Arthur S. Foutz

Cholinergic mechanisms and cataplexy in dogs.

Abstract Narcolepsy is a disabling neurological disease characterized by excessive daytime somnolence and sudden attacks of partial or complete flaccid paralysis called cataplexy. The disease is known to affect humans as well as dogs. Nineteen dogs diagnosed as narcoleptic were used in this study, which utilized the food-elicited cataplexy test. This test is based on the cataplexy-eliciting effect of food. The results of this study showed that the anticholinesterase physostigmine salicylate (0.05 mg/kg i.v.) and the muscarinic cholinomimetic arecoline hydrochloride (0.15 mg/kg s.c.) significantly increased the amount of cataplexy. Two muscarinic blockers, atropine sulfate (0.1 mg/kg i.v.) and scopolamine hydrobromide (20 μg/kg i.v.), were both effective in significantly reducing the amount of cataplexy. Neostigmine (0.05 mg/kg i.v.), atropine methylnitrate (0.1 mg/kg i.v.), and scopolamine methylnitrate (20 μg/kg i.v.), which do not penetrate the bloodbrain barrier, were ineffective. Nicotine (0.03 mg/kg i.v.) and the nicotinic blocker mecamylamine (0.3 and 1 mg/kg i.v.) were also ineffective. The results of this study suggest that central muscarinic cholinergic receptors are critically involved in the mechanism which produces the motor inhibition of cataplexy.

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.

Canine model of narcolepsy: Genetic and developmental determinants

Abstract Genetic and developmental factors were investigated in 101 cases of canine narcoleps-cataplexy syndrome. An experimental breeding program documented a probable autosomal recessive mode of inheritance in two dog breeds, Doberman pinschers and Labrador retrievers, and in crosses between those breeds. Age of onset of cataplexy and severity of symptoms were remarkably similar in 73 subjects with the genetically transmitted form of narcolepsy. Efforts to demonstrate genetic factors by breeding affected poodles and beagles were unsuccessful. Furthermore, a positive family history could not be documented in these animals or in narcoleptic dogs of 13 other breeds. Both age of onset and severity of symptoms were highly variable within this population. We suggest there may be multiple etiologies in canine narcolepsy syndrome: (i) inheritance via a single autosomal recessive gene, which appears to be identical in at least two unrelated dog breeds; (ii) nongenetic mechanisms which may include developmental or traumatic accidents. Preliminary findings of parallel neurochemical studies in narcoleptic dogs indicate distinct abnormalities in central neurotransmitter concentration and turnover. We theorize that these neurochemical abnormalities may result from inherited disorders of synthesis and/or metabolism which are present from birth or from nongenetic neurochemical abnormalities appearing later in life. The observation that symptom characteristics are identical in all affected dogs suggests that a common neurochemical disorder may underlie both genetic and nongenetic variations of the disease.

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.

Respiratory effects of the N-methyl-D-aspartate (NMDA) antagonist, MK-801, in intact and vagotomized chronic cats.

The effects on respiration of MK-801, an N-methyl-D-aspartate (NMDA) non-competitive antagonist, were studied in awake chronic cats by means of the plethysmographic technique. MK-801 (0.01-3.0 mg/kg) was first given i.v. in cumulative doses. The protocol was repeated 10-15 days later in the same animals after bilateral vagotomy. MK-801 selectively increased the duration of inspiration, causing an apneustic respiration but had no effect on the duration of expiration. The maximal inspiratory duration brought about by MK-801 in the intact cat (4.3 s; control 0.9 s) increased 4-fold after bilateral vagotomy (16.4 s; control 1.7 s). Such results suggest that the termination of the inspiratory phase in normal awake cats results from an interaction of pulmonary vagal afferent inputs (inactive on NMDA receptors) with a central mechanism in which NMDA-type glutamate receptors are activated by dicarboxylic amino acid neurotransmission.

Respiratory survival mechanisms in acetylcholinesterase knockout mouse

Cholinergic neurotransmission ensures muscle contraction and plays a role in the regulation of respiratory pattern in the brainstem. Inactivation of acetylcholinesterase (AChE) by organophosphates produces respiratory failure but AChE knockout mice survive to adulthood. Respiratory adaptation mechanisms which ensure survival of these mice were examined in vivo by whole body plethysmography and in vitro in the neonatal isolated brainstem preparation. AChE−/− mice presented no AChE activity but unaffected butyrylcholinesterase (BChE) activity. In vivo, bambuterol (50–500 µg/kg s.c.) decreased BChE activity peripherally but not in brain tissue and induced apnea and death in adult and neonate AChE−/− mice without affecting littermate AChE+/+ and +/− animals. In vitro, bath‐applied bambuterol (1–100 µm) and tetraisopropylpyrophosphoramide (10–100 µm) decreased BChE activity in the brainstem but did not perturb central respiratory activity recorded from spinal nerve rootlets. In vitro, the cholinergic agonists muscarine (50–100 µm) and nicotine (0.5–10 µm) induced tonic activity in respiratory motoneurons and increased the frequency of inspiratory bursts in AChE+/+ and +/− animals. These effects were greatly attenuated in AChE−/− animals. The results suggest that, in mice lacking AChE, (i) BChE becomes essential for survival peripherally but plays no critical role in central rhythm‐generating structures and (ii) a major adaptive mechanism for respiratory survival is the down‐regulated response of central respiratory‐related neurons and motoneurons to muscarinic and nicotinic agonists.

The bulbar network of respiratory neurons during apneusis induced by a blockade of NMDA receptors

SummaryOur aim was to study the mechanisms producing the transition from the inspiratory phase to the expiratory phase of the breathing cycle. For this purpose we observed the changes affecting the discharge patterns and excitabilities of the different types of respiratory neurons within the respiratory network in cat medulla, after inducing an apneustic respiration with the N-methyl-D-aspartate (NMDA) antagonist MK-801 given systemically. Respiratory neurons were recorded extracellularly through the central barrel of multibarrelled electrodes, in the ventral respiratory area of pentobarbital-anesthetized, vagotomized, paralyzed and ventilated cats. Inhibitions exerted on each neuron by the presynaptic pools of respiratory neurons were revealed when the neuron was depolarized by an iontophoretic application of the excitatory amino-acid analogue quisqualate. Cycle-triggered time histograms of the spontaneous and quisqualate-increased discharge of respiratory neurons were constructed in eupnea and in apneusis induced with MK-801. During apneustic breathing, the activity of the respiratory neuronal network changed throughout the entire respiratory cycle including the post-inspiratory phase, and the peak discharge rates of all types of respiratory neurons, except the late-expiratory type, decreased. During apneusis, the activity of the post-inspiratory neuronal pool, the post-inspiratory depression of other respiratory neurons, and the phrenic nerve after-discharge were reduced (but not totally suppressed), whereas the discharge of some post-inspiratory neurons shifted into the apneustic plateau. The shortened post-inspiration (stage 1 of expiration) altered the organization of the expiratory phase. Late-expiratory neurons (stage 2 of expiration) discharged earlier in expiration and their discharge rate increased. The inspiratory on-switching was functionally unaffected. Early inspiratory neurons of the decrementing type retained a decrementing pattern followed by a reduced discharge rate in the apneustic plateau, whereas early-inspiratory neurons of the constant type maintained a high discharge rate throughout the apneustic plateau. Inspiratory augmenting neurons, late-inspiratory and “offswitch” neurons also discharged throughout the apneustic plateau. During the apneustic plateau, the level of activity was constant in the phrenic nerve and in inspiratory neurons of the early-constant, augmenting, and late types. However, progressive changes in the activity of other neuronal types demonstrated the evolving state of the respiratory network in the plateau phase. There was a slowed but continued decrease of the activity of early-inspiratory decrementing neurons, accompanied by an increasing activity and/or excitability of “off-switch”, postinspiratory and late-expiratory neurons. In apneusis there was a decoupling of the duration of inspiration and expiration. The variability of inspiratory duration increased five-fold whereas the variability of expiration was unchanged. We conclude that in the apneustic state, (1) inspiratory on-switching and the successive activation of the different inspiratory neuronal types are preserved; (2) near the end of the inspiratory ramp, the reversible phase of inspiratory off-switching is prolonged, producing the apneustic plateau, and (3) the irreversible phase of offswitching is impaired by a reduced activity of postinspiratory neurons. These results support the 3-phase model of respiratory rhythm generation, in which key roles are played by early-inspiratory and post-inspiratory neurons.

Combined blockade of NMDA and non-NMDA receptors produces respiratory arrest in the adult cat.

We studied the effects of the non-NMDA antagonist NBQX and of the NMDA antagonist dizocilpine (MK-801), administered separately or together, on the respiratory function of conscious cats. NBQX (20 mg kg-1) did not affect minute ventilation nor the timing of inspiratory and expiratory phases, but the addition of a small dose of dizocilpine (0.15 mg kg-1) induced inspiratory pauses and respiratory arrest in the inspiratory phase (apneusis). Similarly, larger doses of either NBQX or dizocilpine did not induce apneusis but the addition of a small dose of the other compound provoked an apneusis. Thus, a blockade of either non-NMDA or NMDA receptors is well tolerated, but the combined blockade of both receptor types severely disrupts the respiratory function in the cat.

Brainstem neurons projecting to the rostral ventral respiratory group (rVRG) in the medulla oblongata of the rat revealed by co-application of NMDA and biocytin

Groups of neurons in the medulla and pons are essential for the rhythm generation, pattern formation and modulation of respiration. The rostral Ventral Respiratory Group (rVRG) is thought to be a crucial area for rhythm generation. Here we co-applied biocytin and NMDA in the rVRG to label retrogradely brainstem neurons reciprocally connected to a population of inspiratory neurons in the rat rVRG. The procedure excited rVRG neurons in multi-unit recordings and led to a Golgi-like labelling of distant cells presumably excited by efferents from the rVRG. Injection of biocytin without NMDA did not label neurons in distant structures. Several brainstem ipsi- and contralateral structures were found to project to the rVRG, but three major respiratory-related structures, the nucleus of the solitary tract (NTS), the parabrachialis medialis and Kölliker-Fuse nuclei (PB/KF) and the caudal VRG, which are known to project bilaterally to the rVRG, were exclusively labelled ipsilaterally, suggesting an ipsilateral excitation of these structures by the rVRG. The pathways of efferent axons from labelled neurons in the rVRG were traced rostrally towards the pons and caudally to the spinal cord. Terminal axonal arborizations were seen in the same regions where retrogradely filled neurons were found as well as in a few other motor nuclei (the dorsal vagal motor nucleus and XII nucleus). Moreover, in the NTS and the PB/KF, efferent terminal varicosities were seen closely apposed to the soma and proximal dendrites of labelled neurons, suggesting monosynaptic connections between the rVRG and these nuclei.

Sleep cycle organization in narcoleptic and normal dogs.

Abstract The normal sleep-wake patterns of four narcoleptic and four normal dogs were recorded electrographically for 48 hours in a laboratory setting. The amount of the various stages of vigilance (wake, light slow wave sleep, deep slow wave sleep and REM sleep) were similar for both sets of dogs except that narcoleptic dogs has significantly less REM sleep and had an additional state scored as cataplexy. Mean episode durations and the number of episodes per 48 hr were not significantly different except that narcoleptic dogs had fewer REM episodes and a trend toward more waking episodes. There was a significant difference in the state progression between normal and narcoleptic dogs before and after REM episodes. Narcoleptic dogs had an increase in LSWS and a decrease in DSWS during the last 5 min before REM sleep onset in comparison to the reverse pattern in controls. In the stages following REM there was a marked elevation of wakefulness in narcoleptic dogs. These data support the notion that narcolepsy is a sleep disorder characterized by a disruption of the normal sleep-wake cycle. These results parallel similar findings in humans and provide strong evidence that this disease entity is similar in man and dog.