Role of peripheral and central chemosensitive afferents in the control of depth and frequency of breathing

Peripheral oxygen-sensing cells directly modulate the output of an identified respiratory central pattern generating neuron

Breathing is an essential homeostatic behavior regulated by central neuronal networks, often called central pattern generators (CPGs). Despite ongoing advances in our understanding of the neural control of breathing, the basic mechanisms by which peripheral input modulates the activities of the central respiratory CPG remain elusive. This lack of fundamental knowledge vis-à-vis the role of peripheral influences in the control of the respiratory CPG is due in large part to the complexity of mammalian respiratory control centres. We have therefore developed a simpler invertebrate model to study the basic cellular and synaptic mechanisms by which a peripheral chemosensory input affects the central respiratory CPG. Here we report on the identification and characterization of peripheral chemoreceptor cells (PCRCs) that relay hypoxia-sensitive chemosensory information to the known respiratory CPG neuron right pedal dorsal 1 in the mollusk Lymnaea stagnalis. Selective perfusion of these PCRCs with hypoxic saline triggered bursting activity in these neurons and when isolated in cell culture these cells also demonstrated hypoxic sensitivity that resulted in membrane depolarization and spiking activity. When cocultured with right pedal dorsal 1, the PCRCs developed synapses that exhibited a form of short-term synaptic plasticity in response to hypoxia. Finally, osphradial denervation in intact animals significantly perturbed respiratory activity compared with their sham counterparts. This study provides evidence for direct synaptic connectivity between a peripheral regulatory element and a central respiratory CPG neuron, revealing a potential locus for hypoxia-induced synaptic plasticity underlying breathing behavior.

PAF and the role of the vagus nerve in the breathing pattern of the pig

In 14 anesthetized, spontaneously breathing pigs we examined the changes in breathing pattern, in respiratory mechanics and in systemic and pulmonary vascular parameters after i.v. PAF administration. In another 3 pigs, the effects of PAF were also examined after bilateral vagotomy. In intact pigs, PAF induces apnea, bronchoconstriction, pulmonary hypertension and systemic hypotension. Our results also show that administration of PAF alters the phasic vagal activity, modifying the slope of VT vs TI and TE vs TI relationships and the TI0/TI ratio. These effects and apnea are vagus-dependent. The central excitatory timing effect of PAF on inspiratory duration (TI0) was correlated with a decrease in passive compliance but not with active compliance. We postulate that the activation by vagal input strengthens the mechanisms that counteract the bronchoconstrictor effect of PAF.

Properties of a sympatho-inhibitory and vasodilator reflex evoked by superior laryngeal nerve afferents in the cat

The background discharge of sympathetic preganglionic neurones shows a marked inspiration-synchronous component which is known to originate from within the central nervous system. The contribution of this component to total neurogenic vasoconstrictor tone is unknown. In order to estimate its extent we have exploited the inspiration-suppressing effect of a group of low threshold afferent fibres in the superior laryngeal nerve. The electrical activities of the cervical sympathetic trunk and of the phrenic nerve were recorded in pentobarbitone-anaesthetized, paralysed, artificially ventilated, sino-aortic denervated and vagotomized cats, together with the perfusion pressure of an innervated hind limb perfused at a constant flow rate. Repetitive stimulation of the superior laryngeal nerve at an intensity just sufficient to suppress phrenic nerve activity inhibited the inspiration-synchronous sympathetic discharge and caused hind limb vasodilatation. This vasodilatation was abolished by hexamethonium or phentolamine, but was not affected by atropine or propranolol. Following the elimination of phrenic nerve activity and inspiration-synchronous sympathetic discharge by systemic hypocapnia, repetitive stimulation of the superior laryngeal nerve either failed to affect the residual sympathetic activity and hind limb perfusion pressure, or caused an increase of both. Stimulation of the superior laryngeal nerve with short (0.2 s) trains of stimuli, delivered at selected times of the respiratory cycle for several consecutive cycles, had similar effects on phrenic nerve bursts, inspiration-synchronous sympathetic discharge and hind limb perfusion pressure. Stimulation at progressively earlier times during inspiration produced a graded reduction in all three variables, while stimulation during late inspiration or early expiration had no effect on any of them. The results suggest that the vasodilator reflex, elicited by inspiration-suppressing afferents in the superior laryngeal nerve, results from selective abolition of the excitatory input which causes the inspiration-synchronous discharge of sympathetic neurones. The magnitude of the hind limb vasodilatation can therefore be taken as an indication of the extent of control of hind limb vasoconstrictor tone exerted by this particular input. By comparing the magnitude of the reflexly evoked vasodilatation with that of the vasodilatation resulting from ganglionic blockade, it was estimated that 24.2% of the neurogenic vasoconstrictor tone of the hind limb was attributable to the inspiration-synchronous component of sympathetic discharge.

Ventilatory responses to hypocapnic vertebral artery perfusion in intact and carotid body denervated dogs

The ventilatory responses to step changes in vertebral artery PCO2 were investigated in intact and carotid body denervated dogs. The steady-state ventilatory responses of the denervated dogs were less than those of intact dogs. However, when expressed as a ratio to the control ventilation there was no difference between the two groups. While the arterial PCO2 was held at 56 mm Hg by adding CO2 to the inspired air the perfusion of the vertebral arteries was switched from the dog's own arterial supply to hypocapnic blood. The ventilation of the denervated dogs decreased at a faster rate (half time = 130 +/- 9 sec) to a level less than the room air control ventilation. The ventilation in the intact dogs decreased at a slower rate (half time = 184 +/- 23 sec) and was maintained above the room air control level after ten minutes of hypocapnic perfusion. Increasing the medullary blood flow, as measured with radiolabeled microspheres, augmented the rate of decline of ventilation in intact dogs. We conclude, (1) the influence of the peripheral chemoreceptors appears to increase as central drive is decreasing, and (2) the remaining time course of the decrease in ventilation is related to the rate of brain stem perfusion.

The role of the pneumotaxic mechanism in the tachypnea of pulmonary vagal origin

The tachypnea due to stimulation of pulmonary vagal irritant and J.receptors with histamine (5%, aerosol) and phenyldiguanide (150 mu g, i.v.) was studied in 11 pentobarbitone anesthetized cats before and after uni (UL) and bilateral (BL) lesion of sites of the pneumotaxic mechanism. BL prolonged the timing parameters of spontaneous and occluded breaths (no phasic input from pulmonary stretch receptors) more under drug stimulation than in control: thus the shortening effect of drug stimulation increased going from intact to BL. Drug stimulation caused a decrease in volume threshold for inspiratory cut-off and this effect was reduced by about 50% by BL. Under hyperoxic iso-PCO2 conditions drug stimulation reduced the respiratory output (indexed by the rate of change of the pressure developed in the airways during occluded breaths) and this effect was left unaltered by BL. The results suggest that the pneumotaxic mechanism plays a role in the reflex respiratory excitatory frequency effect resulting from activation of irritant and J receptors while it seems to be excluded from a control on respiratory output.

Role of hypotension induced by PGI2 on the control of depth and frequency of breathing

We studied the respiratory response to infusion of prostacyclin (PGI2) in 7 anaesthetized pigs in an attempt to understand whether the changes in pattern of breathing were due to a direct action of the substance or to the concomitant hypotensive effect. The depth and frequency of breathing were analyzed in terms of the threshold-inhibition curve for termination of inspiration (VT/TI relationship) and of bulbo-pontine rhythm, estimated from inspiratory and expiratory time during occlusion of the airways at the end expiratory level (TE/TI relationship). This provide the central respiratory rhythm when the phasic lung volume receives no input from pulmonary stretch receptors. The hypotension induced by PGI2 increased pulmonary ventilation mainly through a change in VT and caused a slight rightward displacement of VT/TI relationship without modifying the slope of the curve. This effect seems to be vagally mediated. PGI2 also changed the bulbo-pontine rhythm. Our results show that PGI2 modifies the vagal discharge and bulbo-pontine rhythm by two opposite mechanisms: hypotension and an apparent direct action on carotid and aortic baroreceptors.

Analysis of hemodynamic and respiratory effects of PGI2 in the pig

Cardiovascular and pulmonary effects of prostacyclin (PGI2) were investigated in the anaesthetized pig before and after vagosympathectomy at 1',2',3',4',5',10',15' and 20' after the beginning of the infusion. The animals were perfused with PGI2,2ug/kg/min for 4 min. We measured the following parameters : mean arterial pressure, heart rate, cardiac output, stroke volume, mean pulmonary arterial pressure, mean wedge pressure, left stroke work, total systemic resistance, total pulmonary resistance, tidal volume, respiratory frequency, minute volume, lung resistance and compliance, end-tidal CO2, transpulmonary pressure. Our findings suggest that the hypotension observed is due to a reduction in total peripheral resistance by a direct effect of PGI2 on the vascular bed without inhibition of the sympathetic nervous system. Prostacyclin does not directly modify cardiac function and produces a decrease in preload by venous dilation. The changes in pattern of breathing are related to the hypotensive effect of PGI2.

Control of breathing during acute hemorrhage in anesthetized cats

We studied in anesthetized cats the ventilatory response to acute blood loss under hyperoxic iso-capnic conditions. From a blood pressure, of approximately 150 down to approximately 70 mm Hg ventilation increased on the average by 2.5 times. The ventilatory response was characterized by two phases: there was an initial phase (down to a PB approximately 100 mm Hg) of increase in frequency due to an excitatory effect on bulbo-pontine respiratory timing which was estimated from the duration of breaths following occlusion of the airways at the end expiratory volume. This effect was mainly due to the withdrawal of inhibitory afferents from the baroreceptors of the aortic arch. To this phase corresponded the phase of vasomotor compensation. Subsequently there was a phase of excitatory effect on the respiratory output which was estimated from the rate of change of the pressure developed in the airways during occluded breaths. This effect was mainly due to afferents from carotid sinuses; a minor role was due to the decrease in inhibitory afferents from carotid baroreceptors while the greater part was likely to be due to the hypoxic stimulation of glomus cells due to reduced blood flow. Following vagotomy above the superior laryngeal and sinus denervation the excitatory effect on respiratory timing and output were reduced to about 30% of that observed in intact cats.

Central respiratory effects of carbon dioxide, and carotid sinus nerve and muscle afferents

1. In anaesthetized and paralysed cats, artificially ventilated and with the vagi and carotid sinus nerves cut, the effects of (a) changing end-tidal CO2, (b) stimulation of carotid sinus nerves, and (c) squeezing the calf muscles, on neural respiratory output have been studied. 2. The peak value of integrated phrenic activity, which is the neural equivalent of tidal volume (VTn), the rate of rise of integrated phrenic activity (Slope), inspiratory duration (TI), expiratory duration (TE), and total cycle length (Ttot) were measured. It was found that Slope-VTn, VTn-Ttot and VTn-TE relationships were similar whether respiration was stimulated by carotid sinus nerve stimulation or by increasing end-tidal CO2. However, when respiration was stimulated by squeezing the calf muscles, a given Slope was associated with a smaller VTn and a given VTn was associated with smaller Ttot and TE values than obtained with the other two inputs. 3. It is concluded that these results are compatible with the hypotheses that (1) inputs from central and peripheral chemoreceptors have similar actions at various sites in the respiratory centre complex, and (2) afferents activated by squeezing the limb muscles have additional facilitatory actions on mechanisms terminating inspiration and expiration.

Histamine and phenyldiguanide induced tachypnea in hypercapnia and hypoxia

The rapid shallow breathing of pulmonary vagal origin following administration of histamine (H) and phenyldiguanide (PDG) was studied at different levels of hypercapnic and hypoxic stimulation. At all levels of chemical drive H and PDG caused an excitatory effect on timing of breathing and an inhibitory effect on the respiratory output. The latter was evaluated from the mean inspiratory flow rate and the mean rate of change of pressure developed in the lungs during an inspiratory effort against closed airways. The timing effect was greater at low than at high PaCO2 while the opposite was true for the output effect. At all PaCO2, H and PDG decreased the volume-threshold for termination of inspiration (leftward displacement of the VT vs TI relationship) of the VT Vs TI relationship). Hypoxia increased the respiratory output in control as much as with drug stimulation. Moreover, hypoxia did not affect the volume-threshold curve both in control and with H and PDG. We concluded that vagal afferents stimulated by H and PDG (irritant and/or J receptors) interfere with the timing and output response to central chemoreceptors stimulation (CO2 sensitivity) without affecting the response to peripheral chemoreceptors stimulation (mainly hypoxic chemosensitivity).

CO2-dependent component of the neurogenic vascular tone in the cat

Complete vascular isolation of the hindlimbs was performed in vagotomized cats under Sodium Pentobarbital anesthesia. The hindlimbs were perfused at constant flow with blood kept at a constant and physiological PO2, PCO2, and pH values by means of a specially designed pump-oxygenator system. The animals were hyperventilated with different CO2 mixtures (0%, 5%, 7% and 10% in O2) thereby changing blood gases and pH levels in the upper body but not in the hindlimb vascular bed. At body PaCO2 (mm Hg) of 13.7 +/- 1.0 (means +/- SE), 30.6 +/- 1.05, 40,4 +/- 0,9 and 58.4 +/- 2.9 the hindlimb perfusion pressure (mm Hg) was, respectively 124 +/- 7.6, 138 +/- 7.4, 156 +/- 11.9 and 187 +/- 15.1. These changes in perfusion pressure were still present after complete peripheral chemoreceptor denervation but were abolished after section of the spinal cord at the T5 level. Since hindlimb perfusion pressure fell when body PaCO2 was lowered below physiological levels it is concluded that part of the neurogenic vascular tone of the hindlimbs is maintained by a CO2 mediated stimulation of supraspinal structures.

The mechanism of rapid shallow breathing due to histamine and phenyldiguanide in cats and rabbits

In anaesthetized cats and rabbits we analyzed the rapid shallow breathing following exposure to histamine aerosol (mainly an irritant receptor stimulant) and i.v. injection of phenyldiguanide (mainly a J receptor stimulant). Both drugs caused a marked leftward displacement of the tidal volume (VT) vs inspiratory time (TI) relationship (Hering-Breuer threshold curve) without a corresponding increase in inspiratory flow rate so that inspiration was cut off at a lower VT and TI. The leftward displacement of the VT vs TI relationship occurred with a great shortening of the duration of inspiration during occluded breaths (T0I) accompanied by a shortening of the expiratory phase (T0E). These parameters monitored the central respiratory rhythm in absence of the phasic lung volume related vagal loop. It is suggested that the increased central respiratory frequency was due to the augmented firing of fibers from stimulated irritant and J receptors. Stimulation of these endings also caused the TE vs TI relationship to become steeper in cats and to be displaced downwards in rabbits.