Fluoresceinated peanut agglutinin (PNA) is a marker for live O(2) sensing glomus cells in rat carotid body.

Experiments using live dissociated carotid body (CB) cells for patch clamping, [Ca(++)](i) or other measurements require positive identification of the cell being recorded. At present, cell morphology is usually employed, but several cell types within the carotid body evidence similar morphologic characteristics. Therefore, we sought to develop a method utilizing a vital dye to identify glomus cells before and during experiments that require live cells, such as patch clamp studies. It was previously reported that the binding sites for peanut agglutinin (PNA) were highly expressed by all neuroendocrine-derivatives of the sympathoadrenal neural crest, including glomus cells, small, intensely fluorescent cells, PC-12 cells, and adrenal chromaffin cells in situ (katz et al. 1995). By utilizing the binding characteristics of galactose-specific lectin peanut agglutinin (PNA) on the outer cell membrane, we tested the possibility that the fluoresceinated PNA may preferentially bind to CB glomus cells. The results to date show: (1) Rhodamine tagged PNA (Rhod-PNA) binds to the live dissociated glomus cells in less than one hour incubation and can be visualized in superfused cells; (2) Rhod-PNA labeled cells are perfectly matched with tyrosine hydroxylase (TH) positive glomus cells; (3) Rhod-PNA did not interfere with Fura-2 for Ca(++) imaging; (4) Rhod-PNA bound to glomus cells in [Ca(++)](i) studies does not affect O(2) response of glomus cells. Thus fluoresceinated PNA may be a useful marker for live CB glomus studies, without adversely affecting their physiologic response.

Time-dependence of hyperoxia-induced impairment in peripheral chemoreceptor activity and glomus cell calcium response.

In mammals, transient exposure to hyperoxia for a period of weeks during perinatal life leads to impairment of the ventilatory response to acute hypoxia, which may persist long beyond the duration of the hyperoxia exposure. The impairment of the ventilatory response to hypoxia is due to hyperoxia-induced reduction of carotid chemoreceptor sensitivity to hypoxia. We previously demonstrated that hyperoxia exposure in rats, from birth to two weeks of age, profoundly reduced carotid chemoreceptor single axonal responses to acute hypoxia challenge. However, the time course and mechanisms of this impairment are not known. Therefore, we investigated the effect of hyperoxia (FiO(2) = 0.6) on neonatal rats after 1, 3, 5, 8, and 14 days of exposure, starting at postnatal day 7. Carotid chemoreceptor single unit activities, nerve conduction time and glomus cell calcium responses to acute hypoxia were recorded in vitro. After 1 day in hyperoxia, single unit spiking rate in response to acute hypoxia was increased compared to controls. After 5 days in hyperoxia, the spiking response to acute hypoxia was significantly reduced compared to controls, nerve conduction time was lengthened and the glomus cell calcium response to acute hypoxia was reduced compared to controls. We conclude that perinatal exposure to hyperoxia, in rats, impairs the glomus cell calcium response (pre-synaptic) and the afferent nerve excitability (post-synaptic). The time course indicates that hyperoxia exerts these effects within days.

Hypoxia transduction by carotid body chemoreceptors in mice lacking dopamine D(2) receptors.

Hypoxia-induced dopamine (DA) release from carotid body (CB) glomus cells and activation of postsynaptic D(2) receptors have been proposed to play an important role in the neurotransmission process between the glomus cells and afferent nerve endings. To better resolve the role of D(2) receptors, we examined afferent nerve activity, catecholamine content and release, and ventilation of genetically engineered mice lacking D(2) receptors (D(2)(-/-) mice). Single-unit afferent nerve activities of D(2)(-/-) mice in vitro were significantly reduced by 45% and 25% compared with wild-type (WT) mice during superfusion with saline equilibrated with mild hypoxia (Po(2) approximately 50 Torr) or severe hypoxia (Po(2) approximately 20 Torr), respectively. Catecholamine release in D(2)(-/-) mice was enhanced by 125% in mild hypoxia and 75% in severe hypoxia compared with WT mice, and the rate of rise was increased in D(2)(-/-) mice. We conclude that CB transduction of hypoxia is still present in D(2)(-/-) mice, but the response magnitude is reduced. However, the ventilatory response to acute hypoxia is maintained, perhaps because of an enhanced processing of chemoreceptor input by brain stem respiratory nuclei.

Electrochemical detection of catecholamine release from rat carotid body in vitro.

Neurotransmitter secretion from carotid body glomus cells is hypothesized to be an essential element of chemotransduction. To address one aspect of this hypothesis, catecholamine release in response to hypoxic hypoxia and histotoxic hypoxia was examined using electrically treated carbon-fiber microelectrodes placed in rat carotid bodies in vitro. Carotid bodies of mature rats were removed, along with a portion of the sinus nerve, and suspended in oxygenated (95% O2-5% CO2) Ringer saline at 35 degrees C. The microelectrode differential current after a 50-mV step was recorded over the potential range of -300 to +500 mV. In some preparations, a suction electrode applied to the sinus nerve recorded single-fiber chemoreceptor afferent activity. Stimulation by severe hypoxia (Po2 approximately 0-10 Torr for 3 min, n = 10) and cyanide (2 mM for 2 min) caused an increase in sinus nerve activity and an increase in the carbon-fiber electrode current at a potential corresponding to the oxidation potential of dopamine. As measured in the amperometric mode (constant voltage), tissue catecholamine was 0.35 +/- 0.05 microM (n = 6) and increased to 1.64 +/- 0.43 microM by 1 min of severe hypoxia or to 1.06 +/- 0.17 microM at 2 min of moderate hypoxia (Po2 approximately 50 Torr). Exposure to calcium-free Ringer saline before hypoxia ablated the increase in electrode current, and the response was restored after reperfusion with calcium-containing saline. Repeated exposures to hypoxia (3-min duration) every 15 min resulted in significantly smaller nerve and catecholamine responses. By the third hypoxia exposure, nerve and catecholamine responses were diminished by 30-50%.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemoreceptor nerve excitation may not be proportional to catecholamine secretion.

Enhanced catecholamine secretion from the carotid body glomus cells is hypothesized to play an essential role in mediating the peripheral chemoreceptor response to hypoxia. To test aspects of this hypothesis, the relationship between catecholamine secretion and nerve activity was examined during repetitive hypoxia stimuli and after catecholamine depletion with reserpine. Single-fiber afferent serve activity was measured along with an estimate of free tissue catecholamine by using Nafion-coated carbon-fiber microelectrodes placed in rat carotid bodies in vitro. Baseline and stimulated nerve and catecholamine levels were quantified during repetitive stimulation (anoxia of 1-min duration; PO2 = 0 Torr at nadir, repeated each 200 s). Peak stimulated catecholamine progressively decreased from 26.4 +/- 2.6 microM for the first stimulus to 7.5 +/- 0.9 microM for the fifth stimulus (n = 15), but peak nerve activity was much less affected (23.0 +/- 1.9 Hz, first trial; 19.9 +/- 1.4 Hz, fifth trial). An exposure to moderate hypoxia (approximately 80 Torr) before the repetitive anoxia stimuli produced catecholamine levels comparable to those obtained during repetitive anoxia, but peak nerve activity was significantly less (22.5 +/- 3.4 vs. 12.7 +/- 2.1 Hz). Pretreatment with reserpine (1 mg/100 g) resulted in a large reduction in the average hypoxia-induced catecholamine response (1.4 +/- 0.3 microM, n = 9), but peak nerve activity was not different from nontreated controls. These results demonstrate an independence between carotid body catecholamine secretion and nerve activity, suggesting that nerve excitation is, at least, partially mediated through pathways independent of granule secretion.

Developmental aspects of oxygen sensing by the carotid body.

The carotid body chemoreceptors, the major hypoxia sensory organs for the respiratory system, undergo a significant increase in their hypoxia responsiveness in the postnatal period. This is manifest by a higher level of afferent nerve activity for a given level of arterial oxygen tension. The mechanism for the enhanced sensitivity is unresolved, but most work has focused on the glomus cell, a secretory cell apposed to the afferent nerve ending and believed to be the site of hypoxia transduction. The glomus cell secretory response to hypoxia increases postnatally, and this is correlated with an enhanced calcium rise in response to hypoxia and an increase in oxygen-sensitive potassium currents. These changes are sensitive to the level of hypoxia in the postnatal period, and significant impairment of organ function is observed with postnatal hypoxia as well as postnatal hyperoxia. Although many questions remain, especially with regard to the coupling of glomus cells to nerve endings, the use of cellular and molecular techniques should offer resolution in the near future.

Respiratory changes induced by prolonged laryngeal stimulation in awake piglets.

To examine the role of the laryngeal reflex in modulating cardiorespiratory function, we stimulated the superior laryngeal nerves (SLN) bilaterally in unanesthetized, chronically instrumented piglets (n = 10, age 5-14 days). The SLN were placed in cuff electrodes and wires were exteriorized in the neck for stimulation. A cannula placed in the aorta was used for blood pressure recording and arterial blood sampling. During each experiment, 1-2 days after surgery, ventilation was recorded using whole-body plethysmography, and electroencephalogram and electrocardiogram were recorded after acute subcutaneous electrode placement. After base-line recordings, the SLN were electrically stimulated for 1 h. During this period, mean respiratory frequency decreased by 40-75% and apneas of 10-15 s were regularly interspersed between single breaths or clusters of breaths. Periods of breathing were always associated with opening of the eyes and generally with head and body movements, an awakening that occurred every 10-15 s. At 1 h into the stimulus period, minute ventilation had decreased by 57 +/- 7% (mean +/- SE), arterial partial pressure of O2 (PaO2) by 68 +/- 3 Torr, and arterial partial pressure of CO2 (PaCO2) had increased by 19 +/- 2 Torr. Throughout the entire stimulus period, mean blood pressure and average heart rate were maintained within 12% of base line. We suggest that: low-threshold SLN afferents exert primarily respiratory effects and only minor cardiovascular effects; breathing during laryngeal reflex activation is sustained by an arousal system; and the laryngeal reflex does not pose an imminent threat to the unanesthetized, awake, young animal.

Diaphragmatic failure during loaded breathing: role of neuromuscular transmission.

To determine whether central or peripheral mechanisms are responsible for diaphragmatic failure during loaded breathing, phrenic nerve activity (iENG), diaphragm muscle electromyogram (iEMG), and transdiaphragmatic pressure (Pdi) were measured in unanesthetized chronically instrumented sheep during inspiratory flow-resistive (IFR) loaded breathing. After placement of the IFR load, Pdi increased initially and remained relatively stable for 10-30 min [Pdi = 69.9 +/- 6.3 (SE) cmH2O, n = 6]; arterial PCO2 also increased from baseline (35.8 +/- 0.9 Torr) to 55.1 +/- 4.7 Torr. During IFR loading, iEMG and iENG also increased from baseline, but during the plateau phase of Pdi, iENG continued to increase at the same time while iEMG was stable, and the M wave, evoked by phrenic nerve stimulation, decreased during this period. After the plateau phase, Pdi decreased and arterial PCO2 increased, at which point the study was terminated (at 82.1 +/- 20.6 min). The observation that iENG increased while Pdi and iEMG were stable demonstrates a reduced efficiency of neuromuscular transmission and suggests that the neuromuscular junction is an important site of diaphragmatic failure in unanesthetized sheep during IFR loaded breathing.

Single-unit recordings of arterial chemoreceptors from mouse petrosal ganglia in vitro.

A preparation was developed that allows for the recording of single-unit chemoreceptor activity from mouse carotid body in vitro. An anesthetized mouse was decapitated, and each carotid body was harvested, along with the sinus nerve, glossopharyngeal nerve, and petrosal ganglia. After exposure to collagenase/trypsin, the cleaned complex was transferred to a recording chamber where it was superfused with oxygenated saline. The ganglia was searched for evoked or spontaneous unit activity by using a glass suction electrode. Single-unit action potentials were 57 +/- 10 (SE) (n = 16) standard deviations above the recording noise, and spontaneous spikes were generated as a random process. Decreasing superfusate PO(2) to near 20 Torr caused an increase in spiking activity from 1. 3 +/- 0.4 to 14.1 +/- 1.9 Hz (n = 16). The use of mice for chemoreceptor studies may be advantageous because targeted gene deletions are well developed in the mouse model and may be useful in addressing unresolved questions regarding the mechanism of chemotransduction.

Effect of Na+ and K+ channel blockade on baseline and anoxia-induced catecholamine release from rat carotid body.

Ionic membrane currents are hypothesized to play a major role in determining secretion from carotid body glomus cells, and increased secretion likely mediates the increase in nerve activity in response to hypoxia. The hypothesis that Na+ and K+ channels play an important role in determining secretion and nerve activity was tested by measuring single-fiber afferent nerve activity along with an estimate of free tissue catecholamine using Nafion-covered carbon-fiber micro-electrodes placed in rat carotid bodies in vitro. Baseline and anoxia-stimulated (1 min duration; PO2 of approximately 0 Torr at nadir) levels were quantified. Sham treatment had no significant effect. Tetrodotoxin (2 microns) ablated the nerve activity and reduced peak catecholamine (19.5 +/- 3.1 to 14.5 +/- 3.4 microM; P < 0.05). Cesium (10 microns) had no effect on catecholamine but reduced the nerve response (19.8 +/- 2.7 to 7.8 +/- 2.0 Hz; P < 0.05). 4-Aminopyridine (4 mM) significantly reduced the nerve response (17.2 +/- 3.7 to 4.9 +/- 1.9 Hz; P < 0.05) and increased the baseline (0.9 +/- 0.2 to 3.1 +/- 0.8 microM; P < 0.05) and reduced the peak catecholamine (10.0 to 4.3 +/- 0.8 microM; P < 0.05) levels. These results demonstrate that Na+ and K+ channels play an important role in modulating the secretory and nerve responses. However, channel blockers do not emulate severe hypoxia, suggesting that hypoxia transduction procedes, at least in part, through an alternate pathway.

Prolonged apnea and impaired survival in piglets after sinus and aortic nerve section.

We examined the effects of carotid body denervation (CX, n = 9), CX + aortic nerve section (CAX, n = 9), and sham surgery (SHAM, n = 7) on cardiorespiratory and metabolic function in young piglets (less than 9 days). For comparison, 1-mo-old pigs were also studied. Studies were performed 1 day after surgery, during which time ventilation (barometric plethysmography), heart rate, blood pressure, arterial blood gases, and electroencephalogram were recorded under normoxia. CX and CAX piglets hypoventilated (arterial PCO2 = 47.1 +/- 2.6 and 45.4 +/- 3.1 Torr, respectively) compared with SHAM piglets (arterial PCO2 = 36.4 +/- 1.5 Torr). CX piglets had an average of 8.0 +/- 3.0 apneas/h, lasting, on average, 26 +/- 3 s. CAX piglets averaged 17.2 +/- 7.9 apneas/h, lasting 30 +/- 5 s. Such long apneas were never observed in SHAM animals. Mean heart rate and blood pressure in denervated piglets were not significantly different from those in SHAM piglets. In animals followed up poststudy, significantly high mortality was observed in CX (5 of 9) and CAX (6 of 9) piglets by 7 days after surgery but not in SHAM animals (0 of 7) despite identical environmental and feed conditions (P less than 0.05; chi 2). One-month-old denervated animals showed periodic breathing and hypoventilation, but none died. These results suggest that in the newborn piglet 1) peripheral chemoreceptors have an active role in maintaining normal ventilation and avoidance of prolonged apnea and 2) survivability in early life is critically dependent on peripheral chemoreceptors.

Autoresuscitation: a survival mechanism in piglets.

Piglets were studied to determine 1) the cardiovascular and neurophysiological effects of prolonged laryngeal-induced respiratory inhibition (n = 7) and 2) whether these effects were modulated by autonomic blockade (n = 6). Respiration, electrocardiogram, electroencephalogram (EEG), and blood pressure were recorded, and blood gases were measured. During continuous laryngeal stimulation in the presence of light anesthesia, apnea was interrupted every 1-2.5 min by clusters of two to six breaths. Compared with control, these breaths had a significantly greater tidal volume (430 +/- 30% of control), shorter inspiratory time (87 +/- 5%), and longer expiratory time (124 +/- 15%) and, thus, were of a gasping nature. With each cluster of gasps, arterial PO2 increased from 15 +/- 2 to 56 +/- 5 Torr, heart rate from 84 +/- 7 to 161 +/- 5 beats/min, and mean blood pressure from 48 +/- 4 to 106 +/- 6 mmHg. The EEG became flat by 1 min after the onset of apnea and remained isoelectric throughout the stimulus period. Cyclical gasps were not affected by sympathetic or parasympathetic blockade. These data show that, despite EEG silence, piglets can autoresuscitate by initiating gasps that are not dependent on autonomic integrity. These gasps markedly improve cardiovascular status and may sustain animals for a prolonged period of time.

Biophysical properties of hypoglossal neurons in vitro: intracellular studies in adult and neonatal rats.

A brain stem slice preparation from adult and neonatal (less than or equal to 12 days old) rats and intracellular recordings were used to examine the cellular properties of neurons within the hypoglossal (HYP) nucleus. Resting membrane potential (Vm) for adult hypoglossal neurons was -80 +/- 2 (SE) mV. Rheobase was 2.1 +/- 0.4 nA, and input resistance (RN) was 20.8 +/- 1.5 M omega and decreased during the hyperpolarizing period ("sag"). Compared with adult HYP cells, newborn HYP neurons had significantly lower resting potentials (Vm = -73 +/- 2 mV), lower rheobase (0.7 +/- 0.2 nA), and higher RN (27.6 +/- 3.9 M omega). Single action potentials, elicited by short depolarizing-current pulses, were followed by a slow afterhyperpolarization in adult [6.4 +/- 0.3 mV, time constant (tc) 31.0 +/- 1.2 ms] and newborn cells (7.4 +/- 0.2 mV, tc 37.2 +/- 8.2 ms). Prolonged outward current (2 s) produced little spike frequency adaptation in either adult or newborn neurons. Onset of spike activity was not delayed by hyperpolarizing pulses preceding depolarizations. In addition, pharmacological experiments showed that HYP neurons have a tetrodotoxin-sensitive Na+ current and a delayed and an inward rectifier current but no major Ca2+ current. We conclude the following. 1) Electrophysiological membrane properties mature postnatally in HYP neurons; some of these developmental changes can be ascribed to an increase in soma size and dendritic outgrowth but others cannot. 2) Adult HYP neurons, compared with other brain stem neurons (i.e., vagal cells or cells in the nucleus tractus solitarius), are not endowed with major Ca2+ currents or K+ currents such as the A current and the Ca2(+)-activated K+ current.

Haloperidol-induced suppression of carotid chemoreception in vitro.

Effects of antagonism of endogenous dopamine with haloperidol on single-unit frequency, interspike interval distribution, and interval serial dependency of the cat sinus nerve were tested using an in vitro carotid body-sinus nerve superfusion technique. A dose dependency of inhibition by haloperidol (0.05-2.0 microgram/ml) was observed. Superfusion with 1-2 microgram/ml haloperidol significantly reduced frequency within 5 min (P less than 0.05) and caused a complete cessation of firing within 25 min in 5 of 10 chemoreceptor units. Frequency recovered to control during drug washout. Acetylcholine (10-micrograms/ml superfusion or 500-micrograms bolus) increased sinus nerve activity under control conditions but not during superfusion with haloperidol. No effect of haloperidol on impulse serial dependency was detected. However, interval distribution was significantly altered by haloperidol in five of six chemoreceptor units. Our results suggest an excitatory role for dopamine in carotid chemoreception.

Interspike interval dependency from arterial chemoreceptors.

The carotid body impulse generator has been previously characterized as a Poisson-type random process. We examined the validity of this characterization by analyzing sinus nerve spike trains for interspike interval dependency. Fifteen single chemoreceptive afferents were recorded in vivo under hypoxic-hypercapnic conditions, and approximately 1,000 consecutive interspike intervals for each fiber were timed and analyzed for serial dependence. The same set of intervals placed in shuffled order served as a control series without serial dependence. The original spike interval trains showed significantly negative first-order serial correlation coefficients and less variability in joint interval distributions than did the shuffled interval trains. These results suggest that the chemoreceptor afferent train is not random and may reflect a negative feedback system operating within the carotid body that limits variation about a mean frequency.

Patch clamp recording from the intact dorsal root ganglion.

A method for patch-clamp recording from intact dorsal root ganglion (DRG) cells in rat is described. The L4 and L5 DRGs with sciatic nerve attached were excised from rats (10-15 days old) and placed in a recording chamber after removing the ganglion sheath and dissolving the connective tissue with dilute collagenase. The somata of individual cells were exposed by gentle surface cleaning through a perfusion micropipette. Somata were classified as Abeta, Adelta or C based on the cell size and the shape of the action potential (AP). Under current clamp, axonal conduction velocity (CV) was calculated from the distance between a stimulating electrode and the center of the ganglion divided by the latency of the AP elicited by stimulation of the sciatic nerve. CVs ranged from 0.2-0.8 m/s for C cells, 0.8-2.4 for Adelta and 3.2-5.0 for A/beta cells. AP threshold occurred at a significantly more positive potential in C cells than in Adelta and Abeta cells. Under voltage clamp, sodium currents were recorded from C cells. Both TTX-resistant (TTX-R) and TTX-sensitive (TTX-S currents) were demonstrated in the present study. The results demonstrate the feasibility of patch-clamp recording from intact, identified DRG cells in vitro.

Response to cyanide of two types of glomoid cells in mature rat carotid body.

Cells belonging to glomoids of mature rat carotid bodies were studied using the whole-cell patch clamp technique following acute dissociation. The recorded population encompassed two subtypes: one type (n = 202), termed G(out), was characterized by a small voltage-dependent inward current (43 +/- 9 pA, mean +/- S.E.M.), large outward current (671 +/- 31 pA @ +40 mV), high membrane resistance (1910 +/- 110 M omega) and low capacitance (5.1 +/- 0.1 pF). A second subtype (n = 56), termed G(in), had significantly lower membrane resistance (177 +/- 35 M omega), higher membrane capacitance (15.0 +/- 1.0 pF) and little voltage-dependent current. Neither subtype supported generation of multiple action potentials during depolarization in the current clamp mode. Intracellular staining of the recorded cells by Lucifer yellow showed co-localization of both subtypes to clusters of cells which stained positively for catecholamines. Somal diameter was slightly, but significantly, larger for G(in) cells (8.7 +/- 0.4 microM, n = 7) compared to G(out) cells (7.8 +/- 0.2 microM, n = 31) and all cells had fine cytoplasmic processes extending around neighboring cells. During recordings using the perforated patch technique, histotoxic hypoxia significantly decreased a voltage-dependent outward current in G(out) cells by 113 +/- 60 pA (n = 13), and decreased the holding current by 10 +/- 4 pA (n = 13) from a control value of -32 +/- 6 pA. In G(in) cells, cyanide significant decreased membrane resistance and decreased holding current by 55 +/- 28 pA from a control value of +120 +/- 42 pA (n = 7), but caused no significant change in outward current. These results show that glomoids of mature rat carotid bodies contain at least two types of cells which differ in their morphologic and electrophysiologic characteristics. The subtypes rapidly respond to histotoxic hypoxia and thus may mediate separate roles in the organ response to chemostimuli.

Failure to generate action potentials in newborn diaphragms following nerve stimulation.

Action potentials were recorded intracellularly from single diaphragmatic fibers, in vitro, of newborn (3-10 d, n = 18) and older (> or = 21 d, n = 10) rats using flexible microelectrodes. At 20 and 50 Hz phrenic nerve stimulation (1 s duration), action potential transmission failure was significantly higher in the newborn than in the older fibers. During the failure periods, small and highly variable depolarizations were observed which were most likely EPPs. These results show that failure of action potential transmission across the neuromuscular junction is more prevalent in the newborn, and we speculate that this failure is due to inadequate release of neurotransmitter in newborn muscle fibers.

Effects of contralateral superior laryngeal nerve stimulation on dorsal medullary inspiratory neurons.

The effects of superior laryngeal nerve (SLN) stimulation on phrenic (PHR) nerve activity and on activity of dorsal respiratory group (DRG) inspiratory (I) neurons contralateral to the stimulus were examined in decerebrate, paralyzed cats. Stimulation caused bilateral PHR suppression followed by recovery at ca. 30 ms. Most DRG neurons (70%) contralateral to the stimulus were inhibited, but average onset of inhibition lagged that of PHR suppression. This contrasts sharply with the observation in an earlier study that inhibition of ipsilateral I neurons on the average preceded PHR suppression. The remaining neurons (30%) were not inhibited. Only 22% of contralateral neurons were excited by SLN stimulation, in contrast to 52% of ipsilateral neurons. Thus, contralateral DRG I neurons do not mediate the onset of bilateral PHR suppression by SLN stimulation and are probably inhibited through a longer pathway than that for the ipsilateral unit responses.

Dorsal medullary inspiratory neurons: effects of superior laryngeal afferent stimulation.

In decerebrate paralyzed cats ventilated with a cycle-triggered pump, we examined the responses of inspiratory (I) neurons in the region of the ventrolateral nucleus tractus solitarius (NTS) to single electrical stimuli delivered to the ipsilateral superior laryngeal nerve (SLN). Sixty-five I neurons were classified as: I(-), I(0), I(+, early), I(+, late) or I(other) on the basis of responses to lung inflation, and as I(bulbophrenic) or I(non-bulbophrenic) on the basis of evidence of an excitatory projection to the contralateral phrenic motoneuron pool. The peristimulus histograms of contralateral phrenic activity showed an early peak of excitation with average latency of 4.9 +/- 0.1 ms (mean +/- S.E.M.), followed by depression at 7.3 +/- 0.2 ms, start of recovery from depression at 22.7 +/- 1.0 ms, and recovery to control levels at 28.4 +/- 1.1 ms. The peristimulus histograms of ipsilateral I unit activity showed an initial excitation (latency 2.9 +/- 0.3 ms), followed by spiking silence (latency 6.0 +/- 0.6 ms) and recovery to control discharge frequency at 38.8 +/- 3.6 ms. This time of inhibition was significantly longer than the time of phrenic depression, suggesting that other bulbophrenic excitatory projections are able to rapidly compensate for decreased NTS output. Subgroups of I neurons, as classified by lung inflation tests, did not differ significantly with respect to these timing variables. In contrast, latencies of excitation for I(bulbophrenic) neurons were significantly less than for I(non-bulbophrenic) neurons.