, respiratory rhythmicity was disappeared completely. 5) Propranolol (0.3-1.0 mg/kg i.v.), which have been recognized to have 5-HT1 antagonistic activity, reduced the hyperpolarizing potentials of the post-stimulus averaged potentials, and methysergide (0.3-1.0 mg/kg i.v.), 5-HT1 and 5-HT2 antagonist, reduced both depolarizing and hyperpolarizing potentials. These phenomena would suggest strongly that hyperpolarizing and depolarizing potentials are related to the 5-HT1 and 5-HT2 receptors, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)