Respiratory responses to chemical pulses in the cerebrospinal fluid of cats

Respiratory and circulatory effects of centrally administered phencyclidine in anesthetized dogs

1. Anesthetized dogs were given phencyclidine HCl (PCP) by intracerebroventricular (i.c.v.) injection. 2. Physiological parameters were monitored after consecutive doses of 0.5, 1.0 and 2.0 mg of PCP. 3. Dose-related changes seen, including bradycardia, hypotension and bradypnea, were opposite to those produced by i.v. doses. 4. Single doses of 1.0 or 2.0 mg of PCP confirmed the prior observations, and the latter provided the baseline for further observations on dogs receiving PCP before various i.c.v. pretreatments--atropine, haloperidol, phentolamine or propranolol--in efforts to characterize the central neurotransmitter system(s) involved in the PCP effects.

Target sites for anticholinesterases on the ventral surface of the medulla oblongata: hypotension elicited by organophosphorus agents

Sensitivity of the ventral surface of the medulla oblongata to organophosphorus agents, oxime reactivators, and muscarinic antagonists was examined in order to delineate sites of cholinergic activity in the central nervous system. The exposed ventral surface of the medulla oblongata in anaesthetized cats was treated with the organophosphorus anticholinesterase agents soman and (7-nitro-2-oxa-1,3-diazole) aminopentyl methylphosphonofluoridate (NBD-AP-MFP), a fluorescent active centre-selective probe of acetylcholinesterase. Topical application of soman (1-5 micrograms) or NBD-AP-MPF (5-120 micrograms) elicited a profound (80-90 mm Hg), long-lasting (0.5-3 h), dose-dependent vasodepression with only minor changes in heart rate and respiration. The vasodepression was rapidly reversed (7-10 min) upon topical application of muscarinic antagonists (atropine methylnitrate, atropine sulphate) and the bisquaternary oxime HI-6; systemic administration was without effect. Reversal of the hypotension by HI-6 occurred irrespective of whether the organophosphorus agent was NBD-AP-MPF, which forms conjugates with acetylcholinesterase that undergo no aging, or soman, which forms conjugates that undergo extensive aging rendering the enzyme refractory to oxime reactivation. Hence, oxime efficacy for reversal of the physiologic hypotension was not dependent solely on the fraction reactivatable enzyme. By virtue of the fluorescence distribution of NBD-AP-MPF the chemosensitive sites were estimated to reside no deeper than 50 microns into the medulla oblongata, providing a direct indication for localization of the chemosensitive cells on the superficial surface.

Effects of bronchoconstriction on breathing during normoxia and hypoxia in anesthetized cats

The effect of an increase in bronchomotor tone on control of breathing during both normoxia and hypoxia, and the role of vagal afferents in regulating these responses were studied in 15 anesthetized cats. Minute ventilation (VE) was measured with a pneumotachograph connected in series with a tracheal cannula. Total diaphragmatic EMG activity per minute (means p X f, peak EMG moving average X respiratory frequency) was measured to assess the central inspiratory drive. Bronchoconstriction was generated by inhalation of methacholine aerosol (10-30 breaths, 0.5% solution) which increased total lung resistance to approximately 400% of the control value. Transient hypoxia was induced by allowing the cats to rebreathe a hypoxic gas mixture (4.5% O2 balanced N2) for approximately 1 min. During normoxia, bronchoconstriction increased VE from a baseline of 100 to 129 +/- 7% (mean +/- SEM; P less than 0.05) and increased (means p X f) from 100 to 174 +/- 16% (P less than 0.01). During hypoxia, the response of (means p X f) to bronchoconstriction (404 +/- 40%) was still greater than without bronchoconstriction (306 +/- 35%; P less than 0.01), but the responses of VE were not significantly different between these two conditions (P greater than 0.05). After sectioning both vagus nerves the bronchoconstriction-induced increase in central inspiratory drive was either reduced (during normoxia) or abolished (during hypoxia). These results suggest that stimulation of vagal bronchopulmonary afferents are involved in regulating the ventilatory responses to bronchoconstriction. Other non-vagal factors, such as intrinsic properties and reflex responses of the respiratory muscles, may also contribute, in part, to the observed responses.

Involvement of alpha- and beta-adrenoreceptors in the central action of norepinephrine on temperature, metabolism, heart and respiratory rates of the conscious primate

The effect of the central blockade of alpha- and beta-adrenoreceptors on the action of norepinephrine (NE) infused by the intracerebroventricular (ICV) route was examined in the unanesthetized primate. After cannulae for ICV infusion were implanted sterotaxically, each monkey (Macaca fasicularis) was placed in a restraint chair fitted with a transparent dome for measurement of O2 uptake. During an experiment, the following responses were monitored simultaneously on a polygraph: skin and colonic temperatures, heart and respiratory rates, and metabolic rates as reflected by O2 uptake. Following an ICV infusion of 150 micrograms NE, colonic temperature, heart and metabolic rates declined while skin temperature increased and respiratory rate remained unchanged. Pretreatment with ICV phentolamine (100 micrograms), an alpha-adrenoreceptor antagonist, attenuated the changes in temperature, heart and metabolic rates and reduced respiratory rate. Similar ICV infusion of 100 micrograms propranolol, a beta-adrenoreceptor antagonist, tended also to attenuate the responses induced by NE, but overall was much less efficacious than phentolamine. Our results indicate that although catecholaminergic neurons mediate these vital physiological processes principally by central alpha-adrenoreceptors, a substantial beta-adrenoreceptor component also participates in the mechanism responsible for specific functional changes.

Apneustic respiration normalized by an atypical cholinergic drug action in cats

Cats were anesthetized with a mixture of pentobarbital and barbital given intraperitoneally and vagotomized. All animals were pretreated with atropine i.v. which blocked conventional muscarinic receptors and at the same time revealed atropine-insensitive sites whose stimulation by the cholinergic drug, bethanechol, was earlier found to produce respiratory analepsis. Respiration was recorded by pneumotachograph and intrapleural cannula. Apneustic breathing was produced by placing electrolytic lesions in the pneumotaxic area or by injecting a local anesthetic agent into the cerebrospinal fluid. Experiments were performed under isocapnic conditions. Bethanechol, administered intracerebro-ventricularly, restored regular breathing and increased the tidal volume in a dose-related manner within minutes of its injection. In cats with lesions, the expiratory pause generally decreased first and was followed by shortening of the inspiratory plateau. In cats made apneustic by intracerebro-ventricular injection of procaine, bethanechol acted competitively to normalize the distorted waveshape. The central excitant drug pentylenetetrazol was given intravenously for comparison with bethanechol. Apneustic breathing was partially corrected by pentylenetetrazol through a facilitatory influence on the brain stem interpreted to be independent of its convulsant action. These experiments demonstrate correction of pathological breathing by two chemically unrelated agents which share the ability to increase central excitatory state.

A cholinergic mechanism involved in the respiratory chemosensitivity of the medulla oblongata in the cat

1. Cholinomimetic and adrenomimetic substances were tested on the chemosensitive zones of the ventral surface of the medulla oblongata using a plexiglas ring method. Tidal volume and respiratory frequency, arterial pressure and heart frequency were observed. 2. The increase of ventilation and the depression of arterial blood pressure by locally applied acetylcholine could be blocked by previous local application of atropine. It is therefore assumed that the acetylcholine receptors have muscarinic properties. 3. Nicotine in a small dose raises arterial pressure and with higher doses a drop is observed. The responses of respiration and of arterial pressure to nicotine were blocked by previous intravenous administration of hexamethonium. 4. Local application of atropine in the caudal (L) and rostral (M) chemosensitive zones reduced resting ventilation and the slope of the ventilatory response to CO2-inhalation. Physostigmine in these areas enhanced resting ventilation leaving unchanged the slope of the ventilatory response to CO2-inhalation. 5. With high concentrations of (L)-noradrenaline and (L)-adrenaline a slight increase of arterial pressure was seen while serotonin caused a drop. 6. These results together with those of Fukuda and Loeschcke (1978) suggest that a cholinergic transmission in the surface layer of the ventral medulla is a component in the respiratory and circulatory control systems.

Topography of the respiratory and circulatory responses to acetylcholine and nicotine on the ventral surface of the medulla oblongata

1. Acetylcholine and nicotine were superfused on the ventral medullary surface between the ponto-medullary border and C1 in anaesthetized cats in order to determine the topical distribution of their actions on respiration and circulation. 2. Acetylcholine (10(-4) g . ml-1 = 5.5 . 10(-4) mMol . ml-1) produced an increase in respiration and a lowering of blood pressure. The magnitude and the time course of the responses varied according to the points of superfusion on the surface. 3. Nicotine (10(-4) g . ml-1 = 6.2 . 10(-4) mMol . ml-1) elicited hyperventilation and more often an increase in arterial pressure on unilateral superfusion of the surface. In some cases, however, a drop in blood pressure was also observed. 4. The responsive regions of the surface on which nicotine acted and elicited hyperventilation, bear a close resemblance to the regions responsive to acetylcholine. 5. The topographical distribution of the respiratory effects elicited by the above-mentioned drugs were similar to the distribution of the responses to changes in pH on the ventral medullary surface or to electrical stimulation. 6. Procaine (2 . 10(-2) g . ml-1 = 7.3 . 10(-2) mMol . ml-1) applied bilaterally in the intermediate zone (S) caused profound inhibition of respiration and of arterial pressure. Procaine at this concentration also inhibited respiratory hyperventilation caused by nicotine (10(-4) g . ml-1 = 6.2 . 10(-4) mMol . ml-1) applied to the caudal and rostral areas.

Respiratory and circulatory effects of saxitoxin in the cerebrospinal fluid

1 In cats anaesthetized with pentobarbitone, saxitoxin and, on a few occasions, tetrodotoxin were injected into a lateral cerebral ventricle or into the subarachnoid space of the lower brain stem. Observations were made on frequency and tidal volume of breathing, on CO(2) responsiveness and on electrical responsiveness of the respiratory centre. Effects on the blood pressure were observed simultaneously.2 A single large dose of toxin, e.g., 250 ng, produced within minutes apneustic breathing and a rise in blood pressure which were converted rapidly to respiratory failure and hypotension. In contrast, repeated small doses, e.g., 25 ng, yielded only progressive slowing of the respiration together with circulatory hypotension. Bulbar depression was produced as effectively by subarachnoid injection as by intraventricular injection of the toxins. Onset of action was detectable within seconds.3 Slowing of the respiration occurred independently of change in tidal volume and whether or not the vagus nerves were cut. The reduction in breathing frequency is attributed to direct toxin-induced depression of the central respiratory oscillator.4 Steady-state measurements of tidal volume at controlled levels of alveolar CO(2) pressure in intermediate stages of respiratory depression showed that the toxins produced an increase in CO(2) stimulation threshold as well as a reduction in gain of CO(2) responsiveness, whether or not the vagus nerves were cut. Carotid arterial chemoreceptor reactivity to O(2) was demonstrable when central sensitivity to CO(2) was depressed. These effects are attributed to a direct influence of the toxins upon the brainstem CO(2)-tidal volume controller.5 Responsiveness of the medullary inspiratory centre to electrical stimulation persisted after the failure of spontaneous breathing was caused by the toxins. Conversely, restitution of electrical responsiveness preceded the reappearance of spontaneous respiratory activity in the recovery phase of toxic depression. Circulatory effects paralleled the changes in respiratory behaviour.6 On the basis of the relatively prompt and discrete alterations in the central respiratory and circulatory control mechanisms produced by saxitoxin and tetrodotoxin placed in the cerebrospinal fluid, it is concluded that the agents rapidly penetrated to deep target loci in the lower brain stem.

Pressor response induced by local anaesthetics perfused through the cerebral ventricles of dogs

1 The antihistamines mepyramine and halopyramine which have local anesthetic properties, produced a rise in arterial blood pressure followed by a fall, similar to that seen with procaine, when perfused in a 10 mg/ml solution from the lateral ventricle to the cisterna magna in dogs. 2 The pressor response produced by procaine perfused through the cerebral ventricle is due to an action on structures situated in the caudal half of the floor of the fourth ventricle because a pressor response was obtained when a few microlitres of a 50 mg/ml solution of procaine were applied to a small area of the exposed floor of the fourth ventricle in a region 2 to 4 mm rostral to obex, and after cauterization of this area procaine perfused through the cerebral ventricles no longer raised arterial blood pressure. 3 Bilateral denervation of the carotid sinus did not affect the pressor response obtained with procaine perfused through the cerebral ventricles. 4 The pressor response to procaine is mediated through the sympathetic nervous system. It results partly from an increased adrenaline discharge from the adrenal glands and partly from increased sympathetic vasomotor tone, because the response is abolished or attenuated after removal of the adrenal glands and intravenous injections of hexamethonium and phentolamine.

A central vasodepressor effect of Dyflos

1 Cats were anaesthetized with pentobarbitone sodium and atropinized peripherally by intravenous injection of atropine methyl nitrate; the effect was examined of topical bilateral application of dyflos to the ventral surface of the medulla oblongata at a region lateral to the pyramids and caudal to the trapezoid bodies. Dyflos was applied by means of perspex rings; the volume of fluid placed in each ring was 10 mul.2 The topical application of dyflos (1-20 mg/ml) produced a fall in arterial blood pressure without changes in heart rate and, in experiments without artificial ventilation, tachypnoea with dissociation of thoracic and abdominal respiration.3 Atropine methyl nitrate (50 mg/ml) applied topically in the same way as dyflos, prevented or abolished its vasodepressor effect.4 The two reactivators of acetylcholinesterase, obidoxime (100-200 mg/ml) and pralidoxime mesylate (100-200 mg/ml), applied topically in the same way as dyflos, abolished its vasodepressor effect. The reactivator compound 30 (100 mg/ml), also a pyridinium aldoxime, did not have this effect.5 Obidoxime and pralidoxime mesylate also reversed the vasodepression produced by carbachol applied to the ventral surface of the brain stem but not the vasodepression produced by glycine similarly applied.6 The problem is discussed as to whether the reversal of the dyflos and carbachol-induced vasodepression by obidoxime and pralidoxime is due to acetylcholinesterase reactivation by dephosphorylation and decarbamylation respectively, to a central atropine-like action of these compounds or to a combination of both.