Effect of tilting on the activity of neurons in the vestibular nuclei of the cat

Responses of monkey vestibular-only neurons to translation and angular rotation

Single-unit recordings were obtained from central vestibular neurons in three monkeys during passive head movements. Neurons that discharged in relation to head translation or changes in head orientation, but not eye movement ("vestibular-only," n = 154), were examined in detail. Neuronal discharge rates were analyzed during four stimulus conditions: sinusoidal head translation in the horizontal plane (0.2-4 Hz, 0.2 g peak acceleration), static head tilt in the vertical plane (+/-20 degrees ), oscillatory head tilt (0.5-2 Hz), and sinusoidal angular rotation about an earth-vertical axis (0.5 or 1 Hz). Vestibular-only cells were divided into two groups based on the regularity of their spontaneous discharge rates (CV*). One group (low-sensitivity units) exhibited regular discharge rates (CV* < 0.2), weak discharge modulation during head translation (<25 spikes . s(-1) . g(-1) at f = 1 Hz), and persistent discharge rates related to static head tilt (0.68 spikes . s(-1) . degrees (-1) of head tilt). The second group (high sensitivity neurons) exhibited irregular discharge rates (CV* > 0.2), strong discharge modulation during head translation ( approximately 100 spikes . s(-1) . g(-1) at f = 1 Hz), and little or no change in discharge rate during static head tilt (0.32 spikes . s(-1) . degrees (-1)). The firing rates of some neurons in both groups were modulated during rotation about an earth-vertical axis (42%), but the modulation was greater for neurons classified as high sensitivity units. Previous reports have described neurons similar to the high sensitivity group; however, the low sensitivity or tilt neurons have not previously been characterized. Significantly, recent theoretical models have predicted neurons with discharge patterns similar to those of low- and high-sensitivity neurons.

Response patterns of vestibulospinal neurons to lumbar rotation in decerebrate rats

In vestibulospinal neurons projecting to the lumbar enlargement, activities were modulated in response to alternate tilts of the caudal body around the vertebral axis. Three types of neurons were found: Type I (37.8%) showed faster firing during the ipsilateral side-down tilt; type II (51.1%) showed the reverse pattern; type III were unaffected. Types I and II may be important in the tonic lumbar reflex.

Role of the vestibular nuclei in endothelin-1-induced barrel rotation in rats

The fourth or lateral ventricular injection of endothelin-1 resulted in a dose-dependent increase in the barrel rotation and produced marked induction of c-Fos-positive cells in the vestibular nuclei. The doses of the former injection were lower and had shorter mean latent periods compared with the later injection. c-Fos expression after endothelin-1 injection was prevented by the pretreatment with the endothelin ET(A) receptor antagonist, cyclo(D-alpha-aspartyl-L-propyl-D-valyl-L-leucyl-D-tryptophyl) (BQ-123), the glutamate NMDA receptor antagonist, dizocilpine maleate (MK-801), or the L-type Ca(2+) channel antagonist, verapamil, in addition to the incidence of the rotational behavior. There was a significant difference in c-Fos expression between the right and left medial vestibular nuclei, and the number of c-Fos-labeled neurons in the medial vestibular nucleus was markedly increased on the opposite side of the rotational direction. These results suggest that the elicitation of the barrel rotation may be mediated by endothelin ET(A) receptors, glutamate NMDA receptors, and L-type Ca(2+) channels. The changes in the receptor and channel systems induced by endothelin-1 injections appeared to exert crucial influences on the vestibular nuclei and then on the maintenance of equilibrium. The direction of the barrel rotation has a deep connection with the imbalance of neuronal activity in the left and right medial vestibular nuclei.

Cholinergic and glutamatergic transmission in medial vestibular nucleus neurons responding to lateral roll tilt in rats

The responses of the medial vestibular nucleus (MVN) neurons to lateral tilt and the neurotransmitters mediating otolith information to MVN neurons were investigated using rats. A computer-operated goniometer was tilted 20 degrees clockwise and counterclockwise at an angular speed of 5 degrees /s and paused in the inclined positions for 10 s to record neuronal responses in the static phase. The 185 MVN neurons recorded were classified into eight types according to their responses to tilt (alpha, beta, gamma, delta, epsilon, zeta, eta and theta). A majority showed increased firing in response to ipsilateral tilting and decreased firing in response to contralateral tilting (alpha type: 31.4%) or exhibited the reverse pattern (beta type: 36.8%). Further, other groups of neurons increased (gamma type) or decreased (delta type) firing rates to either side tilting and increased (epsilon and zeta type) or decreased (eta and theta type) firing only on one side. Atropine or L-glutamic acid diethyl ester hydrochloride (GDEE) applied microiontophoretically antagonized tilt-induced firing of alpha type neurons in 58.8% or 60.0%, respectively, and of beta type neurons in 66.7% or 58.3%, respectively. When the effects of atropine and GDEE were examined in the same neurons, antagonizing effects of both drugs on tilt-induced firing were obtained in 28.6% and 40.0% of alpha and beta type neurons, respectively. These results suggest that both acetylcholine and glutamate act as neurotransmitters in the transmission of otolith information to most MVN neurons.

The vestibular nuclei in the macaque monkey

The topography and the main features of the cytoarchitecture of the vestibular nuclear complex in the macaque monkey have been studied in serially cut, Nissl-stained, transverse, parasagittal and horizontal sections. In addition to the four main vestibular nuclei, the topographically closely related small cell groups (f, l, x, y, and z), distinguished by Brodal and Pompeiano ('57) in the cat, have been considered and illustrated. The vestibular nuclear complex in the macaque in general corresponds in topography and architecture to the situation described in some other mammals on which information is available, such as opossum, rabbit, cat, Galago, and man. Some dissimilarities in detail are found. For example, in man the lateral vestibular nucleus differs somewhat from the general pattern, especially in its position, and the small group f, fusing with the descending nucleus, appears to be indistinct; likewise the group y. The latter and the group z appear to be particularly well developed and easily distinguished in the macaque. The question of whether cytoarchitectonic areal differences within the vestibular nuclear complex can be correlated with differences in connections is discussed. Also in this respect there appears to be a general similarity between observations in the macaque and in other mammals. A correlation is most evident in the superior vestibular nucleus, and is rather clear in the medial and lateral vestibular nuclei and for the groups f,x,y, and z, whereas no such correlation can be found in the descending (inferior) nucleus. For several reasons it is difficult to draw reliable conclusions about comparative anatomical trends in the phylogenesis of the vestibular nuclear complex.

Response of central vestibular neurons to horizontal linear acceleration in the rat

Responses of central vestibular neurons to horizontal sinusoidal translation (F:0.25Hz) were recorded in albino rat. 57.5% of vestibular neurons were responding to this stimulation by a modulation of their firing rate, the mean phase angle of the response, averaged from the whole population being 22 +/- 79 deg. lag, relative to the peak of contralateral acceleration. Dynamic characteristics of phase and gain were studied and appeared to be different from previous reports on primary afferents: the gain decreased or was flat with increasing acceleration at one frequency, and the phase lag which was flat in the same conditions increased with increasing frequency. A phase lead of some units has been observed at low frequency (0.1 Hz). Regarding the convergence between otolith and canal inputs on nuclear vestibular neurons, it was shown that the major pattern of convergence is between canal and otolith inputs of same polarity.