Monique Denavit-Saubié

Catecholaminergic depressant effects on bulbar respiratory mechanisms

On the basis of histochemical and pharmacological studies, catecholamines have been implicated in central mechanisms controlling respiration. This hypothesis was tested in iontophoretic studies on neurones located in bulbar respiratory centres. Adrenaline and noradrenaline had a predominantly depressant effect on respiratory as well as on closely situated non-respiratory units. These depressions were mimicked by the application of isoproterenol and clonidine; acetylcholine and serotonin had inconsistent effects on these neurones. In control experiments, microinjections, using a Hamilton syringe, were made in the area of bulbar respiratory centres: noradrenaline, but not serotonin, depressed the central respiratory activity reflected in the phrenic nerve discharge. These results suggest that specific adrenergic and noradrenergic depressant mechanisms could affect both respiratory and other physiological centres at the bulbar level.

Involvement of amino acids in periodic inhibitions of bulbar respiratory neurones

As previously demonstrated, spontaneously firing bulbar inspiratory neurones are periodically inhibited either at the beginning of, or throughout expiration, while bulbar expiratory neurones are inhibited during inspiration. The aim of the present study was to test the hypothesis that amino acids act as transmitters of these periodic inhibitions. The study was performed using iontophoretic applications of drugs on bulbar respiratory neurones. On these neurones GABA and glycine-sensitive sites were identified and differentiated on the basis of the actions of agonist (muscimol) or antagonists (bicuculline, picrotoxin and strychnine). Using competitive antagonists (nipecotic acid, beta-alanine) mechanisms responsible for GABA uptake were found in the close vicinity of respiratory-related neurones. Some but not all types of periodic inhibition were found to be reduced following application of GABA or glycine antagonists. Strychnine was found to reduce periodic inhibitions occurring at the beginning of expiration in inspiratory neurones. GABA antagonists had an effect on those periodic depressions which were prolonged throughout expiration. A different and complementary role of glycine-like and GABA-like systems in central respiratory mechanisms is proposed.

NMDA and non-NMDA receptors may play distinct roles in timing mechanisms and transmission in the feline respiratory network

1. Activation of N‐methyl‐D‐aspartate (NMDA) glutamate receptors in the brainstem network of respiratory neurones is required to terminate inspiration in the absence of lung afferents, but it is not required in the inspiratory motor act of lung inflation. In the present study we examined the involvement of non‐NMDA ionotropic glutamate receptors in these two mechanisms in the adult mammal. 2. Adult cats were either decerebrated or anaesthetized with sodium pentobarbitone, paralysed and ventilated. Inspiratory motor output was recorded from the phrenic nerve and central respiratory activity from neurones in the bulbar ventral respiratory group. 3. In decerebrate vagotomized cats, ionophoretic application of 2,3‐dihydroxy‐6‐nitro‐7‐sulphamoylbenzo(F)quinoxaline (NBQX) onto single respiratory neurones decreased their spontaneous discharge rate and abolished the excitatory effect of exogenously applied (RS) alpha‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazoleproprionic acid (AMPA) but not NMDA. 4. In these animals, intravenous infusion (12 mg kg‐1) of the non‐NMDA receptor blockers GYKI 52466 (1‐(4‐aminophenyl)‐4‐methyl‐7,8‐methylene‐dioxy‐5‐H‐2,3‐benzodi aze pine) or NBQX: (1) decreased (in 10/15 cats) or abolished (in 5/15 cats) the inspiratory‐related discharge of the phrenic nerve; (2) did not prolong the inspiratory phase; (3) reduced or abolished the spontaneous discharge of respiratory neurones; and (4) profoundly decreased the excitatory effects of AMPA but not NMDA ionophoresed onto these neurones. When both the phrenic nerve and the recorded respiratory neurone were silenced, neuronal excitation by ionophoretic application of NMDA first revealed a subthreshold respiratory modulation without lengthening of the inspiratory phase, then respiratory modulation became undetectable. 5. Additional blockade of NMDA receptors by a small dose (0.15 mg kg‐1) of dizocilpine (MK‐801), abolished the phrenic nerve activity which persisted after NBQX (apnoea), but the discharge or the subthreshold modulation of the bulbar respiratory neurones showed a lengthening of the inspiratory phase (apneusis). 6. Elevation of FA,CO2 increased or re‐established phrenic nerve discharges after blockade of non‐NMDA receptors or of both NMDA and non‐NMDA receptors. 7. Small doses of NBQX or GYKI 52466 induced apnoea in five of five cats anaesthetized with sodium pentobarbitone. 8. In decerebrate animals with intact vagi, GYKI 52466 and NBQX depressed the Hering‐Breuer expiratory‐lengthening reflex. 9. The results suggest that: (1) there is a specialization of different classes of glutamate receptors participating in timing mechanisms and transmission within the mammalian respiratory network. Neural transmission predominantly involves activation of non‐NMDA receptors, acting in synergy with NMDA receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibitions mediated by glycine and GABAA receptors shape the discharge pattern of bulbar respiratory neurons

Experiments were performed to identify the glycinergic or GABAergic nature, and the timing of discharge, of the neurons which produce chloride-dependent inhibitions on other bulbar respiratory neurons (RNs) during their silent and active phases. RNs recorded extracellularly in pentobarbital-anesthetized or decerebrate cats, were subjected to iontophoretic applications of glutamate, of the glycine antagonist strychnine, and of the GABAA receptor antagonist bicuculline. Both antagonists induced discharge or increased discharge frequency in restricted parts of the respiratory cycle without affecting the discharge frequency in other parts of the cycle. Strychnine most often elicited activity in late-inspiration and early-expiration, but also in early inspiration and in late expiration. Bicuculline was most often effective throughout the entire discharge period of each neuron with no effect during the silent period, although it also acted selectively during late-inspiration in inspiratory neurons, an effect attributed to GABAA receptor blockade. The convergence of glycinergic afferent inputs during late inspiration and early expiration suggests that glycinergic neurons may play an important role in the inspiratory to expiratory phase transition.

Different effects of μ and δ opiate agonists on respiration

Abstract The involvement of different opiate receptor subtypes in opiate-induced respiratory depression was studied in the unanaesthetized rat. Synthetic opioid agonists, specific for μ or δ receptors, were administered intraperitoneally in freely moving rats while respiratory parameters were recorded by means of the whole body plethysmographic method. TRIMU-4 (Tyr-D-Ala-Gly-NH-CH(CH 3 )-CH 2 -CH(CH 3 ) 2 ), a specific agonist of the μ receptor, reduced the tidal volume and did not change the respiratory frequency. DSLET (Tyr-D-Ser-Gly-Phe-Leu-Thr), a relatively specific agonist of the δ receptor subtype, reduced respiratory frequency and was significantly less effective on tidal volume than was TRIMU-4. It is concluded that the respiratory depression occurring after the administration of opiates in clinical practice is a dual complementary effect involving μ and δ receptors.

N-Methyl-d-aspartate (NMDA) receptors control respiratory off-switch in cat

Functionally active N-methyl-D-aspartate (NMDA) receptors on cat medullary respiratory neurones were revealed by local iontophoretic application of DL-2-amino-7-phosphonoheptanoic acid (AP7). Blockade of NMDA receptors by systemic administration of NMDA antagonists (MK-801, phencyclidine, ketamine, AP7) in vagotomized cats increased the duration of inspiration (Ti) without increasing expiration and caused an apneustic breathing pattern. The increase in Ti which followed systemic MK-801, was accompanied by a shift and complete reversal of early expiratory neuronal discharge in relation to phrenic nerve discharge.

Inhibitory effect of cholecystokinin octapeptide on neurons in the nucleus tractus solitarius

Abstract We investigated the effect of the cholecystokinin octapeptide (CCK8) applied locally to neurons of the nucleus tractus solitarius (NTS). Results demonstrate an inhibitory effect of CCK8 on spike discharges including those related to respiration. It is suggested that CCK8 acts at this level through specific receptor mechanisms since CCK8-induced inhibitions were not reproduced by application of related peptides and were resistant to antagonists of different inhibitory transmitters.

Excitatory effects of iontophoretically applied substance P on neurons in the nucleus tractus solitarius of the cat: lack of interaction with opiates and opioids

The effects of iontophoretically applied substance P (SP), (D-Pro2, D-Trp7,9)-SP and (D-Pro4, D-Trp7,9,10)-SP were studied on neurons identified by their histological location in the nucleus tractus solitarius (NTS), their response to vagal or carotid sinus nerve stimulation and eventually their functional correlation with the central respiratory drive. Potent and consistent excitatory effects of SP were found supporting its role as a putative excitatory transmitter in the NTS. The effects of SP and L-glutamate (Glu) were differentiated by the relative insensitivity of SP-induced excitations to levorphanol and Met-enkephalin.

Localization of μ- and δ-opioid receptors in cat respiratory areas: an autoradiographic study

Abstract Autoradiography after in vitro binding with selective ligands for either μ ([3H](Tyr- d -Ala-Gly-(NMePhe)-Gly-ol)) or δ ([3H](Tyr- d -Thr-Gly-Phe-Leu-Thr)) opioid receptor types revealed the presence of variable amounts of radioactive labeling in the cat brainstem. Areas involved in the respiratory rhythmogenesis were among the most prominently labeled structures. The pneumotaxic center, including the nucleus parabrachialis medialis and the Kolliker-Fuse nucleus, contains a very high density of δ binding sites while the dorsal respiratory nucleus which corresponds to the nucleus tractus solitarius, is more heavily labeled by the μ ligand. The neuroanatomical differences in the distribution of opioid receptors correlates well with the pharmacological responses induced by administration of specific μ- or δ ligands.

O2-sensing after carotid chemodenervation: hypoxic ventilatory responsiveness and upregulation of tyrosine hydroxylase mRNA in brainstem catecholaminergic cells

Ventilatory responses to acute and long‐term hypoxia are classically triggered by carotid chemoreceptors. The chemosensory inputs are carried within the carotid sinus nerve to the nucleus tractus solitarius and the brainstem respiratory centres. To investigate whether hypoxia acts directly on brainstem neurons or secondarily via carotid body inputs, we tested the ventilatory responses to acute and long‐term hypoxia in rats with bilaterally transected carotid sinus nerves and in sham‐operated rats. Because brainstem catecholaminergic neurons are part of the chemoreflex pathway, the ventilatory response to hypoxia was studied in association with the expression of tyrosine hydroxylase (TH). TH mRNA levels were assessed in the brainstem by in situ hybridization and hypoxic ventilatory responses were measured in vivo by plethysmography. After long‐term hypoxia, TH mRNA levels in the nucleus tractus solitarius and ventrolateral medulla increased similarly in chemodenervated and sham‐operated rats. Ventilatory acclimatization to hypoxia developed in chemodenervated rats, but to a lesser extent than in sham‐operated rats. Ventilatory response to acute hypoxia, which was initially low in chemodenervated rats, was fully restored within 21 days in long‐term hypoxic rats, as well as in normoxic animals which do not overexpress TH. Therefore, activation of brainstem catecholaminergic neurons and ventilatory adjustments to hypoxia occurred independently of carotid chemosensory inputs. O2‐sensing mechanisms unmasked by carotid chemodenervation triggered two ventilatory adjustments: (i) a partial acclimatization to long‐term hypoxia associated with TH upregulation; (ii) a complete restoration of acute hypoxic responsivity independent of TH upregulation.