Myoclonus developing after vermisectomy in photosensitive Papio papio

Reflex epilepsy in the Papio-papio baboon, particularly photosensitive epilepsy

Papio-papio baboons may present two types of reflex paroxysmal manifestations: --Myoclonia and generalized seizures are induced by intermittent light stimulation in predisposed animals; this photosensitive epilepsy resembles that observed in some human patients; it involves mainly the cerebral cortex during myoclonia which are accompanied by EEG paroxysmal discharges, and the mesencephalic reticular formation during seizures; --Myoclonia of a different type, never accompanied by EEG paroxysmal discharged and never evolving into seizures, may occur during movement or agitation of predisposed animals; these myoclonia are considered "non-epileptic" since they do not involve the cerebral cortex but probably the lower brain stem; they resemble that observed in startle disease or in some human degenerative disorders. The paper demonstrates that these manifestations constitute two different entities with clinical and electrophysiological characteristics as well as pharmacological reactivities completely different one from the other. Their "epileptic" or "non-epileptic" nature is discussed.

The cholinergic system-dependent myoclonus of the baboon Papio papio is a reticular reflex myoclonus

Neurophysiological studies were performed on four Papio papio baboons presenting with nonepileptic myoclonus (a startle response resembling stimulus-sensitive jerk). Investigations of the EEG, back-averaged EEG, and somatosensory evoked potentials revealed the absence of cortical correlates preceding the jerks, and exclusion of cerebral cortex involvement. No long-latency reflexes could be recorded in these animals. The jerks were symmetric when evoked by unilateral stimulation in normal baboons as well as in a split-brain animal. Polymyographic records showed that the first muscle involved during the jerk was the trapezius; other muscles were involved with latencies increasing in both cranial and caudal directions. From these data, nonepileptic myoclonus of baboons can be classified as a reticular reflex myoclonus. The involvement of cranial nerves did not follow the layout of the nuclei in the brainstem, indicating that the jerk is most likely generated as a complete movement. The generating structure is probably under cholinergic control. Finally, the Papio papio baboon, which was already known as a model for cortical myoclonus elicited by intermittent photic stimulation in predisposed animals, can also be considered a model for the study of the reticular reflex myoclonus.

Stimulus-sensitive myoclonus of the baboon Papio papio: pharmacological studies reveal interactions between benzodiazepines and the central cholinergic system

The baboon Papio papio develops a nonepileptic myoclonus 20 to 30 min after i.m. benzodiazepine injection. It is characterized by bilateral jerks involving mainly the neck and the trunk, by the absence of any correlative EEG paroxysmal discharge, and by its facilitation during movement or agitation. This myoclonus resembles the intention myoclonus of human patients as seen, for example, after anoxia. We found in experiments on 10 adolescent baboons that atropine alone induced the myoclonus for several hours, that physostigmine completely antagonized the benzodiazepine-induced as well as the atropine-induced myoclonus, and that the peripherally acting cholinergic antagonist, methyl-QNB, and agonist prostigmine had no action on the myoclonus, suggesting that the benzodiazepine-induced myoclonus in this species depends on a strong depression of the central cholinergic system by benzodiazepine. The benzodiazepine-induced myoclonus was mediated by benzodiazepine receptors as it was blocked by the specific benzodiazepine receptor antagonist, Ro 15-1788, which did not block atropine-induced myoclonus; latency to myoclonus after benzodiazepine was longer than after atropine. These facts suggest that benzodiazepines, by an as yet unknown mechanism, induce a depression of the cholinergic system which in turn leads to the development of myoclonus. Finally, the benzodiazepine-induced myoclonus of the baboon can be considered as a good model for testing drugs that act on the muscarinic cholinergic system and also for testing benzodiazepine-acetylcholine interactions.

Physiological sleep myoclonus in baboons

Physiological sleep myoclonus (sleep onset myoclonus and myoclonus during paradoxical sleep) was recorded from the tails of 6 baboons (2 Papio hamadryas and 4 Papio papio) and its relationship with spontaneously occurring EEG paroxysmal discharges was studied. It was observed that during light slow wave sleep tail myoclonus (sleep onset myoclonus) was almost always associated with EEG paroxysmal discharges predominating in the fronto-rolandic cortex. During deep slow wave sleep the number of EEG paroxysmal discharges decreased and tail myoclonus disappeared. In contrast, during paradoxical sleep tail myoclonus became more frequent even in the absence of cortical paroxysmal discharges. Though at present we do not know whether sleep onset myoclonus is triggered or not by cortical paroxysmal discharges, we propose that sleep myoclonus (sleep onset myoclonus and myoclonus of paradoxical sleep) is independent of these discharges but that, at a given moment, the two phenomena may be coupled by a hypothetical subcortical pacemaker.

Physostigmine antagonizes benzodiazepine-induced myoclonus in the baboon, Papio papio

The antagonism of some benzodiazepine (Bz) actions by physostigmine was investigated in 4 Papio papio baboons. As a model of these actions, the myoclonus induced in this species by clonazepam i.m. administration was used. The baboon develops, 20-30 min after Bz i.m. injection, a non-epileptic myoclonus characterized by clinical symptomatology (jerks involving mainly the neck and the trunk bilaterally), by the absence of any correlative EEG discharge, and by its facilitation during movement. This Bz-induced myoclonus resembles the intention myoclonus of human patients, as seen for example after anoxia. In the present series, the effect of physostigmine i.v. injection on the frequency of clonazepam-induced myoclonus was tested. Physostigmine produces a rapid and total abolition of the myoclonus, and this effect lasts for a period which corresponds to the pharmacological activity of physostigmine. On the contrary, atropine i.v. injection considerably increases the amount of Bz-induced myoclonus. These results allow the existence of an anticholinergic action of benzodiazepines, reversed by physostigmine, and the theory that the myoclonus would be the consequence of a cholinergic system depression to be hypothesized.

Action of suriclone in naturally and allyglycine-induced photosensitive baboons, Papio papio

Suriclone (SC) is a new anxiolytic compound with a chemical structure different from that of benzodiazepines (BZD) but SC possesses a high affinity for the so-called BZD receptors. SC was tested in the naturally or the allylglycine-treated photosensitive baboon Papio papio. As active BZD administered by the intramuscular or oral route, this new compound possesses a marked protective effect against the excessive photosensitivity of the baboons, it blocks myoclonus following EEG spike and waves discharges induced by intermittent light stimulation; as BZD again, SC can induce spontaneous myoclonus not accompanied by EEG spike and wave discharges.

Differential effects of the benzodiazepine antagonist Ro 15-1788 on two types of myoclonus in baboon Papio papio

Certain benzodiazepines (BZs) like lorazepam, diazepam or clonazepam induce myoclonus jerks in photosensitive and non-photosensitive baboons. Papio papio, which are not accompanied by EEG paroxysmal discharges (type B). The effect of the selective BZ antagonist R0 15-1788 was evaluated in this myoclonus. Ro 15-1788 completely blocked type B myoclonus without decreasing the level of vigilance in the two types of baboons, and reversed the antiepileptic action of the BZs in the photosensitive ones, permitting the reappearance of myoclonus following EEG paroxysmal discharges (type A). L-5-hydroxytryptophan and progabide also blocked type B myoclonus, but the blockade was only transiently effective and was always accompanied by slight drowsiness.

Opposite effects of lorazepam on two kinds of myoclonus in the photosensitive Papio papio

The action of lorazepam was studied in photosensitive baboons. Animals were either naturally very photosensitive or rendered photosensitive by a previous injection of allylglycine. Intravenous administration of varying doses, from 0.05 to 0.5 mg/kg, of lorazepam blocked the myoclonus induced by intermittent light stimulation in all the animals. However, in the naturally photosensitive baboon the injection of lorazepam favoured the appearance of spontaneous myoclonus with no important EEG modification. This myoclonus is different from that induced by intermittent light stimulation, which is always preceded by spike-wave cortical discharges. Lorazepam-induced myoclonus appears during the period when the animal is not photosensitive and its origin is probably in the medulla or in the brain stem.