Jai H. Ryu

Cis-diamminedichloroplatinum (II) ototoxicity in the guinea pig.

Cochleas from 12 guinea pigs were evaluated using light, scanning, and transmission electron microscopy after systemic administration of cis-diamminedichloroplatinum (cis-DDP). Administration of cis-DDP resulted in loss of the Preyer reflex and degeneration of outer hair cells (OHC) with increased dose. The OHC degeneration was most pronounced in the basal turns of the cochlea with greatest severity in the inner row. Ultrastructural evidence of OHC degeneration included dilatation of the parietal membranes, softening of the cuticular plate, increased vacuolization and increased numbers of lysosome-like bodies in the apical portion of the cell. Supporting cells appeared more sensitive than OHC. Alteration of supporting cell ultrastructure preceded detectable change in OHC. Injury to the supporting cells was noted with intracellular vesiculation and increased autophagocytosis.

Survival of implanted irradiated cartilage.

The survival of irradiated homograft cartilage was investigated in cats at three- to nine-week and 14- to 16-month intervals. Compared to fresh homograft and autograft cartilage grafts, irradiated homografts showed significantly higher resorption and bony replacement. It was also clear that irradiated cartilage survived as a nonviable implant.

The effect of diazepam on vestibular compensation

The effect of diazepam (Valium) on vestibular compensation was studied in the cat. Twelve hemilabyrinthectomized cats were given diazepam (0.4 mg/kg/day — physiological dose; 1.6 mg/kg/day — toxic dose) chronically during the compensation period. Thirteen additional hemilabyrinthectomized animals were used as controls. The neural activity of the medial vestibular nucleus was monitored as well as the observations made on nystagmus and postural co‐ordination. The results indicate that diazepam does not delay vestibular compensation in the cat.

The effect of blood pressure on compression-induced facial nerve neuropraxia in the cat.

The facial nerves of 21 adult anesthetized cats were exposed from the parotid gland to the orbicularis oculi muscle. The summated action potential (SAP) of the orbicularis oculi muscle was recorded. A calibrated pressure block was applied to the intact facial nerve between the stimulating electrode and the muscle. Pressures of between 150 and 200 mm Hg caused a rapid stable neuropraxia. In ten animals the blood pressure was elevated by a constant levarterenol infusion. In 11 animals the blood pressure was elevated by coarcting the abdominal aorta and volume overloading the rostral vascular system. In both groups, in all animals, when the systolic blood pressure exceeded the pressure applied to the nerve, a substantial increase in SAP amplitude was noted. If the systolic pressure was allowed to fall below the pressure on the nerve, the neuropraxia rapidly returned. This data suggest that within a physiologic pressure range of 150 to 200 mm Hg there is a reversible ischemic phase of compression neuropraxia and it is in complete accord with the earlier work of Devriese.

Effects of ketamine on the adaptive responses of second-order vestibular neurons of the cat

The adaptive characteristics of 80 neurons in the vestibular nuclei of ketamine-anesthetized cats were investigated. All types I and II neurons responded to stimulation of the horizontal semicircular canals by an 8 degree/sec2 stepwise angular acceleration of 40 sec. For the purpose of this study, vestibular adaptation was defined as a decline in response at a rate of greater than -0.01 spikes/sec/sec. According to this criterion, 71 neurons (89 per cent) behaved as adapting neurons, and nine (11 per cent) showed either no adaptation or reverse adaptation. The rate of adaptation varied from neuron to neuron; the average rate was -0.312 spikes/sec/sec. The average resting discharge rate, the maximum response level, and the average adaptation rate were compared with similar neural responses in barbiturate-anesthetized cats. Although the average resting discharge rates of the two groups showed no significant difference, the average adaptation rate and the maximum response level of the ketamine-anesthetized group were significantly higher than those of the barbiturate-anesthetized group. These data suggest that the gain of the second-order neurons is higher and more actively modulated in more alert (ketamine-anesthetized) cats than in barbiturate-anesthetized cats.

Second-Order Vestibular Adaptation and Cupular Dynamics

The magnitude of initial response and slope of continued response of 45 neurons in the vestibular nuclei of cats to constant and interrupted accelerations were investigated. Eighty-eight percent exhibited adaptation, the magnitude of which was proportional to the stimulus. Comparison of slope and intercept of the response to continuous acceleration before and after a superimposed inhibitory velocity change suggests that the adaptation phenomenon is unrelated to cupular position.

Responses to step and repeated impulse accelerations in second order vestibular neurons of the cat

The characteristics of response of 98 vestibular nuclei neurons were investigated in the barbiturate anesthetized cat. All type I and type II neurons (tonic and kinetic) responded to stimulation of the horizontal canals with step and impulse accelerations. Seventy-seven (79 per cent) neurons of all types demonstrated adaptation, while the remaining 21 (21 per cent) neurons showed either reverse adaptation or no adaptation. When the interval between impulses with impulse accelerations reached a critical minimum, the averaged response was remarkably like the response to step acceleration, including the rate of adaptation. A previously undescribed response decline to repetitive impulse acceleration was seen, which, while sharing some characteristics with habituation, appears to be more closely related to adaptation. Furthermore, the data suggest that adaptation seen at the vestibular nuclear level is more likely a central phenomenon rather than a peripheral one.

Second-order neural responses after contralateral vestibular nerve sectioning

The characteristics of the response to an 8 degrees/sec2 acceleration of 41 neurons in the medial vestibular nucleus of the cat following vestibular nerve sectioning were compared with those of 73 control neurons at the same site. In 14 of the vestibular nerve-sectioned animals the same neuron was recorded before and after sectioning. No difference was found between the resting discharge rates, maximal response rates, and rates of adaptation of the two groups. Following contralateral vestibular nerve sectioning Type II neurons were still plentiful, suggesting many receive ipsilateral labyrinthine innervation.

NONLINEAR CHARACTERISTICS OF SINGLE NEURONS IN THE VESTIBULAR NUCLEI

The characteristics of response of 99 vestibular nuclei neurons were investigated in the cat anesthetized with pentobarbital sodium. All neurons responded to stimulation of the horizontal semicircular canals (HSCC) with prolonged 2, 4, 6, or 8 degrees/second2 angular acceleration. Neural response was recorded utilizing the single-neuron recording technique. The vestibular neurons responded nonlinearly to stimulation of the HSCC. Specifically (1) the time required to reach the maximum response level decreased nonlinearly with an increase in stimulus magnitude (SM); (2) the maximum response level increased nonlinearly with the increase in SM: (3) the neural response to excitatory stimulus was not the mirror image of that to an inhibitory stimulus. Several existing mathematical models were simulated by utilization of the IBM Continuous System Modeling Program, account for the nonlinear response of the vestibular nuclei neurons. The nonlinearities observed in the responses may be due to (1) nonlinear behavior of the end organ, (2) the influence of the CNS including the vestibular efferent system, or (3) the inherent neural properties of the vestibular nuclei.