Jehuda Gutman

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.

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.

Blood Pressure Responses to Electrical Stimulation of Peripheral Nerves and their Modification by Nembutal

Abstract1. Blood pressure changes, evoked by stimulation of peripheral nerves, decline if the frequency of stimulation passes beyond the optimal range. Instability of a response develops progressively and is ascribed to the increasing influence of inhibitory components in a given circuit.2. Unstable reactions can be converted into sustained responses by two methods: a by intermittent stimulation, which prevents the inhibitory effects to summate to their threshold value; and b by application of barbiturate, which produces selective block of inhibitory transmission.3. With graded doses of nembutal, a double reversal of the blood pressure response to sciatic stimulation can be produced. This phenomenon is interpreted on the basis of the complex composition of the nerve, which sends fibres to autonomic centres in the hypothalamus and the medulla.

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.

Central nystagmus under continuous and intermittent stimulation

Abstract Central nystagmus is produced by stimulation in the nystagmogenic area, situated in the mesodiencephalon of the rabbit. The response depends on the frequency of stimulation: Beyond the optimal frequency range the reaction is not sustained, but declines. Intermittent stimulation with appropriate spacing prevents this decay. Afternystagmus varies not only with the frequency of stimulation, but also with the length of the stimulation period: The after-reaction decreases if stimulation is extended beyond its optimal span. These observations can be explained by the assumption that stimulation in the nystagmogenic area activates both excitatory and inhibitory components of the nystagmus circuit, temporal summation of the inhibitory part displaying a lag relative to the excitatory component. The divergence in the time course of the two contributions to the nystagmus response may be accounted for by anatomical or functional differences.

Comparison of the influence of graded doses of phenothiazines and pentobarbitone on central nystagmus.

Abstract The effect of phenothiazines on central nystagmus depends on dosage. Small amounts (10–25 μg/kg) depress the response, while doses above 100 μg per kilogram produce a biphasic reaction, i.e., enhancement for 1 or 2 hours followed by prolonged depression. Similar gradation is found with pentobarbitone; its effects are however restricted to shorter periods. Different mechanisms are involved with these two types of drugs, as phenothiazines restore the nystagmus, suppressed by large doses of barbiturate, and vice versa. Interpretation of the results is based on the heterogeneity of the nervous elements in the nystagmus circuit.

The effect of pentobarbitone on vasomotor responses to brain stem stimulation.

Abstract1. The effect of pentobarbitone on vasomotor responses to stimulation in the brain stem was investigated in wake, immobilized rabbits.2. Pressor reactions were either reduced or converted into depressor responses. Depressor reactions were not affected or even intensified by small doses of barbiturate.3. Blood pressure fall after barbiturate may be ascribed to preferential block of pressor centers. The implications of these findings regarding the mechanism of central blood pressure regulation and the treatment of hypertension are discussed.

The influence of chlorpromazine on central nystagmus

Abstract Beyond the optimal frequency range of excitation, central nystagmus is not sustained, but dies out during stimulation. Small doses of chlorpromazine (250 μg/kg) shift the optimum to higher frequencies and prolong the after-nystagmus. Large doses of the drug diminish the intensity of the nystagmus response. This differential action of chlorpromazine is interpreted as a selective effect of small doses on the inhibitory components of the nystagmus circuit, which are distinguished from the excitatory elements by their electrophysiological properties.

The mechanism of nystagmus induced by anesthetics

Abstract Nystagmus induced in rabbits by ether, barbiturate, or intravenous xylocaine depends on head position; it is present in either side position but absent in the prone position. The direction of the nystagmus is opposite for left and right side position. The direction of barbiturate-induced nystagmus is inverse to that of ether-induced nystagmus. Bilateral labyrinthectomy diminishes considerably the positional nystagmus following anesthetics, but does not abolish it completely. After transection of the spinal cord at C1–C2, block of neck proprioceptors by local anesthetic or transection of the optic nerves positional nystagmus can be elicited. It is suggested that placement of the normal animal in side position also produces stimuli capable of initiating positional nystagmus, but the response is blocked by various inhibitory mechanisms which can be eliminated by anaesthetics or by appropriate lesions.