Hypoglossal and phrenic responses to cholinergic agents applied to ventral medullary surface

Application of cholinergic agents on the ventrolateral surface of the medulla in areas in which the "central chemoreceptors" are believed to be located stimulates breathing. It is also known that cranial nerves, such as the hypoglossal, have respiratory modulated activity (greater in inspiration than expiration) that responds to many of the same stimuli which affect breathing. In the present study we compared effects of cholinergic agents (acetylcholine, carbachol, methacholine, eserine) directly applied to chemoreceptive areas on the ventral medullary surface on phrenic and hypoglossal nerve activity. Studies were performed in paralyzed, anesthetized, and artificially ventilated cats after vagotomy. All cholinergic agents increased hypoglossal activity significantly more than phrenic activity in animals ventilated with O2 or 7% CO2 in O2 whether or not the carotid sinus nerves were intact. Atropine applied topically to the same medullary area blocked the respiratory effects of locally administered acetylcholine. The results suggest that cholinergic agents applied centrally can increase both phrenic and hypoglossal activity. Moreover the effects of the drugs are relatively greater on XII nerve activity than on phrenic discharge, suggesting that the drive from medullary cholinergic structures is distributed with unequal weight to different respiratory motoneurons.

Responsiveness of hypoglossal neurons to testosterone in pre-pubertal rats

We have shown previously that administration of testosterone propionate (TP) to adult rats accelerated regeneration of the hypoglossal nerve. This study was made to determine whether TP had a similar effect in pre-pubertal rats, in order to establish the ontogeny of the responsiveness of the hypoglossal neurons to androgenic stimulation. It was found that TP significantly shortened the time course of the regeneration of the hypoglossal nerve in the 4-weeks old but not in the 2- and 3-weeks old rats; and that males of 4-weeks of age responded to TP a few days earlier than females of the same age. The possible causes of the failure of the hypoglossal neurons of younger ages to respond to TP were discussed.

Anesthesia selectively reduces hypoglossal nerve activity by actions upon the brain stem

Alcohol as well as anesthetic and sedative agents depress the respiratory activity of the hypoglossal-genioglossal system more than that of the phrenic-diaphragmatic system. The mechanisms for this selective depression remain unclear. To evaluate the contribution of pathways traversing the spinal cord, the response of phrenic and hypoglossal nerve activities to 0.5% halothane was obtained in decerebrate cats before and after transection of the spinal cord at T1 and again following transection at C1. Halothane produced a much greater decrease in hypoglossal than phrenic activity before and after spinal cord section at T1. Following cord section at C1, which eliminates phrenic activity, 0.5% halothane still produced a marked depression of hypoglossal activity. Therefore, the selective depression of the hypoglossal-genioglossal system does not depend on spinal mechanisms and appears to be mediated in the brain stem.

Selective depression by ethanol of upper airway respiratory motor activity in cats

We studied the effects of systemically administered ethyl alcohol on the respiratory motor activity of the phrenic, hypoglossal and recurrent laryngeal nerves in unanesthetized, decerebrate cats. Some of the cats were studied after carotid sinus nerve section. In addition, parallel studies were done in intact, awake cats with chronic electromyographic electrodes in the diaphragm, genioglossus, and posterior cricoarytenoid (PCA) muscles. In decerebrate animals, alcohol induced a significant reduction of hypoglossal and recurrent laryngeal nerve activities at doses that had little or no effect on the phrenic nerve discharge. Similar changes were observed in chemodenervated cats. In awake animals, genioglossal and PCA muscle activities were depressed by alcohol, whereas diaphragm activity showed no consistent change. Alcohol caused a significant increase in respiratory frequency in awake cats and reduced the responses of genioglossal and PCA muscle activities to hypercapnia and normocapnic hypoxia. We conclude that alcohol induces a selective reduction in upper airway respiratory motor activity by an action that does not require intact suprapontile structures, vagal afferents, or peripheral chemoreceptors. This reduction may contribute to the alcohol-induced exacerbation of obstructive sleep apnea.

Effects of dopamine, isoproterenol, and lobeline on cranial and phrenic motoneurons

Recent studies have suggested that the upper airway muscles receive a substantial portion of their excitatory input from the peripheral chemoreceptors. We examined the responses of the phrenic, hypoglossal, and recurrent laryngeal nerves to agents that are known to modify carotid body activity. Dopamine, an inhibitor, and isoproterenol and lobeline, two stimulators of carotid body activity, were administered into the lingual arteries of 14 anesthetized, paralyzed cats artificially ventilated with 100% O2. Dopamine decreased the activity of all three nerves (P less than 0.001) but inhibited the hypoglossal nerve more than the phrenic and recurrent laryngeal nerves (P less than 0.001). Isoproterenol and lobeline increased the activity of all three nerves (P less than 0.001) but stimulated the hypoglossal nerve more than the phrenic and recurrent laryngeal nerves (P less than 0.001). These relatively greater effects of all three agents on the hypoglossal nerve compared with the phrenic and the recurrent laryngeal nerves persisted under both hypoxic and hypercapnic conditions. After midcervical vagotomy, the responses of the hypoglossal to all three drugs remained larger than those of the phrenic nerve. After combined vagotomy and carotid sinus nerve section, the stimulatory effects of lobeline and the inhibitory effects of dopamine were no longer apparent. We conclude that it is possible to preferentially alter respiratory-related activity of the hypoglossal nerve compared with that of the phrenic nerve using agents that modify peripheral chemoreceptor activity.

Respiratory-related hypoglossal nerve activity: influence of anesthetics

In decerebrate, vagotomized, paralyzed, and ventilated cats, phrenic and respiratory-related hypoglossal discharges were evident at normocapnic normoxia or hyperoxia. Both increased progressively in hypercapnia or hypoxia. With increasing drive, onset of inspiratory hypoglossal activity began earlier relative to phrenic onset; an early expiratory hypoglossal burst was also observed. Following subanesthetic doses of chloralose, halothane, ketamine, or pentobarbital, hypoglossal activity was depressed much more than phrenic discharge. In moderate hypercapnia or hypoxia, phrenic activity increased more than hypoglossal, whereas, at high drive, the latter rose more sharply in some cats. Electromyograms of the diaphragm and genioglossus were recorded in intact awake cats to determine if their responses and those of decerebrates are comparable. Respiratory-related genioglossal discharge was evident in normocapnia. We conclude that anesthesia suppresses hypoglossal motor activities much more than those of the bulbospinal-phrenic system. Data for decerebrate cats and unanesthetized cats or humans provide no evidence of a differential distribution of chemoreceptor afferents on hypoglossal and bulbospinal-phrenic neurons, as suggested by results in anesthetized animals.

Role of electrical activity in axotomy-induced increased glucose use

Glucose use as demonstrated by 2-deoxyglucose autoradiography increases in the hypoglossal nucleus after axotomy of the hypoglossal nerve. There are several possible explanations for this increase, including increased energy metabolism to support increased protein, RNA, and lipid synthesis during regeneration or, alternatively, increased electrical activity associated with the site of nerve transection or alterations in the neurolemma. To evaluate the possible role of electrical activity, we recorded spike activity in the hypoglossal nucleus after axotomy and evaluated glucose uptake in the hypoglossal nucleus after xylocaine application to the proximal nerve stump. There was no evidence from these experiments that an increased number of action potentials is responsible for the increased glucose use after axotomy.

Selective inhibition of peripheral cholinergic transmission in the cat produced by AF64A

The neuropharmacologic effects of ethylcholine aziridinium ion, AF64A, were studied in cats, using various physiological techniques, to ascertain its synaptic site of action and to determine whether it may act as a cholinergic specific neurotoxin in vivo. Nictitating membrane contractions elicited by preganglionic nerve stimulation (1-16 Hz) were diminished in a dose-dependent manner after injection of AF64A into the carotid artery. Contractions due to injection of l-norepinephrine, tetramethylammonium or acetylcholine were not changed. Postganglionic action potentials from the superior cervical ganglion evoked by preganglionic stimulation were also abolished by AF64A, whereas the postganglionic firing produced by tetramethylammonium was unchanged. Neither the nictitating membrane nor ganglionic responses on the contralateral side of the animal were affected by AF64A treatment. In the same animals, twitch tension in the tongue produced by stimulation of the ipsilateral hypoglossal nerve (1-16 Hz) was gradually reduced and in most experiments completely blocked by AF64A. Repetitive stimulation of either the autonomic or somatic nerves at high frequencies (greater than 10 Hz) magnified and accelerated the onset of neurotoxic effects of AF64A. The suppression of ganglionic and neuromuscular transmission by AF64A was irreversible during the course of the experiments (12-18 hr). From these results, we can conclude that AF64A produces in the peripheral nervous system a longlasting inhibition of cholinergic transmission, without interfering with adrenergic transmission. Moreover, because AF64A did not block the postganglionic responses elicited by cholinergic nicotinic and muscarinic agonists, the inhibitory effects of AF64A must be mediated by a presynaptic action on cholinergic nerve terminals.

Acceleration of the regeneration of the crushed hypoglossal nerve by testosterone

The effect of testosterone propionate (TP) on regeneration of the crushed hypoglossal nerve was studied in young adult rats. The TP treatment promoted axonal outgrowth as measured by an increase in the proportion of hypoglossal neurons labeled by retrograde transport of horseradish peroxidase injected into the tongue at selected postlesion periods compared with that in controls. The result indicated that TP exerts its effect on regeneration primarily by increasing the metabolic activity in hormone target neurons. The somatotopic organization of the hypoglossal nucleus and its projection into the major nerve branches were unaltered subsequent to crush injury. The maintenance of specificity was attributed to the intact perineurial and endoneurial sheaths which guided regenerating fibers to their original targets.

Dopaminergic mechanisms in the nucleus tractus solitarius and effects on blood pressure

The actions of dopamine (DA) agonists and antagonists upon spontaneously active neurons in the nucleus tractus solitarius (NTS), dorsal motor nucleus of the vagus (nX), and the nucleus nervi hypoglossi (nXII) were studied. DA was applied microiontophoretically to 42 neurons within the NTS and nX. Sixteen of these cells were stimulated by DA and 9 neurons were depressed. In neurons localized in the nXII nucleus, DA inhibited 19 cells out of 23. Noradrenaline (NA) excited 11 out of 30 cells in the NTS and nX. In the area of nXII nucleus, NA inhibited 6 out of 19 cells and did not modify 11 cells. Acetylcholine (ACh) stimulated the firing of 18 out of 22 neurons. The effects on NA and DA were directly compared on 21 neurons in the NTS. On 16 cells of this group the responses to DA were quantitatively or qualitatively different from those to NA. The DA receptor blockers sulpiride and fluphenazine antagonized the effects of DA but not those due to NA or ACh. Bilateral microinjections of DA (50 nmole) in the area of the NTS induced an increase in systemic blood pressure and heart rate. ACh induced similar effects. NA in the NTS decreased blood pressure and produced bradycardia. The effects of DA on heart rate and blood pressure were blocked by sulpiride or fluphenazine. The cardiovascular role played by DA in the NTS is discussed.

Effect of vasopressin and oxytocin perfusion of the cerebral ventricles on evoked tongue jerks

The effect of increased concentration of vasopressin and oxytocin in the cerebrospinal fluid on the excitability of the hypoglossal nerve nucleus was investigated. The experiments were carried out on rats under chloralose anaesthesia. Retractory jerks of the outstretched tongue were evoked by supra- or infraorbital nerve stimulation during perfusion of the cerebral ventricles with McIlwain-Rodnight solution. The solution contained synthetic arginine vasopressin 0.05 U/ml or synthetic oxytocin 0.05 U/ml. Perfusion of the ventricles with vasopressin increased and perfusion with oxytocin decreased the evoked tongue jerks.

Ultrastructural responses of the hypoglossal nucleus to the presence in the tongue of botulinum toxin, a quantitative study

The ultrastructural effects of local injection of botulinum toxin into the left half of the tongue of the rat, were studied quantitatively 35 days postoperatively in the left hypoglossal nucleus. The results showed (1) a decrease in somatic and neuropil bouton numbers because of loss of boutons with symmetrical synapses and clear spherical synaptic vesicles, (2) a decrease in the numbers of dendrite profiles in the neuropil, (3) an increase in the proportion of dendrites and boutons with unusual inclusions, suggestive of profile retraction, (4) an increase in the proportion of profiles which were unusually electron-dense, (5) an increase in the amount of astrocyte, and a growth of astrocyte sheaths around bouton-free neurone surfaces, (6) the presence of occasional microglia, and (7) subastrocytic subsurface cisterns. Control rats injected with boiled toxin had no responses except (3) and (4) above, and then only to a modest extent, possibly due to mechanical damage of a few axons or terminals at the time of injection, or to insufficient inactivation of the toxin by boiling. The results were compared with those at 35 days after axotomy, and it was concluded that botulinum toxin, which interrupts neuromuscular transmission, elicits the same responses in the hypoglossal neurones, as does transection of the hypoglossal nerve, even though earlier studies had discovered no glial replication after botulinum toxin, in contrast to axotomy.

[Reccurrent lingual hemiatrophy]

The author reported a 45-year-old woman with numerous developmental anomalies in whom lingual hemiatrophy preceded by strong occipital headaches followed tonsillectomy. After treatment considerable regeneration of tongue nuscle took place. The course of the disease was recurrent. In the differential diagnosis the author considered inflammatory processes, neoplastic diseases, developmental anomalies and toxic effect of procaine used in tonsillectomy as well as the possibility of deeply situated adhaesions in the vicinity of the hypoglossal nerve.

Muscle enzymatic changes induced by blockage of axoplasmic transport

The activity of malic dehydrogenase, pyruvic kinase, and phosphorylase b was measured in the geniohyoid muscle of the cat after injection of 10 10 mM colchicine into the hypoglossal nerve. Experiments performed 1-60 days after the injection showed that the activity of the three enzymes gradually decreased (day 4-5), reached a maximum fall (day 10-25), and subsequently returned to control values (day 30-60). Concomitantly to these enzymatic alterations, the muscles showed fibrillation and ACh hypersensitivity; however, in contrast to denervation, the drug had no effect on nerve conduction, effective neuromuscular transmission, and ultrastructure of motor end plates. Experiments with [3H]colchicine indicated that the observed changes were brought about by the drug acting directly on the motor axons rather than on the muscle cells. The transsynaptic effects induced by colchicine treatment to the nerve can be ascribed to a temporary interruption of axoplasmic transport. It is suggested that neurotrophic regulation of some muscle-soluble enzymes partly depend on the normal operation of the axoplasmic transport system.

Stereochemical preferences for curarimimetic neuromuscular junction blockade I: enantiomeric monoquaternary amines as probes

Seven pairs of monoquaternary enantiomeric neuromuscular junction blocking agents were prepared in which the carbon asymmetry is adjacent to the quaternized nitrogen moiety. The tertiary amines from which the blocking species were obtained are carnegine, laudanosine, N-methylpavine, corydine, isocorydine, glaucine, and boldine. Curarimimetic potencies, obtained with an in vivo cat tongue-hypoglossal nerve preparation, were obtained for the enantiomeric methiodides of each of these amines. Possible contributions to activity be preferential binding to blood components or by selective inhibition of acetylcholinesterase also were studied. The combined studies indicate that there is a modest preference by the neuromuscular junction of the cat for monoquaternary blockers with the (s)-confirguation.

Inhibition of hypoglossal motoneurons by stimulation of the jaw-opening muscle afferents

The synaptic linkage of jaw-opening muscle afferents with the retractive and the protrusive motoneurons (R-Mns and P-Mns) in the hypoglossal motor nucleus was explored in anesthetized cats. It was found that stimulation of low-threshold afferents arising in the jaw-opening muscles (m. mylohyoideus and m. digastricus) evoked IPSPs in both R-Mns and P-Mns with about 10 to 15 msec latencies. By injecting Cl ions into R-Mns or P-Mns, the hyperpolarizing postsynaptic potentials evoked by jaw-opening muscle afferent impulses were reversed to depolarizing potentials. The lingual-IPSPs of hypoglossal motoneurons were concluded to be dependent on increase of Cl conductance. Stimulation of low threshold lingual afferents evoked a reflex discharge in the retractive nerve fibers with a 6.0 msec latency and in the protrusive nerve fibers with a 5.5 msec latency. Inhibition of the linguohypoglossal reflex by stimulation of the jaw-opening muscle afferents was completely removed by an intravenous injection of strychnine (0.1 mg/kg) or picrotoxin (6 mg/kg). It is concluded that the low threshold afferents arising in the jaw-opening muscles are connected to both R-Mns and P-Mns and P-Mns through inhibitory synapses via a polysynaptic pathway.

Stereochemical preferences for curarimimetic neuromuscular junction blockade II. enantiomeric bisquaternary amines as probes

Two pairs of bisquaternary enantiomeric neuromuscular junction blocking agents as well as their diasteriomeric mesoforms were prepared in which the carbon asymmetry is adjacent to the quaternary center. The tertiary amines from which the blocking species were obtained by methyl iodide treatment were N-methylpavine and 1,1'-dodecamethylenebis(6,7-dimethoxy-2-methyl-1,2,3,4-tetrahydroisoquinoline). Blocking potencies were determined by the mouse inclined screen assay and by the cat tongue-hypoglossal nerve technique. The mouse assay showed no statistical difference between the enantiomeric probes derived from N-methylpavine and only a modest superiority of the (R-R) isomer over the (S-S) isomer in the case of the tetrahydroisoquinoline compounds. The cat assay showed a modest statistically significant (R-R) greater than (S-S) difference in potencies in both kinds of probes. The diastereomeric meso-compounds were less active than the enantiomers in mice but were of intermediate activity in the cat determination. Acetylcholinesterase-inhibiting activity was determined for each probe to discount potency differences from this source, and no significant differences in blocking potency attributable to preferential enzyme inhibition by the probes were noted.

Sterochemical preferences for curarimimetic neuromuscular junction blockade III: enantiomeric bisquaternary amines related to benzoquinonium as probes

Enantiomeric neuromuscular junction blocking agents, which are of the benzoquinonium type but which have a methyl group introduced adjacent to the quaternary moieties to provide an asymmetric center were synthesized and tested to determine whether the neuromuscular junction exhibits the relatively modest (R) greater than (S) superiority shown toward previously tested bisquaternaries. Testing included a mouse inclined screen assay and an in vivo cat hypoglossal nerve-tongue preparation, as well as standard estimations of anticholinesterase activity since the candidate compounds are known to have such a component in their activity spectrum. The observed 2:1 difference in blocking activity favoring the compound with an (R)-configuration is the same as that for previously tested bisquaternaries, both in direction and magnitude. Furthermore, it cannot be accounted for by preferential inhibition of acetylcholinesterase by the (S)-enantiomer. Absolute configurations of the enantiomers were assigned on the basis of comparison with compounds of known configuration.

The effect of injury on two hydrolases in the hypoglossal nucleus, with quantitative data on N-acetyl-beta-glucosaminidase

N-acetyl-beta-glucosaminidase and acid phosphatase were studied histochemically in sections of hypoglossal nuclei of normal adult male albino rats, and rats from 1 to 56 days after axotomy of the left hypoglossal nerve, or after injection of botulinum toxin into the left side of the tongue. Both axotomy and botulinum caused a diminution of neurone perikaryal glucosaminidase reaction product on the operated side, but an increase of reaction product elsewhere in the operated nucleus. Whereas the effect of axotomy lasted from 5 to 42 days post-operatively, that of botulinum ceased between 28 and 42 days. Axotomy and botulinum both caused an increase of neuronal acid phosphatase (after a short incubation time) from 2 to 21 days post-operatively in injured nuclei. Glucosaminidase reaction product measured microdensitometrically in isolated cells showed a statistically significant diminution in injured neurones at 18 days after axotomy. There was no change in the amount of reaction product per cell in pericytes, astrocytes, or non-astrocyte glia. Sections scanned quantitatively showed that normal and operated nuclei had a similar amount of reaction product, thus the neuronal diminution must be balanced by the increase elsewhere, either in the elevated numbers of non-astrocyte glia, or in the neuropil.