Integration of chemoreceptor stimuli by caudal pontile and rostral medullary sites

Ventilatory regulation by pontile pneumotaxic and apneustic centers and by rostral medullary sites was evaluated in intercollicular decerebrate cats. Following pneumotaxic center ablation, PAco2 was significantly elevated. Moreover, in response to hypercapina or hypoxia, frequency responses were significantly diminished whereas tidal volume responses were unchanged or elevated. Interruption of apneustic center function by caudal pontile transection or radiofrequency lesions in the caudal pons and/or rostral medulla resulted in significant decreases of tidal volume responses and significant elevations of frequency responses to both hypercapnia and hypoxia. Neither minute volume responses nor the PAco2 level was altered. It is concluded that the apneustic center exercises a primary role in the brainstem definition of tidal volume responses for both peripheral and central chemoreceptor afferent stimuli. The apneustic center is also considered to exert an impoetant function in the definition of respiratory frequency. A medially placed pathway in the rostral medulla is proposed to interconnect the apneustic center with the medullary respiratory nuclei.

Modification of aortic barorecptor resetting in the spontaneously hypertensive rat

Aortic barorecptor function was studied in spontaneously hypertensive rats (SHR) of various ages and normotensive Wistar rats. The aortic arch was isolated and perfused, and the activity of the left aortic nerve was recorded. The threshold pressure to elicit barerecptor firing was 80-120 mmHg in normotensive Wistar rats. Resetting of barorecptors (threshold pressure 160-180 mmHg) was found in all untreated SHR of 35-70 wk of age. Resetting of barorecptors was prevented in SHR by starting treatment with antihypertensive agents at the age of 11 wk. Treatment of 32-wk old SHR with antihypertensive agents for 4-6 wk resulted in reversal of barorecptor resetting in 50% animals. The percentage of SHR showing complete reversal of resetting did not increase even when the duration of treatment was tripled. In 52- to 64-wk old SHR, treated with antihypertensive agents, reversal of baroceptor resetting was seen in only 30% animals. It was concluded that baroceptor resetting in SHR was secondary to hypertension. Hypertension, in turn, induced hypertrophy of the tunica media of the aorta. Histological studies showed a close correlation between aortic hypertrophy resetting. Aortic hypertrophy may, therefore, be one of the important factors involved in baroceptor resetting.

Effects of diphenylhydantoin on cardiac arrhythmias induced by carotid occlusion in the cat

The effects of diphenylhydantoin (DPH) on cardiac arrhythmias induced by occlusion of the common carotid arteries were studied in vagotomized cats under light chloralose anesthesia. It was found that the mean effective dose of DPH via the i.v. route was 11.0 mg/kg. When DPH was injected into the vertebral artery, the carotid artery or the 4th cerebral ventricle, the mean effective dose was 0.8, 1.2 and 0.32 mg/kg, respectively, When DPH was injected directly into the posterior hypothalamic regions, the mean effective dose was even smaller, i.e., 0.14 mg/kg. On the other hand, when DPH was injected into the left ventricle of the heart, the mean effective dose was 1.7 mg/kg and the dose was significantly increased to 4.4 mg/kg when cerebral circulation was temporarily occluded. The data suggest that DPH exerts a potent antiarrhythmic effect through its action on the central nervous system. For the cardiac arrhythmias induced by occlusion of the common carotid arteries, it appears likely that DPH acts on the posterior hypothalamus.

Studies on the localization of central cough mechanism; site of action of antitussive drugs

The localization of the central cough mechanism was studied by electrically stimulating the lower brainstem in cats lightly anesthetized with pentobarbital sodium or in unanesthetized midcollicular decerebrate preparations. Cough responses were recorded with the aid of a microphone. The cough responsive region was concentrated in an area dorsomedial to the trigeminal tract and nucleus. The sites of action of antitussive agents (dextromethorphan, codeine, clonazepam, diazepam and caramiphen) were studied on the centrally induced cough responses. Each of these drugs was administered intravenously to determine the dorsal levels for cough suppression. In other series, the threshold dose via the intravertebral route was determined first. Because the agent necessary to prevent cough via this route was so small in amount, recovery usually occurred in 20 to 90 minutes. Then the same agent was given intravenously in an attempt to obtain an effective dose which was close to the minimum dose for blocking the cough. The mean effective doses of these agents to abolish the cough via the vertebral artery were only about 1/20 of those required via intravenous injection. The findings suggest that these agents act centrally to suppress the cough responses. Clonazepam was found to be the most potent antitussive among these agents, the mean effective dose being about 1/35 of that of codeine. The antitussive potency of benzodiazepines is not well correlated with their muscle relaxant activity. For instance, clonazepam and diazepam have the same potency in depressing polysynaptic spinal reflexes, whereas the former is 6 times more potent than diazepam as an antitussive. This finding indicates that clonazepam has a high specificity as an antitussive.

Leukocytic pyrogen and sodium acetylsalicylate on hypothalamic neurons in the cat

Leukocytic pyrogen and sodium acetylsalicyclate (NaASA) were microinjected into the preoptic/anterior hypothalamic (PO/AH) area of cats to examine the direct effects of these agents on identified thermoregulatory neurons. The pyrogen and NaASA were administered under thermoneutral conditions to preparations that displayed peripheral or peripheral and central thermoreceptor input. The majority of neurons studied with proximate injection of pyrogen responded in a manner consistent with the set-point hypothesis; i.e., units responding to heating with increased activity were depressed and those showing a decreased discharge with the heat test were excited by the pyretic agent. Injection of NaASA without pyrogen pretreatment caused no significant modification of thermoregulatory neuron discharge in most cases. However, when NaASA was administered after pyrogen, it uniformly antagonized the pyretic effect causing a return of the discharge to the control rate. It may be concluded that pyrogen and NaASA act directly in the PO/AH area to produce fever and antipyresis, respectively, by appropriately offsetting the activity of thermoregulatory neurons.

Respiratory stimulant effects of ethamivan and picrotoxin

The respiratory stimulant effects of ethamivan and picrotoxin were studied in unanesthetized decerebrate cats. It was found that neither compound exhibited selective stimulant action on the respiratory neurons. Ethamivan evoked increases in respiratory rate but not in tidal volume, whereas picrotoxin profoundly altered both of these variables. The increases in respiratory rate evoked by ethamivan required intact vagus nerves since midcervical vagotomy abolished this effect. It is conceivable that ethamivan stimulated pulmonary chemoreflexes which then led to increased respiratory rate. Picrotoxin had no discernible effect on peripheral chemoreflexes. It altered, however, the central respiratory rhythmicity, or rate, depth and rhythm of respiration. There was a marked effect on central respiratory control which led to cycling between slow and deep, and rapid and shallow breathing. These were interspersed with periods of rapid and deep respiration.

CNS site of antiarrhythmic action of diphenylhydantoin (DPH) in the cat

Diphenylhydantoin (DPH) is known to be a potent anticonvulsant agent, useful in treating and preventing grand mal seizures. More recently, DPH was reported also to be a potent antiarrhythmic agent acting by means of a depressant action on the heart. The present experiments demonstrated that DPH has also a potent antiarrhythmic action when administered to the CNS. The posterolateral hypothalamus was stimulated in cats to evoke cardiac arrhythmias of varying severity both during and after stimulation. In general, it was found that the post-stimulus arrhythmias were obtained more readily than those during stimulation. The mean effective dose of DPH required to prevent the arrhythmias via the i.v. route was 11.9 mg/kg, and that via the vertebral artery route and via the fourth ventricular route was only 1.9--1.4 and 1.4 mg/kg, respectively. These results suggest that though DPH has identifiable antiarrhythmic action on the heart itself it has a strong antiarrhythmic effect via the central nervous system as well.

Mechanism of morphine-induced miosis in the dog

It was observed that dogs under 50 percent nitrous oxide and succinylcholine exhibited a moderately large pupil, maintained a good pupillary light reflex and had a relatively high sensitivity to the miotic effects of morphine. A cumulative dose of 1 mg/kg. i.v. of morphine caused marked and sustained miosis in these animals. Morphine, 1 mg, was found to have no pupillary effect by intraocular administration. Optic nerve section and cervical sympathectomy did not interfere with the miotic response in either acute or chronic preparations. Conversely, a cumulative i.v. dose of 30 mg/kg of morphine failed to cause pupillary constriction when oculomotor innervation had been interrupted. In addition, morphine, 0.2 to 0.6 mg/kg i.v., caused marked miosis in dogs whose occipital lobes or cerebral hemispheres had been removed. These findings suggest that morphine acts on a subcortical region causing constriction of the pupil. The possible location was ascertained by unit recording with microelectrodes. It was observed that pupilloconstrictor neurons in the visceral nuclei of the oculomotor nuclear complex responded to morphine, 0.2 mg/kg i.v., by increased frequency of discharge. Other neurons in the pupillary light reflex pathway showed depressed activity. Levallorphan, 0.05 mg/kg i.v., but not phenylephrine (locally applied to the conjunctival sac) antagonized all of the actions of morphine on the pupilloconstrictor neurons. The present findings demonstrate that the miosis induced by morphine is accomplished by an excitatory action of the narcotic on the visceral nuclei of the oculomotor nuclear complex.

The activity of respiratory neurons before and during panting in the cat

The effects of heating the preoptic/anterior hypothalamic (PO/AH) region on medullary respiratory neurons were studied in urethane-anesthetized, spontaneously breathing cats. The efferent phrenic nerve discharge or the pneumotachogram served as an indicator of central respiratory periodicity. In each animal, heating of the PO/AH area caused panting, defined as an increase of respiratory rate over 100 breaths per minute. During polypnea similar changes in the discharge patterns of both inspiratory and expiratory neurons were observed. There was a significant decrease in the duration of the discharge phase and the number of impulses per burst so that a reciprocal relationship existed between these parameters and respiratory rate. However, the average impulse frequency within a burst was higher during panting and could be shown to be a linear function of respiratory rate. Due to the concomitant decrease in inspiration and expiration times, the average discharge frequency per cycle time also increased in both inspiratory and expiratory medullary neurons. For continuously discharging neurons which displayed a higher frequency during the inspiration period (frequency modulated discharge), the phasic linkage remained unchanged during polypneic panting. From our results it is concluded that local heating of the PO/AH region shifts the entire respiratory system to a higher level of activity which can be correlated with ventilatory changes during panting.