A. Costin

A low threshold convulsive area in the rabbit's mesencephalon ☆

Abstract 1. 1. In the mesencephalic reticular formation of the rabbit, a circumscribed low threshold convulsive area (LTCA) has been dafined, which is clearly separated from the close nystagmo-genic region. Stimulation of the LTCA produces tonic-clonic convulsions, accompanied by loss of corneal and pupillary reflex. 2. 2. Seizures evoked from the LTCA produce cortical arousal, while the hippocampus and sometimes also the reticular formation respond with a synchronization pattern. 3. 3. Simultaneous stimulation of the caudate nucleus raises the convulsive threshold of the LTCA, especially for low frequencies of stimulation. Transections between the LTCA and the caudate nucleus lower the threshold. 4. 4. Small doses of chlorpromazine reduce the convulsive threshold, apparently by blocking the communication between LTCA and caudate nucleus. Large doses of the drug raise the threshold.

Nystagmus evoked by stimulation of the optic pathways in the rabbit

Abstract Horizontal nystagmus can be regularly evoked in the rabbit by electrical stimulation applied to the optic pathways or directly to the cornea. The rapid phase of the nystagmus is directed toward the eye in which the stimulated fibers take origin. The response varies with the frequency of stimulation, with an optimum at about 40 cycle/sec. The nystagmus response is enhanced by optokinetic stimulation and by side position of the head. Continuous illumination of the eyes inhibits the eye movements, while sectioning of the optic chiasm enhances them. The relation of electrical stimulation of the optic nerve to various modes of excitation of the retina is discussed.

Nystagmus evoked by intermittent photic stimulation of the rabbit's eye

Monokuläre Reizung des Kaninchens durch Lichtblitze ruft einen Nystagmus hervor, dessen Richtung identisch ist mit der Richtung der Augenbewegungen, die durch elektrische Stimulierung des ipsilateralen Nervus opticus verursacht werden. Die anhaltende Beleuchtung des kontralateralen Auges hemmt die Nystagmusreaktion.

Constancy of hippocampal afterdischarge under various conditions of stimulation

Abstract The effect of stimulation parameters on duration of afterdischarge (AD) evoked from the dorsal hippocampus has been studied in the non-anaesthetized rabbit. Repeated hippocampal stimulation of an animal on a single day frequently resulted in prolongation of AD in spite of constant conditions of stimulation. Within short series of stimulations, in which the response remained constant, the following observations were made: 1. 1. Variation of the frequency of stimulation from 10–100 c/sec resulted in AD of the same duration. 2. 2. Increase in voltage, beyond the threshold, did not substantially influence the duration of AD. 3. 3. Hippocampal discharge is observed during stimulation. Under prolonged stimulation, hippocampal discharge comes to an end while stimulation continues. The period from the beginning of stimulation to the end of the hippocampal discharge is constant and not dependent on duration of stimulation. 4. 4. In a given animal AD evoked from different points of both dorsal hippocampi i of the same duration. 5. 5. The duration of hippocampal AD appears to be independent of stimulation parameters over a wide range. It is suggested that this may be a general characteristic of epileptic afterdischarge.

Seizure activity evoked by implantation of ouabain and related drugs into cortical and subcortical regions of the rabbit brain

Abstract Ouabain, digitoxin, ethacrynic acid and mercuric acetate, all of which block active sodium transport by inhibition of membrane ATPases, were implanted into various regions of the rabbit brain. The epileptogenic effect of these compounds was measured by electroence-phalographic recordings from various subcortical and cortical structures. Application of the drugs to the hypothalamus evoked seizure activity first in the hippo-campus and later in other subcortical regions and in the cortex. Conversely, implantation into the hippocampus elicited epileptic discharges first in the hypothalamus and later in other parts of the brain. The sensitivity to ouabain varied greatly from one region to another. The dorsal hippocampus responded to doses of 1 μg or less with spiking activity; in the thalamus, lateral geniculate body and caudate nucleus, 1–2 μg of the glycoside were necessary to evoke hippocampal and cortical seizures; however, in the mesencephalic reticular formation and in the frontal lobe, only doses of 50 μg were able to cause hippocampal and cortical discharges. On the other hand, the lethal effect of ouabain was about the same when applied to various subcortical structures ( ld 50 , about 50 μg), but in the frontal lobe only much larger doses proved fatal. Deposition of ouabain in solid form represents a useful method for the study of intracerebral connections. The present results suggest that in the rabbit brain, both the lateral geniculate body and the caudate nucleus are linked to the contralateral hippocampus.

NYSTAGMUS EVOKED FROM THE SUPERIOR COLLICULUS OF THE RABBIT.

Abstract From the superior colliculus of the rabbit, contraversive horizontal nystagmus can be evoked only under the influence of enhancing factors, such as exclusion of light, simultaneous optokinetic stimulation or application of subnarcotic doses of pentobarbitone. It is concluded that the superior colliculus forms part of the pathway of central as well as optokinetic nystagmus. Stimulation of the mesencephalic reticular formation enhances both central and optokinetic nystagmus. This effect is unspecific, i.e., direction-independent.

Interaction of central and flash nystagmus

Abstract Central nystagmus is evoked in the rabbit by electrical stimulation of one optic pathway. The response is strongest in the dark and is depressed by illumination of the heterologous retina. Flash nystagmus results from intermittent photic stimulation of one eye. Combination of central nystagmus with synergistic flashing potentiates the response markedly. Combination with antagonistic flashing at a rate of 25–30/sec depresses central nystagmus strongly, but at 5–10/sec it breaks the inhibitory effect of illumination on central nystagmus and may raise the response to the level of the dark control.

Influence of low-rate flashing on optic nystagmus

Abstract In the rabbit, low-rate flashing (5–10/sec) of one eye enhances the response, evoked from the heterologous optic pathway by flashing, optokinetic or electrical stimulation. Central nystagmus, elicited by the third form of excitation, is improved by intermittent photic stimulation of either retina at low frequencies, but greater enhancement is obtained by flashing of the heterologous eye. It is assumed that low-rate flashing exerts essentially an unspecific, i.e., non-directional effect.

Effect of cerebellar stimulation on central nystagmus

Abstract The effect of cerebellar stimulation on central nystagmus, evoked from the mesodiencephalon of the rabbit was studied. Inhibition of central nystagmus was found upon stimulation of superficial layers in the rostral part of the vermis. The inhibition was especially evident at low voltage and frequencies (1–10 cycle/sec) of cerebellar stimulation. Stimulation of the caudal part of the vermis or deeper structures in its anterior half as well as stimulation of the lateral lobes of the cerebellum usually resulted in enhancement. Stronger cerebellar stimulation (higher voltage and frequencies of 40–100 cycle/sec) was required for enhancement than for inhibition of central nystagmus. The similarity of cerebellar influence on central nystagmus and on motor reflexes, and the dependence of inhibition and facilitation on parameters of stimulation are discussed.

Influence of lesions in the lateral geniculate body of the rabbit on optic nystagmus

Abstract Division of a single optic nerve of the rabbit does not induce eye movements. However, lesions in a single lateral geniculate body (LGB) cause spontaneous nystagmus toward the injured side. The influence of side position of the head proves that these eye movements are due to stimuli passing along the intact visual pathway. Electrical stimulation of either optic nerve now evokes nystagmus to the same side, viz., toward the operated LGB, in contrast to the responses of the intact rabbit. The spontaneous eye movements cease after a few days. Subsequent injury to the second LGB causes spontaneous nystagmus toward the second lesion. We conclude that the suppression of spontaneous eye movements either in the intact animal or after unilateral damage to the LGB is brough about by mutual central inhibitory processes.