Search

We examined the responsiveness of rat vestibular ganglion neurons (VGNs) to exogenous neurotrophic factors, BDNF, NT-3, and GDNF, during postnatal development in dissociated cultures. VGNs were obtained from postnatal days (P) 1, 3, 7, and 14. After two days in culture, the survival of VGNs in control cultures without any exogenous neurotrophic factors was greater in younger (P1 or 3) rats than older (P7 or 14) rats. None of the three neurotrophic factors used facilitated survival of VGNs from older rats while only BDNF facilitated survival of VGNs from younger rats. BDNF was also effective both for increasing neurite sprouting in VGNs from younger rats and for neurite extension in VGNs from each of the postnatal ages. VGNs from P1 rats showed responsiveness to all three neurotrophic factors in their neurite sprouting. Developmental changes of VGNs in their responsiveness to exogenous neurotrophic factors should be considered in treating or preventing neuronal degeneration caused by peripheral vestibular lesions., (Copyright © 2011 Elsevier B.V. All rights reserved.)

Multiunit activity during horizontal sinusoidal motion was recorded from pairs of oculomotor, trochlear, or abducens nerves of an in vitro turtle brainstem preparation that received inputs from intact semicircular canals. Responses of left oculomotor, right trochlear and right abducens nerves were approximately aligned with leftward head velocity, and that of the respective contralateral nerves were in-phase with rightward velocity. We examined the effect of sectioning or injecting lidocaine (1-2 microL of 0.5%) into the right vestibular nerve. Nerve block caused a striking phase shift in the evoked response of right oculomotor and left trochlear nerves, in which (rightward) control responses were replaced by a smaller-amplitude response to leftward table motion. Such "phase-reversed" responses were poorly defined in abducens nerve recordings. Frequency analysis demonstrated that this activity was advanced in phase relative to post-block responses of the respective contralateral nerves, which were in turn phase-advanced relative to pre-block controls. Phase differences were largest (approximately 10 degrees) at low frequencies (approximately 0.1 Hz) and statistically absent at 1 Hz. The phase-reversed responses were further investigated by eliminating individual canal input from the left labyrinth following right nVIII block, which indicated that the activation of the vertical canal afferents is the source of this activity.

Extracellular unit responses were recorded from the vestibular nucleus (VN) and medial longitudinal fasciculus during horizontal head rotation of an in vitro turtle brainstem in which the temporal bones remained attached. Units were characterized as type I or type II based on the responses to ipsiversive or contraversive rotation, respectively. Lidocaine injections (0.5-2 microl of 0.5%) into the root of the eighth cranial nerve within the cranium caused rapid effects on unit responses to head rotation. Responses of type I units were reduced by ipsilateral injection but enhanced following contralateral injection. On the other hand, type II units had their responses increased by ipsilateral injections yet decreased by contralateral injections. In approximately half of the type II cells, decrease of the contraversive response was accompanied by the appearance of latent ipsiversive activity. Our findings not only confirm that each eighth nerve has afferents that drive ipsiversive excitation of both vestibular nuclei but also suggest that both nerves compete to dominate a central neuron's vestibular response. These results may be inconsistent with the push-pull vestibular model in which each nerve drives the central neuron with a complementary response that enhances the vestibular output. An alternate model is described in which vestibular neurons receive bilateral excitation, and that excitatory input is antagonized by crossed inhibition during contraversive motion.

The objective of this study was to determine which general brainstem regions contain interneurons that are critical for mediating vestibulo-sympathetic responses in decerebrate cats, as a prelude for future cell recording studies. Large injections of kainic acid into the lateral reticular formation at levels caudal to the obex abolished sympathetic nerve responses elicited by electrical stimulation of the vestibular nerve. The same lesions also diminished excitatory and some inhibitory components of somato-sympathetic responses elicited by stimulation of the sciatic nerve, raising the possibility that a common pool of interneurons integrates signals from muscle, skin and the vestibular system that reflect body position in space. Large lesions of other brainstem regions containing interneurons involved in regulation of sympathetic activity, including nucleus tractus solitarius, the lateral tegmental field rostral to the obex, and the parabrachial nucleus, had no appreciable effect on the amplitude of either vestibulo-sympathetic or somato-sympathetic responses. Thus, interneurons critical for relaying vestibular and other somatic signals to descending cardiovascular-regulatory pathways appear to be located in the caudal and lateral parts of the medulla.

The generator potential in sensory afferents of frog crista ampullaris was extracellularly recorded from the cut end of the posterior ampullary nerve by means of suction electrodes. A servocontrolled turntable allowed suitable rotatary stimulations. The analysis of the recorded generator potential revealed a different time course from that predicted on the basis of the pendulum model. Adaptation and undershoots in the responses to velocity ramps, steps and sinusoids, were mainly responsible for the deviations, which became very evident only when fairly high acceleration rates were applied. Both adaptation and undershoots were produced presumably by the activation of an electrogenic pump, probably located in nerv terminals contacting the hair cells. In fact, the time course of the generator potential became much more consistent with the predictions from the pendulum model under treatments capable of hindering the ion pump activity.

Spontaneous activity of semicircular canal afferents in the isolated labyrinth of the frog is altered by bath application of cholinergic agonists. Muscarinic agonists can produce an increase in action potential frequency of individual afferents. This increase develops slowly and is prolonged in the time course of its action. Nicotinic agonists can either increase (most cases) or decrease afferent activity. These effects occur rapidly and decay during the period of activation, suggesting desensitization. Muscarinic effects are blocked by prior administration of atropine and nicotinic effects (both increases and decreases in action potential frequency) by curare. Intracellular recordings reveal that the nicotinic effects on afferent action potential frequency are the result of alterations in the frequency of spontaneous synaptic potentials, indicating a presynaptic site of action on the hair cells for these compounds. This conclusion is supported by the fact that in the presence of high Mg2+/low Ca2+, which blocks hair cell release of transmitter, cholinergic agonists do not affect the resting membrane potential of the vestibular afferent. Electrical stimulation of the VIIIth cranial nerve can result in either an increase or a decrease in spontaneous synaptic potential and action potential frequency of an afferent. These effects are blocked by prior administration of curare or of nicotinic agonists. Repetitive or continuous stimulation of the VIIIth nerve results in a reversible reduction of the evoked response, suggesting desensitization. Transection of the VIIIth cranial nerve two weeks prior to recording eliminates these actions of electrical stimulation, but not the responses to cholinergic agonists, indicating that the effects of electrical stimulation are mediated by centrally arising efferents. These findings confirm that acetylcholine is probably the transmitter released from centrally arising vestibular efferents, and, in addition, demonstrate that efferent-mediated effects are predominantly expressed through nicotinic receptors. Studies comparing the effects of isolation of the semicircular canal alone versus the intact labyrinth suggest that the method of isolation may be an important factor in determining whether efferent activity results in a predominant increase or decrease in afferent activity.

It is well known that eye movements are influenced by an animal's state of arousal. Alterations in the dynamic characteristics of the horizontal vestibulo-ocular reflex of adolescent rhesus monkeys induced by changes in the animal's state of arousal were studied using linear system analysis employing both single frequency sinusoidal and white noise rotational stimulations. Arousal changes were induced by a behavioral task and/or the administration of amphetamines (0.5 mg/kg). Results indicate that highly alert animals display vestibulo-ocular reflex gains significantly different from less alert animals. Specifically, the gain of the vestibulo-ocular reflex is closer to unity over a wider range of frequencies in more alert animals. These changes were independent of the method used to maintain a high level of arousal.